Abstract：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.
蔡鹏程，李 霜，蔡红星，谭 勇，石 晶，苗馨卉. 脉冲激光辐照氮化硅陶瓷损伤阈值的光谱测量[J]. 光谱学与光谱分析, 2019, 39(11): 3433-3437.
CAI Peng-cheng, LI Shuang, CAI Hong-xing, TAN Yong, SHI Jing, MIAO Xin-hui. Study on Measurement of the Damage Threshold of Silicon Nitride Ceramics under Pulsed Laser Irradiation by Spectroscopy. SPECTROSCOPY AND SPECTRAL ANALYSIS, 2019, 39(11): 3433-3437.
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