《复合材料 Composites》课程教学资源（学习资料）第五章 陶瓷基复合材料_LQS-SiC-TiC

J.Am. Ceran.So,9的2189-219(2007) DOl:10.l11551-2916.2007.0166x c 2007 The American Ceramic Society urna Microstructure phase and thermoelastic Properties of Laminated Liquid-Phase-Sintered Silicon Carbide-Titanium Carbide Ceramic composites John H. Liversage, David S. McLachlan, and lakovos Sigala Centre of Excellence in Strong Materials, University of the Witwatersrand, Johannesburg, South Africa Fraunhofer Institute for Ceramics and Sintered Materials. Dresden. German Hot-pressed silicon carbide-titanium carbide(sic--TiC)com- fabrication temperatures of around 2000.C and pressures in ex posites sintered with liquid-phase forming AlzO3 and Y2O3 mix- cess of 50 MPa. An approach used in the production of Sic-TiC tures have been studied Samples were fabricated by successively that is growing in popularity involves the use of liquid-phase stacking tape-cast sheets of a single composition, resulting in a forming additives such as Al2O3 and Y2O3. Several independe laminated body of uniform composition. This approach required reports exist on the properties of Sic-TiC composites sintered the development of a technology easily transferable into the pro- with the aid of specifically AlO3 and Y,O3 mixtures, ,,with duction of functionally graded SiC-TiC materials The effects of the work described in these reports generally involving the pre his processing route on the resultant microstructures and phases sureless sintering of Sic-TiC composites at temperatures as low were explored in detail. Additionally, because of the consequenc- as 1800.C In the present study, the use of liquid-phase forming es for graded materials, the effects of Tic proportion on the sintering aids was considered to be a generally more feasible thermal expansion coefficients, Youngs modulus, and Poissons route for the production of the multilayer materials. The current ratios for several SiC-TiC composites were also determined paper embodies a comprehensive phase, microstructural and thermoelastic property characterization of eight dense, non-grad ed Sic-TiC materials ranging in composition from monolithic TiC (excluding sintering aids) to monolithic SiC (excluding sinte- OVER th past there decade, since the ene ficia characteris 14.0 vol% mixture of the oxide phases Al-O, and Y,03, which a number of techniques have been developed for the production of graded ceramic materials. One such technique is the method Several multilayer samples, each comprised of 44 layers of a of tape casting and laminating, which has in the past enabled the guideline. Results obtained from this investigation would aid in of layers with varying compositions. Among the vast number of generating a valuable properties library, thus enabling further yer materials from which functionally graded bodies could be suc- ired for an cessfully produced, two of the more promising candidates in- accurate prediction of the stress states and behaviors of compo- clude silicon carbide (SiC) and titanium carbide(TiC). The ability to construct a graded Sic-TiC material enables one to produce a body that captures the excellent wear properties Sic while making use of any desirable properties of TiC, such as its high thermal expansion coefficient with respect to SiC. In order to qualify the technology required for the production of Recourse was made in this work to the tape-casting method for the fabrication of ach a class of materials it is necessary to first understand the laminates. Eight layers required for building multilayer physical and chemical interactions that may exist between SiC slurries were prepared, each with a nd TiC, specifically for those materials sintered with the aid of different SiC. TiC ratio corresponding to one of those oxide-based liquid-phase forming additives. An attempt was listed in Table I. The ceramic powders used were an a-SiC (UF15SIC, H.C. Starck, Berlin, Germany), TIC (TIC CAS, H.C. made in this study to characterize nongraded material produced Starck), and as liquid - bace forming additives, Al203(AKP50 via the same lamination technique that one may typically use when constructing graded bodies. To this end a series of C. H.C. Starck). Before the tape-casting slurry preparation, the nongraded laminates were produced and characterized in terms of their microstructures, chemical phases, and thermoelastic were mixed together in appropriate quantities and sub- jected to a high-energy planetary ball milling process, using There exist a number of reports concerning the effect of TiC AlO3 milling media. There was consequently some degree of articles dispersed in a Sic matrix. These reports mainly deal dditional AlO3 uptake in the powders, which was taken into with SiC-TiC composites that have been produced by means of consideration when calculating the final batch compositions lid-state sintering process, using sintering aids such as C, AL, Additional uptake of Al2O3 in this way accounts for the slight andB4c.comPositesproducedinthiswaytypicallyrequire batch-to-batch variation in the Al2O3 contents and correspond- g Y2O3 contents listed in Table I were used in the slurry formulations including poly vinyl-but P. Becher-contributing editor ral (PVB: Sigma-Aldrich, Milwaukee Wn) as a binder trieth- ylene glycol (TEG: Sigma-Aldrich, Steinheim, Germany)as a plasticizer, and Menhaden fish oil (MFO: Sigma-Aldrich, Stein- Manuscript No. 22151. Received August 18, 2006; approved March 4. 200 heim, Germany), which has historically been thought to act as a "Author to whom correspondence shoul addressede-mail:johnliversage(ac6.com dispersant for the present ceramic powders. 4 An azeotropic 2189

