Optical properties and laser damage characteristics of thin-film aluminized Kapton® were investigated. Spectral absorptance of virgin and irradiated samples was measured from the Kapton side of multilayered insulation over 0.2 to 15 µm wavelengths at both room temperature and 150°C. The laser-damage parameters of penetration time and maximum temperature were then measured in a vacuum environment at laser wavelengths of 1.07 and 10.6 µm. Differences in damage behavior at these two wavelengths were observed due to differences in starting absorption properties at these wavelengths. During laser irradiation, the Kapton thin film was observed with a calibrated FLIR® thermal imager in the 8 to 9.2 µm band to determine its temperature evolution. Spectral radiance throughout the mid- and long-wave infrared was also observed with a Fourier transform spectrometer, allowing temperature-dependent spectral emittance to be determined. Kapton emittance increased after the material heated past approximately 500°C, and continued to increase as it cooled posttest. This evolving temperature-dependent spectral emittance successfully predicts the increasing absorptance that led to shortened penetration times and increased heating rates for the 1.07 µm laser. For tests with constant absorptance and no material breakdown, a simplified one-dimensional thermal conduction and radiation model successfully predicts the temporally evolving temperature.
Optical properties and laser damage characteristics of thin-film aluminized Kapton were investigated. Optical
absorption of virgin and irradiated samples was measured from the Kapton side using a Cary 5000 Grating
Spectrophotometer and an ABB/Bomem MB157S FTIR Spectrometer with a combined range of 0.2 to 15 μm at both
room-temperature and 150°C. Laser-induced damage parameters of penetration time and maximum temperature were
measured in a vacuum environment using an IPG Photonics continuous-wave solid-state laser operating at 1.07 μm and
an electric-discharge CO<sub>2</sub> laser operating at 10.6 μm. Rather large differences in damage behavior at the two
wavelengths were observed due to the variability in starting absorption properties between the NIR and LWIR.
A FLIR Systems Quantum Well Infrared Photometer at 8-9.2 μm was used to remotely examine the thin-film
temperature evolution based on a known LWIR band of nearly-constant emissivity. A dual-detector FTIR spectrometer
was also employed during testing in order to extract spectral emittance information from high-temperature irradiation
exposures. Surface emittance was found to change after the material heated past approximately 500°C and during
subsequent post-test cooling. This evolving spectral emittance with temperature successfully predicted increases in
absorption that led to more rapid penetration times and higher heating rates at increased 1.07-μm laser power. A
simplified one-dimensional thermal conduction and radiation model replicated the remotely-sensed temperature as a
function of time in tests with constant absorptance and no material breakdown. With the result of evolving emittance
data, this model could be modified to capture more realistic heating trends at higher irradiances whereby damage occurs
and absorption properties vary spectrally.
A major problem in the regular maintenance of aerospace systems is the removal of paint and other protective coatings from surfaces without polluting the atmosphere or endangering workers. Recent research has demonstrated that many organic coatings can be removed from surfaces efficiently using short laser pulses without the use of any chemical agents. The lasers employed in this study were repetitively-pulsed neodymium YAG devices operating at 1064 nm (15 - 30 ns, 10 - 20 Hz). The efficiency of removal can be cast in terms of an effective heat of ablation, Q* (kJ of laser energy incident per g of paint removed), although, for short pulses, the mechanism of removal is believed to be dominated more by thermo- mechanical or shock effects than by photo-ablation. Q* data were collected as a function of pulse fluence for several paint types. For many paint types, there was a fairly sharp threshold fluence per pulse near 1 J/cm<SUP>2</SUP>, above which Q* values dropped to levels which were a factor of four lower than those observed for long- pulse or continuous laser ablation of paint. In this regime, the coating is removed in fairly large particles or, in the case of one paint, the entire thickness of the coating was removed over the exposed area in one pulse. Hardware for implementing short-pulse laser paint stripping in the field is under development and will be highlighted in the presentation. Practical paint stripping rates achieved using the prototype hardware are presented for several paint types.
There is a critical need to replace ozone-depleting substances and hazardous chemicals that, in the past, have been used routinely in aerospace maintenance operations such as precision cleaning of metal surfaces. Lasers now offer the potential for removal of many organic materials from metals without the use of any solvent or aqueous cleaning agents. This paper presents quantitative results of laser-cleaning process-development research with a pulsed Nd:YAG laser and several common metals and organic contaminants. Metal coupons of Stainless Steel 304, Aluminum 5052, and Titanium were contaminated with known amounts of organic oils and greases at contamination levels in the 5 to 200 (mu) g/cm<SUP>2</SUP> range. A fiber-optic-delivered 1064-nm pulsed laser beam (20-Hz repetition rate) was scanned over the coupons with different overlap and pulse fluence conditions. Measurements of mass loss revealed that all levels of initial contamination could be removed to final cleanliness levels less than 3 (mu) g/cm<SUP>2</SUP>, at which point the mass loss measurements became uncertain. Pulse fluence thresholds for initial cleaning effects and practical cleaning rates for several metal and contaminant combinations are reported. From the totality of the results, an overall picture of the contaminant removal mechanism is emerging. For semi-transparent films, it is conjectured that a thermo-mechanical effect occurs wherein the laser energy is absorbed predominantly in the metal substrate which expands on the nanosecond time scale. This rapid expansion, in combination with some material evaporation at the film/metal interface, is believed to eject the contaminant film directly into aerosol droplets/particles which can be swept away and collected for recycle or cost- effective disposal in a compact form. Evidence for this mechanism will be presented.