Laser cleaning is a subject that has taken internet media by storm in the last 3 years. Videos of laser cleaning anything, from nuclear reactor components to ovens and dishes are posted online on a daily basis. The ease of applying the process has attracted huge interest by the wide public, regardless of the audience’s technical background. In industry, cleaning and preparing surfaces for the next value adding process typically consumes 15% of the process time, while for some components this can exceed 80%.
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Surface preparation is quality critical requiring surfaces with no blemishes, without any traces of oils, debris, or tiny particles that human vision cannot possibly distinguish. Follow up processes can vary from welding or adhesively joining to achieve adequate structural strength for constructing bridges, trains aircrafts and even spaceships, applying corrosion protective coatings with the intension of lasting for decades, and coating with high value design or architect decorative coatings. Moreover, recovery of high value and performance critical items to original state has become a necessity in the last 80 years. Mainly due to advancing recognition of the importance of environmental preservation and reduction of natural resource expenditure as well as cultural conservation of important buildings, sculptures and structures.
China right now, happens to be in the epicentre of this hurricane of events. In 2017, under Li Keqiang it announced it’s interest in changing from a position of an industrial manufacturer into a powerhouse of cutting edge science and technology in all areas. This is reflected by the increase in funding provided for research in ambitious research programs, extending from lunar exploration to quantum telecommunications. Infrastructure and industrial manufacturing will also follow the trend with an extended high speed train network in the works, manufacturing hundreds of civil aircrafts by 2021, expanding the fleet electric taxi fleet and it’s tablet and mobile giants preparing to overtake in performance, the US, Korean and Japanese competitors. In parallel, the government enhanced its commitment towards environmental protection and pollution reduction. Evidently, novel processes like laser cleaning that enhance quality and reduce environmental pollution are expected to play a key role in achieving all of the above.
Nanosecond pulsed lasers have been used in laser cleaning since the late 80s, when q-switching was introduced in a number of then, low power commercial lasers. By switching efficiency of the resonating laser cavity (q-switching), energy build up is halted, allowing the laser gain medium to charge up like a capacitor and then release it’s full energy within a few tens of nanoseconds, reaching peak powers of megawatts. With such photonic intensity it is possible to annihilate and shock away, in a single shot, most coatings, particles, contaminants and in general surface deposits of different nature to a continuous substrate.
However, looking deeper into the process, the key to unlocking high speed laser cleaning is a process called detachment. Light penetrating through transparent or semi-transparent materials, interacts at the substrate-coating interface and detaches the coating via a combination of enhanced interface evaporation and acoustical shock. Indeed, most like polymer paints and organic deposits are semi-transparent at the fundamental Nd:YAG or Nd:glass emission wavelength of 1064nm, used by such lasers. This phenomenon allows for significantly less energy to effectively conduct the process of removal, as the energy is not spent on evaporating large masses, but only interacting with interfacial material in the nanometer range. It also ensures minimum to zero impact on substrates. However, in order to achieve best effects, each pulse must deliver adequate energy for the interfacial propulsion forces to overcome adhesive and sheer cohesion forces holding the coating and contaminants in place.
This excerpt was taken from the Powerlase written Physics World article: Powerlase sets process physics right with laser cleaning article.
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