Tribology International 44 (2011) 101–105

Contents lists available at ScienceDirect

Tribology International

0301-67

doi:10.1

n Corr

E-m

journal homepage: www.elsevier.com/locate/triboint

Tribological performance of graphite containing tin lead bronze–steelbimetal under reciprocal sliding test

Liu Ru-Tie a,n, Xiong Xiang a, Chen Fu-Sheng b, Lu Jin-zhong b, Hong Li-Ling b, Zhang Yi-Qing b

a State Key Laboratory for Powder Metallurgy, Central South University, Changsha 410083, PR Chinab Fujian Longxi Bearing Co. Ltd., Zhangzhou 363000, PR China

a r t i c l e i n f o

Article history:

Received 28 November 2009

Received in revised form

26 August 2010

Accepted 28 September 2010Available online 7 October 2010

Keywords:

Solid self-lubrication

Micro-friction

Reciprocal slide

Bimetal

9X/$ - see front matter & 2010 Elsevier Ltd. A

016/j.triboint.2010.09.012

esponding author. Tel.: +86 13974870967; fa

ail address: llrrtt@csu.edu.cn (R.-T. Liu).

a b s t r a c t

As a solid self-lubricating material to serve under heavy load and low velocity, graphite containing tin

lead bronze–steel bimetal composites were prepared using the powder metallurgy (P/M) technique.

Effects of graphite content on tribological performance under reciprocal sliding were studied using the

UMT-2MT tribo-meter. The optimal performance of average friction coefficient, maximum friction

coefficient, friction coefficient amplitude and wear resistance can be achieved at the graphite content of

�3 wt%. Appropriate graphite content and hardness are the two most crucial factors to achieve a good

quality lubricating film on the worn surface and hence the desired solid lubrication performance.

& 2010 Elsevier Ltd. All rights reserved.

1. Introduction

Metal based self-lubricating composites are a typical solidlubricant material. Some of the most noteworthy propertiesinclude low friction coefficient, high melting point, good mechan-ical strength, improved ductility and elongation, good heat andelectrical conductivity, good dimensional stability, good long-term reliability, good moisture resistance, and as well as goodmachinability [1–3]. Self-lubrication copper based composites,which consists of one of the most important components of thismaterial class, are widely used in many oil-deficiency and evenoil-free applications [4,5]. Particularly interesting are tin bronze–steel bimetals, which are not only good at mechanical strength,heat conductivity, abrasive resistance and load bearing capacity,but also save the use of relatively more expensive non-ferrousmetals. Tin bronze–steel bimetals have now been employedwidely in many industrial fields, such as automotive, aerospace,construction machinery, etc [6]. Powder metallurgy (P/M)technique, which is able to mix non-metallic substances withmetals in any proportions to obtain desired performance, is themost preferred method to fabricate such metal based solid self-lubrication materials [7,8].

Graphite, as a solid lubricant, is most often used in self-lubricating copper base composites, due to its good anti-frictionperformance and chemical–physical stability over a wide

ll rights reserved.

x: +86 731 88710855.

temperature range [9–11]. Even though there is an increasingdemand for lead-free materials [12], the lead can be still hardlyreplaced as an additive in load bearing components, particularlyunder heavy load and low velocity conditions. Furthermore, thereexists a good synergic lubrication effect between graphite and leadwhen they both serve as solid lubricants together [13–15]. In thepresent work, tin lead bronze–steel bimetal composites have beensynthesized using the P/M technique. Effects of graphite content onmaterial hardness, microstructure and tribological performanceunder reciprocal sliding motion have been investigated anddiscussed, in order to understand friction and wear mechanismsof such composites under heavy load and low velocity.

2. Experiment details

2.1. Materials

Tin lead bronze–steel bimetal composites were fabricated with�100 mesh (with sizes of less than 154 mm) CuSn10Pb10 (massfraction) and natural graphite powders. The graphite content of1%, 2%, 3%, 4%, 5%, 6.5%, and 8% (mass fraction, unless otherwisestated) were used, respectively. The back was made with high-grade carbon steel electroplated with copper.