Microstructure, Phase and Thermoelastic Properties of Laminated Liquid-Phase-Sintered Silicon Carbide–Titanium Carbide Ceramic Composites John H. Liversage,w David S. McLachlan, and Iakovos Sigalas Centre of Excellence in Strong Materials, University of the Witwatersrand, Johannesburg, South Africa Mathias Herrmann Fraunhofer Institute for Ceramics and Sintered Materials, Dresden, Germany Hot-pressed silicon carbide–titanium carbide (SiC—TiC) com￾posites sintered with liquid-phase forming Al2O3 and Y2O3 mix￾tures have been studied. Samples were fabricated by successively stacking tape-cast sheets of a single composition, resulting in a laminated body of uniform composition. This approach required the development of a technology easily transferable into the pro￾duction of functionally graded SiC–TiC materials. The effects of this processing route on the resultant microstructures and phases were explored in detail. Additionally, because of the consequenc￾es for graded materials, the effects of TiC proportion on the thermal expansion coefficients, Young’s modulus, and Poisson’s ratios for several SiC–TiC composites were also determined. I. Introduction OVER the past three decades, since the beneficial characteris￾tics of functionally graded materials were first recognized,1 a number of techniques have been developed for the production of graded ceramic materials.2 One such technique is the method of tape casting and laminating, which has in the past enabled the production of sintered multilayer bodies comprised of a number of layers with varying compositions. Among the vast number of materials from which functionally graded bodies could be suc￾cessfully produced, two of the more promising candidates in￾clude silicon carbide (SiC) and titanium carbide (TiC). The ability to construct a graded SiC–TiC material enables one to produce a body that captures the excellent wear properties of SiC while making use of any desirable properties of TiC, such as its high thermal expansion coefficient with respect to SiC. In order to qualify the technology required for the production of such a class of materials it is necessary to first understand the physical and chemical interactions that may exist between SiC and TiC, specifically for those materials sintered with the aid of oxide-based liquid-phase forming additives. An attempt was made in this study to characterize nongraded material produced via the same lamination technique that one may typically use when constructing graded bodies. To this end a series of nongraded laminates were produced and characterized in terms of their microstructures, chemical phases, and thermoelastic properties. There exist a number of reports concerning the effect of TiC particles dispersed in a SiC matrix.3–12 These reports mainly deal with SiC–TiC composites that have been produced by means of a solid-state sintering process, using sintering aids such as C, Al, and B4C.3,4,6 Composites produced in this way typically require fabrication temperatures of around 20001C and pressures in ex￾cess of 50 MPa. An approach used in the production of SiC–TiC that is growing in popularity involves the use of liquid-phase forming additives such as Al2O3 and Y2O3. Several independent reports exist on the properties of SiC–TiC composites sintered with the aid of specifically Al2O3 and Y2O3 mixtures,9,11,12 with the work described in these reports generally involving the pres￾sureless sintering of SiC–TiC composites at temperatures as low as 18001C. In the present study, the use of liquid-phase forming sintering aids was considered to be a generally more feasible route for the production of the multilayer materials. The current paper embodies a comprehensive phase, microstructural and thermoelastic property characterization of eight dense, non-grad￾ed SiC–TiC materials ranging in composition from monolithic TiC (excluding sintering aids) to monolithic SiC (excluding sinte￾ring aids). Each composite was sintered with the aid of a nominal 14.0 vol% mixture of the oxide phases Al2O3 and Y2O3, which were themselves mixed in a volume ratio of 1Y2O3:3Al2O3. Several multilayer samples, each comprised of 44 layers of a single SiC:TiC ratio, were produced with these specifications as a guideline. Results obtained from this investigation would aid in generating a valuable properties library, thus enabling further numerical model development that would be required for an accurate prediction of the stress states and behaviors of compo￾sitionally graded SiC–TiC multilayer materials.13 II. Experimental Procedure Recourse was made in this work to the