**6. The application fields of the coatings**

The thermodynamic background presented above is very useful for production of the coatings based on tungsten, tungsten alloys with Re, Mo, Nb, Ta, V and tungsten compounds (for example tungsten carbides). The tungsten coatings have found wide application in thin-film integral circuits when preparing the Ohmic contacts in the production of the silicon-, germanium-, and gallium-arsenide-based Schottky-barrier diodes. The tungsten selective deposition technology is perspective in the production of conducting elements at dielectric substrates [36]. Tungsten films are used for covering hot cathodes, improving their emission characteristics, and as protective coatings for anodes in extra-highpower microwave devices. The CVD-tungsten coatings are used as independent elements in electronics.

The X-ray bremsstrahlung in modern clinical tomographs and other X-ray units is obtained by using tungsten or W–Re coatings at rotating anodes made of molybdenium or carbon– carbon composite materials. In the nuclear power engineering, tungsten was shown to be a good material for enveloping nuclear fuel particles because of low diffusion permeability of the envelope for the fuel. The tungsten- and W–Re alloy-coatings [2, 3, 5] are extremely stable in molten salts and metals used as coolants in high-temperature and nuclear machinery, e.g., in heat pipes with lithium coolant and in thermonuclear facilities. Tungsten emitters with high emission uniformity, elevated high-temperature grain orientation and microstructure stability are of interest for their use in thermionic energy converters.

High-temperature technical equipment cannot go without tungsten crucibles, capillaries, and other works that can be easily prepared by the CVD techniques. Tungsten is used as a coating for components of jet engines, fuel cell electrodes, filters and porous components of ion engines, etc. [2] The CVD-alloying of tungsten coatings with rhenium allows to improve significantly their operating ability, especially under the temperature or load cycling. Tungsten compounds have a wide field of application. The tungsten-carbide composites deposited by using the fluoride technology occupy a niche among coatings with a thickness of 10 to 100 mkm; they are unique in respect of strengthening practically any material, starting with carbon, tool, and stainless steels, titanium alloys, and finishing with hard alloys. CVD method permits to coat complicated shape components (which cannot be coated using PVD-method or plasma sputtering of carbide powders with binder). Below we list the most promissing fields of applications [37].

In the first place we can mention the strengthening of the oil and gas and drilling equipment (pumps, friction and erosion assemblies). The problems of hydrogen- sulfide corrosion, wear of movable units, and erosion of immobile parts of drilling bits operating underground take special significance because their replacement is very expensive. The carbide coatings can be deposited inside cylinders and on the outer surfaces of components of rotary or piston oil pumps. Numerous units in the oil and gas equipment, for example, block bearings, solution-supplying channels in drilling bits, backings directing the sludge flow, etc. require the strengthening of their working surfaces.

Another application in this field is the coating of metal–metal gaskets in the high- and ultrahigh-pressure stop and control valves. In addition to intense corrosion, abrasion and erosion wear, the working surfaces of ball cocks and dampers are subject of seizing under high pressure; W–C-coatings prevent the seizure. An important advantage of the carbide coatings is their accessibility for the quality of surface polishing, due to the initial smooth morphology. The examples mentioned above relate not only to oil and gas but also to chemical industry. The W–C-coatings are promising for working in contact with hydrogensulfide-rich oil, acids, molten metals, as well as chemically aggressive gases. Due to their high wear and corrosion resistance, these coatings can be use instead of hard chromium.

The abrasion mass extrusion and the metal shape draft require expensive extrusion tools; the product price depends on the working surface quality and life time. The extrusion tools must often have sophisticated shape inappropriate for coating with PVD or PACVD methods. Therefore, W–C-coating prepared by a thermal CVD-method is promising in strengthening these tools. Strengthening of spinneret for drawing wires or complicated section of steel, copper, matrices for aluminum extrusion, ceramic honeycomb structures for the porous substrate of catalytic carriers may give the same effect. Also, very perspective is the deposition of strengthening coatings onto components of equipment for the pressing of powdered abrasion materials. One may also mention the strengthening of knife blade used for cutting paper, cardboard, leather, polyethylene, wood, etc [38].

In addition to the surface strengthening, the W–C-coatings can function as high-temperature glue for mounting diamond particles in a matrix when preparing diamond tools or diamond cakes (conglomerates) in drilling bits [39]. The above-given examples demonstrate the variety of applications for tungsten, its alloys and carbides in mechanical engineering, chemical, gas and oil industry, metallurgy, and microelectronics.
