Supertough wear-resistant coatings with ‘chameleon’ surface adaptation

The chameleon's ability to change skin color depending on environment to increase its chances of surviving served as an inspiration in the development of self-adaptive supertough wear-resistant coatings. Surface chemistry, structure and mechanical properties of these thin (0.5 μm) coatings reversibly change with applied load and environment, providing the best wear protection. Coating designs developed in-house are reviewed together with a critical analysis of design reports in the literature. ‘Chameleon’ coatings were prepared using novel nanocomposite structures, consisting of crystalline carbides, diamond-like carbon (DLC), and transition metal dichalcogenides. Various mechanisms were activated to achieve surface self-adaptation and supertough characteristics. They included: transition of mechanical response from hard and rigid to quasi plastic by grain boundary sliding at loads above the elastic limit; friction induced sp3→sp2 phase transition of the DLC phase; re-crystallization and reorientation of the dichalcogenide phase; change of surface chemistry and structure from amorphous carbon in humid air to hexagonal dichalcogenide in dry nitrogen and vacuum; and sealing the dichalcogenide phase to prevent oxidation. These mechanisms were demonstrated using WC/DLC, TiC/DLC, and WC/DLC/WS2 coatings. The hardness of WC/DLC and TiC/DLC composites was between 27–32 GPa and scratch toughness was 4–5 fold above that of nanocrystalline carbides. The WC/DLC/WS2 composites survived millions of sliding cycles in vacuum and air under 500–1000 MPa loading, and exhibited excellent friction recovery in humid↔dry environmental cycling. Their friction coefficients were about 0.1 in humid air, 0.03 in vacuum, and as low as 0.007 in dry nitrogen. The proposed ‘chameleon’ concept can dramatically increase wear-resistant coating applicability, durability, and reliability.