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Article

Lubricating Performance of Lanzhou Lily Crude Extract as Natural Additive

by
Min Xie
1,*,
Chenghua Lai
1,
Juanjuan Chen
2,
Feng Cao
1 and
Ying Ma
1
1
School of Chemical Engineering, Lanzhou City University, Lanzhou 730070, China
2
State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China
*
Author to whom correspondence should be addressed.
Lubricants 2025, 13(1), 34; https://doi.org/10.3390/lubricants13010034
Submission received: 27 November 2024 / Revised: 29 December 2024 / Accepted: 30 December 2024 / Published: 14 January 2025
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Abstract

:
Due to environmental concerns and multifunctional requirements, natural additives are a promising alternative to traditional enhancers like metal nanoparticles. Lanzhou lily crude extract (LLCE) was investigated as an additive in PEG400 base oil. Firstly, different contents of LLCE as an additive notably improved physiochemical properties, such as thermal stability, viscosity, and the viscosity index. With 5–10 wt.% of LLCE added in PEG400, the thermal degradation temperature increased by 37.3–49.5 °C. Secondly, the addition of LLCE significantly reduced the friction coefficient and wear rate of the steel disc at 50–150 N compared to pure PEG400 base oil with a reduction degree of up to 30% and 92%. The optimum additive content was 7 wt.%, and further increasing the content of the additive did not bring about obvious improvements, even worsening the product in some cases. Thirdly, the lower friction coefficient and wear rate achieved through the addition of LLCE may be due to the higher viscosity facilitating a thicker lubricating film and the higher polarity providing better chemical affinity with the metallic surface. In summary, LLCE is a promising additive that significantly improves the physicochemical properties and lubricating performance of PEG400 base oil.

