July 2007 Properties of Laminated Liquid-Phase Sintered Sic-TiC Ceramic Composite 2191 counterparts. This conclusion is supported in part by the ob- 0.980 rved absence of free Ti and si in the microstructures. micro structural observations of polished cross sections indicated these inclusions to be lying within the grain boundary regions, and a 0.975 having no particu iC-TiC, SiC-SiC, or TiC-SiC). This, in part, supports as an origin of the features the organic residues arising from the debinding process. Regarding the images taken on the cross section of the les, one can arguably discern a directional aspect to the m crostructure. This is particularly evident for the carbonaceous grains, which are apparently elongated in a horizontal direction the current set of micrographs. The presence of a directionally 0.960 homogeneous microstructure in the plane normal to the hot- pressing direction and a textured microstructure in a cross- sectional plane is consistent with the axial symmetry imposed 0,955 the system through uniaxial hot pressing. In Fig 3, the cross- ection images have been oriented such that the hot pressing direction corresponds to the vertical axis. Grain orientation may Fig. 2. Relative densities of LPS.ST composites as a function of the have also been exacerbated in the present system as a result of the tape-casting process, due to the torsional forces imparted elongated grains by the doctor blade Figure 4 shows a region of the microstructure that was examined by means of EDS, as well as the points at which the COdons between adjacent laminates. However, no discernabl elemental compositions were evaluated. Evidence for the pres- sity or phase differences could micro- ence of Al was also found at points 3 and 4 in the eds patterns. structures around the interfacial region and in far from orresponding to the Sic phase. Although there was some ind ly interface. In fact, the interfacial regions themselves could ation that Y may also be present in these regions, it was not be readily identified via simple FESEM observations of the difficult to conclusively confirm this to be the case, due to the sample cross sections, thereby indicating a well-bonded lami rence of signals generated by s with signals nate material. It was, therefore, concluded that the laminating generated by YLn and YL excitations. The possibility of process would not have played a significant role in determining excitations in neighboring material is also unavoidable, d the final densities of the sintered bodies to the size scale of the microstructural features relative to the The FESEM images of Fig 3 show the microstructures of FESEM spot size, which further complicates any attempts elected composites taken in a plane normal to the hot-pressing to quantitatively establish the exact elemental make-up of the irection and a corresponding set of images taken in a cross- arious pl sectional plane Images taken in a plane oriented normal to the igure 5 shows an optical microscope image depicting the hot-pressing direction suggest an equiaxed grain morphology microstructural regions that were evaluated by means of a m reasonably homogeneous distribution of the various phas cro-Raman technique. With this technique one can attain a spa es in the materials. There is also qualitative visual evidence for tial resolution of <I rogation depth would be there being a low degree of porosity, and there is no indication limited to typically 100 nm in opaque materials. Vibrational coarsening. At higher magnifica ctra ted from points 2 to 7 in Fig. 5 have been plotted tions and with alternate brightness and contrast settings, poros- in Fig. 6. Point l, corresponding to a TiC region, showed no ity associated with voids(incomplete sintering) could be visuall Raman activity and its spectrum was therefore omitted from the differentiated from the dark regions in the microstructures current discussion. The absence of Raman activity for this re which were invariably found to contain greater or lesser n is in agreement with the lack of Raman vibrations expect of microporous matter. Median grain sizes for TiC and Sic for the cubic NaCl structure of stoichiometric TiC. Figure 6(a) particles found within sintered microstructures were determined shows the typical Raman spectra seen for points 2-4, which has with the aid of an image analysis of a number of appropriate been plotted alongside reference spectra relating to B-SiC micrographs. These were found to be about 3 um for TiC p (cubic) and a-SiC(hexagonal). The spectral characteristics of ticles and around I um for SiC particles. Apart from an addi- the experimentally obtained data, and in particular the presence tional phase contrasted as a dark region in all the micre of the 970 cm line, point to the presence of a-phase in the Sic les with mixed TiC and sic fractions (Figs. 3(cHD), clear gions, which is in agreement with