Introduction Thin-film structures consisting of many alternating layers can exhibit high hardness when the layer repeat period  is in the nanometer range. 1,2 In these nanolayered structures, enhanced hardness with respect to the constituent materials typically re- sults from the high interface density im- peding dislocation motion. Early work on nanolayered hard coatings such as TiN/VN and TiN/NbN demonstrated enhanced hardness (to 50 GPa) compared with monolithic coatings of these materials (20 GPa), 2 making them potentially in- teresting for protective coating applications. 3 The nanolayers are typically prepared by multiple-target magnetron sputtering depo- sition, using either moving substrates 4 or a movable shutter 5 to modulate the sput- tered fluxes and produce nanolayers. The moving-substrate technique involves a re- volving cylindrical substrate holder placed midway between two sputtering targets. The revolving substrate is alternately ex- posed to each target, resulting in a nano- layered thin film. This article reviews two topics of re- cent interest in hard nanolayered coatings: high-temperature stability and epitaxial stabilization. Much of the early work in nanolayers was performed on miscible layered materials with similar structures (rock-salt cubic, in the example of nitrides). While stable coherent interfaces formed in these cases, the layers interdiffused rap- idly at elevated temperatures, resulting in a loss of hardness. 6,7 More recent work focused on combinations that are thermo- dynamically stable with respect to each other (i.e., immiscible) and typically have different crystal structures (i.e., are non- isostructural). In some cases, the different equilibrium structures can form low-energy coherent interfaces; this feature is impor- tant 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 high- temperature stability. Two types of non- isostructural coatings are described: rock-salt nitride/hexagonal boride nano- layers, such as TiN/TiB 2 and ZrN/ZrB 2 , 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 1000°C for several hours showed good stability. The nanolayer hard- ness generally increased after annealing to values approaching 50 GPa (Figure 1), a considerable enhancement over the hard- ness of the constituent layer materials (10–30 GPa after annealing). Because these nanolayers retain their high hard- ness at elevated temperatures, they are of practical interest as protective coatings for applications such as cutting tools. In our discussion on the epitaxial stabi- lization 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 zinc- blende cubic layers form in AlN/W. 17 This stabilization results from minimizing the MRS BULLETIN/MARCH 2003 169 S tability 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. Keywords: annealing, hardness testing, superhard coating materials, thin films. Figure 1. Representative hardness results from epitaxial and polycrystalline nanolayers, both as-deposited and after annealing. The corresponding as-deposited rule-of-mixtures values for the different combinations are Mo/NbN, 12 GPa; W/NbN, 22 GPa; W/ZrN, 30 GPa; ZrN/ZrB 2 , 30 GPa; and TiN/TiB 2 , 36 GPa. Rule-of-mixtures values for the annealed films were lower.  is the layer repeat period. www.mrs.org/publications/bulletin