Characterization of Individual Microneedles Formed on Alloy Surfaces by Femtosecond Laser Ablation
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Ultrashort pulse laser ablation has been known to produce clean, precise, and more reproducible processing of materials, due to nearly direct solid-to-vapor transformation even at low pulse energies, and a reduced heat-affected zone.[1–3] Formation of periodic structures on materials surfaces by femtosecond laser ablation has received tremendous attention, because of the ability of these structures to create super-hydrophobic surfaces for self-cleaning applications and enhance surface absorptance for stealth applications such as sensors and detectors.[4–7] Among these, fabricators of micron/nano-sized column/cone/spike/pillar-shaped periodic surface structures have so far extensively investigated the effects of pulsed laser parameters (pulse width, repetition rate, wavelength, and fluence) and processing media (air, vacuum, gas, and water) on the structures’ origin, evolution, and growth.[8–11] Microstructural characterization studies have been limited to
SUDIP BHATTACHARYA, Research Fellow, LIJUN SONG, Research Investigator, and JYOTIRMOY MAZUMDER, Robert H. Lurie Professor of Engineering, Professor of Mechanical Engineering, and Professor of Materials Science and Engineering, are with the Center for Laser Aided Intelligent Manufacturing (CLAIM), University of Michigan, Ann Arbor, MI 48109. Contact e-mail: Mazumder@umich. edu DONG HYUCK KAM, formerly Research Fellow, with the Center for Laser Aided Intelligent Manufacturing (CLAIM), University of Michigan, is now Senior Engineer, with the LED Product & Technology, LED Business, Samsung Electronics Co., Ltd., San #24 Nongseo-Dong, Giheung-Gu, Yongin-City, Gyeonggi-Do, South Korea. Manuscript submitted December 14, 2011. Article published online May 23, 2012 2574—VOLUME 43A, AUGUST 2012
elemental analysis and crystallinity of micron/nanosized structures formed on semiconductor surfaces.[9,12] Sakabe et al.[13] investigated the formation mechanism of surface gratings or fringes on metal surfaces by femtosecond pulsed laser irradiations. However, no reported microstructural characterization of micron/ nano-sized structures formed on metals and alloy surfaces by femtosecond laser ablation; more importantly, no definite physical mechanism of their formations is available in the open literature. The current investigation was undertaken to characterize the microstructure of individual micron-sized structures formed on three key engineering alloy surfaces, Al 5754, AISI 4340 steel, and Ti6Al4V, by femtosecond laser ablation with both low (near threshold) and high laser fluence radiations. Femtosecond laser processing parameters were optimized through a full factorial design of experiments to precisely control size and number density of micron-sized structures formed on the aforementioned alloy surfaces. Throughout the article, these micron-sized structures are referred to as microneedles. The underlying physical processes for the formation of microneedles under two fluence regimes are explored. Microneedles were formed by ablation of alloy surfaces with a FCPA lJewel D-10K (Ytte
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