Effect of Microstructure of Silicon Nitride Ceramics on Wear Mechanism

A silicon nitride sintered body with uni-directionally aligned β-Si3N4 elongated grains (UA-SN) was fabricated and the effect of microstructural anisotropy on wear properties was investigated. Block-on-Ring tests without lubricants were carried out using the UA-SN as block specimens and commercially supplied Si3N4 as ring specimens. In addition, scratch tests were also carried out using a diamond stylus having 25μm of tip radius with the normal load increasing continuously up to 1.47N. For UA-SN, tribological properties were evaluated in three directions with respect to the grain alignment; in the direction normal to the grain alignment on the plane normal to the grain alignment (N), and in the direction parallel (L) to and perpendicular (T) to the grain alignment in the side plane. The N direction exhibited the highest wear resistance, and the worn surface of this plane was quite smooth, in contrast to the other specimens whose surfaces were irregular owing to grain removing. For the scratch test, there was an inflexion point in friction force during increasing load for each specimen. The starting states of wear, such as grain removing, were observed at this point and the loads at this point among specimens correlated with the result of wear resistance obtained from wear tests. Microfracture of the silicon nitride induced by scratching started at the grain boundary as microcracks. Wear was caused by subsequent grain removing following propagation and combination of these microcracks. It is suggested that the high wear resistance achieved in the plane normal to the grain alignment is attributable to the inhibition of crack propagation along the sliding surface by the stacked elongated grains normal to the sliding surface.