tape-casting method for the fabrication of the thin layers required for building multilayer laminates. Eight powder slurries were prepared, each with a different SiC:TiC volume ratio corresponding to one of those listed in Table I. The ceramic powders used were an a-SiC (UF15SiC, H.C. Starck, Berlin, Germany), TiC (TiC CAS, H.C. Starck), and as liquid-phase forming additives, Al2O3 (AKP50, Sumitomo Chem. Co., Tokyo, Japan) and Y2O3 (Y2O3 Grade C, H.C. Starck). Before the tape-casting slurry preparation, the powders were mixed together in appropriate quantities and sub￾jected to a high-energy planetary ball milling process, using Al2O3 milling media. There was consequently some degree of additional Al2O3 uptake in the powders, which was taken into consideration when calculating the final batch compositions. Additional uptake of Al2O3 in this way accounts for the slight batch-to-batch variation in the Al2O3 contents and correspond￾ing Y2O3 contents listed in Table I. Several organic additives were used in the slurry formulations including poly vinyl–but￾yral (PVB; Sigma-Aldrich, Milwaukee, WI) as a binder, trieth￾ylene glycol (TEG; Sigma-Aldrich, Steinheim, Germany) as a plasticizer, and Menhaden fish oil (MFO; Sigma-Aldrich, Stein￾heim, Germany), which has historically been thought to act as a dispersant for the present ceramic powders.14 An azeotropic P. Becher—contributing editor w Author to whom correspondence should be addressed. e-mail: john.liversage@e6.com Manuscript No. 22151. Received August 18, 2006; approved March 4, 2007. Journal J. Am. Ceram. Soc., 90 [7] 2189–2195 (2007) DOI: 10.1111/j.1551-2916.2007.01666.x r 2007 The American Ceramic Society 2189

2190 Journal of the American Ceramic Society--Liversage et al Vol. 90. No. 7 Table 1. Ceramic Powder Proportions Used in the Production of the eight Multilayer Laminates 450c ex fTc(vol %) ST0000.0084.7311433.84 MFO ST008685779711 200 mLmin- ST020171367.8211.193.87 ST03429.1355.97 05°Cmin ST05042.8642.4110.82 3.91 50.26 ST06656.5928.85 3.93 ST08371.20144310.42 3.95 83.15 ST10085.81 100.00 20min/0s℃mi refers to the volume fractions of the corresponding materials and oric to the TiC volume fraction with respect to the total carbide content of the composites. system. T of ethanol and trichloroethylene was used as a solvent This particular choice of organic additives was based on Fig 1. The temperature cycle used in the debinding of the tems.5.I6 The additive quantities used in the preparation of the inated bodies. slurries were decided on by means of a multivariate optimization exercise, which was carried out in order to minimize green-tape The methodology employed for this purpose assumes the test piece to be an isotropic medium. The validity of this while simultaneously maximizing the density and homogeneity n the context of the present system will be addressed further the casting of the tapes, which enabled the production of green layers with a nominal thickness of 120 un Forty-four oblong sheets were fashioned from each of the II. Results and discussion Bonding of the lb. equently laminated together, giving a body For liquid-phase-sintered SicC-TiC(LPS-ST) composites pro- reen tapes with a thickness of 5 mm and an area of 42 mm x 32 mn duced without an applied pressure, relative densities -f greater of individual layers, which was achieved with a light application thermore, in cases where pure SiC has been pressureless sintered by-layer stacking in this way, a light clamping pressure was ap with AlO3 and Y2O3 additives, densities in excess of 99% have been previously reported. In both of the above situations the plied to the resultant multilayer body, and the clamping assem- additive proportions used were similar to those used in the pres- bly was then placed into a container holding a solvent-rich ent work. Despite these prior successes, it was nevertheless de- cided that a moderate pressure application would be required subjected to the low-temperature furnace treatment illustrated for a more efficient sintering of the present set of samples.The in Fig. I, in order to remove the remnant organic additives. The application of pressure during sintering was indeed found to re- debinding procedure employed in the present work was devel- sult in laminate LPS-ST bulk densities of greater than 96%for ped through a careful study of the decomposition characteristics lI TiC fractions. This value is somewhat lower than that pre- of the organic additives used here, and entailed operating tem- viously seen for similar systems, and is possibly the result f micro-porous carbon inclusions in the present system. The tions of around 2. 