Correlation of Plasma Sprayed Coating Deposition Efficiency with Volume Flux Measurements by Phase Doppler Anemometry (PDA)

The use of phase-Doppler anemometry (PDA) to characterize the detail in-flight plasma sprayed particle behavior has been demonstrated previously [Ma et al. Plasma Chem Plasma Process 24(1):85; 25(1):56] The present articles shows further that a direct relationship may exist between the PDA measured particle volume flux and the coating microstructure and deposition efficiency (DE). In the situation when the precise particle temperature information is not available, the PDA measured particle volume flux may provide an alternative to predict quantitatively the variation of the coating microstructure and the DE. By monitoring the in-flight particle volume flux variations, instead of the particle velocity, size and temperature individually and simultaneously, the optimal settings of the plasma spraying parameters may also be determined conveniently. However, it is noted that the effective applications of such approach depend largely on the particle surface morphology and the pre-determination of the particle size range.     

New In-Situ Sampling and Analysis of the Production of CeO<Subscript>2</Subscript> Powders from Liquid Precursors in a rf Thermal Plasma Reactor. Part 2: Analysis of CeO<Subscript>2</Subscript> Powders,Fuel Addition,and Image Analysis

A novel reactor design, sampling probe and wet collection system were used to investigate the combined effects of plasma operating parameters and particle collection mechanisms on the synthesis of CeO₂ particles from liquid precursors. The sampling of particles in-flight and the collection of particles at several reactor regions were used to provide experimental evidence of particle size at different reactor locations at various plasma operating conditions, i.e., power and plasma gas flow rates. This information provided a picture of how CeO₂ particles were formed and how these particles were collected in various locations. The effect of adding water-soluble fuels (alanine and glycine) to the original cerium nitrate solutions was also investigated. Fuel addition decreased the temperature of CeO₂ formation by acting as a local heat source as a result of fuel auto-ignition. Photographs of the particles in-flight were taken using a fast speed CCD camera.     

An Extreme Learning Machine Algorithm to Predict the In-flight Particle Characteristics of an Atmospheric Plasma Spray Process

A robust single hidden layer feed forward neural network (SLFN) is used in this study to model the in-flight particle characteristics of the atmospheric plasma spray (APS) process with regard to the input processing parameters. The in-flight particle characteristics influence the structure and properties of the APS coating and, thus, are considered important parameters to comprehend the manufacturing process. The training times of traditional back propagation algorithms, mostly used to model such processes, are far slower than desired for implementation of an on-line control system. Use of slow gradient based learning methods and iterative tuning of all network parameters during the learning process are the two major causes for such slower learning speed. An extreme learning machine (ELM) algorithm, which randomly selects the input weights and biases and analytically determines the output weights, is used in this work to train the SLFNs. Performance comparisons of the networks trained with ELM algorithm and standard error back propagation algorithms are presented. It is found that networks trained with ELM have good generalization performance, much shorter training times and stable performance with regard to the changes in number of hidden layer neurons. The trends represent robustness of the trained networks and enhance reliability of the application of the artificial neural network in modelling APS processes.     

Artificial Intelligence Computation to Establish Relationships Between APS Process Parameters and Alumina–Titania Coating Properties

Modeling the behavior of air plasma spray (APS) process, one of the challenges nowadays is to identify the parameter interdependencies, correlations and individual effects on coating properties, characteristics and influences on the in-service properties. APS modeling requires a global approach which considers the relationships between coating characteristics/ in-service properties and process parameters. Such an approach permits to reduce the development costs. This is why a robust methodology is needed to study these interrelated effects. Artificial intelligence based on fuzzy logic and artificial neural network concepts offers the possibility to develop a global approach to predict the coating characteristics so as to reach the required operating parameters. The model considered coating properties (porosity) and established the relationships with power process parameters (arc current intensity, total plasma gas flow rate, hydrogen content) on the basis of artificial intelligence rules. Consequently, the role and the effects of each power process parameter were discriminated. The specific case of the deposition of alumina–titania (Al₂O₃–TiO₂, 13% by weight) by APS was considered.     

