纳米孪晶铜摩擦行为研究

Fretting wear behavior of nanocrystalline surface layer of pure copper

under oil lubrication

Y.S.Zhang a and Z.Han a,*

a

Shenyang National Laboratory for Materials Science,Institute of Metal Research,Chinese Academy of Sciences,72Wenhua Road,Shenyang,

110016,P.R.China

Received 7December 2006;accepted 5February 2007;published online 2March 2007

Lubricated fretting tests in mineral oil were performed with a nanocrystalline surface layer on a pure bulk Cu prepared by surface mechanical attrition treatment (SMAT)against a WC-Co ball.It was found that the nanocrystalline surface layer exhibited a markedly enhanced fretting wear resistance and higher friction coe cient relative to the coarse-grained (CG)form.The wear volume of the SMAT Cu is one order of magnitude lower than that of the CG Cu.The friction coe cient of the SMAT Cu increases with an increasing load and frequency,while for the CG Cu,the friction coe cient increases with an increasing fretting frequency up to 100Hz and thereafter decreases.The higher hardness of the SMAT Cu is suggested to be the main factor causing its improved wear resistance and higher friction coe cient.A discontinuous metal transfer layer can be found on the WC-Co ball only after fretting against the SMAT Cu,which may partly account for the higher wear resistance of the SMAT Cu in comparison with the CG Cu.

KEY WORDS:Nanocrystalline metals,Oil lubrication,Fretting,Transfer,Copper

1.Introduction

It is well-known that nanocrystalline materials exhibit unique and often enhanced physical,chemical,and mechanical properties.During the last decade,tribo-logical properties of nanostructured materials have been paid more attention and many experimental results have shown a signi cant enhancement in friction and wear properties of nanostructured materials, e.g.[1–5].However,it should be mentioned that most investiga-tions on wear properties of nanostructured materials have been focused on sliding wear mode.To our knowledge,only few experimental data [6–8]have been reported yet concerning the fretting wear behavior of nanostructured materials.

Fretting is described as the oscillatory relative tan-gential movement which contacting surfaces may expe-rience as a result of vibration or cyclic stressing of one of the surfaces [9].It is often the origin of catastrophic failures or loss of functionality in many industrial applications,including bolted mechanical joints,stacks of objects in transport,and electrical connectors in vibrating machinery.In recent decades,how to improve the fretting wear resistance of material has always been a huge challenge for material scientists due to its impor-tance in industrial application.

In our previous studies [1,10],a nanocrystalline surface layer on a pure Cu plate was achieved by means

of surface mechanical attrition treatment (SMAT),of which the sliding behavior was investigated.The objec-tive of this work is to evaluate its fretting wear resistance under oil condition,a situation more common in prac-tical engineering applications.The bene cial e ect of a nanocrystalline surface layer of pure Cu on fretting wear behavior and the involved mechanisms responsible for this e ect are also discussed.

2.Experimental

Details of the SMAT process were described previ-ously [11,12].AISI52100steel balls with a diameter of 8mm were placed at the bottom of a cylinder-shaped chamber that was vibrated by a generator,causing the balls to impact onto the sample surface to be treated at the upper side of the chamber.Due to severe plastic deformation at a very high strain rate,grains in the surface layer were e ectively re ned to the nanometer scale.

A copper plate (100Â100Â3mm in size)with a purity of 99.99wt.%was subjected to the SMAT pro-cess.Before treatments,the Cu plate was annealed at 923K for 2h to obtain homogeneous coarse grains.The samples were treated under vacuum at room tempera-ture for 30min with a vibration frequency of 50Hz.The cross-sectional optical and transmission electron microscope (TEM)observations on the Cu sample after the SMAT processing for 30min have been carried out in our previous work [1,10].The microstructure of the top surface layer of this kind of Cu sample is

*To whom correspondence should be addressed.E-mail:zhonghan@imr.ac.cn

1023-8883/07/0700-0053/0Ó2007Springer Science +Business Media,LLC

Tribology Letters,Vol.27,No.1,July 2007(Ó2007)53

DOI:10.1007/s11249-007-9204-2