Deformation of nanostructured materials: insights from experimental observations

Abstract

Increased interest in nanoscale deformation has been spurred by the rapid miniaturization of features and components in microelectronic devices, magnetic storage media, and MEMS/NEMS, i.e., microand nanoelectromechanical systems, respectively. Materials for these applications are often chosen because of their electronic, optical, or magnetic properties. Frequently, however, the mechanical properties are the performance-limiting factor. Until recently, the techniques used to produce samples resulted in a high number of induced artefacts and, hence, irregular mechanical properties. Thanks to progress in processing methods, we can now make nanograin samples of high purity and high density that, under experimental conditions, show reproducible characteristics. Such fully dense materials are excellent candidates for fundamental mechanical property studies as well as for high-performance applications. Compared with their microcrystalline counterparts,nanostructured metals with grain size smaller than 100nm possess attractive properties such as high yield and fracture strength and improved wear resistance. But evidence also suggests that miniaturization may influence their microstructural and mechanical stability, resulting in undesirable shape change, mechanical response, thermal behavior, and electrical performance. These instability phenomena are the main focus of our research efforts. Small sample size generally rules out standardized tests, such as tensile and compression tests. But advances in methods such as nanoindentation accommodate very small amounts of material. Traditional nanoindentation testing measures the elastic modulus and hardness of engineering materials by pushing a probe into the material while monitoring load and penetration depth. Now, instrumentation offers new possibilities. For example, we perform cyclic nanoindentation tests with up to a million load Figure 1. Shown is the grain structure of nickel following 10 loading cycles using a pyramidal tip. The initial grain size was 30nm. Some areas of the indented region (along the edge as indicated in the insert) show significantly larger grains and increased surface roughness after cyclic contact loading, as marked by arrows.