Fracture mechanics and design principles for metal-reinforced ceramics

The principal reason for reinforcing a ceramic with a metal is to enhance its toughness. With metal reinforcement, the work of fracture of a ceramic can be enhanced by two orders of magnitude or more, as discussed in the review in Chapter 2. This is much more than can be achieved by any other toughening method, such as ceramic fibers or transformation toughening. In cermets (metal particle reinforcement), the most important toughening mechanism is energy absorption by plastic deformation of the ductile metal phase. Crack deflection can also have a significant role. In metal fiber-reinforced ceramics, energy absorption by plastic deformation of the ductile metal phase, crack bridging, and fiber pull out are all important mechanisms. Crack deflection can also have a significant role. Fracture mechanics of brittle ceramics, and fracture mechanics of ductile metals are both well-developed disciplines. This chapter is concerned with the specifics of fracture mechanics for metal-reinforced ceramics, a unique class of CMCs that are midway in their fracture mechanics behavior between ceramics and metals, and therefore have unique fracture mechanics attributes and require specific design principles for optimizing their toughness. This chapter is focused on both these issues: fracture mechanics and design principles, as they relate to metal-reinforced ceramics.