Introduction

Nanocrystalline composites have been classified into groups as follows [168, 175, 324, 382, 579, 580]:

• Nanocrystals of hard materials embedded into an amorphous matrix of originally hard materials such as Si3N4, AlN, and diamond-like carbon

• Nanocrystals of hard materials embedded into an amorphous matrix of relatively soft metals such as Ni, Cu, and Y

• Nanocrystals of hard materials embedded into a matrix of low-dimensional materials such as carbines [21, 175] and carbon nanotubes [581-586].

Nanocrystalline composites (Fig. 2) have a number of advantages over nanolayered composites (Fig. 1). First, nanomodulation of the nanocrystalline composite coatings occurs automatically under optimal conditions [577]. Second, nanocrystalline composites have a much more isotropic structure than nanolayered composite coatings [577]. Third, interdiffusion, which can decrease hardness, can be avoided by employing low-temperature deposition conditions [577]. Fourth, coating intricately shaped substrates by nanocrystalline-amorphous composite coatings should be easier than that by nanolayered composite coatings [325, 579, 587].

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