Browsing by Author "Vinodh, M. S."

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On the initial oxidation of MgAl alloys
(2006) Vinodh, M. S.; Mittemeijer, Eric (Prof. Dr. Ir.)
On the initial oxidation of MgAl alloys The initial thermal oxidation of bare, polycrystalline Mg-based MgAl alloys has been investigated by angle-resolved X-ray photoelectron spectroscopy (AR-XPS) and real-time, in-situ, spectroscopic ellipsometry (RISE). To this end, bare MgAl substrates of various bulk Al alloying contents (i.e. 2.63, 5,78 and 7.31 at.%) were oxidized by exposure to pure O2 gas for various oxidation times at T = 304 K and various partial oxygen pressures (pO2) of 10^-6 – 10^-4 Pa. As demonstrated in this study for the quantitative analysis of the Al 2p and O 1s photoelectron lines recorded from an α-Al2O3 reference, ignoring the effects of the anisotropy of photoionization cross-section and elastic scattering can introduce errors as high as 16% in the compositional analysis of AR-XPS spectra recorded in parallel data acquisition mode. Cumbersome corrections for these effects can be avoided by adopting values for the relative sensitivity factors as a function of the angles alpha and phi determined experimentally from a reference solid of known composition. A novel procedure has been developed for the quantitative analysis of the measured AR-XPS spectra of the bare and oxidized MgAl substrates. Therefore, a spectral evaluation procedure was developed to retrieve the total metallic Mg 2p and Al 2p, oxidic Mg 2p and Al 2p and oxygen O 1s PZL intensities from the measured XPS spectra of the bare and oxidized MgAl substrates. The expressions and calculation schemes for the determination of the oxide-film thickness, composition and the effective depth distribution of the resolved species were then deduced from the principle equations for the corresponding photoelectron intensities. The measured changes in amplitude ratio and phase shift dependent parameters, Δ(λ) and ψ(λ) as a function of oxidation time, as recorded using RISE, could be accurately fitted by adopting a compositionally inhomogeneous, Al-doped MgO oxide film developing on the MgAl alloy surface. To describe an inhomogeneous depth distribution of Al within this Al-doped MgO layer, a 3-node graded EMA layer approach was used. Application of such modelling to the measured spectra of ψ(λ) and Δ(λ) yielded a linear relationship between the change in Δ and the change in oxide-film thickness for thicknesses up to about 3 nm. A corresponding linear relationship between δψ and the oxide film thickness is not evident from the present study, indicating that ψ is insensitive to the presence of a thin-film phase between the ambient and substrate media. The initial drop and subsequent steep increase of ψ during the initial, fast oxidation stage is governed by oxygen incorporation in the alloy surface concurrently with nucleation and growth of Al-doped MgO islands on the bare MgAl surface. Upon formation of a closed oxide-film, the gradual increase in ψ at high pO2 is mainly governed by compositional changes in the MgAl subsurface region due to the competing processes of chemical segregation of Mg from the interior of the alloy to the alloy/oxide interface and the overgrowth of a Mg-rich oxide by the preferential oxidation of Mg. Due to the concurrent processes of preferential sputtering of Mg from the alloy surface and bombardment-enhanced Gibbsian segregation of Mg from the interior of the alloy during the sputter-cleaning treatment (to remove the native oxide), a strong Al-enrichment exists in the alloy subsurface region prior to oxidation. Since at the onset of oxidation, the preferential oxidation of Mg from the alloy surface is much faster than the rate of supply of Mg from the interior of the alloy towards the reacting alloy/oxide/oxygen interface, it then follows that the bare alloy surface is almost instantaneously depleted of Mg upon exposure to oxygen gas. Consequently, a relatively Al-rich MgO-type of oxide phase is formed during the initial stages of oxide-film growth (i.e. Al-rich as compared to the Al-doped MgO-type of oxide formed during the subsequent, slow oxidation stage). Subsequent growth after the formation of a closed oxide film proceeds by the overgrowth of Mg-rich oxide by preferential oxidation of Mg from the alloy. The structure of the oxide films resembles an Al-doped MgO-type of oxide phase. The resulting oxide films are deficient of anions and enriched of cations (with respect to MgO). With increasing pO2, the overall anion molar density increases and the overall cation molar density decreases, while maintaining a constant ratio of Mg to Al cations in the oxide film. As evidenced from the reconstructed effective depth plot, the oxide films are enriched of Al in the regions of the alloy and the oxide film adjacent to the alloy/oxide interface. The LBE O 1s species is attributed to oxygen anions in the interior of the oxide film, whereas the HBE O 1s component has been shown to originate from a defect oxide structure in the surface-adjacent region of the oxide film. After the formation of a 'closed' oxide film on the bare alloy surface, this defect oxide structure becomes partly transformed into 'bulk' oxide upon further growth by the outward diffusion of Mg cations under influence of the surface-charge field setup up by chemisorbed oxygen species. The associated depletion of Mg in the alloy subsurface region during the slow growth stage is governed by the competing processes of preferential oxidation of Mg and oxidation-induced, chemical segregation of Mg from the interior of the alloy towards the alloy/oxide interface.