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Source: Reprinted with permission from [175], G. M. Demyashev et al., in "Handbook of Organic-Inorganic Hybrid Materials and Nanocomposites" (H. S. Nalwa, Ed.), Vol. 2, Chap. 13. American Scientific Publishers, Stevenson Ranch, CA, 2003. © 2003, American Scientific Publishers.

holder [430, 432], which helps to regulate the sputtered fluxes for the deposition of nanolayered composites.

In reality, the nanolayered composites are not so perfect [397]. As they need to reach high hardness and have a well-defined bilayer repeat period, sharp interfaces between adjacent nanolayers are often realized [432]. It was demonstrated that interdiffusion between adjacent layers greatly affects the perfection of the nanolayered coatings [436]. High-quality interfaces for dissimilar materials require that deposition rates should be both well controlled (±1%) and well known [573]. An in-situ monitoring method based on using a soft X-ray reflectometer has been developed [574], which allows one control to accurately the thickness of each nanolayer.

Monocrystalline-type TiN/NbN nanolayered coatings have defects such as dislocations, low-angle grain boundaries, and elongated weak-phase defects [397]. These defects can affect the microhardness of the nanolayered composite coatings [436, 575].

Synthesis of nanolayered composites is a long-term process because of the inherent low rate of growth. The deposition of a few micrometers of such coatings takes a few hours, which affects the interdiffusion nanolayers and leads to the intermixing of nanolayers [397]. Extensive X-ray investigation of the interdiffusion in TiN/NbN nanolayered composites has demonstrated [436] that up to 15% metal substitution occurs within the nanolayers and that the interface width is 0.4-2.0 nm [436, 576].

It is difficult to obtain uniform nanolayered composites of an intricate shape because the technologies are based on the "line-of-sight" phenomenon. Flat samples have been used as substrates [345, 377, 397, 430, 432]. In addition, the nanolayered composites have anisotropic properties [577]. The nanolayered coatings are usually absent in the first 300400 nm above the substrate [397].

Precise control of the applied sputtering power and the nitrogen partial pressure in front of each target needs to be realized in order to obtain a suitable thickness of nitride nanolayers as well as a stoichiometric composition. For example, NbN deposits require higher nitrogen partial pressure than TiN deposits at the same power level [430, 432].

Modulation of the TiN/NbN composition is not constant. The ratio hTiN/A varies with depth in the nanocomposite coatings between 0.1 and 0.3 in different areas [397]. Target poisoning is a very sensitive issue and results in a spontaneous reduction of the bilayer repeat period [578].

Preparation of substrates for deposition of nanolayered composites must be precise [345, 377, 397, 434]. The complicated process of cleaning the workpieces includes a computer-controlled automated cleaning line, several ultrasonic baths, deionized water baths, and vacuum dryers [479].

Summarizing, it is imperative to point out that the super-hard nanolayered composites, as well the technologies for their deposition, have a number of weaknesses:

Difficult to cover complicated-shape substrates Sometimes not compatible with polycrystalline substrates

Difficult to support optimal bilayer period Line-of-sight limitations of used methods Strong regulation of nitrogen partial pressure for supporting stoichiometry of nanolayers Difficult to support the ratio hTiN/A Complicated preparation of substrates Poisoning of the target surface during processing Weak adhesion between substrate and nanolayered coatings (special interlayer should be incorporated) Highly sensitive total pressure control [472] Flatter and better defined interfaces between nanolayers [471]

The above-mentioned current technologies for the deposition of nanolayered composite coatings do not lessen the prospects of the coatings. On the contrary, they stimulate further developments of novel processes for the synthesis of nanolayered composite coatings.

First, synthesis of nanocrystalline composite coatings, which have similar microhardness as nanolayered coatings, should be relatively simpler from the standpoint of deposition processes than synthesis of nanolayered coatings, and, second, nanocrystalline composite coatings are more compatible with polycrystalline substrates. Nanocrystalline composites, which are the subject of the next section, are expected to be more convenient for practical applications.

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