Tribological behavior of biolubricant base stocks and additives

Renewable and Sustainable Energy Reviews 93 (2018) 145–157

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Tribological behavior of biolubricant base stocks and additives ⁎

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T

Chung-Hung Chan , Sook Wah Tang, Noor Khairin Mohd, Wen Huei Lim , Shoot Kian Yeong, Zainab Idris Advanced Oleochemical Technology Division, Malaysian Palm Oil Board, 43000 Kajang, Selangor, Malaysia

A R T I C LE I N FO

A B S T R A C T

Keywords: Vegetable oil Biolubricant Additives Tribology Friction coefficient Wear

Biolubricants are gaining popularity and acceptance globally due to their sustainable and environmentally friendly properties; being derived from feedstocks from vegetable oils. Indeed, the potential for biolubricants to eventually replace conventional lubricants is currently viewed in the literature as a real possibility. This study will provide valuable information pertaining to the formulation of biolubricants, by assessing and evidencing the tribological performances of various types of biolubricant base stocks and their related additives. This study begins with a presentation of the basic tribological parameters in lubrication. Following that, the criteria for the molecular structure of biolubricant base stocks for high tribological performance are discussed, based both on the tabulation of the friction coefficient and on the measurement of wear scar from experimental studies. The biolubricant base stocks under review in this study include vegetable oils (VO), epoxidized VO, ring-opened products from epoxidized VO, estolides, and polyol esters. This review also discusses recent advances in ecofriendly tribological additives such as plant-derived compounds and polymers, particulate and layered materials, and ionic liquids. The performance and various applications of these additives are also reviewed.

1. Introduction Biolubricants, or bio-based lubricants, are generally perceived as environmentally friendly, biodegradable and non-toxic lubricants which are derived from renewable resources such as plant oils and animal fats. Most biolubricant products currently on the market are made up either entirely or partially from bio-based oils, providing these oils fulfill the requirements of international standards in terms of renewability, biodegradability, toxicity and technical performance. According to European standards [1,2], a certified biolubricant must have a bio-based carbon content of at least 25% and biodegradability of at least 60%. The biolubricant must also be non-toxic to the environment and fit for the purpose of an application. The market demand for biolubricants is driven by various factors, which include: consumer environmental awareness, government directives and the global demand for lubricants. A long-running and increasingly urgent issue is the environmental cost of hazardous and non-degradable lubricants entering the ecosystem through total loss applications, spillages and so on [3]. It is, in fact, well-attested that about half of the total production of lubricants in the world accumulates in the environment every year [3,4]. Continuing efforts have been made to fight this crisis through various government directives and environmental legislation. For instance, the European Union (EU) Renewable Energy Directives 2009/

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28/EC were mandated to promote the use of energy from renewable sources in various sectors including fuels and lubricants [5]. At a consumer level, environmental awareness is being raised by the awarding of environmental labels such as Blue Angel, Nordic Swan and European Eco-label for lubricant products [3], with the aim of highlighting the consequences of consumer purchasing and hiring decisions for the planet. In addition to the above initiatives, the market growth of biolubricants is further substantiated by the increase in global lubricant demand due to the specific development of end-user industries in China, India, South Africa, Brazil and Iran [6,7]. In the current scenario, the global market for lubricants is growing at approximately 2% annually; amounting to approximately 144.45 billion USD in 2015 [7]. Indeed, the global biolubricant market has always been rising steadily at 10% annually, in spite of it constituting only about 1% of the total market in lubricants [8,9]. From this perspective, the shift towards biobased lubricants represents a global trend in the global lubricant market; as part of the global effort to mitigate the environmental impact of hazardous and non-degradable conventional lubricants. The successful formulation of any lubricant product is built upon a multi-objective optimization of the types and concentrations of base stocks and additives, to meet specific application specifications and requirements [10]. On average, a typical lubricating oil consists of 93% base stock and 7% additives, though the additive content may vary

Corresponding authors. E-mail addresses: [email protected], [email protected] (C.-H. Chan), [email protected] (W.H. Lim).

https://doi.org/10.1016/j.rser.2018.05.024 Received 17 March 2017; Received in revised form 27 March 2018; Accepted 13 May 2018 1364-0321/ © 2018 Elsevier Ltd. All rights reserved.

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2. Basic tribological parameters

from 1%, as in simple compressor oils, or up to 30%, as in gear oil and metal-working fluids [11,12]. In any lubricant formulation, base stock is viewed as the essential part of the lubricating oil, as base stock possesses the vital lubricant physical properties such as viscosity, thermal stability, lubricity and so forth. Among all base stocks, vegetable oil (VO) is the simplest choice for a biolubricant [13–19]. It has in fact always been the alternative to mineral oil due to its superior lubricity, viscosity index, and wear resistance; not to mention its biodegradability and the sustainability of its feedstock [13,20–22]. Unfortunately, VO is known to have poor thermal and oxidative stability [23–25], but by means of chemical modifications, VO may be converted into better-performing products such as epoxidized VO [26–28], ringopened products from epoxidized VO [23,29–31], estolides [32–35], and polyol esters [36–42]. These base stocks have been seen as promising for use in biolubricant formulations. Further improvement to VO-based lubricant performance is possible through additivation. Additives are able to enhance the physical properties of the lubricant by providing additional features, including anti-corrosion, metal scavenging, and anti-wear to the base stock [12]. The most important group of additives are tribological additives, as far as lubrication is concerned, since the ability to reduce wear and damage to machines is ultimately the goal. Indeed, the aim of any new formulation is to achieve better performance, higher energy efficiency, and a longer life cycle for the machinery, as well as longer intervals for the replacement of the lubricating oil [43]. It should be mentioned here, of course, that not all conventionally-used additives are compatible with biolubricant base stock [12], but tribological additives for biolubricants have so far been advanced in the field with favorable regard to their lubricity, wear protection and load-carrying capacity. The additives from this group which are currently enjoying most research focus are those containing sulfur and phosphorus [44–47], environmental-friendly polymers [15,48–50], plant-derived compounds [35,51], particulates [52–56], two-dimensional (2D) layered materials [57–60], and ionic liquids [61–64]. In practice, the varying performance of different lubrications in terms of friction and wear is usually assessed based on the coefficient of friction (COF) and wear scar diameter (WSD), or volume (WSV), respectively [13,35,65,66]. COF denotes the ratio of friction force between two bodies and the normal force pressing the bodies together [67], whereas WSD and WSV are respectively the diameter and volume of wear after sliding activity. These measurements are often used in sub-scale machine or laboratory tests to investigate the tribological behavior of lubricant products in the workings of real machinery. When evaluating and comparing the tribological behavior of different lubrications, it is important to specify the type of lubricant, the tribosystem, and the lubrication conditions to minimize the discrepancy between the laboratory test and the field performance [65,67]. In the context of biolubricants, most of the literature simply discusses overviews, developments, synthesis and applications of biolubricants in a very general context [24,28,68–76], or in specific applications [77–87], while other studies focus only on the development and performance of a specific type of additive [88–95]. In fact, a thorough search of the relevant research yielded no reviews which provided any adequate detail about aspects of tribological behaviors. This review, therefore, seeks to address this gap in the literature by providing comprehensive evidence and valuable discussion pertaining to the tribological performances of recent biolubricant base stocks, and related additives. This study focuses on their influencing parameters, molecular structure criteria, synergisms, and mechanisms; concerning lubricity, wear protection, and load-carrying capacity. In addition, a detailed tabulation of friction and wear responses of various biolubricants is presented for comparison purposes. The aim is for interested parties to be able to utilize the tribological information presented in this review as a guide to selecting and formulating biolubricants for their own applications.

