Influence of Feedstock and Thermal Spray Process on the Phase Composition of Alumina Coatings and Their Sliding Wear

Suspension thermal spraying is an emerging coating technology that enables the deposition of dense-structured ceramic coatings. As wear resistance is a main application field of alumina (Al2O3) coatings, this study aimed to evaluate the dry reciprocating sliding wear resistance of suspension sprayed high velocity oxy-fuel (S-HVOF) alumina coatings and to compare it with atmospheric plasma sprayed (APS) and HVOF coatings. Coatings were analyzed in the as-sprayed state and post-treated at 910 degrees C (hot isostatically pressed, HIPed) conditions. Wear tests were conducted using a tribometer, following the ASTM G133-02 standard and a sintered WC-6 wt.% Co ball as the counterbody. Coating characterization was done using scanning electron microscopy, x-ray diffraction and nanoindentation technique. Results indicate that the HVOF, HVOF-HIP and S-HVOF coatings had a high alpha-Al2O3 content, whereas the APS and APS-HIP coatings had a high gamma-phase content together with high porosity. Sliding wear resistance was an order of magnitude higher for the S-HVOF and HVOF coatings than the APS and APS-HIPed coatings. This difference in wear performance was attributed to the high nanohardness, elastic modulus, dense microstructure and relatively high alpha-Al2O3 content in the HVOF, S-HVOF and HVOF-HIP coatings. Results are discussed in terms of the wear mechanism and structure-property relationship.