Surface Structuring with Lasers

Table of Contents

ADHESIVE AND PAINT BONDING ENHANCEMENT
HYGROPHOBICITY
MULTI-MATERIAL THERMAL BONDING
TISSUE AND CELL ADHESION
FRICTION INCREASE
AESTHETIC FINISH

In the last 30 years, surface structuring technology has become relevant to a large range of manufacturing sectors. This technology is spread across many industries and areas of engineering, making the process of adopting this technology slow. Not all applications have been fully developed yet. This article describes the uses and advantages of lasers in surface structuring for metals.

surface structuring technology

ADHESIVE AND PAINT BONDING ENHANCEMENT

Before applying paint or coating to a surface, the surface has to be abraded. This can also be necessary when the un-treated surface does not have a large enough adhesive strength. Surface abrasion increases the surface area and removes oxides and other contaminations. Q-switched lasers are well-suited for this task, since they concentrate energy within pulses that last around 10 ns. These pulses have powers exceeding 1 MW, which is enough the evaporate a few microns from any metallic surface or remove any surface contaminants such as oils, particles, dirt, oxides, etc.

This process generates a fresh and clean metallic surface where the adhesive surfactants can attach as desired. If more powerful ablation is employed, then the process can increase the surface area by a factor of 5. It can also be used to create certain surface features, which creates a better adhesion. The Rigel and Centauri lasers developed by Powerlase produce thousands of pulses per second and can abrade surfaces at a high rate. Galvanic scanners and beam profile shaping optics can be used to achieve the desired surface patterns and roughness levels. This can increase the bond strength by a factor of 2 to 8. Depending on the depth of the required surface features, the abrasion rate can range from 0.005 to 0.1 m2/min.

ADHESIVE AND PAINT BONDING ENHANCEMENT

HYGROPHOBICITY

There is currently a significant scientific interest in hydrophobic surfaces in nature, such as the leaves of the lotus plant. Scientists and engineers are trying to develop synthetic materials which imitate this behavior. Hydrophobic surfaces are resistant to wetting and droplets are observed to slide off the surface. A signature of this property is the large contact angle between the water droplet and the surface. The are a multitude of nanoparticles and coatings that have been engineered to exhibit this behavior. Surface structuring is an alternative method to achieve this.

HYGROPHOBICITY

By creating micro-structured features such as protrusions, ridges, tubules, rodlets, cones and hemispheres, a surface can be made hydrophobic or hydrophilic, depending on the size of these features. If the feature size ranges from 1 to 30 µm, the surface tends to display hydrophobic behavior, whereas for sizes between 30 and 150 µm, the surface becomes more hydrophilic. Since the hydrophobicity is created by the surface structure rather than the chemical composition of the material, these surfaces will repel any type of liquid, be it oil based, water based or a liquid based on other bipolar molecules. They are therefore best described as hygrophobic surfaces.

The opposite behavior, hygrophilicity, is more difficult to achieve. In this case, a liquid attaches to the surface because of the associated surface energy gain. Using microstructuring to achieve hygrophilicity is more complicated, since it depends more strongly on the relationship between the substrate and the wetting liquid. The q-switched lasers developed by Powelase can create surface patterns with sizes as small as several tens of microns to make a surface preferentially hygrophobic or hygrophilic. The automation systems such as galvanic scanners or mechanized positioning systems can be used to apply the procedure to selected areas.

The Powerlase lasers have high pulse repetition rates and high pulse energies, making them very effective at modifying the surfaces. They can achieve rates between 0.05 and 0.4m2/min. Also, they can induce direct lithographic patterns on any material.

MULTI-MATERIAL THERMAL BONDING

The two main methods for attaching metals to polymers or composite materials are adhesive bonding and mechanical fastening. When both methods are unsuitable, surface structuring with Powerlase ns pulsed laser can be the solution. The lasers emit short pulses with high pulse energies that can create microstructure of the metallic surface. The polymer or composite material is then heated to just below the melting point and pressed again the metal. The microstructure penetrates the polymer and locks onto it mechanically. Powelase offers high power IR laser such as the 1600 W Rigel i1600E than can be used to attain structuring speeds up to 0.03 m2/min.

TISSUE AND CELL ADHESION

YAG and fiber based solid-state lasers have wavelengths of the same order as the size of a eukaryotic cell (10-100 µm). They can therefore be used to create surface features of similar sizes. Features can be designed on the surfaces of implant materials such as titanium, stainless steel, nickel, cobalt, tantalum, zirconium, magnesium and hydroxyapatite. These can then be used as scaffolding for cells to adhere to. In this way, the build-up of tissue can be controlled. Using beam profile shaping optics or strong focusing optics, the features on the surface of these materials can engineered at rates of several cm2/min. If an application necessitates small, high definition designs, then the second and third harmonic emission lines at 532 nm and 355 nm respectively are a cost-effective solution, that allows for high processing speeds. On the other hand, for ceramic, polymer and hydroxyapatite based materials, the 3rd harmonic at 355 nm that emitted by the Rigel u90 laser has a high absorption rate and is therefore more efficient in creating small, well-defined shapes.

FRICTION INCREASE

Powerlase Rigel and Centauri IR lasers can be used for controlling friction. An important application of friction increase is the scribing of automotive brake discs. In other cases, it can be desirable to only increase the friction of certain areas of a surface. Pulsed lasers generate shallow features on a surface, thus increasing roughness. Automated beam positional systems let the user apply the procedure only to specific areas of a surface. The laser creates repeatable patters, for example for automotive brake disks, radial or spiral lines are usually chosen. The farrows or dimples generated, together with spatter material and the Marangoni displaced material on the rim of the ablated sites contribute to the finished material surface. The process rates achieved reach from 0.002 to 0.04 m2/min. Metals such as copper, gold, brass, silver, aluminum and polymers are more difficult to process and the Rigel g400 and u180 lasers are best suited for these applications.

AESTHETIC FINISH

Surface and material design is becoming more important than ever in the consumer good industry. Nanosecond pulsed lasers provide a simple and competitive solution that let the manufacturer apply surface structuring to product surfaces to improve their properties. The process is also universal: Powerlase lasers reach peak powers over 1 MW and can therefore ablate most materials. Compared to competitors, Powerlase offers the highest average power for an ns pulsed laser. This significantly increases the processing rate, which can reach up to 2 m2/min. The speed depends on the particular pattern that is imprinted on the surface.

Andritz

This information has been sourced, reviewed and adapted from materials provided by Andritz Powerlase Limited.

For more information on this source, please visit Andritz Powerlase Limited.

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