Stainless steel medical devices such as surgical instruments have special requirements when it comes to laser marking. For example, it's important that marks are corrosion resistant to withstand processes such as sterilization. So, what is the best way to laser mark the alloy?

Answering this question is best done by first looking at the different kinds of marking lasers, which generally fall into two main categories. Solid-state, near infrared lasers emit beams close to the wavelength of visible light (1.06 micron). This wavelength is commonly used because it produces a wide range of marks on metal. On the other hand, far infrared CO2 lasers have a longer far infrared wavelength at 10.6 micron, which works well on non-metallic materials.

Fiber lasers are a new type of solid state lasers. They came on the scene in early 2000. Later, a range of pulsed lasers called Q-switched fiber lasers specifically targeted laser marking. Recent advances include our YLP fiber lasers featuring 200kHz pulse-repetition rates. YLPs produce pulses in the nanosecond range and combine a high focusability (often known as “brightness”), high pulse energy, and short pulse-length lead to produce a high power density (fluence) on the surface of the target.

304 stainless steel

Probably the most widely marked material in the medical device industry is stainless steel. The focus here is on the alloy 304.

Low-power, relatively low-cost fiber lasers are not considered as “exciting” as multikilowatt devices, so they are rarely the subject of serious study. Indeed, the marks they produce are rarely even shown at high magnification. However, when it comes to power density, the performance of the lasers on 304 stainless steel starts to get interesting, both from practical and theoretical standpoints.

In a fiber laser, what controls the size of the laser spot at the target is the brightness of the laser beam. And the power density or, more correctly, peak power density, measured in MW/cm2, controls the reaction of the surface to the laser beam. Power density is defined as:

(MW/cm2)=J/τ/πr2

where: J = pulse energy in joules

τ = pulse duration in milliseconds

r = radius of laser spot in centimeters

Because fiber lasers have a high brightness, power density is high, and hence melt ejection happens easily. Thus, the lasers make marks at 1 m/s that are clearly visible to the naked eye. At lower scribing speeds, lasers dig a deeper trench, removing more material. Slower speeds are widely used when permanent scratch-resistant marks are needed.

Although deeply scribed marks tolerate abrasion, they do have disadvantages. First, more heat is conducted into the part. This can distort parts and might negatively affect the component's metallurgy. Second, the rough surfaces can be unacceptable in certain medical applications.

However, it has been shown over a period of years that fiber lasers can produce a more appropriate, different type of mark specifically intended for medical devices. The term currently in use is an anneal mark. In metallurgical terms, annealing involves heat treating that alters the microstructure of a material, causing changes in properties such as strength and hardness. Our studies have shown that anneal marking actually consists of growing a dense cohesive oxide coating on the mark surface and that melting is unnecessary. Hence, in our view the term is technically incorrect and misleading. We suggest the term dark mark instead.

The major challenge: Producing a dark mark that is easily visible and has excellent corrosion resistance, necessary, because many stainless-steel medical devices are often passivated after marking. Marks must withstand the process, otherwise, they might be ruined or removed. Corrosion resistance also ranks as important because marks often must resist multiple centrifuging during their lifetime.

However, there is little detailed information to be found on the corrosion-resistance of marks on 304 stainless steel. What do marks consist of and how are they produced? How much corrosion resistance is needed? We set out to answer these questions and our work took on a wider form, turning into an empirical scientific study.

Testing corrosion

The general-purpose marking of metals is typically performed using 20kHz,1mJ parameters. However, increasing the laser's pulse-repetition rate at a fixed average power of 20W lowers the pulse energy and, as a result, the laser produces more subtle effects on the mark's surface.

As repetition rates go above 100 kHz, the pulse energy decreases to 0.2 mJ. Here, without any change of the optical set up, no visible melting takes place on the surface of the mark. When scan speed slows to <50mm/s, the overlap of the laser spots approaches 100%. In this scenario, a slight surface discoloration of the mark is apparent. Under high magnification, it can be seen that oxide growth appears at the grain boundaries of the metal.

Our early trials showed that when the laser beam is raster scanned for multiple passes over a well-defined area of the target surface at the conditions described above, the surface oxide on 304 stainless steel grows from the grain boundaries to cover the whole target surface. The oxide layer changes color as the layer thickness increases. The order in which the colors appear is gold, red, blue, blue/green, grey, black. Further, coloration can be determined by controlling the heat input in a single pass, or by using multiple passes. One trial produced all these colors on the same sample with identical processing parameters simply by increasing the number of passes.

A comprehensive series of tests produced a mark with excellent corrosion resistance. Although the mark appears light brown under optical magnification, the naked eye sees it as black and shiny.

These marks work well to identify surgical tools because they withstand passivation, centrifuging, and autoclaving. Our test showed that a non-corroding smooth dark mark can be produced at high speed using higher repetition rates. In fact, 200kHz appears optimum for this application.

How standards stack up

Standards for testing corrosion resistance of medical devices include:

  • ISO 10555-1 describes a test method for testing corrosion resistance for single-use intravascular catheters. To permit more rapid feedback on test results, IPG Photonics Corp., Santa Clara, CA, developed an in-house modified ISO 10555-1 procedure using stronger reagents that gave accelerated feedback on the corrosion properties of each series of trials. This standard specifies a relatively weak saline solution and long exposure times. To accelerate the development of the dark mark process, IPG modified the standard by increasing the strength of the saline solution to 0.8% WV and by operating at close to the boiling point of this saline solution, 76°C. Placing laser-marked sample coupons in the solution gave an excellent reproducible assessment of corrosion resistance within 10 min. Initial trials on commercially available dark marks gave complete removal of the oxide coating within 10 min. The criterion for a successful test was the capability to withstand 1 hr at 76°C without complete loss of protective oxide. Dark marks made with optimized laser parameters comfortably met this target.

  • ASTM F 1089 covers general test procedures and evaluation criteria for the corrosion resistance of surgical instruments intended for reuse in surgery and fabricated from stainless steel. This specifies a copper sulfate corrosion test that was developed to detect chromium depletion at the grain boundaries of austenitic material but it also can detect the presence of free iron. A copper sulphate corrosion test was used after our early marking trials. IPG found that any of the rougher marks corroded very rapidly. The conclusion: Any significant disruption of the surface produced free iron.

  • ASTM A 967 Standard Specification for Chemical Passivation Treatments for Stainless Steel Parts. The generally agreed view is that passivation removes contamination from iron or iron compounds from the surface of stainless steel by means of an acidic chemical dissolution, but does not significantly affect the stainless steel itself. Alternatively, passivation is described as enhancing the spontaneous formation of the protective passive film. Due to environmental considerations, the citric-acid based technique is used almost exclusively now. Tests on citric acid in the semiconductor industry have shown that the chrome-oxide ratio of the surface of stainless steel is as high as 12.5: 1. Also, typically, the top 25 to 30 Å (1.0 × 10-10 m) of the surface is highly chromium enriched. Passivation is essential on any medical device because, even with austenitic stainless steel, such as the 300 series with their self-generating passive oxide film, once a corrosion site has started, continuous and self-catalyzing corrosion happens.