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

Abstract

α- and γ-Al2O3 coatings, are often used for high temperature applications T ≥ 800 °C. While stable α-Al2O3 is typically synthesized by chemical vapor deposition (CVD) due to its high formation temperature T ≥ 1000 °C, conventional physical vapor deposition (PVD) is limited to T ≈ 600 °C. 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 α- and γ-Al2O3 coatings at T ≈ 780 °C. 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 °C 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 ≈ 33 % and Y ≈ 40 % at THT-NI = 650 °C and THT-NI = 750 °C, 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 °C, which can extend the application of these coatings to higher temperatures.