Pulsed CVI

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... 

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. 

Sugiyama K, Nakamura T (1987) Pulse CVI of porous carbon. J Mater Sci Lett 6 331-333... 

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

Figure 5.41 is printed with kind permission from Springer Science+Business Media Journal of Materials Science, Reinforcement and antioxidizing of porous carbon by pulse CVI of SiC,Vol. 24, 1989, pp. 3756-3762, K Sugiyama and E Yamamoto, Figure 1. 

Naslain R Pailler R., Bourrat X., Bertrand S., Heurlevent F., Dupel P. and Lcmouroux F. Synthesis of Highly Tailored Ceramic Matrix Composites by Pressure-pulsed CVI // Solid State Ion. 2001. V. 141. P. 541-548. 

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]. 

The strength and tensile behavior, at room and high temperatures, as well as the structure of three dimensional earbon fiber/SiC composites, fabricated by the slurry pulse/CVI combined process, were eharacterized by Suzuki et al [207-209]. Carbon fiber preforms, constructed with 4-step braid, 4-step/axial braid, 2-step braid and orthogonal weave, were used as reinforcements of the composites. The composites were fabrieated by a process consisting of slurry and dissolved organosilicon polymer infiltrations, followed by the application of pulse CVI. 

Suzuki K, Nakano K, Structure and characterisation of SiC matrix composite via slurry and pulse CVI joint process. Ceramic Eng Sci Proc, 18(3), 427-433, 1997. 

Fig.2 TEM micrographs (a) inter-fiber configuration (b) multilayered interfacial coatings Typical microstructures of the interiacial layers fabricated by T-pulsing CVI were characterized by transmission electron microscopy (TEM), Fig.2. Fig.2a shows the inter-fiber configuration of the composites, in which multilayered interphases can be observed clearly.

Fig.3 Fractography of the composite showing the interfacial debonding The flexural strength is about 474.0 36.9MPa, indicating that T-pulsing CVI may provide another way to design and fabricate CMCs with well controlled interfacial layers and improved...

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