Ternary compounds denominated by the structure formula Mn+1AXn (MAX) phases comprise an extended family of layered, hexagonal transition metal carbides and nitrides. Due to their unique crystal chemistries and layered atomic structures, MAX phase materials combine many unique attributes of both ceramics and metals. These include low density, easy machinability, high thermal and electrical conductivities, thermal shock and damage tolerance, and excellent high-temperature resistance to oxidation and corrosion. These remarkable properties attract growing interest on the fabrication, characterization and implementation of MAX phase materials considered for structural and nonstructural applications under extreme conditions in the form of both bulk materials and thin films.
Alumina-forming MAX phases, like Ti2AlC and Cr2AlC with self-healing capabilities, have shown great potential in applications as high-temperature oxidation and corrosion resistant coatings. However, deposition of phase-pure polycrystalline MAX phase coatings, either by spraying technology or physical vapor deposition (PVD), often remains a challenge. The retention of stable competing phases, like binary carbides and intermetallics, degrade the performance of the coatings. In this study, a two-step approach has been established, i.e. first magnetron sputtering of nanoscale elemental multilayer stacks and subsequently thermal annealing in argon, for potential growth of three phase-pure MAX phase carbide (Ti2AlC, Cr2AlC and Zr2AlC) coatings. The temperature-dependent phase and microstructural evolutions during annealing were systemically investigated by high-temperature XRD, HRTEM and Raman spectroscopy. Dense and phase-pure Ti2AlC and Cr2AlC coatings were successfully fabricated, while growth of Zr(Al)C carbide rather than Zr2AlC MAX phase was confirmed in the Zr-C-Al system. In addition, the high-temperature oxidation behaviors of the three coatings were examined in steam.