The Relationship Between Near-Surface Mechanical Properties, Loading Rate and Surface Chemistry
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of the surface leads to the repeated making and breaking of nano-asperity contacts. INTRODUCTION
The importance of surface films and adsorbate layers in determining both tribological properties and near-surface mechanical properties has been known for many years [1-4], but a clear understanding of the physical and chemical processes which are of importance has remained elusive. This can be attributed to the complexity of the physical and chemical mechanisms which are operating and, additionally, the large range of time and length scales which determine how the near-surface region deforms. Due to this diversity of phenomena, studying the deformation of a surface necessitates experimental testing across a range of time and length-scales. This must include studies at the near atomic-scale as well as at the macroscopic scale where the material behaves like a continuum [e.g. 5]. Over the past 30 years a succession of researchers have examined the effects of surface chemistry on the deformation of nanocontacts [6-12]. Simultaneously, instruments which had been used previously to examine micro-scale surface deformation have been refined to examine properties on the atomic-scale. These new instruments include Scanning Probe Microscopes (SPMs) [13-16] and nanoindentation devices [17-19]. Thus, for the first time ever, we have the capability to examine the mechanical properties of a surface on every length-scale from the macro to atomic. Given the experimental techniques which are now available, and recent advances in the theoretical modeling of nano-scale mechanical properties [e.g. 20-23], it may appear that we are now in a position to develop a full understanding of tribological properties and near-surface 307 Mat. Res. Soc. Symp. Proc. Vol. 505 ©1998Materials Research Society
mechanical properties. However, the main emphasis of this paper is that both the theoretical modeling and experimental methods currently in use need to be adapted and expanded to take account of the importance of time and temperature dependent phenomena in mechanical deformation on the nanoscale. Most importantly phenomena like the diffusion of atoms and defects over time scales in the range of milliseconds to seconds must be considered. To some extent this has already been demonstrated by recent nanoindentation results [24-28] and Molecular Dynamics simulations [e.g. 22] which show that diffusion can be so prevalent that the contact resembles a liquid [29]. In this paper we present experimental results that demonstrate how the large stresses which result from the loading of a nanocontact (whether via an external load or the action of attractive surface forces) magnify the time-dependence of mechanical deformation. Further to this, the results show that the time-dependence of nanoscale deformation on time-dependent phenomena is very closely linked to the role of surface chemistry in determining mechanical and tribological properties. Specifically, the chemistry of the surface is important in determining the stability of nanocontacts and by conseq
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