(a) Cross-sectional FE-SEM image of the structure. Photographs of ultrablack films deposited on magnesium alloy (b) without interlayer and (c) with interlayer. (d) Absorption spectra of the ultrablack films on magnesium alloy. Photographs of the magnesium alloy parts (e) before and (f) after ultrablack films’ deposition. Credit: Jianfei Jin et al.
Researchers have developed an ultraback thin-film coating that can be applied to aerospace-grade magnesium alloys. The coating provides excellent durability, even in harsh conditions, while absorbing 99.3% of light.
Astronomy and precision optical devices are commonly coated with black paint to absorb stray light and improve performance. With every bit of stray light potentially degrading performance, manufacturers regularly seek out the blackest blacks available to coat their devices. The coating developed by researchers from the University of Shanghai for Science and Technology and the Chinese Academy of Sciences provides excellent light absorbance while providing the durability that is commonly lacking in other coatings for use in harsh environments such as the vacuum of space.
“Existing black coatings like vertically aligned carbon nanotubes or black silicon are limited by fragility,” said author Yunzhen Cao. “It is also difficult for many other coating methods to apply coatings inside a tube or on other complicated structures. This is important for their application in optical devices as they often have significant curvature or intricate shapes.”
To remedy these durability concerns the team employed atomic layer deposition (ALD), in which the target is contained within a vacuum chamber while sequentially exposed to specific gasses that adhere to the surface in multiple layers.
“One big advantage of the ALD method lies in its excellent step-coverage ability, which means we can obtain uniform film coverage on very complex surfaces, such as cylinders, pillars, and trenches,” added Cao.
The coating itself is comprised of alternating layers of aluminum-doped titanium carbide (TiAlC) and silicon nitride (SiO2). “TiAlC acted as an absorbing layer, and SiO2 was employed to create an anti-reflection structure,” said Cao. “As a result, nearly all of the incident light is trapped in the multilayer film, achieving efficient light absorption.”
In the study, published in the Journal of Vacuum Science & Technology A, analysis revealed that the coating absorbed 99.3% of light across a range of wavelengths from 400 nanometers to 1,000 nanometers. To optimize the coating for use in magnesium alloys, a commonly used but easily corroded material, the team employed a specialized barrier layer.
Future applications of the coatings could include use on space telescopes and other optical hardware employed in harsh environments. The team intends to further optimize the coating to absorb more wavelengths in the future. “Now that the film can absorb over 99.3% of incoming visible light, we’re hoping to expand its light absorption range even further to include ultraviolet and infrared regions,” said Cao.