Micro-electromechanical systems (MEMS), micro-electronic packaging, and micromechanical sensors and actuators commonly use thin films and multilayers whose behaviors are found to be significantly different from that of the bulk materials from which they are made. As such, there is a need to deal with design and manufacturing issues at micron levels, and even down to nanometer levels. Experimental evidence shows that the microscale behavior manifests itself as a strong size effect on the plastic flow stress of metals and ceramics when the characteristic length scale of deformation is on the order of several microns or less. Conventional theories of plasticity are unable to explain and predict these effects, which reflect some inherent nature of the material. The cause of these size effects is rooted in the dependence of plastic flow stress on not only the plastic strain, but also on the gradient of plastic strain. Plastic strain gradients are, in general, inversely proportional to the length scale over which plastic deformation occurs so that strain gradient effects tend to become important at these small scales. This proposal is aimed at understanding the deformation mechanisms behind large plastic strain gradients in thin films and multilayers and using continuum strain gradient plasticity theories to characterize the deformation. Large strain gradients will be generated through innovative nano- and microscale bending and indentation tests. The proposal will provide a thorough, experiment-based examination of the basic deformation behavior and mechanisms for various thin films and wires at nano- and microscales, and a development of the strain gradient plasticity theory with emphasis on determining the values of pertinent material properties. The proposal includes the study of (i) strain gradient plasticity in monolithic materials using bending tests for thin foils and wires, and indentation tests; and (ii) strain gradient plasticity and layer thickness effects in nano- and micro-layered materials. The former will include the development of new experimental methods for nanomechanics and will validate current theories of strain gradient plasticity for nanoscale deformation. The latter will examine material length scales associated with both strain gradient plasticity and with layer thickness.
