Diffusion Of Nitrogen Atoms Research Articles

Microstructure and fretting behavior of hard TiN-based coatings on surgical titanium alloys

An original nitriding method [powder immersion reaction assisted coating (PIRAC)] for improving wear resistance of Ti-based surgical alloys was developed. CP Ti and Ti–6Al–4V alloy samples were annealed in sealed stainless steel containers that allow selective diffusion of nitrogen atoms from the atmosphere. The microhardness-microstructure and nitrogen concentration relationship were studied for TiN-based PIRAC coatings grown at 850–1100°C. Phase identification and microstructure characterization of nitrided samples were performed by XRD, SEM/EPMA and HRSEM. The effect of PIRAC coating on fretting behavior of Ti–6Al–4V was studied in vitro under conditions simulating in vivo conditions at the interface of modular orthopedic implants. A significant reduction in friction coefficient and fretted areas was measured for the coated samples.

Growth of Bulk GaN Single Crystals by the Pressure-Controlled Solution Growth Method

We are the first researchers to have developed the pressure-controlled solution growth (PC-SG) method. Using this method, supersaturation of growing crystals is realized by applying an over pressure without any temperature decrease. We applied this method to the growth of GaN crystals for examining the validity of the PC-SG method, using a high-pressure furnace. It was determined that the crystal growth rate increased as a function of temperature with a maximum at 1475°C under a nitrogen pressure of 0.98 GPa. The dependency of the growth rate on the Ga metal weight strongly supports the fact that the dominant growth process is by diffusion of nitrogen atoms in the Ga melt. GaN single crystals with a maximum size of 10 mm diameter were reproducibly grown by the PC-SG method and it was determined that these crystals had good crystallinity.

Thermal Stability of Stainless Steel Surfaces Nitrided by Plasma Immersion Ion Implantation

The thermal stability and the magnetic nature of metastable nitrided surface layers produced by plasma immersion ion implantation at 300°C on an austenitic X6CrNiTil810 (f.c.c.) and a ferritic X10CrA118 (b.c.c.) stainless steels were examined. The surface phase analysis was performed by the Conversion Electron Mossbauer Spectroscopy (CEMS) and the results were correlated with X-ray diffraction and microhardness measurements. The dominant phase in the as-nitrided surfaces of both steels was found to be expanded austenite (S-phase). The first structure changes of phase composition of the surface of the ferritic X10CrA118 steel were detected after an annealing at 200 °C while in the case of the austenitic X6CrNiTi1810 steel at 250°C. The lower stability of the ferritic steel is ascribed to higher diffusion of nitrogen atoms in b.c.c. structure and to a lack of other austenite stabilising elements besides nitrogen.

Surface modification of titanium alloy orthopaedic implants via novel powder immersion reaction assisted coating nitriding method

Abstract There is increasing interest in using surface modification technology to improve the wear properties of titanium alloys used in total joint replacements. In the present work, a simple and original Power Immersed Reaction Assisted Coating (PIRAC) nitriding method suitable for surface modification of large complex shape orthopaedic implants has been developed. CP Ti and Ti-6Al-4V alloy samples were annealed at 850–1100°C in sealed stainless steel containers that allow selective diffusion of nitrogen atoms from the atmosphere. The relationship between the microstructure, nitrogen concentration and microhardness was studied employing X-ray diffraction (XRD), scanning electron microscopy (SEM)/electron probe X-ray microanalysis (EPMA) and high resolution SEM (HRSEM). PIRAC nitrided surfaces were found to have a layered structure with a TiN/Ti 2 N coating followed by nitrogen-stabilized α-Ti. In contrast to previous investigations of surface nitrided Ti-6Al-4V alloy, a Ti 3 Al intermetallic phase was detected at the Ti 2 N/α–Ti interface acting as a barrier for nitrogen diffusion. Importantly for biomedical applications, no toxic Al or V were detected in the surface layer of PIRAC nitrided Ti-6Al-4V alloy.

