ROLLING-CONTACT FATIGUE FAILURE MECHANISMS IN PLASMA AND HVOF SPRAYEDWC-CO COATINGS

The rolling contact fatigue (RCF) behaviour of thermally sprayed WC-Co coatings with nominal compositions of WC-12%Co, WC-10%Co-4%Cr and WC- 17%Co was studied with a two-roll configuration roll-against-roll test ing apparatus under 420-600 MPa Hertzian contact stresses in unlubrica ted pure rolling conditions. The coatings were prepared by atmospheric plasma spray (APS) and two high-velocity oxyfuel (HVOF) spray process es. In the APS sprayed WC-12%Co coating the RCF damage was dominated b y an increased surface roughness due to spallation of flakes and a for mation of a network of cracks within the coating layer. HVOF sprayed W C-12%Co and WC-10%Co-4%Cr coatings were damaged either by the formatio n of vertical, linear cracks or pitting of the contact surface. The fo rmation of pits in the HVOF sprayed coatings was significantly less th an that found in the APS sprayed coating. The HVOF sprayed WC-17%Co co ating showed the best RCF behaviour among the studied coatings with un changed surface roughness, no formation of cracks and only a few pits were found on the contact surface. The good resistance of this coating against formation of failure in the RCF testing is caused by its high er ductility and fracture toughness due to a higher metallic binder co ntent in comparison with the other coatings. Other characteristics suc h as a low amount of brittle Co-W-C carbides and a dense microstructur e are also believed to be beneficial for a RCF resistant coating. (C) 1997 Elsevier Science S.A.