Studies On The Ablative Mechanism Of C_f/E Irradiated By Re-frequency Laser And Anti-laser Coatings

In this paper,the ablation mechanisms of carbon fiber reinforced E-51 epoxy composites(abbreviated as C_f/E) and E-51 epoxy irradiated by re-frequency Nd:YAG laser whose wavelength is 1064nm have been studied,also the anti-laser performance of several ablative coatings has been analysised,and one of the coatings has been modificated.The optical measurements of E-51 epoxy and C_f/E composites show that the transmission of E-51 epoxy is more than 90%under low density laser irradiation, while the transmission of C_f/E composites become zero because of the intervention of the carbon fiber.The ablation of E-51 epoxy irradiated by re-frequency laser is caused by thermal effects which is arosed by its surface absorbing of laser and the ablative process includes pyrolysis of resin,combustion of pyrolysis gas,charring of resin and vaporization of residual carbon.The ablative process of C_f/E composites includes the ablation of resin matrix and carbon fiber.The ablation of carbon fiber is caused by vaporization when the temperature reaches to its vaporization point,while the avoidance of carbon oxidation attributes to the protection of pyrolysis gas.The resin matrix begins to ablate irradiated at the conditions that duty ratio is 0.02 and maximum power density is 210W·cm^-2,while the temperature of the surface of the composites reaches to the pyrolysis temperature of epoxy which is as low as 300℃.The vaporization ablative threshold of carbon fiber is decided by the maximum power density,and has no business with the re-frequency and irradiation time.The value decided by experiment is 20kW·cm^-2 when the pulse width is 1ms.The thermal-ablated rates of C_f/E composites and E-51 epoxy are decided by their thermal coupling coefficients.With the increasing of irradiation time,the thermal-ablated rate of E-51 epoxy increases gradually,then comes to be stable when the tier of residual carbon formes.The thermal-ablated rates of C_f/E composites and E-51 epoxy increase with the increasing of the maximum power density,then turn to be a stable value,which is 12×10^-2mg·J^-1.When the re-frequency is small to the extent that the irradiation area is unable to form a stable ablation.The thermal-ablated rate of C_f/E composites is low.The stable thermal-ablated rates of these two materials are independent to re-frequency when the irradiation area is able to form a stable ablation.The anti-laser performances of three different kinds of the ablative coatings are researched.The results show that the coating whose inorganic filler is KAl₃Si₃O₁₁ and TiO₂ has the best anti-laser performance.The adiabatic index and the thermal-ablated rate of the coating reach to 4.4s·mm^-1 and 6.4×10^-2mg·J^-1 respectively,while the maximum power density is 8.0kW·cm^-2,duty ratio is 0.02 and irradiation time is 10s. Adding anti-ablated or reflecting solid filler to ablative coating will improve its ablative performance,but deteriorate its adiabatic performance by increasing the conduction of coating.Adding flake graphite into the ablative coating can improve the anti-laser performance because the reflectivity and ablative threshold of flake graphite are high and the vertical conduction is small.