Highly Anti-reflective Surface on TC4 Alloy Fabricated with Nanosecond and Femtosecond Lasers

The anti-reflection properties of a material surface can improve the coupling utilization of specific incident electromagnetic
waves, identify electromagnetic wave signals, and shield unwanted electromagnetic waves. Such properties offer good prospects for
photoelectric conduction, infrared imaging, and military stealth materials. Titanium is a transition metal with excellent
biocompatibility, making it highly favorable in the medical field. Due to its high temperature resistance and corrosion resistance, it is
also widely used in aerospace and military fields. According to Fresnel's law, the anti-reflective performance of the titanium alloy
surface is mainly determined by the light-absorption capability of the substrate and the number of times that light is reflected by the
micro-nano structures on metal surfaces. Based on the characteristics of nanosecond laser processing with high efficiency in surface texturing but processing accuracy that is inferior to a femtosecond laser for micromachining, a method for preparing a highly anti-reflective metal surface using a nanosecond laser and a femtosecond laser was developed. The geometric microstructures and surface reflectivity of the TC4 alloy surface were characterized by a scanning electron microscopy and a spectrophotometer. The
nanosecond laser was used to etch the trough structure on the surface of the TC4 alloy. The thickness of the inner wall of the trough
structure was changed by controlling the filling spacing of the laser scanning. The reflectivity of the structure was measured as 5.76%
for a wavelength range of 200-2 500 nm; the averaged reflectivity was reduced to 3.5% after femtosecond laser scanning. The
anti-reflection performance of the metal surface was further optimized on the fabricated trough structures. The spacing of the
micropores on the surface of the hybrid structures was changed by controlling the scanning speed of the laser beam. The number of
micropores per unit area was proportional to the light-absorptivity of the surface. An optimal averaged reflectivity of 1.87% was
obtained for a wavelength range of 200-2 500 nm. Finally, a laser scanning route was designed and verified to prepare a stable
honeycomb-like structure. The optimal average reflectivity of the hybrid structure with honeycomb-like holes could be reduced to
1.63% for wavelengths of 200-2 500 nm. According to the excitation resonance effect of the particles and the different sizes of the
nanoparticle clusters, the absorption peak was widened from a single frequency to a frequency band. Thus, the light-absorption
capability of the metal surface was improved. In addition to surface topography, it has also been found that the surface element
components of titanium alloy influence light-absorption performance. At room temperature, titanium alloy is chemically stable.
However, the temperature of the sample surface increases with laser irradiation, enhancing the activation performance of titanium. As
a result, oxidation occurs on the surface of titanium alloy to form TiO2; a dense oxide film is generated, which further improves the
anti-reflection performance of the substrate. This study combines the high efficiency of nanosecond laser processing and the high
accuracy of femtosecond laser processing to effectively improve the anti-reflection capability of metal surfaces. Furthermore, by
independently designing the laser processing path, a honeycomb-like hybrid structure can be stably prepared. The reflectivity of the
titanium alloy surface is further reduced, to a minimum of 1.63% for a wide range of wavelengths (200-2 500 nm). Using both
nanosecond and femtosecond lasers can produce a highly anti-reflective titanium substrate that reaches or surpasses the performance with single femtosecond or nanosecond laser processing. This research may also provide suggestions for similar metals