Evolution of microstructural and mechanical properties of lanthanum zirconate thermal barrier coatings at high temperature

Ceramic thermal barrier coatings (TBCs) are usually adopted to protect hot section components of aircraft and gas turbine engines from extreme temperatures and environmental attack promoted by oxygen and corrosive media. Their applications allows to increase the durability as well as to improve the efficiency and to reduce the pollution. In this work, thick lanthanum zirconate (La2Zr2O7) TBCs were deposited by Air Plasma Spraying (APS) and characterized about their microstructure by Scanning Electron Microscopy (SEM). The coatings, composed of single pyrochlore structure, exhibited a lamellar microstructure with embedded pores, cracks and splat boundaries. High-temperature exposure ( 50 hours at 1200 and 1350 °C, in air) assisted the partial densification of this microstructure. The mechanical properties of as-sprayed and heat treated coatings were measured at various temperatures up to 1500 °C in air by employing four-point bending tests, properly optimized for free-standing porous coatings. The elastic modulus measured at room temperature was found to be 16 GPa and changed as the temperature of testing increased. This study was also addressed to analyze the influence of sintering phenomena on the high-temperature behaviour of the same TBCs. The thermal aging caused the partial densification of the microstructure, thus increasing the elastic modulus. However, this increase was less pronounced when the coatings were tested at higher temperature. In addition the effect of densification was clearly observable in terms of deformation at break and modulus of rupture The evolution of thermal properties such as thermal expansion coefficient and heat capacity after high-temperature exposure was also investigated. The results demonstrated that the TBCs herein employed are promising for applications at extreme operating conditions due to enhanced capability and resistance to sintering. They can be also used in multilayered systems in order to further improve their potentiality.