Investigation on in-flight particle velocity in supersonic plasma spraying

0Introduction Asakindofsurfaceengineeringtechnology,thermal sprayingcanprovideprotectiveorfunctionalcoatings whicharewidelyusedinmanyindustrieslikechemistryin dustry,papermaking,electricengineering,powerplant, aviation,automobileproducing,steelmill,glass…     

In-situ particle temperature, velocity, and size measurements in DC Arc plasma thermal sprays

Particle temperature, velocity, and size measurements in DC arc plasma thermal sprays are reported in this article. Experiments were performed using a conventional DC are argon-hydrogen plasma with two 7 wt % yttria-stabilized zirconia powders injected transversely into the plasma jet. Measurements were performed along the axis of the plasma jet as well as at a number of radial locations at several axial positions. It was found that transverse injection of the powder results in the aerodynamic size classification of the powder with the large particles penetrating further across the plasma jet than the smaller particles, which were more readily swept by the high momentum of the plasma jet. Consequently, the particle temperatures were influenced by their degree of penetration into the core of the plasma jet. Average particle temperatures showed a good degree of uniformity radially and decayed with increasing downstream distance. When nanoclustered particles were injected into the plasma, significant differences in particle velocities and temperatures were observed in comparison to the conventional powder under the same plasma operating and particle injection conditions. These differences were attributed to the penetration characteristics of the powder into the plasma jet and the consequent effects on the particle heat up. Hence, axial injection of powder into plasma jets may provide more uniform and axisymmetric particle property distributions compared to the transverse injection schemes.     

Effect of plasma fluctuations on in-flight particle parameters

The influence of arc root fluctuations in direct current (DC) plasma spraying on the physical state of the particle jet is investigated by correlating individual in-flight particle temperature and velocity measurements with the instantaneous voltage difference between the electrodes. In-flight diagnostics with the DPV-2000 sensing device involve two-color pyrometry and time-of-flight technique for the determination of temperature and velocity. Synchronization of particle diagnostics with the torch voltage fluctuations are performed using an electronic circuit that generates a pulse when the voltage reaches some specific level; this pulse, which can be shifted by an arbitrary period of time, is used to trigger the acquisition of the pyrometric signals. Contrary to predictions obtained by numerical modeling, time-dependent variations in particle temperature and velocity due to power fluctuations induced by the arc movement can be very large. Periodic variations of the mean particle temperature and velocity, up to ΔT=600 °C and Δv=200 m/s, are recorded in the middle of the particle jet during a voltage cycle. To our knowledge, this is the first time that large time-dependent effects of the arc root fluctuations on the particle state (temperature and velocity) are experimentally demonstrated. Moreover, large fluctuations in the number of detected particles are observed throughout a voltage cycle; very few particles are detected during parts of the cycle. The existence of quiet periods suggests that particles injected at some specific moments in the plasma are not heated sufficiently to be detected.     

In-flight particle velocity measurements with DPV-2000 in cold spray

The measurements of in-flight particle velocities and deposition efficiency at cold spraying of two stainless 316L powders having different morphology and almost the same size distributions were carried out. It was found that the angular particle had faster velocity than spherical one and resulted in greater deposition efficiency. The critical velocity of both powders was almost the same and did not depend on their micro-hardness. Therefore, the deposition efficiency was not affected by the particle micro-hardness. The critical velocity significantly depended on He gas temperature and decreased as the temperature increases, but little depended on N₂ gas temperature. The critical velocity little depended on operating gas pressure. The cause why the angular particle morphology gives faster velocity in supersonic gas flow compared with the spherical one has to be investigated and clarified. The drag coefficient appears to depend on particle morphology. An angular particle seems to have a larger drag coefficient than a spherical particle.     

Computational study and experimental comparison of the in-flight particle behavior for an external injection plasma spray process

A three-dimensional computational fluid dynamic (CFD) analysis using Fluent V5.4 was conducted on the in-flight particle behavior during the plasma spraying process with external injection. The spray process was modeled as a steady jet issuing from the torch nozzle via the heating of the are gas by an electric are within the nozzle. The stochastic discrete model was used for the particle distribution. The particle temperature, velocity, and size inside the plasma plume at a specified standoff distance have been investigated. The results show that carrier gas flow rate variation from 2 standard liters per minute (slm) to 4.0 slm can increase the centerline particle mean temperature and mean velocity by 10% and 16%, respectively, at the specified standoff distance. A further increase of the carrier gas flow rate to 6 slm did not change the particle temperature, but the particle velocity was decreased by 20%. It was also found that an increase in the total arc gas flow rate from 52 slm to 61 slm, with all other process parameters unchanged, resulted in a 17% higher particle velocity, but 6% lower particle temperature. Some of these computational findings were experimentally confirmed by Kucuk et al. For a given process parameter setting, the kinetic and thermal energy extracted by the particles reached a maximum for carrier gas flow rate of about 3.5–4.0 slm.     

