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Surface structural transformation of sintered alumina induced by excimer laser irradiation

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4 Author(s)
Dupas-Bruzek, C. ; Laboratoire de Physique de l’Etat Solide, Université de Mons-Hainaut, 23 avenue Maistriau, 7000 Mons, Belgium ; Laude, L.D. ; Langenhorst, F. ; Kolev, K.

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Laser surface processing of ceramics is important for several structural, tribological, optical and electronic applications. However, little is known on the transformation mechanism of alumina under excimer laser irradiation. In this work, a sintered alumina type was typically irradiated at 0.8 J cm-2 and cooled at two drastically different rates. Optical investigations on the irradiated specimens allow to conclude that the amount of transformation products is very much dependant on the cooling rates. Transmission electron microscopy observations show that the transformation product is constituted of an amorphous matrix in which are embedded either (i) copper grains (size up to 1 μm) in slowly cooled samples or (ii) smaller copper or CuAl2O4 spinel grains (size up to 100 nm) in rapidly cooled samples. The cement that is present in the starting material at grain boundaries is found to play an important role on the formation of the new amorphous or crystalline materials. All the copper grains exhibit a <120> common zone axis perpendicular to the irradiated surface and parallel to the bulk-to-surface thermal gradient, which drives that preferred orientation. These results reveal the details of the phenomenology of the laser-induced material transformation. This latter decomposes into (i) the incipient formation of tiny γ-Al2O3 nuclei, (ii) integration of Cu ions present in the melt into these tiny γ-Al2O3 nuclei that transform into CuAl2O4 spinel, and (iii) further sedimentation of Cu ions onto the resulting spinel grains acting as nucleation centers and finally formation of larger copper-rich grains. The minimum nucleation rate of cry- stalline grains is estimated to be equal to 1024cm-3s-1, while the growth rate is of the order of 0.1 m s-1 in both samples. Both bulk thermal diffusive and convective transports control the transformation of matter. © 2003 American Institute of Physics.

Published in:

Journal of Applied Physics  (Volume:93 ,  Issue: 8 )

Date of Publication:

Apr 2003

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