Selective laser sintering of amorphous metal powder

For the first time, selective sintering of amorphous PtCuNiP powder with a pulsed Nd:YAG laser has been studied. Upon pulsed interaction, the grains melt only superficially to build necks between the grains. Depending on the laser parameters, the sintered material can be crystallized or retained amorphous. By contrast with crystalline powder, laser sintering of amorphous powder is achieved at substantially lower pulse energies due to its low melting point. The obtained results are compared with previous results from selective laser sintering of titanium powder. PACS 61.43.Dq; 81.20.Ev, 81.05.Rm     

Titanium and aluminum nitride synthesis via layer by layer LA-CVD

The possibility of the layer-by-layer synthesis of 3D parts from nitrides of titanium or aluminum by selective laser sintering/melting is discussed. The relationship between laser processing parameters and structure and phase content of sintered/melted samples are studied by means of optical metallography, X-ray diffraction, scanning electron microscopy and energy dispersive X-ray analysis. Optimal parameters of SLM process for AlN and TiN synthesis are determined. Solid 3D parts containing a TiN phase are produced from Ti powder. Distortion of the crystalline lattice of AlN and TiN phases is observed with the laser energy input.     

Laser/sol–gel synthesis: a novel method for depositing nanostructured TiN coatings in non-vacuum conditions

In this work results of experiments on the in situ production of titanium nitride by the reaction of titania sol–gel with a nitrogenous admixture under laser irradiation are reported. A diode laser beam at different powers and traverse speeds was applied to the mixture placed on EN43 mild steel and 316L stainless steel substrates. Composite coatings of titanium nitride and titanium oxide with a hardness of 17–21 GPa have been achieved by this new method. Surface morphology and microstructure of the deposited coatings and substrate surface layers were examined using optical microscopy, scanning electron microscopy, and field-emission gun scanning electron microscopy. Chemical composition was determined by energy-dispersive X-ray analysis. The phases were identified by X-ray diffraction. Results of microhardness and nanohardness at the top surface were evaluated. PACS 81.15.Fg; 81.20.Fw; 81.05.-t     

Production of amorphous tin oxide thin films and microstructural transformation induced by heat treatment

X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and high resolution transmission electron microscopy were used to study tin oxide thin films deposited on Si(100) substrates at room temperature using pulsed laser deposition techniques with a sintered cassiterite SnO₂ target and subsequently heat-treated. X-ray diffraction and scanning electron microscopy results demonstrated that the as-prepared thin films consisted of an amorphous matrix as well as plume-like features, which are shown many micropores. The thin films that were heat treated for 2 h at 150 °C had tetragonal rutile nanocrystalline SnO₂ structures. The microstructural evolution of the tin oxide thin films during the heat treatment is discussed in the paper. PACS 81.15.Fg; 73.61Jc; 81.05.Gc; 81.40.Ef     

Porosity effect on the electrical conductivity of sintered powder compacts

A new equation for calculating the electrical conductivity of sintered powder compacts is proposed. In this equation, the effective resistivity of porous compacts is a function of the fully dense material conductivity, the porosity of the compact and the tap porosity of the starting powder. The new equation is applicable to powder sintered compacts from zero porosity to tap porosity. A connection between this equation and the percolation conduction theory is stated. The proposed equation has been experimentally validated with sintered compacts of six different metallic powders. Results confirm very good agreement with theoretical predictions. PACS  72.15.Eb; 72.90.+y; 81.05.Rm; 81.20.Ev     

Ultraviolet laser surface treatment for biomedical applications of β titanium alloys: morphological and structural characterization

To improve the surface properties of titanium alloys developed for biomedical applications we have recently suggested a methodology involving laser-assisted nanostructuration. This strategy would benefit from superficial laser heat treatment since laser annealing displays many advantages as compared to the conventional methods: high resolution, high operating speed, low cost and retaining the initial bulk properties. Therefore, this paper reports results concerning the laser treatment of titanium alloys under vacuum. Our interest has been focused on the excimer laser single-pulse irradiation (=248 nm) of a model titanium alloy (Ti6.8Mo4.5Fe1.5Al). The threshold laser fluences corresponding to transus, melting and ablation temperatures as well as the resulting modification depth were first approached theoretically. KrF laser annealings were then carried out in vacuum, varying the fluence conditions from submelting heating to ablation regimes. Subsequent atomic force microscopy and scanning electron microscopy observations were performed to follow the structural and topographical modifications of as-treated specimens and were then discussed as regards the above-mentioned theoretical parameters . PACS 81.05.-t; 68.37.Ps; 68.55.Jk     

