Atomic mixing of Ni2O3/SiO2, NiO/SiO2, and Ni/SiO2 interfaces induced by swift heavy ion irradiation

We have investigated the interface mixing of Ni₂O₃/SiO₂, NiO/SiO₂, and Ni/SiO₂ induced by the irradiation with Ar, Kr and Xe ions of energies ranging from 90 MeV to 260 MeV. Since these energies are in the electronic stopping regime, atomic transport processes will not be directly initiated by elastic ion–target collisions, but need to be excited by secondary processes like electron–phonon coupling or Coulomb explosion. Nevertheless, we have observed a strong mixing effect in the ceramic systems if the electronic energy loss exceeds a certain threshold value. Estimation of an effective diffusion constant indicates that diffusion takes place in the molten ion track. In contrast to the ceramics, the metallic Ni layer is still insensitive even for the highest electronic stopping power used (S_e=28 keV/nm) and does not exhibit mixing with its SiO₂ substrate. In addition, NiO/SiO₂ and Ni/SiO₂ were irradiated in the nuclear stopping regime with 600 keV Kr and 900 keV Xe–ions. Here the intermixing effect is in good agreement with the assumption of ballistic atomic transport. Received: 5 February 2002 / Accepted: 11 February 2002 / Published online: 3 May 2002 RID="*" ID="*"Corresponding author. Fax: +49-711/685-3866, E-mail: bolse@ifs.physik.uni-stuttgart.de     

Precision laser ablation of dielectrics in the 10-fs regime

Received: 4 January 1999 / Accepted: 5 January 1999     

Femtosecond ablation of ultrahard materials

Several ultrahard materials and coatings of definite interest for tribological applications were tested with respect to their response when irradiated with fs laser pulses. Results on cemented tungsten carbide and on titanium carbonitride are reported for the first time and compared with outcomes of investigations on diamond and titanium nitride. The experiments were carried out in air, in a regime of 5–8 J/cm² fluences, using the beam of a commercial Ti:sapphire laser. The changes induced in the surface morphology were analysed with a Nomarski optical microscope, and with SEM and AFM techniques. From the experimental data and from the calculated incident energy density distributions, the damage and ablation threshold values were determined. As expected, the diamond showed the highest threshold, while the cemented tungsten carbide exhibited typical values for metallic surfaces. The ablation rates determined (under the above-mentioned experimental conditions) were in the range 0.1–0.2 μm per pulse for all the materials investigated. Received: 31 August 2001 / Accepted: 3 December 2001 / Published online: 20 March 2002     

Influence of adsorbates on UV-pulsed laser melting of Si in ultra-high vacuum

The dynamics of laser melting of atomically clean Si is investigated in ultra-high-vacuum (UHV) by transient reflectivity with single-pulse sensitivity in the presence of monitored amounts of chlorine, oxygen or propene. Adsorption of one monolayer (1 ML) leads to measurable variations of the melting dynamics, which are strongly adsorbate-dependent. The variations differ qualitatively and quantitatively from those observed with heavy exposures to gases. The melting dynamics returns to that of clean Si upon subsequent irradiation by laser pulses without readsorption. The required number of pulses for return to clean Si dynamics depends strongly on the type of adsorbate. Adsorbate-induced changes of absorption and reflectivity, and/or incorporation of adsorbates into the substrate, do not explain the results. By contrast, the variations of the melting dynamics are correlated to the photoemitted electron yield, suggesting that laser melting is sensitive to the presence of electrons in the conduction band. These results show that accurate modelling of laser melting of Si interacting with gases should take into account the presence of the gases. Received: 12 September 2000 / Accepted: 9 January 2001 / Published online: 27 June 2001     

Structural evolution and metastable phase formation in Ni-W multilayers upon ion irradiation

A crystal-to-amorphous structural transition was induced in the Ni₂₅W₇₅ and Ni₃₅W₆₅ multilayers by ion irradiation at room temperature. More interestingly, prior to complete amorphization, a sequential disordering of first Ni and then W crystalline lattices was observed in the Ni₂₅W₇₅ sample with increasing ion dose. Such sequence in disordering is attributed to the difference in melting points between the two constituent metals. In another two multilayered samples with overall compositions of Ni₆₀W₄₀ and Ni₇₈Nb₂₂, ion irradiation under similar conditions resulted in the formation of two Ni-based fcc solid solutions, respectively. In comparison, the same Ni-based fcc solid solution was formed in the Ni₃₅W₆₅ multilayered sample upon solid-state reaction at 500 °C. Solid-state reaction at 550 °C resulted in the formation of a new W-rich metastable hcp phase in the Ni₂₅W₇₅ multilayered sample and the bcc–hcp transition was thought to be realized through a shearing mechanism. A Gibbs free-energy diagram, including the free-energy curves of the newly formed metastable crystalline phases, of the Ni-W system was calculated based on Miedema’s model and it can give a reasonable explanation of the observed sequential disordering. The calculated results also showed that the free-energy difference between the amorphous and metastable crystalline phases was quite small, leading to a situation that the phase selection, namely which phase was more favored to be formed eventually than its competitors, was influenced or even determined by the kinetics involved in the respective processes. Besides, the growth kinetics of the MX phases was also discussed. Received: 26 January 1999 / Accepted: 8 March 1999 / Published online: 14 June 1999     

