Tribological Behavior of Si₃N₄ Ceramics and M50 Steel with Limited Oil at High Temperature

To address the issue of high-temperature lubrication of high-temperature resistant bearings, a high-temperature oil-controlled friction and wear experiments was conducted on Si₃N₄ ceramics and M50 steel using the SRV-Ⅳ fretting wear testing machine, followed by a high-temperature oil-controlled and oil-starved friction and wear experiment for different mating methods. The results of the oil-controlled experiment showed that under the condition of 200 ℃ and sufficient oil lubrication, the friction coefficient and disk wear rate of Si₃N₄ (ball)-M50 (disk) mating pair was lower than that of M50 steel self-mating pair, and the friction coefficient of Si₃N₄ (ball)-M50 (disk) was 0.03 on average lower than that of M50 steel self-mating pair, and the ball wear rate was one ten thousandth of that of M50 steel self-mating pair, which had better friction and wear performance. The surface of the wear marks of the M50 self-mating disk was dominated by obvious strip grooves and deep grooves, indicating that the main form of wear was abrasive wear; the wear marks of the Si₃N₄ (ball)-M50 (disk) were dominated by dense point-like spalling, and there were obvious residues of reactive materials around the wear marks. The white light morphology reveals that the M50 (ball) self-matching disk, under 10 μL and 5 μL oil volume, has a deeper furrow on the disk surface in the friction process, which leads to the increase of the distance between the ball and disk surfaces in the region, the decrease of the oil film pressure, the weakening of the cutting effect of the lubricant containing abrasive particles on the ball surface, and the generation of more obvious strip-like bumps on the ball surface, which reduces the wear rate of the M50 steel ball. The EDS analysis revealed that the P element content of the M50 disk surface of the Si₃N₄ (ball)-M50 (disk) mating increased from 0% to 1.41% (mass fraction) and the O element content increased by 2.01% (mass fraction) before and after wear. The chemical composition of the worn area of the M50 disk was further determined by XPS analysis, and a spin orbit of BE (Fe 2p_3/2) = 712.8±0.2 eV was found in the Fe elemental profile, indicating the existence of Fe-P-O bonds in the worn area, proving the involvement of P-containing additives in the lubricant in the chemical reaction; the P elemental profile showed that P elements existed mainly in the form of P-O bonds in the friction reaction film. In addition, the wear rate of Si₃N₄ (ball)-M50 (disk) mating pair and M50 steel self-mating pair were compared separately, and it can be found that the wear rate of Si₃N₄ (ball)-M50 (disk) mating pair was basically not affected by the lubricant quantity, while the wear rate of M50 steel self-mating pair was more affected by the lubricant quantity. This was due to the fact that the surface of M50 disk of Si₃N₄ (ball)-M50 (disk) mating pair generates stable phosphorus-containing friction reaction film during the friction process, and the wear rate was small; while the wear of M50 steel self-mating pair was mainly abrasive wear, with the increase of lubricant quantity, the density of abrasive particles in unit volume of lubricant decreased, the cutting area gradually became sparse, and it was difficult to concentrate in the same area, and the wear rate also decreased. The results of comparison experiments of different mating pairs showed that only the M50 (ball)-Si₃N₄ (disk) mating pair did not have lubrication failure for 1 h under 200 ℃ and starved oil (3 μL) volume, and could maintain a low wear rate, besides the protective effect of the reaction film produced during the friction process, the chemical reaction film on the surface of M50 steel ball had a smaller film area, and the film formation was not easily affected by the lack of oil conditions.