The most important consideration for selecting a suitable lubricant is that it fulfills the lubrication requirements of the particular application, especially in its tribological aspects. Tribological performance in lubrication concerns lubricity, friction and wear. Generally speaking, lubricity is associated with the formation of the lubrication layer, or tribofilm, on sliding surfaces. High lubricity reduces the direct contacts of the surface asperities and thereby lowers friction and energy loss [11,96]. High lubricity is not always accompanied by better wear protection, since the mechanism of forming a protective layer on the sliding surfaces occurs through the adsorption of the surface active materials of the lubricant, which can be certain types of base stock or additives [97]. Whenever there is surface asperity contact during the sliding activity, there is the possibility that three types of mechanical wear are produced: abrasion, adhesion, and fatigue [46,97]. Abrasive wear (e.g. grooves) is produced via the removal of a certain volume of surface material by hard asperities on the sliding surface [98]; adhesive wear (e.g. attachment of wear debris) is the result of plastic deformation and strong bonding of the interface materials [98]; fatigue wear (e.g. crack, fracture) is due to the weakening of surface materials after repeatedly contact with high local stress for a large number of times [98]. The obvious solution to the wear problem is to employ thicker tribofilm, though this is not always efficient and desirable in some applications, such as metalworking. Prior to any discussion of tribological performance, it is essential to understand what are known as the three basic tribological parameters: the mechanical properties of a tribosystem; the lubrication regime and its conditions; and also the physical properties and tribochemistry of the lubricant as illustrated in Fig. 1. The details of each parameter are elucidated as follows. 2.1. Mechanical properties of a tribosystem The mechanical characteristics of a tribosystem can essentially be described as the material hardness, the surface roughness, the contact geometry, and the sliding mechanism of the interacting part and its counterpart, as presented in Fig. 1(A). Limiting the discussion to the laboratory tribometer, most sliding materials have a 30–64 Rockwell C Hardness (HRC) and a 0.01–1.0 µm surface roughness, depending on the following: the type of material (e.g. steel, magnesium, copper), the hardening process (e.g. nitriding, carburizing), and the coating (e.g. carbon-based coating, ultra-high molecular weight polyethylene) [15–17,26,37,66,99]. Basically, a higher hardness value provides a lower wear rate, while a lower roughness value provides better lubricity [100]. Comparing tribological results across different types of tribometers is not possible due to varying sliding geometries (e.g. four-ball, pin-on-disk, ball-on-disk) and the particular sliding mechanism (e.g. unidirectional or reciprocating, sliding to rolling ratio) [65,67]. For a valid comparison, the measurements should be conducted using a similar type of machine and method (e.g. ASTM D5183, ASTM D4172, and ASTM D6079). Even if all of these conditions are closely matched, researchers are required to reproduce the measurements in their instruments to ensure accuracy. 2.2. Lubrication regimes and conditions The operating conditions of any tribosystem play a significant role in characterizing tribofilm and, subsequently, tribological behavior. According to the famous Stribeck curve [67,101] there are three lubrication regimes, namely: boundary; mixed and elastohydrodynamic (EHL); and hydrodynamic lubrication regimes, as illustrated in Fig. 1(B). Boundary lubrication usually occurs at high contact load or low sliding speed lubrication, whereby thin or unstable tribofilm is formed, causing the contacting surfaces to rub against each other and generate high friction and wear [102]. As the film thickness in this regime is less than the surface roughness [102], the presence of surface 146

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Fig. 1. Three basic parameters affecting friction and wear behaviors of a lubricated tribosystem.

ultimately of tribological performance. For as more viscous lubricant creates thicker tribofilm but gives lower efficiency due to higher flow resistance [3], the selection of lubricant viscosity for an application is a trade-off between lubrication and energy performance. In cases where elevated temperature and high contact load are involved, a high viscosity index and high pressure-viscosity coefficient are preferred in lubricants to ensure stable tribofilm [3]. Other properties that affect the strength and the stability of tribofilm include: pour point, volatility, thermal degradation temperature, and oxidative stability [3,98]. On the other hand, where the tribochemical properties of a lubricant are concerned; lubricity, wear protection, and load carrying capacity are exhibited by so-called surface active materials. These surface active materials mainly act as a friction modifier (FM), an anti-wear additive (AW), and an extreme pressure additive (EP). More generally, FM promotes softer and easily sheared protection layers which reduce sliding friction [20,103], whereas AW and EP actually form a sacrificial protective layer on the sliding surface to minimize wear [45,46,103]. The typical COF of an FM layer (0.01–0.05) is generally lower than that of EP and AW layers (0.06–0.15), but higher if compared to tribofilm in a hydrodynamic regime (< 0.001) [103]. The differences between AW and EP are that the latter has a higher reactivity and its protective layer is tougher and thicker, allowing for a higher load tolerance [103]. The above additives are mostly employed to minimize wear and enhance lubricity for boundary lubrication and mixed/EHL lubrication regimes.

active materials is vital in order to protect the contacting surfaces from sliding damage. Further increase in the sliding speed or decrease in the contact load shifts the lubrication to a mixed/EHL regime, and then to a hydrodynamic lubrication regime. Along with this transition, the associated friction and wear rate decrease since the contacting surfaces are now separated by a thicker tribofilm due to hydrodynamic lift [102]. The film thickness is about 1–3 times that of the surface roughness under mixed/EHL regime, and 3 times greater than the surface roughness when under a hydrodynamic lubrication regime [102]. In the latter regime, the sliding contacts are now fully lubricated and the associated friction is dominated by the viscous drag and resistance of the lubricant. In this manner, there is no significant mechanical wear on the moving parts except fatigue [98]. According to rough tabulations [97], a typical boundary lubrication has a coefficient of friction (COF) value greater than 0.1, whereas mixed/EHL lubrication produces a COF in the range of 0.01–0.10, and hydrodynamic lubrication gives COF less than 0.01. Note that the sliding speed and contact load to achieve a certain lubrication regime vary according to both the tribosystem and the physical properties of the lubricant; especially the physical property of viscosity. Other influential operating parameters such as lubrication temperature and assessment time also need to be specified. In most tribological assessments, the lubrication operates within a boundary or mixed/EHL regimes, at an operating temperature which closely matches the required equilibrium temperature of an application. The friction and wear measurements are taken after reaching the steady state, which is about an hour of operation, as adopted in most standard practices.

3. Molecular structure criteria of biolubricant base stocks The physical and tribochemical properties of biolubricant base stocks depend very much on the molecular structure of their constituent compounds. In most biolubricant base stocks, there are polar and nonpolar regions in the molecules [12]. The polar region is responsible for adsorption or adhesion on a sliding surface, whereas the non-polar region is responsible for the strength and oiliness of the lubricating fluid [3]. Some examples of the molecular structure of the base stocks can be found in Fig. 2. The simplest biolubricant base stock of all is vegetable

2.3. Physical and tribochemical properties of lubricant The physical properties and the tribochemical properties of any lubricant are the most crucial parameters for tribological performance, as depicted in Fig. 1(C). In view of the physical properties of base stock, viscosity and its relationship with temperature and pressure is the primary property responsible for the thickness of tribofilm, and 147

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Fig. 2. Molecular structures of biolubricant base stocks and their basic property-structure relationships. Molecular structure: (1) triglyceride, (2) oleic acid, (3) ricinoleic acid, (4) epoxidized oleic acid, (5) epoxidized ricinoleic acid, (6) oleic estolide, (7) 9-hydroxy-10-acyloxyoctadecanoic acid, (8) octyl 9-hydroxy-10acyloxyoctadecanoate, (9) octyl 9-(lauroyloxy)-10-(acyloxy)octadecanoate, (10) 10,12-dihydroxy-9-acyloxystearic acid, (11) octyl 10,12-dihydroxy-9-acyloxystearate, (12) trimethylolpropane (TMP) ester, (13) pentaerythritol (PE) ester, (14) neopentyl glycol (NPG) ester.

