Extracting reliable mechanical properties using the nanoindentation technique

Abstract

Extracting the mechanical properties of thin films and small volumes of bulk materials through the use of nanoindentation is a well established technique but getting good data from all types of test sample is not always easy. Factors such as surface roughness and oxidation, density/porosity of the material, adhesion/detachment of a thin film, pile-up/sink-in, the presence of the substrate, as well as grain size and its distribution have a significant effect on the observed mechanical properties (e.g. Young’s modulus and hardness). Considerable differences between predicted and observed performance can be seen depending on the material tested and how it has been prepared. This thesis concerns developing test protocols to get good nanoindentation data and reliable measurements of the properties for a range of material types (chiefly metals and ceramics). Firstly, this work highlights the effect of crystallographic anisotropy, grain size, shape and orientation on the mechanical response of metallic thin films such as copper used for semiconductor metallisation. Results obtained on highly polished semiconductor materials were compared with those from engineering surfaces with much higher roughness which show increased scatter in results across the complete range of contact scales. Further studies were carried out on hard coatings and bulk materials such as titanium carbide, zirconium nitride and tungsten. The scatter in data obtained at low tests loads is dominated by anisotropy and grain size effects but disappears at higher loads. For soft materials such as copper, the appearance of pile-up was shown to be significant when compared with harder materials which tend to sink-in. Secondly, to assess the effect of creep (time-dependent behaviour) and also grain boundary effects on the measured mechanical properties, soft materials with a range of grain sizes have been examined. Different indentation control cycles (load and displacement control, single indent and multicycling tests) have been investigated to determine what is most suitable with displacement control being essential in most cases. To study the effect of the density/porosity of the sample and its surface roughness on mechanical properties, the work was carried out on porous coatings of tin, copper and copper-tin alloy coatings with a low density. To further understand the behaviour of porous materials and their mechanical properties, finite element analysis was also used to compare the experimental results with a numerical model. The size, shape and location of porosity with respect to the indenter is critical in determining the mechanical properties of a porous material obtained from nanoindentation analysis. Finally, fully processed engineering surfaces were investigated at the component scale to compare with idealised flat plate samples. Titanium-based and cobalt-chrome alloys in the form of femoral heads and stems for replacement hips have been used to assess the effect of in service oxidation on mechanical properties. These have been studied to look at the effects of sample fixturing and support and surface contact in worn and virgin regions of the sample surface. The extent of oxidation and the mechanical properties of the oxide produced are critical in dictating performance.