W. Yang et al. /Ceramics International 31(2005)525-531 indentation testing system with a Berkovich type diamond Composite I.D. Interphases(nm) Density(mg/m) pyramidal indenter. The maximum load of the indenter is TSA-SL 2.74±0.02 0.88 N. Detailed description of TSA-ML FC58sic140±25/C50+1M262± experimental procedure can be found elsewhere [19]. The pushout specimens were cut from the composites with one of the fiber bundles perpendicular to the cut surfaces, and were carefully ground and polished at both surfaces with diamond paste to reduce the thickness of the specimens to 50 pr The final polish grain size was l um. For each composite, 20 isolated fibers perpendicular to the polished surface were pushed out to extract the Iss, which was defined as Matrix where F is the onset load for fiber pushout to occur, and D and t are the fiber diameter and specimen thickness, Second PyC layer C laye 3. Results and discussion Fiber First PyC layer 500nm 3. 1. Density and interlayer structures of the composites Fig. 2. The PyC SiC PyC tri-layer interphase structures in composite Fig. I shows the SEM images of the cross-section and TSA-ML. pores distribution in composite TSA-SL, which shows that several relatively large inter-fiber bundle pores(a)were left dense and uniform matrix densification in the composite by in the matrix while the intra-fiber bundle area(b) was rather he present CVI conditions. Composite TSA-ML showed dense deposited. The relatively large inter-fiber bundle pores slightly lower density, 2.62+0.03 mg/m3 originated from the large pores at the intersections of crossed The interlayer structures and thickness were examined by fiber bundles in the preform before the matrix densification. high magnification SEM images. Fig. 2 shows the interlayer More fabrication process observations found that more structure of composite TSA-ML, which clearly shows very appropriate arrangement between the fiber cloth layers in the thin PyC/SiC/PyC tri-layer interphase between the fiber and preform would result in less and smaller inter-fiber bundle matrix in the composite. The measured thickness and pores. The composite densities are as in Table 1. The average standard deviations from six areas over the cross-section of density oml posite TSA-SL is 2.74+ 0.02 mg/m' both composite TSA-ML and TSA-SL are given in Table 1 (corresponding to a porosity of M10%), indicating quite The average thickness of the single Pyc layer and PyC/sic/ lmm Oum Gas flowing Fig. 1. Cross-section and inter/intra-fiber bundle pores in composite TSA-SL.the most direct measurement of ISS [17–19]. The single fiber pushout tests were performed using a load controlled micro￾indentation testing system with a Berkovich type diamond pyramidal indenter. The maximum load of the indenter is 0.88 N. Detailed description of the system and the experimental procedure can be found elsewhere [19]. The pushout specimens were cut from the composites with one of the fiber bundles perpendicular to the cut surfaces, and were carefully ground and polished at both surfaces with diamond paste to reduce the thickness of the specimens to
50 mm. The final polish grain size was 1 mm. For each composite, 20 isolated fibers perpendicular to the polished surface were pushed out to extract the ISS, which was defined as ISS ¼ F pDt (1) where F is the onset load for fiber pushout to occur, and D and t are the fiber diameter and specimen thickness, respectively. 3. Results and discussion 3.1. Density and interlayer structures of the composites Fig. 1 shows the SEM images of the cross-section and pores distribution in composite TSA-SL, which shows that several relatively large inter-fiber bundle pores (a) were left in the matrix while the intra-fiber bundle area (b) was rather dense deposited. The relatively large inter-fiber bundle pores originated from the large pores at the intersections of crossed fiber bundles in the preform before the matrix densification. More fabrication process observations found that more appropriate arrangement between the fiber cloth layers in the preform would result in less and smaller inter-fiber bundle pores. The composite densities are as in Table 1. The average density of composite TSA-SL is 2.74  0.02 mg/m3 (corresponding to a porosity of
10%), indicating quite dense and uniform matrix densification in the composite by the present CVI conditions. Composite TSA-ML showed a slightly lower density, 2.62  0.03 mg/m3 . The interlayer structures and thickness were examined by high magnification SEM images. Fig. 2 shows the interlayer structure of composite TSA-ML, which clearly shows very thin PyC/SiC/PyC tri-layer interphase between the fiber and matrix in the composite. The measured thickness and standard deviations from six areas over the cross-section of both composite TSA-ML and TSA-SL are given in Table 1. The average thickness of the single PyC layer and PyC/SiC/ W. Yang et al. / Ceramics International 31 (2005) 525–531 527 Fig. 1. Cross-section and inter/intra-fiber bundle pores in composite TSA-SL. Table 1 Composite densities and interphase structures Composite I.D. Interphases (nm) Density (mg/m3 ) TSA-SL F/C120  21/M 2.74  0.02 TSA-ML F/C58  9 /SiC140  25/C50  12/M 2.62  0.03 Fig. 2. The PyC + SiC + PyC tri-layer interphase structures in composite TSA-ML