Magnetron Sputtering and Corrosion of Ti-Al-C and Cr-Al-C coatings for Zr-alloy Nuclear Fuel Cladding

The disaster at the Fukushima Daiichi Nuclear Power Plant in March 2011 bought renewed focus to the issue of corrosion in nuclear fuel cladding applications. This thesis reports on the background behind these issues, the investigation strategy, and the analysis of experiments focused on mitigating oxidation of Zr-alloy fuel cladding. This thesis seeks to develop magnetron sputtered Ti-Al-C and Cr-Al-C coatings for Zr-alloy substrates and characterize the as-deposited and corroded samples.

Ti-Al-C and Cr-Al-C coatings were deposited onto ZIRLO, Si, and Al₂O₃ [Aluminum Oxide] substrates under various sputtering conditions. A combinatorial sputtering method was employed to refine the sputtering parameters. Following each deposition, the coatings were characterized using grazing incidence x-ray diffraction. The crystal structures of the as deposited Ti-Al-C coatings are consistent with TiC [Titanium Carbide] with aluminum substituting onto carbon sites and TiAl₃ [Titanium Aluminide]. The as deposited Cr-Al-C coatings were determined to be Cr₂AlC [Chromium Aluminum Carbide] MAX phase.

After characterization of the as-deposited coatings, corrosion tests in water were performed on ZIRLO coupons, in a 360 degrees Celsius autoclave. In this thesis, each of the coatings exhibited significant weight gain compared to conventional Zr-alloy claddings. Following corrosion, the samples were analyzed using grazing incidence x-ray diffraction.

Results of corrosion testing show the Ti-Al-C coatings of this study are not fit for the application coatings for nuclear cladding, as multiple oxide and hydroxide phases were produced.

Much of the Cr-Al-C the coating spalled off the substrate during corrosion testing. Cr-Al-C coatings that survived the corrosion testing produced phases consistent with Cr₂AlC MAX phase, Al₂O3, and Cr₂O₃ [Chromium Oxide].

In addition to the corrosion study, an in-situ high temperature grazing incidence x-ray diffraction study was performed on the Cr-Al-C on Al₂O₃ sample. Analysis of the high temperature data suggest the Cr₂AlC MAX undergoes a disordered to ordered transformation of the phases Cr and Al sites at increasing temperatures.