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JTTEE5 18:272–283 DOI: 10.1007/s11666-009-9295-4 1059-9630/$19.00 Ó ASM International

Manufacturing Process of Nanostructured Alumina Coatings by Suspension Plasma Spraying Changjun Qiu and Yong Chen (Submitted April 16, 2008; in revised form July 3, 2008) This paper describes the formation process of nanostructured alumina coatings and the injection system obtained by suspension plasma spraying (SPS), an alternative to the atmospheric plasma spraying technique in which the material feedstock is a suspension of the nanopowder to be sprayed. The nanoscale alumina powders (d
20 nm) were dispersed in distilled water or ethanol and injected by a peristaltic pump into plasma under atmospheric conditions. Optical microscopy (OM), scanning electron microscopy (SEM), and x-ray diffraction (XRD) analyses were performed to study the microstructure of the nanostructured alumina coatings. The results showed that the nanoscale alumina powders in suspension were very easily adsorbed at the inner surface of injection, which caused the needle to jam. The rotation of the pump had a great effect on the suspension injection in the plasma. The very small resistance of the thin plasma boundary layer near the substrate can drastically decrease the impacting velocity of nanosize droplets. The concentration of suspension also has a significant influence on the distribution of the size of the droplet, the enthalpy needed for spraying suspension, and the roughness of the coating surface. The phase structures of alumina suspension coatings strongly depend on the plasma spraying distance. A significant nanostructured fine alumina coating was obtained in some areas when ethanol was used as a solvent. The microstructures of the coating were observed as a function of the solvent and the spraying parameters.

Keywords

nanoscale alumina powders, nanostructured alumina coatings, suspension plasma spraying

1. Introduction Alumina ceramics are widely used as wear-resistant and insulating coatings. Atmospheric plasma spraying (APS) is a common method for preparing Al2O3 coatings (Ref 1). A new interest in nanostructured coatings, with grain sizes smaller than 100 nm, has recently emerged. This is due to the enhanced properties of the nanoscale effect. Depending on the processing conditions, they can exhibit superior performance when compared with conventional coatings. The processing of nanostructured coatings has also reached the field of thermal spray, in which they have also shown superior performance when compared with conventional coatings (Ref 2-6). Many articles and much information about nanostructured coatings formed by chemical vapor deposition (CVD) and plasma vapor deposition (PVD) can be found. When their crystallite size is less than 100 nm, the nanocrystallite coatings demonstrate superior mechanical performance when compared with conventional coatings Changjun Qiu and Yong Chen, College of Mechanical Engineering, University of South China, Hengyang 421001, China. Contact e-mail: [email protected].

272—Volume 18(2) June 2009