Synthesis and high-temperature oxidation of ternary carbide coatings on zirconium-based alloy cladding

Abstract

Zirconium-based alloy claddings used for current light water reactors (LWRs) possess a variety of desirable features in steady-state normal operation, however, constraints regarding fast degradation, rapid exothermic reaction with high-temperature steam associated with hydrogen generation in accident scenarios motivate the requisite to develop enhanced accident tolerant fuel (ATF) claddings. One reasonable solution to improve the accident tolerance of the zirconium alloy cladding in accidental conditions while preserving its excellent behavior under normal operating conditions is external surface modification via such as coatings deposition. In addition, protective coatings applied to the zirconium alloy claddings offer the potential benefits of drastically reduced corrosion and degradation during normal operation, which are expected for application within the design framework of both current and future generation LWRs. The Mn+1AXn (MAX) phase materials comprise an extended family of layered, hexagonal ternary carbides and nitrides. They combine many attractive properties of both ceramics and metals stemming from their unique layered crystal structures and bonding characteristics; certain Al-MAX phases also possess excellent high-temperature oxidation resistance and chemical compatibility with select coolants such as hot water and molten lead. The objectives of this work are to synthesize and to evaluate three Al-containing MAX phase carbides (Ti2AlC, Zr2AlC and Cr2AlC) as potential protective coatings on Zircaloy-4 substrates with an emphasis on their high-temperature oxidation performance in steam. Oxidation of one commercial bulk Al-MAX phase Ti2AlC (Maxthal 211®), as reference/benchmark material, in steam in the temperature range of 1400°C - 1600°C was investigated to validate its high-temperature oxidation resistance and ascertain its potential as protective coatings. Oxidation of bulk Ti2AlC MAX phase ceramic at 1400°C and 1500°C formed a continuous coarse α-Al2O3 scale with randomly distributed Al2TiO5 isolated areas on the surface. The oxide scale thickening rate of Ti2AlC is more than three orders of magnitude lower than that of Zircaloy-4 at 1400°C and the maximum tolerance temperature of Ti2AlC in steam is approximately 1555°C. Therefore, these findings hold great promise of Al-containing MAX phase carbides, especially Ti2AlC, as oxidation resistant coating on zirconium-based alloy claddings. An innovative two-step approach has been established, i.e. first magnetron sputtering of nanoscale elemental multilayer stacks and subsequently thermal annealing in argon, for potential growth of phase-pure MAX phase coatings. The crystallization behavior and phase evolution of the as-deposited multilayers during annealing were systematically investigated using a diverse range of characterization and analytical techniques. The mechanical properties of designated coatings were evaluated by means of scratch tests and nanoindentation. Thermal annealing of the nanoscale elemental multilayer stacks (transition metal layer/carbon layer/aluminum layer) in argon revealed that onset crystallization temperatures of the Ti2AlC and Cr2AlC MAX phase from competing binary carbides and intermetallic phases locate at around 660°C and 480°C, respectively. Phase-pure Ti2AlC and Cr2AlC coatings were successfully fabricated, but the formation of a mixed ternary Zr(Al)C carbide rather than the Zr2AlC MAX phase was confirmed. Both MAX phase coatings have a basal-plane preferred orientation with the c-axis perpendicular to the sample surface and the multilayer stacks. The Zr/C/Al coatings crystallized to a cubic, solid solution Zr(Al)C phase with a B1 NaCl crystal structure. The phase evolution during annealing appears associated with the thermodynamic stability of corresponding MAX phases and their counterpart binary carbides. Coatings of altered designs with respect to introducing diffusion barrier or overlayer were deposited on Zircaloy-4 substrates. The coating thicknesses are 5~6 μm. Oxidation performance and degradation of these coatings during exposure in steam at elevated temperatures were investigated by thermogravimetric analysis and examining the microstructural evolution of the coating-substrate system. Steam oxidation tests found no protective effect of the Ti2AlC and Zr(Al)C based coatings with significant spallation and cracking from around 1000°C. Growth of an Al2O3-rich layer with TiO2 or ZrO2 layer beneath for the Ti2AlC and Zr(Al)C based coatings, respectively, was observed rather than a dense alumina layer. The failure of the Ti2AlC and Zr(Al)C based coatings from 1000°C can be attributed to the low thickness of the coatings, high interdiffusion rate between coating and substrate and potential phase transformation of the oxide products. The Cr2AlC-based coatings possess superior oxidation resistance up to at least 1200°C and autonomous self-healing capability with a thin and dense α-Al2O3 layer growth. Another design with a thin Cr overlayer above the Cr2AlC coating was further developed to eliminate potential fast hydrothermal dissolution of Al during normal operation. Moreover, first neutron radiography investigations of hydrogen permeability through the Ti2AlC and Cr2AlC MAX phase coatings on Zircaloy-4 substrates were reported. Hydrogen permeation experiments through non-oxidized and pre-oxidized Ti2AlC and Cr2AlC MAX phase coatings on Zircaloy-4 evidenced that both coatings are robust hydrogen diffusion barriers and impede hydrogen permeation into the matrix efficiently. The unique microstructural features of the coatings, namely free of columnar growth and highly basal-plane textured grains owing to the two-step approach, improve their efficiency in limiting hydrogen permeation as a barrier.