Influence of Post-treatment on the Tribo-mechanical properties of Cermet Coatings R. Ahmed, S. Stewart, V. Stoica Heriot-Watt University, UK Y. Itsukaichi Fujimi Inc., Japan HIPing Post-treatment – Previous Investigations 1) Kuribayashi, H., Suganuma, K., Miyamoto, Y., & Koizumi, M., “Effect of HIP treatment on plasma sprayed coating onto stainless steel”, American ceramic society bulletin, 65(9), 1306-1310, (1986). Al 2 O 3 , ZrO 2 and TiC plasma spray coatings HIPed (Capsulated) at 1100 to 1300 o C for 1hour at 100MPa - Remarkable improvements in hardness and tensile strength. 2) H. Ito, Nakamura, R., Shiroyoma, M., & Sasaki, T., “Post-treatment of plasma sprayed WC-Co coatings by hot isostatic pressing”, NTSC, CA, 233- 238, (1990). WC-Co plasma spray coatings HIPed (Capsulated) at 500 to 1000 o C for 30 minutes at about 5MPa – Remarkable improvements in hardness and abrasive wear resistance. Lamellar to granular transformation. 3) Khor, K.A. & Loh, N. L., “Hot isostatic pressing of plasma sprayed Ni-base alloys”, JTST, 3(1), 57-62, (1994). Numerous studies on Capsulated and uncapsulated HIPing of plasma sprayed NiCrAl and ZrO 2 -Y 2 O 3 coatings – Improvement in hardness and modulus, reduction in porosity. 1
Project Background S. Tobe, Y. Ando, R. Ahmed and V. Stoica, “ Enhancement of wear and mechanical properties of thermally sprayed WC-Co coatings by HIPing post-treatment”, Tribology in Environmental Design, Second International conference, Bournemouth, UK, ISBN 1 860584152, 119-127, 2003. HIP Code HC-1 HC-2 HC-3 HC-4 HC-5 AS Capsulation Yes No Yes Yes Yes As- Spray ed Temperature 850 850 900 900 1000 (Not ( o C) HIPe Pressure (MPa) 150 150 150 150 150 d) Holding Time 60 60 60 120 60 (minutes) Project Background (WC-Co coatings) Holding Time = 1 hour Holding Time = 1 hour a) As sprayed sample (AS) b) HC-1 sample c) HC-2 sample As-Sprayed Coating HIPed @800 o C (Capsulated) HIPed @800 o C (Not Capsulated) Holding Time = 1 hour Holding Time = 2 hour Holding Time = 1 hour e) HC-4 sample f) HC-5 sample HIPed @900 o C (Capsulated) d) HC-3 sample HIPed @900 o C (Capsulated) HIPed @1000 o C (Capsulated) 2
Aims of Current Investigation • Consider Economical and Design Factors • Higher Temperatures? • Influence of pressure? (HIPing vs. VHT) • Develop structure property relationship for tribo-mechanical applications Methodology of Investigation POST-TREATMENT POWDER HIPed (850 & 1200 o C HVOF @ 150MPa for 1 hour) MANUFACTURE SPRAYING (Uncapsulated) (WC-12%Co) (JP5000) Vacuum Heating Agg. & Sint. @1200 o C for 1 hour •Coating Microstructure (SEM, XRD) •Mechanical Strength (Modulus, Hardness) •Residual Stress using Neutron Diffraction •Sliding Wear Resistance (Ceramic and Metallic couples) •Rolling Contact Fatigue Testing 3
Hot Isostatic Pressing Unit HIPing Conditions Temperature 850 °C , 1200 °C Pressure 150 MPa PRESSURE Environment Argon VESSEL Heating 4 °C /minute FURNACE /Cooling rate Encapsulation No WORK- PIECE SEM – As-sprayed vs. HIPed@850 o C As-Sprayed HIPed @850 o C 4
SEM – As-sprayed vs. HIPed or VHT @1200 o C XRD – As-sprayed vs. HIPed @850 o C HIPed or VHT As-Sprayed 10 10 Intensity (Counts/sec) -Co 6 W 6 C 20 20 -Co 6 W 6 C 30 30 -WC -WC -WC -Co 6 W 6 C -W 2 C -WC -W 2 C -W 2 C -W 2 C 40 40 -Co 6 W 6 C -W -Co 2 W 4 C -Co 6 W 6 C -Co 6 W 6 C -WC -WC 50 50 -W 2 C HIPed@850 o C 60 60 -Co 6 W 6 C As-sprayed -W 2 C -WC -WC -WC -WC -W 2 C 70 70 -Co 6 W 6 C -WC -WC -Co 6 W 6 C -WC -WC -WC -WC 80 80 -WC -WC 90 90 5
