Growth Of Nitride Layers Research Articles

Modelling nitriding of iron: From thermodynamics to residual stress

The present article presents a few selected aspects of the modelling of gaseous nitriding of pure iron. After descriptions of the thermodynamics of the gas phase and the reactions at the gas/solid interface, a model description of the thermodynamics of $\gamma'-Fe_{\rm 4}N{\rm 1-x}$ is given, which takes the long-range ordering of nitrogen atoms into account. Subsequently, the kinetics of nucleation and growth of iron nitride layers is described in terms of the rates of the surface reactions and solid state diffusion. Thereafter, the mechanisms of stress generation in $\gamma'-Fe_{\rm 4}N{\rm 1-x}$ layers during nitriding are summarized. Finally, the model for stress development in $\gamma'-Fe_{\rm 4}N{\rm 1-x}$ layers is compared with published experimental work.

Surface Nitriding of SUS 304 Austenitic Stainless Steel by a Molten Salt Electrochemical Process

Electrochemical surface nitriding of SUS 304 austenitic stainless steel was investigated in a molten system at 773 K, and surface hardening by this molten salt electrochemical process without any special pretreatment was confirmed. From X-ray photoelectron spectroscopy and X-ray diffraction analyses, it was found that the surface nitrided layer contained CrN precipitates in Fe-Ni matrix. The square of the thickness of the surface nitrided layer was proportional to the electrolysis time. The rate constant, increased as the applied potential was more positive. This finding indicated that the chemical potential of adsorbed nitrogen on SUS 304 stainless steel electrodes depended on the applied potential and it affected nitrided layer growth. Furthermore, surface nitriding of a SUS 304 stainless steel tube was achieved. It was confirmed that adhesive coherent nitrided layers formed both on the outside and inside of the tube. These results suggested that this electrochemical surface nitriding process can be applied to specimens of various shapes. © 2004 The Electrochemical Society. All rights reserved.

RF nitriding of severely deformed Armco iron and St2K50

The nitriding behaviour of specimens of severely deformed Armco iron and St2K50 steel was compared with that of as-received specimens of the same materials. Radio frequency (RF) nitriding with −1 kV bias was performed for 3 h at 350 and 400 °C in pure nitrogen atmosphere at a pressure of 10 −5 bar. Results of scanning electron microscopy, glancing angle XRD, as well as hardness and corrosion measurements are presented and discussed with regard to the influence of severe deformation on the nitriding behaviour, particularly nitride formation and nitride layer growth, and on the mechanical and corrosion properties. The results show a remarkable increase of the nitrogen inward diffusion and enhanced corrosion resistance of severely deformed material.

Electrochemical surface nitriding of SUS 430 ferritic stainless steel

Electrochemical surface nitriding of SUS 430 ferritic stainless steel was investigated in a molten LiCl–KCl–Li 3N system at 773 K, and surface hardening by this electrochemical surface nitirding process was confirmed. From the results of XPS and XRD analyses, it was found that the surface nitrided layer contained CrN precipitates in α-Fe matrix. The square of the thickness of the surface nitrided layer was proportional to the electrolysis time. The rate constant, K P , increased as the applied potential was more positive. This finding indicated that the chemical potential of adsorbed nitrogen (N ads) on SUS 430 stainless steel electrodes depended on the applied potential and it affected nitrided layer growth.

Growth and Microstructure of Iron Nitride Layers and Pore Formation in ϵ-Fe3N

Abstract Layers of ϵ-Fe3N and γ′-Fe4N on ferrite were produced by nitriding iron single crystals or rolled sheets of iron in flowing ammonia at 520°C. The nitride layers were characterised using X-ray diffraction, light microscopy as well as scanning and transmission electron microscopy. The compound layer consists of ϵ-Fe3N at the surface and of γ′-Fe4N facing the ferrite. After 4 h of nitriding, pores develop in the near surface region of ϵ-Fe3N showing more or less open porosity. Growth of the entire compound layer as well as of the massive and the porous ϵ-Fe3N sublayer is diffusion-controlled and follows a parabolic growth rate. The γ′-Fe4N layer is formed as a transition phase within a narrow interval of nitrogen activity, and it shows little growth in thickness. The transformation of γ′-Fe4N to ϵ-Fe3N is topotactic, where the orientation of the closed-packed iron layers of the crystal structures is preserved. Determination of lattice plane spacings was possible by X-ray diffraction, and this was co...

