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.