ReF6 3H2 Re 6HF

The reaction temperature ranges from 500 to 900 °C (930 to 1650 °F). The best deposits are obtained at 700 °C (1290 °F) and at low pressure (<2.7 kPa, or 20 torr).

Another common reaction is the pyrolysis of the chloride:

The reaction temperature ranges from 1000 to 1250 °C (1830 to 2280 °F). The best deposits are obtained at low pressure (<2.7 kPa, or 20 torr). This reaction usually gives a more ductile and purer material than the reaction given in Eq 4, although a higher temperature is necessary.

Tungsten (Ref 23, 24, 25) is usually obtained by the hydrogen reduction of the halide:

The reaction temperature ranges from 300 to 700 °C (570 to 1290 °F), and pressure ranges from 1.3 to 101 kPa (10 to 760 torr). Deposition at a lower temperature (500 °C, or 930 °F) gives a finer grain structure with high strength (83 MPa, or 12 ksi) than deposition at a high temperature (700 °C, or 1290 °F).

Another deposition reaction for tungsten is the hydrogen reduction of the chloride:

The reaction temperature ranges from 900 to 1300 °C (1650 to 2370 °F), and the pressure ranges from 2.0 to 2.7 kPa (15 to 20 torr). This reaction, which yields high-purity deposits, is used to coat x-ray targets and to produce structural parts.

Graphite (Ref 26). The CVD of graphite, which is relatively simple, is obtained by the thermal decomposition of a hydrocarbon. The most common precursor is methane, which is generally pyrolyzed at 1100 °C (2010 °F) and at a pressure ranging from approximately 0.25 to 101 kPa (2 to 760 torr):

Another common precursor is acetylene (C2H2), which decomposes at temperatures between 300 and 750 °C (570 and 1380 °F) and at pressures up to 101 kPa (760 torr) in the presence of a nickel catalyst. A third precursor is propylene (C3H6), which decomposes at temperatures between 1000 and 1400 °C (1830 and 2550 °F) and at low pressure (13 kPa, or 100 torr).

Diamond (Ref 26, 27, 28). The CVD of diamond requires the presence of atomic hydrogen, which selectively removes graphite and activates and stabilizes the diamond structure. To dissociate hydrogen requires a high-energy source. In addition to the need for atomic hydrogen, other factors, such as energy input and the presence of oxygen, have been shown to be important, as well.

The deposition mechanism is complex and not fully understood at this time. The basic reaction involves the decomposition of a hydrocarbon, such as methane:

CH4 ® C(diamond) + 2H2 The reaction can be activated by microwave plasma, thermal means (hot filament), or plasma arc.

Diamond-like carbon (DLC) represents a new form of carbon coating that is neither diamond nor graphite. A common deposition method is a high-frequency RF gas discharge (13.56 MHz) generated in a mixture of hydrogen and a hydrocarbon, such as methane (CH4), «-butane (C4Hi0), or acetylene (C2H2).

The technology of CVD diamond and DLC has progressed considerably, and applications are reaching the production stage in electronics, optics, and tribology. Cutting tools coated with CVD diamond have performed remarkably well and should enter the market soon.

The deposition of ceramics usually involves titanium diboride, boron carbide, silicon carbide, titanium carbide, boron nitride, silicon nitride, titanium nitride, or alumina, each of which is described below.

Titanium diboride (Ref 29) is deposited by the hydrogen reduction of the halides. A typical reaction is:

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