Nitriding Kinetics Research Articles

Opportunity to achieve an efficient SiC/SiO2 interface N passivation by tuning the simultaneous oxidation modes during the SiC surface nitridation in N2 + O2 annealing

We investigated a possible guideline to design the SiC surface nitridation process for an efficient interface N passivation in high-temperature N2 + O2 annealing, based on the understandings of SiC surface nitridation kinetics. Based on the findings of the time-dependent transition of the simultaneous SiC surface oxidation mode from passive to active during the annealing with reduced O2 partial pressure condition, it was proposed that an efficient SiC surface nitridation is achievable by following these steps: triggering the passive-to-active transition of the simultaneous surface oxidation by tuning the thermodynamic conditions and SiO2 thickness, and terminating the surface reaction at the beginning of the active-oxidation-dominant period by controlling the annealing time. It is confirmed that the observed interface passivation would be attributable to the N introduction to SiC/SiO2 interface, suggesting an opportunity to achieve an efficient interface N passivation following the proposed guideline.

Isothermal reduction and nitridation kinetics of ilmenite concentrate in ammonia gas

Low-temperature chlorination is considered a suitable process for ilmenite containing high contents of MgO and CaO. However, reduction carbonization or nitridation is essential to convert titanium oxide to carbide or nitride, which can be enriched by magnetic separation. In this paper, the isothermal reduction–nitridation kinetics of Panzhihua ilmenite concentrate in an ammonia atmosphere was investigated. Both model–free and model–fitting methods were used to compare the isothermal kinetics. The reaction process can be divided into two successive stages: the reduction reaction and the nitridation reaction. The average apparent activation energy for the reduction reaction was 139.63 kJ/mol, obtained by the model–free method. The reduction process can also be subdivided into two stages by the model–fitting method. The chemical reaction (G(α) = 1–(1–α)1/3 = kt) and three–dimensional diffusion (G(α) = [1–(1–α)1/3]2 = kt) were identified as the mechanism function for the reduction process. The activation energies of these two stages were 121.73 kJ/mol and 141.40 kJ/mol, respectively. The function G(α) = 1–(1–α)1/3 = kt, which also corresponds to the chemical reaction mechanism (R3), exhibited the best fit for the nitridation stage. The corresponding activation energy was 32.10 kJ/mol. Metallic iron and Ti3O5 were formed in the reduction stage, while Ti3O5 was then transformed to Ti(N,O) in the nitridation stage.

Metallic Powder Promotes Nitridation Kinetics for Facile Synthesis of (Oxy)Nitride Photocatalysts.

Nitrogen-containing semiconductors (including metal nitrides, metal oxynitrides, and nitrogen-doped metal oxides) have been widely researched for their application in energy conversion and environmental purification because of their unique characteristics; however, their synthesis generally encounters significant challenges owing to sluggish nitridation kinetics. Herein, a metallic-powder-assisted nitridation method is developed that effectively promotes the kinetics of nitrogen insertion into oxide precursors and exhibits good generality. By employing metallic powders with low work functions as electronic modulators, a series of oxynitrides (i.e., LnTaON2 (Ln = La, Pr, Nd, Sm, and Gd), Zr2 ON2 , and LaTiO2 N) can be prepared at lower nitridation temperatures and shorter nitridation periods to obtain comparable or even lower defect concentrations compared to those of the conventional thermal nitridation method, leading to superior photocatalytic performance. Moreover, some novel nitrogen-doped oxides (i.e., SrTiO3- x Ny and Y2 Zr2 O7- x Ny ) with visible-light responses can be exploited. As revealed by density functional theory (DFT) calculations, the nitridation kinetics are enhanced via the effective electron transfer from the metallic powder to the oxide precursors, reducing the activation energy of nitrogen insertion. The modified nitridation route developed in this workis an alternative method for preparing (oxy)nitride-based materials for energy/environment-related heterogeneous catalysis.

