Comparison of WC/DLC/WS₂ and YSZ/Au/DLC/MoS₂ “Chameleon” Coatings for Tribological Applications

Chameleon coatings are nanocomposite systems that adapt their tribological performance to changes in environmental conditions such as humidity and temperature. In this research we have investigated the tribological properties of two nanocomposite “chameleon” coatings and compared their properties. The two material systems of interest included: (i) Diamond like carbon (DLC) matrix with nanocrystalline WC and WS₂ inclusions, and (ii) Yttrium Stabilized Zirconia (YSZ) in an Au matrix with DLC and MoS₂ nanoparticle inclusions. The coating design approach included formation of nanocrystalline hard carbide or oxide particles for wear resistance, embedding them into an amorphous matrix for toughness enhancement, and inclusion of nanocrystalline and/or amorphous solid lubricants for friction adaptation to different environments. The coatings were produced using a combination of laser ablation and magnetron sputtering. Chemical and structural analysis of the coatings included x-ray photoelectron spectroscopy, x-ray diffraction, transmission electron microscopy, and micro-Raman spectroscopy. Mechanical properties such as coating hardness and toughness were investigated using nanoindentation, scratch, and indentation adhesion tests. It was observed that both YSZ and WC are valuable in enhancing film toughness. The chemical analysis was used to ascertain a correlation between chemical bonding of species and frictional properties. Friction measurements were studied by cycling between humid air and dry nitrogen conditions. The graphitic carbon component imparted low friction in humid air, MoS₂ and WS₂ were excellent for dry N₂ conditions, and Au was valuable for low friction at elevated temperatures. The direct comparison among coatings demonstrates that similar “chameleon” behavior can be achieved with different material systems, validating the universal nature of the design approach.