2.2. Fabricating process

Fig. 1 shows our fabrication process flow of the bimetals. First,bronze and graphite powders were mechanically mixed. Then, the

Fig. 3. Effects of graphite content on the hardness.

R.-T. Liu et al. / Tribology International 44 (2011) 101–105102

mixed powders were laid on the copper-electroplated steel backevenly prior to the first sintering process. The first-stage cold-rolling was then carried out, followed by the second-stagesintering, precision cold-rolling and finally leveling. Thesintering temperature was between 750–830 1C. The sinteringtime is 1 hr. The sintering atmosphere is hydrogen.

2.3. Test and analysis

The Brinell hardness of the bronze coating surface was testedaccording to Standard GB10453-89 (issued by the StandardizationAdministration of the People’s Republic of China), using a 5.0 kgfload, 30 s dwell time and F-1.00 mm steel ball indenter. Friction andwear tests were conducted on the UMT-2MT tribo-meter with aball-on-disk configuration without lubrication. The ball material wasa quenched chromium steel with a Rockwell hardness of 60–63 anda diameter of 9.5 mm. The disk was vertically fixed while the ballwas reciprocal sliding on the disk. All tests were performed at roomtemperature under a fixed load of 100 N. The relative humidity waskept at 50–60%. The reciprocal sliding had an amplitude of 10 mmand a vibrating frequency of 1 HZ. The dimensions of the samplesare 20 mm in length, 12 mm in width and 12 mm in height. Everyfriction and wear test lasted 60 min. The wear data of the bimetalcomposites was determined by the width of the grinding cracksmeasured by a scale optical microscope. The microstructures wereobserved on LEICA optical microscope (Germany). Scanning electronmicroscopy (SEM, JEOL-6360L Japan) equipped with energy dis-persive spectroscopy (EDS) was used to characterize the morphol-ogies and chemical compositions of the worn surfaces.

Fig. 4. Curves of average friction coefficient and grinding crack width versus

graphite content.

3. Results and discussions

3.1. Microstructures

During sintering processes, graphite and bronze form apseudo-alloy on the surface of the bimetal because graphitedoes not react with any elements of CuSn10Pb10. Fig. 2 comparesthe unetched optical microstructures of bimetals with graphitecontent of 1, 3, and 5 wt%, respectively. The bright parts in thepictures are metal base and the grey parts are mainly graphite.Porosities are not obvious under such a low magnification. At thebottom of the pictures shows the steel back. It is clearly seen thatthe graphite content gradually increases from Fig. 2(a)–(c). Due tothe significant difference in weight, graphite segregation in the

Mixing Laying First

singtering

First

rolling

Fig. 1. Fabrication process flow of the bimetals.

Fig. 2. Metallographic images of the materials. (a) 1 w

bronze host can easily occur during the mechanical mixing. Thissegregation becomes more evident with the increase in content ofgraphite, and the bimetal with 5 wt% graphite presents the mostsevere segregation, and possibly related, the lowest adhesion

t% graphite (b) 3 wt% graphite (c) 5 wt% graphite.

Fig.5. Curves of maximum friction coefficient versus graphite content.

R.-T. Liu et al. / Tribology International 44 (2011) 101–105 103

between the surface alloy and the steel back. However, theadhesion will not further be discussed in this paper though it mayaffect the use of the bimetal to some extent.

3.2. Effects of graphite content on Brinell hardness

The graphite phase is much softer than the bronze phase. Aswe expect, with the increase in graphite content, the hardness of

Fig. 6. Curves of micro-friction for different graphite contents at the stable and

last periods.

Fig.7. SEM of material with 1 wt% grap

the pseudo-alloy on the surface of the bimetal compositedecreases. In Fig. 3, the Brinell hardness reduces to slightlyabove 40 when the graphite content reaches 8 wt%.

