Abstract
In this contribution, chemical, structural, and mechanical alterations in various types of femtosecond laser-generated surface structures, i.e., laser-induced periodic surface structures (LIPSS, ripples), Grooves, and Spikes on titanium alloy, are characterized by various surface analytical techniques, including X-ray diffraction and glow-discharge optical emission spectroscopy. The formation of oxide layers of the different laser-based structures inherently influences the friction and wear performance as demonstrated in oil-lubricated reciprocating sliding tribological tests (RSTTs) along with subsequent elemental mapping by energy-dispersive X-ray analysis. It is revealed that the fs-laser scan processing (790 nm, 30 fs, 1 kHz) of near-wavelength-sized LIPSS leads to the formation of a graded oxide layer extending a few hundreds of nanometers into depth, consisting mainly of amorphous oxides. Other superficial fs-laser-generated structures such as periodic Grooves and irregular Spikes produced at higher fluences and effective number of pulses per unit area present even thicker graded oxide layers that are also suitable for friction reduction and wear resistance. Ultimately, these femtosecond laser-induced nanostructured surface layers efficiently prevent a direct metal-to-metal contact in the RSTT and may act as an anchor layer for specific wear-reducing additives contained in the used engine oil.
Highlights
The fabrication of laser-induced periodic surface structures (LIPSS) on metals, semiconductors, and dielectrics has been experimentally demonstrated when irradiated with linearly polarized high-intensity ultrashort laser pulses
Electron-based chemical analyses such as energy-dispersive X-ray analysis (EDX), Auger electron spectroscopy (AES), or X-ray photoelectron spectroscopy (XPS) may reveal the presence of specific chemical elements at those high roughness structures; these techniques generally suffer from constraints regarding the analytical information from both the depth and the topographic artifacts, which limits the quantification of elements and their actual distribution in depths, for EDX [30, 31], where the information depths typically range between some hundreds of nanometers and a few micrometers [30]
Low-spatialfrequency LIPSS are displayed in Fig. 1b, where it is possible to resolve a pattern of parallel lines all perpendicular to the laser beam polarization indicated by a yellow arrow
Summary
The fabrication of laser-induced periodic surface structures (LIPSS) on metals, semiconductors, and dielectrics has been experimentally demonstrated when irradiated with linearly polarized high-intensity ultrashort laser pulses. The Grooves and Spikes may be superimposed by LIPSS (generated in the low fluence wing of the Gaussian beam distribution at local fluences close to the ablation threshold), forming jointly together a hierarchical heterogeneous surface structure. LSFL are the most desired type of structures for many applications, due to the simplicity to produce them on different materials and the rather moderate overall fabrication time required These structures can be analyzed by different surface analytical techniques including optical and chemical surface characterization. Taking benefit of its time and cost efficiency, in this work we apply GD-OES in order to analyze the depth extent of the laser-induced oxide layer generated upon femtosecond laser processing of three characteristic surface morphologies (LIPSS, Grooves, and Spikes) on Ti6Al4V titanium alloy and a polished (non-irradiated) sample as a reference. Chemical effects during the formation of various types of femtosecond laser‐generated surface
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