Wear 255 (2003) 657–668 Wear performance of oil lubricated silicon nitride sliding against various bearing steels L. Wang a,∗ , R.J.K. Wood a , T.J. Harvey a , S. Morris a , H.E.G. Powrie b , I. Care c a Surface Engineering and Tribology Group, School of Engineering Sciences, University of Southampton, Southampton SO17 1BJ, UK b Smiths Aerospace Electronic Systems, School Lane, Chandlers Ford, Southampton, Hampshire SO53 4YG, UK c Rolls-Royce plc, ML-77, PO Box 31, Derby DE24 8BJ, UK 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 3 N 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 3 N 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 3 N 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 3 N 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. © 2003 Elsevier Science B.V. All rights reserved. Keywords: Silicon nitride; Wear; Electrostatic sensing; Bearing steel 1. Introduction Silicon nitride ceramics were developed for use in rolling bearings and hybrid ceramic ball bearings, with Si 3 N 4 balls and steel rings, and have been used in machine tool spindles under high speed operation for approximately 10 years [1]. Silicon nitride has also been proposed for very high temper- ature rolling element bearings on aircraft engines [2]. Yui et al. [3] have tested hybrid ball bearings under high tem- perature (300 ◦ C) and high speed (3.5 million DN—bearing inner race rib diameter (D) × number of revolutions (N)) conditions. The results of the tests confirmed that the bear- ings function normally with tripentaerythritol ester oil at high temperatures that exceed the limiting temperature of conventional ester-based gas turbine engine oils, and that the sintered powder carburising steel is well suited to bear- ing applications in which present levels of temperature and ∗ Corresponding author. E-mail address: ling.wang@soton.ac.uk (L. Wang). speed are exceeded. The properties of the selected steels for bearing races are also very critical to the performance of hybrid bearings. To improve the rolling contact fatigue (rcf) resistance, wear and scuffing resistance, and tolerance to poor lubrication conditions of the well-established bearing steels M50 and M50NiL, a plasma nitriding process was de- veloped to enhance the surface hardness and microstructure. There is some evidence that the resistance is proportional to hardness according to Archard wear law [4]. Plasma ni- triding results in the formation of a diffusion zone below the surface and, depending on the nitrogen concentration during nitriding, in the formation of a so called compound layer (white layer) on the surface. The compound layer, which is predominately iron nitride and only several microns thick, is very hard and brittle, and thus is prone to break down under the influence of the high cyclic stresses expe- rienced by a bearing during service. Therefore, this white layer must be removed by grinding from the race of a bear- ing. The surface hardness of M50 DH and M50NiL DH achieved by plasma nitriding is about 1000Hv, the normal 0043-1648/03/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0043-1648(03)00045-0