On the Transition from Static to Dynamic Boundary Friction of Lubricated PEEK for a Spreading Adhesive Contact by Macroscopic Oscillatory Tribometry

Sherif Ahmed Abdel-Wahed,Raimund Jaeger,Christof Koplin,Matthias Scherge

doi:10.3390/lubricants5030021

Sherif Ahmed Abdel-Wahed, Raimund Jaeger + Show 2 more

Open Access

https://doi.org/10.3390/lubricants5030021

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Abstract

The tribology of lubricated poly(ether ether ketone) (PEEK)—steel tribosystems was investigated in the static and boundary friction regime. Pentaerythrite ester and trimellitic acid ester were used as lubricants. The lubricants differed in their molecular structure but showed only minor differences in their rheological and cohesive energetic properties. In order to investigate the effect of the lubricants on static and dynamic friction, macroscopic oscillatory tribometry experiments and gliding experiments were carried out. The surface and interfacial energies of PEEK, lubricant, and steel, which can be used to determine e.g. the spreading tendency of the lubricant, were employed to explain the tribological characteristics of the system. The gliding experiments exhibited a velocity dependence of the coefficient of friction which indicates a “rate-and-state” type of friction. Trimellitic acid ester resulted in the lowest static and dynamic friction values due to its high tendency to spread. A different static friction behavior was found for pentaerythrite ester, which indicates a possible lubricant-induced mobilization of the polymer surface chains. This finding was supported by an increased interaction energy measured by contact angle measurements. The macroscopic oscillatory tribometry was shown to be a unique and precise method to investigate the transition from static to dynamic friction and to quantify macroscopic adhesive friction. Both types of experiments used a polished steel surface as a frictional partner, which favors the adhesive component of friction. However, the results for the lubricated contacts of polished steel surfaces in the boundary lubrication regime can give some insight into effects of the polymer transfer to rough steel surfaces in the mixed lubrication regime.

Highlights

Thermoplastic polymers are increasingly used to replace other materials in tribologically loaded components, e.g., in the automotive industry or in mechanical engineering
The results for the lubricated contacts of polished steel surfaces in the boundary lubrication regime can give some insight into effects of the polymer transfer to rough steel surfaces in the mixed lubrication regime
The energetic point of view neglects the effect of the normal force, The overall velocity dependence of the coefficient of friction (COF) confirms a “rate-and-state” friction behavior in the boundary friction regime for all contact conditions

Summary

Introduction

Thermoplastic polymers are increasingly used to replace other materials in tribologically loaded components, e.g., in the automotive industry or in mechanical engineering. This development is largely driven by the need to reduce weight in order to obtain more energy efficient products. A profound understanding of the friction and wear mechanisms is a prerequisite for developing reliable tribological systems. While the interaction between polymer and steel already has been studied in detail for non-lubricated tribological systems, only limited information is available on lubricated tribological systems. Polymer transfer [1] or “temperature hot spots” of the polymer during frictional loading in lubricated systems indicate that the interaction between polymer and its frictional partner for lubricated contacts is complex and includes adhesive interactions. In tribological tests in Lubricants 2017, 5, 21; doi:10.3390/lubricants5030021 www.mdpi.com/journal/lubricants

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