Abstract
Low shear strength (30 MPa) organic films were grown in situ on Pt0.9Au0.1 surfaces via cyclic sliding contact in dry N2 with trace concentrations of ambient hydrocarbons. We present a systematic investigation of the stress- and time-dependent film formation. Steady-state friction coefficients were found to be as low as µ ~ 0.015 and inversely proportional to contact pressure, revealing non-Amontonian behavior. Above a Hertzian contact pressure of ~500 MPa, shear strength dropped, indicating an activated process. Raman spectroscopy identified non-uniformity in areal coverage and relative order with contact pressure. Regions of steady-state low-friction behavior exhibited spectra similar to DLC coatings. Atomic force microscopy was used to study the formation and growth of films at the nanoscale. Stress- and time-dependent measurements suggested a sublinear increase of film volume with time, and a transition from growth to wear at a Hertzian contact pressure of ~1.2 GPa.
Highlights
Investigations of organic film formation on catalytic metal surfaces began as early as 1958 with Hermance and Egan’s work on mated palladium sliding contacts in vapor or liquid hydrocarbon environments.[1]
In dry N2 sliding conditions, we have shown that carbonaceous tribofilms readily form on Pt-Au thin films, resulting in steady-state friction coefficients as low as l $ 0.016 at a maximum Hertzian contact pressure of about 1 GPa
Raman spectroscopy demonstrated that an increase in contact pressure led to an increase in coverage, concentration, and disorder of the tribofilm
Summary
Investigations of organic film formation on catalytic metal surfaces began as early as 1958 with Hermance and Egan’s work on mated palladium sliding contacts in vapor or liquid hydrocarbon environments.[1]. Subsequent characterization showed that the film was an amorphous, high molecular weight material, that they referred to as a ‘‘frictional polymer.’’ studies of contact force-dependent and speed-dependent formation were carried out, the mechanisms of formation were not clearly defined. Chaikin’s work on frictional polymer formation attempted to better understand film formation though the lens of catalysis and charged particle emission.[6] this study showed that electrostatic interactions played a role, it was noted that catalyst inhibitors were inconsequential in the production of frictional polymers. There is consensus, that the mechanisms for forming a frictional polymer in situ require at least three conditions: the presence of organics, a catalytically-active substrate, and shear between surfaces (i.e., sliding contacts).[4]
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