Al2O3:Cr3+ Coatings on Tungsten Substrate Synthesized by Plasma Electrolytic Oxidation: Photoluminescence and Temperature Sensing Applications

Al2O3:Cr3+ coatings were synthesized on tungsten substrates by plasma electrolytic oxidation in a phosphate-aluminate electrolyte containing dispersed Cr2O3 nanoparticles, and their structural, photoluminescent, and temperature-sensing properties were investigated. The coatings exhibited a typical porous PEO morphology with a uniform thickness of approximately 31 μm, and EDS analysis confirmed the incorporation of Cr species from the electrolyte, with Cr content increasing with the concentration of Cr2O3 particles. XRD analysis showed that the coatings were composed predominantly of α-Al2O3, with minor contributions from metastable γ-Al2O3, confirming that our previously established process for forming the thermodynamically stable α-Al2O3 phase directly on a non-aluminum substrate remains robust upon the introduction of dopant nanoparticles. The Al2O3:Cr3+ coatings displayed characteristic ruby-like photoluminescence, including broad excitation bands associated with the 4A2⟶4T1 and 4A2⟶4T2 transitions and sharp R-line emission arising from the spin-forbidden 2E⟶4A2 transition. The strongest emission was obtained for coatings prepared with 0.05 g/L Cr2O3, while higher concentrations resulted in concentration quenching. Temperature-dependent photoluminescence revealed two complementary thermometric mechanisms: R-line spectral shifting and thermally induced redistribution between the 2E and 4T2 emissions. The deconvolution-based intensity-ratio approach provided a stronger temperature response than simple spectral partitioning, demonstrating the potential of PEO-derived Al2O3:Cr3+ coatings on tungsten as robust luminescent temperature-sensing layers.