Diffusion Coefficient Of Nitrogen Research Articles

Nitrogen Diffusion and Stresses during Expanded Austenite Formation in Nitriding

Low-temperature nitriding of austenitic stainless steels or chromium containing alloys can produce expanded austenite, known as S-phase, with combined improvement in wear and corrosion resistance. In the paper a critical review of various models for nitrogen diffusion during nitriding is presented. A special attention is paid to the expanded austenite growth. A new model based on bi-velocity method and including stresses is presented. Basic equations and boundary conditions are discussed. Composition dependent nitrogen diffusion coefficient is assumed. Numerical solutions are obtained for the growth of the S-phase layer in steel. The results are compared with previous experiment and calculations.

The Modeling of Nitrogen Mass Transport in CoCr Alloys

Abstract The kinetics of plasma nitriding of CoCr alloy below temperatures of nitrides formation and mechanisms of nitrogen penetration are analyzed by proposed kinetic modeling in this article. Proposed nitrogen diffusion model is based on the trapping – detrapping (TD) model and developed taking into account the effect of the concentration dependent diffusivity of nitrogen, nitrogen adsorption on the surface of alloy and surface swelling process. The model indicates the influence of chromium atoms to nitrogen atoms diffusivity. The model consists of time and depth dependent diffusion, which is described by a partial differential equation, and it is solved by using Crank – Nicolson finite difference method. By fitting of experimental nitrogen depth profiles, it is shown that nitrogen diffusion coefficient varies with nitrogen concentration according to Einstein-Smoluchowski relation. Nitrogen depth profiles in plasma nitrided medical grade CoCr alloy (ISO 5831 – 12) at T = 400 °C for 1, 4 and 20 hours calculated on the basis of this model are in good agreement with experimental nitrogen profiles. Furthermore, the swelling process is showed and analyzed, derived the dependency of swelling rate on nitriding duration – the swelling rate is inversely proportional to the square root of nitriding duration. The obtained diffusion coefficient value and the swelling process rates satisfy the experimental data form Ref. The derived model explains physical processes during plasma nitriding and allows obtaining nitrogen depth profiles for any requisite nitriding duration.

Study on the diffusion coefficients for ammonia nitrogen and nitrite and nitrate in PVA gels.

In order to quantify the proliferation of polyvinyl alcohol (PVA) gels in a matrix and optimize the performance of mass transfer, activated carbon (AC) and CaCO3 were selected as adding materials in this experiment. For the performance of mass transfer, the optimal conditions were analyzed using response surface method (RSM) considering the inter-correlated effects of the amount of AC and CaCO3. For RSM, 13 trials resulted in a partial cubic polynomial equation, which best predicted the amount of residual debris after homogenization. The results of the study show that the effective diffusion coefficient test device can analysis the diffusion rate nitrogen, nitrite and nitrate within the PVA gels quantitatively; adding appropriate amounts of AC and CaCO3 in the biological active filter can improve the performance of mass transfer effectively; the maximum effective diffusion coefficient of nitrogen and nitrite and nitrate in the packing were 1.3637 × 10-9 and 1.0850 × 10-9 and 1.0199 × 10-9 m2/s, respectively, at optimal addition amount.

Kinetics and enhancement mechanism of plasma oxynitriding for AISI 1045 steel

Abstract Plasma oxynitriding (PON) with air addition of 0.2 L/min was carried out at temperatures ranging from 783 K to 843 K and holding time ranging from 30 min to 240 min in the plasma nitriding furnace for AISI 1045 steel, and plasma nitriding (PN) without air addition was also conducted for comparison. The microstructure, surface morphology and phase compositions of compound layers were investigated by means of optical microscopy, SEM and X-ray diffraction, and then the kinetics for forming compound layer was analyzed. The results show that air addition can promote the diffusion of active nitrogen atoms toward the substrate, thus enhance the nitriding efficiency. In other words, plasma oxynitriding can shorten the holding time and decrease the treating temperature to get the expected compound layer thickness comparing with normal plasma nitriding (PN) due to a loose oxide layer formed on the outmost surface. Kinetics analysis found that the diffusion coefficient of nitrogen in PON is much higher, and the activation energy for nitrogen atoms diffusion is reduced from 227.25 kJ/mol to 138.57 kJ/mol in PON. Kinetics of PON with air addition of 0.2 L/min at temperatures ranging from 783 K to 843 K was obtained as the following formula: d = 4.16 × 10 − 3 exp − 1666.67 / T t . Finally, kinetics verification results confirmed that the obtained kinetics is in good agreement with the tested results; therefore, it can be used in real applications to predict the compound layer thickness for given process parameters of temperature and time; or to determine the parameters of temperature and time for a designed compound layer thickness.

