Investigation of the thermal microstructural effects of CO^sub 2^ laser engraving on agate via X-ray microtomography

CO2 laser micro-machining is a precise and versatile tool for brittle, low-conductive materials such as agate, a quartz mineral variety of chalcedony. Laser engraving on agate’s polished surface generates a permanent white mark, noticeable to the unaided eye, resulting from several thermal microstructural effects originated by the laser’s focalized heat that rapidly melts, vaporizes, and solidifies specific regions. However, few studies have attempted evaluating and quantifying such micro-scale effects during laser processing. We used a non-destructive, high-resolution method to reconstruct and analyze a 3D model of the laser engraved agate surface based on microtomography (µCT) images. Intrinsic water found in agate’s microstructural defects was identified as the major cause for the development of micro-cracks, fractures, and internal porosity due to the resultant vapor pressure. Molten SiO2 ejection also generated an external porosity composed of large, open cavities, forming the mark on the gemstone’s surface. µCT images allowed the evaluation of the absolute and relative amounts of material ejected and vaporized, and both molecular water (H2O) and hydroxyl bonds on silanole groups (SiOH), within the material’s microstructure, should be considered for a precise and uniform laser engraving. The results show that µCT analysis is an accurate method for evaluating and quantifying the microstructural effects of laser engraving on the surface of silica-based materials.