Main Issues for a Fully Predictive Plasma Spray Torch Model and Numerical Considerations

Plasma spray is one of the most versatile and established techniques for the deposition of thick coatings that provide functional surfaces to protect or improve the performance of the substrate material. However, a greater understanding of plasma spray torch operation will result in improved control of process and coating properties and in the development of novel plasma spray processes and applications. The operation of plasma torches is controlled by coupled dynamic, thermal, chemical, electromagnetic, and acoustic phenomena that take place at different time and space scales. Computational modeling makes it possible to gain important insight into torch characteristics that are not practically accessible to experimental observations, such as the dynamics of the arc inside the plasma torch. This article describes the current main issues in carrying out plasma spray torch numerical simulations at a high level of fidelity. These issues encompass the use of non-chemical and non-thermodynamic equilibrium models, incorporation of electrodes with sheath models in the computational domain, and resolution of rapid transient events, including the so-called arc reattachment process. Practical considerations regarding model implementation are also discussed, particularly the need for the model to naturally reproduce the observed torch operation modes in terms of voltage and pressure fluctuations.     

Comparative Study of Flow Characteristics Inside Plasma Torch with Different Nozzle Configurations

A numerical analysis of the influence of different nozzle configurations on the plasma flow characteristics inside D.C plasma torches is presented to provide an advanced nozzle design basis for plasma spraying torches. The assumption of steady-state, axis-symmetric, local thermodynamic equilibrium, and optically thin plasma is adopted in a two-dimensional modeling of plasma flow inside the plasma torch. The PHOENICS software is used for solving the governing equations, i.e. the conservation equations of mass, momentum, and energy along with the equations describing the K-epsilon model of turbulence. The calculated arc voltages are consistent with the experimental results when arc current, gas inflow rate, and working gas are the same as the experimental parameters. Temperature, axial velocity contours inside plasma torches, profiles along the torch axis and profiles at the outlet section are presented to show the plasma flow characteristics. Comparisons are made among those torches. The results show that torches with different anode nozzle configurations produce different characteristics of plasma flows, which suggest some important ideas for the advanced nozzle design for plasma spraying. In order to validate the model and to show its level of predictivity, a comparison of the model with experimental results encountered in the literature is presented in the last part.     

Preliminary spectroscopic experiments with helium microwave induced plasma produced in air by use of a new structure: the axial injection torch

In this experimental study we have used spectroscopic methods to characterize helium plasma obtained by means of a novel waveguide-fed microwave plasma torch at atmospheric pressure, the axial injection torch. This device produces a “plasma flame” by coupling high frequency (HF) power at 2.45 GHz to the discharge. Various flame parameters (namely the electron density number and the electron and gas temperatures) have been determined by using spectroscopic diagnostic techniques that provided an estimate in terms of the helium flow rate, absorbed HF power and axial position in the experiments. These preliminary results suggest some departure from local thermodynamic equilibrium (LTE) and seem to indicate the utility of the discharge as an excitation source for emission spectroscopy. Comparison with other microwave torches already described in the literature is made in terms of the electron density and the electron and gas temperature.     

Melting Refining Mechanisms in Supersonic Atmospheric Plasma Spraying

In recent years, Yttria-stabilized zirconia based thermal barrier coatings (TBCs) are deposited by newly-developed high-efficiency supersonic atmospheric plasma spraying (SAPS) technology. The final microstructure of the plasma-sprayed coatings is strongly dependent on the size distribution of spray particles. It has been corroborated through experiments that there is a special phenomenon of particle melting refining in SAPS, as compared with the conventional atmospheric plasma spraying (APS). This phenomenon greatly affects the final particle size and distribution, which has not been explained reasonably up to now. Therefore, it is necessary to investigate the melting refining behavior of in-flight particles to control the particle size and to analyze the coating properties. In this paper, the breakup of particle is presented to characterize the phenomenon of particle melting refining, and the peak of size distribution becomes bigger with increasing the spray distances, which is explained by collision-coalescence. Furthermore, based on the maximum entropy formalism, the particle-size distribution is calculated and the result is in good accordance with the plasma spraying experiment results, which verifies the mechanism analysis presented in this paper. This work could provide more efficient applications of the SAPS technology in high-performance TBCs.     

