Cobalt- and Chromium-Oxide-Based Coatings: Thermally Spraying a Glaze Layer

The recent demand for more efficient gas turbine engines has led to a growing need for new high performance materials and engineered surfaces. Consequently, there has been recent development of thermally sprayed coatings capable of withstanding harsh environments to advance these engines. For instance, oxide-based coatings exhibit high temperature stability, making them potential coating candidates for applications at elevated temperatures, thereby further improving gas turbine engines' efficiency. In particular, cobalt- and chromium- based oxides have previously been shown to be beneficial in terms of reducing friction and wear in high temperature environments. However, limited work has been performed on the deposition of such coatings by means of thermal spray processes. Therefore, the main purpose of this study is to develop and critically evaluate thermally sprayed cobalt- and chromium-based coatings for extreme environments. More specifically, the coatings were deposited by means of suspension plasma spray (SPS) and characterized before and after ball-on-flat tests at different temperatures. The coatings developed in this study have demonstrated high resistance to wear when tested against IN718. In all cases, most of the wear was observed on the counterballs. The CoO coating exhibited the lowest combined wear when compared to the other coatings. Ex-situ Raman analysis revealed the formation of Co₃O₄ for the worn cobalt oxide-based coatings tested at 450 °C, which correlates well with the lower wear rates.