Microstructure and dielectric properties of Al₂O₃–Cr₂O₃ composite coatings fabricated by wide-velocity range high-energy plasma spraying

The presence of γ-phase in alumina (Al₂O₃) coatings has been widely acknowledged as a detrimental factor compromising dielectric performance, presenting a longstanding challenge in advanced coating design. To overcome this limitation, a series of Al₂O₃-Cr₂O₃ composite coatings were fabricated by wide-velocity range high-energy plasma spraying (WPS) in this study. The systematic investigation of chromium oxide (Cr₂O₃) doping effects on dielectric properties of as-sprayed coatings was conducted through dielectric breakdown tests. The results suggested that the incorporation of Cr₂O₃ effectively reduced porosity and increased α-Al₂O₃ content in all the as-sprayed coatings. Compared to pure Al₂O₃ coatings, the Al₂O₃-Cr₂O₃ composite coatings exhibited significantly lower porosity and higher α-Al₂O₃ content. There is a marked decrease of porosity (1.3–1.7 %) occurred when the Cr₂O₃ concentration increased to 4 % and 8 %. Notably, the α-Al₂O₃ content reached a maximum of 53.2 % at 4 wt% Cr₂O₃ incorporation, while the coating with 8 wt% Cr₂O₃ showed the highest breakdown voltage (35.2 V/μm), representing a 7.8 V/μm improvement (28.5 % increase from 27.4 V/μm for pure Al₂O₃) due to its high α-Al₂O₃ content and low porosity (4 %). Although various studies have addressed the retention of the α-Al₂O₃ in thermally sprayed Al₂O₃ coatings, achieving a balance between the α-Al₂O₃ and porosity remains a crucial challenge. Our findings provide valuable insights for the development of high-performance plasma-sprayed Al₂O₃-based insulating coatings, offering a viable pathway to enhance dielectric properties through controlled phase composition and microstructure engineering simultaneously.