Design of nanostructured thin films for tribological applications

Nanoscaled coatings for tribological applications can be subdivided into nanostructured multilayer films, nanomodulated superlattice films, nanocrystalline films, nanostabilized single and multilayer films and nanograded films. The interface volume, grain size, single layer thickness, surface and interface energy, texture, and epitaxial stress and strain are principal factors, besides materials selection and deposition characteristics, determining constitution, properties, and performance of these coatings. The functional and structural design of multilayer coatings can result in tailored multifunctional coatings as is shown for nanoscaled TiN/TiAlN, TiC/TiB2, TiN/MoS2 and TiC/C multilayer films. The influence on properties of the modulation of composition, structure, stress and strain in so-called superlattice films is discussed as a function of the modulation period and materials selection. Nanocrystalline coatings (e.g. TiC/TiB2, TiC/C or TiN/MoS2 films) are well suited to combining film materials of similar or extremely different bonding characteristics into multifunctional composite coatings. New structures for thin film materials or specific film textures can be stabilized by epitaxial growth in nanostabilized multilayer coatings. The stabilization of fee AIN, fee SiC, crystalline C3N4, and wurtzite-type BN stands for materials either used successfully or under development. New metastable nanocrystalline films can be deposited by vapor quenching. Thermodynamic and kinetic modelling results in new film materials (e.g. TiBCN), designed to requirements of constitution, properties and performance. The concept of nanograded films lends itself particularly well to improving the adhesion and critical loads of failure of superhard covalent bonded C- and BCN-films.