Growth of nitrided layer after cathode sputtering

The authors present results of the investigations on the growth of nitrided layer formed during gas nitriding after the surface activation. Cathode sputtering was used as an activation treatment which increases the number of activated centres. The surface of the samples was bombarded by nitrogen or argon ions.The experiments were carried out on the samples made of α-Fe. The parameters of the sputtering were as follows: voltage—700–1350V, current density—1–4mA/cm2, time—10–60min. The nitriding atmosphere was composed of ammonia and dissociated ammonia.For every sample the following investigations were made: metallography of cross-section of nitrided layers and layer thickness measurements (the compound layers), surface observation by SEM and XRD analysis.It was observed that cathode sputtering improves the kinetic of gas nitriding and this effect was observed for both sputtering gases. The mathematical relationship between the thickness of nitride layer and voltage and current density of sputtering was also established.

Improved nitrogen transport in surface nanocrystallized low-carbon steels during gaseous nitridation

In the thermochemical process of low-carbon steel nitridation, improvements of mechanical properties are governed by the nature (hardness, thickness, etc.) of the compound layer since the compound layer will be kept under working conditions. Because of low nitrogen diffusivity in steel and low mass transfer between gaseous reaction and steel, long duration and high temperature are classically used for thermal nitridation. The present study shows that improved nitrogen transport can be obtained by gaseous nitridation after the samples have been surface nanocrystallized by ultrasonic shot peening (USSP). Comparing to the untreated samples, the nitridation efficiency of treated samples has been improved dramatically. The growth of γ′ nitride layer conforms to a parabolic relationship with nitridation duration during the whole nitridation process in treated samples, while in untreated samples, the growth of nitride layer could be traditionally divided into two stages: the first linear growth stage and the second parabolic growth stage. This difference clearly indicated that the whole nitridation process is diffusion-controlled in treated samples instead of interface reaction-controlled stage and then diffusion-controlled stage in untreated samples. The high nitridation efficiency obtained may be explained by an increased volume fraction of grain boundary and an enhanced reactivity of surface atoms of the steel samples due to the nanostructure layer generated by surface nanocrystallization of USSP.

A New Surface Treatment by Pulsed Plasma Nitriding for Chromium Plated Austenitic Stainless Steel.

The present paper deals with the methodology adopted for pulsed plasma nitriding of chromium plated type 316LN austenitic stainless steel. The influence of nitriding temperature, time and gas mixture ratio of nitrogen to hydrogen on the nitriding behaviour of chromium plated type 316 stainless steel has been investigated. The results indicated that the nitriding temperature plays a dominant role in obtaining hardness and case depth in this material. Plasma nitriding at a temperature of about 833 K produced a case depth of about 5 µm and surface hardness of about 550 HV. At temperatures more than 1 073 K, a large fraction of chromium has been found to be converted to chromium nitride with hardness exceeding 1 000 HV. Nitriding at an intermediate temperature of 913 K for 45 h has been found to produce nitrided layer of optimum properties. Calculation of diffusion co-efficients and activation energy for nitrogen diffusion are presented to demonstrate that the nitride layer growth in chromium plated stainless steel is a diffusion controlled process.

Microstructures of GaN and InxGa1-xN Films Grown by MOCVD on freestanding GaN Templates

AbstractWe summarize structural properties of thick HVPE GaN templates from the point of view of their application as substrates for growth of nitride layers. This is followed by the results of optical and structural studies, mostly transmission electron microscopy, of nitride layers grown by MOCVD on top of the HVPE substrates. The results indicate high structural quality of these layers with a low density of threading dislocations (in the range of 106 cm-2). Convergent beam electron diffraction studies showed that the MOCVD GaN films have Ga-polarity, the same polarity as the HVPE GaN substrates. Structural studies of an InGaN layer grown on top of the MOCVD GaN film showed the presence of two layers, which differed in lattice parameter and composition. The upper layer, on the top of the structure had a clattice parameter about 2 % larger than that of GaN and contained 10.3 ± 0.8 % of In. Values measured for the thinner, intermediate layer adjacent to the GaN layer were about 2.5 times lower.

