Nano-scale Wear: A Review

The definition of wear has been proposed as a progressive loss of materials due to mechanical action of two bodies while in contact. Several researchers have also suggested the definition of wear as the progressive loss of substance from the operating surface of a body occurring as a result of relative motion against a solid, liquid or gaseous counter body at the surface. Thus, it can be stated that wear encompasses a change in surface topography due to plastic deformation without any loss of material as well as removal of material from the surface due to physical interactions. In a broader sense, it may also involve chemical reactions between surfaces such as oxidation and hydration. It may be stated that wear occurs as a consequence of energy input to the interface of sliding contact caused by an externally applied force to the components in a given environment. Wear can be found in almost all machines with moving parts and it is often the limiting factor in determining the life of the machine. In addition to its effect on the life of the system, wear has a direct impact on the system performance. In this regard, the economic impact of wear is tremendous for any nation where technology constitutes a significant portion of its economy. It has been reported that high fiscal spending as much as several percent of the national GNP is typically wasted due to various tribological problems. In this regard, economic benefits as well as technological advantages controlling friction and wear would be quite profound. Particularly, the scientific contributions of minimizing tribological problems can lead to performance maximization of automobiles, information storage devices, and other precision systems. Also, environmental issues will be eased by extending the life of machines and reducing the need of lubricants, and hence, the consumption of lubricants in various machines may be minimized. As effects of wear has a deep affinity with a variety of technologies, researches on wear phenomena have been extensively conducted over decades. It has been widely accepted that there are several mechanisms of wear such as abrasive, adhesive, corrosive, fatigue and fretting. However, in many tribological systems wear can occur by more than one mechanism simultaneously which leads to difficulties in predicting the characteristics of wear. In order to predict the amount of wear, numerous models have been proposed over the years. However, the most broadly applicable law is the Archard’s wear equation expressed as: