摘要
飞秒激光与材料相互作用的物理机制是飞秒激光微纳加工的基础。深入分析并理解飞秒激光烧蚀金属与合金材料,对提高加工效率与加工质量,全面优化加工工艺有很好的指导作用。本文采用结合双温模型与分子动力学模拟的方法研究飞秒激光与金属及合金材料相互作用所涉及的温度场演化,内部应力分布和原子位型变化等瞬态物理过程。论文的主要内容总结如下:
(1)分析总结了飞秒激光与金属材料相互作用的一般物理过程。在超快时间领域内激光能量的沉积过程,材料对能量的吸收以及转移转化等过程。描述材料在不同的特征时间内产生的特定物理现象。具体阐述了结合双温模型的分子动力学模拟方法的执行步骤。
(2)从一维双温模型出发,分析了材料热物理参量,包括电子热容,电子热传导率模型和电子声子耦合因子等对飞秒激光烧蚀金属材料温度场演化的影响。研究发现电子热容较大的金属需要较高的能量才能发生烧蚀行为,而不同的电子热传导率模型对飞秒激光作用下材料的热影响区域深度至关重要。电子声子耦合因子则对材料内部电子晶格间能量非平衡性持续时间起决定作用。还探讨了不同激光参量对温度场演化的影响,发现激光能量越高,则温度越高,温度梯度越明显,电子声子耦合时间变长。而脉宽越短,作用过程越激烈,激发的电子温度越高。
(3)采用结合双温模型的分子动力学方法,开展了飞秒激光与过渡金属镍,轻质金属铝和贵金属银等典型金属作用的研究。过渡金属镍有较大的电子热容和较小的电子热传导率,因此热影响区域深度较小,且发生烧蚀行为所需的激光能量较高。飞秒激光与金属材料作用机理可描述为当能量足够大时,材料形成均匀的熔化固液界面向材料内部推进,当继续增加能量密度,可能诱发气相形核甚至达到材料的临界点温度,从而可能发生相爆炸现象。若继续增加能量密度,则可直接导致材料表面稠密等离子的形成。
(4)首次初步建立了飞秒激光与B2结构镍钛合金相互作用的结合双温模型的分子动力学模型。分析不同能量密度和脉冲宽度的激光对烧蚀结果的影响。测得了材料在吸收能量密度为29.3mJ/cm2下发生烧蚀行为。烧蚀发生在激光诱导产生的拉应力波传播过后的材料次表层区域。材料内部压力波传播的速度与材料的声速相当。低能量密度下,烧蚀产物包含一个较大的液体团簇,而高能量密度下,烧蚀产物还有小液滴和单个粒子的存在。激光诱导的应力波在材料后表层形成一个应力集中区域,可能会造成材料在该区域的机械力破碎现象。
Mechanism of femtosecond laser interaction with materials has been widely investigated in recent years. It is the foundation of femtosecond laser micromachining. In this paper, a hybrid simulation combined two-temperature model and molecular dynamics simulation is applied to investigate femtosecond laser interaction with metals and alloy. The physical process including temperatrure evolution, pressure distribution and atomic configuration is described in detail. The main work is summarized as follows:
(1) The general physical process is analyzed and summarized in the domain of femtosecond laser interaction with metals, which includes the deposition of optic energy, the absorption and transfer energy in the material. The characteristic time is resolved in the special physical phenomenon. The basic calculation methods have been performed in detail.
(2) On the foundation of one dimension two-temperature model, the influence of material properties and laser parameters are investigated on femtosecond laser interaction with typical metals. The simulation results show that the metal which has a bigger electron heat capacity needs more fluence to occur to ablation behavior. The different electron thermal conductivity model is the key to the depth of heat affect zone. The nonequilibrium time between electron and lattice under femtosecond laser is strongly dependent on the electron-phonon coupling factor. The stronger laser fluence is, the higher the temperature and the longer the coupling time. It is also observed that by the shorter pulse duration, the electron temperature is higher and the process is stronger.
(3) We research on femtosecond laser ablation of transition metal (Ni), light metal (Al) and noble metal (Ag) using the hybrid modeling combined two-temperature model and molecular dynamics simulation, respectively. The results show that transition metal has the small and thin heat affect zone because of bigger electron heat capacity and less electron thermal conductivity. The mechanism of femtosecond laser interaction with metals can be described that at certain laser fluence, heterogeneous nucleation increases and surface material begins melting. The phase explosion may occur while increasing laser fluence and temperature reaching to critical point. If the laser fluence is high enough, it directly results in forming plasma.
(4) The modeling of femtosecond laser ablation of B2 type NiTi alloy is resolved by combined two-temperature model and molecular dynamics simulation. The simulation results show that ablation threshold is near absorbed fluence of 29.3mJ/cm2 and the speed of laser-induced pressure wave is related to velocity of sound in material. Thermal ablation occurs in subsurface while the tensile wave passes through the subsurface of material. At low laser fluence, ablated material is formed by a big liquid cluster, however, at high laser fluence, the ablated component consists of small cluster, liquid droplets and single particles. A focused region of negative pressure is formed in the substrate. The fragmentation may occur by this negative pressure. The novel ablation mechanism results from laser induced thermomechanical wave.
引文
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