4
Anadolu University
Materials Science and Engineering Department
MLZ 306
Materials Processing
Laboratory-II
2015-2016
Course Instructors
Laboratory Instructors
Prof. Dr. Cemail AKSEL
Prof. Dr. Ferhat KARA
Prof. Dr. Gürsoy ARSLAN
Assoc. Prof. Dr. Dilek TURAN
Assoc. Prof. Dr. A. Tuğrul SEYHAN
Assist. Prof. Dr. Emrah DÖLEKÇEKİÇ
Assist. Prof. Dr. G. İpek NAKAŞ
Res. Assist. Özlem TUZLACI
Res. Assist. Umut SAVACI
Res. Assist. K. Burak DERMENCİ
Res. Assist. S. Çağrı ÖZER
Res. Assist. Hande MARULCUOĞLU
Res. Assist. H. Şule ÇOBAN
Res. Assist. Burak DEMİR
Celal ÇELEBİ
Ramazan KALE
Merve GEÇGİN
Ufuk AKKAŞOĞLU
Ayşegül AKYÜREKLİ
Gülden TOK
Şükran GÜRCAN
Course Coordinator
Dr. H. Boğaç POYRAZ
MLZ 306
Materials Processing Laboratory-II
Instructors:
Coordinator:
MLZ 306 P
Assoc. Prof. Dr. A. Tuğrul SEYHAN
MLZ 306 S
Prof. Dr. R. Ferhat KARA
MLZ 306 W
Prof. Dr. Gürsoy ARSLAN
MLZ 306 X
Assoc. Prof. Dr. Dilek TURAN
MLZ 306 Y
Assist. Prof. Dr. Emrah DÖLEKÇEKİÇ
MLZ 306 Z
Prof. Dr. Cemail AKSEL
MLZ 306 T
Assist. Prof. Dr. G. İpek NAKAŞ
Dr. H. Boğaç POYRAZ
GRADING TABLE
Exam
II.
MIDTERM
Exam Type
6 Quizzes*
(Exp#1, Exp#2, Exp#3,
50 %
Exp#4, Exp#5, Exp#6)
6 Reports*
FINAL
Percentage of
Exam
(Exp#1, Exp#2, Exp#3,
50 %
Exp#4, Exp#5, Exp#6)
*No quiz and report for Experiment #7.
IMPORTANT NOTE : Minimum qualifying grade in this course is 50.
Experiment ?
Time ?
a) Heat Treatment of Steels
1 – 2 pm
b) Microstructure Analysis in Metallic Materials
2 – 3 pm
c) Jominy – End Quench Test
3 – 4 pm
d) Hardness Testing Techniques
4 – 5 pm
EXP #1
EXP #2
a) NDT Experiments
2 – 3.30 pm
b) Fatigue-Charpy Impact-Tension Testing
3.30 – 5 pm
Where ?
MatSE Metallography Lab.-2
#MLZ 118
Instructors ?
Umut SAVACI
Özlem TUZLACI
MatSE Metallography Lab.-1
#MLZ 119
Celal ÇELEBİ
Faculty of Aeronautics and Astronautics
A Block, Materials Lab.
Ramazan KALE
MatSE Lab.
#MLZ 232
MatSE Lab.
#MLZ/S 207
Merve GEÇGİN
S. Çağrı ÖZER
Hande MARULCUOĞLU
Ufuk AKKAŞOĞLU
MatSE Process Lab.
#MLZ 123
K. Burak DERMENCİ
H. Şule ÇOBAN
a) Sonic Modulus
1 – 3 pm
b) Porosimetry
3 – 5 pm
a) Electrolytic Reduction of Zinc
1 – 3 pm
b) Corrosion and Cathodic Protection
3 – 5 pm
a) Electrical & Structural Characterization of Thin Films and Coatings
1 – 3 pm
MatSE Thin Film Lab.
#MLZ 129
Burak DEMİR
b) Electrical Property Measurements
3 – 5 pm
MatSE Lab.
#MLZ 232
Ayşegül AKYÜREKLİ
Burak DEMİR
EXP #6
Polymer Processing
1 – 5 pm
MatSE Polymer Lab.
#MLZ 130
Şükran GÜRCAN
EXP #7
Standard Tests
2 – 5 pm
Ceramic Research Center,
Standard Tests Lab.
Gülden TOK
EXP #3
EXP #4
EXP #5
MLZ 306 Materials Processing Laboratory-II
SCHEDULE
12.02.2016
Lab Meeting (MLZ M1)
All groups
19.02.2016
Technical Excursion – (BİLECİK DEMİR ÇELİK A.Ş.)
All groups
Group P - NDT Experiments,Fatigue-Charpy Impact-Tension Tests
Group S - Standard Tests
Rotative Experiments
Group W - Polymer Processing
26.02.2016
Group X - Elec. & Struc. Charac. of Thin Films and Coatings - Electrical Property Measurements
Report Submission Due By
Group Y - Electrolytic Reduction of Zinc - Corrosion and Cathodic Protection
04.03.2016
Group Z - Sonic Modulus - Porosimetry
Group T - Heat Treatment-Microstructure Analysis-Jominy-Hardness Testing
Group P - Heat Treatment-Microstructure Analysis-Jominy-Hardness Testing
Group S - NDT Experiments, Fatigue-Charpy Impact-Tension Tests
Rotative Experiments
Group W - Standard Tests
04.03.2016
Group X - Polymer Processing
Report Submission Due By
Group Y - Electrical & Structural Characterization of Thin Films - Electrical Property Measurements
11.03.2016
Group Z - Electrolytic Reduction of Zinc - Corrosion and Cathodic Protection
Group T - Sonic Modulus - Porosimetry
Group P - Sonic Modulus - Porosimetry
Group S - Heat Treatment-Microstructure Analysis-Jominy-Hardness Testing
Rotative Experiments
Group W - NDT Experiments, Fatigue-Charpy Impact-Tension Tests
Group X - Standard Tests
11.03.2016
Report Submission Due By
Group Y - Polymer Processing
25.03.2016
Group Z - Electrical & Structural Characterization of Thin Films - Electrical Property Measurements
Group T - Electrolytic Reduction of Zinc - Corrosion and Cathodic Protection
1st Midterm Week (14-19 March, 2016)
Group P - Electrolytic Reduction of Zinc - Corrosion and Cathodic Protection
Rotative Experiments
25.03.2016
Group S - Sonic Modulus - Porosimetry
Group W - Heat Treatment-Microstructure Analysis-Jominy-Hardness Testing
Group X - NDT Experiments, Fatigue-Charpy Impact-Tension Tests
Report Submission Due By
Group Y - Standard Tests
01.04.2016
Group Z - Polymer Processing
Group T - Electrical & Structural Characterization of Thin Films - Electrical Property Measurements
Group P - Electrical & Structural Characterization of Thin Films - Electrical Property Measurements
Group S - Electrolytic Reduction of Zinc - Corrosion and Cathodic Protection
Rotative Experiments
01.04.2016
Group W - Sonic Modulus – Porosimetry
Group X - Heat Treatment-Microstructure Analysis-Jominy-Hardness Testing
Report Submission Due By
Group Y - NDT Experiments, Fatigue-Charpy Impact-Tension Tests
08.04.2016
Group Z - Standard Tests
Group T - Polymer Processing
Group P - Polymer Processing
Group S - Electrical & Structural Characterization of Thin Films - Electrical Property Measurements
Rotative Experiments
08.04.2016
Group W - Electrolytic Reduction of Zinc - Corrosion and Cathodic Protection
Group X - Sonic Modulus - Porosimetry
Report Submission Due By
Group Y - Heat Treatment-Microstructure Analysis-Jominy-Hardness Testing
15.04.2016
Group Z - NDT Experiments, Fatigue-Charpy Impact-Tension Tests
Group T - Standard Tests
Group P - Standard Tests
Group S - Polymer Processing
Rotative Experiments
15.04.2016
Group W - Electrical & Structural Characterization of Thin Films - Electrical Property Measurements
Group X - Electrolytic Reduction of Zinc - Corrosion and Cathodic Protection
Report Submission Due By
Group Y - Sonic Modulus - Porosimetry
06.05.2016
Group Z - Heat Treatment-Microstructure Analysis-Jominy-Hardness Testing
Group T - NDT Experiments, Fatigue-Charpy Impact-Tension Tests
2nd Midterm Week (25-30, April, 2016)
06.05.2016
Technical Excursion – (not confirmed, to be announced later)
All groups
13.05.2016
Technical Excursion – (not confirmed, to be announced later)
All groups
Final Week (14-27, 2016)
GENERAL INSTRUCTIONS FOR
MLZ 306 MATERIALS PROCESSING LABORATORY-II
1. There will be one midterm and one final exam regarding to this course. All quizzes and
experiment reports will contribute to the overall grade. The midterm exam will cover “6
quizzes” (Exp#1, Exp#2, Exp#3, Exp#4, Exp#5, Exp#6) and the final exam will cover “6
reports” (Exp#1, Exp#2, Exp#3, Exp#4, Exp#5, Exp#6). There will be no quiz and report
for the Exp#7.
2. It is extremely important that you read each experiment and basic references prior to the
lab. The lab instructor will ask general questions during the lab to test your understanding
of the lab.
3. It is obligatory to wear laboratory apron unless the lab instructor tells otherwise. Students
without aprons (lab coats) will not be admitted to the laboratory.
4. The lab groups must be present in the room/building where the lab will take place (stated
in the lab manual) 5 minutes before the lab starts. Students are obliged to learn the location
of the labs before the labs begin.
5. All labs (except Exp#7) will be evaluated with a quiz and a report. It’s definitely obligatory
to have the lab. manual with you and read the corresponding lab. chapter before coming
to the lab.
6. Before each lab. (except Exp#7) starts, there will be a quiz composed of 2-3 questions.
7. Students are responsible to submit the lab reports to the corresponding lab instructor’s box
on Friday until 12:00. These are not formal reports although neatness, organization, ect.,
of the report, as well as proper execution of the experiment will count towards your grade.
The lab instructor will grade the lab. reports within a week and will post the results. If you
wish to discuss the grade, make an appointment to see the lab instructor at his/her
convenience. A copy of the graded reports will be handed to you upon your request if
needed.
8. The nature of working in groups implies that there should be cooperation and discussion
between members of the group and the lab instructor. It is, however, expected that when
students prepare their reports, that they do so individually using their own words and
interpretation. Plagiarizing or blatant copying of a report or reference will result in an
automatic zero for that lab for the first offense. A second offense will result in an automatic
FF grade for the course.
9. Students must attend each lab on the specified group. The students will be admitted to the
class within the first half an hour. Lab reports must be handed in on time;
otherwise 10% will be deducted from the mark for each day late.
10. Minimum qualifying grade for this course is determined as 50.
11. Lab. manuals will be available on the department web-site: http://matse.anadolu.edu.tr
12. Instructions for Informal Lab Reports
(a) Each experiment should be organized in the report neatly and carefully;
For example;
Student Name
:
Student Number
:
Lab. Group Number :
I.
II.
III.
IV.
V.
VI.
VII.
VIII.
Date:
Experiment Name
Equipment & Materials
Experimental Methods & Procedures
Important Parameters
Results and Discussion
Conclusions
Notes
References (if used)
(b) All the above will be neatly written, pasted, taped, etc. into the lab. report.
(c) The reports must be written in English and will be prepared by handwriting, handdrawing, and/or by the use of a computer. This will be announced by the lab instructor
at the end of each experiment.
(d) The reports will not exceed “2 PAGES”, excluding Tables and Figures. Reports
exceeding 2 pages will not be taken into account (unless the lab instructor requests
otherwise).
(e) The reports will cover only 1-page of background information, if necessary. The
second page will include the activities performed during the laboratory hour,
comments, and the answers to the lab instructor’s questions and demands, if any.
*The requests of the lab instructors may be different from what is written in “General
Instructions”. In that case, the reports will be prepared according to the requests of the lab
instructors. If the lab. report is prepared by the use of a computer, both the hardcopy and
digital copy will be handed to the corresponding lab. instructor.
EXPERIMENT # 1: HEAT TREATMENT OF STEELS –
MICROSTRUCTURE ANALYSIS IN METALLIC MATERIALS –
JOMINY - END QUENCH TEST – HARDNESS TESTING TECHNIQUES
(a) HEAT TREATMENT OF STEELS
Objective:
Investigation of conventional heat treatment procedures used to tailor the
properties of steels. Effects of heat treatment and different cooling
conditions on microstructure and hardness.
Materials:
AISI 4140 alloy steel, heat treatment oil (Petrofer isorapid 277hm)
Equipment:
Box furnace, oil and water baths.
BACKGROUND
Metals and alloys for many structural and mechanical applications need to be hard,
strong and tough. These required properties are dependent mainly on composition, and
microstructure of the steel. In microstructure, the size and distribution of the phases and grains
controls the mechanical properties. For a metal finer grains in the microstructure yields with
greater hardness, strength and toughness. But the mechanical properties depend strongly on the
phases present than the grains.
