E. V. Golosov

Femtosecond laser writing of subwave one-dimensional quasiperiodic nanostructures on a titanium surface

One-dimensional quasiperiodic structures whose period is much smaller than the wavelength of exciting radiation have been obtained on a titanium surface under the multipulse action of linearly polarized femtosecond laser radiation with various surface energy densities. As the radiation energy density increases, the one-dimensional surface nanorelief oriented perpendicularly to the radiation polarization evolves from quasiperiodic ablation nanogrooves to regular lattices with subwave periods (100–400 nm). In contrast to the preceding works for various metals, the period of lattices for titanium decreases with increasing energy density. The formation of the indicated surface nanostructures is explained by the interference of the electric fields of incident laser radiation and a surface electromagnetic wave excited by this radiation, because the length of the surface electromagnetic wave for titanium with significant interband absorption decreases with an increase in the electron excitation of the material.

Ultrafast changes in the optical properties of a titanium surface and femtosecond laser writing of one-dimensional quasi-periodic nanogratings of its relief

One-dimensional quasi-periodic nanogratings with spacings in the range from 160 to 600 nm are written on a dry or wet titanium surface exposed to linearly polarized femtosecond IR and UV laser pulses with different surface energy densities. The topological properties of the obtained surface nanostructures are studied by scanning electron microscopy. Despite the observation of many harmonics of the one-dimensional surface relief in its Fourier spectra, a weak decreasing dependence of the first-harmonic wavenumber (nanograting spacing) on the laser fluence is found. Studies of the instantaneous optical characteristics of the material during laser irradiation by measuring the reflection of laser pump pulses and their simulation based on the Drude model taking into account the dominant interband absorption allowed us to estimate the length of the excited surface electromagnetic (plasmon-polariton) wave for different excitation conditions. This wavelength is quantitatively consistent with the corresponding nanograting spacings of the first harmonic of the relief of the dry and wet titanium surfaces. It is shown that the dependence of the first-harmonic nanograting spacing on the laser fluence is determined by a change in the instantaneous optical characteristics of the material and the saturation of the interband absorption along with the increasing role of intraband transitions. Three new methods are proposed for writing separate subwave surface nanogratings or their sets by femtosecond laser pulses using the near-threshold nanostructuring, the forced adjustment of the optical characteristics of the material or selecting the spectral range of laser radiation, and also by selecting an adjacent dielectric.

Sub-100?nanometer transverse gratings written by femtosecond laser pulses on a titanium surface

One-dimensional transverse (perpendicular to the laser polarization) gratings with periods Λ ≈ 50–60 nm were observed on a titanium surface within 150 nm wide, micrometer-long regular surface modification longitudinal stripes fabricated by multiple 744 nm Ti:sapphire femtosecond laser shots, occurring at a repetition rate of 10 Hz. In the center of the surface laser spot these stripes are oriented strictly perpendicular to the laser polarization, in accordance with the plasmon-polaritonic model, and appear as ablative longitudinal trenches centered along the main stripe axes, which are precursors of longitudinal common ripples with a 500 nm period. At the low-fluence periphery of the laser spot, the stripes appear not as ablative longitudinal trenches, but as linear arrays of sub-ablative transverse nanoripples with periods down to 50 nm. The appearance of such superfine transverse nanoripples is related to incomplete spallation of the laser–molten surface layer, periodically modulated at the nanoscale through coherent sub-surface cavitation.

Formation of periodic nanostructures on aluminum surface by femtosecond laser pulses

One-dimensional periodic nanostructures have been produced on the surface of an aluminum specimen using femtosecond laser pulses at wavelengths of 744 and 248 nm. The nanostructurization of the specimen has been conducted in water and in air in the preablation regime. We investigate the dependence that the surface topology has on the parameters of laser radiation (wavelength, fluence, and number of pulses), as well as on the medium in contact with the specimen surface. A calculation of the optical characteristics of aluminum as they depend on the electron temperature is performed that is good at describing the dependence that the reflection of the p-polarized infrared femtosecond pulses of pumping has on the fluence. Using these optical characteristics of the photoexcited aluminum within the interferential model, periods of the aluminum surface nanogratings are estimated which are in good agreement with the periods measured experimentally.