Catalytic gas nitriding of M50NiL steel by LaFeO3 pervoskite oxide under low-pressure conditions

To generalize the application of the LaFeO3 oxide in catalytic nitriding of steels, the M50NiL steel, a typical high-alloy bearing steel, was nitrided under low-pressure conditions. The catalytic effect of the LaFeO3 oxide on nitriding kinetics of M50NiL steel was evaluated and a new understanding of the catalytic mechanism was proposed. The microstructure, phase composition, surface morphologies and depth of nitrided layers were characterized by optical microscope, X-ray diffraction, scanning electron microscope and microhardness tester, respectively. The wear resistance of nitrided layers was evaluated by a pin-on-disk wear tester and the chemical bonds on nitrided surfaces were identified by X-ray photoelectron spectroscopy to analyze the process of nitriding reactions. The results show that the LaFeO3 catalytic nitriding was suitable for M50NiL steel. The LaFeO3 catalytic nitriding could dramatically improve the toughness of nitrided layer via the suppression of nitrides formation. The adsorption and decomposition of NHx species on the LaFeO3 oxide were alternatively occurred on La-O terminated surfaces and Fe-O terminated surfaces, resulting in the fluctuate nitrogen concentration and rapid diffusion of active nitrogen atoms. The nitriding kinetics greatly depended on the oxidation state of nitrided surfaces. Namely higher oxidation capacity corresponded to higher nitriding efficiency.

High-Temperature Nitriding Kinetics of Zr–U-Based Alloys

High-Temperature Nitriding Kinetics of Zr–U-Based Alloys

Kinetics of High-Temperature Nitridation of Zr–Nb Solid Solutions

Kinetics of High-Temperature Nitridation of Zr–Nb Solid Solutions

Influence of ferrite-austenite distribution in 2205 duplex stainless steel on high-temperature solution nitriding behaviour

Duplex stainless steel 2205 was subjected to annealing treatments in the range of 1000–1200 °C to adjust the phase fractions of ferrite and austenite. Annealed steels were subsequently subjected to high-temperature solution nitriding, which transformed the surface-adjacent region into austenite under the influence of nitrogen ingress. Microstructural characterization of the pre-annealed state and the solution-nitrided cases were performed with X-ray diffractometry, light-optical microscopy, electron back-scatter diffraction and hardness indentation. The phase fractions of austenite and ferrite have a significant influence on the nitriding kinetics. A relatively high ferrite phase fraction results in finer grains in the nitrogen-stabilized austenite case. The developing austenite case and the evolution of the microstructure during solution nitriding are discussed in a computational thermodynamics and kinetics context. Slower grain growth in the austenite cases for higher ferrite phase fractions can be understood in terms of phase distribution, alloying element partitioning, and possibly, pinning effect by M2N nitrides.

Duplex Treatment of AISI 420 Steel by RF-ICP Nitriding and CrAlN Coating Deposition: The Role of Nitriding Duration

The duplex treatment of AISI 420 steel samples by nitriding in a radiofrequency inductively coupled plasma (RF-ICP) discharge of Ar + N2 + H2 atmosphere followed by CrAlN coating deposition was performed in this study. The influence of plasma nitriding (PN) duration (10, 20, 40, and 60 min) on the structural and functional properties of the duplex-treated samples was determined. A non-linear dependence of AISI 420 steel nitriding kinetics was found on the square root of the PN duration. The thicknesses of the compound layer (CL) and nitrogen diffusion zone (DZ) in the samples and their phase composition resulted in different critical loads of coating failures under adhesion tests. Increasing the load-bearing capacity by the PN caused coating hardening in duplex-treated samples. The role of the PN duration on the wear characteristics of the AISI 420 steel samples after the duplex treatment has been discussed. Corrosion tests of AISI 420 steel demonstrated the significant enhancement (5–67 times) of its corrosion resistance in a 3.5 wt.% NaCl solution after duplex treatment.

