On the Difference in the Action of Anti-Wear Additives in Hydrocarbon Oils and Vegetable Triglycerides

Abstract

This paper examines the difference between the effects of anti-wear additives on vegetable and hydrocarbon-based oils. Knowledge of the specific influence of additives on the anti-wear properties of vegetable oils is necessary to increase the efficiency of the development of biodegradable lubricating oils. In addition, this is interesting from the point of view of clarifying the mechanism of action of AW/EP additives. The effect of non-toxic additives—adipic acid monoester and hexadecanol—on hydrocarbon hydrocracking oil and vegetable oil was compared. The comparison was carried out in rolling contact with sliding, sensitive to the separating ability of the oil. It was found that in hydrocarbon oil, the additive affects the parameters of the hydrodynamic friction regime. When adding an additive to vegetable oil, the hydrodynamic parameters do not change. The additive acts in the same way in both oils during mixed and transient modes. The obtained results are compared to available data, and an explanation of the difference is proposed based on the AW/EP mechanism of action. It is concluded that there is little chance of enhancing vegetable oil properties for hydrodynamic bearings. Search criteria for additives that effectively influence the antifriction and anti-wear properties of vegetable oils in mixed and boundary friction modes are proposed.

1. Introduction

Due to the globally recognized importance of environmental and hygienic aspects concerning the operation of various machinery, there is currently a wide range of biodegradable lubricants being developed and utilized. Leading manufacturers such as BECHEM (Hagen, Germany) and PANOLIN (Madetswil, Switzerland), among others, provide lubricants for various purposes.

In recent years, global production and research focused on environmentally friendly lubricants and fluids made from renewable sources, including vegetable oils and compositions of vegetable and mineral lubricating oils, have been heavily developed [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15]. The study of specialized literature on this topic demonstrates these advancements. These materials are primarily used in industries such as forestry, agriculture, construction, and small boat engines. Vegetable oils find extensive use in hydraulic systems, gearboxes, and bearings that operate in diverse mechanisms, machines, and equipment used in the processing industry, land reclamation, as well as the plowing and harvesting of food crops. They hold immense functional significance for equipment in its entirety. The lifespan of hydraulic machines and systems is primarily determined by the characteristics of their working fluid, including rheological parameters, resistance to oxidation, and anti-wear properties. The issue of substituting mineral oils with eco-friendly options has been resolved through two approaches: partially substituting mineral oils with vegetable-mineral compositions mixed in specific ratios and fully replacing them with vegetable oils that contain functional additives. The popularity of utilizing vegetable oils as lubricants is attributed to their main benefits over hydrocarbon oils, which include being nontoxic, biodegradable, renewable, and easily accessible [14,15,16,17].

It is established that vegetable oils can only meet the physico-chemical and tribological requirements (anti-wear and anti-seize) of certain machines under specific conditions. It is important to note that technical abbreviations must be explained when first introduced. These oils, including rapeseed, sunflower, linseed, castor, palm, and cotton, contain organic surfactants in the form of various unsaturated fatty acids, such as oleic, stearic, erucic, linoleic, and mono- and di-ethers of these acids, among others. The surfactant layers adsorbed onto the metal surface exhibit a damping effect on the friction contact and inhibit direct interaction between the friction surfaces. Organic surfactants found in vegetable oils are natural anti-wear additives, particularly in the context of combining vegetable and mineral oils. Nevertheless, synthetic additives developed for hydrocarbon oils, such as dialkyl dithiophosphates of zinc, are significantly more effective than natural anti-wear components. Vegetable oils typically have a higher viscosity index (VI) of around 200 compared to mineral oils.

