This section summarizes the experimental data that reflect the two major distinguishing characteristics of superhard nanolayered composites, namely:

• Superhardness effect

• Stabilization effect

The anomalous increasing hardness of nanolayered composite coatings when the thicknesses of the nanolayers reach optimal values is summarized in Section 3.1.2.

The nanostructured state allows stabilization of some phases in a nanoscale range [418]. The possibility of stabilizing metastable phases by trapping them within nanolayers is considered to be an advantage of nanolayered composite coatings. A few examples of such stabilization are discussed below. It was noticed [419, 420] that zirconium and titanium, having a hexagonal close-packed lattice, can be stabilized with a body-centered cubic (bcc) lattice in the nanolayered composite coating of Nb/Zr and Nb/Ti. The similar phenomenon of pseudomorphic stabilization was observed in nanolayers of Al/Ti [384, 421, 422] and Ag/Ti [423]. At the smallest bilayer repeat period, both Al and Ti nanolayers were of hexagonal closed-packed structure; for intermediate periods, both Al and Ti nanolayers were of face-centered cubic (fcc) structure. A thermodynamic model was proposed [421] to account for the effect of coherency strains on phase stability. The stabilization of the face-centered cubic Ti phase in a nanolayered composite of Ni/Ti was reported as well [424]. Fe nanolayers in a nanolayered composite of Fe/Ni with equal thickness of Fe and Ni nanolayers are in an fcc and bcc lattice for a bilayer repeat period of 2.76 nm and 12 nm, respectively [425]. A novel epitaxially stabilized CoSi phase with CsCl-type lattice was determined [426]. A crystalline-to-amorphous transition phenomenon is found for a nanolayered composite of Ni/a-Si with a bilayer repeat period below 5 nm [427] and for a Cu/a-Cu4Zr3 nanolayered composite [428]. The stabilization phenomenon in nanostructured materials was used to stabilize hexagonal niobium nitride nanolayers, the hypothetical superhard phase j8-C3N4, the cubic form of aluminum nitride (c-AlN), cubic boron nitride (c-BN), and the metastable cubic form of ^-WC1-x. The stabilization phenomenon in superhard nanolayered composites is discussed in Section 3.1.3.

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