14 research outputs found
Silicon based oxidation-resistant coatings on Ti6242 alloy by dynamic ion mixing
The influence of SixCy and SixNy amorphous coatings on the oxidation resistance of a Ti6242 (Ti–6Al–2Sn–4Zr–2Mo) alloy was investigated. They were produced at room temperature by the dynamic ion mixing technique combining physical vapour deposition with simultaneous bombardment with 120 keV Ar+ ions. Isothermal oxidation tests were carried out at 600 °C in 1 atm flowing synthetic air (80% N2, 20% O2) demonstrating a considerable reduction (not, vert, similartwo orders of magnitude) of the oxidation rate for at least 100 h. The structural modifications after oxidation were investigated by XPS, XRD, SEM, SIMS. The formation of SiO2 is detected as the main oxidation product in the coating and the formation of Ti–Si compounds is also observed in the coating/substrate interface region. The crystallisation of SixNy is not detected and for SixCy only some traces of β-SiC could exist. The improvement of oxidation resistance of Ti6242 is discussed in relation with the intrinsic properties of the coatings and with the interface mixing and ion beam densification
The Kinetic Model of Diffusion and Reactions in Powder Catalysts during Temperature Programmed Oxygen Isotopic Exchange Process
The mathematical model of diffusion in powder oxide catalysts during the process of temperature programmed oxygen isotopic exchange is proposed. The diffusion is considered together with the homogeneous and heterogeneous oxygen isotopic exchange processes. The matrix forms of exchange rate equations of simple and complex heteroexchange, and homoexchange reactions which obtain symmetrical forms are analyzed. The quantitative values of model parameters are found from the fitting of experimental data taken from literature of temperature programmed oxygen isotopic exchange process in catalysts ZrO2 and CeO2. The fittings show a good matching of model results with experimental data. The shapes of kinetic curves registered during temperature programmed oxygen isotopic exchange process are analyzed and the influence of various process parameters such as activation energies of simple and complex heteroexchange, oxygen surface concentration of catalyst, ratio of catalysts surface and volume of reactor, diffusion activation energy is considered. The depth profiles of diffusing oxygen species in oxide catalysts powder are calculated
Modeling of Nitrogen Penetration in Medical Grade CoCrMo Alloy during Plasma Nitriding
<p>For analysis of plasma nitriding process and nitrogen penetration into CoCrMo alloy the trapping-detrapping model is applied. This model is commonly used for analysis of stainless steel nitriding, however, in this work it is shown that the same nitrogen penetration mechanism takes place in CoCrMo alloys. From the fitting of experimental curves, taken from literature, it is found by the proposed model that diffusion coefficient depends on nitrogen concentration according to Einstein-Smoluchowski relation <em>D</em> µ 1/<em>C<sub>N</sub></em>. The diffusion coefficients for 400<sup>o</sup>C temperature nitriding of in CoCrMo are calculated. The shape of nitrogen depth profile curves are analyzed showing influence of different parameters such as detrapping activation energy, chromium concentration, etc.</p><p>DOI: <a href="http://dx.doi.org/10.5755/j01.ms.20.1.3458">http://dx.doi.org/10.5755/j01.ms.20.1.3458</a></p
Stress Induced Nitrogen Diffusion in Nitrided Austenitic Stainless Steel
The nitrogen transport mechanism in plasma nitrided austenitic stainless steel at moderate temperatures (around 400 °C) is explained by non-Fickian diffusion model. The model considers the diffusion of nitrogen in presence of internal stresses gradient induced by penetrating nitrogen as the next driving force of diffusion after concentration gradient. For mathematical description of stress induced diffusion process the equation of barodiffusion is used, which involves concentration dependant barodiffusion coefficient. For calculation of stress gradient it is assumed that stress depth profile linearly relates with the nitrogen concentration depth profile. The calculated nitrogen depth profiles in an austenitic stainless steel are in good agreement with experimental nitrogen profiles. The diffusion coefficient D = 1.68-10 -12 cm2/s for nitrogen in a plasma source ion nitrided 1Cr18Ni9Ti (18-8 type) austenitic stainless steel at 380 °C was found from fitting of experimental data. It is shown that nitrogen penetration depth and nitrogen surface concentration increases with nitriding temperature nonlinearly.http://dx.doi.org/10.5755/j01.ms.17.1.241</p
Kinetic Modeling of Grain Boundary Diffusion: The Influence of Grain Size and Surface Processes
Based on rate equations, the kinetics of atom adsorption, desorption, and diffusion in polycrystalline materials is analyzed in order to understand the influence of grain boundaries and grain size. The boundary conditions of the proposed model correspond with the real situation in the electrolytes of solid oxide hydrogen fuel cells (SOFC). The role of the ratio of grain boundary and grain diffusion coefficients in perpendicular and parallel (to the surface) concentration profiles is investigated. In order to show the influence of absolute values of grain and grain boundary diffusion coefficients, we select four different cases in which one of the diffusion coefficients is kept constant while the others vary. The influence of grain size on diffusion processes is investigated using different geometrical models. The impact of kinetic processes taking place on the surface is analyzed by comparing results obtained assuming the first layer as a constant source and then involving in the model the processes of adsorption and desorption. It is shown that surface processes have a significant influence on the depth distribution of diffusing atoms and cannot be ignored. The analytical function of overall concentration dependence on grain and grain boundary volume ratio (Vg/Vgb) is found. The solution suggests that the concentration increases as a complementary error function while Vg/Vgb decreases
Fizika. Mechaninė ir molekulinė fizika
Vytauto Didžiojo universiteta
Kinetics of Ni:C Thin Film Composition Formation at Different Temperatures and Fluxes
In this work analysis considering Ni:C thin films growth on thermaly oxidized Si substrate by proposed kinetic model is presented. Model is built considering experimental results where microstructure evolution as a function of the substrate temperature and metal content of Ni:C nanocomposite films grown by hyperthermal ion deposition is investigated. The proposed kinetic model is based on the rate equations and includes processes of adsorption, surface segregation, diffusion, chemical reactions of constituents. The experimental depth profile curves were fitted by using proposed model. The obtained results show a good agreement with experiment taking into account concentration dependent diffusion. It is shown by modeling that with the increase of substrate temperature the process of nickel surface segregation becomes most important. <p>DOI: <a href="http://dx.doi.org/10.5755/j01.ms.19.3.5234">http://dx.doi.org/10.5755/j01.ms.19.3.5234</a></p
The anisotropic stress-induced diffusion and trapping of nitrogen in austenitic atainless ateel during nitriding
Plasma nitriding of austenitic stainless steels at moderate temperatures is considered in the presented work. The anisotropic aspects of stress-induced diffusion and influence of nitrogen traps are investigated by kinetic modeling based on rate equations. The model involves diffusion of nitrogen in the presence of internal stress gradients induced by penetrating nitrogen as the next driving force of diffusion after the concentration gradient. The diffusion equation takes into account the fact that nitrogen atoms reside in interstitial sites and in trapping sites. Stress-induced diffusion has an anisotropic nature and depends on the crystalline orientation while trapping–detrapping is isotropic. The simulations are done considering the synergetic effects of both mechanisms and analyzing the properties of both processes separately. Theoretical curves are compared with experimental results taken from the literature. Good agreement between simulated and experimental results is observed, and gives the possibility to find real values of parameters needed for calculations. The nitrogen depth profile shapes, the dependences of nitrogen penetration on nitriding time and on diffusivity, are analyzed considering crystalline orientation of steel single crystal