1 Introduction

Apart from stationary equipment (e.g., pumps, compressors, etc.), approximately 90% of the internal parts of earthmoving machinery are lubricated [1,2,3,4]. Nearly 50% of all lubricants sold worldwide are wasted through the total loss of applications, volatility, spills, or accidents and pollute the environment. More than 95% of these lubricants are mineral-based [1]. Mineral lubricants are produced from crude oil via conventional oil refining processes and contain many classes of chemical components, including paraffin, naphthene, aromatic compounds, etc. [5]. Because of their high ecotoxicity and low biodegradability, mineral lubricants pose a serious threat to the environment and different life forms. According to a report published by the Department of Ecology in the State of Washington [6], the potential harm of oil spills to the environment can be categorized as follows:

  1. 1.

    Acute toxicity: the ability to cause severe biological harm or death after exposure. Volatile compounds can readily dissolve in water and soil and kill plants and animals. Studies have shown that cancer, skin diseases, and allergic reactions can also be linked to long-term exposure to the disposed mineral oils in the environment [1, 7]. For example, crude oil contains up to 40% of acutely toxic compounds.

  2. 2.

    Mechanical injury: this is a measure of how much harm oil causes owing to its physical characteristics. This is also called the “coating” effect because thick and sticky layers of oil tend to coat plants and animals to a level that causes physical injury. For example, a film of crude oil-based lubricants on water not only causes disturbances in the oxygen–gas and heat exchange between the water and atmosphere but may also reduce the penetration of light into the depths, which may in turn disrupt the photosynthesis process [8]. Such phenomena endanger aquatic plants and animals and lead to ecosystem disorders. Substances that are extremely viscous and very sticky present a high mechanical injury threat. The levels of mechanical injury caused by crude oil, minerals, and biodegradable lubricants can be severe, moderate, and low, respectively [6].

  3. 3.

    Persistence: a measure of how long the lubricant stays in the environment before it starts to be broken down by microorganisms. Biodegradability test data indicate that crude and mineral oils (depending on their composition) can take 1–10 years to decompose in the environment [6]. The degradation period of biodegradable lubricants (e.g., butter, lard, fish oil, and vegetable oil) has been reported to vary from 4 to 48 weeks, which is noticeably shorter than that of their mineral counterparts. Figure 1 summarizes the potential environmental hazards associated with minerals and biodegradable lubricants.

Fig. 1
figure 1

Summary of the environmental impacts of mineral and biodegradable lubricants. The threat level of crude oil is given as a reference measure. (e.g. Crude oil: has high persistence lasting in the environment for 5–10 years, has high viscosity that causes severe mechanical injury, and contains volatile and reactive toxic compounds). Credits: middle right photo by Magda Ehlers on Pexels; bottom right photo by Colby Winfield on Unsplash

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Despite mounting pressure to use more environmentally friendly lubricants, especially in ecologically sensitive areas, regulatory and practical obstacles must be overcome. According to the legislation passed by the European Union, a lubricant can still be considered biodegradable even if it contains up to 50% petroleum oil [8]. This can be quite misleading for users deciding to purchase biodegradable lubricants, and, therefore, they will be unaware of the potential harm to health and the environment upon exposure. As can be noted, the term “biodegradable” is ambiguous as no objective definition has been developed. Moreover, the assessment of lubricant biodegradability is not straightforward. The composition of lubricants varies depending on the production process and set of refining additives; therefore, selecting an appropriate testing method and sample preparation are crucial for a comprehensive assessment of biodegradability. The Organization for Economic Co-operation and Development (OECD) uses a set of tests to assess biodegradation (e.g., 301 A–F, 302 A–C, etc.). According to OECD requirements, “a lubricant is considered biodegradable if it reaches a level of biodegradation of 60–70% within 28 days. During this period of 28 days, the time taken for 10% of the lubricant to biodegrade must not surpass 10 days”. Current testing methodologies do not provide 100% biodegradability. “The assumption is made that the complete breakdown of the remaining portion will occur in a time period that is calculated based on the data obtained from the first 28 days” [9]. However, this assumption is only valid for synthetic and natural oils. Most components derived from crude oil degrade slowly and undergo various chemical reactions. Therefore, degradation lasting longer than 28 days should be monitored. Conducting biodegradability tests is time-consuming and expensive. In practice, it is very difficult to use them in the “daily” technical inspection of oils, where often permission to use oil should be issued on a regular basis [8].

