www.elsevier.com/locate/apsusc

Applied Surface Science 253 (2007) 7549–7553

Surface restoration induced by lubricant additive of natural minerals

Yang Yu a,*, Jialin Gu a, Feiyu Kang a, Xianqing Kong b, Wei Mo b

a Department of Materials Science and Engineering, Laboratory of Advanced Materials, Materials Research Center,

Tsinghua University, Beijing 100084, Chinab Department of Automotive Engineering, Tsinghua University, Beijing 100084, China

Received 12 October 2006; accepted 25 March 2007

Available online 4 April 2007

Abstract

The effect of a new-fashioned lubricant additive is studied. The additive is prepared out of natural minerals containing flaky silicate, schungite

and some other catalyzers. Applications of the additive obviously improve the surface mechanics properties of steel–steel friction pairs, and the

nanohardness and the modulus of the friction surface are increased by 67 and 90%, respectively. The friction surface is especially examined with

the high resolution transmission electron microscope (HRTEM), and an amorphous restoration film mostly made up of C with some Si or Si–O

amorphous structure doped was found. Considering all research results about the restoration film, this study suggests the film is a sort of diamond-

like carbon film (DLC film).

# 2007 Elsevier B.V. All rights reserved.

PACS : 81.40.Pq; 82.45.Jn

Keywords: Friction; Lubricant additive; Schungite; Silicate; Restoration; DLC film

1. Introduction

With the development of the machine industry in modern

society, it is becoming more and more dissatisfactory only

depending on lubricant oils. Plentiful and thorough investiga-

tions have been made on diversified synthetical or artificial

lubricant oil and grease additives to test their tribological

properties and application effects. A new-fashioned lubricant

additive, PBC [1,2], is applied in China in recent years. The

additive is made of powder with grain size of 0.3–3 mm. The

powder is prepared out of natural minerals, which are chosen

from the flaky silicate with addition of catalyzers on the basis of

the schungite and rare-earth metals. An approximate compo-

nent contains 90–95% ophite, 4.8–9.8% schungite, 0.10–0.15%

rare-metals, and other natural flaky silicate admixtures, and all

raw materials of the powder can be obtained from the nature.

Some studies have been carried on the effects of the additive,

and results show excellent performances of friction reduction

* Corresponding author at: Room 1410, Department of Materials Science and

Engineering, Tsinghua University, Beijing 100084, China.

Tel.: +86 10 6277 3791; fax: +86 10 6277 1160.

E-mail address: yangyu03@gmail.com (Y. Yu).

0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.apsusc.2007.03.061

and anti-wear of the additive. Generally, these studies

proclaimed a restoration film which contributes to the

outstanding tribological properties forms on the friction surface

but rarely mentioned the detailed composition, microstructure

and formation mechanism of the film.

The purpose of this paper is to examine the surface of

friction pairs processed by the additive and analyze the

mechanic properties, morphology, microstructure and forma-

tion mechanism of the restoration film by the nanoindentation

and the HRTEM.

2. Experimental

The friction pairs are made of the 45# steel quenched. The

contacting stress and the relative moving velocity between

friction pairs are respectively about 10 N/mm2 and 2.2 m/s. The

friction pairs were immerged into the 30# mechanical oil into

which the PBC additive was added by 0.9 wt% concentration;

another normal test was also made on the same experimental

parameters but only with the mechanical oil used in order to

characterize the effect of the additive.

The surfaces of the friction pairs were examined by the MTS

nanoindentation and the JEM-2010 HRTEM. In order to

accurately reveal the composition and microstructure of the

Fig. 1. Preparation of specimens. (a) Schematic diagram of preparation of sectional specimen. (b) Schematic diagram of preparation of vertical specimen.

Y. Yu et al. / Applied Surface Science 253 (2007) 7549–75537550

restoration film, the film was observed in both vertical and

sectional directions. Fig. 1a shows how the sectional specimens

were prepared. Two same specimens were adhered to each

other to protect the friction surfaces, and slicing and thinning

were carried out as Fig. 1a displays. Fig. 1b shows that the

vertical specimen was thinned only from one single side with

the friction surface protected. Cu rings were adhered to the

specimens to strengthen them consequently.

3. Results

3.1. Nanohardness and elastic modulus

Eight points were chosen at random on the surface of every

friction sample to measure their nanohardness and elastic

modulus, and the nanohardness- or modulus–depth curves are

showed in Fig. 2. The maximum of every curve is chosen to

represent the value of the point. The surface nanohardness of

the normal friction pairs ranges from 5 to 15 GPa and gets an

average of 9 GPa which is in accord with the normal quenched

steel. But the surface nanohardness of the friction pairs

processed by the PBC additive ranges from 10 to 20 GPa and

gets an average of 15 GPa which is about 67% higher than that

of the normal friction pairs. Similarly, the surface elastic

modulus of the normal friction pairs gets an average of 200 GPa

which is almost equal to the elastic modulus of the 45# steel.

However, the surface elastic modulus of the friction pairs

processed by the PBC additive can get an average of 370 GPa

which is 90% higher than that of the normal friction pairs.

3.2. Morphology and microstructure

Fig. 3 shows a sectional view of the restoration film. Fig. 3a

shows the morphology and energy dispersive spectra (EDS) of

B area of the restoration film, and Fig. 3b shows the HRTEM

image of A area. From the sectional view, it can be easily read

that the thickness of the film is about below 25 nm. The

HRTEM image vividly contrasts typical amorphous micro-

structure of the film with the orderly atom array of the Fe

substrate, and the EDS indicates the film is made up of nearly

pure carbon with only a little Si doped (Cu peak comes from

the Cu ring, and without regard to the Fe substrate). In

addition, the HRTEM image also reveals that there is an ideal

combination between the restoration film and the substrate,

and the film can fill up the rough surface of the substrate to

make it smoother.

