In-flight Particle Temperature Research Articles

Thermal Sprayed Protective Coatings for Bipolar Plates of Hydrogen Fuel Cells and Water Electrolysis Cells

As one core component in hydrogen fuel cells and water electrolysis cells, bipolar plates (BPs) perform multiple important functions, such as separating the fuel and oxidant flow, providing mechanical support, conducting electricity and heat, connecting the cell units into a stack, etc. On the path toward commercialization, the manufacturing costs of bipolar plates have to be substantially reduced by adopting low-cost and easy-to-process metallic materials (e.g., stainless steel, aluminum or copper). However, these materials are susceptible to electrochemical corrosion under harsh operating conditions, resulting in long-term performance degradation. By means of advanced thermal spraying technologies, protective coatings can be prepared on bipolar plates so as to inhibit oxidation and corrosion. This paper reviews several typical thermal spraying technologies, including atmospheric plasma spraying (APS), vacuum plasma spraying (VPS) and high-velocity oxygen fuel (HVOF) spraying for preparing coatings of bipolar plates, particularly emphasizing the effect of spraying processes on coating effectiveness. The performance of coatings relies not only on the materials as selected or designed but also on the composition and microstructure practically obtained in the spraying process. The temperature and velocity of in-flight particles have a significant impact on coating quality; therefore, precise control over these factors is demanded.

Active learning-driven uncertainty reduction for in-flight particle characteristics of atmospheric plasma spraying of silicon

The first-of-its-kind use of the active learning (AL) framework in thermal spray is adapted to enhance the prediction accuracy of the in-flight particle characteristics. The successful AL framework implementation via Bayesian Optimisation is beneficial in, first, reducing the maximum uncertainty, which greatly improves the prediction accuracy and informativeness of the existing database. Second, it reduces local uncertainty around a contrived test point that offers the capability to find improvement in a limited search area, allowing an accurate prediction around a desired test point. The dataset for Machine Learning (ML) training consists of 26 atmospheric plasma spray (APS) parameters of silicon and a further six AL-guided spray runs carried out to reduce maximum uncertainty in the initial database. On average, a 52.9% improvement (error reduction) of RMSE and an R2 increase of 8.5% were reported on the predicted in-flight particle velocities and temperatures after the AL-driven optimisation. Furthermore, the contrived test point optimisation to predict the best possible characteristics in a limited search space resulted in a three-fold increase in prediction accuracy compared to the non-optimised prediction. The AL-driven optimisation proved to be greatly beneficial for resource-intensive thermal spraying, as the framework not only allowed an accurate prediction of the in-flight particle characteristics but also found expected improvement around a desired in-flight characteristic. Furthermore, the framework uses the Gaussian Process (GP) ML model as a surrogate that generalises a global solution without necessarily involving physical and underlying mechanisms, thus extending the framework to other thermal spraying methods.

Tailoring microstructural features of Cr3C2-25NiCr coatings through diverse spray variants and understanding the high-temperature erosion behavior

Thermal sprayed Cr3C2-NiCr coatings are widely used to protect industrial components from ambient or elevated erosive degradation. Various thermal spraying techniques such as plasma spray, detonation spray (DS), high velocity oxy-fuel (HVOF) and high velocity air-fuel spray (HVAF) are widely used to deposit Cr3C2-NiCr coatings. The choice of thermal spraying technique governs the inflight particle velocity and temperature, resulting in unique coating microstructural features, characteristics and erosion performance. In this study, Cr3C2-NiCr coatings are deposited through HVAF, DS and axial plasma spray. A comprehensive comparative analysis was performed to understand the process-structure-property relationship of Cr3C2-NiCr coatings, which correlated with the room and high temperature erosion performance. HVAF spray deposited Cr3C2-NiCr coatings with preferred microstructural features and properties, ensuring superior (high/room temperature) erosive resistance. Cr3C2-NiCr coatings deposited via DS spray performed second best, while plasma sprayed exhibited defective microstructure with inferior mechanical properties, resulting in substandard erosion performance.

