Many industries require markings to be corrosion resistant. According to the EU's regulation on medical devices, implants or surgical instruments must be be assigned a traceable UDI code (Unique Device Identification), for example. Black marking allows for extremely various types of markings to be made, which remain easily legible, even after numerous cleaning and passivation cycles.
Black marking – permanent, deep-black, and corrosion-resistant markings
Black marking is a procedure in laser processing which causes extremely dark, high-contrast markings on a surface without material ablation. Extremely short laser pulses cause structures on the surface at the nanometer level. The rough surface ensures that light scattering is reduced, and creates a marking with constant depth and a consistent black colour. If the laser pulses used for this marking are ultra short, this means that the color change in specific parameter ranges is also corrosion resistant. The reason for this is that the use of ultrashort pulse lasers means that the heat-affected zone is extremely small, meaning that enough free chrome remains on the surface that a self-healing oxide film can form.
An overview of your advantages
- Surface structure: Ultrashort pulse lasers with a pulse duration in the range of pico or femtoseconds provide the basis for corrosion-resistant black marking. They make it possible for materials to be processed with virtually no thermal or mechanical influences. This is because the laser pulse, and therefore the duration of the energy input, is so short that temperature transport to the neighboring atoms does not even occur. This also prevents, for example, thermal stress cracks, which could occur in conventional annealing. This is why this is referred to as "cold processing". The laser structures the material at the nanometer level.
- Oxide film: In addition to surface structuring, a chromium oxide film plays the second central role in corrosion-resistant black marking. The low exposure to heat in comparison to annealing with short-pulse lasers allows for a sufficient amount of chrome to remain on the surface, which promotes the self-healing process of the passive film. This creates more corrosion-resistant films with chromite (Fe2+Cr2O4) and magnetite (Fe3O4), as well as films consisting of a mixed phase: FeFe2-xCrxO4 (iron-chrome-spinel).
- Passivation: Marking is followed by the cleaning of the medical products. The legibility and durability of laser marking can be affected by prolonged exposure times, aggressive cleaning agents, or high temperatures. This is why a targeted passivation procedure is often opted for when it comes to refinishing. In this process, an acid bath consisting of saltpeter or citric acid removes highly reactive elements (e.g. free iron ions) from the surface, and supports the clean, fast formation of a new chrome oxide film for even better corrosion resistance. At the same time, the surface is also cleaned and sulfides removed during this process.
Applications examples for black marking
Laser-marked valves for pressure compensation in the brain
Permanently easy-to-read markings are fundamentally important for the unique identification and traceability of implants. The shunt shown is used to treat hydrocephalus ("water on the brain") and it guides the excess fluid in the brain from the ventricles under the skin to the abdomen.
Emesis basins made of precious metals are provided with a uniquely traceable, corrosion-resistant UDI code (Unique Device Identification) through black marking.
Extreme peak pulse powers mean that even surgical equipment such as scalpels or clamps are marked with deep-black, corrosion-resistant UDI codes which can withstand numerous cleaning cycles.