Nanoengineered Plasma Polymer Films for Biomaterial Applications

Plasma polymerization has evolved in an important technology for generation of thin films that have found numerous advanced applications. The orthodox view to plasma polymers is as continuous, homogeneous and pinhole-free coatings. However, there is an emerging trend towards creating more advanced films though engineering them at the nanoscale. This paper presents a summary of our work and published studies, from other groups, which demonstrate the potential of plasma polymerization to generate advanced nanostructured interfaces. The focus is on applications in the area of biomaterials. Strategies for generation of antibacterial coatings through inclusion of metallic nanoparticles in plasma polymer films are described. Drug delivery platforms developed via templating and incorporation of drug particles are outlined. A record of recent progress in fabrication of cell guidance surfaces facilitated by nanoengineering of plasma polymer film is also included. The paper concludes with the author’s view to the future outlook of the niche area of nanoengineered plasma polymer films.     

OES Use and Vaporization Modeling for Fly-Ash Plasma Vitrification

Results are presented of optical emission spectroscopy (OES) application asa control tool to improve fly-ash plasma vitrification. A twin-torch plasmasystem has been used for the fly-ash processing, and a new OES method hasexamined metallic vapors above the melt. The method allows the study ofnonhomogeneous optically thin plasmas exhibiting a symmetry plane withoutsophisticated tomographic systems. The dc arc torches are mounted above acold crucible filled with a synthetic glass. The arc intensity is from200 to 400 . Argon is introduced into the torches along the cathodeand the anode, while argon, oxygen or hydrogen are injected through thelance between the torches. Local plasma temperatures above the melt havebeen evaluated using measured relative intensities of spectral lines ofthe plasma-forming gas. Metallic vapor concentration in the plasma isdeduced from the intensity ratio of the metal–gas spectral lines. Leadoxide has been used to study heavy-metal behavior at the fly-ash plasmavitrification. Distribution of the lead along the crucible surface,depending on the plasma-forming gas composition as well as the concentrationevolution with time, have been examined. The elemental analysis of theresultant glass has been measured by scanning electron microscopy (SEM)with energy-dispersive spectrometry (EDS). A predictive model has beenadapted to simulate the noncongruent vaporization of heavy metals from themelt. According to the data obtained, steep variations of the volatility ofthe elements depend strongly on reducing properties of gases controllingthe plasma composition near the melted surface. In addition, the melttemperature and the redox potential of the gas phase are found to be themost critical parameters.     

Electric Arc Fluctuations in DC Plasma Spray Torch

Direct current plasma torches for plasma spraying applications generate electric arc instabilities. The resulting fluctuations of input electrical power hamper a proper control of heat and momentum transfers to materials for coating deposition. This paper gives an overview of major issues about arc instabilities in conventional DC plasma torches. Evidences of arc fluctuations and their consequences on plasma properties and on material treatments are illustrated. Driving forces applied to the arc creating its motion are described and emphasis is put on the restrike mode that depends on the arc reattachment and the boundary layer properties around the arc column. Besides the arc root shown as a key region of instability, the Helmholtz oscillation is also described and accounts for the whole plasma torch domain that can generate pressure fluctuations coupled with voltage ones.     

Application of infrared thermography for online monitoring of wall temperatures in inductively coupled plasma torches with conventional and low-flow gas consumption

Inductively coupled plasma (ICP) sources typically used for trace elemental determination and speciation were investigated with infrared (IR) thermography to obtain spatially resolved torch temperature distributions. Infrared thermographic imaging is an excellent tool for the monitoring of temperatures in a fast and non-destructive way. This paper presents the first application of IR thermography to inductively coupled plasma torches and the possibility to investigate temperatures and thermal patterns while the ICP is operating and despite background emission from the plasma itself. A fast and easy method is presented for the determination of temperature distributions and stress features within ICP torches.     