2. Features of Vegetable Oils and Application Problems

The effectiveness of using pure vegetable oils in friction units is hindered by their low thermal and oxidative stability, as well as their high solidification temperature. To increase thermal-oxidative stability, it is necessary to reduce the content of unsaturated fatty acids and free acids in the oils. Although, in doing so, the anti-wear properties are also diminished. The majority of research is focused on technical rapeseed oil, which is currently the most common lubricant. It should be noted that the benefits of biodegradable oils are most evident when used in technical equipment that may potentially leak lubricants into the natural environment, such as soil or water bodies. Such equipment often includes gears, hydraulic systems, bearing units, and leak-prone moving joints. Most of these machines are characterized by high loads and require lubricants with excellent anti-wear properties. For most gear and transmission oils and greases, the ASTM D4172 measurement for anti-wear properties requires a wear track diameter of 0.45 to 0.5 mm at an axial load of 392 N. Meanwhile, typical values for hydraulic oils range from 0.5 to 0.6 mm. For vegetable oils, specifically rapeseed oil, the wear track diameter ranges from 0.7 to 0.8 mm. Sunflower, cotton, and castor oils typically require a wear track diameter of 0.8 to 1.0 mm or even more. In this regard, many biodegradable oils derived from vegetable fats possess anti-wear additives. Occasionally, they may be enhanced with antioxidant additives [18,19,20].

Significant improvement in the anti-wear properties of lubricants based on vegetable triglycerides presents a considerable challenge. A broad spectrum of anti-wear additives developed for hydrocarbon oils are either unsuitable for vegetable oils or ineffective when used in them. The most prevalent additive type, zinc dialkyldithiophosphates (ZDDP), undergoes chemical reactions with organic acids and their esters. Alkylsalicylates can be quite costly and offer limited effectiveness. Alkyltriazoles do not significantly improve the anti-wear properties of vegetable oils. Certain additives, such as organic disulfides and polysulfides, as well as derivatives of alkylphenols and alkylsulfonates, can greatly decrease the biodegradability of oils.

Three methods can enhance anti-wear properties: adding solid lubricating components to the lubricating oil, as shown in [7,8,9,10,11], modifying vegetable oils chemically, as demonstrated in [12,13], and adding oil-soluble anti-wear components, as detailed in [14,15,16,17,18,19,20]. Solid lubricating components are applicable only to grease lubricants. The use of nanosuspensions made from metals and other inorganic components that are resistant to settling can cause deterioration in oil filterability. Oils are a crucial parameter for the functioning of hydraulic systems and engines. Operators need to be aware that the composition of oils can change due to operational influences. The presence of water and the oxidation of oil can both notably impair the stability of nanosuspensions. Modifying vegetable oils chemically, such as by creating thioesters, can lead to a reduction in biodegradability and the creation of corrosive products during operation.

To preserve the advantages of vegetable oils, such as their non-toxicity, lack of odor, and ease of production, the authors suggest that the most promising method to boost tribotechnical performance involves incorporating completely soluble functional additives. This approach has yielded outstanding outcomes for lubricants that rely on hydrocarbon oils. For more information on such studies, please refer to articles [14,15,16,17,18,19,20]. Although certain AW/EP components developed for hydrocarbon oils [16,17,18] have been successfully applied, studies have shown that the efficiency of these additives differs between hydrocarbon and vegetable oils. The enhanced adsorption of molecules of vegetable oils and esters on the metal surface hampers the adsorption of molecules of conventional AW/EP components. Therefore, substances that are more polar than those used in hydrocarbon oils are found to be more effective.

The primary areas of application for biodegradable oils include engines utilized for water transport in enclosed water bodies, hydraulic systems and transmissions of agricultural machinery, and equipment used in food processing. The development of anti-wear additives for these specific uses is primarily connected to the adsorption mechanism of the additive’s action. Oil-soluble surfactants with a large polar group that can easily adsorb onto metal surfaces and long hydrocarbon radicals for intermolecular interaction with the base oil are expected to have high efficiency.