Investigation of particle in-flight characteristics during atmospheric plasma spraying of yttria-stabilized ZrO2: Part 1. Experimental

A detailed investigation of the relationship between the parameters of the spray process and the in-flight properties of the particles was carried out using a multivariate statistical approach. A full factorial designed experiment concerning the spray process was performed, the spray gun parameters’ current, argon flow rate, hydrogen flow rate, and powder feed rate being selected to control the process. The particle properties, viz. velocity, temperature, and diameter, were determined using an optical measurement system, DPV 2000. In addition, the standard deviations of, and the correlations between, the measured particle properties were analyzed. The results showed current to have the strongest impact on particle velocity and particle temperature and argon flow rate to be the only parameter with an inverse effect on velocity and temperature.     

Investigation of particle in-flight characteristics during atmospheric plasma spraying of yttria stabilized ZrO2: Part 2. modeling

Yttria stabilized ZrO₂ particle in-flight characteristics in an Ar-H₂ atmospheric plasma jet have been studied using analytical and experimental techniques. In the previous article,^[1] the primary gas flow, plasma composition, current, and powder feed rate were systematically varied and particle surface temperatures, velocities, and size distributions measured and statistically analyzed. In this paper, a mathematical model for the plasma flow and particle characteristics is presented. Model predictions are compared with the experimental results in Ref 1 and a reasonable correlation is found. A statistical investigation (composite cubic face (CCF)) is performed on the particle predictions, giving fast and simple relationships between gun parameters and particle in-flight properties. The statistical and theoretical models that are presented here combine to form a powerful and cost-effective tool, which can be used in the evaluation and optimization of spray parameters off-line.     

Spray parameters and particle behavior relationships during plasma spraying

Using laser anemometry, laser fluxmetry, and statistical two-color pyrometry, the velocity, number flux, and surface temperature distributions of alumina and zirconia particles in dc plasma jets have been determined inflight for various spraying parameters. The flux measurements emphasized the importance of the carrier gas flow rate, which must be adjusted to the plasma jet momentum depending on the arc current, nozzle diameter, gas flow rate, and gas nature. It has also been shown that the particle trajectories depend both on the particle size and injection velocity distributions and that the position and tilting of the injector plays a great role. The particle size drastically influences its surface temperature and velocity, and for the refractory materials studied, only the particles below 45 μm in diameter are fully molten in Ar-H₂ (30 vol%) plasma jets at 40 kW. The morphology of the particles is also a critical parameter. The agglomerated particles partially explode upon penetration into the jet, and the heat propagation phenomenon is seriously enhanced, particularly for particles larger than 40 μm. The effects of the arc current and gas flow rate have been studied, and the results obtained in an air atmosphere cannot be understood without considering the enhanced pumping of air when the plasma velocity is increased. The Ar-He (60 vol%) and Ar-H₂ (30 vol%) plasma jets, when conditions are found where both plasma jets have about the same dimensions, do not result in the same treatment for the particles. The particles are not as well heated in the Ar-He jet compared to the Ar-H₂ jet. Where the surrounding atmosphere is pure argon instead of air (in a controlled atmosphere chamber), he radial velocity and temperature distributions are broadened, and if the velocities are about the same, the temperatures are higher. The use of nozzle shields delays the air pumping and increases both the velocity and surface temperature of the particles. However, the velocity increase in this case does not seem to be an advantage for coating properties.     

Measurement and numerical simulation of particle velocity in cold spraying

The velocity of cold spray particles was measured by a diagnostic system designed for thermal spray particles that is based on thermal radiation. A laser beam was used to illuminate the cold spray particles in cold spraying to obtain a sufficient radiant energy intensity for detection. The measurement was carried out for copper particles of different mean particle sizes. The particle velocity was also estimated using a two-dimensional axisymmetric model developed previously. The simulated velocity agreed well with the measured result. This fact indicates that particle velocity in cold spraying can be predicted reasonably by simulation. Therefore, it is possible to optimize the cold spray process with the aid of the simulation results. This article was originally published inBuilding on 100 Years of Success, Proceedings of the 2006 International Thermal Spray Conference (Seattle, WA), May 15–18, 2006, B.R. Marple, M.M. Hyland, Y.-Ch. Lau, R.S. Lima, and J. Voyer, Ed., ASM International, Materials Park, OH, 2006.     

