Tensile Behavior of Air Plasma Spray MCrAlY Coatings: Role of High Temperature Agings and Process Defects

Abstract

$\beta$-$\gamma$ MCrAlY coatings generally exhibit a brittle mechanical behavior below 600 °C. When exposed at elevated temperatures, the microstructure of such coatings evolves, leading to an increasing content of $\gamma$ phase, i.e., the ductile phase, and a decreasing content of $\beta$ phase, i.e., the brittle phase. Therefore, the evolution of the mechanical properties of such environment-protective materials is worth of investigation. In the present study, a $\beta$-$\gamma$ NiCoCrAlY coating was processed by air plasma spray (APS) technology. $150\,\mu \text {m}$-thin freestanding specimens were prepared then aged in an oxidative atmosphere at high temperatures (950 ºC up to 1150 ºC) for different durations to simulate in-service degradation of the coatings. Microtensile testings were conducted at room temperature for all the aging variants and the mechanical properties of the aged specimens were found to evolve as follows: (i) an increase in both Young’s modulus and tensile strength and a loss in ductility for aging temperatures below 1050 °C, (ii) a decrease in Young’s modulus and a gain in ductility for aging temperatures above 1050 °C, and (iii) a significant scatter in mechanical properties for high temperature agings. The low ductility observed for high temperature agings was related to intruded oxides developing during the aging treatment, heterogeneously distributed in the volume of the coating. The gain in ductility was mainly attributed to the $\beta$-phase decrease, the loss in interconnection between $\beta$ phases compared to the as-received microstructure and a topological inversion of the $\beta$-$\gamma$ microstructure.