In addition to the abovementioned regulatory obstacles, the applicability of biodegradable lubricants to various industrial and engineering tasks has limitations based on their properties. The major drawbacks compared to mineral oils are their high pour point (high solidification temperatures), poor oxidation, thermal stability (high coefficient of friction at elevated temperatures), which limits the operating temperature to approximately 120 °C, and hydrolytic instability [10,11,12]. Vegetables are the major source of biodegradable lubricants. However, not all types of vegetable oils are suitable for lubrication because certain requirements (e.g., good lubricity, good corrosion protection, and compatibility with other materials) must be fulfilled. The properties and sources of biodegradable lubricants are discussed in detail in the following sections. Finally, another factor that prevents the widespread use of biodegradable lubricants across different industries is their price, which is generally higher than that of mineral lubricants. However, in many cases, the increased cost can be justified as the spill remediation process is an expensive and time-consuming process. The average cleanup and damage cost for an oil spill in the United States is approximately $16 per gallon [13]. The extreme examples of oil spills in history reportedly cost millions of dollars (e.g., Exxon Valdez oil spill in Alaska: $3.8 billion; oil spill in El Capitan State Beach, California: $62 million; etc.) [13, 14]. Therefore, a bio-based lubricant should be considered for any application where leaks and spills are a real possibility and containment cannot be easily achieved.

It is important to emphasize that the process of fully switching to biodegradable lubricants will be a gradual and complicated process that will require government support and close collaboration among different sectors, including agriculture, industry, and academia. In the following sections, we introduce various sources of biodegradable lubricants, their properties, current states, and applications. We also discuss the most recent advances and future trends in improving the tribological performance of biodegradable lubricants.

2 Sources of Biodegradable Lubricants: Properties and Applications

Unlike crude oil, which has finite global reserves, biodegradable lubricants are derived from renewable sources. In general, pure biodegradable lubricants are either plant- or animal-based. Plant-based lubricants are composed of triglycerides, which consist of glycerol and fatty acids (saturated or unsaturated) with different chain lengths [15]. In the synthetic mode, they may contain different base stocks such as polyalphaolefins (PAOs), polyalkalene glycols (PAGs), and esters. PAOs are readily biodegradable and exhibit good low-temperature performance, hydrolytic stability, and low volatility. They are a good choice for applications where low viscosity is required. PAGs are highly biodegradable owing to their chemical composition and are highly soluble in water. They are primarily used as fire-resistant fluids. PAGs are incompatible with other oils and can cause problems if they are inadvertently mixed with non-PAG oils. Regarding synthetic esters, although they have lower biodegradability scores than other compounds, they have quickly become one of the main base stocks of biodegradable lubricants owing to their excellent low-temperature performance and low volatility. Synthetic esters are typically added to vegetable-oil-based lubricants to improve their low-temperature properties. Esters and PAGs are also used to boost performance when pure vegetable oils cannot satisfy the performance requirements. The most common sources of biodegradable lubricants and a summary of their applications and annual production are listed in Table 1.

Table 1 Sources, applications, and estimated worldwide production of biodegradable lubricants. The applications are partially adapted from reference [19] with permission
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As can be noted, lubricants from some sources (e.g., canola, castor, olive, rapeseed, palm, and soybean) have more industrial applications than others [16,17,18]. This arises from the necessity of fulfilling specific criteria to ensure optimal lubrication performance. As previously indicated, not all oils obtained from various sources are suitable for use as lubricants. In general, some of the most important criteria are [12]:

  1. 1.

    Low volatility under operating conditions;

  2. 2.

    Satisfactory viscosity characteristics (e.g., a high viscosity index is desirable for engine oils);

  3. 3.

    High stability to maintain desirable characteristics during use;

Lubricant stability is influenced by temperature, oxygen, and contaminants, such as unburned fuel fragments and corrosive acids. All lubricating oils react with oxygen in the air, eventually forming acidic or sludge products, which in turn limit the lifecycle of a lubricant. The oxidative stability of vegetable oils depends on the levels of unsaturated compounds. The lower the unsaturation, the better the oxidative stability, but the higher the melting point [12].

  1. 4.

    Compatibility with mating materials and other additives. The main industrial applications and technical specifications of the commonly used biodegradable lubricants are discussed below.

Canola oil is a genetically modified rapeseed plant. It contains approximately 12% α-linolenic acid (omega-3), approximately 65% monounsaturated fatty acids (oleic acid), and a small amount of saturated fatty acids (< 7%) compared with other common vegetable oils. Canola oil is the only known vegetable oil with a sulfur atom in some fatty acid structures, which is responsible for its sulfur flavor [20]. It can be converted into diesel fuel for use in regular diesel engines with little or no modification. It can also be modified for use as a transmission gear and hydraulic oil [21]. The reported viscosity of canola oil at 40 °C is 40.6 cSt [22]. The density of the refined canola oil is 0.91 g/cm3.

Rapeseed oil contains various combinations of fatty acids, phospholipids, and unsaponifiable components. The extraction method plays a significant role in determining phosphorus content and phospholipid profiles. Refined and cold-pressed rapeseed oils are valuable owing to their high α-linolenic acid content (~ 10%) and low saturated fatty acid content (6%) [23]. It is widely used in hydraulic systems and power transmissions. It can be used as diesel fuel, either as biodiesel, directly in heated fuel systems or blended with petroleum distillates for powering motor vehicles [12]. Rapeseed oil can also be used to compound mineral oils or as a viscosity modifier. The density of the oil is 0.92 g/cm3. The viscosities of refined rapeseed oil at 40 °C and 100 °C are 7.5 and 34.3 cSt, respectively [24].

