Transient self-dewetting of steels after pulsed electron beam melting
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.E. Martı´nezb) Facultad de Ciencias Exactas y Naturales de la Universidad de Buenos Aires, Ciudad Universitaria, Buenos Aires, Argentina (Received 27 December 2000; accepted 28 May 2001)
Experimental evidence of transient self-dewetting of metallic surfaces is presented. Steel surfaces are melted by a pulsed electron gun, and the subsequent fast cooling against its substrate gives rise to the formation of characteristic patterns that we attribute to the dewetting of the liquid film. The patterns formed are similar to those obtained by spinodal dewetting, that is, when the dewetting action develops from a nonlinear instability on the liquid surface, and not from holes nucleation. High-purity iron does not show a similar behavior, indicating that the origin of the instability is due to the influence of the sulfur in the temperature dependence of the surface tension of the melt, which gives rise to a Be´nard-Marangoni instability. I. INTRODUCTION
The dewetting of thin films has received great attention due to both the interest in the basic mechanisms involved and the technological impact of the studies of adherence failure of coatings. The dewetting evolves following characteristic spatial patterns. Two different mechanisms have been proposed as originating the pattern formation during the dewetting process: (i) The first is heterogeneous nucleation and subsequent growth of holes over preexisting defects in the substrate, with the formation of ridges due to the material accumulation. The pattern is predetermined by the distribution of nucleation sites. (ii) The second is film destabilization by thermally activated surface waves. For this mechanism, the instabilities that produce the film rupture happen for surface modulation wavelengths characteristic of each system. This mechanism has been described by Vrij1 and is named spinodal dewetting, due to the analogy with the spinodal decomposition that gives rise to the separation between two phases. Prior work on this type of problem has been mainly with polymer and organic liquid films over dielectric substrates.2–5 Recently Bischof et al.6 have demonstrated the formation of similar patterns in very thin metallic films (20–50 nm) over dielectric substrates, melted and cooled rapidly in times below 1 s. In a recent publication, Herminghaus et al.7 have demonstrated that very different systems give rise to similar morphologies and
a) b)
e-mail: [email protected] e-mail: [email protected] J. Mater. Res., Vol. 16, No. 8, Aug 2001
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that different morphologies can appear within the same spinodal dewetting mechanism.8 The process evolves as follows: (i) A nonlinear surface destabilization gives rise to liquid–vapor surface undulation. (ii) When the surface touches the substrate, isolated holes appear with ridges accumulating the material. (iii) The holes grow giving a characteristic pattern when an advanced degree of coalescence is reached. (iv) The final equilibrium structure is that of isolated drops. None of these w
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