Friction and wear of lubricated M3 Class 2 sintered high speed steel with and without TiC and MnS additives

M3/2 sintered high speed steel and composite materials processed by initial admixing of 5 wt.% TiC (to decrease wear) and 5 wt.% MnS (to minimise friction) powders, singly and in combination, were assessed in pin-on-disc tribometers specially constructed to simulate use in the automotive valve train. Pins were of the sintered materials and the mating tribological material discs of T1 high speed steel. For comparison with existing conventional materials, identical tests were performed with discs and pins of two types of spheroidal cast iron. Testing at ∼110 °C, employing a few drops of fresh Shell Helix Standard SAE: 15W-40, API: SJ/CF oil, in daily segments of 5000 m of sliding distance was carried out until the break of the boundary film and appearance of the early signs of seizure. Accordingly the conditions were initially elastohydrodynamic, then mixed lubrication, then boundary, and finally decaying boundary. In comparison with the baseline cast iron system, the friction, wear and lifetime performance of all the high speed steel systems was markedly superior. MnS further lowered the coefficient of friction and TiC increased the load carrying capacity of M3/2 steel. The lifetime, test distance until seizure, was the most discriminating parameter between the high speed steel systems, being 1.5–3 times longer for the unmodified M3/2 than the composites and 10 times longer than that of the cast irons system.