<i>In Situ</i> Observations of the Formation of Surface Fine Crevice Structures Created by Laser Irradiation

Abstract

We used laser irradiation to create surface fine crevice structures, which induced region-selective super-spread wetting. These crevice structures enabled joining of CuCu. We recently reported that laser irradiation can form surface fine crevice structures, not only on Cu but also on Fe surfaces. However, the formation mechanism of such structures is not yet understood. To clarify this mechanism, we performed in situ observations of Cu and Fe surfaces during laser irradiation, with the use of a high-speed camera and laser illumination. In this way we directly observed swelling and spattering caused by ejection of molten metal from laser-irradiated regions. We conclude that the formation of the surface fine crevice structure was caused by the accumulation of ejected molten metal, that is, entanglement of protuberances and spatter.

Copper Surface Treatment Method with Antibacterial Performance Using “Super-Spread Wetting” Properties

“…The substrate had a surface with a smooth and even structure ( Figure 2 a). Our previous work confirmed that laser irradiation melts the metal and causes swelling and spattering, and as a result, the liquid metal accumulates and forms a fine crevice structure ( Figure 2 b) [ 38 ]. Before the coating process, the fine crevice structure formed on all and 48% of the substrate by laser irradiation ( Figure 2 c,d).…”

“…First, a fine structure was formed on the surface to allow liquid copper to spread on the surface. Our previous research confirmed that surfaces with a fine crevice structure can be created by laser irradiation [ 38 ]. As shown in Figure 1 a, a continuous Nd: YAG laser (ML-7062A, Miyachi Corporation, Tokyo, Japan) was used to fabricate the fine crevice structure with two types of patterns: covering all (10 × 10 mm square) and 48% (0.8 mm × 9.9 mm × 6 pcs rectangle arranged at intervals of 1.0 mm) of the substrate, respectively.…”

“…However, our previous works have shown that liquid copper unusually penetrates and spreads on a surface with fine crevice structures formed by capillary action [ 35 ]. The resulting phenomenon, named “super-spread wetting”, caused by the capillary characteristics of the liquid metal and metal surface with a fine crevice structure, differs from an ordinary occurrence [ 36 , 37 , 38 ]. In addition, using the super-spread wetting property, the liquid copper is able to flow to the desired target point [ 39 ].…”