Wear behaviour of α-Si 3N 4 ceramics reinforced by rod-like β-Si 3N 4 grains

Abstract

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-Si 3N 4 with various amounts of in-plane aligned rod-like β-Si 3N 4 grains were fabricated by hot pressing the tape cast and stacked green bodies with a small amount of rod-like β-Si 3N 4 seeds. The effect of anisotropy in microstructure and β-Si 3N 4 content on tribological properties was investigated by means of block-on-ring tests without lubricant using commercially supplied Si 3N 4 as ring specimens. Three sliding directions were set for the texured Si 3N 4 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 Si 3N 4 with less than 20% β-Si 3N 4 content, but it became larger with the increasing β-Si 3N 4 content. The C-para specimen exhibited larger wear rate than the other specimens of textured Si 3N 4 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 Si 3N 4 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 Si 3N 4 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.