J. Cent. South Univ. Technol. (2007)05−0673−06 DOI: 10.1007/s11771−007−0129−6

Preparation of tungsten disulfide motor oil and its tribological characteristics

SHI Chen(石 琛)，MAO Da-heng(毛大恒)，FENG Hao(俸 颢)

(School of Mechanical and Electrical Engineering, Central South University, Changsha 410083, China)

Abstract: Through using mineral oil and synthetic oil to deploy the semisynthesis base oil, modifying the surfaces of ultrafine tungsten disulfide grains by surface chemical embellishment and adsorption embellishment to make them suspended steadily in the base oil as solid lubricating additive, and adding some function additives, the tungsten disulfide motor oil was prepared. The tribological characteristics of this kind motor oil and the well-known motor oils in our country and overseas were studied. The results show that the oil film strength of this kind of motor oil is respectively 1.06 and 1.38 times of that of shell helix ultra motor oil and great wall motor oil, and its sintering load is 1.75 and 2.33 times of that of them, and when tested under 392 N, 1 450 r/min and 30 min, the friction coefficients of friction pairs lubricated by the tungsten disulfide motor oil decrease with the increase of time, meanwhile, the diameter of worn spot is small, and the surface of worn spot is smooth, and no obvious furrows appear. The experiments indicate that the tungsten disulfide motor oil has the better antiwear, antifriction and extreme pressure properties than the well-known motor oils. Key words: tribology; motor oil; tungsten disulfide; lubricating property 1 Introduction

In modern society, considered as the blood of

automobile industry, motor oil has been advancing with the development of automobile industry. In the developed country, the wastage of motor oil occupies 50% of that of all the lubricating oil. Therefore, the research and development of motor oil have become the most interesting item in the lubricating oil industry[1−2]. Recently, because of the environmental protection and energy saving, the requirement of industry to the level of motor becomes higher and higher, and so do that to the motor oil[3]. Consequently, many kinds of motor oil which are environment-protective and energy-saving have been developed[4].

The PetrolMoly Company in America prepared a new type of motor oil named PetrolMoly, which contains the solid lubricative grain molybdenum disulfide. After traffic test, the results showed that compared with traditional motor oil, PetrolMoly could decrease the NOx emission by 73% and decrease the oil wastage by 37%, increasing the fuel economy by about 10%. Now PetrolMoly has been set down as one of the 125 items of new technology and new product which are recommended to the member countries by UN[5−6]. Tungsten and molybdenum both are the VIB subgroup elements with the same chemical property. As a new kind of solid lubricative material, tungsten disulfide also has excellent lubricating properties, but because of the high

price, it’s mostly used as the type of solid film lubrication in the field of spaceflight, aviation and military affairs. Presently, some researchers used the ultrafine tungsten disulfide powders in high temperature lubricating grease in our country[7], but few researchers used the tungsten disulfide as the additive of motor oil overseas and in our country[8].

SHI et al[9] found that tungsten disulfide has better lubricating properties than molybdenum in motor oil, with the same content, tungsten disulfide’s oil film strength is 1.1 times of that of molybdenum disulfide, and the friction coefficient is 0.9 times of that of molybdenum. Consequently, it can be inferred that tungsten disulfide has better application foreground. Upon that, after series of deployment experiments and optimizing dispensation, the tungsten disulfide motor oil was prepared by using the ultrafine tungsten disulfide powders made in Changsha Huajing Powdery Material Technological Company. The tribological characteristics of this kind of motor oil and the other motor oils in our country and overseas were studied in this paper. 2 Prepare of tungsten disulfide motor oil 2.1 Preparation of materials 2.1.1 Ultrafine tungsten disulfide powders

Tungsten disulfide doesn’t dissolve in water, oil, alcohol, grease and other organic impregnants, and doesn’t dissolve in ecumenic liquors of acid or alkali, except nitric acid, hydrofluoric acid and the admixture

Received date: 2007−03−20; Accepted date: 2007−04−23 Corresponding author: SHI Chen, Doctoral candidate; Tel:+86-731-8877915; E-mail: shichenmail@163.com

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acid of nitric acid and hydrochloric, whose oxygenation abilities are strong. In the atmosphere, tungsten disulfide will be disassembled at 510 ℃ and oxygenized rapidly at 539 ℃. It can be lubricative for a long time below 425 ℃. In vacuum, its disassembly temperature is 1 150 ℃. Tungsten disulfide’s intensity of antipressure is 21 MPa. Tungsten disulfide’s radio resistance is better than that of graphite and molybdenum disulfide. It has nicer lubricating capabilities, not only applicable to the usual lubricative condition, but also to the rigorous work environment, such as high temperature, high pressure, high vacuum, high load, radioactive and mordant medium. Basal physics parameters of tungsten disulfide used are shown in Table 1.

