Thermal stability and oxidation behavior of quaternary TiZrAlN magnetron sputtered thin films: Influence of the pristine microstructure

Quaternary TiZrAlN coatings deposited by reactive magnetron sputtering from elemental targets have recently been shown to offer tailored nanostructural design with enhanced mechanical properties by fine-tuning the Al content at fixed N2 partial pressure. Here, the influence of the microstructure of as-deposited (Ti,Zr)1 − x − yAlxNy coatings on their thermal stability and oxidation resistance is studied in details by scanning electron microscopy and X-ray diffraction. At low Al content ('type I' microstructure, 0 ≤ x ≤ 0.05), single-phase, stoichiometric (Ti,Zr,Al)N solid solutions with cubic structure are formed. These films are thermally stable after vacuum annealing at 600 °C, the main structural changes being related to a defect annihilation and crystal recovery, leading to the development of a net tensile stress. Nanocomposite (Ti,Zr)1 − x − yAlxNy films with 'type II' microstructure (0.06 ≤ x ≤ 0.11; 0.34 ≤ y ≤ 0.39), consisting of cubic (Ti,Zr,Al)N nanocrystals embedded in an amorphous matrix, showed partial crystallization already at 600 °C. At 950 °C, phase decomposition takes place via the formation of cubic ZrN-rich and TiN-rich domains for both film-types. A decrease of the onset temperature for thermal decomposition is evidenced with increasing Al content. However, films with higher Al content delaminated after annealing at temperature higher than 800 °C, suggesting that nitrogen deficiency is an important factor influencing the thermal stability. Oxidation experiments yield fully oxidized TiZrAlN coatings at 950 °C, with the formation of a porous morphology and significant swelling (~ 200% increase in film thickness) and local blistering. Amorphous films with 'type III' microstructure (0.14 ≤ x ≤ 0.24; 0.24 ≤ y ≤ 0.31) show the best oxidation resistance, as the temperature for the formation of orthorhombic TiZrO4 oxide layer is increased by ~ 150 °C compared to Al-free coatings. However, films with higher Al content underwent extensive film flaking after air annealing.