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What are the different mechanisms by which a laser can mark a surface?

   

Laser marking is usually achieved using Q switched Nd:YAG lasers of modest average power (60-100W) focused to small (~100-150µm) diameter spots. Due to the Q switching, even at these low average powers, available peak power densities can vaporise material. Excimer and CO2 lasers are also used for some marking applications. If the material is marked simply by the effects of heat and an oxidising atmosphere, the process is known as laser annealing marking. Close to the melting point, many metals show a change in absorption and colour, mainly to black. The high speed and small spot size of the laser beams used limit the colour change to lines ~100µm wide with effected depths of ~5µm, and little material is accumulated near to the mark. High power densities are used if marking involves visible material removal (up to about 100µm depth), in a process similar to the laser drilling of rapidly connected blind holes. With Nd:YAG lasers, some of the material ejected is re-cast at the sides of the mark. Most metals, many plastics, some ceramics and even gemstones respond to this process.

Chemical transformation of the surface of plastics by laser beam can be used to produce a mark by way of a colour change in the plastic material. This can be caused either by the reaction of pigments (such as titanium oxide and mica) in the plastic to the laser light being used, or by the local expansion of the plastic material causing a colour change perceived by reflection of a different wavelength of visible light. For line widths of up to 200µm, the colour can be changed up to approximately the same depth. Painted, inked and anodised surface layers can all be removed cleanly at moderate vaporisation intensities to reveal a high contrast base layer.

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