Improving the Properties of Plasma and HVOF Sprayed Alumina Coatings by Chromia Addition

Alumina is a relatively low-cost material, easily processable by thermal spraying into wear and corrosion resistant coatings. However, thermally sprayed alumina coatings show inferior corrosion resistance versus chromia coatings, particularly in low and high pH electrolytes. Further, alumina possesses decreased mechanical properties in the as-sprayed state. These are due to the presence of metastable phases of alumina, like γ or δ, instead of the stable alpha alumina. The most desired form of alumina would be the α-phase (corundum) and some attempts have been reported in the open literature in order to stabilise the alpha phase in the thermally sprayed coatings. Most of the studies have been carried out using water stabilized plasma (WSP) process, while effect of the chromia additions to plasma and HVOF sprayed coatings has remained unanswered.

The aim of this study was to increase the amount of α-phase in the APS and HVOF sprayed alumina coatings by the means of chromia addition. The addition was carried out in the spray drying manufacturing process of the powder. An increase in the α-phase content of the coatings is expected to result in improved chemical resistance and mechanical properties of the coatings. The coatings were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The corrosion resistance of the coatings was evaluated by immersion tests at elevated temperatures in highly acidic and basic electrolytes. The mechanical properties of the coatings were evaluated by abrasion wear tests and hardness measurements. Alumina based coatings with greatly improved corrosion and wear properties were obtained by relatively small chromia additions by both studied spray methods. The results will be presented and discussed.