| Abstract [eng] |
Materials surface hardness measurements in the industry are mainly done by means of deformation (Vickers, Brinell etc.) and result in visible craters as well as require special material preparation, as specific specimen form requirements have to be met. Because of these limitations, applied method for remote and rapid materials hardness measurements could yield advantages in fields like space engineering and nuclear power technology maintenance. A number of articles have shown great promise for LIBS (laser induced breakdown spectroscopy) use in materials surface hardness analysis applications. The method for applying LIBS for hardness measurements comes from searching for either plasma excitation temperature values, or ionic/atomic spectra line intensity ratio, as both of these values are shown to increase with surface hardness of a specimen for a variety of different materials. The use of LIBS in materials surface hardness has many advantages to traditional methods: available remote analysis; rapid analysis of surface hardness; not needed material preparation as the method does not rely on specimen form; as well as high relative accuracy comparing with Vickers method. Most articles, however, tend to analyze the applicability of LIBS on the matter using nanosecond pulses and very few articles analyze the problem using ultrashort pulses. Successfully applied ultrashort pulses for hardness measurements could result in wider applicability as they tend to be used in materials micromachining for their precise and controlled ablation characteristics. In our work we analyze the applicability of fs-LIBS for surface hardness measurements by performing LIBS measurements on five steel samples with known Vickers hardness values. The plasma was excited using Pharos femtosecond laser 201fs pulsewidth, using first (1030nm) and third harmonic (343nm) irradiation as the excitation source for the plasma. We compared the results exciting the plasma with UV and IR irradiation. The results show clear dependancies of plasma temperature and ionic/atomic line intensity ratio in relation to materials surface hardness in the case of fs-UV irradiation. However the plasma excitation temperature tends to have and opposite dependency comparing with longer pulse LIBS results – with materials surface hardness increasing, the plasma temperature decreases. This opposite dependency compared to nanosecond and picosecond pulse irradiation might be explained by taking into the account the different ablation and plasma plume formation dynamics in the case of fs pulse irradiation. In the case of fs-IR irradiation, no clear dependencies were observed. In order to exclude the possibility of error in the experiment, we conducted extra measurements increasing the pulsewidth of irradiating pulses. The pulsewidth was increased to 3.9ps, 7.3ps and 15ps. In all of the picosecond pulsewidth irradiation cases, clear linear increases of plasma excitation temperature in relation to steel samples surface hardness were observed. For further analysis a shadowgraph scheme was used to measure plasma shockwave speed. The lasers first harmonic (1030nm) irradiation was used for plasma excitation and second harmonic (515nm) was used for shockwave imaging. The measured shockwave speed tend to rise with steel samples surface hardness. In conclusion the results show that only in the case of UV irradiation did we manage to get clear dependencies, needed for applications of fs-LIBS for surface hardness measurements. |
