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.l81mixture 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