Tribo-surface charge and polar lubricant molecules on friction and lubrication under multiple 3D asperity contacts H.T. Zhu a,n , X. Zheng a,b , P.B. Kosasih a , A.K. Tieu a a School of Mechanical Materials and Mechatronic Engineering, University of Wollongong, Northfield Avenue, Wollongong, NSW 2522, Australia b Research Institute of Zhejiang University – Taizhou, Shifu Avenue, Taizhou, Zhejiang, 318000, China article info Article history: Received 15 September 2014 Received in revised form 22 January 2015 Accepted 2 February 2015 Keywords: Boundary lubrication Asperity contact Polyethylene oxide Molecular dynamics simulation abstract Lubricant is usually a mixture of polar and non-polar molecules, and tribo-metal surfaces can be oxidised and have surface atoms with charges. A molecular dynamics simulation of bcc iron with additional charge as rough tribo-surface and polarisable PEO polymer as lubricant was proposed to investigate the tribo-surface charge and polar molecules on friction and lubrication during the compression and sliding. Tribo-surface roughness is crucial for asperity interaction because it affects real contact area, friction, wear and lubrication. Random Midpoint Displacement algorithm was introduced to generate 3D multiple asperities with irregular shape at upper and lower wall surfaces. The results show that compared to non-charged tribo-surfaces where the amount of lubricant and surface roughness determine the friction and lubrication, the charged tribo-surface attracts the polar PEO molecules to form a coated like layer; thereby resulting in a different tribology behaviour. The polar PEO polymers separate the positive charged sliding tribo-surfaces and reduce the direct asperity contact; hence significantly lowering the friction force. & 2015 Elsevier B.V. All rights reserved. 1. Introduction In the boundary/mixed lubrications, two tribo-surfaces interact through an ultra-thin lubricant layer and asperity contact which results in elastic–plastic deformation and possible fracture of the asperities. However, the macroscopic laws of friction break down in nano-scale contact [1]. The influence of tribo-surface morphology on friction and wear during surface compression and sliding has attracted the significant attentions in the research of nano-tribology. Atomic tribo-surface roughness has been found to have a dramatic effect on the real contact area and contact stress [2–4]. By adopting a single- asperity model with the presence of self-assembly stearic acid layer on the iron surfaces, Eder et al. [5] proposed a smooth particle approach to define and calculate the asperity contact area and lubricant cavity volume. Zheng et al. [6] demonstrated that long chain-length alkanes significantly reduce the friction and wear between rough surfaces. However, Zheng's model only considered a regular sinusoidal asperity existed on tribo-surfaces. Molecular dynamics (MD) simulations of multi-asperity contact have been carried out by Spijker et al. [7,8] with a main conclusion that the normal load, surface roughness, and adhesion force collectively determine the contact area. However, the lubrication was ignored in Spijker's MD modelling. With the con- sideration of 3D rough tribo-surfaces lubricated by n-alkanes, Zheng et al. [9,10] recently proposed a MD model to investigate nano-contact mechanism of boundary/mixed film lubrication. An increase in friction force has been found for the partially-lubricated condition due to the combined contributions of asperity contact and lubricant flow resis- tance to sliding. But, the electrostatic influences on the tribo-surfaces and polar charges within the lubricant molecules were not considered. Real metal tribo-walls usually is oxidised and have surface atoms with charges, e.g. Fe 2 O 3 . Berro et al. [11,12] investigated tribological performance of lubricant mixture containing hexadecane and zinc dithiophospate (ZDDP) confined by smooth Fe 2 O 3 surfaces. They found that the charge on iron and oxygen have a significant effect on the rheology behaviour of polar lubricant molecules. As the flat walls were introduced, the asperity contact and boundary lubrication mechanism were not measured in Berr's model. A new class of fluid, Ionic Liquids (ILs), is emerging in tribology research, which is being increasingly studied due to their potential to enable a step-change in lubricant design as either base lubricant or additives [13]. Metal surface in aqueous solution will acquire a charge due to either chemical dissociation of surface groups, pre- ferential physicochemical adsorption of electrolyte ions, or, preferential desorption or adsorption of ions [14]. For common metal oxides, H þ is the potential-determining ion. The charged surface is a major deter- minant of the adhesion of surfaces, the transport properties of slurries, the self-assembly of amphiphiles and many other phenomena. Polyethylene oxide polymer, known as PEO, is widely used in manufacturing due to its high wetting and spreading ability. It can form a uniform coating on contact surfaces which results in low Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/wear Wear http://dx.doi.org/10.1016/j.wear.2015.02.045 0043-1648/& 2015 Elsevier B.V. All rights reserved. n Corresponding author. Tel: þ61 242214549. E-mail address: hongtao@uow.edu.au (H.T. Zhu). Wear 332-333 (2015) 1248–1255