Dry sliding wear behavior of hard-chrome and zinc-phosphate conversion coatings on steel substrates

We have studied tribological properties of conversion coatings, hard chrome on AISI 430 stainless steel, and zinc phosphate on AISI 1044 carbon steel. These substrates were chosen due to their extensive industrial applications. The goal of the present study is to improve the wear resistance of these substrates through the coatings. We performed nano-indentation and ball-on-disc tribometer experiments to study mechanical and tribological behavior of substrates and coatings. We find that when zinc-phosphate coating is applied to AISI 1044 steel, a new phase Zn2Fe(PO4)2·4H2O (phosphophyllite) is formed on the surface that increases the surface hardness. We further find that coefficient of friction decreases approximately by 8% due to hard-chrome coating on AISI 430 steel and by 48% due to zinc-phosphate coating on AISI 1044 steel under load and speed conditions applied in the present study. We attribute our findings to formation of: (1) an adhering layer of chromium in the case of chrome–steel contact, leading to chrome–chrome contact at the interface of test-specimen and counter-surface, and (2) a glossy layer at interface in the case of phosphate-steel contact, owing to the compaction and crushing of phosphate wear debris. Our wear studies show maximum reductions in specific wear rate to be around 80% and 89% for chrome and zinc-phosphate coatings, respectively. We note that adhesion and abrasion are the primary modes of wear for chrome and zinc-phosphate conversion coatings, respectively at the initial stage. However, adhesive wear switches to delamination wear at a later stage for hard-chrome coating.