Mechanical properties of mass-produced nanostructured titanium

The structure and mechanical properties of nanostructured titanium VT1-0 derived using an ingenious method which combines helical and longitudinal rolling are studied in comparison with the properties of commercial titanium alloys VT6 and VT16, as well as VT1-0 in a coarse-grained state. The mechanical properties of these materials are studied using quasi-static tensile and torsion tests (including finished products, i.e., implants for osteosynthesis), as well as fatigue tests. It is shown that the use of the developed method of severe plastic deformation is an efficient mode for the formation of a high-strength nanostructured state in titanium VT1-0, which exhibits sensitivity to stress concentrators under cyclic loading that is characteristic of titanium alloys and an extremely high reserve of torsional plasticity.

Femtosecond laser modification of titanium surfaces: direct imprinting of hydroxylapatite nanopowder and wettability tuning via surface microstructuring

Femtosecond laser modification of titanium surfaces was performed to produce microstructured hydrophilic and biocompatible surface layers. Biocompatible nano/microcoatings were prepared for the first time by dry femtosecond laser imprinting of hydroxylapatite nano/micropowder onto VT6 titanium surfaces. In these experiments HAP was first deposited onto the titanium surfaces and then softly imprinted by multiple femtosecond laser pulses into the laser-melted surface metal layer. The surface relief was modified at the nano- and microscales depending on the incident laser fluence and sample scanning speed. Wetting tests demonstrated that the wetting properties of the pristine Ti surface can be tuned through its laser modification in both the hydrophobic and hydrophilic directions.

Orientation, substructure, and optical properties of rutile films

The orientation, optical properties, and substructure of rutile films prepared by thermal and pulsed photon-assisted oxidation of single-crystal Ti films were investigated by transmission electron microscopy, optical spectroscopy, and high-energy electron diffraction. Crystallographic orientation relationships at the contact between titanium and rutile were established, and the energy band gap was determined. It was shown that the luminescence intensity decreased significantly with decreasing grain size from the submicrolevel to nanolevel.

Effect of cryorolling on the structure and the mechanical properties of ultrafine-grained nickel

Grain-boundary misorientations and mechanical properties of ultrafine-grained (UFG) nickel produced by equal-channel angular pressing are investigated after rolling at a temperature of −196°C. It is shown that cryogenic rolling increases the fraction of high-angle grain boundaries almost two times in UFG nickel. The approach based on electron backscatter diffraction analysis is proposed to estimate the relative energy of high-angle boundaries in polycrystalline metals.

Structure and mechanical properties of a eutectic high-temperature Nb-Si alloy grown by directional solidification

The structure and the short-term high-temperature strength of Ni-18.7 at % Si (Nb-Nb3Si eutectic) alloys fabricated by vacuum electron-beam zone melting and induction melting in an argon atmosphere are studied. The structure of the samples prepared by vacuum electron-beam zone melting is characterized by the presence of primary Nb5Si3 intermetallic precipitates and the absence of its secondary precipitates. The structure of the samples prepared by induction melting in an argon atmosphere has two characteristic zones, namely, eutectic and eutectoid ones.

Surface Modification of Titanium by Pulsed Laser Radiation of Femtosecond Duration

The effect of an infrared pulse laser with femtosecond radiation (λ = 744 nm, τ ≈ 120 fs, E ≤ 8 mJ) on the surface of Ti of commercially purity in the submicrocrystalline state is investigated using scanning electron microscopy (SEM). Laser irradiation leads to the formation of a surface nanorelief consisting of alternating projections (bars) and hollows (grooves). After surface treatment under a water layer, cube-shaped particles are formed on Ti surface; they are composed of many tightly contiguous rectangular plates.