Study On The Mechanism And Method Of CW-Femtosecond Combined Laser Ablation Of Alumina Ceramic Materials

As an important material of the armor protection parts,alumina ceramics have low laser absorption,high ablation threshold and other characteristics that make it difficult to achieve hard damage by the existing high-energy CW laser,and how to regulate the laser ablation efficiency and performance to achieve efficient destruction has become a pressing problem.Femtosecond laser can achieve"kilometer"into the filament,auxiliary CW laser can achieve high energy,high peak power combined laser output,explore the CW-femtosecond combined laser ablation mechanism of alumina ceramic materials,build combined laser ablation efficiency control method,to provide theoretical guidance for the practicalization of high-energy laser equipment.The main research contents of this thesis are as follows:(1)A multiphase hydrodynamic model for ablation of CW-femtosecond laser targets was constructed to solve the problem of solving the ablation process of CW and femtosecond lasers across multiple time scales.Based on the heat transfer and fluid dynamics theories,a numerical model of CW laser-matter interaction is established to study the ablation mechanism of CW laser on alumina ceramics;a numerical model of femtosecond laser-matter interaction is established by the dual-temperature equation to study the ablation mechanism of femtosecond laser on alumina ceramics.Because the CW laser,which is mainly melting and sputtering,and the femtosecond laser,which is mainly ionizing,are completely different physical processes,they cannot be directly coupled to solve.Further,based on Sedov-Taylor theory,femtosecond laser is treated as an assisted equivalence of shock waves,the multiphase fluid dynamics simulation of CW-femtosecond combined laser alumina ceramic ablation was realized,and the dynamic processes of material heating,phase change,melt pool flow,impact and sputtering of the target material during CW-femtosecond combined laser ablation were elucidated,laying the foundation for the theoretical study of alumina ceramic ablation by combined laser.(2)The experimental system of CW/femtosecond single-beam laser ablation has been set up,and the limiting characteristics of the high inverse property of alumina ceramics and the power clamping effect of femtosecond laser filament formation on the CW and femtosecond single-beam ablation efficiency were clarified by theoretical analysis,shadow imaging and numerical simulation.The experiments showed that the CW laser achieved stable ablation of alumina ceramics with the assistance of coating,and the highest ablation efficiency was about 2.6×10~7μm~3/J at CW laser power of 800 W with an ablation time of 5 ms,focal length 50 mm,and the ablation efficiency obtained at an ablation time of 20 ms is about1.13×10~7μm~3/J;the femtosecond laser with a pulse energy of 1 m J produces a plasma shock wave pressure of about 10-30 MPa and an effective duration of about 80 ns.Due to the filament power clamping effect,the pulse energy has less influence on the intermediate ablation efficiency of the filament,and the plasma shock wave is beneficial to the ablation efficiency,and the highest ablation depth is obtained in the intermediate ablation of the filament among all parameters,while the ablation efficiency is about 2.2×10~6μm~3/J at the pulse energy of 1m J.(3)A compbined laser ablation experimental system with simultaneous CW-femtosecond output was built to achieve effective and stable ablation of alumina ceramics without surface treatment by CW laser with the assistance of femtosecond laser filamentation.The central role of the assisted femtosecond laser filamentation was manifested by the preparation of defects to enhance the CW laser absorption,while the excited plasma shock wave enhanced the sputtering of the melt pool.It is shown that without any surface treatment,the CW-femtosecond combined laser ablates alumina ceramics for 20 ms and obtains an ablation efficiency of about 1.51×10~7μm~3/J with focal length 50 mm,which is about an order of magnitude improvement relative to the same parameters of the femtosecond laser alone and about 37%improvement relative to the coating-assisted CW laser alone.High-speed shadow imaging shows that the sputtering effect of the plasma shock wave on the melt pool gradually decreases with increasing ablation depth;the pattern of melt pool depth and temperature evolution in the numerical simulation results indicates that the femtosecond laser shock wave effect decreases with increasing ablation depth,which is consistent with the results monitored by shadow imaging.(4)The influence of combined laser spatial and temporal parameters on the ablation efficiency was systematically investigated,and the optimal spatial and temporal parameters matching for combined laser ablation were obtained.The optimal parameters are:400 W for the continuous laser,25ms for the ablation time,focal length 50 mm,and the combined ablation efficiency of about 1.79×10~7(μm)~3/J in the middle of the filament formation.Detailed analysis of the CW-femtosecond combined laser ablation was carried out in four aspects,namely,the combined laser spot size,different spatial positions of the femtosecond laser,different femtosecond laser pulse energies,and different ablation times.The effect of the spatial and temporal parameters of the composite laser on the ablation of alumina ceramics is clarified,and the method of regulating the ablation efficiency of CW-femtosecond composite laser alumina ceramics is mastered.In summary,this thesis proposes a CW-femtosecond combined laser ablation method for alumina ceramics,solving the problems of hard destruction of alumina ceramics by CW laser and low ablation efficiency by femtosecond laser due to filament formation power clamping,exploring the mechanism of CW-femtosecond combined laser ablation of alumina ceramic materials,and constructing a combined laser ablation efficiency regulation method,which can provide theoretical reference for the practicalization of high-energy laser equipment.