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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.

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.

growth, has been defined by energy per deposited atom (Ep) [693]:

where Us is the substrate bias, is is the substrate ion current density, aD is the deposition rate, and e is the elementary charge.

For efficient control of the nanostructure, the value of the substrate ion current density (is) should be greater than 1 mA/cm2 [382]. Low-energy ion bombardment is a nonequi-librium process, called atomic scale heating, and is accompanied by (1) heating the growing deposit at an atomic scale and (2) applying an extremely fast cooling rate of about 1014 K/s [327, 382].

For deposition of nanostructured coatings, researchers usually try to increase the rate of nucleation and decrease the growth rate. Low-energy ion bombardment stops grain growth and allows the formation of a nanostructure [382]. Ion bombardment should be used for nanostructure formation cautiously because the residual stress increases with increasing ion bombardment energy [694].

The mixing process is the second method allowing for control of nanostructure deposits [324, 327, 382]. The main parameters that control a nanostructure include the amount and type of additional elements [382]. The following factors play a vital role in the formation of nanocrystalline structures [382]:

• Mutual miscibility/immiscibility of deposit elements

• Ability of the elements to form solid solutions or inter-metallics

• Negative/positive enthalpy in alloy formation

The mixing process in comparison with low-energy ion bombardment has no substrate bias, but external heating is necessary [327]. The formation of nanocomposite structures is explained by the co-deposition of two phases. A segregation in binary (or ternary) systems, which is a driving force for the spontaneous self-organization of nanostructure formation [692], is realized by means of narrow boundaries consisting of an amorphous phase.

The revised superhard nanocomposites were prepared by various evaporation/sputtering techniques [161, 168, 169, 170, 272, 281, 324, 345, 353, 377, 378, 397, 401, 430, 432435, 448, 454, 455-459, 471, 472, 479-483, 496, 509, 526, 535, 539, 541, 543, 557, 559, 568, 573, 579, 580, 605, 606, 609, 615-618, 624, 640-642, 661, 663, 665, 667, 668, 670-672, 675, 677], vacuum arc evaporation [552, 570, 571, 644, 646, 678], ion beam-assisted deposition [555], pulsed laser deposition [625, 637], ion beam implantation [639], plasma-assisted chemical vapor deposition based on different sources of power [1, 469, 577, 597, 598, 600, 643], conventional chemical vapor deposition [21, 45], and so forth. All applicable technologies realize nonequilibrium conditions [579], which can produce the nanostructured state of materials. Comprehensive reviews of vapor processing of nanostructured materials have been published elsewhere [695, 696].

Commonly used methods to produce nanocomposite coatings are plasma-assisted techniques, which provide [45, 577, 697]:

• Low deposition temperatures

• Possibility to control the ion bombardment of the growing coating in order to minimize grain size and com-pressive stress

• High rate of nucleation and low rate of grain growth simultaneously

• Superfast cooling rates reaching up to 1014 K/s at an atomic level

Magnetron sputtering technology [698-701] is the most frequently used technique for deposition of nanostructured coatings [702, 703]. The magnetron sputtering systems can have a number of configurations such as direct-current diode electrical circuit, alternating-current sputtering, and radio frequency (13.56 MHz) diode sputtering [696].

The principles of nanocomposite deposition by magnetron sputtering have been considered elsewhere [579, 692]. Magnetron sputtering is a nonequilibrium process with extremely high cooling rates of up to 1014 K/s that is similar to a quenching process [579]. The direct-current magnetron sputtering process includes [696, 704-706]:

• Sputtering processes

• Direct-current plasma formation

• Transport of species

• Kinetics of coating growth

• Chemical interaction on target and coating surfaces

Direct-current magnetron sputtering (DCMS) systems have been used for growing superhard nanolayered composite coatings such as TiN/VN and TiN/NbN.

If the permanent magnetic fields of magnets located under the target are unbalanced, the magnetic field lines flair toward the substrate, which enhances the ion bombardment of the substrate. This design is referred to as unbalanced magnetron sputtering (UMS).

Unbalanced magnetron sputtering has been used for the deposition of superhard nanolayered composite coatings such as TiN/NbN [432] as well as superhard nanocrystalline composite coatings such as nc-TiN/a-Si3N4 [605]. Usually, two pairs of rectangular targets, for example, Nb and Ti targets, are installed in a cylindrical vacuum chamber. A rotation shaft with hanging workpieces [605] has been used as a holder. To obtain nanolayered composite coatings, a bilayer repeat period is controlled by the rotation speed of the workpiece holder.

The UMS has a number of distinctions. The separated targets allow independent regulation of each target in order to adjust the ratio of depositing atoms. The UMS configuration also allows both a high sputtering rate and a high plasma density in the zone of deposition. The planetary rotation of the workpieces provides uniform exposure of the growing coatings [605].

The main advantages of UMS systems are as follows [327]:

• Nonequilibrium process at atomic level

• No problems with sputtering of alloys and their nitrides, carbides, etc.

