Polymer impregnation/pyrolysis

Compared with conventional techniques for fabricating ceramic-matrix composites, such as hot pressing (HP), reactive melt infiltration (RMI) and polymer impregnation and pyrolysis (PIP), CVI techniques have distinct advantages, which can be summarised as follows [8, 9] ... 

Silicon carbide (SiC) matrix composites have been fabricated by chemical vapor infiltration (CVl), polymer impregnation and pyrolysis (PIP), and reaction sintering (RS). The RS process can be recognized as an attractive technique, because it offers a high density and good thermal conductivity, compared to those of CVl and PIP process. In general, the fabrication of fiber reinforced SiC matrix composites by reaction sintering involves melt infiltration (Ml) or liquid silicon infiltration (LSI). However, the fabrication of continuous fiber reinforced SiC matrix composites by RS focused in melt infiltration (Ml) such as liquid silicon infiltration (LSl) Vapor silicon infiltration was rarely used for SiC matrix composites. 

Since the oxidation resistance of SiC is much better than that of carbon, SiC/SiC composites have been developed for aerospace application such as propulsion and high velocity systems. Similar to carbon/carbon composites, the SiC/SiC continuous fiber composites consist of a fiber architecture made of silicon car-hide fibers in a matrix of silicon carbide. The matrix is usually produced by CVl or preceramic polymer impregnation and pyrolysis. 

S.M. Dong, Y. Katoh, A. Kohyama, S.T. Schwab, and L.L. Snead, Microstructural Evolution and Mechanical Performances of SiC/SiC Composites by Polymer Impregnation/Microwave Pyrolysis (PIMP) Process, Ceram, ht., 28,899-905 (2002). 

Carbon—Carbon Composites. Above 300°C, even such polymers as phenoHcs and polyimides are not stable as binders for carbon-fiber composites. Carbon—carbon composites are used at elevated temperatures and are prepared by impregnating the fibers with pitch or synthetic resin, foUowed by carbonization, further impregnation, and pyrolysis (91). 

Domier has developed a production route for continuous fiber-reinforced ceramics based on the impregnation and pyrolysis of Si-polymers. This process is related to the manufacturing of fiber-reinforced plastics and allows the cost-effective production of large and complex CMC-structures. 

C(/SiC-BN composites and C(/SiC-ZrC composites were fabricated to improve the oxidation resistance and high temperature performance of C /SiC composites through the modification of matrix. SiC-BN matrix was formed through an in-situ reaction of active filler boron and protective gas N2 in the active-filler-controlled polymer pyrolysis (AFCOP). The oxidation performance of C(/SiC-BN composites was greatly improved when oxidized at 1000°C compared to that of C /SiC composite. Meanwhile, SiC-ZrC matrix was fabricated using the ZrC particles as inert filler. Both C(/SiC-BN composites and Ci/SiC-ZrC composites show non-catastrophie ftaeture behavior. The microstructures were also characterized by SEM and EDS. It was shown that the fiber reinforcement hindered the impregnation of solid particles into the fiber bundles so that most of the fillers remained in the inter-bundle matrix and most of the intra-bundle matrices were composed of Sic that resulted from the decomposition of polycarbosilane (PCS). 

Basically, processing can be divided into the powder and chemical routes. The powder route includes sintering, slurry impregnation and reaction bonding, whilst the chemical route includes CVI, direct oxidation, sol gel and polymer pyrolysis. Final consolidation and densification must be achieved at high temperature. The powder route produces higher density ceramic matrix composites and the chemical route produces better quality. 

For the further increase of toughness, continuous hber reinforcement has been employed. In this category, silicon carbide reinforced with silicon carbide hbers, which is usually fabricated by the densification process using chemical vapor infiltration (CVl) or impregnation and pyrolysis of organosilicon polymers, is one of the most attractive materials. In these materials, the interface is also essentially important. As shown in Figure 9.1.12, appropriate thickness of carbon interface introduced by CVl process leads to a nonlinear fracture. 

---