Evaluation of boron mobility on the phases FeB, Fe2B and diffusion zone in AISI 1045 and M2 steels

The present study evaluates the growth kinetics of boride layers at the material surface on AISI 1045 and M2 steels during the paste boriding process. This surface hardening technique produces on the material two characteristic phases FeB, Fe₂B and a transition zone, denominated diffusion zone, in the layer/substrate interface. The thermochemical treatment was done at three different temperatures: 1193, 1223 and 1273 K with two treatment times: 2 and 6 h for the 1045 steel, and 1223, 1253 and 1273 K with the same treatment times for M2 steel, modifying the boron potentials in equilibrium at the substrates surfaces. Using the mass balance equation, and assuming a linear concentration profile at the interfaces, the mobility of boron was determined on both types of steels. The influence of boron potential, treatment time and temperatures is clearly observed on the growth kinetics of boride layers.     

Growth kinetics of borided layers: Artificial neural network and least square approaches

The present study evaluates the growth kinetics of the boride layer Fe₂B in AISI 1045 steel, by means of neural networks and the least square techniques. The Fe₂B phase was formed at the material surface using the paste boriding process. The surface boron potential was modified considering different boron paste thicknesses, with exposure times of 2, 4 and 6 h, and treatment temperatures of 1193, 1223 and 1273 K. The neural network and the least square models were set by the layer thickness of Fe₂B phase, and assuming that the growth of the boride layer follows a parabolic law. The reliability of the techniques used is compared with a set of experiments at a temperature of 1223 K with 5 h of treatment time and boron potentials of 2, 3, 4 and 5 mm. The results of the Fe₂B layer thicknesses show a mean error of 5.31% for the neural network and 3.42% for the least square method.     

Effect of boron paste thickness on the growth kinetics of polyphase boride coatings during the boriding process

The growth kinetics of FeB and Fe₂B phases forming on AISI M2 steel by paste boriding was studied using different values of paste thickness, treating temperature and exposure time. The growth of iron boride layers is described by the mass balance equation between phases in thermodynamic equilibrium, assuming that the boron concentration at the interfaces remain constant during the treatment. The experimental results show that boron mobility and growth kinetics of iron borides are considerably increased when the paste thickness is increased at constant values of temperature and exposure time.     

The pack-boronizing of pure vanadium under a controlled atmosphere

Pack boronizing of pure vanadium was performed at 1100 °C for 4, 8, 12 and 16 h under a controlled atmosphere. Characterization of the boride formed on the surface of pure vanadium was carried out by metallographic techniques, profilometry, SEM-EDS, XRD and microhardness measurements. The metallographic studies revealed that a single boride layer with dense, compact and relatively smooth morphology was formed on the surface of pure vanadium. The interface between boride layer and base metal was wavy in nature. The formation of only the VB₂ phase on pure vanadium was confirmed by surface and cross-sectional XRD analysis. The microhardness of the boride layer was approximately 3700 HV for all boriding times. Fracture toughness of the boride layer was evaluated using Vickers indentation, giving the value of 2.1-5.9 and 1.7-3.4 MPa m^1/2 for Palmqvist and median/radial approaches, respectively. Thickness of the boride layer increased almost parabolically from about 23 to 50 μm with boriding time. Surface roughness of the coating was relatively increased from approximately 0.58 to 2.25 μm by boriding duration.     

Growth kinetics of iron boride layers: Dimensional analysis

Dimensional analysis is presented as a powerful tool in the study of the paste boriding process. In particular, a dimensional method is used to study the growth kinetics of the boride layers FeB and Fe₂B. Experiments were performed in AISI 1045 steel and AISI M2 steel, to test the suggested model. Samples of 1045 steel were prepared and treated using boron paste thickness of 3-5 mm, at temperatures of 1193, 1223 and 1273 K, with 2, 4 and 6 h of treatment time. The M2 specimens had boron paste thickness of 3 and 4 mm and temperatures of 1223, 1253 and 1273 K for 2 and 6 h. Results indicate that the growth of boron layers obeys power laws of the form y = αx^β, where α and β constants are a function of the material and the interface of interest. Validation of the model was carried out using experimental data with an average error percentage of 7.6% for Fe₂B in 1045 steel, 15.8% for FeB and 3.4% for Fe₂B in M2 steel.     

Laser controlled melting of pre-treated zirconia surface

Laser treatment of pre-prepared zirconia surface is carried out. The pre-prepared surface, prior to laser treatment, consists of 50 μm carbon film and 7% titanium carbide particles, which are imbedded in the carbon film. The microstructural and morphological changes in the laser treated surface layer are examined using optical and scanning electron microscopes, energy dispersive spectroscopy, and X-ray diffraction. The fracture toughness of the laser treated surface is measured and the residual stress formed at the surface vicinity is determined from the X-ray diffraction technique. It is found that the microhardness of the laser treated surface increased slightly due to the dense layer formed at the surface vicinity. However, the laser treatment process reduces the fracture toughness of the surface due to improved surface hardness and the residual stress formed in the surface vicinity.     

