$n$-Octadecylphosphonic acid functionalized tool surfaces for metal-forming applications

Prünte, Stephan; Schneider, Jochen M. (Thesis advisor); Jansson, Ulf (Thesis advisor)

Aachen (2020)
Book, Dissertation / PhD Thesis

In: Materials chemistry dissertation 34 (2020)
Page(s)/Article-Nr.: 1 Online-Ressource (XII, 91 Seiten) : Illustrationen, Diagramme

Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2020


This thesis critically appraises the suitability of employing functionalized tool surfaces in liquid lubricant-free aluminum forming applications. Functionalizing metal-oxide surfaces with n-octadecylphosphonic acids results in single layers of distal n-octadecyl moieties chemisorbed by P-O-metal bonds. Initially, it was demonstrated that functionalizing tool steel die surfaces with n-octadecylphosphonic acid decreases friction during Al cold forming. Specifically, functionalization leads to a 1.9-fold decrease in time-averaged torque during tribological compression-torsion wear tests utilizing functionalized AISI H11 steel tools. X-ray photoelectron spectroscopy suggests that weak van der Waals interaction between aluminum and the distal alkyl-termini of the phosphonic acid molecules anchored to the AISI H11 steel surface via P‑O-metal bonds lubricate the aluminum-steel interface. While the adhesive volume of aluminum on the AISI H11 tool steel was significantly reduced by the surface functionalization, minor improvements due to surface functionalization of AISI D2 tool steel were identified. Investigations of microstructure and surface chemistry by Auger electron spectroscopy implied that the molecular coverage of the surface functionalization was significantly lower on the chromium carbide precipitations of the AISI D2 tool steel compared to the AISI H11 steel, where higher molecular coverages were observed by Auger electron spectroscopy. Based on these results, the initial surface functionalization process was modified: Polished steel tools were first coated with a thin copper-film intentionally oxidizing providing a homogenous as well as reactive surface for the n-octadecylphosphonic acid reactant. Subsequently, the molecular coverage was varied by increasing the evaporation time of the n-octadecylphosphonic acid during functionalization of cooper-coated steel surfaces. The sliding wear behavior of these surfaces was evaluated against aluminum in ball-on-disk tribometer experiments at contact pressures of 250 MPa. After 5 m of sliding, the friction coefficient of the functionalized sample with maximum molecular coverage was ≤ 0.3 ± 0.1 compared to the 3-4-fold higher friction on non-functionalized surfaces. Surfaces with lower coverage mitigated friction and wear as well exhibiting initially similar low friction coefficients but revealed breakdown of lubrication for sliding distances < 5 m. The length of the low friction sliding distance before breakdown scaled with the molecular coverage of n-octadecylphosphonic acids on the copper-coated steel. Applying the best-performing functionalization cycle on copper-coated Si-wafers 2 and 4 times resulted in further increase of the molecular coverage but in a variation of the density of anchoring bonds as well: While both, friction and wear, are reduced compared to non-functionalized copper-surfaces in reciprocal sliding experiments at contact pressures in the range of 140 – 280 MPa against aluminum, no significant differences in terms of load-carrying capacities could be established. Minor surface damage and transfer films observed on tribopairs with constantly low friction indicate the necessity to renew the surface functionalization on forming tools after processing a small number of aluminum-workpieces. Hence, lubricating the open tribosystem of aluminum forming processes solely by a surface functionalization has been shown to be unrealistic. Conclusively, this work contributes to the understanding of the tribological behavior of functionalized surfaces lubricating aluminum contacts. As the failure of the molecular thin functionalizations most likely emerged from asperity contacts, a future protective surface design should include a thicker coating (> 100 nm) beneath the functionalization to lubricate asperity contacts by the > 100 nm coating.