Wear behaviour of α-Si3N4 ceramics reinforced by rod-like β-Si3N4 grains

The microstructure of silicon nitride ceramics has a significant affect on their mechanical and tribological properties. The grain morphology, in particular, is a significant factor for controlling wear resistance. In this study, α-Si3N4 with various amounts of in-plane aligned rod-like β-Si3N4 grains were fabricated by hot pressing the tape cast and stacked green bodies with a small amount of rod-like β-Si3N4 seeds. The effect of anisotropy in microstructure and β-Si3N4 content on tribological properties was investigated by means of block-on-ring tests without lubricant using commercially supplied Si3N4 as ring specimens. Three sliding directions were set for the texured Si3N4 used as the block specimens in the friction test; directions perpendicular to and parallel to the casting direction on the casting plane (referred to as C-perp and C-para, respectively), and a direction parallel to the casting plane on the stacking plane (referred to as S). The effect of sliding direction on wear rate was small for the textured Si3N4 with less than 20% β-Si3N4 content, but it became larger with the increasing β-Si3N4 content. The C-para specimen exhibited larger wear rate than the other specimens of textured Si3N4 with more than 30% β content. The worn surface of the C-para specimen was irregular owing to grain pull-out, in contrast to the other specimens whose surfaces were quite smooth. In addition, for the textured Si3N4 with more than 30% β content the wear resistance of the S specimen was higher than of the C-perp specimen. The wear behaviour of the textured Si3N4 with high β content can be explained by the toughness anisotropy, assuming that wear proceeds through generation of microcracking, followed by their propagation mainly parallel to the sliding surface.