the phase composition of the dicate the presence of only two major phases. This obser- precursory Sic powder. Typical spectra obtained fc tt5-7 found in the SiC-TiC pseudo h cipated lack of solid solutions corresponding to the carbonaceous regions, have been plotted in vation is consistent with the ar Fig 6(b)alongside reference spectra for carbon exhibiting var One particular concern is the nature and origin of the mate- ious levels of crystallographic disorder. The presence of a broad rial represented as the darkest regions in the micrographs. The peak at about 1356 cm in the experimentally obtained spec- presence of bonded material in these regions suggests that these trum is a strong indicator of a type of partially disorder inclusions were not the result of incomplete pore closure during graphite most likely arising from a pyrolytic conversion- ntering, nor the result of grain pullout resulting from micro- This result confirms the earlier suggestion that the inclusions phic sample preparation. The low secondary-electron image nay have arisen from an incomplete removal of the organic intensity associated with these regions suggests elemental carbon additives used in the slurry formulation. to be the most likely candidate for the material present therein XRD diffraction patterns for monolithic Sic and monolithic As far as the likely sources for carbonaceous inclusions within TiC samples have been plotted in Figs. 7 and 8, respectively. The he microstructure are concerned, the most probable source peaks in Fig. 7 were labeled according to JCPDS reference pat would be residues resulting from an incomplete pyrol terns that were found for three of the most common Sic poly of the organic components used in the precursory processing types, namely 6H, 4H, and 15R. Although there is a substantial of the ceramic powders. As already alluded to, the absence erlap between several peaks there is at least one peak for each of solid solutions in the SiC-TiC pseudo-binary system rules of the polytypes that is unique to that particular polytype. It out the possibility that these inclusions could have arisen from can, therefore, be concluded that all three polytypes are present a dissociation of either of the metals from their carbide in the monolithic SiC sample. Furthermore, from a qualitativeregions between adjacent laminates. However, no discernable porosity or phase differences could be observed in the micro￾structures around the interfacial region and in regions far from any interface. In fact, the interfacial regions themselves could not be readily identified via simple FESEM observations of the sample cross sections, thereby indicating a well-bonded lami￾nate material. It was, therefore, concluded that the laminating process would not have played a significant role in determining the final densities of the sintered bodies. The FESEM images of Fig. 3 show the microstructures of selected composites taken in a plane normal to the hot-pressing direction and a corresponding set of images taken in a cross￾sectional plane. Images taken in a plane oriented normal to the hot-pressing direction suggest an equiaxed grain morphology and a reasonably homogeneous distribution of the various phas￾es in the materials. There is also qualitative visual evidence for there being a low degree of porosity, and there is no indication of any inhomogeneous grain-coarsening. At higher magnifica￾tions and with alternate brightness and contrast settings, poros￾ity associated with voids (incomplete sintering) could be visually differentiated from the dark regions in the microstructures, which were invariably found to contain greater or lesser degrees of microporous matter. Median grain sizes for TiC and SiC particles found within sintered microstructures were determined with the aid of an image analysis of a number of appropriate micrographs. These were found to be about 3 mm for TiC par￾ticles and around 1 mm for SiC particles. Apart from an addi￾tional phase contrasted as a dark region in all the micrographs, samples with mixed TiC and SiC fractions (Figs. 3(c)–(f)), clear￾ly indicate the presence of only two major phases. This obser￾vation is consistent with the anticipated lack of solid solutions found in the SiC–TiC pseudo-binary system.21 One particular concern is the nature and origin of the mate￾rial represented as the darkest regions in the micrographs. The presence of bonded material in these regions suggests that these inclusions were not the result of incomplete pore closure during sintering, nor the result of grain pullout resulting from micro￾graphic sample preparation. The low secondary-electron image intensity associated with these regions suggests elemental carbon to be the most likely candidate for the material present therein. As far as the likely sources for carbonaceous inclusions within the microstructure are concerned, the most probable