Thermal Modification of Microstructures and Grain Boundaries in Silicon Carbide
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Lutgard C. De Jonghe Materials Sciences Division, Lawrence Berkeley National Laboratory, and Department of Materials Science and Mineral Engineering, University of California, Berkeley, California 94720 (Received 23 May 2003; accepted 5 September 2003)
Polycrystalline SiC samples hot pressed with aluminum, boron, and carbon sintering additions (ABC-SiC) were characterized using transmission electron microscopy. The study focused on the effects of high-temperature treatment on microstructure. Three temperatures, at which considerable microstructural changes took place, were found to be critical. At a threshold temperature of approximately 1000 °C, 1-nm-wide, amorphous intergranular films started to crystallize. At approximately 1300 °C, lattice diffusion in SiC grains resulted in nanoprecipits, which could diffuse into grain boundaries and significantly altered composition. Quantitative microanalysis revealed doubled Al content in intergranular films after annealing at 1300 °C. Except for crystallization in intergranular films and nano-precipitation in matrix grains, microstructure remained stable until 1600 °C, when microstructural changes with volatile features occurred. A brief holding at 1900 °C brought marked changes in microstructure, including structural change in intergranular films, dissolved nanoprecipitates, unit cell dilation, and cracking. The results indicate that ABC-SiC is highly promising in structural applications at up to 1500 °C.
I. INTRODUCTION
SiC possesses many attractive properties such as low density and thermal expansion coefficient, good in thermal conductivity, hardness, elastic modules, flexural strength, and thermal shock resistance. However, the use of SiC to date has been limited by its low fracture toughness (KIc, ∼2–5 MPa m1/2 for commercially available materials). Various attempts have been made to improve toughness, including reinforcement with alumina-coated SiC platelets,1 and in situ toughening. The in situ toughening mechanism, which was effective for Si3N4 ceramics,2,3 also led to significantly enhanced toughness for SiC.4 A microstructure containing elongated, platelike SiC grains was produced by hot pressing a mixture of ␣–SiC with Al2O3 and Y2O3,5 while –SiC hot pressed with aluminum, boron, and carbon additives (ABC-SiC) led to further enhancement of the fracture toughness.4 In the latter, liquid phases were formed at processing temperatures, leading to the formation of residual intergranular films (IGFs).4,6–11 The nature of these amorphous intergranular films was shown to consist mainly of Al–O–Si–C11 or Al–O–C,12,13 and to provide a path for intergranular crack propagation. Elastic bridging and J. Mater. Res., Vol. 18, No. 12, Dec 2003
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pullout of the interlocked SiC grains4,5,14 contributed to the toughening,2,15–17 leading to fracture toughness of as high as 9 MPa m1/2 for ABC-SiC.4 In general, it can be expected that glassy grainboundary films in ceramic materials benefit room temperature toughness but degr
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