Effect of pressure: pyrolytic graphite coatings with more uniformity, better coverage, and improved quality are generally obtained at low deposition pressure. Pressure controls the thickness of the surface boundary layer and consequently the degree of diffusion. By operating at low pressure, the diffusion process can be minimized and surface kinetics becomes rate-controlling. Low pressure deposition tends to be isotropic.
At high pressure, the reactant gas must be diluted with an non-reactive gas such as hydrogen or argon to prevent uncontrolled vapor-phase precipitation, while generally no dilution is necessary at low pressure. However, atmospheric pressure reactors are simpler and cheaper, and, with proper control of the deposition parameters, satisfactory deposits can be obtained.
Effect of C/H ratio: The C/H ratio of the gas mixture entering the reaction chamber is an important factor in the control of the nature of the deposition. Higher C/H ratios favor laminar deposition and lower ratios favor isotropic deposition.
Effect of temperature: generally, isotropic deposits are obtained at higher temperatures and laminar and columnar deposits at lower temperatures.
In summary, isotropic deposits are obtained at higher temperature, low pressures, and low carbon-to-hydrogen ratio. The opposite conditions favor the deposition of laminar and columnar deposits.