products from epoxidized VO, estolides, and polyol esters [69]. Epoxidized VO is actually produced by forming a highly reactive oxirane ring at the unsaturation site of the fatty acid chain [105]. Subsequently, the oxirane ring reacts with suitable organic acid or aliphatic alcohols to produce poly-functional molecules [105,106]. Estolide, on the other hand, is produced through homopolymerization of fatty acids at its unsaturation sites by ester linkage. It is characterized by the degree of polymerization or estolide number [32,107]. Lastly, polyol esters can be produced from VO methyl ester or fatty acid through transesterification or esterification with polyols, including neopentyl glycol (NPG), trimethylolpropane (TMP) and pentaerythritol (PE) [108–111].

oil (VO). VO is triglyceride consisting of three straight chained fatty acids attached to a glycerol backbone by ester linkage. Typically, the fatty acid chains in VO triglyceride have a 14–24 carbon atoms chain length, with varying degrees of unsaturation [24], although coconut oil has mostly 12 carbon atoms in its fatty acid chain [104] and castor oil has an additional hydroxyl group (-OH) in the fatty acid chain (i.e. ricinoleic acid) [33]. Due to the intrinsic instability of the β-hydrogen atom in the glycerol backbone coupled with the unsaturation site at the fatty acid chains in VO, it is accepted that chemical modifications on such sites are possible with the aim of generating better oxidative and thermal stabilities for products such as, epoxidized VO, ring-opened 148

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[39], where incorporating a longer chain of alkyl (C5 to C18) into the ester actually enhances both its pour point (−75 °C to −42 °C) and viscosity index (80–208). A different length of alkyl chain can tailor the ester to a specific application [39]: Valeric acid-TMP ester (C5) is suitable, as cutting fluids at low-temperature application due to low viscosity; Caprylic acid–TMP ester (C8) is used for high viscosity and stability applications like dielectric coolants and rail wheel lubricants; in addition to TMP-oleate (C18), which is mostly employed for hydraulic fluid. It is important to know that the above relationship between the carbon chain length and the pour point is not applicable for biolubricant molecules that have significant steric hindrance, such as base stocks from ring-opening of epoxidized VO. In this case, the relationship is in reverse: the increase in the chain length creates a larger steric barrier to inhibit the molecule from crystallization [29,120], thus improving its cold flow property [105].

The performances of the aforementioned base stocks can essentially be based on four criteria: degree of unsaturation, carbon chain length, functional group(s) and branching effect [112,113]. A schematic diagram showing the effects of the molecular structure criteria on the physical and tribological properties of biolubricant base stocks is presented in Fig. 2. The details of each criterion will be elucidated as follows. 3.1. Degree of unsaturation Degree of unsaturation refers to the number of unsaturation sites in the carbon chains of a molecule. This criterion is often used to assess the non-polar regions of VO molecules, since the oil contains different compositions of saturated and unsaturated fatty acid chains. A thorough study performed by Reeves et al. [16] found that the higher the degree of unsaturation of VO, the higher were the COF and WSV produced. This is because the unsaturation site disrupts and prevents the carbon chains of VO from forming a densely packed bundle, weakening the stability of the tribofilm [16,17,114]. It is also evidenced that highly unsaturated VO tends to have a higher composition of free fatty acids, which may induce chemical wear on the sliding material due to its reactive carboxyl group (RCOOH) [17,49]. Nevertheless, at extreme pressure lubrication, the reactive carboxyl group of the free fatty acid may have an excellent anti-wear effect [18], as it allows better coupling of the fatty acid with the sliding material to produce low friction carboxylate soap protective film [115]. As a whole, the effect of unsaturation of VO on tribological performance is significant only in metallic-to-metallic contact, since the metallic oxide layer allows chemisorption of VO or free fatty acid [98]. Otherwise, it does not result in any substantial difference, as is reported in the lubrication of ultrahigh-molecular-weight polyethylene (UHMWPE) sliding contacts using sesame and Nigella sativa oils [66]. In spite of the fact that a low degree of unsaturation is preferred in tribological performance, and also for better thermal and oxidative stabilities, it is desirable to keep certain degree of unsaturation, so that the unsaturation sites disrupt the stacking of molecules and crystallization to prevent any hardening of the base stock and subsequently to improve the cold flow property (e.g. a low pour point) [116]. It is therefore agreed that a monounsaturated carbon chain like the oleic acid chain is the best tradeoff in most base stocks, including VO [16,19,29] and other synthetic esters [109,117,118].

3.3. Functional groups The polarity in the polar region of a biolubricant molecule affects its intermolecular interaction and its interaction with sliding surfaces. This polarity influences both the tribological behavior and the physical properties of the biolubricant molecule. The most common functional group in biolubricants is the ester group (RCOOR). The ester group has only a mild polarity, but it is able to provide sufficient adhesive strength to form tribofilm with a sliding surface. It exhibits better antifriction capability [13,17,37,99] and promotes lower operating temperature in gear compared to mineral oils [121–123]. However, the ester group does not have any superior anti-wear effect on the boundary lubrication regime, due to its mild polarity and adhesiveness [13,14]. Another functional group which is commonly found in biolubricant molecules is the hydroxyl group (OH). The hydroxyl group creates a thick and viscous fluid as a result of hydrogen intermolecular bonding. A biolubricant with a hydroxyl group such as castor oil is suitable for applications which operate in a mixed/EHL and hydrodynamic lubrication, such as hydraulic fluid [15,48]. Other than strong intermolecular bonding, the presence of a hydroxyl group favors microcrystal particle solubilization [124], but it compromises the cold flow property and oxidation stability, unless the molecule possesses steric hindrance [29,106]. Among all groups, the strongest polar group available in biolubricant base stocks is the carboxyl group (RCOOH) such as fatty acids and their derivatives [106,120]. The molecule with the carboxyl group has a strong adhesion to metallic surfaces in addition to possessing good anti-wear properties in boundary lubrication [16,20,125,126]. If the oxide layer of a sliding surface is worn off and there is no chemisorption site for adhesion, a chemical attack on the material may occur in which the reactive carboxyl group causes softening and melting of the metallic soap [98]. The above effects of functional groups can be multiplied and intensified with the increase of the number of functional group in the molecule. Taking the ester group as an example, ring-opened products from epoxidized oil that have more ester functional groups provide greater resistance to shear force and better wear protection [119].

3.2. Carbon chain length and molecular weight Carbon chain length in the non-polar region of biolubricant base stock molecules usually amounts to 8–22 carbon atoms chain length. Longer chain length molecules tend to create thicker tribofilm due to their increase in molecular weight and viscosity. Longer chain length also reduces the contact of asperities while increasing the protected surface area. With the increase in molecular weight and viscosity, long chain length base stocks tend to shift the lubrication regime (as in Fig. 1(B)) to the right to give lower COF, WSD and WSV [66,119]. While increasing the chain length of biolubricant molecules is desirable for better tribological performance, it nonetheless affects some of the physical properties of the biolubricant. Usually, a long chain length in biolubricant molecules improves tribological performance and viscosity index, but at the cost of both pour point and oxidative stability [34,39]. These effects are not obvious for VOs, since their average chain length spans only a narrow range, typically in between 16 and 18 carbon atoms [17,104], but the effects are profound and significant in estolides, polyol esters and the ring-opened products from epoxidized fatty acids. For example, estolide capped with shorter chain fatty acids (C2 to C10) gives a better pour point (−12 to −30 °C) [34,107] when it is capped with a long chain fatty acid or when it undergoes a large degree of polymerization and its anti-wear property is enhanced due to viscous tribofilm [35]. Another example can be seen in the case of TMP ester