Study of Injection Angle and Carrier Gas Flow Rate Effects on Particles In-Flight Characteristics in Plasma Spray Process: Modeling and Experiments

This paper investigates the influence of particle injection angle on particle in-flight behaviors and characteristics at different primary and carrier gas flow rates through an integrated modeling and experimental approach. Particle in-flight status such as temperature, velocity, size and their distribution are analyzed to examine particle’s melting status before impact. Results from the experiments and numerical simulations both show that, when carrier gas flow rate is fixed, a small injection angle favors the particle melting and flattening. This behavior is independent of primary and secondary gas flow rates, spray distance and carrier gas flow rate. When both carrier gas flow and injection angle vary, a high carrier gas flow rate and a small injection angle are recommended for high particle temperature and velocity.     

Particle Injection in Direct Current Air Plasma Spray: Salient Observations and Optimization Strategies

External injection of high-melting point low thermal conductivity ceramics orthogonal to a typical direct current thermal plasma jet plays a vital role in determining the in-flight state of the particles and the process downstream. The interactions between low density ceramic particles and high temperature plasma jet is quite complex, which influences the spray process and associated deposition. Detailed in-flight particle diagnostics as well as spray stream visualization have significantly enhanced our capability to diagnose and control the process. In this paper we present some salient observations on the role of key variables on particle injection. A number of experiments were conducted using a 7MB torch (Sulzer Metco, Westbury, NY) with both Ar–H₂ and N₂–H₂ plasma gases, where the carrier gas flow to inject Yttria Stabilized Zirconia (YSZ) was varied systematically and the resulting in-flight particle state was captured using an array of particle and spray stream sensors arranged in a 3D set-up. A notable observation is the existence of a “sweet-spot” in the plasma jet where the particle temperatures and velocities achieved a maximum. This sweet-spot can be characterized by the plume position (location of centroid of the spray stream) rather than carrier gas flow rate and is independent of primary gas flows and other process/material conditions. This result suggests a possible approach to optimize particle injection independent of plasma-forming-torch-parameters. Controlling particle injection at this sweet-spot has shown to benefit the overall process efficiency (in terms of melting) and process reliability (both in-flight measurement and coating build-up) with concomitant application benefits.     

Numerical Modeling in Radio Frequency Suspension Plasma Spray of Zirconia Powders

A comprehensive model was developed to investigate the suspension spraying for a radio frequency (RF) inductively coupled plasma torch. Firstly, the electromagnetic field is solved with the Maxwell equations and validated by the analytical solutions. Secondly, the plasma field with different power inputs is simulated by solving the governing equations of the fluid flow coupled with the RF heating. Then, the suspension droplets embedded with nano particles are modeled in a Lagrangian manner, considering feeding, collision, heating and evaporation of the suspension droplets, as well as tracking, acceleration, melting and evaporation of the nano or agglomerate particles. The non-continuum effects and the influence of the evaporation on the heat transfer are considered. This particle model predicts the trajectory, velocity, temperature and size of the in-flight nano- or agglomerate particles. The effects of operating conditions and intial inputs on the particle characteristics are investigated. The statistical distributions of multiple particles’ size, velocity, temperature are also discussed for the cases with and without consideration of suspension droplets collision.     

Powder Loading Effects of Yttria-Stabilized Zirconia in Atmospheric dc Plasma Spraying

Powder loading effects have been reexamined for various yttria-stabilized zirconia powders under atmospheric dc plasma spraying. A laser illumination method was utilized to observe powder injection into the plasma jet, while single particle and ensemble methods to measure particle state parameters. Statistical temperature distributions of in-flight particles suggested a rapid increase in the number of semi-molten particles above a certain powder loading rate. Despite drops in the particle temperature and velocity due to the powder loading effect, the deposition efficiency tends to have increased in some cases. Reliability of the single particle and ensemble methods has also been examined at various powder feed rates. Particle temperature measurement by the ensemble method at low powder feed rates could cause a significant error, which may affect powder injection optimization. Particle plume trajectory was not affected as much by the powder loading, which hence had only a limited effect on the particle diagnostics.     

