氮化硅陶瓷具有耐高温、 耐腐蚀和耐磨损等优异性能, 可应用于金属材料和高分子材料难以胜任的极端工作环境。 但具备这些优良特性的同时也给其加工带来了不便, 传统的磨削加工方法效率低, 设备损耗严重, 激光辅助加工为其提供了一种新途径。 将等离子体光谱法和显微成像法相结合, 对脉冲激光辐照氮化硅陶瓷的损伤阈值进行了测量, 并分析了损伤机理。 实验选用热压烧结氮化硅陶瓷为靶材, 参考ISO21254国际损伤阈值测试标准搭建试验系统, 采用1-on-1法利用Nd3+∶YAG固体脉冲激光分别在纳秒和微秒脉宽下辐照氮化硅陶瓷, 两种脉宽分别选取10个能量密度梯度进行激光辐照, 每个能量密度辐照10个点。 利用光纤光谱仪采集光谱信息, 利用金相显微镜获取显微图像信息, 将光谱结果与显微成像结果对比分析, 发现纳秒脉宽下材料一旦损伤光谱上就会出现等离子体峰, 通过分析光谱中等离子体峰, 元素指认是否含有材料中特征元素即可判断损伤, 为了区别空气电离击穿同时测量了空气等离子体光谱对比分析剔除干扰。 微秒脉宽下显微图像观察到刚开始损伤时, 光谱中只出现较强热辐射谱线并未出现等离子体谱线, 进一步增加激光能量密度, 光谱中会出现少量等离子体峰, 因此不能直接以等离子体峰判断材料损伤阈值。 利用金相显微镜观察损伤形貌, 纳秒脉宽下在损伤区域内部观察到明显的烧蚀冲击状损伤, 光谱呈现出大量等离子体谱线, 说明纳秒激光辐照氮化硅损伤机制主要为等离子体冲击波引起的力学损伤效应。 微秒脉宽在辐照区域边缘发现热烧蚀痕迹, 损伤区内观察到大量熔融物, 出现明显热辐射光谱, 说明微秒激光辐照氮化硅损伤机制主要是由于长脉宽热积累引起的热损伤效应, 随着能量密度增加热辐射谱上叠加有等离子体峰, 等离子体峰值强度与损伤程度一致。 利用零几率损伤阈值法对两种方法测得结果进行了拟合, 分析发现等离子体光谱法更适用于纳秒脉宽下损伤阈值测量, 得到结果为0.256 J·cm-2; 显微成像法适用于微秒脉宽下损伤阈值测量, 得到结果为6.84 J·cm-2。

Silicon nitride ceramics have high temperature, corrosion and wear resistance; therefore, they are good candidates to be used in extreme working environments where metals and polymers are difficult to handle. Unfortunately, besides these excellent properties, these materials are difficult to process. The traditional grinding method is inefficient, and the mechanical damage of the material is serious. In this regard, laser-assisted machining is a new promising way for the efficient processing of silicon nitride ceramics. In this paper, we combined plasma spectroscopy and microscopic imaging methods to measure the damage threshold of pulsed laser irradiated silicon nitride ceramics to analyze the damage mechanism. For this experiment, we selected a hot-pressed sintered silicon nitride ceramic as the target material and built a test system with reference to the ISO21254 international damage threshold tests standard. Silicon nitride ceramics were irradiated bysolid-state Nd3+∶YAG pulsed laser at nanosecond and microsecond pulse duration using 1-on-1 method. The two pulse widths were respectively selected from 10 energy density gradients for laser irradiation, and with each fluence 10 points were irradiated. The spectral information was acquired using a fiber optic spectrometer, and the microscopic image information was acquired by using a metallographic microscope. Under the nanosecond pulse irradiation, damage will occuronce the plasma peak appearing on the spectrum. Analyzing the plasma peak on the spectrum, we could identify whether it contains the characteristic elements of the material to determine the damage. In order to distinguish air ionization breakdown, the interference was eliminated by comparing and analyzing the air plasma spectrum. Under the microsecond pulse irradiation, the microscopic imaging showed that at the beginning of the damage, there was a strong thermal radiation line of the spectrum but no plasma spectrum line. Further increasing the laser fluence, we observed a small amount of plasma peaks appearing on the spectrum. Therefore, the material damage threshold cannot be directly judged upon the plasma peaks. The damage morphology was observed with the metallographic microscope: and obvious ablation impact was visible inside the damage area after the nanosecond pulse irradiation. A large number of plasma lines appearing on the spectrum indicate that in case of the nanosecond pulse irradiation, the damage of the silicon nitride is mainly mechanical caused by plasma shock wave. The microsecond pulses, create hot ablation marks on the edge of the irradiated area with a large amount of molten material in this zone. The spectrum shows obvious thermal radiation features, which indicates that in this case the damage is mainly thermal, caused by the long pulse duration and the corresponding heat accumulation. As the energy density increases, a plasma peak is superimposed on the thermal radiation spectrum. The degree of damage after the appearance of the plasma peak on the spectrum is consistent with the peak intensity of the plasma. The results of plasma spectroscopy and microscopic imaging were compared and analyzed. The measured spectra were fitted with the zero probability damage threshold model. Thefit result showed that the plasma spectroscopy method is more suitable for the damage threshold measurement at nanosecond pulse width, and the corresponding damage threshold is of 0.256 J·cm-2. On the other hand, the microscopic imaging is more suitable for measuring the damage threshold at the microsecond pulse width; the corresponding damage threshold is of 6.84 J·cm-2.