Castor oil is non-edible and can be extracted from castor seeds. It is a triglyceride (glycerol molecule) in which each of its three hydroxyl groups is esterified with a long-chain fatty acid. Castor oil is a potential alternative to petroleum-based starting chemicals for producing materials with various properties. It has been used in biodiesel production, racing cars, and early rotary-type aero-engines [12]. It is inexpensive and has a high content (85–90%) of abundant fatty acids (e.g., ricinoleic acid), high viscosity and molecular weight, and a low coefficient of friction at mid-to-low temperatures (< 60 °C) [25]. Castor oil has low thermal stability and poor oxidation properties [26]. It is soluble in alcohol but not petroleum. The density and viscosity of castor oil at 40 °C are 0.95 g/cm3 and 37 cSt, respectively [27].

Olive oil is obtained by pressing the olive fruit and is primarily composed of saturated (~ 15%) and unsaturated (~ 85%) fatty acids esterified almost entirely with glycerol to form triacylglycerols (~ 99%). Olive oil is an excellent but costly lubricant with an oleic acid content of ~ 74% and is a good candidate for biodiesel production. Owing to its large amount of monounsaturated fatty acids, olive oil has better oxidation stability, which makes it a suitable lubricant [28, 29]. The density and viscosity of olive oil at 40 °C are 0.91 g/cm3 and 33.5 cSt, respectively [30].

Palm oil is the most widely produced edible oil and is obtained from the fruits of palm trees. It is cheaper than canola, soybean, or rapeseed oils. Palm oil is very stable towards oxidation because of the maximum proportion (49.9%) of saturated bonds compared to all vegetable oils [31]. The amounts of monounsaturated and polyunsaturated (linoleic) fatty acids in palm oil are 38.7% and 10.5%, respectively. The application of palm oil (pure or mixed with other lubricants) as a biodiesel and industrial lubricant has increased rapidly over the past few years [32]. The density and viscosity of palm oil at 40 °C are 0.88 g/cm3 and 39.6 cSt, respectively [33].

Soybean oil is extracted from soybean seeds and is one of the most important and widely produced vegetable oils. Unmodified soybean oil is a triglyceride composed of saturated (approximately 15%), monounsaturated (22.6%), and polyunsaturated (57.7%) fatty esters [34]. Soybean oil is used in a variety of industrial applications, such as the production of biodiesel, industrial resins, caulks, and mastics, which are useful as adhesives or sealants. It can perform well in lubricating pump components. However, when not modified, the lack of oxidative stability causes an increase in viscosity during use in hydraulic systems [35]. The density and viscosity of soybean oil at 40 °C are 0.91 g/cm3 and 32.6 cSt, respectively [33].

Sunflower oil is extracted from sunflower seeds and is the fifth most common edible vegetable oil worldwide. It comprises approximately 10% saturated, 62.9% monounsaturated, and 20.7% polyunsaturated fatty acids. However, depending on the type, it can contain high or low levels of oleic acid [36]. Sunflower oil, either degummed or transesterified, can be used as a substitute for diesel. It also exhibits excellent high-temperature properties as a synthetic gear lubricant [37]. The density and viscosity of sunflower oil at 40 °C are 0.91 g/cm3 and 33.9 cSt, respectively [33].

The composition of the natural oils in terms of the percentages of saturated, monounsaturated, and polyunsaturated fatty acids differs significantly. Previous studies have shown that the thermal-oxidative stability of natural oils requires a low percentage of polyunsaturated fatty acids. In other words, oxidative stability increases with decreasing amounts of polyunsaturated fatty acids [38, 39]. Furthermore, monounsaturated fatty acids with one double bond improve their oxidative stability while simultaneously providing good low-temperature properties and superior tribological properties [40]. In the following section, we discuss the overall effect of the type of fatty acid on the tribological properties of natural oils. Moreover, recent studies on the tribological performance of commonly used biodegradable lubricants are reviewed and discussed.

