In recent years, small to medium size optical lenses are produced by precision glass modeling technology which is a cost-effective method in comparison to conventional grinding, lapping and polishing processes. An application of BK7 optical glass is in production of optical lenses. These lenses have been produced using glass molding in its transitional temperature. Due to the difference in the thermal expansion of glass and mold material, the produced lens does not have the geometry of mold. Currently, manufacturers use trial and error method for making molds which is a time-consuming and expensive method. Simulation can be a solution to tackle this problem, but a realistic simulation needs knowing the frictional and material behavior of glass at elevated temperatures. Today, the challenge in glass molding is designing the geometry of model to achieve the right form of lens within the micron accuracy. So, the simulation can save the time and cost of modeling. The lack of knowledge in frictional and material behavior of glass in transition temperature range brings difficulty to the realistic simulation in glass modeling technology process. At elevated temperatures, glass shows different frictional behavior in contact with a metal surface (mold surface) since it becomes a viscoelastic material at these temperatures. One of the characteristics of friction is stick-slip frictional phenomena. The spring-damper system was used to describe the stick-slip frictional behavior at elevated temperatures. To find spring and damper coefficients, a subroutine which was composed using MATLAB was employed. In order to design the experiments, three factors including rotational speed, temperature and vertical load, each in three levels, were considered. To determine the behavior in glass transition temperature range, temperatures above and below the glass transition temperature were used. High temperature pin on disk device was used for performing the experiments at high temperatures under dry friction. The pin in the experiments is made of alumina. The experimental results show that with increasing temperature up to the transitional temperature, frictional force increases. Further increase in the temperature results in a decrease in the friction force. To check the wear behavior at different temperatures using Archard equation, intended to determine the coefficient of wear. Based on the data obtained from the experimental results, it was observed that with increasing temperature to glass transition temperature, wear coefficient increased and with increasing temperature to 630° C to melt the glass, the wear coefficient is reduced. Both experimental and theoretical results are reasonably compatible with each other since the model is selected correctly. It is observed that in BK7 optical glass, the friction force and wear volume are a function of sliding speed, temperature, and pressure. Key Words: Glass Transition Temperature, Wear, BK7 Optical Glass, Friction, Viscoelastic, Pin on Disc, Stick-Slip Behavior.
