PSI - Issue 74

Kipkurui Ronoh et al. / Procedia Structural Integrity 74 (2025) 77–84 Kipkurui Ronoh / St ructural Integrity Procedia 00 (202 5 ) 000 – 000

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surfaces were Fe, Cr, C, and O. Fig. 4b) presents a high-resolution C 1s spectrum of 699 XA after laser irradiation with the laser fluence of 1 J/cm 2 . Similar spectra were observed at fluences of 4, 8 and 10 J/cm 2 , indicating that the carbon present likely originated from the same source, adventitious carbon due to air exposure (Ronoh et al., 2025). As seen in Fig. 4b, the C 1s spectrum is fitted with four peaks that indicate the presence of carbon in four distinct chemical states corresponding to C-C/C-H, C-O-C, C-O and O-C=O bonds. The C-C/C-H dominates the composition on the surfaces. Comparable C 1s spectra were observed on the Kanthal® AF and MONEL® alloy 400 samples. The carbon content for the three alloys, as a function of laser fluence, is shown in Table 4. The amount of carbon content present on the laser-ablated samples is lower than on the polished samples, and this could be attributed to irradiation, which destroys the adsorbed carbon on the surface (M. A. Khan et al., 2024). As shown in Table 4, carbon content decreases as the laser fluence increases, consistent with previous findings (Li et al., 2010; Ronoh et al., 2024a). Higher laser fluences generate more heat, effectively destroying adsorbed carbon. Surface topography significantly affects adventitious carbon adsorption. Polished, smoother surfaces promote the formation of a uniform thin carbon layer, which aligns well with the XPS probing depth, enhancing photoelectron yield from carbon. In contrast, rough or laser-ablated surfaces show less uniform carbon coverage due to nanoroughness, leading to shadowing effects and variable take-off angles, which reduce the overall carbon signal detected by XPS (Artemenko et al., 2009). An exception to the trend was noted for the 699 XA alloy, where the lower carbon content at 4 J/cm² is recorded in comparison to at 8 J/cm 2 . This is likely to higher Sa which magnify roughness-induced effects.

Table 4: Effect of laser fluence on carbon content on the surfaces of the alloys Percentage of carbon content (% at) Laser Fluence (J/cm²) 699 XA Kanthal® AF MONEL® Alloy 400 Polished 70.62 55.88 69.88 1 48.50 42.44 44.12 4 35.10 32.35 30.93 8 38.91 24.39 25.88 10 28.07 22.03 26.19

3.4. Influence of the laser parameters on wettabilit y Fig. 5 and Fig. 6 show static contact angle measurements as a function of the laser fluence (1 – 10 J/cm 2 ) and hatching distance (5 – 100 µm), respectively. The contact angles of the polished samples were greater than 90 degrees, indicating hydrophobic properties. After laser irradiation and atmospheric exposure of the samples, the contact angles increased significantly, ranging from 136° and 146°, exhibiting strong hydrophobicity. The increment is attributed to surface roughening induced by laser ablation, as evidenced in Fig. 1, Fig. 2, and from the Sa data in Table 3. The increased roughness increased the hydrophobicity of the samples as stated by Wenzel's model. According to the model,

Fig. 5: Static contact angle (CA) measurements on the surfaces of the alloys as a function of laser fluence (LF).

Fig. 6: Static contact angle (CA) measurements on the surfaces of the alloys as a function of hatching distance (HD)