Investigation of the morphologies of chelate flame-sprayed metal oxide splats

This paper reports an investigation of Er2O3 splats formed by metal–ethylenediaminetetraacetic acid (EDTA) complex particles that were applied onto aluminum alloy substrates by flame spraying. The splat morphologies and coating microstructures were analyzed under different conditions. The effects of the in-flight particle temperature of the carrier gases and the impact particle deposition temperature on solidification of the molten droplets were evaluated with a rotating dodecahedron stage operated at velocities of 30, 75, and 90 rpm and with different temperatures and angles of incidence. Various techniques were used to analyze the surfaces and cross sections of the splats and coatings formed under the different conditions. Most of the splats underwent transitions from disk shapes (ratio > 85 %) to irregular shapes (circularity of 0.61–0.80) with increasing rotational velocity. The spray process provided a coating with a porosity of 23.3 % at a powder flow rate of 20 g/min. The temperature reached during the chelate flame spraying (CFS) process and the condensing velocity of the deposited splat, rather than the in-flight particle temperature, were found to control the splat morphology. This synergistic condensation effect paves the way to successful deposition of high-temperature structural ceramic coatings such as Er2O3 with the CFS method.

Ni-Cr Powders Modified with Rhenium as a Novel Coating Material-Physical Properties, Microstructure, and Behavior in Plasma Plume.

The aim of this work was to develop a new coating material based on Ni20Cr alloy modified with up to 50%wt. rhenium. The modification was carried out by the mechanical mixing of the base powder and ammonium perrhenate with the subsequent thermoreduction in an H2 atmosphere. The obtained powder consists of a nickel–chromium core surrounded by a rhenium shell. The characterization of the powders—including their microstructure, phase and chemical composition, density, flowability, particle size distribution, and specific surface area—was performed. The influence of plasma current intensity and hydrogen gas flow on in-flight particle temperature and velocity were investigated. The results indicate that there is interdiffusion between the base Ni20Cr and the rhenium shell, resulting in intermediary solid solution(s). The modified powders have a higher specific surface area and a lower flowability, but this does not prevent them from being used as feedstock in plasma spraying. In-flight measurements reveal that increasing the content of rhenium allows for the higher temperature of particles, though it also reduces their speed.

Understanding the role of in-flight particle temperature and velocity on the residual stress depth profile and other mechanical properties of atmospheric plasma sprayed Al2O3 coating

This report deals with the influence of particle temperature and velocity on the microstructure, mechanical properties and residual stress depth profile of plasma sprayed alumina coating. The coatings were produced by varying the particle temperature while maintaining a relatively fixed particle velocity, and vice versa. Residual stress profiles were acquired by measuring the stress in successive layers using X-ray Sin2ψ technique. A substantial increase in hardness and indentation modulus by 76% and 64%, respectively were observed with an increase in particle temperature. With an increase in velocity, coating porosity was found to decrease initially. However, at a high velocity, porosity again increased to a limited extent. The mechanical properties were found to depend strongly on porosity. An increase in particle temperature resulted in an increase in the tensile residual stress in the coatings. Moreover, partial recovery of the grit blasted compressive substrate occurred during spraying owing to an annealing effect.

Tailoring the Mechanical, Tribological and Corrosion Behavior of Fe-Based Metallic Glass Coating Synthesized Using Atmospheric Plasma Spraying

Since the last few decades, plasma spraying has gained its reputation to produce excellent metallic glass coatings because of its high process controllability. Present work studies the effect of various plasma spray parameters on the mechanical, tribological and the corrosion properties of plasma sprayed Fe-based bulk metallic glass (BMG) coating. The in-flight particle velocity and temperature were monitored using the accuraspray diagnostic sensor. Among different sets of parameters, combination of higher power at lower feed rate and higher gas flow provided the maximum density of the coating due to better melting of the particles. XRD analysis showed that high-temperature exposure of the in-flight particles incepted few traces of oxides and carbides in the coating, apart from the intrinsic α- and γ-phases of the Fe-based feedstock. The corrosion resistance of the coatings was highly influenced by the stand-off distance, keeping all the other parameters at higher values. The reason may be attributed to the more number of powder particles melting at higher power and impinging on the substrates at quite lesser time. Current work concludes that the various properties of the Fe-based BMG coating are seriously tailored by different sets of plasma parameters that urge to govern the overall coating properties.

