Plasma Spraying of Ceramics with Particular Difficulties in Processing G. Mauer, N. Schlegel, A. Guignard, M.O. Jarligo, S. Rezanka, A. Hospach, and R. Vaßen (Submitted June 4, 2014; in revised form July 17, 2014) Emerging new applications and growing demands of plasma-sprayed coatings initiate the development of new materials. Regarding ceramics, often complex compositions are employed to achieve advanced material properties, e.g., high thermal stability, low thermal conductivity, high electronic and ionic conductivity as well as specific thermo-mechanical properties and microstructures. Such materials however, often involve particular difficulties in processing by plasma spraying. The inhomogeneous dissociation and evaporation behavior of individual constituents can lead to changes of the chemical composition and the formation of secondary phases in the deposited coatings. Hence, undesired effects on the coating characteristics are encountered. In this work, examples of such challenging materials are investigated, namely pyrochlores applied for thermal barrier coatings as well as perovskites for gas separation membranes. In particular, new plasma spray processes like suspension plasma spraying and plasma spray-physical vapor deposition are considered. In some cases, plasma diagnostics are applied to analyze the processing conditions. Keywords atmospheric plasma spraying, dissociation, evap- oration, phase composition, plasma spray-physical vapor deposition, stoichiometry, suspension plas- ma spraying 1. Introduction In plasma spraying, the inhomogeneous dissociation and evaporation behavior of individual feedstock constit- uents can lead to changes of the chemical composition and the formation of secondary phases in the deposited coat- ings. In principle, this is undesired since important coating characteristics might be affected like thermo-mechanical performance or electronic and ionic conductivities. A typical example for materials with such difficulties in processing is the pyrochlore lanthanum zirconate La 2 Zr 2 O 7 (LZO), which is a promising candidate for thermal barrier coatings (TBCs). The processing of LZO by atmospheric plasma spraying (APS) is challenging because La 2 O 3 is prone to evaporate in the plasma plume (Ref 1). It shows a considerably higher vapor pressure than ZrO 2 (Ref 2, 3). The same is the case for gadolinium zirconate Gd 2 Zr 2 O 7 (GZO) and the inhomogeneous evaporation of Gd 2 O 3 and ZrO 2 , however to a smaller extent than for LZO. Such decomposition is expected to be detrimental for TBC performance since non-stabilized zirconia could be formed in the coating which is subjected to phase transformations during TBC operation combined with considerable changes of the specific volume (Ref 4). La 1x Sr x Co 1y Fe y O 3d (LSCF) is a perovskite mixed ionic and electronic conductor which is applied for oxygen transport membranes (Ref 5) and electrode functional layers. Because of the different vapor pressures of the constituents, LSCF is prone to decomposition. Single phase LSCF deposits can be obtained by APS, only if the process is optimized concerning moderate plasma power, short particle flight times, and increased coherence of the feedstock agglomerates (Ref 6). Such functional layers must be as thin as possible to achieve low resistance. But on the other hand they have also to be gas-tight. Thus, if using APS, a minimum thickness is required since typical coatings show considerable amounts of voids and cracks. This dilemma is an important motivation for the devel- opment of novel plasma spray processes. One of them is suspension plasma spraying (SPS). For conventional APS processes, well-flowable powders are required, which can be melted in a very short period of time (typically less than 1 ms). Hence, the size range at APS is usually about 10-100 lm. The use of liquid feed- stock such as suspensions however, yields higher flexibility as even submicron- to nano-sized particles can be pro- cessed (Ref 7). This allows to achieve porous segmented or columnar-structured TBCs or thin and dense mem- brane layers. But, it must be considered that the feedstock agglomerates being formed, when the suspension carrier liquid is evaporated, show significantly larger specific surface areas compared to APS since their size is in the This article is an invited paper selected from presentations at the 2014 International Thermal Spray Conference, held May 21-23, 2014, in Barcelona, Spain, and has been expanded from the original presentation. G. Mauer, N. Schlegel, S. Rezanka and R. Vaßen, Institut fu ¨r Energie- und Klimaforschung (IEK-1), Forschungszentrum Ju ¨ lich GmbH, 52425 Ju ¨ lich, Germany; A. Guignard, tsd Technik- Sprachendienst GmbH, Cologne, Germany; M.O. Jarligo, Department of Chemical and Materials Engineering, University of Alberta, Edmonton, AB, Canada; and A. Hospach, Siemens AG, Corporate Technology, Research & Technology Center, Munich, Germany. Contact e-mail: [email protected]. JTTEE5 DOI: 10.1007/s11666-014-0149-3 1059-9630/$19.00 Ó ASM International Journal of Thermal Spray Technology Peer Reviewed