Hardness of oxynitride glasses: topological origin.

Oxynitride glasses are mixed-anion systems, in which the 2-fold coordinated oxygen atoms have been partially substituted by 3-fold coordinated nitrogen atoms. This so-called nitridation process introduces additional bonds and thereby constrains and compacts the glass network and consequently alters the glass hardness. To explore how and why hardness varies with the degree of nitridation, we have derived a topological model of oxynitride glass hardness using temperature-dependent constraint theory, by which the scaling of glass hardness with nitrogen content can be predicted. A linear model has been derived based on the assumption that the substitution of oxygen atoms with nitrogen atoms is responsible for the hardness increase due to the increase in the number (n) of bond-bending and bond-angular constraints. It turns out that the model agrees with the experimental observation, i.e., an approximate positive linear trend of the hardness change with nitrogen content is observed for a wide range of glass compositions. The topological model may thus be useful for designing new oxynitride glass compositions with targeted hardness values.