G.H. Zhou et al. /Ceramics International 33(2007)1395-1398 carboxymethyl cellulose( CMC) as a binder and isopropyl alcohol as a dispersant, and then ball-milled with agate balls The prepreg was prepared by infiltrating the continuous carbon Fiber Bundle Failure fiber into the slurry and then dried, stacked in a graphite die and hot-pressed at 1350C and 20 MPa in a N2 atmosphere. For omparison, fused silica and unidirectional carbon fiber reinforced fused silica composites were also prepared. The t content of carbon fiber was approximately 30 vol %o in the Zigzag composites Density measurements were performed based on Archi medes principle. The specimens were machined into bars of 36 mm x 4 mm x 3 mm to measure the flexural strength by the three-point bending method with a span of 30 mm and a cocracking cross-head speed of 0.5 mm/min at room temperature (RT in air. Single-edge notched-beam (SENB) samples were 0000.250.50 fabricated by notching the segments of tested flexure specimens nt/m with a 0.20 mm thick diamond wafering saw. The Fig. 1. Load-displacement curves: (a) uni-CHSiO2 and (b) uni-Cr 30 mm x 6 mm x 3 mm SENB samples were tested in SiOz+ 20wt% SiCE three-point loading with a span of 24 mm and a cross-head speed of 0.05 mm/min Fracture toughness Kic was calculated from 18.0 to 54.3 MPa, and the fracture toughness also had a by the AsTme 399-74 formula[ 13]. The flexural strength and slight increase from 20.1 to 21.9 MPa m". This is because the the fracture toughness measurements were conducted viscosity of fused silica increased Instron-1195 testing machine. Five specimens were tested for would retard the process of densification at the same sintering each sample. The fracture surface of composite was observed temperature Density measurements showed the relative density y electron probe X-ray microanalyser(EPMA, JXA-8100). (R D )of uni-CuSiO2 composite was 97.6%. However, R.D. of The microstructural features of interface were examined using uni-C/SiO2 +20 wt %o SiCp composite was 95.0%. The lower transmission electronic microscope (TEM, Model 200CX, R D. was responsible for the decrease of flexural JEOL, Japan). parallel to the fiber direction for uni-CHSiO2+20 wt. omposite. On the other hand, the increase of flexural st 3. Results and discussion perpendicular to the fiber direction may be attributed to the enhancement of the fiber/matrix interfacial bonding strength by Table I is the mechanical properties of several SiO2-based the addition of SiCp, which was similar to the case of 10 vol composites. Enhanced by unidirectional carbon fiber, the chopped CrSiOz+ 5 vol. Si3N4 composite [14] flexural strength and the fracture toughness of the hot-pressed Fig. la and b show the typical load-displacement curves of fused silica increased from 54.8 to 667.3 MPa and 1.0 to uni-CrSiO2 and uni-Cr/SiO2+ 20 wt SiCp composites, 20.1 MPa m"parallel to the fiber direction(Cr), respectively. respectively. With the increase of the load, the two samples Nevertheless, the flexural strength of uni-C.SiO2 composite exhibit elastic response in the initial stage, and then a deviation was only 18.0 MPa perpendicular to the fiber direction (1C+). appears at a load of about 170 MPa, indicating occurrence of Parallel to the fiber direction, the reinforcing effect of C was microcracking in the matrix. After that, the second elastic significant and the continuous fiber played an important role in response appears up to the maximum load where a significant carrying the load. On the contrary, perpendicular to the fiber drop in load occurs, which is attributed to fiber bundle failure. direction, there was no Cr to carry the load and the interstices The final stage is the non-linear region-the tail of curve, between carbon fibers resulted in the decrease of the revealing fiber pull-out, bridging, and sliding [6, 15. There are mechanical properties, to values lower than the matrix itself. two differences in the load-displacement curves of the two After SiCp addition, it was seen that though the flexural strength samples. Firstly, the second elastic response of uni-CrSio parallel to the fiber direction decreased from 667 3 to composite is smooth(Fig. la), whereas, that of uni-CH 431.8 MPa, that perpendicular to the fiber direction increased SiO2+ 20 wt SiCp composite is zigzag, which is ascribed to Table 1 Properties of SiOz-based composites Properties Hot-pressed Uni-C小SiO2 Uni-C/SiO2+ 20 wt% SiC 10 vol %o chopped CH fused silica SiO2+5vol%SiN4[14】 2.02 2.15 Flexural strength(MPa) 6673±33.9(C ±283(|Cp 73 180±20(⊥Cr Fracture toughness, KIc(MPa m 1.0 20.1±2.0(|C 219±14(|C 2.4carboxymethyl cellulose (CMC) as a binder and isopropyl alcohol as a dispersant, and then ball-milled with agate balls. The prepreg was prepared by infiltrating the continuous carbon fiber into the slurry and then dried, stacked in a graphite die and hot-pressed at 1350 8C and 20 MPa in a N2 atmosphere. For comparison, fused silica and unidirectional carbon fiber reinforced fused silica composites were also prepared. The content of carbon fiber was approximately 30 vol.