A Critical Assessment of Particle Temperature Distributions During Plasma Spraying: Experimental Results for YSZ

Plasma sprayed coatings of Yttria Stabilized Zirconia (YSZ) have been studied extensively through the years to understand variations in coating properties as well as to achieve control on microstructure of the coatings. The requirement for microstructural control and reliability have become all the more important as coatings have now become part of an integrated “prime reliant” design strategy aimed at increasing turbine inlet temperature and associated efficiencies. One of the important thrusts in monitoring and controlling the process has been the application of process sensors that measure spray stream characteristics, notably particle temperature and velocity. Although single particle-based measurements have been available for some time, in general control strategies based on particle state rely on average values of temperature and velocity. In this study, a detailed examination of particle temperature distributions is presented. When systematically examined over a wide range of operating conditions of the resulting range of particle temperatures, a significant structure in the statistical distribution has been observed. A close inspection of the data indicates that this distribution can be interpreted as melting state indicator for YSZ. A characteristic peak at the melting point of ZrO₂ (error in absolute T-measurement is ≈ ±10%) can be used as an indicator for re-solidified particles. In the past, control strategies based on process diagnostic sensors have been based on average particle temperatures and velocities. Although the average values seem to be promising as control parameters, it has been shown through our results that different melting states could be demonstrated for the same average T and V settings. The melting state in turn has an important bearing on the coating structure and properties. It therefore implies that a process control strategy (to maintain coating quality) based on in flight particle sensors will have to take these findings into account. As an example, one strategy of process control would not only define the process in terms of the average particle temperature and velocity but also include the effect that parameter changes have on distributions.     

A Thermodynamic Analysis of Nonequilibrium Argon Plasma Torches

The nonequilibrium process of argon plasma torches is analyzed theoretically. Thermodynamic diagrams of different degrees of ionization are developed to aid in understanding and analyzing the transition from chemical equilibrium to frozen flow in dc plasma torch operations. A thermodynamic model is developed to describe the nonequilibrium processes in a dc argon plasma torch. In the model the ionization process is approximated as a constant-pressure heating process, with little deviation from the equilibrium state upon completion of heating. If the plasma flow is frozen shortly after heating, the entropy increase is small during the transition from equilibrium to frozen flow. In this case the frozen flow will have nearly the same composition and entropy as the flow at the heating section exit. For singly ionized argon plasmas in the entropy range relevant to dc torch operation, the frozen flow solutions on the affinity–pressure diagram are found to be insensitive to entropy change. Therefore the present model predicts that argon plasmas generated at different power levels will have almost identical affinity at the torch exit for the same operating pressure. This prediction agrees with experimental observations except for very low torch power levels.     

Improved thromboresistance and analytical performance of intravascular amperometric glucose sensors using optimized nitric oxide release coatings

In this work, nitric oxide (NO) release coatings designed for intravenous amperometric glucose sensors are optimized through the use of a polylactic acid (PLA) layer doped with a lipophilic diazeniumdiolated species that releases NO through a proton-driven mechanism. An Elast-Eon E2As polyurethane coating is used to both moderate NO release from the sensor surface and increase the sensor''s linear detection range toward glucose. These sensors were evaluated for thromboresistance and in vivo glucose performance through implantation in rabbit veins. By maintaining NO flux on a similar scale to endogenous endothelial cells, implanted glucose sensors exhibited reduced surface clot formation which enables more accurate quantitative glucose measurements continuously. An in vivo time trace of implanted venous sensors demonstrated glucose values that correlated well with the discrete measurements of blood samples on a benchtop point-of-care sensor-based instrument. The raw measured currents from the implanted glucose sensors over 7 h time periods were converted to glucose concentration through use of both a one-point in vivo calibration and a calibration curve obtained in vitro within a bovine serum solution. Control sensors, assembled without NO release functionality, exhibit distinctive surface clotting over the 7 h in vivo implantation period.     

A novel silica aerogel microspheres loaded with ammonium persulfate gel breaker for mid-deep reservoirs

Sol–gel based transparent functional coatings such as easy-to-clean coatings, anti-glare coatings, anti-reflective coatings, and anti-fingerprint coatings have been developed and commercialized for display applications. These coatings can be applied by a spray process on the glass substrates followed by a low temperature curing at 100° to 150?°C. Hydrophobic easy-to-clean coatings exhibit high water contact angles at ≥115°, with an excellent wear durability by passing 10,000 cycles using #0000 steel wool under 1?kg load. In addition, easy-to-clean coatings show a silky touch feel with a coefficient of friction of less than 0.03. Anti-glare coatings without particles have a gloss value between 50 to 100 gloss units, an adhesion of 5B, a pencil hardness ≥8?H, and no sparkling issues on touch screen displays. A combination of easy-to-clean coatings on top of anti-glare coatings significantly increases the stylus pen durability. Single-layer anti-reflective coatings show a transmittance increase of 1.5% to 2.0% by spray coating on a single side of the glass substrate, or a transmittance increase of more than 4.0% by dip-coating on a polycarbonate substrate. Anti-fingerprint coatings show less obvious fingerprints on the coated glass surface compared to those on uncoated glass surface. These coating solutions have been scaled up to 1000?kg batch in PPG’s Tianjin plant, China.

