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Laser energy deposition removes coatings via thermochemical ablation: The surface coating is quickly heated to a temperature at which vaporization and chemical decomposition occur.

Through the use of the Q-switched laser, GLC is able to create surface conditions for thermochemical ablation while virtually eliminating conduction of energy into the underlying material. The Q-switched laser, working through a unique Lasertronics delivery tool, applies an intense burst of laser energy over a very short time interval. Surface heating is so intense that ablation temperatures are achieved before heat conduction into the substrate material can become significant. The surface ablation event from the Q-switched pulse of laser energy occurs faster than the heat conduction time constant of the material.

With this very rapid heating of the coating, coating compounds vaporize and chemically dissociate, generating gas phase pressure waves that drive material from the surface at explosive speed. These pressure waves also eject condensed phase reaction products and non-volatile components of coatings from the surface as particulate matter.

The unique Lasertronics workhead system delivers laser ablation pulses to small surface spots, typically 0.3 to 0.9 mm diameter, with duration of about 100 microseconds, or 1/10,000 second, per pulse. In Lasertronics tools, the pulse beam moves rapidly using a set of scanning optics to optimize the ablation process and remove only a small fraction of the coating thickness with each pulse.

A Lasertronics-engineered mechanism directs the laser beam from one surface location to another to provide stripping over a large area. Other advantages derive from moving the beam rather than dwelling on one surface location, including:

  1. Subsequent laser pulses do not pass through the particulate ablation products of earlier pulses;
  2. Any residual substrate heat is spread over a large area;
  3. Most importantly, the coating removal process can be controlled to allow for the possibility of tapering or removing topcoat while leaving primer.

By focusing ablation energy on one small spot, we can scale the process speed depending on throughput requirements without changing ablation dynamics. For a particular laser power level and pulse frequency (number of pulses per second), the Lasertronics system selects the appropriate spot area to achieve the ideal power density (intensity) for the desired ablation performance. Furthermore, the laser ablation process that Lasertronics selects for a particular application can be scaled to a higher throughput rate simply by specifying a laser with higher average power and adjusting the scanning optics to hold constant laser pulse width and spot power density.

Industrial lasers are available with a wide variety of characteristics to meet various requirements. For laser ablation, the most important characteristics are:

  • Average Power,
  • Wavelength of the light produced,
  • Pulse Width, or length of time that the laser discharges during each pulse cycle. Pulse width can vary over a wide range from femto-seconds (10-15) to milli-seconds ( 10 -3). A particular laser will have a pulse width somewhere in this range, but it typically cannot be changed; and
  • Peak Power. This attribute is related to average power, pulse width, and pulse frequency. The formula is:
  • Peak Power = Average Power / (Pulse Width X Pulse Frequency).
    High peak power is required to rapidly heat the coating to its removal threshold temperature before heat diffuses into the substrate. If the laser’s peak power is not high enough, the coating and substrate heat to the point that the coating simply chars. Obviously, these temperatures can damage many substrates. Higher peak power enables photoablation, in which the coating itself absorbs the vast majority of the energy. This process removes the coating without significantly heating the substrate.

    Ablation dynamics vary considerably with pulse width. Shorter pulses (higher peak power) generally remove more coating material per average watt of laser power and create less substrate warming, because less time exists for the heat to diffuse in. As explained below, however, lasers that produce very short pulses have other drawbacks.

    Types of Industrial Lasers