On the Role of Grain-Boundary Films in Optimizing the Mechanical Properties of Silicon Carbide Ceramics
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On the Role of Grain-Boundary Films in Optimizing the Mechanical Properties of Silicon Carbide Ceramics R. O. Ritchie,1,2 X.-F. Zhang 1 and L. C. De Jonghe 1,2 1 Materials Sciences Division, Lawrence Berkeley National Laboratory, and 2Department of Materials Science and Engineering, University of California, Berkeley, CA 94720 ABSTRACT Through control of the grain-boundary structure, principally in the nature of the nanoscale intergranular films, a silicon carbide with a fracture toughness as high as 9.1 MPa.m1/2 has been developed by hot pressing β-SiC powder with aluminum, boron, and carbon additions (ABC-SiC). Central in this material development has been systematic transmission electron microscopy (TEM) and mechanical characterizations. In particular, atomic-resolution electron microscopy and nanoprobe composition quantification were combined in analyzing grain boundary structure and nanoscale structural features. Elongated SiC grains with 1 nm-wide amorphous intergranular films were believed to be responsible for the in situ toughening of this material, specifically by mechanisms of crack deflection and grain bridging. Two methods were found to be effective in modifying microstructure and optimizing mechanical performance. First, prescribed post-annealing treatments at temperatures between 1100 and 1500oC were seen to cause full crystallization of the amorphous intergranular films and to introduce uniformly dispersed nanoprecipitates within SiC matrix grains; in addition, lattice diffusion of aluminum at elevated temperatures was seen to alter grain-boundary composition. Second, adjusting the nominal content of sintering additives was also observed to change the grain morphology, the grain-boundary structure, and the phase composition of the ABC-SiC. In this regard, the roles of individual additives in developing boundary microstructures were identified; this was demonstrated to be critical in optimizing the mechanical properties, including fracture toughness and fatigue resistance at ambient and elevated temperatures, flexural strength, wear resistance, and creep resistance. INTRODUCTION The nature of the grain boundaries, in particular the presence of nanoscale intergranular films, is of paramount importance in controlling the properties of many ceramics. This is especially important for silicon carbide ceramics, which despite offering many desirable properties, including a high melting temperature, low density, high elastic modulus and hardness, excellent wear resistance, and low creep rates, are rarely used structurally because of their poor toughness. Consequently, an imperative in the structural application of SiC is high fracture toughness, which for commercially available SiC is typically on the order of 3 MPa.m1/2, well below that of grain-elongated Si3N4 and yttria-stabilized ZrO2, for example. The toughening of ceramic materials can best be induced by crack bridging mechanisms (i.e., extrinsic toughening by crack-tip shielding), which can result from crack paths bridged by unbroken reinforcem
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