In-flight Particle Velocity 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.

Prediction of In-Flight Particle Properties and Mechanical Performances of HVOF-Sprayed NiCr-Cr3C2 Coatings Based on a Hierarchical Neural Network.

High-velocity oxygen fuel (HVOF) spraying is a promising technique for depositing protective coatings. The performances of HVOF-sprayed coatings are affected by in-flight particle properties, such as temperature and velocity, that are controlled by the spraying parameters. However, obtaining the desired coatings through experimental methods alone is challenging, owing to the complex physical and chemical processes involved in the HVOF approach. Compared with traditional experimental methods, a novel method for optimizing and predicting coating performance is presented herein; this method involves combining machine learning techniques with thermal spray technology. Herein, we firstly introduce physics-informed neural networks (PINNs) and convolutional neural networks (CNNs) to address the overfitting problem in small-sample algorithms and then apply the algorithms to HVOF processes and HVOF-sprayed coatings. We proposed the PINN and CNN hierarchical neural network to establish prediction models for the in-flight particle properties and performances of NiCr-Cr3C2 coatings (e.g., porosity, microhardness, and wear rate). Additionally, a random forest model is used to evaluate the relative importance of the effect of the spraying parameters on the properties of in-flight particles and coating performance. We find that the particle temperature and velocity as well as the coating performances (porosity, wear resistance, and microhardness) can be predicted with up to 99% accuracy and that the spraying distance and velocity of in-flight particles exert the most substantial effects on the in-flight particle properties and coating performance, respectively. This study can serve as a theoretical reference for the development of intelligent HVOF systems in the future.

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.

Influence of Plasma Torch Power on the Plasma Jet Properties and Microstructure of Alumina Coatings

In this study, alumina coatings were formed using atmospheric plasma spraying, increasing the torch power from 29.4 to 45.1 kW. The surface morphology of the coatings was determined using scanning electron microscopy; the elemental composition was examined using energy-dispersive X-ray spectroscopy (EDS); phase composition was investigated using X-ray diffraction; and surface roughness was determined using a profilometer. The steel surface temperature was measured using a type-K thermocouple, and the plasma jet temperature, at a distance of 70 mm, using a type-B thermocouple. Alumina particle velocity was calculated by analyzing high-speed camera footage using ImageJ software. The results indicate that plasma jet temperature, speed, and in-flight particle velocity increased with plasma torch power. Furthermore, the amount of γ-Al2O3 phase in the coating increased, and the α-Al2O3 decreased with increasing plasma power. The surface roughness (Rq) of the Al2O3 coatings decreased from 7.13 to 5.54 μm, with an increase in torch power. The EDS measurements indicate that the increase in torch power did not affect the elemental composition of as-sprayed coatings. The results provide a wider understanding of an atmospheric plasma spray technique, optimizing and controlling the parameters using air as a primary gas.

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.

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.

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.

Particle Velocimetry, CFD, and the Role of Particle Sphericity in Cold Spray

Implementation of the cold spray (CS) technology in manufacturing and repair creates a demand for reliable quality control and process monitoring measures. In this regard, particle size and impact velocity are undoubtedly the most important control parameters in CS. Several in-flight measurement systems for particle velocimetry are now available commercially for CS. These systems provide great potential to be used as a diagnostic tool for validating CS system performance in industrial applications. However, post processing the velocimetry data is required in many instances for achieving a complete understanding of the particle flow field. In this study, particle velocimetry is used in conjunction with computational fluid dynamics (CFD) simulations to: (i) identify the physical factors that dictate the particle velocity and its variability; (ii) develop high fidelity CFD models to accurately predict particle flight parameters that cannot be measured by available experimental tools; and (iii) present the capabilities of state-of-the-art velocimeters as a reliable diagnostic tool for measuring the consistency and repeatability of CS systems in manufacturing settings. In-flight particle size, location, and velocity are measured using a commercially available velocimeter for aluminum and copper particles sprayed with supersonic nozzles using helium, nitrogen, and air by two high pressure CS systems. As a result of this work, particle sphericity was clearly identified to have strong effects on particle velocity and to be one of the main factors of the variability of particle velocity. Furthermore, methods for building a high-fidelity 3D-CFD model was presented. CFD models were validated using particle velocimetry and schlieren imaging. Finally, particle velocimetry is shown to be a valid diagnostics tool for CS with systems capable of measuring in-flight particle velocities along with particle sizes. This article also outlines steps necessary for conducting cold spray process diagnostics repeatably and reliably.

