8.2 Materials The reaction bonded ceramic examined in this study was provided by M-Cubed Technologies (Trumbull, CT, USA). The final ceramic composition was 75 % B4C, 8 % SiC and 17 % residual Si. Figure 8.2 shows a representative micrograph of the reaction bonded boron carbide ceramic. The microstructure was made up of coarse B4C particles (dark grey), whiskerlike SiC particles (black), and residual Si pockets (light grey). Specimens of dimensions 3.4 3.4 5.0 mm were prepared by PremaTech Advanced Ceramics (Worcester, MA, USA). Surfaces were polished using standard ceramographic procedures down to a final polishing step with 1 μm diamond paste. Exact details of the manufacturing process for the examined material are proprietary and were not disclosed. The processing of composite materials can lead to the development of thermal mismatch stresses. Given that the weakest phase in the composite was Si, it was of particular interest to determine the level of thermal mismatch between the Si and the B4C and SiC phases. The coefficients of thermal expansion (CTE) for B4C, SiC, and Si are approximately 6.0, 4.8, and 3.7 10 6/ C [4], respectively. Thus, it was expected that during cooling from the processing temperature the reaction bonded ceramic would develop residual compressive stresses in the Si phase due to thermal mismatch Fig. 8.1 Schematic of the reaction bonding process. (a) Beginning with a porous preform, the preform is (b) placed into a vacuum furnace, which is then raised above the melting temperature of Si. Molten silicon infiltrates the preform and reacts with carbon in the preform to form SiC. (c) The final product is a fully dense, fully infiltrated B4C–SiC–Si ceramic composite Fig. 8.2 Optical micrograph of the reaction bonded boron carbide ceramic, which was made up of coarse B4C particles (darkgrey), whiskers-like SiC (black), and residual Si pockets (light grey) 40 P. Jannotti and G. Subhash
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