Guidelines for the synthesis of molybdenum nitride: Understanding the mechanism and the control of crystallographic phase and nitrogen content

Bulk (Mo2N) and supported molybdenum nitride (Mo2N/TiO2, Mo2N/CeO2) were synthesized by reduction-nitridation of bulk and supported MoO3 under a mixture of N2/H2. We investigated the mechanism of formation of molybdenum nitrides by coupling elemental analysis, in situ and ex situ XRD, Raman spectroscopy, in situ XPS/UPS spectroscopy and DFT calculation. We showed that it is possible to control the crystallographic phase and degree of nitridation (i.e. nitrogen vacancies) by adjusting the parameters of the synthesis, including the heating rate and final temperature, the nature, composition and flow of gas during reduction/nitridation and cooling. We demonstrated that β-Mo2N and γ-Mo2N go through the same intermediates: molybdenum bronze, MoO2 and Mo. The final crystallographic phase of the nitride depends mainly on the temperature rate and gas hourly space velocity employed during the synthesis, as they impact the crystallite size of the intermediates and the kinetics of reduction and nitridation. Molybdenum nitride clusters (<0.5 nm) are formed on TiO2.

Practical Aspects of Plasma Nitriding Kinetics for 17-4 PH Stainless Steel

Abstract Plasma nitriding is very effective in removing the passive layer of chromium oxide formed naturally on the surface of stainless steel, but controlling the kinetics is key for optimal results.

Non-isothermal nitridation kinetics of Ti6Al4V in Al2O3-based refractories

To establish a kinetic model of nitridation of Ti6Al4V in Al2O3-based refractories, the non-isothermal nitridation of Ti6Al4V–Al2O3 composite refractories at various heating rates was investigated using a thermogravimetric (TG) analyzer for large samples. The activation energy (E) and kinetic model (G(α)) for the nitridation of Ti6Al4V were determined using the isoconversional and master plots methods, respectively. The nucleation and growth of nitriding products of the TiN solid solution was the controlling step in the nitridation of Ti6Al4V in Al2O3-based refractories. The Avrami-Erofeev kinetic model, depicted by the G(α) = [-ln (1-α)]4 equation, is the most rational kinetic model. The values of E and A for the nitridation of Ti6Al4V were calculated to be 214.99 kJ/mol and 1.46 × 107 (S−1), respectively.

Promoted low-temperature plasma nitriding for improving wear performance of arc-deposited ceramic coatings on Ti6Al4V alloy via shot peening pretreatment

In the present work, the combination of shot peening (SP), low-temperature plasma nitriding (PN) and arc deposition (AD) is proposed to improve the wear performance of Ti6Al4V alloy. The low-temperature rapid nitriding of Ti6Al4V alloy was realized by SP pretreatment to obtain a desired nitrided interlayer. The high-density structural defects produced by SP pretreatment promote the nitriding kinetics at low temperature. The nitrided interlayer consists of an outermost TiN/Ti2N compound layer and an inner diffusion layer, showing a diffusion-typed hardness-profile, which is beneficial for the enhancing of adhesion strength and load-bearing capacity. Moreover, the improvement of adhesion strength is associated closely with TiN/Ti2N compounds which can provide heterogeneous nucleation sites for AD coating with a high hardness of about 3100 HV. The high bearing-load capacity and adhesion strength can ensure the excellent wear resistance of SP-PN-AD-coated sample when compared to single AD or PN-AD samples, especially under intensive wear load conditions. The present work provides a new insight into surface hardening of Ti alloys.

Nitrogen Solubility and Gas Nitriding Kinetics in Fe–Cr–Mo–C Alloy Melts under Pressurized Atmosphere

The precise control of nitrogen content in high nitrogen martensitic stainless steel is the guarantee of its excellent performance, and is a crucial issue for its manufacturing process using pressurized gas nitriding. In this paper, the effects of nitrogen pressure, temperature and alloy composition on nitrogen solubility and pressurized gas nitriding kinetics of Fe–Cr–Mo–C alloy melts were investigated. In order to correct the deviation of nitrogen solubility from Sieverts’ law under high nitrogen pressure, the first- and second-order interaction parameters of nitrogen on itself were obtained; accordingly, the nitrogen solubility model under pressure was established and well verified. The nitrogen solubility decreased with increasing temperature and C content, and increased with increasing Cr and Mo contents. The nitrogen mass transfer in liquid phase was the nitriding rate-determining step under different nitrogen pressures. The apparent mass transfer coefficient of nitrogen was approximately 0.0218 to 0.0230 cm·s−1, and showed a weak dependence with pressure. The apparent mass transfer coefficient of nitrogen increased significantly with the increase of temperature and electromagnetic stirring, and was rarely affected by the content of Cr, Mo and C. Considering the influence of pressure change on nitriding rate in pressurization stage, a novel kinetic model for gas nitriding under high pressure was established and exhibited effectiveness in practice.

