Mikro-Plastizität und -Tribologie von metallischen Gläsern
Zusammenfassung der Projektergebnisse
The nanomechanical properties of metallic glasses have been studied by high-resolution force microscopy in ultrahigh vacuum, allowing truly nanometer-scale experiments on well-defined surfaces. Procedures for surface preparation of metallic glasses have been developed and validated, including controlled oxidation. We have found a nanometer-scale lower size limit for discrete plasticity in the indentation of a Ptbased metallic glass. The plastic deformation of crystalline platinum occurs in form of discrete steps and AFM-based indentation allows the observation of even single dislocation nucleation in a sudden sub-nanometer excursion of the indenting tip. In contrast, the serrated flow of a Pt-based metallic glass at larger scale changes over to a homogeneous viscous flow for nanometer scale indentation. The oxidation of the Zr60Cu30Al10 metallic glass proceeds by preferential oxidation of Zr and Al, the aggregation of Cu into crystalline oxide particles at the surface embedded in an amorphous oxide, and a sub-surface copper depletion. The oxidation process leads to a sharp interface between oxide layer and metallic glass substrate and to delamination of the complete oxide films when scratching the sample with micro-tips. At the nanometer scale, atomic stick-slip friction experiments reveal the atomic and the cluster structure of the metallic glass and its oxide. Careful choice of appropriate scan areas allow for a comparison between the shear strength of clean and oxidized surface, which was found to be seven times higher for the oxidized surface. Nanometer-scale wear on clean metallic glass surfaces proceeds by homogeneous plowing, while the wear mechanism on oxidized surfaces depends critically on the relation between tip sharpness and particulate surface structure. High-resolution friction force microscopy was used to quantify the effects of annealing in a Ni60Nb40 metallic glass at temperatures below the crystallization threshold. Atomic stick-slip friction maps directly revealed the increase in closed-packed compactness for the annealed glass. Additionally, an increased wear resistance and higher friction were found for the annealed glass. The experimental results demonstrate the capability of high-resolution force microscopy to reveal microscopy mechanisms in friction and plastic deformation of metallic glasses. Beyond this demonstration of feasibility, we were able to identify changes in the microscopic structure and the related mechanical properties upon relevant processes such as surface oxidation and annealing of the free excess volume.
Projektbezogene Publikationen (Auswahl)
- Lower nanometer-scale size limit for the deformation of a metallic glass by shear transformations revealed by quantitative AFM indentation. Beilstein Journal of Nanotechnology 2015, 6, 1721-1732
A. Caron and R. Bennewitz
(Siehe online unter https://doi.org/10.3762/bjnano.6.176) - Importance of surface oxide for the tribology of a Zr-based metallic glass. Friction (2017) 5: 115-122
S.J. Kang, K.T. Rittgen, S.G. Kwan, H.W. Park, R. Bennewitz, and A. Caron
(Siehe online unter https://doi.org/10.1007/s40544-017-0149-7)
