Evolution of multilayered scale structures during high temperature oxidation of ZrSi 2
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Robert Mariani and David Bai Idaho National Laboratory, Idaho Falls, Idaho 83402, USA
Kumar Sridharana) Department of Engineering Physics, University of Wisconsin-Madison, Madison, Wisconsin 53706, USA (Received 13 June 2016; accepted 15 September 2016)
The oxidation behavior of bulk ZrSi2 at 700, 1000, and 1200 °C in ambient air has been investigated. Parabolic to cubic oxide layer growth kinetics was confirmed by weight gain measurements and the average oxide layer thickness was 470 nm, 6.7 lm, and 37 lm at 700 °C, 1000 °C, and 1200 °C, respectively, after 5 h oxidation tests. Evolution of compositionally modulated nano/micro structures was confirmed in the oxide layer. At 700 °C, Si diffusion resulted in discontinuous Si-rich oxide phases in amorphous Zr–Si–O matrix. At 1000 °C, complex multilayered structures such as fine and coarse irregular spinodal structures, wavy Si-rich oxide, and Si-rich islands evolved. At 1200 °C, additional nucleation of nanoscale ZrO2 particulate phase was observed. The spinodal structures were confirmed to be crystalline ZrO2 and amorphous SiO2, and the thermodynamic driving force for phase evolution has been explained by extension of liquid miscibility gap in the binary ZrO2–SiO2 phase diagram.
I. INTRODUCTION
In light water reactors (LWR), zirconium-alloy (Zr-alloy) fuel claddings have been widely used for over four decades due to the excellent neutron transparency of Zr, and combination of corrosion resistance and mechanical properties in normal operating conditions.1 However, in high temperature environments (1000 °C or higher) as may be the case during accident scenarios, significant degradation of the Zr-alloy cladding occurs. Profuse exothermic oxidation occurs (with associated hydrogen production) leading to a loss in mechanical integrity of the cladding. Furthermore, zirconium-oxide undergoes phase transformations as the temperature rises, which promotes its spallation and exacerbates cladding degradation.2–5 Efforts to address this challenge have been aimed at replacing Zr-alloy as cladding material with other materials such as FeCrAl alloys and SiC–SiC composites.2,6–9 Another approach involves deposition of oxidation-resistant coatings on Zr-alloy cladding. In this regard, chromium,10,11 Ti2AlC,12 and TiN13 coatings have been investigated. It is well known that transition metal-silicides have outstanding oxidation resistance and structural properties at high temperatures. For example, MoSi2 has been extensively Contributing Editor: Yanchun Zhou a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2016.363
studied on account of its outstanding oxidation resistance.14 However, this material has been reported to suffer from accelerated pest oxidation at low temperatures resulting in a porous and nonprotective oxide scale.15,16 ZrSi2 has been studied to a much lesser extent, but appears to be a more appropriate candidate for a coating for the nuclear fuel cladding application from the standpoints of neutronics and greater compatibility w
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