Mechanochemical Syntheses of Titanium Carbide, Diboride and Nitride.

TiC and TiB2 powders were mechanochemically synthesized from powder mixtures of elemental Ti-C (atomic ratio 1:1) and Ti-B (1:2) in 34h 45min and 15h 10min, respectively, in a planetary ball mill using tungsten carbide balls. TiN powder was also mechanically synthesized from titanium powder for 60h in a high-energy horizontal rotary ball mill (steel balls) equipped with a nitrogen gas flow system. The as-milled mixed powders have been characterized as a function of the milling time by means of X-ray diffraction, scanning electron microscopy and X-ray fluorescence spectroscopy. The reaction processes for TiC and TiB2 formation were initiated by mechanically induced solid-state diffusion in the induction period when carbon or boron atoms diffused into titanium, and a small amount of TiC or TiB2 was formed by solid-state diffusion until a mechanically induced self-propagating reaction occurred. The reaction process for TiN formation proceeded via the surface reaction of titanium particles with molecular nitrogen and the mechanically induced solid-state diffusion of nitrogen atoms into titanium. The lattice parameters obtained for TiC, TiB2 and TiN agree with previously reported measurements.

Excimer laser induced surface nitriding of aluminium alloy

This study describes a technique for the growth of thin nitride layer on aluminium alloy samples by direct laser synthesis with the advantages of good adhesion and localisation. The laser irradiation process is performed using an XeCl excimer laser ( λ=308 nm, 50 Hz) under a nitrogen atmosphere. The laser induced plasma interacts with the melted sample surface resulting in nitrogen atom diffusion and reaction into the depth of the sample. Plasma spots were overlapped by two dimensional laser beam displacement to ensure the complete coating of the surface with a specified laser fluence (1.6 J/cm 2) and number of pulses (500), while not removing of the nitride layer already synthesised. Under these conditions, the diffusion layer penetrates a few μm deep, but its crystalline quality is preserved. Interesting information on the layer formation and composition are drawn from nuclear analysis (RBS and NRA) to determine the nitrogen and oxygen (contaminant) concentration profiles.

Isothermal Kinetic Study of the Decomposition of Nitric Oxide over Rh(111) Surfaces

The kinetics of the thermal decomposition of NO on Rh(111) surfaces was probed both by temperature-programmed desorption (TPD) experiments and by isothermal measurements using an extension of the so-called King and Wells collimated beam method. The TPD studies corroborated previously reported results, including the existence of two distinct N2desorption peaks, the first of which displays apparent first-order kinetics. The isothermal work proved that the adsorption of NO is precursor mediated at low temperatures, and that it is not affected significantly by the presence of coadsorbed nitrogen and/or oxygen atoms at any temperature below 900 K. The rate of molecular nitrogen production was found to be significant above 450 K and to be controlled by the recombination of atomic nitrogen below 600 K, but the experimental data could not be reproduced in a satisfactory manner by any empirical rate law unless the order in nitrogen coverage was set to be less than unity. Such an observation is interpreted here as being the result of the slow diffusion of nitrogen atoms across the surface prior to their recombination. A strong additional effect due to lateral repulsion between nitrogen and/or oxygen atoms was also inferred from the data.

Wear behaviour of TiAIn films deposited on plasma nitrided high speed steel

Titanium aluminium nitride films were deposited on a plasma nitrided high speed steel substrate by reactive sputtering. The hardness and nitriding depth of the plasma nitrided substrates increased with increase in N2/(N2 + H2) flow ratio because of the enhanced diffusion of nitrogen atoms. The hardness and adhesive strength of TiAlN films coated on the plasma nitrided substrate were also found to be higher than those of the unnitrided substrates. In addition, the hardness of the TiAlN films increased with increase in the N2 + H2) flow ratio. The wear resistance, which was principally dominated by the hardness of the TiAlN films, was also enhanced by an increase in theN2/(N2 + H2) flow ratio.

