Influence of NH 4 Cl Powder Addition for Fabrication of Aluminum Nitride Coating in Reactive Atmospheric Plasma Spray Pr
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ohammed Shahien, Motohiro Yamada, Toshiaki Yasui, and Masahiro Fukumoto (Submitted May 7, 2010; in revised form October 9, 2010) Reactive plasma spray is the key to fabricating aluminum nitride (AlN) thermally sprayed coatings. It was possible to fabricate AlN/Al composite coatings using atmospheric plasma spray process through plasma nitriding of Al powders (Al 30 lm). The nitriding reaction and the AlN content could be improved by controlling the spray distance and the feedstock powder particle size. Increasing the spray distance and/or using smaller particle size of Al powders improved the in-flight nitriding reaction. However, it was difficult to fabricate thick and dense AlN coatings with an increase in the spray distance and/or when using fine particles. Thus, the coatings thickness was suppressed because of the complete nitriding of some particles (formation of AlN particles) during flight, which prevents the particle deposition. Furthermore, the excessive vaporization of Al fine particles (due to increased particle temperature) decreased the deposition efficiency. To fabricate thick AlN coatings in the reactive plasma spray process, improving the nitriding reaction of the large Al particles at short spray distance is required to decrease the vaporization of Al particles during flight. This study investigated the influence of adding ammonium chloride (NH4Cl) powders on the nitriding process of large Al powders and on the microstructure of the fabricated coatings. It was possible to fabricate thick AlN coatings at 100 mm spray distance with small addition of NH4Cl powders to the Al feedstock powders (30 lm). Addition of NH4Cl to the starting Al powders promoted the formation of AlN through changing the reaction path to vaporphase nitridation chlorination-nitridation sequences as confirmed by the thermodynamic analysis of possible intermediate reactions. This changes the nitriding reaction to a mild way, so it is more controlled with no explosive mode and with relatively low heating rates. Thus, NH4Cl acts as a catalyst, nitrogen source, and diluent agent. Furthermore, the evolved gases from the sublimation or decomposition of NH4Cl can prevent the Al particles coalescing after melting.
Keywords
aluminum nitride, ammonium chloride, reaction pathway, reactive plasma spray
1. Introduction Among nonoxide ceramics, aluminum nitride (AlN) has received a great deal of interest in recent years because of This article is an invited paper selected from presentations at the 2010 International Thermal Spray Conference and has been expanded from the original presentation. It is simultaneously published in Thermal Spray: Global Solutions for Future Applications, Proceedings of the 2010 International Thermal Spray Conference, Singapore, May 3-5, 2010, Basil R. Marple, Arvind Agarwal, Margaret M. Hyland, Yuk-Chiu Lau, Chang-Jiu Li, Rogerio S. Lima, and Ghislain Montavon, Ed., ASM International, Materials Park, OH, 2011. Mohammed Shahien, Motohiro Yamada, Toshiaki Yasui, and Masahiro Fukumoto, Toyohashi University of Technol
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