Heat treatment is a combination of heating and cooling operations applied to any metal
or alloy in the solid state in order to obtain desired microstructure and mechanical properties.
The microstructure of any heat treated metal is deduced using Continuous Cooling
Transformation (CCT) which is related to the Time-Temperature-Transformation (TTT)
diagrams of the selected metal sample. The CCT diagram for 4140 steel is shown in Figure 1.
EXPERIMENTAL PROCEDURE
Heating the furnace
Placing the steel samples into the furnace
Stop the heating of furnace
Quenching
with water
Quenching
with oil
Cooled
at furnace
Sample Preparation
Microstructure Investigation
Hardness Tests
Cooled
at air
Figure 1. CCT diagram of 4140 steel. [1]
Figure 2. TTT diagram of 4140 steel [2]
References:
[1] Voort, G.F.V., Atlas of Time-Temperature Diagrams for Irons and Steels
[2] ASM, Atlas of isothermal transformation and cooling transformations diagrams.
(b) MICROSTRUCTURE ANALYSIS IN METALLIC MATERIALS
Objective:
The purpose of this laboratory is to acquaint students with the manner in
which metallurgical specimens are prepared for metallographic study, to
correlate the relationship between the schematic sketches and the actual
appearances of the microstructures.
Materials:
A set of polished and etched identified and unidentified metal specimens.
Equipment:
Metallographic preparation facilities and metallurgical microscopes.
BACKGROUND
Introduction
A properly prepared metallographic sample can be aesthetically pleasing as well as revealing
from a scientific point of view. The purpose of this practical is to understand how to prepare
and interpret metallographic samples systematically. The process is illustrated below.
•
•
•
•
•
•
•
Gather information about chemical composition, heat treatment, processing, phase
diagram.
Cut representative sample, noting plane of section relative to prominent features (e.g.
long direction of rod).
Mount sample, grind and polish.
Examine unetched sample.
Etch lightly and examine again.
Etch further if necessary.
Compare with microstructure expected from equilibrium phase diagram
You are provided with these samples: hypo-eutectoid steel, eutectoid steel, hyper- eutectoid
steel, martensite steel, pearlitic-ferritic grey cast iron, laminar (flake) graphite grey cast iron,
Spheroidal (nodular) graphite cast iron, white cast iron, brass (alloy of copper and zinc).
Sample Mounting
Small samples can be difficult to hold safely during grinding and polishing operations, and
their shape may not be suitable for observation on a flat surface. They are therefore mounted
inside a polymer block This can be done cold using two components which are liquid to start
with but which set solid shortly after mixing. Cold mounting requires very simple equipment
consisting of a cylindrical ring which serves as a mould and a flat piece of glass which serves
as the base of the mould. The sample is placed on the glass within the cylinder and the mixture
poured in and allowed to set Cold mounting takes about 40 minutes to complete.
In hot-mounting the sample is surrounded by an organic polymeric powder which melts under
the influence of heat (about 200 oC). Pressure is also applied by a piston, ensuring a high
quality mould free of porosity and with intimate contact between the sample and the polymer.
This is not the case with cold mounting where the lack of proper contact and the presence of
porosity can cause problems such as the entrappment and seepage of etchant during the final
stages of preparation. Consequently, hot-mounting should be the preferred way of
encapsulating specimens assuming that time and resources permit, and assuming that the heat
involved in the process does not influence the sample .
Grinding and Polishing
Grinding is done using rotating discs covered with silicon carbide paper and water. There are a
number of grades of paper, with 180, 240, 400, 1200, grains of silicon carbide per square inch.
180 grade therefore represents the coarsest particles and this is the grade to begin the grinding
operation. Always use light pressure applied at the centre of the sample. Continue grinding
until all the blemishes have been removed, the sample surface is flat, and all the scratches are in
a single orientation. Wash the sample in water and move to the next grade, orienting the
scratches from the previous grade normal to the rotation direction. This makes it easy to see
when the coarser scratches have all been removed. After the final grinding operation on 1200
paper, wash the sample in water followed by alcohol and dry it before moving to the polishers.
The polishers consist of rotating discs covered with soft cloth impregnated with diamond
particles (6 and 1 micron size) and an oily lubricant. Begin with the 6 micron grade and
continue polishing until the grinding scratches have been removed. It is of vital importance that
the sample is thoroughly cleaned using soapy water, followed by alcohol, and dried before
moving onto the final 1 micron stage. Any contamination of the 1 micron polishing disc will
make it impossible to achieve a satisfactory polish.
Etching
The purpose of etching is two-fold. Grinding and polishing operations produce a highly
deformed, thin layer on the surface which is removed chemically during etching. Secondly, the
etchant attacks the surface with preference for those sites with the highest energy, leading to
surface relief which allows different crystal orientations, grain boundaries, precipitates, phases
and defects to be distinguished in reflected light microscopy.
2% Nital Ferric Chloride
Steel
Stainless steel
Copper & alloys
-
Sodium Hydroxide
Aluminium & alloys
Zinc & alloys
Magnesium & alloys
A polished sample is etched using a cotton tip dipped in the etchant. Etching should always be
done in stages, beginning with light attack, an examination in the microscope and further
etching only if required. If you overetch a sample on the first go then the polishing procedure
will have to be repeated.
Metallographic Imaging Modes
The reflected light microscope is the most commonly used tool for the study of the
microstructure of metals. It has long been recognized that the microstructure of metals and
alloys has a profound influence on many of the properties of the metal or alloy. Mechanical
properties (strength, toughness, ductility, etc.) are influenced much more than physical
properties (many are insensitive to microstructure). The structure of metals and alloys can be
viewed at a wide range of levels - macrostructure, microstructure, and ultra-microstructure.
In the study of microstructure, the metallographer determines what phases or constituents are
present, their relative amounts, and their size, spacing, and arrangement. The microstructure is
established based upon the chemical composition of the alloy and the processing steps.
Microstructures
•
Microstructures of Steel
Steel is an alloy consisting mostly of iron, with a carbon content between 0.2 and 1.7 or 2.04%
by weight (C:1000–10,8.67Fe), depending on grade. Carbon is the most cost-effective alloying
material for iron, but various other alloying elements are used such as manganese, chromium,
vanadium, and tungsten. Carbon and other elements act as a hardening agent, preventing
dislocations in the iron atom crystal lattice from sliding past one another. Varying the amount
of alloying elements and form of their presence in the steel (solute elements, precipitated
phase) controls qualities such as the hardness, ductility and tensile strength of the resulting
steel. Steel with increased carbon content can be made harder and stronger than iron, but is also
more brittle. The maximum solubility of carbon in iron (in austenite region) is 2.14% by
weight, occurring at 1149 °C; higher concentrations of carbon or lower temperatures will
produce cementite. Alloys with higher carbon content than this are known as cast iron because
of their lower melting point. Steel is also to be distinguished from wrought iron containing only
a very small amount of other elements, but containing 1–3% by weight of slag in the form of
particles elongated in one direction, giving the iron a characteristic grain. It is more rustresistant than steel and welds more easily. It is common today to talk about 'the iron and steel
industry' as if it were a single entity, but historically they were separate products.
Microstructure of hyper eutectoid steel
•
Microstructures of Cast Irons
Cast irons with a composition equivalent to about 4.3% C solidify as a eutectic. Because cast
irons are not simple binary Fe-C alloys, it is usual practice to calculate the carbon equivalent
(CE) value which is the total carbon content plus one-third the sum of the silicon and
phosphorus contents. If the CE is > 4.3, it is hypereutectic; if it is < 4.3, it is hypoeutectic.
In the Fe-C system, the carbon may exist as either cementite, Fe,C, or as graphite. So the
eutectic reaction is either liquid transforming to austenite and cementite at about 1130°C or
liquid transforming to austenite and graphite at about 1135°C. Addition of elements such as
silicon promote graphite formation. Slow cooling rates promote graphite formation, while
higher rates promote cementite. The eutectic grows in a cellular manner with the cell size
varying with cooling rate which influences mechanical properties.
Gray lron
Figure 1 shows interdendritic flake graphite in a hypoeutectic alloy where proeutectic austenite
forms before the eutectic reaction. This type of graphite has been given many names. In the US
it is referred to as Type D (ASTM A247) or as undercooled graphite. It was thought that the
fine size of the graphite might be useful, but it is not technically useful as it always freezes last
into a weak interdendritic network.
Figure 2 shows more regularly-shaped graphite flakes in an alloy of higher carbon content,
although still hypoeutectic. While flake lengths in Figure 1 are roughly 15-30µm, flake lengths
in Figure 2 are in the 60-120µm range. Figure 3 shows somewhat coarser flakes (250-500µm
length range) in a higher carbon content cast iron .
Other graphite forms are also observed. For example, Figure 4 shows disheveled graphite
flakes in a casting. Note that a few nodules are present. This appears to be a mix of B- and Dtype flakes. Figure 5 shows a hypereutectic gray iron where very coarse flakes form before the
eutectic which is very fine. This is similar to C-type graphite.
Nodular Iron
The addition of magnesium ('inoculation') desulfurizes the iron and causes the graphite to grow
as nodules rather than flakes. Moreover, mechanical properties are greatly improved over gray
iron; hence, nodular iron is widely known as 'ductile iron'.
Nodule size and shape perfection can vary depending upon composition and cooling rate.
Figure 6 shows fine nodules, about 15-30µm in diameter, while Figure 7 shows coarser nodules
(about 30-60µm diameter) in two ductile iron casts . Note that the number of nodules per unit
area is much different, about 350 per mm2 vs.125 per mm2, respectively.
Compacted Graphite
Compacted graphite is a more recent development made in an effort to improve the mechanical
properties of flake gray iron. Figure 8 shows an example where the longest flakes are in the 60120µm length range. Compare these flakes to those shown in Figures 2 and 3.
Figure 1
Figure 2
Figure 3
Figure 4
Figure 5
Figure 6
Figure 7
Figure 8
(c) JOMINY - END QUENCH TEST
Objective:
Understanding the effect of cooling rate on the hardness and formation of
hardenability curve of the selected steel sample.
Materials:
AISI 4140 alloy steel test sample (ASTM A225).
Equipment:
Box furnace, jominy end quench test apparatus
BACKGROUND
The Jominy End-Quench test is a standard procedure in order to measure hardenability
of steel. Hardenability is a term that is used to describe the ability of an alloy to be hardened by
the formation of martensite as a result of a given heat treatment.
The hardenability of a steel depends on:
(1) the composition of the steel
(2) the austenitic grain size
(3) the structure of the steel before quenching.
In general hardenability increases with carbon content and with alloy content. The most
important factor influencing the maximum hardness that can be obtained is mass of the metal
being quenched. In a small section, the heat is extracted quickly, thus exceeding the critical
cooling rate of the specific steel and this part would thus be completely martensitic. The critical
cooling rate is that rate of cooling which must be exceeded to prevent formation of
nonmartensite products. As section size increases, it becomes increasingly difficult to extract
the heat fast enough to exceed the critical cooling rate and thus avoid formation of
nonmartensitic products. Hardenability of all steels is directly related to critical cooling rates
[1].
Experimental Procedure
AISI 4140 steel test sample prepared according to the ASTM A255 [2] test standard
(Figure 1). Preheated samples are placed to the test set-up system and then water is sprayed to
the sample as shown in Figure 2 for maximum of 10 minutes. After cooling to the room
temperature, two flats 180° apart shall be ground to a minimum depth of 0.015 in. (0.38 mm)
along the entire length of the bar and Rockwell C hardness measurements made along the
length of the bar. Hardness measurements are made for the first 50 mm (2 in.) along each flat
for the first 12.8 mm, hardness readings are taken at 1.6-mm intervals, and for the remaining
38.4 mm every 3.2 mm. A hardenability curve is produced when hardness is plotted as a
function of position from the quenched end.
Figure 1. Jominy Test Sample[3]
Figure 2. Jominy Test set-up[2]
References
[1] http://web.itu.edu.tr/~arana/jominy.pdf
[2] ASTM A255-10 standard test methods for determining hardenability of steel
[3] Shackelford, IF, Introduction to materials science
(d) HARDNESS TESTING TECHNIQUES
Objective:
Investigation of conventional heat treatment procedures used to tailor the
properties of steels. Effects of heat treatment and different cooling
conditions on microstructure and mechanical properties.
Materials:
AISI 4140 alloy steel heat treated test samples.
Equipment:
Hardness Test Equipment (EMCOTEST M1)
Rockwell Hardness Testing
Rockwell hardness testing is the most widely used method for determining hardness,
primarily because the Rockwell test is simple to perform and does not require highly skilled
operators. By use of different loads (forces) and indenters, Rockwell hardness testing can
determine the hardness of most metals and alloys, ranging from the softest bearing materials to
the hardest steels. Readings can be taken in a matter of seconds with conventional manual
operation and in even less time with automated setups. Optical measurements are not required;
all readings are direct [1].
Figure 1. Principles of Rockwell Hardness Testing
The Rockwell hardness test is a very useful and reproducible one provided that a number of
simple precautions are observed. Most of the points filled below apply equally well to the other
hardness tests:
•
The indenter and anvil should be clean and well seated.
