Laser induced surface nitriding of niobium: Phase evolution and superconducting behaviour

Laser nitriding is a versatile approach for tailoring the surface properties of metals. To date, its effect on the superconducting response of niobium nitrides remains largely unexplored. In this study, the nitriding of niobium by laser irradiation under controlled nitrogen atmospheres up to 2.50 bar using a nanosecond pulsed laser (λ = 1064 nm) was investigated. Niobium sheets were processed over a wide range of laser conditions, including pulse widths (20–200 ns), repetition frequencies (175–1000 kHz) and scanning speeds (15–250 mm/s). By independently tuning the nitrogen pressure, two-dimensional accumulated fluence ( F 2D ) and laser irradiance, a laser processing map for the formation of β-Nb 2 N (hexagonal) and γ-Nb 4 N 3±x (tetragonal) phases was established. The phase, microstructural and superconducting properties were characterized using X-ray diffraction, scanning electron microscopy, electron backscatter diffraction and SQUID magnetometry. The γ-phase forms in the near-surface layer through melting when F 2D exceeds a threshold (> 50 kJ/cm 2 at 2.50 bar). Beneath this layer, a β-layer was observed, followed by embedded β-grains in the Nb matrix, whose size decreases with depth, indicating thermal gradients and a diffusion-controlled growth mechanism. When the γ-phase becomes predominant, the superconducting critical temperature increases to T c ≈ 15 K, a c companied by magnetic irreversibility. Lower F 2D conditions (≈ 7.5 kJ/cm 2 at 1.50– 2 .50 bar) promote a uniform β-Nb 2 N layer composed of submicron-sized grains, yielding a fourfold enhancement in surface microhardness. These findings establish the first systematic correlation between the laser nitriding parameters and the superconducting properties of Nb–N layers. • Distinct nitride phases generated by pulsed ns laser irradiation of Nb • Laser processing map established for γ-Nb 4 N 3±x and β-Nb 2 N formation • Low-fluence nitriding yields β layers with fourfold microhardness enhancement • Surface melting at high accumulated fluence triggers γ phase, increasing T c to 15 K • Crystallographic ordering in the tetragonal γ phase governs superconductivity