E. Carpene

Laser nitriding of iron with laser pulses from femtosecond to nanosecond pulse duration

Pulsed-laser nitriding is an attractive method to improve metal surface properties, such as hardness, wear, and corrosion resistance, with the advantage of simple experimental setup, rapid treatment, and precise process control. Here, the dependence of the laser nitriding process on the laser pulse duration was investigated over five orders of magnitude in a series of experiments employing pulsed lasers ranging from nanosecond excimer laser pulses (55 ns) and Nd-doped yttrium aluminum garnet (Nd:YAG) laser pulses (8 ns), to ultrashort Ti:sapphire laser pulses (150 fs). The results revealed that for all laser pulse durations and different wavelengths a large nitriding effect was observed. The excimer laser shows the highest nitriding efficiency. The basic processes for the femtosecond pulsed laser are not well understood but seem to result at least partly from processes within the plasma, whereas nanosecond nitriding is based mainly on processes within the liquid/solid surface.

Thermal stability of laser-produced iron nitrides

Laser nitriding is a very efficient method to improve the mechanical properties, surface hardness, corrosion, and wear resistance of iron and steel, with the advantages of a high nitrogen concentration, fast treatment, and accurate position control, and without any undesired heating effect on the substrate. However, the stability of laser-produced iron nitrides is still under investigation. This article reports investigations of the thermal stability of these iron nitrides upon annealing treatments, which were conducted both in vacuum and air. The phase and elemental composition of the nitride layers were deduced from conversion electron Mossbauer spectroscopy, resonant nuclear reaction analysis, and grazing incidence x-ray diffraction. The surface hardness was measured by the nanoindentation method. In laser-nitrided iron, two critical temperatures are found: at 523 K the predominant iron-nitride phase changes from the γ/e to the γ′ phase. When the temperature exceeds 773 K, all of the nitrogen has escap...

Modeling of nitrogen depth profiles in iron after nitriding with a homogenized laser beam

In this letter we propose a phenomenological model to explain the nitrogen depth profile in iron after laser nitriding. The model is based on the one-dimensional diffusion equation and two sets of functions are use to fit the experimental profiles: complementary error function (erfc) and Gaussian. The different nature of these profiles reflects the presence of two stages in the process: the nitrogen is supplied in the sample as an erfc, while the diffusion to larger depths takes place as Gaussians.

Gold nanoclusters on amorphous carbon synthesized by ion-beam deposition

Gold clusters have been deposited by a monoenergetic, mass-selected ion beam with low energies (20–350eV) on amorphous carbon substrates in order to minimize the influence of the surface crystallinity and the ion-induced structural changes. Gold has been used as a model system, due to the poor reactivity with carbon, to study the ion-energy dependence, the temporal evolution, and the influence of the temperature on the cluster distribution. The cluster size is very sensitive to the energy and the mean size strongly decreases from 4 to less than 1nm as the ion energy increases. We can also note that the size distribution becomes broader. For impact energies below 100eV, surface processes dominate the cluster nucleation and growth. If higher energies are used, an increasing number of ions is implanted below the surface and different processes control the cluster formation. When the energy increases above 350eV, the cluster size drastically drops below 5nm. The samples are analyzed with different methods suc...

Laser–plume dynamics during excimer laser nitriding of iron

On the time scale of tens to hundreds of nanoseconds, high intensity pulsed excimer laser irradiation of iron in nitrogen atmosphere produces thin iron nitride layers with high nitrogen concentration. The laser plasma, or laser plume, which plays a crucial role in the complicated interactions within the laser–plasma–metal system, depends strongly on the ambient nitrogen gas pressure. Its influence was investigated in the nitrogen gas pressure range from 0.05 bar to 10 bar. The nitrogen depth profiles were measured via the nuclear resonance reaction 15N(p,αγ)12C, while the phases formed in the surface layer were analyzed by conversion electron Mossbauer spectroscopy and x-ray diffraction. Utilizing sequentially 15N-enriched and natural nitrogen atmospheres, the evolution of the nitrogen depth profiles during the laser nitriding process was traced. The experimental results suggest that the one-dimensional laser-supported combustion wave model reasonably describes the laser–plume dynamics and the nitriding e...

