Effect of Al and Cd sacrificial coatings on the wear of steel substrates

Abstract

Aerospace and automotive grade steels are often coated with sacrificial coatings to mitigate corrosion and increase service life. Some of these coated components are subjected to sliding motion and, for these cases, a sacrificial Cd coating provides the additional benefit of a low shear strength, essentially acting as a solid lubricant. Cd coatings will be replaced due to their toxicity and Al coatings are a candidate with low hardness that could replace Cd for these applications. In this study, a linear reciprocating sliding test is used to measure the wear and friction coefficient with change in relative humidity (RH), with and without Cd and Al sacrificial coatings. These soft coatings do not offer much wear resistance, so the study focuses on how the substrate wears after coating removal compared to uncoated substrates (grit-blasted and as-rolled steel). Two types of Al coatings deposited by aprotic and ion vapor process and a low hydrogen embrittling (LHE) Cd coating were studied. An order of magnitude difference in wear rate of the steel substrate was observed with changes in relative humidity and type of coating. For example, wear rate of steel substrate in dry air decreases from ~3 × 10-4 mm3/N·m to 3 × 10-5 mm3/N·m when Cd coating is applied. Whereas, wear rate of steel substrate increases to ~1.5 × 10-3 mm3/N·m with Al coating. Application of LHE Cd coating decreases the substrate wear irrespective of relative humidity, but IVD Al is only effective during high relative humidity of 60%. Electrodeposited Al coating, irrespective of relative humidity, aggravates substrate wear. After the coating has been worn through, residual third bodies, which were oxides and hydroxides of the coating, affect the wear process of the steel substrate.