Temperature-dependent reflectance of plated metals and composite materials under laser irradiation

Experiments were performed to investigate laser-heating-induced changes in the surface properties of metals and composite structural materials commonly used in aerospace applications. The objective of this test series was to determine the effect of surface heating on the 1,3-mu m-wavelength laser reflectance of these materials. Reflectance measurements were made using a unique hemiellipsoidal reflectometer. Polished metals, plated metals, and composite materials were tested to determine their temperature-dependent reflectance characteristics in vacuum and air environments. This experiment series produced temperature-dependent, 1.3-mu m laser reflectance curves for use in surface degradation analyses. All of the finishes investigated on duplex stainless-steel substrates increase the low-temperature, 1.3-mu m reflectance compared to the unpolished steel. However, the reflectance decreases dramatically at elevated temperatures when the surfaces discolor. The large reflectance transition observed for the polished and plated steel coupons in air also occur in vacuum. The polished aluminum alloys had a higher 1.3-mu m reflectance than the plated aluminum samples throughout the temperature range. The elevated temperature reflectance of S-glass epoxy is irradiance dependent. Low irradiance creates a black char. A higher irradiance can ignite the char and form a lighter, more reflective surface.