How does Laser cleaning work?

Laser Cleaning Technology - WORKING PRINCIPLE

Traditional industrial cleaning methods are often seen as tedious (and for good reasons). Rust removal can be time and labor consuming. Oxide removal may involve hazardous chemicals specific to each material that needs to be removed. In some cases, paint removal by sandblasting can damage the metal underneath.

Dealing with these problems usually comes at a significant cost, but laser cleaning is changing this: it is a cost-effective solution that reduces cleaning times and maintenance.

Laser cleaning technology works by sending millions of nanosecond-length pulses (0,00000001 sec) of laser light towards a surface. When it interacts with contaminants like rust, coating, paint,.. that absorb the pulses of laser light, the contaminants or coating particles will either turn into a gas or the pressure of the interaction will cause particles to free from the surface.

Laser cleaning - or ablation - is the process of clearing away undesired material from a solid surface by irradiating it with a laser beam. By absorbing the energy of the laser beam, the targeted material is heated very quickly, making it evaporate or sublimate. As the surface below does not absorb any energy, it stays untouched.

By manipulating the laser flux, its wavelength and its pulse length, the amount of material that is being removed by a single laser pulse can be controlled with extreme precision. Making laser cleaning equally suitable for rapid and deep rust removal as for removing only one thin layer of paint, without damaging the base coat.

All Materials Have an Ablation Threshold

Ablation: if material change from fixed form in a gas form, without the liquid fase in between!

Laser ablation occurs when a material layer or a coating is removed with a laser beam. This is the process behind all laser cleaning applications. Take as exaple laser rust removal on steel. When the laser beam hits the surface, molecular bonds in the dust or rust layer are broken and the free molecules are ejected from the substrate. In less technical terms: the layer to be removed is simply vaporized by the laser beam.

A simple way to grasp the importance of the ablation threshold is to compare it to throwing a ball over a wall. If you don't throw it high enough, it will never make it over to the other side. Even if you throw the ball a thousand times, you will always fail. The same applies to laser derusting. You can shoot the laser beam a thousand times but as long as the energy is below the ablation threshold of the material you’re working with, nothing will ever be removed.

Now, every material has different properties and thus different molecular bonds. In other words, each material has a specific ablation threshold. To successfully remove a layer from a given material, the energy transferred by the laser beam must be above the ablation threshold of that particular material. And every material has a different threshold.

Can we remove layer by layer?

Is it Possible to Remove a Material layer in a Highly Selective Way?

Let's continue with our analogy of throwing a ball over a wall: Imagine there was a second, higher wall behind the first one and that a ball was thrown with just enough energy to make it over the first wall, but not enough to make it over the second. The ball would bounce off the second wall and fall in between the two walls. Once again, no matter how many times you throw the ball, you will always get the same result. You will make it past the first wall but never the second. So it will never pass at all!

Since there is an ablation threshold for each material, laser cleaning can discriminate between two or more materials when trying to remove an undesired layer from an object. Given a sufficiently large ablation threshold difference between the materials, it is possible to select a material to be removed (i.e., the one with the lower ablation threshold) while leaving the other material untouched.

For example, the rust ablation threshold is much lower than the threshold for common metals like steel and aluminum. The same goes for paint and oil. This vast gap between two values allows contaminants and coatings to be completely vaporized without any risk of damaging the base material underneath. There's just not enough energy for damage to happen to the base material.

Laser Cleaning parameters explained

Laser cleaning or surface cleaning by Flash laser heating using short‐pulsed laser irradiation of a surface is demonstrated to be a promising new approach for effective removal of particulate contaminations of sizes as small as 0.1 μm.

This is very useful because micron‐ and submicron‐sized particulates adhere tenaciously onto a solid surface, and conventional cleaning techniques are inadequate for removal.

We find that laser cleaning with highest efficiency is achieved by choosing a laser wavelength (infrared-A or UV) that is strongly absorbed by the surface together with pulse depositing a water film of thickness on the order of microns on the surface momentarily before the pulsed laser irradiation. This permits the effective removal of particles smaller than ∼20 μm, down to as small as 0.1 μm, from a solid surface using a modest laser power starting at ∼0.1 J/cm².

Beam intensity

Beam intensity is the crucial parameter in the laser cleaning process. If it is not correctly selected, it may damage the substrate. The beam intensity and its effects on the substrate and the product to remove influence a lot on the speed of removing and the way of removing.

Beam size is one of the factors that influence cleaning efficiency and time of cleaning.

Beam Area

A smaller beam size is normally good for laser cleaning process, because of the high removal rate and the less cleaning time. However, if the laser beam area is too small, it may damage the surface. Thus the beam size should be selected carefully

Pulse duration

Pulse duration is the time it takes for the laser to emit one pulse

In the case of long-pulse lasers and continuous lasers (CW), the removal of surface material like pâint, dirt, oil, etc.. is done by vaporization and/or burning.

At the same time, ablation of the material occurs in the case of high-power short-pulse lasers, by acoustic shock waves created by the absorption of high-intensity laser energy. A higher removal rate can be achieved in the case of short pulses for a fixed amount of laser energy.

If the pulse duration is too short, the cleaning is not efficient; at the same time, if the pulse duration is too long, the substrate is affected by heat. Thus appropriate pulse duration should be selected. Therefor are needed laser sources with fine pulse adjusting.

Pulse repetition rate

Pulse repetition is the number of pulses per second.

If dwell time is too short, the dust from the graffiti, dirt, oil,.. and the laser pulse interfere with each other. Hence, the laser pulses do not reach the target. If the pulse repetition rate is too long, the cleaning takes more time. Therefore pulse repetition rate should be selected as appropriate.