July 2007 Properties of Laminated Liquid-Phase Sintered SiC-Tic Cera nic Composite 2193 Fig 4. of interest by Wavenumber(cm) reaction product of AlO3 and Y203 in the monolithic TiC ma- Raman spectra measured at (a) points 2, 3, and 4, and (b)at terial, and a possible AlTiOs phase, which could have been the 5, 6, and 7 in Fig. 4. Reference spectra for the identified materials been plotted alongside these data. result of an oxidation of the TiC. It is, however, more likely the case that the peak at 20 26.5 is associated with the presence of the carbon inclusions, as it was also observed in patterns ob- modulus values of 385 and 450 GPa, and Poissons ratios of tained from samples with zero TiC fraction. 0. 147 and 0.195 for monolithic S Diffraction pattens obtained from intermediate LPS-ST composites exhibited no additional peaks that were not already uid-phase-sintered TiC, respectively, Cho et al. have report observed for the monolithic sic and Tic materials. The absence d Poissons ratios of 0. 17 and 0. 25. Again concerning liquid- of new peaks is a strong indicator for the absence of any addi- phase-sintered SiC-TiC systems, Sand et al. have observed a tional phase formation occurring as a result of high-temperature Youngs modulus of 405 GPa, which was shown to be roughly constant for all TiC fractions. As suggested in the preceding Elastic property data for the various LPS-ST composites paragraph, it is arguable that any hot-press induced microstruc- given in Fig 9 and has been tabulated together wit tural anisotropy and also the carbonaceous inclusions in the hermal expansion coefficient data in Table Il. As mentioned present microstructures may have contributed to some of the earlier, the ultrasonic pulse-echo methodology employed in the discrepancies found between values in the present work and in determination of the elastic properties assumes an isotropic e work of others. If the speculated microstructural anisotropy medium. From a microstructural point of view this condition is the main source of the discrepancy, then the greater discre may not be strictly valid in the present system. However, the ancy seen for the TiC- rich samples can be explained in terms of a geometrically anisotropic characteristics of the present micro- possibly greater degree of anisotropy in the TiC samples, result structures are minor and are manifested mainly in the carbona from a greater degree of plastic deformation of the TiC ceous inclusions. Based on these facts the data are therefore hase at high temperatures and pressures. It is not a sim expected to be a reasonable estimate of the materials true elastic matter to assess the way in which the porous graphite inclusions could have lead to an alteration of the elastic properties of the The elastic property data of Fig 9 are noted to be somewhat present materials. However, based on the small quantities of different from values seen for similar SiC-TiC systems. 4. 0, I e inclusions at typically <5 vol %, it is not likely that most notably for samples having a high TiC fraction Inc there generally exists a great deal of disagreement in the elastic property values quoted in literature dealing with various SiC- TiC systems. Endo et al., who have measured the elastic prop- erties of solid-state-sintered SiC-TiC materials, found Youngs ■49-1428 Moissanite6h Fig. 7. Peak assignments for the monolithic SiC XRD diffraction Fig. 5. Selected regions of an ST034 composite that were examined by several Sic polytypes. The numbers in the legend refer to the applicable JCPDS numbers.phases. These are for an Y3Al5O12 phase, which was an expected reaction product of Al2O3 and Y2O3 in the monolithic TiC ma￾terial, and a possible Al2TiO5 phase, which could have been the result of an oxidation of the TiC. It is, however, more likely the case that the peak at 2y B26.51 is associated with the presence of the carbon inclusions, as it was also observed in patterns ob￾tained from samples with zero TiC fraction. Diffraction patterns obtained from intermediate LPS-ST composites exhibited no additional peaks that were not already observed for the monolithic SiC and TiC materials. The absence of new peaks is a strong indicator for the absence of any addi￾tional phase formation occurring as a result of high-temperature reactions between SiC and TiC. Elastic property data for the various LPS–ST composites has been given in Fig. 9 and has been tabulated together with thermal expansion coefficient data in Table II. As mentioned earlier, the ultrasonic pulse-echo methodology employed in the determination of the elastic properties assumes an isotropic medium. From a microstructural point of view this condition may not be strictly valid in the present system. However, the geometrically anisotropic characteristics of the present micro￾structures are minor and are manifested mainly in the carbona￾ceous inclusions. Based on these facts the data are therefore expected to be a reasonable estimate of the material’s true elastic property values. The elastic property data of Fig. 9 are noted to be somewhat different from values seen for similar SiC–TiC systems,4,10,12 most notably for samples having a high TiC fraction. Indeed, there generally exists a great deal of disagreement in the elastic property values quoted in literature dealing with various SiC– TiC systems. Endo et al.,4 who have measured the elastic prop￾erties of solid-state-sintered SiC–TiC materials, found Young’s modulus values of B385 and B450 GPa, and Poisson’s ratios of 0.147 and 0.195, for monolithic SiC and monolithic TiC, re￾spectively. Furthermore, for a liquid-phase-sintered SiC and a liquid-phase-sintered TiC, respectively, Cho et al. 10 have report￾ed Poisson’s ratios of 0.17 and 0.25. Again concerning liquid￾phase-sintered SiC–TiC systems, Sand et al. 12 have observed a Young’s modulus of 405 GPa, which was shown to be roughly constant for all TiC fractions. As suggested in the preceding paragraph, it is arguable that any hot-press induced microstruc￾tural anisotropy and also the carbonaceous inclusions in the present microstructures may have contributed to some of the discrepancies found between values in the present work and in the work of others. If the speculated microstructural anisotropy is the main source of the discrepancy, then the greater discrep￾ancy seen for the TiC-rich samples can be explained in terms of a possibly greater degree of anisotropy in the TiC samples, result￾ing from a greater degree of plastic deformation of the TiC phase at high temperatures and pressures. It is not a simple matter to assess the way in which the porous graphite inclusions could have lead to an alteration of the elastic properties of the present materials. However, based on the small quantities of graphite inclusions at typically o5 vol%, it is not likely that Fig. 4. Regions of interest investigated by means of an EDS analysis for a ST050 composite. Fig. 5. Selected regions of an ST034 composite that were examined by means of a micro-Raman method. a b Fig. 6. Raman spectra measured at (a) points 2, 3, and 4, and (b) at points 5, 6, and 7 in Fig. 4. Reference spectra for the identified materials have been plotted alongside these data.23 Fig. 7. Peak assignments for the monolithic SiC XRD diffraction pattern for several SiC polytypes. The numbers in the legend refer to the applicable JCPDS numbers. July 2007 Properties of Laminated Liquid-Phase Sintered SiC–TiC Ceramic Composites 2193