Gaseous nitriding of additively manufactured maraging steel; nitriding kinetics and microstructure evolution

The nitriding response of additively manufactured M300 maraging tool steel was investigated through a series of nitriding treatments at different temperatures in the range 360–480 °C for a duration of 40 h. The interaction between nitrogen and alloying element nitrides (Ti and Mo) and intermetallics (Ni 3 Ti and Ni 3 Mo) was studied by comparing nitriding responses of as-solution treated and subsequently aged specimens. The nitriding behavior was found to depend strongly on the initial condition of the material and the nitriding temperature applied. Ultra-low temperature nitriding at 360 °C was found to be feasible as it resulted in a nitrided case, albeit with incomplete precipitation hardening of the bulk structure, thus resulting in a softer core. The application of a post-ageing treatment resulted in full bulk hardening and distinctly changed the appearance of the hardness profile. • Nitriding of solution treated and not aged maraging steel creates deeper case. • Nitride and intermetallic formation competes at increased nitriding temperatures. • Ultra-low temperature nitriding of maraging steel is feasible. • Post-ageing of ultra-low temperature nitriding creates promising hardness profile.

Kinetics of the UNS S32750 Super Duplex Stainless Steel Low-Temperature Plasma Nitriding

This work reports an experimental investigation on the thermodynamic and kinetic view-point of the nitrided layer growth behavior, carried out separately at ferrite and austenite phase grains, for the UNS S32750 super duplex stainless steel (SDSS). For this purpose, and motivated by the DSSs distinct behavior showing formation of two- or single-phase nitrided layers, which apparently depend on the steel substrate chemical composition, two series of treatment were studied here, one varying the nitriding temperature on the 300-450 °C range for 4 h time, and other one varying the time on the 2-8 h range at 350 °C. Microstructural characterization of studied layers by means of SEM and EDS analysis, XRD and microhardness measurements showed precipitation-free nitrogen-expanded austenite layer formation on both steel phases for all samples treated up to 400 °C. The nitriding kinetics study showed that the layer thickness on both steel phases is proportional to the square root of the treatment time and follows an Arrhenius law for the studied treatment temperatures. The nitrogen diffusion activation energy, separately determined from the layer growth on each steel phase, was 115±3.2 and 120±5.4 kJmol-1, for austenite and ferrite grains, respectively.

Influence of rare earths on the gas nitriding kinetics of 32CrMoNiV5 steel at low temperature

This work aims to study the influence of rare earth on nitrogen diffusion kinetics and on surface properties (surface hardness and diffusion depth) of 32CrMoNiV5 steel containing various additions of rare earth (Ce and La). Gas phase nitriding treatments based on ammonia, nitrogen, and hydrogen were carried out by thermogravimetry at 520 °C for 48 h with a constant K N nitriding potential. Several methods of measurement, analysis and observation were used to characterize the nitrided alloys: X-ray diffraction, microhardness, optical and scanning microscopes, and electron microprobe. All the results show that rare earth have a significant influence on the nitrogen diffusion rate: improvement of surface hardness, diffusion depth and gradients of hardness, and nitrogen concentration in the diffusion layer. Secondary electron images, X-ray images and X-ray mappings performed on different particles showed the presence of cerium and lanthanum oxides in the diffusion layer in spheroidal form and of elongated platelets, of size 0.50 to 15 µm, distributed along the grain boundaries and in the grains of the matrix.

