IE 337: Materials & Manufacturing
Processes
Lecture 13:
Ceramics, Glass and
Powder Processing
Chapters 7, 12, 16 & 17
This Time




2
Ceramics
Glass Processing
Powder Processing: Ceramics and Metals
Homework #5 on Thursday (2/25/10)
Ceramics

General properties










Hard
High wear resistance
Brittle
High compressive strength
High elastic modulus
High temperature resistance
Good creep resistance
Low conductivity
Low thermal expansion
Good chemical inertness
Ceramics: Classification
Al2O3-SiO2
ZrO2
SiC
BN
Al2O3
WC
Diamond
Si3N4
ZrO2
Al2O3
AlN
Common Ceramics





Oxides: Al2O3, ZrO2
Nitrides: AlN, Si3N4, BN, TiN
Carbides: WC, SiC, TiC, TaC
Glasses: SiO2 + others
Carbon: Graphite, Diamond
Processed as powders
sinter
Whiteware Ceramics

Clay
 Quartz
 Feldspar

Processing






Water addition, mixing
Air removal
Shaping
Drying
Coating
Firing

Products





Brick
Structural Tile
Drain / sewer pipe
Decorative applications
Bath / kitchen structures
Refractory Material

Retain properties at high
temperature
 Mechanical
 Chemical

Products




7
Fire brick
Insulating fibers
Refractory linings
Coatings



Silica
Alumina
Magnesium Oxide
Abrasives


High hardness

Roughing Applications
 Grinding
 Cutting
Examples
 Silicon carbide
 Aluminum oxide
 Cubic boron nitride


Water-jet
Sawing
 Coatings

Super-Finishing
 Honing
 Lapping
8
Glasses

Amorphous solid
 Vitreous (noncrystalline)
structure
 Amorphous
 Cooled to semi-solid
condition without
crystallization
 Subject to creep

Silica Glass
 Optical properties
 Thermal stability
9

Products




Window glass
Fiber optics
Chemical containers
Lenses
Glass Ceramics

Crystalline solid
 0.1 to 1.0 micron grains
 Use of nucleating agents

Glass Ceramic
 Efficient processing in
glassy state
 Net shape process
 Good mechanical
properties versus glass
 Low porosity
 Low thermal expansion
 Higher resistance to
thermal shock
10

Products
 Cookware
 Heat exchangers
 Missile radomes
Cermets

Combination of metals &
ceramics
 “Cemented” carbides
 Bound with high
temperature metal

11
Properties
 High hardness
 High temperature
resistance
 Improved toughness
 Improved strength
 Improved shock
resistance

Applications
 Crucibles
 Jet nozzles
 High temperature brakes

Production
 Press powder in metal
mold
 Sintering in controlled
atmosphere
WC-Co
GLASS
Shaping Methods for Glass


Methods for shaping glass are different from
those used for traditional and new ceramics
Glassworking: principal starting material is silica
 Usually combined with other oxide ceramics that form
glasses


13
Heated to transform it from a hard solid into a
viscous liquid; it is then shaped into the desired
geometry while in this fluid condition
When cooled and hard, the material remains in
the amorphous state rather than crystallizing
14
The typical process sequence in glassworking:
(1) preparation of raw materials and melting,
(2) shaping, and
(3) heat treatment
Glassworking Processes



15
Piece Ware
Flat and Tubular Glass
Glass Fibers
Piece Ware Shaping Processes




16
Spinning – similar to centrifugal casting
Pressing – for mass production of flat products
such as dishes, bake ware, and TV faceplates
Blow forming – for production of smaller-mouth
containers such as beverage bottles and
incandescent light bulbs
Casting – for large items such as large
astronomical lenses that must cool very slowly
to avoid cracking
Spinning
Spinning of funnel-shaped glass parts such as back sections of
cathode ray tubes for TVs and computer monitors:
(1) gob of glass dropped into mold; and
(2) rotation of mold to spread molten glass on mold surface
17
Pressing
Pressing of flat glass pieces: (1) glass gob is fed into mold from
furnace; (2) pressing into shape by plunger; and (3) plunger is
retracted and finished product is removed (symbols v and F
indicate motion (velocity) and applied force)
18
Blow Forming
19
Blow forming sequence: (1) gob is fed into inverted mold
cavity; (2) mold is covered; (3) first blowing step; (4)
partially formed piece is reoriented and transferred to
second blow mold, and (5) blown to final shape
Casting





20
A low viscosity glass can be poured into a mold
Uses: massive objects, such as astronomical
lenses and mirrors
After cooling and solidifying, the piece must be
finished by lapping and polishing
Casting of glass is not often used except for
special jobs
Smaller lenses are usually made by pressing
Rolling
Starting glass from melting furnace is squeezed
through opposing rolls whose gap determines
sheet thickness, followed by grinding/ polishing
21
Float Process
Molten glass flows onto the surface of a molten
tin bath, where it spreads evenly, into a
uniform thickness and smoothness - no
grinding or polishing is needed
22
Forming of Glass Fibers
Products can be divided into 2 categories:
1. Discontinuous fibrous glass for insulation and
air filtration, in which the fibers are in a
random, wool-like condition

