Laser induced surface nitriding of niobium: phase evolution and superconducting behaviour
Laser nitriding represents a versatile approach for tailoring the surface properties of metals. Up to now, its effect on the superconducting response of niobium nitrides remains largely unexplored. In this work, the nitriding process of niobium by laser irradiation under a controlled nitrogen atmosphere up to 2.50 bar, using a nanosecond pulsed laser with wavelength of 1064 nm has been investigated. By independently tuning the nitrogen pressure, the two-dimensional accumulated fluence ($F_{2D}$) and the laser irradiance, a laser-processing map for the formation of either a combination of $\beta$-Nb$_2$N (hexagonal) and $\gamma$-Nb$_4$N$_{3\pm x}$ (tetragonal) phases or only the $\beta$-phase has been established. Systematic analysis by X-ray diffraction, scanning electron microscopy and electron backscatter diffraction revealed that the nitrogen-rich $\gamma$-phase forms in the near-surface layer through melting when $F_{2D}$ exceeds a certain value ($> 50 \,\mathrm{kJ/cm^2}$ at 2.50 bar). A $\beta$-layer is observed underneath, and further inside, there is a band of embedded $\beta$-grains in the Nb matrix. Their size gradually decreases with increasing distance to surface, suggesting thermal gradients and a diffusion formation mechanism. When the $\gamma$-phase becomes predominant, a significant increase in the superconducting critical temperature is observed, up to $T_c \approx 15\,\mathrm{K}$, and magnetic irreversibility. For low $F_{2D}$ values ($\approx 7.5 \,\mathrm{kJ/cm^2}$ at 1.50-2.50 bar), the formation of a uniform nitride layer composed of sub-micron-sized $\beta$-Nb$_2$N grains results in a ca. fourfold enhancement in surface microhardness. These findings provide fundamental insights into laser-induced nitriding of niobium to engineer mechanically robust and superconducting Nb-N layers.