bluuebunny/crossref_metadata_2025 huggingface dataset:
Acoustic Emission as a Tool for Characterising HVOF-Sprayed WC-Co Coatings
Authors: Driver L.C. , Shipway P.H. , McCartney D.G. , Donohue L.A. , Rickerby D.S.
Published: September 2022

This research examined the use of 4-point bend testing, with in-situ acoustic emission analysis, for the characterization of the deformation of WC-Co coatings sprayed by the high-velocity oxy-fuel method. Coatings were deposited from WC- 17wt.%Co powders using a Praxair JP5000 system. Two sets of gun operating parameters were employed to produce coatings with distinctly different structures. In the present studies the cracking behaviour of the coatings was investigated through analysis of the acoustic emission (AE) data. In particular, AE data recorded has shown clear differences between the two coating types, in terms of the critical strain level and amount of energy released during cracking. The critical strain levels for the different coatings were 0.32% and 0.6% respectively. Analysis of the coatings by scanning electron microscopy and X-ray diffraction has allowed correlations to be made between the principle microstructural features of the coatings and the mechanical behaviour under bending. The AE responses for these cermet coatings were highly reproducible.
Property Evaluation of HVOF-Sprayed Magnetic Thick Films
Authors: Ma X. , Ge S. , Zhang T. , Zhang Y.
Published: September 2022

Spinel soft ferrites such as NiZnFe2O4 are of great interest due to their extensive applications in magnetic-optics, microelectronics, and microwave devices. This study deals with the production of thick Ni0.5Zn0.5Fe2O4 magnetic films (>50 microns) using high velocity oxy-fuel (“HVOF”) thermal spray. With the HVOF process, high density films more than 98% theoretical density were prepared, apparently due to the high velocity of droplets in the process. The influence of the fuel composition on coating structure, deposition efficiency, and magnetic properties of the Ni-Zn ferrite films was investigated. Better magnetic properties were obtained in the as-sprayed film formed with a neutral or oxygen-rich flame. The decomposition of the ferrite material occurred with a fuel-rich flame and resulted in a reduction in magnetic properties. However, the magnetic properties could be improved dramatically after heat treatment. Pellet and toroid samples of these films were produced for evaluation of magnetic properties by saturation magnetization and magnetic permeability. The permeability exhibited a very flat response to high frequencies up to 13 MHz.
Coating Formation by Impact of Molten Metal Droplets with Uniform Size and Velocity
Authors: Dhiman Rajeev , Chandra Sanjeev
Published: September 2022

The adhesion of splats formed by impact of molten metal droplets was studied experimentally. Tin droplets (550 µm diameter) were produced using a drop-on-demand generator. To achieve high impact velocities the stainless steel coupons used as substrates were mounted on the rim of a rotating flywheel and heated using cartridge heaters. To hit a falling droplet with the substrate and photograph its impact, a timing circuit was used to synchronize three events with the position of the substrate: ejection of a droplet, triggering of the camera and a flash to provide illumination. The impact velocity was varied from 10 – 40 m/s whereas the substrate average roughness (2.0 µm) and the droplet diameter (~550 µm) were kept constant. We measured the adhesion strength of splats by a simple pull test. A wire was attached to the upper surface of each splat using epoxy and the force required to separate the splat from the substrate was recorded. A significant increase in adhesion strength was observed as the impact velocity was increased. Coatings were produced by depositing many droplets sequentially. Substrate temperature and impact velocitiy were the main parameters varied. SEM images of cross-sections through coatings showed that increasing impact velocity and substrate temperature produced better adhesion between the coating and substrate.
Splat Profile of Impinging Droplets on Rough Substrates
Authors: Amada S. , Imakawa I. , Aoki S.
Published: September 2022

Thermal sprayed coatings are influenced by the wettabillity of the molten particles to substrates. The contact angles of several molten metal droplets are measured in air and vacuum. The flattening ratio is evaluated by taking into account of wettability. Comparing with the experimental data of the freefall droplet, it was concluded that the wettability to the roughened substrates influenced on the flattening characteristics.
Modeling Residual Stress Build-Up in the Coating Microstructure
Authors: Ghafouri-Azar R. , Chandra S. , Mostaghimi J.
Published: September 2022

