The Effect of Si3N4 Reinforcement on Optical Band Gap and Absorption Behavior in Recycled Glass Frits

The environmental pollution created by waste glass frits generated in industrial production processes makes the conversion of these materials into secondary products important in terms of both sustainability and resource efficiency. In this context, the recycling of waste glasses not only reduces the need for disposal but also offers an alternative way to access high value-added engineering materials. Waste glass frits that generated in enamel applications can be used as the alternative materials. Therefore, in this study, the waste glass frit with SiO2-ZrO2-B2O3-Na2O-ZnO-CaO-Al2O3-K2O-Fe2O3 oxide components generated in enamel applications was transformed into an adaptable composite for advanced applications. For this purpose, Si3N4 was added to the waste frit matrix at rates of 5%, 10%, 15%, and 20% wt., and the samples were shaped and sintered using powder metallurgy processes to obtain the final products. Microstructural (SEM–EDS), mechanical (hardness), and physical (density) characterizations were performed on the produced samples to determine Si3N4 effect on the structural properties. Additionally, the changes in optical absorbance and band gap were evaluated using UV-Vis spectroscopy. It was observed that increasing the Si3N4 content significantly transformed the microstructure by altering the reinforcement-matrix interface within the structure. The hardness and density results revealed that Si3N4 addition directly affected the mechanical behavior, while the changes in optical response showed that the matrix-reinforcement interaction also reshaped the optical performance. In the 15 wt.% Si3N4 sample exhibited improvement on density (2.57 g/cm3) and Vickers hardness (14 GPa), indicating enhanced mechanical consolidation. The same composition also showed a narrowing optical band gap (4.61 eV) compared to the other samples, confirming a clear modification of the electronic structure due to Si3N4 addition.