Chemical vapor deposition (CVD) is both a developed and versatile application method that is utilized to deposit layers of almost any metal onto a substrate material. Other compounds such as metallic oxides, nitrides, carbides and intermetallics can be deposited relatively easy.
Because there are numerous metallic elements and derivatives of these elements that are used in CVD, there are equally as many reactive chemistries. These can include thermal decomposition, oxidation and carburizing.
A CVD reaction is controlled by the following factors:
- Thermodynamic mass transport and kinetic considerations
- Chemistry of the reaction
- Process parameters such as temperature, pressure and chemical activity
When considering the application of a surface-deposition method, it is necessary to exercise good control procedures of the selected method to produce repeated and consistent depositions. It should be remembered that they are depositions and not diffusion processes. This means that pre-cleaning of the part prior to the coating application is of paramount importance. The degree of cleanliness accomplished will determine the success of the entire deposited coat.
With a diffusion process, a volumetric change in size is often seen that could lead to the necessity of further machining after the diffusion treatment. The need for post-machining can be reduced by careful selection of the process temperature.
The deposition methods guarantee that there will be a surface size change. The size change will be dependent on both of time and temperature. Some predictability can allow the engineer some latitude in allowing the part to grow into size. The choice between a deposited coat or a diffused case is one that must be made after careful consideration to the operating conditions and environment that the component will experience.