Growth Kinetics Of Layers Research Articles

Kinetics of borided gear steels

In this study, the case properties and diffusion kinetics of GS18NiMoCr36 (GS18), GS22NiMoCr56 (GS22) and GS32NiCrMo6.4 (GS32) gear steels borided in Ekabor-II powder were investigated by conducting a series of experiments at temperatures of 1123, 1173 and 1223 K for 2, 4 and 6 h. The boride layer was characterized by optical microscopy, X-ray diffraction technique and micro-Vickers hardness tester. X-ray diffraction analysis of boride layers on the surface of the steels revealed the existence of FeB, Fe2B, CrB and Cr2B compounds. The thickness of the boride layer increases by increasing boriding time and temperature for all steels. The hardness of the boride compounds formed on the surface of the steels GS18, GS22 and GS32 ranged from 1624 to 1905 HV0,05, 1702 to 1948 HV0,05, and 1745 to 2034 HV0,05 respectively, whereas Vickers hardness values of the untreated steels GS18, GS22 and GS32 were 335 HV0,05, 358 HV0,05 and 411 HV0,05, respectively. The activation energies (Q) of borided steels were 228.644 kJ/mol for GS18, 280.609 kJ/mol for GS22 and 294.359 kJ/mol for GS32. The growth kinetics of the boride layers forming on the GS18, GS22 and GS32 steels and the thickness of boride layers were also investigated.

Effects of electromagnetic induction on growth kinetics of case hardened layers

Induction heating using medium or high frequency currents has been proposed as a means of achieving improved layer growth kinetics during case hardening. Experimental research on the carburising of 21TiMnCr12 (18HGT) steel in natural gas by induction heating with medium frequency currents (2000 Hz) has demonstrated significant process acceleration. A model of the carburising process has been derived by statistical processing of the experimental data from a designed experimental programme. The diffusion coefficients and the adsorption rate constants in these processing conditions were derived using the Popov methodology, based on criteria solution of Fick’s second law. The diffusion coefficient and adsorption rate constant values obtained were approximately two orders of magnitude higher than those obtained for conventional processing in resistance furnaces (∼10−5 versus ∼10−7 cm2 s−1). The form of the iso-property fields (constant values of the total case hardened depth for different values of the heat and time process parameters) calculated from the mathematical model demonstrates the highly accelerated kinetics obtained during induction carburising.

In situ ceramic layer growth on coated fuel particles dispersed in a zirconium metal matrix

The extent and nature of the chemical interaction between the outermost coating layer of coated fuel particles embedded in zirconium metal during fabrication of metal matrix microencapsulated fuels were examined. Various particles with outermost coating layers of pyrocarbon, SiC, and ZrC have been investigated in this study. ZrC–Zr interaction was the least substantial, while the PyC–Zr reaction can be exploited to produce a ZrC layer at the interface in an in situ manner. The thickness of the ZrC layer in the latter case can be controlled by adjusting the time and temperature during processing. The kinetics of ZrC layer growth is significantly faster from what is predicted using literature carbon diffusivity data in ZrC. SiC–Zr interaction is more complex and results in formation of various chemical phases in a layered aggregate morphology at the interface.

Microstructural evolution during nitriding, finite element simulation and experimental assessment

A finite element simulation of nitriding is proposed in this paper, using the analogy between diffusion and heat conduction, to overcome the shortcomings of the classical internal oxidation model in predicting the kinetics of layer growth and nitrogen distribution during nitriding. To verify the model, a typical gas nitriding has been carried out on an axisymmetric specimen. Treated specimen has been characterized using optical microscopy (OM), scanning electron microscopy (SEM), micro-hardness and X-Ray diffraction (XRD) measurements. It was found that the so-called diffusion zone can be divided into two parts with different influence on the mechanical characteristics including residual stress and hardening. First layer which is a two phase region of ferritic matrix and γ′ (Fe4N) makes further improvement with respect to the second layer which is a solid solution of nitrogen in ferrite. The formation of that two phase region, which is not predicted by classical model, can be efficiently recognized by the proposed model. It is also proved that the model has the ability to consider the geometry dependency of layer growth and formation in nitriding.

