Cross-sectional helium irradiation reveals interface-controlled bubble evolution in Cr/CrAlSiN multilayer coatings on zirconium alloys

The irradiation stability of Cr based protective coatings on zirconium alloys is critical for the development of accident-tolerant fuel claddings. However, conventional surface irradiation often produces shallow, nonuniform damage, obscuring interfacial behavior. In this study, we perform cross-sectional He irradiation to directly examine the interfacial response and He bubble evolution across Cr monolayer and Cr and CrAlSiN multilayer coatings on Zr substrates. Irradiation was carried out at 500 C and 750 C to doses of 2 and 3 dpa, enabling a direct comparison of temperature-dependent microstructural evolution. In the Cr monolayer, He implantation produced a homogeneous distribution of nanoscale bubbles throughout the damaged region and large cavities at the Cr and Zr interface, indicating severe Kirkendall-type voiding and interfacial decohesion at elevated temperature. In contrast, the Cr/CrAlSiN multilayer exhibited a periodically modulated bubble distribution, with bubble fragmentation and transformation into nanoscale platelets at CrAlSiN interfaces. A N-enriched Zr(N) interlayer formed spontaneously at the CrAlSiN and Zr interface, effectively suppressing bubble accumulation and interdiffusion. The nanochannel interfaces acted as He sinks and diffusion barriers, enhancing interfacial bonding and mitigating swelling. This work demonstrates that cross-sectional ion irradiation is a powerful approach for probing interfacial stability in multilayer systems, offering new insights into He-defect interactions and radiation tolerance engineering at buried interfaces. The findings highlight the potential of Cr and CrAlSiN multilayers as advanced coating architectures for high-temperature nuclear environments.

💡 Research Summary

The paper investigates the irradiation stability of chromium‑based protective coatings on zirconium alloy substrates, focusing on the role of engineered interfaces in governing helium (He) bubble evolution under reactor‑relevant conditions. Two coating systems were examined: a monolithic Cr layer and a multilayer architecture consisting of alternating Cr (≈900 nm) and amorphous‑like CrAlSiN (≈150 nm) layers deposited by magnetron sputtering onto Zr‑alloy coupons. To overcome the limitations of conventional surface ion irradiation—namely shallow, non‑uniform damage—the authors performed cross‑sectional He²⁺ irradiation using 300 keV ions at 500 °C and 750 °C, delivering fluences of 1 × 10¹⁷ ions cm⁻², which correspond to peak He concentrations of ~6 at.% and displacement damage of 2.3–2.8 dpa throughout the full coating thickness.

In the Cr monolayer, He implantation produced a homogeneous distribution of nanoscale bubbles across the damaged zone, but large cavities formed at the Cr/Zr interface. These cavities are interpreted as Kirkendall‑type voids arising from Cr diffusion into Zr and the formation of a ZrCr₂ Laves phase, leading to interfacial decohesion and severe loss of adhesion at elevated temperature.

Conversely, the Cr/CrAlSiN multilayer displayed a periodically modulated bubble population. At each CrAlSiN interface, bubbles fragmented and transformed into nanoscale platelets, indicating that the amorphous‑like layers act as efficient He sinks and provide excess free volume that encourages bubble flattening rather than growth. Notably, a nitrogen‑rich Zr(N) interlayer formed spontaneously at the CrAlSiN/Zr interface, serving as a diffusion barrier that suppresses both He accumulation and interdiffusion of Cr and Zr. These “nano‑channel” interfaces thus function simultaneously as He traps, diffusion barriers, and sites for enhanced defect recombination, resulting in markedly reduced swelling and improved interfacial bonding.

The study demonstrates that cross‑sectional ion irradiation is a powerful tool for probing buried interface behavior, allowing uniform damage across multilayer stacks and direct observation of interface‑controlled defect dynamics. The findings highlight three key insights for accident‑tolerant fuel (ATF) cladding design: (1) engineered multilayer interfaces can dramatically mitigate He‑induced swelling by redirecting bubble nucleation and promoting platelet formation; (2) amorphous CrAlSiN layers provide the benefits of nano‑channels without creating continuous pathways that could compromise mechanical integrity or corrosion resistance; and (3) spontaneous formation of a nitrogen‑enriched Zr(N) layer offers an intrinsic diffusion barrier that further stabilizes the coating/substrate interface.

Overall, the Cr/CrAlSiN multilayer architecture outperforms a simple Cr coating in terms of high‑temperature oxidation resistance, mechanical robustness, and radiation tolerance. The work therefore positions Cr/CrAlSiN multilayers as promising candidates for next‑generation ATF claddings, where managing helium production and transport at the nanoscale is essential for long‑term safety and performance.