Near-Edge X-ray Absorption Fine Structure Imaging of Spherical and Flat Counterfaces of Ultrananocrystalline Diamond Tribological Contacts: A Correlation of Surface Chemistry and Friction
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A recently installed synchrotron radiation near-edge X-ray absorption fine structure (NEXAFS) full field imaging electron spectrometer was used to spatially resolve the chemical changes of both counterfaces from an ultra- nanocrystalline diamond (UNCD) tribological contact. A silicon flat and Si3N4 sphere were both coated with UNCD, and employed to form two wear tracks on the flat in a linear reciprocating tribometer. The first wear track was produced using a new, unconditioned sphere whose surface was thus conditioned during this first experiment. This led to faster run-in and lower friction when producing a second wear track using the conditioned sphere. The large depth of field of the magnetically guided NEXAFS imaging detector enabled rapid, large area spectromicroscopic imaging of both the spherical and flat surfaces. Laterally resolved NEXAFS data from the tribological contact area revealed that both substrates had an as-grown surface layer that contained a higher fraction of sp2-bonded carbon and oxygen which was mechanically removed. Unlike the flat, the film on the sphere showed evidence of having graphitic character, both before and after sliding. These results show that the graphitic character of the sphere is not solely responsible for low friction and short run-in. Rather, conditioning the sphere, likely by removing asperities and passivating dangling bonds, leads to lower friction with less chemical modification of the substrate in subsequent tests. The new NEXAFS imaging spectroscopy detector enabled a more complete understanding of the tribological phenomena by imaging, for the first time, the surface chemistry of the spherical counterface which had been in continual contact during wear track formation.
KeywordsFriction mechanisms Solid lubrication friction Spectroscopy Humidity Diamond Carbon Solid lubrication mechanisms Solid lubrication wear Coatings NEXAFS
Funding was provided by Air Force grant FA9550-08-1-0024. This study was partially supported by the Nano/Bio Interface Center through the National Science Foundation NSEC DMR08-32802. Use of the Center for Nanoscale Materials was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. Use of the NSLS, Brookhaven National Laboratory, was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-98CH10886. The LARIAT was developed by Synchrotron Research, Inc. under NIST SBIR funding/Contract No. SB1341-04-C-0021. Certain commercial names are mentioned in this manuscript for purpose of example; this does not constitute an endorsement by the National Institute of Standards and Technology. The authors thank E. Principe for a critical reading of the manuscript. The authors gratefully acknowledge Dr. A.V. Sumant for coating flats and spheres with UNCD.
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