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Anti-glare coatings with irregular surface microstructures formed during a spray process have a gloss value between 50 to 100 gloss units, a pencil hardness ≥8H, and no sparkling issues on touch screen displays. Optical profiler image (with ×50.3 magnification) of the anti-glare coating surface (70 gloss units) shows irregular microstructure patterns which are random locally but uniform in large area. The light scattering on the anti-glare coating surface is caused by these irregular surface microstructures. Thus the specular reflection is significantly reduced, causing glare reduction, gloss decrease, and haze increase.

     

Parameters controlling the generation and properties of plasma sprayed zirconia coatings

D.C. plasma jets temperature and velocity distributions as well as the arc root fluctuations at the anode were studied for Ar-H₂ (25 vol%) plasma forming gases. The parameters were the arc current up to 700 A, the total gas flow rate up to 100 slm, and the nozzle diameter which was varied from 6 to 10 mm. The trajectories of partially stabilized zirconia particles into the jet were studied by a 2D laser imaging technique and two fast (100 ns) two color pyrometers. The results have revealed the difficulty to inject small particles into the plasma flow since most were found to by-pass the jet rather than penetrate it. The results also show the broad trajectory distribution within the jet and the influence of the arc root fluctuations on the mean particle trajectory distribution within the jet. Beside the measurements of the particle surface temperature and velocity distributions in flight, the particle flattening and the cooling of the resulting splats were studied statistically for single particles all over the spray cone. Such studies have emphasized the drastic influence of the substrates or previously deposited layers temperature on the contact between them and the splats. At 200–300°C this contact is excellent (cooling rates of the order of 100 K/μs for 1 μm thick splats) and it results in a columnar growth within the splats and the layered splats of a bead (up to 500 layered splats). This growth can be observed through passes provided the bead surface temperature has not cooled too much (a few tens of K) before the next bead covers it. A/C values up to 60 MPa were achieved with PSZ coatings. The effect of impact velocity of the particles, of substrate preheating temperature, of relative movments torch to substrate, of substrate oxidation on A/C values and splat formation were also studied.     

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.     

Multifunctional cold spray coatings for biological and biomedical applications: A review

A variety of coating techniques are available for medical devices to be tailored with surface properties aimed at optimizing their performance in biological environments. Cold spray, as a member of the thermal spray family, is now being exploited to efficiently deposit micro- to nanometer sized metallic or non-metallic particles on surgical implants, medical devices and surfaces in the healthcare environment to create functional coatings. Cold spray has attracted attention in the context of biomedical applications due to the fact that multiple materials can be combined easily at the surface of these devices, and that oxygen-sensitive and heat-sensitive organic molecules, including bioactive compounds, can be incorporated in these coatings due to the relatively low temperatures used in the process. The ability to maintain material and chemical properties and the ability to create functional coatings make the cold spray process particularly suitable for applications in the MedTech industry sector.This review explores the fabrication of cold spray coatings including the types of materials that have been used for biomedical purposes, provides a detailed analysis of the factors affecting cold spray coating performance, and gives an overview over the most recent developments related to the technology. Cold spray coatings that have been used until this point in time in biomedical applications can be broadly classified as biocompatible coatings, anti-infective coatings, anti-corrosive coatings, and wear-resistant coatings. In addition, this review discusses how these applications can be broadened, for example by providing antiviral effect against coronavirus (COVID-19). While we highlight examples for multifunctional cold spray coatings, we also explore the current challenges and opportunities for cold spray coatings in the biomedical field and predict likely future developments.     