3.4. Branching and steric hindrance Introducing branching around the polar region of a biolubricant molecule helps to shield the molecule from physical and chemical interactions due to steric hindrance. As mentioned previously, biolubricant molecules with steric hindrance such as ring-opened products from epoxidized oil [105,106] and polyol esters [108] are able to inhibit the stacking of molecules and crystallization. At low temperature conditions, the molecules instead form a microcrystalline structure. This structure allows the molecules to tumble and glide over one another to give fluidity [25]. In addition to that, other known advantages of having a branching structure in the molecule include better thermal, oxidative and hydrolytic stabilities [53,116]. In spite of the favorable 149

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Table 1 Tribological performances of biolubricant base stocks. a

Tribosystem

Lubrication conditions

Base stock, viscosity (40 °C)

COF

[16]

Pin-on-disk test: pin (oxygen-free electronic copper C101, 6.35 mm diameter, 50 mm length, hemispherical tip); disk (aluminum alloy 2024, 70 mm diameter, 6.35 mm thickness, 0.3 µm roughness)

10 N, 36 mm/s, 25 °C

[17]

Four-ball test: ball (AISI E-52100 chrome alloy steel, 12.7 mm diameter, extra polish EP grade 25, 64–66 HRC hardness)

2574 MPa, 1200 rpm, 75 °C

[66]

Pin-on-disk test: pin (stainless steel, 0.06 µm roughness); disk (ultra-high-molecular weight polyethylene UHMWP, 0.31 µm roughness) Four-ball test: ball (Chrome alloy steel, 12.7 mm diameter, 0.1 µm roughness, 62 HRC hardness)

20 N, 30 mm/s, 25 °C

soybean oil, 25.9 cP corn oil, 26.8 cP sesame oil, 28.6 cP canola oil, 29.9 cP peanut oil, 31.9 cP genetically-modified safflower oil, 31.9 cP avocado oil, 31.9 cP olive oil, 32.7 cP coconut oil, 24.8 cSt sunflower oil, 27.8 cSt rice bran oil, 40.6 cSt mineral oil, 105 cSt sesame oil, 58.0 cP b Nigella sativa oil, 64.5 cP b dry or no lubricant olive oil, 37.042 cSt commercial SAE15W40 oil, 103 cSt olive oil, 37.042 cSt commercial SAE15W40 oil, 103 cSt olive oil, 37.042 cSt commercial SAE15W40 oil, 103 cSt olive oil, 37.042 cSt commercial SAE15W40 oil, 103 cSt Jatropha Curcas L. oil, 36.61 cSt hydrotreated rapeseed oil (paraffinic hydrocarbon containing free fatty acid and triglycerides), 4.68 cSt rapeseed methyl ester (biodiesel), 2.83 cSt Jatropha Curcas L. oil, 36.61 cSt hydrotreated rapeseed oil, 4.68 cSt rapeseed methyl ester (biodiesel), 2.83 cSt Jatropha Curcas L. oil, 36.61 cSt hydrotreated rapeseed oil, 4.68 cSt rapeseed methyl ester (biodiesel), 2.83 cSt Jatropha Curcas L. oil, 36.61 cSt hydrotreated rapeseed oil, 4.68 cSt rapeseed methyl ester (biodiesel), 2.83 cSt moringa oil, 44.9 cSt passion fruit oil, 31.8 cSt epoxidized moringa oil, 80.4 cSt epoxidized passion fruit oil, 185.7 cSt commercial hydraulic oil A, 80–100 cSt commercial hydraulic oil B, > 150 cSt epoxidized oleic acid, 0.804 g/cm3 c monoester 9-hydroxy-10-octyloxyoctadecanoic acid, 0.813 g/cm3 c 9-hydroxy-10-nonanoxyoctadecanoic acid, 0.819 g/cm3 c 9-hydroxy-10-lauroxyoctadecanoic acid, 0.822 g/cm3 c 9-hydroxy-10-myristoxyoctadecanoic acid, 0.826 g/cm3 c 9-hydroxy-10-palmitoxyoctadecanoic acid, 0.830 g/cm3 c 9-hydroxy-10-stearoxyoctadecanoic acid, 0.833 g/cm3 c 9-hydroxy-10-behenoxyoctadecanoic acid, 0.836 g/cm3 c diester octyl 9-hydroxy-10-octyloctadecanoate, 0.841 g/cm3 c octyl 9-hydroxy-10-nonanoxyoctadecanoate, 0.843 g/cm3 c octyl 9-hydroxy-10-lauroxyoctadecanoate, 0.846 g/cm3 c octyl 9-hydroxy-10-myristoxyoctadecanoate, 0.851 g/cm3 c octyl 9-hydroxy-10-palmitoxyoctadecanoate, 0.855 g/cm3 c octyl 9-hydroxy-10-stearoxyoctadecanoate, 0.858 g/cm3 c octyl 9-hydroxy-10-behenoxyoctadecanoate, 0.862 g/cm3 c Triester octyl 9-(lauroyloxy)−10-(octyloxy)octadecanoate, 0.905 g/cm3 c octyl 9-(lauroyloxy)−10-(nonanoxy)octadecanoate, 0.913 g/cm3 c octyl 9-(lauroyloxy)−10-(lauroxy)octadecanoate, 0.920 g/cm3 c octyl 9-(lauroyloxy)−10-(myristoxy)octadecanoate, 0.925 g/cm3 c octyl 9-(lauroyloxy)−10-(palmitoxy)octadecanoate, 0.931 g/cm3 c octyl 9-(lauroyloxy)−10-(stearoxy)octadecanoate, 0.939 g/cm3 c octyl 9-(lauroyloxy)−10-(behenoxy)octadecanoate, 0.946 g/cm3 c

0.406 0.446 0.318 0.087 0.098 0.494 0.020 0.078 0.101 0.060 0.073 0.117 0.028 0.032 0.290 0.052 0.080 0.061 0.084 0.073 0.090 0.077 0.097 0.126 0.164

0.384 mm3 0.297 mm3 0.215 mm3 0.201 mm3 0.168 mm3 0.195 mm3 0.104 mm3 0.175 mm3 0.601 mm 0.616 mm 0.585 mm 0.549 mm 0.024 mm3 0.028 mm3 0.051 mm3 0.621 mm 0.717 mm 0.850 mm 1.172 mm 1.254 mm 1.640 mm 1.501 mm 2.095 mm < 0.001 mm3 < 0.001 mm3

0.139 0.109 0.134 0.115 0.129 0.153 0.132 0.106 0.134 0.125 0.084 0.060 0.080 0.068 0.123 0.103 0.58

0.001 mm3 0.001 mm3 < 0.001 mm3 0.002 mm3 0.003 mm3 < 0.001 mm3 0.002 mm3 0.010 mm3 < 0.001 mm3 0.008 mm3 0.15 mm 0.076 mm 0.146 mm 0.131 mm 0.181 mm 0.164 mm 0.89 mm

0.47 0.42 0.38 0.27 0.21 0.18 0.14

0.83 mm 0.75 mm 0.68 mm 0.56 mm 0.48 mm 0.35 mm 0.28 mm

0.46 0.40 0.33 0.23 0.17 0.13 0.09

0.66 mm 0.59 mm 0.53 mm 0.46 mm 0.39 mm 0.34 mm 0.25 mm

0.35 0.31 0.25 0.20 0.14 0.08 0.06

0.61 mm 0.53 mm 0.49 mm 0.42 mm 0.31 mm 0.22 mm 0.14 mm

[18]

load, 1200 rpm, 75 °C load = 400 N load = 500 N load = 600 N load = 800 N

[19]

Ball-on-disk (High frequency reciprocating rig (HFRR)): ball (AISI E52100 steel, 10 mm diameter), disk (X210Cr12 steel)

load, speed (8 mm stroke), 20 °C load = 10 N speed = 10 Hz. load = 10 N speed = 20 Hz load = 20 N speed = 10 Hz load = 20 N speed = 20 Hz

[26]

Ball-on-disk test (HFRR): ball (AISI 52100 steel, 570–750 HV hardness, 6 mm diameter); disk (AISI 52100 steel, 190–210 HV hardness, 10 mm diameter, #1200 polished surface)

2 N, 20 Hz (1 mm stroke), 60 °C

[29]

Four-ball test: ball (AISI E-52100 chrome alloy steel, 12.7 mm diameter, extra polish EP grade 25, 64–66 HRC hardness)

392 N, 1200 rpm, 75 °C

WSD / WCV

a

Ref.