Recent development of basic magnetism in interstitially modified rare-earth iron nitrides R 2Fe 17N 3

EPMA-line profiles for N-element in Sm 2Fe 17N x indicate that under low N 2-gas pressure, nitrogen absorption into grain interior is mainly due to diffusion of nitrogen atoms, while under high N 2-gas pressure, the grain growth of fully nitrogenated phase Sm 2Fe 17N 3 becomes more dominant than diffusion, making high-pressure nitrogenation effective for fabricating a high-quality nitride. Neutron diffraction studies of Y 2Fe 17 and Y 2Fe 17N 3.1 prepared by high N 2-gas pressure revealed that the N-atoms fully occupy at the 9e-sites (99.5%) and a small amount of N-atoms locates on the 18g-sites (4%). The introduction of N-atoms into Y 2Fe 17 led to such a strong modification in the Fe magnetic moment that the moment of the 18f-Fe atoms being the nearest to the 9e-N atoms is the smallest (∼2.0 μ B), whereas the 6c-Fe atoms being the farthest from the 9e-N atoms have the largest moment of ∼2.9 μ B at 10 K. The magnetization measurements of Nd 2Fe 17N 3 single crystal clarified a strong enhancement of crystalline electric field (CEF) acting on R-atoms upon nitrogenation, leading to almost three times larger CEF-parameter A 2 0 in nitride than in host compound.

New mechanical grinding process for anisotropic Sm2Fe17Nx powder with a nanocrystalline structure

A new mechanical grinding (MG) process in NH3 and subsequent nitriding in N2 without a crystallization treatment was developed for synthesizing Sm2Fe17Nx powders. This process features a shorter MG time compared to conventional MG to prevent amorpharization of the alloy structure and to avoid hydrogen in the ground powders which promotes a diffusion of nitrogen atoms into their particles. The Sm2Fe17Nx powder with x=3.2 prepared by grinding for 10 h and subsequent nitriding at 723 K for 6 h showed magnetic anisotropy and its magnetic properties were Ir=1.00 T, Hc=0.61i MA m−1, and (BH)max=129 kJ m−3 (16.1 MGOe).

Formation of copper nitrides in the course of implanting copper with large doses of nitrogen ions

The paper presents a mathematical-physical model of the formation of nitride phases in the course of implantation of big doses of nitrogen ions into copper and its alloys. Distributions of nitrogen atoms implanted into copper and brass obtained by means of the RBS method and calculated on the basis of the proposed model were compared and on that basis the value of the coefficient of radiation enhanced diffusion of nitrogen atoms in copper and brass was estimated.

Hardening of WC–Co alloys by ion implantation

Abstract The hardening effect on the surface layers of WC–Co alloys after Ar+- and N+-ion implantation with the fluence in the range 1 ÷ 8.7 × 1017 cm−2 has been investigated at room temperature and under heating with an ion beam. The depth of the Auger distribution profiles and the microhardness of implanted samples were measured. The radiation-stimulated diffusion of nitrogen atoms and the microhardness enhancement were observed. The contribution of the polymorphic Co-phase transformation and the production of Co–N compounds is discussed.

Anisotropic atomic diffusion mechanism of N, C and H into Sm 2Fe 17

The preparation of Sm 2Fe 17N 3 involves the slow diffusion of nitrogen atoms from the surface to the bulk of the material. The atomic diffusion mechanism operating in this case is voidal diffusion. The nitrogen atoms are located inside the 9(e) octahedra, which share Sm corners but no faces between them. Nitrogen atoms migrate by jumping from a 9(e) site into a thermodynamically unstable tetrahedral 18(g) site, and subsequently into a new 9(e) site. In such a migration path, nitrogen atoms have to contend with an enormous energy barrier, accounting for the energy needed to overcome the strong bonding from its nearest neighbors (Fe and, especially, the Sm atoms) and, more importantly, for the strain energy needed to break out through the octahedral face (Fe(f)Sm(c)Fe(h)) and in through the tetrahedral face (Fe(h)Sm(c)Fe(h)). Although the 18(g) sites cannot accommodate nitrogen atoms in an equilibrium fashion, their presence plays a key role for the diffusion of the nitrogen atoms. The diffusion of nitrogen atoms is anisotropic, as a result of the anisotropic crystal structure of Sm 2Fe 17. Bitter domain patterns have been used to map the nitrogen diffusion fields of nitrogenated particles, and clearly show the expected anisotropic behavior of the diffusivity. Grain boundaries with their open structure provide free paths, behaving essentially the same way as free surfaces exposed to nitrogen gas. The presence of hydrogen facilitates nitrogen diffusion but, more importantly, it fractures the particles such that nitrogen effectively penetrates towards the centre of the particles. The use of ammonia gas causes severe morphological changes to the grains, resulting in very distinct fine microstructures.

