Tribo-Mechanical Evaluations of HIPed Thermal Spray Cermet Coatings V. StoicaHeriot-Watt University, UK Rehan Ahmed Heriot Watt University, UK T. ItsukaichiFujimi Inc., Japan S. TobeAshikaga Inst. of Tech., Japan R. Gadow, M. Escribano University of Stuttgart, Germany

INTRODUCTION 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. 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 : Improve intersplat cohesion. Transform the mechanism of coating adhesion from mechanical interlock to metallurgical bonding. Improve the homogeneity and crystallinity of coating microstructure. POWDER MANUFACTURE (WC-NiCrBSiFeC) HVOF SPRAYING HIPing POST- TREATMENT Coating Microstructure (SEM, XRD) Mechanical Strength (Modulus, Hardness, Toughness) Sliding Wear Resistance (Ceramic and Metallic couples) Residual Stress

STARTING POWDER Pre-alloying of WC-NiCrBSiFeC powders. Pre-alloying of WC-NiCrBSiFeC powders. Two different compositions: WC-10%NiCrBSiFeC and WC- 40%NiCrBSiFeC were produced by the agglomeration and sintering. Two different compositions: WC-10%NiCrBSiFeC and WC- 40%NiCrBSiFeC were produced by the agglomeration and sintering. WC- 10%NiCrBSiFeC WC- 40%NiCrBSiFeC Alloy composition: Cr 7.6%, Si 3.6%, Fe 2.4%, B 1.6%, C 0.25%, Ni Bal.

WC-10%Ni alloy (400  m) 440C steel substrate (8mm thick) THERMAL 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. 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: The spraying parameters were as follows: Oxygen flow – 893 lit/min Kerosene flow – lit/min spraying distance – 380 mm Spraying rate – 50 g/min Shot-blasting WC-40%Ni alloy (100  m) Grinding and polishing 32 mm diameter

Two Different HIPing temperatures of 850 o C and 1200 o C were adapted at a pressure of 150 MPa. Cooling and heating rates were optimised to 4 o C/minute. Holding time was 60 minutes. Uncapsulated HIPing conditions. HIPing POST-TREATMENT

COATING MICROSTRUCTURE Substrate WC-40NiCrBSi WC-10NiCrBSi As sprayed coatings Substrate WC-40NiCrBSi WC-10NiCrBSi HIPed at 850 o C coatings Substrate HIPed at 1200 o C coatings 7µm 10µm

-WC -Ni 3 B -W2C-W2C -Ni-W2C-W2C -FeW 3 C -W-W As-sprayed coating -W2C-W2C XRD EVALUATIONS Powder vs. Sprayed Coating -WC -NiB -Ni Starting powder WC

-WC HIPed at 1200 o C -WC -Ni-Ni 2 Si -Ni 2 W 4 C -Ni 3 B -Ni 5 Si 2 -FeW 3 C -Ni 2 W 4 C -Ni 4 B 3 -FeW 3 C WC -WC -Ni -WC -Ni 3 B -W2C-W2C -W2C-W2C-FeW 3 C W -W2C-W2C As-sprayed coating -WC -Ni -Ni 2 W 4 C -FeW 3 C -W2C-W2C HIPed at 850 o C XRD EVALUATIONS Sprayed and HIPed coatings

MIROHARDNESS EVALUATIONS Substrate WC-40%NiCrBSi WC-10%NiCrBSi As-sprayed HIPed at 850 HIPed at 1200 Vickers Hardness Distance from Surface (µm)

INDENTATION MODULUS =E(1- 2 ) As-Sprayed HIPed at 850 o C HIPed at 1200 o C Distance from Surface (µm) Indentation Modulus (GPa) WC-40%NiCrBSiWC-10%NiCrBSi Surface

As sprayed coatings HIPed at 850C coatings HIPed at 1200C coatings SEM observations: HVOF coatings Cryogenic fractured coatings micro-cracks pores

As-sprayed coating INDENTATION TOUGHNESS 200  m HIPed at 850 o C coating 200  m HIPed at 1200 o C coating 200  m Cracks

Sliding direction SLIDING WEAR TESTS Reciprocating ball on plate apparatus Normal load Ball Coating Test conditions Counter Body (balls) 440C Steel Si 3 N 4 ceramic Load12 and 22 N Sliding Speed0.012m/s Dry/LubricatedDry

HIPed at 1200 o C Volume loss (mm 3 ) As-sprayed HIPed at 850 o C Coatings Vs steel, 12N load Coatings Vs steel, 22N load Coatings Vs ceramic, 12N load Coatings Vs ceramic, 22N load SLIDING WEAR: COATING VOLUME LOSS

SLIDING WEAR: WEAR SCARS As-sprayed coating HIPed at 850C coating HIPed at 850 o C coating HIPed at 1200C coating HIPed at 1200 o C coating Three dimensional interferometric plots of the coatings tested against ceramic balls (load – 22N)

Total volume loss, 12N load Total volume loss, 22N load As-sprayedHIPed at 850 o CHIPed at 1200 o C Total volume loss (mm 3 ) SLIDING WEAR: TOTAL VOLUME LOSS Total volume loss – Coatings Vs steel balls

SLIDING WEAR: TOTAL VOLUME LOSS Total volume loss – Coatings Vs ceramic balls As-sprayedHIPed at 850 o CHIPed at 1200 o C Ball volume loss, 12N load Coating volume loss, 12N load Ball volume loss, 22N load Coating volume loss, 22N load Total volume loss (mm 3 ) Why improvement in wear resistance?

FRICTION As-sprayed HIPed at 850 o C HIPed at 1200 o C Friction coefficient Time (mins) Coatings Vs steel balls (load - 22N) Time (mins) Coatings Vs ceramic balls (load - 22N) As-sprayed HIPed at 850 o C HIPed at 1200 o C Friction coefficient

SLIDING WEAR: WEAR MECHANISMS As-sprayed coatings HIPed at 850C coating HIPed at 850 o C coating HIPed at 1200C coating HIPed at 1200 o C coating SEM micrographs within the wear tracks of the coatings tested against steel (load – 12N)

RESIDUAL STRESS MEASUREMENT Residual stress (MPa) Distance from the surface ( µ m) As-sprayed HIPed at 850C HIPed at 1200C

CONCLUSIONS 1. Uncapsulated HIPing can be successfully applied to post-treat thermally sprayed coatings. 2. HIPing post-treatment can improve the sliding wear resistance of thermal spray cermet coatings. 3. Wear resistance improves with the increase in HIPing temperature. 4. Improvement in sliding wear resistance is thought to originate from the increase in coating ’ s hardness, elastic modulus and fracture toughness. 5. HIPed coatings show WC recovery and formation of complex carbides. 6. Results indicate higher elastic modulus after HIPing due to higher bonding between lamellas.

WORK 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