TPT January 2020
AR T I C L E
Black marking – A unique laser method for stainless steel marking By Thorsten Ferbach, Coherent Inc, USA
that preclude the use of ns laser marking, particularly for re-usable medical devices. These limitations stem from inherent aspects of the marking mechanism together with the passivation that provides corrosion-resistant surfaces on stainless steel products. Passivation is employed because mild steels are easily corroded by oxidation (rust). The use of stainless steels (alloys that contain a high chromium content) eliminates this problem because oxidation of the surface chromium atoms leaves a thin protective outer layer of chromium oxide. This passivation can occur naturally, but the thickness and integrity of the passivated layer is usually enhanced by chemical treatment with a mixture of acids (nitric, citric) such as Citrisurf ® . Importantly, the passivated surface is left with no exposed iron atoms.
Picosecond laser-based systems provide a turnkey solution for permanent, high-contrast marking of stainless steel. They are ideal for applications from unique device identifier (UDI) marking of medical devices to consumer appliances, with no negative impact on surface passivation.
Why laser marking? There is a growing need to apply identification, informational and logo marks to stainless steel devices and products, and these marks must meet several strict criteria which largely preclude the use of traditional (non-laser) techniques such as printing or engraving. For medical products, for example, repeated-use devices are legally required to have a unique device identifier (UDI), but a major drawback of printing is that it is not permanent and will fade with repeated sterilisation (autoclaving). In contrast, engraving will compromise the surface passivation requiring chemical reprocessing; plus, it leaves a surface texture that can trap contaminants or, in the case of implantable devices, cause irritation. And for non-medical applications, printed marks can become difficult to read after shipping, handling or storage, and also permit purposeful counterfeiting. There are several well-established approaches to laser marking, and numerous industries have used these techniques for decades. Carbon dioxide (CO 2 ) lasers, solid- state nanosecond pulse width (called DPSS) lasers, and nanosecond fibre lasers are widely used for this purpose, depending on the particular material involved. These diverse laser marking applications involve producing a change inside the bulk of the material, a colour change on a surface, or a macroscopic variation in surface relief (eg engraving) or texture that is easily visible. Marking passivated stainless steel with nanosecond lasers Lasers with nanosecond (ns) pulse widths are sometimes used to create a semi-permanent mark on stainless steel. These high contrast marks provide an affordable solution for single- use medical devices and consumer products where humidity is never encountered. However, there are certain limitations
Figure 1: Nanosecond lasers mark stainless by a thermal process that creates a layer of dark material
In terms of laser technology, a pulse width of tens or hundreds of nanoseconds is relatively long. Moreover, these lasers are limited to a maximum pulse repetition rate of 100kHz, so the high average power needed for fast throughput translates into high pulse energy. As a result the interaction of the laser and material is primarily photothermal, where intense heating produces localised melting and the mark results from chemical/structural transformation of the steel (figure 1). This transformation includes diffusion of the chromium away from the surface layer, oxidation of both chromium and iron atoms generating different oxides of both metals, de-mixing (dilution) of the alloy components, and changes in the phase/grain structure of the re-solidified metal. While this type of chemical/compositional mark is suitable for some stainless applications, it cannot be used for UDIs on re-usable medical devices for several reasons. Most importantly it severely compromises the passivation of the steel surface, as confirmed by the appearance of significant corrosion after a single test cycle: 50°C, 5 per cent salt water spray, for 72 hours.
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