Tribological properties of adaptive nanocomposite coatings made of yttria stabilized zirconia and gold

Composite coating architectures where hard nanocrystalline grains are embedded in an amorphous matrix provide considerable improvement in hardness, toughness, wear resistance, and environmental adaptation. Using this concept, nanocrystalline yttria stabilized zirconia (YSZ) was embedded in an amorphous YSZ/Au matrix to address problems with YSZ ceramics in sliding wear. The coatings were produced by a hybrid of laser ablation of YSZ and magnetron sputtering of Au. Coating composition and microstructure were investigated using a number of analytical techniques, and correlated with results of sliding friction tests at 25 and 500°C. In situ transmission electron microscope imaging of microstructure evolution during a temperature cycling from 25 to 500°C was performed to explain changes in tribological properties. In comparison to YSZ ceramic, YSZ/Au coatings were tougher, formed less wear debris, and reduced friction coefficients from 1.0 to 0.3–0.4 at 25°C and to 0.2 at heating to 500°C. Improvements in tribological properties were related to the microstructure adaptive changes at elevated temperatures and formation of lubricating Au transfer films.