3 in nction with an inert atmosphere used origins and effects of such inclusions will be discussed late during debinding uired for the prevention of TiC oxida- Relative densities determined for the various composites tion, one can naturally expect there to be an incomplete removal have been tabulated in Table Il, and have also been plotted of the organic components. Consequently, as will be discussed in Fig. 2. In the calculation of the theoretical densities it was further on, it would not be surprising to observe some level of residues in the sintered material, typically in the form of free assumed that there was an inherent 2 wt of SiO, situated on the surface of the Sic particles, which would likely have not carbon. The development of a debinding procedure and slurry been reduced during sintering. The sintered densities mostly imization efforts form the subject of a separate study, which agree with those which have been achieved by others in similar will be elaborated on elsewhere. The loosely bonded brown-state systems,,,although they are to some extent lower than ex materials were then transferred to a hBN-coated graph pected, particularly for composites with lower TiC fractions. and die set, which was placed into a uniaxial hot press. All of th One could argue that there may have been a lowering of the mples were sintered in an argon atmosphere at a temperature bulk density as a result of reduced densities in the interfacial of 1900., and under an applied pressure of 40 MPa. Bulk densities of the sintered laminates were determined using the Archimedes method. Phase identification was achieved by means of an XRD analysis of the sintered materials in bot Table II. Density, Elastic and Thermal Expansion Properties dense and pulverized states. Phase identification within the sin for Various TiC fractions tered materials was complemented by means of a SEM and an EDS investigation of the microstructures, and also with the aid ndex x(10-6°C-1) of micro-Raman measurements. Thermal expansion coefficients STO00 3 19 0.96 413.6 0.155 were then determined for each of the different composites by ST008 3.33 0.96392.50.155 5.47 means of direct thermal dilation measurements along a direction ST020 3.51 0.9737090.147 69 in the plane of the laminated interfaces. The elastic property ST034 3.74 0.97 358.2 0.154 1 A-scan ultrasonic pulseecho method. both theelastic modul STe03340983401273 and the Poisson ratios were determined with this technique for a ST083445097354.60.171 797 pulse propagation direction parallel to the hot-pressing d 4.68 tion, which was normal to the planes of the laminated interfa ST100 0.97350.60.l81

mixture of ethanol and trichloroethylene was used as a solvent system. This particular choice of organic additives was based on the findings of earlier studies dealing with similar powder sys￾tems.15,16 The additive quantities used in the preparation of the slurries were decided on by means of a multivariate optimization exercise, which was carried out in order to minimize green-tape property variations between tapes with different TiC fractions, while simultaneously maximizing the density and homogeneity of the green tapes.13 A doctor-blade arrangement was used for the casting of the tapes, which enabled the production of green layers with a nominal thickness of 120 mm. Forty-four oblong sheets were fashioned from each of the green tapes and subsequently laminated together, giving a body with a thickness of 5 mm and an area of 42 mm
32 mm. Bonding of the layers was achieved through the pasting together of individual layers, which was achieved with a light application of solvent between adjacent layers. After completing the layer￾by-layer stacking in this way, a light clamping pressure was ap￾plied to the resultant multilayer body, and the clamping assem￾bly was then placed into a container holding a solvent-rich atmosphere. Once fully cured the multilayer materials were then subjected to the low-temperature furnace treatment illustrated in Fig. 1, in order to remove the remnant organic additives. The debinding procedure employed in the present work was devel￾oped through a careful study of the decomposition characteristics of the organic additives used here, and entailed operating tem￾peratures of up to 4501C. With binder-to-solids volume propor￾tions of around 2:3 in conjunction with an inert atmosphere used during debinding, as required for the prevention of TiC oxida￾tion, one can naturally expect there to be an incomplete removal of the organic components. Consequently, as will be discussed further on, it would not be surprising to observe some level of residues in the sintered material, typically in the form of free carbon. The development of a debinding procedure and slurry optimization efforts form the subject of a separate study, which will be elaborated on elsewhere. The loosely bonded brown-state materials were then transferred to a hBN-coated graphite punch and die set, which was placed into a uniaxial hot press. All of the samples were sintered in an argon atmosphere at a temperature of 19001C, and under an applied pressure of 40 MPa. Bulk densities of the sintered laminates were determined using the Archimedes method. Phase identification was achieved by means of an XRD analysis of the sintered materials in both dense and pulverized states. Phase identification within the sin￾tered materials was complemented by means of a SEM and an EDS investigation of the microstructures, and also with the aid of micro-Raman measurements. Thermal expansion coefficients were then determined for each of the different composites by means