3 Tribology of Biodegradable Lubricants

The tribological characteristics of biodegradable lubricants are significantly affected by the type and concentration of the fatty acids. Reeves et al. [40] performed a comprehensive study in this regard by performing tribological tests using biodegradable natural lubricants. As shown in Fig. 2a, the selected lubricants differed significantly in terms of the type and concentration of fatty acids. Analysis of the friction and wear data (Fig. 2b, c) with respect to the unsaturation number (UN) is a metric used to quantify fatty acid concentrations. The greater the UN, the greater the degree of unsaturation, revealing the presence of correlations. It was found that the friction and wear increased with decreasing percentages of oleic acid (lower UN) and increasing percentages of linoleic acid (higher UN). Safflower is an exception to this trend because of its genetically modified structure. Unlike linoleic acid, which has two double bonds, oleic acid has one double bond and is therefore more densely packed [41, 42]. Owing to its more densely packed structure, an effective fatty acid monolayer can be established using saturated and monounsaturated fatty acids. This monolayer acts as a protective layer, limiting metal-to-metal contact (surface asperities), thereby minimizing friction and wear [40]. Recent studies on the tribological behavior of commonly used biodegradable lubricants are discussed below. For each lubricant, the effects of various parameters such as load, temperature, and testing apparatus on the tribological properties are discussed. The reported values of friction and wear in the literature for a given lubricant vary significantly from one another. This can be attributed to the high dependency of friction and wear phenomena on the mating materials as well as the testing conditions (e.g., load, velocity, and ambient conditions).

Fig. 2
figure 2

Credits: The figure was adapted from reference [40] with permission from Elsevier

a Correlation between the percentage of different fatty acids in bio lubricants; b coefficient of friction and c wear properties of different bio lubricants. The tribological tests were performed in ambient conditions under a load of 10 N. Pin and disk materials were copper and aluminum alloy, respectively.

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3.1 Canola Oil

As discussed, canola oil is a promising substitute for mineral lubricants and therefore can be used in various tribological systems. In a basic study performed by Shalwan et al. [43], the friction and wear behaviors of a mild steel material sliding against a steel tip under canola oil lubrication were investigated. The load and sliding velocity were varied from 10 to 20 N and 0–2 m/s, respectively. The results indicated that the coefficient of friction (COF) and wear increased with increasing load and sliding velocity. A minimum steady-state COF of ~ 0.01 was achieved. In another study by Alvarez et al. [44], the tribological performance of a steel-bearing system under canola oil lubrication was investigated (Fig. 3a). A steel pin was used as the countersurface. The load and sliding velocity were 5 N and 5 cm/s, respectively. A minimum COF of ~ 0.02, and wear rate of 6.33 × 10–7 mm3/N m were achieved (Fig. 3b). Optical analysis revealed the formation of oxide films on the wear track, possibly due to the induced high temperatures. Azhari et al. [45] compared the lubrication properties of canola oil with those of SAE40 commercial mineral oil. The results showed that pure canola oil (without friction modifiers) exhibited a higher COF (~ 0.14) than that of SAE40 mineral oil (~ 0.1). Kumar et al. [46] investigated the lubrication properties of canola oil in a steel-steel contact under high normal loads (150 and 180 N) at rotational speeds ranging from 600 to 1800 rpm (with an interval of 300 rpm). The COF varied in the range of ~ 0.05 to 0.09 depending on the applied load and rotational speed. Canola oil has been used in combination with other coatings. Mobarak et al. [47] studied the tribological performance of hydrogenated amorphous carbon (a-C:H)-coated 440C stainless steel ball pins for engine and drivetrain applications. The thickness of the coating was ~ 1.6 µm. The pin was rubbed against a disk surface that was also made of 440C steel. The tests were performed under 10 N at 80 °C. The a-C:H/steel pair exhibited a COF of 0.08, which is approximately 45% lower than that of the steel/steel pair. The wear loss was reduced by approximately 21%. The positive impact of canola oil on the tribological properties of a-C:H was attributed to the large number of polar components in the canola oil that promoted lubricity by forming a lubricating film with an a‐C:H coating and the graphitization of a‐C:H.

Fig. 3
figure 3

Tribological properties of a steel bearing system under canola oil lubrication. a Configuration of the tribological tests, and b the obtained COF, wear rate, and the optical microscopy analysis of the wear track. The figure was adapted from reference [44] with permission under the creative common attribution license

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3.2 Rapeseed Oil

The tribological properties of rapeseed oil have been widely investigated owing to its vast industrial applications. More than two decades ago, Cao et al. [48] investigated the load-carrying capacity and tribological properties of commercially available rapeseed oil using a four-ball tester. The maximum non-seizure load was 700 N. GCr15 bearing steel balls were used for the tribological tests. An average COF value of 0.065 was recorded for the tests performed under loads of 392 and 588 N. Analysis of the wear scars showed signs of scuffing on the surface. In other studies that similarly used a four-ball tribotester, COFs in the range of ~ 0.07 to 0.1 were achieved [49,50,51]. In all cases, the dominant wear mechanism was adhesion or scuffing, wherein the worn surfaces had a high roughness [52]. However, a recent study has shown that the wear mechanism can change according to the type of sliding motion (unidirectional or reciprocating) [53]. The COF and wear were also affected, wherein the obtained values were higher for unidirectional sliding than for reciprocation sliding when lubricated with rapeseed oil. The tribological properties of rapeseed oil were investigated at high temperatures. In a study performed by Arumugam [54], the tribological properties of rapeseed oil at 120 °C were investigated using a cylinder liner-piston ring tribotester. The reported COF value was ~ 0.4, which is a significant increase compared to that at room temperature. A similar tendency was reported by Opia et al. [53], in which rapeseed oil showed a higher COF at 100 °C than at 50 °C. As stated previously, the rapeseed oil extraction method significantly affects its chemical and tribological properties. Padgurskas et al. [15] investigated the influence of different extraction methods (e.g., cold or hot extraction) and refinement methods on the tribological properties of rapeseed oil. The results showed that the samples lubricated with hot-extracted rapeseed oil experienced less wear than those lubricated with cold-extracted oil. Moreover, unrefined rapeseed oil was found to be more effective than the refined version in preventing wear. The negative effect of oil refinement on wear properties was attributed to the removal of fatty acid groups during the process.

