Effect of heat treatment on hardness and fracture toughness of Si/BSAS/Yb2SiO5 EBCs system under 1200 °C

High-temperature oxidation environments have an important influence on phase transition and thermal/residual stresses of Si/BSAS/Yb2SiO5 environmental barrier coatings (EBCs). To further reveal the failure mechanisms of Yb2SiO5 EBCs, different kinds of Si, BSAS, Yb2SiO5 coatings deposited on Cf/SiC composites were prepared by air plasma spraying (APS) technique, respectively. The content and microstructure variations of all coatings before and after different heat treatments were analyzed by X-ray diffraction and scanning electron microscope methods, respectively. The corresponding Vickers hardness, fracture toughness and residual stresses were evaluated by indentation tests at room temperature. The hardness and fracture toughness of the coating after heat treatment at 1200 °C were systematically studied. It is found that with the increase of heat treatment, the porosity of the coating decreases, and the hardness and fracture toughness of the coating increases. Porosity of the coating is affected by phase transformation and grain growth. The residual compressive stress of the coating is positively correlated with the hardness and fracture toughness of the coating.