title: Thermal stability and oxidation resistance of Ti-Al-N coatings.
abstract: Ti1 - xAlxN coatings are widely used for wear resistant applications due to their excellent mechanical and thermal properties, which depend to a great extent on the Al content. Here, we concentrate on a comparative study of the effect of Al content on crystal structure, thermal stability and oxidation resistance of Ti1 - xAlxN coatings. In agreement to earlier studies, thermal annealing of the individual cubic (c) and wurtzite (w) structured metastable Ti1 - xAlxN coatings induces decomposition into their stable phases c-TiN and w-AlN. The decomposition process for c-Ti1 - xAlxN involves an intermediate formation of cubic Al-rich and Ti-rich domains which results in a hardness increase to 34.7 and 34.4 GPa for x = 0.52 and 0.62 when annealed at 950 and 900 °C, respectively. In general, coatings with an Al content closer to the solubility limit, exhibit an earlier decomposition process, and hence an earlier peak-hardness. During exposure of the Ti1 - xAlxN coatings to ambient air at elevated temperatures Al2O3, TiO2and Al2TiO5are formed. The oxidation resistance of as-deposited single-phase Ti1 - xAlxN coatings, cubic or wurtzite structured, increases with increasing Al content. However, coatings containing Al contents at the metastable solubility limit, which result in a mixed cubic-wurtzite structure, have the worst oxidation resistance of the Al-containing coatings investigated. The single phase wurtzite structured coating w-Ti0.25Al0.75N shows the best oxidation resistance, with only ~0.7 μm oxide scale thickness, after thermal exposure for 20 h at 850 °C in ambient air.
authors.affiliation: Department of Physical Metallurgy and Materials Testing, Montanuniversität Leoben, Leoben, 8700, Austria ; State Key Laboratory of Powder Metallurgy, Central South University, Changsha Hunan, 410083, China.
authors.forename: Li
authors.initials: L
authors.lastname: Chen
authors.forename: Jörg
authors.initials: J
authors.lastname: Paulitsch
authors.forename: Yong
authors.initials: Y
authors.lastname: Du
authors.forename: Paul H
authors.initials: PH
authors.lastname: Mayrhofer
keywords: Age-hardening; AlTiN; Oxidation resistance; Thermal stability; Ti–Al–N