Ultrafast laser selective phase removal for surface modification of nanocomposite materials.

Modeling of inhomogeneous heating behavior in ultrafast laser interaction with dielectric–dielectric nanocomposite with band gap contrast

In order to investigate the response of materials to extreme conditions, there are several approaches to depositing extremely high concentrations of energy into very small volumes of material, including ultrafast laser and swift heavy ion (SHI) irradiation. In this study, crystalline-to-crystalline phase transformations in cubic Gd2O3 and monoclinic ZrO2 have been investigated using ultrafast laser irradiation. The phases produced by the extreme conditions of irradiation were characterized by grazing incidence x-ray diffraction (GIXRD) and Raman spectroscopy. Gd2O3 exhibited a cubic-to-monoclinic phase transformation, as evidenced by the appearance of the monoclinic (402¯), (003), (310), and (112¯) peaks in the GIXRD pattern and of four Ag and three Bg Raman modes. ZrO2 underwent a monoclinic-to-tetragonal phase transformation, as evidenced by the emergence of the tetragonal (101) peak in the GIXRD pattern and of Eg and A1g Raman modes. The new phases formed by ultrafast laser irradiation are high temperature polymorphs of the two materials. No evidence of amorphization was seen in the GIXRD data, though Raman spectroscopy indicated point defect accumulation. These results are identical to those produced by irradiation with SHIs, which also deposit energy in materials primarily through electronic excitation. The similarity in damage process and material response between ultrafast laser and SHI irradiation suggests a fundamental relationship between these two techniques.

Using molecular-dynamics computer simulation, we study the material processes occurring for fixed laser fluence in a thin metal film induced by ultrafast laser irradiation. The simulation algorithm combines molecular-dynamics for the atom motion supplemented by an analytical solution of time dependence of the homogeneous electron dynamics. Our results show that the laser pulse duration influences virtually all material processes in the irradiated film: melting, build-up of compressive and tensile pressure, spallation and cluster formation. In particular, for the same laser fluence, a reduction of the laser pulse duration leads to more pronounced pressure peaks in the film, which may induce thermomechanical spallation of the sample, and—at higher fluences, where the ablated material forms a vapour cloud—to an increase in the number of large droplets in the vapour cloud. If the laser pulse duration is smaller than the electron–phonon relaxation time in the material, the latter quantity becomes dominant in limiting the material processes.

Ultrafast electron crystallography (UEC), combining the ultrafast time resolution with femtosecond lasers and the atomic spacial resolution with electron crystallography, is developed and applied to elucidate the structural dynamics in solids, surfaces and macromolecular systems. The UEC experiments for surface studies were first demonstrated on semiconductor surfaces. Coherent nonthermal motions of atoms following ultrafast laser irradiation were shown with the timescales in picoseconds, and the amplitude of the motions was determined in picometer. Using Langmuir-Blodgett films, two-dimensional crystalline monolayer, bilayers and multilayers of fatty acids and phospholipids were also studied by UEC. The atomic structures under different preparation conditions were determined. The structural dynamics following a temperature jump induced by femtosecond laser on the substrates were obtained and compared to the equilibrium temperature dependence. It was observed that a coherent anisotropic expansion solely along the aliphatic chains happens at picosecond timescale, followed by nonequilibrium contraction and restructuring at longer times. The effects of different molecules, layer thickness and substrate on the dynamics were examined. Unlike monotonic disordering in the equilibrium heating, a transient structural ordering was revealed on the picosecond timescale.

Molecular dynamics simulation of the production of hollow silver nanoparticles under ultrafast laser irradiation

A comprehensive research is conducted to fabricate laser-induced periodic surface structures (LIPSS) with different morphologies on zinc oxide substrates by ultrafast laser. 1-dimensional or 2-dimensional LIPSS can be created depending on types of laser polarizations.

A novel method of fabricating titanium superhydrophobic surfaces by ultrafast laser irradiation is reported. The ultrafast laser irradiation creates self-organized microstructure superimposed with nano-scale roughness, after which a fluoropolymer coating is applied to lower the surface energy of the textured surface and achieve superhydrophobicity. The focus of this study is to investigate abrasion effects on this mechanically durable superhydrophobic surface. The mechanical durability is analyzed with linear abrasion testing and microscopy imaging. Linear abrasion tests indicate that these surfaces can resist complete microstructure failure up to 200 abrasion cycles and avoid droplet pinning up to ten abrasion cycles at 108.4 kPa applied pressure, which roughly corresponds to moderate to heavy sanding or rubbing in the presence of abrasive particles. The wear mechanisms are also investigated and the primary mechanism for this system is shown to be abrasive wear with fatigue by repeated plowing. Although these results demonstrate an advancement in mechanical durability over the majority of existing superhydrophobic surfaces, it exemplifies the challenge in creating superhydrophobic surfaces with suitable mechanical durability for harsh applications, even when using titanium.

