Fig. 3.
Stylus profilometry in the area of the narrow part of the crater.
The bottom of this crater was not flat since it was close to the target surface. The absorption of the plasma is high due to the incident laser radiation. This regime is restored because the density of ionised species is also high. However, in our experiment the pulse duration of the laser was long (about 6 ns), and therefore it can be assumed that the absorption coefficient oscillates in magnitude during laser irradiation. Similar behaviour was described in Refs. [17] and [18]. The crater is caused by the intense radiation near the cathode of the laser tube.
EDS-analysis (Fig. 2) showed that the content of O at the place 1 was 17.54 wt.%, at the place 2–15.44 wt.% and at the place 3–14.32 wt.%. The content of oxygen was higher (∼16%) than when the Cu-surface was cold. Those three locations were chosen at different distances from the crater centre. The content of O decreased with the distance from the centre.
Despite of the fact, that some differences were observed between different sites after 100 pulses, the most outstanding feature was the formation of apparently periodic concentric waves around the craters.
The surface waves indicated in Fig. 2 were investigated in more detail. These regions were also free of any significant disturbances and droplets. The enlarged micrographs of these regions, along with the corresponding FIB cross-sections, are shown in Fig. 4.
Fig. 4.
The waves on a heated copper plate after 100 pulses. (a) A region indicated by A in Fig. 2, (b) the FIB cross-section of the same area. (c) An area indicated by B in Fig. 2, (d) the FIB cross-section of the same area. A thin oxide layer covers the surface. Beneath are equiaxed copper grains, with some twin boundaries.
FIB cross-sections revealed that the whole surface was covered with a thin—few 100 nm thick oxide layer, which was formed due to the interaction between oxygen in the air and molten copper. Beneath the oxide layers, there was a polycrystalline microstructure. It consisted of copper grains separated by high-angle grain boundaries. Twin boundaries were also observed within some grains. It was not possible to determine the thickness of the molten surface layer from the inspection of the microstructure. The waves could be observed on the copper plate, which was slightly exaggerated by the oxide layer. However, the height of waves was less than 1