Title: Residual stress measurement of thermally sprayed cermet coatings using synchrotron x-ray radiation
1Tribo-Mechanical Evaluations of HIPed Thermal
Spray Cermet Coatings
Rehan Ahmed Heriot Watt University, UK
V. Stoica Heriot-Watt University, UK
T. Itsukaichi Fujimi Inc., Japan
S. Tobe Ashikaga Inst. of Tech., Japan
R. Gadow, M. Escribano University of Stuttgart,
Germany
2INTRODUCTION
The aim of this investigation was to integrate
the potential benefits of two process
technologies of thermal spraying and HIPing to
improve coatings tribo-mechanical performance.
- The specific objectives were to improve coating
strength and wear resistance by - Improved intersplat cohesion by HIPing
post-treatment. - Transform the mechanism of coating adhesion from
mechanical interlock to metallurgical bonding. - Improve the homogeneity and crystallinity of
coating microstructure.
3STARTING POWDER
WC-10NiCrBSiFeC
WC-40NiCrBSiFeC
Alloy composition Cr 7.6, Si 3.6, Fe 2.4, B
1.6, C 0.25, Ni Bal.
- Pre-alloying of WC-NiCrBSiFeC powders.
- Two different compositions WC-10NiCrBSiFeC and
WC-40NiCrBSiFeC were produced by the
agglomeration and sintering.
4THERMAL SPRAYING
- Functionally graded coatings were produced by the
HVOF (JP5000) process on 440-C bearing steel
substrate to minimise the mismatch of thermal and
elastic properties.
- The spraying parameters were as follows
- Oxygen flow 893 lit/min
- Kerosene flow 0.321 lit/min
- spraying distance 380 mm
- Spraying rate 50 g/min
440C steel substrate (8mm thick)
5HIPing POST-TREATMENT
- Two Different HIPing temperatures of 850oC and
1200oC were adapted at a pressure of 150 MPa. - Cooling and heating rates were optimised to
4oC/minute. - Holding time was 60 minutes.
- Uncapsulated HIPing conditions.
6COATING MICROSTRUCTURE
7XRD EVALUATIONS Powder vs. Sprayed Coating
8XRD EVALUATIONS Sprayed and HIPed coatings
9MIROHARDNESS EVALUATIONS
10 INDENTATION MODULUS E(1-?2)
11SEM observations HVOF coatings
Cryogenic fractured coatings
pores
micro-cracks
pores
As sprayed coatings
HIPed at 1200C coatings
HIPed at 850C coatings
12INDENTATION TOUGHNESS
200 ?m
As-sprayed coating
13SLIDING WEAR TESTS
Test conditions
Counter Body (balls) 440C Steel Si3N4 ceramic
Load 12 and 22 N
Sliding Speed 0.012m/s
Dry/Lubricated Dry
Coating
Reciprocating ball on plate apparatus
14SLIDING WEAR COATING VOLUME LOSS
0.014
0.012
0.01
0.008
Volume loss (mm3)
0.006
0.004
0.002
0
HIPed at 1200oC
As-sprayed
HIPed at 850oC
15SLIDING WEAR WEAR SCARS
Three dimensional interferometric plots of the
coatings tested against ceramic balls (load 22N)
16SLIDING WEAR TOTAL VOLUME LOSS
Total volume loss Coatings Vs steel balls
1
Total volume loss (mm3)
0.8
Total volume loss, 12N load
0.6
Total volume loss, 22N load
0.4
0.2
0
As-sprayed
HIPed at 850oC
HIPed at 1200oC
17SLIDING WEAR TOTAL VOLUME LOSS
Total volume loss Coatings Vs ceramic balls
Why improvement in wear resistance?
18FRICTION
19SLIDING WEAR WEAR MECHANISMS
SEM micrographs within the wear tracks of the
coatings tested against steel (load 12N)
As-sprayed coatings
HIPed at 1200oC coating
HIPed at 850oC coating
20RESIDUAL STRESS MEASUREMENT
21CONCLUSIONS
- Uncapsulated HIPing can be successfully applied
to post-treat thermally sprayed coatings. - HIPing post-treatment can improve the sliding
wear resistance of thermal spray cermet coatings. - Wear resistance improves with the increase in
HIPing temperature. - Improvement in sliding wear resistance is thought
to originate from the increase in coatings
hardness, elastic modulus and fracture toughness. - HIPed coatings show WC recovery and formation of
complex carbides. - Results indicate higher elastic modulus after
HIPing due to higher bonding between lamellas.
22WORK IN PROGRESS
- Influence of HIPing pressure, HIPing vs. Vacuum
Heat Treatment. - Influence of Coating Materials, especially WC-Co
- Coating Substrate Bonding Mechanism.
- Measurement of Adhesive and Cohesive strength.
- Optimisation of HIPing Parameters
- Influence on Fatigue and Impact performance