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.