Non-equilibrium Surface-Modification of Metals by Laser Irradiation in Various gas Atmospheres

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Pure metals (Al, Fe, Nb, Ni, Ti, W and Zr) have been irradiated in air, N_2 and Ar+NH_3 gas using a high-power pulse laser, and surface products have been identified by X-ray diffracto-metry. The results obtained for irradiation at 3x10^9 W/cm^2 can be summarized as follows.(1) W : Even by low-angle X-ray diffractometry, neither peaks corresponding to oxides nor those corresponding to nitrides were identified.(2) Zr and Nb : Diffraction peaks from ZrN were clearly identified for Zr irradiated in N_2 gas and in air. No oxide peak was identified in specimens irradiated in air. In low-angle X-ray diffraction spectra of Nb irradiated in both N_2 and air, small but clear peaks corresponding to Nb_2N are found.(3) Ti and Al : Peaks from TiN were clearly identified for Ti irradiated in N_2 gas and air. A small peak of TiO_2 was also identified in for Ti irradiated in air. The presence of TiO was not clarified since the peak positions of TiO and TiN are almost overlapped. Because the peak positions of TiN are identical for the specimens irradiated in both N_2 gas and air, TiO did not seem to be produced. Formation of AIN was also identified.(4) Fe and Ni : No peaks corresponding to any nitrides or oxides were identified in specimens irradiated in N_2 gas. On Ni irradiated in air, NiO was identified. On Al and Fe irradiated in air, neither oxide nor nitride was identified, even by low-angle X-ray diffractometry. These metals were heavily damaged by irradiation in N_2 gas and air.(5) Nb, Zr, and Ti, which have higher boiling points than 3500 K, about the maximum surface temperature attained by the present laser irradiation, show the formation of nitrides for irradiation in both N_2 and air. On the other hand, nitride formation is not identified for the other metals, Fe, and Ni, with boiling points lower than 3500 K by X-ray diffractometry.