XRD – As-sprayed vs. HIPed @1200 o C -WC -WC As-sprayed Intensity (Counts/sec) -WC -WC -W 2 C -WC -WC -WC -WC -W 2 C -WC -WC -W 2 C -W 2 C -W 2 C -WC -W 2 C -W -Co 6 W 6 C 10 20 30 40 50 60 70 80 90 HIPed@1200 o C -WC -Co 6 W 6 C -Co 6 W 6 C -Co 6 W 6 C -WC -WC -WC -Co 6 W 6 C -Co 6 W 6 C -Co 6 W 6 C -Co 6 W 6 C -WC -WC -WC 10 20 30 40 50 60 70 80 90 Microstructural Changes -Summary HIPed @1200 o C As-sprayed Kirkendall voids At% Co 6 W 6 C & C W 21% C 48% C Co 5% Co Fe 19% WC Cr 7% coating Diffusion layer substrate Cr Fe • Diffusion of Fe and Cr from • Carbon diffusion substrate towards the substrate • Carbide dissolution to form • Formation of prismatic faces Kirkendall voids • Free Carbon & eta-phases • Diffusion layer 6
Residual Strain –Neutron Diffraction coating Detector 2 substrate 1000 0 Microstrain Diffracted beam Incident Beam -1000 HIPed@1200 o C As-Sprayed coating -2000 Sample HIPed coating As-sprayed -3000 -4000 Detector 1 0 0.5 1 1.5 2 2.5 3 Depth from coating surface (mm) ISIS Facility Set-up, UK Hardness Comparison COATING SUBSTRATE 2000 1800 Vickers Hardness (HV 300 ) 1600 1400 1200 1000 As-sprayed 800 HIPed at 850C 600 HIPed at 1200C 400 200 Heat-treated 0 0 50 100 150 200 250 300 350 400 450 Depth from surface 7
ν 2 Indentation Modulus = E(1 - ν E(1- 2 ) ) 500 ) As-sprayed HIPed @ 1200°C a P HIPed @ 850°C Vacuum heat-treated @ 1200°C G 400 ( s u l u 300 d o M 200 n o i t a 100 t n e d 0 n I Sliding Wear Tests Sliding Wear Tests Test conditions Test conditions Normal load Normal load Counter Body 440C Steel (balls) Si 3 N 4 ceramic Si 3 N 4 ceramic Load 12 and 22 N Ball Sliding Speed 0.012m/s Coating Lubrication Dry Sliding direction Sliding direction Reciprocating ball on plate apparatus Reciprocating ball on plate apparatus 8
Wear Performance - system 80 As-sprayed HIPed at 850C 70 Wear resistance (Nm/mm 3 ) HIPed at 1200C Heat-treated 60 50 40 30 20 10 0 ceramic - 22N steel - 12N steel - 22N ceramic - 12N Rolling Contact Fatigue Tests Drive shaft connected via belt to motor rotates coated disc at 4000 rpm Hertzian Contact Width 2a Coating Air pressure from bellows 0.5a generates required contact Substrate load between balls and disc. Maximum 45 o shear stress 9
RCF Test Conditions Planetary Balls Steel / Ceramic Temperature 25 ºC Hertzian Contact 2.7 GPa Stress Lubricant Full Film Boundary Regime RCF Test Results 4 As-sprayed HIPed @ 1200°C Stress cycles ( × 10 6 ) HIPed @ 850°C Vacuum heat-treated @ 1200°C 3 2 1 0 Steel, 2.7GPa Ceramic, 2.7GPa 10
RCF Failure Modes Micro-cracks WEAR TRACK Vacuum heated@1200 o C HIPed@1200 o C Catastrophic Failure Progressive and Predictable Failure Post RCF Test Analysis as-sprayed 40µm 200 (µm) HIP 1200ºC 20µm 200 (µm) 11
Conclusions 1. Microstructural changes associated with the post-treatment of WC-Co coatings can significantly improve tribo-mechanical performance of components by improving the hardness (phase changes), cohesive strength (interlamellar bonding) and adhesive strength (diffusion zone) of coatings. 2. Improvement in RCF performance was attributed to the diffusion at the coating substrate interface resulting in metallurgical bonding. 3. Sliding wear test results indicate that the overall wear resistance improves with the post-treatment, and best results were obtained for coatings HIPed at 850 o C for ceramic and at 1200 o C for steel counterbody. 4. Residual stress investigations confirmed that not only the post-treated coatings have lower and more uniform compressive strain, but also the strain gradient at the coating substrate interface is minimised after the post-treatment. Acknowledgements •S. Davies at Bodycote HIP, UK (HIPing) •Prof. S. Tobe at Ashikaga Institute, Japan for EPMA analysis •ISIS (Rutherford Lab, UK) for use of neutron diffraction facilities •EPSRC, UK for financial contribution 12
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