Prediction of Phase Composition and Nitrogen Concentration During the Nitriding Process in Low-Alloy Steel

A diffusion/transformation coupled model has been developed which combines finite difference (FD) model with a phenomenological model. The composition of the different iron-nitrogen(Fe-N) hardening phase can be regard as a function of nitriding time and nitrogen concentration. The diffusion model and transformation model are linked by the limiting nitrogen solubilities and the effective diffusion coefficients. The effect of alloy elements (Cr, Mo, Mn, V, Ni etc.) is considered by introducing an alloy coefficient for limiting nitrogen solubilities and diffusion coefficient. The diffusion/transformation model can predict nitrogen concentration, phase composition and hardness distribution. The model is employed to simulate the nitriding process of SCr420H low-alloy steels. The simulated nitrogen concentration and hardness profiles are consistent with the measured ones. In addition, the predicted depth distributions of iron-nitrogen phase agree well with the available experimental results. Therefore, the comparison shows the reliability of the coupled model. It can be applied to improve the nitriding process parameters.

Modelling the evolution of composition-and stress-depth profiles in austenitic stainless steels during low-temperature nitriding

Nitriding of stainless steel causes a surface zone of expanded austenite, which improves the wear resistance of the stainless steel while preserving the stainless behaviour. During nitriding huge residual stresses are introduced in the treated zone, arising from the volume expansion that accompanies the dissolution of high nitrogen contents in expanded austenite.An intriguing phenomenon during low-temperature nitriding is that the residual stresses evoked by dissolution of nitrogen in the solid state, affect the thermodynamics and the diffusion kinetics of nitrogen dissolution. In the present paper solid mechanics was combined with thermodynamics and diffusion kinetics to simulate the evolution of composition-depth and stress-depth profiles resulting from nitriding. The model takes into account a composition-dependent diffusion coefficient of nitrogen in expanded austenite, short range ordering (trapping) of nitrogen atoms by chromium atoms, and the effect of composition-induced stress on surface concentration and diffusive flux. The effect of plasticity and concentration-dependence of the yield stress was also included.

Kinetic analysis of direct current field enhanced salt bath nitriding

A novel rapid salt bath nitriding technology based on normal salt bath nitriding was primarily developed by additionally applying a direct current field (DCF) of 7·5 V between the treated samples, and the corresponding kinetics was investigated. The results showed that DCF could significantly enhance the nitriding efficiency, shorten the holding duration and decrease the treatment temperature. Meanwhile, the diffusion coefficient of nitrogen was increased about two times, and the activation energy was decreased from 184 to 159 kJ mol− 1. The main enhancement mechanism was discussed as well.

Internal stress assisted nitrogen diffusion in plasma nitrided medical CoCr alloys

The role of internal stress on nitrogen diffusion in medical CoCr alloys during plasma assisted nitriding at low temperature is analyzed by stress assisted diffusion model. The nitrogen diffusion model is based on the concept of the interaction between the compositionally induced internal stress and interstitial nitrogen. From a theoretical point of view, this is mean that the driving force for nitrogen diffusion in CoCr alloys is not only the concentration gradient, but also the gradient of the compositionally induced internal stress. This model was used to predict nitrogen behavior during coupled mass diffusion with stress in plasma nitrided CoCr ASTM F75 and L605 alloys. The calculated nitrogen depth profiles agree very well with experimentally observed ones. Good agreement between simulated and experimental results shows that the stress assisted model can describe the phenomenon of an anomalous diffusion of nitrogen in CoCr alloys. The concentration dependent diffusion coefficients of nitrogen in expanded austenite were determined by fitting the nitrogen experimental profiles in different CoCr alloys (ASTM F75 and L605).

