Nanocomposite tribological coatings for aerospace applications

Challenges in aerospace tribology and composite coatings for aerospace applications are briefly reviewed. Attention is given to nanocomposite coatings made of carbide, diamond-like carbon (DLC) and transition-metal dichalcogenide phases. The preparation of such coatings within the W-C-S material system using a hybrid of magnetron sputtering and pulsed laser deposition is described. Coatings consist of 1-2 nm WC and 5-10 nm WS2 grains embedded in an amorphous DLC matrix. These WC/DLC/WS2 nanocomposites demonstrate low friction and wear in tests performed in high vacuum, dry nitrogen and humid air. Coatings are found to adapt to the test conditions, which results in: (I) crystallization and reorientation of initially nanocrystalline and randomly oriented WSI grains, (2) graphitization of the initially amorphous DLC matrix; (3) reversible regulation of the composition of the transfer film between WS2 and graphite with environmental cycling from dry to humid; and (4) possible DLC/WS2 synergistic effects, providing friction reduction in oxidizing environments. These adaptive mechanisms achieve low friction coefficients of 0.02-0.05 and an endurance above two million cycles in space simulation tests. This also provides stable coating performance and recovery of low friction in tests simulating ambient/space environmental cycling. Correlations among WC/DLC/WS2 chemistry, structure, hardness, friction and wear are discussed. The tremendous potential of such composites for aerospace tribology is demonstrated. (C) 1999 Elsevier Science S.A. All rights reserved.