Understanding and Improving the Toughening Mechanisms in Silicon Carbide Ceramics

In order to enhance the utility of advanced silicon carbide materials for use in mechanically demanding environments, this study was conducted to understand the underlying mechanisms of fracture toughness and methods that promise to improve the toughness to acceptable levels. Silicon carbide was sintered using both pressureless and hot pressing methods, with an additive system consisting of 3 wt% Al, 0.6 wt% B, 2 wt% C (designated 3ABC-SiC), with some materials also using the rare earth (RE) additives La, Y, or Yb. The RE dopants were used to decorate the grain boundary phases typically found in liquid-phase-sintered ceramics such as SiC in order to evaluate both the mechanism of crack deflection and the role of these additives in determining the fracture behavior. Using a two-step process to create high density in pressureless-sintered 3ABC-SiC, it was found that the action of nitrogen and the necessarily long sintering times altered both the morphology of the grains and the nature of the grain boundary phase. These changes did not create a favorable situation for crack deflection and enhanced fracture toughness; therefore the pressureless-sintered 3ABC-SiC maintained a low value of toughness. Using hot-pressed 3ABC-SiC with RE additives, it was found that cracks are deflected along the interface between the grain boundary phase and the SiC grains. This process can be described by the He & Hutchinson model for crack deflection at an interface, and is dependent on several important mechanical parameters of the grain boundary phase: the stiffness of the phase and the toughness of its interface with the matrix grains. It was shown that these parameters change with the size of the RE ion added in such a way that large ions favor crack deflection---high toughness---and small ions favor crack penetration through the interface---low toughness. This behavior is seen empirically over many studies. It was also found that the yttrium additive system could display extensive crack deflection or primarily low-toughness transgranular fracture, depending on the processing conditions. Microscopy revealed that crack deflection was enhanced when the Y ions became collected in the triple pockets between grains, and transgranular fracture existed when the Y ions were trapped in the grain boundary phase. The segregation behavior depended on the concentration of the Y ions, and is expected to also vary with heat treatment temperatures and cooling rates.