More successful varieties of esters for vegetable-based lubricating oils include complete and incomplete esters made from bidentate and multibasic organic acids with aliphatic alcohols. However, the development of such additives is complicated by differences in the antifriction and anti-wear actions of additives compared to those used in hydrocarbon oils. As indicated in prior studies [21,22], anti-wear additives concomitantly decrease the test results for wear spot diameter in accordance with the ASTM D4172 method, as well as wear and friction coefficient in experiments utilizing conformal sliding friction units that mimic sliding bearings. Efficiency is demonstrated in hydrodynamic mode by achieving liquid friction at higher contact pressures than unadditivized oil. This allows for the evaluation of additive effectiveness through the standard four-ball method for various friction units under differing contact pressures and speeds, including radial bearings of liquid friction, gears, rolling bearings, and others. In contrast, when testing additives for vegetable oil, there is typically no impact on the hydrodynamic regime parameters, despite a significant reduction in the diameter of the wear trace observed in four-ball tests. Consequently, several critical questions arise that demand in-depth exploration.

Studying the impact of additives on various friction regimes is crucial to resolving multiple problems. One of these problems is identifying the upper limit of anti-wear and/or anti-friction characteristics achievable for the particular base oil. In addition to the viscosity parameter, this level is determined by the structure of base oil molecules, the properties and structure of additive molecules, and the interaction between additives and the base oil. Research on hydrocarbon oils has resulted in notable enhancements in anti-wear properties for engine oils and has made it feasible to use energy-efficient engine oils with viscosity grades SAE 20 and lower instead of traditional SAE 40 and SAE 30 (ACEA 2016). This was only accomplished after extensive and intensive research conducted by major oil companies such as Exxon Mobil, Royal Dutch Shell, BP, and Fuchs, among others. For biodegradable oils made from vegetable triglycerides, this type of research requires significantly less effort and funding. In such circumstances, it is crucial to understand the achievable level of target parameters for the lubricating oil, especially in terms of its anti-wear properties.

The second task pertains to the development of lubricating oils optimized for specific friction units. Lubricating oil requirements vary between hydrodynamic liquid friction bearings and rolling contacts. Similarly, low-speed heavy-duty rolling bearings and gears necessitate distinct oil parameters for optimum performance as compared to high-speed bearings. On the other hand, for hydrocarbon oils, a solution is to utilize a wide range of base oils with varying viscosities [14] in combination with AW/EP/HD additives. The limited range of possible viscosity levels in vegetable oils presents a challenge in adapting to the requirements of certain technical devices. As for vegetable oils, the main method of meeting technical device requirements involves the use of solid or oil-soluble additives. It is important to assess the potential of sourcing additives and utilizing vegetable bases for certain applications of lubricating oil.

The third task pertains to the mathematical modeling of friction units intended for use with biodegradable grease. The most challenging aspect of the calculation of friction units is the mathematical modeling of the lubricating layer. It is a well-established fact that the outcomes of hydrodynamic calculations of sliding bearings align with experimental results only when the thickness of the lubricating layer is sufficiently large. When load and speed values approach their maximum for units like crankshaft bearings in automobile engines, the calculated results start to significantly deviate from experimental results. This occurs when the rheological model of the lubricating fluid solely depends on the dynamic viscosity coefficient related to temperature, pressure, and shear rate. In this case, it is challenging to explain the feasibility of utilizing conventional motor oils with a viscosity rating of SAE 20, such as Toyota 0W-20, in automobiles that demand oils with a viscosity range of SAE 30 and even SAE 40 (typically unsupercharged passenger car engines). Explaining the differences in hydrodynamic regime parameters when using oils with the same viscosity or the base oil and the base oil with an anti-wear additive presents significant challenges. Realistic results in contact interaction models can only be obtained for dry friction conditions. Empirical parameters derived from experimental data are always used in the presence of lubrication, such as those cited in references [23,24,25,26,27].