We used an object oriented finite element numerical method to examine residual stress build-up in a simulated crosssection of coating. The cross-section image of coating was the result of simulation produced by a 3-D stochastic model of thermal spray coatings, which model features of coating microstructure including internal pores and surface roughness. An adaptive meshing technique was used to fit a grid in a section through the simulated coating, and stresses in the coating calculated using a finite element method. The results showed that stresses are sensitive to the thickness and roughness of the coating.
Model of Thermal Spraying Using a Monte-Carlo Approach
Authors: Zhang T. , Liu B. , Bao Y. , Gawne D.T.
Published: September 2022

The motion and heating of a single particle in a plasma jet is strongly influenced by the feedstock injection conditions and can be predicted using computational modelling as described in the literature. In practice, however, the size and initial velocity of a given particle are essentially random variables within the fairly wide limits of the injected feedstock population. This is compounded by the non-uniformity of the plasma jet. As a result, the motion and heating of particles take on a substantial random element. There are three independent variables that jointly affect the particle motion and heating in a given jet: the diameter, the radial coordinate and the azimuthal angle of a particle. Nevertheless, these parameters cannot be specified for every particle in the jet in a deterministic manner owing to the above complexities and so a simulation based on a single particle cannot provide a realistic prediction of the deposition process. In the present study, the random element present in practical spraying is simulated using a Monte-Carlo approach. The distributions of the motion and heating of a population of particles are simulated rather than those for a single particle. The statistical method presented in this paper gives more detailed information on the effects of processing parameters and the assessment of process quality. The results show that this is able to provide a more accurate means of simulating the thermal spraying process.
Plasma Spraying Flow Modeling: Generation of the Plasma Column and Turbulent Flow into a Dense Atmosphere
Authors: Guenadou D. , Meillot E.
Published: September 2022

This paper presents the numerical simulation of the plasma flow into a dense atmosphere. The plasma generation is performed by the simple model developed by Eichert. Two models are exposed to take in account the arc fluctuation inside the anode. They permit us to simulate plasma puffs convected into the flow. The aim of this study is to compare these two models with experiments and to determine which one is the most relevant.
On the use of SYSWELD and PHOENICS for the Computation of Heat Transfer in a Substrate Exposed to an Impinging Plasma Jet
Authors: Li J. , Bolot R. , Liao H. , Coddet C.
Published: September 2022

The study of heat transfers in a substrate exposed to an impinging plasma jet is proposed using two different software products. Thermal exchanges between the plasma jet and the substrate were first calculated using the PHOENICS CFD software in which a two-layer extension to the Chen-Kim k-s model was implemented in order to consider both the turbulent nature of the plasma jet and heat transfer phenomena through the viscous sub-layer formed at the surface of the substrate. The model is supposed to provide accurate predictions of thermal exchanges. However this preliminary step is not described since it is part of some previous studies. In a second step, two different commercial software products are used to perform three dimensional transient calculations of the heat conduction inside the substrate. The first approach consists in the use of the finite element based SYSWELD software whereas the second one consists in the use of the finite volume based PHOENICS software. Numerical results are presented and compared for the case of an impinging plasma jet displacing linearly on the substrate. Additionally, the influence of different parameters such as the substrate sample thickness, the stand-off distance, the displacement velocity or the nature of the substrate is also discussed. The results show a good accordance between numerical predictions obtained using the two methods concerning the maximum temperature observed. These results are useful since the substrate temperature is known to have an important influence on the coating adhesion and properties.
Turbulence Modeling of Inductively Coupled Plasma Flows
Authors: Xue S. , Proulx P. , Boulos M.I.
Published: September 2022

