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 2189Microstructure, 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