Arc plasma nitriding of low carbon steel

Arc plasma nitriding of low carbon steel was carried out in the temperature range 1000–1100°C under different gas (Ar, N 2, H 2) configurations for 15 min each. The plasma nitrided surfaces were characterized by XRD, SEM and microhardness measurement to identify various nitride phases thus grown, observe surface morphology of grains and determine improvement in hardness, respectively. The depth of the nitride layer growth was found to be in the order of 100–120 μm. Microhardness was observed to increase by three to four times that of the original samples taken. Fe 2–3N(ε), Fe 2N 1− x and Fe 2N, etc., were the main phases that grew in the arc plasma process. Experimental details and characterization of arc plasma nitrided low carbon steel are described and discussed in this paper.

Numerical simulation of nitrided layer growth and nitrogen distribution inε-Fe2-3N, γ'-Fe4N and α-Fe duringpulse plasma nitriding of pure iron

The surface phase constitution and the kinetics of layer growthin pure iron pulse plasma nitrided at 520 °C for different times weresystematically studied, and the relative expressions of the nitrogen concentrationprofiles in nitrided layers have been deduced. The results show that thenitrided layer consists of ε-Fe2-3N (ε),γ'-Fe4N (γ') and α-Fe (α) phases,and its growth in thickness follows a parabolic law. In addition, the growthof a compound layer and diffusion layer and the nitrogen concentrationdistribution in the nitrided layers can be precisely predicted by employing themathematical models obtained.

In situ characterization of boron nitride layer growth by polarized FTIR reflection spectroscopy

Layers of almost pure cubic boron nitride (c-BN) have been deposited in a hollow cathode arc evaporation device on silicon and titanium nitride substrates. The layer growth is analyzed by in situ polarized infrared reflection absorption spectroscopy (IRRAS). The spectra are sensitive to determine the phase of the growing film due to the different phonon absorption frequencies of hexagonal BN (h-BN) and c-BN. The optical set-up can be used either as infrared spectroscopic ellipsometer (IRSE) in a rotating analyzer configuration or as IRRAS with variable state of polarization. For a quantitative analysis the spectra have been simulated with a three-layer model consisting of the substrate, the h-BN interface layer and the c-BN layer. To calculate the dispersion relation of the BN layers we used a Lorentz oscillator model. The oscillator frequency is correlated to the stress in the c-BN films, which is one of the delimiting factor for the deposition of thick films. Evaluating the oscillator frequency by a simulation procedure it is possible to determine the stress of the c-BN films in situ as a function of deposition conditions. The data received by IRSE and IRRAS are compared and the error sources are discussed.

Growth of nitrided layers on Fe–Cr alloys

Abstract Chromium is an important alloying element present in numerous commercial steels. A systematic study on the nitriding behavior of Fe–Cr alloys is helpful in predicting the properties of nitrided Cr-alloyed steels. Aspects such as microstructural evolution, growth kinetics, and mechanical properties should be particularly emphasized. Fe–Cr alloys containing 5, 10, and 20 wt.% Cr have been arc melted and subsequently plasma nitrided under a N 2 –80% H 2 atmosphere. The microstructure of the resulting nitrided layers was characterized and the microhardness profiles evaluated. Thicker layers developed on low chromium alloys. Differences in hardness profiles were also observed as a function of chromium contents. Nitriding Fe–5% Cr alloys resulted in two discrete fronts, refereed to as the diffusion front and the transformation front. Transformed regions sustained a decrease in hardness from 1000 down to 600 HV, associated with the conversion of homogeneously dispersed fine precipitates into coarser needle like particles immersed in the ferritic matrix. Similar behavior was not observed for the other alloys, where both fronts developed simultaneously.