Table 1 Basal physics parameters of tungsten disulfide

Parameter Value

Grain diameter/μm 0.3

Relative molecular mass 247.9

Density/(g·cm−3) 7.4−7.5

Heat stability/℃ ≤425

Crystal structure Crystal lattice with six squares

Colour and lustre Gray with luster

Mohs hardness 1−2

Friction coefficient 0.03−0.05

2.1.2 Base oil

As the main constituent of motor oil, base oil plays an important role in deciding the quality of motor oil, so several kinds of synthetic oil were added in the mineral oil to improve the antiwear property, viscosity versus temperature property and biological degradability. After concoction, a semisynthesis base oil was obtained, which can improve the properties and reduce the cost. 2.1.3 Function additives

On the basis of the synthetical consideration of dispersing property of tungsten disulfide in motor oil and the antiwear and antifriction property, oxidative stability, viscosity versus temperature property of motor oil, the following function additives were selected: dressing agent, detersive, dispersant, antioxidant and anti- corrosion, antiwear agent and antifriction, oiliness improver, rust inhibitor, pour inhibitor and kilfoam[10−11]. 2.2 Preparation process

Firstly, the mineral oil and synthetic oil were used to deploy base oil, and the dispensation was optimized through simulation analysis by BP artificial neural network，whose mass and structure have been optimized by genetic algorithm[12−13], then the semisynthesis base oil of motor oil with excellent properties was obtained.

Secondly, the ultrafine tungsten disulfide grains and dressing agent were put into absolute ethyl alcohol and heated to 100 ℃ in the inert gas for 1 h to remove the micro-molecules of water and alcohol. The mixture was disjoined and purified after reaction at 180−200 ℃ for 6 h, and then the surface chemical embellished ultrafine tungsten disulfide grains with excellent dispersibility were obtained.

Thirdly, the chemical embellished ultrafine tungsten disulfide grains, detersive and dispersant were put into three-necked bottle and stirred at 50−60 ℃ for 30 min by motor stirrer to adsorption embellish the surfaces. According to Ref.[9], when the content of tungsten disulfide in the motor oil is 1%, the lubricating properties of motor oil are the best. After that, the base oil was added in and stirred for 1 h.

Finally, the function additives such as antioxidant, anticorrosion, antiwear agent, antifriction, oiliness improver, rust inhibitor, pour inhibitor and kilfoam were added in and stirred to uniformity and cool-down, then the tungsten disulfide motor oil was obtained. During the preparation process, all the dispensations of the additives are optimized. 3 Tribological characteristics test of tungsten

disulfide motor oil 3.1 Experimental materials and equipment

The experimental materials are tungsten disulfide motor oil, Shell helix ultra motor oil and Great wall motor oil.

The experimental equipments are as follows: MRS- 10A type four-ball machine produced by Tsinan Experiment Machine Factory, and the steel balls used are GCr15 bearing steel balls with diameter of 12. 7 mm and rigidities of HRC59 according to nation standard II; GB265 kinematic viscosity tester(made in Dalian Instruments and Meters Company); DMI 5000M metallographic microscope(made in Leica Camera AG); PHI-5100 X-ray Photoelectron Spectroscope(XPS) (made in America Perkin-Elmer company).

3.2 Experimental method

The oil film strength (PB) and sintering load (PD) of the three kinds of motor oil were measured by MRS-10A four-ball machine, and the worn spots and friction coefficient were tested under 392 N, 1 450 r/min and 30 min. The kinematic viscosities of the three kinds of motor oil were measured at 40 ℃ and 100 ℃ by GB265 kinematic viscosity tester, and then their viscosity indexes were calculated. The dispersing condition of

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tungsten disulfide in the motor oil and the surfaces of worn spot of steel ball were observed using DMI 5000M metallographic microscope. The chemical states of the major elements of the worn spot’s surface were analyzed by PHI-5100 X-ray photoelectron spectroscope(XPS) with Mg Kα ray(29.35 eV), taking C1S binding energy as inner mark.