Various turbulent models (Spalart-Allmaras, Standard k-ε, RNG k-ε, Realizable k-ε and Reynolds Stress Models) along with Standard, and two-zonal wall functions are used to simulate inductively coupled plasma flows. The computational results can be classified into two categories: All the turbulent models that include low Reynolds number effects, such as, Low Reynolds number k-ε model, Spalart-Allmaras one-equation model, Standard k-ε model with two-zonal wall function, RNG model with turbulent viscosity determined by a differential equation, RSM etc., give similar modelling results. These models predict almost the same temperature contours which are similar to the one predicted by laminar model. The viscosity ratios in plasma region predicted by these models are very close to zero except for in the wall-neighbouring cells, which means the plasma flow is almost laminar. The other category contains those models that do not include the low Reynolds number effects, such as Standard, RNG and so-called Realizable k-ε models with standard wall function. They predict the plasma flow to be turbulence-dominated. In comparison with the results of experimentally measured heat fluxes to a substrate, the heat fluxes predicted by these models that include low Reynolds number effect are very close to experimental measurements while these models that do not include low Reynolds number effects deviate greatly from experimental measurements. It is found that the Reynolds stress model(RSM) appears to be the best predictive model.
Diagnostic of Supersonic High Frequency (HF) Plasma Flow
Authors: Léveillé V. , Boulos M.I. , Gravelle D.A.
Published: September 2022

In this study, two complementary techniques of diagnostic are used to study the properties of a supersonic HF plasma flow: namely, an enthalpy probe measurement and a flow visualization. A PL-35 induction plasma torch operating with three convergent-divergent Laval-type nozzles generates the plasma: a Mach 3.0 velocity water-cooled (wc) nozzle, a Mach 1.5 velocity wc nozzle and a Mach 2.45 velocity radiation-cooled (rc) nozzle. The plasma plate power is fixed at 20 kW and chamber pressure varies between 1 and 10 kPa. The plasma gas is argon and its flow is fixed at 60 slpm. The enthalpy probe profiles of local enthalpy and stagnation pressure are measured, from which, temperature, velocity and Mach number are obtained. The effect of nozzle design on plasma properties is investigated. The RC nozzle creates a plasma jet hotter with a steeper thermal profile and a higher mean velocity than the wc nozzle. The enthalpy probe calculations imply the assumption that the static pressure of the flow is similar to the chamber pressure. Experimental results show that this assumption is still applicable in the jet fringes, but its value changes strongly along the radial and axial axis. Also, photographs of the oblique shock wave in front of a cone in the plasma flow allow the approximation of the flow Mach number produced by a Mach 1.5 velocity wc nozzle. Its approximate value is 2.0, which is higher than predicted.
Modeling of Transport and Evaporation of Liquid Droplets Sprayed into RF-ICPs
Authors: Shan Y. , Mostaghimi J.
Published: September 2022

Study of the injection of liquid sprays into thermal plasmas has a number of important applications. Examples include Thermal Plasma Chemical Vapor Deposition (TPCVD), hazardous waste destruction and, ICP atomic emission or mass spectrometry in analytical chemistry. The spray is comprised of thousands of single liquid droplets. To calculate the mass, momentum and energy exchange between the gas and the spray, one must account for a distribution of droplet sizes, velocities, and temperatures. For some cases, droplet collision and breakup also are important. This paper developed a model for the transport and evaporation of liquid droplets sprayed into an radio frequency inductively coupled plasma (rf-ICP). The model considers the evolution of spray distribution function as the liquid spray travels through the discharge. Coupling calculations between plasma gas and water spray is performed for a typical rf-ICP torch. This model is capable of predicting the thermal and dynamic behavior of the liquid spray and its effects on the plasma gas for different rf-ICP operating conditions.
Substrate Melting During Thermal Spray Splat Quenching: Case Study for Molybdenum Droplets on Various Substrates
Authors: Li L. , Wang X.Y. , Vaidya A. , Sampath S. , Wei G. , Zhang H.
Published: September 2022

Molybdenum powder has been plasma sprayed on stainless steel, brass and aluminum substrates. The substrate melting phenomenon is observed and investigated by means of scanning electron microscopy (SEM) and scanning white light interferometery (SWLI). It is found that the flower-shape splat morphology is typical for molybdenum on all three substrate materials when the substrate is at room temperature. Notable substrate melting is manifested through the energy dispersion analysis of X-ray (EDAX) map and Robinson back-scattered image of cross-sections of splats. It has been shown that the substrate material plays an important role in substrate melting phenomenon. The lift angle of the petals of splats and the maximum crater depth have been characterized and compared. Both of these increase in the sequence, from stainless steel, brass to aluminum. A ‘volume of fluid’ (VOF) based model coupled with rapid solidification has been used to simulate splat deformation, solidification, substrate melting and resolidification. The numerical & analytical results agree quite well with the experimental data. A substrate melting mechanism is proposed based on the time scales of the droplet solidification and substrate melting to explain the formation of flower like splat morphologies.
Polymerlike C:H Thin Film Coating of Nanopowders in Capacitively Coupled RF Discharge
Authors: Kouprine A. , Gitzhofer F. , Boulos M. , Fridman A.
Published: September 2022