Sensitivity of metal thiolate piezoquartz sensor coatings to hydrocarbons, methanol, and water

Mn(II), Fe(II), Co(II), Ni(II), Cu(II), Zn(II), Ag(I), and Pb(II) octadecylthiolates were used as chemical coatings for piezoquartz (PQR) sensors sensitive to hydrocarbon, alcohol, and water vapors. The sensitivity of these PQR sensors in a stationary flow mode at 25 °C varies from 0.5 to 5 Hz/(mg/L) depending on the composition of the coating and nature of the analyte. An array of eight PQR sensors with these coatings gives a sensor image of the corresponding analyte, which hardly varies with the analyte concentration. The sensitivity of the PQR sensors with the coatings studied changed by no more than 10–15% upon storage for several months. __________ Translated from Teoreticheskaya i éksperimental’naya Khimiya, Vol. 42, No. 6, pp. 371–376, November–December, 2006.     

Thermophysical Properties of H<Subscript>2</Subscript>O–Ar Plasmas at Temperatures 400–50,000 K and Pressure 0.1 MPa

The article presents the calculation of thermophysical properties of the mixture water steam–argon which has been used to further enhance the characteristics of plasma torches stabilized by the water wortex. The calculations were performed at the temperatures 400–50,000 K and at 0.1 MPa. First, the composition and thermodynamic properties are determined by classical methods. Further the calculations of viscosity, electrical conductivity and thermal conductivity of the mixture are computed in the 4th approximation of the Chapman–Enskog method. The computation of collision integrals is described with special respect to the interactions of charged particles where the necessary calculations for the Coulomb potential screened at the Debye length were enlarged to cover the 4th approximation. Then the formulae describing the method based on the variational principle of solving the system of Boltzmann integrodifferential equations are shortly introduced and the transport coefficients are presented.     

电感耦合等离子体原子光谱光源工作气体的现状与发展

电感耦合等离子体(ICP)光源氩气用量通常超过12L/min,是ICP光谱仪运行分析的最主要的消耗品,价格较贵。现介绍并评论多种低氩气耗量ICP光源,包括低气流炬管、水冷炬管、微型ICP炬管、双原子分子气体光源及混合气体光源等。讨论了节省氩气ICP光源技术的最新发展。     

Corrosion studies and interfacial bonding of urea/poly(dimethylsiloxane) sol/gel hydrophobic coatings on AA 2024 aluminum alloy

Bis[(ureapropyl)triethoxysilane] bis(propyl)-terminated-polydimethylsiloxane 1000 (PDMSU), an organic-inorganic hybrid, diluted in either EtOH or a mixture of EtOH-PrOH, was used in thin film form (<200 nm) to inhibit the corrosion of AA 2024 alloy. Potentiodynamic, time-dependent cyclovoltammetric measurements and salt spray tests showed that the corrosion inhibition of the latter was 10 times higher than that of the former films. This was correlated with the higher degree of hydrolysis and the formation of more open polyhedral silsesquioxane species (T2) in the bulk heat-treated PDMSU/EtOH-PrOH xerogels (29Si NMR spectra). The structure of the coatings deposited on AA 2024 Al alloy was deduced from the infrared reflection-absorption (IR RA) spectra, which revealed more extensive urea-urea interactions and more efficient silane-Al interface bonding for the PDMSU/EtOH-PrOH coatings with higher corrosion inhibition. Ex situ IR RA potentiodynamic spectroelectrochemical measurements of PDMSU coatings revealed that their degradation did not proceed via the formation of silanol groups and consequent hydration of the coatings but that they decomposed above E(corr) by forming fragments composed of -CH2- segments in an all-trans conformation.     

Study of the dynamic and static behavior of de vortex plasma torches: Part II: Well-tye cathode

This work was devoted to the study of the dynamic and static behavior of de vortex plasma torch with a well-type cathode (power level below 100 kW). The dynamic behavior of the torch was characterized by the fulctuations of arc voltage and current, plasma jet radiation, and acoustic pressure. Characteristic frequencies of the arc root movement inside the torch were observed. By numerical simulation (with the numerical codeMelodie, it was shown that the position of the erosion diameter) of the axial velocity along the cathode channel near the wall. The static behavior of the torch was inverstigated for different cathode designs. The variations of voltage U with arc current I, gas flow rate G nature of the gas and cathode design were represented by semiempirical relationships established between dimensionless numbers. By dimensional analysis, the behavior of this torch was compared with that of two powerful torches: the Aerospatiale and the Plasma Energy Corporation torches.