Effect of a Gas Pressure on the Growth Rate of AlN Layer

The dependence of the layer growth rate on a gas (argon, nitrogen) pressure inside the reactor has been examined in order to analyze the conditions of growth of AlN thick layers and bulk crystals by the sublimation sandwich-method. It is shown that the layer growth rate steadily increases as the pressure in the reactor decreases in a wide pressure interval 1–0.02 bar. This suggests that a key role in the layer growth kinetics is played by the source-to-substrate transfer of the components (Al, N), rather than the adsorption - desorption processes on the source and substrate surface. In addition the growth rate in argon atmosphere is much higher than in nitrogen one for the high pressures and is practically the same for the lowest (0.05 – 0.02 bar).

Effect of mechanical activation on jell boronizing treatment of the AISI 4140

The article presents the effect of mechanical activation on the growth kinetics of boride layer of boronized AISI 4140 steel. The samples were boronized by ferroboron+(SiO2–Na2O) powders for 873–1173K temperature and 2, 4, 6 and 8h times, respectively. The morphology and types of borides formed on the surface of AISI 4140 steel substrate were analyzed. Layer growth kinetics were analyzed by measuring the extent of penetration of FeB and Fe2B sublayers as function of treatment time and temperature in the range of 873–1173K. High diffusivity was obtained by creating a large number of defects through mechanical activation in the form of nanometer sized crystalline particles through the repeated fracturing and cold-welding of the powder particles, and a depth of 100μm was found in the specimen borided by the 2h MA powders, for 4h and 1073K, where 2000–2350 HV were measured. Consequently, the application conditions of boronizing were improved by usage of mechanical activation. The preferred Fe2B boride without FeB could be formed in the boride layer under 973K boronizing temperature by mechanically activated by ferroboron+sodium silicate powder mixture due to the decrease of the activation energy.

Simulation of the growth kinetics of boride layers formed on Fe during gas boriding in H2-BCl3 atmosphere

The modeling of the boriding kinetics is considered as a necessary tool to select the suitable process parameters for obtaining boride layer of an adequate thickness. Therefore, the simulation of the growth kinetics of boride layers has gained much attention for last years. The majority of the published works described the kinetics of the pack-boriding or paste-boriding. In this study, the model of growth kinetics of two-phase boride layer (FeB+Fe2B) on pure Fe was proposed for gas boriding. Displacements of the two interfaces (FeB/Fe2B and Fe2B/substrate) resulted from a difference of the arrival flux of interstitial boron atoms to one phase and the departure flux of the boron atoms from this phase to the second phase. The mass balance equations were formulated. The measurements of thickness of both zones (FeB and Fe2B), for different temperature of boriding, were used for calculations. Based on the experimental data, the parabolic growth constants AFeB and BFe2B versus the temperature of boriding were determined. The linear relationships were accepted. As a consequence, the activation energies (QFeB and QFe2B) were calculated. The calculated values were comparable to other data derived from gas boriding. The presented model can predict the thicknesses of the FeB and Fe2B zones (XFeB and YFe2B, respectively) formed on pure Fe during gas boriding. Additionally, the diffusion annealing after boriding was analyzed. This process was carried out in order to obtain a single-phase boride layer (Fe2B). The relationship between the reduction in FeB zone (dXFeB) and the growth in Fe2B phase (dYFe2B) was determined. The time tXFeB=0, needed for the total elimination of FeB phase in the boride layer was calculated and compared to the experimental data.

Growth kinetics of the boride layers formed on SAE 1035 steel

Growth kinetics of the boride layers formed on SAE 1035 steel has been investigated during the boriding treatment. This treatment was carried out in slurry salt bath consisting of borax, boric acid and ferrosilicon for temperatures ranging from 1073 to 1273 K and treatment times of 2, 4 and 8 h. The presence of both FeB and Fe2B phases formed on the surface of SAE 1035 steel was confirmed by X-ray diffraction. Scanning electron microscopy (SEM) and optical microscopy examinations showed that the boride layers have a saw-tooth morphology. The thickness of boride layers was found to be increased when the treatment time and the boriding temperature increase, its value ranged from 20 to 387 μm. The average hardness of the boride layer was about 1760 ± 200 HV0.1, while the hardness of un-borided steel was about 225 ± 20 HV0.1. The fracture toughness of boride layers (KC) was found to be ranged between 3.42 and 4.57 MPa m1/2. The kinetic study showed a parabolic relationship between the boride layer thickness and the process time. The value of boron activation energy for the borided SAE 1035 steel was estimated as 227.51 kJ mol-1.

