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.     

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 on in-flight particle velocity in supersonic plasma spraying

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

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.     

Novel method for in-flight particle temperature and velocity measurements in plasma spraying using a single CCD camera

A novel, technically simple imaging system for individual, in-flight particle temperature and velocity measurements for plasma and other thermal spray processes is described. A custom double dichroic mirror is used to add spectral resolving capability to a single, black-and-white, fast-shutter digital charge coupled device (CCD) camera. The spectral double images produced by the individual in-flight particles are processed using specialized image processing algorithms. Particle temperature determination is based on two-color pyrometry, and particle velocities are measured from the length of the particle traces during known exposure times. In this paper, experimental results using the first prototype system are presented. Laboratory tests were performed using rotating pinholes to simulate in-flight particles, and plasma spraying experiments were performed with commercial, standard spraying equipment operated with Al₂O₃ and NiCrAlY powders. The prototype instrument can be readily used to determine velocity and temperature distributions of individual in-flight particles from the imaged region of interest of the plume. Dividing the imaged area into smaller sections, spatial distributions of particle temperature, velocity, and number of detected particles can be studied. The study aims to develop a technically simple, single imaging instrument, which can provide a visual overview of the spray plume in combination with quantitative evaluation of the most important spray particle parameters.     

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.     

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

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.     

On the reproducibility of air plasma spray process and control of particle state

Controlling particle state is important to not only achieve the required microstructure and properties in coatings but also to clearly isolate and understand the role of other clusters of variables (such as the various substrate and deposition conditions) on the aforementioned attributes. This is important to design coatings for high performance applications and in the ongoing efforts toward achieving prime reliance. This study examines the variability in particle state and explores a few strategies to control them for improved reproducibility with the aid of in-flight particle and plume sensors. The particle state can be controlled by controlling the torch parameters or by directly controlling the particle state itself via feedback from particle and plume sensors such as DPV 2000 (Tecnar Automation Ltd, Quebec, Canada) and torch diagnostic system-spray plume trajectory sensor (TDS-SPT) (Inflight Ltd, Idaho Falls, ID). There exist at least a few control protocols to control the particle state (predominantly temperature and velocity) with judicious choice of critical parameters. In the present case particle state has been controlled by varying the critical torch parameters (primary gas flow and arc current) in a narrow range using 8% YSZ of angular morphology (fused and crushed) with 10–75 μm size distributions in conjunction with a N₂-H₂ laminar (nonswirl) plasma. Two important results emerge: (a) The particle state resulting from averaged individual particle measurements (DPV 2000) is surprisingly stable with variability in T<1% and variability in V of <4%. Ensemble approaches yield a somewhat higher variability (5% in temperature). Despite this the variability in basic coating attributes such as a thickness and weight is surprisingly large. (b) Applying a much simpler control strategy to only control the particle injection and hence the particle trajectory results in reduced variability in coating attributes. 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.     

In-situ plasma spraying: Alumina formation and in-flight particle diagnostic

In-situ plasma spraying (IPS) is a promising process to fabricate composite coatings with in-situ formed thermodynamically stable phases. In the present study, mechanically alloyed Al-12Si and SiO₂ powder was deposited onto an aluminum substrate by atmospheric plasma spraying (APS) to obtain a composite coating consisting of in-situ formed alumina reinforced hypereutectic Al-18Si matrix alloy. The effects of spray parameters (arc current and spray distance) and in-flight particle characteristics (temperature and velocity) on in-situ reaction intensity (alumina and silicon) have been investigated. The results show that, in-situ alumina formation and silicon intensity strongly depend on in-flight particle characteristics, spray distance and substrate temperature.     

Artificial neural networks implementation in plasma spray process: Prediction of power parameters and in-flight particle characteristics vs. desired coating structural attributes

Artificial neural networks (ANN) were implemented to predict atmospheric plasma spraying (APS) process parameters to manufacture a coating with the desired structural characteristics.The specific case of predicting power parameters to manufacture grey alumina (Al₂O₃-TiO₂, 13% by wt.) coatings was considered. Deposition yield and porosity were the coating structural characteristics.After having defined, trained and tested ANN, power parameters (arc current intensity, total plasma gas flow, hydrogen content) and resulting in-flight particle characteristics (average temperature and velocity) were computed considering several scenarios. The first one deals at the same time with the two structural characteristics as constraints. The others one deals with one structural characteristic as constraint while the other is fixed at a constant value.     

