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  4. Multiwall carbon nanotubes for solid lubrication of highly loaded contacts
 
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2023
Journal Article
Title

Multiwall carbon nanotubes for solid lubrication of highly loaded contacts

Abstract
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.
Author(s)
MacLucas, Timothy
Saarland University
Klemenz, Andreas
Fraunhofer-Institut für Werkstoffmechanik IWM  
Grünewald, Patrick
INM - Leibniz Institute for New Materials
Presser, Volker
INM - Leibniz Institute for New Materials
Mayrhofer, Leonhard
Fraunhofer-Institut für Werkstoffmechanik IWM  
Moras, Gianpietro  
Fraunhofer-Institut für Werkstoffmechanik IWM  
Suarez, Sebastian
Saarland University
Dienwiebel, Martin  
Fraunhofer-Institut für Werkstoffmechanik IWM  
Mücklich, Frank
Saarland University
Moseler, Michael  
Fraunhofer-Institut für Werkstoffmechanik IWM  
Journal
ACS applied nano materials  
Project(s)
Fluidfreie Schmiersysteme mit hoher mechanischer Belastung
Funder
Deutsche Forschungsgemeinschaft -DFG-, Bonn  
DOI
10.1021/acsanm.2c04729
Language
English
Fraunhofer-Institut für Werkstoffmechanik IWM  
Keyword(s)
  • carbon nanotubes

  • coating materials

  • friction

  • iron

  • wear

  • tribo-induced phase transition

  • molecular dynamics

  • sliding tests

  • solid lubrication

  • multiwall carbon nanotubes

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