Guidelines for increasing the oxidation resistance of Ti-Al-N based coatings

Highlights: • Isothermal oxidation behavior at 850 and 950 °C of arc evaporated Ti_0.5Al_0.5N. • Different oxidation mechanisms when alloying Ti_0.5Al_0.5N with Y, Zr, or Ta. • Combined DSC and TGA investigations allow studying especially the initial minutes. • Zr stabilizes the anatas-TiO₂ phase formation and works best at medium temperatures. • Ta promotes the rutile-TiO₂ phase formation and works also at higher temperatures. -- Abstract: Generally valid guidelines for increasing the oxidation resistance of NaCl type Ti_1-xAl_xN coatings are necessary to widen their field of applications. Therefore, we have studied the influence of alloying Y, Zr, or Ta on the phase formation during oxidation and their potential to form protective oxide scales. By combining differential-scanning-calorimetry, thermo-gravimetric-analysis, and X-ray diffraction we identified that also the anatase (a-TiO₂) to rutile (r-TiO₂) phase transformation within the formed oxide scale has a major impact on the oxidation resistance of these coatings. At intermediate temperatures (up to 850 °C), this transformation can be avoided by alloying Zr, because Zr strongly stabilizes the a-TiO₂ phase. Contrary, alloying Ta strongly promotes the r-TiO₂ phase (reduces the intermediate a-TiO₂ formation, which is typical for Ti_1-xAl_xN coatings). Consequently, high temperature oxidation resistance requires the addition of Ta, as thereby denser TiO₂ based oxide scales form and a direct formation of r-TiO₂ minimizes/avoids the detrimental effects of an a-TiO₂-to-r-TiO₂ transformation.