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How Does Laser Etching Work?

authorIcon By Jerome Landry on December 16, 2019 topicIcon Laser Marking

Laser etching is one of the most popular marking processes. But most people don’t really know what it means to etch a surface, and how it actually works. Look up “etch” in a dictionary and you’ll see engrave listed as a synonym. Any laser expert will tell you that laser etching is not the same as laser engraving.

In scientific terms, here’s a good definition for laser etching:

 

Laser etching generates marks (like data matrix codes, serial numbers, and barcodes) by melting a material’s surface. The melted surface expands, creating bumps of up to 80 microns high, and altering surface roughness to create black and white contrasts. As opposed to laser engraving the material is melted, not vaporized.

There’s a lot going on in that definition. Let’s go step by step to understand how the surface melts, and how bumps and colored markings are created.

Step 1. The laser pulses a beam onto the surface

All marking methods have one thing in common: the laser beam is pulsed, releasing sudden bursts of energy at specific intervals. Within 1 second, a 100W pulsed laser can release 100,000 pulses. Each pulse contains 1 millijoule of energy and can reach 10,000W of peak power.

To control the amount of energy released by the laser, experts can adjust the system and its parameters.

The most important parameters are pulse spacing and line spacing, which adjust the distance between pulses. The closer they are, the higher the concentration of energy. The images below show spacing examples for laser etching and engraving.

 

Pulse and line spacing

Laser Etching Process Laser Engraving Process
Evenly-space circles representing laser pulses for laser etching. Circles are distant compared to laser engraving. Evenly-space circles representing laser pulses for laser engraving. Circles are close compared to laser etching.

As you can see, pulses are a lot closer with laser engraving than they are with laser etching. This is because laser etching is less energy demanding than laser engraving.  As a result, laser etching is much faster.

Step 2. The material absorbs the laser beam’s energy

When the beam hits the surface, the material absorbs its energy, converting it into heat. While the material surface reflects most of the beam’s energy, it also absorbs and converts part of that energy into heat. For laser etching, the material must absorb just enough energy to melt.

Each type of material absorbs that energy according to its absorption spectrum.

A graph that shows the absorption rate of metals in relation with a range of wavelengths. The wavelengths of solid-state lasers and CO2 lasers are also identified in the graph. Some of the metals whose absorption spectrum is shown include aluminum, steel, and iron.

Original work published in Laser Focus World reprinted courtesy of Endeavor Business Media, LLC.

Laser systems release energy by emitting specific wavelengths. As you can see in the graph above, metals absorb more energy from the wavelengths produced by solid-state lasers than from the ones produced by CO2 lasers. For this reason, solid-state lasers are a better choice when marking metals.

Let’s look deeper at solid-state fiber lasers that emit a wavelength of 1,064 nm. For that wavelength, aluminum absorbs ≈5% of the laser beam’s energy, iron absorbs ≈30%, and steel absorbs even more.

Since steel absorbs more energy than aluminum, you may think that etching steel is easier than etching aluminum. But it’s a bit more complex than that. You need to consider other physical properties, including the material’s melting point.

Step 3. The surface melts and becomes malleable

As the beam’s energy is transformed into heat, the temperature of the material increases until it reaches its melting point. At such a high heat, the surface melts and becomes malleable, allowing its shape to change.

 

Melting Point of Different Metals
Material Melting Point
Aluminum 6061 585°C
Aluminum 380 566°C
Carbon Steel 1425—1540°C
Lead 327.5°C
Magnesium 650°C
Stainless Steel Grade 304 1400—1450°C
Stainless Steel Grade 316 1375—1400°C

Step 4. Localized surfaces expand, altering surface roughness

As the material melts and cools down within milliseconds, localized changes occur on the surface. Surface roughness changes, creating permanent markings.

What do changes in roughness look like? Let’s look at magnified images taken using an electron microscope.

Before the Laser Etching Process After the Laser Etching Process
The bare aluminum before the laser etching process, viewed using an electron microscope. The surface is rather smooth. Cells of a data matrix code etched on an aluminum surface, viewed using an electron microscope. Part of the surface shows diffuse reflections caused by small changes in roughness. Another part of the surface shows absorption caused by chaotic changes in roughness.

As you can see, surface roughness is permanently affected. The altered roughness changes the way we perceive colors.

Step 5. High-contrast markings appear on the surface

Color changes appear as a result of different patterns on the surface. For high-quality markings, the black and white colors offer the best contrasts.

The following schemas show how laser etching creates those colors.

Diffuse reflections create white Absorption creates black

Lines show small changes in surface roughness, causing diffuse reflections, or reflections of the lines in all directions.

Where the surface roughness causes light rays to be reflected at different angles (i.e., diffuse reflections), the surface appears white.

A line represents a ray of light that is trapped in the chaotic surface roughness when it is reflected on the surface, creating black

Where the surface roughness causes light rays to be trapped (i.e., absorbed), the surface appears black.

And that’s how high-contrast markings are created using laser etching.

Here’s a video compilation of aluminum laser etching where you can see the black and white colors created during laser etching.

 

Learn More About Laser Marking Processes

It’s best to know the complete range of possibilities before choosing a marking solution. To learn more about other laser technologies, you can read the following posts:

Need a traceability solution for industrial applications? Contact us for guidance.

Or take a look at Laserax laser machines (for turnkey solutions) and laser systems (for OEM integrators).

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Jerome Landry's picture

Jerome Landry

Trained in physics and physical engineering, Jerome has been working in the high-tech industry for more than 4 years. He is currently a technical sales specialist at Laserax. He has hands-on experience with laser processes and their interaction with materials, as well as with industrial traceability standards. This allows him to guide clients toward the best laser solution.