An approach of stand-off measuring hardness of tungsten heavy alloys using LIBS
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Surface hardness and microstructural properties are important parameters of plasma-facing material and play a key role in long pulse operation of fusion reactor. Nowadays, nuclear reactors are facing the problem of change in physical properties especially surface hardness due to dominant extreme conditions. Consequently, it is important to monitor these changes. Laser-induced breakdown spectroscopy (LIBS) has a potential diagnostic ability to monitor in situ surface hardness and their correlation with plasma wall interaction. In this work, the hardness of different tungsten heavy alloy grades is measured by stand-off approach using LIBS. The difference in hardness was attributed to grain size, crystal size, dislocations density and energy band gap (Eg) of materials. These microstructural and electronic structure properties have direct impact on electron temperature in laser-ablated plasma. Plasma electron temperature has been determined using Boltzmann plot method in the range from 1.76 ± 0.01 to 1.90 ± 0.02 eV, while electron density has been derived using Stark broadening spectral profile of (W-I) 429.47 nm line. The obtained direct relation between the ionic to atomic species of (WII/WI) and the material hardness are associated to increase in the value of plasma electron temperature (Te). The energy band gap of these tungsten heavy alloy targets has been observed from 3.24 to 3.59 eV as hardness increases from 314 ± 2.2 to 354 ± 1.1. The results showed that the energy band gap of these targets increases with hardness and have direct relation with plasma electron temperature. Ablation efficiency was also measured as a function of laser irradiance from crater depth analysis. The results showed that average ablation rate is decreased from soft to hard material.
This work was supported by the National Key R&D Program of China (Nos. 2017YFE0301304, 2017YFA0402501, 2017YFA0402502), the National Natural Science Foundation of China (Nos. 51837008, 11475039, 11405022, 11605023, and 11705020), China Postdoctoral Science Foundation (No. 2018M630285) and Lianoing Provincial Natural Science Foundation of China (No. 20170540153).
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