Fracture toughness and crack morphologies in eroded WC–Co–Cr thermally sprayed coatings

The fracture toughness of two thermally sprayed tungsten carbide/cobalt–chromium coatings of nominally identically composition, 86WC–10Co–4Cr, one produced by the Detonation Gun (D-Gun) process and one by the High Velocity Oxy-fuel (HVOF) process, has been determined by the indentation method. Indentation testing revealed that both coatings exhibited anisotropic crack propagation and hence, fracture toughness, crack propagation being much easier parallel to the coating/substrate interface than transverse to it. The sample produced by the D-Gun process had a lower fracture toughness, and greater range of fracture toughness values, both parallel and transverse to the coating/substrate interface, than the HVOF coating reflecting the greater microstructural inhomogeneity of this coating. The subsurface cracks produced by indentation have a similar morphology to those produced on erosion testing and hence, the fracture toughness values obtained by this method are appropriate for use in erosion rate prediction equations. However, the range in fracture toughness values found should be taken into account when modelling erosion as regions of low fracture toughness, where subsurface crack growth will be easier, will tend to control the rate of erosion. The cracks produced by indentation testing and erosion tend to propagate along the boundary between material that has been completely molten during thermal spraying and the inner core of the projected particle.