Due date 28 August
Assignment 1: Describe the fatigue failure of engineering or biological system with example which is associated with surface
Wear and discuss the role of environment ( Not more than 200 words)
Assignment 2 : Why erosion rate for metal is higher at lower impact angle and for ceramic it higher at higher impact angle?
Ma et al. Evaluation and Characterization of a Durable Composite Phase Thermal Barrier Coating in Solid Particle Erosion and Burner Rig Tests.JTST. https://doi.org/10.1007/s11666-020-01103-9
Surface Modification
Mechanical treatment
(surface compression)
Chemical treatment
(Change the surface properties by
Chemical processing)
Mechanical treatment of surface
1.
2.
3.
4.
Modification of surface roughness
Densification of Surface
Shot peening
Laser peening
Mechanical treatment
Shot peening: Shot peening is used to produce a compressive residual stress layer near the surface and modify the
mechanical properties of Materials. It entails striking a surface with shot (round metallic, glass, or ceramic particles) with
force sufficient to create plastic deformation
Wang et al. Effect of high energy shot-peening on the microstructure and mechanical properties of
Al5052/Ti6Al4V lap joints.
https://doi.org/10.1016/j.jmatprotec.2017.12.005
https://osk-kiefer.de/en/technology-delivery-program/shotpeening/
https://www.youtube.com/watch?v=b-T5i9IrOx0
Laser peening or laser shock peening is a surface engineering process used to impart residual stresses in materials. The
deep, high-magnitude compressive residual stresses induced by laser peening increase the resistance of materials to
surface-related failures, such as fatigue, fretting fatigue, and stress corrosion cracking. Laser shock peening can also be used
to strengthen thin sections, harden surfaces, shape or straighten parts (known as laser peen forming), break up hard
materials, compact powdered metals and for other applications where high-pressure, short duration shock waves offer
desirable processing results
 The Laser peening uses dynamic mechanical effects of a shock wave imparted by a laser to modify the surface of a
target material
 Laser peening can be accomplished with only two components: a transparent overlay and a high-energy pulsed laser
system
 The transparent overlay confines the plasma formed at the target surface by the laser beam. It is also often beneficial
to use a thin overlay, opaque to the laser beam, between the water overlay and the target surface
Laser Peening Animation 2021 - YouTube
Chemical treatment
Diffusion technology
1. Carburising
2. Nitriding
3. Cyaniding etc
4. Pack cementation etc
5. Aluminising
6. Cromising
Overlay technology
Diffusion technology
Fick's first law relates the diffusive flux to the gradient of the concentration. It postulates that the flux goes from regions of
high concentration to regions of low concentration, with a magnitude that is proportional to the concentration gradient or
in simplistic terms the concept that a solute will move from a region of high concentration to a region of low concentration
across a concentration gradient.
There are two Laws of ficks
Fick’s First Law
𝑑𝑐
j (mas flux) = -D𝑑𝑥
Fick’s second law
𝜕𝑐
𝜕𝑡
=
𝜕𝑗
− 𝜕𝑥
=
𝜕2𝑐
-D𝜕2𝑥
1.https://www.youtube.com/watch?v=05Xi4zo6I0Y
2. https://www.youtube.com/watch?v=0BaUFmDPp44
3.https://www.youtube.com/watch?v=SgBMoCArNak
D= D0*𝑒𝑥𝑝−𝑄/𝑅𝑡
Diffusion length = 𝐷 ∗ 𝑡
Relative diffusivities of interstitial and substitutional solutes in
iron. After Jack, 1975
Common example of case hardening :
A. Interstitial diffusion
1. Carburising
2. Nitriding
3. Nitrocarburizing
4. Boriding
5. etc
B. Substitutional diffusion :
1. chromising,
2. vanadising
3. Aluminising
4. Etc.
Case hardening which exploits Interstitial Diffusion is done either in gaseous state or Liquid state (salt bath) or using plasma.
https://www.aiheattreating.com/services/gas-nitriding
https://www.nitridingprocess.com/salt_bath_nitriding.html
Nitriding is a heat treating process that diffuses nitrogen into the surface of a metal to create a case-hardened surface. These
processes are most commonly used on low-alloy steels. They are also used on titanium, aluminum and molybdenum.
