2017 Volume 58 Issue 8 Pages 1227-1230
We investigated the detailed mechanism of the unusual wetting of liquid metal bismuth on the surface fine crevice structure of copper metal created with laser-irradiation by focusing on the wetting state during the course of wetting. It was observed that liquid bismuth infiltrates into the fine pore and the interspaces on the surface fine porous structure, and wets on the surface fine crevice structure itself ahead and behind the wetting front, respectively. This revealed that the unusual wetting on the surface of the fine crevice structure proceeded through a transition from microscopic wetting to macroscopic wetting.
Unusual wetting is a phenomenon in which a liquid metal shows spreading wetting on fine complicated surface structures of metal by capillary action1–6). Originally, Tanaka and coworkers1–3) found that unusual wetting occurs on the surface fine porous structure formed by treating atmospheric oxidation–reduction for metal surface, and reported that unusual wetting have a potential to achieve a joining with minimum overlay formation in brazing and soldering. Recently, the authors4–6) have discovered a new way to create the structure “surface fine crevice structure” for unusual wetting by means of laser-irradiation to the surface of copper (Cu) and iron. The surface fine crevice structure can be formed in a target region by the area selectivity of lase-irradiation to realize region-selective unusual wetting which is impossible in the surface fine porous structure by atmospheric oxidation–reduction. The surface fine crevice structure causes the unusual wetting of liquid bismuth (Bi), tin, and tin–lead solder on Cu and liquid tin and indium on iron.
The unusual wetting of a liquid on the surface fine crevice structure is believed to result from capillary action of liquid metal into the laser-irradiated metal surface structure, i.e. splashing-water-like structure with crevices at intervals of several tens of micrometers. However, the above description on the mechanism of unusual wetting was surmised based on only the observation of surface fine crevice structure after laser-irradiation and the cross-sectional view after completion of the unusual wetting4–6). In addition, the influence of oxides generated by laser-irradiation in air on unusual wetting has been ignored in its discussion so far because of its tiny amount guessed from the results of x-ray diffraction. Thus, the existing circumstances are far from a concrete understanding of unusual wetting on surface fine crevice structure
In this study, we focus on clarifying the detailed mechanism of the unusual wetting on Cu with surface fine crevice structure. To achieve this purpose, the state of liquid Bi infiltration into the surface fine crevice structure of Cu during the wetting process was investigated by giving attention to the infiltration states around wetting front of liquid Bi, especially. Through the analysis of infiltration state during the wetting process, moreover, the relationship between the oxides of surface fine crevice structure and unusual wetting was discussed.
A Cu substrate (2 mm thick, 99.96% purity) and Bi (99.999% purity) were used as a base material and liquid metal, respectively. The combination of Bi and Cu is advantageous for wetting of solid Cu by Bi. Intermetallic compounds do not exist in a binary Bi–Cu system, and their mutual solubility is low at low temperature7). The contact angle between solid Cu and liquid Bi is lower than 90° above 600 K; i.e., liquid Bi wets solid Cu at such a temperature3,8). Therefore, unusual wetting involving capillary action in a wetting system can occur without any reaction of the two elements, such as compound formation in a Bi–Cu binary system. Laser treatment of the Cu substrate was conducted using a Q-switched yttrium aluminum garnet laser with a 6.0 kHz pulse (Miyachi Corporation, ML-7062A) in the ambient atmosphere. The wavelength and output power of the laser were 1064 nm and 50 W, respectively. The sample position was adjusted to give a 0.1 mm laser spot diameter. The region shown in Fig. 1 was treated in a reticular pattern with a 10 mm/s scanning rate and 0.05 mm scan interval. The laser-irradiated region consisted of three areas: start area, 4 × 4 mm; goal area, 4 × 4 mm; and the path between them, with a total length of 20 mm and width of 0.5 mm. A piece of Bi was placed in the start area and the sample was positioned in a soaking zone of the furnace. The basic configuration of device is similar to that in Ref. 5). In the present work, a jacket with a window at the top of furnace was newly set up to observe the wetting behavior during experiment. Residual air in the furnace was removed by evacuation and hydrogen (H2) gas (99.995% purity) was introduced into the furnace. H2 was dehydrated using silica gel and magnesium perchlorate prior to entering the furnace. The experiment was carried out under a flow rate of 20 mL/min H2. The sample was heated to 673 K for 40 min. Bi melted completely at 673 K because its melting point is 544.4 K7). Before liquid Bi reached the goal zone, which was observed through the top window of furnace, the sample was cooled to room temperature at a rate of −10 K/min.
