Abstract The porous architecture of coatings has a significant influence on the coating performances and thus should be properly designed for the intended applications. For simulating the coating properties, it is necessary to determine the numerical representation of the coating microstructure. In this study, YSZ coatings were manufactured by suspension plasma spray (SPS). Afterwards, the porous architecture of as-prepared coatings was investigated by the combination of three techniques, imaging analysis, Ultra Small Angle X-ray Scattering (USAXS), and X-ray transmission. A microstructural model for reconstructing the porous architecture of the SPS coating was subsequently computed according to the collected experimental results. Finally, the coating thermal properties were simulated based on the model and were compared with the experimental results.

Abstract Thermal spray is a versatile process that produces high-quality coatings possessing diverse properties such as superhydrophobicity, wear resistance, corrosion resistance, dielectric properties etc. Conventionally, powder feedstock is used in thermal spray, and this process is commercialised in numerous industrial processes. However, liquid feedstock based thermal spray is still in its development phases, due to limited information available on process parameters. Various parameters such as plasma/fuel gas, plasma current, feedrate, feeding angle, type of feedstock (suspension or solution precursor), feedstock concentration, feedstock viscosity, solvent, etc. significantly influence the thermal and kinetic energy exchange between plasma/flame and feedstock material. Suspension plasma spray (SPS) and suspension high velocity oxy-fuel spray (SHVOF), once optimised, can give rise to coatings with multiscale features. An in-depth understanding of the complex interaction between feedstock solution/suspension chemical-physical properties and plasma/flame jet characteristics is essential to understand its specific impact on coating properties and their application. This paper presents comparisons between two different TiO2 coatings, deposited by SPS and S-HVOF, and obtained by varying some of the fundamental spray deposition parameters. The surface morphology and cross-sections of the as-deposited coatings were compared through SEM/EDX. Further, surface wetting properties were analysed through measuring the static and dynamic contact angles.

Abstract Hybrid plasma spraying is emerging as the next potential technology leap in thermal spraying. The combination of high throughput and deposition rates of coatings sprayed from powders with the tailored functionality of liquid-feedstock sprayed coatings appears highly promising for a wide range of applications. Moreover, possible refined mixtures of different materials come readily with the utilization of multiple feedstocks with varying particle sizes. However, the practical aspects of hybrid coatings production are accompanied with several peculiarities not encountered when using distinct feedstocks. To deepen the understanding of this novel route, this paper presents fundamental hybrid coating formation principles and the effect of selected deposition parameters using multiple case-study material systems, such as Al2O3-YSZ, Al2O3-Cr2O3, and Al2O3-TiO2.

Abstract Cascaded plasma torches are becoming increasingly common, but the influence of geometry, notably that of the anode, is relatively unexplored. This work investigates the relationship between anode-cathode distance and plasma voltage fluctuations. The study was conducted using cascaded torches that can be configured with different numbers of neutrodes and commercially available Al2O3 powders. The powders were sprayed at different gas flow rates and current intensities while monitoring voltage fluctuations as well as in-flight particle temperature and velocity. The resulting alumina coatings were characterized based on microstructure, phase composition, porosity, and hardness. A frequency analysis of the arc voltage fluctuations revealed well-defined peaks at 60, 120, and 180 kHz that vary in intensity based on the number of neutrodes. The more neutrodes, the sharper and higher the peak. In contrast, the power spectra of the arc voltage generated by a conventional plasma torch contains no such peaks, indicating a random displacement of the arc root leading to less stability of the arc.

Abstract Thermally sprayed WC-Co coatings provide excellent wear resistance and corrosion protection under heavy loads, but their application usually involves additional grinding and polishing steps, which can be 3-4 times costlier than the spraying process itself. There is thus the motivation to develop a process that produces smooth, near-net-shape carbide coatings. This contribution is an investigation of WC-12Co coatings obtained by suspension HVOF spraying. Significant work was devoted to the development and characterization of water-based hardmetal suspensions synthesized from commercially available WC and Co powders. The suspensions produced were sprayed using the HVOF process, and the resulting coatings were evaluated based on microstructure, hardness, and phase composition.

Abstract The focus of this study is the formation of a solid solution and metallic nickel in the cobalt-nickel mixed oxide coatings during suspension plasma spray (SPS) deposition. The (Co,Ni)O solid solution is a potential material for inert anode applications in aluminum production. SPS coatings and in-flight collected particles are studied to gain further insight into the melting and mixing phenomena of the NiO and CoO powders as well as phase formation in the deposited coatings. Moreover, the role of suspension feedstock particle sizes on the microstructure of coatings is discussed. SEM, EDS and X-ray diffraction studies helped better understanding the formation of different crystalline phases within the as-sprayed coatings. It was found that the formation of metallic nickel is possible in the coatings. The results support the importance of substrate temperature on the formation of metallic Ni, so that keeping the substrate at low temperature results in an increase of the Ni content in the coatings. In this study, possible causes for the formation of metallic Ni during spraying are discussed.