Laser Surface Modification
A wide variety of processes in the context of laser material processing aims at the modification of various properties of surfaces – on metallic parts, on ceramics, glasses and polymers. Such processes often involve substantial heating, but many of them cannot be understood as purely thermal processes. Typically, there is a direct interaction of the laser beam with the workpieces, and sometimes with additional material applied to the surface.
General terms for this area are laser surface modification and laser surface engineering. Depending on the concrete purpose of the process and sometimes on involved physical mechanisms, surface modification techniques are named with specific terms such as laser hardening, laser remelting, varnishing, annealing, laser honing, vitreous enameling or alloying – various such techniques are explained in this article.
In some cases, surface modification can be done with optical radiation, but not generated with lasers. For example, excimer lamps are sometimes used.
Some kind of surface modifications are used for creating visible patterns; such processes are called laser marking. This article, however, concentrates on methods for improving mechanical or chemical surface features.
Advantages of Laser Surface Modification
Partly, laser surface modification techniques compete with traditional non-laser processes, such as mechanical processes, thermal processes with conventional techniques of heating, e.g. with flames, and electrochemical processes. Laser-based processes typically have the following advantages:
- They can be precisely applied to certain parts of surfaces, e.g. to isolated narrow stripes or small rectangles, while neighbored material is largely unaffected.
- The processing time is usually rather short, which is economically advantageous.
- Because in many cases no direct contact is required, contamination and wear-off of tools is avoided.
- The surface modification can be achieved even at locations which are hard to access, for example within drilled holes of small diameter.
- The process can easily be automated, also often fully integrated with other processing steps.
Examples of Laser Surface Modification Techniques
Laser Hardening of Steel and Cast Iron
A particularly important industrial laser application is the hardening of carbon-rich steels and cast iron. Here, the heating of a surface layer to roughly 1000 °C, followed by appropriately rapid cooling, basically changes the way in which carbon is integrated in the steel structure: there is a transformation from austenite, the high-temperature form of steel, into martensite, a non-equilibrium state with much improved hardness. At the same time, the bulk material below the hardened layer (with a thickness of e.g. 1 mm or 2 mm) remains unaffected, which can be advantageous because of the increased brittleness which goes along with the hardening.
The laser hardening process simply involves heating the surface with a moderately intense laser beam for a short while; the heat is then conducted downwards. When the laser beam is turned off or moved away, the surface rapidly cools, mainly by heat conduction into the bulk material.
Typical industrial applications of laser hardening are the fabrication of machine parts which must withstand substantial forces during their operation. For example, that is the case for turbine blades, bending tools and gear wheels, which can become much more long-lived with such hardening treatment of the surface.
See the article on laser hardening for details.
Laser Remelting, Varnishing and Annealing
By depositing more thermal energy, parts can be melted at their surface, and when the laser beam is switched off or moved away, the melt rapidly re-solidifies mainly by heat conduction into the bulk material. The resulting material can then exhibit substantially modified mechanical properties. Such changes can result from different processes:
- Non-metallic substances such as oxides, nitrites or sulfides may be removed by evaporation, effectively changing the material composition.
- Crystal structures or grain sizes and shapes can be changed. One may obtain metastable forms of material, e.g. oversaturated forms which could not be obtained with slower processes.
- For very rapid cooling, some substances can form an irregular glass matrix instead of the crystal matrix; that is called varnishing.
- If the heating occurs over a somewhat longer time, one may achieve annealing, i.e., bring the material closer to its thermal equilibrium. For example, amorphous silicon may be annealed to form a polycrystalline structure; such processes are used in display fabrication.
The resulting cooling rates can widely differ, depending on the circumstances: while in some cases only 100 K/s are reached, dramatic cooling at 108 K/s occurs in extreme cases. Important factors for the cooling rate are the strength and temporal profile of the laser irradiation and the thermal conductivity and heat capacity of the material.
Laser remelting is often applied to metals like cast iron, with various kinds of steel and with aluminum alloys. With accurate process control, much improved mechanical properties can be quickly and reliably achieved.
Laser varnishing (formation of a glass layer) works only with special metal alloys (e.g. iron with carbon, chromium or boron), and with ceramics. However, it can also be applied to other metals if some additional substance, for example in the form of a ceramic powder, is placed on the surface before applying the laser beam. This is called vitreous enameling. Due to the resulting color effects, such techniques are often used in the context of laser marking.
Glasses surfaces can be effectively laser-polished, e.g. with CO2 lasers; this also involves remelting of a thin surface layer. Very smooth glass surfaces can be generated quite quickly that way. In laser polishing, hardly any ablation occurs.
Laser honing is applied to pistons and cylinders of combustion engines in order to improve their durability and reduce friction. Here, one applies intense pulses of ultraviolet light from an excimer laser (e.g. xenon chloride, 308 nm) to the metal surface. This leads to quite intense surface modifications, but only at a small depth of, e.g., 2 μm. The process is done in a nitrogen atmosphere and also involves the incorporation of some nitrogen in the material. A thin, robust layer of material with structures on a nanometer scale is formed.
Laser alloying means that one generates an alloy on the surface by incorporating additional chemical species. Those can be supplied in the form of a powder, for example, but also through a process gas (e.g. N2, NO2 or CO2). When the laser melts a surface layer, the melt can chemically react with the additional species, which are then incorporated when the material solidifies again.
For example, with laser alloying one may obtain hard carbides or nitrides on a metal surface, which make it substantially more resistant against abrasion or corrosion.
Laser Texturing and Patterning
With laser ablation, one can generate fine regular or irregular micro-patterns on surfaces. For example, irradiation of surfaces (e.g. of silicon, diamond or polymers) with femtosecond laser pulses with a fluence near the ablation threshold can lead to nanoripples (laser–induced periodic surface structures), while nanosecond pulse irradiation has led to ripples with larger periods, apparently generated by interference of incident and reflected light. One may also generate patterns in a controlled fashion by irradiating only selected points or lines on a surface.
Such laser texturing can lead to changes of various surface properties, such as friction, adherence to other bodies, wettability, electrical and thermal conductivity, and light absorption. Sometimes, such processes are used as a preparation for further processes, for example the application of a coating.
Other laser-based processes involve other chemical reactions which are initiated at high temperatures. An example is the processing of silicon carbide (SiC) surfaces and an atmosphere of argon and chlorine. Here, the chlorine reacts with some of the silicon, removing it in the form of the SiCl4. The remaining carbon forms grains of graphite. Those grains can serve as a solid lubricant, substantially improving the surface quality in terms of abrasion resistance.
Laser coating may be considered as a method of laser surface modification. After all, the applied coatings often substantially modify service properties like electrical conductivity, hardness, friction, abrasion resistance or chemical resistance. See the article on laser coating for details.
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