Journal of The Surface Finishing Society of Japan

A wind tunnel to produce a uniform air stream and a high-speed camera system assembled for this study enabled direct and simultaneous measurements of sliding droplets' shape and acceleration. Using this system, wind velocity dependence of deformation and the moving acceleration of a water droplet on an inclined octadecyltrimethoxysilane (ODS) coating were investigated. The motions of a water droplet in an air stream are classifiable into three : sliding, stopping, and climbing. In the sliding zone, the sliding acceleration decreases with increasing wind velocity. During stopping, the droplet shape changes from a forward-bent to backward-bent posture against the surface inclination. In the climbing zone under large wind velocity, water droplets have large climbing acceleration with large shape deformation.

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Three-dimensional visualization by serial sectioning and deformation analysis by EBSD were performed for detailed microscopic evaluation of the interface between an electroplated film of tin and a copper substrate. Three-dimensional morphology revealed that the intermetallic compound of Cu₆Sn₅ developed nonuniformly from the tin grain boundaries, which are thought to be preferred nucleation sites of Cu₆Sn₅. Results further showed that the whisker root grain coarsened in the plated plane because of incoming tin atoms and consequent grain boundary movement. Scanning ion microscopy and a kernel average misorientation map showed that copper deforms locally as Cu₆Sn₅ develops at the interface between the electroplated film and substrate. The mode of the local deformation of copper adjacent to Cu₆Sn₅ was evaluated using pattern analysis of EBSD. The tensor, which describes the local deformation, showed that rotation and shear strain were introduced because of the volume change in the Cu₆Sn₅ formation. The nonuniform morphology of Cu₆Sn₅ and resultant local deformation of copper suggest nonuniform deformation in the tin film, which might induce tin atom diffusion and promote whisker growth.

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Tarnished copper foil was investigated before surface treatment. Its surface color changed to band strips (streaks) of soft reddish-brown, dark blue, purple, dark blue, and soft reddish-brown. Its surface morphology was related to the color change. Actually, this surface oxide film formation caused the color change. Surfaces of all samples were composed of CuO and Cu₂O. The fractions of these oxides differed. The soft reddish-brown out-end sample has the lowest CuO content ; all other samples show almost identical CuO contents. The oxygen contents of the out-end sample show the lowest value. Its degree of oxidation was the lowest of all samples. The out-end and center samples show an almost constant CuO content region. In contrast, the CuO content of the middle of the samples decreased gradually with increasing depth from the surface. The color samples, out-ends, and center samples have similar depth profiles of CuO contents, but they have different surface contents of CuO. However, the other color samples, the middle two samples, show almost identical CuO depth profiles and contents. Therefore, it is concluded that the main factor of color change is the CuO depth profile.

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This study investigated the effect of lead on whisker growth of a tin film that was electrodeposited onto a phosphor bronze substrate using FE-SEM, FE-AES, FE-EPMA, GD-OES, and XPS.
Subsequent FE-AES observation revealed that the whisker growth on the electrodeposited tin film varied depending on the oxide film condition. The whisker growth was suppressed concomitantly with thinness or fragility of the oxide film formed on the tin deposits. The whisker growth was also suppressed concomitantly with lead co-deposition in the tin deposits. In the Sn/Sn-Pb double layer, because the lead between the tin film and the substrate was transferred to the surface, the whisker growth was also suppressed. This whisker growth suppression effect caused by the existence of lead in the tin deposits was attributed to oxide film thinning and embrittlement, depending on the lead transfer to the surface at room temperature.

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