Tribological behavior of Mo5Si3 particle reinforced Si3N4 matrix composites

The tribological behavior of Mo5Si3 particle reinforced silicon nitride composites has been investigated by pin-on-disk wear testing under dry conditions. The friction coefficient of the Si3N4 decreased with the incorporation of submicrometer-sized Mo5Si3 particles and also as the content of Mo5Si3 particles increased. The wear mechanism of the Mo5Si3-Si3N4 composites consisted of fatigue between the Si3N4 grains and the amorphous grain boundaries, fracture of Si3N4 grains and grain-boundary phases, debonding of the Mo5Si3 particles from the grain boundaries, and tribochemical reactions. Under dry conditions, in air at room temperature, the detached Mo5Si3 particles reacted with oxygen to form MoO3 and MoO2 phases at the adherent film of the wear scar and exhibited self-lubricating behavior. When the Mo5Si3-Si3N4 composites were oxidized at 700 °C in air, solid-lubricant MoO3 particles were generated on the surface layer. Oxidized Mo5Si3-Si3N4 composites showed self-lubricating behavior, and the average friction coefficient and wear rate of the oxidized 2.8 wt% Mo5Si3-Si3N4 composite were 0.43 and 0.72 × 10−5 mm3/N m, respectively, both values ∼30% lower than those for Si3N4. The formation of MoO3 particles on the surface of the Mo5Si3-Si3N4 composites as a result of the oxidation treatment improved the lubrication condition for the wear pairs, and decreased the wear rate, as well as the friction coefficient.