Investigation of the sliding contact properties of WC-Co hard metals using nanoscratch testing

Abstract

Nanoscratch tests were performed on a range of WC-Co hard metals with varying cobalt content and WC grain size using a Nanoindenter XP. Single and multiscratch tests were conducted with various loads. The scratch friction coefficient for all the samples was found to be approximately 0.4 and was observed to fluctuate due to the hard and ductile phases in the material. The scratch width and depth were found to increase with increasing load for single scratches. Multiscratching with a constant load resulted in the widening and deepening of the scratches at each load with accumulative damage occurring as more tests were performed. Damage was mainly attributed to a brittle mechanism occurring via the formation and interaction of subsurface cracks and the deformation of WC grains via slip. A deformed layer was formed on the surface of several of the hard metals during multiscratching and this was found to contain WC fragments which were formed during testing. Cobalt extrusion also took place and in the case of the 6 and 6.5% cobalt samples, led to the subsequent loss of loosely anchored WC grains. The scratch depth and width were found to increase linearly with load with more severe grain fracture taking place as the load increased.