of direct thermal dilation measurements along a direction in the plane of the laminated interfaces. The elastic property dependence on the TiC fraction was then also examined, using an A-scan ultrasonic pulse-echo method. Both the elastic moduli and the Poisson ratios were determined with this technique for a pulse propagation direction parallel to the hot-pressing direc￾tion, which was normal to the planes of the laminated interfaces. The methodology employed for this purpose assumes the test piece to be an isotropic medium.17 The validity of this requirement in the context of the present system will be addressed further on. III. Results and Discussion For liquid-phase-sintered SiC–TiC (LPS-ST) composites pro￾duced without an applied pressure, relative densities of greater than 98% have been routinely obtained by others.9–12,18 Fur￾thermore, in cases where pure SiC has been pressureless sintered with Al2O3 and Y2O3 additives, densities in excess of 99% have been previously reported.19,20 In both of the above situations the additive proportions used were similar to those used in the pres￾ent work. Despite these prior successes, it was nevertheless de￾cided that a moderate pressure application would be required for a more efficient sintering of the present set of samples. The application of pressure during sintering was indeed found to re￾sult in laminate LPS–ST bulk densities of greater than 96% for all TiC fractions. This value is somewhat lower than that pre￾viously seen for similar systems, and is possibly the result of micro-porous carbon inclusions in the present system. The origins and effects of such inclusions will be discussed later. Relative densities determined for the various composites have been tabulated in Table II, and have also been plotted in Fig. 2. In the calculation of the theoretical densities it was assumed that there was an inherent 2 wt% of SiO2 situated on the surface of the SiC particles, which would likely have not been reduced during sintering. The sintered densities mostly agree with those which have been achieved by others in similar systems,9,11,12 although they are to some extent lower than ex￾pected, particularly for composites with lower TiC fractions. One could argue that there may have been a lowering of the bulk density as a result of reduced densities in the interfacial Table I. Ceramic Powder Proportions Used in the Production of the Eight Multilayer Laminates Index fTiC (vol%) fSiC (vol%) fAl2O3 (vol%) fY2O3 (vol%) jTiC ¼ fTiC ðfTiCþfSiC Þ ST000 0.00 84.73 11.43 3.84 0.00 ST008 6.85 77.97 11.33 3.85 8.08 ST020 17.13 67.82 11.19 3.87 20.16 ST034 29.13 55.97 11.02 3.89 34.23 ST050 42.86 42.41 10.82 3.91 50.26 ST066 56.59 28.85 10.63 3.93 66.23 ST083 71.20 14.43 10.42 3.95 83.15 ST100 85.81 0.00 10.21 3.98 100.00 f refers to the volume fractions of the corresponding materials and jTiC to the TiC volume fraction with respect to the total carbide content of the composites. Temperature (°C) Fig. 1. The temperature cycle used in the debinding of the green lam￾inated bodies. Table II. Density, Elastic and Thermal Expansion Properties for Various TiC Fractions Index r(g/cm3) r/rth E (GPa) n a(1061C1) ST000 3.19 0.96 413.6 0.155 5.11 ST008 3.33 0.96 392.5 0.155 5.47 ST020 3.51 0.97 370.9 0.147 5.69 ST034 3.74 0.97 358.2 0.154 6.26 ST050 3.98 0.98 352.4 0.157 7.03 ST066 4.23 0.98 352.0 0.162 7.33 ST083 4.45 0.97 354.6 0.171 7.97 ST100 4.68 0.97 350.6 0.181 8.40 2190 Journal of the American Ceramic Society—Liversage et al. Vol. 90, No. 7

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 qualitative

regions 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

2192 Journal of the American Ceramic Society--Liversage et al Vol. 90. No. 7 Hot pressing axis Hot pressing axis ewing plane. TiC Sic x uctural features found on face. and edge-sections of four selected pric compositions. (a). (c),(e)and(g)show set of s taken of the face sections of STO00, ST020, ST066, and ST100, respectively, and (b)(d)(f)and (h) depict the edge sections for the corresponding comparison of the intensities of peaks that are uniquely attrib- assigned to polytypes of Sic are likely associated with the pres- utable to different polytypes, it is proposed that the most abu ence of Y-AHO oxide phases dant polytype in the present Sic system is that of the 6H The essential elements of the monolithic TiC diffraction p structure, with there being only minor additional quantities of tern, shown in Fig 8, are consistent with the diffraction peaks of 15R and 4H. This is in line with the more commonly encoun- the khamrabaevite structure In addition to the diffraction peaks ered polymorphic compositions found for liq associated with the khamrabaevite structure are peaks which SiC. Minor peaks in Fig. 7 which could not be conclusively were found to correspond to a possible two different oxide