When modeling friction units lubricated with biodegradable oils that contain additives like esters of two- or three-basic organic acids, the traditional explanation of the anti-wear and antifriction properties arising from the formation of iron sulfides or phosphides on the contact surface is not applicable. Instead, the influence of the adsorbed additive layer and its effect on the rheological parameters of the separating grease layer necessitate adequate modeling. Otherwise, the calculated results are not appropriate for predicting friction parameters under changing conditions. It is worth mentioning that delineating the physical and chemical mechanisms that prompt changes in the lubricant’s state due to adsorption lies beyond the purview of tribology and necessitates the engagement of other scientific fields’ methods [28].

However, the laws of physical chemistry only allow for obtaining general insights about potentially altering liquid viscosity upon contact with a solid surface and the likelihood of a phase transition occurring. This can occur due to a decrease in the entropy of the lubricating layer resulting from adsorption. Quantitative data essential for mathematically modeling frictional contact can only be procured through experimentation, specifically by examining the impact of anti-wear additives on friction parameters in various modes.

3. Methods for Evaluating the Anti-Wear Effect of Additives

Information regarding the impact of anti-wear (AW) additives on lubricating oils can be acquired through traditional test methods, assuming that the friction mechanisms are suitably represented. These tried-and-true approaches encompass establishing correlations between the friction coefficient and the contact pressure while testing on conformal sliding units that simulate a radial liquid friction bearing. The obtained dependencies enable the comparison of lubricating oils based on two critical parameters: the maximum contact pressure value at which friction occurs in hydrodynamic mode and the contact pressure that triggers the transition to the boundary mode of friction. Figure 1 provides a typical example. The data shown in Figure 1 and Figure 2 are published in our studies [28].

The ASTM D4172 method is the most commonly used method for determining anti-wear properties, although its friction conditions differ significantly from those typically found in most technical devices. Balls with a diameter of 12.7 mm made of ball-bearing steel are used as samples in this method.

A more informative approach is to measure mass or linear wear in laboratory friction units that simulate actual friction conditions in specific technical devices. In this instance, it is of interest to determine the friction mode in which the AW additive effect is maximized. This will enable assumptions to be made about the anti-wear mechanism, specifically regarding the influence of the additive on the contact condition. It is evidenced by direct mass wear measurements (Figure 2) in the same contact featured in Figure 1.

When comparing Figure 1 and Figure 2, it is evident that an increase in ZDDP content from 1.25% to 2.5% leads to a substantial enhancement of antifriction parameters between 15 and 20 MPa of contact pressure. However, the same increase in ZDDP content results in a comparatively negligible improvement in anti-wear properties. As contact pressure increases further, sliding properties noticeably contrast, while the distinction in wear values diminishes. In the four-ball test (392 N axial load), incorporating 1.25% ZDDP into the base PAO oil yielded a 2-fold decrease in wear spot diameter compared to the pure oil. However, at 2.5% ZDDP, the wear spot is larger than at 1.25%. In the typical case described, augmenting the AW additive concentration above optimal values results in increased corrosive wear. Consequently, the efficacy of AW additives may significantly differ in various technical frictional units.

Comparing laboratory test results and the performance properties of vegetable oils can be a complicated matter. An instance of this is when a vegetable oil containing 4% wt. trihexadecyl ester of citric acid was subjected to ASTM D4172 tests. At a load of 392 N, this concentration displayed the maximum effect, resulting in a 1.5-fold reduction in the wear spot diameter from 0.75 mm to 0.5 mm. However, our experimental studies of the contact tests that simulated a plain bearing revealed no discernible difference between the original and additive oils (Figure 3) [29].

Thus, a proper evaluation of the effectiveness of the chosen or developed AW/EP additive requires the use of a test method that replicates the primary friction attributes in the desired technical assemblies.

