Effect of laser pulse duration on relative hardness estimation using LIBS

Abstract

As the demand for a quick and remote way of hardness estimation increases, the attention to use LIBS for such application increases. Most of the studies of hardness estimation that have been done used nanosecond laser sources for LIBS while using femtosecond laser has not been studied yet. In this work, a group of five samples with different hardness were analyzed using nanosecond and femtosecond LIBS for relative hardness estimation. Almost the same wavelength was used to decrease the effect of wavelength as much as possible. Considering that the laser produced plasma is in a state of local thermodynamic equilibrium (LTE), the Boltzmann plot method was adopted to calculate the plasma excitation temperature (Te) which is closely correlated to the samples’ hardness. It was found that in case of nanosecond LIBS as the sample hardness increases the excitation temperature Te increases, while for the femtosecond LIBS the behavior was opposite and Te was found to be decreasing as the hardness of the target increases. This result may be attributed to the different plasma plume creation processes and its expansion for both nano- and femto- second lasers.