Grain boundary and triple junction chemistry of silicon carbide sintered with minimum additives for armor applications

Silicon carbide containing a minimum amount of additives for armor application was fabricated by hot pressing. Microstructural development, phase information, mechanical properties, and triple junction and grain boundary chemistry were described. Correlation between the characteristics of the secondary phases and mechanical properties was also presented. The grain boundary and triple junction phases were characterized using high resolution transmission electron microscopy, energy dispersive spectroscopy and energy filtered transmission electron microscopy.; The grain boundary and triple junction phase characteristics in silicon carbide sintered with the addition of aluminum, boron and carbon varied with the amount of aluminum additive. Silicon carbide sintered with the addition of boron and carbon only did not form triple junction and grain boundary phase, while addition of 0.5 and 1 wt. aluminum resulted in the formation of amorphous Al-O-rich triple junction phase.; Crystalline Al2O3 formed at the triple junction pockets upon addition of 1.5 and 4 wt. % Al. Formation of amorphous intergranular films, with thickness of approximately 1 nm, was also observed at these amounts of aluminum additive. An increase in toughness, from 2.7 to 6.1 MPa • m1/2, accompanied the presence of this grain boundary phase, which was also coincidental with the transition from transgranular to intergranular mode of fracture. The increased toughness and the change in fracture mode from transgranular to intergranular fracture were attributed to the residual stresses in the intergranular films.; The triple junction and grain boundary phase characteristics in silicon carbide sintered with Al, AlN, and Al2O3 addition did not vary with the source of Al additive. The triple junction phases were observed to be crystalline Al2O3 while the grain boundary phase composition remained primarily Al-O-rich. The intergranular film also remained amorphous with thicknesses of approximately 1 nm. Toughness value of 6.8 MPa • m1/2 was attained by hot pressing at 2200°C and using 3.3 wt. % Al additive. The resulting microstructure in this material consisted of elongated grains in a matrix of fine equiaxed grains. The use of AlN additive resulted in a retarded the grain growth and equiaxed grain morphology. Addition of B4C and C resulted in increased grain growth.; Heat treatment at 1300°C for 3 hours of select SiC specimens showed that crack healing was possible. Exposure to argon, air and water vapor-containing environments resulted in strength recovery in all materials studied. Decreases in crack sizes were also observed upon crack healing. Toughening in solid state sintered SiC in water vapor-containing environment, due to the possible formation of secondary phases, was also observed.