Mullite Coatings Produced by APS and SPS: Effect of Mullite Powder Morphology and Spray Processing on the Microstructure, Crystallinity and Mechanical Properties

Mullite coatings are being considered as one of the main protective layers of environment barrier coatings (EBCs) used to protect Si-based substrates for the next generation of gas turbine engines. In this study, the influence of the powder morphology and processing conditions on the microstructure (FE-SEM), relative crystallinity (XRD) and mechanical properties (hardness and elastic modulus) of mullite coatings was investigated. To accomplish this objective, coatings produced from two types of mullite powders (spray-dried and spray-dried flame-spheroidized) were deposited by conventional air plasma spray (APS) and mullite suspension coatings were deposited via suspension plasma spray (SPS). The particle state (temperature and velocity values) for all these systems were measured and related to the effects of powder morphology and spray processing conditions with the overall coating characteristics in order to create a process map for mullite coatings. No major differences among the highest relative crystallinity values for the three types of coatings produced were observed in this study. However, phase texture, microstructures and mechanical properties changed considerably according to powder morphology and spray processing conditions.