Temperature sensing using ruby coatings created by plasma electrolytic oxidation

Abstract

Thermographic properties of Cr3+-doped Al2O3 coatings created by plasma electrolytic oxidation (PEO) are investigated. Scanning electron microscopy revealed dense coatings uniformly distributed over the aluminum surface with uniform elemental maps, confirming homogeneous distribution of chromium ions. X-ray diffraction analysis shows that created coatings consist of a mixture of alpha and gamma phase of alumina and that alpha-to-gamma phase ratio increases with PEO processing time. Room-temperature photoluminescent measurements revealed two deep-red, sharp emissions placed at 694 nm (R1 line) and 693 nm (R2 line) typical for Cr3+ in Al2O3 octahedral configuration. No evidence of broad 680−800 nm emission typical for alumina gamma phase is observed and emissions were predominantly attributed to the ruby α-Al2O3:Cr3+. The temperature is obtained from the coating's emission using temperature-induced changes in: i) band shift of the R1 line, (ii) luminescence intensity ratio (LIR) between two Cr3+ R1,2 lines, and (iii) lifetime of the Cr3+ red emission with maximal sensitivities of 0.2 cm−1 K−1 @ 550 K, 4.8 % K−1 @ 20 K and 0.9 % K−1 @ 450 K, respectively.