(continued on next page)

150

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Table 1 (continued) a

Ref.

Tribosystem

Lubrication conditions

Base stock, viscosity (40 °C)

COF

[30]

Ball-on-disk test (HFRR): ball (AISI 52100 steel, 570–750 HV hardness, 6 mm diameter); disk (AISI 52100 steel, 190–210 HV, 10 mm diameter, #1200 polished surface)

200 g, 50 Hz (1 mm stroke), 60 °C

low lubricity diesel fuel (LLDF), n/a blend of LLDF and ring opening products of epoxidized canola oil with n-butanol at ratio 95:5 w/w, 251.7 cSt d blend of LLDF and ring opening products of epoxidized canola oil with amyl alcohol at ratio 95:5 w/w, 190.5 cSt d blend of LLDF and ring opening products of epoxidized canola oil with 2-ethylhexanol at ratio 95:5 w/w, 85.5 cSt d oleic acids-estolides with degree of polymerization of 6.33 (O1), 415.59 cSt ricinoleic acids-estolides with degree of polymerization of 6.92 (R1), 6712.98 cSt blend of high-oleic sunflower oil (HOSO) and O1 at ratio: 85:15 w/ w, 51.31 cSt blend of HOSO and R1 at ratio: 85:15 w/w, 69.15 cSt HOSO, 41.94 cSt Lunaria annua oil-based TMP ester, 29.97 cSt mineral oil, 28.10 cSt palm oil-based TMP ester (POTMP), 40.03 cSt ordinary lubricant (OL), 101.86 cSt blend of OL and POTMP at ratio: 99:1 v/v, 108.75 cSt blend of OL and POTMP at ratio: 97:3 v/v, 107.21 cSt blend of OL and POTMP at ratio: 95:5 v/v, 101.42 cSt blend of OL and POTMP at ratio: 93:7 v/v, 102.15 cSt blend of OL and POTMP at ratio: 90:10 v/v, 97.637 cSt palm oil-based TMP ester, 40.03 cSt palm oil-based PE ester, 68.40 cSt mineral oil, 30.61 cSt commercial fully formulated lubricant, 101.86 cSt palm oil-based NPG ester, 27.92 cSt palm oil-based PE ester, 72.78 cSt palm oil-based PE ester with anti-wear and anti-corrosion additives, 75.37 cSt commercial fully formulated lubricant, 131.76 cSt palm oil-based NPG ester, 27.92 cSt palm oil-based PE ester, 72.78 cSt palm oil-based PE ester with anti-wear and anti-corrosion additives, 75.37 cSt commercial fully formulated lubricant, 131.76 cSt

0.400 0.119

0.656 mm 0.179 mm

0.099

0.142 mm

0.081

0.120 mm

n/a

0.247 mm

n/a

0.174 mm

n/a

0.332 mm

n/a n/a 0.079 0.149 0.081 0.068 0.064 0.052 0.056 0.053 0.065 0.25 0.06 welded 0.08 0.092 0.094 0.063

0.242 mm 0.357 mm 0.269 mm 0.344 mm 0.8053 mm 0.3511 mm 0.3441 mm 0.2789 mm 0.3125 mm 0.3631 mm 0.3898 mm 2.47 mm 2.54 mm welded 0.54 mm 0.659 mm 0.669 mm 0.377 mm

0.087 0.093 0.050 0.046

0.218 mm 1.213 mm 0.584 mm 0.338 mm

0.101

0.434 mm

[35]

[42]

Ball-on-three-plates test: plate (45° inclined, C45E-1.1191 steel, 25–30 HRC hardness); ball (100Cr6 steel, 12.7 mm diameter)

20 N, 400 rpm, 25 °C

Ball-on-disk test (HFRR): ball (detail n/a); disk (detail n/a) Four-ball test: ball (AISI 52–100 steel, 12.7 mm diameter, 64–66 HRC hardness)

1 kg, 50 Hz (1 mm stroke), 25 °C 40 kg, 1200 rpm, 75 °C

[37]

Four-ball test: ball (AISI 52–100 steel, 12.7 mm diameter, 64–66 HRC hardness)

1000 N, 1770 rpm, 25 °C

[36]

Four-ball test: ball (AISI 52–100 steel, 12.7 mm diameter, 64–66 HRC hardness)

40 kg, 1200 rpm, Temperature (T) T= 50 °C

[38]

T= 100 °C

a b c d

WSD / WCV

a

Values obtained at steady state or after sufficiently long sliding duration Viscosity at 20 °C Density is given instead since viscosity data is not available Kinematic viscosity of the ring opening products at 40 °C rather than the mixture

A biolubricant base stock with ester functional groups such as VO is more efficient in terms of its frictional properties and wear protection than mineral oils [18]. VOs with a low degree of unsaturation such as avocado oil (0.985 unsaturation), olive oil (0.948 unsaturation), and peanut oil (1.102 unsaturation) exhibit superior frictional properties compared to high unsaturation VOs such as soybean oil (1.451 unsaturation), corn oil (1.381 unsaturation) and sesame oil (1.232 unsaturation) [16]. Their wear protection effects, nonetheless, are not promising, particularly under extreme pressure conditions in a four-ball test [17]. It is interesting to note that VOs derived from the breeding of genetically-modified organisms (GMOs) such as safflower oil (1.010 unsaturation) in work [16] do not follow the trends of other natural oils, probably due to their distinct structures in terms of their fatty acids. On the other hand, short-chained VOs such as coconut oil are not efficient as far as frictional properties are concerned [17]. Should a thicker fluid be required for an application such as hydraulic fluid, epoxidized VO may be considered, as it has about twice the viscosity value of the original VO [26]. It also performs better in COF and WSD relative to commercial hydraulic oils within the same viscosity range [26]. Another alternative to resolve the low viscosity problem is to incorporate estolide as a thickening agent, as seen in the case whereby oleic acid-based and ricinoleic acid-based estolides can be blended into a low viscosity fluid to improve wear performance [35]. When lubricating at extreme operating temperatures and enduring a heavy

physical properties above, excessive molecular branching puts the tribological properties at risk, as the adhesion of the molecule on the sliding surface could be disrupted and the probability of asperity contact increased [98]. Molecular branching is nonetheless still beneficial in lubrication involving high contact loads or extreme pressure. Indeed, in extreme cases, as observed in polyol esters; particularly TMP and PE ester [38], molecular branching actually improves the load-carrying capacity of the base stocks when benchmarking against mineral oils. This is because molecular branching helps to stabilize the tribofilm from breaking apart [122]. 4. Tribological performances of biolubricant base stocks Base stock is the primary element which determines the final properties of a lubricant product. Even with the best additives, a poorly performing base stock can only be improved with difficulty. The selection of suitable base stock either in its neat form or in a combination of various types of base stocks (blend or emulsion) is usually based on a list of properties, viz. physical properties, material compatibility, renewability, degradability and so on. As one of these properties, tribological performance is the most critical criterion. Recognizing its importance, Table 1 tabulates the tribological performances of various types of lubricant base stocks. Their key tribological behaviors will be elucidated subsequently. 151

