Processing application of ultrafast laser on bulk metallic glass

Recently, an international research team led by Professor Zhang Peilei from the School of Materials Science and Engineering at Shanghai University of Engineering and Technology published a review paper titled "Research status of femtosecond lasers and nanosecond lasers processing on bulk metallic glasses (BMGs)" in the renowned journal Optics&Laser Technology in the field of optics and lasers.

This paper combines the years of experience and achievements of the author team to comprehensively review the current status of femtosecond and nanosecond laser processing on the surface of metal glass, including but not limited to the interaction mechanism between laser and material, the preparation of surface microstructure and nanostructures, and the improvement of different material properties.

In recent years, there has been increasing research on surface modification of metallic glass by laser processing, indicating that the interaction between laser and the surface of metallic glass is very interesting. Short pulse laser and ultra short pulse laser processing are most prominent in this field due to their narrow pulse width, high energy density, and short action time on materials. Especially femtosecond and nanosecond lasers offer enormous possibilities for high-quality, efficient, and low loss processing of metallic glasses (BMGs).

The author summarized this specific research field in the paper, introduced the properties and applications of metallic glass, the processing characteristics of femtosecond and nanosecond lasers, and explained the complex process between laser and metallic glass from a microscopic perspective. 

In addition, this article also analyzed the microstructure and macroscopic morphology, and discussed the influence of microscopic mechanisms on the structure. On the other hand, the performance of micro/nano structures, including mechanical properties, optical properties, wettability, and biocompatibility, was also discussed, which is of great significance for the surface functionalization of metallic glass.

The author team found through experiments and simulations that there is a connection between the high-altitude frequency LIPSS in the direction of laser polarization and the nanoscale roughness on the surface. Using femtosecond laser irradiation on two different surface structures of Zr based metallic glass surfaces, as shown in Figure 1, the scenario of HSFL formation was proposed by analyzing the single particle and multi particle scattering characteristics, and it was explained that this phenomenon depends on the single anisotropic near-field enhancement process and the collective mixing scattering and interference effects driven by the coupling between the incident field and particles. 

Near field enhancement has a significant impact on particle size in the range of 200-400 nm, driving the growth of anisotropic structures along the polarization direction through feedback mechanisms. The potential for local feedback driving effects and collective scattering to be consistent with the evolution of HSFL topology was discussed. And it indicates that scattering and interaction with the incident field contribute to the formation of LSFL.

Figure 1. Scanning electron microscopy images of the surfaces of BMG (a, c) and CA (b, d) samples irradiated with linearly polarized laser pulses of F=0.38 J/cm2 (a, b) and F=0.15 J/cm2 (c, d). The illustration shows the enlarged area of CA shape, as well as the EBSD structural evaluation of amorphous and crystalline phases on the BMG surface. Higher flux values (N=1,2,4) use fewer pulses, while lower flux values use more pulses (N=20,50100). (a) The illustration in provides an example of a higher flux F=0.6 J/cm2 at N=4. LSFL represents the LSFL ripple typically formed perpendicular to the direction of laser polarization. The two-dimensional Fourier transform (FT) representation of LIPSS for N=4 pulses in BMG and CA cases is presented in (a, b), showing the development of LSFL and HSFL and their specific spatial periodicity in the spatial frequency space (K-space).

By changing the laser energy density of nanosecond lasers, the formation of nano patterns was investigated on Fe based metal glasses. Three different laser energy densities were used: 0.85 J/cm2, 1.39 J/cm2, and 1.89 J/cm2. The experiment showed that the distribution of nanoparticles varied with the increase of laser energy density.

They also discussed the formation and evolution of this structure (as shown in Figure 2). When the energy is relatively low, nanoparticles appear in a discrete form in the irradiation area; When the energy is increased, except for the heat affected zone, the irradiated area exhibits a large area of network nanostructures. These network nanostructures are not always completely enclosed, especially at the center of the irradiation area. As the energy further increases, the entire irradiation area has two types of structures: network nanostructures around the heat affected zone and nanoparticles located at the center of the irradiation area.