•
The surface to be tested should be clean and dry, smooth, and free from oxide. A roughground surface is usually adequate for the Rockwell test.
•
The surface should be flat and perpendicular to the indenter.
•
Tests on cylindrical surfaces will give low readings, the error depending on the
curvature, load, indenter, and hardness of the material. Theoretical and empirical
corrections for this effect have been published.
•
The thickness of the specimen should be such that a mark or bulge is not produced on
the reverse side of the piece. It is recommended that the thickness be at least 10 times
the depth of the indentation. The spacing between indentations should be three to five
times the diameter of the indentation.
•
The speed of application of the load should be standardized. This is done by adjusting
the dashpot on the Rockwell tester. Variations in hardness can be appreciable in very
soft materials unless the rate of load application is carefully controlled [2].
Brinell Hardness Testing
The brinell hardness testing is a simple indentation test for determining the hardness of
a wide variety of materials. The test consists of applying a constant load (force) usually
between 500 and 3000 kgf. for a specified time (10 to 30 s) using a 5- or 10-mm-diam hardened
steel or tungsten carbide ball on the flat surface of a workpiece. The time period is required to
ensure that plastic flow of the work metal has ceased. After removal nf the load, the resultant
recovered round impression is measured in millimeters using a low-power microscope [1].
Hardness is determined by taking the mean diameter of the indentation (two readings
at right angles to each other) and calculating the Brinel hardness number (BH) by dividing the
applied load by the surface area of the indentation according to the following formula:
𝐵𝐻 =
𝐹
𝜋
!
!
2 𝐷. (𝐷 − 𝐷 − 𝐷! )
Where;
BH= the Brinell hardness number
F = the imposed load in kg
D = the diameter of the spherical indenter in mm
Di = diameter of the resulting indenter impression in mm
Vickers Hardness Testing
The Vickers indenter (Fig. 2) is a highly polished, pointed, square-based pyramidal
diamond with face angles of 136o. With the Vickers indenter, the depth of indentation is about
one seventh of the diagonal length.
When the mean diagonal of the indentation has been determined the Vickers hardness
may be calculated from the formula, but is more convenient to use conversion tables. The
Vickers hardness should be reported like 800 HV/10, which means a Vickers hardness of 800,
was obtained using a 10 kgf force. Several different loading settings give practically identical
hardness numbers on uniform material, which is much better than the arbitrary changing of
scale with the other hardness testing methods. The advantages of the Vickers hardness test are
that extremely accurate readings can be taken, and just one type of indenter is used for all types
of metals and surface treatments. Although thoroughly adaptable and very precise for testing
the softest and hardest of materials, under varying loads, the Vickers machine is a floor
standing unit that is more expensive than the Brinell or Rockwell machines [3].
Figure 2. Diamond pyramid indenter used for the Vickers test and resulting
indentation in the workpiece
The indenter employed in the Vickers test is a square-based pyramid whose opposite
sides meet at the apex at an angle of 136º. The diamond is pressed into the surface of the
material at loads ranging up to approximately 120 kilograms-force, and the size of the
impression (usually no more than 0.5 mm) is measured with the aid of a calibrated microscope.
The Vickers number (HV) is calculated using the following formula:
136!
2𝐹 sin 2
𝐹
𝐻𝑉 =
→ 𝐻𝑉 = 1.854 !
!
𝑑
𝑑
Where;
F= Load in kgf
d = Arithmetic mean of the two diagonals, d1 and d2 in mm
HV = Vickers hardness
Knoop Hardness Testing
The Knoop indenter (Fig. 3) is a highly polished, rhombic-based pyramidal diamond
that produces a diamond-shaped indentation with a ratio between long and short diagonals of
about 7 to 1. The pyramid shape used has an included longitudinal angle of 172° 30' and an
included transverse angle of 130° 0'. The depth of the indentation is about 1/30th of its length
[1].
Figure 3. Pyramidal Knoop indenter and resulting indentation in the work piece
The diamond indenter employed in the Knoop test is in the shape of an elongated foursided pyramid, with the angle between two of the opposite faces being approximately 170º and
the angle between the other two being 130º. Pressed into the material under loads that are often
less than one kilogram-force, the indenter leaves a four-sided impression about 0.01 to 0.1 mm
in size. The length of the impression is approximately seven times the width, and the depth is
1/30 the length. Given such dimensions, the area of the impression under load can be calculated
after measuring only the length of the longest side with the aid of a calibrated microscope. The
final Knoop hardness (HK) is derived from the following formula [3]:
!
!
𝐻𝐾 = ! = !"!
Where;
P= the applied load, k gf
A= the unrecovered projected area of indentation, mm2
L= the measured length of long diagonal, mm
C= constant to 0.07028
Fracture Toughness
Fracture toughness is an indication of the amount of stress required to propagate a
preexisting flaw. It is a very important material property since the occurrence of flaws is not
completely avoidable in the processing, fabrication, or service of a material/component. Flaws
may appear as cracks, voids, metallurgical inclusions, weld defects, design discontinuities, or
some combination thereof. Since engineers can never be totally sure that a material is flaw free,
it is common practice to assume that a flaw of some chosen size will be present in some
number of components and use the linear elastic fracture mechanics (LEFM) approach to
design critical components. This approach uses the flaw size and
features, component geometry, loading conditions and the material
property called fracture toughness to evaluate the ability of a
component containing a flaw to resist fracture [4].
A parameter called the stress-intensity factor (K) is used to
determine the fracture toughness of most materials. A Roman numeral
subscript indicates the mode of fracture and the three modes of
fracture are illustrated in the image to the right. Mode I fracture is the
condition in which the crack plane is normal to the direction of largest
tensile loading. This is the most commonly encountered mode and,
therefore, for the remainder of the material we will consider KI
The stress intensity factor is a function of loading, crack size,
and structural geometry. The stress intensity factor may be represented
by the following equation [4]:
Figure 4. Fracture toughness
𝐾! = 𝜎 𝜋𝑎𝛽
Where:
KI =is the fracture toughness
σ=is the applied stress in MPa or psi
a=is the crack length in meters or inches
β=is a crack length and component geometry factor that is different for each specimen
and is dimensionless.
References
[1] ASM Metal Handbook Volume 08 Mechanical Testing and Evaluation
[2] http://www.key-to-steel.com/Articles/Art140.htm
[3] http://www.gordonengland.co.uk/hardness/vickers.htm
[4] http://www.ndted.org/EducationResources/CommunityCollege
EXPERIMENT # 2: NDT EXPERIMENTS - FATIGUE-CHARPY
IMPACT-TENSION TESTING
(a) NON-DESTRUCTIVE TESTING (NDT)
Objective:
The objective of the experiment is to introduce the use and the
importance of non-destructive testing methods.
BACKGROUND
Non-destructive tests (NDT) are inspection methods which are usually used to search for the
presence of defects in components, without causing any effects on the properties of the
components. The types of defects detectable are cracks, porosity, voids, inclusions, etc.
Modern NDT is used by manufacturers to: ensure product integrity and reliability; prevent
failure, accidents and saving lives; make profit for users; ensure customer satisfaction; aid in
better product design; control manufacturing process; lower manufacturing costs; maintain
uniform quality level; ensure operational readiness.
The table below shows the types of NDT methods used today.
Commonly Used Methods
Other Methods
Ultrasonics
Visual Methods
Radiography
Acoustic Emission
Dye Penetrant
Thermography
Magnetic Particle Inspection
Holography
Eddy Current
1) Basic Principles Of Ultrasonic Testing
Mechanical vibrations can be propagated in solids, liquids and gases. The actual particles of
matter vibrate, and if the mechanical movements of the particles have a regular motion, the
vibration can be assigned a frequency in cycles per second, measured in hertz (Hz), where 1 Hz
= 1 cycle per second. If this frequency is within the approximate range 10 to 20,000 Hz, the
sound is audible; above about 20 kHz, "the sound" waves are referred to as ultrasound or
ultrasonics.
The ultrasonic principle is based on the fact that solid materials are good conductors of sound
waves. The waves are not only reflected at the interfaces but also by internal flaws (material
separations, inclusions,etc.).
As an example of a practical application, if a disc of piezoelectric materials is attached to a
block of steel (Figure 1a), either by cement or by a film of oil, and a high- voltage electrical
pulse is applied to the piezoelectric disc, a pulse of ultrasonic energy is generated in the disc
and is propagated into the steel. This pulse of waves travels through the metal with some
spreading and some attenuation and will be reflected or scattered at any surface or internal
discontinuity such as an internal flaw in the specimen. This reflected or scattered energy can be
detected by a suitably-placed second piezoelectric disc on the metal surface and will generate a
pulse of electrical energy in that disc. The time- interval between the transmitted and reflected
pulse is a measure of the distance of the discontinuity from the surface, and the size of the
return pulse can be a measure of the size of the flaw. This is the simple principle of the
ultrasonic flaw detector and the ultrasonic thickness gauge. The piezoelectric discs are the
"probes" or "transducers"; sometimes it is convenient to use one transducer as both transmitter
and receiver. In a typical ultrasonic flaw detector the transmitted and received pulses are
displayed in a scan on a timebase on an oscilloscope as shown in Figure 1b.
2) Radiography (Rt)
Radiography involves the use of penetrating gamma or X-radiation to examine parts and
products for imperfections. An X-ray generator or radioactive isotope is used as a source of
radiation. Radiation is directed through a part and onto film or other imaging media. The
resulting shadowgraph shows the dimensional features of the part. Possible imperfections are
indicated as density changes on the film in the same manner as a medical X-ray shows broken
bones.
3) Dye Penetrant
Penetrant testing is used to test non-ferrous materials such as aluminum and stainless steel.
Different test methods are available ranging from visible red/water washable penetrant systems
to complex emulsifier penetrant systems. Various sensitivities are selected based on the product
and customer requirements. Components can be tested in-house or on site. We are expert in
setting up specific testing programs for our clients.
Common uses would be:
•
•
•
•
Non-ferrous parts can readily be tested using this method
Aircraft components
Non-ferrous castings
Finished machined components
4) Magnetic Particle Testing (Mt)
This NDE method is accomplished by inducing a magnetic field in a ferromagnetic material
and then dusting the surface with iron particles (either dry or suspended in liquid). Surface and
near-surface imperfections distort the magnetic field and concentrate iron particles near
imperfections, previewing a visual indication of the flaw.
5) Electromagnetic Testing (Et) Or Eddy Current Testing
Electrical currents are generated in a conductive material by an induced alternating magnetic
field. The electrical currents are called eddy currents because they flow in circles at and just
below the surface of the material. Interruptions in the flow of eddy currents, caused by
imperfections, dimensional changes, or changes in the material's conductive and permeability
properties, can be detected with the proper equipment.
(b) FATIGUE-CHARPY IMPACT-TENSION TESTING
Objective:
The objective of the charpy impact experiment is to evaluate the energy
absorbing characteristics of metal materials at room temperature using
the Charpy impact method. The object of the fatigue test is to
comprehend how fatigue conditions affect metals and to determine the
fatigue life of the specimen. The object of the tension testing is to
determine the tensile properties for various metallic samples using the
American Standard Test Methods (ASTM).
Materials:
V-notch charpy impact samples and fatigue samples. A set of metallic
specimens prepared according to the tension testing standards.
Equipment:
Impact testing machine for charpy impact test. Mechanical testing
machine for fatigue test. Instron Universal Testing Machine for tension.
(a) Charpy Impact Test
Introduction
Charpy impact test is a method for evaluating the toughness and notch sensitivity of
engineering materials. It is usually used to test the toughness of metals, but similar tests are
used for polymers, ceramics and composites. The test measures the energy absorbed by the
fractured specimen. This absorbed energy is a measure of a given material's toughness.
Test specimen
The test specimen is 55 mm long, with cross section 10x10 mm, and has a V-notch 2 mm deep
of 45° included angle and a root radius of 0.25 mm.
Test procedure
1. Pulling the pendulum at a known height and engaging the safety latch
2. Clamping the specimen in the machine
3. Releasing the pendulum
4. Reading the result from the indicator
(b) Fatigue Testing
Introduction:
In many applications, materials are subjected to vibrating or fluctuating forces. The behavior of
materials under such load conditions differs from the behavior under a static load. Designers
are faced with predicting fatigue life, which is defined as the total number of cycles to failure
under specified loading conditions, since the material is subjected to repeated load cycles
(fatigue) in actual use. Fatigue testing data is used to predict cycle number of crack initiation
and crack propagation behavior.
Test specimen
The fatigue test specimen is center crack tension (CCT) type and was made by 2024-T3
aluminum alloy. The width (W), length (L) and thickness (B) of the specimens are 90, 300, 6
mm, respectively. The specimen was machined according to ASTM E 647 recommendation
(Fig. 1).
300
6
17
2a
90
Figure 1: The fatigue crack growth CCT specimen geometry (dimensions mm).
Test procedure
1.
2.
3.
4.
5.
