PAPER REVIEW

Authors: Zhen Yang, Yanling Tian, Yuechao Zhao, Chengjuan Yang

This study investigates the fabrication of super-hydrophobic surfaces on Inconel alloy via nanosecond laser ablation under ambient air. Initially, laser-treated surfaces displayed super-hydrophilicity, but gradually transitioned to super-hydrophobicity over time due to surface chemistry evolution. SEM observations revealed hierarchical micro/nanostructures, while XPS analysis confirmed increased surface carbon content, particularly in C–C(H) bonding, indicating airborne hydrocarbon adsorption as a key factor. The wettability shift was attributed to both the laser-induced topography and subsequent chemical modification, which reduced surface free energy. The apparent water contact angle exceeded 150° after 30 days of exposure, demonstrating durable super-hydrophobicity without chemical coatings. Two wetting states—initial hydrophilic and final hydrophobic—were proposed to explain the transition mechanism. The findings underscore the synergistic role of surface morphology and post-ablation atmospheric reactions in achieving stable, coating-free super-hydrophobicity. This work contributes to understanding the passive chemical conversion mechanism on laser-structured metal surfaces, suggesting a scalable method for functionalizing Inconel alloys for anti-wetting applications. 

Authors: Cong Cui, Xili Duan, Brandon Collier, Kristin M. Poduska 

This study investigates the fabrication of hydrophobic surfaces on 17-4 PH stainless steel using nanosecond laser machining to create microscale structures. Four surface patterns—channels, pillars, and their height-variant counterparts—were designed and machined, generating not only the intended microfeatures but also unintended submicron structures. Wettability was assessed by measuring static contact angles and contact angle hysteresis, with and without a commercial Aculon coating. All laser-treated surfaces exhibited significant hydrophobicity, with static contact angles exceeding 130° even without surface coating. The Cassie–Baxter model, based on fractional liquid–solid contact, accurately predicted the observed contact angles, particularly under assumptions related to partial surface wetting. Surfaces with varied-height structures exhibited enhanced air entrapment and, when coated, demonstrated superhydrophobicity with contact angles over 150°. Although submicron roughness contributed to surface texture, it did not significantly influence static wetting behavior. These results confirm that simple and scalable nanosecond laser processing can effectively generate hydrophobic surfaces on industrial steel without complex or costly microfabrication techniques. 

Authors: C.Y. Cui, X.G. Cui, X.D. Ren, M.J. Qi, J.D. Hu, Y.M. Wang

This study investigates the transient surface oxidation of AISI 304 stainless steel under Nd:YAG pulsed laser irradiation in air. A combination of SEM, HRTEM, XRD, and XPS analyses reveals a distinct distribution of oxides within a single laser spot. The center region predominantly forms Fe₂O₃ nano-spheres due to higher temperature and Fe mobility, while the edge favors Cr₂O₃ hexagonal structures enriched in Cr and depleted in Fe. A minor presence of MnO₂ is also detected. XPS data confirm that most surface elements are in oxidized states, and thermodynamic calculations indicate Cr has a higher affinity for oxygen than Fe. However, due to faster diffusion, Fe migrates through the Cr₂O₃ layer to form Fe₂O₃ at the outer surface, resulting in a duplex oxide structure. The oxidation mechanism is governed by a competition between thermodynamics and kinetics, dependent on localized temperature and element diffusivity. This work provides new insights into ultrafast, non-equilibrium oxidation processes in stainless steels during laser processing.

Authors: S.C. Han, L.H. Wu, C.Y. Jiang, N. Li, C.L. Jia, P. Xue, H. Zhang, H.B. Zhao, D.R. Ni, B.L. Xiao, Z.Y. Ma

Joining metals and non-polar plastics like polypropylene (PP) is challenging due to the lack of interfacial chemical bonding. To address this, the authors applied a surface laser pretreatment on aluminum to create deep porous structures, followed by friction spot welding (FSW) with PP. By varying the number of laser scans, the surface morphology was controlled, and joint characteristics were analyzed. As a result, a maximum joint strength of 29 MPa—equal to the strength of base PP—was achieved, indicating 100% joint efficiency. This strong bonding was attributed to mechanical interlocking from the porous surface and the formation of C–O–Al chemical bonds between thermally oxidized PP and the aluminum oxide layer. No bubbles or defects were observed at the interface, and interfacial bonding improved significantly with increasing laser scan count. The study demonstrates that surface structuring plays a key role in enhancing both mechanical and chemical bonding at metal–polymer interfaces. This approach provides a simple yet effective strategy for fabricating high-strength hybrid joints in lightweight structural applications.