Effect of D.C. electric field on salt bath nitriding for 35 steel and kinetics analysis

A rapid salt bath nitriding technology was primarily developed by additionally applying direct current (D.C.) electric field on the basis of traditional technique (NM). Characterization of the modified surface layers was made by means of optical microscopy, Vickers micro-hardness test and X-ray diffraction analysis. The results showed that D.C. electric field could significantly enhance the nitriding efficiency and the enhancement effect was closely related to the intensity of D.C. electric field. By applying D.C electric field of 7.5V, even a little thicker compound layer could be obtained at only half duration of that in traditional technique, and the thickness of compound layer increased more than 60%, from 18μm up to 29μm at the same treating temperature of 848K and holding duration of 100min. Meanwhile, higher surface hardness, modestly higher sub-surface hardness and superior hardness profile were obtained assisted by D.C. electric field. It was also found that the diffusion coefficient of nitrogen was increased more than 1.9 times and activation energy was decreased from 184kJ/mol to 159kJ/mol enhanced by D.C. electric field. The possible enhancement mechanism is that D.C. electric field can promote chemical reactions and produce more active nitrogen atoms in the salt bath, positively charge the active atoms and force them diffuse directionally toward the surface of the treated specimen, and hence significantly improve the efficiency of the active atoms in the salt bath.

Nitrogen Diffusion in Niobium under Low Pressure

The interaction among heavy interstitial atoms present in metals with bcc structure is studied using anelastic spectroscopy. This technique makes it possible to obtain information on interstitial concentration, precipitation, solubility limit, and diffusion. The diffusion coefficients of nitrogen in niobium were obtained using the relaxation parameters obtained from anelastic spectroscopy measurements for different oscillation frequencies of the system. The results showed the interstitial diffusion of nitrogen present in solid solution in niobium when submitted to different charges of nitrogen at a temperature of 1373 K and a partial pressure in the order of 10-4Torr. The exponential variation of the pressure experimentally in function of the time was thus obtained.

Low pressure line shape study of nitrogen-perturbed acetylene transitions in the ν1+ ν3band over a range of temperatures

Six nitrogen-perturbed transitions of acetylene within the ν1+ ν3absorption band have been recorded using a three-channel diode laser spectrometer. The goal being an improved understanding of both broadening and narrowing effects of low-pressure acetylene under the influence of nitrogen gas. To this end we have examined C2H2spectra using a hard collision Rautian profile over a range of five temperatures (213–333 K) and five pressures (5–40 Torr). From these fits we have obtained the N2-broadening and narrowing coefficients of C2H2and examined their temperature dependence. The experimentally measured narrowing coefficients have been used to estimate the nitrogen diffusion coefficients (D12) and are presented within. The broadening coefficients and corresponding temperature dependence exponents have also been compared to that of calculations completed using a classical impact approach on an ab initio potential energy surface. We have observed good agreement between our theoretical and experimental results.

Effect of nitriding treatment on corrosion behaviour of Co–Cr–Mo alloy in liquid Al

Effects of nitriding treatment on the corrosion behaviour of Co–29Cr–6Mo alloy in liquid Al were investigated. Because of the high solubility and high diffusion coefficient of nitrogen in Co, Cr2N and π phases formed from sample surface into alloy matrix during nitriding at 1273K; however, the nitride layer was not continuously formed on sample surface even in a prolonged process. The Cr2N showed obviously higher resistance to liquid Al compared to alloy matrix, but owing to its discontinuous distribution, the formed Cr2N did not obviously improve the corrosion resistance of the alloy to liquid Al.

