The present article reports the results of studies related to the synthesis of nano-crystalline alumina powder by combustion process using different combustion aids. Aluminium nitrate as oxidant and urea as fuel were used as reagents. Starch and ammonium nitrate were compared as combustion aids. The results showed that employment of starting fuel with combustion aids resulted in synthesizing nano-crystalline alumina powder with small crystalline size (<20 nm) and fine agglomerates. By using combustion aid, the energetics of the combustion reaction has been changed. The different selection of combustion aid determines the characteristics of the as-synthesized powders.

LaAlO3 has gained attention in the last few years because of its favorable microwave dielectric properties, excellent lattice matching and a good matching for thermal expansion. Nanoparticles of LaAlO3 were produced by using gelatin as an organic precursor. The optimum temperature for the phase formation of the precursor sample is found out by TG/DTA analysis. The specific surface area of the powder was measured by the BET technique with nitrogen. X-ray diffraction (XRD) was used to determine the phases present in the calcined powders and to estimate particle size. The morphology of LaAlO3 powder was studied by transmission electron microscopy (TEM). Microwave dielectric properties were measured using a HP8722ET network analyzer. The results demonstrate that pure perovskite LaAlO3 powder formed at 800 °C for 2 h. And the crystallite size was in the range of 17-48 nm. The specimen sintered at 1450 °C for 8 h shows 97.4% of the theoretical density and gave the excellent microwave dielectric properties: εr=23.3 and Qf=32300 GHz.

Alumina matrix composites containing 20 wt % of Ni was sintered by spark plasma sintering (SPS) process. The influence of sintering temperature (1400-1550 °C) on densification and grain size of the composites were investigated (heating rate: 200 °C /min, sintering time: 5 min). Wear behavior of samples obtained by SPS were evaluated in a dry condition with reciprocate ball-on-disk testing machine. The results show that the relative density and alumina grain sizes increase with the increasing sintering temperature, and the fine-grained structure (alumina size controlled in 1–2 µm) are obtained at a temperature lower than 1400 °C; the alumina grain sizes increase to 5 µm at a temperature higher than 1500 °C. The fine-grained structure samples exhibits higher wear resistance.

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Surface coatings; Mechanical alloying; Microstructure; Tribological properties Abstract. The mechanical alloying method process has been innovatively used to prepare Al-Cr coating on the inner wall of steel pipe. The coating thickness was measured from all samples using optical and Scanning Electron Microscope was used to observe the surface microstructure of Al-Cr coating. Microhardness was analyzed by Digital Microhardness Tester. A wear test was performed by high speed reciprocating friction testing machine. The results show that the coating thickness is 20μm and 26μm at the rotating speed of 200 rpm and 300 rpm, respectively. The surface morphology is significantly influenced by the the rotating speed. When the rotating speed was 200 rpm, a heterogeneous coating surface consisting of flattened particles produced by cold welding with less interparticle contact is formed. When the rotating speed was 300 rpm, the coating became denser and a smooth, highly consolidated and dense coating is formed. The hardness of the Al–Cr coating prepared at 200 and 300 rpm are about 250 Hv and 270 Hv. The friction coefficient of the Al–Cr coating prepared at 200 rpm are about 0.37, 0.39 and 0.24 at the frequencies of 3, 4 and 5 Hz. The friction coefficient of the Al-Cr coating prepared at 300 rpm are about 0.3, 0.18 and 0.28 at the frequencies of 3, 4 and 5 Hz.