Reactive Chemical Vapour Deposition of Titanium Carbide from H2-TiCl4 Gas Mixture on Pyrocarbon: A Comprehensive Study

Abstract In Reactive Chemical Vapour Deposition (RCVD), the absence of one element of the deposited carbide in the initial gas phase involves the consumption/conversion of the solid substrate. In this way, the growth of a continuous carbide layer on the substrate requires solid-phase diffusion of the reagent. In this work, a parametric study of the RCVD of titanium carbide from pyrocarbon (PyC) and an H2-TiCl4 mixture has been carried out. Conversion ratio, PyC consumption and carbide layer growth kinetics have been determined at 1000C. The influence of the H2/TiCl4 dilution ratio has been also investigated. The apparent inter-diffusion coefficient of the carbon through the TiC deposited layer and the direct apparent reaction rate were determined from a comparison between simulations based on a Deal-Grove-type model and the experimental results. The study has been completed with FTIR spectrometry analyses of the gases.

Microstructure, Growth Kinetics, and Abrasive Wear Resistance of Boride Layers on Fe–30% Cr Alloy

Two boride layers are found to form at the interface between reacting phases in the course of boriding a Fe–30% Cr alloy in boron powder with KBF4 (activator) in the temperature range of 850–950°C and reaction times 3600–43200 sec (1–12 h). Each of these layers is single-phase structurally (crystallographically) and two-phase compositionally (chemically). The outer boride layer bordering boron consists of the crystals of the (Fe, Cr)B and (Cr, Fe)B compounds, while the inner layer adjacent to the alloy base comprises the crystals of the (Fe, Cr)2B and (Cr, Fe)2B compounds. The characteristic feature of both layers is a profound texture. Diffusional layer-growth kinetics are close to parabolic and can alternatively be described by a system of two non-linear differential equations dx/dt = (kB/x) – (rgkFe/py), dy/dt = (kFe/y) – (qkB/sgx), where x is the outer FeB layer thickness (m), y is the inner Fe2B layer thickness (m), kB is the FeB layer growth-rate constant (m2⋅sec–1), kFe is the Fe2B layer growth-rate constant (m2⋅sec–1), g is the ratio of the FeB and Fe2B molar volumes, p = q = r = 1, and s = 2 (factors from the chemical formulae of FeB and Fe2B). The temperature dependence of the layer growth-rate constants obeys a relation of the Arrhenius type K = Aexp (–E/RT), where K stands for any constant, A is the frequency factor, E is the activation energy, R is the gas constant, and T is the absolute temperature. Application of the least-squares fit method yielded the following equations: kB = 3.42⋅10–8⋅exp(–175.4 kJ × × mol–1/RT) m+⋅sec–1, kFe = 7.45⋅10–9 exp(–144.6 kJ⋅mol–1/RT) m2⋅sec–1. Microhardness values are 18.1 GPa for the outer boride layer, 15.2 GPa for the inner layer, and 1.75 GPa for the alloy base. The dry abrasive wear resistance of the outer boride layer, found from mass loss measurements, is more than 300 times greater than that of the Fe–30% Cr alloy base. Such a huge increase in wear resistance is due to the microstructure of boride layers having a peculiar regular arrangement of enhanced rigidity.

