Abstract

Ceramic coatings inspired by the glaze layers naturally formed on Co-Cr-W superalloys (e.g., Haynes 25) were developed to meet the demands of next-generation gas turbine engines operating at elevated temperatures. Four distinct chemical compositions were investigated: CoO, CoO-21Cr₂O₃, CoO-42Cr₂O₃, and Cr₂O₃, all produced via suspension plasma spraying (SPS) under varying process conditions.
The study began by evaluating the performance of a CoO coating versus Inconel 718 and Haynes 25 serving as baselines. Tribological tests were carried out using a ball-on-flat tribometer against Inconel 718 counterfaces at 600 °C and 800 °C. Results showed that CoO coating provided exceptional protection to the counterface while maintaining low wear rates of the coated surfaces. Inconel 718 exhibited the highest wear among all configurations. In contrast, Haynes 25 demonstrated significantly improved wear resistance at 800 °C due to the in-situ formation of a glaze layer. Interestingly, at 600 °C where Haynes 25 did not form a glaze, and CoO coatings exhibited minimal wear as the formation of a glaze is not required, showing superior high-temperature tribological performance.
To replicate the beneficial effects of the glaze layer, mixed compositions were developed by using different spraying conditions. The stand-off distance was found to significantly influence redox reactions during deposition. Among the oxides tested, CoO exhibited the highest tendency toward oxidation and reduction, leading to the formation of layered microstructures. These redox-driven transformations and their effects on coating microstructure and cohesion were analyzed in detail. The presence of metallic cobalt, was found to compromise coating integrity, suggesting the need for careful control of processing parameters depending on the target application.
Further wear tests at room temperature and 450 °C revealed that while CoO coatings underwent more wear at room temperature compared to Cr₂O₃-based compositions, they provided enhanced protection to the counterface. At 450 °C, CoO coatings demonstrated significant reductions in wear, reinforcing their suitability for intermediate-temperature applications. Furthermore, cracking at the tribofilm formed over the counterfaces at lower temperatures was observed, consistent with the brittle-to-ductile transition behavior of glaze-based materials.
Further tests of CoO-21Cr₂O₃, CoO-42Cr₂O₃, and Cr₂O₃ coatings at higher temperatures (600 °C and 800 °C) were performed. At closer to intended service conditions, all coatings displayed negligible wear. However, spallation was observed for Cr₂O₃ at 800 °C. The addition of cobalt oxide in the mixed compositions (CoO-21Cr₂O₃ and CoO-42Cr₂O₃) mitigated cracking and improved structural integrity. Phase segregation and oxidation phenomena led to the formation of cobalt oxide-rich layers on the surface and within cracks, contributing to enhanced coating cohesion. These layers also played a role in improving tribological performance and hinted at a potential self-healing mechanism, which was further explored and discussed.