Thermal modeling of laser sintering of two-component metal powder on top of sintered layers via multi-line scanning

Laser sintering of a two-component metal powder layer on top of sintered layers, with a moving circular Gaussian laser beam is modeled numerically. The overlap between the adjacent scanning lines, as well as the binding between the newly sintered layer and existing sintered layers underneath through melting, are also considered. The governing equation is formulated by a temperature-transforming model, with partial shrinkage induced by melting taken into account. The liquid flow of the molten low melting point metal powders driving by capillary and gravity forces is formulated by Darcy’s law. The effects of the dominant processing parameters, including the moving laser beam intensity, scanning speed, and the number of the existing sintered layers underneath on the shape of the heat affected zone (HAZ) are investigated. PACS 44.05.+e; 81.20.Ev     

Structural modifications of silicon-implanted GaAs induced by the athermal annealing technique

We have used high-resolution X-ray diffraction and Raman spectroscopy to investigate structural modifications inside and outside the focal region of Si-implanted GaAs samples that have been irradiated at high power by a focused short-pulse laser. Si atoms implanted into the GaAs matrix generate exciton-induced local lattice expansion, resulting in a satellite on the lower-angle side of the Bragg peak. After the laser pulse irradiation, surface features inside and outside the focal spot suggest the presence of Bernard convection cells, indicating that a rapid melting and re-crystallization has taken place. Moreover, the laser irradiation induces a compressive strain inside the focal spot, since the satellite appears on the higher-angle side of the Bragg peak. The stress maximizes at the center of the focal spot and extends far outside the irradiated area (approximately 2.5-mm away from the bulls eye), suggesting the propagation of a laser-induced mechanical wave. The maximum compressive stress inside the focal spot corresponds to 2.7 GPa. Raman spectra inside the focal spot resemble that of pristine GaAs, indicating that rapid melting has introduced significant heterogeneity, with zones of high and low Si concentration. X-ray measurements indicate that areas inside the focal spot and annealed areas outside of the focal spot contain overtones of a minor tetragonal distortion of the lattice, consistent with the observed relaxation of Raman selection rules when compared with the parent zinc-blende structure. PACS 61.72.Vv; 62.50.+p; 71.55.Eq; 79.20.Ds; 81.05.Ea     

Hyperdoping of silicon with deep-level impurities by pulsed YAG laser melting

Hyperdoping with deep-level impurity is a promising method to prepare intermediate band semiconductors. We prepared silicon hyperdoped with deep-level impurities, sulfur and titanium, by ion implantation followed by pulsed YAG laser melting. The processes of sulfur and titanium hyperdoping are comparatively studied. The amorphous sulfur and titanium ion-implanted layers changed to monocrystal by following pulsed laser melting. The depth profile of sulfur impurity after pulsed laser melting is similar to that of ion-implanted sample, while large segregation is observed for titanium hyperdoping. The crystallinity and degree of segregation depend on the laser shot number and initially implanted titanium dose. There is a trade-off between crystallinity and depth profile of impurity for titanium hyperdoping. From a viewpoint material processing, formation of high-quality silicon monocrystal hyperdoped with sulfur is easier than that with titanium. Correlation between the mid-infrared optical absorption and photoconductivity is also discussed for sulfur-hyperdoped sample.     

Laser and plasma nitriding of titanium using CW-CO2 laser in the atmosphere

Surface nitriding of the titanium by the mixing technology with laser and plasma (LPN) in atmosphere has been investigated. Comparing with the technique of laser nitriding, we could obtain the titanium nitride at relatively low laser power density and the oxidation was prevented without the chamber. The synthesized layers comprised of titanium nitrides were about 178 μm depth. The effect of the laser power density, scanning velocity, and plasma flow rate on the components consisting of the material of the nitrided layer was studied. X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) were used to reveal the components consisting of the material of the nitrided layer.     