Alumina-base plasma-sprayed materials—Part II: Phase transformations in aluminas

Aluminum oxide is widely used for plasma spraying. Alumina deposits consist of a number of metastable crystallographic modifications, which at elevated temperatures, transform to the stable α phase. It was shown that additions of various oxides changed the phase composition and shift phase transformation temperatures. This paper addresses the variation of phase compositions and temperatures of the phase changes for plasma-sprayed alumina deposits manufactured with alumina-base materials containing O₂O₃ and TiO₂. This study combines the results obtained from energy dispersive analyzer of X-rays (EDAX) and scanning electron microscopy (SEM) chemical analysis, differential thermal analysis (DTA), and X-ray powder diffrac-tion (XRD) quantitative phase analysis of as-sprayed and annealed samples of alumina deposits and shows how the two additives change the phase composition and the α-phase formation temperature. This transformation temperature varied by nearly 200 °C. The metastable alumina sequences were also influenced by the chemical composition; for example, the content of 6 alumina varied between 0 and 55 wt %. Part I of this article was printed in the September issue of JTST Vol 6 (No. 3), September 1997, p 320–326     

In-flight characteristics of plasma sprayed alumina particles: Measurements,modeling, and comparison

The key phenomena controlling the properties of sprayed coatings are the heat and momentum transfer between the plasma jet and the injected particles. Modern on-line particle monitoring systems provide an efficient tool to measure in-flight particle characteristics in such a way that factors that could affect the coating quality can be identified during the spray process. In this work, the optical sensing device, DPV-2000 from Tecnar, was used for monitoring the velocity, temperature, and diameter of in-flight particles during the spraying of alumina with a Sulzer-Metco F4 plasma torch. Evolution of particle velocity, temperature, diameter, and trajectory showed well-marked trends. Relationships between the position of the in-flight particles into the jet and their characteristics were pointed out, thus delivering valuable information about their thermal treatment. Moreover, a numerical model was developed and predictions were compared with experimental results. A good agreement on particle characteristics was found between the two different approaches.     

Diagnostics and modeling of an argon/helium plasma spray process

The natural instability of the are in direct current (DC) plasma torches used in spray processing is one of the most important causes for variations in heating of sprayed particles, leading to inconsistencies in the final coating quality. A relatively simple diagnostic system has been set up to monitor the plasma jet instability, as well as some important process characteristics. Effects of the operating parameters and the anode condition on properties of plasma jets, particle properties, and coatings have been measured. These results show that the inconsistency caused by the jet instability influences the plasma spray process in several ways. The coating porosity and the deposition efficiency can be correlated to an average jet length obtained from a series of high speed images. Selected frequency peaks in the power spectrum of the acoustic signal are correlated with the average jet length, and these results are used to derive a simple control scheme, which adopts a fuzzy look-up model indicating the condition of the anode. Increasing the are current is the most effective way to counteract the negative effects of anode erosion.     

Arc instabilities in a plasma spray torch

The control over coating quality in plasma spraying is partly dependent on the arc and jet instabilities of the plasma torch. Different forms of instabilities have been observed with different effects on the coating quality. We report on an investigation of these instabilities based on high-speed end-on observation of the arc. The framing rate of 40,500 frames per second has allowed the visualization of the anode attachment movement and the determination of the thickness of the cold-gas boundary layer surrounding the arc. The images have been synchronized with voltage traces. Data have been obtained for a range of arc currents, and mass flow rates for different gas injectors and for anodes displaying different amounts of wear. The analysis of the data has led to quantitative correlations between the cold-gas boundary layer thickness and the instability mode for the range of operating parameters. The arc instabilities can be seen to enhance the plasma jet instabilities and the cold-gas entrainment. These results are particularly useful for guiding plasma torch design and operation in minimizing the influence of plasma jet instabilities on coating properties.     

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.     