Evaluation of the tool life and fracture toughness of cutting tools boronized by the paste boriding process

The present study evaluates the tool life and the fracture toughness of AISI M2 steel cutting tools boronized by the paste boriding process. The treatment was done in selective form on the tool tips of the steels. The temperatures were set at 1173 and 1273 K with 4 h of exposure time and modifying the boron carbide paste thicknesses in 3 and 4 mm. Microindentation fracture toughness method was used on the borided tool at the temperature of 1273 K and a 4 mm paste thickness, with a 100 g load at different distances from the surface. Also, the borided cutting tools were worn by the turning process that implied the machining of AISI 1018 steel increasing the nominal cutting speed, of 55 m/min, in 10 and 25% and maintaining the feed and the depth cut constants. The tool life was evaluated by the Taylor's equation that shows the dependence of the experimental parameters of the boriding process.     

The effect of substrate bias voltages on impact resistance of CrAlN coatings deposited by modified ion beam enhanced magnetron sputtering

CrAlN coatings were deposited on silicon and AISI H13 steel substrates using a modified ion beam enhanced magnetron sputtering system. The effect of substrate negative bias voltages on the impact property of the CrAlN coatings was studied. The X-ray diffraction (XRD) data show that all CrAlN coatings were crystallized in the cubic NaCl B1 structure, with the (1 1 1), (2 0 0) (2 2 0) and (2 2 2) diffraction peaks observed. Two-dimensional surface morphologies of CrAlN coatings were investigated by atomic force microscope (AFM). The results show that with increasing substrate bias voltage the coatings became more compact and denser, and the microhardness and fracture toughness of the coatings increased correspondingly. In the dynamic impact resistance tests, the CrAlN coatings displayed better impact resistance with the increase of bias voltage, due to the reduced emergence and propagation of the cracks in coatings with a very dense structure and the increase of hardness and fracture toughness in coatings.     

Characterization of bilayer coatings of TiN/ZrN grown using pulsed arc PAPVD

Nitride coatings have been used to increase hardness and to improve the wear and corrosion resistance of structural materials. Coatings of TiN/ZrN were grown on stainless steel substrates using a physical vapour deposition system assisted by pulsed arc plasma (PAPVD). The coatings have been characterized by X-ray diffraction (XRD) in order to identify the present phases of the films, microstrain level generated, crystallite size and the variation of the lattice parameter. The results showed plane orientations (1 1 1) and (2 0 0) in both TiN and ZrN films. Morphology surface analysis of the samples were performed using a scanning probe microscope to characterize the grain size and roughness in the mode of the atomic force microscopy (AFM) hence it was observed that the root-mean-squared (rms) roughness for ZrN is smaller than for TiN. Besides elastic and friction properties of the films were characterized qualitatively, and then, they were compared with those of the substrates by using force modulation microscopy (FMM) and lateral force microscopy (LFM) modes. In addition, an elemental analysis of the samples was realized by means of energy dispersive spectroscopy (EDS). Both, XRD and AFM results are given as a function of the number of shots. Chemical states of the TiN and ZrN films were determined by X-ray photoelectron spectroscopy (XPS).     

A simple model for the growth kinetics of Fe2B iron boride on pure iron substrate

The present work evaluates the growth kinetics of Fe₂B iron boride forming on iron substrate by means of a diffusion model in the temperature range 1223-1323 K. The model takes into account the effect of the boride incubation time during the formation of Fe₂B phase. The parabolic growth constant at the (Fe₂B/Fe) interface and the mass gain generated by this treatment were estimated. Likewise a simple relationship was proposed to describe the variation of the parabolic growth constant as a function of both the temperature and the boron content in the Fe₂B phase. Furthermore, the simulation results show a good agreement with our experimental results.     

A nondestructive method for estimation of the fracture toughness of CrMoV rotor steels based on ultrasonic nonlinearity

The objective of this paper is to develop a nondestructive method for estimating the fracture toughness (K(IC)) of CrMoV steels used as the rotor material of steam turbines in power plants. To achieve this objective, a number of CrMoV steel samples were heat-treated, and the fracture appearance transition temperature (FATT) was determined as a function of aging time. Nonlinear ultrasonics was employed as the theoretical basis to explain the harmonic generation in a damaged material, and the nonlinearity parameter of the second harmonic wave was the experimental measure used to be correlated to the fracture toughness of the rotor steel. The nondestructive procedure for estimating the K(IC) consists of two steps. First, the correlations between the nonlinearity parameter and the FATT are sought. The FATT values are then used to estimate K(IC) using the K(IC) versus excess temperature (i.e., T-FATT) correlation that is available in the literature for CrMoV rotor steel.     

