This review presents recent developments in suspension and solution precursor thermal spraying and demonstrates some of the tools available to characterize plasma-liquid interactions and the coatings produced. It compares and contrasts the two methods with conventional thermal spraying routes and identifies areas needing improvement.

This paper provides an overview of the high velocity suspension flame spraying (HVSFS) process that covers spray gun design, suspension and substrate preparation, and process optimization. Examples are given showing how the process is used to produce tribofunctional coatings for engine applications, electrolyte layers for SOFCs, and netshape chromia for high-end scissor blades. The substrates in the three examples are AlSi 9 Cu 3 , Crofer nickel cermet, and low carbon steel.

In this investigation, titanium dioxide and hydroxyapatite (HA) suspensions are plasma sprayed onto stainless steel, titanium, and aluminum substrates and the structure and properties of the resulting layers are correlated with spraying conditions. The suspensions were formulated with fine TiO 2 pigment and HA milled from spray-dried powder or synthesized from calcium nitrate and ammonium phosphate. In some experiments, an atomizer was used to inject the suspensions into the plasma jet, and in others, the suspensions were fed into the jet using continuous stream injection. The deposits are characterized on the basis of morphology, chemical and phase composition, scratch hardness, and dielectric strength.

Suspension plasma spraying (SPS) offers the manufacture of unique microstructures which are not possible with conventional powder feedstocks. Due to the considerably smaller size of the droplets and also the further fragmentation of these in the plasma jet, the attainable microstructural features like splat and pore sizes can be downsized to the nanometer range. Our present understanding of the SPS deposition process including injection, suspension plasma plume interaction, and deposition is outlined in this report. The conclusions drawn are based on microstructure analysis in combination with enthalpy probe and particle temperature and velocity and measurements. Measurements with a water-cooled stagnation probe provide valuable information on the interaction of the carrier fluid with the plasma plume. The examples presented include segmented thermal barrier coatings for turbine components, LSM cathode deposits for SOFCs, and TiO 2 layers for photovoltaic Gratzel cells.

This paper describes the properties and behaviors of alumina and titania coatings that have recently been produced using suspension spraying techniques. It examines coating microstructures and phase compositions are shows how they are influenced by different operating parameters and interactions. A selection of new experimental results obtained by the authors is also presented. In the case of Al 2 O 3 , the goal was to retain a high ratio of the thermodynamically stable α-phase. In the case of TiO 2 , the spraying process was optimized to preserve the anatase phase in order to obtain photocatalytically active layers.

One of the goals of this study is to better understand how suspension plasma spraying parameters, particularly plasma gas mixtures, influence layer formation. Another goal is to produce finely structured layers of Al 2 O 3 -ZrO 2 with a wide range of architectures. To that end, a simple theoretical model is used to describe the operating conditions of the plasma torch and the influence of spraying parameters is expressed in terms of the shape and size of spray beads.

In this study, nickel-zirconia cermet layers are produced by solution precursor plasma spraying (SPPS) and compared with suspension plasma sprayed (SPS) coatings of similar content. Although nickel is uniformly distributed in both coatings, its presence in the suspension caused problems with the SPS process. With the SSPS process, precursor solutions are fragmented into droplets in which Ni, Zr, and Y are intimately mixed, resulting in very fine microstructure without the problems encountered with the SPS process. It was also found that plasma gas enthalpy and spray distance have predominant effects on in-flight pyrolysis of the elements, and that plasma gas mixture has an impact on porosity as well as the oxidation state of the nickel.

In this study, suspension plasma spraying is used to produce cast iron coatings that benefit from a graphite structure. In order to increase the graphite content, different hydrocarbons in the form of liquid suspension (hexane and toluene) and gas precursor (methane) were injected into the plasma stream along with iron powder. Besides promoting the formation of a soot carbon structure, liquid hydrocarbon injection also prevents in-flight particle oxidation, which is a major concern when spraying metals. In addition, it has been observed that using a shroud during spraying significantly increases the amount of soot carbon in cast iron coatings, which can be transformed into graphite by post annealing.

Suspension plasma spraying facilitates the production of thick coatings structured at the submicron or even nanometer scale. Due to the large volume fraction of internal interfaces, nanostructured coatings tend to be superior to their microstructured counterparts. Suspension plasma sprayed oxide ceramics, for example, have higher coefficients of thermal expansion, lower thermal diffusivity and hysteresis, higher hardness and toughness, and better wear resistance. In this work, Y-PSZ thermal barrier coatings are manufactured by means of SPS using two commercial submicron powders with different particle size distributions. By varying spray parameters, several coating architectures and thicknesses were achieved. The coatings were subjected to a series of thermal and isothermal shocks in order to assess the effect of particle size distribution, layer thickness, and substrate roughness on thermomechanical behavior.

In this study, suspension plasma spraying is used to produce self-lubricating titanium oxide coatings. Certain nonstoichiometric titanium oxide phases, called Magneli phases, exhibit a reduction in friction under dry sliding conditions at elevated temperatures. These phases, however, tend to undergo crystal changes during thermal spraying, resulting in the loss of their good friction behavior. In this work, the goal is to stabilize these phases with suitable lattice substitutions for Ti 4+ . The resulting phases are shown to be homologous to Ti n O 2 n -1 , but have the advantages of a three-component system, making them more thermally stable with a broader area of formation.

In this study, a TiO 2 (anatase) nanopowder suspension was processed by high velocity suspension flame spraying (HVSFS). The resulting coatings were characterized and compared to conventional HVOF and atmospheric plasma sprayed layers. It is shown that the HVSFS operating parameters can be adjusted to achieve dense titania with a near nanostructure and homogeneous distribution of anatase and rutile phases. These coatings have lower pore interconnectivity and higher wear resistance than the APS and HVOF layers. Alternatively, large unmelted agglomerates of anatase nanoparticles can be embedded in the coating, increasing the porosity and anatase content for enhanced photocatalytic efficiency.

Numerous works have shown that decreasing the scale of coating structure leads to an improvement in tribological behavior. Suspension plasma spraying has proven particularly effective at producing coatings with submicron even nanoscale structure, while maintaining the versatility of thermal spraying. This paper examines the dry sliding behavior of several ceramic oxide composite coatings produced by suspension plasma spraying. The structural scale and the effect of composition are studied as well.