Influence of hardmetal feedstock powder on the sliding wear and impact resistance of High Velocity Air-Fuel (HVAF)sprayed coatings

The present work aimed to clarify how the characteristics of WC-CoCr hardmetal feedstock powders, namely the grain size of the WC carbides and of the binder and the compressive strength of the sintered aggregates, affect the dry sliding wear and impact resistance of coatings deposited by High Velocity Air-Fuel (HVAF)spraying. Ball-on-Disc tests, which mimic a sliding wear process in the presence of hard asperities as it may occur e.g. in hydraulic seal joints or papermaking components, resulted in mild wear through near-surface microscale plastic flow, the exact nature of which was significantly affected by WC size. Finite element simulations of a single-asperity sliding process indeed showed that large WC grains concentrate contact stresses, thus undergoing very localised deformation. It is experimentally seen that repeated deformation of the carbide grains resulted in their cracking and pull-out. Uniformly distributed, fine carbides allowed the matrix to take on some stress, thus undergoing more homogeneous plastic flow. Block-on-Ring tests elicited adhesive wear as it may happen e.g. in metal-to-metal contacts (e.g. petrochemical valves). This could be effectively restrained by low matrix mean free path. Cyclic impact resistance of coarse-grained coatings was better than that of fine-grained ones, because of better large-scale cohesive strength.