Fretting wear and cracking in sintered metal matrix composites

A methodology, involving fretting tests, to develop wear and crack resistan t materials for tribological applications for automotive valve train parts (e.g. cams, tappets) has been recently reported for high speed steels. Modi fications to one of these sintered steels, M3 Class 2, were effected by add itions, singly and in combination, of 5 wt.% of wear resistant titanium car bide and of solid lubricant manganese sulphide. In our fretting tests alter nate displacements were imposed between the test material (plane) and a chr omium steel or alumina ball. Running conditions fretting and material respo nse fretting maps were constructed for the four materials. Two types of fre tting damage were detected and analysed: cracking or particle detachment an d wear through the tribologicaly transformed structure (TTS). Crack initiat ion, associated with porosity and interfaces, was detected when the maximum tensile stress in the contact reached 1.2 GPa. Cracking analyses were also carried out using static and fatigue mechanical tests and replica scanning electron microscopy. Crack growth and propagation were influenced by detai ls of the microstructure, e.g. TiC was observed to arrest crack growth, whe reas MnS made it easier. Wear analysis included the determination after eac h test of the wear volume, which could be related to the coefficient of fri ction and the cumulative dissipation energy during the fretting test. (C) 2 001 Elsevier Science B.V. All rights reserved.