A key technology to minimize CO 2 -emissions is the production of hydrogen from water electrolysis. The proton exchange membrane water electrolysis (PEMWE) consists of a stacked system out of bipolar plates (BPP), porous transport layers (PTL) and a membrane electrode assembly (MEA). Research activities are ongoing to minimize material input, reduce costs and increase the performance. For example, the BPP on the anodic side of the stack is currently manufactured of bulk titanium and its substitution by a Ti-coated steel substrate is economically interesting. The main requirements for the BPP-coating are a high coating density, a low electrical resistance and a long lifetime in a harsh electrochemical environment. Coating application on substrates of s ≤ 0.5 mm thickness is conducted with three thermal spraying technologies: Cold Gas Spraying (CGS), High Velocity Air-Fuel (HVAF) spraying and High Velocity Oxy-Fuel (HVOF). Substrate preparation is examined as well. Coating development is conducted with regards to coating thickness, density and oxidation. The examination of coatings includes roughness analysis, structural and chemical analysis. The results allow an evaluation of the suitability of thermally sprayed Ti-coatings by the structural properties for the PEMWE application. Among the three tested processes, CGS is the most suitable for this type of application. The three chosen thermal spraying processes are examined for coating application on metal sheets in context of PEMWE for the first time.

The feasibility of processing various polymers by cold spray has been exemplified by depositions with low porosity and properties comparable to the bulk material. However, cold sprayed polymers are generally deposited with low deposition efficiency compared to more extensively studied metal sprays. Low efficiencies in polymer sprays are attributed to characteristic differences in material properties between metals and polymers. Notably, the thermophysical properties of polymers limit heat transfer and promote intra-particle thermal gradients that develop during cold spray processing. These properties (e.g., thermal conductivity, heat capacity, density) and low deposition efficiencies demand alterations to the cold spray process equipment outside typical metal powder spray conditions. Herein, a modified powder feed tube is used to pre-heat powder to temperatures (~84 °C) below the powder melting point, or cool it (~-55 °C) below room temperature before contacting the high velocity carrier gas in the nozzle of a CSM 108 cold spray system. Numerical simulation demonstrated that pre-heating/cooling the powder feedstock is a viable means of adjusting particle temperature upon impact with the substrate; however, this technique has generally not been deliberately utilized in the cold spray of polymers. In the present work, no significant increase in deposition efficiency (~65% for all sprays) was found by increasing the pre-heat temperature. However, pre-heated particles had a mechanical strength 28% higher than particles injected at room temperature and -55 °C. Despite this, scanning electron microscope images indicated no notable differences between the deposit microstructures. Future works are planned to study the effect of pre-heat at higher particle impact velocities and degrees of pre-heat to improve powder consolidation.

Cold gas spraying is a solid-state deposition process developed for metallic powders as feedstock materials. For ceramic materials; such low temperature-high velocity kinetic process is still questionable but could have interesting advantages. In the CERASOL project (ANR-19-CE08-0009); the nature and the architecture of porous ceramic powders involving agglomerated sub-micrometric grains are investigated. To that purpose; three oxide ceramics powders (alumina; zirconia and yttria) have been prepared for cold spray. These powders were analyzed in order to assess their architecture (composition; particle size; porosity; density; crystallite sizes…). Preliminary cold spray experiments were carried out implementing velocities measurements for various stand-off distances and spraying of coupons with line experiments. The characteristics of the deposited layers have been examined by SEM and XRD in order to discuss the role of the powder architecture on the impact behavior of the nanostructured agglomerated particles. The role of the gas stream that affects the kinetic and the trajectory of the particles are also discussed.

In previous studies at McGill University, tin was successfully cold sprayed onto carbon fiber reinforced polymers (CFRPs). A “crack-filling” mechanism was described as the deposition mechanism that allowed deposition of tin onto the CFRP. Improving the coating conductivity for lightning strike protection (LSP) purposes was achieved by adding other metal powders (aluminum, copper, zinc) to tin and cold spraying on the CFRP. At the same time, it was noticed that the addition of this secondary component (SC) provided an increase in deposition efficiency (DE), tamping was initially hypothesized to explain this improvement, thus prompting a study solely on the effect of SC hardness, which is reported elsewhere in this conference. However, it is recognised that other powder characteristics may also be influencing the DE. Thus, in this study, SCs with a wider variety of particle sizes, morphologies, densities and hardness values were mixed with tin and sprayed on CFRPs. The effect of SC properties on tin deposition is discussed and an optimal combination of SC properties for cold spraying of tin is suggested.

