Advertising feature Physics World  September 2018

Powerlase sets process physics right with laser cleaning

(Left figure) Schematic depicting the difference in detachment threshold achieved on a semi-transparent coating with low energy ns pulses and high energy ns pulses. The graph on the right of each schematic indicates laser light transmission intensity through the coating in accordance to the Beer-Lambert law and in comparison with the intensity threshold for coating detachment I each case. (Right figure) Top left – paint detachment during laser irradiation from an automotive component, using 1.6 kW average power and 200 mJ pulse energy. Top right – aluminium coating removal from automotive blank prior to welding, using 1.2 kW average power and 100 mJ. Bottom – early stages of marine paint detachment from a steel plate, using 1.6 kW average power and 160 mJ.

Laser cleaning is a subject that has taken internet media by storm in the last three 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%.

Surface preparation is quality critical and requires surfaces with no blemishes, traces of oils, debris or tiny particles that human vision cannot distinguish. Follow-up processes can vary from welding or adhesively joining to achieve adequate structural strength for constructing bridges, trains, aircraft and even spaceships; applying corrosion protective coatings, with the intention of it 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 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.

Increase in funding in research programs

Right now, China happens to be in the epicentre of this hurricane of events. In 2017, under Li Keqiang, China announced an interest in changing from a position of an industrial manufacturer to 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 aircraft by 2021, expanding the electric taxi fleet and tablet and mobile giants preparing to overtake US, Korean and Japanese competitors in performance. In parallel, the government enhanced its commitment to environmental protection and pollution reduction. Evidently, novel processes like laser cleaning, which enhances quality and reduces 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 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 its 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 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 1064 nm, used by such lasers. This phenomenon allows for significantly less energy to effectively conduct the process of removal, because 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.

Powerlase, a company spin-off from Imperial College, in London, 2000, has always produced nanosecond-pulsed laser sources, offering the best possible combination of high-pulse energy and fast-pulse repetition rate. The product concept matched the exact needs of the process underlying physics. Thus, in 2015 Powerlase decided to construct and market its own fully operational handheld laser system. A market review conducted in 2016 indicated a global market size was then in excess of 3 billion USD. Powerlase has engaged with the local market early on this application. The expression of interest by the busy industrial landscape of the county is remarkable. The applications laboratory in London has been busy with tests performed on applications ranging from oil and gas cleaning and rust removal, to marine refurbishment, aircraft weld preparation, steel sheet manufacturing and low environmental impact bridge maintenance.

Process speeds

With process speeds easily reaching surface coverage of 1 ft2/min when processing with pulse energies above 100 mJ and, for some, material-coating combinations and oxidised metals surpassing 1 m2/min, laser cleaning is adopted by an increasing number of manufacturers and service providers without second thought. As of 2018, the prospect of the Chinese diverse industry becoming the largest user of high-power laser cleaning machines is almost certainly a reality.