1. Introduction

To introduce certain properties into lubricating oil, special additives such as anti-wear additives, extreme pressure enhancers, antioxidants, corrosion inhibitors, detergents, viscosity improvers, etc., have been used to satisfy specific applications. Due to the toxicity of traditional additives, bio-originated and eco-friendly additives are required for various real-world applications. Eco-friendly particulate additives are mainly used as anti-wear and extreme pressure additives. Characteristic examples include hexagonal boron nitride (BN), boric acid, ZnO, graphene or graphene oxide comparable to tungsten disulphide (WS2), molybdenum disulphide (MoS2), and graphite (C) [1].
To meet increasingly diverse applications and stringent environmental concerns, natural vegetable oil has also been introduced in many fields. Numerous natural vegetable oils have been investigated and demonstrated different lubricating performances due to their intrinsic chemical properties. The biolubricant stock has been constantly broadened and chemically modified by different proposals to obtain various types of biolubricants and additives [2]. Bahari et al. found that the lubricating performance of palm oil and soybean oil mixed with SAE 15W40 improved by 25% and 27%, respectively, compared with pure oil at high temperatures in contact pressure reciprocating contact [3]. Shahabuddin et al. reported that the addition of jatropha oil in mineral oil could reduce the friction coefficient and wear scar diameter by 34% and 29% using Cygnus wear and four-ball tribotesting machines [4]. Jabal and Khalefa found a mineral–mustard oil blend to be a promising lubricant with a lower friction coefficient and wear scar diameter using the four-ball tribotester [5]. Farfán-Cabrera discovered that a mixture of mineral oil and jatropha oil (volume ratio of 4:1) showed better tribological performance than pure mineral oil and jatropha oil. The blended oil exhibited an anti-shudder property, enabling the oil’s application as part of an automatic transmission fluid formulation [6]. The addition of transesterified olax oil in petrol diesel not only increased the runtime but also reduced the temperature of the emitted gas during engine tests. The blended oil in the fuel pumps of compression ignition direct injection engines simultaneously played the roles of fuel, coolant, and lubricant [7]. Lee reported that trimethylolpropane trioleate blended in polyalphaolefin (PAO) could significantly increase the viscosity index and reduce the friction power and wear scar diameter of a cast iron sliding pin against a wear disc fabricated from JIS-SKD11 tool steel by 6% and 40%, respectively [8]. Moreover, a kind of mucilage collected from Brasenia Schreberi leaves, an aquatic plant, could dramatically improve the friction reduction and anti-wear properties of synthetic ester (SE) for both steel–steel and steel–aluminum friction pairs [9].
Other natural additives such as ginger, garlic, and black pepper have been shown to enhance thermal stability and lubrication properties and bring down the pour point of coconut oil [10]. Vitamin C, Vitamin E, citric acid derivates, gallic acid ester, and lipid-modified EDTA have been used as natural antioxidants [11]. Succinic acid ester can be used as a corrosion inhibitor to avoid the corrosion and rusting of metal parts on a contacting surface. Natural amino acid derivatives like the Schiff base ester of cystine have strong anti-friction, anti-wear, and anticorrosion effects due to the existence of a disulphide group [12]. Ethylene-vinyl acetate and ethyl cellulose are perfect viscosity-modifying agents that can increase viscosity manyfold with small-quantity additions [13]. Oleogel, created with lignin–castor oil, has shown significant improvements in oxidation resistance, and the wear of steel contacts lubricated with the oleogel was shown to be 64% lower than that with pure castor oil [14].
Due to the multifunctional and biodegradation capabilities mentioned above, natural additives have become a promising alternative to traditional enhancers. Investigating novel unknown vegetable oils, as base oils or additives, is essential for updating multi-purpose biolubricants with excellent properties [15]. Lilium davidii var. unicolor, known as Lanzhou lily or sweet lily, is widely cultivated in Lanzhou, Gansu Province, China, for its nutrition and medicinal usage. Lanzhou lily is in high demand due to its sweet taste, little fibre, no bitterness, and white jade colour. Lanzhou lily is the best-quality and only edible sweet lily in China. Recent statistics show that Lanzhou lily is widely cultivated in Lanzhou, Gansu Province, and annual production is about 1.4 × 105 tons of lily bulb. Meanwhile, waste from the abandoned aerial parts and poor outer parts produced during the growth and production processes of the lily amounted to about a million tons. This could supply a sustainable feedstock of biolubricants or additives. The chemical composition of the lily bulb includes abundant phospholipids, phenols, fatty acids, alkaloids, sulphides, and polysaccharides. Research from our group has suggested that Lanzhou lily crude extract (LLCE) as a biolubricant exhibits good thermo-oxidative stability and an excellent extreme load-carrying capacity [16]. Therefore, LLCE as a sustainable additive was blended in PEG400, the physiochemical and lubricating properties were investigated, and related mechanisms were discussed.

2. Materials and Methods

2.1. Preparation of LLCE and PEG400-LLCE Lubricants

The discarded poor outer parts of the Lanzhou lily bulb were dried in the shade, grounded into power, soaked in ethanol (3 times the weight of the lily bulb), and placed in an ultrasonic extractor for 90 min to collect the chemical ingredients of lily bulbs. Lixivium was collected, and fresh ethanol was added for a second extraction for another 90 min in the ultrasonic extractor. The extraction process was repeated 3 times. The extracts were combined, and the solvent was vaporized with a rotary evaporator to obtain the Lanzhou lily crude extract (LLCE). Three samples of LLCE at 100.0, 140.0, and 200.0 mg were accurately weighted and, respectively, blended with 1.9000, 1.8600, and 1.8000 g of PEG400 base oil in three glass vials. Then, the blends in the vials were sonicated at 40 °C for 40 min up to 1 h using an ultrasonic bath (ultrasound power of 70 W) at a frequency of 80 Hz. Different concentrations of LLCE in PEG400 from 5 wt.% to 10 wt.% were obtained.

