TPT January 2020

AR T I C L E

Coherent Inc

So whereas ns lasers produce a mark consisting of chemically transformed black material, the ps laser creates a surface that appears black, but whose chemical composition and alloy distribution are barely affected.

Several other limitations of ns fibre laser marking reduce its utility for other applications, especially aesthetic (eg, brand logo) applications. First, the colour and contrast of the mark changes with viewing angle. Second, this appearance is very sensitive to process conditions, presumably due to variations in the thickness of the affected layer and the grain size within the transformed layer. Consequently, consistent results can only be obtained in a very limited process window. In addition, the thermal stress from the localised heating can deform thin items such as sheet and tube substrates.

Advantages of black marking Black marking of stainless steels offers a unique combination of advantages that explain the fast growing demand for this process in re-usable medical devices, consumer appliances (‘white goods’) and other products. First, the marks are extremely dark, providing very high contrast that maximises readability for both human and machine vision readers. Just as important, neither the colour nor the contrast is affected by changes in viewing or illumination angles, further adding to their readability.

Figure 2: The short pulses of a picosecond laser create a surface texture that traps light, leaving the underlying material unaffected

Black marking with picosecond lasers Fortunately, a relatively new laser process, called black marking, avoids these limitations. This technique relies on the use of lasers that deliver pulse widths in the 10-20 picosecond range (such as the PowerLine Rapid NX), ie, 10,000 times shorter than typical ns fibre lasers. So even though the pulse energy may be 100X lower than with ns lasers, the peak power (pulse energy/pulse width) can be 100X greater. The combination of high peak power with short pulse duration results in a very different and more subtle transformation of the metal surface, as shown schematically in figure 2. Just as important, the pulsing mechanism – called mode locking – used in these picosecond lasers supports pulse repetition rates as high as 1 MHz. Thus the laser can deliver the high average power (>10 watts and beyond) needed for cost-effective high throughput, but without generating the high pulse energies that characterise ns lasers, and avoiding unwanted thermal effects. When a picosecond laser impacts the steel surface, it creates a high-contrast black mark. While this appears superficially similar to the ns laser marks, it is of a quite different form. Specifically, the short pulse duration minimises the thermal buildup and limits any liquid phase to the outer few layers of atoms. The main result is the formation of a nanoscale surface texture called laser induced periodic surface structure (LIPSS) that acts as a light trapping surface. This is accompanied by minimised diffusion of the metal atoms and limited de-mixing, together with only partial oxidation of surface chromium and iron atoms.

Figure 3. Corrosion resistance. These marks on 1.4301 reference samples show the results of a 72-hour salt spray test for a black mark (left) and a black mark followed by a passivation cycle (right). Both samples are essentially unaffected by the corrosion test For the re-usable medical device market, there are two important passivation benefits: first, the marking process does not compromise a previously passivated surface. Second, where the marking is performed before passivation, subsequent passivation does not cause the marks to fade in any way. This robustness and flexibility to apply the mark at different process steps maximises its value and lowers its cost impact. Figure 3 shows an example of corrosion test reference samples of 1.4301 stainless steel, where both were black marked using identical process conditions, but only one of the samples was re-passivated with Citrisurf ® . Both samples were then subjected to a corrosion test of 72h in 50°C, 5 per cent salt water spray. Clearly, there is virtually no difference in the marks’ ability to withstand corrosion. Other stainless steels

Before passivation

After passivation

Figure 4: These images show a black mark before and after being subjected to a 20-minute Citrisurf ® passivation process. The passivation does not affect mark contrast

78

www.read-tpt.com

JANUARY 2020