Atomic-scale imaging and the effect of yttrium on the fracture toughness of silicon carbide ceramics

In SiC sintered with Al, B and C additions (ABC–SiC), the presence of Y in the Al–Si–O–C grain-boundary phase leads to less frequent crack deflection and lower toughness. When Y is absent from the grain-boundary phase and remains in the triple pockets, crack deflection is restored, and higher toughness results from grain-bridging mechanisms. The observations are consistent with elastic modulus changes in the intergranular phase, which depend on their yttria and silica content, and indicate that these can play an important role in determining crack deflection. While high-toughness ceramics such as ABC–SiC and Si3N4 rely on sintering additives forming crack-deflecting intergranular films, the present case is a striking example where the presence of a segregant in the grain boundary promotes transgranular fracture by raising the modulus of the nanoscale intergranular grain-boundary film.