Stress corrosion cracking behavior of Nd:YAG laser-treated aluminum alloy 7075

Nd-YAG laser surface treatment was conducted on 7075-T651 aluminum alloy with the aim of improving the stress corrosion cracking resistance of the alloy. Laser surface treatment was performed under two different gas environments, air and nitrogen. After the laser treatment, coarse constituent particles were removed and fine cellular/dendritic structures had formed. In addition, for the N₂-treated specimen, an AlN phase was detected. The results of the stress corrosion test showed that after 30 days of immersion, the untreated specimen had been severely attacked by corrosion, with intergranular cracks having formed along the planar grain boundaries of the specimen. For the air-treated specimen, some relatively long stress corrosion cracks and a small number of relatively large corrosion pits were found. The cracks mainly followed the interdendritic boundaries; the fusion boundary was found to be acting as an arrestor to corrosion attacks. In contrast, only few short stress corrosion cracks appeared in the N₂-treated specimen, indicating an improvement in corrosion initiation resistance. The superior corrosion resistance was attributed to the formation of the AlN phase in the surface of the laser-melted layer, which is an electrical insulator. The electrochemical impedance measurements taken during the stress corrosion test showed that the film resistance of the laser-treated specimens was always higher than that of the untreated specimen, with the N₂-treated specimen showing the highest resistance.     

Stochastic effects on dislocation structure development under cascade-damage irradiation

Received: 28 January 1998/Accepted: 24 March 1998     

Incubation of laser ablation in fused silica with 5-fs pulses

The threshold fluences for laser-induced damage of fused silica with single 5-fs pulses from a Ti:sapphire laser system were determined by extrapolating the ablated volume to zero. These thresholds are about 4 times as high as the values previously obtained from multi-shot experiments. This result is interpreted in terms of an irreversible modification of the original material below the single-shot threshold (incubation). Received: 14 June 1999 / Accepted: 24 June 1999 / Published online: 8 September 1999     

Micromachining of complex channel systems in 3D quartz substrates using Q-switched Nd:YAG laser

A novel microchannel fabrication technology for quartz using a Q-switched Nd:YAG laser is presented. Complex 3D channel systems inside quartz substrates can be constructed directly using a laser beam by controlled fracturing, and high-quality microchannels can be fabricated by melting quartz using a laser-induced plasma. The behavior of laser-induced plasmas in drilling microchannels is discussed. The diameter of the microchannels can be controlled from 25 to 200 μm. The average roughness of the interior channel wall is less than 0.2 μm. Currently, microchannels longer than 4 mm in fused-quartz cubes can be achieved using laser-induced plasmas. Received: 15 January 2001 / Accepted: 5 June 2001 / Published online: 30 October 2001     

Experimental and numerical study of surface alloying by femtosecond laser radiation

Here we report on experimental studies of femtosecond laser induced surface metal alloying. We demonstrate that layers of different metals can be mixed in a certain range of laser pulse energies. Numeric simulations demonstrate that the sub-surface melting and mixing is advantaged through the difference in the electron-phonon coupling constants of the metals in the multi-layer system. Dependence of the depth of the mixed layer on the number of laser pulses per unit area is studied. Numeric simulations illustrate physical picture of the laser alloying process.     

Comparative study of microstructural characteristics of electrospark and Nd:YAG laser epitaxially growing coatings

As low-heat input welding processes, electrospark deposition and pulsed Nd:YAG laser cladding can be commonly used to prepare epitaxially growing coatings. However, these two processes have quite different characteristics in the energy input, the amount of materials involved, and the temperature gradient, and hence might result in dissimilar microstructural characteristics. In this paper, a comparative study has been made between microstructural characteristics in epitaxial growth coatings prepared by electrospark deposition and pulsed Nd:YAG laser cladding. Some interesting results have been achieved. Firstly, epitaxial growth coatings can be commonly achieved by these two techniques. Secondly, microstructural morphologies of these two epitaxial growth coatings are obviously different, cellular columnar structure prevails in the electrospark coating while columnar dendritic structure occupies most of the laser coating thickness, more importantly, electrospark coating remains fully columnar in the whole layer whereas laser coating tends to change from columnar to equiaxed at the top of the layer. Thirdly, electrospark coating possesses finer and more homogeneous microstructure than laser coating.     

