J.J. Hu

Nanocomposite tribological coatings with ''chameleon'' surface adaptation

Nanocomposite tribological coatings were designed to respond to changing environmental conditions by self-adjustment of their surface properties to maintain good tribological performance in any environment. These smart coatings have been dubbed “chameleon” because, analogous to a chameleon changing its skin color to avoid predators, the coating changes its “skin” chemistry and structure to avoid wear. The concept was originally developed using WC, diamondlike carbon, and WS2 material combination for adaptation to a humid/dry environment cycling. In order to address temperature variation, nanocomposite coatings made of yttria-stabilized zirconia (YSZ) in a gold matrix were developed with encapsulated nanosized reservoirs of MoS2 and diamondlike carbon (DLC). Coatings were produced using a combination of laser ablation and magnetron sputtering. They were characterized by x-ray photoelectron spectroscopy, x-ray diffraction, transmission electron microscopy, x-ray energy dispersive spectroscopy, and micro-Ram...

Growth and structural characterization of yttria-stabilized zirconia–gold nanocomposite films with improved toughness

Abstract To counteract the natural brittleness of oxide ceramic films and achieve better toughness, nanocomposite films combining yttria-stabilized zirconia (YSZ) and gold were produced. A hybrid of magnetron sputtering and pulsed laser ablation was used to grow crystalline YSZ embedded in an amorphous YSZ/Au matrix at near room temperature. Results from chemical analyses, X-ray and electron diffraction, high-resolution transmission electron microscopy, lateral force microscopy, nanoindentation hardness measurements and toughness estimates are discussed. At a fixed deposition temperature, the film microstructure was critically dependent on the gold content. Three distinct microstructures were produced: an amorphous YSZ/Au coating at less than 8 at.% Au; 5–10 nm YSZ crystals embedded in an amorphous YSZ/Au matrix at 10–15 at.% Au; and 20–100-nm crystallites of (101)-oriented YSZ mixed with micron-sized Au agglomerates at above 20 at.% Au. From these films, only the 5–10 nm nanocrystalline YSZ embedded in the amorphous YSZ/Au matrix provided the improvements in toughness. Coatings with this structure had hardness of 15–20 GPa, elastic modulus of approximately 250 GPa, and did not develop cracks in Vickers pyramid indentation at loads up to 1 kg, showing remarkable ductility. The improvement in toughness of this relatively hard film was explained by grain boundary sliding of YSZ nanocrystals in the YSZ/Au amorphous matrix. The new YSZ/Au nanocomposite films developed in this study may have good potential for surface protection, where a combination of thermal stability, oxidation resistance and improved film toughness are required.

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

Abstract 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.