Selecting test parameters and calculating maximum and average load values
Clamping the specimen
Inputting test parameters to the MAX program controlling the test machine
Starting the test
Monitoring and measuring the crack length by the traveling microscope.
(a) Tensile Testing
Introduction:
When a specimen is loaded so that the resultant force passes through the centroid of the
specimen cross-section, the loading is categorized as axial and can be either tensile or
compressive. Tests determine material properties. When materials for engineering projects are
procured, the engineer often must specify material property requirements to the manufacturer.
After the material is received it is generally good practice, if not mandatory, to perform
acceptance tests to verify the material properties before the materials are used. Therefore, it is
important to understand which material properties are relevant and how those properties are
obtained. Results from simple tension tests, similar to the test described in this experiment, can
provide information from which several material properties can be determined. The
experiments to be completed for Tension I will illustrate the usefulness of the simple tension
test and demonstrate the mechanical behavior of materials.
What is Tension Test ?
Method for determining behavior of materials under axial stretch loading. By pulling on
something, you will very quickly determine how the material will react to forces being applied
in tension. As the material is being pulled, you will find its strength along with how much it
will elongate. Data from test are used to determine elastic limit, elongation, modulus of
elasticity, proportional limit, reduction in area, tensile strength, yield point, Yield Strength
and other tensile properties. Procedures for tension tests of metals are given in ASTM E-8.
Methods for tension tests of plastics are outlined in ASTM D-638, ASTM D-2289 (high strain
rates), and ASTM D-882 (thin sheets).
Why Perform a Tensile Test or Tension Test?
You can learn a lot about a substance from tensile testing. As you continue to pull on the
material until it breaks, you will obtain a good, complete tensile profile. A curve will result
showing how it reacted to the forces being applied. The point of failure is of much interest and
is typically called its "Ultimate Strength" or UTS on the chart.
Hooke's Law For most tensile testing of materials, you will notice that in the initial portion of
the test, the relationship between the applied force, or load, and the elongation the specimen
exhibits is linear. In this linear region, the line obeys the relationship defined as "Hooke's Law"
where the ratio of stress to strain is a constant, or
. E is the slope of the line in this
region where stress (σ) is proportional to strain (ε) and is called the "Modulus of Elasticity" or
"Young's Modulus".
Modulus of Elasticity The modulus of elasticity is a measure of the stiffness of the material,
but it only applies in the linear region of the curve. If a specimen is loaded within this linear
region, the material will return to its exact same condition if the load is removed. At the point
that the curve is no longer linear and deviates from the straight-line relationship, Hooke's Law
no longer applies and some permanent deformation occurs in the specimen. This point is called
the "elastic, or proportional, limit". From this point on in the tensile test, the material reacts
plastically to any further increase in load or stress. It will not return to its original, unstressed
condition if the load were removed.
Yield Strength
A value called "yield strength" of a material is defined as the stress applied to the material at
which plastic deformation starts to occur while the material is loaded.
Strain
You will also be able to find the amount of stretch or elongation the specimen undergoes during
tensile testing This can be expressed as an absolute measurement in the change in length or as a
relative measurement called "strain". Strain itself can be expressed in two different ways, as
"engineering strain" and "true strain". Engineering strain is probably the easiest and the most
common expression of strain used. It is the ratio of the change in length to the original length,
.
Whereas, the true strain is similar but based on the instantaneous length of the specimen as the
test progresses,
, where Li is the instantaneous length and L0 the initial length.
Ultimate Tensile Strength
One of the properties you can determine about a material is its ultimate tensile strength
(UTS). This is the maximum load the specimen sustains during the test. The UTS may or may
not equate to the strength at break. This all depends on what type of material you are testing. .
.brittle, ductile, or a substance that even exhibits both properties. And sometimes a material
may be ductile when tested in a lab, but, when placed in service and exposed to extreme cold
temperatures, it may transition to brittle behavior.
Testing Procedure
We will serve to introduce the Instron testing equipment and testing procedures. For the
experiment, a 8 mm nominal diameter hot rolled steel sample will be tested to failure. Load
versus- strain diagrams will be produced during the test and this diagram will subsequently be
used to determine material properties. The student will learn how to properly conduct a
tension test and obtain the relevant material properties from the results. Further, the student will
discover how different materials behave under similar loading conditions as well as how
material properties differ.
REPORT REQUIREMENTS:
For the material tested please determine and tabulate the following properties:
a. Proportional Limit
b. Yield Strength
c. Ultimate Strength
d. Modulus of Elasticity
e. Percent elongation
f. Percent reduction in area
g. Provide stress versus strain plot, appropriately labeled, for specimen tested
h. Briefly summarize, in words, tension test.
EXPERIMENT # 3: SONIC MODULUS-POROSIMETRY
Objective:
The objective of the experiments is to introduce various testing
techniques available in the department.
(a) Sonic Modulus
There are basically two ways to measure elastic properties, of which Young’s, bulk, and
shear moduli are most common.
The first way is to measure strain in response to some quasi statically applied stress,
commonly in conjunction with strength testing. There are several limitations to this, an
important one is that often only Young’s modulus is obtained. Another is that since most
ceramics have high moduli and hence low stains, using the easiest measurement of strains, i.e.,
from test machine head travel, is generally grossly inaccurate due to substantial parasitic
deflections of the loading train being of similar or greater magnitude as the specimen strains.
The second and generally preferred method of measuring elastic properties is but one of
two sets of wave motion measurements. These are versatile, often being applicable on
components, and generally offer good accuracy and other advantages. One basic way of doing
this by transmission of ultrasonic waves, or transmission and reflection of pulses (i.e.,
pulse echo). Thus, elastic properties can be calculated from the material density (ρ) and
measurable velocities of longitudinal (νl) and shear waves (νs). Another basic way of measuring
elastic properties by wave methods is by resonance vibration of specimens [1].
The velocity of ultrasonic pulses, travelling in a solid material, depends on the density,
elastic properties and on the different phases present inside the material, thus the quality could
be related to its measurement. Ultrasonic testing is applied to measure elastic stiffness,
mechanical properties and defect’ presence in concrete, refractories and ceramic materials, etc
[2].
The greatest advantage of ultrasonic velocity measurements is the nondestructive
determination of elastic moduli. Assuming that the samples used in this analysis are isotropic,
standard velocity-elasticity relations can be used to calculate the various moduli. These
relations are:
E = νl2 ρ(1+σ)(1-2σ)/(1-σ)
G = νs 2 ρ
σ = (1-2b2)/(2-2b2)
K = E/[3(1-2σ)]
where νl is the longitudinal wave velocity (m/s), νs the shear wave velocity (m/s), E the
Young’s modulus (pascals), G the shear modulus (pascals), σ the Poisson’s ratio, K the bulk
modulus (pascals) and b= νs/ νl [3].
The Nature of Sound
Sound waves are mechanical vibrations involving movement within the medium in
which they are travelling. The particles in the medium vibrate, causing it to distort, thus
transferring energy from particle to particle, along the wave path [4].
Longitudinal waves: Particles oscillate parallel to the direction of propagation. In aluminum νl=
6.25 X 103 m/s [5].
Figure 1. Longitudinal waves [6]
Shear waves: Particles oscillate transverse to the direction of propagation. In aluminum νl= 3.1
X 103 m/s [5].
Figure 2. Shear waves [6]
Test Materials
Different ceramic materials (densities must be known).
Test Procedure
1- Propagation time (t) of longitudinal waves’ measurement for pulse-echo mode.
2- Propagation time (t) of shear waves’ measurement for pulse-echo mode.
3- Thickness (d) measurement of ceramic materials.
4- Ultrasonic wave velocity calculation for pulse-echo mode (ν=2d/t).
5- Calculation of elastic properties.
References
1. RICE, R.W., Porosity of Ceramics, M. Dekker, New York, 24-25 (1998).
2. ROMAGNOLI, M., BURANI, M., TARI, G. and FERREIRA, J.M.F., A Non-destructive Method to Assess
Delamination of Ceramic Tiles, Journal of the European Ceramic Society, 27, 1631-1636 (2007).
3. KULKARNI, N., MOUDGIL, B. and BHARDWAJ, M., Ultrasonic Characterization of Green and Sintered
Ceramics: I, Time Domain, American Ceramic Society Bulletin, 73, 146-153 (1994).
4. Ultrasonic Non-destructive Testing, Published by: The Institution of Metallurgist, The Chameleon Press,
London, 10 (1983).
5. Encyclopedia of Applied Physics, Testing Equipment-Mechanical to Topological Phase Effects, Wiley-VCH
Verlag GmbH, 21, 39 (1997).
6. www.cen.bris.ac.uk/projects/eqteach97/waves2.htm (19.10.2007)
(b) Mercury Porosimetry
Figure 1. Mercury porosimetry
Mercury porosimetry characterizes a material’s porosity by applying various levels of pressure
to a sample immersed in mercury. The pressure required to intrude mercury into the sample’s
pores is inversely proportional to the size of the pores.
Analysis Technique
To perform an analysis, the sample is loaded into a penetrometer, which consists of a sample
cup connected to a metal-clad, precision-bore, glass capillary stem. The penetrometer is sealed
and placed in a low pressure port, where the sample is evacuated to remove air and moisture –
the user controls the speed of the evacuation and there’s no need for a separate preparation unit.
The penetrometer’s cup and capillary stem are then automatically backfilled with mercury.
Excess mercury is automatically drained back into the internal reservoir; only a small amount
remains in the penetrometer.
As pressure on the filled penetrometer increases, mercury intrudes into the sample’s pores,
beginning with those pores of largest diameter. This requires that mercury move from the
capillary stem into the cup, resulting in a decreased capacitance between the now shorter
mercury column inside the stem and the metal cladding on the outer surface of the stem.
The instrument automatically collects low pressure measurements over the range of pressures
specified by the operator. Then, the penetrometer is moved to the high pressure chamber, where
high pressure measurements are taken. Data are automatically reduced using the low and high
pressure data points, along with values entered by the operator, such as the weight of the
sample and the weight of the penetrometer loaded with mercury.
Proven Science
Mercury porosimetry is based on the capillary law governing liquid penetration into small
spaces. This law, in the case of a non-wetting liquid like mercury, is expressed by the
Washburn equation:
𝐷=
−2𝛾. 𝑐𝑜𝑠𝜃
𝑃
where D is the pore diameter, P is the applied pressure, γ the surface tension of mercury and φ
the contact angle between the mercury and the sample, all in consistent units. The volume of
mercury V penetrating the pores is measured directly as a function of applied pressure. This PV information serves as a unique characterization of pore structure.
The Washburn equation assumes that all pores are cylindrical. Although pores are rarely
cylindrical in reality, this equation provides a practical representation of pore distributions,
yielding very useful results for most applications.
As pressure increases during an analysis, pore size is calculated for each pressure point and the
corresponding volume of mercury required to fill these pores is measured. These measurements
taken over a range of pressures give the pore volume versus pore size distribution for the
sample material.
If decreasing pressure are included in the analysis, extrusion data are also calculated using the
Washburn equation. Extrusion P-V curves usually differ from intrusion curves because of
mercury entrapment and because there is no driving force to bring the mercury out of the pores
during the extrusion phase of the analysis. Differences between intrusion curves and extrusion
curves can be used to characterize channel restrictions and the structure or shape of pores.
Pore diameters may be offset toward larger values on extrusion curves because receding
contact angles are smaller than advancing contact angles. This results in equivalent volumes of
mercury extruding at lower pressures than those at which the pores were intruded. Also, pore
irregularities, such as enlarged chambers and “ink-well” structures sometimes trap mercury.
Comprehensive Results
Mercury porosimetry can determine a broader pore size distribution more quickly and
accurately than other methods. Comprehensive data provide extensive characterization of
sample porosity and density. Available results include:
•
•
•
•
•
•
•
•
•
•
Total pore volume
Incremental volume
Differential volume
Log-differential volume
% of total volume
Total pore surface area
Incremental area
Median or mean pore diameter
Pore size distributions and % porosity
Sample densities (bulk and skeletal)
EXPERIMENT # 4: a) ELECTROLYTIC REDUCTION OF ZINC
Objective:
The objective of the electrolytic reduction of Zinc experiment is to have
a basic knowledge of electrolytic reduction concept, Faraday Law and
terms of Electrochemical cell, Faraday Efficiency, Polarization,
Hydrogen Overvoltage, EMF Series, Nernst Diffusion Layer. During
experiment, you will be introduced the effects of effective surface area,
impurities present in the solution, temperature and electrolysis time.
Materials:
Zinc sulfate solution (65 g/l Zn+2 containing ZnSO4 ve 130 g/l free
H2SO4), Al cathodes, Pb anodes
Equipment:
-
DC Power Supply
Multimeter
Glass beaker, electrode holders, cables etc.
Electrolysis:
Faraday defined the electrolysis as a chemical work done by electric current generated
in DC power supply and pass through electrolyte. Conductive materials such as electrolytes can
be divided into two main subgroups. One group of conductive materials (metals, alloys, metal
oxides, graphite etc.) conduct electric current without chemically changed. Other group of
materials (ionic conductors) conducts with the help of ionic transport. An electrolytic cell
consists of electrolyte, electrodes (anode, cathode) and DC supply. Figure 1 shows schematic
view of electrolysis system.
