Laser heating and ablation of carbon–carbon composites

Carbon/carbon (C/C) composites are used as heat shield materials on hypersonic vehicles. Localized ablation can be initiated on C/C composites through targeted heating that results in oxidation and sublimation of the material. The effects of sublimation through targeted heating on C/C composites are studied and compared to graphite by ablating both materials using a 2 kW continuous wave laser in a low-pressure, inert environment. The average temperatures on the back face of both materials under multiple experimental conditions show that graphite experiences a higher average temperature on the back face than C/C composites. Scanning electron microscopy (SEM) micrographs of the ablated C/C composite samples show regions where the fibers ablate faster than the matrix. In addition, fibers exhibit needling and thinning in regions where the matrix ablates before the fibers. The surface of samples outside the ablated region developed a surface coating which was analyzed through X-ray photoelectron spectroscopy and determined to be re-deposition of sublimation gases onto the surface of a sample. The in-depth material response near the ablated region was imaged using X-ray computed tomography (XRCT), and no evidence of internal damage was found. The profiles of the ablated craters obtained using XRCT were fit to a bivariate normal distribution to measure the recession profiles. The results showed that the depth of the craters in the C/C composite was lower than the depth of the crater formed on graphite.