Laser-induced structural modifications of FeMoCuB metallic glasses before and after transformation into a nanocrystalline state

The effects of laser treatments on the structural and magnetic properties of metallic ribbons have been studied using the melt-spun Fe76Mo8Cu1B15 alloy in as-quenched and nanocrystalline states. 57Fe Mossbauer effect techniques, comprising transmission geometry measurements (TM) and detection of conversion electrons (CEMS), have been employed in addition to magnetization measurements, differential scanning calorimetry and x-ray diffraction. The Curie temperature of the as-quenched alloy was about 70 °C. The distributions of hyperfine magnetic fields as well as quadrupole splitting obtained from TM and CEM spectra have revealed the possibility of observing laser-induced structural modifications even at room temperature when the system is only weakly magnetic. Consequently, both types of hyperfine interactions have been detected and they are nearly in equilibrium (having the same strength or occurring to the same extent). After treatments with a pulsed XeCl excimer laser (with a homogeneous beam of 5×5 mm2, 308 nm, 55 ns, 1 Hz), the significance of magnetic dipole interactions rises as a function of the number of laser pulses (up to 64) and the laser beam fluence (up to 3 J cm-2). No traces of laser-induced crystallization have been found. In the nanocrystalline Fe76Mo8Cu1B15 alloy, surface crystallization was already completely removed after the first pulse of 1 J cm-2.

Hydrogen incorporation in titanium via laser irradiation

We have applied the technique of direct laser synthesis to the hydrogen–titanium system. Large amounts of hydrogen are incorporated into the sample surface by laser irradiating the samples in a hydrogen atmosphere at elevated gas pressures. The process of “laser hydriding” leads to the formation of TiH2 and the amount of incorporated hydrogen was found to be independent of the hydrogen gas pressure. Similarities to the laser nitriding process are briefly discussed and the results are interpreted with the help of thermodynamic simulations of the laser–material interaction.

Investigation of the Thermal Stability of Laser Nitrided Iron and Stainless Steel by Annealing Treatments

Laser nitriding has revealed to be a very promising and effective treatment to improve the technical properties, like surface hardness and corrosion-wear resistance, of iron and steels. The high nitrogen concentration, the fastness and precision of the treatment and the easy experimental setup make this technique very suitable for applications on industrial scale. Samples of pure iron and austenitic stainless steel have been irradiated with ns laser pulses in the UV radiation range and analyzed by means of Conversion Electron Mossbauer Spectroscopy (CEMS), Resonant Nuclear Reaction Analysis (RNRA), Grazing Incidence X-Ray Diffraction (GXRD) and Microhardness. Mossbauer Spectroscopy, in particular, is capable of detecting the phase composition of the nitrided layer and therefore represents an essential tool for these kind of analysis. The thermal stability of the treated samples have been investigated by subsequent annealings at increasing temperatures in vacuum and in air. For iron samples the annealing treatment at 250°C shows a rather drastic phase transformation from γ phase (fcc) into γ′ (Fe4N) while a strong depletion of N has been observed for 400°C or higher, regardless of the ambient pressure (atmospheric or vacuum). On the other hand, the stainless steel shows a very good thermal stability up to 500°C, but higher temperatures induce a gradual decrease in the nitrogen concentration which seems to be a common feature for both pure iron and stainless steel. Furthermore, annealing in air leads to the formation of a thin oxide layer on the surface of the iron sample which is easily characterized by Mossbauer spectroscopy.

Nitrogen and hydrogen depth profiling with MaRPel

Abstract We report on the first measurements carried out with the new accelerator facility MaRPel (Materials Research Pelletron) in Gottingen. Samples of pure tantalum were irradiated with a pulsed nanosecond excimer laser in the UV wavelength range in a nitrogen atmosphere. The laser-nitrided tantalum samples were perfectly suited for testing a low-level set-up for resonant nuclear reactions analysis. The samples also allowed for a recalibration of the mass separator magnet, which was necessary after moving the 3 MV NEC-Pelletron from the Max Planck Institut fur Kernphysik in Heidelberg to the University of Gottingen. In addition, it is shown that the low-level set-up allows high resolution hydrogen depth profiling.

Laser-produced iron nitrides seen by Mössbauer spectroscopy

Mössbauer spectroscopy is a very powerful tool to investigate technological processes performed mainly at the surface of materials. Nitriding of metals and steel is well established in surface engineering, and gas nitriding is used most frequently. Laser nitriding, i.e. the nitrogen take-up from the ambient gas upon irradiation of a steel surface with short laser pulses, is presented in its application to iron, stainless steel and plain carbon steels. It will be demonstrated how Mössbauer spectroscopy in combination with complementary methods (Rutherford backscattering spectroscopy, Resonant nuclear reaction analysis, Nanoindentation) can help to reveal basic mechanisms in these processes.