Abstract
Generally valid guidelines for increasing the oxidation resistance of NaCl type Ti1-xAlxN 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 Ti1-xAlxN 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.
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