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achieve what the current conventional additives can offer. On the other hand, ethylene-vinyl acetate copolymer (EVA) and ethyl cellulose (EC) are known as viscosity modifiers. Even when constituting only a slight weight percentage of the additives in a base stock, they are able to boost the viscosity of base stock at least by two-fold [15,48]. They are also able to form a thicker film for better frictional, wear, and load-carrying properties [15,50]. In fact, both polymers excel in different applications. EVA helps to reduce friction and wear mainly in the mixed lubrication regime, whereas EC is more effective in extreme boundary lubrication [15]. In view of other improvements [50], EC is able to sustain tribofilm at a high temperature, in addition to acting as an excellent pour point depressant, which is proven to be more effective than EVA and mineral oil additives with a polymethacrylate backbone. The above advantages are nonetheless dependent on the solubility of the additives in the base stock. In one study [50], blending high oleic sunflower oil with a more polar oil, e.g. castor oil, can boost the solubility of EC. This blend became a potentially eco-friendly base stock which was suitable for various applications due to a wide range of viscosity, good low-temperature properties and low degree of temperature-viscosity dependency.

contact load, biolubricant base stocks with significant molecular branching and steric hindrance, namely the ring-opened products from epoxidized oil and the polyol ester, can be employed. The molecular branching resists crystallization at a low temperature and degradation at an elevated temperature and pressure, producing stable tribofilm which are versatile and may be used for various applications [53,116]. According to the data tabulated in Table 1, it is suggested that the dominant molecular structure criteria affecting lubricity and wear protection at boundary lubrication is the degree of molecular branching, followed by molecular chain length, and number of ester functional groups, in descending order. The latter two parameters are more profound in mixed/EHL and hydrodynamic regimes as they affect the viscosity of the fluid and the adhesion of tribofilm, respectively [101]. It should be noted that the effect of viscosity on friction is not significant in boundary lubrication [98]. The above observation is obtained based on the following cases: The triester product of ringopening of epoxidized oleic acid produced lower COF and smaller WSD in boundary lubrication as compared to the monoester and the diester [29]. This comparison was done at the basis of similar chain lengths attached to the ester functional group. In addition, the ring-opened product of epoxidized canola oil with branched alcohol such as 2ethylhexanol exhibits better wear protection when blended into a low lubricity oil, compared with that blended with other more viscous products derived from linear alcohols [30]. In the studies involving polyol esters, PE ester gives better lubricity than TMP and NPG esters, especially at an elevated temperature (e.g. 100 °C) and under extreme pressure (e.g. 100 kg load). This is because PE possesses a higher branching degree and therefore a much more stable molecular structure [36,37]. TMP and NPG esters may perform better at mild operating conditions, or in mixed/EHL lubrication regimes. In actual fact, it is difficult to isolate and investigate the tribological performances of NPG, TMP and PE esters, since their molecular weight and degree of steric hindrance are varying, giving rise to different viscosities, thermal stability, and cold flow property [36,37]. Each type of ester has its own advantages: they can be designed and tailored to give specific physical properties and tribological advantages to suit a specific application.

5.2. Particulate and layered materials Nano-sized particulates such as CuO, TiO2 and ZnO are non-toxic anti-wear additives for biolubricants. With their sizes ranging from around 2 to 120 nm [3], these materials are able to coalesce in asperity valleys, creating a thin, smooth and solid lamellar film between contacting surfaces [52,57]. As a result, these solid particulates provide superior frictional and wear properties [52,57]. Their performance depends very much on the surface roughness of the sliding materials. If the particulate size is slightly smaller than the surface roughness, the wear volume decreases with the decrease of particulate size [54,57]. When the particulate size is extremely small (e.g. five-fold smaller) relative to the surface roughness, the wear volume increases with the decrease in the particle size [57]. In the case whereby particulate size is larger than the surface roughness, grooves from abrasions are produced [57]. Another parameter affecting frictional and wear behavior is the concentration of the additive. The presence of excessive additive particles in the asperities valleys could either give insignificant improvement in tribological performance [56], or produce abrasive wear (ploughing effects) by increasing the shear-concentration at the contact surface [60]. The latter may be due to the interference of the additive particles causing poor adsorption of base stock at the contact area resulting in inadequate lubrication, as observed in the case of nanoparticles in polar oils such as ester-based oil [56] and VO [128]. The above relationships are also applicable for two-dimensional (2D) layered materials, including hexagonal boron nitride (hBN), boric nitride and graphene [54,60]. The nature of 2D layered materials and the particulates is different, whereby the former are known for their low interlayer friction ability and the ability to accommodate relative surface velocity to produce low COF during sliding [57,58]. Taking graphene as an example [58], its atoms lying on the same layer are closely bonded together by strong covalent bonds, while the layers are attached together by weak Van der Waals. When employing nanoparticles and layered materials additives, replenishment of the solid additives is required as the additive may be molten, welded or forced out of the tribosystem after a certain time. Therefore, an effective suspension or colloidal system that is stable and compatible with the solid additives is crucial.

5. Tribological enhancement using biolubricant additives In the current scenario, environmental concerns continue to drive original equipment manufacturers to change their specifications for lubricant performances. Lubricant formulation has been developed in such a way that the elements of chlorine, phosphorus, sulfur, and metals in the formulation are already reduced [47]. This helps to mitigate environmental impact, as it is well known that the toxic and not readily degradable additives may accumulate in the environment and harm the ecosystem [127]. There are various types of additives available nowadays, but most of them are not environmentally friendly. For instance, zinc dialkyl dithiophosphate (ZDDP) has been used in engines both as an anti-wear and antioxidant additive for decades, despite its non-degradability and toxicity in nature [3,98]. As far as environmental friendliness is concerned, the tribological additives for biolubricant which are currently receiving most attention from researchers are plant-derived compounds and polymers, particulate and layered materials and ionic liquids. Their tribological performances are tabulated in Table 2. These additives will be briefly discussed in the following subsections. 5.1. Plant-derived compounds and polymers

5.3. Ionic liquids

Plant-derived compounds such as cystine schiff base ester are potential anti-friction, anti-wear and anti-corrosion additives [51]. They are comprised of disulfide groups, along with amine and carboxylic functionalities, which are capable of forming a surface-complex film, particularly when metal-to-metal contact is involved [51]. As this type of additive is relatively new, its tribological properties are yet to

Ionic liquids have been applied in almost every field, including lubrication, due to their limitless pool of structural variations and properties. Among the variety of ionic liquids, imidazolium-based ionic liquid is claimed to have a great potential and to be a versatile lubricant 152

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Table 2 Tribological enhancement of biolubricants using additives. a

Ref.

Tribosystem

Lubrication conditions

Base stock, viscosity (40 °C)

COF

[15]

Ball-on-disk test (HFRR): ball (AISI 52100 steel, 800 VPN hardness, 6 mm diameter); disk (AISI 52100 steel, 750 VPN, 10 mm diameter)