Microstructural investigation of CaxCo4Sb12 films prepared by pulsed laser deposition

Ca_xCo₄Sb₁₂ skutterudite thin films have been synthesized by pulsed laser deposition from a Nd:YAG laser working at 532 or 355 nm. The influence of deposition temperature, laser fluence and working atmosphere on the structure and morphology of the films has been studied. The characterization has been carried out by X-ray diffraction, atomic force microscopy and scanning electron microscopy. Laser wavelength proved to be the most significant parameter to produce the skutterudite phase. PACS 81.15.Fg; 68.55.Jk; 81.05.Bx     

Modelling of the thermal processes that occur during laser sintering of reacting powder compositions

A two-dimensional mathematical model has been developed to describe simultaneous self-propagating high-temperature synthesis (SHS) and selective laser sintering (SLS) of powder compositions. The models applicability is limited by the magnitude of laser irradiation. It allows a comparison to be made between product geometry and the velocity of the laser irradiation spot. PACS 81.20.ka; 81.05.bx; 81.05.Zx     

Surface treatment of titanium by Nd:YAG laser irradiation in the presence of nitrogen

This work presents the surface treatment of commercial titanium alloy by means of a Nd:YAG (1.064 7m) laser in the presence of nitrogen. The treated surface was characterised by using scanning electron microscopy, atomic force microscopy, X-ray diffraction, secondary ion mass spectrometry, UV-visible spectrophotometry and microindentation. Experimental results show the formation of a nitrogen-rich layer, 500 nm thick, with a surface morphology characterised by the presence of polygonal structures which suggest that melting occurred under the action of the laser. The ' phase of titanium nitride was identified, in addition to a nitrogen in !-titanium solid solution. The reflectance spectrum of the yellow-golden sample is similar to that of deposited titanium nitride thin films. The hardness of the treated surface was measured by microindentation and found to be 14 GPa, a value three times higher than that of the titanium alloy. In scratch tests the nitrided layer detached at a load of 0.9 N.     

Athermal annealing of Mg-implanted GaAs

High-resolution X-ray diffraction scans and electrical resistivity measurements were performed on Mg-implanted GaAs exposed to laser shock annealing on both the implanted and the unimplanted surfaces of the wafer. Measurements on the sample that was subjected to a laser pulse exposure on the implanted side indicate that annealing had occurred within 3 mm of the center of the laser-exposed spot. X-ray scans and the topographs indicate the propagation of a mechanical wave propagating in the radial direction from the laser-exposed spot. No evidence of annealing, however, was observed at any position on the Mg-implanted GaAs sample that was subjected to a laser pulse exposure on the unimplanted side. The Mg implant did not show any redistribution for the laser-shocked annealing, in contrast to the conventional rapid thermal annealing. PACS 61.72.Vv; 62.50.+p; 71.55.Eq; 79.20.Ds; 81.05.Ea     

Laser diffusion nitriding of Ti-6Al-4V for improving hardness and wear resistance

Owing to poor tribological properties, titanium (Ti) alloys are usually surface-treated to enhance their surface properties. Laser surface nitriding, among others, is a common method employed to increase hardness and wear resistance for Ti alloys. Conventional laser nitriding involves surface melting of Ti alloys in a nitrogen atmosphere. This inevitably results in a roughened surface and post-treatment might be required. The present study aims at laser diffusion nitriding Ti alloys without surface melting via carefully selecting the laser processing parameters. The nitrided surface was characterized by X-ray diffractometry (XRD), optical microscopy (OM), scanning-electron microscopy (SEM), and profilometry. The nitride layer formed was about 1.62 μm upon repeated passes. The change in surface roughness resulting from laser diffusion nitriding was only minimal. Nanoindentation measurements revealed that the hardness of the nitride layer was around 11.3 GPa, being about 2.3 times that of Ti-6Al-4V. Ball-on-slab sliding wear test recorded a reduction in wear volume by about 8 times. The results of the present work thus demonstrate the feasibility of diffusion nitriding of Ti-6Al-4V by laser treatment for enhancing its surface properties and performance.     

