Development of a Monitoring Strategy for Laser-Textured Metallic Surfaces Using a Diffractive Approach

Abstract

The current status of research around the world concurs on the powerful influence of micro- and nano-textured surfaces in terms of surface functionalization. In order to characterize the manufactured topographical morphology with regard to the surface quality or homogeneity, major efforts are still required. In this work, an optical approach for the indirect evaluation of the quality and morphology of surface structures manufactured with Direct Laser Interference Patterning (DLIP) is presented. For testing the designed optical configuration, line-like surface patterns are fabricated at a 1064 nm wavelength on stainless steel with a repetitive distance of 4.9 µm, utilizing a two-beam DLIP configuration. Depending on the pulse to pulse overlap and hatch distance, different single and complex pattern geometries are produced, presenting non-homogenous and homogenous surface patterns. The developed optical system permitted the successfully classification of different pattern geometries, in particular, those showing single-scale morphology (high homogeneity). Additionally, the fabricated structures were measured using confocal microscopy method, and the obtained topographies were correlated with the recorded optical images.

Keywords: diffraction analysis; direct laser interference patterning; homogeneity characterization; indirect surface characterization; periodic structures.

Figure 1

(a) Schematic representation of the two-beam DLIP principle for an overlap angle of 2Θ and a laser wavelength λ; (b) representation of the strategy used for processing the samples with: D = spot-diameter; p = spatial period; Ov = pulse to pulse overlap; H = hatch overlap. The upper image represents negatives H and Ov overlaps (−25%), while in the lower image, the spots are overlapped by +25% in both directions.

Figure 2

(a) Schematic optical set-up of the diffraction measurement system (M, mirror; P, linear polarizer; L, lens; BS, beam splitter; WP, λ/4 wave plate; CAM, Camera) including simulated rays diffracted by gratings with different periods (cyan, pink, green and orange lines); (b) Simulated sensor image at the camera (CAM) of the first diffraction orders (DO) for periods of 1.33, 1.66, 2.50 and 10.00 µm; CSG represents the Central Spot Group equivalent to the zero diffraction order; (c) CAD model of the fabricated optical system.

Figure 3

(a) DLIP structured stainless-steel sample with a matrix of 12 × 12 fields. The used laser fluence was 1.2 J/cm² and the hatch (horizontal) and pulse to pulse overlap (vertical) were varied in the range from 0% to 99% (with 10% steps till 90%, and then 95% and 99%); (b–d) Confocal microscopy images for (b) Ov = 0%: H = 0%, (c) Ov = 0%: H = 99%, (d) Ov = 60%: H = 60%. The insets show the recorded diffraction patterns by the CCD camera with m = −2, m = −1, m = 0, m = 1 and m = 2 (in (c)) as indicators for the diffraction orders accountable from the DLIP structures.

Figure 4

Topographical measurements of structure depths (z) as function of overlap and hatch values. (a) Second modulation depth measured along X-orientation Z_sm(X) and (b) Y-orientation Z_sm(Y); (c) mean DLIP structure depth Z_fm; (d) quotient between both depth modulations (Z_sm/Z_fm defined in Equation (3)).

Figure 5

Representation of the spatial period as function of hatch and overlap calculated after calibration of the optical device.

Figure 6

Exemplary selected confocal microscopy images (left) and captured diffraction pattern of the Central Spot Group (CSG) (right) as function of both overlap (Ov) and hatch (H). The blank rectangles indicate that no light intensity could be recorded.

Figure 7

(a) Measured diffraction order count of the second modulation (DOC_sm) and (b) total area illuminated of the Central Spot Group (A_CSG) plotted over overlap and hatch values.

Figure 8

Categorized results of the homogeneity fraction Z_sm/Z_fm (depth of first modulation by the DLIP structure to depth of the second modulation ratio) in comparison to (a) Diffraction Order Count of second modulation (DOC_sm) and (b) Area of the Central Spot Group (A_CSG)); (c) Representation of the different category-definitions as function of hatch H and overlap Ov. The different categories are listed in Table 2.