Chemical Vapour
Deposition (CVD) of films and coatings can provide highly pure materials with structural control at atomic
or nanometer scale. 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 over other
deposition techniques is its non-line-of-sight-deposition capability which allows the coating of complex shapes and the fabrication of nano-devices, carbon–carbon (C–C) composites, ceramic matrix composite (CMCs), free standing shapes.
One of the major applications 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. 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, nano-structured or compositionally graded coating systems containing a combination of TiN, TiC, Ti(C,N) and Al2O3 in different sequences for improved component lifetime. TiC, TiN, Cr–C and TiB2 have been identified for tribological applications such as bearings, valves, nozzles, etc.
Borides (e.g.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. 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 further oxidation upto 1900°C. Oxides (Al2O3, SiO2) have wide potential to provide effective protection for stainless steel in corrosive environments (CO2) up to 1000 C, and 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. Inclusion of hard particles in a ductile matrix are useful for tribological applications while inclusion of Cr and Al containing phases in protective coatings (which form protective oxide layers) are useful for protection against high temperature corrosion.