2.2. Physiochemical Tests

A thermogravimetric analysis (TGA) was carried out for PEG400 and PEG400-LLCE lubricants via simultaneous thermogravimetry. The tests were conducted under a nitrogen atmosphere, and the heating rate was 10 °C/min from room temperature to 600 °C. The dynamic viscosity of different samples was tested by performing a shear rate sweep at fixed temperatures of 25 °C, 40 °C, and 100 °C with a rheometer (HAAKE RS6000, Thermo Fisher, Karlsruhe, Germany). The kinetic viscosity and viscosity index values of each lubricant were calculated according to GBT 265-1988 [17] and GBT 2541-1981 [18] (National Standards of the People’s Republic of China). The density of each lubricant at 25 °C was measured according to GBT 2540-1981 [19].

2.3. Tribology Tests

The lubricating properties of PEG400 base oil with different contents of LLCE were evaluated with an SRV-IV fretting tribometer supplied by Optimol Instruments, München, Germany. Before the test, a disc was fixed on the lower holder that connects to a piezoelectric sensor, and a ball was fixed on the upper holder that connects to horizontal rods. Meanwhile, parameters like the frequency, load, stroke, and duration were set in the computer software. During the test, a horizontal rod was driven by the motor, causing the balls to slide on the discs. The friction coefficient was measured using the piezoelectric sensor and recorded in the software. The lower pair of the SRV-IV was an AISI 52100 disc with a diameter of 24 mm and thickness of 8 mm, and the upper pair was an AISI 52100 ball with a diameter of 9.6 mm. The load-carrying capacities of different lubricants were evaluated with elevated load tests. The loads were increased to 50 N during the first 2 min and to 100 N after another 2 min and then increased by 50 N every 2 min until lubrication failure. The tribological test for each condition was performed in triplicate.
The anti-friction performance of LLCE as an additive at 50 N, 100 N, and 150 N was tested with the temperature, stroke, frequency, and duration time, respectively, fixed at 25 °C, 1 mm, 25 Hz, and 20 min. After the friction tests, the wear volumes of the AISI 52100 discs were measured and 3D profiles were recorded with a non-contact 3D optical profilometer MicroXAM-800 supplied by the KLA-Tencor company, (Milpitas, CA, USA). The wear rate of the AISI 52100 disc under different lubrication conditions was calculated as the worn volume divided by the sliding distance and applied load. The worn surface morphologies of the AISI 52100 lubricated with different lubricants at 50 N and 150 N were examined using a scanning electron microscope.

3. Results and Discussion

3.1. Thermal Stability

The thermostability of lubricants with different contents of LLCE added to PEG400 base oil was measured and compared with that of pure PEG400. Figure 1 shows TGA curves of different lubricants under a nitrogen atmosphere. As shown in Figure 1, PEG400 base oil began to break down at 219.9 °C. The addition of LLCE markedly postponed the degradation of PEG400. In total, 5 wt.% of LLCE added to PEG400 increased the degradation onset temperature to 257.2 °C, which is a rise of 37.3 °C from that of pure PEG400. The degradation onset temperatures for 7 wt.% and 10 wt.% of LLCE in PEG400 were, respectively, 259.6 °C and 269.4 °C, which are higher than that for 5 wt.% of LLCE.

3.2. Viscosity and Viscosity Index

Viscosity is one of the most important properties of lubricants for better lubrication performance. The dynamic viscosity of each lubricant at a shear rate of 50 s−1 is listed in Table 1, and the kinematic viscosity was calculated as the ratio of the dynamic viscosity and density. The viscosity index was calculated as listed in Table 1. The viscosity value of PEG400 base oil at 25 °C was 91.8 mm2/s, which constantly improved as the concentration of LLCE additives increased from 101.0 to 130.1 mm2/s. At 40 °C, the viscosity values of different lubricants followed the same trend, and the values were increased from 42.2 mm2/s to 48.0–58.0 mm2/s through the addition of LLCE. At 100 °C, the viscosity value was 6.7 mm2/s for PEG400; it increased to 9.6 mm2/s with an addition of 5 wt.% of LLCE and remained roughly the same as the content of LLCE further increased from 9.7 to 10.1 mm2/s. The viscosity values above led to a much higher viscosity index for lubricants with 5–7 wt.% of LLCE and a lower viscosity index for the lubricant with the maximum addition of LLCE.