% in the composites. Density measurements were performed based on Archi￾medes principle. The specimens were machined into bars of 36 mm 4 mm 3 mm to measure the flexural strength by the three-point bending method with a span of 30 mm and a cross-head speed of 0.5 mm/min at room temperature (RT) in air. Single-edge notched-beam (SENB) samples were fabricated by notching the segments of tested flexure specimens with a 0.20 mm thick diamond wafering saw. The 30 mm 6 mm 3 mm SENB samples were tested in three-point loading with a span of 24 mm and a cross-head speed of 0.05 mm/min. Fracture toughness KIC was calculated by the ASTME 399-74 formula [13]. The flexural strength and the fracture toughness measurements were conducted by Instron-1195 testing machine. Five specimens were tested for each sample. The fracture surface of composite was observed by electron probe X-ray microanalyser (EPMA, JXA-8100). The microstructural features of interface were examined using transmission electronic microscope (TEM, Model 200CX, JEOL, Japan). 3. Results and discussion Table 1 is the mechanical properties of several SiO2-based composites. Enhanced by unidirectional carbon fiber, the flexural strength and the fracture toughness of the hot-pressed fused silica increased from 54.8 to 667.3 MPa and 1.0 to 20.1 MPa m1/2 parallel to the fiber direction (jjCf), respectively. Nevertheless, the flexural strength of uni-Cf/SiO2 composite was only 18.0 MPa perpendicular to the fiber direction (?Cf). Parallel to the fiber direction, the reinforcing effect of Cf was significant and the continuous fiber played an important role in carrying the load. On the contrary, perpendicular to the fiber direction, there was no Cf to carry the load and the interstices between carbon fibers resulted in the decrease of the mechanical properties, to values lower than the matrix itself. After SiCp addition, it was seen that though the flexural strength parallel to the fiber direction decreased from 667.3 to 431.8 MPa, that perpendicular to the fiber direction increased from 18.0 to 54.3 MPa, and the fracture toughness also had a slight increase from 20.1 to 21.9 MPa m1/2. This is because the viscosity of fused silica increased after SiCp addition, which would retard the process of densification at the same sintering temperature. Density measurements showed the relative density (R.D.) of uni-Cf/SiO2 composite was 97.6%. However, R.D. of uni-Cf/SiO2 + 20 wt.% SiCp composite was 95.0%. The lower R.D. was responsible for the decrease of flexural strength parallel to the fiber direction for uni-Cf/SiO2 + 20 wt.% SiCp composite. On the other hand, the increase of flexural strength perpendicular to the fiber direction may be attributed to the enhancement of the fiber/matrix interfacial bonding strength by the addition of SiCp, which was similar to the case of 10 vol.% chopped Cf/SiO2 + 5 vol.% Si3N4 composite [14]. Fig. 1a and b show the typical load–displacement curves of uni-Cf/SiO2 and uni-Cf/SiO2 + 20 wt.% SiCp composites, respectively. With the increase of the load, the two samples exhibit elastic response in the initial stage, and then a deviation appears at a load of about 170 MPa, indicating occurrence of microcracking in the matrix. After that, the second elastic response appears up to the maximum load where a significant drop in load occurs, which is attributed to fiber bundle failure. The final stage is the non-linear region—the tail of curve, revealing fiber pull-out, bridging, and sliding [6,15]. There are two differences in the load–displacement curves of the two samples. Firstly, the second elastic response of uni-Cf/SiO2 composite is smooth (Fig. 1a), whereas, that of uni-Cf/ SiO2 + 20 wt.% SiCp composite is zigzag, which is ascribed to 1396 G.H. Zhou et al. / Ceramics International 33 (2007) 1395–1398 Table 1 Properties of SiO2-based composites Properties Hot-pressed fused silica Uni-Cf/SiO2 Uni-Cf/SiO2 + 20 wt.% SiCp 10 vol.% chopped Cf/ SiO2 + 5 vol.% Si3N4 [14] Density (g/cm3 ) 2.17 2.02 2.10 2.15 Flexural strength (MPa) 54.8 667.3 33.9 (jjCf) 431.8 28.3 (jjCf) 73.2 18.0 2.0 (?Cf) 54.3 1.9 (?Cf) Fracture toughness, KIC (MPa m1/2) 1.0 20.1 2.0 (jjCf) 21.9 1.4 (jjCf) 2.40 Fig. 1. Load–displacement curves: (a) uni-Cf/SiO2 and (b) uni-Cf/ SiO2 + 20 wt.% SiCp.