3.3 Results and analysis 3.3.1 Dispersing condition of tungsten disulfide in motor

oil For the ultrafine tungsten disulfide powders have

the characteristics of small grain size, large surface area and high surface energy, it’s very easy for the powders to agglomerate spontaneously, make the grain largened and stop the oil circuit system[14−15]. Therefore, the deconcentration of tungsten disulfide in the oil becomes the greatest problem when preparing the tungsten disulfide motor oil. Our research group use dressing agent and dispersant to embellish the surface of tungsten disulfide grain by surface chemical and adsorption. Chemical embellishment is to use dissolvable alkoxy of the dressing agent molecule to chemically react with the lively hydrogen in the hydroxyl of the surface of tungsten disulfide, and the long-chain organic functional group of the other end of resin acceptor molecule is compatible to the base oil. Adsorption embellishment is that the dispersant’s polarity functional group adsorbs the grain’s surface, and long-chain nonpolarity end is compatible to the base oil, then the grain’s agglomerating is prevented effectively. By surface embellishment, tungsten disulfide can disperse in the motor oil steadily, forming steady suspending oil. The dispersing conditions of tungsten disulfide in the motor oil are shown in Fig.1. It can be seen from Fig.1 that the surface of the ground slide carrying oil drop is very clean. The tungsten disulfide particle in unembellished tungsten disulfide is comparatively large, and the agglomerating phenomenon is obvious, while in the embellished tungsten disulfide, the particle is small, dispersing equably, without any agglomerating phenomenon. 3.3.2 Tribological characteristics of three kinds of

motor oil Table 2 lists the property parameters of three kinds

of motor oil. In Table 2, the oil film strength of the tungsten disulfide motor oil is respectively 1.06 and 1.38 times of that of shell helix ultra motor oil and great wall motor oil, and its sintering load is 1.75 and 2.33 times of that of them, and its extreme pressure property is excellent. Meanwhile, the tungsten disulfide motor oil has high viscosity index, and property of viscosity versus temperature is good.

Fig.1 Dispersing conditions of unembellished and embellished

tungsten disulfide in oil (a) Ground slide；(b) Unembellished tungsten disulfide；

(c) Embellished tungsten disulfide Fig.2 shows the surfaces of worn spot of steel balls

lubricated by three kinds of motor oil after rubbing for 30 min, indicating that, when lubricated by the tungsten disulfide motor oil, the worn spots of friction pairs are small and the surfaces are smooth, without obvious furrows, it can be guessed that fatigue wear and friction burn occur between the friction pairs, and the abrasion surfaces are blocked off by friction reaction film and depositional film. When lubricated by shell helix ultra motor oil and great wall motor oil, the worn spots’ diameters are comparatively large, with many deep furrows. It can be inferred that tiny cutting and adhesion abrasion occur between the friction pairs, because it’s difficult to avoid the direct contact of the tiny protruding bodies of friction pairs under traditional boundary lubrication condition, and the tiny protruding bodies

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cause plastic deformation and “cold welding”. Furthermore, the abrasive grains produced during the

rubbing process enter the lubricating medium and aggravate the abrasion of friction pairs.

Table 2 Property parameters of three kinds of motor oil

Items

Kinematic viscosity/(mm2·s−1) Viscosity index PB /N PD /N

Diameter of worn spot(wearing for 30 min)/mm

40 ℃ 100 ℃

Shell helix ultra motor oil 83.13 12.59 131 1 117 1 960 0.45

Great wall motor oil 143 15.6 110 862 1 470 0.47

Tungsten disulfide motor oil 146 18.4 126 1 186 3 430 0.40

Fig.2 Surfaces of worn spot of steel balls lubricated by three kinds of motor oil

(a) Tungsten disulfide motor oil, Lower magnification; (b) Shell helix ultra motor oil, Lower magnification; (c) Great wall motor oil, Lower magnification; (d) Tungsten disulfide motor oil, Higher magnification; (e) Shell helix ultra motor oil, Higher magnification;

(f) Great wall motor oil, Higher magnification

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Fig.3 shows the variation of friction coefficients of

friction pairs with time lubricated by three kinds of motor oil under 392 N. From Fig.3, it shows that when lubricated by shell helix ultra motor oil and great wall motor oil, the friction coefficients of friction pairs increase with time, and that the organic molecules of lubricating medium cause friction decomposition, which makes increase of the tiny protruding bodies of friction pairs’ direct touch and larger the real contact area, leading to the increase of friction coefficients of friction pairs. In the experiment, the friction coefficients appear falling for some time, maybe it’s the result that the P element deposits on the surface of friction pairs for the friction energy’s aggregation. When lubricated by the tungsten disulfide motor oil, the friction coefficients decrease with time at the first 8 min, which may be the result of chemical reaction film and depositional film. Comparing the three curves in Fig.3 shows that the friction coefficients of friction pairs lubricated by the tungsten disulfide motor oil are small all the time, indicating excellent lubricating properties.