Nitrogen absorption and desorption of Sm 2Fe 17 in ammonia and hydrogen atmospheres

Sm 2Fe 17 was nitrided in ammonia and then denitrided in a hydrogen atmosphere at temperatures between 563 and 715 K. The amounts of absorbed and desorbed nitrogen, and the kinetics of the reaction were measured gravimetrically in situ. In flowing ammonia, Sm 2Fe 17 can quickly absorb nitrogen up to about [N]/[Sm 2Fe 17]=10. The high uptake and high absorption rate in ammonia, compared with the uptake in nitrogen, can be explained with thermodynamic arguments. At these high concentrations, the lattice is highly distorted. The rate-limiting step of the absorption is diffusion of nitrogen atoms in the lattice. For desorption in hydrogen, the nitrogen content can be quickly reduced to [N]/[Sm 2Fe 17] = 3−3.5. The desorption rate in hydrogen is much faster than that in argon. The magnetic properties were measured for compounds produced in this way.

Layer-growth kinetics on gaseous nitriding of pure iron: Evaluation of diffusion coefficients for nitrogen in iron nitrides

Models were derived for monolayer and bilayer growth into a substrate in which diffusion of the solute governs the growth kinetics, as in gas-solid reactions, for example. In the models, the composition dependence of the solute diffusivity in the phases constituting the layers was accounted for by appropriate definition of an effective diffusion coefficient for a (sub)layer. This effective diffusion coefficient is the intrinsic diffusion coefficient weighted over the composition range of the (sub)layer. The models were applied for analyzing the growth kinetics of a γ′-Fe4N1-x monolayer on an α-Fe substrate and the growth kinetics of an e-Fe2N1-z/γ′-Fe4N1-x bilayer on an α-Fe substrate, as observed by gaseous nitriding in an NH3/H2-gas mixture at 843 K. The kinetics of layer development and the evolution of the microstructure were investigated by means of thermogravimetry, layer-thickness measurements, light microscopy, and electron probe X-ray microanalysis (EPMA). The effective and self-diffusion coefficients were determined for each of the nitride layers. The composition dependence of the intrinsic (and effective) diffusion coefficients was established. Re-evaluating literature data for diffusion in γ′-Fe4N1-x on the basis of the present model, it followed that the previous and present data are consistent. The activation energy for diffusion of nitrogen in γ′-Fe4N1-x was determined from the temperature dependence of the self-diffusion coefficient. The self-diffusion coefficient for nitrogen in e-Fe2N1-z was significantly larger than that for γ′-Fe4N1-x. This was explained qualitatively, considering the possible mechanisms for interstitial diffusion of nitrogen atoms in the close-packed iron lattices of the e and γ′ iron nitrides.

Storage models for ammonia synthesis over iron catalyst under periodic operation

Two models of possible nitrogen storage mechanisms in an iron catalyst during cycling of ammonia synthesis are examined. A surface storage model assumes two kinds of nitrogen adsorption sites on the catalyst surface. It is also assumed that chemical reaction can take place during the surface diffusion of nitrogen atoms between these sites. A bulk dissolution model also assumes dual nitrogen sites. With a well-mixed gas phase, it is shown that the surface storage model and bulk dissolution model can predict enhancement of the time-average production rate of ammonia over the optimal steady-state rate as well as resonance behaviour.

