Microstructure and physicochemical properties of a thermally sprayed coating depend on the dynamics of the particles interacting with the spray jet. This is especially the case for electrical properties. In this study, different spraying processes were used to spray p-type and n-type half-Heusler powders. Thermoelectric powders, Hf20Zr75Ti05CoSb80Sn20 (p-type) and Hf60Zr40NiSn98Sb02 (n-type), were selected due to their interesting electrical properties. The spray processes were evaluated based on coating composition and mechanical property measurements. The only coatings of practical interest were those that were plasma sprayed and they were examined in detail to assess the effect of process parameters on coating properties.

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.

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.

In thermal spray processes, the characteristics of in-flight particles (velocity and temperature) have a significant effect on coating performance. Although many imaging systems and algorithms have been developed for identifying and tracking in-flight particles, most are limited in terms of accuracy. One key to solving the tracking problem is to get an algorithm that can distinguish different particles in each image frame. As the study showed, when noise and interference are treated, particles are more readily identified in the background, leading to more accurate size and position measurements with respect to time. This approach is demonstrated and the results discussed.

This study investigates the in-flight behavior of particles during cold spraying by means of high-speed shadowgraph photography using a laser imaging system. It also characterizes the particle jet outside the nozzle for different powder sizes (<10 µm to 155 µm) and densities (copper, aluminum). Observations of the jet reveal two low-pressure cold spray flow regimes, one stable, the other unstable, the effect and control of which are discussed.

YSZ coatings were deposited by suspension plasma spraying and a parametric study was performed with different process parameters. Afterward, the porosity of the as-prepared coatings was investigated by SEM imaging and X-ray transmission and a multivariate analysis of the collected data was carried out. The results show that total porosity correlates negatively with suspension mass load, but positively with original powder size, spray step, substrate roughness, and spray distance, which was found to have the greatest impact. A porosity prediction model was also developed and its practical use is discussed.

In this study, pure rutile TiO 2 coatings are deposited on stainless steel substrates by very low-pressure plasma spraying (VLPPS). The spraying system was used in the reactive mode, injecting both titanium powder and oxygen gas to achieve nanosize particles. Optical emission spectroscopy showed that the interaction between Ti particles and O 2 occurred in flight. Coating microstructure and phase composition were characterized at the surface and in the bulk with respect to operating parameters. Coating surfaces show typical cauliflower microstructure with many nanoparticles, while the microstructure below was found to change from binary to columnar as spraying distance increases.

Suspension plasma spray (SPS) is far more complicated than conventional plasma spray and requires a deep knowledge about the influence of process parameters and their correlations. In this study, YSZ coatings were manufactured by SPS with six different process parameters such as plasma power, suspension mass load, original powder size, substrate surface topology, spray distance, and spray step. Afterwards, the porosity of as-prepared coatings was investigated by image method and X-ray transmission technique. A multivariate analysis on the collected experimental data was carried out by employing mathematical statistics methods. The results showed that: 1) Coating porosity has a negative correlation with plasma power and suspension mass load and a positive correlation with the original powder size, spray distance, spray step, and substrate roughness; 2) Spraying distance is the main factor affecting to coating porosity, followed by suspension mass load and substrate surface roughness, respectively. A linear model for porosity prediction was developed and was verified by experiments. The mechanism by which process parameters influence coating porosity is also discussed.

Additive manufacturing offers the ability to produce complex parts or shapes by layer based printing method using 3D modeling software equipment. Among the different technologies, 3D Printing and Rapid Prototyping are well established. However, thermal spraying makes a contribution towards this field as Cold Spray for repairing metal components. VLPPS and PS-PVD are both thermal spray processes using plasma technology in a very low-pressure controlled atmosphere. These conditions allow to obtain different precursor states: molten and/or vapor. As a result, the microstructure of the coating is unique in the community (lower scale elements, pore architecture) and the properties are improved. Furthermore, vapor phase of metal can react with some gases to generate oxides or nitrides. Another opportunity presented in this study is the ability of this vapor phase to fill mold. The objective is to demonstrate that VLPPS process can be employed as an additive manufacturing device to create well-defined objects.

The objective of this work is to investigate the microstructure, composition and hardness of reactive titanium nitride coatings manufactured by Reactive Very Low Pressure Plasma Spraying (R-VLPPS) process. Pure titanium powder is injected into the plasma and a reactive gas is added to generate a reaction with the molten particles /vapors. Firstly the plasma-jet properties were analyzed by means of optical emission spectroscopy (OES). Then titanium nitride coatings were manufactured with a F4-VB low-power plasma gun under a working pressure of 150Pa. Investigations show that according to the radial position of the substrates compared to the plasma jet axis, the resulting coatings microstructures are different (mix of semi-molten particles, liquid spat, clusters and vapors). In addition, the coatings composition is modified with an evolution of TiN 0.3 and TiN phase crystalline phases. Finally, the hardness of the coatings is examined.

This paper aims at improving the adhesive strength of SS 316L coating by substrate preheating (400, 600 and 700°C). The relationships between the adhesive strength of coating/substrate interface and the substrate preheating temperature are discussed. It was found that stronger adhesion is able to occur despite the presence of a thick oxide film on the substrate surface. The preheated substrate surface undergoes a stronger plastic deformation that disrupts the oxide films for obtaining an intimate contact between particle and substrate material. In addition, the oxide films on the substrate surface can prevent the generation of material jet of the substrate. The effects of substrate preheating on the microstructure and hardness were also investigated.