Characteristics of Multi-Phase Alternating Current Arc for Glass In-Flight Melting

An innovative in-flight melting technology with multi-phase AC arc was developed for glass industry. The enthalpy probe and high speed video camera were used to characterize the temperature, velocity, and discharge behavior of multi-phase AC arc. The effects of input power and sheath gas flow rate on arc and melting behavior were investigated. Results show that the temperature and velocity on arc center are increased with input power or sheath gas flow increase. The fluctuation of luminance area ratio and coefficient of variation reflects the change of arc discharge behavior. High temperature of plasma enhances the melting of granulated raw particles during in-flight heating treatment. The shrinkage of particle and the volatilization degree of Na₂O increase under a larger flow rate of sheath gas. The characterized arc behavior agrees with the melting behavior of glass raw materials, which can provide valuable guidelines for the process control of glass melting.     

Gas flow dynamics of an inductively coupled plasma discharge

Temperature and velocity distributions within a low power (0.5–0.75 kW) inductively coupled plasma discharge operating in argon have ben determined for various operating parameters including aerosol gas now rate, plasma gas flow rate, aerosol nozzle diameter, and input power level. All measurements were made without samples. The channel formed in the discharge by the aerosol gas now exhibits temperature and velocity characteristics which distinguishes it from the plasma core and wall regions. Maximum plasma temperature of 8700 ± 300 K and channel gas velocity of 120 $\text{m}\text{s}$ were found.     

The Particle In-Flight Characteristics in Plasma Spraying Process Measured by Phase Doppler Anemometry (PDA)

Detailed measurements of particle in-flight characteristics have been carried out using a PDA system for benchmarking as well as to provide further information to aid the development of simulation models. The parameters studied included four conditions of primary gas flow rate and carrier gas flow rate. The particle velocities, diameters, and the corresponding volume flux at different locations were obtained. Due to the one port particle injection arrangement, it was noted that particles in general sprayed with an angle deviated from the nozzle axis Z_n, to the opposite side of the powder feeder port. The particles would also deviate from the spraying cone axis with a divergence angle (). The deviation and divergence angles were examined under different plasma spraying conditions. The measurement data rates at different cross-sectional planes were also obtained so as to compare the results derived from the volume flux measurement and the actual coating on a substrate at the equivalent standoff distance. It was found that the spraying area obtained from the measurement-data-rate increased with downstream distance and a linear relationship between spraying area and distance was also established. Comparing the integrated results, it was noted that the spraying areas derived from the measurement data rate were close to the actual spraying areas obtained from the coordinate measurement machine (CMM) results.     

Phenomena Involved in Suspension Plasma Spraying Part 2: Zirconia Particle Treatment and Coating Formation

The first part of this paper was devoted to phenomena related to liquid injection in the Suspension Plasma Spraying (SPS) process. This second part emphasizes the solid particles treatment and the coating generation. A simplified theoretical approach is proposed to evaluate the influence of the numerous experimental parameters on heat and momentum transfers from the plasma jet to individual particles. It is shown that small solid particles vanish by vaporization and are very sensitive to plasma arc fluctuations and thermophoresis effect, which makes particles to escape from the jet. This study is completed by experimental investigations concerning, first, in-flight collection of particles and, second, coating elaboration. The importance of the particle size distribution of the injected powder and the parameters for the plasma generation is demonstrated, as their choice can give either porous or dense coatings.     

