Microstructure and mechanical properties evolution in tri-layer Si-HfO2/Yb2Si2O7/Yb2SiO5 environmental barrier coating by PS-PVD during post-annealing

In recent years, the plasma spray-physical vapor deposition (PS-PVD) process has garnered significant attention as an advanced method for producing environmental barrier coatings (EBCs), and the tri-layer Si-HfO2/Yb2Si2O7/Yb2SiO5 coating system as the EBCs of SiCf/SiC has received widespread attention for potential applications in hot sections of aero-engines. Unfortunately, the peeling phenomenon of these EBCs in the process of high-temperature gas erosion is serious and reduces the service life of the coating system. In this research, tri-layer Si-HfO2/Yb2Si2O7/Yb2SiO5 coatings were deposited on SiCf/SiC with PS-PVD. After spraying, the samples were annealed at 1200 °C, 1300 °C and 1400 °C for 10 h, 30 h and 50 h, respectively. The effects of annealing parameters on the microstructure and mechanical properties of the coating were investigated. Annealing resulted in an increase in grain size, healing of microcracks and pores, accompanied by a decrease in surface roughness and porosity. The change in coating microstructure led to variations in the mechanical properties of the coating, including the residual stress, hardness and elastic modulus of the coating. In addition, the diffusion behavior of bonding layer elements was investigated. Furthermore, the bonding strengths of as-sprayed and annealed samples were obtained by tensile testing. High-temperature annealing enhanced the bonding strength between coating and substrate by promoting the diffusion of silicon elements into the substrate to form strong intergranular bonding. The prerequisite for the diffusion of silicon was that it was not oxidized during annealing. However, long-term high-temperature annealing leads to the formation of pores in the bonding layer, which reduce the bonding strength.