Wear performance of oil lubricated silicon nitride sliding against various bearing steels

Abstract

The selection of bearing steel surfaces for use with silicon nitride rolling elements within hybrid bearings is critical to the performance and life of such components, which have potential applications in advanced high speed aircraft. The wear and friction performance of these combinations is a major factor currently being considered for the next generation hybrid bearings. This paper reports on hybrid bearing contacts that have lubricated Si 3N 4 elements, which have been loaded against various bearing steels under pure sliding contact conditions on a fully instrumented pin-on-disc wear test rig. The wear and friction performance of Si 3N 4 has been compared to a baseline case of bearing steel M50 ball sliding against a M50 disc. Both hybrid and steel on steel contacts were lubricated by an aircraft engine oil Mobil Jet II. Wear mechanisms were determined by post-test analysis of the pin wear scars, disc wear surface and wear debris using optical microscopy, surface profilometry and FEG-SEM (scanning electron microscopy). The wear rates of Si 3N 4 sliding against different bearing steels are ranked by performance and related to their wear mechanisms, hardness and microstructure. Typical sliding contact wear mechanisms were found for the steel on steel combination while Si 3N 4 sliding against steel showed that transgranular and sub-micron-cracking mechanisms predominate. Evidence of material transfer (steel onto the silicon nitride) was found. Friction values for the various combinations are also reported and found to be substantially lower ( μ=0.04) than bearing steel on bearing steel combinations ( μ=0.17). The disc and pin wear was monitored on-line by an electrostatic wear sensor, LVDT and laser displacement probe, a friction strain gauge, and an infrared thermometer. Correlations between wear rate and charge generation/level, friction, contact temperature, and disc hardness are presented.