CARBON-DIOXIDE LASER CUTTING OF A CARBON-FIBER SILICON CARBIDE-MATRIXCOMPOSITE

CO2 laser scribing and cutting were studied on a carbon-fiber-silicon carbide-matrix (C/SiC) composite nominally containing 45 vol% of carbo n fibers. The scribing and cutting were performed in continuous-wave ( CW) mode using laser powers between 750 and 1500 W, and specimen trans lation velocities between 0.5 and 4 cm/s. The laser spot size was 300 mum in diameter. The groove width and depth were measured as functions of power and velocity. The results were compared to theoretically pre dicted values obtained by solving the quasi-steady-state heat conducti on equation in three dimensions for a moving body. Reasonably good agr eement between theory and experiment was found. The microstructures of the laser-cut surfaces indicated the formation of redeposit by conden sation from the vapor phase. X-ray diffraction and Raman spectroscopy analyses of the redeposit showed the presence of beta-SiC and graphiti c carbon. The four-point bending strength of the laser-cut composite w as found to be approximately 20% lower than the corresponding strength of the diamond-cut composite. The strength was fully recovered after removing 180 +/- 10 mum of the material from the lased surface by grin ding. The oxidation resistance of the laser-cut and diamond-cut compos ites was studied with a thermogravimetric balance at 1103-degrees, 130 4-degrees, and 1402-degrees-c in air. The oxidation behavior at all in vestigated temperatures for both materials was dominated by a rapid in itial mass loss due to the oxidation of carbon and a possible active o xidation of SiC, followed by a slow mass gain due to the passive oxida tion of SiC. At 1304-degrees-C the rate of passive oxidation of SiC in the laser-cut material was somewhat higher than in the diamond-cut ma terial. At 1402-degrees-C, the diamond-cut surface oxidized more rapid ly than the laser-cut surface. The differences in oxidation rates were attributed to the differences in microstructure.