comparison of the intensities of peaks that are uniquely attrib￾utable to different polytypes, it is proposed that the most abun￾dant polytype in the present SiC system is that of the 6H structure, with there being only minor additional quantities of 15R and 4H. This is in line with the more commonly encoun￾tered polymorphic compositions found for liquid-phase-sintered SiC.20,25 Minor peaks in Fig. 7 which could not be conclusively assigned to polytypes of SiC are likely associated with the pres￾ence of Y–Al–O oxide phases. The essential elements of the monolithic TiC diffraction pat￾tern, shown in Fig. 8, are consistent with the diffraction peaks of the khamrabaevite structure. In addition to the diffraction peaks associated with the khamrabaevite structure are peaks which were found to correspond to a possible two different oxide Fig. 3. A comparison of general microstructural features found on face- and edge-sections of four selected jTiC compositions. (a), (c), (e) and (g) show images taken of the face sections of ST000, ST020, ST066, and ST100, respectively, and (b) (d) (f ) and (h) depict the edge sections for the corresponding set of samples. 2192 Journal of the American Ceramic Society—Liversage et al. Vol. 90, No. 7

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

2194 Journal of the American Ceramic Society--Liversage et al Vol. 90. No. 7 32-1 Fig 8. The diffraction pattern for ST100 showing the peaks corre- sponding to the khamrabaevite structure and apparent oxide phase Fig 10. Thermal sion coefficients as a function of the Tic The numbers in the legend refer to the applicable JCPDS numbers fraction showing a clear linear trend within the expected experimental accuracy nclusions would have lead to discrepancies as extensive as IV. Conclusions actually observed between the present data and previously ublished data. It is unclear at this stage as to the origin of the use of sintering aids in the present laminated Sic-TiC sys- differences between the elastic property values reported here and in conjunction with an applied pressure during sintering, hose observed by Cho et al. and Sand et al., both of whom was found to result in well-densified materials and was found to determined their values in an LPS-ST system, using similar ad ot result in the formation of any additional phases. With the ditives and additive fractions onfirmed absence of additional phase formation between the Thermal expansion coefficients for different TiC-fractions, as SiC and TiC components, one is assured that the system is well determined from thermal dilation measurements between 200% suited for the fabrication of a functionally graded multilayer and 750C, have been presented in Fig. 10. The dilation curve material. This is because the properties of the composite will were found to be strongly linear over this temperature range and ake on values that are dependent on the properties of the con herefore the thermal expansion coefficients were determined stituent phases in a semi-predictable way, and will not be infit directly from the slope of a linear regression fit over the entire enced by the formation of additional ternary phases. In this temperature range of dilation data. Also shown in Fig. 10 egard it is a simple matter to tailor any property variations linear regression of the thermal expansion data, the plac within a functionally graded material, particularly the thermal ement hich suggests an approximately linear trend in the thermal ex oefficient, which was found to a reasonable approx TiC fraction. In imation as being linea dependent on the TiC fraction. One of contrast with the elastic property data, the thermal expansion the shortcomings of the technique used to produce both the data was found to be very much in agreement with values quot layers and the resultant laminated materials is associated with ed for similar SiC-TiC syst 10. the development of appreciable amounts of carbonaceous in- Once again, however, the clusions. It was speculated that these inclusions were a result of occurrence of an anisotropically distorted microstructure and an additional carbonaceous phase may ncomplete removal of the organic additives used in the fabri ve con any slight differences seen between the values herein and those ation of the ceramic tapes. This is rted by the observed quoted by others absence of such inclusions in sintered Sic and TiC materia produced in an identical manner but without making use of or- ganic-binder-based slurries before sintering. Consequently, it ry to out an optimization exercise on the ● Elastic modulus D Poisson ratio binder burnout process before the transfer of the technology into the production of graded multilayer materials References M. B. Bever and P. F. Duwez,"Gradients in Composite Materials. "Mater. Sci. Eng tionally Graded Materials, Mater. Sci. Eng BG.C. Wei and P. F. Becher, "Improvements in Mechanical Properties in Sic by 0,1 .S.-M Dong D.L. Jiang, S.-H Tan, and J.-K Guo. "Mechanical SiC/TiC Composites by Hot Isostatic Pressing, "J. Mater. Sci. L 知mHmM工Mms Fig 9. Youngs moduli and the Poisson ratio dependence on the TiC Sic and of SiC-TiC Composites, " Am. Ceram. Soc. Bull, 65(21 3-

