Extensive growth of state-of-the-art technologies has created a demand for high quality lenses and has driven the industry toward an inexpensive process for manufacturing of aspheric glass lenses called Precision Glass Molding (PGM). Finite Element Analysis (FEA) has been used to predict the right mold geometry. Having a realistic simulation to predict mold geometry depends on the correct model of material behavior and friction coefficient at elevated temperature.
Finding the static and dynamic coefficient of friction experimentally between two flat surfaces at elevated temperature is the subject of this research. The equipment used in this study was originally designed for the Precision Glass Molding (PGM) process and was modified for friction measurement by using molds designed specifically for the friction test. The performance of this apparatus was validated using a steel-steel friction pair at room temperature and a steel-BK7 pair at elevated temperature.
The frictional behavior of two different types of oxide glasses; BK7 and
Soda-Lime-Silica glass have been studied. During trials at which the temperature is above the glass transition temperature, the results show the effect of glass viscoelasticity in the friction data. This effect is in the form of exponential increase in friction force data prior to the onset of sliding. Moreover, the effect of stick-slip phenomenon can be seen as a jump in the position data (in the order of microns in tangential direction). Coulomb's Law has been used to calculate the friction coefficient. An average friction coefficient has been defined and calculated for some trials, providing a quantitative value for dynamic friction coefficient at different process parameters.
The final part of the investigation involved using the Design of Experiment approach to include a broader range of processing parameters and do a sensitivity analysis to find the effect of temperature, normal force, feed rate, and surface finish on dynamic friction coefficient.
The finding from the current investigation demonstrates reasonable changes in dynamic friction of glass due to its viscoelastic properties close to its transition temperature. These friction data can be used to improve the accuracy of simulations of the PGM process.