Schematic diagram of the laser-irradiated Cu plate and experimental setup.
The appearance of the sample after the experiment is shown in Fig. 2. A metallic-colored substance similar to Bi exists on the path of the Cu plate, confirming that liquid Bi passed through the laser-irradiated path from the start area on the Cu plate. The wetting front of liquid Bi did not reach the laser-irradiated goal area, which means that we obtain a sample where the unusual wetting was not completed. Figure 2 also depicts the observation results at several points in the laser-irradiated path obtained by scanning electron microscopy.
Appearance of the sample after the experiment.
Surface images at the starting area side of the wetting front, wetting front, goal area side of the wetting front, and goal area are presented in Fig. 3(a), (b), (c), and (d), respectively. The white part is Bi and the grey part is Cu, as detected by energy-dispersive X-ray spectrometry. Small areas of Bi are scattered in the goal area in Fig. 3(d), closing tiny pores on the surface of the surface fine crevice structure. Considering the wetting front as a boundary, the different wetting states are observed in Fig. 3(a)–(c). On the goal area side of the wetting front, Bi is observed at the boundaries and interspaces of grains. Meanwhile, almost all of the surface fine crevice structure is covered with Bi on the start area side of the wetting front; only the top of the surface fine crevice structure appears above the liquid Bi.
Observation results at several points in the laser-irradiated path: (a) goal area, (b) goal area side of the wetting front, (c) wetting front, and (d) start area side of the wetting front. Grey parts are Cu and white parts are Bi.
In contrast to the previous study4), where the unusual wetting of liquid Bi on the surface of Cu with surface fine crevice structure considered to be attributed simply to its structure, the above result clarified that the unusual wetting has the following successive-transitional mechanism. First, liquid Bi infiltrates into the fine porous structure of the surface fine crevice structure. Then, liquid Bi sequentially infiltrates into the grain boundaries and the valleys in the concave–convex surface of the surface fine crevice structure. Finally, liquid Bi spreads to cover the whole surface fine crevice structure while filling the keyholes formed by laser irradiation. This gradual wetting phenomenon must be controlled by the capillary size dependence on capillary pressure, P, which is expressed as follows:
| \[P = 2 \gamma_{\rm L} \cos \theta /R,\] | (1) |
The fine porous structure on the surface of the surface fine crevice structure strongly contributes to the wetting in the initial stage of the unusual wetting process, as described above. Fukuda et al.4) reported that surface fine crevice structure, i.e., a structure that looks like splashing water on the outermost surface with crevices at intervals of several tens of micrometers below the outermost surface, was formed by splashing of molten Cu in the keyhole region produced during laser irradiation. This is not considered to form the fine porous structure. However, X-ray diffraction confirmed that a small amount of oxides such as CuO and Cu2O existed in the laser-treated area. These oxides can be generated by laser irradiation in the ambient atmosphere. The reducing conditions of H2 gas during the wetting experiment reduces these oxides on the surface fine crevice structure, which can create the fine porous structure, similar to the surface fine porous structure formed by atmospheric oxidation–reduction1).
Figure 4 shows the surface and cross section of the surface fine crevice structure of Cu treated under reducing conditions after laser irradiation. As expected, many fine pores existed on the surface fine crevice structure, especially the surface of the keyhole region and splashed Cu. These pores are thought to be connected with each other to form a three-dimensional network porous structure, which is a factor that contributes to the infiltration of liquid Bi at the start of the unusual wetting on the surface fine crevice structure of Cu. In the previous works4,6), the existence of oxide in surface fine crevice structure is regarded as sort of factor interfering with the unusual wetting because it is generally known that the wettability of oxide by liquid metal is bad. From the above results, it was identified that the oxide generated by laser-irradiation is changed into the fine porous structure under reduction condition during wetting experiment to work as an important contributor to the unusual wetting.
(a) Surface and (b) cross-sectional images of the surface fine crevice structure of Cu treated under reducing conditions.
The unusual wetting behavior of liquid Bi on the surface fine crevice structure of Cu was investigated to clarify the wetting mechanism. The observation of the wetting state in the middle of wetting revealed the following points. The unusual wetting of liquid Bi on the surface fine crevice structure of Cu proceeds via a gradual process of infiltration into micropores and grain boundaries, infiltration into the valleys in the concave–convex surface, and finally spreads to cover the surface fine crevice structure.
