Peculiar wear behavior of soda lime silicate glass in humid air and its implications

Soda lime silicate glass shows unusual wear and scratch behavior as a function of humidity. Under moderate wear conditions, the surface damage produced is entirely through the interfacial shear along a direction tangential to the surface. In contrast, severe wear tends to occur at much larger contact pressures resulting in damage due to indentation and shear stress (e.g. scratching). In dry environments under moderate wear conditions, soda lime glass is easily abraded by harder counter surface materials. Yet, in humid environments, soda lime silicate glass has unusually high wear resistance to counter-surface materials that are significantly harder or more chemically durable. This high wear resistance of soda lime glass can be attributed to mechanically induced chemical reactions that may suppress shear-induced damage. In the past, the high wear resistance of soda lime glass has been attributed to leachable sodium ions that may participate in suppressing shear-induced damage. Yet, recent work casts doubt on this hypothesis. Heat strengthened soda lime glass is highly susceptible to wear and scratching, indicating that wear behavior cannot be exclusively attributed to leachable sodium ions. Instead, it was hypothesized that the strained Si-O network becomes more susceptible to shear-induced hydrolysis resulting in greater damage under moderate wear conditions at high humidity. Under severe wear conditions in the presence of water, heat strengthened glass is also more susceptible to time-delayed chipping compared to annealed glass. It was hypothesized that the more open silicate network structure of heat strengthened glass may increase the transport kinetics of molecular water to critical sub-surface flaws. This may result in sub-critical crack growth of sub-surface lateral cracks resulting in time-delay chipping along the width of the scratch.