• Deposited ions with a high energy

• Magnetron systems easily commercialized

The high energy of deposited atoms enables (1) synthesis of high-temperature phases on unheated substrates owing to high cooling rates (~1014 K/s) and (2) formation of nanocomposite coatings of the nc-MeN/soft-amorphous-phase type [327]. Three basic configurations have been used for sputtering of nanocomposite coatings [327]:

• One magnetron with an alloyed target

• Two magnetrons with targets fabricated from different metals, alloys, or compounds

• Pulse-operated dual magnetron

The major advantage of plasma-assisted chemical vapor deposition (PACVD) operating at low pressure is that it allows one to reduce the deposition temperature as compared to a conventional CVD process [707].

Microwave plasma reactors having a specific strength, such as a nonpolluting electrodeless plasma source, are of interest in surface engineering [708, 709]. Microwave plasma systems are devices in which a microwave discharge is employed to yield plasma of desired physical properties. The microwave source design and operation must satisfy three main objectives [710]:

• Filling the discharge tube with the required gas composition, pressure, and flow rate

• Providing distribution and intensity of the required electromagnetic field in order to initiate and sustain the discharge

• Ensuring an efficient power transfer from the microwave source to the plasma

Different designs of low-pressure microwave plasma reactors can be realized, namely [706, 711-714]:

• Jar-type design where the microwave plasma at low pressure in a dielectric jar (quartz or alumina) is sustained

• Tube-type design where a dielectric tube of the same material with low pressure inside is put through a microwave guide

The other microwave plasma-processing configurations have been reviewed elsewhere [715].

The microwave plasma process at atmospheric pressure without any vacuum restrictions is a favorable approach to perform plasma coating deposition and surface modification in run-through processes of large-sized, low-cost products. There are two types of atmospheric pressure microwave plasma discharge setups. One is a waveguide-based design [716, 717]. The other system exploits the microwave resonance phenomenon in which the selected resonance mode fixes the electric field geometry inside the cylindrical resonance cavity [175, 718, 719] to create a region of strong electric field that is Q times greater than that in a waveguide-type setup (Q is the quality factor of the resonance system). The localized electric field, in turn, allows one to control the location of the breakdown and sustains discharge. The microwave resonance plasma torch developed on the basis of a cylindrical resonance cavity surpasses the conventional plasma spray sources in characteristics and is an emerging process for nanotechnology [720-722].

In particular, the microwave resonance plasma torch is based on the cylindrical resonant cavity operating in the TM013 mode [175, 718, 719]. This mode in the cylindrical waveguide is of interest to plasma applications because it is well suited to producing stable axial microwave plasma discharges [723]. The typical unloaded Q factor of TM013 mode cylindrical resonators is greater than 50,000 at 2.45 GHz, which makes available a required ionizing electric field density for plasma discharge and uses microwave energy to produce expanding plasma directed through a convergent-divergent nozzle [724]. Different precursors (gases, liquids, powders) and carrier gases (N2, Ar, He, etc.) can be utilized for formation of the microwave plasma torch.

The direct-current glow discharge plasma nitriding system allows deposition of superhard nanocomposite coatings [620, 622] and is equipped with both a conventional cathode and an auxiliary cathode. The auxiliary cathode fulfills two functions:

• Intensifying nitriding discharge by additional discharges in the holes of the hollow cathode

• Sputtering auxiliary cathode materials

The sputtered material is integrated onto the surface of nitrided workpieces. The auxiliary cathode is made from materials that should be built in to the nitrided layer in order to form nanocomposite coatings. This technique has been used for the deposition of nc-Mo2C/(a-C+a-Mo2C) super-hard nanocomposite coatings [620].

Chemical vapor deposition, which is deposition processing of solid coatings on heated surfaces as a result of chemical reactions in a vapor phase, is a commercialized versatile technology [725-727] that has been widely adopted. Hot-wall chemical vapor deposition (HWCVD) is a well-established process [728] that is commonly used for surface engineering. In HWCVD, exterior heating of the reactor walls activates chemical heterogeneous/homogeneous reactions capable of coating deposition on substrates. Therefore, the reaction chamber, substrates, and a reactant gas mixture are under identical thermal conditions. HWCVD is an alternative process to CVD where the reaction chamber remains cold and a substrate is heated [729]. One of the significant strengths of CVD processing is that the technology is not based on the line-of-sight principle, thus, allowing one to coat work-pieces of intricate shapes. HWCVD can operate under both atmospheric and low-pressure conditions.

CVD is usually regarded as a high-temperature process, typically taking place above 1000 °C, which is necessary for the activation of chemical reactions. Efforts to eliminate the weaknesses associated with high-temperature CVD have been attempted. A medium temperature CVD, which is carried out at 700-900 ° C and use metallo-organic precursors, has been developed. It is possible to carry out low-temperature CVD at 350-700 °C with both inorganic and metallo-organic precursors [730]. Further diminution of the CVD temperature, based on the plasma phenomenon, allows a reduction in temperature of 200-400 °C [731].

The hot-wall reactor can have various designs, depending on the tasks to be performed. In principle, the hotwall chamber can represent a quartz (or stainless steel) tube located within a resistively heated furnace. For cylindrical reactors, the deposition kinetics has been well developed [732]. The precursors are directed along the axis of the chamber tube and low-temperature CVD processing running at 400-700 °C has been used to obtain nanocomposites of nc-W3C/nc-carbynes [45, 175].

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