Scale effects in abrasive wear of composite sol–gel alumina coated light alloys

Chemically-bonded composite sol–gel (CB-CSG) coatings have been developed that incorporate calcined ceramic powder particles (filler) dispersed in a sol–gel (SG) matrix (binder). As a result, they have significantly reduced densification strains compared to conventional SG coatings. The CB-CSG coatings, which are several orders of magnitude thicker than typical micron-thick SG films, can be produced easily by spray depositing onto Al and Mg alloys, followed by a short low temperature (300°C) heat treatment. The abrasive wear performance was determined for Al2O3(binder)/Al2O3(filler), Al2O3(binder)/Al2O3-SiC(fillers) and a hybrid Al2O3(binder)/Al2O3(filler)+Al(40 μm particle filler) coatings on commercial Al and Mg alloys. In two-body abrasion tests, the coated alloys were abraded by bonded SiC abrasives of increasing coarseness with comparative experiments also being performed on the uncoated substrate alloys. Coating wear rates exhibited an order of magnitude increase when the scale of indentation increased above a critical depth, whereupon wear rates approached those of the substrate alloy. Similar observations were made in three-body microabrasion experiments using a ball-cratering technique to abrade the coatings with very fine and coarse alumina slurries. The modes and mechanisms of coating damage were characterized by SEM and optical surface profilometry techniques. Normalized parameters for hardness, abrasive wear and indentation depth, were used in interpreting and comparing coating tribological performance.