Fund Project:Project supported by the National Natural Science Foundation of China (Grant No. 11772354).
Received Date:28 November 2018
Accepted Date:10 January 2019
Available Online:01 March 2019
Published Online:05 March 2019
Abstract:Femtosecond laser ablation possesses a variety of applications due to its better control, high power density, smaller heat-affected zone, minimal collateral material damage, lower ablation thresholds, and excellent mechanical properties. The non-Fourier effect in heat conduction becomes significant when the heating time becomes extremely small. In order to analyze the femtosecond laser ablation process, a hyperbolic heat conduction model is established based on the dual-phase-lag model. Taken into account in the model are the effect of heat source, laser heating of the target, the evaporation and phase explosion of the target material, the formation and expansion of the plasma plume, and interaction of the plasma plume with the incoming laser. Temperature-dependent optical and thermophysical properties are also considered in the model due to the fact that the properties of the target will change over a wide range in the femtosecond laser ablation process. The effects of the plasma shielding, the ratio of the two delay times, and laser fluence are discussed and the effectiveness of the model is verified by comparing the simulation results with the experimental results. The results show that the plasma shielding has a great influence on the femtosecond laser ablation process, especially when the laser fluence is high. The ratio between the two delay times (the ratio B) has a great influence on the temperature characteristic and ablation characteristic in the femtosecond laser ablation process. The augment of the ratio B will increase the degree of thermal diffusion, which will lower down the surface temperature and accelerate the ablation rate after the ablation has begun. The ablation mechanism of femtosecond laser ablation is dominated by phase explosion. The heat affected zone of femtosecond laser ablation is small, and the heat affected zone is less affected by laser fluence. The comparison between the simulation results and the experimental results in the literature shows that the model based on the dual-phase-lag model can effectively simulate the femtosecond laser ablation process. Keywords:femtosecond laser ablation/ dual-phase-lag model/ hyperbolic heat conduction equation/ plasma shielding
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2.1.两个不同阶段的热传导方程
当飞秒激光辐照金属靶材时, 部分激光被反射, 剩余的激光被靶材吸收; 靶材吸收激光能量后被加热, 从而导致靶材的蒸发、相爆炸, 该过程被称为飞秒激光烧蚀. 飞秒激光烧蚀过程可分为两个阶段: 在第一个阶段, 靶材被激光加热并且其表面温度低于沸点, 此时不用考虑靶材蒸发和等离子体屏蔽的影响, 其示意图见图1; 在第二个阶段, 靶材表面被激光加热至高于沸点, 靶材开始被烧蚀并且在靶材表面形成等离子体羽流, 然后等离子体羽流会膨胀并吸收部分激光能量, 其示意图见图2. 本文针对以上两个阶段不同的物理过程, 将DPL模型分别与不同阶段的能量守恒方程相结合建立相应的双曲型热传导方程, 然后在双曲型热传导方程中耦合等离子体膨胀和屏蔽模型, 由此建立了一种飞秒激光烧蚀金属模型. 此外, 由于飞秒激光烧蚀金属靶材时, 靶材的吸收深度远远小于激光束的直径, 因此三维热传导问题可以简化为激光辐照方向的一维热传导问题. 图 1 蒸发开始前激光与靶材相互作用示意图 Figure1. Schematic of laser interaction with target before the initiation of the evaporation.
图 2 蒸发开始后激光与靶材相互作用示意图 Figure2. Schematic of laser interaction with target after the initiation of the evaporation.
由方程(19)可得到激光强度与最大激光强度的比值随时间的变化, 如图3所示, 设置为半峰值脉宽(full width at half maximum, FWHM)的激光脉宽也示于图3. 本文中除了3.5节的模型验证, 其他部分的计算结果所用激光脉宽都为170 fs. 图 3 激光强度与最大激光强度的比值随时间的变化(tp = 170 fs (FWHM)) Figure3. The variation of the ratio of laser intensity to maximum laser intensity with time (tp = 170 fs (FWHM)).
不同比值B时, 激光能量密度为0.2 J/cm2、时间为400 fs时的温度沿靶材深度分布情况如图7所示. 靶材表面温度随着比值B的增大而减小, 而热传导的深度随着比值B的增加而增加. 这是因为, 比值B的增加会加快热扩散的程度, 使热量更快地向靶材内部传导, 从而导致在相同的激光能量密度下比值B较高时的表面温度低于比值B较低时的表面温度, 而相应的热传导深度加深[2]. 图 7 不同比值B (${\tau _q}$不变)时, 温度沿靶材深度的分布(Ffluence = 0.2 J/cm2) Figure7. Distribution of temperature along the target depth at different ratios B (${\tau _q}$ is constant) (Ffluence = 0.2 J/cm2).
