In-flight Particle Properties Research Articles

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.

Suspension Spraying Tip: High Molecular Weight Solvent

In suspension spraying, two most frequently used solvents are water and ethanol. In this study, we test the potential of using alternative, high molecular weight solvent and demonstrate the associated advantages. For that, two organic solvents are directly compared: ethanol (serving as a benchmark, suspension formulated at 10 wt.% solid load) and di-propylene glycol methyl ether (two suspensions at 10 wt.% and 20 wt.% solid load). As a model material, $$\hbox {Al}_{2}\hbox {O}_3$$ is selected, a frequently sprayed ceramics employed in many industrial sectors. Sub-micron 100% alpha-alumina powder is used to formulate the suspensions. Identical spray conditions are then used to deposit the coatings using hybrid water-stabilized plasma torch. Shadowgraphy monitoring of the suspension fragmentation as well as in situ measurement of the particle in-flight properties is employed, showing no significant differences between the three series. Further, it is shown that the ethanol- and ether-based-feedstock coatings are fully comparable in terms of their microstructure, porosity content, surface roughness as well as hardness and adhesion to the substrates. Importantly, the ether-based coatings exhibit slightly higher levels of $$\alpha$$ - $$\hbox {Al}_{2}\hbox {O}_3$$ phase when compared to their ethanol-based counterpart (17 wt.% vs. 6 wt.%). The use of 20 wt.% solid load in the ether solvent leads to twofold increase in the deposition rate while (as opposed to ethanol) successfully retaining a dense microstructure. Lastly, the ether is significantly cheaper and safer to handle than ethanol.

High-Speed Video Analysis of the Process Stability in Plasma Spraying

In plasma spraying, instabilities and fluctuations of the plasma jet have a significant influence on the particle in-flight temperatures and velocities, thus affecting the coating properties. This work introduces a new method to analyze the stability of plasma jets using high-speed videography. An approach is presented, which digitally examines the images to determine the size of the plasma jet core. By correlating this jet size with the acquisition time, a time-dependent signal of the plasma jet size is generated. In order to evaluate the stability of the plasma jet, this signal is analyzed by calculating its coefficient of variation cv. The method is validated by measuring the known difference in stability between a single-cathode and a cascaded multi-cathode plasma generator. For this purpose, a design of experiment, covering a variety of parameters, is conducted. To identify the cause of the plasma jet fluctuations, the frequency spectra are obtained and subsequently interpreted by means of the fast Fourier transformation. To quantify the significance of the fluctuations on the particle in-flight properties, a new single numerical parameter is introduced. This parameter is based on the fraction of the time-dependent signal of the plasma jet in the relevant frequency range.

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.

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.

Cold-Sprayed Al6061 Coatings: Online Spray Monitoring and Influence of Process Parameters on Coating Properties

Process optimization and quality control are important issues in cold spraying and coating development. Because the cold spray processing is based on high kinetic energy by high particle velocities, online spray monitoring of particle inflight properties can be used as an assisting process tool. Particle velocities, their positions in the spray jet, and particle size measurements give valuable information about spraying conditions. This, in turn, improves reproducibility and reliability of coating production. This study focuses on cold spraying of Al6061 material and the connections between particle inflight properties and coating characteristics such as structures and mechanical properties. Furthermore, novel 2D velocity scan maps done with the HW CS2 online spray monitoring system are presented as an advantageous powder and spray condition controlling tool. Cold spray processing conditions were similar using different process parameters, confirmed with the online spray monitoring prior to coating production. Higher particle velocities led to higher particle deformation and thus, higher coating quality, denser structures, and improved adhesions. Also, deposition efficiency increased significantly by using higher particle velocities.

