“Superhard” coating materials are defined by hardness values that exceed 40 GPa. In this issue of MRS Bulletin, we focus on noncarbon-based superhard coatings, with the exception of a review of carbon nitride (CN) materials. Nanometer-scale multilayered nitride coatings were the first to show the superhard property, and these coatings have quickly made their way into industry as protective coatings for cutting-tool operations. Nanocomposite thin films also exhibit superhardness, and some of these materials have hardnesses approaching that of diamond. Cubic boron nitride (c-BN), which is naturally superhard, has proven very difficult to deposit at thicknesses exceeding 0.1 μm, but it is now reported that chemical vapor deposition techniques based on fluorine chemistries can produce c-BN films up to 20 μm thick. The search to produce cubic β-CN has led to the development of noncubic, fullerene-like forms of CN that are both hard and elastic, a very interesting combination of properties that has already been put to use in the hard-disk industry. Overall, the development of hard and superhard coatings during the past 20 years has been remarkable. We have progressed from trying (and often failing) to deposit hard coatings to now designing new nanometer-scale multilayered and nanocomposite coatings that exhibit excellent hardness properties and other high-performancecharacteristics.

This article reviews two topics related to the stability of hard coatings composed of nanometer-thick layers: epitaxial stabilization and high-temperature stability. Early work on nanolayered hard coatings demonstrated large hardness increases as compared with monolithic coatings, but it was subsequently found that the layers interdiffused at elevated temperatures. More recently, it has been shown that nanolayers exhibit good stability at elevated temperatures if the layer materials are thermodynamically stable with respect to each other and are able to form low-energy coherent interfaces. This article discusses metal/nitride, nitride/nitride, and nitride/boride nanolayers that exhibit good high-temperature stability and hardness values that are maintained (or even increase) after high-temperature annealing. Epitaxial stabilization of nonequilibrium structuresin thin layers is a well-known phenomenon that has been applied to hard nitride materials. In particular, AlN, which crystallizes in the hexagonal wurtzite structure in bulk form, was stabilized in the rock-salt cubic structure in nitride/nitride nanolayers (e.g., AlN/TiN). These results and the current understanding of epitaxial stabilization in hard nanolayers are discussed.

Nanoscale multilayered (superlattice) hard coatings can be manufactured in a plastic hardness range (HP) between 25GPa and 55 GPa by a combination of cathodic arc evaporation and unbalanced magnetron sputtering (arc bond sputter technology). Using large-scale industrial physical vapor deposition (PVD) equipment and a sufficiently high pumping speed, multilayered coatings can be deposited by simultaneously operating cathodes without special shutter and shielding facilities in a common reactive-gas atmosphere. The efficiency of the process is in many cases identical to that of TiN and CrN. Temperature-resistant, wear-resistant, and corrosion-resistant coatings of various compositions have been produced under industrial conditions. So far, the main applications concentrate on metal-forming and on cutting die steel, Inconel, stainless steel, and titanium. Applications have also been found in the chemical, textile, medical, and automotive industries.

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.

This article reviews recent progress in the deposition of thick, adherent, cubic boron nitride (c-BN) films. Most of the previous work applied ion-assisted physical vapor deposition methods to deposit c-BN films. The ion impact was successful in nucleating c-BN crystallites, but it resulted in a very small crystallite size and introduced stress, which caused the delamination of films thicker than 100 nm. Recent efforts to reduce the stress and obtain thicker films are described. The limited success of these attempts motivated us to explore chemical vapor deposition methods based on fluorine chemistry. We review this work, detailing the success of depositing thick (>20 μ), stress-free, adherent films with a larger crystallite size and significantly better crystalline quality.

Oxide-based ceramic materials are rapidly proving to have exciting potential for application as hard coatings. The most commonly known oxide material is silica, with a well-known variety of polytypes. Many other oxides, especially those involving metals, are now also proving to be important for a variety of applications where, because of the chemical role played by metal d electrons, large oxygen coordinations can be sustained. Properties of a variety of oxides—from silica to metal-based oxides—are discussed, and the role of predictive computer modeling is shown to be valuable in guiding the search for potentially new superhard oxides.

Carbon nitride is an emerging material for wear-resistant coatings. The fullerene-like CNx compounds generally exhibit extreme elasticity in combination with a low work of indentation hardness. Yet CNx shows a low-to-moderate resistance to penetration, depending on deposition conditions. Since the deformation energy is predominantly stored elastically, the material possesses an extremely resilient character. This new class of materials consists of sp2-coordinated basal planes that are buckled from the incorporation of pentagons and cross-linked at sp3-hybridized C sites, both of which are caused by structural incorporation of nitrogen. Carbon nitride thus deforms elastically due to bending of the structural units. The orientation, radius of curvature of the basal planes, and the degree of cross-linking between them defines the structure and properties of the material. Due to the unique deformation behavior, the hardness requires special care to assess, but can be very high for films with a large degree of cross-linking. This article is a review of the research on CNx films depositedby reactive magnetron sputtering, with examples from our recent work. The findings are significant for the design of fracture-tough materials.