Supertough wear-resistant coatings with 'chameleon' surface adaptation

The chameleon's ability to change skin color depending on environment to in crease its chances of surviving served as an inspiration in the development of self-adaptive supertough wear-resistant coatings. Surface chemistry, st ructure and mechanical properties of these thin (0.5 mu m) coatings reversi bly change with applied load and environment, providing the best wear prote ction. Coating designs developed in-house are reviewed together with a crit ical analysis of design reports in the literature. 'Chameleon' coatings wer e 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 su pertough characteristics. They included: transition of mechanical response from hard and rigid to quasi plastic by grain boundary sliding at loads abo ve the elastic limit; friction induced sp(3) --> sp(2) phase transition of the DLC phase; re-crystallization and reorientation of the dichalcogenide p hase; change of surface chemistry and structure from amorphous carbon in hu mid air to hexagonal dichalcogenide in dry nitrogen and vacuum; and sealing the dichalcogenide phase to prevent oxidation. These mechanisms were demon strated 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 roughness 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 lo ading, and exhibited excellent friction recovery in humid <--> dry environm ental cycling. Their friction coefficients were about 0.1 in humid air, 0.0 3 in vacuum, and as low as 0.007 in dry nitrogen. The proposed 'chameleon' concept can dramatically increase wear-resistant coating applicability, dur ability, and reliability. (C) 2000 Elsevier Science S.A. All rights reserve d.