Multiwall Carbon Nanotubes for Solid Lubrication of Highly Loaded Contacts
- Timothy MacLucas*
- Andreas Klemenz
- Patrick GrünewaldPatrick GrünewaldINM─Leibniz Institute for New Materials, 66123Saarbrücken, GermanyMore by Patrick Grünewald
- Volker PresserVolker PresserINM─Leibniz Institute for New Materials, 66123Saarbrücken, GermanyDepartment of Materials Science & Engineering, Saarland University, Campus D2.2, 66123Saarbrücken, GermanySaarene─Saarland Center for Energy Materials and Sustainability, 66123Saarbrücken, GermanyMore by Volker Presser
- Leonhard MayrhoferLeonhard MayrhoferFraunhofer Institute for Mechanics of Materials IWM, MicroTribology Center μTC, Woehlerstr. 11, 79108Freiburg, GermanyMore by Leonhard Mayrhofer
- Gianpietro Moras
- Sebastian SuarezSebastian SuarezChair of Functional Materials, Saarland University, Campus D3.3, Saarbrücken66123, GermanyDepartment of Materials Science & Engineering, Saarland University, Campus D2.2, 66123Saarbrücken, GermanyMore by Sebastian Suarez
- Martin DienwiebelMartin DienwiebelFraunhofer Institute for Mechanics of Materials IWM, MicroTribology Center μTC, Woehlerstr. 11, 79108Freiburg, GermanyKarlsruhe Institute of Technology (KIT), IAM─Institute for Applied Materials, MicroTribology Center μTC, Straße am Forum 7, 76131Karlsruhe, GermanyMore by Martin Dienwiebel
- Frank MücklichFrank MücklichChair of Functional Materials, Saarland University, Campus D3.3, Saarbrücken66123, GermanyMore by Frank Mücklich
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- Michael Moseler*Michael Moseler*Email: [email protected]Fraunhofer Institute for Mechanics of Materials IWM, MicroTribology Center μTC, Woehlerstr. 11, 79108Freiburg, GermanyModelling of Functional Nanomaterials Group, Institute of Physics, University of Freiburg, Hermann-Herder-Str. 3, 79104Freiburg, GermanyFreiburg Materials Research Center, University of Freiburg, Stefan-Meier-Str. 21, 79104Freiburg, GermanyCluster of Excellence livMatS, Freiburg Center for Interactive Materials and Bioinspired Technologies, University of Freiburg, Georges-Köhler-Allee 105, 79110Freiburg, GermanyMore by Michael Moseler
When lubrication of rolling bearings with oil or grease is not possible, for example because the lubricant evaporates in vacuum, solid lubrication by multiwall carbon nanotubes (MWCNT) is a viable alternative. To understand the mechanisms underlying MWCNT lubrication of highly loaded contacts, we combine an experimental approach with large-scale molecular dynamics (MD) simulations. Tribometry is performed on ground iron plates coated with two different types of MWCNTs by electrophoretic deposition. Although structural differences in the MWCNT materials result in slightly different running-in behavior, most of the tests converge to a steady-state coefficient of friction of 0.18. The resulting wear tracks and tribolayers are subjected to structural and chemical characterization and suggest a tribo-induced phase transformation resulting in tribolayers that consist of MWCNT fragments, iron oxide, and iron carbide nanoparticles embedded in an amorphous carbon matrix. Covalent bonding of the tribolayer to the iron surface and low carbon transfer to the alumina counter body indicate sliding at the tribolayer/ball interface as the dominant mechanism underlying MWCNT solid lubrication. MD simulations of nascent a-C tribofilms lubricated by MWCNT bundles and stacks of crossed MWCNTs reveal two different sliding regimes: a low-load regime that leaves the MWCNTs intact and a high-load regime with partial collapse of the tube structure and formation of a-C regions. The critical load for this transition increases with the filling ratio of the MWCNT and the packing density of the stacks. The former determines the stability of the MWCNT, while the latter controls the local stresses at the MWCNT crossings. For both MWCNT materials, the high-load regime is predicted for the experimental loads. This is confirmed by a remarkable agreement between transmission electron microscopy (TEM) and atomistic simulation images. Based on the findings of this work, a multistep lubrication mechanism is formulated for MWCNT coatings rubbing against alumina on an iron substrate.
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