Influence of annealing treatments on microstructure and toughness of liquid-phase-sintered silicon carbide
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The possibility of an in situ toughening through the  → ␣ phase transition was evaluated on liquid-phase-sintered SiC (with Al2O3 and Y2O3 as additives). Dense hot-pressed materials with an equiaxed morphology were annealed at 1850, 1900 and 1950 °C for 1 to 4 h so the effect of time, temperature, composition, and amount of second phase could be investigated. Microstructure features revealed that 1900 °C was the best temperature in producing a high percentage of elongated grains with limited grain coarsening and reduction of second-phase pockets. Mean grain size and aspect ratios increased from 0.5 to 1.5 m and to 5, respectively, during the first 2 h of treatment at 1900 °C and then maintained a constant value. The amount and composition of second phase influenced the rate of transformation from equiaxed to elongated grain morphology. Toughness increased from 3.3 to 5.5 Mpa ⭈ m1/2 in 1900 °C annealed samples due to a crack-deflection mechanism.
I. INTRODUCTION
Silicon carbide is one of the promising ceramic candidates for high-temperature structural applications because of the combination of its mechanical properties, particular hardness, and wear resistance. However, the most important features that limit the application of silicon-carbide-based ceramics are their brittleness and low fracture toughness. In recent years, improvement of fracture toughness has been one of the most important goals. Silicon carbide ceramics can be densified by solidstate sintering using boron and carbon as sintering aids.1 The resulting materials have a very high temperature resistance, but a low strength ( 1850 °C), the  → ␣ transition took place giving rise to ␣–SiC elongated grains. In Fig. 2(a) second grain growth and elongation in sample SAY64 via a solutionreprecipitation mechanism is shown; the original equiaxed grain is apparent with its dark core and the first shell. During heat treatment at 1900 °C/4 h there was a significant growth and development of an elongated external shell. The contrast gradient on the micrograph was probably due to a different amount of oxygen and aluminum in solid solution between the external shell, the internal one, and the core, resulting in different plasmaetching rates.27 (iii) Grain coarsening through coalescence both of grains and second-phase pockets, with creations of new large, platelike grains. These grains contained either second-phase pockets and traces of the pre-existing grain boundary phase or round or faceted pores resulting from depletion of second-phase pockets. In Fig. 2(b) an example of grain coalescence in SAY23 is shown; at least four elongated grains originally separated by a grain boundary phase coalesced during thermal treatment at 1900 °C/4h and traces of second phase are still apparent. This phenomenon occurred mainly in (a) regions of the samples poor in liquid phase (therefore for long time treatments or in close-to-surface areas), where a solutionreprecipitation mechanism was no longer possible and where the probability of grains impingement was high, and in (b) re
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