High velocity solid particle erosion behaviour of CVD boron carbide on tungsten carbide

The superior mechanical properties of boron carbide (B13C2 phase) make it an attractive candidate for application as a wear resistant coating in tribological applications. The present work aims to determine the solid particle erosion behaviour of 12–18 μm thick CVD boron carbide coatings on cemented tungsten carbide substrates. The erosion tests were performed on a high-energy air solid particle erosion rig using 150–420 μm spherical soda-lime glass beads and 90–355 μm angular quartz silica sand under normal impact, at impingement velocities between 132 and 250 m s−1 and a flux rate of 0.5 kg m−2 s−1. The erosion rates and mechanisms are presented and discussed in terms of coating thickness, particle velocity, particle shape and size. The eroded surfaces were examined using 2- and 3D white light optical interferometry profiling and scanning electron microscopy (SEM) with EDS mapping to investigate the erosive wear damage mechanisms of the CVD boron carbide. The nature of post-test erodent fracture was determined using laser diffraction particle size analysis. The results indicate that erosion of CVD boron carbide occurs predominantly through a single-stage mechanism by the formation of lateral–radial crack systems that propagate outwards towards the free CVD surface and extend into the coating substrate interface. This was confirmed by optical depth profiling and from the presence of substrate and interlayer peaks in the EDS map from the vertex of the radial–lateral crack systems. The damage mechanism observed appears to be independent of the erodent shape. Adjacent lateral cracks intersect, resulting in further material loss as erosion progresses. The density of the lateral–radial crack systems is higher at the centre of the wear scar compared to the outer regions. Similar failure mechanisms for brittle coatings have recently been predicted through fracture mechanics considerations and also observed experimentally by other authors. Some evidence of surface micro- and nanochipping is also observed in other regions of the eroded CVD boron carbide surface. Comparisons have been made with previous investigations into the erosion behaviour of CVD diamond coatings under similar test conditions.