**1. Introduction**

The material Ti6Al4V is one of the most widely used titanium alloys (e.g., in the aerospace industry and for implants in medicine) that are usually restricted in tribological applications due to the low surface hardness, the rather high coefficient of friction and the low abrasive wear. By means of surface modification technologies, the aforementioned shortcoming can be overcome. One of the most promising approaches is based on the use of lasers, which allow a contactless, fast, and reliable surface functionalization.

Ultrafast laser processing can be used to generate various surface morphologies, employing either direct contour-shaping or, for larger surface areas, functionalization via the generation of "self-organized" micro- and nanostructures [1]. This allows researchers to induce different surface functionalities in the fields of optics [2], fluidics [3], medicine [4,5], and tribology [6] and can be scaled up to the current demands for sizes and production rates in industrial applications.

The exposure to high-intensity pulsed laser radiation excites the treated materials into extreme conditions, which then return to equilibrium through various relaxation channels. Potentially, these involve phase transitions such as melting, evaporation (ablation), recrystallization, or accompanying chemical alterations in the ambient atmosphere (e.g., oxidation), etc., finally featuring different types of surface topographies [7]. One specific kind of these topographies currently gaining attention in various fields of applications is called laser-induced periodic surface structures (LIPSS, ripples). They are formed either perpendicular or parallel to the linear laser beam polarization and usually exhibit spatial periods in the sub-micrometer range [8]. These laser-induced nanostructures can be produced in a very reliable manner in a contactless single-step approach.

In a series of previous publications, we have explored the tribological performance of two different types of these surface nanostructures, high spatial frequency LIPSS (HSFL) [9] and low spatial frequency LIPSS (LSFL) [6,10,11], in the regime of mixed friction, reporting for the LSFL a remarkably positive effect on the coefficient of friction (CoF) and the associated wear of LIPSS-covered titanium or titanium alloy surfaces. They were tested in linear reciprocating sliding tribological tests (RSTT) in a fully formulated engine oil against a ball of hardened 100Cr6 steel and compared to polished reference surfaces. Via contrasting juxtaposition to identical RSTT conducted in non-additivated paraffin oil, it was speculated that this positive effect may be attributed to the presence of the additive (zinc dialkyl dithiophosphate, ZDDP [12]) contained in the engine oil (VP1) [10], able to efficiently bond to the laser-structured but not to the polished surfaces. These ZDDP molecules could then cover the laser-treated surfaces and, therefore, prevent a direct contact of the tested metals.

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 TiO2 and micro-crystalline Ti2O3 [13] may help to stabilize mechanically the oxygen containing near the surface region, preventing its delamination during the tribological demands/stresses. Moreover, with some hundreds of nanometers, the ball-sample deformation during the RSTT is of similar magnitude as the surface modulation depth of the LSFL [10], allowing a confinement of the lubricant in the tribological contact area during the RSTT.

In this work, we test our previous hypothesis that an additivated, fully formulated engine oil in combination with a laser-oxidized surface is beneficial for the tribological performance of Ti surfaces [10]. Specific emphasis is laid on the disclosure of the relevance of some specific anti-wear additives on the tribological performance of fs-laser-processed titanium alloy (Ti6Al4V) surfaces. By replacing the commercial engine oil (VP1) by an admixture of its base oil (VPX) with a single additive—here, the anti-wear additive (2-ethylhexyl zinc dithiophosphate)—its crucial role can be specifically addressed under our standard tribological testing conditions. Moreover, since the oils and additives are optimized for metallic (steel) surfaces, by varying the counterbody material, the role of the surface chemistry is further elucidated. Despite the use of engine oil, this work is an academic study and not one aimed at a specific engineering problem.

#### **2. Experimental**

Commercial grade-5 titanium alloy (Ti6Al4V) was purchased from Schumacher Titan GmbH (Solingen, Germany). The rods of 25 mm diameter were cut into circular slabs of 8 mm thickness. The top surfaces of the slabs were subsequently mechanically polished resulting in surface roughness parameters *R*<sup>a</sup> = 5 nm and *R*rms = 6 nm.

For large area surface processing, a linearly polarized commercial Ti:sapphire laser amplifier system was used (Femtolasers, Compact Pro, Vienna, Austria: τ = 30 fs pulse duration, λ = 790 nm central wavelength, ν = 1 kHz pulse repetition rate). The titanium alloy samples were mounted on a motorized *x*-*y*-*z* linear translation stage and placed perpendicular to the incident laser beam, realizing at the surface a Gaussian-like beam profile with a radius of *w*<sup>0</sup> (1/*e*2) ~140 μm. Square-shaped areas of 5 <sup>×</sup> 5 mm2 were processed upon meandering movement of the sample (*v*<sup>x</sup> = 5 mm/s scan velocity, Δy = 0.1 mm line-offset) under the focused laser beam at (35 ± 2) μJ/pulse. These laser-processing conditions correspond to a laser peak fluence of ϕ0~0.11 J/cm2 in front of the surface and an effective number of laser pulses per beam spot diameter of *N*eff ~56 in the direction of the processed lines. Once processed, the samples were ultrasonically cleaned in acetone for 5 min. All samples were stored in a desiccator (up to a few months prior to the laser irradiation or the surface characterizations

following the tribological tests). Given the very high temperatures of the titanium surface transiently exceeding 5000 K (estimated in [13]), repetitively reached during the fs-laser irradiation process, a significant storage-related post-oxidation of the laser treated samples at room temperature cannot be expected here.