Improving plasma sprayed Raney-type nickel\u2013molybdenum electrodes towards high-performance hydrogen evolution in alkaline medium

Rationally designed free-standing and binder-free Raney-type nickel–molybdenum (Ni–Mo) electrodes produced via atmospheric plasma spraying (APS) are developed by correlating APS process parameters with the microstructure of electrodes and their electrochemical performance in alkaline media. The results revealed that the electrode morphology and elemental composition are highly affected by the plasma parameters during the electrode fabrication. It is found that increasing plasma gas flow rate and input plasma power resulted in higher in-flight particle velocities and shorter dwell time, which in result delivered electrodes with much finer structure exhibiting homogeneous distribution of phases, larger quantity of micro pores and suitable content of Ni and Mo. Tafel slope of electrodes decreased with increasing the in-flight particles velocities from 71 to 33 mV dec−1 in 30 wt.% KOH. However, beyond a critical threshold in-flight velocity and temperature of particles, electrodes started to exhibit larger globular pores and consequently reduced catalytic performance and higher Tafel slop of 36 mV dec−1 in 30 wt.% KOH. Despite slightly lower electrochemical performance, the electrodes produced with highest plasma gas flow and energy showed most inter-particle bonded structure as well as highest stability with no measurable degradation over 47 days in operation as HER electrode in 30 wt.% KOH. The Raney-type Ni–Mo electrode fabricated at highest plasma gas flow rate and input plasma power has been tested as HER electrode in alkaline water electrolyzer, which delivered high current densities of 0.72 and 2 A cm−2 at 1.8 and 2.2 V, respectively, representing a novel prime example of HER electrode, which can synergistically catalyze the HER in alkaline electrolyzer. This study shows that sluggish alkaline HER can be circumvented by rational electrode composition and interface engineering.

Microstructure and mechanical property of high velocity oxy-fuel sprayed WC-Cr3C2-Ni coatings

WC-Cr3C2-12Ni coatings, with Cr3C2 weight content of 10% and 30%, were fabricated on steel substrate by high velocity oxy-fuel (HVOF) flame spraying technique, and microstructure, phase composition and mechanical properties of WC-Cr3C2-12Ni coatings prepared by different oxygen flows were investigated. The results reveal that phase compositions of WC-Cr3C2-12Ni coatings were mainly influenced by the Cr3C2 content rather than the oxygen flow. WC-Cr3C2-12Ni coatings demonstrated low decarburization and decomposition of WC due to coarse WC gains and high in-flight particle velocities during HVOF. WC-Cr3C2-12Ni coatings exhibited low porosities and typical microstructures, where a mixture of fine and coarse WC grains was uniformly embedded in the Ni/Cr3C2 matrix. The microhardness, indentation fracture toughness and elastic modulus of WC-Cr3C2-12Ni coatings were improved by coarse WC grains, Cr3C2 addition and dense microstructure, which resulted in high wear resistances of WC-Cr3C2-12Ni coatings. The abrasive wear features with groove, peel-off and exposed/fractured WC grains were observed in WC-Cr3C2-12Ni coatings after dry-sliding wear. The relationship between microstructure and mechanical property of WC-Cr3C2-12Ni coatings was discussed from the viewpoints of powder properties and process parameters.

