Superhard Nanocomposites

Only technologies that were used for the synthesis of super-hard nanocomposites are highlighted here.

The conditions for the deposition of nanolayered composites have been defined as [573]:

where R is the deposition rate, N0 is the number of sites for absorption having equal absorption energy, v is the atomic vibration frequency (1012-1013 s-1), E2 is the adatom-adatom bond energy, k is Boltzmann's constant, and T is the absolute temperature. Nanolayered composites can be deposited using any method in which the structure is formed by means of atom-by-atom processing [573], and, as consequence, they have a relatively low rate of deposition (Table 1).

Practical rules for the synthesis of nanocrystalline-amorphous composites are as follows [163, 208, 577, 587, 692]:

• A ternary system should be used, which allows for the formation of nanocrystalline-amorphous composites with sharp interfaces.

• The cohesive energy of the interfaces should be high.

• The phases forming the nanograins should be hard.

• Low-temperature deposition should be used in order to avoid interdiffusion.

• The matrix should be amorphous and thin to reduce crack formation/propagation.

Many transitional metal nitrides, borides, and carbides are hard and can serve as nanocrystalline components.

As has been stated [168, 324, 327, 382, 579, 580], there are two basic processes for controlling the grain size in nanocomposite coatings:

• Low-energy ion bombardment

• Mixing by incorporation of additional elements into a base material

Low-energy ion bombardment of growing nanocompos-ites can effectively change the deposition mechanism. The effect of low-energy ion bombardment, which restricts grain

Table 3. Summary of the technological characteristics for the synthesis of superhard nanocomposites.
0 0

Post a comment