Cold spray deposition of Ti2AlC coatings for improved nuclear fuel cladding
Introduction
Zirconium-alloy fuel claddings have been used successfully in Light Water Reactors (LWR) for over four decades, where they have exhibited good irradiation and corrosion performance [1], [2]. However, there are a few well-acknowledged limitations of zirconium-alloy claddings, such as the formation of a thick oxide layer and associated hydriding effects and susceptibility to fretting wear at grid spacers [3], [4]. In the event of a Loss of Coolant Accident (LOCA), zirconium alloys at elevated temperatures can oxidize via an exothermic reaction that is accompanied by hydrogen production as a result of reaction of zirconium with steam [5]. One approach that is being investigated to address this problem is to develop oxidation resistant coatings for zirconium-alloy cladding that can provide the necessary protection during an off-normal high temperature or LOCA conditions, while potentially enhancing performance under normal operating conditions.
MAX phase compounds have the general stoichiometry Mn+1AXn (n = 1, 2, 3) where, ‘M’ represents an early transition metal, ‘A’, a group IIIA or IVA element, and ‘X’ is either carbon or nitrogen [6], [7], [8]. These compounds have a nanolaminate structure, and possess many attractive properties such as high thermal conductivity, toughness, and machinability. MAX phase compounds, particularly those containing aluminum have been shown to have outstanding high temperature oxidation resistance due to the formation of a protective alumina layer on the surface of the coating. In the proposed research, we have explored the deposition of Ti2AlC MAX phase compounds using a low temperature powder spray process, also known as the cold spray process. The cold spray coating process, a commercial technology, involves propulsion of powder particles at supersonic velocities (Mach 2–3 or higher) on a part surface to form coatings with specific functionalities [9], [10], [11]. The particle temperature in cold spray process is low and deposition occurs in solid state. This confers a number of benefits: coatings relatively free of oxidation, porosity, compositional segregation, and thermal decomposition effects during spraying. Additionally, low deposition temperatures allow for the process to be used on temperature-sensitive substrates with no heat affected zone (HAZ).
Section snippets
Experimental
Powders of Ti2AlC MAX phase in the size range of less than 20 μm were procured from Sandvik. A commercial 4000-34 Kintetik system at the University of Wisconsin was used to deposit these powders using nitrogen propellant gas at preheat temperature of 600 °C and 35 bar. Deposition was performed on Zircaloy-4 (referred to hereafter as Zry-4) test flat substrates which were surface ground using 320 grit SiC paper and thoroughly cleaned prior to deposition. Detailed examination of the powders and
Results and discussion
Fig. 1a shows the SEM image of the as-received particles. The powders were produced by mechanical attrition and therefore have an irregular-shaped morphology. Fig. 1b shows the elemental EDS X-ray mapping of the three main elements: Ti, Al, and C.
Fig. 2a shows a cross-sectional SEM image the Ti2AlC coating on the Zry-4 substrate. Deformation and compaction effects of the powder due to high velocity particle impact are clearly observed in the coatings. Fig. 2b shows the EDS X-ray elemental maps
Conclusions
Coatings of MAX phase Ti2AlC, an oxidation resistant material have been successfully deposited on Zry-4 substrates using cold spray deposition process using powder particle sizes of <20 μm. The coatings exhibited high density and were well adhered to the Zry-4 substrate. X-ray diffraction analysis showed the phase purity of the coatings to be identical to the starting powders indicating that no oxidation or phase transformation of the powder material had occurred during the deposition process.
Acknowledgements
The authors would like to thank Josh Mueller and Elliot Busta at the University of Wisconsin, Madison for their assistance in this work. The authors acknowledge use of instrumentation supported by the UW MRSEC (DMR-1121288) and the UW NSEC (DMR-0832760).
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