Solution Precursor Plasma Spraying of Cr-Doped Al2O3 Thermochromic Coatings

Solution precursor plasma spraying (SPPS) presents a modern route to deposit coatings directly from liquid feedstock. In this study, unique ability of SPPS to intermix different precursors on the atomic scale was employed to deposit pure and Cr-doped Al2O3 (technically, synthetic ruby) from several mixtures of aluminum and chromium nitrates. Highly porous coatings (> 60% porosity) were deposited by high-enthalpy WSP-H torch. Homogeneous distribution of Cr atoms in Al2O3 resulted in coatings color ranging from pink to grayish-green correlated with increasing Cr content and change of lattice parameters as observed by x-ray diffraction. High content of over 80 wt.% of α-alumina phase was reached for all coatings. For the Cr-doped Al2O3 coatings, distinct and fully reversible thermochromic behavior was observed by temperature-resolved colorimetry. Further increase in α-phase content (up to 95 wt.%) and coating densification were achieved by additional surface remelting by plasma torch.