JOURNAL OF MATERIALS SCIENCE 28(1993) 4421-4426 Effect of rigid inclusions on the sintering of mullite synthesized by sol-gel processing D.-Y. JENG*, M. N. RAHAMAN University of Missouri-Roll& Ceramic Engineering Department, Roll& MO 65401, USA The effect of crystalline particulate inclusions of mullite or zirconia on the sintering and crys- tallization of a mullite powder matrix was investigated as a function of the inclusion volume fraction and size. The mullite powder was synthesized by sol-gel processing and, within the limits of X-ray diffraction, was amorphous. Composites containing up to 22.5 vol % zirconia reached almost full density after sintering at 1500~ for 1 h. Under identical conditions, the sintered density of the composites containing crystalline mullite inclusions was considerably lower. The zirconia inclusions were inert but the mullite inclusions enhanced the independent nucleation and growth rate of the mullite crystals in the matrix. The lower sintering rate of the matrix reinforced with crystalline mullite is attributed to the enhanced matrix crystallization. 1. Introduction Ceramic matrix composites are required to meet the demands of many technological applications ranging from advanced heat engines to electronic substrates. The most recognized path to the formation of poly- crystalline ceramics involves powder compaction into a porous body that is made dense by heat treatment. In the formation of polycrystalline matrix composites however, the presence of second-phase inclusions (e.g. particles, whiskers or fibres) leads to a drastic reduc- tion in the matrix densification rate. Thus con- siderable difficulties are often encountered in the formation of polycrystalline matrix composites by conventional, pressureless sintering. While techniques such as hot pressing and hot isostatic pressing are effective for fabricating composites with high density, they may have severe limitations for a number of applications. It will be useful, therefore, to explore techniques that have the potential for producing high- density composites by pressureless sintering. As outlined earlier [1], densification prior to crys- tallization (i.e. a glass-ceramic route) is expected to be particularly important for forming high-density com- posites by conventional pressureless sintering. At an equivalent inclusion content, glass matrices have been shown to exhibit a much higher sinterability com- pared to polycrystalline matrices [2-7]. The aim of the glass-ceramic route is to exploit the beneficial sintering characteristics of amorphous (or glass) ma- trices. After densification, it will be necessary to con- trol the crystallization of the matrix to obtain the required microstructure. The sintering and crystallization of a mullite pow- der synthesized by sol-gel processing has been reported in an earlier paper [1]. Within the limits of detection by X-ray analysis, the synthesized powder was amorphous. The compacted powder densified appreciably prior to the onset of crystallization and was therefore considered to be a good model powder for investigating the usefulness of the glass-ceramic route for the formation of dense composites by conventional sintering. In the work reported in the present paper, the effect of rigid particulate inclusions on the sinterability of the mullite powder was investigated. Crystalline inclu- sions of two different compositions were used: zirconia and mullite. For the sintering temperatures employed, the zirconia inclusions were expected to be inert, while the crystalline mullite inclusions were expected to enhance the independent nucleation and growth rate for the crystallization of the matrix phase. The effects of inclusion size and volume fraction on the sintering and crystallization of the matrix were investigated. 2. Experimental procedure Mullite powder, synthesized by a sol-gel route de- scribed earlier [1], was used as the matrix phase in the formation of the composites. The composition of the powder corresponded to that for stoichiometric mul- lite (3A1203.2SIO2) and, within the limits of X-ray diffraction, was amorphous. Crystalline zirconia (Grade SC; Magnesium Electron, Flemington, N J, USA) with an average size of 2.5 gin and crystalline mullite (Catapal XM; Vista Chemical Company, Houston, TX, USA) with two different average sizes of 0.3 and 2 gm, were used as the particulate inclusion phase. The inclusions were heated at 1600 ~ for 1 h to densify any agglomerates and then milled prior to incorporation into the matrix phase. Composites containing 0, 7.5, 15, and 22.5 vol % inclusion phase were formed by conventional tech- niques. The matrix powder and the inclusion phase were mixed in isopropanol, stir dried, and then ground *Present address: Department of Materials Science and Engineering, University of California, Los Angeles, CA 90024, USA. 0022-2461 9 1993 Chapman & Hall 4421