3.3. Lubrication Behaviors and Mechanisms

Figure 2 shows the friction coefficient vs. load curves of PEG400 modified with different contents of LLCE, revealing the extreme pressure capacity of the lubricants. Without modification, PEG 400 exhibited an extreme pressure capacity of 150 N. In contrast, the addition of LLCE significantly improved the extreme pressure capacity of PEG 400, indicating that the LLCE content was positive for extreme pressure capability. It was found that the extreme pressure load of PEG400 modified with 5 wt.%, 7 wt.%, and 10 wt.% of LLCE increased to 200 N, 250 N, and 300 N, respectively.
After the screening of the load-carrying tests, the performance of the additive at a low load was tested from 50 N to 150 N. Figure 3 shows the friction coefficients of the AISI 52100 steel sliding pairs lubricated with PEG400 modified with different contents of LLCE. The lubricity of PEG400 was enhanced by the natural lily extract, correlating with the LLCE content and applied load. At the applied load of 50 N, the friction coefficients of the AISI 52100 steel sliding pairs lubricated with PEG400, PEG400-5 wt.% LLCE, PEG400-7 wt.% LLCE, and PEG400-10 wt.% LLCE were 0.133, 0.110, 0.095, and 0.105, respectively, which indicated that the lubricity improved by about 20%-30%. At 100 N, the friction coefficient for PEG 400 lubrication was about 0.137, which then decreased to 0.133, 0.129, and 0.124 when the content of LLCE in PEG 400 was 5 wt.%, 7 wt.%, and 10 wt.%, where the reduction degree was up to 10%. At 150 N, the friction coefficients decreased from 0.140 to 0.123, with the LLCE content increasing from 0 to 10 wt.%, presenting a reduction degree of about 10%. These results suggest that the enhancement of the LLCE additive was more pronounced at low applied loads, and the best additive content was about 7 wt.%.
Figure 4 shows the wear rates of the AISI 52100 steel discs lubricated with PEG400 modified with different contents of LLCE at different loads. Over the entire load range of 50–150 N, the wear rates of the AISI 52100 discs lubricated with PEG400 were at their maximum and stable at about 10.4 × 10−8 mm3/Nm. In comparison, the wear rates of AISI 52100 discs lubricated with PEG400-5 wt.%LLCE, PEG400-7 wt.% LLCE, and PEG400-10 wt.% LLCE were in the ranges of (4.35–8.39) × 10−8 mm3/Nm, (0.86–5.42) × 10−8 mm3/Nm, and (2.10–6.08) × 10−8 mm3/Nm, respectively. The lubricity improvement of the LLCE additive could be ranked as follows: 7 wt.% > 10 wt.% > 5 wt.%. In accordance with the friction coefficient, the wear reduction effect of the LLCE additive was notable at low loads. The wear rate for PEG-7 wt.% LLCE lubrication was about one magnitude of order lower than that for PEG 400 lubrication at 50 N, whereas it decreased by about 50% at 100 N and 150 N.
Figure 5 and Figure 6 show the worn surface morphologies, EDS compositions, and 3D profiles of the AISI 5200 steel discs lubricated with PEG400 modified with different contents of LLCE at 50 N. The worn surface of an AISI 52100 disc lubricated with PEG400 seemed to be rough, and there were many cracks and local peeling pits, indicating its severe wear. Three-dimensional images confirmed that the worn surface was rough and deep. EDS compositions of the AISI 52100 discs lubricated with neat PEG400 base oil showed that the composition varied greatly by region. However, the worn surfaces of the AISI 52100 disc lubricated with PEG400-LLCE were smooth and presented slight grooves. Three-dimensional images suggested that the furrows of the wear scars were very tiny. The EDS compositions of the AISI 52100 discs showed that there were no obvious differences in different regions lubricated with PEG400 modified with LLCE. These indicate that the wear mechanism of the AISI 52100 discs lubricated with the lubricants was abrasion wear and that the PEG400 modified with LLCE provided good lubricity.
Figure 7 and Figure 8 show the worn surface morphologies and 3D profiles of the AISI 52100 steel discs lubricated with PEG400 modified with different contents of LLCE at 150 N. All the worn surfaces of the AISI 52100 discs lubricated with PEG400 and PEG400 modified with LLCE were characterized by grooves, indicating abrasion wear. Through a comparison of the 3D profiles of the wear scars, it was also found that the wear scar of the AISI 52100 discs lubricated with PEG400 was deeper and wider than that of discs lubricated with PEG400-LLCE lubricants. The EDS compositions of the AISI 52100 discs lubricated with PEG400 at 150 N varied greatly by region and those lubricated with PEG400-LLCE lubricants showed relatively uniform constituents.
The specific chemical composition of lily extract enables its promising performance as a lubricant and additive. Lily extract is mainly composed of large amounts of phenol derivates, fatty acids, phospholipids, alkaloids, and organic sulphide, enabling it to serve as a natural multifunctional additive by forming a protective tribo-film [16]. Phenol derivates are natural compounds found to exhibit satisfactory antioxidant properties and reduce wear and friction [20,21]. Fatty acids and their derivates as phospholipids or amides are all amphiphile molecules with higher polarity and greater affinity to metal surfaces than traditional vegetable oil composed of triglyceride; they form a protective tribo-film as a fatty acyl iron layer [22,23], with FePO4, Fe3(PO4)2, Fe2O3, and Fe3O4 (Figure 9) [24]. Alkyl sulphide analogues as additives could provide superior load-carrying ability by forming FeS and FeS2 [25].