Estimation of Particle Mass Flow Rate in Free Jet Using In-Flight Particle Diagnostics in Plasma Spraying

In-flight particle diagnostics have enhanced our understanding of thermal spraying and improved coating reproducibility. However, no methodology to verify the measured in-flight particle properties has been proposed in the literature yet. This methodology requires describing the entire free jet from accurate measured values. This study deals with a novel method to verify the measured in-flight particle sizes and velocities by estimating the particle mass flow rate (PMFR) in the free jet. To this end, the entire free jet cross section was divided into several non-overlapping focal planes, and the size and velocity of the in-flight particles were measured by optical diagnostics at these focal planes. The PMFR of the powder feeder was used as a reference to validate the determined PMFR in the free jet. The results showed a good agreement with the PMFR of the powder feeder and could be replicated with different feedstock powders, demonstrating the capability of the developed method. Furthermore, the determined PMFR distribution in the entire free jet, referred to as digital footprint, agreed well with the height of the experimental footprints of the spray jet on a substrate. Consequently, it can be concluded that the spatial PMFR distribution was also properly measured.

Influence of cold gas spray processing conditions on the properties of 316L stainless steel coatings

ABSTRACTIt is well known that particle velocity before deposition is one of the most important parameters for fabrication of the high-quality cold sprayed coatings. In this work, 316L stainless steel powders were deposited on the substrate with the same material. The influence of particle size, morphology and processing parameters such as gas pressure (up to 4 MPa) and temperature (up to 1000°C), on particle velocity have been investigated using a DPV-2000 in-flight particle velocity monitoring system, followed by the measurement and analysis of deposition efficiency and coating mechanical properties including tensile strength and micro-hardness of the coatings. The results show that a higher particle velocity can be achieved by using particles with an angular morphology and a finer particle size. Higher particle velocities, resulting from higher working gas pressure and temperature, yield much denser coatings with higher deposition efficiency, micro-hardness and tensile strength.

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.

Effect of HVOF Process Parameters on TiO2 Coatings: An Approach Using DoE and First-Order Process Maps

High-velocity oxy-fuel deposition stages of titanium oxide coatings were analyzed and reported here. For the first stages of deposition, process maps were used to evaluate the influence of deposition parameters such as stand-off distance (SOD), powder feed rate (PFR) and fuel/oxygen ratio (F/O) on velocity and temperature of in-flight particles. These parameters were then combined with particle size distribution (PSD) to further optimize the deposition process considering the spray trace width and the area covered by splats and non-molten particles as responses. Furthermore, the structural, morphological and mechanical characteristics either of single splats, clusters of semi- and/or non-molten particles or coatings were obtained by, x-ray diffraction, Raman spectroscopy, scanning electron microscopy and nanoindentation. During coating build-up, lower PFR and F/O induce more splat particles and fewer clusters of semi- and/or non-molten particles, whereas the PSD affects the width of the spray trace. The combination of F/O and SOD upper levels was related to a high percentage of TiO2 splat deposition. Finally, the proposed methodology allowed to obtain dense TiO2 coatings with a higher content of rutile phase and an average thickness of 65 µm.

Warm-particle peening assisted HVOF spraying: A new process to improve the coating performances

In order to improve the coating quality to a higher level, a method of warm-particle peening assisted thermal spraying was put forward. This work presents a study based on a warm-particle peening assisted high-velocity oxy-fuel (WPPA-HVOF) spray system, in which the spraying powder was injected from the port near the gun nozzle throat and the peening media was injected from the port outside the gun barrel exit. A type of FeBSiNb amorphous coating was prepared by the WPPA-HVOF process and compared with that of the traditional HVOF process. The in-flight particle velocity and temperature were tested using a high-speed thermal spray process monitoring system, which manifests that the velocity and temperature of the awaiting deposit powder are hardly affected by the addition of peening particles. Through characterization of the deposit by methods such as SEM, EDS, and FIB, it was found that the peening particle is rebounded after impacting on the substrate. As a result, there were no peening particles mingled in the deposited coating even when the feed rate was as high as 140 g/min. A by-product is that the coating structure becomes dense, e.g. the porosity can be as low as 0.7%. The WPPA-HVOF process results in a decrease in coating surface residual stress to compressive state. The properties such as microhardness, cohesive strength and wear resistance increase to varying degrees consequently. It is hypothesized that this is attributed to the large compressive residual stress because of warm-particle peening during the deposition process.