Chemical Vapour Deposition (CVD) of films and coatings involve the chemical reactions of gaseous reactants on or near the vicinity of a heated substrate surface. This atomistic deposition method can provide highly pure materials with structural control at atomic or nanometer scale level. Moreover, it can produce single layer, multilayer, composite, nanostructured, and functionally graded coating materials with well controlled dimension and unique structure at low processing temperatures. The unique feature of CVD and its variant techniques (such as thermally activated CVD, plasma enhanced CVD, photo-assisted CVD, atomic layer epitaxy process, metallorganic assisted CVD) over other deposition techniques such as the non-line-of-sight-deposition capability allows the coating of complex shape engineering components and the fabrication of nano-devices, carbon–carbon composites, ceramic matrix composite (CMCs), free standing shape components. The versatility of CVD has led to rapid growth and it has become one of the main processing methods for the deposition of thin films and coatings for a wide range of applications, including semiconductors for microelectronics, optoelectronics, energy conversion devices; dielectrics (e.g. SiO2, AlN, Si3N4) for microelectronics; refractory ceramic materials (e.g. SiC, TiN, TiB2, Al2O3, BN, MoSi2, ZrO2) used for hard coatings, protection against corrosion, oxidation or as diffusion barriers; metallic films (e.g. W, Mo, Al, Au, Cu, Pt) for microelectronics. Nanostructured materials refer to materials with dimensions less than 100 nm. These materials have potential applications in areas such as electronics, magnetism, optics, energy storage, electrochemistry and biomedical sciences. Nanostructured materials in the form of nanotubes have been gaining considerable technological interests because they are stronger yet lighter than steel. These materials are being explored for applications in advanced electronic devices. CVD has been used to synthesise carbon nanotubes and in combination with microfabrication techniques, have been used to synthesise ordered carbon nanotube architectures on surfaces.Another application of CVD is to deposit thick ceramic coatings (e.g. SiC, TiC B4C, TiN, BN, Si3N4, TiB2, MoSi and Al2O3) to protect engineering components against chemical diffusion, wear, friction, oxidation and corrosion. There are other competing deposition methods for protective coatings such as physical vapour deposition and plasma spraying. However, plasma spraying tends to produce a splat-like structure with a high degree of porosity and microcracks and surface roughness. The PVD method is a line-of-sight process. A range of ceramic coating materials such as TiN, TiC and Al2O3 are suitable to use as protective coatings for cemented carbide tools against wear, abrasion, corrosion and oxidation. Future of the protective coatings involve multilayer or compositionally graded coating systems containing a combination of TiN, TiC, Ti(C, N) and Al2O3 in different sequences for improved component lifetime. In addition, TiC, TiN, Cr–C and TiB2 have also been used in other tribological applications such as bearings, valves, nozzles, etc. Borides such as TiB2 are generally resistant to molten metals, especially molten aluminium, and are commonly used to protect crucibles in the evaporation of metals. Carbides have good resistance to sulphuric acid, sea water and industrial wastes.
For example, chromium carbide is very corrosion resistant and is used widely as a passivation interlayer. SiC coated molybdenum heating pipes can provide high temperature corrosion protection in an oxidising atmosphere at 830–1130 C.Nitrides such as boron nitride is one of the most corrosion resistant coating materials and is used widely as protective coating materials or CVD of free standing crucibles for molten metals, ceramic and glass processing.Silicides, especially MoSi2, have good high temperature oxidation resistance by forming thin layer adherent oxides, which protect silicides from further oxidation upto 1900°C. Therefore, it has been used as high temperature corrosion protection coatings for engineering components that are exposed to corrosive atmospheres such as CO2, SO2 and N2O. Oxides such as Al2O3, SiO2 have wide potential to provide effective protection for stainless steel in corrosive environments such as CO2 up to 1000 °C, as well as oxidation protection for carbon steel at elevated temperatures. Composite coatings in-situ can be produced by simultaneous injection of powder/particle into the gas phase during the deposition or formed via homogeneous gas phase reactions. For example, the inclusion of hard particles in a ductile matrix for tribological applications and inclusion of chromium and aluminium containing phases in protective coatings which form protective oxide layers that are useful for protection against high temperature corrosion have reported a variety of particle reinforced ceramic matrix coatings for high temperature protection and tribological applications. CVD is also used widely to coat nuclear fuel particles such as fissile U-235, U-233 and fertile Th-232 with pyrolytic carbon. The carbon coating of fuel particle is normally performed in a CVD fluidised bed reactor. Carbon is deposited from the decomposition of a hydrocarbon precursor such as propylene (C3H6) at 1350 °C. The carbon coated fuel particles are subsequently consolidated into fuel rods and assembled to form fuel elements. The role of the carbon coating is to contain the by-product of the fission reaction and thus minimise the use of shielding. This also serves as a protective coating against corrosion during subsequent processing. Isotropic carbon is completely biologically inert. It has low density (2.1 g/cm3) and high strain to failure (1.2%). CVD is the main deposition process for isotropic carbon for medical implants such as heart valves, ear prosthesis, dental implants etc. Isotropic carbon is usually deposited using the pyrolysis of hydrocarbon (e.g. methane) at 1200–1500 °C to deposit carbon with a turbostratic structure consisting of randomly oriented small crystallites with very little ordering.