Papers by Keyword: Diffusion Coating

Abstract: The intrinsic diffusion coefficients in diffusion aluminide coatings based on Fe-30Cr were determined at 1000oC. The diffusion fluxes were given by the Nernst Planck formulae and the Darken method for multicomponent systems was applied. This paper summarizes some numerical results to determine the composition dependent diffusivities in Fe-Cr-Al systems. The method presented in this study to obtain average intrinsic diffusion coefficients is as an alternative to the Dayananda method. Our method based on empirical parameters allowed us to predict the concentration profile during the interdiffusion process.

Abstract: Samples of 316L austenitic steel were submitted to a thermochemical treatment which implies surface diffusion of Al and Ti. The technique of pack cementation with NH4Cl as activator was employed. The powder mixture was made of aluminium, titanium, aluminium oxide and ammonium chloride. The same ratio of Al : Ti = 1 : 5 was used in all experiments. The variables were temperature and time. As a function of these parameters, diffusion layers of different thicknesses were obtained. The samples were analyzed by optical microscopy, scanning electron microscopy (SEM) and energy dispersive X-ray microanalysis (EDX), X-ray diffraction and Vickers microhardness trials. All layers were formed by diffusion with reaction and present two zones with different structures and compositions and therefore different properties. The Ti3NiAl2N compound was identified by X-ray diffraction. The presence of this compound in the diffusion coatings increases the superficial hardness of the samples.

Abstract: This paper presents a numerical method to determine the composition dependent diffusivities and to predict the concentration profile during the interdiffusion process. The intrinsic diffusion coefficients in diffusion aluminide coatings (Fe-Al) were determined at 1000oC. The obtained diffusion coefficient for iron in Fe3Al or FeAl is in the range 10-10 to 10-9 cm2.s-1. The aluminum diffusion coefficient varies from 10-11 to 10-7 cm2.s-1 in the same phases.The present approach also permits to model the reactive diffusion in the Fe-Al systems.

Abstract: Spherical Al particles sized in the range of 2 to 5 μm were deposited with an organic binder by brushing on the austenitic steel X6 CrNi 18-10 (Alloy 304H). The coated samples were annealed in air at 400°C for 1 h in order to expel the binder. For studying the oxidation behaviour in air, isothermal experiments were performed at 700°C and 900°C with oxidation times of 5 h, 100 h and 1000 h. The oxide formation was studied in situ by high temperature X-ray diffraction (HTXRD) up to 100 h. Field emission scanning electron microscopy (FE-SEM) was applied to investigate the surface and the cross-section of the particle coating. During oxidation, the stable α-Al2O3 was identified in situ by HT-XRD on all studied samples at both temperatures. No meta-stable alumina phases were found. In the initial state, 2 h at 900°C, the Al particles are completely oxidised to hollow alumina spheres, controlled predominantly by the reaction due to the small particle size and relatively high surface portion. Simultaneously, the Alrich diffusion layer is formed in the substrate. On further exposure, a thin protective alumina scale continues growing on the top of the diffusion layer. After exposure to both 700°C and 900°C, a coating structure was encountered, which consists of a quasi-foam top coat from conjoint hollow spherical alumina particles and an Al-rich diffusion layer below. The quasi-foam top coat has the potential to effectuate as thermal barrier by gas phase insulation, while the diffusion layer below serves as protective coating against oxidation. The approach by particle size processing opens a potential for obtaining a complete thermal barrier coating system in one manufacturing step. The coating properties can be adjusted by parameters like selection of source metal/alloy, particle size, substrate, binder and heat treatment.