The effect of pre-oxidation on microstructure, mechanical properties and high-temperature tribological behaviors of HVOF-sprayed NiCoCrAlYTa coating

The HVOF-sprayed NiCoCrAlYTa coating would generate a dense oxidative layer under high temperature so that the microstructure, chemical component, mechanical properties and high-temperature tribological behaviors of the coatings surface were changed significantly. The as-sprayed NiCoCrAlYTa coating is mainly composed of the phases of gamma-Ni and beta-NiAl as well as gamma'-Ni3Al. The Al-rich phase of beta-NiAl in the post-oxidized coating has disappeared, replaced by the emergence of alpha-Al2O3. The formation of the oxidative layer with alpha-Al2O3 columnar crystal could greatly improve the mechanical properties of the NiCoCrAlYTa coating. Along with the elevated temperature, the friction coefficient and wear rate of as-sprayed coating gradually decreased, and the primary mechanism changed from abrasive wear to adhesive wear. Although the friction coefficient of the post-oxidized coating is slightly higher than that of as-sprayed coating, the wear rate decreases, obviously. This is because the oxidative layer of post-oxidized coating exerts a great influence on the wear resistance, especially under the high temperature.