Measurement of temperature and absorptance for laser processing applications

Abstract

Absorption measurements by laser calorimetryThe 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 CO 2 laser wavelength of 10.6 pm. 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 CO 2 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.

Temperature-Dependent Absorptances of Ceramics for Nd:YAG and CO2 Laser Processing Applications

“…Each experiment was carried out at different power levels, to ascertain that there were no serious changes of reflectance with irradiation levels. It was observed, as done by Ramanathan and Modest [16] before, that the ablation temperature of each material increases somewhat with irradiation level; this is in qualitative agreement with an ablation rate according to an Arrhenius relation. Because of the limited number of runs, and because the maximum temperature reached in each experiment (but on the same material) was different, determination of standard deviations was deemed inappropriate.…”

“…The rapid increase of absorptance can be attributed to a band shift at elevated temperatures. The present result is considerably different from that of Ramanathan and Modest [16], who reported that the absorptance increases only moderately as the temperature increases from 1300K to 2200K. As discussed earlier, due to the slow detector used in their experiment, their apparatus apparently did not respond fast enough to the rapidly changing signal.…”

“…The hot-pressed silicon nitride used in this study contains 6wt% Y 2 O 3 and 2wt% Al 2 O 3 as additives and was purchased from GTE Inc. Absorptance data for Si 3 N 4 at 10.6 m and 1.06 m are shown in Figs.2 and 3 for temperatures up to its ablation temperature of approximately 2170K [18,16]. The absorptance of Si 3 N 4 at 1.06 m remains almost constant at 0.85 for temperatures all the way up to 2170K.…”

“…The heating process took 100ms, which is the minimum to ensure temperature uniformity in the surface sublayer which absorbs and reflects thermal radiation from the probe laser. A simpler approach was used by Ramanathan and Modest [16], who used a single laser beam as both heating and probe light source. To smoothen out fluctuations in laser power, a thermopile detector was used for the reflectance measurements at the cost of temporal resolution.…”

“…In the scheme adopted by Ramanathan and Modest [16], their thermopile detector had a response time of 0.3s while the heating process took only 1s. This made it difficult for the detector to keep up with the changing signal, i.e., the detector may have lagged behind the changing signal, giving too flat a response.…”