Functionally Gradient SiC Coatings Produced by Chemical Vapor Reaction
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251 Mat. Res. Soc. Symp. Proc. Vol. 363 ©1995 Materials Research Society
EXPERIMENTAL
C-C composites used in this work were comprised of a T-300 8 harness satin carbon fabric in a 1k and 3k tow size. Five different densification approaches were utilized to produce the carbon matrix: i) phenolic resin densification, ii) low-pressure pitch impregnation (LoPIC), iii) Chemical Vapor Infiltration (CVI), iv) hybrid pitch-CVI densification, and v) pitch + green carbon coke (GCC) densification (the Japanese composite). The CVR process was carried out using a SiO2/Si generator at a high temperature to generate SiO which reacts with the carbon substrate to produce a SiC coating. Surface and cross sections of CVR converted C-C coupons were examined by optical microscopy and Scanning Electron Microscopy (SEM). RESULTS AND DISCUSSION Figure la shows a SEM micrograph of the cross section of the SiC CVR coated C-C composite made with the 3k fabric and a phenolic- derived matrix. Conversion appears to be non-uniform with poor coating adhesion and well-developed cracking. Figure lb shows the SEM micrograph corresponding to the surface of the same composite. A clear cracking pattern can be observed with about 30pm cracking frequency. Similar features were observed on the surface of the SiC CVR converted C-C composite made with the 1k fabric and phenolic-derived matrix.
a)
b)
Figure 1. SEM micrograph of CVR coating on the phenolic derived C-C composite made with the 3k fabric: a) cross section, b) surface.
resin-
Figure 2a shows the SEM micrograph of the cross section of the CVR SiC converted C-C composite made with the 3k fabric and a
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pitch-derived matrix. A non-uniform conversion is observed combined with some coating cracking. Figure 2b shows the SEM micrograph of the surface of the composite shown in Figure 2a. Wide cracks can be observed (area A in Figure 2b) with about 100 Pm cracking frequency. The CVR converted composite made with the 1k fabric and pitch derived matrix exhibited similar features to that shown in Figures 2a and 2b.
a) Figure 2.
b)
SEM micrograph of CVR coating on the pitch-derived C-C composite made with the 3k fabric: a) cross section, b) surface.
Figure 3a shows the SEM micrograph of the cross section of the CVR SiC converted C-C composite made with the 1k fabric and a CVI derived matrix. A uniform, dense, 10-20 pm thick SiC coating is observed on the surface (area A in Figure 3a) combined with no cracking. The underlying section of the composite appears to be uniformly converted to about 100 pm depth with no evidence of cracking. Figure 3b shows the SEM micrograph of the surface of the composite shown in Figure 3a. No cracking is observed in the surface layer. The CVR coating on the C-C composite made with the 3k fabric and CVI derived matrix looks similar to that of the composite made with the 1K fabric and CVI derived matrix. Figure 4a shows the SEM micrograph of the cross section of the CVR SiC coating on the C-C composite made with a hybrid matrix. A uniform 10 pm thick surface layer
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