2.
Long continuous filaments suitable for fiber
reinforced plastics, yarns, fabrics, and fiber
optics

23
Produced by centrifugal spraying
Produced by drawing
Drawing
Continuous glass
fibers of small
diameter are
produced by
pulling strands of
molten glass
through small
orifices in a
heated plate
made of a
platinum alloy
24
Heat Treatment
Annealing to eliminate stresses from
temperature gradients
 Annealing temperatures are around 500C
followed by slow cooling
Tempering to make the glass more resistant to
scratching and breaking due to compressive
stresses on its surfaces
25
 Heating to a temperature above annealing,
followed by quenching of surfaces by air jets
Finishing Operations


Glass sheets often must be ground and
polished to remove surface defects and scratch
marks and to make opposite sides parallel
Decorative and surface processes performed
on certain glassware products include:
 Mechanical cutting and polishing operations; and
sandblasting
 Chemical etching (with hydrofluoric acid, often in
combination with other chemicals)
 Coating (e.g., coating of plate glass with aluminum
or silver to produce mirrors)
26
Powder Processing Parts
Figure 16.1 A collection of powder metallurgy parts
(photo courtesy of Dorst America, Inc.).
27
Powder Processing
1.
2.
3.
28
The Characterization of Engineering Powders
Production of Metallic Powders
Conventional Pressing and Sintering
Powder Metallurgy (PM)
Metal processing technology in which parts are produced
from metallic powders
 Usual PM production sequence:
1. Pressing - powders are compressed into desired shape to
produce green compact
 Accomplished in press using punch-and-die tooling
designed for the part
2. Sintering – green compacts are heated to bond the particles
into a hard, rigid mass
 Performed at temperatures below the melting point of the
metal
29
Why Powder Metallurgy is Important



PM parts can be mass produced to net shape
or near net shape, eliminating or reducing the
need for subsequent machining
PM process wastes very little material - ~ 97%
of starting powders are converted to product
PM parts can be made with a specified level of
porosity, to produce porous metal parts
 Examples: filters, oil-impregnated bearings and
gears
30
More Reasons Why PM is Important

Certain metals that are difficult to fabricate by other
methods can be shaped by powder metallurgy
 Tungsten filaments for incandescent lamp bulbs are made by
PM

Certain alloy combinations and cermets made by PM
cannot be produced in other ways
 Non-equilibrium microstructures possible


31
PM compares favorably to most casting processes in
dimensional control
PM production methods can be automated for
economical production
Engineering Powders
A powder can be defined as a finely divided
particulate solid
 Engineering powders include metals and
ceramics
 Geometric features of engineering powders:
 Particle size and distribution
 Particle shape and internal structure
 Surface area
32
Measuring Particle Size


Most common method uses screens of
different mesh sizes
Mesh count - refers to the number of openings
per linear inch of screen
 A mesh count of 200 means there are 200 openings
per linear inch
 Since the mesh is square, the count is equal in both
directions, and the total number of openings per
square inch is 2002 = 40,000
 Higher mesh count = smaller particle size
33
Screen Mesh
Figure 16.2 Screen mesh for sorting particle sizes.
34
Particle Shapes in PM
Figure 16.3 Several of the possible (ideal) particle shapes in powder
metallurgy.
35
Observations





36
Smaller particle sizes generally show greater
friction and steeper angles
Spherical shapes have the lowest interpartical
friction
As shape deviates from spherical, friction between
particles tends to increase
Easier flow of particles correlates with lower
interparticle friction
Lubricants are often added to powders to reduce
interparticle friction and facilitate flow during
pressing
Particle Density Measures

True density - density of the true volume of the
material
 The density of the material if the powders were
melted into a solid mass

Bulk density - density of the powders in the
loose state after pouring
 Because of pores between particles, bulk density is
less than true density
37
Packing Factor
Bulk density divided by true density
 Typical values for loose powders range between
0.5 and 0.7
 If powders of various sizes are present, smaller
powders will fit into spaces between larger ones,
thus higher packing factor
 Packing can be increased by vibrating the
powders, causing them to settle more tightly
 Pressure applied during compaction greatly
increases packing of powders through
rearrangement and deformation of particles
38
Porosity
Ratio of volume of the pores (empty spaces) in
the powder to the bulk volume
 In principle
Porosity + Packing factor = 1.0


39
The issue is complicated by possible existence
of closed pores in some of the particles
If internal pore volumes are included in above
porosity, then equation is exact
Chemistry and Surface Films

Metallic powders are classified as either
 Elemental - consisting of a pure metal
 Pre-alloyed - each particle is an alloy

Possible surface films include oxides, silica,
adsorbed organic materials, and moisture
 As a general rule, these films must be removed prior
to shape processing
40
Production of Metallic Powders



In general, producers of metallic powders are
not the same companies as those that make
PM parts
Any metal can be made into powder form
Three principal methods by which metallic
powders are commercially produced
1. Atomization
2. Chemical
3. Electrolytic