Preparation of bulk-like La0.8Sr0.2Ga0.8Mg0.2O3-δ coatings for porous metal-supported solid oxide fuel cells via plasma spraying at increased particle temperatures

While porous metal-supported solid fuel oxide cells (PMS-SOFCs) have the potential to decrease the cost and increase the start-up speed of power units, the available fabrication processes remain too cumbersome for industrial production. In this study, we prepared bulk-like strontium and magnesium-doped lanthanum gallate (LSGM) coatings using atmospheric plasma spraying (APS) at an increased particle temperature. The large equiaxed grains inside the coatings indicated the epitaxial growth of the splat interfaces and improvement in the coating quality. With increased particle temperature, the conductivity of dense LSGM coatings directly deposited by APS was comparable to that of the bulk material, and cell performance was also significantly enhanced. The maximum power density of the PMS-SOFC at 700 °C was 831 mW cm−2 and 596 mW cm−2 when high and low particle temperatures were used, respectively. These results indicated that the quality of the coating was improved by increasing the in-flight particle temperature.

Implementation of Artificial Neural Networks for Forecasting the HVOF Spray Process and HVOF Sprayed Coatings

In the high velocity oxygen fuel (HVOF) spray process, coating properties are sensitive to the characteristics of in-flight particles, which are mainly determined by process parameters. Obtaining a comprehensive multi-physical model or analysis of the HVOF process remains challenging because of the complex chemical and thermodynamic reactions that occur during the deposition procedure. This study proposes to develop a robust methodology via the artificial neural networks (ANN) to solve this problem for the HVOF sprayed coatings under different operating parameters. Two ANN models were developed and implemented to predict coating’s performances (microhardness, porosity and wear rate) and to analyze the influence of operating parameters (stand-off distance, oxygen flow rate, and fuel flow rate) while considering the intermediate variables (temperature and velocity of in-flight particles). A detailed procedure for creating and optimizing these two ANN models is presented in this work, which encodes the implicitly physical phenomena governing the HVOF process. Results show that the developed implicit models can satisfy the prediction requirements and clarify the interrelationships between the spraying conditions, behaviors of in-flight particles, and the final coating performances, resulting in providing better control of the HVOF sprayed coatings.

In-flight particle states and coating properties of air plasma sprayed ytterbium disilicates

The effects of processing parameters on the in-flight particle behavior, phase composition, and properties of air plasma sprayed ytterbium disilicate (Yb2Si2O7) coatings were investigated in this study. With the increase in plasma enthalpy, in-flight particle temperature increases. Higher particle temperature caused an increase in silica (SiO2) evaporation from the Yb2Si2O7 particles and, as a result, additional Yb2SiO5 phase appears in the Yb2Si2O7 coating. Thermal expansion tests showed the coating containing a higher volume of Yb2SiO5 phase has a larger coefficient of thermal expansion (CTE). Thermal cycling tests in a steam environment indicated that the Yb2SiO5 phase content in the APS Yb2Si2O7 coatings has a significant effect on the EBC's durability. Thermal cycling lives decrease with the increase of Yb2SiO5 phase content in APS Yb2Si2O7 coatings. Analysis of the failed coatings indicated that the predominant EBC failure mechanisms are due to growth of the thermally growth oxides (TGO) as well as the stresses induced by CTE mismatch. Furthermore, Calcium–magnesium–alumina-silicate (CMAS) corrosion tests showed that the recession rate decreases with the increase of Yb2SiO5 phase content in APS Yb2Si2O7 coatings.

