Thermal stability of zirconia/alumina thin coatings produced by magnetron sputtering

Thin ZrO2-Al2O3-Y2O3 coatings were deposited by reactive magnetron sputtering technique onto steel substrates. Annealing at 1273 K the ceramic ZrO2-Al2O3 mixtures in air crystallise in the tetragonal phase and grain growth is observed. The Al2O3 content limits the maximum observed grain size between 25 and 30 nm during the observation time of this study of about 1.8 x 106 s (500 h). In contrast, the ZrO2-Y2O3 coatings revealed tetragonal grains with a size of about 200 nm. Diffusion processes cause the grain growth as well as the observed decrease of the intrinsic coating stress. Zirconia coatings stabilised by Al2O3 (without Y2O3) generally show tetragonal grains but prolonged annealing at 1273 K decreases the effect of the rigid Al2O3 matrix and the zirconia transforms into its monoclinic phase modification. Multilayered coatings of ZrO2 with layer thickness lower to 6 nm also retained the tetragonal phase. Nevertheless, 1-nm thick Al2O3 spacing layers are not sufficient to suppress the diffusion, growth and subsequent phase transformation of the zirconia. (orig.)