Figure 10. EDS analysis of the change of the weight percentage (wt%) relation between the samples treated by laser radiation in different gases and untreated surfaces for C (black bars) and O (cyan bars) elements after 30 days of ageing in atmospheric air. Subscripts of bars denote the gas type (Ar, CO2, N2, SF6, O2) and VAC corresponds to the vacuum conditions. HP and CP correspond to horizontal and circular polarizations, similar to Fig. 7. Numbers 1 and 2 indicate the scan speeds used (50 and 300 mm/s, respectively).
For SF6 bars (from SF6 HP1 to O2 CP2 bars in Fig. 10, panel b) we also observed relatively high presence of the carbon. Meanwhile, only in this case among all gases we observed the relatively high presence of one of the component of SF6 gas, fluorine F (up to 8 wt% in case of 50 mm/s scanned sample) such increasing of fraction of light elements (C, O, F) compare to main heavy Fe element leads to common increase of wt% for all light elements and this can explain relatively high presence of C not accompanied with increasing of WCA. The presence of F can lead to formation of the metal-fluoride compounds, which may explain the unstable behavior of the samples. For other gases (Ar, N2, CO2) (from Ar HP1 to CO2 CP2 bars in Fig. 10, panel a and bars N2 HP1 to N2 CP2 in Fig 10, panel b) and vacuum (bars VAC HP1 to VAC CP2 in Fig 10, panel c), the common features were the small additional oxidation and lesser presence of carbon, which is well correlates with lesser values of WCA measurements shown in Fig. 9. Also notice on the insignificant difference for results produced at linear (HP) and circular (CP) polarizations either in the case of WCA or EDS measurements.
Conclusions
Our studies have revealed the role of different gas environment at which stainless steel meshes were laser-textured for long-term air ageing. We also introduced the surface morphology difference by producing two types of structures (ripples and spikes) when the accumulated laser fluence was varied by changing the scanning speed. We did not observe a significant dependence of the low-scale LIPSS morphology, while observing a weak dependence on the scanning speed variations in the studied range of variations of this parameter. After 30 days of ageing the samples produced in atmospheric air and in pure O2have demonstrated the transition towards large contact angles (WCA > 145°), while the samples prepared in Ar, CO2, N2, SF6 and vacuum maintained their WCA below or at around 140°. EDS analysis have demonstrated the correlation of the amount of O and C with the observed hydrophobicity. Probably, gases, except O2, reduce the final WCA after 30 days of aging, which was attributed to lesser oxidation preventing the formation of phobic layer. For the stainless steel mesh, O2 and hence oxidation play important role in providing optimal conditions for transformation to hydrophobic state.
Funding: These studies were supported by FRG AS1801 grant and the Common Research Facility at the American University of Sharjah.