不同比值B情况下, 激光能量密度为10.0 J/cm2时靶材表层的温度变化如图8所示. 不同比值B情况下, 靶材表层温度先缓慢上升, 当温度超过熔点后, 靶材表层温度上升的速度大大加快, 这是因为超过熔点后, 靶材的吸收率提高, 而导热系数降低. 同时可以发现, 比值B越大, 靶材表面温度开始上升的时间越晚, 且上升速度越缓慢. 当靶材表层温度达到0.9Tcr时, 该层靶材由于发生相爆炸而被烧蚀掉. 图 8 比值B (${\tau _q}$不变)对表层温度的影响(Ffluence = 10.0 J/cm2) Figure8. The effect of ratios B (${\tau _q}$ is constant) on temperature of surface layer (Ffluence = 10.0 J/cm2).
不同比值B情况下, 激光能量密度为10.0 J/cm2时靶材表面温度变化如图9所示. 需要说明的是, 此时的靶材表面是指烧蚀后的实时表面位置, 它会随着烧蚀深度的变化而变化. 不同比值B情况下, 靶材表面温度都快速上升到0.9Tcr并诱导相爆炸, 由于激光能量的持续注入, 相爆炸会持续发生, 因此温度会维持在0.9Tcr的位置. 当激光能量减小到一定值时, 表面温度不能维持在0.9Tcr, 这意味着相爆炸结束, 靶材表面温度开始下降. 由于模型中使用的激光脉宽为FWHM (如图3所示), 因此在FWHM之外的部分时间内仍然有较强的激光强度, 由此导致了图9中的相爆炸维持时间超过了170 fs. 图 9 比值B (${\tau _q}$不变)对表面温度的影响(Ffluence = 10.0 J/cm2) Figure9. The effect of ratios B (${\tau _q}$ is constant) on surface temperature (Ffluence = 10.0 J/cm2).
对比不同比值B条件下的表面温度变化可知, 比值B的改变对靶材温度上升阶段和下降阶段的影响较大, 但是对相爆炸维持时间的影响不大. 在温度下降阶段, 由于B较大时, 热量向内部传导的较多, 内部与表面之间的温度梯度较低, 因此温度下降速度较慢. 不同比值B条件下, 激光能量密度为10.0 J/cm2时烧蚀深度随时间的变化如图10所示. 随着比值B的增加, 烧蚀深度增加. 这是因为在相爆炸开始后, 表面温度会维持在相同的温度下(0.9Tcr), 而比值B增大时, 热量向内部传递的速度加快, 这会使内部温度升高越多, 从而加快烧蚀速度. 图 10 比值B (${\tau _q}$不变)对烧蚀深度的影响(Ffluence = 10.0 J/cm2) Figure10. The effect of ratios B (${\tau _q}$ is constant) on ablation depth (Ffluence = 10.0 J/cm2).
23.3.${{\tau}_{\bf T}}$不变时, 比值B的影响 -->
3.3.${{\tau}_{\bf T}}$不变时, 比值B的影响
由于${\tau _q}$和${\tau _{\rm{T}}}$分别代表热传导过程中类波行为和类扩散行为的强度, 当比值B大于1时, 类波行为的强度会衰减, 类扩散行为的强度会增强, 从而使得热传导以超扩散的方式传播[88], 因此通过改变${\tau _q}$的大小来改变比值B的大小对烧蚀过程的影响与3.2节大体相同. 图11和图12分别为比值B对表面温度和烧蚀深度的影响. 由图11可知, 随着比值B的增加, 温度达到相爆炸温度的时间越晚, 相爆炸结束后温度下降得越缓慢. 由图12可知, 随着比值B的增加, 烧蚀深度增加. 图 11 比值B ($\tau _{\rm{T}}$不变)对表面温度的影响(Ffluence = 10.0 J/cm2) Figure11. The effect of ratios B ($\tau _{\rm{T}}$ is constant) on surface temperature (Ffluence = 10.0 J/cm2).
图 12 比值B($\tau _{\rm{T}}$不变)对烧蚀特性的影响(Ffluence = 10.0 J/cm2) Figure12. The effect of ratios B ($\tau _{\rm{T}}$ is constant) on ablation depth (Ffluence = 10.0 J/cm2).