Figure 1. Electrolysis system
The EMF cell converts the chemical reaction energy into electrical energy. The
following equation gives the relation between Gibbs free energy change (ΔG) and potential (E):
∆𝐺 = −𝑛 × 𝐹 ×𝐸
where n is the number of electrons passing through cell and F is Faraday constant
(96500 Coulomb). Since metals always give electrons n must be positive digit as well as
Faraday constant. If ΔGcell = 0, it states the equilibrium. Positive ΔGcell states that reaction is
impossible without any external manipulation. Electrolysis makes positive Gibbs free energy
change reactions possible with the help of applied current so they convert electrical energy into
chemical energy.
The potential difference between two plates (anode/cathode) dipped into electrolyte is
called cell voltage. The potential difference between one of the electrodes and electrode with
known potential (reference electrode) is called single electrode potential or half-cell voltage.
Standard potentials of elements given in EMF series are potentials measured with respect to
Standard Hydrogen Reference Electrode (SHE).
Faraday Law gives the relation between consumed electrical energy and chemical
work. It shows the equivalent-gram metal collected in the cathode by 1 Faraday (96500
Coulomb) current passing through electrolyte or molten salt and the equation:
𝑚 𝑔 =
m:
AZn:
I:
t:
z:
𝐴!" × 𝐼 × 𝑡
𝑧 × 96500
the mass of collected metal in the cathode (g)
Atomic weight of Zn (g/mole)
Applied current (A)
Electrolysis time (s)
Valence electron of collected metal
Positive (+) pole of DC current supply is called anode and negative (-) pole is cathode.
During electrolysis, anions (negatively charged ions) move towards anode, and cations
(positively charged ions) move towards cathode. Oxidation occurs in anode (anodic reaction)
whereas reduction takes place in cathode (cathodic reaction).
Total cell voltage (ET) in an electrolytic cell:
𝐸! = 𝐸! + 𝐸! + 𝜂! + 𝜂! + (𝐼×𝑅)
EA :
EK :
ηA :
ηA :
(I x R) :
Standard electrode potential of anode
Standard electrode potential of cathode
Overvoltage of anode
Overvoltage of cathode
Resistive losses
If current is passing in the electrolytic system, electrode potential (Ei) will be different
comparing to its original potential (E0). This phenomenon is named Overvoltage (polarization)
and can be formulized:
𝜂 = 𝐸! − 𝐸!
Main overvoltage types are;
•
Diffusion Overvoltage
•
Chemical Reaction Overvoltage
•
Charge Transfer Overvoltage
•
Crystallization Overvoltage
Metal ions can travel through cathode via diffusion, convection and migration.
When the
electrode is polarized, the surface concentration of the species that is either being oxidized or
reduced falls to zero. Additional material will then diffuse to the electrode (cathode) surface
towards this region of lower concentration. This lower/higher concentration region is called
Nernst Diffusion Layer (δ). Resulting concentration-distance gradient at the electrode surface is
represented in Figure 2. The width of the Nernst diffusion layer is not a function of current.
Increasing current density reduces the bulk solution concentration – electrode surface
concentration difference as well as it reduces Nernst diffusion layer.
Figure 2. Nernst Diffusion Layers on Electrodes and Concentration Gradient Between Electrodes
Metallic Zinc Production
More than %90 of total Zinc production is from sulphureted ores. Concentrated sulphureted
ores are first calcined and leached with sulphuric acid. Iron in the solution then precipitated via
jarosite, hematite and/or goethite processes. After that, solution is purified by removing
impurities (Cd, Ni, Co, Cu etc.) via cementation with Zn powder. Solution treatment and
purification are crucial steps for electrowinning of zinc from sulphureted ores because 4 main
groups of materials can present in the solution:
Ø Elements below Zn in EMF series: These have higher electronegativity than Zn (K, Na,
Ca, Mg, Al, Mn). However, they’re impossible to collect on cathode surface. Molten
salt electrolysis is used to produce these metals.
Ø Elements between Zinc and Hydrogen:
o Pb, Cd, Ta, Sn: Easily reduce on cathode. Impure the cathode material
o Fe, Co, Ni: Difficult to reduce on cathode because hydrogen overvoltage on
these metals is lower than Zn.
Ø Elements above H in EMF Series: Reduces Hydrogen overvoltage (Cu, As, Sb, Te, Ge,
In etc.)
Ø Metals resolve Zinc into the solution producing local cell: (Ni, Cu, Co, As, Sb)
The final step for metallic Zinc production is electrolytic reduction. Reactions below take
place in electrodes:
Zn2+ + 2e- = Zn0 (cathodic reaction)
-0.76V (vs. SHE)
SO42- + H2O = SO42- + 2H+ + 2e- + ½ O2 (anodic reaction)
2+
2-
0
2-
+
-
Zn + SO4 + H2O = Zn + SO4 + 2H + 2e + ½ O2 (total reaction)
+1.23V (vs. SHE)
-1.99V (vs. SHE)
Excluding kinetic factors, the voltage needed from DC source is 1.99 Volt.
Thermodynamically, it’s impossible to reach metallic zinc from aqueous solutions.
Because, elements below Hydrogen in EMF series, result in reduction of H+ ions to H2. The
overvoltage needed to form gaseous H2 in the system can be defined as Hydrogen Overvoltage.
It makes electrolytic reduction of Zn kinetically possible. Three main steps of gaseous H2
formation is electron transfer in Hydrogen ion (Volmer reaction), formation of Hydrogen
molecule (Tafel Reaction) and transition to gaseous phase.
•
Effect of Impurities on Electrolytic Reduction of Zinc:
Cleanliness of electrolytic solution is the most crucial part of electrolytic reduction
systems. Impurities in the solution can be divided into 2 sub-groups of impurities more noble
than Zinc and impurities more active than Zinc according to EMF Series. If impurities more
noble than Zinc are presented in the solution, they will be reduced before Zn, when external
current is applied. It reduces purity and current efficiency of collected Zn metal in cathode.
However, impurities more active than Zn don’t cause any problem because they’re impossible
to be reduced on cathode. They need specific production methods (e.g. Molten Salt
Electrolysis) to collect.
•
Effect of Temperature on Electrolytic Reduction of Zinc:
Temperature strongly effects kinetics of electrolytic systems. Metallic Zn collection and
H2 formation occurs at the cathode side of electrolytic reduction of Zn. Increasing
temperature enhances the reaction rate of Hydrogen Overvoltage. In other words, with
increasing temperature, more spaces in active cathode surface occupied with H2
formation and less spaces to collect metallic Zn.
Electrolytic reduction of Zn in industrial scale requires very high current density levels
between 600-1000 A/m2.
Experimental Procedure:
Figure 3. Experimental Setup
In Figure 3, 300 ml electrolyte is put into three 500 ml glass beakers that contain one Pb
anode and one Al cathode and 65 g/l Zn+2 containing ZnSO4 based electrolyte of each. First cell
will be at 60°C whereas second and third cells will be set up at room temperature. The
electrolyte at the third cell has 1 g/l Fe3+ or 3 g/l Cu2+ ions. Three electrolysis cells are
connected in series and current densities between 600-1000 A/m2 are applied. Here the wetted
(effective) surface areas of the electrodes should be especially paid attention. Each hour, Al
cathodes are washed, dried in drying oven and weighed to find out the amount of deposited
metal.
EMF SERIES (vs SHE)
Au/Au+
Pt/Pt2+
Pb/Pb4+
Ag/Ag+
Rh/Rh2+
Cu/Cu2+
As/As3+
Sb/Sb3+
Bi/Bi3+
H2/2H+
Fe/Fe3+
Pb/Pb2+
Sn/Sn2+
Ni/Ni2+
Co/Co2+
Cd/Cd2+
Fe/Fe2+
Zn/Zn2+
Mn/Mn3+
Al/Al3+
Ti/Ti2+
Ca/Ca2+
Li/Li+
1.70
1.20
0.80
0.799
0.6
0.34
0.30
0.24
0.20
0.0
-0.04
-0.126
-0.140
-0.23
-0.27
-0.402
-0.44
-0.763
-1.05
-1.66
-1.75
-2.84
-3.01
Hydrogen
Over
Voltage
b) CORROSION AND CATHODIC PROTECTION
Objective: The objective of Corrosion and Cathodic Protection experiment is to have a basic
knowledge on corrosion, corrosion cell, corrosion current, electrical double layer and effect of
cathodic surface area on corrosion current.
Materials:
Ø 3 Zn plates with 2 cm2 effective area
Ø Cu plates with 2, 10 and 30 cm2 effective area
Ø %3.5 NaCl solution
Equipments:
Ø Calomel electrode
Ø Multimeter
BACKGROUND
Metals are found in nature as a metallic compounds. Production of a metals and alloys is a
matter of “material-energy-knowledge”. However, metals have high tendency to form its
metallic compounds giving a reaction in its environment. As a result of that, physical, chemical,
mechanical and electrical properties will change. We often call that “damage”. Corrosion is
defined as degradation and damage of materials in its environment. Corrosion is a self-driven
reaction. We don’t need to give energy from outsource. Corrosion in atmosphere, aqueous
solutions, even under soil, is called aqueous corrosion. If there’s no water (e.g. High
temperature, gas-metal reactions), this type of corrosion is high temperature corrosion. Besides
them, there are also corrosion of organic liquids and molten metal corrosion.
Corrosion is a kind of electrochemical reaction. It’s based on electron transfer, so the
electron transfer system in corrosion is named corrosion cell. Typical corrosion cell consists of;
•
Anode, the metal which is oxidized,
•
Cathode, the metal that is reduced or consume the produced electrons at surface,
•
Electrolyte, solution which provides ionic conduction,
•
Interface layer, the layer that oxidation and reduction reactions take place
•
Electrical conductor transfers electrons transporting from anode to cathode.
The current that flow in corrosion cell is named as corrosion current (icor). In corrosion
cell, rate of oxidation and reduction reaction are equal to each other (ianode = icathode = icor). If
there is no reduced material in the cell, since the produced electrons are not consumed,
corrosion does not take place. In other words, if there is no cathodic reaction or cathodic
reaction is prevented, corrosion reaction does not occur. Additionally, corrosion reaction does
not take place if there is no;
•
electrical connection between anode and cathode,
•
contact of anode or cathode with the solution,
•
conductor.
Dissolution rate of metal (corrosion rate) is determined by the reduction rate. The
corrosion rate (icor) decreases when the amount of reduced material in solution decreases, since
corrosion rate is controlled by the cathodic reaction rate (icathode).
In corrosion, if anode and cathode is apart from each other, metal only decomposed at
anode. In this situation selective or regional corrosion occur and this type of corrosion cell is
named as macrocorrosion cell. At some situations even an atomic size point defects act as both
anode and cathode, as a result of this metal corrodes uniformly.
Ø Corrosion Prevention
The corrosion behavior of metal surfaces can be changed either by coating or by
cathodic protection. Cathodic protection methods are based on 2 approaches. First one is using
sacrificial anode that more active metal (sacrificial anode) is presented in the system. Other
approach is using external current source that changes anode to cathode in the electrochemical
system.
Experimental Procedure:
Surfaces of the three Zn plates are cleaned, dried and weighed. Surface of the Cu plate
with 2 cm2 area is cleaned, dried and weighed. One of the 2 cm2 active surface area Zn plate is
weighed, put into a 400 ml beaker with % 3,5 NaCl solution. This sample is taken out of the
solution, washed, dried and weighed.
Another weighed 2 cm2 Zn plate is coupled with 10 cm2 Cu plate and put into a 400 ml
beaker with % 3,5 NaCl solution. During this operation current is recorded for 15 minutes by
zero resistant amperemeter. At the end of the experiment Zn plate is washed, dried and
weighed.
One of the other weighed 2 cm2 Zn plate is coupled with 30 cm2 Cu plate and put into a
400 ml beaker with %3,5 NaCl solution. During this operation current is recorded for 15
minutes by zero resistant amperemeter. At the end of the experiment Zn plate is washed, dried
and weighed. The experimental setup is shown in Figure 1.
Figure 1. Experimental setup for corrosion and cathodic protection
EXPERIMENT #5 - ELECTRICAL & STRUCTURAL
CHARACTERIZATION OF THIN FILMS &
COATINGS
- ELECTRICAL PROPERTY MEASUREMENTS
(a) Electrical & Structural Characterization of Thin Films
Objective:
The objective is to learn thin film deposition processes (e.g. sputtering).
Main purpose of the experiment is to learn what is vacuum and why it is
necessary for thin film deposition. Understanding the main important
parameters effecting thin film properties (Table 7.5.1 in the pdf file). It is
also the basis of a number of scientific instrumental techniques with
atomic resolution, the scanning probe microscopy techniques such as
STM, AFM.
Equipment:
- Low vacuum sputtering tool (Au coating)
- High vacuum sputtering tool
- FPP- Four point probe
- ATM - STM
Materials:
Au/Pd deposition on glass, Si, SiO2 and insulating ceramic substrates
using low and high vacuum tools.
