The exceptional properties of Ti-6Al-4V of high strength, lightweight, corrosion resistance and machinability make it one of the most widely used alloys in in the aerospace industry. Significant efforts are underway to establish powder bed additive manufacturing (AM) technologies for Ti-6Al-4V. There are also increasing attempts to use thermal and cold spray to build near net shape parts with buildup rates orders of magnitude higher than powder bed. Thermal spraying, such as HVOF, can oxidize and degrade the alloy due to the high processing temperature. Lowering the flame temperature through inert gas addition in full-size HVOF systems is a possible approach to retain solid state deposition of the feedstock particles, thereby limiting oxidation and detrimental α-case formation, while providing sufficient heat input for particle softening and plastic deformation at impact. Novel miniaturized HVOF systems, with spray jets of only a few millimetre in width, may further offer the possibility to improve the spatial resolution of the buildup for near net shape forming. The process parameter range for solid state deposition of Ti-6A-4V, using the liquid fuelled TAFA Model 825 JPid and the novel hydrogen fuelled Spraywerx ID-NOVA MK-6 with the addition of nitrogen will be discussed. Build-ups at over 80% deposition efficiency generally yield as-sprayed porosities below 3% and hardness above 200 HV100gf. Attainable microstructures and oxygen content as a function of spray parameters are delineated. Recrystallization and beta annealing of selected samples lowered the residual porosity and created equiaxed α and intergranular ß-phases. Ultimate tensile strengths of up to 1100 MPa were attained, however, at limited elongation.

Suspension plasma spraying is gaining greater interest for emerging applications such as new thermal barrier coatings, next generation environmental barrier coatings and ceramic membranes as in solid oxide fuel cells. Mettech developed an axial injection plasma process coupled with an automatic suspension feed system, and demonstrated its capability to overcome the complexities of the process and deliver quality coatings. This paper aims at determining the durability and stability of the gun, suspension feeder and their components. A 120-hour duration test was performed, and the plasma torch and suspension feed parameters and performances were recorded. The test results indicate that the equipment and process are stable and reliable, and ready for industrial applications.

Yttrium oxide (Y 2 O 3 ) coatings have been prepared with high power axial injection plasma spraying using fine powder slurries. It is clarified that the coatings have high hardness, low porosity and high erosion resistance against CF4 contained plasma in the previous study. This suggests that the plasma spraying of Y 2 O 3 with slurry injection techniques is applicable to fabricating equipments for semiconductor devices, such as dry etching. Surface morphologies of the slurry coatings with splats are almost similar to conventional plasma-sprayed Y 2 O 3 coatings, identified from microstructural analysis by field emission SEM in this study. However, no lamellar structure has been seen from cross sectional analysis, which is apparently different from the conventional coatings. It has also been found that crystal structure of the slurry Y 2 O 3 coatings mainly composed of metastable phase of monoclinic structure, whereas the powders and the conventional plasma spray coatings have stable phase of cubic structure. Mechanism of coating formation by plasma spraying with fine powder slurries will be discussed based on the findings.

In this study, high-power axial-injection suspension plasma spraying is used to synthesize yttrium oxide coatings from fine powder slurries. The coatings are assessed based on microstructure, hardness, and porosity and compared with coatings produced by other spraying methods. Resistance to erosion from CF 4 containing plasma is also investigated. Test results show that the suspension plasma sprayed Y 2 O 3 coatings are superior in terms of density, hardness, uniformity, and plasma erosion resistance. They also retain a smoother surface when exposed to plasma that contains CF 4 .

An axial injection suspension plasma spray system has been used to produce layers of fully stabilized yttria-stabilized zirconia (YSZ) that could be used as solid oxide fuel cell (SOFC) electrolytes. Suspension plasma spraying is a promising technique for the rapid production of coatings with fine microstructures and controlled porosity without requiring a post-deposition heat treatment. This new manufacturing technique to produce SOFC active layers requires the build up of a number of different plasma sprayed SOFC functional layers (cathode, electrolyte and anode) sequentially on top of each other. To understand the influence of the substrate and previously-deposited coating layers on subsequent coating layer properties, YSZ layers were deposited on top of plasma sprayed composite lanthanum strontium manganite (LSM)/YSZ cathode layers that were first deposited on porous ferritic stainless steel substrates. Three layer half cells consisting of the porous steel substrate, composite cathode, and suspension plasma sprayed electrolyte layer were then characterized. A systematic study was performed in order to investigate the effect of parameters such as substrate and cathode layer roughness, substrate surface pore size, and cathode microstructure and thickness on electrolyte deposition efficiency, cathode and electrolyte permeability, and layer microstructure.

