ARCHITECTURE OF MULTILAYER NANOCOMPOSITE COATINGS WITH SUPER-HARD DIAMOND-LIKE CARBON LAYERS FOR WEAR PROTECTION AT HIGH CONTACT LOADS

Super-hard and low-friction diamond-like carbon (DLC) coatings deposit ed at low temperatures are currently of great interest for wear protec tion and friction reduction. However, their high hardness (50-80 GPa), intrinsic stresses, and poor adhesion limit their use to applications where contact pressures are below 1 GPa and the coating thickness is below 0.5 mu m to prevent cracking and delamination. These negative ef fects are especially pronounced when the coatings are applied to relat ively soft substrates, such as steels. The limitations were removed by a multilayer design, where metal and ceramic layers were used to incr ease the load support capability, improve the adhesion strength, and i ncrease the thickness of DLC layers. Existing approaches to the design of tough multilayer coatings were considered critically and a coating architecture was suggested using the following concepts: (i) formatio n of a load support and adhesion promoting underlayer with mechanical characteristics varied gradually from the substrate to the DLC layer; (ii) separation of hard DLC layers with interlayers of softer material to reduce stresses and brake cracks; (iii) use of crystalline interla yers with thicknesses permitting operation of dislocation sources for stress relaxation and deflection of cross-sectional cracks. The develo pment of these concepts is discussed sequentially from bilayer ceramic /DLC coatings to functionally gradient metal/carbide/DLC coatings, and finally to nanocomposite coatings consisting of stacks of Ti/DLC, TiC / DLC, and CN/DLC layers with individual thicknesses within 10-60 nm d eposited onto a gradient Ti-TiC-DLC underlayer. The coatings were depo sited onto stainless steel substrates by a hybrid of magnetron sputter ing and pulsed laser deposition. They had a 1-2 mu m total thickness o f super-hard (60-70 GPa) DLC layers and resisted delamination to 50-80 N loads in scratch adhesion tests. In ball-on-disk sliding tests, the se coatings supported Hertzian contact pressures above 2 GPa and had f riction coefficients around 0.1. Their wear lives exceed 10(6) cycles under initial contact pressures of 1.4 GPa. The conceptual architectur e of multilayer nanocomposite coatings presented extends the range of wear resistant applications for super-hard DLC materials. (C) 1997 Els evier Science S.A.