Novel Online Diagnostic Analysis for In-Flight Particle Properties in Cold Spraying

In cold spraying, powder particles are accelerated by preheated supersonic gas stream to high velocities and sprayed on a substrate. The particle velocities depend on the equipment design and process parameters, e.g., on the type of the process gas and its pressure and temperature. These, in turn, affect the coating structure and the properties. The particle velocities in cold spraying are high, and the particle temperatures are low, which can, therefore, be a challenge for the diagnostic methods. A novel optical online diagnostic system, HiWatch HR, will open new possibilities for measuring particle in-flight properties in cold spray processes. The system employs an imaging measurement technique called S-PTV (sizing-particle tracking velocimetry), first introduced in this research. This technique enables an accurate particle size measurement also for small diameter particles with a large powder volume. The aim of this study was to evaluate the velocities of metallic particles sprayed with HPCS and LPCS systems and with varying process parameters. The measured in-flight particle properties were further linked to the resulting coating properties. Furthermore, the camera was able to provide information about variations during the spraying, e.g., fluctuating powder feeding, which is important from the process control and quality control point of view.

Response surface regression analysis on FeCrBSi particle in-flight properties by plasma spray

This work discusses the interactive effects between every two of argon flow rate, voltage, and spray distance on in-flight particles by plasma spray and constructs models that can be used in predicting and analyzing average velocity and temperature. Results of the response surface methodology show that the interactive effects between voltage and spray distance on particle inflight properties are significant. For a given argon flow rate, particle velocity and temperature response surface are obviously bending, and a saddle point exists. With an increase in spray distance, the interactive effects between voltage and argon flow rate on particle in-flight properties appear gradually and then weaken. With an increase in voltage, the interactive effects between spray distance and argon flow rate on particle in-flight properties change from appearing to strengthening and then to weakening.

Effect of suspension characteristics on in-flight particle properties and coating microstructures achieved by suspension plasma spray

This paper focuses on the influence of suspension properties on the manufacturing of coatings by suspension plasma spraying (SPS). For this purpose, alumina suspensions were formulated with two different liquid phases: water and ethanol. Suspensions were atomized with a twin-fluid nozzle and injected in an atmospheric plasma jet. Suspension injection was optimized thanks to shadowgraphy observations and drop size distribution measurements performed by laser diffraction. In-flight particle velocities were evaluated by particle image velocimetry. In addition, splats were collected on glass substrates, with the same conditions as the ones used during the spray process. Scanning electron microscopy (SEM) and profilometry analyses were then performed to observe the splat morphology and thus to get information on plasma / suspension interactions, such as particle agglomeration. Finally, coatings were manufactured, characterized by SEM and compared to each other.

The influence of substrate temperature on properties of APS and VPS W coatings

The effect of substrate temperature on integral properties of W coatings was investigated. Cooled sample holder enabling simultaneous deposition on substrates with different temperatures was used. The spraying was performed by atmospheric plasma spray (water stabilized torch) and vacuum plasma spray (RF inductively coupled torch). Three torch power levels were used for vacuum plasma spray to obtain different spray particle in-flight properties. It was observed that the substrate temperature influenced the porosity, coating modulus, hardness and thermal conductivity of the deposits. The strongest influence of the substrate temperature was observed for the optimal spray condition resulting in the lowest porosity deposit. For the other deposits, the presence of W oxides, condensed W phase, unmelted feedstock particles and splashed splats decreased the values of all abovementioned coating properties.

The influence of particle in-flight properties on the microstructure of coatings deposited by the supersonic atmospheric plasma spraying

In this work, yttria-stabilised zirconia (YSZ) coatings were deposited by a high-efficiency supersonic atmospheric plasma spraying (SAPS) system. During spraying, the in-flight properties (including velocity and surface temperature) of the sprayed particles were diagnosed on-line by a commercially available Spray Watch 2i system positioned normal to the spraying axis. The effect of the in-flight properties of the sprayed particles on the microstructure of the coatings was studied, with the aim of achieving improved knowledge on the accurate control of their coating quality. The VT value, defined as a function of the in-flight velocity of the particles and the surface temperature of the substrate, was observed to be directly proportional to the reciprocal of the porosity and structure uniformity of the coatings, which indicated that the structure of the SAPS-coatings can be tailored by adjusting the VT value of the sprayed particles.

