Femtosecond Laser Induced Surface Morphological, Structural, and Mechanical Modifications of Brass

The present study investigates the effects of femtosecond (fsec) laser irradiation on the surface morphology, composition, microstructure, and nanohardness of brass under vacuum and oxygen environments. Brass targets were irradiated using a Ti:sapphire fsec laser with a central wavelength of 800 nm, pulse duration of 30 fsec, repetition rate of 1 kHz, and constant fluence of 1.27 J cm−2 for 30, 50, 100, 300, 600, and 1000 pulses. The surface morphology and elemental composition of the irradiated brass targets were examined using scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS), respectively, while structural modifications were investigated using X-ray diffraction (XRD) and Raman spectroscopy. Nanohardness measurements were performed to evaluate the corresponding mechanical response. The characterization results show that the surface morphology, measured composition, microstructure, and mechanical response depend on both the number of laser pulses and the surrounding atmosphere. Under vacuum, the surface initially develops fine cellular and resolidified structures accompanied by increased XRD peak broadening and a decrease in apparent Scherrer crystallite size up to 100 pulses. The value of nano-hardness increases from 2.19 GPa at 30 pulses to a maximum of 2.43 GPa at 100 pulses. Further irradiation promotes thermal accumulation, microstructural recovery, crystallite coarsening and the development of larger cavities and melt-flow structures, resulting in a decrease in hardness to 1.90 GPa at 1000 pulses. In oxygen, the measured surface oxygen content reaches a maximum of 25.50 wt.% at 100 pulses. XRD shows irradiation associated structural and oxide related changes, while Raman spectroscopy of the 100-pulse oxygen-treated sample provides additional evidence of oxide related surface modification. The oxygen-treated surface exhibits a maximum mean hardness of 2.57 GPa at 100 pulses, compared with 2.43 GPa under vacuum at the same pulse condition. At higher pulse numbers, the oxygen concentration decreases and the initially refined microstructure progressively coarsens, accompanied by a reduction in hardness to 2.23 GPa at 1000 pulses. The combined SEM, EDS, XRD, Raman, and nanoindentation results reveal a clear correlation between pulse dependent morphological, compositional, structural, and mechanical changes under vacuum and oxygen environments. Among the investigated pulse conditions, the maximum mean nanohardness in both environments was obtained at 100 pulses. Further pulse accumulation was accompanied by XRD peak narrowing, an increase in apparent Scherrer crystallite size, surface coarsening, and a reduction in nanohardness, suggesting possible cumulative thermal effects and partial structural recovery. These findings show that pulse accumulation and the surrounding environment are associated with systematic changes in the surface morphology, composition, microstructure, and mechanical response of femtosecond laser irradiated brass. These correlated changes demonstrate the potential of femtosecond laser processing for controlled surface modification and hardening of brass components.