Nanoindentation of metallic materials:
Time-dependent and crystallographic effects
PhD Project
Reza Rastegar Tohid
Coatings are commonly used by industry for different engineering purposes such as wear resistance, corrosion resistance, and thermal resistance. Reliable mechanical data for coatings can be difficult to obtain since the coatings can have different microstructure to bulk materials, and therefore bulk material data can not always be used in design calculations. Nowadays thin film and coating technologies have been widely developed and, as many films have been deposited to ever smaller thicknesses, the characterization of their mechanical properties is an increasing challenge. In this study nanoindentation techniques (Hysitron Triboindenter® and Nanoindenter® II fitted with a Berkovich or cube corner tip) have been used to investigate mechanical properties of soft metallic coatings and bulk materials; since soft materials have a high creep rate or show time-dependent behaviour, it is important to have a sound understanding of their instrumented indentation behaviour since this will differ considerably from that of the harder, stiffer materials for which the techniques were originally developed.
It is well known that the standard Oliver and Pharr analysis method does not produce accurate results when it is applied to anisotropic materials or soft materials with a high creep rate. Therefore in addition to continuously recording indentation techniques atomic force microscopy (AFM), scanning electron microscopy (SEM), x-ray diffraction (XRD) and electron backscatter diffraction (EBSD) have been used in conjunction with a design of experiments (DOE) approach to characterize the basic mechanical properties of aluminium, zinc, a zinc-nickel coating, a hot dip galvanized zinc coating and a copper thin film. These metals show a range of anisotropy and creep behaviours.
In the first part of this study the mechanical properties and time-dependent behaviour of aluminium and copper thin films has been investigated. These are fcc materials with different anisotropy. After pile-up correction the measured Young’s modulus is similar to standard tensile test value for bulk materials; it was observed that the measured Young’s modulus values for a copper thin film was much lower than the reported values for bulk copper. This is related to the microstructure of the thin film.
The second part of this study was focused on getting accurate nanoindentation data for time-dependent materials by optimizing all the influencing test parameters. The nanoindentation results can be affected by material-related or experimental-related parameters such as surface roughness of the sample, loading and unloading rate, holding time, thermal drift correction, tip calibration and control strategy (load control, displacement control, or open loop). It has been shown that the displacement control strategy can be successfully used in the testing of materials which exhibit time-dependent behaviour. In this study a design of experiments approach (DOE) has been used to optimize the feedback gains (proportional, integral, derivative, and adaptive) used as part of the displacement control strategy.
The next part of the study addresses the behaviour of bulk zinc and zinc coatings in the nanoindentation experiments. The vast majority of existing studies have focussed on almost isotropic materials but in certain systems such as zinc crystallographic orientation effects have a considerable influence on the indentation response. Polycrystalline zinc has hexagonal close packed structure and therefore exhibits a great degree of anisotropy. Also it has been observed that, in comparison to the bulk materials, thin films exhibit stronger anisotropy in mechanical properties and thus highly anisotropic behaviour can be expected in the hot dip galvanizing sample. The evaluation of the mechanical properties of anisotropic materials usually requires investigation of different single crystals responses. In this study instead of using this classical approach, the electron backscattered diffraction (EBSD) technique has been used; by providing the grain orientation map of bulk zinc and the hot dip galvanized coating the crystallographic effects on the nanoindentation results have been fully examined. It has been observed that the wide range of mechanical properties can be achieved due to the different crystal orientation and the indentation angle with respect to the basal plane (0001).
At the end of the project a zinc-nickel coating which is a common coating in car industry with a bcc crystal structure has been tested in order to compare its nanoindentation response with the hot dip galvanized coating. Zn-Ni shows little or no anisotropic behaviour and has a similar E/H ratio to the hot-dip galvanized coating and is expected to behave similarly in mechanical application.
Selected publications:
[1] Getting accurate nanoindentation data from zinc: Time dependent and microstructural effects, R.R. Tohid and S.J. Bull, Int. J. Mat. Res. (Z. Metall.), 98 (2007) 353-359.