Crystallization behavior of amorphous GST films under an ultrafast laser irradiation

Self-organized micro/nano structures in metal surfaces by ultrafast laser irradiation

Accurate prediction of band gap of materials using stacking machine learning model

Group IVB carbides have been applied in extreme aerospace environments as hard ceramic coatings; ZrC is being considered as a replacement for SiC in nuclear reactors. Therefore, a thorough understanding of the laser irradiation response of group IVB carbides is of clear significance. However, the existing knowledge on the fundamental properties of IVB group carbides is limited and insufficient with regard to both irradiated and non-irradiated characteristics. We investigate the effect of ultrafast laser irradiation on the lattice stability of ceramic materials (IVB group carbides) using the density functional perturbation theory (DFPT). The calculated phonon frequencies of TiC and ZrC at the ground state are in good agreement with previous calculations and experimental values. The phonon frequencies of IVB group carbides are positive, even though the electronic temperature reached 5 eV. Thus, IVB group carbides are more stable under ultrafast laser irradiation, which has greater benefits in nuclear and aeronautical applications compared to metals (W, Na), semimetals (Bi), and semiconductors (Si, SiC). The thermodynamic properties of ZrC are calculated as functions of their lattice temperature at different electronic temperatures. The elastic shear constants of IVB group carbides satisfy the Born stability criteria at Te = 5 eV. In addition, a comparison of the predicted melting temperatures of IVB group carbides, reveal that HfC is better suited for extreme high-temperature environments.

Calculation of the three-body recombination rate is reformulated in a manner in which the velocity contribution is more explicit and the recombination rate is spectrum analysed for electron velocity distributions under ultra-fast laser irradiation. Electron distribution functions are calculated by using a two-dimensional Fokker-Planck simulation. The shape of the recombination rate spectrum with respect to the velocity is found to be well described by vf(v) where f(v) is the pitch-angle-averaged velocity distribution. It is shown visually that the increase in the slow electron population effectively enhances the recombination rate. The effect of a model energy loss on the recombination rate is also discussed.

Ternary zirconia-alumina coatings with different compositional ratios, ranging from pure zirconia to 50% alumina content, were deposited by reactive sputtering from two targets, Zr and Al, in argon-oxygen mixtures. The coating composition was controlled by the Zr/Al target power ratio provided by two pulsed-DC power supplies. The coatings were ~1 µm thick and they were deposited on floating potential substrates at a temperature of 650±3K. XRD indicated that the pure zirconia coatings possessed a monoclinic structure with a grain size of 35-40 nm. Adding alumina to the zirconia coating stabilized the cubic zirconia phase and decreased the grain size to 10-15 nm. The alumina phase in the coatings remained amorphous. The hardness of the nanocomposite structure increased from 11.6±0.5 GPa to 16.1±0.5 GPa for an alumina content of 17%. At higher alumina concentrations, the zirconia phase became amorphous and the hardness decreased to 10-11 GPa. Structure stability of the zirconia-alumina coatings was studied by measuring the coating structure and hardness after annealing at temperatures up to 1173 K. Pure zirconia (m-ZrO2) coatings had low structure stability; the hardness reached a maximum value of 18±1 GPa after annealing at a temperature of 773-873K; however, at higher annealing temperatures the hardness decreased, reaching a minimum value of 12.3±0.6 GPa after annealing at 1173K. The hardness of the nanocomposite ZrO2/Al2O3 coating with various compositions increased with annealing temperature. The hardness of a coating with an alumina content of 17% reached a high value of 19.2±0.5 GPa after annealing at 1073-1173 K. Measurements of post annealing XRD analyses indicated that the stabilization of the coating structure with c-ZrO2/a-Al2O3 phases is the reason for the higher structure stability. From the analyses of phase stability and hardness before and after annealing, we conclude that adding alumina to the zirconia phase promotes the formation of nanocomposite c-ZrO2/a-Al2O3 coatings with a markedly higher stability than single-phase m-ZrO2. Highlights: 1. ZrO2/Al2O3 nanocomposite coatings were deposited by co-sputtering from Zr and Al targets. 2. Adding alumina to the zirconia coating stabilized the cubic zirconia phase. 3. ZrO2-17% Al2O3 coatings had a grain size of 10-15 nm and a hardness of 16.1±0.5 GPa. 4. ZrO2/Al2O3 coatings maintained a high hardness after annealing at 1173K with a high value of 19 GPa for alumina content of 17%. 5. The ZrO2/Al2O3 nanocomposite coatings were crack-free after annealing at 1173K.

It is challenging to selectively remove a thin film with a high melting temperature from an underlying one with a low melting temperature where precise stops at layer interfaces are required. In this study, an ultrashort laser pulse is used to cleanly ablate a 40 nm thin molybdenum layer from an aluminium thin film in a commercially relevant heterostructure. We observe clean delamination of molybdenum from aluminium. We observe that columnar nanostructures in Mo are generated well below the damage threshold fluence. Both delamination and nanostructure formation are considered in terms of electromechanical forces. At low fluence, only selective removal of the very near surface of the Mo thin film was observed. At higher fluences, ablation of the Mo film occurred without melting of the underlying Al layer. Our computational results confirm the important role of thermionic electron emission and near surface electron-phonon coupling in nanostructure generation. We introduce a thermal boundary resistance effect in the heat diffusion by electrons to more accurately describe the release of the Mo from the Al layer. The comparison of experimental and simulation results confirms that electromechanical stresses can be an important mechanism in selective thin film ablation.

Ultrafast laser-induced simultaneous carbonization and texturization of titanium alloy surface and its tribological properties