Abstract
Superlubricity, i.e., coefficient of friction (COF) below 0.01, was earlier limited to microscale in controlled environments in the earlier literature and more recently realized for macroscale sliding of ceramic or carbon surfaces lubricated by water or other polar fluids. However, there is lack of report of superlubricity for the most common bearing system, i.e., steel-steel contact in non-polar oil lubrication. Here we present ultra-low COF of 0.001–0.007 by using a sacrificial coating composed of vertically-aligned carbon nanotubes (CNTs) for macroscale steel-steel sliding under minimum quantity lubrication (MQL) of a polyalphaolefin (PAO) oil in the ambient environment. Raman spectroscopy and electron microscopy analyses detected graphene-containing tribofilms on both the steel surfaces, which was produced by the fractured CNT flakes and metallic wear debris during running-in. The in situ formed graphene-graphene contact interface presumably possesses a low shear resistance leading to superlubricity. The presence of oil, despite as little as one droplet, has proven to be crucial. Such a superlubricity performance has shown good sustainability in extended testing of more than 500,000 cycles and strong ability of accommodating changes in sliding conditions. Results here demonstrate feasibility with fundamental insights for achieving ambient environment macroscale superlubricity.
Original language | English |
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Article number | 100297 |
Journal | Materials Today Nano |
Volume | 21 |
DOIs | |
State | Published - Mar 2023 |
Funding
Notes : This manuscript has been authored by UT-Battelle, LLC under Contract No. DE-AC05-00OR22725 with the U.S. Department of Energy. The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, world-wide license to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes. The Department of Energy will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan (http://energy.gov/downloads/doe-public-access-plan). The research was supported by the Laboratory Directed Research and Development Program , Oak Ridge National Laboratory (ORNL) and the Solar Energy Technologies Office and Vehicle Technologies Office , Office of Energy Efficiency and Renewable Energy , Department of Energy . We thank P. Menchhofer (retired) and K. Cooley from ORNL for technical assistance and discussion for CNTs synthesis, C. Parish from ORNL for setting up STEM and EDS examination, and J. Arregui-Mena from ORNL for assistance with TEM data acquisition. The CNT-coated SS disk surface were analyzed using a Hitachi 4800 scanning electron microscope (SEM). Samples suitable for transmission electron microscopy (TEM) were prepared by drop-casting a toluene solution of CNTs onto lacey carbon films supported on a copper grid (300 mesh), allowing slow solvent evaporation. The grid was stored in vacuum overnight before analysis. TEM images were acquired using a JEOL JEM 2200-FS aberration corrected TEM at 200 kV. The contact angles of water and oil on the CNTs coated SS disk were measured using a DataPhysics Instruments coupled with SCA20 software (version 5) in the ambient environment at room temperature.Such a graphene-containing tribofilm is supported by the cross-sectional STEM images and EDS elemental maps (see Fig. 5 and S5) that clearly show a mixture of carbon and metallic elements (primarily Fe and Cr) with a good match with the oxygen distribution. Raman spectra (see Fig. 6) suggest that the carbon content in the tribofilm is rich of graphene and graphene oxides. The carbon nanoparticles inside the tribofilm are rather fine, <10 nm, compared with the 40–50 nm diameter and 20–30 mm length of the unfractured CNTs. No complete circular CNT sections or long graphene ribbons were observed in the tribofilm. This implies that only the finer CNT debris was able to participate in the tribofilm formation. Some previous studies also reported CNTs broken into graphene flakes under tribotesting. For example, The fate of CNTs at the sliding interface was examined by analyzing the wear debris after the test and broken CNTs, twisted CNTs, and graphene flakes were observed [20].The research was supported by the Laboratory Directed Research and Development Program, Oak Ridge National Laboratory (ORNL) and the Solar Energy Technologies Office and Vehicle Technologies Office, Office of Energy Efficiency and Renewable Energy, Department of Energy. We thank P. Menchhofer (retired) and K. Cooley from ORNL for technical assistance and discussion for CNTs synthesis, C. Parish from ORNL for setting up STEM and EDS examination, and J. Arregui-Mena from ORNL for assistance with TEM data acquisition. Notes: This manuscript has been authored by UT-Battelle, LLC under Contract No. DE-AC05-00OR22725 with the U.S. Department of Energy. The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, world-wide license to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes. The Department of Energy will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan (http://energy.gov/downloads/doe-public-access-plan).
Keywords
- CNTs
- Graphene
- Minimum quantity lubrication
- Steel-steel sliding
- Ultra-low friction