Comparative analysis of the lubrication performance of functionalized copolymers interacting with silicon, cobalt, and silver doped diamond-like carbon

TL;DR

This study investigates how doping diamond-like carbon coatings with silicon, cobalt, or silver affects their lubrication performance with a specific polymer, revealing that silicon doping significantly improves friction and wear due to stronger chemical interactions.

Contribution

It provides a comparative analysis of dopant effects on DLC coatings' tribological behavior and combines experimental and first-principles calculations to elucidate the role of chemical bonding in boundary lubrication.

Findings

Si-DLC exhibits lowest friction coefficient (~0.045) and 45% lower wear.

Strong chemical bonding between dopants and polymer fragments influences tribofilm formation.

Adsorption strength correlates with tribological performance, with Si > Co > Ag.

Abstract

This study examines the tribological behavior of diamond-like carbon (DLC) coatings doped with silicon (Si), cobalt (Co), or silver (Ag) in the presence of an amine-functionalized block copolymer lubricant. Under boundary lubrication, Si-doped DLC (Si-DLC) exhibited the lowest coefficient of friction ($\approx$0.045) and nearly 45% lower wear than undoped DLC. Co-DLC showed moderate improvement, while Ag-DLC provided no significant benefit. Cross-sectional FIB-TEM revealed thin tribofilms, 12-17 nm in thickness, on Si- and Co-doped surfaces. As reported for Si-DLC, these films incorporate copolymer-derived fragments, suggesting a similar composition for Co-DLC. These results indicate that dopant-polymer interactions are key to the development of self-organized boundary layers. To gain atomic-level insight, first-principles calculations were carried out on the adsorption of the dimethylaminoethyl methacrylate (DMAEMA) unit, the copolymer's functional group. The calculated adsorption energies were $-$2.27 to $-$0.57 eV for Si-DLC, $-$1.73 to $-$1.49 eV for Co(0001), and $-$1.21 to $-$1.08 eV for Ag(111). The order of stability (Si $>$ Co $>$ Ag) was consistent with the experimental tribological ranking. Chemical bonding dominated for Si-DLC, while Ag showed mainly weak physisorption. Simulated pull-off forces further reflected this hierarchy, with N-Si bonds requiring about twice the force of N-Co and nearly five times that of N-Ag. The correspondence between adsorption strength and tribological response highlights the decisive role of dopant species in tribofilm formation. These findings provide guidance for designing durable low-friction surfaces in applications such as electric drivetrains and precision mechanical systems.