Nanopowders of amorphous SiO2, with typical particle sizes of 30-80 nm, were treated under non-equilibrium plasma conditions created by a capacitively coupled (CC) RF discharge in pure methane or ethane. The gas flow rate was varied between 0.02-0.06 slpm, with reactor pressures maintained between 1000 and 5000 Pa, and applied RF power inputs between 700 and 1500 W. The plasma properties were monitored through measurements of the C2 rotational and the atomic hydrogen excitation temperatures. The compositions of the gases that passed through the plasma were analyzed by mass-spectrometry. In spite of the evidence indicating the presence of CnH2n+2 and CnH2n (n=1-3) species, as well as acetylene, in the discharge, the homogeneous formation of soot was not observed. At the same time, introduced nanoparticles acted as centers for the inception and growth of C:H thin coatings in the form of polymer-like hydrocarbon layers, whose thickness lay between < 5 - 30 nm. The results of TEM, IR spectroscopy, thermo-gravimetric and precision calorimetric analyses performed on the plasma treated powders provided evidence for the formation of an amorphous, high density C:H matrix on particles' surfaces.
Correlations Between Processing Parameters, Coatings Properties and In-Flight Particle Characteristics
Authors: Guessasma S. , Montavon G. , Coddet C.
Published: September 2022

The reproducibility of the coating properties is the result of an efficient control of the thermal spray process. Actually, it is not possible for the sprayer to guarantee the temporal reproducibility of the coating properties without online control. However, adjusting the system fluctuations with regards to the temperature, velocity and size of particle could alter some properties that depend or not on the in-flight particle characteristics. This study aims at discussing the existing correlations between the processing parameters and the coating properties established with a Neural Network methodology. It demonstrates the segmentation of the correlations by comparing the result of the merging procedure of two sub-network structures and the direct correlation from the processing parameters to the coating properties. The sub-structures are built considering respectively the in-flight particle characteristics relationship with the operating conditions and the in-flight particle characteristics relationship with the coating properties.
Investigations of In-Flight Particle Characteristics Through DPV Measurements and Correlation with Impact Analysis and Coating Properties
Authors: Planche M.P. , Lakat A. , Liao H. , Coddet C.
Published: September 2022

While twin wire arc spraying, strong dependences are usually noticed between elaboration process parameters and coating properties. Therefore, control is needed for increasing coating performances. The aim of this paper is to investigate the coating properties in relation with in-flight particle characteristics and with the impact modes. Measurements were achieved on iron particles when changing either the current intensity or the air flow rates. In-flight particle characteristics were determined by using the DPV diagnostic system 200 mm from the gun exit and for three different radial locations. The same locations were kept for the impact analysis. Then, the morphology of the splats was evaluated by its flattening and shape factors when the substrate is preheated or not. Finally, the coating properties were characterized in terms of porosity, oxide contents, microhardness and thickness. Through the determination of related interactions, a better knowledge of coating elaboration conditions will lead to improve reliability and properties of the deposits.
Influences on the Kinematics of the APS-Process Investigated by Means of Particle Image Velocimetry
Authors: Bach Fr.-W. , Möhwald K. , Drößler B. , Prehm J. , Rothardt T. , Copitzky T.
Published: September 2022

