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 N-2 partial pressure. Here, the influence of the microstructure of as-deposited (Ti,Zr)(1-x-y)AlxNy 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 degrees 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-y)AlxNy films with 'type II' microstructure (0.06 <= x <= 0.11; 034 <= y <= 0.39), consisting of cubic (Ti,Zr,Al)N nanocrystals embedded in an amorphous matrix, showed partial crystallization already at 600 degrees C. At 950 degrees 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 degrees C, suggesting that nitrogen deficiency is an important factor influencing the thermal stability. Oxidation experiments yield fully oxidized TiZrAlN coatings at 950 degrees C, with the formation of a porous morphology and significant swelling (similar to 200% increase in film thickness) and local blistering. Amorphous films with 'type III' microstructure (0.14 <= x <= 024; 0.24 <= y <= 031) show the best oxidation resistance, as the temperature for the formation of orthorhombic TiZrO4 oxide layer is increased by similar to 150 degrees C compared to Al-free coatings. However, films with higher Al content underwent extensive film flaking after air annealing. (C) 2013 Elsevier B.V. All rights reserved.