4. Method and Results of Comparative Tests of Biodegradable Additive in Hydrocarbon and Vegetable Base Oils

The authors have selected the primary testing method with two assumptions in mind. Firstly, evaluating the effectiveness of oils with AW and AW/EP additives based on their ability to separate rubbing surfaces at various contact pressures and surface displacements is rational. Secondly, out of all the potential uses of biodegradable oils, rolling contacts achieve the maximum contact loads. The most demanding friction conditions occur in gear contacts where rolling with slip is present. Tests involving rolling contact with slips are of particular interest. The contact utilized in these tests involves two steel rollers with respective diameters of 90 mm and 45 mm, both rotating at equal angular velocities. The large roller was made from tractor engine crankshaft material with appropriate processing. These rollers are fitted onto two driving shafts of the II-5018 friction machine, as depicted in Figure 4. This method of friction contact ensures that measurement results are highly stable and reproducible.

The width of the larger roller is 20 mm, while the width of the smaller roller is 10 mm. The working surfaces of the rollers have moduli of elasticity of 230,000 MPa and 200,000 MPa, respectively. The test conditions include a rotational speed of ω = 200 min⁻¹, a stepwise increase of pressing load from 0 to 2500 N with an interval of 100 N, a roughness of roller surfaces of R_a = 0.2, an initial temperature of 50 ± 4 °C, and a final temperature of 65…72 °C. The lubricating oil supply to the contact zone is excessive and drips.

The calculated parameters are as follows:

-

The friction force (F) is calculated using the equation F = M/0.045, where M represents the friction torque and 0.045 is the radius of the roller on the shaft with the torque sensor.

-

The friction coefficient (f_fr) can be determined by the formula f_fr = F/P, where P is the load.

-

The maximum contact stress (σ_n) can be found using Equation (1):

$${\sigma }_n=\sqrt{\frac{q}{{\rho }_{pr}}}·\frac{E_{pr}}{2\pi \left(1-{\mu }^2\right)},$$

(1)

where q represents the distributed load (q = P/10 mm), ρ_pr denotes the reduced roller radius, E_pr represents the reduced modulus of elasticity, and µ indicates Poisson’s ratio (in this particular instance, taken to be 0.28 for solid surfaces).

Based on the data we obtained and calculated, we have plotted the f_fr(1/σ_n) dependencies.

We selected two base oils for our analysis: VHVI-4 hydrocarbon hydrocracking base oil and refined sunflower oil. We used a monoester of adipic acid and hexadecyl alcohol as an additive and decided to use hydrocracking base oil as it is a better solvent for monoesters of bivalent acids, which are not very soluble in PAO and mineral oils. We added the additive to both base oils at a concentration of 2% by weight.

The results of the hydrocarbon oil tests are displayed in Figure 5.

According to Figure 5 data, adding ashless biodegradable additives to hydrocarbon oil results in a reduced friction coefficient in the hydrodynamic region. This effect increases as the contact pressure rises and the lubrication layer gets thinner, with the hydrodynamic regime region extending to higher contact pressure values. In the case of vegetable oil (Figure 6), there is a minor reduction in the coefficient of friction in the hydrodynamic regime. As the contact pressure rises, the additive’s impact is only noticeable in the expansion of the stable coefficient of friction values to higher contact pressure zones compared to pure oil. In transient mode areas, the additive has no impact on the friction coefficient.

5. Discussion

Several prior studies by the authors [28,29,30,31] support the concept of the adsorption mechanism of anti-wear additives in hydrocarbon oils. The models developed are based on the physico-chemical principles of secondary polymolecular adsorption of hydrocarbon liquid on metal surfaces that have been modified by surfactants, specifically AW additives. The model comprises a conditional viscosity µ_S for a slim adsorbed layer next to the metal surface and a characterization l_h for the exponential drop in viscosity as distance increases from the surface owing to the effect of thermal motion on the adsorbed layer. The technical terms are thoroughly explained, and there is a clear, logical flow of information. In the lubricating layer with a thickness of h that separates the surfaces of the tribocoupling, the conditional viscosity µ_i at a distance h_i from the surface can be ascertained utilizing the formulas for one (2) and two surfaces (3).

$${\mu }_i={\mu }_0+{\mu }_S\left(T\right)\exp \left(-\frac{h_i}{l_h\left(T\right)}\right),$$

(2)

$${\mu }_i={\mu }_0+{\mu }_S\left(\exp \left(-\frac{h_i}{l_{h_1}}\right)+\exp \left(-\frac{h-h_i}{l_{h_2}}\right)\right),$$

(3)

where µ₀ is the value of initial oil viscosity without the influence of surfaces.

