Femtosecond lasers are ultra-fast pulsed lasers, which are used to remove material. The material is not heated, but immediately turns into a plasma. In addition, the technology is extremely suited for the very precise application of functionalities via textures.
Over the past ten years, laser technology has progressed very rapidly. New developments have made lasers cheaper, more compact, more energy efficient, enabling the emergence of new production technologies such as additive manufacturing, laser ablation, laser welding and laser curing. One of these recent developments is that, due to the drop in price, femtosecond pulsed lasers (10-15 (s) could be commercialised and are now within the reach of manufacturing companies and research institutes.
Femtosecond lasers are ultra-fast pulsed lasers, which are used to remove material. The difference with CW, micro and nanosecond lasers is that a femtosecond laser does not heat the material, but immediately transforms it into a plasma. This is because the pulses are faster than the speed with which heat can disperse into the material by means of phonon transport (i.e. transport through vibrations of the individual atoms). In micro and nanosecond lasers, material is removed by melting and evaporation, leading to microcracks, melting zones, higher surface roughness and residual material on the surface.
These (often unwanted) side effects do not occur in femtosecond lasers, which makes them particularly interesting for precise work, such as cutting small components or applying surface functionalities by means of textures. These lasers also work on almost all materials (as long as they absorb the light), from plastics to hard ceramics.
Principle of femtosecond laser processing (right) versus microsecond laser processing (left)
Surface functionalities have long been applied to consumer goods, tools and machine components in order to increase the value of the piece and/or enable a certain functionality. Examples include hard coatings on tools to increase the life span or water-repellent coatings to keep surfaces clean. However, applying these functionalities by means of textures has some fundamental advantages. Textures wear off much slower; if a piece crumbles off, no foreign material is introduced into the environment (e.g. the pharmaceutical and food processing industries) and the level of functionality that can be achieved is superior to a coating and/or the use of specific substrate materials. For example, textures can reach contact angles with water that are higher than 160° and can even cause droplets to splash.
Typical examples of functionalities that can be achieved by means of textures are:
- (Super) hydrophobicity: a self-cleaning effect, anti-ice formation, anti-biofilm
- Haptic effects ‘soft touch', a better (micro-)grip
- Tribological functionalities: higher or lower customised friction coefficients, stick/slip effects
- Aero/hydrodynamic functionalities: lower coefficients of friction, better flow
- Optical functionalities: diffraction effects, micro-lensing
- Thermal functionalities: increasing a specific surface area, for example to improve cooling
- Aesthetic effects: gloss, optical black, patterns
The lotus leaf effect: raised papillae increase the angle of contact with water to over 150°
Classic methods vs laser
In today's industry, classic methods such as etching, micro- and nanosecond lasers, EDM, ECM and micro milling are used to apply textures. However, these are either too slow and complex to become competitive (etching, ECM, micro milling), or they are not precise enough (EDM, micro and nanosecond lasers) to provide the desired functionalities. Femtosecond lasers provide a solution. With spot sizes up to a few micrometres, very precise structures can be applied, without any thermal damage to the material and/or waste accumulation on the sides of the structures. A very nice, clean cut can be achieved, and this in a quick and easy way. Such a result is impossible to achieve with, for example, nano and microsecond lasers.
Line pattern of 30 µm deep and wide in brass, applied by means of the Sirris femtosecond laser machine