WEAR ELSEVIER Wear 210 (1997) 27-38 Wear mechanisms in oil-lubricated and dry fretting of silicon nitride against bearing steel contacts M. Kalin a, .I. Vi~intin ""*, S. Nov~k b, G. DraWl6 b " University of Ljubljana, Faculty of Mechanical Engineering, Bo~i~it~'era 8, IO00 Ljubljana. Slovenia b Jo~efStefim Institute. Jamova 39, I000 I.j.~pljana, Slovenia Received I I June 1996: revised 24 March 1997: accepted 7 April 1997 gbstract The wear and friction behaviour of silicon nitride against bearing steel was investigated under lubricated and dry freuing conditions as a function of amplitude and test duration. Tests were performed on a high frequency fretting tester. Silicon nitride bearing balls were used as the upper oscillating specimens while the lower stationary flats were standard specimens of bearing steel. Amplitudes in the intermediate 5 to 50 p,m range and a test duration from l0 to 360 min were studied. In lubricated conditions a commercial lubricant, ISO VG 220. was used. Light microscopy, scanning electron microscopy ( SEM ), energy dispersive spectroscopy ( EDS ), Auger spectroscopy (AES) andtransmission electron microscopy (TEM) were employed to determine the wear mechanisms. Under lubricated conditions transition from high to low wear volumes was recognised with increasing amplitude. At lower amplitudes and in ~e early stage of fretting tests at moderate amplitudes, mechanical wear dominated. Cracks on the stick-slip boundary and spalling of a thin tribolayer was observed. Under these conditions the highest wear in lubricated fretting was obtained. In the final stage of fretting tests at moderate amplitudes, and from the beginning at higher amplitudes, tribochemicai wear is suggested as the dominant wear form. A 0.2 ltm thick tribolayer was observed on the contact, containing inclusions with different Fe and Si contents. A very high concentration of carbon, formed by oil degradation, was also determined in this layer, confirming the critical influence of oil on the wear behaviour. Quite a different wear mechanism is proposed for dry fretting conditions. Results of AES analysis showed a layer an order of magnitude thicker than in lubricated fretting, also having a remarkably different chemical composition. TEM analysis confirmed that the reaction layer consisted of a silica-rich amorphous phase containing small inclusions of Fe20, and Fe~O.~. In contrast to lubricated conditions, where the layer created was ductile, in the case of dry fretting the layer was brittle. The continuous process of forming and spalling the brittle tribolayer caused much higher wear rates and wear losses than under lubricated fretting conditions. No transition in wear behavionr was observed as was the case in lubricated fretting. © 1997 Elsevier Science S.A. Ke~a,'ords: Fretting wear: Silicon nitride: Bearing steel: Ceramics: Oil: Lubricated conditions: Dry conditions 1. Introduction Hybrid ceramic bearings are finding increased applications in machine tool spindles and aerospace vehicles. They have steel rings assembled with ceramic balls, Hot isostatically pressed ~,ilicon nitdde was found to be the most advantageous material for the ceramic rolling elements. The advantages of hybrid bearings are most pronounced in high-speed applica- tions because of reduced centrifugal force, since the density of ceramic balls is 60% lower than that of steel balls, it is known that the strength and corrosion resistance of ceramic * Corresponding author. Tel.: +386 61 125 9200: fax: +386 61 125 ! 188. a Presented at the 11th International Conference on Wear of Materials. San Diego, CA, USA, 20-23 April 1997, 0043-1648/97/$17.00 © 1997 Elsevier Science S.A. All rights reserved Pil S0043-1648 ( 97 ) 00082- 3 materials is maintained over a much greater temperature range than for typical beating steel. This allows hybrid bear- ings to operate at over 500 °C, and all-ceramic bearings at over 1000 °C [ I-3], while the limiting temperature forcon- ventional bearings is significantly lower. Many rolling bear- ings are subjected to marginal lubrication which causes interactions of the asperities on mating surfaces, leading to microplastic deformation, micropitting and finally bearing failure [41. Ceramics have a high yield strength and can prevent severe damage on rolling surfaces with marginal lubrication or in other cases of lubricant starvation. One of the wear forms which significantly limits the oper- ating life of rolling-element bearings and other machine com- ponents and structures is also fretting wear. in recent decades studies of different aspects of the subject have been made. but quite different and incompatible results have been