Fig. 2. Nanohardness and elastic modulus of the surfaces of friction pairs. (a) Nanohardness of normal friction pairs. (b) Nanohardness of friction pairs processed by

PBC additive. (c) Elastic modulus of normal friction pairs. (d) Elastic modulus of friction pairs processed by PBC additive.

Y. Yu et al. / Applied Surface Science 253 (2007) 7549–7553 7551

Fig. 4 shows a vertical view of the restoration film. This

group of pictures displays a restoration film made up of C, Si

and O elements (EDS of C area), which possesses an

amorphous microstructure indicated by the HRTEM image.

In addition, considering the C is lighter than the other elements,

the C peak is so high that it can be concluded that the film is

mostly made up of C with some Si and O doped.

In conclusion, the results of the HRTEM analysis make it

clear that the restoration film is mostly made up of C with a few

Si or Si and O doped, and its microstructure is amorphous.

4. Discussion

Considering the nanohardness, elastic modulus, composi-

tion and microstructure of the restoration film, it is suggested

that the film is a sort of DLC film.

DLC films are well known for their amorphous micro-

structure, high hardness, low friction, and they show excellent

wear resistance in dry, water- and oil-lubricated conditions. The

hardness and elastic modulus of the synthetic DLC films in

laboratories respectively range from 10 to 60 GPa and 210 to

700 GPa [3–5]. Researches manifest that the DLC films are

effective in decreasing the friction coefficient and improving

the wear resistance of steel, magnesium alloy, and many other

materials [6,7]. To improve the adhesion of DLC film, some

other elements, such as Si, have been included in DLC films.

Study results show that 1–2% of Si doped is suitable for

improving the adhesion of the films and reducing the internal

stress while maintaining the surface hardness of DLC films [8].

Furthermore, the structural modification of pure DLC films has

been attempted by the addition of Si–O structures into the DLC

films. In this kind of film, two random interpenetrating

amorphous networks of carbon and Si–O structures mutually

stabilize each other, which releases the residual stress and

permits a thicker film [9].

The HRTEM examination suggests that the composition and

microstructure of the restoration film are in accord with DLC

films; furthermore, the measure of the nanohardness and elastic

modulus especially confirms that the restoration film possesses

the same mechanics properties as synthetic DLC films.

Fig. 3. Morphology and structure of restoration film of sectional specimen. (a) Morphology and EDS of area B of the restoration film. (b) HRTEM image of area A in a.

Y. Yu et al. / Applied Surface Science 253 (2007) 7549–75537552

Considering both the experimental results of this paper and the

investigations of other researchers, a conclusion can be drawn

that a sort of DLC film with a little Si or Si–O structures doped

forms on the surface of the steel–steel friction pairs processed

by the PBC additive.

The DLC film forming on the friction surface is made up of

C which comes in two ways. Firstly, the carbon comes from the

schungite which is one catalyzer of the PBC additive. The

schungite is a kind of ungraphitized carbon, which is metastable

and of the sphericity hyper-molecule microstructure. The

Fig. 4. Morphology, structure and composition of restoration film of vertical

specimen.

schungite is a kind of natural mineral, and it is different from

the graphite because of its noncrystal structure which is the

same as DLC film and from the coal and the bitumen because of

its low volatile content. Secondly, the carbon comes from the

carbon offsprings which are produced by the lubricant oil

decompounding at high temperature under friction conditions

and mostly made up of the graphite. Apparently, the Si and O

elements come from the silicate minerals.

The formation mechanism of this sort of DLC film as

restoration film is very complicated, and the formation

mechanism of the carbon nanotube can be helpful to discuss

the film. The friction brings fresh surfaces of the steel where the

activity of Fe as catalyzer is enhanced, and the locations are

offered for the active schungite to assemble and form the core of

the carbon film. As the friction goes on, the carbon offsprings

coming from the decompounded lubricant oil increase and

supply much carbon for the growth of the carbon film.

Remarkably, this kind of carbon offsprings can exist in all

lubricant systems, but only in the certain system containing the

schungite does the DLC film form. Therefore, it can be inferred

that the carbon film transforms into a sort of DLC film not only

with the high energy provided by the friction but also especially

with the catalysis of the schungite. Actually, further work needs

to be done to accurately present the details of the formation of

the film.

5. Conclusions

(1) A sort of DLC film with Si or Si–O structures doped forms

on the surface of the steel–steel friction pairs after applying

the PBC lubricant additive.

(2) T

his kind of DLC film restores the rough friction surface

and contributes to the excellent mechanics properties of the

friction surface.

Y. Yu et al. / Applied Surface Science 253 (2007) 7549–7553 7553

(3) T

he carbon making up of the DLC film comes from the

schungite and the carbon offsprings produced by lubricant

oil decompounding at high temperature under friction

conditions. The Si and O elements come from the silicate

minerals.

(4) T

he schungite not only forms the core of the carbon film but

also makes the film transform into the DLC film because of

its catalysis.

Acknowledgements

The authors would like to express their appreciation to

Beijing PBC Science and Technology Development Co. Ltd.

for supporting this study and supplying the new-fashioned

additive used in the test.

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