High temperature friction characterization for viscoelastic glass contacting a mold

Computational simulations of glass forming processes when interface slip occurs require a precise characterization of friction since friction, like viscosity, affects how much load is required to deform glass at a given rate. Friction therefore affects the processing time in the case of a load controlled process such as precision glass molding (PGM) and complicates the determination of viscosity when using parallel plate viscometry (PPV). In this combined experimental and computational study the ring compression test, where a “washer” shaped glass specimen is compressed between two flat molds at high temperature, is conducted and simulated. This test is very similar to PGM and PPV, making it ideal to quantify friction for these processes. The Coulomb friction model is used due to observed glass slipping behavior and the relatively low values of shear stress encountered in these processes. For glass at high temperature where viscosity is in the range of about 107–1011 Pa · s, it is demonstrated that the outcome of the test has a very weak dependence on material properties, which is significant since the stress and structural relaxation properties of glass within the transition temperature region are temperature dependent and difficult to obtain. The presented friction calibration curves are therefore material independent when the proposed processing procedure is followed. Sensitivity analysis is performed with respect to various factors, such as rate of deformation, magnitude of loading, temperature non-uniformity and contact of the inner and outer cylindrical surfaces with the mold surfaces, providing the experimentalist with guidelines to conduct valid tests. Using this friction characterization method, the viscosity range for PPV can be increased by accurately correcting for interface slip.