Synthesis of Al 2 O 3 /AlN composite powders by plasma processed Al 2 O 3 with various additives
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A novel method was developed to produce Al2O3/AlN composite powders. The composite powders were synthesized by the direct nitridation of Al2O3 using various additives by plasma processed with two different types of gas (Ar/N2 and N2 plasma). The effects of these two different plasma gases and additives on the formation of the Al2O3/AlN composites were studied. The x-ray diffraction results in the composite powders showed that the cubic AlN was converted to the major phase while transient ␥–Al2O3 was the minor phase when the composite powder was fully melted using nitrogen as the sole working gas. In contrast, the main phase was still ␣–Al2O3 when the composite powder was not well melted using the mixture of argon and nitrogen as the working gas. Carbon was found as an effective conversion additive to increase the cubic AlN of this composite powder up to 49 vol%. The reaction mechanism of forming this composite powder was also analyzed.
I. INTRODUCTION
Thermal plasma spraying techniques have been used for many years to produce layered coatings for many different applications.1–3 Such coatings have been increasingly used in the automobile, aerospace, textile, biomedical, electrical, and optical industries to impart properties of wear resistance, thermal barrier, corrosion resistance, biocompatibility, and electrical insulation.4–6 Although it was initially intended for coating purposes, plasma spraying has widened its scope to perform powder processing, which allows in situ production of ultrafine powders7,8 and ceramic-matrix composite powders,9 and opened up new approaches to material processing. Thermal spraying is characterized by its high temperatures (∼10000 K), high specific energy densities, and high cooling rates.10,11 The use of thermal spray methods to synthesize particulate systems in contrast has only been developed recently. Early attempts included the use of a direct current (dc) plasma reactor to form carbides from metal powders and gaseous precursors.12 Fine SiC powders were also synthesized using SiO2 particles and CH4 gas in a dc plasma jet reactor.13 Arc plasma methods were also used for the direct production of ultrafine silicon powers and nitrides/carbides of silicon, titanium, and tungsten.14–16 Nanocrystalline zirconia powders were produced using zirconium butoxide solutions by thermal spray pyrolysis.17 These recent works have demonstrated
the feasibility of producing fine powders by reactive plasma spray processing, which is able to vary the extent of the chemical reaction in the plasma environment, allowing a reasonable method by which to prepare varying composite powder system. Composite materials have the ability to improve mechanical, chemical, and thermal behavior by combining materials with distinctive or supplementary properties.18–20 Recent works have shown how, using such a plasma spray method, a Al2O3/AlN integral composite powder system can be formed, which can be cost effectively sintered to give a pure AlN powders as well as monoclinic AlN substrate.21,22 In the case of the l
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