Stress Relaxation and Refractive Index Change of As₂S₃ in Compression Molding

As₂S₃ is one of the chalcogenide glasses that have attracted increasing interests for compression molding applications. This article aimed to evaluate the stress relaxation behavior of As₂S₃ above its glass transition temperature and calculate its refractive index change during cooling. First, creep tests were conducted with cylinder glass specimens at three different temperatures, in order to deduce the shear stress relaxation function by using the relationship with creep compliance function. In addition, the shift factor for thermo-rheological simplicity using Williams–Landel–Ferry equation was obtained. Then, finite element simulation was implemented to verify the calculated shear stress relaxation function. The acquired shear stress relaxation function needs to be modified to compensate the influence of friction on the thickness change in the experiments, so that the simulation results using the modified shear stress relaxation would match the experiments better. Finally, the refractive index changes of As₂S₃ at different cooling rates were modeled by using the Tool–Narayanaswamy–Moynihan (TNM) model for structural relaxation behavior. It is confirmed that the slower the cooling rate is, the less the refractive index drop will be. It was also demonstrated that the refractive index drop is strictly dependent on the cooling rate logarithmically by using TNM model. In summary, the results presented in this article can provide reliable references for viscoelastic characterization of As₂S₃ glass, crucial for compression molding or similar applications.