This study investigates the effect of preheating on the dynamic flowability of HVOF powders, including conventional WC-Co, nano WC-Co, WC-FeCrAl, and Cr 3 C 2 -NiCr. The results show that the flowability of WC-Co powders can be significantly improved with a two-hour preheat at 200 °C. One explanation for the improvement is that moisture absorbed by the powder is released during pretreatment, but further study is required as it was found that dynamic density influences flow behavior as well.

This study assesses the microstructure and properties of SiC-based coatings deposited using liquid and gas-fueled HVOF spraying techniques and a recently developed SiC-YAG ceramic powder. The coatings are shown to be superior to plasma and high-frequency pulse deposition sprayed SiC in terms of density and microstructure and comparable in terms of adhesion values. SEM and EDX analysis of the coatings shows that hard SiC particles are retained in a YAG binder, forming a composite that exhibits good sliding wear and erosion behaviors. Due to its low density (< 4 g/cm 3 ), the SiC composite may be an alternative to coating materials such as WC-CoCr and Cr 3 C 2 -NiCr in weight-sensitive applications.

This study assesses the influence of atmospheric plasma spraying parameters on splat stacking and porosity formation in bioglass coatings prepared from commercial powders. Coating samples were deposited on stainless steel substrates using spraying parameters established through numerical simulations. Different Ar-H 2 mixtures were used as the forming gas, and plasma current and spraying distance were varied. Coating microstructure and phase composition were determined by SEM and XRD analysis. Although numerical simulations for each parameter set predicted a suitable Sommerfeld number for proper splat stacking, Na 2 O and P 2 O 5 volatilization occurred during spraying, promoting the formation of porosity in the coatings. Denser coatings were obtained, however, by adjusting the gas mixture ratio, plasma current, and spraying distance such that enthalpy of the plasma jet is sufficient to overcome the glass transition temperature of the powder and at the same time avoid the evaporation of volatile oxides.

Despite the wide application of powder metallurgy in the field of additive manufacturing, a general understanding of the spreadability of powder particles in electron beam powder bed fusion (EB-PBF) is lacking. This paper presents the results of a literature review on particle flowability and spreading in additive processes. Different flowability tests are described and spreading mechanisms for different powder-bed processes are reviewed. A technique is proposed to study spreadability in which a single layer of powder is ‘frozen’ in the as-spread condition by contact sintering and then characterized using white-light interferometry. A standard method to calculate powder-bed density is defined based on this approach, and correlations between density, packing factor, and flowability are established.

In this study, NiCrBSi powders with a size range of 30-50 μm were deposited on mild steel substrates by self-fusing atmospheric plasma spraying. Particle temperatures exceeded 2400 °C and the deposits were remarkably dense with low oxygen content. Based on the results, a novel strategy is proposed to directly deposit dense, oxide-free coatings by plasma spraying without the need of post-spray fusing processes.

In this study, laser glazing is used to densify plasma-sprayed YSZ coatings on carbon steel substrates. Melt pool characteristics are assessed for different laser settings and treatment conditions, including substrate preheating. SEM examination of coating surfaces and cross-sections before and after laser treatment shows how microstructure responds to process parameters. It also shows how preheating widens the melt pool, deepens the laser-glazed layer, and reduces the surface density of cracks, thus improving coating quality.

This study demonstrates a novel method for improving the corrosion resistance of cold sprayed Al6061 coatings. Large stainless steel particles were added to a commercial Al6061 powder and the mixture was deposited on Mg alloy AZ31B substrates using nitrogen gas at low working pressure and temperature. It is shown that the stainless steel particles had a shot-peening effect, thus increasing the density as well as the corrosion resistance of Al6061 coatings. SEM examination showed that no stainless steel particles were incorporated in the coating.

In this study, zirconia coatings were fabricated by vacuum plasma spraying using hollow spherical and fused and crushed YSZ powders. Relationships between spray parameters and in-flight particle velocities and temperatures were investigated in real time and correlated with coating microstructure and density obtained under vacuum as well as atmospheric spraying conditions. The results indicate that plasma sprayed particles reach higher velocities under vacuum and slightly higher temperatures in atmospheric conditions. Powder morphology and structure play a major role in determining coating microstructure and porosity, especially in vacuum spraying. The fused and crushed powder yielded the densest coatings under the vacuum process conditions employed.

This study investigates the influence of particle size, nozzle diameter, gas flow rate, and stand-off distance on the microstructure and density of suspension plasma sprayed yttrium-oxide coatings and the intermediate effect of particle characteristics. Three ethanol suspensions were prepared, one with coarse Y 2 O 3 , one with fine Y 2 O 3 , and one with submicron YSZ. The suspensions were injected vertically into the plasma jet downstream of the nozzle and a thermal spray sensor was used to measure in-flight velocity and temperature. The coatings were found to have columnar and dense vertically cracked (DVC) microstructure, varying in hardness and density. Text results and examination findings are presented and correlated with spray parameters, particle properties, and possible coating formation mechanisms.