Gas Nitriding : In gas nitriding the donor is a nitrogen-rich gas, usually ammonia (NH3), which is why it is sometimes known
as ammonia nitriding. When ammonia comes into contact with the heated work piece it dissociates into nitrogen and
hydrogen.
Salt bath nitriding: In salt bath nitriding the nitrogen donating medium is a nitrogen-containing salt such as cyanide
salt. The salts used also donate carbon to the workpiece surface making salt bath a nitrocarburizing process. The
temperature used is typical of all nitrocarburizing processes: 550 to 570 °C. The advantages of salt nitriding is that it
achieves higher diffusion in the same period of time compared to any other method.
Plasma Nitriding: Plasma nitriding, also known as ion nitriding, plasma ion nitriding or glow-discharge nitriding, is an
industrial surface hardening treatment for metallic materials.
In plasma nitriding, the reactivity of the nitriding media is not due to the temperature but to the gas ionized state. In this
technique intense electric fields are used to generate ionized molecules of the gas around the surface to be nitrided. Such
highly active gas with ionized molecules is called plasma, naming the technique.
The effect of processing time on the solute concentration profile (gradient) produced by
the carburising or nitriding of low-alloy steels. A reverse-S-shape profile created beneath a
constant surace solute chemical potential (C) maintained by the process. The
concentration gradient becomes less steep with increasing time
Typical hardness-depth profiles for prior hardened and tempered low-alloy (0.2%c 3.2% Cr) steel (722M24) and austenitic
stainless (17% Cr, 12% Ni, 2%Mo) steel (316516) after plasma nitriding in a cracked ammonia atmosphere for 10 hours at
500°C. The profile for 722M24 is fuzzy', whilst that for 316S16 is sharp
Substitutional Diffusion Processes:
Cementation
CVD
Pack cementation : Pack cementation is a process that has been used to produce corrosion and wear resistant coatings
Aluminizing
Chromizing
Siliconizing
The traditional pack mainly consists :
Substrate or parts to be coated
Master alloy (Cr and/or Al, Cr and/or Si)
Halide salt activator
Relatively inert filler powder (Al2O3, SiO2, or SiC)
Typical pack compositions used to produce a range of metallic coatings are given in table below. Of these the two
diffusion-coating processes that are most widely used are “aluminising” and “chromising”. The pack-aluminising process
will be used as an example to illustrate a typical process cycle. During the aluminising process material from the pack is
transferred to the component surface through the formation of intermediate volatile aluminium monohalide gas and
therefore the coating process is probably more accurately described as a chemical vapour deposition process.
Interdiffusion between the depositing aluminium and the substrate alloy results in the formation of the intermetallic
coating, primarily NiAl or CoAl, depending on the alloy base, but containing to a degree most of the elements present in
the base alloy either in solution or as dispersed phases.
Several approaches have been utilized for NiAl coating
fabrication. Early attempts employed pack cementation
to create thick (>25 m) coatings [10–12]. This process is
schematically illustrated in the figure. In this method, a nickel aluminide
coating is formed by the interdiffusion of aluminum
and nickel. The aluminum is supplied from the vapor phase by
a chemical vapor reaction with solid-state aluminum sources
in the pack. The deposition of Ni and Al multilayers followed
by reaction–diffusion heat treatments was later explored as a
method for synthesizing thin film nickel aluminides [13].
Z. Yu et al. / Materials Science and Engineering A 394 (2005) 43–52
Pack aluminising to form bond coat :
N. Padutre Thermal barrier coatings for gas turbine engine application. Science.
Chemical vapour deposition (CVD):
Coating Technology to Modify Surfaces.