Modern thermal spray processes require wide use of diagnostics to gather an extensive process understanding. Today's diagnostic results provide the basis for future designs and advancements, particularly and increasingly on basis of computational approaches. Due to the measuring area in the square centimeter range and its quick and accurate results, the Particle Image Velocimetry (PIV) represents an enriching for thermal spray process diagnostics. Our experimental results obtained from PIV are in accordance to present theoretical and empirical derivations of some kinematic parameters of thermal spray processes. Beneath verification of well-known dominant parameters (for example powder fraction or carrier gas mass flow), this procedure enables the detection and characterization of ancillary influences on the process due to its high accuracy. By statistical analysis of our experiments, using multiple parameter variations per experiment according to the technique of "Design of Experiments" (DoE), we possibly found some hints on interactions between ancillary parameters which shall be analysed in further works carefully. In combination with detailed simulations on plasma - particle interactions and powder injection it should be possible to develop methods for thermal spray processes to minimize the particle flow expansion for an optimization of deposition rate and energy efficiency in the future.
Diagnostics at Thermal Coating Processes – Research Results of the DFG-Founded Project
Authors: Bach Fr.-W. , Henne R. , Landes K. , Lugscheider E. , Copitzky T. , Prehm J. , Droessler B. , Ernst F. , Fischer A. , Seemann K. , Streibl T. , Zimmermann S. , Bouyer E.
Published: September 2022

The aim of the project group consisting of four research centers and founded by the DFG (German Research Society) is to characterize the plasma spraying process by means of diagnostic methods so that, based on the requirement profile of the coating, appropriate adjusting of the process parameters can be realized. For this purpose, different, partly newly-developed diagnostic tools, like Particle Shape Imaging, Laser Doppler Anemometry, Streak Technique, Particle Image Velocimetry, Enthalpy Probe, DPV 2000 and Thermography were qualified and adjusted to each other. The new results presented in this article are limited to the areas of particle injection and substrate which are difficult to handle with diagnostic methods.
On-line Optical Diagnostics of a Rotating Internal Diameter Plasma Spray Gun Used for Coating of Cylinder Bores in Automotive Industry
Authors: Hämäläinen E. , Kriikka N. , Barbezat G.
Published: September 2022

Coating of cylinder bores using plasma spraying is a growing application of thermal spraying in automotive industry. The Sulzer Metco RotaPlasma manipulator provides continuous rotation of the internal diameter plasma gun and enables a cost-efficient and reliable coating of the cylinder blocks. The development towards more advanced quality control systems and closed-loop spray control systems requires an on-line measurement of the most important particle and spray parameters such as particle temperature, velocity and flux. However, the small dimensions of the plasma plume of an internal diameter spray gun together with the rotation of the gun makes the measurement difficult, if not impossible, for many traditional diagnostics equipment. In this work, SprayWatch, an imaging CCD camera based diagnostics system from Oseir Ltd. was used for measuring particle and spray parameters during the rotation of an internal diameter plasma spray gun. The results show that the SprayWatch camera system is a promising tool for both development and production monitoring of cylinder bore coating process, as well as for realising a closed-loop spray control system.
Experimental Study of Substrate Thermal Conditions at APS and HVOF
Authors: Zagorski A. , Szuecs F. , Belashchenko V. , Ivanov A. , Siegmann S. , Margadant N.
Published: September 2022

Systematic measurements of heat fluxes into the substrate under the conditions of APS and HVOF have been performed with the use of a specially designed calorimeter. Results of measurements are presented in terms of the heat absorbed by a 150 mm diameter disk related to the electrical input power (EIP) and/or to the plasma/gas enthalpy. Depending on the process conditions heat flux into the substrate could reach as much as 20 per cent of the torch input power. Influences of gas composition, torch power and standoff distance on the heat transfer efficiency as well as the radial distributions of heat fluxes have been studied in detail and compared for three types of guns. Data obtained from the measurements of the radial distribution of heat fluxes in the plasma plume allowed estimating effective plasma jet diameter and the heat flux density, respectively. Rates of jet expansion differ significantly between the F4 and PlazJet plasma torches. Direct measurements of the substrate temperatures with the use of thermal paints have been performed and temperature patterns were compared with the results of 3D heat transfer modeling. Experimental values of heat fluxes were used in the calculations. Comparison showed that the experimental data on heat transfer efficiency could be used for accurate predictions of the substrate thermal conditions during thermal spraying.
DC Plasma Diagnostics for Improvement of Plasma Spraying Process under Soft Vacuum Conditions
Authors: Rat V. , Bouyer E. , Henne R.H. , Mayr W.
Published: September 2022