Moreover, as the laser energy density increases, the number of such nanoparticles continues to increase. When the number is too high, they are connected to form nanowires or nanogrid structures, which can be attributed to laser-induced elemental enrichment (i.e. amorphous erbium oxide) and mismatched wettability with the substrate. In addition, under the combined influence of recoil pressure and surface morphology, the diffusion and connection of nanoparticles can lead to the formation of network nanostructures. These nanoparticles and nanogrids have a wide range of applications, such as as as reinforcement materials.

Figure 2. The left image shows the SEM morphology of the iron based MG surface after laser irradiation at different laser energy densities: (a, b) 0.85 J/cm2, (c, d) 1.39 J/cm2, and (e, f) 1.89 J/cm2; The diagram on the right shows the formation process of nanoparticles on Fe based metal glass substrates.

They explored the effect of nanostructures on bacterial adhesion using femtosecond lasers, and generated cluster like nanoparticle structures and different LIPSS structures on four BMG surfaces using different laser parameters, as shown in the left figure of Figure 3. When measuring the surface roughness, surface contact angle, and surface energy of BMG, it was found that the surface roughness and surface energy of the polished surface and LIPSS structure were lower outside the cluster like nanoparticle structure, exhibiting good hydrophobicity.

In the bacterial experiment, fluorescence microscope images were used to test the surface of Escherichia coli and Staphylococcus aureus on the specimen. It was found that when the surface hydrophobicity of BMG was good, the antibacterial ability was also better. In addition, they also explained bacterial adhesion from both microscopic and morphological perspectives, as shown in the right figure of Figure 3. From a microscopic perspective, the energy between two surfaces during the adhesion process of free bacteria is explained by van der Waals forces, Coulomb forces, and Brownian motion. From a morphological perspective, the curvature of nanostructured surfaces and the radius of curvature of bacteria determine the quality of antibacterial performance.

Figure 3. The left image shows the morphology of femtosecond laser nanostructured surfaces: (a) the polished surface of V105s; (b) The surface structure of V105s nanoparticles; (c) LIPSS for V105; (d-g) Nanoparticle structure surface of Zr BMGs; (h-k) LIPSS of Zr BMG. The right image shows the surface contact state of Staphylococcus aureus on (a) polished surface, (b) nano particle structure surface, and (c) LIPSS. Schematic diagram of surface bacterial adhesion state (d) on polished surfaces, (e) on nanostructured surfaces, and (f) on LIPSS.

In summary, as an emerging material, metallic glass has many advantages compared to traditional metallic materials, which also makes it have huge potential in the industrial or medical industry. The emergence of femtosecond and nanosecond lasers has made the application of metallic glass more widespread.

In addition, based on the understanding of the above research, the author team will further study the direction of nanosecond laser and femtosecond laser processing of metal glass, propose more mechanisms of their interaction, and demonstrate them through simulation, providing more powerful mechanism explanations for this direction. In addition to the two laser processing methods mentioned above, what interesting phenomena will occur when attosecond laser, as a laser with shorter pulse time, interacts with metallic glass? Currently, research is still needed, and it is believed that this will provide important assistance for the development of metallic glass.

Author Introduction
Professor Zhang Peilei
Graduated from Shanghai Jiao Tong University in 2010 with a doctoral degree. I have conducted visiting research at Central South University and the ILT Laser Technology Research Institute in Frankfurt, Germany. We have been committed to the research of laser material interaction and laser intelligent manufacturing systems. At present, more than 100 SCI and EI indexed papers have been published in internationally renowned journals in the field of laser intelligent manufacturing, and 12 national invention patent authorizations and 5 utility model patent authorizations have been obtained. Served as an editorial board member for SVOA Materials Science&Technology, a special issue editor for Coatings, a commentary editor for Frontiers in Metals and Alloys, a youth editorial board member for Welding Magazine, and an editorial board member for Metal Processing (Hot Working).

Zhang Weilin
Master's degree student at Shanghai University of Engineering and Technology, mainly engaged in ultra fast laser processing of micro and nano structures on the surface of metallic glass.

Source: Photoelectric sink