Demands, Potentials, and Economic Aspects of Thermal Spraying with Suspensions: A Critical Review

JTTEE5 24:1143–1152 DOI: 10.1007/s11666-015-0274-7 1059-9630/$19.00 Ó ASM International Filofteia-Laura Toma, Annegret Potthoff, Lutz-Michael Berger, and Christoph Leyens (Submitted January 28, 2015; in revised form June 22, 2015) Research and development work for about one decade have demonstrated many unique thermal spray coating properties, particularly for oxide ceramic coatings by using suspensions of fine powders as feedstock in APS and HVOF processes. Some particular advantages are direct feeding of fine nano- and submicron-scale particles avoiding special feedstock powder preparation, ability to produce coating thicknesses ranging from 10 to 50 lm, homogeneous microstructure with less anisotropy and lower surface roughness compared to conventional coatings, possibility of retention of the initial crystalline phases, and others. This paper discusses the main aspects of thermal spraying with suspensions which have been taken into account in order to produce these coatings on an economical way. The economic efficiency of the process depends on the availability of suitable additional system components (suspension feeder, injectors), on the development and handling of stable suspensions, as well as on the high process stability for acceptance at industrial scale. Special focus is made on the development and processability of highly concentrated water-based suspensions. While costs and operational safety clearly speak for use of water as a liquid media for preparing suspensions on an industrial scale, its use is often critically discussed due to the required higher heat input during spraying compared to alcoholic suspensions. Keywords concentrated suspension, economic aspects, hardware development, process stability, suspension thermal spraying 1. Introduction For about one decade, modified thermal spraying processes using suspensions of fine submicron- and nanosizedpowders as feedstock materials have continuously gained increasing interest in the scientific world. Extensive development efforts reflected by an important number of papers and reviews made over the last years have uncovered the potential of thermal spraying with suspensions, e.g., Ref 1-6. Compared with conventional thermal spray methods, the suspension spraying technique presents some advantages: direct feeding of fine nano- and submicronscale particles; tailored coating architecture that can be adapted to the given application; less anisotropy and lower surface roughness of the coating; retention of the initial Filofteia-Laura Toma, Fraunhofer Institute for Material and Beam Technology (IWS), Winterbergstrasse 28, 01277 Dresden, Germany; Annegret Potthoff and Lutz-Michael Berger, Fraunhofer Institute for Ceramic Materials and Systems (IKTS), Winterbergstrasse 28, 01277 Dresden, Germany; and Christoph Leyens, Fraunhofer Institute for Material and Beam Technology (IWS), Winterbergstrasse 28, 01277 Dresden, Germany and Technische Universitát Dresden (TUD), Helmholtzstrabe 10, 01069 Dresden, Germany. Contact e-mail: . Journal of Thermal Spray Technology crystalline phases (i.e., a-Al2O3, anatase modification of TiO2, hydroxyapatite) resulting in improved or new coating properties. Moreover, the technique allows thick and thin, finely (nano)-structured coatings to be prepared. Suspensions are used as feedstock for both atmospheric plasma spraying (APS) and for high velocity oxy-fuel (HVOF) spraying. The processes can be abbreviated analogously to powder and wire flame spray processes as ‘‘S-APS’’ and ‘‘S-HVOF’’ to identify them as processes using suspensions as feedstocks. Abbreviations as ‘‘SPS’’ (suspension plasma spraying) and ‘‘HVSFS’’ (high velocity suspension flame spraying) are also used in the literature. The use of suspensions allows a direct processing of nanopowders. However, an important advantage is the direct use of finely dispersed oxide powders commonly applied in the production of sintered technical ceramics. For this reason, thermal spraying with suspensions is primarily seen as a technology for the preparation of ceramic oxide coatings. Except oxides (Al2O3, TiO2, Cr2O3, YSZ), biomaterials (hydroxyapatite, bioglasses) and perovskites were studied to produce suspensions for spraying (i.e., Ref 4, 7-18). In the case of metals, only the chemically prepared metallic powders are good candidates for suspensions. However, the preparation of coatings from composites, such as WC-Co (Ref 19-21), or oxide-coated SiC (Ref 22) is much more difficult as from plain oxides. Using aqueous suspensions of different WC-Co powders, coatings with a hardness of up to 1000 HV0.3 and a good sliding wear resistance were obtained (Ref 20, 21). Before the technology can be transferred to industry, industrial-grade hardware, i.e., suspension feeders and Volume 24(7) October 2015—1143 Peer Reviewed Demands, Potentials, and Economic Aspects of Thermal Spraying with Suspensions: A Critical Review Peer Reviewed injectors, as well as modified or specially designed spray guns must be available. All issues related to use of suspensions as feedstock, including preparation or commercial availability, transport, handling, storage, and operational safety must be clarified, too (Ref 5). The choice of the powders for the suspensions has to be made carefully, because the specific material properties play a more significant role than in conventional spraying. In this paper, the suspension characteristics and the options for suspension supply are discussed, and specific hardware components are presented. Economic aspects (suspension concentration and feed rates, deposition efficiencies) together with the appropriate hardware components and long-time process stability are the basis for costeffective coating manufacturing by suspension spraying to meet the industrial expectations. 2. Suspensions for Thermal Spraying: Demands and Processability High process stability and reliability are indispensable for use of suspension spraying at industrial scale, with suspension properties playing a main role. Thus, the suspension development should be tailored, which includes selection and dispersion of the raw material in the liquid to enable all requirements to be met: – Requirements of the spray process: homogeneity, low viscosity (good flowability), high content of solids, high stability of the suspension (neither sedimentation nor modification of the suspension composition), compatibility with the hardware components (avoidance of corrosion, abrasion, or clogging), long-term process stability (constant suspension flow rate). – Requirements to achieve tailor-made coating properties specific for each application: material, phase composition, crystallinity, tolerance concerning the impurities, primary particle sizes, dispersant choice and content. – Expectations of the industry are availability, low price, reproducibility of batches, safety of transport and handling, low environmental impact, long-term storage, high deposition e (...truncated)