Nuclear reactor cladding enhancement via magnetron sputtering: Examining the effects of increasing Mo content in high-entropy coating on Zircaloy-4

Single-metal interlayer coating systems reliably protect standard Zircaloy tubes, making them suitable candidates for accident-tolerant fuel (ATF) cladding in nuclear service environments. However, they also deplete during interfacial metallic interactions (e.g., during Nb-Zr interdiffusion) and co-precipitate (e.g., when Mo interlays between Cr and Zr), thereby losing their availability as protective coatings for long-term use. In the quest to address these shortcomings, the present study explores a multiple-metal-engineered high-entropy alloy (HEA) coating (CrFeNiMoAl), proposed as a reliable alternative, particularly based on Mo as a refractory metal, for testing under high-thermal conditions. Four coating variants with Mo content ranging from 0 to 30 % (HEAMo0-3) were deposited on Zircaloy-4 via magnetron sputtering. To examine their corrosion behavior, these coatings were comparatively tested under conditions simulating Pressurized Water Reactor (PWR) cooling systems via electrochemistry in LiOH/H 3 BO 3 , by accelerated annealing (1200 °C at atmospheric pressure for 5 min), and subcritical corrosion tests at 360 °C in 18.6 MPa steam. In addition to material strengthening, improved barrier performance is attributed to the optimal Mo content, which supports the presence of predominantly body-centered cubic (BCC) nanocrystalline multiphase structure. Barrier performance depends largely on Mo content in the following order of increasing strengths: HEAMo1 (with 10.1 % Mo) ˃ HEAMo2 (with 20.7 % Mo) ˃ HEAMo3 (with 30 % Mo) ˃ HEAMo0 (with 0 % Mo). Higher Mo content oversaturated the internal coating structure, leading to coating degradation, increased Mo-ion release, and the formation of additional pathways for corrosive ions to reach the Zr surface. This work does not aim to investigate interfacial elemental diffusion, as ample research already addresses this topic. The essential insight from the present study concerns the impact of coolant chemistry and coating content on the performance and durability of ATF-cladding materials for next-generation nuclear reactor fuel systems.