Growth kinetics of titanium borides produced by CRTD-Bor method

In this study, a new boriding method called CRTD-Bor (Cathodic Reduction and Thermal Diffusion based boriding) was applied to titanium in a systematic manner and the effects of boriding process parameters on the morphological and chemical state of boride layers were explored. The presence of both TiB and TiB2 phases was confirmed by the X-ray diffraction technique. The cross-sectional examination of borided titanium verified the boride layers consisted of a homogeneous TiB2 phase on the top and TiB whiskers toward the substrate. Moreover, the growth kinetics of the TiB2 layers forming on titanium substrates was investigated during CRTD-bor which was performed at a constant current density of 200mA/cm2 in a borax based electrolyte at temperatures ranging from 900 to 1100°C for periods of 15 to 120min. The rate of the TiB2 layer formation was found to have a parabolic character at all applied process temperatures. The activation energy (Q) and the pre-exponential factor (Ko) of the TiB2 layer were determined as 189.9kJ/mol and 4.66×10−7m2s−1, respectively. The specific empirical equation that can be used to estimate the thickness of the TiB2 layers (dTiB2) for the conditions explored in the study is dTiB2=682.67exp−22833/Tt1173K≤T≤1373K;200mA/cm2.

Al-Cd Alloy Formation by Aluminum Underpotential Deposition from AlCl3+NaCl Melts on Cadmium Substrate

Aluminum was incorporated into a polycrystalline cadmium electrode surface by underpotential deposition from equimolar AlCl3+ NaCl melt at 473 K, 523 K, and 573 K (200 °C, 250 °C, and 300 °C). The process was studied by linear sweep voltammetry and potentiostatic deposition/galvanostatic striping. The deposits were characterized X-ray diffraction (XRD), Auger electron spectroscopy (AES), and electron probe microanalyzer (EPMA). The electrochemical measurements showed evidence of Cd-Al alloys being formed but they could not be identified. The growth kinetics of the Cd-Al layers of various proportion and depths that depended on temperature and deposition time were described.

Modeling the Gas Nitriding Process of Low Alloy Steels

The effort to simulate the nitriding process has been ongoing for the last 20 years. Most of the work has been done to simulate the nitriding process of pure iron. In the present work a series of experiments have been done to understand the effects of the nitriding process parameters such as the nitriding potential, temperature, and time as well as surface condition on the gas nitriding process for the steels. The compound layer growth model has been developed to simulate the nitriding process of AISI 4140 steel. In this paper the fundamentals of the model are presented and discussed including the kinetics of compound layer growth and the determination of the nitrogen diffusivity in the diffusion zone. The excellent agreements have been achieved for both as-washed and pre-oxided nitrided AISI 4140 between the experimental data and simulation results. The nitrogen diffusivity in the diffusion zone is determined to be constant and only depends on the nitriding temperature, which is ~5 × 10−9 cm2/s at 548 °C. It proves the concept of utilizing the compound layer growth model in other steels. The nitriding process of various steels can thus be modeled and predicted in the future.

Modeling the Nitriding of Steel by Compound Layer Growth Model

The effort to simulate the nitriding process has been ongoing for the last 20 years. Most of the work has been done to simulate the nitriding process of pure iron. In the present work, the compound layer growth model has been developed to simulate the nitriding process of AISI 4140 steel. In this paper, the fundamentals of the model are presented and discussed including the kinetics of compound layer growth and the determination of the boundary conditions. The experimental verification of the model is also presented. It proves the concept of utilizing the compound layer growth model in other steels. The nitriding process of various steels can thus be modeled and predicted in the future.

Nucleation and growth kinetics of graphene layers from a molten phase

Nucleation and growth of graphene layers from Ni–C melts were investigated. It is shown that upon cooling of supersaturated liquids, graphite will grow either with flake or sphere morphology depending on the solidification rate and degree of supersaturation. At small solidification rates, graphite crystals are normally bounded by faceted low index basal and prismatic planes which grow by lateral movement of ledges produced by 2D-nucleation or dislocations. At higher growth rates, however, both interfaces become kinetically rough, and growth becomes limited by diffusion of carbon to growing interface. The roughening transition from faceted to non-faceted depends on the driving force and nature of growing plane. Due to high number of C–C dangling bonds in prismatic face, its roughening transition occurs in smaller driving force. As such, at intermediate rates, the prismatic interfaces become rough and grow faster while the basal plane is still faceted, leading to formation of flake graphite. At higher growth rates, both interfaces grow with a relatively similar rate leading to initiation of graphite sphere formation, which later grow by a multi-stage growth mechanism. An analytical model is developed to describe the size and morphology of graphite as a function of solidification parameters.

