On The Origin of Laser-Induced Surface Activation of Ceramics
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iii shown in figure 1 were taken at increasing depths. They demonstrate that as the distance to the surface increases the chemistry changes from A120 3 to AIN, as expected.
(iii)
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(ii) (i)
AILVV Peak position from A12 0
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AILVV Peak position from AIN
30 3ý +b45 50 Y5 66 6 70 -'5Kinetic Energy (eV)
L
Figure 1. Al LVV peak spectra from the surface (i) and the subsurface (ii and iii, at increasing depth), of as-received AIN substrate. To obtain the subsurface AES peaks the surface was gently sputter-etched in situ. The spectrum from the surface is that of stoichiometric alumina (i). As sputter etching advances to deeper substrate layers, the surface chemistry changes from aluminum oxide to aluminum nitride. Figure 2 shows the Al LVV peaks from the surface and from the interior of an AIN substrate laser-irradiated at 2 J/cm 2 in a reducing atmosphere (Ar-4%H2). The surface is mostly A120 3 and, Al LVV peak position
from AIN
(60) (i)
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ALIL WPek position from
Al20
AILVV Peak position
3
from metallic aluminum 40
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Kinetic Energy (eV) Figure 2. Al LVV peak spectra from the surface (i) and subsurface (ii and iii) of AIN laserirradiated in Ar-4%H 2 at 4J/cm 2 . Al LVV from the surface (i) is a superposition of peaks from A12 0 3 and AIN, while sputter etching reveals that the subsurface peaks are from both AIN and metallic aluminum (ii and iii).
520
as the surface layers are removed, metallic aluminum appears together with A120 3 . Deeper inward the substrate consists of metallic aluminum and AIN. The amount of metallic aluminum at level iii) is larger that the amount of aluminum in AIN form (Figure 2). The oxide layer after laser irradiation is one to two monolayers thick, whereas that of as-received AIN was much thicker, but varied depending on the area. After laser irradiation in a reducing atmosphere AIN decomposes and a relatively thick layer of aluminum is formed. This implies that the laser irradiation increases the temperature above the decomposition value (12). When the specimen is exposed to air the aluminum is oxidized forming aluminum oxide. The decomposition of AIN is thermally induced and, hence, it takes place even if the irradiation is performed in air or in pure oxygen. When the irradiation is performed in an oxygen-rich atmosphere, however, the oxide layer is significantly thicker. This is not surprising because the aluminum layer that results from the decomposition is exposed to oxygen at high temperatures. Auger mapping (13) and cross sectional TEM (9) revealed that metallic aluminum produced by laser -induced decomposition clustered in the AIN matrix. An AES study was conducted in alumina in a similar manner to the study in aluminum nitride. Alumina substrates were irradiated in oxygen and in Ar-4%H 2 at 2 J/cm 2 . The surface chemical composition of the specimen irradiated in oxygen corresponds to A120 3 , while in the subsurface regions metallic aluminum and/or substoichiometric aluminum oxide is observed (Figure 3). AILVV Peak
(ii
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