Sliding and abrasive wear of composite sol–gel alumina coated Al alloys

Alumina coatings, 60-μm thick (HV10g 647±144 kg mm−2), were deposited using a novel sol–gel technique on 6061 Al. The samples were subjected to dry sliding wear tests against hard bearing steel balls (SAE 52100 steel) and softer mild steel pins (AISI 1018 steel) at different sliding speeds and contact loads. Wear at low contact loads and sliding speeds was characterized by steel oxide (Fe2O3) transfer to the coating. Coating rupture arose at a critical transition boundary of sliding contact loads and speeds which were approximately twice as high for mild steel as against the harder bearing steel. The coated alloy was also subjected to abrasive wear against SiC abrasives of increasing coarseness. Coating wear rates approached those of the substrate alloy as the indentation depth increased. A similar reduction in coating hardness was observed for Vickers diamond pyramid indentations at greater depths. When abrasion indentation depths were greater than ∼20% of the coating thickness, a rapid degradation in wear resistance arose and the same effect was seen for scratch test indentation depths. The use of normalizing parameters for sliding wear, hardness, abrasive wear and indentation depth were found to be useful in interpreting coating wear performance.