Spectroscopic temperature measurements in direct current arc plasma jets used in thermal spray processing of materials

An experimental study was conducted to determine the plasma temperature field and its parametric variation with respect to plasma operating conditions using emission spectroscopy. The focus of our study was the direct current (DC) arc plasma systems used in thermal spray processing of ceramic materials. A commercial plasma system (Metco 9M series) was operated with mixtures of argon and hydrogen in the power input range from 12 to 36 kW. Temperature measurements were based on the detection of emission line intensities from Ar-I neutral species. Spatially resolved measurements were obtained of the plasma temperatures in axisymmetric plasma jets using Abel deconvolution. The variation of plasma axial and radial temperature distributions was measured as a function of the plasma input power, the total gas flow rate, and the binary gas composition of argon and hydrogen. Time-averaged plasma gas temperatures were found to increase with increasing plasma input power, increasing hydrogen content of the plasma gas, and decreasing total gas flow rate. Plasma temperatures decrease progressively with increasing distance from the nozzle exit. The peak temperatures near the nozzle exit are in the range of 12,500 to 14,000 K. The radial temperature profiles show an approximately self-similar decay in the near field of these plasma jets. It was also determined from time resolved intensity measurements that there are significant fluctuations in the argon emission intensity with increasing hydrogen fraction in the mixture. These fluctuations with a typical frequency of 5.2 kHz are attributed to the arc root instabilities observed before. Finally, the measured plasma temperature field is empirically correlated in terms of radial and axial coordinates, plasma electrical input power, plasma efficiency, and gas composition. These temperature data can be used to validate numerical simulations as well as in choosing locations where different materials can be introduced into the plasma jets. This is particularly important for “nanostructured” materials, which loose their structure upon melting as a result of being exposed to high plasma temperatures.     

Influence of particle parameters at impact on splat formation and solidification in plasma spraying processes

A measurement system consisting of two high- speed two- color pyrometers was used to monitor the flattening degree and cooling rate of zirconia particles on a smooth steel substrate at 75 or 150 °C during plasma spray deposition. This instrument provided data on the deformation behavior and freezing of a particle when it impinged on the surface, in connection with its velocity, size, and molten state at impact. The results emphasized the influence of temperature and surface conditions on particle spreading and cooling. When the substrate temperature was 150 °C, the splats had a perfect lenticular shape, and the thermal interface resistance between the lamella and the substrate ranged from 10^{− 7} to 10^{− 8} W/m² · K. The dependence of the flattening degree on the Reynolds number was investigated.     

Calibration of a two-color imaging pyrometer and its use for particle measurements in controlled air plasma spray experiments

Advances in digital imaging technology have enabled the development of sensors that can measure the temperature and velocity of individual thermal spray particles over a large volume of the spray plume simultaneously using imaging pyrometry (IP) and particle streak velocimetry (PSV). This paper describes calibration, uncertainty analysis, and particle measurements with a commercial IP-PSV particle sensor designed for measuring particles in an air plasma spray (APS) process. Yttria-stabilized zirconia (YSZ) and molybdenum powders were sprayed in the experiments. An energy balance model of the spray torch was used to manipulate the average particle velocity and temperature in desired ways to test the response of the sensor to changes in the spray characteristics. Time-resolved particle data were obtained by averaging particle streaks in each successive image acquired by the sensor. Frame average particle velocity and temperature were found to fluctuate by 10% during 6 s acquisition periods. These fluctuations, caused by some combination of arc instability, turbulence, and unsteady powder feeding, contribute substantially to the overall particle variability in the spray plume.     

Scaling analysis and prediction of thermal aspects of the plasma spraying process using a discrete particle approach

On the basis of a discrete particle approach, a scaling analysis was used to predict features of the thermal plasma spraying process. Correlations were obtained using the analysis and they were subsequently used to predict two important features: the state of the particle at the moment of impact on the substrate, and the nature of solidification process. Limitations and restrictions were also identified in the development of the analysis that can be used to infer the resulting structure of coating. The correlations that were developed might be utilized in optimizing the thermal plasma spraying process, as well as in producing new types of coatings.     

Computer aided measurement and compensation system for focal length of lenses in camera manufacture based on the MTF performance using the line CCD sensor

A computer aided system has been developed for the focal length measurement/compensation in camera manufacture. Signal data proportional to light intensity are obtained and sampled very rapidly from the line CCD. Based on the measured signal, the MTF performance is calculated, where the MTF is the ratio of magnitude of the output image to the input image. In order to find the optimum MTF performance, an efficient algorithm has been implemented using the least squares technique. The developed system has been applied to a practical camera manufacturing process, and demonstrated high productivity with high precision