In-depth Analysis of Laser Blackening of Stainless Steel

In laser marking services, blackening of plastics and metals are commonly used functions. Metal blackening is further divided into stainless steel blackening and anodized aluminum blackening. Today, we’ll share the general principles of three methods for blackening stainless steel: burn-in blackening, high-temperature oxidative blackening, and micro/nano-structure blackening.

1. Burn-in Blackening: Generally not recommended. It’s caused by carbonized dust particles adhering to the metal surface after a violent reaction between the laser and the metal, resulting in a rough finish that is easily damaged or detached.
2. High-Temperature Oxidation Blackening: This refers to the black oxide layer formed when the metal absorbs laser light and generates high temperatures, then reacts with oxygen, such as iron(III) oxide (Fe3O4). This is similar to the principle of laser color marking, involving a chemical reaction, and is the most commonly used laser blackening process on the market. However, this type of black is not corrosion-resistant and generally fails quality control tests.
3. Micro/Nano-Structure Blackening: This method utilizes the ultra-high peak power of an ultrashort pulse laser to rapidly construct nano-scale pits and grooves on the metal surface through a non-thermal ablation process—the so-called micro/nano structure. Incident visible light is affected by microstructures through:

A. Multiple reflections and absorptions: Incident visible light first encounters the “protrusions” of the microstructure. Some is reflected, but more enters the “pits.” The light entering the pits undergoes multiple reflections, with each reflection being absorbed by the metal material. Ultimately, almost all visible light is absorbed and cannot be reflected back to the surface.

B. Enhanced light scattering: The size of the micro/nanostructure (50-500nm) is close to the wavelength of visible light (400-760nm), triggering Mie scattering—light no longer travels in a straight line but is scattered in various directions. Most of this scattered light is absorbed again by the surrounding microstructures, further reducing reflection.

C. Surface plasmon resonance (SPR): Electrons on the metal surface resonate with the electromagnetic field of the incident light (i.e., plasmon resonance). This resonance significantly enhances the metal’s absorption of specific wavelengths of light, especially in the visible light band, resulting in a deeper black color on the surface.

The combined effect of these three processes reduces the reflectivity of visible light to below 5%, which can be understood as the visible light being absorbed by the microstructure. Like a black hole, it visually presents a uniform, delicate, and pure black. Micro-nano structured black is a stable structure physically integrated with the substrate, resistant to friction, salt and alkali, and does not fade. It is suitable for all metallic materials and boasts the fastest processing efficiency among all laser blackening processes. It can be considered an essential laser blackening method for high-end casting.

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