3.3 Castor Oil

Many researchers have investigated the tribological properties of castor oil owing to its attractive properties. Owing to its high molecular polarity, which enhances the formation of a transfer layer, castor oil exhibits better lubrication and oxidation resistance than other vegetable oils [55]. Asadauskas [56] investigated the lubrication properties of castor oil as a potential basestock for biodegradable lubricants. Castor oil was found to have lower volatility than other vegetable and mineral oils and good lubrication properties. Bhaumik et al. [57] compared the tribological properties and load-bearing capacities of neat castor oil to those of mineral oils. In this instance, neat castor oil exhibited a higher COF and lower load-bearing capacity than mineral oil. Hussain et al. [58] reported that neat castor oil exhibited a lower COF but higher wear than SAE40 mineral oil. The governing mechanisms were not discussed in this study. The effect of load on the tribological properties of castor oil (steel-steel contact) was investigated by Ouyang et al. [59]. As the load increased from 5 to 20 N, the COF increased marginally from ~ 0.05 to ~ 0.06. However, the wear volume drastically increased from 10,000 to 30,000 mm3. A similar phenomenon was reported in another study in which the COF increased from ~ 0.02 to ~ 0.07 with an increase in the load from 10 to 120 N [60]. The maximum wear depth also increased tenfold. According to the experimental analysis, the change in the lubrication regime was the responsible mechanism. Singh et al. [61] studied the effects of sliding speed on the friction and wear behavior of an aluminum alloy-steel contact under castor oil lubrication. Reductions of ~ 40% in the COF and wear rate were observed when the sliding speed increased from 100 to 400 rpm.

3.4 Olive Oil

Olive oil has good oxidative stability owing to its high monounsaturated fatty acid content. Kalam et al. [62] analyzed the thermal stability and tribological characteristics of olive oil and compared them with those of SAE15W40 commercial oil. Because of the higher content of fatty acids, the average friction coefficient of olive oil was ~ 26% lower than that of SAE15W40 under a 400 N load at 75 ℃. A schematic of the 4-ball tester setup and the obtained COF values are shown in Fig. 4.

Fig. 4
figure 4

Comparison of the tribological characteristics of olive oil with SAE 15W40 commercial oil. a Schematic of the experimental setup of the four-ball tester. b Average friction coefficient of olive oil and SAE 15W40. The figure was adapted from reference [62] with permission under the creative common attribution license

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Murakami et al. [63] conducted the four-ball test with increasing temperature (at a rate of 10 ℃/min) to investigate the lubrication mechanism of olive oil. The results showed that the friction coefficient of olive oil was low at low temperatures but increased as the temperature increased. Additionally, friction tests were conducted by varying the rotational speed. At high speeds, olive oil formed an elastohydrodynamic film more effectively, and the double-bond concentration in the oil increased. This led to a higher load-carrying capacity of the olive oil at higher speeds (116 mm/s). Solea et al. [64] also conducted a four-ball test to assess the influence of the rotation speed on the dynamic viscosity of olive oil. Carcel et al. [65] evaluated the tribological performance of olive oil as a lubricant during stamping. Tribological behavior is important to achieve high-quality stamping operations that ensure part integrity and surface quality. Olive oil showed a friction coefficient between 0.11 and 0.13, which was lower than that of commercial oil. Cesiulis et al. [66] studied the influence of the chemical and physical properties of olive oil on tribological performance and corrosion protection. According to the electrochemical impedance spectroscopy data, olive oil formed a porous film on the steel surface, which improved the corrosion resistance. They also conducted a friction test with four types of olive oil according to the acid number (Ifri, Khonguem Ohedjidid, and Tourough). Owing to the different fatty acid structures, Ohedjidid showed the lowest mean friction coefficient and wear scar diameter (0.063 and 0.7 mm, respectively).