Influence of in-flight particle characteristics and substrate temperature on the formation mechanisms of hypereutectic Al-Si-Cu coatings prepared by supersonic atmospheric plasma spraying

Hypereutectic Al-Si-Cu coatings were prepared by supersonic atmospheric plasma spraying to enhance the surface performance of lightweight alloys. To find out optimum process conditions and achieve desirable coatings, this work focuses on the influence of three important parameters (in-flight particle temperature, impact velocity, and substrate temperature) on the collected splats morphology, coatings microstructure and microhardness. Results show that appropriate combinations of temperature and velocity of in-flight particles cannot only completely melt hypereutectic Al-Si-Cu particles, especially the primary Si phase, but also provide the particles with sufficient kinetic energy. Thus, the optimized coating consists of 98.6 % of fully-melted region with nanosized coupled eutectic and 0.9 % of porosity. Increasing the substrate deposition temperature promotes the transition from inhomogeneous banded microstructure to homogeneous equiaxed microstructure with a lower porosity level. The observations are further interpreted by a newly developed phase-change heat transfer model on quantitatively revealing the solidification and remelting behaviors of several splats deposited on substrate. Besides, phase evolutions including the formation of supersaturated α-Al matrix solid solution, growth of Si and Al2Cu phases at different process conditions are elaborated. An ideal microstructure (low fractions of unmelted/partially-melted regions and defects) together with solid solution, grain refinement, and second phase strengthening effects contributes to the enhanced microhardness of coating. This integrated study not only provides a framework for optimizing Al-Si based coatings via thermal spraying but also gives valuable insights into the formation mechanisms of this class of coating materials.

Effect of in-flight particle behavior on the morphology of flame sprayed Er2O3 splats

The morphology and microstructure of splats impact the comprehensive capability of a new coating methodology called chelate flame spraying (CFS). This study addresses the quantitative characterization of the spread morphologies of flame sprayed Er2O3 splats directly deposited under different spray conditions on aluminum alloy substrates with a mirror finish. The influence of the in-flight particle temperature and velocity, carrier gas type, and carrier gas ratio on the solidification mechanism of molten droplets was investigated. Image analysis methods were employed to identify single splats from the morphology observed with field-emission scanning electron microscopy (FE-SEM). In addition, Er2O3 films were synthesized on an Al–Mg alloy (A5052) substrate using N2 or O2 as the carrier gas. When O2 was used as the carrier gas, 109-μm-thick films were deposited on the A5052 substrate. The cross-sectional porosity of the films was 3.8%. In contrast, films with 101-μm thickness were synthesized on the A5052 substrate when N2 was used as the carrier gas. The cross-sectional porosity of these films was 13.8%. The results showed that the carrier gas type (N2) and carrier gas ratio had a significant effect on the flattening behavior of the molten droplets. A spraying method combined with multidimensional modes is proposed to control the morphology of the splats.

Novel liquid fuel HVOF torches fueled with ethanol: relationships between in-flight particle characteristics and properties of WC-10Co-4Cr coatings

Commercial WC-10Co-4Cr powder was sprayed by a novel HVOF process with ethanol used as fuel. Ethanol benefits from environmentally friendly features and less consumption of oxygen compared to conventional fossil liquid fuels (i.e. kerosene). Investigations were carried out to determine the influence of different oxygen/fuel ratios on the evolution of the velocity and temperature of in-flight particles in correlation with the properties of the resulting WC-CoCr coatings. The phase compositions, microstructures, porosity data, microhardness and fracture toughness were tested for comparation. The velocity and temperature of particles strongly depend on the total flow rate of oxygen and fuel. The variation of the ethanol flow rate has a more obvious influence on the velocity and temperature of the particles than that of oxygen. Detailed correlation of particle parameters and the coating properties are elaborated in order to deduce particle parameter ranges providing the coatings exhibiting optimum properties.

