B. Gaković

Surface modification of stainless steels by TEA CO2 laser

Abstract The interaction of a transversely excited atmospheric (TEA) CO 2 laser with austenitic stainless AISI 316 and high-speed tool AISI M2 steel is considered. The results have shown that both types of steel were surface modified by the laser beam, for the laser energy densities used. Morphological features on the AISI M2 were more prominent in comparison to a AISI 316 steel. On the AISI M2 steel crater like forms, solidified boundary and hydrodynamical modifications were observed, while on the AISI 316 one, corrugation, cracking and resolidified areas were registered. The morphology changes of steel surfaces in dependence of laser pulse temporal shape were analyzed especially. The pulse with a tail for difference of a tail-free pulse, for the same incident peak power density, as a rule caused higher level of damage.

Femtosecond laser-induced periodic surface structure on the Ti-based nanolayered thin films

Laser-induced periodic surface structures (LIPSSs) and chemical composition changes of Ti-based nanolayered thin films (Al/Ti, Ni/Ti) after femtosecond (fs) laser pulses action were studied. Irradiation is performed using linearly polarized Ti:Sapphire fs laser pulses of 40 fs pulse duration and 800 nm wavelength. The low spatial frequency LIPSS (LSFL), oriented perpendicular to the laser polarization with periods slightly lower than the irradiation wavelength, was typically formed at elevated laser fluences. On the contrary, high spatial frequency LIPSS (HSFL) with uniform period of 155 nm, parallel to the laser light polarization, appeared at low laser fluences, as well as in the wings of the Gaussian laser beam distribution for higher used fluence. LSFL formation was associated with the material ablation process and accompanied by the intense formation of nanoparticles, especially in the Ni/Ti system. The composition changes at the surface of both multilayer systems in the LSFL area indicated the intermixing between layers and the substrate. Concentration and distribution of all constitutive elements in the irradiated area with formed HSFLs were almost unchanged.

Efficient small-scale TEA CO2 laser for material surface modification

An efficient small scale TEA (Transversely Excited Atmospheric) CO2 laser has been considered. The laser was a low flowing, UV preionized, pulsed system. Specific design of the head as well as the electrical circuit ensure the laser operation with high efficiency and reliability. Nonconventional CO2/Y, Y equals N2/H2; H2/He and H2 gas mixtures showed relatively high energy output. CO2/N2/H2 mixture as the most superior one produces output energy and peak power of 220 mJ and 1.40 MW, respectively. High-energy output of the laser was employed for surface modification of austenitic stainless steel AISI 316 and titanium nitride (TiN) coating deposited on the same steel substrate. Used laser peak power densities of 100 and 170 MW/cm2 have induced the morphology changes of AISI 316 steel and TiN coating, respectively.

Nickel-based super-alloy Inconel 600 morphological modifications by high repetition rate femtosecond Ti:sapphire laser

AbstractTheinteractionofTi:sapphirelaser,operatingathighrepetitionrate of75 MHz,with nickel-basedsuper-alloyInconel600was studied. The laser was emitting at 800 nm and ultrashort pulse duration was 160 fs. Nickel-based super-alloy surfacemodification was studied in a low laser energy/fluence regime of maximum 20 nJ–15 mJ/cm 2 , for short (10 s) and longirradiation times (range of minutes). Surface damage threshold of this material was estimated to be 1.46 nJ,i.e., 0.001 J/cm 2 in air. The radiation absorbed from Ti:sapphire laser beam under these conditions generates at thesurface a series of effects, such as direct material vaporization, plasma creation, formation of nano-structures and theirlarger aggregates, damage accumulation, etc. Laser induced surface morphological changes observed on Inconel600 were: (1) intensive removal of surface material with crater like features; (2) material deposition at near and fartherperiphery and creation of nano-aggregates/nano-structures; (3) sporadic micro-cracking of the inner and outer damagearea. Generally, features created on nickel-based super-alloy surface by high repetition rate femtosecond pulses arecharacterized by low inner/outer damage diameter of less than 11 mm/30 mm and relatively large depth on the orderof 150 mm, in both low (10 s) and high (minutes) irradiation time regimes.Keywords: Femtosecond; Laser-matter interaction; Nickel-based super-alloy Inconel 600; Ti:sapphire laser

Superficial changes on the Inconel 600 superalloy by picosecond Nd:YAG laser operating at 1064, 532, and 266 nm: Comparative study

AbstractA comparative study of superficial changes on the superalloy Inconel 600, induced by a picosecond Nd:YAG laseroperating at 1064, 532, and 266 nm, is presented. All of the laser wavelengths, as well as the used fluences of 2.5(1064 nm), 4.3 (532 nm), and 0.6 J/cm 2 (266 nm) were found to be adequate for inducing surface variations. Quitedifferent surface features were produced depending on the laser wavelength used. The measured surface damagethresholds were 0.25, 0.13 and 0.10 J/cm 2 for 1064, 532, and 266 nm, respectively. Drastic differences, in function ofthe wavelength used, were recorded for the crater depths, as well the appearance of hydrodynamic effects and periodicsurface structures. Differences in crater depths were explained via an easier propagation of the first harmonic laserradiation (1064 nm) through the ejected material and plasma compared to a radiation at 532 and 266 nm. Finally,changes in the surface oxygen content caused by ultrashort laser pulses were considered.Keywords: Laser ablation; Periodic surface structures; superalloy Inconel; Surface morphology

