Abstract The erosion by Al 2O 3 particles of WC-6wt.%Co alloys with WC grain size in the range 0.57–3.25 μm was determined as a function of impinging particle size (63–405 μm), velocity (35–93 m s −1) and angle of impact (20°–90°). The angle at which the maximum in erosion rate occurred decreased from 90° as the particle size and velocity increased and the WC grain size decreased. Scanning electron microscopy observations revealed that the erosion mode depended on the relative size of the impact crater with respect to the WC grain size. When the crater encompassed less than 10 WC grains, the erosion mode was of the brittle type; when it encompassed more than 100 WC grains it had the characteristics of the ductile type. The principal material removal mechanism in the brittle mode was by cracking and crushing of the WC grains; that in the ductile mode by plastic deformation and fracture of the binder phase, the WC grains remaining relatively intact. Some plastic deformation of the WC grains occurred in both modes. The results for the ductile mode were in accord with a modification of the plastic indentation model, and those for the brittle mode with the elastic-plastic fracture models. Fragmentation of the impacting particles occurred and its magnitude and significance are discussed.