Enhanced Characterization of Naproxen Formulation by Near Infrared Spectroscopy

Near infrared spectroscopy in combination with appropriate chemometric methods is an effective technique for quantitative analysis of parameters of interest for the pharmaceutical industry. In this study, the artificial neural network (ANN) was applied to monitor critical parameters (compression force, tablet hardness, mean particle size, and active pharmaceutical ingredient concentration of tablets) in the process of naproxen pharmaceutical preparation. The performance of ANN was compared to linear methods (partial least squares regression (PLS) and synergy interval partial squares (siPLS)). The ANN models for compression force, tablet hardness, mean particle size, and active pharmaceutical ingredient concentration of tablets yielded the low root mean square error of prediction (RMSEP) values of 0.936 KN, 0.302 kg, 4.49 mg, and 2.14 µm, respectively. The predictive ability of the PLS model was improved by siPLS with selection of spectral regions and the best performance among all calibration methods was showed by the nonlinear method (ANN). Effective models were built by using these approaches using near infrared spectroscopy.     

Heat Generation and Particle Injection in a Thermal Plasma Torch

The operation of plasma guns used for plasma spraying involves a continuous movement of the anode arc root. The resulting fluctuations of voltage and thermal energy input introduce an undesirable element in the spray process. This paper deals with the effects of these arc instabilities on the plasma jet, and the behavior of particles injected in the flow. The first part refers to the formation of the plasma jet. Measurements show that the static behavior of the arc depends strongly upon the plasma-forming gas mixture, especially the mass flow rate, of the heavy gas, injection mode, nozzle diameter, and arc current. These parameters control the electric field in the arc column, the arc length, its stability, and the gas velocity and temperature. The dynamic behavior of the arc is examined to determine how the tempeature and velocity of the plasma gas vary with voltage variations. Relationships between the gas velocity at the nozzle exit and the lifetime of the arc roots, and the independent operating parameters of the gun can be established from a dimensional analysis. The second part discusses the interaction between the plasma jet and the particles injected into the flow. The parameters controlling particle injection and trajectory are examined to determine how injection velocity must vary with particle size and density to achieve a given trajectory. The effect of the transverse injection of the powder carrier gas is investigated using a 3-D computational fluid dynamics code. Finally, the effect of the jet fluctuations on particle trajectory is studied under the assumption that the jet velocity follows the voltage variation. The result is a continuous variation of the particle spray jet position in the flow. Experimental observations confirm the model predictions.     

Influence of velocity and surface temperature of alumina particles on the properties of plasma sprayed coatings

In this paper are described the main characteristics of the plasma spraying process of alumina deposits, i.e., the temperature and flow field of the plasma jets obtained with the classical spraying torches, the injection of the particles into the plasma jet, the particle surface temperature and velocities in the plasma (measured for calibrated alumina particles), and the coating generation. The measurements on the alumina particles are compared with the predictions of a mathematical model. The experimental and computed particle velocities are in rather good agreement. However, this is not the case for the particle surface temperature. Possible reasons for the discrepancy are proposed (influence of the carrier gas, thermophoretic forces, and poor penetration of the particles into the plasma core even for an injection velocity twice that of the optimal calculated one, as shown by recent measurements). Finally the correlations between the particle velocities and surface temperature, and the properties of the alumina coating (porosity, crystal structure, mechanical properties) are studied.     

Application of tertiary amines for arsenic and selenium signal enhancement and polyatomic interference reduction in ICP-MS analysis of biological samples

Water soluble tertiary amines enhance signals and decrease polyatomic chloride interferences in the direct inductively coupled plasma – mass spectrometric (ICP-MS) determination of As and Se in biological samples. Preliminary experiments with amine concentrations and nebulizer flow rates produced element and interference signal intensity changes. Arsenic and Se ICP-MS determination parameters have been optimized by a simplex procedure with amines in an argon plasma or without amines but with addition of N₂ to the Ar. Variables include RF (radio frequency) power, nebulizer gas flow rate, intermediate gas flow rate, and amine concentration or nitrogen gas flow rate. Detection limit, minimization of polyatomic ion intensities, and reproducibility have been evaluated as reponse factors. The signal enhancement and element-to-molecular interference ratios differ to some extent with analyte intensity optimum operating conditions. The detection limits with addition of nitrogen (16 pg mL^–1 for As and 180 pg mL^–1 for Se) or of amines (8 pg mL^–1 for As and 120 pg mL^–1 for Se) and the extent of chloride interference minimization were compared. Amines addition was more beneficial. Biological standard reference materials and food and fecal samples were analyzed following different sample dissolution procedures.     