Plasma spraying was successfully applied for Thermal Barrier Coatings (TBCs), which typically possess a lamellar structure with a porosity of 5-20%. Control of the plasma process presents a big challenge when the goal is to achieve fully dense coatings, as required for some emerging applications such as solid oxide fuel cells (SOFCs) and dense TBCs. Fine powders produce finer lamellae, and result in denser coatings. However, powders finer than 10 microns are very difficult to feed consistently into a plasma torch. Liquid slurries offer a means to deliver fine particles to thermal spray torches. In this paper, an automatic slurry feed system was developed to consistently deliver micro and nano powder slurries. The slurries were injected axially into a high energy/high velocity plasma torch to generate dense coatings. The effects of plasma parameters and different feedstocks on coating microstructures are investigated; dense coatings for various applications are demonstrated.

Atmospheric plasma spraying has emerged as a cost-effective alternative to traditional sintering processes for solid oxide fuel cell (SOFC) manufacturing. However, the use of plasma spraying for SOFCs presents unique challenges, mainly due to the high porosity required for the electrodes and fully dense coatings required for the electrolytes. By using optimized spray conditions combined with appropriate feedstocks, SOFC electrolytes and electrodes with required composition and microstructure could be deposited with an axial plasma spray system. In this paper, the challenges for manufacturing SOFC anodes, electrolytes, and cathodes are addressed. The effects of plasma parameters and different feedstocks on coating microstructure are discussed, and examples of optimized coating microstructures are given.

Yttria stabilized zirconia (YSZ) is the most commonly used electrolyte material for solid oxide fuel cells (SOFCs), due to its pure ionic conductivity and chemical stability. Standard electrolyte fabrication techniques for planar fuel cells involve wet ceramic techniques such as tape-casting or screen printing, which require sintering at temperatures above 1300°C. Plasma spraying (PS) may provide a more rapid and cost efficient method of producing SOFCs without requiring high temperature post-deposition heat treatments. However, it is difficult to produce plasma sprayed layers that are both thin (<20µm) and completely dense. It is of utmost importance to have a dense electrolyte to prevent the mixing of cathode and anode reactant gases. This study investigates the use of spin coated sol gel derived YSZ precursor solutions to fill the pores present in plasma sprayed YSZ layers.

Suspension plasma spraying is a promising modification to traditional plasma spray techniques that may allow plasma sprayed layers with finer microstructures and better porosity control to be produced. The fine microstructures and controlled porosity of these layers, combined with plasma spraying’s ability to produce layers rapidly without requiring a post-deposition heat treatment, makes this an interesting new manufacturing method to produce solid oxide fuel cell (SOFC) active layers. This study uses an axial injection suspension plasma spray system to produce thin, high-density layers of fully stabilized yttria-stabilized zirconia (YSZ) for use as an SOFC electrolyte. Three different aqueous feedstock suspensions with varying solid contents were sprayed, which resulted in coatings with splat thicknesses of approximately 0.5 µm and some intersplat porosity. Total coating thickness increased as the suspension solid content was increased, but suspension flow rates and deposition efficiencies decreased.

A porous composite cathode containing (La 0.8 Sr 0.2 ) 0.98 MnO 3 (LSM) and yttria stabilized zirconia (YSZ) for use in a solid oxide fuel cell has been produced by air plasma spraying. Deposition was carried out using axial powder injection for increased deposition efficiency and composition control. A plasma composed of argon and nitrogen was used to decrease processing costs and avoid decomposition of the cathode material during deposition. Preliminary investigations focused on determining the range of plasma conditions under which each of the materials could be successfully deposited separately. A set of conditions was thereby determined that were suitable for the deposition of a composite cathode from pre-mixed LSM and YSZ powders. Graphite pore former was added to the powder mixture in order to achieve sufficient porosity in the final coating. A tape cast YSZ electrolyte was used as the substrate for the deposition of the cathode and also as the mechanical support layer in the finished cell. Following deposition of the cathode, an anode was produced by traditional wet ceramic processing techniques. Plasma sprayed cathode was characterized by SEM, EDX, and XRD, and the electrochemical performance of the full fuel cell was evaluated.

Aluminum coatings minimum 1.8 mm thick are applied to water-cooled proton beam collimators used in the manufacture of medical isotopes on the TRIUMF TR30 cyclotrons in Vancouver, British Columbia, Canada. The sprayed surface of the collimators is made from silver. These collimators are used to trim the proton beam so that only a designated area on the isotope production target is irradiated with protons. Aluminum is used because its activation products at the energies used have short half-lives, thus minimizing the amount of collateral radioactivity produced. The aluminum is sprayed using an Axial III plasma spray torch. In service, the collimators are subject to high heat fluxes due to the proton beam. Service life, heat transfer and application data are provided in this paper.

Northwest Mettech Corporation has recently developed a new high power torch for spraying internal surfaces, the Axial III ID. The Axial III ID has been successfully operating at 135 kW while spraying inside a drum with an inner diameter of 34 cm. The three-electrode axial injection configuration of the new torch is based on the current Axial III torch. Various coatings were sprayed inside a drum while monitoring the torch temperature. The results of these experiments are presented in this paper.