Stress-induced phase transformation and amorphous-to-nanocrystalline transition in plasma-sprayed Al2O3 coating with relative low temperature heat treatment

In the present study, the relationship between phase transformation and stress-inducing is established. The residual stress would decrease the phase transition temperature of the γ-Al2O3 to α-Al2O3. In order to further ensure the reliability of stress-induced phase transformation, the as-sprayed Al2O3 coating was heat treated at 600 °C for 24 h. The Al2O3 coatings were investigated using X-ray diffraction and transmission electron microscopy. The relative low temperature heat treatment facilitates the growth of γ-Al2O3 nanocrystals, amorphous-to-nanocrystalline transition and stress-induced phase transformation in the Al2O3 coating. The heat treated Al2O3 coating has greater Vickers hardness and bending strength than the as-sprayed Al2O3 coating. However, the fracture toughness descent might be attributed to the pore-toughening effect reduction due to phase transition (from γ-Al2O3 to α-Al2O3) accompanied by a certain extent volume shrinkage and the brittleness of new generated α-Al2O3.