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Biomimetic Surface Composite Effects Achieved by Interfering Laser Beams

K.W.Liu, C.Daniel, Z.F.Liu and F.Mücklich
Functional Materials, Department of Materials Science, Saarland University, DE

Keywords: Biomimetic, Composite Effect, Laser Interference Structuring

Abstract:
It is observed from nature that certain insects possess the optimised surface fixing configurations against sliding and the tree body consists of different kinds of micro/nano-tissues with optimised properties to prevent breaking by wind. The effect that the performance of the material are strongly enhanced by combination of materials with different properties in a well distributed manner can be called the composite effect. One prominent characteristics of these natural structures is their micro/nano-scaled periodicity. If such topological and micro-structural structures can be created on surface, the material would be expected to exhibit superior surface composite effect. The objective of this work is to show how the biomimetic surface composite effects in different materials can be achieved by interfering laser beams. Laser irradiation could induce phase transformation and formation due to thermal interaction with sample surface. Laser thermal energy could generate evaporation, melting and thermal conduction. With laser interference, the laser power input could be distributed at interference dots or lines. Due to the short duration of thermal exposure by pulse laser, the sample surface could be modified under precise control of the laser irradiation. The schematic drawing of a two-beam interference is shown in Fig. 1. Material systems are employed in this work to demonstrate that both the topographic and phase surface structuring formed by laser interfering beams could optimise the overall function of the materials. Periodic structured B2-NiAl intermetallic thin films are synthesised from the pre-sputtered Ni and Al multi-layers. After suitable laser interference irradiation, the B2 phase can be formed in the area where laser interference maximum works while the other areas remain the original phase. The combination of a hard B2 NiAl with the rest soft phase in a nearly perfect periodicity optimise the surface toughness as a whole. In the hot dipped tined CuSn4 sample, x-ray diffraction indicates that there are a layer of pure tin, a Cu6Sn5 and a very thin Cu3Sn film on Cu matrix. After the laser structuring, the Cu3Sn film is observed to have grown and the Cu6Sn5 layer thickness have decreased. It can be seen with AFM and nano-indentation measurements that this phase transformation is local and periodical (Fig. 2). The soft tin phase is detected on topological peaks and the hard intermetallic phase is grown on valley. Controlled crystallisation of amorphous Si film on substrate of glass is achieved by laser interference. The surface morphology is investigated by AFM (Fig. 3). It can be seen that nano-grained Si crystals are generated along the periodic lines. There are called “cold line” between the crystallised zones where the laser interference intensity is minimum. The grain size of the crystallite increase gradually from a few tens of nanometer at the edge of the line structure to some hundred nanometer in the middle of the line.

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