EROSIVE WEAR OF THERMALLY SPRAYED TUNGSTEN COATINGS

A detailed study has been made of the erosion behaviour of air plasma sprayed (APS) and vacuum plasma sprayed (VPS) tungsten coatings. The erosion rates observed under a variety of test conditions are discussed in terms of wear mechanisms, coating microstructure and mechanical properties. Both coating systems exhibited similar trends in wear behaviour as a function of impingement angle, particle velocity and size. However, the steady-state erosion rates of APS coatings were an order of magnitude greater than those of either VPS coatings or bulk material tested under similar conditions. For APS tungsten coatings, the higher erosion rates are attributed to the loss of individual splats by brittle boundary fracture which is caused by the presence of an oxide layer at the intersplat boundaries. In contrast, VPS tungsten coatings were well bonded and oxide free and the material removal mechanisms involved plastic deformation, i.e. ploughing of the surface of splats. Scratch adhesion and tensile test measurements were of the coatings in terms of bulk modulus and intersplat cohesion.