Size effects and microstructure evolution in textured metal surfaces during reciprocating sliding
Final Report Abstract
Correlating the microstructure of a material with its properties is one of the most central questions in materials science and engineering. For sliding interfaces, there is a significant lack of knowledge about the mechanics of the materials involved and how exactly the microstructures evolve under tribological loading. Tribological contacts have significant industrial relevance in all moving mechanical parts and gaining more knowledge about the materials science governing friction and wear thus will have wide spread impact. This is why we systematically investigated the elementary mechanisms of the microstructure evolution under a tribological load. We reduced the complexity of a sliding contact to a sapphire sphere reciprocating on high purity copper without lubrication. We found that the microstructure and the mechanical properties of the surface layer are determined during the very first loading cycle. During this first trace and due to the complex stress field under the sliding contact, roughly 150 nm under the surface dislocation self-organization occurs, forming a line of very high dislocation density, parallel to the sliding surface. This feature, called the dislocation trace line, introduces the first discontinuity into the microstructure. It most likely determines where parts of the surface later dissociate from the rest of the material in the form of wear particles and was found over a wide range of sliding speeds, normal loads and counter body sizes. The dislocation trace line evolves into the known microstructural features with increasing sliding distance. We followed these processes closely and elucidated the sequence of elementary mechanisms. After the formation of the dislocation trace line at the very beginning of the sliding contact, it further evolves into a small angle grain boundary. Additionally, and deeper into the material, networks of geometrically necessary dislocations begin to appear and self-organize to form sub-grains. As an additional elementary process, we revealed the formation of amorphous/nanocrystalline copper oxide clusters on the surface after roughly 100 sliding cycles. The growth of the microstructurally altered layer follows a square root growth law. For C85 pearlitic steel we could successfully correlate the growth of this layer with the strain in the material through the adaptation of a high pressure torsion-based model. When investigating a possible size effect for the static friction of brass samples with morphological surface textures in the form of round dimples, we found that smaller dimples indeed give rise to larger static friction forces. This size effect could successfully be explained by viewing the transition from static to dynamic friction as a mode II crack moving through the interface between the two contacting materials. This crack is associated with a line tension and in analogy with the Orowan model for dislocations, it interacts with the morphological surface textures. Through the design and setting-up of a high-speed insitu tribometer, we were able to follow the crack as it moves through the interface and even see it bend around the dimples. In lubricated systems, we also investigated a possible size effect and found that due to a competition between different mechanisms governing friction, there exists an optimal dimple diameter for which friction forces can be reduced by up to 80%. In dry contacts and through applying laser surface texturing to generate bio-inspired snake skin-like surface morphologies, we successfully reduced friction by up to 80%. All of these results combined help to gain a much deeper understanding of sliding contacts from a materials science point of view and aim at assisting to rationally design tribological systems with tailored microstructures and surface morphologies in the future. Since the Interim Report, the main surprise was how interesting the dislocation trace line results really turned out to be. The additional focus on tribologically induced oxidation also was not foreseen.
Publications
- “Laser textured surfaces for mixed lubrication: Influence of aspect ratio, textured area and dimple arrangement”, Lubricants (2017), 5(3), 32
J. Schneider, D. Braun, C. Greiner
(See online at https://doi.org/10.3390/lubricants5030032) - “Transparent, abrasion-insensitive superhydrophobic coatings for real-world applications”, Scientific Reports (2017), 7(1), 15078
D. Helmer, N. Keller, F. Kotz, F. Stolz, C. Greiner, T. Nargang, K. Sachsenheimer, B. Rapp
(See online at https://doi.org/10.1038/s41598-017-15287-8) - “Characterization of the microscopic tribological properties of sandfish (Scincus scincus) scales by atomic force microscopy”, Beilstein Journal of Nanotechnology (2018), 9, 2618–2627
W. Wu, C. Lutz, S. Mersch, R. Thelen, C. Greiner, G. Gomard, H. Hölscher
(See online at https://doi.org/10.3762/bjnano.9.243) - “Friction reduction through biologically inspired scale-like laser surface textures”, Beilstein Journal of Nanotechnology (2018), 9, 2561-25727
J. Schneider, V. Djamiykov, C. Greiner
(See online at https://doi.org/10.3762/bjnano.9.238) - “Glassomer – Processing fused silica glass like a polymer”, Advanced Materials (2018), 1707100
F. Kotz, N. Schneider, A. Striegel, A. Wolfschläger, N. Keller, M. Worgull, W. Bauer, D. Schild, M. Milich, C. Greiner, D. Helmer, B. Rapp
(See online at https://doi.org/10.1002/adma.201707100) - “Microstructure evolution and deformation mechanisms during high rate and cryogenic sliding of copper”, Acta Materialia (2018), 161(12), 138-149
X. Chen, R. Schneider, P. Gumbsch, C. Greiner
(See online at https://doi.org/10.1016/j.actamat.2018.09.016) - “Stages in the tribologically-induced oxidation of high-purity copper“, Scripta Materialia (2018), 153(8), 114-117
Z. Liu, C. Patzig, T. Höche, S. Selle, P. Gumbsch, C. Greiner
(See online at https://doi.org/10.1016/j.scriptamat.2018.05.008) - “The origin of surface mutation in sliding friction”, Scripta Materialia (2018), 153(8), 63-67
C. Greiner, Z. Liu, L. Pastewka, R. Schneider, P. Gumbsch
(See online at https://doi.org/10.1016/j.scriptamat.2018.04.048) - “Insights into tribology from in situ nanoscale experiments”, MRS Bulletin (2019), 44(6), 478-486
T.D.B. Jacobs, C. Greiner, K. Wahl, R.W. Carpick
(See online at https://doi.org/10.1557/mrs.2019.122) - “Solids under Extreme Shear: Friction-mediated Subsurface Structural Transformations”, Advanced Materials (2019), 1806705
C. Greiner, J. Gagel, P. Gumbsch
(See online at https://doi.org/10.1002/adma.201806705)
