The effect of hard phases grain size on Ti(C,N) and WC based cermets fracture toughness was studied based on Palmqvist indentation toughness measurements. It was revealed that, when the composition and sintering process were the same, the fracture toughness of Ti(C,N) based cermet increased with the decrease of hard phase grain size, but the fracture toughness of WC based cermet increased with the increase of WC grain size. It may be explained by the differences existing between Ti(C,N) and WC crystal structures: There are eighteen independent slip systems in the two possible ｛110｝<11 0> and ｛111｝<11 0> slip families of face centered cubic Ti(C,N) crystal, but there are only four independent slip systems in the ｛101 0｝<111 3> slip family of close-packed hexagonal WC crystal. The propagating crack can easily seek a favorably oriented cleavage plane in Ti(C,N) grains when it crosses a big Ti(C,N) grain boundary, so it can easily propagate through contiguous Ti(C,N) grains and without markedly directional change. However, a markedly directional change occurred when a propagating crack passes through a WC crystal owing to strong anisotropy slip. With the increase of WC particle size, its crack deflecting and branching effect increases, which elevates toughness by increasing the area of fracture surface.
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