Spectral properties of laser-induced plasma in confined conditions during thermal hardening treatment

Thermal hardening of metals under laser irradiation with an auxiliary absorbing layer occurs due to the formation and propagation of a plasma plume in confined conditions. Most post factum studies are focused on the hardness values achieved by processing, while the characteristics of the plasma plume in confined conditions stay deficiently investigated. However, it can reasonably be assumed that the contribution of the plasma plume determines energy reallocation, the development of shock wave processes, and the formation of surface structures with improved functional properties. The aim of this work is to identify the relationship between the properties of laser-induced microplasma during the laser hardening of titanium and the laser irradiation parameters as well as the target configuration. Patterns of plasma plume formation were investigated in the work using the method of optical emission spectroscopy with variable parameters of nanosecond laser radiation with a wavelength of 1064 nm. Spectral lines were recorded in the range of 380–740 nm using an “Avantes” fiber-optic spectrometer. Technically pure titanium plates were used as the object of the study and were examined in three target configurations: titanium plate, a titanium plate under glass, and a titanium plate under a layer of graphite powder and glass. The processing of the recorded emission spectra of the plasma plume included the background signal subtraction, correction for the curve of spectral sensitivity of the spectrometer and the curve of transmission spectrum of the glass, and analysis of spectral lines using the NIST Atomic Spectra Database. Numerical values of surface hardness were determined by performing Vickers microhardness measurements under a low load. It was demonstrated that during laser processing with an impulse power density above 4.1∙10¹¹ W/m², neutral atom and singly ionized atom lines can be identified for all processing configurations. This indicates that the ablation limit of the titanium target is reached, and a partially ionized plasma plume is formed. The introduction of absorbing (graphite) and confining (glass) layers leads to a visible decrease in the registered absolute emission intensity while keeping the characteristic spectral lines. At the same time, the surface hardness values increase from 185 HV to 520 HV. It was demonstrated that the target configuration significantly influences the formation patterns of the plasma plume and the hardness of the titanium surface layer within the target assembly. The identified trends may indicate the implementation of a combined thermomechanical (shock) hardening mechanism, in which efficient conversion of laser energy absorbed by the graphite layer located beneath the glass plate results in heating and strengthening of titanium to a greater extent than in heating of graphite particles ablated into the plasma plume. The results of this work can be used to optimize laser processing regimes of titanium for controlled formation of surfaces with improved hardness values.