A Novel Decarburizing-Nitriding Treatment of Carburized/through-Hardened Bearing Steel towards Enhanced Nitriding Kinetics and Microstructure Refinement

Decarburization is generally avoided as it is reckoned to be a process detrimental to material surface properties. Based on the idea of duplex surface engineering, i.e., nitriding the case-hardened or through-hardened bearing steels for enhanced surface performance, this work deliberately applied decarburization prior to plasma nitriding to cancel the softening effect of decarburizing with nitriding and at the same time to significantly promote the nitriding kinetics. To manifest the applicability of this innovative duplex process, low-carbon M50NiL and high-carbon M50 bearing steels were adopted in this work. The influence of decarburization on microstructures and growth kinetics of the nitrided layer over the decarburized layer is investigated. The metallographic analysis of the nitrided layer thickness indicates that high carbon content can hinder the growth of the nitrided layer, but if a short decarburization is applied prior to nitriding, the thickness of the nitrided layer can be significantly promoted. The analysis of nitriding kinetics shows that decarburization reduces the activation energy for nitrogen diffusion and enhances nitrogen diffusivity. Moreover, the effect of decarburization in air can promote surface microstructure refinement via spinodal decomposition during plasma nitriding.

Comparison of nitriding kinetics of austenitic stainless steel assisted with hollow cathode discharge in different potentials

In the present research 304 austenitic stainless steels were nitrided using hollow cathode discharge-assisted plasma and the effect of potential conditions on the nitriding kinetics was evaluated. The results indicate that nitriding kinetics is significantly promoted in cathode potential. CrN in the anode specimen is more difficult to form than those in the floating and cathode specimens and the temperature affects the modified layers seriously.

Nitriding of Sintered VT1-0 Titanium Alloy

Components produced from sintered titanium alloys are increasingly used in mechanical engineering but need to be protected for operation in corrosive environments because of porosity. Nitriding is one of the effective protection methods. The objective of the study was to analyze the nitriding kinetics of a sintered VT1-0 titanium alloy at atmospheric (105 Pa) and reduced (1 Pa) nitrogen pressure at 800, 850, and 900°C for 5, 10, and 20 h versus a wrought titanium alloy of the same composition. The study employed discrete gravimetry, hardness measurement, optical and electronic metallography, profilometry, and X-ray diffraction. The nitriding kinetics of the VT1-0 titanium alloy sintered from titanium powder alone and from a mixture of titanium and titanium hydride powders was studied in comparison with wrought titanium. Under the nitriding parameters in question, the weight gain of all samples was described by a parabolic law and the weight gain of the sintered samples was higher than that of the wrought ones. The kinetic parameters of nitriding for the sintered and wrought VT1-0 titanium alloys were determined for the first time in the test temperature–time and gas dynamic conditions. Changes in the surface and near-surface layers depending on metal porosity were shown. After nitriding, the surface microhardness of the sintered titanium was lower than that of the wrought titanium samples because of porosity, and the strengthened surface layer was thicker. The nitride film formed on the sintered titanium alloy was thinner and less distorted than that on the wrought titanium alloy. The results can be used to develop nitriding conditions for sintered VT1-0 titanium products.

Control of kinetics of plasma assisted nitriding process of Ni-base alloys by substrate roughness

AbstractThe present study investigated the effect of surface roughness on plasma assisted nitriding (PAN) process kinetics of Ni-base alloys. Two model alloys, namely Ni-10Cr and Ni-14Cr-4Al (wt.%) and commercial Rene 80, were examined. To elucidate the effect of surface roughness on nitriding kinetics, three methods of surface preparation were used, (1) polishing up with 1 μm diamond suspension, (2) grinding up to 220 grit sand-paper, and (3)grit blasting. The samples from each type of material were nitrided under the same conditions and investigated after processing. It was found that increase in roughness results in decreasing nitriding kinetics. The decrease of nitriding kinetics depends on alloy chemical composition, namely more complex chemistry resulted in smaller decrease of kinetics. Moreover, grit-blasting was found to be an improper method for surface preparation for PAN. The responsible mechanism for the effect of surface roughness on PAN kinetics of Ni-base alloys was proposed.