Hardness Determination of Thin Film Structural Materials Using Laser-Induced Breakdown Spectroscopy

The hardness of a thin film structural material is a direct reflection of its comprehensive mechanical properties, which are dependent on the film thickness. In recent years, laser-induced breakdown spectroscopy (LIBS) has been gradually applied to the determination of the hardness of materials; however, there is no relevant studies have been conducted on thin film structural materials. In this study, Q235 steels with different copper plating thicknesses on the surface were used to determine the correlation between the hardness and spectroscopic parameters. Nanoindentation was used for the hardness analysis. First, a linear relationship was obtained between the sample hardness and copper film thickness and the goodness-of-fit value was 0.9961. Second, we calibrated the plasma temperature and Cu ionic-to-atomic line intensity ratio of the surface spectra to the hardness and found that the spectroscopic parameters of the surface spectra of the samples were not sufficient to characterize their hardness. Finally, the number of shots reaching the Cu-substrate interface is determined by the similarity between the spectra obtained from two adjacent shots, 2, 5, 21, and 96, respectively. We determined a more accurate linear relationship between the plasma temperature, ionic-to-atomic line intensity ratio of the spectra at the film-substrate interface, and sample hardness. The goodness-of-fit values were 0.9923 and 0.9973. This demonstrates that the spectra at the film-substrate interface are more suitable for sample hardness analysis and LIBS is a potential method for the estimation of the hardness of thin film structural materials.