3.3. Effects of graphite content on tribological performance

Graphite has a layered structure with a hexagonal lattice ofcarbon. It is valued for its extremely low friction coefficient,superior heat and electrical conductivity, and stable chemicalproperties. Owing to these advantages, graphite has long beenrecognized as one of the most widely used solid lubricants inindustry. Fig. 4 shows the dependence of the average frictioncoefficient and grinding crack width on the graphite content.When graphite is added into the bronze alloys, the solidlubrication effect is enhanced as a function of the graphitecontent, and hence the average friction coefficient decreases. Atthe content of 1 wt% graphite, there is only a slight reduction infriction coefficient as compared with the graphite-free tin leadbronze alloy. When the graphite content reaches 2 wt%, theaverage friction coefficient drops significantly. For the graphitecontent of more than 2 wt%, up to the content of 8 wt%, theaverage friction coefficients maintain a low value of 0.100–0.125.Within the same range of graphite content, the wear of thebimetal composite reduces first but then increase gradually withthe graphite content. For the content of 6.5 wt% and above, thewear becomes even worse than in the graphite-free case. Thelowest wear is found at the content of 3 wt% graphite.

Fig. 5 reports the maximum friction coefficient duringreciprocal sliding in micro-friction tests versus the graphitecontent for tin lead bronze–steel bimetal composites. Thosedata were obtained at the maximum amplitudes of reciprocalsliding motions when frictions change from static to dynamic, andtherefore are obviously larger than the average friction coeffi-cients measured in Fig. 4. During the future service of the bimetalcomposites, the average friction coefficient would oftendetermine the energy dissipation in friction. Namely, the lowerthe average friction coefficient is, lesser the energy dissipationdue to friction. Nevertheless, the maximum friction coefficient isalways an important design parameter of products. It measuresthe maximum starting torque to initiate a movement. Fig. 6 showsthe real-time micro-friction curves of bimetal composites withdifferent graphite content of 1, 3, 5, and 8 wt% during the laststeady-state periods in micro-friction tests (the whole curves aretoo wide to be presented here). It is clear that the curve of 3 wt%graphite has the minimum amplitude while the curve of 1 wt%graphite has the maximum amplitude. In comparison, the bestand most comprehensive tribological performance can beachieved with the addition of 3 wt% graphite.

hite content. (a) �100 (b) �500.

Fig. 8. SEM of material with 3 wt% graphite content. (a) �100 (b) �500.

Fig. 9. SEM of material with 5 wt% graphite content. (a) �100 (b) �500.

Element Wt% At%

CK 16.88 57.37

PbM 20.01 03.94

SnL 06.30 02.17

FeK 00.66 00.48

CuK 54.66 35.11

ZnK 01.48 00.93

Matrix Correction ZAF

Fig. 10. EDS results of worn surface of material with 3 wt% graphite content.

R.-T. Liu et al. / Tribology International 44 (2011) 101–105104

3.4. Tribological mechanism analysis

Graphite has a very low shear strength. It reduces the frictioncoefficient by adhering itself to the wear surface and forming a thinlubricating film to prevent a direct contact between two componentsin contact. When the graphite content is low, a considerablyintegrated lubricating film can hardly form on the wear surface,and consequently the bimetal composite exhibits only a limitedlubricating ability (even though there is indeed a slight improvementcompared with the graphite-free one). It is seen in Fig. 7 that for the1 wt% graphite content, the worn surface is rather rough, on whichobvious sticking and fatigue wear are evident. In Fig. 8, for the 3 wt%graphite content, the worn surface is much smoother, in either thelower or the higher magnification view, which suggests a goodcontinuously integrated lubricating film has formed on the wornsurface. When the graphite content increases to 5 wt% or above, asignificant reduction in hardness is resulted, and hence under thehigh contacting pressure (estimated to be more than 70 MPaaccording to the load and the grinding crack width), an integratedsolid lubricating graphite film can hardly be supported andmaintained on the worn surface. As a result, fractures and flakescan easily form on the worn surface as can be seen in Fig. 9. Thisexplains the reason why the bimetal composite with 5 wt% graphitehas even worse tribological performance than one with 3 wt%graphite under certain micro-friction testing conditions. It can beconcluded now that when the graphite content exceeds 3 wt%, thereduced hardness of the surface alloy of the bimetal composite doesnot allow a well support of solid lubricating film on the worn surface.The maximum friction coefficient may increase and wear becomemore pronounced, even though the average friction coefficient could

R.-T. Liu et al. / Tribology International 44 (2011) 101–105 105

remain still low. Fig. 10 presents the EDS results to show that theconcentrations of graphite and lead on the worn surface (of thebimetal composite with 3 wt% graphite) are 16.88 and 20.01 wt%,which are significantly higher than those inside the bulk alloy (withalmost 3 wt% graphite and 10 wt% lead). It is evident that graphiteand lead can segregate to form a lubricating film on the worn surface,so as to serve as an effective solid lubricant.

