An investigation of microstructure and sliding wear in thermally sprayed WC-Co coatings
Four types of WC-Co cermet powders (conventional WC-12 wt% Co, two different types of conventional WC-17 wt% Co and nanoscale WC-12 wt% Co powders), which have differences in terms of Co content, powder manufacturing process and WC grain size, have been sprayed to form coatings with both the high velocity oxygen-gas fuel (HVOGF) and the high velocity oxygen-liquid fuel (HVOLF) processes. The carbide grain size, powder particle size distribution and phase volume fraction of the starting powders were measured. The as-sprayed coatings were characterised by optical microscopy (OM), scanning electron microscopy (SEM), xray diffraction (XRD), transmission electron microscopy (TEM) and microhardness and surface roughness measurements. The HVOGF coatings all displayed a lamellar splat structure characteristic of thermal spraying. HVOLF coatings had less well defined splat structures which suggested lower degrees of particle melting. A number of phases (WC, W2C. W and amorphous) were observed in the coatings by XRD, SEM and TEM analysis indicating phase decomposition and oxidation during spraying. The degree of decomposition was increased by higher gas temperatures, longer particle residence times in the jet, small carbide grain size in the powder particles and an open powder structure. By suitable selection of the spraying system, spraying parameters and powder feedstock, particle decomposition during spraying could be minimised. All coatings obtained were subjected to sliding wear against an alumina ball; a sintered WC-11 wt% Co solid disc was also tested for comparison. The decomposition of the original WC-Co powder structure in the coatings was found to be deleterious to wear resistance since the tungsten rich binder phase that now existed was significantly more brittle than the pure cobalt binder in the feedstock powder from which it was derived. However, optimisation of the conditions to reduce decomposition did not result in an increase in wear resistance. Instead, deposition of powder particles that had a significant portion of solid phase resulted in fragmentation and debonding of the carbides resulting from deformation during impact and causing increased wear. The sintered WC-Co exhibited the highest wear resistance of all materials examined, whilst the HVOGF sprayed WC-12 wt% Co coating, which exhibited significant decomposition, was the most wear resistant of all the coatings examined.