Fracture and deformation in brittle solids: A perspective on the issue of scale

A perspective on the issue of scale in the fracture and deformation properties of ordinarily brittle covalent–ionic solids (ceramics) is presented. Characteristic scaling dimensions for nanomechanical properties of this class of solids are identified—specimen size or layer thickness, microstructural scale, and contact dimension. Transitions in mechanical damage processes occur as the characteristic dimensions diminish from the macroscale to the submicroscale. Such transitions generally preclude unconditional extrapolations of macroscopic-scale fracture and deformation laws into the nanomechanics region. Strength of brittle solids tends to increase while toughness tends to decrease as the scaling dimensions diminish. The nature of flaws that control strength in the submicroscale region also undergoes fundamental changes—even flaws without well-developed microcracks can be deleterious to strength.