Y Liu et al. /Materials Science and Engineering A 475(2008)217-22 Table 2 shown in Table 2. fracture toughness and micro-hardness val Fracture toughness and micro-hardness value as a function of the second-step ues changes with the second-step pressure. The highest fracture roughness is 7. 11 MPamin2 The second-step pressure(MPa) Fig 4 shows strength measurements of SiC(py/Sic Micro-hardness(GPa) Fracture roughness(Mpam"2) 25.83 25.72 26 17 24.39 ites as a function of temperature. The room temperature strength is around 284 MPa and the value is almost retained up to 1600C This is due to SiC(PSiC composites can be fabricated by CV with virtual elimination of sintering additives, this is consistent with the study of Sic ceramics and its composites with intergral ular glassy phase free grain boundaries, whose high temperature 20 mechanical properties are much superior to that of Sic ceramic and particle reinforced SiC matrix composites with intergranular glassy phase [7] 3.3. Microstructure and properties of Si3 N4(p/Si3 N4 Fig 5 shows the typical microstructure of the Si3N4(p)/Si3N4 Fig. 4. Flexural strength with temperature as a function of SiCp/SiC composites used 15 MPa the first step pressure and 3 MPa second step pressur composites fabricated by CVI. There are large amounts of silicon nitride infiltrated inter-agglomerations and intra-agglomerations as shown in Fig. 5(a). Known from the cross-section morphol the second-step pressure changes from 3 to 5 MPa, then the ogy of composites, the composites is dense, but the residual strength increases when the second-step pressure changes from pores with an average size of 200-300 um inter-agglomerations 5 to 7 MPa, finally the strength decreases when the second- exist as shown in Fig. 5(a). The composite edge is denser than step pressure is above 7 MPa. The highest flexural strength is the inner because the edge is more favorable for gas transport 284 MPa, when the first-step pressure is 15 MPa and the second- and deposition reaction than the inner( Fig. 5(b). There is also step pressure is 7 MPa. The same function of strength changing residual pore intra-agglomeration as shown in Fig. 5(c). In some exists when the first-step pressure is 20 MPa and the highest intra-agglomeration pores, the CVi Si3 N4 whiskers also grow at strength is 265 Mpa. the low super-saturation(Fig. 5(d)), which is around 5 um. Fracture toughness and micro-hardness values, determined The flexural strengths and dielectric constants of sev by means of Vickers indentation method with a load of 196N, are eral Si3N4 materials are shown in Table 3. Compared with Coating Agglomeration (c) CvISin4 whisker Intra-agglomerations pore 5.0 um Fig. 5. Typical microstructure of Si3 N4(P/Si3N4 composites(a)cross-section; (b)composites edge coating; (c)intra-agglomerations: (d)CVI Si3N4 whisker.Y. Liu et al. / Materials Science and Engineering A 475 (2008) 217–223 221 Table 2 Fracture toughness and micro-hardness value as a function of the second-step pressure The second-step pressure (MPa) 3579 Micro-hardness (GPa) 25.83 25.72 26.17 24.39 Fracture roughness (Mpa m1/2) 6.93 6.87 7.11 6.74 Fig. 4. Flexural strength with temperature as a function of SiCP/SiC composites used 15 MPa the first step pressure and 3 MPa second step pressure. the second-step pressure changes from 3 to 5 MPa, then the strength increases when the second-step pressure changes from 5 to 7 MPa, finally the strength decreases when the second￾step pressure is above 7 MPa. The highest flexural strength is 284 MPa, when the first-step pressure is 15 MPa and the second￾step pressure is 7 MPa. The same function of strength changing exists when the first-step pressure is 20 MPa and the highest strength is 265 Mpa. Fracture toughness and micro-hardness values, determined by means of Vickers indentation method with a load of 196 N, are shown in Table 2. Fracture toughness and micro-hardness val￾ues changes with the second-step pressure. The highest fracture roughness is 7.11 MPa m1/2. Fig. 4 shows strength measurements of SiC(p)/SiC compos￾ites as a function of temperature. The room temperature strength is around 284 MPa and the value is almost retained up to 1600 ◦C. This is due to SiC(P)/SiC composites can be fabricated by CVI with virtual elimination of sintering additives, this is consistent with the study of SiC ceramics and its composites with intergran￾ular glassy phase free grain boundaries, whose high temperature mechanical properties are much superior to that of SiC ceramic and particle reinforced SiC matrix composites with intergranular glassy phase [7]. 3.3. Microstructure and properties of Si3N4(p)/Si3N4 composites Fig. 5 shows the typical microstructure of the Si3N4(p)/Si3N4 composites fabricated by CVI. There are large amounts of silicon nitride infiltrated inter-agglomerations and intra-agglomerations as shown in Fig. 5(a). Known from the cross-section morphol￾ogy of composites, the composites is dense, but the residual pores with an average size of 200–300m inter-agglomerations exist as shown in Fig. 5(a). The composite edge is denser than the inner because the edge is more favorable for gas transport and deposition reaction than the inner (Fig. 5(b)). There is also residual pore intra-agglomeration as shown in Fig. 5(c). In some intra-agglomeration pores, the CVI Si3N4 whiskers also grow at the low super-saturation (Fig. 5(d)), which is around 5 m. The flexural strengths and dielectric constants of sev￾eral Si3N4 materials are shown in Table 3. Compared with Fig. 5. Typical microstructure of Si3N4(P)/Si3N4 composites (a) cross-section; (b) composites edge coating; (c) intra-agglomerations; (d) CVI Si3N4 whisker.