Measurement of temperature and absorptance for laser processing applications

The use of CO2 lasers has been successfully demonstrated for several manufacturing processes such as cutting, drilling, scribing, etc., of a wide range of materials. The absorptance of a material at the laser wavelength and at the material removal temperature substantially affect the efficiency of the laser machining process. Some materials have absorption bands in the mid-infrared and the absorptance changes drastically with temperature at the CO2 laser wavelength of 10.6 μm. Additionally, the absorptance of a material can change due to high-temperature effects such as decomposition, oxidation, etc., necessitating the measurement of absorptance at temperatures close to the material removal temperature and, preferably, during the laser machining process. A relatively simple experimental set-up is presented for the measurement of material removal temperature and of the absorptance of ceramic materials at the CO2 laser wavelength and at temperatures close to the material removal temperature. Experimental results have been obtained for hot-pressed silicon nitride and two continuous-fiber ceramic matrix composites, namely, carbon fibers embedded in a silicon carbide matrix (C/SiC) and silicon carbide fibers in a silicon carbide matrix (SiC/SiC). Temperatures were measured using an optical pyrometer while the absorptance measurements were performed with an integrating sphere. The material removal temperatures show good agreement with extrapolated equilibrium decomposition data; the absorptance data were found to be repeatable to within ±0.03.