Under the condition of minimizing the fluid friction force F_fr, the shear rate dV/dh takes zero values in high-viscosity layers h_S1 and h_S2, which are adjacent to the surface. As a result, the average viscosity of the layer takes on a value known as the equivalent viscosity:

$$\overline{\mu }=\frac{1}{h-2h_S}\underset{h_{S1}}{\overset{h-h_{S2}}{\int }}\left({\mu }_0+{\mu }_S\left(\exp \left(-\frac{h_i}{l_h}\right)+\exp \left(-\frac{h-h_i}{l_h}\right)\right)\right)d{h}_i,$$

(4)

where the values of h_S1 and h_S2 are found by the condition:

$$F_{fr}=\min \left(\frac{V_0}{{\left(h-2h_{S1}\right)}^2}\underset{h_{S1}}{\overset{h-h_{S2}}{\int }}\left({\mu }_0+{\mu }_S\left(\exp \left(-\frac{h_i}{l_h}\right)+\exp \left(-\frac{h-h_i}{l_h}\right)\right)\right)d{h}_i\right),$$

(5)

where V₀ is the velocity of displacement of the surfaces.

Further model development is necessitated considering the destruction of the polymolecular adsorption layer at a critical shear rate value for each current value of µ_i. This condition, linked to liquid structurization and similar to liquid crystals’ properties, poses significant challenges for description. For the sake of convenience in hydrodynamic calculations, the authors suggest incorporating the impact of shear stresses on the l_h parameter via a smooth function as follows:

$$l_{h_1}=l_{h0}\exp \left(\frac{F_0}{F_1}\right),$$

(6)

where F₀ and F₁ are two load values, and l_h0 and l_h1 are their corresponding values of the parameter l_h. Accordingly, the current value µ_i in Equation (3) takes the following form:

$${\mu }_i={\mu }_0+{\mu }_S\left(\exp \left(-\frac{h_i}{l_{h0}\exp \left(\frac{F_0}{F_1}\right)}\right)+\exp \left(-\frac{h-h_i}{l_{h0}\exp \left(\frac{F_0}{F_1}\right)}\right)\right).$$

(7)

It is important to note that the mechanical parameters of the adsorbed layer of lubricating oil are represented by the gradient of dynamic viscosity. This representation was chosen for the sole purpose of convenience in hydrodynamic calculations for sliding bearings with liquid lubrication. The calculations based on the aforementioned model are presented in [31] and reveal the coincidence of both calculated and experimental dependencies of force and friction coefficient on contact pressure.

The impact of an ashless biodegradable additive on hydrocarbon oil aligns seamlessly with the proposed model and concepts of the AW additive adsorption mechanism. The lessening of force and coefficient of friction in transient modes primarily stems from the wider load distribution in comparison to additive-free oil. In addition, the lubricating layer separates surfaces at higher levels of contact pressure.