Sorption and diffusion of carbon dioxide and nitrogen in poly(methyl methacrylate)

Molecular dynamics simulations are performed to determine the solubility and diffusion coefficient of carbon dioxide and nitrogen in poly(methyl methacrylate) (PMMA). The solubilities of CO2 in the polymer are calculated employing our grand canonical ensemble simulation method, fixing the target excess chemical potential of CO2 in the polymer and varying the number of CO2 molecules in the polymer matrix till establishing equilibrium. It is shown that the calculated sorption isotherms of CO2 in PMMA, employing this method well agrees with experiment. Our results on the diffusion coefficients of CO2 and N2 in PMMA are shown to obey a common hopping mechanism. It is shown that the higher solubility of CO2 than that of N2 is a consequence of more attractive interactions between the carbonyl group of polymer and the sorbent. While the residence time of CO2 beside the carbonyl group of polymer is about three times higher than that of N2, the diffusion coefficient of CO2 in PMMA is higher than that of N2. The higher diffusion coefficient of CO2, compared to N2, in PMMA is shown to be due to the higher (≈3 times) swelling of polymer upon CO2 uptake.

Effect of Gas Composition on Nitriding and Wear Behavior of Nitrided Titanium Alloy Ti-15V-3Cr-3Al-3Sn

Titanium alloy, Ti-15V-3Cr-3Al-3Sn was nitrided at different temperatures with low pressure plasma with 100% nitrogen, and nitrogen diluted with hydrogen and argon. The nitrided layers were characterized for hardness, structure, and composition. Nitrided samples show weight gain that depended on temperature and duration of nitriding. EDS results show that intake of nitrogen is significant at temperatures above 750 °C. Hydrogen dilution increases intake of nitrogen. Samples nitrided with hydrogen dilution have lower surface roughness and higher nitrogen concentration. Depth profiling by XPS shows the formation of nitride in the near-surface region and also that nitrogen concentration in the interior of the nitrided layers is higher at higher temperatures. Micro Raman shows that formation of nitride takes place at higher temperatures. XRD shows that the nitrided layers consist predominantly of alpha Ti and Ti2N. This is reflected in the hardness increase and hardness profile in the nitrided samples. The low intake of nitrogen by the alloy is attributed to the low solubility of nitrogen in beta alloy and low diffusion coefficient of nitrogen. Reciprocating wear studies showed a lower coefficient of friction and lower wear loss for nitrided samples compared to that of substrate.

Nitriding behaviour of maraging steel: experiments and modelling

The microstructure and the kinetics of growth of the nitrided zone of a Mo-containing maraging steel were investigated by performing gaseous nitriding at temperatures between 713 K (440 °C) and 793 K (520 °C) and at nitriding potentials up to 0.5 atm−1/2 for both solution-annealed and precipitation-hardened specimens. The microstructure of the nitrided zone was investigated by means of X-ray diffraction (phase constitution; crystal imperfection). Fine, initially largely coherent Mo2N-type precipitates developed in the nitrided zone. The elemental concentration-depth profiles were determined employing glow discharge optical emission spectroscopy (GDOES). The nitrogen content within the nitrided zone exceeds the nitrogen content expected on the basis of the molybdenum content and the equilibrium solubility of nitrogen in a (stress-free) ferritic matrix: excess nitrogen occurs. A numerical model was applied to predict the nitrogen concentration-depth profile within the nitrided layer. The model describes the dependence on time and temperature of the nitrogen concentration-depth profiles with, as fit parameters, the surface nitrogen concentration, the diffusion coefficient of nitrogen in the matrix, a composition parameter of the formed nitride and the solubility product of the nitride-forming element and dissolved nitrogen in the matrix. Initial values for the surface nitrogen concentration and the composition parameter were determined experimentally with an absorption isotherm and fitted to the measured nitrogen concentration-depth profiles. The results obtained revealed the striking effects of the amount of excess nitrogen and the extent of precipitation hardening on the developing nitrogen concentration-depth profile.