Lanthanum gallate doped with strontium and magnesium (LSGM) is a good electrolyte candidate for Intermediate-temperature solid oxide fuel cells (IT-SOFCs). In this study, low-temperature sintering is used to increase the density of LSGM coatings prepared by vacuum cold spraying (VCS). LSGM layers with different thickness were deposited by VCS on NiO-YSZ substrates. In order to suppress chemical reactions between Ni and LSGM, the substrates were coated with gadolinium-doped ceria by tape casting. After sintering at 1200 °C, the coatings were found to be denser in most regions due to grain growth, which appears to be accompanied by cracking, particularly in thicker layers. A second layer was deposited on the annealed coatings to seal the cracks and the two-layer structure was further sintered. Gas permeability test results show that the multilayer films are dense enough to consider their use as electrolyte membranes in IT-SOFCs.

This paper introduces a new way to inject nanoparticles into a plasma flame and demonstrates its use in the deposition of dense ceramic coatings. Instead of suspensions or pastes, nanoparticles are dispersed in micro resin fragments. In this case, zirconia particles with an average diameter of 200 nm were mixed with a thermosetting acrylic liquid resin and the mixture was solidified, crushed, and screened. Micro resin fragments are fed into the plasma flame using a conventional powder feeder. The resin content mostly burns away in the plasma jet, which heats and propels the nanoparticles into the substrate. SEM analysis of the zirconia deposits shows that they are free of microcracks and pores, although carbon contamination was detected by thermogravimetry. Coating hardness tests were also conducted and the results are presented.

In this study, a nickel-coated Al 2 O 3 powder, produced by hydrothermal hydrogen reduction, is cold sprayed with the aim of increasing the volume fraction of alumina particles in the nickel matrix. Spray trials were conducted to investigate the effect of oxidation temperature on the microstructure and hardness of as-sprayed as well as heat treated Ni-Al 2 O 3 composite coatings. Results show that oxidizing has a significant effect on microstructure and hardness and that the morphologies of oxide films differ depending on the heat treatment.

This paper presents the decision-making process used to simultaneously optimize the thickness and density of copper coatings produced by low-pressure cold spraying. Utility values based on both process parameters are analyzed for optimization using a Taguchi approach. The selected input parameters of powder feeding arrangement, substrate material, air stagnation pressure, air stagnation temperature, and standoff distance are shown to significantly improve the utility function of interest. The percentage contributions of the input parameters to optimize coating density and thickness are substrate material (50.03%), standoff distance (28.87%), air stagnation pressure (6.41%), powder feeding arrangement (4.68%), and air stagnation temperature (2.64%).

This study compares the morphology, porosity, and purity of yttria powders produced by spray drying, spray drying and sintering (SDS), and spray drying and plasma fusion (SDPF). The surface morphology of each type of powder is examined by SEM. Pore volume and density are determined by Hg porosimetry, and impurity concentrations are assessed via glow discharge mass spectrometry (GDMS). Coatings made from the powders by means of air plasma spraying are evaluated based on porosity, spray time, powder consumption, and embedded fine particles.

In this work, alumina coatings are produced by air plasma spraying (APS) using dense powders ranging in size from 3 to 36 µm and one porous powder with an average particle size of 33 µm. Two spray systems were used, one rated at 40 kW, the other at 100 kW. The powders were applied to grit-blasted Al 6061 and low-carbon steel substrates. Coatings applied to Al 6061 using the high-power sprayer and 3 µm powder peeled off, likely due to thermal shock and mismatch. For all other coatings, the microstructure was examined by cross-sectional SEM, porosity was estimated via optical microscopy, and dielectric strength and volume resistivity were measured. Coatings formed from 3 µm powder were found to be dense with a mostly γ-phase crystal structure. Surprisingly, however, their volume resistivity was lower than that of more porous coatings with high amounts of α-phase. The findings show that, in the case of resistivity, spray equipment has a bigger influence than particle size, but with coating density, the opposite is true.

This paper describes the basic design and operation of a low-pressure plasma spraying (LPPS) system in use at Sandia National Laboratories. To demonstrate the versatility of the system, Sandia engineers, working in collaboration with the New Mexico Institute of Mining and Technology, produced thin (< 100 μm), dense yttria-stabilized zirconia coatings using three deposition mechanisms: liquid droplet, vapor, and mixed mode (vapor and droplet). Despite slight differences in equipment configuration, the work duplicates many of the results obtained in previous investigations, confirming the advantages of LPPS over other thin film deposition techniques.