Heat diffusion and debris screening in Er:YAG laser ablation of hard biological tissues

Received: 25 March 1997/Revised version: 15 October 1997     

Observation of negative alkali ions from alkali halides during 248-nm laser irradiation

Below laser fluences where a plasma is formed (the so-called plasma or plume formation threshold) a number of fundamental phenomena can occur where particles such as atomic and molecular ions, atoms and molecular neutrals, and electrons can be emitted. An understanding of such processes is necessary to develop predictive models for material removal from laser irradiated surfaces—at the foundation of laser etching, machining, and pulsed laser deposition. We have reported on a number of the mechanisms for such emission processes. Here, due to space limitations, we present a summary of our studies on the formation of negative alkali ions from single crystal KCl during exposure to pulsed 248-nm radiation at fluences well below the threshold for plasma formation. Despite the high electron affinities of the corresponding halogen atoms, negative halogen ions were not detected. Significantly, the positive and negative alkali ion distributions overlap strongly in time and space, consistent with K formation by the sequential attachment of two electrons to K⁺. Negative alkali ions are also observed under comparable conditions from LiF, NaCl, and KBr. In each material, the strong overlap between the positive and negative alkali ion distributions, and the lack of detected negative halogen ions, suggest that negative ion formation involves a similar mechanism.     

Surface modification of Ti dental implants by Nd:YVO4 laser irradiation

Surface modifications have been applied in endosteal bone devices in order to improve the osseointegration through direct contact between neoformed bone and the implant without an intervening soft tissue layer. Surface characteristics of titanium implants have been modified by addictive methods, such as metallic titanium, titanium oxide and hydroxyapatite powder plasma spray, as well as by subtractive methods, such as acid etching, acid etching associated with sandblasting by either AlO₂ or TiO₂, and recently by laser ablation. Surface modification for dental and medical implants can be obtained by using laser irradiation technique where its parameters like repetition rate, pulse energy, scanning speed and fluency must be taken into accounting to the appropriate surface topography. Surfaces of commercially pure Ti (cpTi) were modified by laser Nd:YVO₄ in nine different parameters configurations, all under normal atmosphere. The samples were characterized by SEM and XRD refined by Rietveld method. The crystalline phases αTi, βTi, Ti₆O, Ti₃O and TiO were formed by the melting and fast cooling processes during irradiation. The resulting phases on the irradiated surface were correlated with the laser beam parameters. The aim of the present work was to control titanium oxides formations in order to improve implants osseointegration by using a laser irradiation technique which is of great importance to biomaterial devices due to being a clean and reproducible process.     

Nanofoaming dynamics in biopolymers by femtosecond laser irradiation

We have recently shown that irradiation of self-standing films of the biopolymers collagen and gelatine with single femtosecond laser pulses produces a nanofoaming layer with regular bubble size which can be controlled by wavelength selection. Following these initial studies, here we report on the temporal evolution of the foaming effect by measurements in situ and in real time of the change in the transmittance of a cw probe HeNe laser through the irradiated films. Self standing films of the biopolymers were irradiated with 90 fs laser pulses at 800, 400, and 266 nm. For fluences below and above the modification threshold a permanent attenuation of the transmission occurs (increasing with fluence). The initial decay of the transmission is fast (around few tens of ns), and is followed by dynamics in the longer timescale (micro and milliseconds). The temporal evolution of the transmission measured upon fs laser irradiation is similar with that determined in the irradiation of the biopolymer films at 248 nm with 25 ns laser pulses. The method allows separating in time the different processes occurring after irradiation that lead to a permanent nanofoaming structure, while the results allow us to understand the mechanisms of femtosecond laser processing of the biopolymers and their interest in biomedical applications.     

Comparative laser study of Nd:KGW and Nd:YAG near 1.3 μm

A comparative study of Nd:KGW and Nd:YAG laser crystals pumped by flashlamp has been conducted near 1.3 μm with output energy up to 1 J and at a repetition rate up to 50 Hz. An average power of 23 W for KGW in free-running mode was achieved with a total efficiency better than 2.8 % for the Nd:KGW and 1.8 % for the Nd:YAG. Received: 9 December 1996 / Revised version: 10 February 1997     

Different alloying behaviors of Co-Mo multilayered films upon ion irradiation and thermal annealing

The Co-Mo system is characterized by a small negative heat of formation being −7 kJ/mol, which provides a small driving force for alloying. In the case of the initial multilayered films with closest-packed semi-coherent interfaces, solid-state interfacial reaction is frustrated upon thermal annealing even at a temperature up to 600 °C. However, amorphization through interfacial reaction was achieved upon ion irradiation as the intensive ion irradiation can elevate the initial energetic state by the irradiation energy as well as destroy the semi-coherent interfaces and thus drive atomic mixing in the films eventually becoming disordered. Received: 7 June 1999 / Accepted: 1 November 1999 / Published online: 8 March 2000     

Growth and spectroscopic properties of RE, Mn:YAP (RE=Yb and Ce) photorefractive crystals

RE, Mn:YAP (RE=Yb and Ce) crystals with dimension of Φ 25×60 mm were successfully grown by the Czochralski method. The spectroscopic properties of RE, Mn:YAP (RE=Yb and Ce) crystals before and after γ-irradiation were investigated at room temperature. The results show that the content of Mn⁴⁺ ions was increased with the Yb³⁺ ions co-doping, but decreased by Ce³⁺ ions co-doping. Thermoluminescence (TL) spectra of the crystals indicate three steps of recombination, and the probable recombination processes were discussed.