High-temperature friction characteristics of N-BK7 glass and their correlation with viscoelastic loss modulus

Abstract

It is fundamentally important to investigate the interfacial friction between softened glass and rigid molds for controlling the product quality in glass hot forming techniques. However, the underlying mechanism has not been fully understood in terms of the contribution of viscous dissipation to the viscoelastic friction of softened glass. In this paper, a dry rotary sliding scheme using the ball-on-disc pair is employed to investigate the high-temperature friction characteristics of a typical borosilicate crown glass N-BK7 from 500 °C to 660 °C. Experimental results show that the friction coefficients in the steady stage remarkably increase with the applied temperatures. For softened glass, friction could originate from adhesion, ploughing and dissipation. Furthermore, with careful estimations on both the scratch deformation frequency and the induced flash temperature during sliding, the increase in friction coefficient with temperature is well correlated with the loss modulus ratio, which presents the viscous energy dissipation by macroscale ploughing. Additionally, the friction coefficients at 500 mm/s are slightly larger than those at 250 mm/s, because the effect of more frictional heat prevails. Lastly, the influences of temperature and frequency shifting on the loss modulus ratio are summarized to explain the experimental evolution of friction coefficient. It is confirmed for N-BK7 glass that the dependences of friction coefficients on temperature and sliding velocity, also follow the classical superposition master curve of viscoelastic friction.