The characteristics of the surface ripples were previously studied in detail on an identically processed sample revealing low-spatial frequency LIPSS (LSFL) with periodicities of (620 ± 80) nm along with height modulation depths of ±150 nm [10], typically manifested in a sine-like surface modulation. The modulation depths of the LSFL are similar to the ball-sample deformation during the RSTT (see Table 1). This may help to confine the lubricant in the tribological contact area of the nanostructured sample surface. For further details on the fs-laser processing and sample characterization, the reader is referred to [10].

RSTT measuring the coefficient of friction by the dissipated energy method were conducted with an in house-built tribometer [15] by sliding the LSFL-covered samples against either a hardened and polished steel ball (100Cr6, ø = 10 mm, *R*<sup>a</sup> = 8 nm), a polished polycrystalline aluminium oxide ball (Al2O3, ø = 10 mm, *R*<sup>a</sup> = 10 nm), or a polished polycrystalline silicon nitride ball (Si3N4, ø = 10 mm, *R*<sup>a</sup> = 4 nm) as counterbodies. The normal load applied by dead weight is so small (1 N) that the stiffness of the holders is not compromised. The tribological tests were carried out in mixed lubrication conditions to have both, the effects of the laser-modified surface and the applied oil involved. The range of uncertainty in the friction coefficient measurements is ±0.02. Due to the limited availability of fs-laser-structured samples, each test was done once. The same RSTT conditions as in [10] were used in order to have directly comparable results. All relevant experimental parameters for the RSTT are summarized in Table 1.


**Table 1.** Linear reciprocating sliding tribological test conditions.

The tribological tests were conducted using VPX oil as base lubricant, i.e., a variant of the polyalphaolefin (PAO) based factory fill Castrol engine oil SAE 0W-30 "VP1" (Castrol, Liverpool, UK) containing only anti-oxidation and anti-corrosion additives but no friction modifiers or wear protecting constituents. As a second variant, the commercial anti-wear additive RC 3180 purchased from LANXESS Deutschland GmbH (Business Unit Rhein Chemie, Cologne, Germany) was added

to the base oil VPX. This additive consists of 2-ethylhexyl zinc dithiophosphate containing zinc, phosphorous, and sulphur by 9.5, 8, and 16 wt %, respectively. The optimum percentage of the additive content resulting in the lowest CoF and wear was identified as 0.5 wt % RC 3180 addition. That optimized mixture was then used for all RSTT experiments involving RC 3180-additivated VPX oil. The results for synthetic paraffin oil and for the fully additivated engine oil VP1 were already published in [10]. However, we will show them here along with the new results for direct comparison. After the RSTT, the samples were cleaned in benzene (petroleum ether) for 15 min using an ultrasonic bath in order to remove the residual lubricants.

The corresponding wear tracks were inspected by optical microscopy (OM, Carl Zeiss, Discovery V20/Keyence, VHX 5000, Oberkochen, Germany) and scanning electron microscopy (SEM, Carl Zeiss, Gemini Supra 40, Oberkochen, Germany). The volume of large wear scars was calculated by measuring the cross-sectional area and the width/length of the scars by means of a 3D confocal profilometer (Nanofocus, μ-surf Expert, Oberhausen, Germany) and subsequently applying analytical equations following the procedure described in the ASTM D7755-11 (2017) standard [16]. However, on the LSFL-covered and not completely worn surfaces the wear volume was estimated by assuming that roughly half of the wear track area (only at the peaks of the topography and not at the valleys) was removed over the depth measured by the profilometry (i.e., a few tens to hundreds of nanometers only).

In addition, selected parts of a wear track were analysed by scanning transmission electron microscopy (STEM). The sample was prepared by a focused ion beam (FIB) milling machine (FEI, Quanta 3D, Waltham, MA, USA) using an in situ lift-out technique. The preparation of the TEM lamella involved the deposition of a protective Pt cap layer at the region of interest, before thinning it by Ga-ions up to electron-transparency. The Pt layer protects the covered sample surface from being damaged during the FIB preparation of the lamella. This lamella of ~100 nm thickness was then characterized in a scanning transmission electron microscope (JEOL, JEM 2200FS, Akishima, Tokyo, Japan) operated at 200 kV with a point resolution better than 0.25 nm. The system is equipped with a field emission gun, an in-column energy filter and an energy-dispersive X-ray (EDX) spectroscopy system for elemental studies.