4. Conclusions

Based on our experimental results and discussions, thefollowing conclusions can be drawn:

(1)

For a graphite containing tin lead bronze–steel back bimetalcomposite, the hardness decreases and the microstructurehomogenization deteriorates gradually as the graphite con-tent increases.

(2)

With increasing graphite content, the tribological perfor-mance improves first and then degrades. The optimalperformance of average friction coefficient, maximum frictioncoefficient, friction coefficient amplitude, and wear resistancecan be obtained at 3 wt% graphite content.

(3)

Appropriate graphite content and hardness are the two mostcrucial factors to achieve a good quality lubricating film onthe worn surface and thus the desired solid lubricationperformance.

References

[1] Wang Jing-bo, Lu Jin-jun, Ning Li-ping. Study on the tribological behavior ofbronze-matrix self-lubricating composites. Tribology 2001;21:110–3(in Chinese).

[2] Owen KC, Wang MJ, Persad C, et al. Preparation and tribological evaluation ofcopper–graphite composites by high rate powder consolidation. Wear1987;120:117–21.

[3] Johnson LB, Kuhlman-Wilsdorg D. Performance characteristics of silver–graphite electrical brushes. Materials Science and Engineering 1983;58:1–4.

[4] Ding Huadong, Hao Hongqi, Jin Zhihao. Influence of graphite content onsintering expansion of copper base alloy. The Chinese Journal of NonferrousMetal 1996;6:106–10 (in Chinese).

[5] Ma W, Lu J, Wang B. Sliding friction and wear of Cu–graphite against2024,AZ91D and Ti6Al4V at different speeds. Wear 2009;266:1072–81.

[6] Yin Yan-guo, Liu Jun-wu, Zheng Zhi-xiang. Effect of graphite on the frictionand wear properties of Cu alloy-matrix self-lubricating composites atelevated temperature. Tribology 2005;25:216–20 (in Chinese).

[7] Moustafa SF, El-Badry SA, Sanad AM, et al. Friction and wear of copper–graphite composites made with Cu-coated and uncoated graphite powders.Wear 2002;253:699–710.

[8] Sanad AM. Effect of copper coating on consolidation and sintering of copper–graphite composites. Powder Metallurgy 1997;40:201–6.

[9] Kestursatya M, Kim JK, Rohatgi PK. Wear performance of copper–graphitecomposite and a leaded copper alloy. Materials Science and Engineering2004;A339:150–8.

[10] Margam C, Paramanand S. Sintered iron–copper–tin–lead antifriction materials-effect of temperature. Materials Science and Engineering 2000;A 292:26–33.

[11] Hirotaka K, Masahiro T, Yoshiro I, et al. Wear and mechanical properties ofsintered copper–tin composites containing graphite or molybdenum dis-ulfide. Wear 2003;255:573–8.

[12] Arwed Uecker. Lead-free carbon brushes for automotive starters. Wear2003;255:1286–90.

[13] Da HH, Rafael M. A novel electrical contact material with improved self-lubrication for railway current collectors. Wear 2001;249:626–36.

[14] Liu Xuefeng, Zhao Jinsong, Xie Jianxin. Evaluation of stainless steel back tin–bronze gradient self-lubricating composites, Acta Material Composite Sinica2008; 25:131–6 (in Chinese).

[15] Chen Sui-yuan, Xing Rui-rui, Liu Chang-sheng, Zhang Shuang-ding. Prepara-tion of a new Cu-based self-lubricating composite. Journal of NortheasternUniversity(Natural Science) 2007;28:1285–8 (in Chinese).