Effect of oxygen/fuel ratio on the in-flight particle parameters and properties of HVOF WC-CoCr coatings

High Velocity Oxy-Fuel (HVOF) spray techniques can produce high performance alloy and cermet coatings for applications that require wear resistant surfaces. In HVOF process, the particle velocity and temperature determine the resultant coating properties and in many cases enables a better understanding of the process.The aim of this study is to investigate influences of different oxygen/fuel ratios on velocity and temperature of flying particles as well as properties of the HVOF thermal sprayed WC-CoCr coatings. Particle parameters were recorded just prior to impact on the substrate using in-flight particle diagnostic tool Accuraspray-g3®. Detailed correlation of particle parameters and the coating properties are evaluated in order to deduce particle parameter ranges providing coatings with optimum properties.     

喷涂工艺参数对硅灰石涂层结构的影响

采用等离子喷涂方法,在不同喷涂距离、主气流量和喷涂功率下制备硅灰石涂层.使用扫描电镜观察了涂层的微观形貌,研究了喷涂工艺参数对涂层结构的影响.结果表明,在较大主气流量下,随着喷涂距离增加,涂层粒子扁平化程度降低,涂层内孔隙逐渐增多;在较小主气流量下,涂层粒子扁平化程度随喷涂距离增加呈现先增加后减小的趋势.主气流量增加,涂层致密,粒子扁平充分.喷涂功率增加,粒子熔化好,涂层致密;但随喷涂功率进一步增加,涂层中出现较多的圆形孔隙.喷涂工艺参数对涂层结构的影响主要通过影响熔融粒子的温度和速度所致.     

Arc voltage fluctuations: Comparison between two plasma torch types

In plasma spray process, the arc root lifetime at the anode wall is limited and new arc roots are continuously created according to restrike, takeover or mixed modes. It results in voltage fluctuations at frequencies in the few thousands hertz range. Such phenomena play a key role in the spray process through the fluctuations of particle trajectories and correspondingly their velocity and temperature time-variations at impact. They have been intensively studied for Ar-H₂ plasmas produced by Sulzer-Metco PTF4 and Praxair plasma torch. However, the arc movement depends on several parameters such as the plasma spray parameters, the plasma gas mixture composition, the electrode geometry and the plasma forming gas injector design. In this paper, two commercial plasma torches, PTF4 and 3MB, running both with argon-hydrogen gas mixtures, have been studied. This research work shows the geometry effect and the cold boundary layer thickness on the arc voltage fluctuations, as well as the particle thermal treatment and the resulting coating properties.     

等离子熔射过程中射流与粒子流的加热效应

采用CCD图像采集系统与图像处理技术提取等离子射流长度;以红外测温仪检测的单位时间内基体温度变化来衡量加热效应,研究不同熔射距离与射流长度条件下射流和粉末粒子流对基体的加热效应特点.结果表明,当熔射距离不大于射流长度时,基体温升主要来至于射流加热效应;随着熔射距离增大,射流对基体的加热效应迅速减弱;当熔射距离大于射流长度时,粒子流加热效应比较明显.提出射流长度可以作为合理选择熔射距离的特征评价指标,并通过不同熔射距离条件下熔射皮膜的截面尺寸以及形貌进行验证.     

Effect of particle morphology on flow characteristics of a composite plasma spray powder

The effects of BaF₂-CaF₂ particle morphology on National Aeronautics and Space Administration (NASA) PS304 feedstock powder flowability were investigated, BaF₂-CaF₂ eutectic powders were fabricated by comminution (producing an angular morphology) and by gas atomization (producing a spherical morphology). The fluoride powders were added incrementally to the other powder constituents of the NASA PS304 feedstock, (Ni-Cr, Cr₂O₃, and Ag powders). A linear relationship between flow time and concentration of the BaF₂-CaF₂ powder was found. The flow of the powder blend with spherical BaF₂-CaF₂ was better than that with angular BaF₂-CaF₂. The flowability of the powder blend with angular fluorides decreased linearly with increasing fluoride concentration. However, the flow of the powder blend with spherical fluorides was independent of fluoride concentration. The results suggest that for this material blend, particle morphology plays a significant role in flow behavior, offering potential methods to improve powder flowability and enhance the commercial potential. These findings may be applicable to other difficult-to-flow powders such as cohesive ceramics.