The vacuum plasma spraying process has to be optimized for each task in order to obtain the required mechanical and electrical properties for the desired coatings. This paper deals with the characterization of plasma and powder spray jets at deposition conditions for Solid Oxide Fuel Cells layers. First, DC plasma jets under soft vacuum conditions are characterized by using an enthalpy probe system and a Schlieren optic installation. The influence of the inner contour of the plasma spray torch anode on the temperature and velocity profiles as well on the shape of the plasma jets are investigated. Second, Laser Doppler Anemometry (LDA) measurements were performed for (8 mol %) yttria stabilized zirconia (YSZ) powder (-20+5 µm) spray jets for two chamber pressures, different argon carrier gas flow rates and injection modes. The results show that a M3 Laval nozzle and a F4V nozzle with conical inner profile allow to obtain a larger plasma volume and a more uniform plasma than with a standard F4 anode nozzle resulting in a better treatment of solid particles in the plasma. LDA measurements, using a M3 anode nozzle, show that the penetration and the acceleration of particles in the core of the plasma jet have their optimum at 10 kPa with an inclined injection with respect to the plasma jet axis for a 3.75 slpm carrier gas flow rate.
Examination of the Dry Ice Removal Process for Thermal Sprayed Coatings by Particle Image Velocimetry
Authors: Redeker C.F. , Bach Fr.-W. , Copitzky T.
Published: September 2022

The life time of some thermal sprayed coatings is limited. Flaking or partial removal of the coating reduces its protective effect. To allow further use of the component the coating has to be reworked before the part gets heavily damaged. The damaged coating must be removed with minimum impact on the surface of the component. Many removal methods as chemical, electrochemical or waterjet techniques have to be used with great care to prevent damaging of the substrate or are environmentally hazardous. An alternative technique which removes the coating selective with minimum impact on the substrate is the use of dry ice. Pellets of solid carbon dioxide are accelerated by means of compressed air. During the impact the carbon dioxide sublimates at a temperature of –78°C. The sublimation generates a large expansion (~600 times) of the carbon dioxide. The coating is strongly cooled while the shock wave of the sublimation destroys the cohesion. To support this process and to enlarge the thermal stresses in the coating a laser can be used. In this contribution the pellet velocity is measured and compared for three different dry ice jet devices by particle image velocimetry. Analysed parameters which are supposed to have an influence to the particle velocity are the mass flow of the compressed air, nozzle geometry and the working principles of the devices. The pellet velocity and the resulting kinetic energy are important process parameter. The results are used to optimise the laser assisted dry ice jet process.
Highly Porous Deposits of Cr2O3 Prepared by Plasma Co-spraying of Cr2O3 and Al
Authors: Neufuss K. , Kolman B. , Ctibor P. , Chráska P. , Laakso J. , Ahmaniemi S. , Vuoristo P. , Mäntylä T.
Published: September 2022

By modification of parameters in plasma spraying it is possible to alter porosity of the deposits only in a relatively small range. Industrial applications may require to adjust the porosity in wider range, to double or triple the original value in some cases. Such changes can be achieved only by special procedures. One of them is plasma spraying of ceramic/metal deposit followed by removal of the metal within the coating. The material removal is performed by dissolving, by leaching or by its extraction with an appropriate process. The paper describes preparation of very porous Cr2O3 coatings by this method from a composite Cr2O3/Al deposit. The initial porosity of approximately 13% (total porosity) was increased to 37% or 51% respectively by changing the initial volume of aluminum in the ceramic/metal plasma-sprayed composite. The discussion is complemented by observation of the resulting coating structure and detailed characterizations of the pore structure and porosity.
Interconnected Porosity of Plasma Sprayed Alumina Coatings: Evaluation and Modification
Authors: Haure T. , Denoirjean A. , Tristant P. , Desmaison J. , Fauchais P. , Maître A.
Published: September 2022

