Stability of Nanometer-Thick Layers in Hard Coatings
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Stability of
Nanometer-Thick Layers in Hard Coatings
Scott A. Barnett, Anita Madan, Ilwon Kim, and Keith Martin Abstract 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 structures in 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.
coherent interfaces; this feature is important for maintaining a stable nanolayered structure at elevated temperatures. In other cases, the different layer structures do not readily form low-energy coherent interfaces, a situation that can lead to the epitaxial stabilization of one layer into a nonequilibrium structure. In the next section, we describe recent results on immiscible, non-isostructural nanolayers that exhibit good hightemperature stability. Two types of nonisostructural coatings are described: rock-salt nitride/hexagonal boride nanolayers, such as TiN/TiB2 and ZrN/ZrB2,8,9 and bcc metal/rock-salt nitride nanolayers, such as Mo/NbN, W/NbN, and W/ZrN.10–13 The nanolayered structures were typically stable—even 1-nm-thick layers annealed at 1000C for several hours showed good stability. The nanolayer hardness generally increased after annealing to values approaching 50 GPa (Figure 1), a considerable enhancement over the hardness of the constituent layer materials (10–30 GPa after annealing). Because these nanolayers retain their high hardness at elevated temperatures, they are of practical interest as protective coatings for applications such as cutting tools. In our discussion on the epitaxial stabilization of nonequilibrium nanolayered structures, the primary example we use is AlN, which crystallizes in the hexagonal wurtzite structure in bulk form. AlN has been stabilized in the rock-salt cubic structure in AlN/TiN, AlN/VN, and AlN/NbN nanolayers,14–16 whereas zincblende cubic layers form in AlN/W.17 This stabilization results from minimizing the
Keywords: annealing, hardness testing, superhard coating materials, thin films.
Introduction Thin-film s
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