Abstract
The influence of corrosion upon the nanoscale topography and friction response of a hydrogenated amorphous carbon film (a-C:H) was investigated. Electrochemical atomic force microscopy was used to characterise topographical changes to the coating at two oxidative potentials. Corrosion of the coating at 1.5 V (corrosion rate 0.5 nm h−1) resulted in no changes to the nanoscale topography; whereas corrosion at 2.5 V (corrosion rate 26.4 nm h−1) caused the root mean square roughness of the a-C:H film topography to decrease, but the local fine-scale irregularity or ‘jaggedness’ of the surface to increase. X-ray photoelectron spectroscopy revealed that corrosion at both potentials oxidised the a-C:H surface to form alcohol, carbonyl and carboxyl groups. Lateral force microscopy and adhesion force measurements showed that both the friction force and surface adhesion of the coating increased upon corrosion. The outcome was attributed to the surface oxidation that had occurred at both oxidative potentials, resulting in several potential mechanisms including increased attractive intermolecular interactions and capillary forces. The highest friction coefficient was observed for the a-C:H film corroded at 2.5 V, and identified as a consequence of the jagged surface topography promoting an interlocking friction mechanism.
Highlights
Diamond-like carbon (DLC) is a class of thin film material that exists in several forms
For the lower resolution spectra acquired at a pass energy of 300 eV, no correction was performed with regard to the small variations that could arise from the differences in probing depth at the characteristic kinetic energies of the C 1s and Cl 2p region compared to the O 1s region
Where M is the molar mass (12 g molÀ1), jcorr is the corrosion current density (A cmÀ2), r is the material density (1.9 g cmÀ3), z is the number of electrons that participate in the chemical reaction (4 eÀ, vide infra), F is the Faraday constant (C molÀ1) and K is a conversion factor (3.6 Â 1010) to nm hÀ1
Summary
Diamond-like carbon (DLC) is a class of thin film material that exists in several forms. Depending upon the synthesis method, DLC will contain different fractions of carbon-carbon sp, carbon-carbon sp, and carbon-hydrogen bonding; with its two major variants classified as tetrahedral hydrogen-free amorphous carbon (ta-C) and hydrogenated amorphous carbon (a-C:H) [1,2] This chemical and structural versatility makes DLC a highly sought after material as a protective coating, owing to its many desirable properties including extreme mechanical hardness, excellent wear resistance, low friction coefficient, and high thermal and chemical stability [3,4]. The influence of corrosion on the nanoscale topography of a hydrogenated amorphous carbon film (a-C:H) and its subsequent influence upon the nanoscale friction response is as yet unexplored These findings are likely to be of interest to the nanotechnology sector, in particular with regard to MEMS/NEMS applications. The a-C:H film topographical changes are characterised statistically, and ex situ XPS and lateral force microscopy are used to examine the effect of corrosion on a-C:H film surface chemistry and friction at the nanoscale
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