This poster was presented at the National Metallurgists' Day-Annual Technical Meeting (NMD-ATM), 2016, held at IIT-Kanpur

1. Shalaka Shinde1, Abhilash Agnihotri2*, Hariharan S3, Dheepa Srinivasan3,
Vishwanathan Venkatachalapathy4 and Biju Dasan3
1Materials Engineering, State University of New York, Stony Brook, New York, USA,
2Metallurgical and Materials Engineering, NITK-Surathkal, India.,
3GE Power, GE India Technology Center Pvt. Ltd, Bengaluru, India,
4GE Manufacturing Technology Center, Dammam, Saudi Arabia
*Presenting Author; (E-mail: agnihotri.abhilash1@gmail.com)
Objectives
The objectives of this study are,
1. To compare ICP results for HVOF and APS processes and see if it
reflects the fundamental difference between the processes.
2. To study the effect of pre-heating passes on the remnant stress
from grit blasting.
3. To study the effect of number of coating passes, process type,
substrate thickness and substrate type on various stresses.
4. To develop the In-situ Coating Processing Sensor (ICP) for
applications via a study of the evolving stress during deposition
and a quantification of overall residual stress, in order to be able to
obtain good quality thermal sprayed coatings on components and
reduce the need for destructive analysis/metallography .
Comparison of Residual Stresses in HVOF and APS MCrAlY
Coatings using In-situ Coating Property (ICP) Sensor
Data Acquisition using the ICP Sensor
Introduction
Thermal spray coatings are extensively used in gas turbine hot gas path
components as oxidation and thermal protection coatings. MCrAlY
(M=Ni, Co, Fe) coating thickness range between 150 – 350 µm and are
applied using either via high velocity oxy fuel (HVOF) or air plasma
sprayed (APS) processes. Coating performance is heavily dependent on
the residual stress generated during these processes. The origin of
residual stress in thermally sprayed coatings is mainly due to three
effects:
• Quenching (Intrinsic) Stress:
Due to the hindered contraction of the individual splats as they cool
from the melting temperature to the temperature of the underlying
substrate.
• Differential Thermal Contraction Stress:
Arises due to the mismatch in CTE (Co-efficient of Thermal
Expansion) of the substrate and the deposit.
• Through Thickness Thermal Gradient Stress:
The transient injection of relatively high heat fluxes onto the substrate
lead to high thermal gradients which in turn generate differential
thermal expansion at different depths and changes to the local misfit
strains.
These techniques have their own merits and demerits. In-Situ Coating
property (ICP) sensor measures macroscopic curvature changes and
calculates stress using the same. During the coating process, the
substrate (to be coated) is grit blast and is given pre-heating passes to
relieve the stress due to the same. The number of such passes is
empirically decided.
Methods and Materials
1. Beams of IN718 and SS316 of dimension 9’ X 1’ X (1.6 mm/2.4
mm) were used as substrates to carry out the experiments.
2. These beams were grit blast with -85 mesh alumina powder.
3. In-situ Coating Property Sensor-8 of ReliaCoat Technologies was
used for measuring residual stresses
4. Weight and thickness before and after coating were measured as
these are required in the calculation of residual stresses.
5. Amdry 962 from OC Oerlikon, which is a NiCrAlY based powder
was used to produce the coating.
6. The points of interest in the data acquired (Start of spraying; Start
of evolving stress; End of evolving stress; End of spraying; End of
cooling) were chosen and the residual stress values were obtained
from the software.
Results
Effect of Pre-Heating (PH) Passes:
-------------------------------------------------------------------------------------
Effect of Coating Passes
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Effect of Process Type:
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Effect of Substrate Thickness:
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Effect of Substrate Type:
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Effect of Powder Type: Ni211-4 vs Metco 4198
Process Steps
As received beam
Grit blasted
1.Grit blasting using Alumina
(-85 mesh)
2.Pre-heating,deposition
and acquisition of data
Data
Acquisition
51.9
38.4
-50
0
50
100
10 CP 15 CP
Stress(MPa)
No. of coating passes
HVOF
Evolving Stress Deposition
Thermal Residual
61.7
70.8
-20
0
20
40
60
80
100
10 CP 15 CP
No. of coating passes
APS
Evolving Stress Deposition
Thermal Residual
30
40
50
60
70
80
90
10CP 15CP 20CP
ResidualStress
(MPa)
Coating Thickness
HVOF
Before
APS
Before
51.9
87.7
-50
0
50
100
150
1.6 mm 2.4 mm
Stress(MPa)
Substrate Thickness
HVOF
Evolving Stress Deposition
Thermal Residual
0.9
23.4
-100
-50
0
50
100
1.6 mm 2.4 mm
Substrate Thickness
APS
Evolving Stress Deposition
Thermal Residual
51.9
26.3
-200
-100
0
100
200
IN718 SS316
Stress(MPa)
Substrate Type
HVOF
Evolving Stress Deposition
Thermal Residual
35
16.5
-60
-40
-20
0
20
40
60
80
IN718 SS316
Substrate Type
APS
Evolving Stress Deposition
Thermal Residual
Microstructures
Microstructures:
Qualifying Coating:
Evolving stress : 61.5 MPa
Coating with Unmelts:
Evolving stress : -7.1 MPa
Residual Stresses calculated using the XRD Sin2
ψ technique
Sample Average Std. Deviation
SS316 2PH -99.46 48.43
SS316 4PH -20.42 18.09
Conclusions
1. Four pre-heat passes relieve stresses better than two pre-heat passes
(as seen quantitatively and also from the XRD pattern).
2. Increasing coating passes decreases residual stresses for HVOF
coatings while increases the same for APS coatings.
3. In both HVOF and APS coatings increasing substrate thickness
increased residual stresses.
4. SS316 beams have lesser residual stresses than in IN718 in both the
processes.
The evolving stress calculated by the ICP reflects the nature of the
adhesion of the incoming particles on the existing ones. This can be an
important asset in industrial spray booths as a quality control parameter.
Design of Experiments
HVOF / APS
IN718 SS316
1.6 mm 2.4 mm 2.4 mm1.6 mm
10 CP 20 CP
10 CP
10 CP 20 CP
10 CP
Process Type
Substrate
Substrate
Thickness
Coating
Passes
SS316
2 PH 4 PH Pre-Heat Passes
Temperature sensor
Displacement sensorCooling water
Curvature sensor Substrate
Robotic arm
Spraying gun
Spray Booth Setup for ICP
HVOF APS
100 µm 100 µm
100 µm 100 µm
180 µm
220 µm
90 µm
125 µm
Residual stress decreases with increasing coating passes for HVOF coatings and increases for
APS coatings.
Residual stress increases with increasing substrate thickness for both HVOF and APS
coatings.
Residual stress is lesser for SS316 substrates than IN718 ones for both HVOF and APS
coatings.
XRD peak of the 2 pre-heat passes sample is not sharp, indicating the presence of macro
strain. This is also seen quantitatively, in contrast to 4 pre-heat passes sample.
Acknowledgements
The authors would like to thank GE Power, Repair Development
Center, Bangalore, for enabling carrying out this project.
Intensity(counts)
2PH 4PH
1400
1200
1000
800
600
400
200
2θ0
5000
4000
3000
2000
1000
2θ0