Strain and stress analyses on thermally annealed Ti-Al-N/Mo-Si-B multilayer coatings by synchrotron X-ray diffraction

Abstract

In order to analyse the main failure mechanism of multilayered coating material in oxidative environments, we separately investigated the cross-sectional strain/stress evolution induced by thermal loads and oxidation for the Ti-Al-N/Mo-Si-B model system. The bilayer period (Λ) of the crystalline Ti-Al-N/amorphous Mo-Si-B layers was varied between 26, 130, 240, and 1085 nm. The stress state was characterised by synchrotron X-ray nano-diffraction, using monochromatic X-ray radiation with a beam size of around 200 × 300 nm2. This allows for analysing the spatially resolved strain/stress evolution of the as-deposited state as well as after thermally treated coatings – either 1 h annealed in vacuum or ambient air at 900 °C. For small bilayer periods, the alteration of face centred cubic Ti-Al-N by amorphous Mo-Si-B layers effectively reduces the as-deposited compressive strain profile along the growth direction. Furthermore, for Λ ≤ 130 nm, the decomposition of Ti-Al-N to form hexagonal structured AlN as well as the crystallisation of the Mo-rich layers towards the intermetallics Mo5SiB2 and Mo5Si3 is significantly delayed. After oxidation, the oxide scale grows in the low-compressive stress regime, while the intact multilayer shows similar microstructural changes as the vacuum annealed coatings.