The effects of high-flux broadband photoirradiation on metallic surfaces: Nickel and stainless steels

The effects of high flux ({dollar}geq{dollar}1 MW/m{dollar}sp2{dollar}) broad band (h{dollar}nu{dollar} {dollar}approx{dollar} 0.5-6.2 eV) radiation on metallic surfaces were studied for nickel and a variety of stainless steels. This was done by exposing samples of these materials in air to a concentrated photon beam at sample temperatures 350-500{dollar}spcirc{dollar}C. Analysis and characterization were done by Auger electron spectroscopy (AES) combined with ion sputter depth profiling, x-ray photoelectron spectroscopy (XPS), mass spectrometry (MS), thermal desorption spectroscopy (TDS), low energy electron diffraction (LEED) and scanning electron microscopy (SEM). The results were compared with reference samples of similar materials heated under the same conditions of temperatures and times but using infrared radiation.; Oxidation under high flux photo-irradiation was found to be significantly enhanced for all samples. For nickel, the oxide thickness was found to be up to 320% greater under 3.3 MW/m{dollar}sp2{dollar} flux irradiation than under infrared heating. Also, the oxide thickness was found to increase linearly with increasing flux. The wavelength dependence of the photo-enhanced oxidation was studied using lasers and it has shown increasing oxidation with decreasing wavelength with a threshold wavelength around {dollar}lambda{dollar} = 720 nm. Stainless steels, depending on their composition, have shown varying amounts of photo-enhanced oxidation, with oxide thickness increases of up to 874% under high flux irradiation for type 304. The oxide thickness was found to increase with increasing flux but the dependence, however, was found to be quadratic in flux. Also, the oxide thickness was found to increase with decreasing wavelength and a threshold wavelength around {dollar}lambdaapprox{dollar} 413 nm was observed. The microstructure and morphology of the oxides produced under photo-irradiation and infrared heating were also studied and were found different. Finally, the surface temperature under high flux photo-irradiation was measured for Ni(100) using LEED. A maximum increase of 63{dollar}spcirc{dollar}C in surface temperature over bulk temperature was observed at sample temperature of 503{dollar}spcirc{dollar}C and 1.9 MW/m{dollar}sp2{dollar} flux.; The observed enhanced oxidation of nickel under high flux photo-irradiation is believed to be due to induced photo-excitation of molecular oxygen adsorbed at the surface to the more reactive singlet {dollar}sp1sumsbsp{lcub}rm g{rcub}{lcub}+{rcub}{dollar} state. In stainless steels, in addition to this photo-excitation of molecular oxygen, the photodesorption of CO and CO{dollar}sb2{dollar} from the surface was found to be active at wavelengths {dollar}lambda{dollar} {dollar}leq{dollar} 425 nm leading to reduction of the protective oxides {dollar}rm Crsb2Osb3{dollar} and {dollar}rm FeCrsb2Osb4{dollar} and increased oxidation.