Papers by Author: Sung Hee Lee

Abstract: There has been a great progress in the development of heat-resistant silicon carbide ceramics, owing to the better understanding of composition-microstructure-properties relations. Based on the progress, it has been possible to fabricate heat-resistant SiC ceramics with improved fracture toughness. In this paper, three rare-earth oxides (Re2O3, Re=Er, Lu, and Sc) in combination with AlN were used as sintering additives for a β-SiC containing 1 vol% α-SiC seeds. The effect of intergranular phase, using Re2O3 and AlN as sintering additives, on the microstructure and mechanical properties of liquid-phasesintered, and subsequently annealed SiC ceramics were investigated. The microstructure and mechanical properties were strongly influenced by the sintering additive composition, which determines the chemistry and structure of IGP. The strength and fracture toughness of the Lu2O3-doped SiC were ∼700 MPa at 1400oC and ∼6 MPa.m1/2 at room temperature, respectively. The beneficial effect of the new additive compositions on high-temperature strength was attributed to the crystallization of the intergranular phase.

Abstract: The effect of glassy-phase, using AlN and Lu2O3 as sintering additives, on the microstructure and mechanical properties of liquid-phase-sintered, and subsequently annealed SiC ceramics was investigated. The microstructure was strongly influenced by the sintering additive composition, which determines the intergranular phase (IGP). The average thickness of SiC grains increased with increasing the Lu2O3 /(AlN + Lu2O3) ratio, whereas the average aspect ratio decreased with increasing the molar ratio. The homophase and heterophase boundaries of the SiC ceramics were completely crystalline in all specimens. The room temperature (RT) strength decreased with increasing the molar ratio whereas the RT toughness showed a minimum at the molar ratio of 0.6. The best results at RT were obtained when the molar ratio was 0.2. The flexural strength and fracture toughness of the ceramics were >700 MPa and ~6 MPa.m1/2 at RT. The high temperature strength was critically affected by the chemistry, especially the content of Al in the IGP. The best strength at temperatures ³ 1500oC was obtained when the molar ratio was 0.5. Flexural strengths of the ceramics at 1500oC and 1600oC were 610 ± 80 MPa and 540 ± 30 MPa, respectively. The beneficial effect of the new additive compositions (Lu2O3-AlN) on high-temperature strength of SiC ceramics was attributed to the crystallization or removal of IGP and introduction of Al into SiC, i.e., removal or reduction of Al content from the IGP, resulting in an improved refractoriness of the IGP.