HVOF spraying of granulated SiO{sub}2/Al-Mg powder accompanied by in-process reaction

摘要

Thermodynamically instable hard particles such as WC in composite spray materials are ready to decompose to W2C and tungsten during thermal spray process. so that composite coatings containing nitrides or carbides as a reinforcement have been recently fabricated by reactive thermal spraying based on in-process reactions. In the present study, instead of DC and RF plasma spraying, HVOF spraying characterized by high flame velocity and low flame temperature is applied to the formation of in situ composite coatings. Granulated particles of SiO{sub}2/Al-Mg with a SiO{sub}2·Na{sub}2O binder are sprayed onto an aluminum substrate. Through the reduction of SiO{sub}2 accompanied by an exothermic reaction, the composite coatings are expected to consist of MgAl{sub}2O{sub}4 and Al-Si alloy matrix. However, the sprayed coatings actually consist of MgAl{sub}2O{sub}4, Al-Si and additional Mg{sub}2Si. This is because the reaction rate of Mg{sub}2Si is much faster compared with that of MgAl{sub}2O{sub}4, although MgAl{sub}2O{sub}4 is more thermodynamically stable than Mg{sub}2Si. The amount of formed MgAl{sub}2O{sub}4 decreases with an increase in the spray distance, though the long spray distance gives a long dwelling period of droplets. Hence, the in-process reaction to form MgAl{sub}2O{sub}4 mainly occurs on a substrate in the present composite system.