Diamond coating of mills by the hot-filament CVD method.
||In chemical vapor deposition the components are present as gas. The film material is formed by chemical reactions which take place in the gas phase as well as on the substrate surface. To make surface reactions possible, increased substrate temperatures are required. These may be as high as 1000 °C. Even these processes can be plasma-assisted; this increases the rate of reaction and the deposition temperature can then be lowered (PACVD).
A so-called activation is required for the CVD deposition of diamond. This splits the hydrogen gas into hydrogen radicals which are very reactive. Activation can be achieved by two different methods. In the first, temperatures exceeding 2000 °C are applied in the vicinity of the substrate surface. This is normally carried out with the aid of hot wires which are heated by passing a current through them (hot-filament or hot-wire CVD). The other method is to ignite a plasma by using the applied ionization energy to split the hydrogen atoms (PACVD). The plasma can be created by direct current discharge or by microwave irradiation.
The hydrogen radicals then start a sequence of chemical reactions which, in combination with the input gas which contains carbon (in most cases, methane CH4), results in carbon attaching itself to the substrate surface. If the correct process parameters are provided, this carbon deposits in the crystalline form of diamond.