ELECTRICAL AND STRUCTURAL
CHARACTERIZATION OF THIN FILMS & COATINGS
Anadolu University, Faculty of Engineering, Department of Materials Science and Engineering
Supervisor and Instructor: Assoc. Prof. Dr. Ing. Ramis Mustafa ÖKSÜZOĞLU
Authors: Associated Prof. Dr. Ramis Mustafa ÖKSÜZOĞLU, M.S. Phys. Eng. Mustafa YILDIRIM
Page Number
CONTENTS....................................................................................................................... iii
1. INTRODUCTION .......................................................................................................... 1-2
2. VACUUM SCIENCE AND TECHNOLOGY ............................................................... 2-4
3. THIN FILM FORMATION ............................................................................................ 4-6
4. THIN FILM DEPOSITION TECHNIQUES .................................................................. 6-7
5. SPUTTERING PROCESS ........................................................................................ 7
5.1. DC Sputtering .............................................................................................................. 7-8
5.2. RF Sputtering ............................................................................................................... 8-9
5.3 Magnetron Sputtering ................................................................................................... 9
6. THIN FILM CHARACTERISATION TECHNIQUES ................................................. 9-10
6.1. Atomic Force Microscope (AFM) ............................................................................... 10-11
6.2. X-Ray Reflectometry (XRR) ...................................................................................... 11-12
6.3. Four Point Probe (FPP) (Sheet Resistance (R) and Resistivity (ρ) ............................. 13
7. REFERENCES ............................................................................................................... 13
1. INTRODUCTION
Thin films are fabricated by the deposition of individual atoms on a substrate. A thin film is defined
as a low-dimensional material created by condensing, one-by-one, atomic/molecular/ionic species of
matter. The thickness is typically less than several microns. Thin films differ from thick films. A thick
film is defined as a low-dimensional material created by thinning a three-dimensional material or
assembling large clusters/aggregates/grains of atomic/molecular/ionic species. Historically, thin films
have been used for more than a half century in making electronic devices, optical coatings,
instrument hard coatings, and decorative parts. The thin film is a traditional well-established material
technology. However, thin film technology is still being developed on a daily basis since it is a key
in the twenty-first century development of new materials such as nanometer materials and/or a
man -made super lattices.
Thin film materials and devices are also available for minimization of toxic materials since the
quantity used is limited only to the surface and/or thin film layer. Thin film processing also saves on
energy consumption in production and is considered an environmentally benign material technology for
the next century. Thin film technology is both an old and a current key material technology [3]. In all
deposition techniques and application of thin films the vacuum technologies are imperative.
2. VACUUM SCIENCE AND TECHNOLOGY
Virtually every thin-film deposition and processing method or technique employed to characterize
and measure the properties of films requires either a vacuum or some sort of reduced-pressure ambient.
For example, there are plasma discharges, sustained at reduced gas pressures, in which many important
thin-film deposition and etching processes occur. Evacuated spaces are usually populated by uncharged
gas atoms and molecules [2].
The net result of the continual elastic collisions and exchange of kinetic energy is that a steady-state
distribution of molecular velocities emerges given by the celebrated Maxwell - Boltzmann formula.
f (v) = 1/n (dn / dv) = 4/π1/2(M/2RT)3/2 v2exp – Mv2/2RT
(2.1)
This centrepiece of the kinetic theory of gases states that the fractional number of molecules f(v)
here n is the number per unit volume in the velocity range v to v + dv , is related to their molecular
weight (M) and absolute temperature (T). In this formula the units of the gas constant R are on a permole basis.Momentum transfer from the gas molecules to the container walls gives rise to the forces that
sustain the pressure in the system. Kinetic theory shows that the gas pressure, P, is related to the meansquare velocity of the molecules and thus, alternately to their kinetic energy or temperature.
P = nMv-2/3NA = nRT/NA
(2.2)
where NA is Avogadro's number. From the definition of n, n/NA is the number of moles per unit
volume and therefore, (2.2) is an expression of the perfect gas law. Definitions of some units together
with important conversions include
1 atm = 1.013 x 106 dynes/cm2 = 1.013 x 105 N/m2 = 1.013 x 105 Pa
1 torr = 1 mm Hg = 1.333 x 103 dynes/cm2 = 133.3 N/m2 = 133.3 Pa
1 bar = 0.987 atm = 750 torr.
The mean distance travelled by molecules between successive collisions, called the mean-free path,
λ mfp, is an important property of the gas that is dependent on the pressure. One collision will occur under
the condition that πdc2 λmfp n=1, For air at room temperature and atmospheric pressure, λmfp ~ 500Å,
assuming dc ≈ 5 Å.
A molecule collides in a time given by λmfp/v, and under the above conditions, air molecules make
about 1010 collisions per second. As a result of collisions, they are continually knocked to and fro,
executing a zigzag motion and accomplishing little net movement. Since n is directly proportional to P, a
simple relation for ambient air is [1,2]:
λmfp = 5 x 10-3 / P
(2.3)
with λmfp given in centimeters and P in torr. At pressures below 10-3 torr, λmfp is so large that
molecules effectively collide only with the walls of the vacuum chamber. A most important quantity
that plays a role in both vacuum science and vapor deposition is the gas impingement flux Φ . It is a
measure of the frequency with which molecules impinge on or collide with a surface, and should be
distinguished from the previously discussed molecular collisions in the gas phase. The number of
molecules that strike an element of surface, perpendicular to a coordinate direction, per unit time and
area is given by
Φ = 0 ∫ vx dnx
∞
(2.4)
A useful variant of this formula is
Φ = 3.513 x 1022 P/(MT)1/2 molecules / cm2-s
(2.5)
when P is expressed in torr.
As an application of the preceding development, consider the problem of gas escaping the vessel
through a hole of area A into a region where the gas concentration is zero. The rate at which molecules
leave is given by ΦA and this corresponds to a volume flow per second (V) given by ΦA/n cm3/s. Upon
substitution of (2.2) and (2.5), we have
V = 3.64 x 103 (T/M)1/2 A cm3 / s.
(2.6)
For air at 298 K this corresponds to 11.74 liters/s, where A has units of cm2. In essence we have just
calculated what is known as the conductance of a circular aperture. As a second application, consider
the question of how long it takes for a surface to be coated by a monolayer of gas molecules. This is
an issue of great importance when attempting to deposit or grow films under extremely clean conditions.
The same concern arises during surface analysis of films, which is performed at very low pressures in
order to minimize surface contamination arising from the vacuum chamber environment. This
characteristic contamination time, tc, is essentially the inverse of the impingement flux. Thus, for
complete monolayer coverage of a surface containing some 1015 atoms/cm2, the use of (2.5) yields
tc = 1015/3.513 x 1022 . (MT)1/2/P = 2.85 x 10-8 / P. (MT)1/2s
(2.7)
with P measured in torr. In air at atmospheric pressure and ambient temperature a surface will
acquire a monolayer of gas in 3.49 x 10-9s assuming all impinging atoms stick. A condensed
summary of the way system pressure affects the gas density, mean free path, incidence rates, and
monolayer formation times is conveniently displayed in Fig.2.1.
The pressure scale is arbitrarily subdivided into corresponding low, medium, high, and ultrahigh vacuum
domains, each characterized by different requirements with respect to vacuum hardware (e.g., pumps,
gauges, valves, gaskets, feed throughs). Of the film deposition processes, evaporation requires a vacuum
mainly in ultrahigh regimes, whereas sputtering between the high and low-pressure and chemical vapour
depositions are accomplished at the border between the medium and high vacuum ranges. In the lowpressure molecular-flow regime the situation is little changed from that of an isolated chamber where the
perfect gas law applies. At the higher pressures in the viscous-flow regime, gas flow is governed by the
laws of compressible fluid dynamics. Conductance, throughputs, and pumping speeds are now
complicated functions of pressure. With the exception of chemical vapour deposition this labour script is
exclusively concerned with the easier-to-model low- pressure chambers. Attaining low system pressures
is largely a matter of connecting a high vacuum-backing pump combination to the chamber via highconductance ducts. Oil diffusion, turbo molecular, cryo-, and ion pumps produce the low pressures,
while rotary, Roots, and sorption pumps are necessary to provide the necessary fore pressure for
operation of the former. Assorted valves, cold traps, and gauges round out the complement of required
vacuum hardware [2].
3. THIN FILM FORMATION
Any thin-film deposition process involves three main steps: 1. Production of the appropriate atomic,
molecular, or ionic species. 2. Transport of these species to the substrate through a medium. 3.
Condensation on the substrate, either directly or via a chemical and/or electrochemical reaction, to form
a solid deposit. The Steps in Film Formation: 1. Thermal accommodation; 2. Binding; 3. Surface
diffusion;
4. Nucleation; 5. Growth; 6. Coalescence; 7. Continued growth. Depending on the
thermodynamic parameters of the deposit and the substrate surface, the initial nucleation and growth
stages (4 and 5) of the many observations of film formation have pointed to three basic growth modes
may be described as (a) island type, called Volmer-Weber type, (b) layer type, called
Fig. 2.1 Molecular density, incidence rate, mean free path, and monolayer formation time as a function of pressure.
Frank-van der Merwe type, and (c) mixed type, called Stranski-Krastanov type [3]. These are illustrated
schematically in Fig.3.1. Island growth occurs when the smallest stable clusters nucleate on the substrate
and grow in three dimensions to form islands. This happens when atoms or molecules in the deposit are
more strongly bound to each other than to the substrate. Many systems of metals on insulators, alkali
halide crystals, graphite, and mica substrates display this mode of growth [1]. The opposite
characteristics are displayed during layer growth.
Here the extension of the smallest stable nucleus occurs
overwhelmingly in two dimensions resulting in the
formation of planar sheets. In this growth mode the atoms
are more strongly bound to the substrate than to each other.
The first complete monolayer is then covered with a
somewhat less tightly bound second layer. Providing the
decrease in bonding energy is continuous toward the bulk
crystal value, the layer growth mode is sustained [1].
Fig. 3.1 Thin film growth models
The layer plus island or Stranski-Krastanov (S.K.)
growth mechanism is an intermediate combination of the aforementioned modes. In this case, after
forming one or more mono layers, subsequent layer growth becomes unfavorable and islands form. The
transition from two- to three-dimensional growth is not completely understood, but any factor that
disturbs the monotonic decrease in binding energy characteristic of layer growth may be the cause. For
example, due to film-substrate lattice mismatch, strain energy accumulates in the growing film. When
released, the high energy at the deposit-intermediate layer interface may trigger island formation. This
growth mode is fairly common and has been observed in metal-metal and metal-semiconductor systems
[1].
At an extreme far removed from early film formation phenomena is a regime of structural effects
related to the actual grain morphology of polycrystalline films and coatings. This external grain structure
together with the internal defect, void or porosity distributions frequently determines many of the
engineering properties of films. For example, columnar structures, which interestingly develop in
amorphous as well as polycrystalline films, have a profound effect on magnetic, optical, electrical, and
mechanical properties [1].
4. THIN FILM DEPOSITION TECHNIQUES
Typical deposition processes are physical and chemical. The physical process is composed of the
physical vapour deposition (PVD) processes, and the chemical processes are composed of the chemical
vapour deposition (CVD) process (Fig. 4.1).
Fig. 4.1 Schematic representation of physical and chemical vapor deposition
In particular by the necessity to deposit insulating and passivating films, the CVD is served as a
powerful processing method. PVD processes (often just called thin film processes) are atomistic
deposition processes in which material is vaporized from a solid or liquid source in the form of atoms or
molecules, transported in the form of a vapour through a vacuum or low pressure gaseous (or plasma)
environment to the substrate where it condenses. Typically, PVD processes are used to deposit films
with thicknesses in the range of a few nanometers to micrometers; however they can also be used to
form multilayer coatings, graded composition deposits, very thick deposits and freestanding structures.
The substrates can range in size from very small to very large such as the 10” x 12” glass panels used for
architectural glass. The substrates can range in shape from flat to complex geometries such as
watchbands and tool bits. Typical PVD deposition rates are 10–100Å (1–10 nanometers) per second.
PVD processes can be used to deposit films of elements and alloys as well as compounds using reactive
deposition processes. The main categories of PVD processing are thermal evaporation, sputter
deposition, ion plating and arc vapour deposition. In this script the sputtering process is used as a main
thin film deposition process in the Thin film Laboratory.
5. SPUTTERING PROCESS
For convenience sputtering processes are divided into four major categories: namely, DC, RF and
magnetron. Magnetron sputtering is practiced in DC and RF as well as reactive variants and has
significantly enhanced the efficiency of these processes. Because magnetron sputtering is now the
dominant method of physically depositing films by plasma methods, it merits a section all its own. The
thin films that deposit are derived from target cathodes which play an active role in the plasma. Mainly
sputtering process described in fig.5.1 [2].