3.5 Palm Oil

The saturated fatty acid content in palm oil is approximately 40–50%, which is higher than in olive and sunflower oils. Owing to its high proportion of saturated fatty acids, palm oil can easily create boundary lubrication conditions, leading to reduced friction and wear [67]. Mannekote et al. [68] evaluated the possibility of using palm oil as a lubricant in a four-stroke engine and compared its performance with that of commercial engine oil. Fuel consumption calculations showed that the engine could potentially consume less fuel with palm oil as the lubricant than with commercial engine oil. Rahim et al. [69] studied the possibility of applying palm oil to the drilling of titanium alloys. The results indicated that palm oil can effectively reduce the cutting force and temperature of the workpiece. Haseeb et al. [70] analyzed the tribological properties of palm oil as a biodiesel with increasing temperature. Biodiesel is an alternative fuel derived from vegetable oils or animal fats and has various advantages, such as reduced exhaust emissions and toxicity. Palm biodiesel exhibits increased friction and wear with increasing temperatures because of the decreasing viscosity of palm oil at high temperatures. Razak et al. [71] investigated the wear characteristics of curved surfaces with hard-soft materials such as ball bearings and acrylonitrile butadiene styrene (ABS) when palm oil was used as a lubricant. The average friction torque of palm oil (0.71 Nm) was found to be lower than that of mineral oil (0.74 Nm). The concentration of unsaturated fatty acids in palm oil plays an important role in reducing friction and wear. Syahrullail et al. [72] evaluated the application of palm oil as a lubricant in the cold forward extrusion process and compared its performance with that of additive-free paraffinic mineral oil. The extrusion load of palm oil decreased, and the surface roughness of the product with palm oil decreased compared to that of additive-free paraffinic mineral oil.

3.6 Soybean Oil

Soybean oil has high thermal stability and is therefore attractive to many researchers [73]. Peng [74] investigated the tribological properties of soybean oil using a ball-on-disk tribometer and derived friction and wear reduction mechanisms using optical microscopy. The wear scar diameter decreased when soybean oil was used, and the boundary lubrication condition improved because of the fatty acids in soybean. Fatty acids can form thick molecular layers that reduce friction and wear. Bahari et al. [75] studied the friction and wear characteristics of gray cast iron under soybean oil lubrication. The friction coefficient was reduced by approximately 24%, and wear decreased by approximately 7%, regardless of the hardness of the gray cast iron. Guo et al. [76] used a castor/soybean oil mixture for minimum quantity lubrication (MQL) and analyzed its tribological characteristics. As the soybean oil concentration in the mixture increased, the grinding force decreased, reaching a minimum value at the maximum soybean oil concentration. The quality of the workpiece surface improved at high concentrations of soybean oil, and the surface exhibited a clear texture, narrow and light wear tracks, and narrow stripped wear debris. Siniawski et al. [77] conducted a wear test to analyze the effect of humidity on the tribological performance of soybean and mineral oils. When mineral oil was used, wear rate increased as humidity increased; however, in the case of soybean oil, wear rate maintained its value under high-humidity conditions, similar to low-humidity conditions. This suggests that the tribological performance of soybean oil is stable regardless of the humidity level; therefore, soybean oil is a more suitable lubricant for high-humidity conditions. A summary of the tribological properties is shown in Fig. 5.

Fig. 5
figure 5

The effect of humidity on tribological properties. a The friction coefficient and b wear rate of different lubricants. The figure was reprinted from reference [77] with permission under the creative common attribution license

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Radulescu et al. [78] studied the relationship between the rheological and tribological properties of soybean oil. The results showed that soybean oil has low thixotropy and non-Newtonian fluid properties. The oleic acid in soybean oil can reduce the friction coefficient and wear rate because it forms a dense protective layer at the interface.

3.7 Sunflower Oil

Sunflower oil easily adheres to metallic surfaces owing to its long-chain fatty acid groups. It also exhibits good wettability and lubrication characteristics. Therefore, it is generally considered a potential small-quantity lubricating medium [79]. Siniawski et al. [80] studied the concentrations of fatty acids that affect the tribological performance of sunflower oil. When sunflower oil was used, the wear rate decreased. This result can be attributed to the long-chain hydrocarbon fatty acids in sunflower oil, which reduce wear rate and friction compared to mineral oils. Anand et al. [81] analyzed the surface characteristics of Inconel 718 using sunflower oil-based MQL-assisted micro end milling and compared them with dry machining. As a result, the average width and height of the burrs were reduced by ~ 36% compared to dry conditions. When sunflower oil was used, the friction decreased and the quality of the surface of the finished products remained good, even though the feed/tooth range increased. Fox et al. [82] tested oxidized sunflower oil under boundary lubrication conditions using a reciprocating wear ring. They found that the destruction of triglyceride fatty acids and higher levels of hydroperoxides strongly affected wear. Thus, in the boundary lubrication region, the chemical properties of the lubricant significantly affect tribological performance. Jabal et al. [83] investigated the tribological characteristics of sunflower oil using a four-ball tester and four-stroke single-cylinder diesel engine. The wear diameter and flash temperature decreased by 9.51% and 13.06%, respectively, compared with those of mineral oil. When the normal load was approximately 290 N, the friction coefficient of sunflower oil decreased by 2.7% owing to the presence of a long fatty acid chain.

