Sliding wear and friction characteristics of six Zn-based die-casting alloys

Abstract

The friction and wear characteristics of zinc-based alloys in warm, oil-lubricated, sliding contact against mild steel were investigated. The alloys, their compositions, and their microstructures included: pure Zn (100% Zn, η zinc), Zamak 3 (Zn4Al0.1Cu, η zinc plus η α eutectic), ZA 27 (Zn31Al2.3Cu, α aluminum plus η ga eutectic), ACuZinc 5 (Zn3.0Al5Cu) in die cast form, and ACuZinc 10 (Zn3.5Al10Cu, epsilon dendrites plus peritectic η plus η/α/ϵ ternary eutectic) in die cast and squeeze form. The test temperature, sliding speed, duration, and apparent contact pressure were 50°C, 0.15 m s −1, 7 h, and 6.9 MPa (1000 psi). The mass wear rates of the zinc-based alloys generally correlated directly with coefficient of friction and inversely with hardness. They varied by a factor of 27. The relatively high wear resistance of the ACuZinc alloys is attributed largely to the hard, copper-bearing, epsilon phase. In addition to high mass wear rates, the softer alloys showed significant changes in dimension and shape due to plastic flow, which appeared to consist of both lateral exudation of microscopic layers, and gross “mushrooming”. Zinc-rich transfer particles were found on the worn steel rings, associated with fine axial cracks on those rings. Macroscopic fibrous steel wear debris was generated in tests involving the harder zinc-based alloys. The hypothesized formation of such debris by multiple ploughing events is discussed briefly.