Abstract
(Re0.67Al0.10)B2 and (Re0.74Al0.11)B2 solid solution as well as Re0.85B2 thin films were deposited by hybrid RF-DC magnetron sputtering. X-ray diffraction (XRD) showed that all films exhibit the ReB2 (P63/mmc) crystal structure. X-ray photoelectron spectroscopy (XPS) analyses performed on atmosphere exposed thin film surfaces suggest that ReB2 corrodes, consistent with literature, by forming perrhenic acid (HReO4) already after two days, while (Re0.74Al0.11)B2 forms a self-passivating Al-oxide layer preventing corrosion in a time period ≥ 60 days. Hence, it is evident that Al additions to ReB2 significantly increase the chemical stability during atmosphere exposure.
Highlights
The increasing number of highly specialized industrial machining applications creates a demand for suitable new hard coating material systems
No other Re-B phases or Al-borides are detected by X-ray diffraction (XRD); it is reasonable to assume that the 3.5 at.% Al in (Re0.67Al0.10)B2 is dissolved in the ReB2 crystal lattice and a solid solution is formed
While the Al concentration induced changes in lattices parameters measured by XRD (Δa = +0.31%, Δc = −0.31%) are opposite in trend to the values predicted by density functional theory (DFT) (Δa = −0.08%, Δc = +0.25%), it has to be noted that the magnitude of the obtained deviations are according to Paier et al.[30] in line with the exchange-correlation functionals employed here
Summary
Diffractograms of both Re0.85B2 and (Re0.67Al0.10)B2 films (Fig. 1., bottom and top curves, respectively) utilized for further mechanical characterization only display peaks attributable to the ReB2 crystal structure (JCPDS card 00-11-5081). High resolution XPS spectra of the B 1 s, Re 4 f, and Al 2 s transitions of an (Re0.74Al0.11)B2 film after different air exposure durations are shown, respectively. The oxidized component in the Re 4 f spectrum, i, is increased to only 5.6% of the total integrated intensity after 66 days atmosphere exposure, so exhibiting approximately a 3.83 × t0.09 dependence, where t is the oxidation time in days. Re0.85B2 reacts in humid air and forms perrhenic acid already after two days of atmosphere exposure, as shown by XPS. This corrosive reaction renders unalloyed ReB2 coatings unsuitable for application in humid air. It is evident that Al additions to ReB2 significantly increase the chemical stability during atmosphere exposure
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