Thermal stability of thick α- and γ-Al2O3 coatings deposited by high-speed PVD

alpha- and gamma-Al2O3 coatings, are often used for high temperature applications T >= 800 degree celsius. While stable alpha-Al2O3 is typically synthesized by chemical vapor deposition (CVD) due to its high formation temperature T >= 1000 degree celsius, conventional physical vapor deposition (PVD) is limited to T approximate to 600 degree celsius. Nevertheless, this method may result in metastable or amorphous Al2O3 phases, which limits their thermal stability and application at higher temperatures. Previous studies showed that High-Speed PVD can synthesize thick alpha- and gamma-Al2O3 coatings at T approximate to 780 degree celsius. This study investigates the thermal stability of these coatings deposited by HS-PVD. High-temperature X-ray diffraction (HT-XRD) measurements show no phase transformations of coatings up to T = 1200 degree celsius in a vacuum or atmosphere. The coating morphology remained unchanged, and no cracks were detected by scanning electron microscopy (SEM) after annealing during HT-XRD. In situ, HT nanoindentation (HT-NI) revealed a reduction in indentation hardness of X approximate to 33 % and Y approximate to 40 % at THT-NI = 650 degree celsius and THT-NI = 750 degree celsius, respectively. However, hardness values remained similar to the as-deposited state after annealing in HT-XRD. These findings reveal a high thermal stability of crystalline Al2O3 coatings deposited by HS-PVD up to T = 1200 degree celsius, which can extend the application of these coatings to higher temperatures.