Infrared thermometry and interferential microscopy for analysis of crater formation at the surface of fused silica under CO2 laser irradiation

Abstract

In situ spatial and temporal temperature measurements of a fused silica surface heated by a 10.6 μm CO2 laser were performed using an infrared camera. These measurements were derived from heat flux emission of the fused silica. High temperature measurements—in the range 400–2500 K—were performed at the surface of a semi-transparent media with a high spatial resolution. Particular attention was given to the experimental conception and to the calibration of the infrared device. Moreover, both conventional and interferential microscopes were used to characterize the silica surfaces after CO2 laser irradiation. By associating these results with thermal camera measurements we identified the major surface temperature levels of silica transformation when heated during 250 ms. Surface deformation of silica is observed for temperatures <2000 K. This is consistent with other recent work using CO2 laser heating. At higher temperatures, matter ejection, as deduced from microscope observations, occurs at temperatures that are still much lower than the standard boiling point. Such evaporation is described by a thermodynamical approach, and calculations show very good agreement with experiment.