P-CVI

According to the controlling parameters used in the process, CVI approaches can be classified into five typical categories [9] isothermal/isobaric CVI (I-CVI), forced-flow CVI (F-CVI), thermal gradient/isobaric CVI (TG-CVI), pulsed CVI (P-CVI) and liquid immersion CVI (LI-CVI). There are more than ten CVI techniques if a particular method is coupled with plasma, microwave or a catalyst to enhance the process [10], Some representative techniques are discussed in this chapter as follows. 

Based on pulsed CVD (P-CVD) by Bryant [55] in 1976 Sugiyama [56, 57] proposed the pulsed CVI method (P-CVI). The aim is to accelerate the in-diffusion of fresh precursor species and the out-diffusion of the by-products to and from the pores of the preform and thereby reduce the total infiltration time and density gradient along the thickness of the composites. The instantaneous introduction of precursor gases ensures the uniformity of the precursor gas composition along the depth of the pores within the preform and leads to uniform dense composites. 

The above three-phase (namely gaseous reactant injection, deposition and evacuation) cycle is repeated for the whole duration of the P-CVI process until a product is finished. The inlet and outlet valves opening and closing are typically controlled with a computer system. 

As described above P-CVI works on the principle of alternative injection of reactant gases, deposition and evacuation of CVI exhaust gases. This is required to rapidly transport reactant species into, and by-product gases out of, the preform at isothermal conditions. For each cycle the detailed sequence generally consists of the following essential and sequential steps, as shown in Figure 5.42 ... 

The P-CVI processing parameters include the deposition temperature and pressure, precursor injection time, holding time, evacuation time and the number of pulses. 

The overall model of a P-CVI process is as shown in Figure 5.43. During a P-CVI process the mass transport of gaseous species can be divided into two stages. In the first stage mass transport takes place by forced convection within a very short period of a few hundredths or tenths of a second. In the second stage of the duty cycle the mass transport is dominated by the diffusion from the free space of the reaction chamber into the pores of the fibre preform. The temperature is kept... 

Figure 5.43. Overall model of a P-CVI process (a) during gas introduction and (b) during gas evacuation...

Figure 5.44. SEM graphs of multilayered matrix composite by P-CVI (a) details of four sequences on a single fibre [23], (b) fracture surface of 2.5-D C/SiC composite [59], (c) fracture surface of a single fibre [59]...

In order to overcome the above limitations of the F-CVI technique, alternative techniques using thermal gradients or pressure gradients have been examined for several years [11]. In the thermal gradient process, the core of the fibrous preform is heated in a cold wall reactor. The heat loss by radiation is favorable to get a colder temperature in the external surface. The densification front advances progressively from the internal hot zone toward the cold side of the preform. In the P-CVI process, the source gases are introduced during short pulses [11]. The P-CVI process is appropriate to the deposition of thin films. 

Finally, pressure-pulsed-CVI (P-CVI) has recently been presented as a means of engineering, at the micrometer (or even nanometer) scale, either the interphase or the matrix. Based on this technique, multilayered selfhealing interphases and matrices (combining crack-arrester layers and glass-former layers) have been designed and produced, through a proper selection of chemical composition of the layers [539]. 

Since Kasn is P/Cvi, the ratio of chemical vapor pressure to water solubility, it can also be expressed as H/ RT) where H is the Henry s law constant or with units of... 

Naslain R, Pailler R, Bourrat X, Bertrand S, Heurtevent F, Dupel P, Lamouroux F (2001) Synthesis of highly tailored ceramic matrix composites by pressure-pulsed CVI. Solid State Ion 141-142 541-548... 

Golecki I (2003) Industrial carbon chemical vapor infiltration (CVI) processes. In Delhaes P (ed) Fibre and composites. Taylor Francis, London, ppl 12-138... 

Dupel P, Bourrat X, Pailler R (1995) Stucture of pyrocarbon infiltration by pulse-CVI. Carbon 33 1193-1204... 

---