source would be residues resulting from an incomplete pyrolysis of the organic components used in the precursory processing of the ceramic powders. As already alluded to, the absence of solid solutions in the SiC–TiC pseudo-binary system rules out the possibility that these inclusions could have arisen from a dissociation of either of the metals from their carbide counterparts. This conclusion is supported in part by the ob￾served absence of free Ti and Si in the microstructures. Micro￾structural observations of polished cross sections indicated these inclusions to be lying within the grain boundary regions, and as having no particular preference to the type of grain boundary (TiC–TiC, SiC—SiC, or TiC–SiC). This, in part, supports as an origin of the features the organic residues arising from the debinding process. Regarding the images taken on the cross section of the sam￾ples, one can arguably discern a directional aspect to the mi￾crostructures. This is particularly evident for the carbonaceous grains, which are apparently elongated in a horizontal direction in the current set of micrographs. The presence of a directionally homogeneous microstructure in the plane normal to the hot￾pressing direction and a textured microstructure in a cross￾sectional plane is consistent with the axial symmetry imposed on the system through uniaxial hot pressing. In Fig. 3, the cross￾section images have been oriented such that the hot pressing direction corresponds to the vertical axis. Grain orientation may have also been exacerbated in the present system as a result of the tape-casting process, due to the torsional forces imparted to elongated grains by the doctor blade. Figure 4 shows a region of the microstructure that was examined by means of EDS, as well as the points at which the elemental compositions were evaluated. Evidence for the pres￾ence of Al was also found at points 3 and 4 in the EDS patterns, corresponding to the SiC phase. Although there was some indi￾cation that Y may also be present in these regions, it was difficult to conclusively confirm this to be the case, due to the interference of signals generated by SiKa excitations with signals generated by YLZ and YLl excitations. The possibility of excitations in neighboring material is also unavoidable, due to the size scale of the microstructural features relative to the FESEM spot size, which further complicates any attempts to quantitatively establish the exact elemental make-up of the various phases. Figure 5 shows an optical microscope image depicting the microstructural regions that were evaluated by means of a mi￾cro-Raman technique. With this technique one can attain a spa￾tial resolution of o1 mm2 , and the interrogation depth would be limited to typically 100 nm in opaque materials. Vibrational spectra generated from points 2 to 7 in Fig. 5 have been plotted in Fig. 6. Point 1, corresponding to a TiC region, showed no Raman activity and its spectrum was therefore omitted from the current discussion. The absence of Raman activity for this re￾gion is in agreement with the lack of Raman vibrations expected for the cubic NaCl structure of stoichiometric TiC.22 Figure 6(a) shows the typical Raman spectra seen for points 2–4, which has been plotted alongside reference spectra23 relating to b-SiC (cubic) and a-SiC (hexagonal). The spectral characteristics of the experimentally obtained data, and in particular the presence of the 970 cm1 line, point to the presence of a-phase in the SiC regions, which is in agreement with the phase composition of the precursory SiC powder. Typical spectra obtained for points 5–7, corresponding to the carbonaceous regions, have been plotted in Fig. 6(b) alongside reference spectra for carbon exhibiting var￾ious levels of crystallographic disorder. The presence of a broad peak at about 1356 cm1 in the experimentally obtained spec￾trum is a strong indicator of a type of partially disordered graphite most likely arising from a pyrolytic conversion.23,24 This result confirms the earlier suggestion that the inclusions may have arisen from an incomplete removal of the organic additives used in the slurry formulation. XRD diffraction patterns for monolithic SiC and monolithic TiC samples have been plotted in Figs. 7 and 8, respectively. The peaks in Fig. 7 were labeled according to JCPDS reference pat￾terns that were found for three of the most common SiC poly￾types, namely 6H, 4H, and 15R. Although there is a substantial overlap between several peaks there is at least one peak for each of the polytypes that is unique to that particular polytype. It can, therefore, be concluded that all three polytypes are present in the monolithic SiC sample. Furthermore, from a qualitative Fig. 2. Relative densities of LPS-ST composites as a function of the TiC fraction. July 2007 Properties of Laminated Liquid-Phase Sintered SiC–TiC Ceramic Composites 2191