Growth of normally-immiscible materials (NIMs), binary alloys, and metallic fibers by hyperbaric laser chemical vapor deposition

This work demonstrates that two or more elements of negligible solubility (and no known phase diagram) can be co-deposited in fiber form by hyperbaric-pressure laser chemical vapor deposition (HP-LCVD). For the first time, Hg-W alloys were grown as fibers from mixtures of tungsten hexafluoride, mercury vapor, and hydrogen. This new class of materials is termed normally-immiscible materials (NIMs), and includes not only immiscible materials, but also those elemental combinations that have liquid states at exclusive temperatures. This work also demonstrates that a wide variety of other binary and ternary alloys, intermetallics, and mixtures can be grown as fibers, e.g. silicon-tungsten, aluminum-silicon, boron-carbon-silicon, and titanium-carbon-nitride. In addition, pure metallic fibers of aluminum, titanium, and tungsten were deposited, demonstrating that materials of high thermal conductivity can indeed be grown in three-dimensions, provided sufficient vapor pressures are employed. A wide variety of fiber properties and microstructures resulted depending on process conditions; for example, single crystals, fine-grained alloys, and glassy metals could be deposited. PACS 81.15.Fg; 81.05.Bx; 81.05.Je; 81.15.Gh     

X-ray, scanning electron microscopy and electrical properties of synthetic fresnoite (Ba2TiSi2O8) ceramics

Polycrystalline ceramic samples of fresnoite (Ba₂TiSi₂O₈ or BTS) have been prepared by a standard solid-state reaction method using high-purity oxides and carbonates. For one set of compounds, in stoichiometric ratio BaCO₃, TiO₂ and SiO₂ were melted at 1300°C and then sintered into pellet form, whereas the other set of compounds have been prepared without melting and sintered into pellet form at 1250°C. The formation of the single-phase compound and its structural parameters were investigated by X-ray diffraction followed by Rietveld refinement and scanning electron microscopic (SEM) techniques. A better agreement between the observed and calculated X-ray diffraction patterns was obtained by performing the Rietveld refinement with a structural model using the non-centrosymmetric space group P4bm. A better agreement between observed and calculated d-values shows that the lattice parameters calculated using the Rietveld refinement analysis are better than that of the earlier report so far. The activation energies of both compounds were calculated by measuring its dc electrical conductivities. The results are discussed in detail.     

Morphology and oxidation of Zr-based amorphous alloy ablated by femtosecond laser pulses

Femtosecond laser ablation of an amorphous alloy in air, including single-pulse ablation, multi-pulse drilling and trenching has been investigated. Laser-induced ablation and related effects were examined by means of scanning electron microscopy, energy dispersive X-ray analysis, transmission electron microscopy and electron diffraction. Oxidation was observed in the multi-pulse ablation region surface. With selected parameters, molten trace, spatter and crystallization can be avoided in the vicinity of the machining area. The results show that femtosecond laser ablation with selected parameters is a promising method for micromachining amorphous alloys. PACS 79.20.Ds; 87.80.Mj; 81.05.Bx; 81.65.Mq; 61.82.Bg     

Synthesis and characterizations of hollow spheres and nanospheres of Au

Hollow spheres and nanospheres of Au have been prepared by a simple reaction of HAuCl₄·4H₂O, NaOH and (NH₂OH)₂·H₂SO₄ in the presence of gelatin. The role of gelatin and the effect of the temperature of the reaction in producing the spherical particles of Au are discussed. The products were characterized by powder X-ray diffraction (XRD), transmission electron microscopy and UV–Vis absorption spectroscopy. The sizes of the nanospheres of Au were estimated by the Debye–Scherrer formula according to the XRD spectrum. PACS  81.05.Bx; 81.05.Rm; 81.07.-b; 81.16.Be; 81.20.Fw     

Laser sintering of silver nanoparticle thin films: microstructure and optical properties

We present a method for the sintering of silver (Ag) nanoparticle thin films by millisecond pulsed laser irradiation. The microstructure of sintered thin films and sintering behaviors of nanoparticles were systematically investigated in this paper. Absorption spectra of sintered thin films showed blue-shifted surface plasmon resonances (SPR) from 500 nm to 480 nm and red-shifted from 480 nm to 550 nm when laser power was varied from 100 W to 140 W and from 140 W to 200 W, respectively. This indicates a new technique to control light absorption through joining nanoparticles with laser sintering. According to theoretical calculations based on a heat diffusion model, the melting temperature of these Ag nanoparticles was estimated to be 440 °C during laser irradiation.