The Nanomechanical Behavior of Metallic Glass near Its Surface
Student thesis: Doctoral Thesis
Related Research Unit(s)
Metallic glass (MG) is an excellent candidate structural material for fabricating micro/nano-devices due to their unique structural features and outstanding mechanical, physical and chemical properties. At micro- and nano-scale, surface effect overwhelms bulk effect and becomes the dominant factor to determine the mechanical properties of MGs. Therefore, elucidating the nature of the mechanical behavior of MGs near its surface region not only expands our understanding of MG at fundamental research level, but also advances the current technology of nano-manufacturing based on crystalline.
In this thesis, we firstly studied in details the nanoscale plasticity in MGs near its surface region via dynamic nanoindentation. Our experiments clearly show that nano-scale shear-banding in different MGs undergoes a transition from the “distributed” to “localized” mode when the resultant plastic flow extends over a critical length scale. In the “distributed” regime, multiple shear bands with a constant but small shear offset are activated while in the localized regime one or a few dominant shear bands with large and varying shear offsets prevail. Through the extensive experimental and theoretical efforts, we unveil an intrinsic interplay between elasticity and fragility that governs the nanoscale plasticity crossover in MGs.
In the second part we focused on the surface dynamic features of MGs. By employing atomic force microscopy as a “forcing” and “imaging” tool, we provide the experimental report of how plasticity initiates and proceeds on an Au-Si metallic glass surface. A four-step evolution process was revealed, which starts from a stochastic process featured with local structural rearrangements, towards a deterministic process of structural relaxation (densification & hardening) till reaching an apparent dynamic equilibrium, and finally to a deterministic process of structural rejuvenation (dilation & softening). This surface plasticity resembles the general behavior of soft glassy materials and demonstrates that the surface dynamics of MG is faster than the interior.
Finally, we studied the effect of the fast surface dynamics on the friction behavior of MGs by nano-scratch experiments. We find that the mobile surface layer acts as a lubricating layer in friction experiments, which leads to a significant reduction of frictional coefficient by almost a factor of 2. This obvious reduction arises from the homogenous plastic flow of the mobile surface layer, which facilitates self-lubrication at MG surface. When the plastic flow extends over a critical length scale, localized shear bands prevail and result in stick-slip instabilities in friction data. Through the extensive experimental efforts, we finally unveil an interplay between the critical length scale and the homologous temperature T/Tg, in which Tg is glass transition temperature.
Our work sheds qualitative light on the nanomechanical behavior of metallic glass near its surface and suggests that nano-sized MG can be further explored for important applications, such as self-healing and self-lubrication.