Anomalous diffusion of nitrogen in nitrogen-implanted silicon

A secondary-ion mass spectrometry analysis of the coimplantation of nitrogen, carbon, and oxygen into float-zone silicon followed by rapid thermal annealing for 10 s at different temperatures is used to study the anomalous diffusion behavior of nitrogen in silicon. The results may be only partially explained by a model of paired nitrogen atom diffusion. The complexity of the diffusion of nitrogen in ion-implanted samples, with and without coimplants, and the expectation that the nitrogen after annealing may be in many different forms, suggest that studies which use nitrogen implantation for basic understanding of nitrogen-related defects may be misleading.

Nitrogen implantation in GaAs1−xPx (x=0.4; 0.65)

Nitrogen ions were implanted in GaAs1−xPx (x=0.4; 0.65) at room temperature at various doses from 5×1012 cm−2 to 5×1015 cm−2 and annealed at temperatures from 600°C up to 950°C using a sputtered SiO2 encapsulation to investigate the possibility of creating isoelectronic traps by ion implantation. Photoluminescence and channeling measurements were performed to characterize implanted layers. The effects of damage induced by optically inactive neon ion implantation on photoluminescence spectrum were also investigated. By channeling measurements it was found that damage induced by nitrogen implantation is removed by annealing at 800°C. A nitrogen induced emission intensity comparable to the intensity of band gap emission for unimplanted material was observed for implanted GaAs0.6P0.4 after annealing at 850°C, while an enhancement of the emission intensity by a factor of 180 as compared with an unimplanted material was observed for implanted GaAs0.35P0.65 after annealing at 950°C. An anomalous diffusion of nitrogen atoms was found for implanted GaAs0.6P0.4 after annealing at and above 900°C.

U + N 2 Reaction layer growths

Reaction layers were formed on uranium at various temperatures up to 2400 °C in nitrogen (0.003 to 2.0 atm). Reaction with solid uranium is propagated by spalling and is most rapid near 800 °C. Reaction with liquid uranium occurs by two parallel processes, diffusion through and fracturing of UN layers. From observed thicknesses of fracture-free layers, concentration-gradient chemical interdiffusion coefficients are calculated. Marker experiments indicate that the UN layers form by inward diffusion of nitrogen atoms from which are deduced nitrogen self-diffusion coefficients represented by the equation D ̄ S N[cm 2/sec] = 12exp−(120 ± 25)[kcal/mole]/RT . Nitrogen selfdiffusion coefficients are also deduced from estimated nitrogen activity gradients and these exhibit a dependence on nitrogen pressure. Uranium-saturated UN compositions range from N/ U = 0.991 to 0.997 ± 0.006 at temperatures above 1600 °C. Change in N/U ratio across the UN phase is 0.005 at about 1600 °C.

Thermometric Study of the Frozen Products from the Nitrogen Microwave Discharge

Thermometric observations have been made of the phenomena accompanying the stabilization and the recombination of free nitrogen atoms in a solid molecular nitrogen matrix at liquid helium temperatures. Condensation from the gas phase may occur at temperatures considerably above the 4.2°K coolant temperature as the result of the normal change in heat content and the heat of atom recombination. Estimates, made from the latter effects, of the free atom concentration in the gas phase were as high as 4 atom %. Qualitative confirmation of the presence of an energetic species stabilized in the solid phase was made by detection of heat release upon warmup. From the latter effect a minimum nitrogen atom concentration in the solid of 0.03 atom % was estimated. For the conditions used the initiation of atom recombination occurs at a temperature of about 9°K. The upper limit of about 36°K for stabilization coincides with the known thermal transition in solid nitrogen. The data suggest that at least above 9°K diffusion of nitrogen atoms occurs throughout the solid matrix during deposition from the gas phase. If recombination during warmup is diffusion-controlled, unusual dependence upon temperature is indicated.