Mysteries of Friction and Wear Unfolding: CMS Advances the Field of Tribology

The field of tribology defined in Webster’s Dictionary as “a science that deals with the design, friction, wear and lubrication of interacting surfaces in relative motion” has a long and important history in science and technology[1]. The ancient Egyptians, for example, used water to lubricate the path of sleds employed to transport extremely heavy objects. The first scientific studies of friction were carried out in the 16th century by Leonardo da Vinci who observed that the weight, but not the shape of an object influences its frictional force. Da Vinci was also the first to introduce the concept of a friction coefficient as the ratio of frictional force to normal load. This and similar observations led 200 years later to the development of Amonton’s Law, that states that the frictional force between two objects is proportional to the normal load and independent of the apparent contact area. In the 18th century, Coulomb verified these observations and clarified the difference between static and dynamic friction[1]. Our current understanding of the tribological properties of engineering materials recognizes that macroscale surfaces are never in flat contact, but rather are in contact through asperities, typically of varying sizes and shapes (Figure 1). At very low loads, the friction and wear properties of macroscale systems are dominated by surface forces. These include van der Waals interactions, hydration forces, and electrostatic or double-layer forces, depending on the materials and conditions of contact[2]. At higher loads, frictional forces reflect the properties of the asperities. The interactions between asperities during sliding may result in energy loss through elastic deformation, plowing, and formation of wear debris. In steels for example, asperities under certain sliding conditions may undergo a Martensitic transformation to hard and brittle structures. These brittle structures may break off during sliding to form interfacial debris that produce wear tracks[3]. Volume 5, Number 3 Mysteries of Friction and Wear Unfolding: CMS Advances the Field of Tribology