Mass removal by oxidation and sublimation of porous graphite during fiber laser irradiation

• Published in: Optical engineering 2017-01, Vol.56 (1), p.011013-011013
• Main Authors: Phillips, Grady T, Bauer, William A, Fox, Charles D, Gonzales, Ashley E, Herr, Nicholas C, Gosse, Ryan C, Perram, Glen P
• Format: Article
• Language: English
• ISSN: 0091-3286; 1560-2303
• DOI: 10.1117/1.OE.56.1.011013

The various effects of laser heating of carbon materials are key to assessing laser weapon effectiveness. Porous graphite plates, cylinders, and cones with densities of 1.55 to 1.82  g/cm3 were irradiated by a 10-kW fiber laser at 0.075 to 3.525  kW/cm2 for 120 s to study mass removal and crater formation. Surface temperatures reached steady state values as high as 3767 K. The total decrease in sample mass ranged from 0.06 to 6.29 g, with crater volumes of 0.52 to 838  mm3, and penetration times for 12.7-mm-thick plates as short as 38 s. Minor contaminants in the graphite samples produced calcium and iron oxide to be redeposited on the graphite surface. Dramatic graphite crystalline structures are also produced at higher laser irradiances. Significantly increased porosity of the sample is observed even outside the laser-irradiated region. Total mass removed increases with deposited laser energy at a rate of 4.83  g/MJ for medium extruded graphite with an apparent threshold of 0.15 MJ. At ∼3.5  kW/cm2, the fractions of the mass removed from the cylindrical samples in the crater, surrounding trench, and outer region of decreased porosity are 38%, 47%, and 15%, respectively. Graphite is particularly resistant to damage by high power lasers. The new understanding of graphite combustion and sublimation during laser irradiation is vital to the more complex behavior of carbon composites.