Self-Bonding Effect Development for Plasma Spraying of Stainless Steel Coating Through Using Mo-Clad Stainless Steel Powders

The limited inter-lamellar bonding in the conventional thermal-sprayed 304SS coating usually leads to much lower corrosion and wear resistance than their bulk counterparts. In this study, Mo-clad stainless steel 304SS-17Mo powders prepared by mechanical alloying were used for plasma spraying to generate ultra-high-tempertature droplets to deposit the coatings with enhanced inter-lamellar bonding. The temperature at the interface between the molten splats and the stainless steel was calculated by numerical simulation, and the surface temperature of in-flight particles was measured by a commercial thermal spray particle diagnostic system. The microstructures of the coatings after etching were characterized to reveal inter-lamellar bonding. The adhesive and cohesive strengthes of the coatings were estimated by tensile and scratch tests. The measurement of the in-flight particle temperature reveals the possibility of creating a self-bonding effect, which is confirmed by microstructure examination, high adhesion over 66 MPa, and cohesion of 221 MPa.

Determination of local deposition efficiency based on in-flight particle diagnostics in plasma spraying

Increasing the deposition efficiency (DE) and consequently reducing the manufacturing costs has been always one of the central goals in thermal spraying. Hence, the effect of modifying various spraying parameters on DE has been widely investigated in the literature. However, there has not been a methodology so far to correlate the in-flight particle properties with their DE on the substrate locally. In this study, a method is developed to determine the local deposition efficiency (LDE) of the plasma spraying process based on optical particle diagnostics of the in-flight particles. To this end, the entire free-jet transverse section is divided into several non-overlapping focal planes and the size and velocity of the particles are measured at these focal planes. Furthermore, the in-flight particle temperatures are captured in a measurement grid normal to the gun axis. The experiments are conducted with alumina spray powder and a three-cathode plasma gun. The LDE is calculated based on the spatial distribution of the in-flight particle mass flow rate (PMFR) and the local deposited mass on the substrate. Subsequently, the interdependencies between the in-flight particle properties and the calculated LDE are investigated using a nonlinear regression model.

Online Diagnostic System to Monitor Temperature of In-Flight Particles in Suspension Plasma Spray

Suspension plasma spray (SPS) is going through a transition phase from research and development to daily use on production lines. Improving repeatability and reproducibility of coating elements and parameters makes SPS a replacement of former well-developed processes. This transition can be achieved by using in-flight particles diagnostic systems to monitor and control key parameters that influence the coating microstructure. Temperature and velocity of the in-flight particles are among the most critical parameters that should be monitored. However, accurately characterizing the in-flight particles in SPS is particularly challenging due to the small particle size of coating materials, harsh spray conditions, considerably shorter spray distances compared to APS, possible interference from the solvent, and limitations of previous measurement systems. In this study, different strategies were investigated to improve the accuracy of temperature measurements of in-flight particles in SPS. For this purpose, two light collection configurations (double-point and single-point measurement) were investigated along with the influence of plasma radiation. The results were evaluated by collecting and studying splats. The size and shape of splats were correlated with the temperature of in-flight particles in order to confirm the accuracy of the sensor’s temperature measurements. In addition, the sensitivity of temperature measurements to the optical filter used for two-color pyrometry, reflection of plasma radiation from surrounding objects, and direct radiation from plasma were investigated. The results showed that the single-point measurement configuration was well adapted for SPS.

Modelling Knudsen number effects in suspension high velocity oxy fuel thermal spray

Suspension high velocity oxy fuel thermal spray is a system characterized by supersonic velocities and length scales of particles of the order of nm – µm. As the effects of rarefication become significant the assumptions within the continuum models begin to collapse, the effects of rarefication can be evaluated through the flow Knudsen number. Modifications to the numerical modelling must be made to incorporate the effects of rarefaction. This study looks to include the effects of rarefication into the computational fluid dynamics (CFD) models for the suspension high velocity oxy-fuel (SHVOF) thermal spray process. A model for the heat transfer coefficient that take into account the Knudsen and Mach number effects is employed. Finally, the Ranz-Marshall correlation for the Nusselt number is compared to the Kavanau correlation and a compressible Nusselt number correlation. The model is validated through comparisons of particle temperatures which are obtained from two colour pyrometry measurements using a commercially available Accuraspray 4.0 diagnostic system. This study shows that there is a significant improvement in the prediction of inflight particle temperatures when accounting for the effects of compressibility and the effects of rarefication on the Nusselt number.

