Oxide layer reduction and formation of an aluminum nitride surface layer during femtosecond laser surface processing of aluminum in nitrogen-rich gases

Femtosecond laser surface processing (FLSP) is a unique material processing technique that can produce self-organized micro/nanostructures on most materials including metals, semiconductors, and dielectrics. These structures have demonstrated the enhancement of surface properties such as heat transfer and broadband light absorption. The chemical composition and morphology of FLSP structures is highly dependent on processing parameters including background gas composition, pressure, laser fluence, and number of laser pulses. When the laser processing is carried out in open atmosphere, a thick oxide layer forms on the FLSP surface structures due to the high reactivity of the surface with the environmental constituents immediately after laser processing. In this work, N2 and forming gas are used during laser processing in an effort to form a metal nitride on the surface of aluminum. Aluminum nitride is a promising material for enhancing the heat transfer performance of surfaces because of its thermal conductivity, which can be as high as 285 W/m-K, whereas aluminum oxide has a low thermal conductivity (30 W/m-K). Aluminum nitride incorporation into FLSP surfaces has the potential to act as a passivation layer to decrease the oxygen content and increase the thermal conductivity of the surface. Nitrogen incorporation is studied by applying FLSP in air, N2, and a 95% N2/5% H2 mixture. The chemical composition of the FLSP surfaces is determined by X-ray photoelectron spectroscopy (XPS) and energy-dispersive X-ray spectroscopy (EDS). Cross-sectional analysis of the FLSP microstructures is performed using ion beam milling.

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