Abstract
Nanosecond laser ablated metallic surfaces showed initial super-hydrophilicity, and then experienced gradual wettability conversion to super-hydrophobicity with the increase of exposing time to ambient air. Due to the presence of hierarchical structures and change of surface chemistry, the laser-induced Inconel alloy surfaces showed a stable apparent contact angle beyond 150° over 30-day air exposure. The wetting states were proposed to elucidate the initial super-hydrophilicity and the final super-hydrophobicity. The basic fundaments behind the wettability conversion was explored by analyzing surface chemistry using X-ray photoelectron spectroscopy (XPS). The results indicated that the origins of super-hydrophobicity were identified as the increase of carbon content and the dominance of C–C(H) functional group. The C–C(H) bond with excellent nonpolarity derived from the chemisorbed airborne hydrocarbons, which resulted in dramatic reduction of surface-free-energy. This study confirmed that the surface chemistry is not the only factor to determine surface super-hydrophobicity. The laser-induced super-hydrophobicity was attributed to the synergistic effect of surface topography and surface chemical compositions. In this work, the corresponding chemical reaction was particularly described to discuss how the airborne hydrocarbons were attached onto the laser ablated surfaces, which reveals the generation mechanism of air-exposed super-hydrophobic surfaces.
Highlights
As a precipitation-hardenable super-alloy, Inconel 718 alloy (IN718) has attracted more attention due to its significant potentials in gas turbines, rocket motors and spacecraft
Previous literatures have demonstrated that surface modification with a super-hydrophobic property plays an important role in various applications, for instance, self-cleaning [4,5], anti-bacteria [6], anti-corrosion [7], enhanced heat transfer [8], and drag reduction [9,10]
By exploring the fresh and aged pristine IN718 surfaces as well as the laser ablated IN718 surfaces, we investigated the mechanism of time-dependent apparent contact angle and contact angle hysteresis, and found that the surface chemistry was by no means the single factor to determine laser-induced super-hydrophobicity
Summary
As a precipitation-hardenable super-alloy, Inconel 718 alloy (IN718) has attracted more attention due to its significant potentials in gas turbines, rocket motors and spacecraft. This material shows perfect high strength, thermal and wear resistance in serious conditions [1,2]. It is urgent to find alternative method to manufacture IN718 alloy with super-hydrophobic property to expand its value in potential applications. By investigating materials from natural creatures including lotus leaves and butterfly wings, two main factors for achieving super-hydrophobic surfaces are rough micro/nano structures and the low-free-energy coatings [11].
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