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

Thin ZrO 2-Al 2O 3-Y 2O 3 coatings were deposited by reactive magnetron sputtering technique onto steel substrates. Annealing at 1273 K the ceramic ZrO 2-Al 2O 3 mixtures in air crystallise in the tetragonal phase and grain growth is observed. The Al 2O 3 content limits the maximum observed grain size between 25 and 30 nm during the observation time of this study of about 1.8 × 10 6 s (500 h). In contrast, the ZrO 2-Y 2O 3 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 Al 2O 3 (without Y 2O 3) generally show tetragonal grains but prolonged annealing at 1273 K decreases the effect of the rigid Al 2O 3 matrix and the zirconia transforms into its monoclinic phase modification. Multilayered coatings of ZrO 2 with layer thickness lower to 6 nm also retained the tetragonal phase. Nevertheless, 1-nm thick Al 2O 3 spacing layers are not sufficient to suppress the diffusion, growth and subsequent phase transformation of the zirconia.