The oxidation of Al-particles down to nano-scale was investigated by TG, SEM and in-situ X-ray diffraction. Al particles are usually coated by a 2 ± 4 nm layer of Al 2 O 3 which can be derived from the degree of weight increase on complete oxidation by TGcurves. The low temperature oxidation of Al particles occurs at least in two steps. The first step builds a layer of 6 to 10 nm thickness composed of crystallites of the same size independent on the initial particle size. This reaction is dominated by chemical kinetics and converts a substantial fraction of the particle if the particle sizes decrease below 1 mm, an effect carefully to be taken into account for nano-particles because of safety reasons. The second step combines diffusion and chemical reaction and proceeds therefore slowly, the slower the bigger the particles are. The kinetic parameters of these two steps can be obtained by a model taking into account both reaction steps, chemical kinetics and diffusion for spherical particles when fitting it to TG-curves. X-ray diffraction shows that particles smaller than 1 mm build gand q-Al 2 O 3 in the first step with nano-crystalline structures which are then transformed to a-Al 2 O 3 .

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Micro-sized spherical Al particles have recently attracted interest for the development of a new concept for coatings based on their capability to form hollow alumina spheres and aluminized diffusion zones in the substrate. For understanding better their oxidation behaviour, spherical µm-Al particles with different sizes were oxidized in air on heating up to 1300°C and under isothermal conditions at 800°C and 850°C. The oxide formation was studiedin situby high temperature X-ray diffraction and the oxidised particles were analysed by scanning electron microscopy. On heating the µm-Al particles begin to form a g-Al2O3scale before reaching the melting point and the molten Al is kept within the g-Al2O3shell. On further heating q-Al2O3is detected, which forms simultaneously with the g-Al2O3. The g-Al2O3/ q-Al2O3scale is stable and protective under isothermal conditions up to 800°C within the investigated times. On further heating the g-Al2O3and q-Al2O3transform simultaneously to a-Al2O3in a temperature range of 850°C to 1100°C. Under isothermal conditions the g à a-Al2O3transformation is observed after 160 min at 850°C. During the g à a-Al2O3transformation shrinkage occurs that leads to formation of pores. A model is proposed describing the mechanism that leads to the formation of the observed whiskers morphologies during the g à a-Al2O3transformation.

Mechanical sensitivity of energetic materials is discussed as a function of particle size and lattice defects. Therefore an approach is made to quantify and characterize lattice defects of the energetic powders RDX and HMX using X-ray diffraction. The mechanical properties of these cyclic nitramines are supposed to be dominated by different defect mechanisms-deformation twinning and dislocation slip. The energetic powders and the line-shape standard SRM 660a ͑NIST͒ were measured on a Bragg-Brentano diffractometer D5000 ͑Bruker-AXS͒, and the diffraction patterns were evaluated by Rietveld-and peak-profile analysis using Williamson-Hall plots. Additionally, preparing samples as thin powder layers was tested in order to reduce line broadening due to low absorption. The investigations reveal interesting details. Characteristic line broadening was found for RDX and HMX pointing to a comparably higher microstrain in HMX. Anisotropic line broadening was found for RDX and discussed in terms of strain fields of dislocations. © 2005 International Centre for Diffraction Data.

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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.

Micro-sized spherical aluminium particles deposited as slurry by brushing or spraying on a substrate alloy oxidise at high temperatures to form a top coat from sintered hollow alumina spheres whilst forming an aluminised diffusion zone. The top coat has the potential to be effective as a thermal barrier by gas phase insulation. The formation of the diffusion zone and the adherence of the top coat are influenced not only by the parameters of the heat treatment, but by factors such as particle size and surface finishing. Coated samples of Alloy 321 were cured and heat treated at 650 degrees C for 5 h and were analysed by field emission scanning electron microscopy. Coating samples of Alloy 321 containing spherical aluminium particles with the sizes 0.3-0.7 mu m, 2-3 mu m, 5 mu m, and 30-50 mu m and with a surface finishing of 120#, 500# and 1200# revealed that the size range of 2-5 mu m and a surface finishing of 500# are more suitable for forming the coating structure of the diffusion zone with an adherent top coat. Using aluminium particles with a size of 30-50 mu m, the temperature range where diffusion predominates at the expenses of the alumina sphere formation is wider and top coat spallation is observed