Tribology of Silicate Glasses. Friction and Surface Damage

It is well established that surface damage to silicate glass determines the mechanical strength and durability of glass products. A succession of damages leads to the generation of defects of increasing size, which gradually lower the mechanical strength. In this article, we present a brief overview of the main concepts involved in understanding the causes of such damage, from contact-induced cracking to scratches and spalling. We illustrate these principles with a few examples taken from different types of glass products, in order to highlight the generality of the ideas we present. Finally, we describe some of the known effects of the environment on surface mechanics and show how they help to understand the phenomenon of controlled material removal, i.e. polishing.

Fragile" solids such as silicate glasses offer a model case for contact cracking, and we therefore focus primarily on contact mechanics. We deal with the morphology of cracks generated by contact between rounded indenters such as blunt points. This is classically explained in terms of the stress fields that develop under such contact. We will first discuss contact under a force normal to the surface, then show why sliding contact is much more severe. In the same way, we'll then look at contact with a pointed solid. The characteristic feature of indentation is to stress the plastic response. This concept of plastic response is less common in amorphous silicates than in metals, and will therefore be the subject of a brief introduction. Without delving into the more controversial details of these phenomena, we focus on the role of plasticity in crack morphologies at the smallest scales, and provide an overview of the modes of scratch cracking. Finally, we briefly mention the less understood aspects of the issue, namely the nature of early surface defects and crack initiation mechanisms.

In the second part, we discuss the processes used to reduce surface damage. Without seeking to be exhaustive, our examples are chosen to illustrate the variety of solutions that have been developed to control glass surface damage in mass production processes, depending on the nature of the glass products and their manufacturing methods. These include float glass, glass fiber reinforcement, optical fibers and glass bottles.

In the third section, we highlight a final feature of silicate tribology. These inorganic materials undergo a rich chemistry in the presence of water, i.e. in the ambient environment. What's more, the coupling between silicate reactivity and surface mechanical stress is strong, and the mechanochemical mechanisms are therefore very active. These are, on the one hand, stress corrosion mechanisms, which condition the impact of the environment on the speed of crack propagation, and...