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Figure 19. Maximum oxidation temperature of the CrN/NbN nanolay-ered composite coatings as a function of nitrogen flow rate. Reprinted with permission from [472], P. E. Hovsepian et al., Surf. Coat. Technol. 116-119, 727 (1999). © 1999, Elsevier Science.

the drills by more than 50 times [541], which is significantly higher in comparison with conventional TiN coatings which prolong the tool life of coated drills by about 20 times.

Superhard Nanocrystalline Composite Coatings The deformation behavior of nanocrystalline composites is a crucial factor in their applications. A few deformation mechanisms [842, 843] and models [844] for nanocomposites were recently proposed, namely, grain boundary sliding [842, 843], stress-induced mass transfer [333, 368, 845], and rotation of nanograins [846, 847].

The mechanical failure of nanocrystalline-amorphous composites occurs through the formation and propagation of microcracks [328, 848], as illustrated in Figure 20. A critical stress (o"c) should be applied for the growth of microcracks:

where C is a constant depending on the shape of the microcrack, ys is the surface energy, B is Young's modulus, and amax is the maximum size of the microcrack. This results from the fact that smaller cracks require higher critical stress for growing.

The function of an amorphous phase between separated grains in nanocrystalline composite coatings is very important [849]. First, plastic deformation of the nanocrystalline composite coatings is realized in the intergranular amorphous phase, and, second, the existence of the intergranular amorphous phase allows one to maximize the hardness of the nanocrystalline composite coatings.

It has been noted [850] that diffusivity in nanocrystalline copper at room temperature is 14-20 orders of magnitude higher than that of normal microcrystalline copper. The room-temperature diffusivity was measured as being 10-20 m2/s, which is higher than the grain boundary diffusivity (~10-24 m2/s) and significantly higher than the lattice diffusion (~10-40 m2/s). Therefore, high diffusivity in nanocrystalline materials was suggested [365, 851] as an additional mode of plastic deformation leading to softening at very fine grain sizes.

Figure 20. Deformation mechanism of nanocomposite materials: (a) formation of a stacking fault (arrow) and a pore and (b) crack nucle-ation at stacking fault 1 and crack propagation and debonding of nanocrystals 2. Reprinted with permission from [281], C. Mitterer, et al., Surf. Coat. Technol. 120-121, 405 (1999). © 1999, Elsevier Science.

Figure 20. Deformation mechanism of nanocomposite materials: (a) formation of a stacking fault (arrow) and a pore and (b) crack nucle-ation at stacking fault 1 and crack propagation and debonding of nanocrystals 2. Reprinted with permission from [281], C. Mitterer, et al., Surf. Coat. Technol. 120-121, 405 (1999). © 1999, Elsevier Science.

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