F. Landry

Influence of the spatial laser intensity distribution on laser nitriding of iron

Laser nitriding of iron and other metals is governed by the complicated interplay of the laser–plasma–solid interactions which lead to a superposition of several mechanisms. This work reports on the drastic influence of the spatial laser intensity distribution on the nitriding process. The effects of the lateral laser intensity on the nitrogen lateral and depth profiles, the phase formation, the surface topology, and the microhardness are revealed by resonant nuclear reaction analysis, Mossbauer spectroscopy, surface profilometry, and nanoindentation. Homogeneous laser beams lead to a strong reduction or almost the absence of the piston mechanism, thus confining the nitriding and the transport processes to the laser spot and avoiding the fallout. The details are discussed in relation to the results obtained for the raw-beam irradiations. Much higher nitrogen saturation concentrations can be achieved with a homogenized beam, but the surface hardness and the hardening depth are lower than in the case of irr...

ORIGIN OF NITROGEN DEPTH PROFILES AFTER LASER NITRIDING OF IRON

In spite of its technological importance, the basic mechanisms of laser nitriding of metals and alloys are hardly understood. The nitrogen depth profiles achieved by laser nitriding of pure iron were measured with high accuracy by resonant nuclear reaction analysis and described by two superimposed diffusion profiles. Using simple estimates, together with the results of marker experiments and laser treatments in 15N-isotopically enriched atmospheres, the development of these profiles with the number of pulses can be simulated in excellent agreement with the experimental results.

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.

Laser nitriding of iron: influence of the spatial laser intensity distribution

Abstract Laser nitriding of metals and alloys has attracted technological interest. Nevertheless, the basic nitriding mechanisms are hardly understood. Here, a detailed analysis of the nitrogen profiles, laterally and in depth, the surface profiles and the microhardness is given in their respective dependence on the spatial laser intensity profile. Irradiation with a homogenised laser beam results in a more homogeneous lateral nitrogen distribution and a smooth surface as compared to irradiation with the inhomogeneous raw beam. Furthermore, the nitrogen saturation concentrations reach over 10 at.% in the case of the homogenised beam, which is significantly higher than the 3–4 at.% found for the raw beam. On the other hand, the hardness and the hardening depth are considerably larger for the raw beam treatment, which is explained by the high amount of the e-nitride phase formed only in this case.

Simulation and deconvolution program WinRNRA for depth profiling of light elements via nuclear resonance reactions

Abstract In this paper, a Windows PC-program is described that performs the transformation of measurement spectra obtained by resonant nuclear reaction analysis (RNRA) into concentration depth profiles. For the shape of the resonance a simple Breit–Wigner form is implemented. For the calculation of the straggling both the Bohr and the Lindhard–Scharff models can be used. The program can be used to evaluate depth profiles of an element in an otherwise homogeneous target. A nanometer depth scale is obtained by considering the concentration-dependent densities of the phases. This is exploited for the investigation of nitrogen profiles via the resonance reaction 15 N ( p , αγ ) 12 C . For demonstration purposes the program is used to determine nitrogen depth profiles of laser nitrided iron.

Correlation of the microhardness with the nitrogen profiles and the phase composition in the surface of laser-nitrided steel

Abstract The effect of the nitrogen take-up upon irradiation of iron or steel with excimer laser pulses in air or nitrogen atmosphere is well established. The resulting nitrogen depth profiles and phase compositions were measured by a combination of Rutherford backscattering spectrometry, resonant nuclear reaction analysis and conversion electron Mossbauer spectroscopy for various laser parameters. The accuracy of the nitrogen profiling via the 15 N(p, xy) 12 C reaction allowed the detection of very low nitrogen concentrations and also the resolving of them laterally across the laser spot. The laser-induced changes in surface topography were observed with optical microscopy and surface profilometry. The microhardness across the laser spot and as a function of the depth is compared with the nitrogen depth and lateral profiles and the phase composition. Thus the mechanical properties can be related to the microstructure and phase composition in the laser modified surfaces of iron and steel. A comparison is made between the effects for irradiation of pure iron and the carbon steel C80.

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.

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.

Laser nitriding investigated with Mössbauer spectroscopy

The effect of the nitrogen take‐up upon irradiation of iron or steel with excimer laser pulses in air or in nitrogen atmosphere is well established. The resulting phase compositions and nitrogen depth profiles were measured by a combination of simultaneous Conversion Electron Mössbauer Spectroscopy (CEMS), Conversion X‐ray Mössbauer Spectroscopy (CXMS), and Resonant Nuclear Reaction Analysis (RNRA) as a function of the nitrogen gas pressure during irradiation. A maximum nitrogen content and a maximum fraction of the ɛ-nitride was found at 0.1 MPa. This result is in accordance with hardness measurements performed by the nanoindentation technique.