Fracture and deformation in brittle solids: A perspective on the issue of scale
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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.
I. INTRODUCTION
Materials technology is witnessing an ever-continuing miniaturization in microelectromechanical systems (MEMS) and nanoelectromechanical systems (NEMS), computer chips, sensors and actuators, microfluidics and bioengineering devices, and so on. Figure 1, a polysilicon MEMS device fabricated at the Sandia National Laboratories, is an illustrative example—characteristic dimensions of individual components, of component/ component contacts, and of the underlying grain microstructures all lie in the submicrometer range. Questions inevitably arise as to how valid it is to extrapolate our knowledge base downward from the large scale as such characteristic dimensions diminish. We might expect to find fundamental differences between conventional responses at the macroscale (governed by continuum laws), microscale (governed by discrete defects—dislocations, microstructural interfaces, microcracks), and nanoscale (governed by interatomic force laws). Feynman, in his celebrated 1959 lecture “There Is Plenty of Room at the Bottom,” pointed out that properties will inevitably change on approaching the nanoscale, not just because of quantum effects but also from a shifting balance between competing classical forces as the surface/volume ratio increases. In this view, intrinsic size effects may be expected to constitute the rule rather than the exception in materials properties. Size effects are no less true of mechanical properties. The mechanical properties of materials is a wellestablished field of study at the macroscale level. But 22
http://journals.cambridge.org
J. Mater. Res., Vol. 19, No. 1, Jan 2004 Downloaded: 03 Apr 2015
how valid are the conventional laws of fracture and deformation at the nanoscale—the realm of nanomechanics? How does diminishing separation between boundaries (external surfaces, internal grain or interlayer interfaces) increasingly constrain deformation and fracture processes? Are there fundamental transitions in underlying mechanisms en route between scaling limits? For instance, metals tend to become more brittle as grain or interlayer size diminishes.1 This is attributable at least in
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