Terbium as an alternative for luminescence sensing of temperature of thermal barrier coating materials

Previous work has demonstrated the utility of europium doping into 7YSZ for luminescence sensing of temperature in TBC systems by the luminescence decay time of the europium chromophore after pulsed laser excitation, with sensitivity up to 1100 °C. Based on relative energy gaps and a multi-phonon relaxation model, terbium is predicted to yield longer decay times than europium and allow extension of the temperature sensitivity to higher temperatures. Therefore, terbium doping as a chromophore for luminescence sensing of current thermal barrier coatings is investigated. Terbium doping is found to be ineffective in both yttria-stabilized zirconia and gadolinium zirconate (GZO) because of the formation of Tb4+. However, terbium doping in gadolinium aluminum perovskite (Tb:GaAlO3) gives rise to strong luminescence bands. Luminescence decay measurements indicate that Tb:GaAlO3 has potential as a thermographic sensor to at least 1250 °C. Further, the brightness of Tb:GdAlO3 at moderate and high temperatures is found to exceed that of Eu:7YSZ. The phase compatibility of Tb:GaAlO3 with alumina and gadolinium zirconate is suitable for luminescence temperature sensing both at a TGO interface and on the surface of a GZO coating.