Tribological behavior of silicon nitride materials under unlubricated sliding between 22°C and 1000°C

Abstract

Friction and wear of 13 different materials with silicon nitride (Si 3N 4) matrix were investigated under unlubricated conditions. Tests were performed in laboratory air at sliding velocities between 0.03 and 5 m s −1 and at temperatures in the range of 22 to 1000°C. These materials were tested in the same test rig by means of the pin-on-disc configuration with self-mated couples to evaluate tribological influences of composition, structure and other properties, which were varied to reach friction coefficients below 0.2 and wear coefficients below 1 × 10 −6 mm 3 N −1 m −1. For monolithic silicon nitride materials, high friction coefficients between 0.4 and 0.9 and wear coefficients between 5 × 10 −6 and 1 × 10 −2 mm 3 N −1 m −1 were measured. By adding boron nitride, the friction coefficient could be reduced to 0.1 at room temperature. Simultaneously, after running-in, the wear rate was also reduced by a factor of five in comparison to monolithic silicon nitride. Through the addition of titanium nitride, the wear was reduced by a factor of five at room temperature and even by two orders of magnitude at a high temperature of 800°C. All materials and worn surfaces as well as wear debris were extensively characterized by means of SEM, plus energy-dispersive X-ray, X-ray diffraction, small-spot electron spectroscopy for chemical analysis, electron-probe microanalysis, Auger electron spectroscopy, Fourier transform infrared spectroscopy and transmission electron microscopy before and after the tribological tests. Combined with theoretical estimations on mechanical and thermal stresses, which are based on calculations of hot-spot (“flash”) and bulk temperatures, wear processes and wear models are proposed. The monolithic silicon nitrides investigated reveal a mechanically dominated wear mechanism with fatigue between the grains and the amorphous grain boundaries. Tribochemical reaction layers (SiO 2, SiO x, N y, Si 3N 4 · Y 2O 3) did not tend to decrease friction and/or wear in a wide range of test conditions. Only at higher temperatures above 400°C and higher sliding velocities (above 0.3 m s −1 at 1000°C and above 1 m s −1 at 400°C) was a more pronounced wear reduction caused by the tribochemical formation of protective layers and their plastic deformation. The addition of titanium nitride (TiN) or boron nitride (BN) led to microstructures which were mainly free from glassy grain-boundary phases. More important, these silicon nitride composites are able to form relatively soft lubricious oxides like TiO 2−x and H 3BO 3 or an intrinsically solid lubricant like BN · H 20 on a hard substrate, so that then a tribochemical wear mechanism dominates. Considering tribological and surface analysis results as well as theoretical calculations, wear maps are given for the various monolithic silicon nitride materials and for the composites Si 3N 4-TiN and Si 3N 4-BN. They give indications for the further development of ceramic materials.