Studies of separation of vapours and gases through composite membranes with ferroferric oxide magnetic nanoparticles

Poly(2,6-dimethyl-1,4-phenylene oxide) composite membrane containing the different amount of nano-sized magnetic iron oxide (magnetite) was used in gas separation (N2/O2) experiments and vapour permeation (ethanol/water) studies. Presence of the magnetic particles additive has an impact on the separation properties of the polymer matrix. Using this type of membranes in separation of gaseous mixtures restrains permeation of paramagnetic constituent and promotes diffusivity of the diamagnetic component. Diffusion coefficient of nitrogen increased from 8.66×10−4 to 42.00×10−4cm2s−1 respectively for pure PPO membrane and loaded with 10w/w% of Fe3O4. In case of the vapour permeation studies, increase both flux and diffusivity of less polar component (ethanol) is observed. Flux changed from 1.07×10−12 to 53.2×10−12cm3(STP)cm−2s−1 and diffusion coefficient raised from 2.01×10−10 to 84.2×10−10cm2s−1 respectively for pure PPO membrane and loaded with 10w/w% of Fe3O4.

Thermochemical nitridation of sapphire substrates of different crystallographic orientations

This paper presents the results of the study of the influence of physicochemical parameters of thermochemical nitridation of sapphire substrates of different crystallographic orientations in an N2 + (CO,H2) reducing atmosphere on structural characteristics, composition, and kinetics of AlN layer formation. The effective diffusion coefficients of nitrogen in sapphire at T = 1473−1773 K have been determined. The temperature dependences of the structural quality, thickness, and roughness of the AlN layer surface have been found; their strong dependence on the composition and reduction potential of the nitridation atmosphere has been shown. The structural models of matching the AlN and sapphire lattices during thermochemical nitridation have been proposed.

Nitrate–Nitrogen Transport in Horizontal Soil Columns of the Yellow River Delta Wetland, China

AbstractHorizontal movement of nitrate–nitrogen was simulated in horizontal soil columns from a typical tidal flooding wetland and a short‐term flooding wetland of the Yellow River Delta. The primary objectives of this study were to investigate the changes in transport fluxes of nitrate–nitrogen with increasing movement distances and to identify the relationship between transport fluxes of nitrate–nitrogen and water diffusion coefficients. The results showed that the transport fluxes of nitrate–nitrogen were significantly and negatively correlated with movement distances in both wetlands, and decreased exponentially with increasing distances. However, the transport fluxes of nitrate–nitrogen increased exponentially with increasing water diffusion coefficients in both wetlands. The higher transport fluxes of nitrate–nitrogen in surface soils were observed compared to deeper soils, while they were higher in surface soils at the distances near the tracer water head. The findings of this study can contribute to water quality protection by regulating buffer zone width based on the horizontal movement of nitrate–nitrogen.

Bonding and diffusion of nitrogen in the InSbN alloys fabricated by two-step ion implantation

We report bonding and diffusion behavior of nitrogen incorporated into InSb wafer by two-step implantation. Three nitrogen-containing regions, i.e., a surface accumulation region, a uniform region, and a tail region, were observed in the samples after post annealing. X-ray photoelectron spectroscopy measurements at different depths reveal that majority of the nitrogen forms In-N bonds in the uniform region but exists as interstitial defects in the tail region. The diffusion coefficients of nitrogen in InSb were obtained by fitting the modified Fick’s law with experimental data and the activation energy of 0.55 ± 0.04 eV extracted confirms the interstitial dominating diffusion of nitrogen in the InSb wafer.

Stress induced and concentration dependent diffusion of nitrogen in plasma nitrided austenitic stainless steel

The nitrogen transport mechanism in austenitic stainless steel during plasma nitriding at moderate temperatures (around 400 °C) is considered by stress induced diffusion model. The model involves diffusion of nitrogen in presence of internal stresses gradient induced by penetrating nitrogen as the next driving force of diffusion after concentration gradient. Furthermore, in the present work it was found that nitrogen diffusion coefficient vary with nitrogen concentration according to well-known Einstein–Smoluchowski relation D(CN) = f(1/CN). Nitrogen depth profiles in nitided AISI 316L steel at T = 400 °C for 1, 3 and 8 h calculated on the basis of this model are in good agreement with experimental nitrogen profiles. The dependencies of nitrogen flux and nitriding time on nitrogen concentration, nitrogen surface concentration and penetration depth are analyzed by proposed model. It is shown that, with the increase of nitriding time the compositionally-induced stresses and thickness of stressed steel layer increases.