Various laser-aided techniques have been developed for applying thin coatings to different materials. For example, metal surfaces of machine parts are coated for increased resistance to abrasion or corrosion and thus for achieving increased lifetimes. A more general term for possible purposes is the functionalization of the surface; this can relate to other properties such as hydrophobic or electrostatic behavior.
In contrast to laser cladding, the deposited coating material is not necessarily metallic. Indeed, one often applies materials like carbides to metallic surfaces. Also, there are coating processes which are applied to non-metallic base materials. Besides, the thickness of coatings is usually substantially less than the thickness of claddings.
Note that lasers are also used for removing coatings. See the article on laser cleaning for details.
Laser Coating Processes
A typical laser coating process involves a slow movement of a laser processing head over the surface while the coating material is supplied to the region of the beam focus. Absorption of the laser light causes that material to melt. When the laser beam has moved away, it solidifies, forming a solid coating on the base material. A stable mechanical connection is created by some small amount of inter-diffusion of the materials; only a rather thin bonding zone is usually observed when grinding test pieces.
The coating material is often supplied in the form of a powder, which is usually transported by a process gas. (That gas may at the same time protect the involved materials against oxidation.) The material feed may be fully integrated into the laser processing head (coaxial feed), while in other cases it is connected to the head from the side (lateral feed).
Instead of a powder, one may also supply the coating material in the form of a wire, tape or paste.
Alternative coating methods, particularly suitable for thicker layers, involve spraying the coating material to the surface. Here, the material is not melted immediately at the surface, but converted into a spray well before it hits the surface. That can be done with conventional air plasma spraying methods, not involving lasers, but improved laser-aided methods have been developed. While in some cases one uses a laser post-processing for improving the coating quality in a second step, in other cases the laser radiation is immediately involved in the coating deposition process.
A number of process parameters like laser power, spot size and shape, speed of movement and the amount of supplied coating material need to be optimized for best results with a given combination of materials. Automated laser coating machines can automatically control such parameters, possibly also adapting parameters for variable speeds of movement. Additional facilities for process monitoring may also be used in automated processes.
A completely different method of applying coatings is pulsed laser deposition. Here, the coating material is vaporized with intense laser pulses hitting a target, and then deposited on a nearby substrate. It is usually used for making rather thin coatings.
A wide range of coatings can be applied with laser-aided techniques. Metallic materials are used, but also carbides, tungstates and others.
Metallic coatings are often alloys rather than pure metals – for example, nickel- or cobalt-based alloys. They feature electrical conductivity and also enhanced thermal conductivity. Some of them are somewhat soft (e.g. gold coatings), while others provide substantial hardness. With laser coating processes, they can be applied with a strong metallurgical bond, i.e., stable adhesion even under aggressive environmental conditions. Mixing with the underlying base metal is largely avoided by the relatively short exposure time to the laser heating.
Another class of coating materials is of ceramic (polycrystalline) nature. For example, one widely uses carbide materials for coating steel surfaces. Such carbide coatings can exhibit high mechanical strength as well as good resistance to chemical attack and to extreme heat. In medical technology, one applies titanium nitride (TiN) coatings on titanium parts.
In some cases, small solid particles such as carbide grains are integrated into the new structure rather than being dissolved. This is called laser dispersing.
Modern developments also focus on the use of nanoparticulate materials for functional layers. Another interesting aspect is laser coating with ultrashort pulses (from ultrafast lasers), which in some cases allows the deposition under moderate temperature conditions, and of much higher temperatures required with non-laser processes. Thus, coatings can be applied to temperature-sensitive parts which could not tolerate conventional coating processes.
Advantages of Laser Coating
Some of the used coating materials could also be applied with non-laser processes. However, laser coating processes often exhibit specific advantages:
- The achievable coating quality (e.g. in terms of uniformity, thickness control, porosity, corrosion resistance etc.) is often particularly high.
- The potential for highly flexible application (e.g. only to specific areas) is important in some applications.
- A wide range of coating materials can be applied.
- In some cases, the substrate temperature can be much lower than in alternative processes, so that temperature-sensitive substrates can be coated.
The processing speed is not necessarily high, depending e.g. on the available laser power.
Applications of Laser Coating
Laser coating is mainly applied in industrial manufacturing processes. Some examples:
- Turbine blades are often exposed to extreme temperature and pressure conditions, possibly also to abrasive stress. Their lifetime can be substantially increased with robust laser-applied coatings. This also allows the use of even more extreme operation temperatures, which in turn lead to increased power efficiencies and thus to savings of fuels.
- Brake discs for cars and heavier vehicles can be coated with tungsten carbide, for example, for reducing the wear-off and thus substantially increasing the lifetime. At the same time, the emission of problematic particulate matter into the air is reduced.
- Various types of mechanical tools such as cutting, drilling or milling blades are also equipped with hard coatings, mainly for improved abrasion resistance.
- In medical technology, one applies titanium nitride (TiN) coatings to obtain improved biocompatibility of medical implants.
Mostly, laser coatings are applied in the original manufacturing process. In other cases, however, the technology is used in the context of refurbishing or repair of machine parts.
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