Papers by Author: Maria Juez-Lorenzo

Abstract: Previous work on the oxidation of nano- and micro-sized Al particles revealed a particle size window, where no meta-stable alumina phases were observed. Depositing such particles on an austenitic substrate, diffusion layers with reduced Al contents were obtained. These findings opened new perspectives for investigating the potential impact of the Al particle size and shape on the formation of diffusion aluminide coatings. Spherical Al particles sized in the range of 2 to 5 µm were deposited with a binder by brushing on the austenitic steel X6 CrNi 18-10 (Alloy 304H). For the curing process, the samples were annealed in air at 400°C for 1h. The diffusion effect of Al into the base material was studied in isothermal experiments at 700°C and 900°C with exposure times up to 2000 h in air. The sample surfaces and the diffusion aluminide coatings in cross-section were analysed by field emission scanning electron microscopy (FE-SEM). The results show in the initial state the formation of a diffusion layer consisting of a less aluminium-rich Fe(Cr)-Al phase containing a Fe(Cr)-Al phase with higher content of Al in the region beneath the surface. On further exposure a double-layered structure is found with Kirkendall-pores between the two layers, which may lead to a complete separation of the outer layer. A thin adherent alumina scale is observed on the remaining diffusion layer after 1000 h and 2000 h at both temperatures, however overgrown by Cr2O3 at 900°C. The structure of the diffusion zone beneath agglomerates of Al particles reveals the influence of the particle size on the Al supply for the formation of the aluminide diffusion zone.

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.

Abstract: In order to contribute to a better understanding of the processes, which occur in the structure of FeCrAl alloys during oxidation, in situ – studies by two-dimensional high temperature X-ray diffraction (2D-XRD) using a global area detector and grazing incidence with a monocapillary have been performed. The 2D-XRD yields simultaneously with the identification of the oxides and their formation kinetics information about the grain size, grain shape, stresses, texture as well as grain movements during the oxidation process of both oxide and metal. Two commercial FeCrAl alloys with different reactive element additions were investigated in the temperature range of 850°C to 1100°C. In the range of 1100°C already in the first 5 min the alloy grains become coarse and appear as single spots along the lateral profile in the 2D-XRD pattern. Dynamic displacement of these spots along the 2θ – axis during the exposure indicates the formation of stresses, which differ from grain to grain. Initially, re-crystallisation and grain growth dominate and grains disappear and new grains appear. On further exposure the grains twist continuously with 1° to 3° per hour, depending on the alloy. The “dancing grain” effect of the alloy is probably related with growth stresses in the oxide scale and influenced by the bulging of the foil. Simultaneously, α-Al2O3 is detected from the first pattern after 5 min and shows an enhanced formation rate in the first 15 min of the oxidation. The α-Al2O3 grains are with 0.3 to 0.4 4m extremely fine and, a dense well adherent scale is observed even after 1 h.