Functionalization of surfaces by engraving micro or nanotextures has become a broad field of research with potential for industrial deployment. Particularly, laser-based methods offer a unique combination of flexibility, high resolution, high throughput, and environmental-friendly by-products. Among these methods, Direct Laser Interference Patterning (DLIP) is able to fabricate periodic surface structures with resolutions down to the micro- or submicron scale with throughputs approaching 1 m2/min [1]. One of the current challenges for this technique is to guarantee a sufficiently good texture homogeneity over large areas, so that the produced functionality can be the controlled on the entire workpiece. In this context, reliable methodologies for quantifying the homogeneity of periodic textures are required. Recently, a statistical analysis based on the Gini coefficient was developed for periodic surfaces [2], whereby the homogeneity can be assessed in terms of arbitrary user-defined attributes, such as the structure height. Although this method has the potential to become a robust tool for surface characterization, it has not been exploited thoroughly. Thus, in this contribution DLIP-structured coatings with varying aspect ratios were analysed with the Gini coefficient theory to establish a correlation between the DLIP-irradiation conditions and the resulting surface homogeneity.

Three studied coatings systems, namely CrN (chromium nitride), WC (tungsten carbide) and a-C:H (amorphous hydrogenated carbon), were deposited on stainless steel adding a thickness between 20 µm and 30 µm to the substrates. The samples were structured by two-beam DLIP, yielding line-like textures with a spatial period of 5.8 µm for all samples and different structure heights, depending on the used laser parameters. Details on the coatings deposition and DLIP process parameters can be found elsewhere [3]. The surfaces were characterized by optical confocal microscopy and post-processed to level the surface, to eliminate high-frequency noise, and to fill non-measured pixels, if any. The Gini analysis was implemented in MATLAB with a self-developed script. The chosen attributes for the homogeneity assessment are structure height, skewness, kurtosis, and real-to-projected area ratio, as they can reveal valuable information, such as amplitude, shape, volume, and effective area, of the analysed texture. To each of these attributes a homogeneity value ranging from 0 to 100% can be assigned. Furthermore, the global homogeneity was defined as a combination of the individual homogeneity values.

From the results, several trends between laser parameters and homogeneity can be extracted. For instance, it has been observed that the lower the average skewness, the higher is the homogeneity in terms of the skewness as well as globally. For all studied materials the higher overlaps of 90%, 95% and 97% allow for more homogeneous periodic structures. Increasing the laser fluence improves the homogeneity solely for those sample treated with the lower overlaps. In conclusion, the used method allows for an effective quantification of the texture homogeneity and can provide insights on the impact of process parameters and topography.

[1] V. Lang, T. Roch and A.F. Lasagni, World record in high speed laser surface microstructuring of polymer and steel using direct laser interference patterning, Proc. Vol. 9736, Laser-based Micro- and Nanoprocessing X; 97360Z (2016).

[2] B. Lechthaler, C. Pauly and F. Mücklich, Objective homogeneity quantification of a periodic surface using the Gini coefficient. Sci. Rep. 10, 14516 (2020).

[3] F. Kuisat, T. Abraham, T. Schmidt, M. Weber, M. Demmler, G. Bräuer and A.F.Lasagni, Surface modification of forming tools for aluminum sheet metal forming, J. Laser Micro/nanoeng. 15, 1 (2020).