Fig.3 Variation of friction coefficients of three kinds of motor oil with time under 392 N

1—Great wall motor oil; 2—Shell helix ultra motor oil; 3—Tungsten disulfide motor oil

3.3.3 Lubricating mechanism analysis

The microstructure of tungsten disulfide crystal is layered hollow sphere with hexagonal, its surface forms hexagonal network with S―W―S molecular, layers are connected by van der Waals Force[16]. This weak binding force is easy to be sheared on the middle of layers, which puts up low friction coefficient. Consequently, during the lubricating process, ultrafine tungsten disulfide powder is a lubricating system microcosmically with steady performance, excellent quality and low consumption. Fig.4 shows the SEM image of ultrafine tungsten disulfide powders. It can be seen that tungsten disulfide colloidal powder is lamellar structure, bedding surface is so smooth, length and breadth size are much greater than thickness, slipping occurs easily between layers, which lead to low friction coefficient.

Fig.4 SEM image of ultrafine tungsten disulfide powders

In order to study the lubricating mechanism of

ultrafine tungsten disulfide powders in motor oil, the main chemical constitution of the worn surface was analyzed by using X-ray photoelectron spectroscope(XPS). The analysis results are shown in Fig.5. It can be seen that the spectrum peaks of S2P and Fe2P with low intensities appear at binding energy of 161.3 and 710.3 eV, respectively, which are correspondence with the divalent S and divalent Fe of FeS. This shows that FeS is formed in the reaction between the tungsten disulfides and metal surface during the friction process, and then the FeS surface film is formed on the surface of worn spot, which improves the lubricating properties of the motor oil.

Fig.5 XPS spectra of S and Fe on worn surface

(a) S2P; (b) Fe2P

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The lubricating process of tungsten disulfide in the motor oil is the synergism process of mechanical action and chemical action. When the surfaces of friction pairs just contact each other, ultrafine tungsten disulfide grains form a film on the surfaces of friction pairs, then tungsten disulfide grains deposit in the low valleys of the friction pairs’ surfaces, and become a surface depositional film, with the effect of filling and leveling up which reduce the surface roughness and the sliding friction. When the load increases and friction is aggravated, the chemical action between tungsten disulfide and the base material of friction pairs happens and the FeS film is formed, which has good lubricating properties and can prevent the surfaces of friction pairs from contacting directly, reducing the abrasion between surfaces of friction pairs remarkably. Under high temperature, tungsten disulfide is oxidated slowly and becomes tungsten oxide (WO3), which can prevent tungsten disulfide from being oxidated further, besides, tungsten oxide has low friction coefficient and the same lubricating properties as tungsten disulfide, so it also has the ability to reduce the abrasion between surfaces of friction pairs.

All above show that taking the tungsten disulfide motor oil as lubricating medium, the surface of friction pairs can form a mixed protective film, which is not a single physical adsorbed film, not a single chemical reactive film or a single tungsten disulfide depositional film, but a synthesis with synergistic effect of these single films. Consequently, the tungsten disulfide motor oil displays excellent antiwear, antifriction and extreme pressure properties. 4 Conclusions

1）By surface embellishing the ultrafine tungsten disulfide powders, tungsten disulfide was applied into motor oil, and one kind of oil, the tungsten disulfide motor oil in which tungsten disulfide powders are suspended steadily was prepared.

2）The oil film strength of this kind of motor oil is respectively 1.06 and 1.38 times of that of shell helix ultra motor oil and great wall motor oil, and its sintering load is 1.75 and 2.33 times of that of them, and when testing under 392 N, 1 450 r/min and 30 min, the friction coefficients of friction pairs lubricated by the tungsten disulfide motor oil decrease with the increase of time. Meanwhile, the diameter of worn spot is small, and the surface of worn spot is smooth, without obvious furrows. These indicate that the tungsten disulfide motor oil has better lubricating properties than well-known motor oil

in our country and overseas. 3）When taking the tungsten disulfide motor oil as

lubricating medium, the surface of friction pairs can form a mixed protective film, which includes physical adsorbed film, chemical reactive film and tungsten disulfide depositional film. The mixed protective film makes the tungsten disulfide motor oil display excellent antiwear, antifriction and extreme pressure properties, and this kind of motor oil can reduce the exhaust emission and oil wastage, enhance the fuel economy and improve the environment, so it is a kind of environment- protective and energy-saving motor oil. References [1] WANG Yu-min, WANG Heng. Practical Automobile Lubricating

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(Edited by ZHAO Jun)