475 Pure Appl. Chem., Vol. 77, No. 2, pp. 475–485, 2005. DOI: 10.1351/pac200577020475 © 2005 IUPAC Thermal plasma deposition from thick to thin coatings and from micro- to nanostructure* P. Fauchais ‡ , M. Vardelle, J. F. Coudert, A. Vardelle, C. Delbos, and J. Fazilleau SPCTS-UMR 6638 CNRS, University of Limoges, France Abstract: This paper is devoted to the presentation of our actual knowledge in plasma spray- ing. It presents successively: the parameters controlling the impact of the molten particles onto the substrate and resulting splat formation followed by splats layering and coating for- mation; the engineering of nano- or finely structured coatings with different possible routes; and the actual possibilities for the on-line control of the spray process. Keywords: plasma spraying; suspension plasma spraying; plasma torches; particle flattening; splats; coating formation. INTRODUCTION Plasma spraying is part of thermal spraying processes in which finely divided metallic and nonmetallic materials are deposited in a molten or semi-molten state on a prepared substrate [1]. The base mate- rial/coating combination can be tailored to provide resistance to heat, wear, erosion, and/or corrosion, as well as unique sets of surface characteristics. Coatings are also used to restore worn or poorly ma- chined parts to original dimensions and specifications. In conventional plasma spraying, more than 97 % of coatings are manufactured with direct current (dc) arcs, and less than 3 % with radio frequency (rf) discharges [2]. Plasmas are mainly used to spray refractory materials, superalloys, and more gen- erally high-added-value materials either in air or controlled atmosphere. Typical coating thickness ranges between 50 μm and a few millimeters. At impact onto the substrate, the molten particles flatten and form lamellae (splats) which layering forms the coating. Splats have columnar or equiaxed struc- tures with grain sizes between 50 and 200 nm. However, this fine structure is altered by grain size ef- fect and large volume fraction of internal interfaces [3]. During the last three decades, many efforts have been devoted to a better understanding of the mass, momentum, and heat transfer between plasma and particles in order to adapt plasma working con- ditions and particle size and morphology to the desired impact velocities and temperatures [2]. This was achieved thanks to measuring devices [4], making it possible to back models [5]. Such measurements have allowed the development, during the last decade, of simplified on-line control, or, more precisely, monitoring systems that are now used in industrial spray booths [6]. During this last decade, numerous studies were devoted to splat formation [7], but none to coating formation. Finally, during recent years, different techniques were tested for engineering nano- or finely structured coatings [8]. In this paper, we present our actual knowledge of the splat and coating formation, engineering of nano- or finely structured coatings, optimization of the spray torches, and particle injection to achieve such coatings with possible on-line control of the deposition process. *Paper based on a presentation at the 16 th International Symposium on Plasma Chemistry (ISPC-16), Taormina, Italy, 22–27 June 2003. Other presentations are published in this issue, pp. 345–495. ‡ Corresponding author