Nanoindentation studies of simplified nuclear glasses using molecular dynamics

Abstract

Indentation experiments on complex borosilicate glass used for the immobilization of long-lived radionuclides have demonstrated that its hardness is decreased up to a specific value after being subjected to alpha disintegrations. In this study we have performed multi-million atom nanoindentation simulations using molecular dynamics in order to better understand the atomic mechanisms that produce this decrease. The glasses we selected to simulate are three simplified sodium borosilicates in a pristine form, as well as a “disordered” form, which is analogous to the real irradiated glass. The profiles obtained after the end of the nanoindentation simulations exhibit sink-in for all cases. The calculated hardness values agree with those previously published and were found to follow the decreasing trend which has been experimentally observed after irradiation. These values were found to depend on the composition and more specifically on silica, three-coordinated boron and non-bridging oxygen content. During nanoindentation, silicon atoms retain their coordination numbers, while there is an increase in boron and sodium coordination during loading and a partial reversal during unloading. Finally, nanoindentation induced an increased number of small rings and a decrease of larger ones, the effect being more notable in the case of disordered glasses.