Simplified Model for Description of HVOF NiCr Coating Properties Through Experimental Design and Diagnostic Measurements

The use of factorial design in process parameter development allowed determination of the contribution of key process variables, such as flame energy (combustion pressure and O2/F), spray distance, and feed rate, on in-flight particle properties. The significance of each parameter was used to construct a simple model which enabled the description of particles’ temperature and velocity. Particles with velocities ranging by as much as 300 m/s and temperatures ranging up to 350 °C were used to produce an array of coatings on an in situ curvature sensor enabling the determination of the evolving—during spraying—and residual stress at the end of the process, correspondingly. These diverse particle states combined with the flame impingement on the substrate, resulted in coatings of similar thickness, but significantly different stress states. Real time evolving stresses—during deposition—and coating properties such as, microhardness, modulus, and corrosion behavior were correlated to particle in-flight properties and, via the use of the introduced model, to spray parameters.

Particle In-Flight and Coating Properties of Fe-Based Feedstock Materials Sprayed with Modern Thermal Spray Systems

New developments in the field of thermal spraying systems (increased particle velocities, enhanced process stability) are leading to improved coatings. Innovations in the field of feedstock materials are supporting this trend. The combination of both has led to a renaissance of Fe-based feedstocks. Using modern APS or HVOF systems, it is now possible to compete with classical materials for wear and corrosion applications like Ni-basis or metal-matrix composites. This study intends to give an analysis of the in-flight particle and spray jet properties achievable with two different modern thermal spraying systems using Fe-based powders. The velocity fields are measured with the Laser Doppler Anemometry. Resulting coatings are analyzed and a correlation with the particle in-flight properties is given. The experiments are accompanied by computational fluid dynamics simulations of spray jet and particle velocities, leading to a comprehensive analysis of the achievable particle properties with state-of-the-art HVOF and APS systems.

Heat Diffusion in Solidifying Alumina Splat Deposited on Solid Substrate under Plasma Sprayed Conditions: Application to Coating Formation

A plasma-sprayed coating is built up by the layering of individual splats. The latter are formed by spreading and solidification of molten particles sprayed onto a solid substrate. The coating properties depend on its microstructure and the quality of contact between the splats and the underlying layer and between the piled-up splats. This work deals with a 1D model of heat transfer between plasma-sprayed alumina splat and smooth substrate. The model is based on heat diffusion in the solidifying splat and substrate and includes undercooling phenomenon, heterogeneous nucleation and crystal growth kinetics. It assumes that splat spreading and solidification are two independent processes. The model predicts splat cooling and solidification taking into account, as far as possible, the in-flight particle properties drawn from the literature in order to study their effect on splat thermal history. The effect of the quality of contact between the splats as well as the already-deposited and solidified layer thickness on the grain size distribution and front solidification velocity is investigated.

Effect of in-flight particle properties on deposition of air plasma sprayed forsterite

Abstract Properties of forsterite coatings deposited by two DC-arc plasma spray guns were studied. The guns generate different types and shapes of plasma jets, resulting in different particle/plasma interactions and different microstructures in the coatings due to the different in-flight particle histories. The particle histories are characterized by cross-sectional maps of the plasma jet showing particle temperature, velocity, and particle size distributions and the number of particles correlated with the coating microstructures.

Comparison of In-Flight Particle Properties, Splat Formation, and Coating Microstructure for Regular and Nano-YSZ Powders

The relationship between atmospheric pressure plasma spray parameters and in-flight particle characteristics was determined. The morphologies of individual splats and the coating microstructure were studied for different stand-off distances and arc currents. Coating cross-sectional analysis showed that the total porosity of the coating increased with decreasing arc current, and increasing stand-off distance. Two different materials were used: the regular (r-YSZ) feed stock and the nano size (n-YSZ) agglomerated powder. The results illustrate that the r-YSZ coating has higher total porosity at higher arc currents than n-YSZ coating. The splat flattening degree and circularity was examined at different substrate temperatures for both powders. The results indicate that the flattening degree increased at high temperatures for the two materials, but the values for n-YSZ were higher than those for the r-YSZ. This study showed the operating regimes in which the use of n-YSZ yields improved coating properties.

