Measuring Local Mechanical Properties using FIB Machined Microcantilevers
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1185-II02-08
Measuring Local Mechanical Properties using FIB Machined Microcantilevers David E.J Armstrong1, Angus J. Wilkinson1, and Steve G. Roberts1 1 Department of Materials Science, University of Oxford, Parks Road Oxford, OX1 3PH, United Kingdom ABSTRACT Micro-scale Focused Ion Beam (FIB) machined cantilevers were manufactured in single crystal copper, polycrystalline copper and a copper-bismuth alloy. These were imaged and tested in bending using a nanoindenter. Cantilevers machined inside a single grain of polycrystalline copper were tested to determine their (anisotropic) Young’s modulus: results were in good agreement with values calculated from literature values for single crystal elastic constants. The size dependence of yield behavior in the Cu microcantilevers was also investigated. As the thickness of the specimen was reduced from 23µm to 1.7µm the yield stress increased from 300MPa to 900MPa. Microcantilevers in Cu-0.02wt%Bi were manufactured containing a single grain boundary of known character, with a FIB-machined sharp notch on the grain boundary. The cantilevers were loaded to fracture allowing the fracture toughness of grain boundaries of different misorientations to be determined. INTRODUCTION Much recent work on measuring mechanical properties on the microscale has been carried out using focused ion beam (FIB) machining to manufacture a wide range of different tests specimens[1-3] which are then tested using a nanoindenter as a loading device. This is of great interest as it allows mechanical properties to be measured from much smaller volumes than are required for traditional mechanical tests. It is now possible to measure directly the mechanical properties of individual microstructural features such as grains, grain boundaries or thin layers, such as ion-implanted layers in bulk specimens [4]. A recent paper [5] by the present authors has demonstrated that testing of microcantilevers with a triangular cross section, machined into the surface of single crystal copper, can be used to measure the anisotropy of Young’s modulus with respect to crystallography. This paper describes the extension of such techniques to measure plastic and fracture properties as well as Young’s modulus. EXPERIMENT Materials Three materials were chosen as model systems for developing microcantilever-testing techniques. For measuring elastic properties high purity polycrystalline copper was chosen. This was easily prepared and is well known for having highly anisotropic elastic properties. Single crystal copper was used for carrying out measurements into size effects in plastic properties. This allowed all cantilevers to be made with the same crystallography to avoid the influence of crystal anisotropy. For measuring grain-boundary fracture properties Cu-Bi was chosen, as it is well
known that a small amount of bismuth (less then 0. 1wt%) [6] causes grain boundary embrittlement at room temperature. Bulk Sample Preparation The single crystal copper (99.9999%) was prepared by first mechanically polishing with diamond
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