High Temperature Oxidation of Amorphous Zr-B-C-N Thin Films Grown by Electron Beam Evaporation

Boron carbonitride films have been shown to yield enhanced hardness and oxidation resistance, making them potentially attractive as protective coatings for high temperature applications. Most previous work has focused on thick films (>1μm) grown by magnetron sputtering. In this study, 200 nm thick ZrBCN films with a range of stoichiometries were grown on r-sapphire substrates at 850^oC using e-beam evaporation of ZrC and B sources in a nitrogen plasma. As-deposited films were found to be amorphous by XRD, and the nitrogen content was relatively constant independent of ZrC or B evaporation rates. XPS analysis of the films after air annealing over the range 500-800^oCshowed that B and N were entirely depleted at the surface, while ZrO₂ grains nucleated within the amorphous film matrix. XRD showed the formation of a tetragonal-ZrO₂ phase at 600^oC that became replaced by a monoclinic-ZrO₂ phase at 800^oC as the ZrO₂ grains grew in size. SEM analysis showed film delamination and cracking during high temperature oxidation to relieve film stress. The oxidation resistance of the 200 nm ZrBCN films is much less than reported for thicker sputtered films, indicating that the oxidation resistance mechanism is strongly dependent on film thickness.