Simulation of the growth kinetics of the (FeB/Fe2B) bilayer obtained on a borided stainless steel

The present work is an attempt to simulate the growth kinetics of the (FeB/Fe₂B) bilayer grown on a substrate made of AISI 316 stainless steel by the application of the powder-pack boriding process, and using four different temperatures (1123, 1173, 1223 and 1273 K) and five exposure times (2, 4, 6, 8 and 10 h). The adopted diffusion model solves the mass balance equation at each growth front: (FeB/Fe₂B or FeB/substrate) under certain assumptions and without considering the diffusion zone. To consider the effect of the incubation times for the borides formation, the temperature-dependent function ?(T) was incorporated in the model. To validate this model, a computer code written in Matlab (version 6.5), was developed with the purpose of simulating the kinetics of the boride layers. This computer code uses the following parameters as input data: (the boriding temperature, the treatment time, the upper and lower limits of boron concentration in each iron boride, the diffusion coefficients of boron in the FeB and Fe₂B phases as well as the ?(T) parameter). The outputs of the computer code are the parabolic growth constant at each growth front and the thicknesses of the FeB and Fe₂B layers. A good agreement was obtained between the experimental parabolic growth constants taken from a reference work [I. Campos-Silva et al., Formation and kinetics of FeB/Fe₂B layers and diffusion zone at the surface of AISI 316 borided steels, Surf. Coat Technol., 205 (2010) 403-412] and the simulated values of the parabolic growth constants (k_FeB and k₁). The present model was also able to predict the thicknesses of the FeB and Fe₂B layers at a temperature of 1243 K during 3 and 5 h.In addition, the mass gain at the material surface was also estimated as a function of the time and the upper boron content in each iron boride phase. It was shown that the simulated values of the generated mass gain are very sensitive to the increase of both temperature and the upper boron contents in the FeB and Fe₂B phases.     

Kinetics of the formation of Fe2B layers in gray cast iron: Effects of boron concentration and boride incubation time

The growth kinetics of Fe₂B layers formed at the surface of gray cast iron were evaluated in this study. The pack-boriding process was applied to produce the Fe₂B phase at the material surface, and the variables included three temperatures (1173, 1223 and 1273 K) and four exposure times (2, 4, 6 and 8 h). Taking into account the growth fronts obtained at the surface of the material and the mass balance equation at the Fe₂B/substrate interface, the boron diffusion coefficient on the borided phase was estimated for the range of treatment temperatures. Likewise the parabolic growth constant, the instantaneous velocity of the Fe₂B/substrate interface, and the weight gain in the borided samples were established as a function of the parameters τ(t) and α(C), which are related to the boride incubation time (t₀(T)) and boron concentration at the Fe₂B phase, respectively. Observation of the growth kinetics of the Fe₂B layers in gray cast irons suggest an optimum value of boron concentration that is in good agreement with the set of boriding experimental conditions used in this work.     

On the growth strain origin and stress evolution prediction during oxidation of metals

High temperature oxidation of metals leads to residual stresses in the metal and in the oxide. In this work, we try to predict the evolution of the residual stresses in the growing oxides layers, during isothermal oxidation. The origin of these stresses is based on the microstructural model of Clarke, however, another justification is proposed, assuming a proportional dependence of the growth strain with the oxide layer thickness. Using the mechanics of thin layers, as well as the analysis proposed to describe the growth strain, a system of equations are deduced that predict the stresses evolution with oxidation time. Numerical analysis is performed, leading to a set of theoretical curves.     

太阳光谱辐射计的信号传递与性能表征模型研究

太阳光谱辐射计作为一种专用光谱辐射测量仪器,对于其宽谱段和大动态范围的测量特点,准确完整地表征分光性能是十分重要的。研究内容旨在为太阳光谱辐射计的研制与验证提供明确的理论依据和测试方法,给出清晰而准确的机理模型和指标模型对光谱仪器系统的设计和评估提供指导。因此,重点阐述了光谱仪器系统的信号传递模型到性能指标模型的推导和建立过程。线谱扩展函数这种串联卷积模型能够综合反映仪器的各个元件对系统的影响,而且很容易通过窄带线谱光源测试而得到,并且线谱扩展函数矩阵能够清晰完整地给出光谱仪器的分光性能细节特性。而在线谱扩展函数的基础上再进一步提取关键几何特征,通过简单的算法定义即可得到半高全宽(FWHM),带外抑制和带外辐射这三项示性函数指标,可以很好地对分光系统的性能进行定量表征,是十分有效的光谱仪器系统性能评估的指标模型。     