these inclusions would have lead to discrepancies as extensive as those actually observed between the present data and previously published data. It is unclear at this stage as to the origin of the differences between the elastic property values reported here and those observed by Cho et al. 10 and Sand et al.,12 both of whom determined their values in an LPS–ST system, using similar ad￾ditives and additive fractions. Thermal expansion coefficients for different TiC-fractions, as determined from thermal dilation measurements between 2001 and 7501C, have been presented in Fig. 10. The dilation curves were found to be strongly linear over this temperature range and therefore the thermal expansion coefficients were determined directly from the slope of a linear regression fit over the entire temperature range of dilation data. Also shown in Fig. 10 is a linear regression of the thermal expansion data, the placement of which suggests an approximately linear trend in the thermal ex￾pansion coefficient with respect to a changing TiC fraction. In contrast with the elastic property data, the thermal expansion data was found to be very much in agreement with values quot￾ed for similar SiC–TiC systems.10,12 Once again, however, the occurrence of an anisotropically distorted microstructure and an additional carbonaceous phase may have contributed to any slight differences seen between the values herein and those quoted by others. IV. Conclusions The use of sintering aids in the present laminated SiC–TiC sys￾tem, in conjunction with an applied pressure during sintering, was found to result in well-densified materials, and was found to not result in the formation of any additional phases. With the confirmed absence of additional phase formation between the SiC and TiC components, one is assured that the system is well suited for the fabrication of a functionally graded multilayer material. This is because the properties of the composite will take on values that are dependent on the properties of the con￾stituent phases in a semi-predictable way, and will not be influ￾enced by the formation of additional ternary phases. In this regard it is a simple matter to tailor any property variations within a functionally graded material, particularly the thermal expansion coefficient, which was found to a reasonable approx￾imation as being linearly dependent on the TiC fraction. One of the shortcomings of the technique used to produce both the layers and the resultant laminated materials is associated with the development of appreciable amounts of carbonaceous in￾clusions. It was speculated that these inclusions were a result of incomplete removal of the organic additives used in the fabri￾cation of the ceramic tapes. This is supported by the observed absence of such inclusions in sintered SiC and TiC material produced in an identical manner but without making use of or￾ganic-binder-based slurries before sintering. Consequently, it may be necessary to carry out an optimization exercise on the binder burnout process before the transfer of the technology into the production of graded multilayer materials. References 1 M. B. Bever and P. F. Duwez, ‘‘Gradients in Composite Materials,’’ Mater. Sci. Eng., 10, 1–8 (1972). 2 B. Kieback, A. Neubrand, and H. Riedel, ‘‘Processing Techniques for Func￾tionally Graded Materials,’’ Mater. Sci. Eng., A362, 81–106 (2003). 3 G. C. Wei and P. F. Becher, ‘‘Improvements in Mechanical Properties in SiC by the Addition of TiC Particles,’’ J. Am. Ceram. Soc., 67 [8] 571–4 (1984). 4 H. Endo, M. Ueki, and H. Kubo, ‘‘Microstructure and Mechanical Properties of Hot-Pressed SiC–TiC Composites,’’ J. Mater. Sci., 26, 3769–74 (1991). 5 S.-M Dong, D.-L. Jiang, S.-H Tan, and J.-K Guo, ‘‘Mechanical Properties of SiC/TiC Composites by Hot Isostatic Pressing,’’ J. Mater. Sci. Lett., 15, 394–6 (1996). 6 D.-L. Jiang, J.-H. Wang, Y.-L. Li, and L.-T. Ma, ‘‘Studies on the Strength￾ening of Silicon Carbide-Based Multiphase Ceramics I: The SiC–TiC System,’’ Mater. Sci. Eng., A109, 401–6 (1989). 7 M. A. Janney, ‘‘Microstructure Development and Mechanical Properties of SiC and of SiC–TiC Composites,’’ Am. Ceram. Soc. Bull., 65 [2] 357–62 (1986). Fig. 8. The diffraction pattern for ST100 showing the peaks corre￾sponding to the khamrabaevite structure and apparent oxide phases. The numbers in the legend refer to the applicable JCPDS numbers. ν Fig. 9. Young’s moduli and the Poisson ratio dependence on the TiC fraction. ° Fig. 10. Thermal expansion coefficients as a function of the TiC fraction showing a clear linear trend within the expected experimental accuracy. 2194 Journal of the American Ceramic Society—Liversage et al. Vol. 90, No. 7

