Unlubricated friction and wear behavior of low-pressure plasma-sprayed ZrO₂ coating at elevated temperatures

The unlubricated friction and wear behavior of low pressure plasma-sprayed (LPPS) ZrO₂ coating were studied by using a standard SRV wear test system with reciprocating motion against sintered Al₂O₃ ceramic sphere from room temperature to 800°C. Microstructural observations showed that ZrO₂ splats appeared as fine lamellae structures together with small amounts of Al₂O₃ and SiO₂ inclusions with different gray levels in the back-scattered electron (BSE) mode. The friction and wear of ZrO₂ coating showed a strong dependence on temperature, changing from a low to a high wear regime with the increase of temperature. ZrO₂ coating exhibited low friction and wear at room temperature and 200°C. However, when temperature was increased to above 400°C, friction and wear of the coating increased rapidly and reached a maximum at 600°C together with significant noise and distinct vibration caused by severe surface fracture. Intersplat fracture and surface fatigue of ZrO₂ splats were demonstrated to be the dominated wear mechanism at room temperature. Delamination caused by subsurface crack propagation and inclusion cracking was the dominated wear mechanism at 600°C. Above 700°C, recrystallization and abrasive wear in the form of removal of fine ZrO₂ grains became the dominated wear mechanism together with local plastic flow and viscous deformation.