Chemical vapor deposition (CVD) is a thermochemical process. In a typical application, such as coating cutting tools with titanium nitride (TiN), the tools are placed on a graphite tray and heated at 950° to 1050°C at atmospheric pressure in an inert atmosphere. Titanium tetrachloride (a vapor), hydrogen, and nitrogen are then introduced into the chamber. The chemical reactions form titanium nitride on the tool surfaces. For a coating of titanium carbide, methane is substituted for the other gases.
Schematic illustration of the chemical-vapor-deposition process
Deposited coatings usually are thicker than those obtained with Physical vapor deposition. A typical cycle for CVD is long, consisting of
- three hours of heating,
- four hours of coating, and
- six to eight hours of cooling to room temperature.
The thickness of the coating depends on the flow rates of the gases used, the time, and the temperature.
The types of coatings and the workpiece materials allowable are relatively unrestricted in CVD. Almost any material can be coated and any content can serve as a substrate, although bond strength may Vary.
The CVD process is also used to produce diamond coatings without binders, unlike polycrystalline diamond films, which use 1 to 10% binder materials. A more recent development in CVD is Medium-Temperature CVD (MTCVD). This technique results in a higher resistance to crack propagation than CVD affords.
