Contents lists available at ScienceDirect Tribology International journal homepage: www.elsevier.com/locate/triboint Nano-tribological characterisation of palm oil-based trimethylolpropane ester for application as boundary lubricant S.H. Hamdan a,b , W.W.F. Chong c,d,* , J.-H. Ng b,c,e , C.T. Chong c,d , H. Zhang f a Faculty of Mechanical and Manufacturing, University Kuala Lumpur Malaysia France Institute, Bangi, Selangor, Malaysia b Faculty of Engineering and the Environment, University of Southampton Malaysia Campus, Iskandar Puteri, Johor, Malaysia c UTM Centre for Low Carbon Transport in Cooperation with Imperial College London, Universiti Teknologi Malaysia (UTM), Johor, Malaysia d Faculty of Mechanical Engineering, Universiti Teknologi Malaysia (UTM), Johor, Malaysia e Energy Technology Research Group, University of Southampton, Southampton, UK f Department of Complex System Science, Graduate School of Informatics, Nagoya University, Nagoya, Japan ARTICLE INFO Keywords: Nano-scale friction Boundary lubrication Biodegradable lubricant Palm oil-based trimethylolpropane ester ABSTRACT To isolate shearing of boundary film from direct surface-to-surface asperity interactions, the study determines boundary lubrication properties of palm oil-based TMPE (PTMPE) using Lateral Force Microscopy coupled with fluid imaging. PTMPE is produced through a series of 3-step transesterification processes, converting palm oil (PO) into palm methyl ester (PME) and finally into PTMPE. It is shown that PME generates the lowest friction. However, using Eyring thermal activation energy approach, PME is shown to possess less desirable load bearing property, portraying a form of stiction or adhesive nature. Even though friction is higher, PTMPE exhibits better load bearing ability, demonstrating the onset of lubricant laminar flow due to increased hydrodynamic effect, which is not observed for the PO and PME measurements. 1. Introduction Approximately 35.7% of the total fuel energy supplied to a typical passenger car is used to overcome friction [1]. From these amount of losses, engine friction, mainly generated by piston rings sliding along engine cylinder liner, contributes 45%. If new technological advance- ments in the field of tribology are being introduced to typical passenger cars, these frictional losses could be reduced by at least 18% [2]. Using a Stribeck curve given in Fig. 1, it is realised that the operating lu- brication regime for piston ring lubrication system is typically between mixed and hydrodynamic lubrication regimes. Along hydrodynamic lubrication regime, friction is governed by viscous shearing, where lu- bricant bulk properties play an important role in affecting lubrication performance. However, the underlying mechanism for friction along mixed lubrication regime is a combination of viscous shearing and surface asperity interaction. Reduction of friction arising from the latter mechanism requires boundary lubrication, where an ultra-thin layer of protective film is typically formed through adsorption of boundary active elements onto opposing surfaces. In view of the various operating lubrication regimes for piston ring- liner conjunction, it is only imperative that an effective lubricant con- sists of various additives, blended with base oils to attain specific tri- bological performance-improving characteristics. These additives include extreme-pressure and anti-wear agents, friction modifiers and viscosity index improvers. They are typically added to engine lubricants up to 30-vol% [3]. If the lubricant is properly formulated and opti- mized, reduction in frictional losses could be achieved, leading to sig- nificant fuel economy improvements [4]. Statistically, the global demand for lubricants are estimated to be around 39 million tonnes in the year 2017, with around 24 million tonnes coming from the automotive sector [5]. This is in-line with the need for the automotive sector in reducing frictional losses, especially for passenger cars. However, 50% of these lubricants are expected to end up in the environment, where 1 L of mineral oil could contaminate up to 1 million litres of water [6]. On top of this, a total of 193 kilo tons of additives, such as anti-wear additive (100 kilo tonnes) and friction modifier (93 kilo tonnes) are added to such an amount of lubricant [7]. Most of these additives are synthetic base. It is brought to light that these additives could also potentially harm the environment if their uses are not properly regulated. A recent study by Pirjola et al. mea- sured particle emissions from modern turbocharged gasoline direct in- jection passenger car running on different engine lubricants. They found that highest emission factors originated from using lubricants with higher concentrations of additives containing zinc, magnesium, phosphorous and sulphur [8]. An additive example with relation to this observation is zinc dialkyldithiophosphate (ZDDP), which is commonly https://doi.org/10.1016/j.triboint.2018.05.036 Received 28 February 2018; Received in revised form 9 May 2018; Accepted 27 May 2018 * Corresponding author. UTM Centre for Low Carbon Transport in Cooperation with Imperial College London, Universiti Teknologi Malaysia (UTM), Johor, Malaysia. E-mail address: william@mail.fkm.utm.my (W.W.F. Chong). Tribology International 127 (2018) 1–9 Available online 30 May 2018 0301-679X/ © 2018 Elsevier Ltd. All rights reserved. T