Marangoni Convection during Free Electron Laser Nitriding of Titanium
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UCTION
THE nitriding and carbonizing of surfaces are wellknown methods for improving tribological properties of different metallic compounds, in particular, of titanium and its alloys. The established methods of metals surface treatments are plasma and gas nitriding.[1] The linked processes, mainly diffusion, have been described by many authors. They are based on the diffusion-like process of the nitrogen in the matrix in millisecond time regimes. Alternatively, it is possible to treat the surface with laser radiation, to synthesize hard coatings directly; in general, this has been done by Nd:YAG[2] and CO2[3] lasers. In this work, coatings several microns in thickness were synthesized. In other experiments,[4,5] TiN coatings were generated for the first time by means of a free electron laser (FEL). Due to its high power and the flexibility in its temporal shaping, this type of laser could be the right tool. The coatings show quite good tribological properties such as hardness. In order to understand the various physical processes, it is necessary to make in-situ investigations. The nitrogen transport and corresponding coating properties are determined by the time of treatment, for which diffusion will be assisted DANIEL HO¨CHE and SVEN MU¨LLER are with the Universita¨t Go¨ttingen, II. Physikalisches Institut, 37077 Go¨ttingen, Germany. GERD RAPIN is with the Institut fu¨r Numerische und Angewandte Mathematik, 37083 Go¨ttingen, Germany. MICHELLE SHINN is with the Thomas Jefferson National Accelerator Facility, Free Electron Laser Group, Newport News, VA 23606. ELVIRA REMDT, MAIK GUBISCH, and PETER SCHAAF are with the TU Ilmenau, Institut fu¨r Werkstofftechnik, FG Werkstoffe der Elektrotechnik, 98684 Ilmenau, Germany. Contact e-mail: [email protected] Manuscript submitted June 24, 2008. Article published online May 2, 2009. METALLURGICAL AND MATERIALS TRANSACTIONS B
by the Marangoni convection for extended time regimes. The film thickness mainly depends on the melting depth and the nitrogen profile resulting from the ratio of the diffusive and convectional transport. The modeling of similar laser treatments is thoroughly discussed in the literature; different approaches are available for describing processes such as welding, deep penetration welding, drilling, cladding, and alloying. It is sometimes necessary to use moving interface approaches to describe the physics of the conditioning. Different numerical models could be used to describe the physics of gas-liquid interfaces, especially for laser melt pools; the volume-of-fluid,[6] Lagrangian–Euler,[7] and level set[8] methods are among the most popular. The latter method was used to describe the keyhole development of iron treatment,[9] the cladding of stainless steel,[10] and solidification.[11] II.
THEORETICAL BACKGROUND
A. Beam Properties of FEL Experiments were performed at the Jefferson Lab (Thomas Jefferson National Accelerator Facility, Newport News, VA). The FEL operates like a synchrotron and can be adjusted in different time regimes and wavelengths. Detailed inf
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