Structures appearing in roughened steel surfaces obtained by self-dewetting with electron beams
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I. INTRODUCTION
IN recent works, a new surface-roughening technique was presented for metal surfaces based on self-dewetting and freezing after pulsed electron-beam melting of a thin layer. The particular structures generated had characteristics that differed from those obtained from other surface modification techniques such as blasting, etching, or laser and spark ablation.[3,4] Considering that different adhesion and tribological properties can arise from these new structures, a deeper understanding of the dynamics and conditions of the onset of the instabilities and final structure is desirable. The melting produced by an electron gun is based on a pulsed glow discharge delivering a 20 A peak current in a 20-s pulse duration (full-width, half-medium) of near 20 keV electron energy.[5,6] The beam is focused geometrically and by the Lorentz force in a spot a few millimeters wide. The surface melts and rapidly cools, dissipating the excess heat into the substrate. While melted, a complex structure developed that was attributed to the onset of nonlinearities originated in the anomalous surface-tension temperature dependence caused by the sulfur content of the sample.[1] The surface patterns created are similar to those obtained in transient stages of the spinodal dewetting of thin layers of liquids laying on surfaces that the liquid does not wet.[7–10] The spatial pattern was found to depend on the sulfur content of the sample (which gives rise to an anomalous temperature dependence of the surface tension of the liquid) and on the heating-pulse duration, energy, and focusing, but not on the underlying microstructure (grain size and basematerial phases). The resulting structures appear as ridges [1,2]
U. CROSSA ARCHIOPOLI, Ph.D. Student, and N. MINGOLO, Professor, are with the Laboratory of Haces Dirigidos, Department of Physics, Facultad de Ingeniería, Universidad de Buenos Aires, C1063ACV Buenos Aires, Argentina. Contact e-mail: [email protected] O.E. MARTÍNEZ, Professor, is with the Laboratory of Electrónica Cuántica, Department of Physics, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, C1428EGA Buenos Aires, Argentina. Manuscript submitted July 5, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS A
of a rapidly cooled melt (martensitic) surrounding valleys of unmodified substrate, a structure attributed to a transient dewetting of the melt.[1,2] Several questions remain unanswered: one is if there is a contribution to the instability due to the rapid ejection of gases after the evaporation temperature has been exceeded at the surface. As during pulsed laser melting, a strong evaporation coexists; this rapid gas ejection initiates different types of instabilities such as Kelvin–Helmholtz hydrodynamic instabilities (originating from the large tangential velocity of the ejected gas[3]) or instabilities of the evaporation front.[3] If a similar gas ejection would appear during electron-beam melting, this could also be the source of the melt destabilization reported before.[1,2] Hence, althoug
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