Evolution and Process Control

For each phenomenon, there are also many elements involved which determine the behaviour of each phenomenon. These phenomena are described by a wide range of characteristic time and length values. For the case of CVI fabrication of fibre-reinforced ceramic-matrix composites, the diameter of a molecule and the thickness of the interfacial phase are about 10 1 run and 102nm respectively, whilst the sizes of the substrate/component and the reaction are around 1 m. In addition, elementary chemical reactions occur in a time range of 10 to 10 4 s, the time for heat transfer and mass transfer is around 1 s to 10 min. By contrast, the total densification time for one CVI run is as long as approximately 102 h. In such cases, it is necessary to establish multiscale models to understand and optimise a CVD process. 

Processing parameters (direct) Processing variables (indirect) Deposit properties (results) 

Nature of precursor Sticking coefficient Crystalline nature 

Substrate/preform geometry Residence time Residual stress 

As listed in Table 6.1, some parameters such as temperature, pressure, flow rate, etc. are considered processing parameters because they can be directly used to control a CVD process and to determine the microstructure and properties of the deposit. The processing variables include temperature field, velocity field and so forth, which are the results of the system response. These are known as indirectly controlling parameters which are very important for understanding the deposition mechanism. 

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