In the case of vegetable oil, the additive’s introduction has negligible effects on the hydrodynamic regime’s parameters. For hydrocarbon oils, the secondary adsorption on the additive’s adsorbed monomolecular layer’s surface results in a rise in the number of dispersion bonds between molecules, thereby structuring the fluid. This effect is sustained for a specific number of molecular layers, which gradually weaken because of the effects of thermal motion. Hydrocarbon oils with varying structures of hydrocarbon molecules demonstrate varying effectiveness in terms of their additive acceptability. Triglyceride molecules, which constitute vegetable oils, contain polar oxygen-containing groups and large hydrocarbon radicals of carboxylic acids. Due to the diversity, complex structure, and high number of potential conformations of these molecules, along with the presence of polar ester groups, it is exceedingly challenging to create new intermolecular bonds even with restricted motion. Therefore, when describing the separating layer of vegetable oil, these factors should be taken into account. Acceptance of smaller values of parameter l_h (4)–(5) and its greater dependence on shear stresses (6)–(7) is recommended. The diagram presented in Figure 6 depicts the substantial impact of additives on anti-wear properties during ASTM D4172 tests, as well as their ability to ensure separability at high contact pressures. These results also suggest the influence of the additive on the lubrication parameters used in boundary friction models, specifically the µ_S parameter in the referenced model.

Thus, the model is intended to describe the real rheological parameters of the lubricating layer separating the surfaces. In accordance with this, the model contains parameters that reflect the individual properties of the lubricant. They depend on both the base oil and additives. The influence of the additive structure is described in [29]. The model provides only a qualitative assessment; specific parameters are obtained empirically. This article discusses the effect of base oil (additive acceptance). The presented results make it possible to determine which of the model parameters should be obtained experimentally to perform the correct calculations. This is new information that allows us to predict the results of research to improve anti-wear properties. Carrying out calculations and their verification requires a large amount of work and will be described in the following works.

As you know, hydrodynamic calculations of sliding bearings give results significantly different from experimental ones. For smooth surfaces with an Ra less than 0.1, the experimental regime of fluid friction occurs at loads at which the calculated thickness of the lubricating layer becomes equal to 0 or negative, but only if an effective lubricant is used.

Anti-wear additives introduced into the lubricating oil sharply increase the maximum load up to which the liquid regime is implemented. Moreover, this is also observed for additives that do not contain sulfur or phosphorus. A huge number of facts are known to suggest that thin layers of liquid lubricant separating the friction surfaces have properties that differ from the usual properties of lubricating oil. Of all the possible physico-chemical mechanisms for changing the properties of a liquid upon contact with a solid surface, the adsorption mechanism provides the greatest effect.

It lies in the fact that if liquid molecules are firmly adsorbed on the surface, then the thermal movement of these molecules is limited. Since liquid molecules interact with each other and the thermal movement of the adsorbed layer is limited, the energy of the bonds between it and the adjacent liquid layer increases. The same happens with the next layer. As a result, the change in the thermodynamic parameters of the liquid that occurred as a result of adsorption is transmitted to a certain layer of the liquid, resulting in a gradual weakening of this effect. This effect has been studied to the greatest extent for water adsorbed on polar solid surfaces. The Brunauer–Emmett–Teller (BET) theory is widely used in the engineering calculations of desiccants and water absorbers. As applied to other liquids, the theory is poorly developed.

As stated in the article, the description of the mechanical properties of the lubricating property only in terms of viscosity was introduced to simplify the application of calculations. Currently, the model is being developed to take into account the existence of a limiting shear stress that destroys the structure of the layer. This will make it possible to extend the model to cases of mixed friction.

6. Findings and Conclusions

The results suggest that the addition of anti-wear additives is not a favorable option when utilizing vegetable oils in hydrodynamic friction units. However, such additives may enhance the operational characteristics of vegetable-based lubricants in both gears and rolling bearings. In technical devices like gasoline saws, engines of small vessels, chain transmissions, and distributive devices of hydraulic systems, boundary and transient friction modes are prevalent. The application of AW/EP additives has proven to be effective in these cases.

According to the adsorption mechanism of action, optimal efficiency can be expected from additives containing free acid groups. However, acid groups’ enhanced activity toward metals may result in corrosive wear. An alternative solution may be monoesters of tri-basic organic acids with aliphatic alcohols or (preferably) alkylated biphasic acids, such as free alkylsalicylates.