4 research outputs found
Understanding the hydrogen and oxygen gas pressure dependence of the tribological properties of silicon oxide-doped hydrogenated amorphous carbon coatings
Silicon oxide-doped hydrogenated amorphous carbons (a–C:H:Si:O) are amorphous thin films used as solid lubricants in a range of commercial applications, thanks to its increased stability in extreme environments, relative to amorphous hydrogenated carbons (a–C:H). This work aims to develop a fundamental understanding of the environmental impact on the tribology of a–C:H:Si:O. Upon sliding an a–C:H:Si:O film against a steel counterbody, two friction regimes develop: high friction in high vacuum and low gas pressure (oxygen pressure < 10 mbar; hydrogen pressure < 50 mbar), and a low friction regime at higher gas pressures (10 mbar < oxygen pressure < 500 mbar; 50 mbar < hydrogen pressure < 1000 mbar). Scanning electron microscopy (SEM) revealed that the tribological behavior of a–C:H:Si:O is governed by adhesive junctions at the sliding interface. At low gas pressures, material transfer from the steel pin to the a–C:H:Si:O flat occurs. At higher gas pressures, a tribofilm forms on the steel countersurface. Raman and near edge X-ray absorption spectroscopy (NEXAFS) spectroscopies demonstrate that upon sliding under the higher gas pressure, low friction regime, a surface layer with an elevated fraction of sp2-bonded carbon atoms forms. These changes indicate that these gases favor the release of the adhesive junctions by dissociatively reacting with the mechanically-stressed sp2 carbon-rich surface layer
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Structure-property relationships from universal signatures of plasticity in disordered solids
When deformed beyond their elastic limits, crystalline solids flow plastically via particle rearrangements localized around structural defects. Disordered solids also flow, but without obvious structural defects. We link structure to plasticity in disordered solids via a microscopic structural quantity, "softness," designed by machine learning to be maximally predictive of rearrangements. Experimental results and computations enabled us to measure the spatial correlations and strain response of softness, as well as two measures of plasticity: the size of rearrangements and the yield strain. All four quantities maintained remarkable commonality in their values for disordered packings of objects ranging from atoms to grains, spanning seven orders of magnitude in diameter and 13 orders of magnitude in elastic modulus. These commonalities link the spatial correlations and strain response of softness to rearrangement size and yield strain, respectively