Tribological behavior of Mo 5Si 3 particle reinforced Si 3N 4 matrix composites

Abstract

The tribological behavior of Mo 5Si 3 particle reinforced silicon nitride composites has been investigated by pin-on-disk wear testing under dry conditions. The friction coefficient of the Si 3N 4 decreased with the incorporation of submicrometer-sized Mo 5Si 3 particles and also as the content of Mo 5Si 3 particles increased. The wear mechanism of the Mo 5Si 3–Si 3N 4 composites consisted of fatigue between the Si 3N 4 grains and the amorphous grain boundaries, fracture of Si 3N 4 grains and grain-boundary phases, debonding of the Mo 5Si 3 particles from the grain boundaries, and tribochemical reactions. Under dry conditions, in air at room temperature, the detached Mo 5Si 3 particles reacted with oxygen to form MoO 3 and MoO 2 phases at the adherent film of the wear scar and exhibited self-lubricating behavior. When the Mo 5Si 3–Si 3N 4 composites were oxidized at 700 °C in air, solid-lubricant MoO 3 particles were generated on the surface layer. Oxidized Mo 5Si 3–Si 3N 4 composites showed self-lubricating behavior, and the average friction coefficient and wear rate of the oxidized 2.8 wt% Mo 5Si 3–Si 3N 4 composite were 0.43 and 0.72 × 10 −5 mm 3/N m, respectively, both values ∼30% lower than those for Si 3N 4. The formation of MoO 3 particles on the surface of the Mo 5Si 3–Si 3N 4 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.