Surface modification of W-Ti coatings induced by TEA CO2 laser beam

In this work, the interaction of a transversely excited atmospheric (TEA) CO 2 laser with tungsten–titanium (W-Ti) alloy deposited on austenitic stainless steel is considered. The W-Ti alloy as a refractory material possesses very good physicochemical characteristics such as thermochemical stability and high melting temperature. Studying of interactions of different energetic particles or laser beams with W-Ti coatings has both application and fundamental importance. The morphological features of the W-Ti coating, deposited on austenitic stainless steel AISI 316, induced by a TEA CO 2 laser after multipulse cumulative laser action, have been considered. The laser pulses with tail (FWHM = 120 ns, tail = 2 μs) and free-tail pulses (FWHM = 80 ns) have been employed. Laser pulses used in the experiment had equal peak power density I = 120 MWcm −2 . For the given peak power density, excessive surface changes on the coating were registered. From direct observation on a microscopic scale (OM, SEM), it can be concluded that W-Ti coatings show different behavior under laser irradiation with various temporal pulse shapes.

Picosecond laser ablation of nano-sized WTi thin film

Interaction of an Nd:YAG laser, operating at 532 nm wavelength and pulse duration of 40 ps, with tungsten-titanium (WTi) thin film (thickness, 190 nm) deposited on single silicon (100) substrate was studied. Laser fluences of 10.5 and 13.4 J/cm2 were found to be sufficient for modification of the WTi/silicon target system. The energy absorbed from the Nd:YAG laser beam is partially converted to thermal energy, which generates a series of effects, such as melting, vaporization of the molten material, shock waves, etc. The following WTi/silicon surface morphological changes were observed: (i) ablation of the thin film during the first laser pulse. The boundary of damage area was relatively sharp after action of one pulse whereas it was quite diffuse after irradiation with more than 10 pulses; (ii) appearance of some nano-structures (e.g., nano-ripples) in the irradiated region; (iii) appearance of the micro-cracking. The process of the laser interaction with WTi/silicon target was accompanied by formation of plasma.

Modification of multilayered TiAlN/TiN coating by nanosecond and picosecond laser pulses

A multilayered TiAlN/TiN coating deposited on H11 work-steel was irradiated by a TEA CO2 laser (ns pulses) and a Nd:YAG laser (ps pulses), and the effects compared. The coating was 2.17 µm thick and consisted of 45 layers. The laser-induced modifications showed dependence on laser pulse duration, pulse count and laser wavelength. The conditions for coating ablation in both cases were determined. The experiment has revealed laser-induced periodic surface structures (LPSS) on nanometre and micrometre scales, depending on the laser wavelength used. Sample surfaces were characterized before and after laser irradiation by an optical microscope, scanning electron microscope (SEM), focused ion beam (FIB) microscope and profilometry.

Partial ablation of Ti/Al nano-layer thin film by single femtosecond laser pulse

The effects of ultra-short laser pulses on reactive Ti/Al nano-layered thin film were investigated. The thin film composed of alternated titanium and aluminium nano-layers, was deposited by ion-sputtering. Single pulse irradiation was conducted in the air with focused and linearly polarized femtosecond laser beam - of 1026 nm wavelength and pulse duration of 170 fs. Laser induced composition and morphological changes, using different microscopy techniques and energy dispersive X-ray spectroscopy, were investigated. Following results were obtained: (i) one step partial/selective ablation of upper Ti layer from nano-layer Ti/Al at low laser fluence and (ii) two step ablation or entire ablation of nano-layer Ti/Al at higher laser fluence. Single pulse selective ablation of the upper Ti layer was confirmed based on profiling (AFM) along the ablation steps and reduction of Ti concentration (EDX) in the ablated areas. Ablation threshold was estimated using well known procedure for ultra-short laser pulses - spot diameter square versus logarithm of pulse energy. To interpret the experimental observations, simulations have been performed to explore the thermal response of the multiple layered structure (Ti(5x(Al/Ti))) after irradiation with a characteristic value of single laser pulse of fluence F = 320 mJ/cm2. The results are in agreement with the calculations.

Transversely excited atmospheric CO2 laser-induced damage of TiN and (Ti, Al)N coatings

Abstract When an energy-carrying beam strikes a solid surface, the surface is sputtered and many effects on the atomic, microscopic and macroscopic scale can be observed. The morphological changes induced on solid surfaces, the shape and size of damaged area depend on the beam and target characteristics. Laser beams are frequently used in modern technology to obtain materials with certain bulk properties and different surface properties. During the last years, there has been an interest for advanced technology application of implemented ceramic thin films/coatings deposited by various methods. In this work the microscopic changes induced by laser beam on TiN and (Ti,Al)N coatings deposited on steel were studied. For this purpose we used pulsed, Transversely Excited Atmospheric (TEA) CO 2 laser, operating with typical and non-typical gas mixtures. The deposited coatings and laser-induced damage after 20–340 pulses were analyzed by X-ray, optical and scanning electron microscopy (SEM). It was shown that the morphological features and damages induced during the laser irradiation depend on the pulse shape, total deposited energy and type of targets. From direct observation on microscopic scale, it can be concluded that similar coatings TiN and (Ti,Al)N showed different behaviour under pulsed laser irradiation.