Effect of crystal orientation of AlN coatings on its high temperature oxidation in wet air

Aluminum nitride is discussed for different high temperature applications, due to its unique properties. Lately it also received attention as a coating for different substrate materials. Within the present study oriented AlNcoatings with two different crystal orientations were produced via reactive magnetron sputtering and a high temperature CVD process on SiC-based substrates. The oxidation resistance of these coatings in comparison to bulk AlN was investigated in wet synthetic air at 1200 degrees C. Both, the bulk AlN and (101 over bar 0)-oriented AlN-coating formed a porous Al2O3-scale, which was not protective and resulted in high oxidation rates. In contrast, the (0001)-oriented coatings were able to form a thin oxide scale and remained protective for the total exposure time of up to 200 h at 1200 degrees C. The underlying mechanism of the oxidation of AlN in wet air was discussed, considering the strong influence of the crystal orientation.