Fretting fatigue properties of WC-Co thermal sprayed NiCrMo steel

Fretting fatigue tests of NiCrMo steel (JIS SNCM439) sprayed with tungsten carbide with additive of 12% cobalt (WC-Co) by HVOF (High Velocity Oxygen Fuel) were carried out to study the effect of WC-Co coating on fretting fatigue behavior. Since the fatigue strength of the present alloy steel was reduced to about 27% by the effect of fretting, fretting fatigue strength was improved by approximately 30% by WC-Co spraying onto the specimen. The tangential force coefficient, which is tangential force measured during the fretting fatigue tests divided by contact load, behaviors of WC-Co sprayed specimen were different form those of the non-sprayed specimen. Especially, the tangential force of WC-Co specimen in an early stage of the test indicated significant low value compared with that of non-sprayed specimen and this was caused from good wear resistance of WC-Co layer. Therefore, the initiation of a fretting crack in case of WC-Co sprayed specimens delayed, and crack propagation rate was lower than that of non-sprayed specimen. These were main reasons for the improvement of fretting fatigue strength by-WC-Co. spray.