X-ray photoelectron spectroscopy study of the growth kinetics of biomimetically grown hydroxyapatite thin-film coatings

Hydroxyapatite (HA) thin-film coatings grown biomimetically using simulated body fluid (SBF) are desirable for a range of applications such as improved fixation of fine- and complex-shaped orthopedic and dental implants, tissue engineering scaffolds and localized and sustained drug delivery. There is a dearth of knowledge on two key aspects of SBF-grown HA coatings: (i) the growth kinetics over short deposition periods, hours rather than weeks; and (ii) possible difference between the coatings deposited with and without periodic SBF replenishment. A study centred on these aspects is reported. X-ray photoelectron spectroscopy (XPS) has been used to study the growth kinetics of SBF-grown HA coatings for deposition periods ranging from 0.5 h to 21 days. The coatings were deposited with and without periodic replenishment of SBF. The XPS studies revealed that: (i) a continuous, stable HA coating fully covered the titanium substrate after a growth period of 13 h without SBF replenishment; (ii) thicker HA coatings about 1 μm in thickness resulted after a growth period of 21 days, both with and without SBF replenishment; and (iii) the Ca/P ratio at the surface of the HA coating was significantly lower than that in its bulk. No significant difference between HA grown with and without periodic replenishment of SBF was found. The coatings were determined to be carbonated, a characteristic desirable for improved implant fixation. The atomic force and scanning electron microscopies results suggested that heterogeneous nucleation and growth are the primary deposition mode for these coatings. Primary osteoblast cell studies demonstrated the biocompatibility of these coatings, i.e., osteoblast colony coverage of approximately 80%, similar to the control substrate (tissue culture polystyrene).     

Comparison of growth stress measurements with modelling in thin iron oxide films

High temperature oxidation of metals leads to residual stresses both in the metal and in the growing oxide. In this work, the evolution of this residual stresses is theoretically predicted in the growing oxide layers. The origin of these stresses is based on a microstructural model. Using experimental results providing from the oxidation kinetics, and an analysis proposed to describe the growth strain occurring in the thin layers, a set of equations is established allowing determining the stresses evolution with oxidation time. Then, the model is compared with experimental results obtained on both α-Fe and phosphated α-Fe, oxidised at different temperatures. Numerical data are extracted from experiments either with an asymptotic formulation or with an inverse method. These two methods give good agreement with experiments and allow extracting the model parameters.     

甲烷固体氧化物燃料电池多物理场模拟及积碳分析

建立了相关实验电池多物理场模型,该模型包括了电化学反应的电化学势平衡方程、甲烷水汽重整的通用速率方程和描述阳极复杂组分气体输运的菲克定律形式尘气模型. 该理论模型的电流～电压 曲线与实验数据很好地吻合,验证了理论模型的有效性. 理论分析发现,低水汽含量甲烷重整反应中水汽的反应级数为1. 理论模型的数值仿真计算还给出多个物理量的详细信息. 据此对碳沉积机制进行深入分析,清楚地描述了工作电流对抑制焦炭生成的机制. 分析比较了积碳活性的两个表达式,发现它们都可以正确地定性反映积碳活性变化趋势,但定量数值并不确切;阳极扩散层降低积碳临界电流的机制也获得了解释. 值得指出的是,虽然积碳活性模型只是定性正确,但分析表明积碳临界电流的降低却可以通过积碳活性模型进行定量解释.     

Theoretical description of adatom migration in two-dimensional highly-ordered states

Analytical expressions for chemical, jump, and tracer diffusion coefficients are obtained for interacting lattice gases on a square lattice. Strongly repulsive nearest neighbor interactions cause the formation of a highly-ordered c() state in the vicinity of half coverage. It is shown that only strongly correlated successive adatom jumps contribute to the particle flow. This allows to describe the adatom kinetics by considering an almost ideal lattice gas of defects. Two types of defects are considered, adatoms in the empty sublattice and vacancies in the filled sublattice of the c() ordered state. The diffusion equations for these defects are developed considering the generation and recombination of defects. In addition we have considered adatom transport caused by the motion of defect pairs (dimers). Dimer transport mechanism prevails in the high coverage region. The characteristic features of the various diffusion coefficients near half coverage are analyzed and discussed. The theory is compared with the results of sophisticated Monte-Carlo simulations which have been executed with the use of a fully parallelized algorithm on a Cray T3E (LC784-128). The agreement between theoretical and MC results is excellent if the motion of dimers at is taken into account. Received 24 June 1998