The potential of WC-Co hardmetals and HVOF sprayed coatings to combat water-droplet erosion

It is well known that impingement of water-droplets at high velocities causes degradation of materials in a wide range of environments, from steam turbines to aircraft components. In very aggressive environments, even highly wear resistant materials will suffer some damage. In this work, six sintered WC-Co hardmetals (varying both in the binder phase fraction and in the size of the carbides) were exposed to a very high velocity stream of water droplets; an HVOF sprayed WC-Co coating was examined alongside together with a titanium alloy for comparison. It was found that all the sintered materials exhibited an incubation period where no damage was observed up to a certain level of exposure, but beyond this exposure, erosion damage increased rapidly. Both grain size and carbide fraction in the sintered hardmetals influenced the incubation period, with the most resistant materials being composed of the smallest carbides and the highest carbide fractions; however, the microstructural features of the hardmetals had little influence on the subsequent development of wear. The incubation period is characterized by the initiation of damage and the removal of the cobalt binder phase. The subsequent wear is associated with hydraulic penetration and removal of the carbides. Unlike the sintered hardmetals, the WC-Co coating wore rapidly in the initial stages of exposure, and thus exhibited no discernable incubation period. The differences in behaviour between this and the sintered materials are associated with the nature of the matrix phase in the thermally-sprayed coatings which is brittle, thus leading to a different mechanism of erosion governed by fracture. In comparison to the titanium alloy, the best of the sintered hardmetals exhibited a much longer incubation period to the onset of erosion, but the then proceeded to erode at a much faster rate. The HVOF sprayed coating exhibited much poorer erosion resistance than the titanium alloy in all cases.

► Wear and damage to WC-Co hardmetals and coatings in water droplet erosion
► Incubation period extended only by having small carbides and low cobalt fraction
► Coatings damaged rapidly under the aggressive conditions employed
► Mechanisms of damage and wear highlighted
► All hardmetals exhibited higher erosion resistance than titanium alloy