Hard DLC Growth and Inclusion in Nanostructured Wear-protective Coatings

Wear-resistant coatings using hard hydrogen-free and tetrahedral bonded ta-DLC offer a unique combination of high hardness, low friction, and extremely low wear rates. The related coating deposition techniques of such wear-protective coatings are reviewed, including filtered vacuum arc and pulsed laser deposition (PLD), where process parameters and plasma characteristics are correlated to ta-DLC bonding and mechanical characteristics. Advancements in hybrid plasmadeposition methods and process-control arrangements allow production of hard ta-DLC with high adhesion and fracture resistance through the development of graded and multilayer coating architectures. Further enhancement came with nanocrystalline carbide/amorphous DLC compositions, where high toughness is achieved by using grain boundary sliding for strain accommodation without brittle fracture. Multiple crystalline/amorphous interfaces help to divert and split nanocracks, avoiding coating macro-cracking even when relatively softer substrates experience a high degree of deformations. Since ta-DLC is exceptionally good in ambient conditions, adaptive “chameleon” composites were developed to use this tribological benefit of ta-DLC, while introducing additional solid lubricant phases for other environments and/or protecting DLC material from degradation in extreme environments, including dry air or inert gas, high vacuum, and elevated temperatures in air. DLC is an important part of today's tribological coating applications, where a high wear resistance and the absence of lubricating fluids are critical, e.g., aerospace, dry machining, and MEMS. The coatings with hard DLC phases can provide friction reduction and wear-life extension without the need of lubrication systems, reducing mechanism complexity, costs, weight, and environmental impact.