硬质相粒度对金属陶瓷断裂韧性的影响

EFFECT OF HARD PHASES GRAIN SIZE ON CERMETS FRACTURE TOUGHNESS

用压痕法测定了具有不同粒度硬质相的Ti(C,N)基和WC基金属陶瓷的断裂韧性,结果发现,Ti(C,N)基金属陶瓷的断裂韧性随硬质相粒度的增大而减小,而WC基金属陶瓷的断裂韧性随硬质相粒度的增大而增大。产生上述现象的主要原因与硬质相的晶体结构有关:在室温条件下,面心立方结构的Ti(C,N)晶体中可能存在{110}<110>和{111}<110>两个滑移族(含18个独立滑移系);裂纹从一个Ti(C,N)颗粒扩展至另一个Ti(C,N)颗粒时很容易形成取向有利。当Ti(C,N)颗粒较粗时,极易发生穿晶断裂,并且裂纹连续穿晶扩展时亦不会发生显著的偏转或分叉,Ti(C,N)呈现较强的脆性断裂特征。而密排六方结构的WC晶体只有{1010}<1123>一个滑移族(含4个独立的滑移系);由于取向不利,裂纹难以连续穿晶扩展,且随WC粒度的增大,其对裂纹的偏转和分叉作用增强,从而导致断裂面表面积增大而增韧。

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} and {111} slip families of face centered cubic Ti(C,N) crystal, but there are only four independent slip systems in the {1010} 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.