Desorption of Al and Phase Transformation of Ti2AlN MAX Thin Film upon Annealing in Ultra-High-Vacuum

Phase stability of single-crystalline Ti2AlN thin film in ultra-high vacuum has been studied in situ by X-ray photoelectron spectroscopy as a function of annealing temperature and ex situ by atomic force microscopy, secondary ion mass spectroscopy, energy-dispersive X-ray spectroscopy, X-ray diffraction, and nanoindentation. Ti2AlN is stable up to 600 °C. At 700 °C, Al is preferentially desorbed from the surface and becomes nearly undetected at 900 °C by XPS, where single-crystalline Ti2AlN with terrace morphology transforms into polycrystalline δ-TiN1–x and ξ-TiN0.75–y phases with voids on the surface and reduced film thickness. Mechanical properties including hardness and Young's modulus are also observed to have deteriorated. Density functional theory calculation shows that Al atoms prefer to diffuse out from the Ti2AlN horizontally along the Al basal planes. The subsequent desorption of Al from surface due to its high vapor pressure results in the decreased Al composition, the void formation on the surface, and the decomposition of Ti2AlN. A kinetic model involving diffusion and desorption processes is proposed to describe the Al behavior and voids formation above 700 °C.