The friction and wear behaviour of two ZrO2-Al2O3 composite materials, ADZ (20 wt% Al2O3 dispersed in yttria-doped ZrO2 matrix) and ZTA (Al2O3-15 wt% ZrO2), were investigated. Sliding wear tests were carried out on these materials under dry conditions using a ball-on-plate configuration. The effects of grain size and zirconia phase transformation on friction and wear behaviour were examined. Sinter forging was applied to determine the role of grain boundaries on the wear process. It was found that for ADZ composites the wear resistance increased with decreasing grain size. Only limited grain pull-out occurs in these systems, which also explains the fact that in this case sinter forging has no positive effect on wear resistance. For ZTA composites a compressive surface caused by zirconia phase transformation has a beneficial effect on wear resistance. Here grain pull-out is the main wear mechanism which explains the positive effect of sinter forging ZTA on wear resistance.