Architecture of multilayer nanocomposite coatings with super-hard diamond-like carbon layers for wear protection at high contact loads

Abstract

Super-hard and low-friction diamond-like carbon (DLC) coatings deposited at low temperatures are currently of great interest for wear protection 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 I GPa and the coating thickness is below 0.5 μm to prevent cracking and delamination. These negative effects are especially pronounced when the coatings are applied to relatively soft substrates, such as steels. The limitations were removed by a multilayer design, where metal and ceramic layers were used to increase the load support capability, improve the adhesion strength, and increase 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) formation 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 interlayers with thickness permitting operation of dislocation sources for stress relaxation and deflection of cross-sectional cracks. The development 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 thickness within 10–60 nm deposited onto a gradient TiTiC-DLC underlayer. The coatings were deposited onto stainless steel substrates by a hybrid of magnetron sputtering and pulsed laser deposition. They had a 1–2 μm total thickness of 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, these coatings supported Hertzian contact pressures above 2 GPa and had friction coefficients around 0.1. Their wear lives exceed 10 6 cycles under initial contact pressures of 1.4 GPa. The conceptual architecture of multilayer nanocomposite coatings presented extends the range of wear resistant applications for super-hard DLC materials.