Abstract
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.
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