Effects on numerical calculations of in-flight particle trajectories and temperatures considering multiple particle size and shape

Abstract In this work the effects on numerical calculations of in-flight particle motion and heating within a semi-industrial scale shaft furnace are presented. Therefore, experiments and numerical calculations with a type of boiler slag powder were conducted. It consists of particles with various size and shape. The obtained results were used for validation purposes of the proposed numerical model. This model differs in two main modifications compared to the current state-of-the-art CFD models. First, the application of a numerically inexpensive combustion model with a detailed chemical reaction mechanism enabled a significant reduction of the computational demand. Furthermore, the used burner type in the furnace with its characteristics can be modeled with sufficient accuracy. Second, various particle sizes and shapes were considered within the numerical model by application of corresponding drag formulations and empirically determined drag model parameters. Latter were obtained using a particle classification method proposed in a previous study. With this approach influences due to particle drag, rotation, oscillation and surface friction of different particle sizes and shapes were considered. Nevertheless, particle trajectories and heat transfer between particles and the fluid can be modeled with a low computational effort. The obtained results were compared to a commonly used approach, considering only one particle shape factor and one particle injection. Resulting particle trajectories and temperatures reveal significant differences between the latter and the proposed approach. Finally, influences on the heating characteristics due to preheated combustion air were investigated. It was shown that particle trajectory and temperature calculations in the shaft furnace are mainly affected by considering various particle shapes. Furthermore, the numerical results were improved by applying empiric model parameters. Using preheated combustion air as oxidizer results in a higher amount of spheroidized particles.

Understanding the microstructural evolution of high entropy alloy coatings manufactured by atmospheric plasma spray processing

Atmospheric plasma spray (APS) of mechanically alloyed equiatomic AlCoCrFeNi high entropy alloy (HEA) results in a complex alloy-oxide coating. All the constituent phases have been identified via extensive microscopy and spectroscopy at various length scales. This microstructural characterization along with the in-flight particle size and temperature measurements and single-pass studies have been used to decode the particle-plasma-atmosphere interaction that resulted in the observed coating microstructure. Particles finer than 5 µm diameter are expected to melt, spheroidize and oxidize completely in-flight when closer to the plasma plume core, whereas those larger than 15 µm only exhibit softening and surface oxidation. Molten particles splat on impact resulting in typical lamellar microstructure, while the unmelted particles either get embedded in the coating or bounce off the substrate. Equiatomic AlCoCrFeNi powder oxidizes differently under APS environment than the cast alloy does during isothermal oxidation; resulting in multiple oxides – alumina, chromia, spinels and residual unoxidized alloy cores. Understanding these phenomena in conjunction with each other enables us to tailor feedstock and spray parameters to obtain the desired coating properties.

Tailoring the bimodal zone in plasma sprayed CNT reinforced YSZ coating and its impact on mechanical and tribological properties

Carbon nanotube (1 wt%) reinforced nano YSZ coatings was deposited using plasma spraying technique. Four ‘key parameters’ i.e., plasma power (P), primary gas flow rate (G), powder feed rate (F) and stand-off distance (S) were varied and a total of 24 experiments were carried. In-flight particle temperature and velocities were recorded and analysed to obtain optimized parameters. The bimodal structures were analysed using FE-SEM and correlation between nano grains and the parameters were established. Substantial improvement in the mechanical, adhesion and tribological properties were also observed after reinforcing CNTs in n-YSZ coatings.