Glasses are important materials for a wide range of applications, due to their high optical transparencies and minimal interactions with other materials. High-strength, fracture-resistant glasses are now routinely available, finished with a toughening (or tempering) process of chemical modification, where traditional Si-rich soda-lime glass is exposed to calcium-doped potassium nitrate (Ca-KNO3) and heated. Sodium ions at the surface of the glass are swapped for potassium and calcium ions in the solution, which propagate into the bulk through diffusion. The result of this ion exchange is a surface held in a state of static compression, with the core in a state of tension; thus, any microcracks (e.g., scratches) forming at the surface are discouraged from propagating into larger-scale fractures.
While the finishing process is well developed and widely used, a gap exists in understanding the local compositions resulting from the chemical modification. In particular, it is suspected that different tempering procedures may lead to different compositional profiles, but probing this is a challenging task. Glass is a bulk insulator, making it difficult to characterize using electron-based probe sources, while conventional photonic techniques lack the spatial resolution for precise detection of local changes. Recent advancements in X-ray source and optical technologies, however, are now facilitating spatially-resolved chemical analysis with X-ray fluorescence (XRF). These advancements enable spatial resolutions in the single micrometers with detection sensitivities in the parts-per-billion (attogram) regime, providing comparable detection sensitivities to techniques such as ICP-MS but with considerably more streamlined workflows. The detection range and spatial resolution coupling are complimentary to other popular techniques, such as EDS and SIMS, while the non-destructive nature of X-rays preserves the specimen for further analysis using correlative microscopy routines.
Here, we present a comparative study performed on three different commercial tempered glasses using a novel micro-XRF spectrometer for characterization. Each specimen was commercially sourced with Mohs hardness of 9H; however, by studying the elemental profile of Si, K, and Ca across each material’s cross-section, several interesting observations were made. First, it was found that one specimen had local concentrations of Ca, suggesting an incomplete chemical modification in those regions. Furthermore, it was found that two had very similar compositional profiles while the third exhibited a different profile. These results suggest that two of the manufacturers may be using similar overall strategies for fabrication, while the third may be using a different approach. In this presentation, we will briefly introduce the technique of micro-XRF in the context of other local probing techniques, then detail the results of this study as a practical commentary on the present and future of XRF.