LLCE as an additive significantly improved the physicochemical properties and lubrication performance of PEG400 base oil. With 5–10 wt.% of LLCE added to PEG400, the thermal degradation temperature was raised by 37.3–49.5 °C, guaranteeing service under elevated temperatures (Figure 1). There was a significant increase (37.3 °C) when adding 5 wt.% of LLCE compared with PEG400 base oil and a slight improvement (2.4 °C, 12.2 °C) with the additive increasing to 7–10 wt.% compared with the PEG400-5 wt.% LLCE lubricant.
As depicted in Table 1, the viscosity and viscosity index were also dependent on the concentration of LLCE in PEG400 base oil. The viscosity values of the PEG400 lubricant at 25 and 40 °C improved constantly as the concentration of the LLCE additives increased, and there was notable improvement in the maximum concentration. At 100 °C, the viscosity value of PEG400 markedly increased with 5 wt.% of LLCE, but there was no obvious increase with the content of LLCE further increasing. The viscosity indexes were ranked as follows: PEG400-7 wt.% LLCE > PEG400-5 wt.% LLCE > PEG400-10 wt.% LLCE > PEG400. Vegetable oil commonly exhibits high viscosity, which makes it a promising biolubricant or additive. Viscosity is vital for determining the discrete grade of a lubricant. High viscosity could increase the drag and temperature, and low viscosity may increase the wear rate. The viscosity index describes the variation in viscosity with temperature. A high viscosity index is essential for a good lubricant, indicating a low change in viscosity with temperature. A lubricant with a high viscosity index suggests viscosity stability, proper tribo-film maintenance, and effectivity at different temperatures [26]. Therefore, the increase in the viscosity and viscosity index is positively correlated with an excellent lubricating performance.
Load-carrying tests demonstrated that the extreme pressure capacities of the lubricants gradually improved as the content of the additive increased. Friction tests showed that under the tested loads of 50–150 N, different contents of LLCE in PEG400 could lead to a reduction in the average friction coefficient. At the lowest load, the anti-friction property was most effective. The reduction degree of the average friction coefficient at 50 N was about 20–30%, and the optimum addition content of LLCE was 7 wt.% obtaining a nearly 30% reduction. At 100 N and 150 N, the average friction coefficient was gradually reduced as the content of LLCE was increased, and the reduction degrees were both about 10% at the maximum addition of lily extract.
As an aspect of the wear rate, the anti-wear property of LLCE was most obvious at the lowest load. With the addition of 5 wt.% LLCE, the wear rate could be reduced to 42% of the base oil. The optimum content of the LLCE additive was 7 wt.%, and the wear rate was as low as 8 wt.% of PEG400 base oil. As the addition of LLCE was further increased, the wear rate was about 20% of that of the PEG400 base oil and slightly higher than that of the PEG400-7 wt.% LLCE lubricant. At 100 N and 150 N, the wear rates under lubricants of different LLCE contents were about 50–60% compared with base oil, except for the slightly high wear rate with the PEG400-5 wt.% LLCE lubricant.
The worn surfaces of AISI 52100 lubricated with PEG400 lubricant modified with LLCE were smoother, and its wear scars were tinier. The EDS compositions also indicated that the addition of LLCE ensures a more uniform coverage for the lubricant. The worn surface morphologies, EDS compositions, and 3D profiles of different lubricants were in accordance with the lubricating properties.
The optimum content of the LLCE additive was 7 wt.%, and the reductions in the friction coefficient and wear volume were 10–30% and 50–92%. This is comparable with the values of the natural derivates, ionic liquids, covalent organic frameworks, and other types of well-designed additives listed in Table 2. Naturally derived N-octadecyl D-gluconamide (NOG) gelator is an effective additive, with an optimal concentration of 4% leading to the coefficient of friction and wear volume (WR) decreasing by 13.5% and 87% [27]. A total of 13 ionic liquid (IL) additives out of 23 ILs prepared showed a friction-reducing performance in PEG400 with a reduction ratio of up to 45.1% [28]. An imidazolium ionic liquid (ImIL) additive reduced friction by around 20% and obtained narrower and shallower wear scars compared with neat PEG [29]. Two kinds of triazine-based covalent organic frameworks (COFs)—Ton-COFs (a triazine ring and lactam interconnected through C-N bonds) and Tol-COFs (the triazine ring through oxygen bridges)—were well designed and synthesized as additives of the polar PEG 400 base oil. Consequently, excellent friction reduction (41.2%) and anti-wear property (97.4%) rates were achieved for the Ton-COFs compared to pure PEG 400 oil at a low concentration of 0.3 wt.%; moreover, rates of 28.6% and 79.0%, respectively, for Tol-COFs at the essential concentration of 0.7 wt.% were achieved [30]. Solvent-free carbon spherical nanofluids (C-NFs) were prepared as an additive with an average coefficient of friction (ACOF) reduction of 20.8% and wear volume reduction of 49% at the optimum concentration of 5–10 wt.% [31]. Lithium bis (oxalic acid) borate (LiBOB) through in situ coordination with PEG also performed well in terms of friction reduction and anti-wear properties by 31.3% and 96%, respectively [32].

