Effect of isothermal heat treatment on nanoindentation hardness and Young's modulus of 4 mol% Y₂O₃-ZrO₂ EB-PVD TBCs

4 mol% Y₂O₃–ZrO₂ (4YSZ) is commonly used in thermal barrier coatings (TBCs) for high-temperature applications such as gas turbine engines. This study was investigated the effects of heat treatment at 1400 °C for 0, 2, 20, and 100 h on the hardness and Young's modulus of 4YSZ coatings prepared by electron-beam physical vapor deposition. Nanoindentation measurements were performed on the top and side surfaces of the coating. The as-coated and the heat-treated coatings exhibited strong anisotropy in the mechanical properties; the hardness and Young's modulus of the side surface of the coatings were lower than those of the top surface. Increasing the heat treatment time was found to increase the hardness and Young's modulus in both the directions. This is correlated with the decrease in porosity with increase in the heat-treatment time and closure of gaps between the columns and arms of their (as-coated) featherlike microstructures, which are known to deleteriously affect both the toughness and thermal insulating properties of TBCs.