Fig. 5.1 Sputtering Principle
5.1. DC SPUTTERING: The DC sputtering is also known as diode or cathodic sputtering. It is
worthwhile, however, to note how the relative film deposition-rate depends on sputtering pressure and
current. At low pressures, the cathode sheath is wide, ions are produced far from the target, and their
chances of being lost to the walls is great. The mean free electron path between collisions is large, and
electrons collected by the anode are not replenished by ion-impact-induced secondary-electron emission
at the cathode. Therefore, ionization efficiencies are low and selfsustained discharges cannot be
maintained below about 10 mtorr. As the pressure is increased at a fixed voltage, the electron mean free
path is decreased, more ions are generated, and larger currents flow. But if the pressure is too high, the
sputtered atoms undergo increased collusion scattering and are not efficiently deposited. Trade-offs in
these opposing trends, where optimum operating conditions are shaded in and include the relatively high
operating pressure of ~ 100 mtorr. During DC-diode sputtering the atoms that leave the target with
typical energies of 5 eV undergo gas scattering events in passing through the plasma gas; this is so even
at low operating pressures. As a result of repeated energy-reducing collisions they eventually thermalize
or reach the kinetic energy of the surrounding gas. This happens at the distance <xth>, is the mean
distance from the cathode sputtered atoms travel before they become thermalized, where their initial
excess kinetic energy, so necessary to provide bombardment of the depositing film, has dissipated. No
longer directed, such particles now diffuse randomly. Not only is there a decrease in the number of
atoms that deposit, but there is little compaction or modification of the resulting film structure. By virtue
of lower operating pressures the magnetron sputtering
removes these disadvantages.
Despite the fact that simple DC sputtering was historically
the first to be used and has an appealing simplicity, it is no
longer employed in production environments. The reasons are
not hard to see. For example, film deposition rates are simply
too low, e.g., a few hundred Å /min at most for many metals.
These rates cannot be appreciably raised at higher operating
pressures, because in addition to more gas scattering, increased contaminant levels of O2 and H2O in
chamber gases can oxidize cathodes. Thin insulating layers that form on the target further reduce the
current and deposition rates.A fundamental problem is the small ionization cross sections at typical
electron energies in the plasma. Even at optimum operating conditions, secondary electrons emitted from
the cathode have an appreciable probability of reaching the anode or chamber walls without making
ionizing collisions with the sputtering gas [2, 10].
5.2. RF SPUTTERING: In contrast dealing with the issue of sustaining AC discharges in an
electrode-less environment, our present concern is with sputter deposition. Building on our experience
with sputtering metal films, suppose we now wish to deposit thin SiO2 films by using a quartz disk of
thickness d as the target in a conventional DC diode sputtering system. For quartz the resistivity ρ is
~1016 Ω-cm. In order to draw a current density j of 1 mA/cm2, a voltage V = pjd would have to be
dropped across the target. Assuming d = 0.1cm, substitution gives an impossibly high value of 1012 V;
the discharge extinguishes and DC sputtering will not work. If a convenient level of V = 100 V is set, it
means that a target with a resistivity exceeding 106 Ω-cm could not be DC sputtered. This impasse can
be overcome, however, if we recall that impedances of dielectric filled capacitors drop with increasing
frequency. Therefore, high-frequency plasmas ought to pass current through dielectrics the way DC
plasmas do through metal targets. And since the sputter yields are essentially similar for both target
materials, sputtering of a dielectric cathode in an AC plasma should be feasible. The trick is to sustain
the plasma while ensuring positive-ion bombardment of the cathode [2, 10].
5.3. MAGNETRON SPUTTERING: For reasons
given earlier as well as others that follow, magnetron
sputtering is the most widely used variant of DC
sputtering. Important implications of this are higher
deposition rates or alternatively, lower voltage operation
than for simple DC sputtering. Another important
advantage is reduced operating pressures. At typical
magnetron-sputtering pressures of a few millitorr, sputtered
atoms fly off in ballistic fashion to impinge on substrates.
Avoided are the gas phase collisions and scattering at high
Fig. Fig. 5.2.1 RF Sputtering
pressures which randomize the directional character of the
sputtered atom flux and lower the deposition rate.
Therefore, for the same electrode spacing and minimum
target voltage a stable discharge can be maintained at lower pressures.
6. THIN FILM CHARACTERISATION TECHNIQUES
For the first half of this century interest in thin films centred on optical applications. The role played
by films was largely a utilitarian one necessitating measurement of film thickness and optical properties.
At first single films on thick substrates were involved. However, with the explosive growth of thin-film
utilization in microelectronics there was an important need to understand the intrinsic nature of films in
more complex materials environments. Increasingly, the benefits of multilayer film structures have been
realized in an assortment of high technology applications. Examples include multilayer metal and
insulating films in microelectronics, compound semiconductor films in optoelectronics, dielectric-film
stacks for optical coatings, and ceramic film layers in hard coatings [2]. A measurement of thin-film
properties is indispensable for the study of thin-film materials and devices. The chemical composition,
crystalline structure, and optical, electrical, and mechanical properties must be considered in evaluating
thin films [3]. Experimental techniques and applications associated with determination of “film
thickness”, “film surface morphology and structure”, “film and surface composition”, “electrical
properties of films”.
These represent the common core of information required of all films and multilayer coatings
irrespective of ultimate application. Within each of these three categories only the most important
techniques will be discussed. Beyond these characteristics there are a host of individual properties (e.g.,
thickness, structure, surface morphology and electrical conductivity) which are specific to the particular
application [2].
6.1. ATOMIC FORCE MICROSCOPE (AFM): The Atomic Force Microscope (AFM), which is
sometimes called the Scanning Force Microscope (SFM), is based on the forces experienced by a probe
as it approaches a surface to within a few angstroms. A typical probe has a 500 Å radius and is mounted
on a cantilever which has a spring constant less than that of the atom-atom bonding. This cantilever
spring is deflected by the attractive van der Waals (and other) forces and repulsed as it comes into
contact with the surface (“loading”). The deflection of the spring is measured to within 0.1 Å. By
holding the deflection constant and monitoring its position, the surface morphology can be plotted.
Because there is no current flow, the AFM can be used on electrically conductive or non- conductive
surfaces and in air, vacuum, or fluid environment. The AFM can be operated in three modes: contact,
noncontact and “tapping.” The contact mode takes advantage of van der Waal’s attractive forces as
surfaces approach each other and provides the highest resolution. In the non-contacting mode, a
vibrating probe scans the surface at a constant distance and the amplitude of the vibration is changed by
the surface morphology. In the tapping mode, the vibrating probe touches the surface at the end of each
vibration exerting less pressure on the surface than in the contacting mode. This technique allows the
determination of surface morphology to a resolution of better than 10 nm with a very gentle contacting
pressure (Phase Imaging). Special probe tip geometries allow measuring very severe surface geometries
such as the sidewalls of features etched into
surfaces [4].
Fig. 6.1.1 Schematic illustration of AFM. The tip is
attached to a cantilever, and is raster-scanned over a surface.
The cantilever deflection due to tip-surface interactions is
monitored by a photodiode sensitive to laser light reflected at
the tip backside
[9].
6.2. X-RAY REFLECTOMETRY (XRR):
Most technological applications of thin films
require films of definite thickness. This is because
most properties of thin films are thickness
dependent. Hence, determination of film thickness with high precision is very crucial for these
technologies. XRR is a non-destructive and non-contact technique for thickness determination between
2-200 nm with a precision of about 1-3Å. In addition to thickness determination, this technique is also
employed for the determination of density and roughness of films and also multilayers with a high
precision [5]. XRR method involves monitoring the intensity of the x-ray beam reflected by a sample at
grazing angles. A monochromatic X-ray beam of wavelength λ irradiates a sample at a grazing angle ω
and the reflected intensity at an angle 2θ is recorded by a detector, see figure 6.2.1. This figure illustrates
specular reflection where the condition ω = 2θ / 2 is satisfied. The mode of operation is therefore θ/2θ
mode which make sure the incident angle is always half of the angle of diffraction. The reflection at the
surface and interfaces is due to the different electron densities in the different layers (films), which
corresponds to different reflective indexes in the classical optics. For incident angles θ below a critical
angle θc , total external reflection occurs. The critical angle for most materials is less than 0.3º. The
density of the material is determined from the critical angle. Above θc the reflection from the different
interfaces interfere and give rise to interference fringes. The period of the interference fringes and the
fall in the intensity are related to the thickness and the roughness of the layer (layers in case of
multilayers). The reflection can be analyzed using the classical theory (Fresnel equation). The typical
range for these measurements are between 0º and 5º in θ [5].
Film Thickness: For incident angles greater than the X-ray beam penetrates inside the film.
Reflection therefore occurs at the top and the bottom surfaces of the film. The interference between the
rays reflected from the top and the bottom of the film surfaces results in interference fringes
Fig. 6.2.1 θ/2θ Scan: The condition of
incident angle ω = (2θ)/2 = θ = outgoing
angle is satisfied. The detector D rotates at
twice the speed of the sample P. This
arrangement is sensitive only to the planes
parallel to the surface of the sample. The
beam makes an incident angle ω with the
surface of the sample P. The reflected
intensity at angle of 2θ is measured. Both
the rotation of the sample ω and the detector
(2θ)
are
about
the
same
axis
MP
(perpendicular to the drawing). The sample
is adjusted so that the rotation axis lies on the sample surface. The Detector circle is fixed through the (programmable)
detector slit (PRS, programmable receiving slit). The anode focus, F of the tube lies on the detector circle.
which does not depend on the frequency like in the case of optical spectroscopy but are angle dependent.
Due to the low amplitude reflection coefficient (ρv,h ~ 1 / sin2 θ => Rv,h = |rv,h| ~ 1 / sin4 θ ≈ 1 / θ4 ) of
interface between adjacent layers, contributions of multiply reflected beams can be neglected. The m-th
interference maximum for a path difference Δ = mλ, is located at
mλ = Δ = 2dΝx,1(θm) ≈ 2d(θ2m-2δ)-1/2 mit m∈Ν ⇔ θ2m ≈ m2λ2/4d2+2δ = m2λ2/4d2+θ2c
(6..1)
If the substrate is optically denser than the film, a phase difference of π occurs at the reflection film /
substrate interface and m is substituted with m+1/2. Employing equation (6.1) and the difference
between two neighboring maxima and minima, the thickness can be determined and is given by
d ≈ λ / { 2(θ2m+1 - θ2C)-1/2 – (θ2m - θ2C )-1/2 } ≈ λ / {2(θm+1 - θm } , θm >> θ2C
(6.2)
The thickness is often determined with a precision better than 1 Å for measurements exhibiting
interference fringes in a bigger angular range. [5]
6.3 FILM RESISTANCE (FOUR POINT PROBE)
The purpose of the 4-point probe is to measure the resistivity or average resistance of a thin layer or
sheet or any semiconductor material by passing current through the outside two points of the probe and
measuring the voltage across the inside two points (see Fig. 6.3.1).
Fig. 6.3.1 A view of Four-Point Probe
measuring geometry: x1 and x2 are positions,
s is distance between probes.
Typical positions of the probe is shown above in Fig. 6.3.1 in travel and measurement
positions.
Sheet resistance measurement principles
The resistance R of a rectangular block of uniform bulk resistivity is given by: R = ρ x L / A, where
A = t x W and ρ: resistivity of the sample. R = (ρ / t) x (L / W) = Rs x (L / W), where Rs: sheet
resistance of a layer of this material. The sheet resistance is expressed in Ohm.square though strictly
speaking it should be in Ohm. Rs can be determined by four probe measurement by applying a DC
current in between the 2 outer current probes and measuring the voltage at the 2 inner voltage probes: Rs
= V/I x CF. The correction factor CF is dependent on the sample size and shape.
8. REFERENCES
[1] Ohring, M., The Materials Science of Thin Films First Edition, Academic Press, USA, 1992.
[2] Ohring, M., Material Science of Thin Films Deposition & Structure Second Edition, Academic Press, USA,
2002.
[3] Wasa, K., Kitabatake, M., Adachi H., Thin Film Materials Technology Sputtering of Compound Materials,
William Andrew Publishing, USA, 2004.
[4] Mattox, Donald M., Handbook of Physical Vapor Deposition (PVD) Processing Film Formation, Adhesion,
Surface Preparation and Contamination Control, Noyes Publication, USA, 1998.
[5] http://ia.physik.rwth-aachen.de/methods/xray/www-xray-eng.pdf
[6] http://www.bal-tec.com/products/pdf/MEDGRE.PDF
[7] http://smif.lab.duke.edu/pdf/Pt_Melting_OperatingProcedure.pdf
[8] http://www.bal-tec.com/products/pdf/QSG100.PDF
[9] http://www.farmfak.uu.se/farm/farmfyskem-web/instrumentation/afm.shtml
[10] http://www.mrsec.harvard.edu
(b) Electrical Property Measurements
Objective:
Understanding the theory and application of electro-ceramic materials,
especially piezoelectric materials, and characterization of electroceramic materials by various equipment.