4. Conclusions

In conclusion, LLCE, as an additive with different contents, notably improved the physiochemical properties of thermal stability, viscosity, and the viscosity index compared to PEG400 base oil. In terms of anti-friction and anti-wear effects, the additive significantly improved the lubricity of PEG400 base oil, typically at a low load of 50 N. The optimum content of the LLCE additive was 7 wt.%, and further increasing the contents of the additive did not bring about obvious improvements or worsen the lubricant. The lower friction coefficient and wear rate obtained through the addition of LLCE may be due to the higher viscosity facilitating the formation of a thicker lubricating film and the higher polarity providing better chemical affinity for metallic surfaces. The plant additive of LLCE was extracted from agricultural wastes with ethanol, which can be recycled. Therefore, the additive is environmentally friendly and economical, with a good potential application in the agricultural industry, such as in chaws, punch presses, etc.

Author Contributions

Conceptualization, funding acquisition, project administration, supervision, writing—original draft preparation, M.X.; validation, resources, writing—review and editing, J.C.; investigation, C.L., F.C. and Y.M.; visualization, C.L. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the National Natural Science Foundation of China (32200325), Longyuan innovation and entrepreneurship talent project of Gansu Province (2024QNTD32), Feitian Scholar Program of Gansu Province, and discipline construction project of Lanzhou City University.

