Structure and properties of HVOF and plasma sprayed ceramic alumina-chromia coatings deposited from fused and crushed powders

Different feedstock powder compositions of the alumina-chromia system (75/25, 60/40 and 25/75 in wt.-%) were deposited by conventional plasma spraying, 3-anode plasma spraying and by HVOF spraying. The powders for the plasma spray processes had particle sizes of -38+10 mm and for HVOF spraying -25+5 mm and -25+10 mm. The coatings were evaluated by their microstructure, phase composition, wear and corrosion properties as well as electrical properties. The study showed that the effect of the spray process is crucial for the wear properties and revealed that particularly the HVOF spray process results in coatings with dense structures and excellent wear properties. Despite of still high porosity 3-anode plasma coatings show higher hardness than conventional APS coatings and can be sprayed with higher feed rates. The coating properties appear not to have a linear dependence on the chromia content.