Structural damage in graphene oxide coatings onto Nb substrates upon laser irradiation

In this work graphene oxide (GO) coatings were obtained on Nb substrates by electrophoretic deposition (EPD), using a ramp of stepped potential and variable deposition time, to evaluate the feasibility as a protective coating against the exposition to a 20 W Yb laser (1070 nm). Laser irradiation constitutes a first approximation to the potential damage by local high heat loads that could occur in a Tokamak-type fusion reactor. GO coatings can act as the first barrier against exposure to the fusion reactor plasma, but the coating may experience local high heat loads and simultaneous exposure to energetic particles. The GO/Nb coatings were irradiated with different laser power-frequency settings. With X-ray photoelectron spectroscopy, the bonding evolution of the coatings was followed for each deposition time. Scanning electron microscopy was used to evaluate the coating morphology before and after laser irradiation, whereas Raman spectroscopy was used to evaluate the structural evolution of GO coatings, crystallinity, and disorder in the graphene oxide. An increase of the crystallite size due to sp2 restoration was observed for the films prepared up to 10 s/V. Atomic force microscopy was used to study the film morphology and to estimate the film thickness by comparing the z-offset between the substrate and coating topography images. Film thickness reduces from ca. 280 nm to 100 nm with the increase in the time at each voltage step. Coating tolerance against laser-induced damage was observed up to 34% of the full laser power, where the coating was damaged and local metal melting was observed. The coating damage occurs between 4.9 × 108 MW/m2 and 2.5 × 109 MW/m2 and higher power levels triggered the occurrence of melting in the metallic substrate. The reduction in laser damage is attributed to the enhanced thermal dissipation by the sp2 dominion as the crystallite size increases. GO coatings prepared by electrophoresis are shown to be promising to protect nuclear components against damage for high heat loads.