4 Current and Future Improvement Trends

As discussed previously, the shortcomings of biodegradable lubricants prevent their full utilization in industrial applications. Hence, to sustainably utilize biodegradable lubricants, their properties should be improved using various innovative methods. Current and future trends in improving the properties and performance of biodegradable lubricants can be divided into two categories: mixing with additives and genetic modification. The relevant references for each category are reviewed in the following sections.

The use of additives to improve the tribological properties of lubricants is a well-established approach. Mineral lubricants for commercial applications contain 5–25% substances known as additives [84, 85]. These additives have different functionalities (extreme pressure, anti-wear, corrosion inhibitors, oxidation inhibitors, defoamers, and viscosity index improvers) and play a major role in determining and improving the overall performance of lubricants. They can also be used to tailor the properties of the base oil for specific applications. Friction reduction and anti-wear behavior depend on the characteristics of the additives, such as their size, shape, and concentration [49, 86]. A variety of additive materials are available, including 2D-based materials (WS2, MoS2), metal oxides and nitrides (copper, nickel, titanium, aluminum, etc.), and carbon-based materials (CNTs, graphene) and many more to choose from. However, considerations regarding biodegradability exist in the selection of additive materials for biodegradable lubricants. In other words, the additives must be biodegradable to avoid compromising the overall biodegradability of the mixture. For example, traditional additives, such as the most widely used commercial additives, zinc dialkyldithiophosphates (ZDDPs), face strong environmental challenges because they are toxic to water bodies and land soils through the decomposition and generation of S- and/or P-containing poisonous compounds [48]. Using such additives as modifiers defeats the initial purpose of using biodegradable lubricants to minimize the environmental footprint. Table 2 provides a comprehensive summary of the types and optimum concentrations of additives used in biodegradable lubricants. Details regarding the tribosystem and friction improvement percentage are provided for each lubricant. As can be noted, ZDDPs and metal oxides have been widely used as additives in various biodegradable lubricants. Similar to ZDDPs, other metal oxides are toxic and potentially harmful to plants and animals [87]. Therefore, despite their positive effects on tribological properties, incorporating such additives into biodegradable lubricants is not plausible. Future research should focus on the development of biodegradable additives to enhance tribological performance without compromising the degree of biodegradability. Carbon-based modifiers are great examples of biodegradable additives. Multiple types of microbes (both bacteria and fungi) have the ability to degrade CNTs, graphene, and their derivatives [88]. CNTs are enzymatically degraded by peroxidases, neutrophils, and macrophages through enzymatic oxidation. It has been also reported that CNTs undergo biodegradation, and their byproducts are not cytotoxic in vitro or in vivo [89, 90]. It is worth mentioning that many factors, such as CNT type, length, impurities, and surface functionalization, may affect the biodegradation process. Regarding tribological properties, when used in canola, rapeseed, and castor oils, studies have shown that CNT and graphene can improve frictional properties by up to 53% [52, 53, 55, 58, 91, 92]. The mechanisms by which CNT and graphene enhance tribological properties involve the formation of physically absorbing protective films between rubbing surfaces. Omrani et al. [91] compared the tribological performance of graphene and graphite mixed with canola oil. The experiments showed that, in this instance, graphene was more effective than graphite at improving friction and wear properties. The graphite particles were larger than graphene and thus could not easily penetrate the contact interface. The results also showed that an increase in concentration resulted in the aggregation and coagulation of graphene, deteriorating its tribological properties. Ouyang et al. [59] compared the tribological performance of a blend of 3D hierarchical porous graphene (3D HPG) and multi-walled carbon nanotube (MWCNT) additives in castor oil under heavy loads and low-speed sliding conditions. An investigation of the wear mechanism revealed that the 3D HPG was torn into 2D graphene nanosheets owing to its easy shear performance, which formed 2D graphene nanosheets. On the other hand, MWCNT improves the friction properties by filling the grooves and pits and making the interfaces smoother.

Table 2 Additives and their effect on the frictional properties of bio-lubricants
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This phenomenon accelerates the formation of a protective film by the suspended 3D HPGs. The study reported that the rolled bearing-like synergistic effect could further reduce friction and wear compared with cases in which 3D HPGs and MWCNT were used individually in castor oil. Figure 6 presents the scanning electron microscopy (SEM) analysis and schematics of the governing wear mechanisms.

