Nanocomposite Hard Coatings for Wear Protection
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Nanocomposite
Hard Coatings for Wear Protection Jörg Patscheider
Abstract Nanocomposite thin films successfully promote hardness, oxidation resistance, improved wear behavior, and other properties relevant for wear-reducing coatings. Such coatings are composed of nanocrystalline grains of transition-metal nitrides or carbides surrounded by an amorphous hard matrix. The properties of nanocomposite coatings, especially hardness, are directly linked to nanostructure. The codeposition of the amorphous and nanocrystalline phases of different compositions results in different morphologies, which in turn affect the coating’s properties. A maximum hardness ranging from 30 GPa to reported values above 60 GPa has been observed for most nanocomposite coatings. To obtain enhanced hardness, the domain size of the nanocrystalline phase must be below 10 nm, while the thickness of the amorphous layer separating the nanocrystals must be maintained at only a few atomic bond lengths. The prime reason for the hardness enhancement is the absence of dislocation activity. Keywords: nanocomposites, elastic properties, superhard coating materials, thin films.
Introduction The use of protective coatings in modern applications where friction is involved has been instrumental in new developments such as energy-efficient car engines and dry-machining technologies. The protective action of such coatings is, on the one hand, based on reduced wear due to increased hardness and thus less severe deformation. On the other hand, reducing the shear forces between two sliding surfaces is a way to improve the performance of a tribosystem (a system involving two or more surfaces, together with liquids, debris, and other material, all participating in relative shear motion). In short, high hardness and low friction coefficients are two key parameters for coatings used for wear reduction. Nanocomposite thin films are a new class of materials that meet these requirements to a large extent. Nanostructured thin films have attracted considerable attention as wear-protective coatings due to their outstanding properties relative to single-phase materials. Their increased hardness makes them interesting candidate materials for applications under extreme conditions. In addition to high hardness and low friction coefficients, other aspects such as thermal stability, coat-
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ing toughness, and interface toughness are decisive parameters for their usability as protective coatings.
Routes to Increased Hardness Perhaps the most intensively investigated route to enhanced performance in protective coatings is increasing their hardness. Hardness is defined as the resistance of a material to plastic deformation. Plastic deformation of crystalline materials occurs predominantly by dislocation movement under applied load; hence, a material with enhanced hardness has a higher resistance to dislocation movement. For a single-phase material, this means that the shear modulus increases.1 TiN is a classic multipurpose coating material, with a hardness value of 23 GPa. Inc
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