Chemically Bonded Ceramics as an Alternative to High Temperature Composite Processing
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opportunity offered by cement-based ceramic composites is that the processing is completed at much lower temperatures than the usual ceramic sintering temperatures. Thus one does not have to worry about the high temperature compatibility of the different components in the composite. The solid body is held together by the chemical cementitious bonds. The reactions leading to a solid structure are completed at near ambient temperatures. A further advantage that the cements have is their ability to be cast in molds. High alumina cements (HAC's) have long been used as the basis of castable refractory cements in the steel industry. Though they are satisfactory for their intended use, their mechanical properties are far from being suitable for high technology, structural or electrical ceramic applications, due to the existence of large flaws. It was shown that these macropores can be minimized, and hence the strength can be increased, by pressing the hydrating cement composite at room temperature, or even better, at elevated temperature during early stages of hydration [3]. By this method, the strength was at least doubled. The combination of the castability of HAC's with improved mixing, forming and compaction techniques offers a new alternative route to near-net-shape composite fabrication at near ambient temperatures. The idea of using cement as a thin binder layer to hold the larger filler blocks together in the construction of a composite material is as old as ancient civilizations. However, scaling it down from a brick wall to a micro composite requires a fine enough binder, in order to maintain the size ratio between the second phase and the cementitious binder. Commercially available cements have particles as large as a couple of hundred microns and are not suitable for fabricating composites mixed on the micron level. In this communication we report on the study of processing and microstructure of a composite between fine, chemically synthesized monocalcium aluminate powder and large zirconia particles. 511 Mat. Res. Soc. Symp. Proc. Vol. 346. 01994 Materials Research Society
EXPERIMENTAL PROCEDURES Calcia stabilized zirconia (ZrO2) (TAM Ceramics, Niagara Falls, NY) and chemically synthesized, fine and single phase calcium aluminate (CaA1204) powders [4, 5] were used in this study. The coarse zirconia powders (-325 mesh with a median size of 10 gim) were classified by sedimentation, and only particles larger than 5 gim were selected. Submicron sized, chemically synthesized, calcium aluminate powders were obtained by ball milling and sedimentation. Unless otherwise specified, only the submicron calcium aluminate powders were used in these batches. Composites with three different zirconia to calcium aluminate weight ratios were prepared (Table I). Weighed proportions of ZrO 2 and CaA120 4 were mixed in a paint shaker for half an hour to obtain a well dispersed batch. Specified amounts of de-ionized water were added to the batches. The mixes were sheared rigorously in a mortar and pestle for 3 minutes or until a
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