Tribological performance of titanium samples oxidized by fs-laser radiation, thermal heating, or electrochemical anodization

Abstract

Commercial grade-1 titanium samples (Ti, 99.6%) were treated using three alternative methods, (i) femtosecond laser processing, (ii) thermal heat treatment, and (iii) electrochemical anodization, respectively, resulting in the formation of differently conditioned superficial titanium oxide layers. The laser processing (i) was carried out by a Ti:sapphire laser (pulse duration 30 fs, central wavelength 790 nm, pulse repetition rate 1 kHz) in a regime of generating laser-induced periodic surface structures (LIPSS). The experimental conditions (laser fluence, spatial spot overlap) were optimized in a sample-scanning setup for the processing of several square-millimeters large surface areas covered homogeneously by these nanostructures. The differently oxidized titanium surfaces were characterized by optical microscopy, micro Raman spectroscopy, variable angle spectroscopic ellipsometry, and instrumented indentation testing. The tribological performance was characterized in the regime of mixed friction by reciprocating sliding tests against a sphere of hardened steel in fully formulated engine oil as lubricant. The specific tribological performance of the differently treated surfaces is discussed with respect to possible physical and chemical mechanisms.

On the Role of a ZDDP in the Tribological Performance of Femtosecond Laser-Induced Periodic Surface Structures on Titanium Alloy against Different Counterbody Materials

“…Hence, it is likely that the laser-induced oxidation and the interplay with surface topography are the crucial aspects here. Inferring that a minimum oxygen-containing layer thickness and an enlarged surface area are required together with the additives ZDDP [14] to form a sufficiently thick anti-wear layer during For a direct comparison, Figure 5a displays again the results obtained for the 100Cr6 steel balls previously shown in Figure 2 (left column). The CoF's measured in the fs-laser-processed regions (B, blue curves) with both ceramic counterbodies (Al 2 O 3 , Si 3 N 4 ) are as small as those obtained for the metallic steel balls, see Table 3.…”

“…For a deeper characterization of a wear track obtained with an anti-wear additive, we extended our previous STEM analysis [14] by adding spatially information on the elemental contribution underneath the laser-irradiated and worn surface. The corresponding wear track was obtained in the same processed LSFL area as previously shown in Figure 2d,e(B),f(B), but it was tribologically tested parallel to the ablation lines, i.e., perpendicular to the LSFL, with otherwise unchanged RSTT conditions.…”

“…Hence, it is likely that the laser-induced oxidation and the interplay with surface topography are the crucial aspects here. Inferring that a minimum oxygen-containing layer thickness and an enlarged surface area are required together with the additives ZDDP [14] to form a sufficiently thick anti-wear layer during RSTT, which prevents a direct contact of the two sliding tribological bodies. These findings are supported by Figure 6, which visualizes the values gathered in Table 3.…”

“…Apart from the topographic alterations, spatially and depth-resolved chemical analyses also revealed a fs-laser-induced oxidation process during the LIPSS formation on titanium [13]. In comparative oxidation experiments (thermal oxidation vs. anodic oxidation vs. LIPSS-induced oxidation) a beneficial tribological effect was found only for sufficiently large oxide layer thicknesses (>150 nm) and for high temperature surface oxidation [14]. The graded oxide layer present for the LSFL consisting mainly of amorphous TiO 2 and micro-crystalline Ti 2 O 3 [13] may help to stabilize mechanically the oxygen containing near the surface region, preventing its delamination during the tribological demands/stresses.…”

“…(d) magnified detail of (c), of which energy-dispersive X-ray (EDX) spectroscopy elemental maps were taken for Fe (e), Ti (f) and Zn (g), (h) is an overlay of (d) to (g). Images (a)-(c) are taken from[14].…”