41
In addition, mechanical methods are
occasionally used to reduce powder sizes
Coventional PM Sequence
42
Figure 16.7 Conventional powder metallurgy production sequence:
(1) blending, (2) compacting, and (3) sintering; (a) shows the
condition of the particles while (b) shows the operation and/or
workpart during the sequence.
Blending and Mixing of Powders

For successful results in compaction and
sintering, the starting powders must be
homogenized
Blending - powders of same chemistry but
possibly different particle sizes are
intermingled
 Different particle sizes are often blended to reduce
porosity

43
Mixing - powders of different elements/alloys
are combined
Compaction



44
Application of high pressure to the powders to form
them into the required shape
Conventional compaction method is pressing, in which
opposing punches squeeze the powders contained in a
die
The workpart after pressing is called a green compact,
the word green meaning not yet fully processed
The green strength of the part when pressed is
adequate for handling but far less than after sintering
Conventional Pressing in PM
Figure 16.9 Pressing in
PM: (1) filling die cavity
with powder by
automatic feeder; (2)
initial and (3) final
positions of upper and
lower punches during
pressing, (4) part
ejection.
Press for Conventional Pressing in PM
Figure 16.11 A 450 kN
(50-ton) hydraulic press
for compaction of PM
parts (photo courtesy of
Dorst America, Inc.).
46
Sintering
Heat treatment to bond the metallic particles,
thereby increasing strength and hardness
 Usually carried out at between 70% and 90%
of the metal's melting point (absolute scale)
 Generally agreed among researchers that the
primary driving force for sintering is reduction
of surface energy
 Part shrinkage occurs during sintering due to
pore size reduction
47
Sintering Sequence
48
Figure 16.12 Sintering on a microscopic scale: (1) particle bonding is
initiated at contact points; (2) contact points grow into "necks"; (3) the
pores between particles are reduced in size; and (4) grain
boundaries develop between particles in place of the necked regions.
Sintering Cycle and Furnace
Figure 16.13 (a) Typical heat treatment cycle in sintering; and (b)
schematic cross section of a continuous sintering furnace.
Limitations and Disadvantages

High costs
 High tooling and equipment costs
 Metallic powders are expensive
 Typically requires a unique material or geometry to justify

Problems in storing and handling metal powders
 Degradation over time, fire hazards with certain metals

Limitations on part geometry because metal powders
do not readily flow laterally in the die during pressing
 This is true for traditional punch and die

50
Variations in density throughout part may lead to yield
issues especially for complex geometries
Interparticle Friction and Powder Flow


51
Friction between particles affects ability of a
powder to flow readily and pack tightly
A common test of interparticle friction is the
angle of repose, which is the angle formed by a
pile of powders as they are poured from a
narrow funnel
Angle of Repose
Figure 16.4 Interparticle friction as indicated by the angle of repose
of a pile of powders poured from a narrow funnel. Larger angles
indicate greater interparticle friction.
52
Powder Injection Molding
powder
final
dry/
debind
flow
shape
53
sinter (firing)
CERAMICS
54
Ceramics Processing

55
(a) shows the workpart during the sequence, while (b)
shows the condition of the powders
Slip Casting
A suspension of ceramic powders in water,
called a slip, is poured into a porous plaster
of paris mold where the water from the mix is
absorbed to form a firm layer of clay
The slip composition is 25% to 40% water
 Two principal variations:
 Drain casting - the mold is inverted to drain excess
slip after a semi-solid layer has been formed, thus
producing a hollow product
 Solid casting - to produce solid products, mold not
drained
56
57
Sequence of steps in drain casting, a form of slip casting:
(1) slip is poured into mold cavity, (2) water is
absorbed into plaster mold to form a firm layer, (3)
excess slip is poured out, and (4) part is removed from
mold and trimmed
SLIP CASTING
58
Tape Casting
Fabrication process for thin ceramic sheets
Doctor Blade
Polyester Film
Carrier
Dried Tape
Slip
Polyester Film Roll
59
Miniaturization of Complex Circuits





60
High Temperature Co-Fired Ceramic (HTCC)
Low Temperature Co-Fired Ceramic (LTCC)
Thick film metal traces are printed on several tape layers of ceramic and
are co-fired
Tape layers are electrically connected through vias
Significant miniaturization of circuit form factor with this technology
Extrusion


61
Compression of clay through a die orifice to
produce long sections of uniform
cross-section
Products: hollow bricks, shaped tiles, drain
pipes, tubes, drill bit blanks, and insulators
Extruder Sectional View
Components and features of a (single-screw) extruder for
plastics and elastomers
62
Ceramic Extrusion: Examples
cordierite
catalytic converter
50 cells/cm2
63
Powder Injection Molding (PIM)

64
Ceramic particles are mixed with a
thermoplastic polymer, then heated and
injected into a mold cavity. Polymer provides
flow characteristics for molding
Mold-Filling Interactions
Jetting
Weld-line
Air trap
Short shot
Fillerpolymer
separation
65
Flashing
Die Pressing
66
Semi-Dry Pressing
Semi-dry pressing: (1) depositing moist powder into die cavity, (2)
pressing, and (3) opening the die sections and ejection
67
Next Time

Joining
Chapter 30 & 31
68