Experimental Testing of a Curvilinear Gas Shroud Nozzle for Improved Plasma Spraying

The performance of a new gas shroud nozzle attachment for plasma spraying was tested experimentally using particle diagnostics, flow visualization, and coating characterization techniques. A nozzle attachment with curvilinear inner walls was tested and compared with a commercially available conical nozzle. Particle temperatures were measured with a high-speed ratio pyrometer and particle velocities were measured with an intensified camera and a two-laser illumination system. Flow visualization was performed by seeding the surrounding air with smoke. Particle temperatures at the spraying distance were 300 K higher with the curvilinear insert. The plasma jet was narrower but the particle velocity distribution at the spraying distance was unchanged. Higher temperatures and improved particle melting with the curvilinear insert resulted in a reduction in coating porosity (from 7.0 to 7.2 to 4.5–5.1%) and an increase in coating adhesion strength (from 27.2 to 42 MPa). Shrouding as injected through a circular slot around the nozzle exit was also seen to provide better protection than gas injected with the standard sixteen-port configuration.     

Comparisons Between Numerical Model and Experiments for a Direct Current Plasma Flow

The governing equations describing a flowing stream of a hydrogen plasma encountered in applications, such as diamond deposition, and in devices, such as arcjet thrusters, are solved numerically using the linearized implicit (LBI) Method of Briley and McDonald. The results of simulations under the assumption that the plasma can be described by a single temperature are compared with detailed experimental measurements of flow characteristics and species concentrations in a 1 kW arcjet. These comparisons show that by formulating the problem in terms of known experimental operating conditions, such as mass flow rate, power, and current levels, it is possible to predict many of the characteristics of the flowing plasma. As expected, predictions from this one-temperature model show that some deviations from the experimental results occur near the exit plane of the channel, where unequal electron and heavy particle temperatures are encountered because of lower pressures.     

Laser Doppler anemometry under plasma conditions. Part I. Measurements in a d.c. plasma jet

A study is presented on the use of laser Doppler anemometry (LDA) techniques for the measurement of the gas and particle velocities under plasma conditions. Experimental data is presented for a d.c. plasma jet in which alumina particles are injected under different operating conditions. The results reveal that the plasma velocity at the exit of the jet is of the order of 200–300 m/s. The intensity of turbulence is as high as 30 to 40% in the free shear layer and the particle velocity distribution is shown to be asymmetric, with particle dispersion in the plane of injection considerably more important than that in the perpendicular direction. The average particle velocity depends on the composition of the plasma gas, the torch current, and power.     

A Computational Examination of the Sources of Statistical Variance in Particle Parameters During Thermal Plasma Spraying

Computational modeling is used to systematically examine many of the sources of statistical variance in particle parameters during thermal plasma spraying. Using the computer program LAVA, a steady-state plasma jet typical of a commercial torch at normal operating conditions, is first developed. Then, assuming a single particle composition(ZrO₂) and injection location, real world complexity (e.g., turbulent dispersion, particle size and density, injection velocity, and direction) is introduced ``one phenomenon at a time to distinguish and characterize its effect and enable comparisons of separate effects. Calculations are also performed wherein all phenomena are considered simultaneously to enable further comparisons. Both nonswirling and swirling plasma flow fields are considered. Investigating each phenomenon separately provides valuable insight into particle behavior. For the typical plasma jet and injection conditions considered, particle dispersion in the injection direction is mostsignificantly affected by (in order of decreasing importance): particle size distribution, injection velocity distribution, turbulence, and injection direction distribution or particle density distribution. Only the distribution of injection directions and turbulence affect dispersion normal to the injection direction and are of similar magnitude in this study. With regards to particle velocity and temperature, particle size is clearly the dominant effect.