4 N, 50 Hz (1 mm stroke), T

high-oleic sunflower oil (HOSO), 38.5 cSt HOSO with 4% w/w ethylene-vinyl acetate copolymer (EVA), 168.14 cSt HOSO with 1% w/w ethyl cellulose (EC), 50.6 cSt castor oil (CO), 242.5 cSt CO with 4% w/w EVA, 427.1 cSt CO with 1% w/w EC, 546.7 cSt HOSO, 9.9 cSt b HOSO with 4% w/w EVA, 29.6 cSt b HOSO with 1% w/w EC, 15.1 cSt b CO, 21 cSt b CO with 4% w/w EVA, 50.9 cSt b CO with 1% w/w EC, 40.3 cSt b HOSO, 39.0 cSt HOSO with 1% w/w EC, 51 cSt CO, 242 cSt CO with 1% w/w EC, 547 cSt blend of HOSO and CO at ratio 1:1 w/w, n/a blend of HOSO and CO at ratio 1:1 w/w with 1% w/w EC, 174 cSt HOSO, 39.0 cSt HOSO with 1% w/w EC, 51 cSt CO, 242 cSt CO with 1% w/w EC, 547 cSt blend of HOSO and CO at ratio 1:1 w/w, n/a blend of HOSO and CO at ratio 1:1 w/w with 1% w/w EC, 174 cSt HOSO, 39.0 cSt HOSO with 1% w/w EC, 51 cSt CO, 242 cSt CO with 1% w/w EC, 547 cSt blend of HOSO and CO at ratio 1:1 w/w, n/a blend of HOSO and CO at ratio 1:1 w/w with 1% w/w EC, 174 cSt HOSO, 10 cSt b HOSO with 1% w/w EC, 15 cSt b CO, 21 cSt b CO with 1% w/w EC, 40 cSt b blend of HOSO and CO at ratio 1:1 w/w, n/a blend of HOSO and CO at ratio 1:1 w/w with 1% w/w EC, 26 cSt b HOSO, 10 cSt b HOSO with 1% w/w EC, 15 cSt b CO, 21 cSt b CO with 1% w/w EC, 40 cSt b blend of HOSO and CO at ratio 1:1 w/w, n/a blend of HOSO and CO at ratio 1:1 w/w with 1% w/w EC, 26 cSt b HOSO, 10 cSt b HOSO with 1% w/w EC, 15 cSt b CO, 21 cSt b CO with 1% w/w EC, 40 cSt b blend of HOSO and CO at ratio 1:1 w/w, n/a blend of HOSO and CO at ratio 1:1 w/w with 1% w/w EC, 26 cSt b polyol base oil, n/a polyol base oil with 1000 ppm cysteine schiff base ester prepared via two step synthesis using 3,5-di-t-butyl-4-hydroxybenzaldehyde followed by lauroyl alcohol (CySBE), n/a polyol base oil with 2000 ppm CySBE, n/a polyol base oil with 3000 ppm CySBE, n/a polyol ester oil, 40.03 cSt polyol ester oil with 1% w/w oleic acid, 39.27 cSt polyol ester oil with 1% w/w CuO nanoparticles, 40.25 cSt polyol ester oil with 1% w/w MoS2 nanoparticles, 40.96 cSt polyol ester oil with 1% w/w CuO nanoparticles and 1% w/w oleic acid, 40.17 cSt polyol ester oil with 1% w/w MoS2 nanoparticles and 1% w/w oleic acid, 40.09 cSt polyol ester oil, 40.03 cSt polyol ester oil with 1% w/w oleic acid, 39.27 cSt polyol ester oil with 1% w/w CuO nanoparticles, 40.25 cSt polyol ester oil with 1% w/w MoS2 nanoparticles, 40.96 cSt polyol ester oil with 1% w/w CuO nanoparticles and 1% w/w oleic acid, 40.17 cSt polyol ester oil with 1% w/w MoS2 nanoparticles and 1% w/w oleic acid, 40.09 cSt

0.049 0.047

0.18 mm 0.19 mm

0.075 0.087 0.057 0.050 0.085 0.067 0.071 0.043 0.085 0.034 0.034 0.028 0.033 0.030 0.027 0.028 0.021 0.031 0.034 0.030 0.025 0.026 0.021 0.022 0.027 0.024 0.024 0.025 0.052 0.016 0.051 0.051 0.049 0.032 0.034 0.016 0.022 0.027 0.016 0.016 0.011 0.013 0.015 0.014 0.012 0.013 0.068 0.075

0.20 mm 0.34 mm 0.16 mm 0.17 mm 0.20 mm 0.22 mm 0.41 mm 0.14 mm 0.55 mm 0.16 mm n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a 0.90 mm 0.91 mm

0.068 0.055 n/a n/a n/a n/a n/a

0.81 mm 1.00 mm 0.58 mm 0.58 mm 0.55 mm 0.48 mm 0.54 mm

n/a

0.51 mm

n/a n/a n/a n/a n/a

welded welded 3.26 mm 3.04 mm 3.14 mm

n/a

2.88 mm

T = 40 °C

T = 100 °C

[50]

Ball-on-disk test (minitraction machine (MTM)): ball (AISI 52100 steel, 19.05 mm diameter 11 nm roughness); disk (AISI 52100 steel, 50 mm diameter, 11 nm roughness)

30 N, entrainment speed (50% slide-to-roll ratio), T T = 40 °C Entrainment speed = 0.01 m/s T = 40 °C Entrainment speed = 0.1 m/s

T = 40 °C Entrainment speed = 1 m/s

T = 100 °C Entrainment speed = 0.01 m/s

T = 100 °C Entrainment speed = 0.1 m/s

T = 100 °C Entrainment speed = 1 m/s

[51]

[52]

Four-ball test: ball (AISI 52–100 steel, 12.7 mm diameter, 64–66 HRC hardness)

Four-ball test: ball (AISI 52–100 steel, 12.7 mm diameter, 64–66 HRC hardness)

198 N load, 1200 rpm, 75 °C

load, 1770 rpm, 25 °C

load = 40 kg

load = 160 kg

WSD / WCV

a

(continued on next page)

153

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Table 2 (continued) a

Ref.

Tribosystem

Lubrication conditions

Base stock, viscosity (40 °C)

COF

[56]

Pin-on-disk test: pin (AISI 1020 steel, 7.92 mm diameter); disk (AISI 52100 steel, 100 mm diameter, 64–66 HRC roughness)

load, 0.5 m/s, T T = 40 °C

mineral oil, 103.55 cSt mineral oil with 0.3% w/w Cu nano-additive, 107.10 cSt mineral oil with 3.0% w/w Cu nano-additive, 107.98 cSt synthetic ester, 107.54 cSt synthetic ester with 0.3% w/w Cu nano-additive, 108.50 cSt synthetic ester with 3.0% w/w Cu nano-additive, 110.88 cSt mineral oil, 103.55 cSt mineral oil with 0.3% w/w Cu nano-additive, 107.10 cSt mineral oil with 3.0% w/w Cu nano-additive, 107.98 cSt synthetic ester, 107.54 cSt synthetic ester with 0.3% w/w Cu nano-additive, 108.50 cSt synthetic ester with 3.0% w/w Cu nano-additive, 110.88 cSt mineral oil, 11.55 cSt b mineral oil with 0.3% w/w Cu nano-additive, 11.87 cSt b mineral oil with 3.0% w/w Cu nano-additive, 12.06 cSt b synthetic ester, 14.70 cSt b synthetic ester with 0.3% w/w Cu nano-additive, 14.81 cSt b synthetic ester with 3.0% w/w Cu nano-additive, 15.05 cSt b mineral oil, 103.55 cSt mineral oil with 0.3% w/w Cu nano-additive, 107.10 cSt mineral oil with 3.0% w/w Cu nano-additive, 107.98 cSt synthetic ester, 107.54 cSt synthetic ester with 0.3% w/w Cu nano-additive, 108.50 cSt synthetic ester with 3.0% w/w Cu nano-additive, 110.88 cSt canola oil mixed with 5% w/w of 5μm hexagonal boron nitride (hBN) particles, n/a canola oil mixed with 5% w/w of 1.5μm hBN particles, n/a canola oil mixed with 5% w/w of 0.5 μm hBN particles, n/a canola oil mixed with 5% w/w of 0.07μm hBN particles, n/a

0.063 0.014 0.006 0.009 0.004 0.008 0.067 0.012 0.080 0.027 0.006 0.090 0.111 0.106 0.107 0.077 0.074 0.078 n/a n/a n/a n/a n/a n/a 0.123

n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a 0.88 mm 0.70 mm 0.74 mm 0.79 mm 0.86 mm 1.11 mm 0.132 mm3