Application of backward propagation network for forecasting hardness and porosity of coatings by plasma spraying

Plasma spray forming has been showing overwhelming advantages in rapid fabricating parts and molds, in which the forming quality is determined by so many processing parameters. A robust methodology that takes into account the relationship between the spray parameters and processing variables is urgently needed to control the forming quality. In this study, two backward propagation (BP) neural network models have been designed. For the first model, the temperature and velocity distribution regularities of in-flight particles have been investigated. The second model has been created to build up the nonlinear relationship between the spray parameters and the coating porosity and hardness, which has a dramatic effect on the forming quality. In order to train the BP structures and to check the validity of the methodology, plasma spraying of WC—12% Co powder was implemented according to the method of orthogonal experiments, and particle in-flight properties were monitored by an optical monitoring system of CCD camera. The predicted results were found in good agreement compared with the experimental data. The research provides an effective way to the adaptive control of the plasma spray forming and shows the effect of great significance to optimize the performance of coating.

Diagnostics for advanced materials processing by plasma spraying

Abstract Advanced coatings deposited by plasma spraying are used in a large variety of industrial applications. The sprayed coatings are employed typically in industry to protect parts from severe operating conditions or to produce surfaces with specific functions. Applications are found in many industrial sectors such as aerospace, automobile, energy generation, and biomedical implants. Coatings are built by the successive deposition of molten or partially molten particles that flatten and solidify upon contact on the substrate, forming lamellae. The coating properties are intimately linked to the properties of these lamellae, which in turn depend on in-flight particle properties as well as substrate temperature during spraying. Consequently, the development of diagnostic tools for monitoring and controlling these spray parameters will help provide the necessary information to study the coating formation process, optimize the coating properties, and, eventually, control the spray process in production. In this paper, a review of some recent developments of optical diagnostic techniques applied to monitor plasma-sprayed particles is presented. In the first part of the paper, two different sensing techniques for in-flight particle measurement are described. First, time-resolved diagnostics on individual particles is described. This technique is used to study the instabilities of the particle characteristics associated with the plasma fluctuations. Secondly, a technique adapted for use in an industrial production environment for measuring the particle jet characteristics as an ensemble is presented. In the second part of the paper, the use of an optical system to study the influence of the substrate temperature on the flattening and solidification of sprayed particles impacting on a flat substrate is described. The last part of this paper describes the optimization of nanostructured coatings based on a tight control of the temperature and velocity of the plasma-sprayed particles.

INFLUENCE OF SPRAY TECHNOLOGY ON IONIC CONDUCTIVITY OF YTTRIA STABILIZED ZIRCONIA

In this paper the influence of plasma spray parameters of both Vacuum Plasma Spraying (VPS) and Air Plasma Spraying (APS) processes onto Yttria Stabilized Zirconia (YSZ) coating microstructures were studied in order to establish a correlation with the ionic conductivity. Investigations of the relationships between spraying conditions and the spreading of particles were carried out thanks to a study of in-flight particle properties. The in-flight properties were determined using the DPV 2000 and the lamellae were studied by optical microscopy and 3D analyzes. Coatings were then elaborated. Coating microstructures were evaluated by Scanning Electron Microscopy (SEM) and their electrical properties by Impedance Spectroscopy (IS) in order to establish a relationship between interlamellar contacts and ionic conductivity. It has been shown that with these results, it has been possible to increase the ionic conductivity of YSZ coatings by improving their interlamellar contacts.

Study of HVOF spraying of WC–CoCr using on-line particle monitoring

Tungsten carbide–cobalt based spray coatings are widely used in industry for applications requiring abrasion, sliding, fretting and erosion resistance. High velocity oxy-fuel (HVOF) flame spraying has been used for the production of high quality carbide coatings. In this study, a WC–CoCr powder was sprayed using the HVOF process. The spray parameters were varied to study their influence on the particle in-flight properties and the coating properties using on-line particle monitoring. It was found that the spray parameters such as the total gas flow rate, the spray distance and the powder feed rate could strongly influence the particle in-flight properties and the coating properties. In general, a trend existed that higher total gas flow rates, shorter spray distance and lower powder feed rates resulted in higher particle velocity and temperatures and denser and harder coatings.