Modeling the Nitriding Process of Steels

Abstract This work describes the development of a model to simulate the compound layer growth kinetics for nitriding AISI 4140 steel based on a customized Lehrer diagram. The model can be used to calculate concentration profiles of nitrogen as a function of time and position during the nitriding process and to predict the thickness of the compound layer. Winner of the 2012 HTS-Bodycote Best Paper in Heat Treating Award endowed by Bodycote Thermal Process-North America.

Kinetics of growth of superhard boride layers during solid state diffusion of boron into titanium

Solid state boriding of titanium is a very effective and inexpensive way to create surface layers that can impart high hardness and wear resistance. In this work, the growth kinetics of hard titanium boride layers created by the solid-state boron (B) diffusion through titanium (Ti) surface at various temperatures has been determined. The hard boride layers comprised of a monolithic titanium diboride (TiB2) layer at the top and a titanium boride (TiB) sub-layer that mostly consisted of TiB whiskers with high aspect-ratio growing normal to the surface. The structure and the crystallography of the layers were also confirmed by X-ray diffraction. Experimental results show that the growth rates of the TiB2 as well as the composite (TiB2+TiB) coating layers follow the parabolic kinetics. It is shown that the thickness development as a function of time can be predicted reasonably accurately, using the error function solutions for the two simultaneously growing boride layers developed in our previous work. The maximum degree of growth as dictated by the diffusivity of B in the boride phases was achieved using a solid state B powder pack in an ambient furnace atmosphere.

Chemical compatibility of uranium based metallic fuels with T91 cladding

Studies related to development of fast reactor fuels based on ternary U–Pu–Zr and binary U–Pu alloys has been initiated in India for building a data base on thermo-physical and thermodynamic properties, fuel-clad compatibility etc. which are very useful to the fuel-designer to optimize the design feature and to predict the in-reactor fuel behaviour. Fuel-clad chemical compatibility is considered as one of the major concerns for metallic fuels. In the present investigation, the performance of Zr as fuel-clad chemical interaction (FCCI) barrier layer between U and T91 was evaluated by diffusion couple experiments. The growth kinetics of reaction layers at U/Zr and Zr/T91 interfaces were established. The growth kinetics of the reaction zone at both the U/Zr and Zr/T91 interfaces were determined at 973K from the plot of log (width) versus log (time). The value of reaction index n was found to be around 2 at both the U/Zr and Zr/T91 interfaces. The reaction constant (k) for the growth of reaction layer at the U/Zr interface was determined to be 2.07×10−8ms−1/2 at 973K. Similarly, the rate constant at the Zr/T91 interface was found to be 1.95×10−8ms−1/2 at 973K. The activation energy Q for the reaction at the Zr/T91 interface was determined and was found to be 54.7kJmole−1. The fuel-clad chemical compatibility between U–6Zr alloy and T91 steel was also investigated in the present study by diffusion couple experiments. The interdiffusion between U–6Zr and T91 at 973K resulted in the formation of three different layers at the interface. The mechanism of formation of these layers was analysed in detail.

Conditions of hydrochemical synthesis, composition, and structure of tellurium films

Conditions of the hydrochemical synthesis of thin tellurium fi lms by reduction of an aqueous solution of tellurous acid with hydrazine were determined. The redox potentials of tellurous acid and hydrazine and their difference in relation to pH of the medium and concentration of tellurite ions were calculated. The kinetics of tellurium layer growth on glass-ceramic substrates and the structure, composition, and morphology of the layers were studied.

The Influence of Chemical Composition of Stainless Steel on the Formation of Low Temperature Nitrided Layer

This paper presents results of investigations on gas nitride layers of stainless steel. Steel with ferritic structure, containing different chromium and nickel content was used in the experiments in order to compare the influence of these two main alloying elements on layer growth kinetics and layer structure. Particular attention was paid to formation of Sphase and expanded martensite which usually are formed during lowtemperature nitriding of stainless steel. It was observed, that chromium and nickel content does not have a significant influence on layer growth kinetics but the treatment temperature does. It was also stated that Sphase formation is possible both in nickel alloyed and nickelfree steel depending on treatment parameters.