Data Availability Statement

The original contributions presented in the study are included in the article; further inquiries can be directly contected with the corresponding author.

Conflicts of Interest

The authors declare no competing financial interests.

Abbreviations

Lanzhou lily crude extract (LLCE), polyethylene glycol (PEG), boron nitride (BN), tungsten disulphide (WS2), molybdenum disulphide (MoS2), Society of Automotive Engineers (SAE), polyalphaolefin (PAO), Japanese Industrial Standards-Steel Kougu Dice (JIS-SKD), synthetic ester (SE), ethylene diamine tetraacetic acid (EDTA), thermogravimetric analysis (TGA), American Iron and Steel Institute (AISI), energy-dispersive spectrum (EDS), N-octadecyl D-gluconamide (NOG), wear volume (WR), ionic liquids (ILs), imidazolium ionic liquid (ImIL), covalent organic frameworks (COFs), solvent-free carbon spherical nanofluids (C-NFs), average coefficient of friction (ACOF), and lithium bis (oxalic acid) borate (LiBOB).

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Lubricants 13 00034 g001
Figure 1. TGA curves of PEG400 base oil with different quantities of LLCE under nitrogen atmosphere.
Figure 1. TGA curves of PEG400 base oil with different quantities of LLCE under nitrogen atmosphere.
Lubricants 13 00034 g001
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Figure 2. Load-carrying capacities of PEG400, PEG400-5 wt.%LLCE, PEG400-7 wt.%LLCE, PEG400-10 wt.%LLCE respectively as (a–d).
Figure 2. Load-carrying capacities of PEG400, PEG400-5 wt.%LLCE, PEG400-7 wt.%LLCE, PEG400-10 wt.%LLCE respectively as (a–d).
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Figure 3. Friction coefficient of AISI 52100 self-mated sliding pairs lubricated with PEG400 modified with LLCE additive at different applied loads.
Figure 3. Friction coefficient of AISI 52100 self-mated sliding pairs lubricated with PEG400 modified with LLCE additive at different applied loads.
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Figure 4. Wear rates of the AISI 52100 discs lubricated by PEG400 modified with LLCE additive at different applied loads.
Figure 4. Wear rates of the AISI 52100 discs lubricated by PEG400 modified with LLCE additive at different applied loads.
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Figure 5. Worn surface morphologies and EDS compositions of the AISI 52100 discs lubricated by PEG400, PEG400-5 wt.%LLCE, PEG400-7 wt.%LLCE, PEG400-10 wt.%LLCE respectively as (a–d) at 50 N.
Figure 5. Worn surface morphologies and EDS compositions of the AISI 52100 discs lubricated by PEG400, PEG400-5 wt.%LLCE, PEG400-7 wt.%LLCE, PEG400-10 wt.%LLCE respectively as (a–d) at 50 N.
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Figure 6. 3D profiles showing the worn surfaces of the AISI 52100 discs lubricated by PEG400, PEG400-5 wt.%LLCE, PEG400-7 wt.%LLCE, PEG400-10 wt.%LLCE respectively as (a–d) at 50 N.
Figure 6. 3D profiles showing the worn surfaces of the AISI 52100 discs lubricated by PEG400, PEG400-5 wt.%LLCE, PEG400-7 wt.%LLCE, PEG400-10 wt.%LLCE respectively as (a–d) at 50 N.
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Figure 7. Worn surface morphologies of the AISI 52100 discs lubricated by PEG400 modified with PEG400, PEG400-5 wt.%LLCE, PEG400-7 wt.%LLCE, PEG400-10 wt.%LLCE respectively as (a–d) at 150 N.