0.078 0.056 0.035

0.080 mm3 0.069 mm3 0.025 mm3

canola oil mixed with 5% w/w of 5μm hBN particles, n/a canola oil mixed with 5% w/w of 1.5μm hBN particles, n/a canola oil mixed with 5% w/w of 0.5 μm hBN particles, n/a canola oil mixed with 5% w/w of 0.07μm hBN particles, n/a jatropha oil-based TMP triester, 16.87 cSt jatropha oil-based TMP triester with 0.05% w/w of 2–5 μm hBN particles, 17.13 cSt jatropha oil-based TMP triester with 0.1% w/w of 2–5 μm hBN particles, 17.20 cSt jatropha oil-based TMP triester with 0.5% w/w of 2–5 μm hBN particles, 17.56 cSt commercial synthetic ester, 19.05 cSt Ionic liquid emulsion consists of 6% w/w I-butyl-3-methylimidazolium tetrafluoroborate [BMIM][BF4], 19% w/w castor oil, 60% w/w Tx-100 surfactant, 15% w/w 1-butanol, n/a Commercial lubricant base oil, n/a Ionic liquid emulsion consists of 6% w/w [BMIM][BF4], 19% w/w castor oil, 60% w/w Tx-100 surfactant, 15% w/w 1-butanol, n/a Commercial lubricant base oil, n/a Ionic liquid emulsion consists of 6% w/w [BMIM][BF4], 19% w/w castor oil, 60% w/w Tx-100 surfactant, 15% w/w 1-butanol, n/a Commercial lubricant base oil, n/a trimethylolpropane oleate (TMPTO), 49.6 cSt TMPTO with 3% 1-tetradecyl-3-(2-ethylhexyl)imidazolium bis(2ethylhexyl) phosphate [TDEHIM][DEHP], 51.23 cSt TMPTO with 5% [TDEHIM][DEHP], 52.5 cSt TMPTO with 7% [TDEHIM][DEHP], 53.37 cSt polyol ester oil, 45.82 cSt polyol ester oil with 2% w/w aspartic acid and glutamic acid derived ionic liquids (ILs) 1, 46.28 cSt polyol ester oil with 2% w/w ILs 2, 46.38 cSt

0.072 0.073 0.090 0.093 0.022 0.022

0.359 mm3 0.392 mm3 0.465 mm3 0.434 mm3 1.280 μm 1.238 μm

0.026

1.514 μm

0.029

1.590 μm

0.090 0.079

1.553 μm 0.652 mm

0.107 0.054

0.801 mm 0.859 mm

0.098 0.064

0.842 mm 0.901 mm

0.088 0.143 0.122

0.883 mm 0.005 mm3 0.0098 mm3

0.114 0.112 0.132 0.0681

0.0057 mm3 0.0095 mm3 0.954 mm 0.750 mm

0.0681

0.562 mm

load = 392 N

T = 40 °C

load = 588 N

T = 100 °C

load = 588 N

[57]

[60]

[63]

Four-ball test: ball (AISI 52100 steel, 12.7 mm diameter, 64–66 HRC hardness)

392 N, 1200 rpm, 75 °C

Pin-on-disk test: pin (oxygen-free electronic copper C101, 6.35 mm diameter, 50 mm length, hemispherical tip); disk (aluminum alloy 2024, 70 mm diameter, 6.35 mm thickness, Ra roughness) Ra = 0.22 μm Ra = 0.43 μm

10 N, 33 mm/s, 25 °C

Four-ball test: ball (AISI 52–100 steel, 12.7 mm diameter, 64–66 HRC hardness

392 N, 1200 rpm, 75 °C

Four-ball test: ball (GCr15 steel, 61–64 HRC hardness)

load, 1450 rpm, 75 °C load = 392 N

load = 588 N

load = 784 N

[62]

[64]

a b

Ball-on-disk test (HFRR): ball (steel); disk (steel)

Four-ball test: ball (AISI 52–100 steel, 12.7 mm diameter, 64–66 HRC hardness)

100 N, 25 Hz (1 mm stroke), 150 °C

392 N, 1200 rpm, 75 °C

WSD / WCV

a

Values obtained at steady state after sufficient sliding duration Viscosity at 100 °C

microemulsion [61,63]. For example, ionic liquid microemulsion comprising of 1-butyl-3-methyl-imidazolium tetrafluoroborate, [BMIM] [BF4], a surfactant, and a vegetable oil, produced better tribological results than conventional base stock [63]. In the latest development, the dual roles of ionic liquid can been seen in the synthesis of polyol ester base stocks, whereby ionic liquid, namely 1-tetradecyl-3-(2-ethylhexyl) imidazolium bis(2-ethylhexyl) phosphate [TDEHIM][DEHP], acts as an efficient catalyst and at the same time works as an in situ anti-wear

which is applicable to various types of sliding materials [129,130]. On top of its unique properties such as low volatility, non-flammability, negligible vapor pressure, and superior chemical and thermal stabilities [130], this ionic liquid possesses excellent lubricating properties, including friction reduction, anti-wear and high load-carrying capacity [130–132]. Nevertheless, the immiscibility of ionic liquid in non-polar mediums like fatty hydrocarbon and vegetable oil is the major drawback. This technical restriction can be overcome with ionic liquid 154

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Table 3 Tribological influences of biolubricant base stocks and additives. Effectsa

Base stocksb Increase unsaturation

Tribofilm characteristics Film thickness Adhesion strength Low temperature flow Viscosity index Thermo-oxidative stability Tribological performance Lubricity Wear protection Load-carrying capacity (extreme pressure) Application Boundary lubrication Mixed lubrication High temperature a b c d e

– +

Additivesc Increase chain length (molecular weight)

Increase polarity

+

+ + –

– +

–

+

– +

– – –

+ +

Increase branching degree (steric hindrance)

Plant-derived polymer (EC, EVA)d

–

Ionic liquid

+

+ +

– +

+ +

+ + +

+ + +

+ +

+ +

+ +

+

+ +

Particulate and layered materialse

+ + –

+ +

Positive effect [+ ]; negative effect [-]; unknown or insignificant effect []. Base stocks: vegetable oil (VO), epoxidized VO, ring opening products from epoxidized VO, estolides and polyol esters. Effectiveness depends on solubility and compatibility. EC: ethyl cellulose; EVA: ethylene-vinyl acetate copolymer. Effectiveness depends on topological condition of contact surface.

References

additive, eliminating the need for catalyst separation from the synthesized ester [62]. In addition, biodegradable and non-toxic amino-acid based ionic liquids have been synthesized as anti-wear additive and friction modifier. They are able to form a stable homogeneous dispersion with polyol ester base stock [64].

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6. Final remarks It is clear, then, that biolubricants derived from renewable energy sources like plant oils are becoming more viable with the development of various types of environmentally friendly base stocks and additives. They are able to perform as well as conventional lubricants, and can even be applied and incorporated into broader applications, including biofuels and bioenergy. A successful formulation of biolubricant relies on the selection of suitable biolubricant base stocks and additives to meet a list of specifications required by an application. In terms of the tribological aspect, a good biolubricant base stock can be ensured by optimizing two characteristics: the stability and the adhesiveness of its generated tribofilm within the lubricating conditions, provided that the viscosity range of the lubricant and the lubrication regime are suitable and tally with the application requirements. Tribological additives can be then added to further improve the base stock. In summary, this review offers an in-depth discussion and comprehensive evidence of the tribological behaviors of biolubricant base stocks and additives. The influences of biolubricant base stocks and additives on tribological performance are summarized in Table 3. It is hoped that the tribological data presented in this review may assist interested parties in preparing and formulating biolubricants which fulfill basic lubrication requirements. Acknowledgement The author thanks the Director-General of MPOB for permission to publish this work. Declarations of interest none 155

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