July 2007 Properties of Laminated Liquid-Phase Sintered Sic-TiC Ceramic Composite 2195 K. w. Chae, K. Niihara, and D.-Y. Kim""Improvements i 7-Advanced Technical Ceramics-Monolithic mEchanical Proper- operties of TiC by the Dispersion of Fine SiC Particles. "J Mater. Sci. Lett. 14 ties at Room Temperature. Part 2. Determination PY -w. Kim. S.-G. Lee, and Y.-I. Lee "Pressureless Sintering of SiC-TiC Com- S Cho. H -J. Choi. -G. Lee and Y -w.Ki crostructure and frac- tre Toughness of In-Situ Toughened SiC-liC Ce es”J. Mater.Sci.,17 K -S. Cho. Y.-w. Kim. H.-J. Choi, and J.-G. Lee. ""SiC-liC and SiC-TiB, ntering. " J. Mater. Sci, 31, 6223-8(15 >S. K. Lee and C. H. Kim, "Effects of a-SiC Versus B-SiC Starting Powders on K-S. Cho. Y-W. K hoi and J -G. Lee. "In-Situ Tough Microstructure and Fracture Toughness of SiC Sintered with Al OrY,O, Addi- Carbide-Titanium Carbide Composites, "J Am Ceram Soc. 79 [6]1711-3( 1996). tives, J. Am. Ceran. Soc., 77[6] 1655-8(1994) CN. P. Padture "In-Situ Toughened Silicon Carbide. "J. Am. Ceram. Soc.77 Materials with a Three Dimensional Composi 251923(194 runajatesan and A. H. Carim, ""Synthesis 308-311. Fimctionally Graded Materials 1998, Proceedings of the Sth international Symposium on FGM, Dresden, Germany, Edited by w.A.Kaysser Theory and Experiments. Vol ll. Edited by M. Cardona, P Fulde K.von J. H. Liversage "Mechanical Property Enhancements of Functionally Graded Klitzing. and H.-J. Queisser. Springer-Verlag, Berlin, 198 Liquid-Phase Sintered SiC-TT V. Huong. ""Structural Studies of Diamond Films and Ultrahard Materials by Raman and Micro-Raman Spectroscopies, Diam. Rel. Mater, 1 Casting”;p.155-74in 33-41(1991) openka and J. D. Pasteris. "Structural Characterization of Kerogens to Granulite-Facies Graphite: Applicability of Raman Microprobe Spectroscopy, n,“ Toughness Properti a silicon carbid With an In-Situ Induced Heterogeneous Grain Structure. J. Am. Ceram. Soc., 77 J Shanefield and R. E Mistler, " "Fine Grained Alumina Substrates: I, the [10 2518-22(1994) Manufacturing Process, "Am. Ceran. Soc. Bull, 53 [5]416-20(1974)

8 K. W. Chae, K. Niihara, and D.-Y. Kim, ‘‘Improvements in the Mechanical Properties of TiC by the Dispersion of Fine SiC Particles,’’ J. Mater. Sci. Lett., 14, 1332–4 (1995). 9 Y.-W. Kim, S.-G. Lee, and Y.-I. Lee, ‘‘Pressureless Sintering of SiC–TiC Com￾posites with Improved Fracture Toughness,’’ J. Mater. Sci., 35, 5569–74 (2000). 10K.-S. Cho, Y.-W. Kim, H.-J. Choi, and J.-G. Lee, ‘‘SiC–TiC and SiC–TiB2 Composites Densified by Liquid-Phase Sintering,’’ J. Mater. Sci., 31, 6223–8 (1996). 11K.-S. Cho, Y.-W. Kim, H.-J. Choi, and J.-G. Lee, ‘‘In-Situ Toughened Silicon Carbide–Titanium Carbide Composites,’’ J. Am. Ceram. Soc., 79 [6] 1711–3 (1996). 12C Sand, J. Adler, and R. Lenk, ‘‘A New Concept for Manufacturing Sintered Materials with a Three Dimensional Composition Gradient Using a Silicon Car￾bide–Titanium Carbide Composite’’; pp. 65–70 in Materials Science Forum, Vols. 308–311, Functionally Graded Materials 1998, Proceedings of the 5th International Symposium on FGM, Dresden, Germany, Edited by W. A. Kaysser. Trans. Tech. Publications, Utikon-Zurich, Switzerland, 1999. 13J. H. Liversage ‘‘Mechanical Property Enhancements of Functionally Graded Liquid-Phase Sintered SiC–TiC Ceramic Composites.’’ PhD Thesis, University of the Witwatersrand, Johannesburg, South Africa, 2005. 14D. J. Shanefield, ‘‘Competing Adsorptions in Tape Casting’’; pp. 155–74 in Advances in Ceramics Vol. 19, Multilayer Ceramic Devices, Edited by J. B. Blum, and W. Roger Cannon. The American Ceramic Society, Columbus, OH, 1986. 15R. E. Mistler, D. J. Shanefield, and R. B. Runk, ‘‘Tape Casting of Ceramics’’; pp. 411–88 in Ceramic Processing Before Firing, Edited by G. Y. Onoda Jr., and L. L. Hench. Wiley and Sons, NY, 1978. 16D. J. Shanefield and R. E. Mistler, ‘‘Fine Grained Alumina Substrates: I, the Manufacturing Process,’’ Am. Ceram. Soc. Bull., 53 [5] 416–20 (1974). 17‘‘Advanced Technical Ceramics—Monolithic Ceramics—Mechanical Proper￾ties at Room Temperature. Part 2. Determination of Elastic Moduli,’’ DDENV 843-2:1996 18K.-S. Cho, H.-J. Choi, J.-G. Lee, and Y.-W. Kim, ‘‘Microstructure and Frac￾ture Toughness of In-Situ Toughened SiC–TiC Composites,’’ J. Mater. Sci., 17, 1081–4 (1998). 19S. K. Lee and C. H. Kim, ‘‘Effects of a-SiC Versus b-SiC Starting Powders on Microstructure and Fracture Toughness of SiC Sintered with Al2O3–Y2O3 Addi￾tives,’’ J. Am. Ceram. Soc., 77 [6] 1655–8 (1994). 20N. P. Padture, ‘‘In-Situ Toughened Silicon Carbide,’’ J. Am. Ceram. Soc., 77 [2] 519–23 (1994). 21S. Arunajatesan and A. H. Carim, ‘‘Synthesis of Titanium Silicon Carbide,’’ J. Am. Ceram. Soc., 78 [3] 667–72 (1995). 22P. Bru¨esch, ‘‘Experiments and Interpretation of Experimental Results’’ in Phonons: Theory and Experiments. Vol II. Edited by M. Cardona, P. Fulde, K. von Klitzing, and H.-J. Queisser. Springer-Verlag, Berlin, 1986. 23P. V. Huong, ‘‘Structural Studies of Diamond Films and Ultrahard Materials by Raman and Micro-Raman Spectroscopies,’’ Diam. Rel. Mater., 1, 33–41 (1991). 24B. Wopenka and J. D. Pasteris, ‘‘Structural Characterization of Kerogens to Granulite-Facies Graphite: Applicability of Raman Microprobe Spectroscopy,’’ Am. Mineral., 78 [5–6] 533–57 (1993). 25N. P. Padture and B. R. Lawn, ‘‘Toughness Properties of a Silicon Carbide With an In-Situ Induced Heterogeneous Grain Structure,’’ J. Am. Ceram. Soc., 77 [10] 2518–22 (1994). & July 2007 Properties of Laminated Liquid-Phase Sintered SiC–TiC Ceramic Composites 2195

Copyright of Journal of the American Ceramic Society is the property of Blackwell Publishing Limited and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder s express written permission. However, users may print, download, or email articles for individual use