Wear mechanism maps for thermal-spray steel coatings

Wear mechanisms in low-carbon-steel-based thermal-spray coatings deposited on aluminum alloys using a plasma-transfer wire-arc (PTWA) process and a high-velocity oxy-fuel (HVOF) process were studied. The coatings investigated consisted of PTWA 1020, HVOF 1020 deposited from a low-carbon steel wire stock, and HVOF 1020-2.5 pct A1 produced using a wire stock with 2.5 wt pct A1 added to the base AISI 1020 composition. Wear tests were performed using a pin-on-disc-type tribometer equipped with an environmental chamber within a load range of 5 to 75 N and a sliding-speed range of 0.2 to 2.5 m/s against tool steel pins in a dry air atmosphere (10 pct relative humidity). The wear rates of the three types of thermal-spray coatings and the micromechanisms that control the wear rates at different loads and sliding speeds were presented in the form of wear maps. Under dry sliding conditions, two basic wear and surface-degradation mechanisms were identified, consisting of (1) mechanical wear, which involved severe plastic deformation of the iron splats on the contact surfaces and their fracture and (2) oxidational wear that took place by the formation of various iron oxides, whose compositions and thicknesses depended on the loading conditions. The wear rates of PTWA 1020 and HVOF 1020-2.5 pct A1 decreased with increasing sliding speed, as they showed a transition from mechanical wear to mild oxidation wear. The wear rates of HVOF 1020 also decreased with increasing speed, but they increased again, once they passed through a minimum where a transition from mild to severe oxidational wear occurred. An improvement in the wear resistance of HVOF 1020 was observed, in particular at high loading conditions, as a result of the addition of 2.5 pct A1 to the wire feed stock. The wear maps demonstrated that the wear rates were sensitive to the compositions of the coatings and, hence, to the thermal-spray technique used in their production.