Abstract

Sialon ceramics with two types of additive elements and varying amounts of sintering aid have been studied with regard to their microstructural behaviour and their tribological performance. In this investigation, neodymium and ytterbium have been chosen as stabilising elements because of their different ionic radii. According to the general sialon formula MxSi12-m-nAlm+nOnN16-n (x = m / 3) the starting compositions have in values of 0.5 and 1.0, respectively, and n values of 1.0. With gas pressure sintering the sinterability strongly depends on the fraction of liquid phase and thus the amount of additive in the starting composition. Here, excess additive means a larger portion of rare earth oxide than calculated from the sialon formula. Excess additive contents change the microstructure in pure alpha-sialon ceramics from equiaxed to needle-like and increase the indentation fracture toughness up to 6.3 MPam(1/2). Reciprocating sliding tests were performed with a cylinder on-disk apparatus and a Hertzian pressure of 180 MPa. Two neodymia containing sialon materials with different alpha:beta-sialon ratios are compared with commercial alumina and silicon nitride materials. Isooctane is used as lubricant for Simulating a fuel injection pump and shows the lowest friction coefficients of 0.3 for the sialon parts. With water as lubricating liquid the neodymium based alpha/beta-sialon reveals a friction coefficient of 0.08 compared to 0.4 for alumina and 0.95 for silicon nitride after a running-in distance of 250 m.