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

Abstract

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.

Alumina-zirconia coatings obtained by suspension plasma spraying from highly concentrated aqueous suspensions

“…In spite of this limitation suspensions with a solid loading significantly higher than those reported in previous works can be injected. It is an important achievement because, as explained above in the introduction section, an increase of the suspension solids content involves energy saving and consequently an easier industrial scaling [9,10,14,15].…”

“…Besides suspension viscosity cannot be too high for the suspension to properly flow through the injector. However it is interesting to note that solids loading of the suspension should be as high as possible so as to improve the deposition efficiency [9,10].…”

Fabrication, Structure, Properties and Application of Gradient Sintered Carbide-Steels with HS6-5-2 Matrix

“…Plasma spraying (PS) was patented already in 1960 and 1962. In this method, a material constituting a layer is melted in a stream of plasma and the melted particles are directed at high velocity by a stream of plasma gas onto an appropriately prepared surface [2,57,58]. Plasma spraying can take place in two variants.…”

Microstructure and Sliding Wear Resistance of Plasma Sprayed Al2O3-Cr2O3-TiO2 Ternary Coatings from Blends of Single Oxides

“…The homogeneity is lowest in case of the powder blends from single oxide powders and highest in the case of agglomerated and sintered powders, where finely dispersed oxide powders are mixed in the manufacturing process. Suspensions as feedstock become increasingly important, but are currently limited to single oxides [5]. Suspensions with two components are under investigation [6].…”

“…An increase of the α-Al 2 O 3 content in the coatings is described as an important measure to improve the coating properties, such as wear, electrical, and corrosion resistance. Except the addition of other oxides (e.g., Cr 2 O 3 [8][9][10]), there are several technological measures for this, as the selection of special spray process conditions [11], use of suspensions [1,5] or heat post-treatments [12] as well as plasma-electrolytic oxidation of arc-sprayed and flame-sprayed aluminum coatings [13]. However, each of these technological measures has certain limitations and the stabilization of the α-Al 2 O 3 by a tailored powder would be favored.…”