Effects of a low-energy ion beam on the growth of titanium nitride layers near the interface of a polyimide

In this letter two different growth methods, reactive evaporation and ion-assisted reactive evaporation, were used to grow titanium nitride films on the polyimide biphenyl tetracarboxylic acid dianhydride-para phenylene diamine. The films were examined with x-ray photoemission spectroscopy and it was found that the chemical properties of the film depend on the distance from the polyimide/titanium nitride interface. While both techniques produced very similar films at distances greater than 5 nm from the interface, only the ion-assisted grown film had a significant amount of nitride near the interface. This is due to the ability of low-energy (100 eV) nitrogen ions to break the initial titanium/polyimide bonds which would otherwise prevent the growth of titanium nitride at the interface.

Fabrication of aluminium nitride/diamond and gallium nitride/diamond SAW devices

Abstract Surface acoustic wave (SAW) devices have been fabricated from thin films of gallium nitride and aluminium nitride deposited on a range of chemical vapour deposition (CVD) diamond substrates. The growth of aluminium nitride and gallium nitride layers on diamond by chemical beam epitaxy (CBE) is reported for the first time. Triethyl gallium and ethyldimethylamine alane precursors were used in conjunction with nitrogen from an RF atom source to deposit the gallium nitride and aluminium nitride layers at substrate temperatures in the range 540 to 575 °C. These layers have been characterised by Raman spectroscopy and atomic force microscopy. The SAW structures were completed by the deposition of gold or aluminium interdigitated electrode structures on the as-deposited nitride surfaces. Preliminary testing indicates that these devices operate as bandpass filters with characteristics consistent with the propagation of acoustic waves at very high phase velocities within the nitride–diamond multi-layer substrate.

Growth of thin transparent titanium nitride layers by reactive laser ablation

Transparent and conductive thin layers of TiN have been grown on Corning glass and silicon substrates by the reactive pulsed laser deposition method. An excimer laser (KrF, λ=248 nm, 4.0 J/cm 2) was used to ablate a massive, metallic Ti target in a N 2 atmosphere. Under optimised conditions, continuous polycrystalline films of fcc TiN exhibiting a lattice parameter a=0.4242 nm very close to the bulk value, an optical transmittance higher than 70% in the 350–1100 nm range, a flat morphology and an electrical conductivity around 550 μΩ cm have been deposited at a substrate temperature of only 400°C. The grown films also posses a good chemical and wear resistance as their properties have not changed after exposure to the ambient atmosphere for 6 months.

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.

Growth of columnar aluminum nitride layers on Si(111) by molecular beam epitaxy

Single crystalline aluminum nitride (AlN) thin films are deposited by molecular beam epitaxy (MBE) using thermally evaporated aluminum and RF-plasma excited nitrogen gas. In this paper we report on films grown on Si(111) at substrate temperatures of 800° with growth rates between 65 and 350 nm h −1. All layers consist of hexagonal and exactly c-axis oriented AlN crystals with column-like structure. For the smoothest layers surface roughness (rms) around 1 nm is obtained. In the XRD-spectra ( ω-scan) we have achieved a minimum FWHM of 0.4° (=25′) for the AlN(00.2) reflex. At maximum growth rates (350 nm h −1) for AlN a transition zone of about 200 nm is formed with high defect density compared to the subsequent growth. For lower growth rates (65 nm h −1) no transition zone exists. Application of a substrate nitridation leads to a partial loss of epitaxial relation between AlN layer and Si(111)-substrate.

Kinetics of titanium nitriding under CW CO 2 laser radiation

Solid-phase nitriding of titanium by a CO 2 laser was carried out in order to investigate the growth of nitrided layers under CO 2 laser irradiation. The results show that the nitriding kinetics obey parabolic laws, and are therefore controlled by species diffusion. Direct irradiation of the substrate with CO 2 photons does not produce any specific assisted nitride growth. From the evolution of the thickness of the nitrided layers with time, the nitrogen diffusion coefficients are estimated. The nitrogen concentration depth profiles calculated using Fick's equation are compared to experimental nitrogen depth profiles.

Physical model of the transition from a hexagonal to a cubic structure during growth of boron nitride under nitrogen and argon ion irradiation

A model based on original experimental data is proposed to describe the transition of the structure from hexagonal to cubic during the growth of boron nitride layers. It is postulated that boron and nitrogen ions entering the growing layer form additional atomic planes parallel to the (ab) planes of the hexagonal structure or complete existing dislocations in the growing crystal, causing the boron nitride to undergo a transition to a cubic structure.