Fig. 6
figure 6

SEM analysis of the worn surfaces and schematics of the wear mechanisms under lubrication with castor oil containing a 0.5% HPG, b 0.7% MWCNT, and c HPG:MWCNT (0.5:0.7). The figure was adapted from reference [59] with permission from Elsevier

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Several possibilities exist for the development of carbon-based additives for biodegradable lubricants to improve their tribological performance. Graphene and CNT can be functionalized with various functional groups to enhance their tribological properties. In a recent study, Ge et al. [112] reported the functionalization of graphene-oxide nanosheets with amino groups (NH2) to facilitate macroscale superlubricity. GO-NH2 showed a drastic improvement of 50% in friction compared to GO. Functionalization enabled a robust adsorption layer on the worn surfaces owing to the high adsorption of amino groups. This robust GO-NH2 adsorption layer not only protects the contact surfaces and contributes to low wear but also causes the shearing plane to transform constantly from solid asperities (high friction) into GO-NH2 interlayers (weak interlayer interactions), resulting in superlubricity. Using molecular dynamics simulations, Cui et al. [113] investigated the effects of functional group type and degree of functionalization on the mechanical and tribological properties of CNTs. It was found that among the different surface-modified CNTs (i.e., carboxyl, hydrazide, and ethylenediamine), the amide-functionalized CNT exhibited the greatest improvement in its mechanical and tribological properties. Interestingly, a higher degree of sidewall functionalization (> 10%) reduces the enhancement effect by destroying the perfect CNT structure. Experimental observations of the positive effects of CNT functionalization on tribological properties by grafting carboxyl and amino groups have also been reported [114].

The next category regarding the future improvement trend of biodegradable lubricants is their genetic modification. Genetic modification can be used to increase resistance to herbicides and to modify fatty acids that in turn improve the oxidative stability and tribological behavior of biodegradable lubricants. Transesterification, Epoxidation, and Hydroxylation (reaction) are the procedures to change the ester ratio, chain length, and unsaturation ratios [115,116,117]. The ratio of ester in biodegradable oil affects its biodegradability and thermal properties. Talib et al. [118] studied the tribological properties of jatropha oil according to the various molar ratios of methyl ester. The transesterification process was carried out to change the methyl ester ratio in jatropha oil. The effect of jatropha methyl ester (JME) and trimethylolpropane (TMP) on tribological characteristics was investigated using a four-ball tester at different JME and TMP ratios. The friction coefficient decreased, and the viscosity increased as the JME ratio increased. The ratio of 3.5:1 (JME:TMP) showed the highest viscosity and the lowest average friction coefficient. Figure 7 presents a schematic of the chemical modification process and the obtained COF values with respect to molar ratio of methyl ester.

Fig. 7
figure 7

a Chemical modification process. b Friction coefficient of jatropha oil according to the molar ratio of methyl ester (CJO (Crude jatropha oil)) and MJO (Modified jatropha oil), JME:TMP; 3.1:1), MJO3 (JME:TMP; 3.3:1), MJO5 (JME:TMP; 3.5:1). The figure was adapted from reference [118] with permission from Elsevier

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The length of fatty acid chains also affects the tribological properties. The longer fatty acid chain in oil can reduce friction and wear through formation of high-strength lubricant films at the contact interface. To change the fatty acid chain length, the epoxidation process has to be conducted by replacing or ring opening. Thampi et al. [119] analyzed the tribological properties of rice bran oil (RBO) through epoxidized (ERBO) using a four-ball tester. The applied normal load and the rotational speed were 392N and 1200 rpm, respectively. The average COF of RBO and ERBO were 0.09 and 0.047 that indicates a reduction of 48% after the epoxidation process. Also, the wear scar diameter was reduced from 555 to 547 µm.

Manipulating the ratio of unsaturated bonds is another way of altering the tribological properties. The ring-opening reaction is the process of introducing hydroxyl groups into unsaturated bonds. The oxirane rings in the oil must be open to introduce hydroxyl groups. This chemical process takes place through the cleavage of one of the carbon–oxygen bonds in the oxirane rings and is a useful procedure for creating hydroxy ester compounds [120]. Arumugam et al. [121] conducted a hydroxylation process to improve the tribological properties of rapeseed oil. To evaluate tribological properties, a four-ball test was used. The average COF values of raw and modified rapeseed oil were 0.09 and 0.04, respectively. Through the hydroxylation process, the structure of rapeseed oil becomes a long fatty chain. These changes made the rapeseed oil to remain at the contact interface for a prolonged period, thus improving the wear resistance.

5 Conclusions

In this paper, we introduced various sources of biodegradable lubricants and reviewed their properties and applications with the aim to gain a better understanding regarding their advantages and limitations. Emphasis was then given to the tribological properties of biodegradable lubricants. The studies show that the chemical composition of biodegradable lubricants can significantly affects their tribological properties. Based on the extensive literature surveyed, although biodegradable lubricants show better tribological properties than mineral lubricants in some instances, there are still drawbacks to overcome to enable the full implementation of biodegradable lubricants in various industries. Lastly, current status and future improvement trends of biodegradable lubricants were discussed. To be able to picture a sustainable future for biodegradable lubricants, it is imperative to conduct extensive researches to improve their properties by means of developing novel bio-additives and genetic modification. In summary, with the growing environmental concerns and consistent technological advances, biodegradable lubricants are expected to gain more attention from various industries.