Figure 7. Worn surface morphologies of the AISI 52100 discs lubricated by PEG400 modified with PEG400, PEG400-5 wt.%LLCE, PEG400-7 wt.%LLCE, PEG400-10 wt.%LLCE respectively as (a–d) at 150 N.
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Lubricants 13 00034 g008
Figure 8. 3D profiles showing the worn surfaces of the AISI 52100 discs lubricated by PEG400, PEG400-5 wt.%LLCE, PEG400-7 wt.%LLCE, PEG400-10 wt.%LLCE respectively as (a–d) at 150 N.
Figure 8. 3D profiles showing the worn surfaces of the AISI 52100 discs lubricated by PEG400, PEG400-5 wt.%LLCE, PEG400-7 wt.%LLCE, PEG400-10 wt.%LLCE respectively as (a–d) at 150 N.
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Figure 9. Schematic showing the lubrication mechanisms of LLCE additive.
Figure 9. Schematic showing the lubrication mechanisms of LLCE additive.
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Table 1. Viscosity of lubricants with different contents of LLCE in PEG400 base oil.
Table 1. Viscosity of lubricants with different contents of LLCE in PEG400 base oil.
PEG400PEG400-5 wt.%
LLCE		PEG400-7 wt.% LLCEPEG400-10 wt.% LLCE
25 °Cη (mPa·s)103.4112.0122.4143.5
cSt (mm2/s)91.8101.0110.8130.1
40 °Cη (mPa·s)47.553.254.063.9
cSt (mm2/s)42.248.048.958.0
100 °Cη (mPa·s)7.510.611.210.7
cSt (mm2/s)6.79.610.19.7
Viscosity index113190200152
Density1.1261.1091.1051.103
Table 2. Performances of different additives in PEG400 base oil.
Table 2. Performances of different additives in PEG400 base oil.
LubricantsFriction Coefficient Wear Volume/10−5 mm3
Steel-steel friction pairs, 100 N, 25 Hz [27]PEG4000.13316
2–5 wt.% NOG0.115–0.1202–5
Steel 52100 pairs, 25 °C, 98 N [28]PEG 4000.1101--
1 wt.% N-containing heterocyclic ionic liquid0.0604–0.1985--
Steel-steel contacts, 40 °C, 7.92 N [29]PEG4000.11--
1–5 wt.% Imidazolium ionic liquid (ImIL)0.09–0.083 --
AISI 52100 pairs, 100 N, 25 Hz, 25 °C [30]PEG4000.1480
0.3–1 wt.%: Ton-COFs and Tol-COFs- PEG4000.08–0.09;
0.10–0.11		2–20;
20–70
Steel balls-GCr15 steel block, 60 N [31] PEG 4000.1234.4
0.5–10.0 wt.% C-NFs0.116–0.1072.2–3.3
AISI 52100-AISI 52100, 200 N [32]PEG4000.16393.4
3 wt.% LiBOB0.1115.6
this workPEG40050 N0.13331.4
100 N0.13763.7
150 N0.14094.2
5–7 wt.% LLCE50 N0.095–0.1102.6–13.1
100 N0.124–0.13331.7–50.4
150 N0.124–0.13144.4–56.2
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Xie, M.; Lai, C.; Chen, J.; Cao, F.; Ma, Y. Lubricating Performance of Lanzhou Lily Crude Extract as Natural Additive. Lubricants 2025, 13, 34. https://doi.org/10.3390/lubricants13010034

AMA Style

Xie M, Lai C, Chen J, Cao F, Ma Y. Lubricating Performance of Lanzhou Lily Crude Extract as Natural Additive. Lubricants. 2025; 13(1):34. https://doi.org/10.3390/lubricants13010034

Chicago/Turabian Style

Xie, Min, Chenghua Lai, Juanjuan Chen, Feng Cao, and Ying Ma. 2025. "Lubricating Performance of Lanzhou Lily Crude Extract as Natural Additive" Lubricants 13, no. 1: 34. https://doi.org/10.3390/lubricants13010034

APA Style

Xie, M., Lai, C., Chen, J., Cao, F., & Ma, Y. (2025). Lubricating Performance of Lanzhou Lily Crude Extract as Natural Additive. Lubricants, 13(1), 34. https://doi.org/10.3390/lubricants13010034

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