Friction and wear performance of newly developed automotive bearing materials under boundary and mixed lubrication regimes

Abstract

Global legislation on emissions and on recyclability of vehicles makes it necessary to run main and connecting rod bearing systems in automobile internal combustion (IC) engines with lower friction using lower oil flows and less viscous oils leading to more contact between bearing and shaft. At the same time lead must be removed from the system. In order to design suitable bearing materials for these new applications it is important to understand the frictional behaviour of candidate materials in a range of conditions covering the boundary and mixed regimes. Three plated plain bearing materials with overlays; a conventional lead based material, a newly developed bismuth based material, and a newly developed tin based system were tested on a tribometer specifically designed and built for this task to compare their performance in the boundary and mixed lubrication regimes under conditions that would be expected in an automobile engine. The resulting data was plotted as a series of Stribeck curves. The bismuth based material had slightly improved performance over the lead based material. The tin based material gave significantly higher friction, when compared with the other two materials. The wear properties of the materials were also tested and the variation of wear factor with testing time was plotted for each of the material at two different operating conditions. All materials experienced more severe wear in the initial stages of operation. The tin based material had the most severe wear, and showed signs of incipient melting of the overlay. The bismuth and lead based materials showed similar wear with the bismuth based material being superior under the most severe conditions, whereas the lead based material was superior under the other conditions tested. The bismuth material showed that it may be used as a suitable lead-free alternative material in IC engine applications under the conditions tested. The tin based material could not be used under such extreme conditions and failed because of thermal effects.