Diffusion Technology
Overlay Technology
Overlay
Atomic nucleation and Growth
CVD, PVD, Electroless or electro deposition etc.
Direct attachment
Agglomerations and Consolidation
Thermal Spray, Cold Spray, Sol. gel etc
 Coating can be crystalline or amorphous
Different Coating Processes:
1.
2.
3.
4.
5.
6.
Physical vapour deposition (PVD)
Chemical Vapor Deposition (CVD)
Thermal Spray
Plating (Electro and auto catalic )
Slurry
Hot Dip
Thermal spray :
1. Flame spray
2. Electric arc spray
3. Plasma spray (Open atmosphere as well as controlled atmosphere)
4. High velocity oxy –fuel or High velocity air fuel
5. Detonation (D gun)
6. Cold spray (New Addition)
https://www.medacta.com/EN/mectagrip
 The common feedstock are powder, rod , wire etc
 It involves projection of molten or semi molten against the surface of materials to be coated (substrate)
 The impinging particle may spread like pancake.
https://empoweringpumps.com/tstcoatings-microstructures-science-thermal-spray-coatings/
Air Plasma Spraying (APS) to Make TBC
 Powder is fed in Plasma which melts and propel the powder toward substrate and form coating
 Coating contains ~15-20 % porosity which provided addition benefits by reducing elastic modulus and thermal conductivity
Inter columnar porosity
Cross-section of splat
Columnar grain
1 µm
APS Coatings has microcracks and pores of length scale
(~ 20 nm to ~ 20 µm )
APS is attractive due to its low cost and high productivity
32
Influence of Direct Splat-Affecting Parameters on the Splat-Type Distribution, Porosity, and Density of Segmentation Cracks
in Plasma-Sprayed YSZ Coatings
DOI.10.1007/s11666-021-01180-4
1. https://www.youtube.com/watch?v=-mcWhRg5w2A
2. Flame spray: https://www.youtube.com/watch?v=W7wJZTK8ec8
3. Arc spray: https://www.youtube.com/watch?v=xw-v9vnhd7Q
Factors affecting the Coating Properties:
1.
2.
3.
4.
5.
6.
7.
8.
9.
Chemical uniformity of feedstock
Particle shape
Spray Particle size distribution
Particle heating
Particle velocity
Atmosphere in flight
Substrate condition
Angle of spray
Coating thickness
Bending Modulus Variation Due to Sintering
Figure 1
Figure 2
Figure 3
 Coatings were sinter between 800 oC-1300 oC for different time
 Over all there is increase in stiffness of the coating after high temperature and reduction hysteresis loss(see Fig.1 and 2)
which is signature of reduction in porosity
 The increase in stiffness reduces thermal shock resistance, which eventually leads to failure of the coating after prolong
service in land based gas turbines
 The thermal cycle leads to reduction stiffness and can eventually leads to cracking in Gas turbiens used for aviation
propulsion after prolong service
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
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References:
Devi Lal, Praveen Kumar, Sanjay Sampath and Vikram, Jayaram: Low temperature stiffening of 7 wt.% Y2O3 stabilized ZrO2 APS Coating: J. Am. Ceram. Soc. 103 (2020) 276–289
Devi Lal, Praveen Kumar, Sanjay Sampath and Vikram Jayaram, Hysteresis and time dependent deformation of plasma sprayed Zirconia ceramic Acta Mat. 194 (2020) 394 – 402
Devi Lal, Vyshnavi ramanandan, Praveen Kumar, Sanjay Sampath and Vikram Jayaram, Damage accumulation in plasma sprayed Zirconia under cyclic loading. : J. Am. Ceram. Soc.
42
(2022), 1–12.
Evaluation of major factors influencing the TBC topcoatformation in axial suspension plasma spraying (SPS)
Curvature evolution during hvof