Equipment-Materials: In electro-ceramics laboratory, it is possible to design piezoelectric,
capacitive and resistive transducers, produce prototypes and measure
their properties. Also behaviour of transducers under AC and DC
voltages can be investigated. The equipment present in this laboratory is
as follows: Agilent-54624A Osciloscope MTI-2000 Fotonic Sensor HP4194A Impedance / Gain-Phase Analyzer WF 1944 Multifunction
Synthesizer HSA-4011 High Speed Bipolar Amplifier SR830 DSP
TREK-Model 6100 Piezo d33 Tester.
Background
While ceramics have traditionally been admired for their mechanical and thermal
stability, their unique electrical, optical and magnetic properties have become of increasing
importance in many key technologies including communications, energy conversion and
storage, electronics and automation. Such materials are now classified under electro-ceramics,
as distinguished from other functional ceramics such as advanced structural ceramics.
Historically, developments in the various subclasses of electro-ceramics have paralleled
the growth of new technologies. Examples include: Ferroelectrics - high dielectric capacitors,
non-volatile memories; Ferrites-data and information storage; Solid Electrolytes - energy
storage and conversion; Piezoelectrics - sonar; Semiconducting Oxides - environmental
monitoring.
Dielectric materials used for construction of ceramic capacitors include zirconium
barium titanate, strontium titanate (ST), calcium titanate (CT), magnesium titanate (MT),
calcium magnesium titanate (CMT), zinc titanate (ZT), lanthanum titanate (TLT), and
neodymium titanate (TNT), barium zirconate (BZ), calcium zirconate (CZ), lead magnesium
niobate (PMN), lead zinc niobate (PZN), lithium niobate (LN), barium stannate (BS), calcium
stannate (CS), magnesium aluminum silicate, magnesium silicate, barium tantalate, titanium
dioxide, niobium oxide, zirconia, silica, sapphire, beryllium oxide, and zirconium tin titanate.
Ferroelectricity is a physical property of a material whereby it exhibits a spontaneous
electric dipole moment, the direction of which can be switched between equivalent states by the
application of an external electric field. Ferroelectrics are key materials in microelectronics.
Their excellent dielectric properties make them suitable for electronic components such as
tunable capacitors and memory cells.
Piezoelectricity is the ability of some materials (notably crystals and certain ceramics)
to generate an electric potential in response to applied mechanical stress. This may take the
form of a separation of electric charge across the crystal lattice. The piezoelectric effect is
reversible in that materials exhibiting the direct piezoelectric effect (the production of
electricity when stress is applied) also exhibit the converse piezoelectric effect (the production
of stress and/or strain when an electric field is applied). For example, lead zirconate titanate
crystals will exhibit a maximum shape change of about 0.1% of the original dimension. The
effect finds useful applications such as the production and detection of sound, generation of
high voltages, electronic frequency generation, microbalances, and ultra fine focusing of optical
assemblies.
EXPERIMENT # 6 POLYMER PROCESSING AND
CHARACTERIZATION
Objective:
In this experiment, objective is learning how thermoplastics and
thermosets are processed and understand thermal analysis techniques of
polymers.
Equipment:
Extrusion
Electrospinning
Dynamic Scanning Calorimeter
Thermal Gravimetric Analysis.
Materials:
Low density polyethylene ( LDPE),Epoxy
a) POLYMERS
Polymers can be divided into three groups - thermoplastics, thermosets, and elastomers.
Thermoplastics
Molecules in a thermoplastic are held together by relatively weak intermolecular forces so that
the material softens when exposed to heat and then returns to its original condition when
cooled. Thermoplastic polymers can be repeatedly softened by heating and then solidified by
cooling - a process similar to the repeated melting and cooling of metals. Most linear and
slightly branched polymers are thermoplastic. All the major thermoplastics are produced
by chain polymerization.Thermoplastics have a wide range of applications because they can be
formed and reformed in so many shapes. Some examples are food packaging, insulation,
automobile bumpers, and credit cards.
•
Polyethylene (Low Density) LDPE, LLDPE
Properties
Semi-rigid, translucent, very tough, weatherproof, good chemical resistance, low water
absorption, easily processed by most methods, low cost.
Applications
Squeeze bottles, toys, carrier bags, high frequency insulation, chemical tank linings, heavy duty
sacks, general packaging, gas and water pipes.
Thermosets
A thermosetting plastic, or thermoset, solidifies or "sets" irreversibly when heated. Thermosets
cannot be reshaped by heating. Thermosets usually are three-dimensional networked polymers
in which there is a high degree of cross-linking between polymer chains. The cross-linking
restricts the motion of the chains and leads to a rigid material. A simulated skeletal structure of
a network polymer with a high cross-link density is shown at the right.
Thermosets are strong and durable. They primarily are used in automobiles and construction.
They also are used to make toys, varnishes, boat hulls, and glues.
•
Epoxy (EP)
Properties
Rigid, clear, very tough, chemical resistant, good adhesion properties, low curing, low
shrinkage.
Applications
Adhesives, coatings, encapsulation, electrical components, cardiac pacemakers, aerospace
applications.
•
Unsaturated Polyester (UP)
Properties
Rigid, clear, chemical resistant, high strength, low creep, good electrical properties, low
temperature impact resistance, low cost.
Applications
Boat hulls, building panels, lorry cabs, compressor housing, embedding, coating.
Elastomers
Elastomers are rubbery polymers that can be stretched easily to several times their unstretched
length and which rapidly return to their original dimensions when the applied stress is released.
Elastomers are cross-linked, but have a low cross-link density. The polymer chains still have
some freedom to move, but are prevented from permanently moving relative to each other by
the cross-links. To stretch, the polymer chains must not be part of a rigid solid - either a glass
or a crystal. An elastomer must be above its glass transition temperature, Tg, and have a low
degree of crystallinity. Rubber bands and other elastics are made of elastomers
b) Melt Extrusion Process
Melt extrusion plays a prominent part on the plastics industry. Extrusion, unlike moulding, is a
continuous process, and can be adapted to produce a wide variety of finished or semi-finished
products, including pipe, profile, sheet, film and covered wire.
In order to produce satisfactory extrudate it is necessary to apply heat to the granules in order to
soften them and make the resulting melt capable of flow under some pressure. This is carried
out rotated in the barrel by means of gear box and variable screw drive .
Therefore the screw barrel has following functions:
•
pumping
•
heating
•
mixing
•
pressurizing
In order to make each function as effective as possible it is normal practice to divide the screw
into 3 zones:
1. feed zone at hopper end.
2. compression zone (transition) at the middle.
3. melt zone (melting zone) at the die end.
The function of the feed zone is to collect granules from the feed hopper and transport (pump)
them up the screw channel. At the same time the granules should begin to heat up and compact
and build up pressure as they advance towards screw tip (die end). For efficient pumping the
granules must not be allowed to lie in the screw channel. They must therefore show high degree
of slippage on the screw channel surface and a low degree of slippage on the barrel.
Figure 1. A principal scheme of an Extrusion Machine is shown in the picture.
Extrusion is used mainly for thermoplastics, but elastomers and thermosets are also may be
extruded. In this case cross-linking forms during heating and melting of the material in the
extruder.
c) Electrospinning
Electrospinning uses an electrical charge to draw very fine (typically on the micro or nano
scale) fibres from a liquid. Electrospinning shares characteristics of both electrosprayingand
conventional solution dry spinning of fibers.[1] The process does not require the use of
coagulation chemistry or high temperatures to produce solid threads from solution. This makes
the process particularly suited to the production of fibers using large and complex molecules.
Electrospinning from molten precursors is also practised; this method ensures that no solvent
can be carried over into the final product.
Process
When a sufficiently high voltage is applied to a liquid droplet, the body of the liquid becomes
charged, and electrostatic repulsion counteracts the surface tension and the droplet is stretched;
at a critical point a stream of liquid erupts from the surface. This point of eruption is known as
the Taylor cone. If the molecular cohesion of the liquid is sufficiently high, stream breakup
does not occur (if it does, droplets are electrosprayed) and a charged liquid jet is formed.
As the jet dries in flight, the mode of current flow changes from ohmic to convective as the
charge migrates to the surface of the fiber. The jet is then elongated by a whipping process
caused by electrostatic repulsion initiated at small bends in the fiber, until it is finally deposited
on the grounded collector.The elongation and thinning of the fiber resulting from this bending
instability leads to the formation of uniform fibers with nanometer-scale diameters.
Figure 2. Fibre formation by electrospinning
Parameters
d) Differential Scanning Calorimeter
Introduction
Thermal analysis is a term used to cover a group of techniques in which a physical property of
a substance and/or its reaction product(s) is measured as a function of temperature. DSC
measures the heat required to maintain the same temperature in the sample versus an
appropriate reference material in a furnace. Enthalpy changes due to a change of state of the
sample are determined. A number of important physical changes in a polymer may be
measured by DSC. These include the glass transition temperature (Tg), the crystallization
temperature (Tc), the melt temperature (Tm), and the degradation or decomposition temperature
(TD). Chemical changes due to polymerization reactions, degradation reactions, and other
reactions affecting the sample can be determined. A typical DSC trace showing these
transitions is shown in Fig. 3.
Table 1. Specific DTA and DSC Applications
Figure 3. A Generic DSC curce depicting several transition types
Glass Transition
From a thermodynamic and mechanical point of view, the glass transition, Tg, is one of the
most important parameters for characterizing a polymer system. Consequently, the
determination of the Tg is usually one of the first analyses performed on a polymer system. A
polymer may be amorphous, crystalline, or a combination of both. Many polymers actually
have both crystalline and amorphous regions, i.e., a semicrystalline polymer. The Tg is a
transition related to the motion in the amorphous regions of the polymer. Below the Tg, an
amorphous polymer can be said to have the characteristics of a glass, while it becomes more
rubbery above the Tg . On the molecular level, the Tg is the temperature of the onset of motion
of short chain segments, which do not occur below the Tg. The glass transition temperature can
be measured in a variety of ways (DSC, dynamic mechanical analysis, thermal mechanical
analysis), not all of which yield the same value . This results from the kinetic, rather than
thermodynamic, nature of the transition. Tg depends on the heating rate of the experiment and
the thermal history of the specimen. Also, any molecular parameter affecting chain mobility
effects the Tg. Table 16.2 provides a summary of molecular parameters that influence the Tg.
From the point of view of DSC measurements, an increase in heat capacity occurs at Tg due to
the onset of these additional molecular motions, which shows up as an endothermic response
with a shift in the baseline.
Table 2. Structural Factors Affecting Tg
Melting and Crystallization
The most common applications of DSC are to the melting process which, in principle, contains
information on both the quality (temperature) and the quantity (peak area) of crystallinity in a
polymer. The property changes at Tm are often far more dramatic than those at Tg, particularly
if the polymer is highly crystalline. These changes are characteristic of a thermodynamic firstorder transition and include a heat of fusion and discontinuous changes in heat capacity,
volume or density, refractive index and transparency. All of these may be used to determine
Tm.
Generally, the crystalline melting point of a polymer corresponds to a change in state from a
solid to a liquid and gives rise to an endothermic peak in the DSC curve. The equilibrium
melting point may be defined as
where ΔH, is the enthalpy of fusion and ΔS, is the entropy of fusion. In addition to determining
the melting point and heat of fusion from DSC, the width of the melting range is indicative of
the range of crystal size and perfection . Because crystal perfection and crystal size are
influenced by the rate of crystallization, Tm depends to some extent on the thermal history of
the specimen.
e) Thermal Gravimetric Analysis
Thermogravimetric analysis (TGA ) uses heat to drive reactions and physical changes in
materials. TGA provides a quantitative measurement of any mass change in the polymer or
material associated with a transition or thermal degradation. TGA can directly record the
change in mass due to dehydration , decomposition , or oxidation of a polymer with time and
temperature . Thermogravimetric curves are characteristic for a given polymer or compound
because of the unique sequence of the physicochemical reaction that occurs over specific
temperature ranges and heating rates and are a function of the molecular structure. The
changes in mass are a result of the rupture and/or formation of various chemical and physical
bonds at elevated temperatures that lead to the evolution of volatile products or the formation
of heavier reaction products . From TGA curves, data concerning the thermodynamics and
kinetics of the various chemical reactions, reaction mechanisms and the intermediate and final
reaction products are obtained. Other processes that can be studied by TGA are adsorption and
desorption phenomena , reactions with purge gases , ash content analysis , quantitative
determination of additives (including plasticizers in polymers) , solid-state reaction
composition of filled polymers , rates of evaporation , and sublimation . TGA has also been
used to estimate the flame retardancy of polymers, as enhanced flame retardancy is often
paralleled by increased amounts o f residual char at high temperature . Similarly, antioxidant
effectiveness can be gauged by the degree to which the degradation of the polymer is pushed
to higher temperatures in the air purge . The temperature at which 5 % of the starting mass has
been lost is a convenient benchmark for comparing antioxidant efficiency.
References
http://www.substech.com/dokuwiki/doku.php?id=injection_molding_of_polymers
http://faculty.uscupstate.edu/llever/Polymer%20Resources/Classification.htm
http://www.pitfallsinmolding.com/extrusion1.html
Polymer Synthesis and Characterization Book