For electrical or thermal insulation, the porosity of an air plasma sprayed (APS) coating is an important property to control. Moreover in aggressive environment the interconnected porosity is responsible for the substrate corrosion. To solve, at least partially, this problem, deposition by mutitechniques (APS and a PECVD) was used to close interconnected and opened porosities. In this study, titanium alloy (TA6V) substrates were coated by alumina using either one or both deposition processes. Electrochemical characterization technique was used to evaluate the open porosity in alumina coatings. It consists of evaluating the polarization resistance of the reference sample surface (uncoated substrate) and to compare it to coated ones. After different tests for selecting the electrolyte solution, the influences of different parameters (thickness and relative position) of each deposition process on coating porosity were examined.
Evaluation of the Photodecomposition Efficiency of the Nanostructured TiO2 Sol Precursor Plasma Coatings
Authors: Choi Hanshin , Han Changmin , Lee Changhee , Kim Hyungjun
Published: September 2022

Two kind of the feedstock materials were used to produce the atmospheric plasma sprayed coatings. One is the spraying dried powder and the other is the nano-particle dispersed sol precursor. Coating properties were evaluated by the XRD, Raman spectroscopy, SEM and TEM. Also, the photodecomposition efficiency was examined by measuring the decomposition rate of the MB(methylene blue) in a water-cooled solution during the UV radiation and compared to the decomposition efficiency of the optimum coating produced using spraying dried particle. Both the anatase phase fraction and the photodecomposition rate of the coating using the sol precursor were higher than that of the optimum coating using the spraying dried powder.
Microstructure and Electrical Properties of RF and DC Plasma-Sprayed TiO2 Coatings
Authors: Branland N. , Meillot E. , Fauchais P. , Vardelle A. , Gitzhofer F. , Boulos M.
Published: September 2022

Relationships between the electrical properties of thermally sprayed titania coatings and their microstructure have been investigated. As far as possible, a broad range of microstructures was produced by using various processes of plasma spraying with different powder size ranges and variations of the plasma operating parameters. The two spraying processes consisted of DC plasma spraying and RF plasma spraying. Physical properties of plasma-sprayed coatings are generally influenced by their microstructure. But the electrical properties of plasma-sprayed titania coatings are known to be strongly influenced by their stoichiometry. It is the reason why coatings with identical stoichiometry were compared. It was found that electrical resistivity was directly linked to the quality of the contact between the splats and their density through the titania plasma-sprayed coatings.
Influence of Plasma Spray Parameters on Microstructural Characteristics of TiO2 Deposits
Authors: Berger - Keller N. , Bertrand G. , Coddet C. , Meunier C.
Published: September 2022

Titanium dioxide (TiO2) has emerged as an excellent photocatalyst material for environmental purification about two decades ago. In order to avoid the use of TiO2 powder, which requires later separation from the treated liquid, few researchers reported on ways of fixing TiO2. In these cases, many difficulties must be overcome such as maintaining the photoactive anatase phase as well as the mechanical stability and avoiding the mass transfer limitations. Thermal spraying seems to be an appropriate process to provide ceramic coatings exhibiting such characteristics. The present work assesses the influence of several spray parameters on the morphology of deposits, dominated by two void systems – interlamellar pores and interlamellar cracks – and on their crystalline structures. TiO2 was atmospherically plasma sprayed by systematically varying the process conditions including torch power, plasma gas mixture Ar/H2 and Ar/He, and cooling device. An analytical procedure based on XRD patterns was established for the determination of the relative amounts of anatase and rutile in the coatings. An Ar/He mixture as compared to an Ar/H2 one seems to enhance the formation of anatase from 38vol% to 44vol%. In both cases, a reduction of the plasma power increases the amount of anatase. Several methods of porosity determination were performed: image analysis, Archimedean porosimetry and calculation from phases, mass and thickness. Generally, the results indicate a total porosity varying between 15 and 25% depending on the spraying conditions.
Structure and Properties of Plasma Sprayed Wollastonite Coatings
Authors: Liu Xuanyong , Zeng Yi , Ding Chuanxian
Published: September 2022

Wollastonite coating deposited on Ti-6Al-4V substrate was prepared using atmospheric plasma spraying technology. Scan electron microscopy, transmission electron microscopy and X-ray diffraction were used to investigate the microstructure, surface morphology and phase compositions of coatings. Simulated body fluid tests were carried out to evaluate the bioactivity of the coating. Osteoblasts culturing test was used to examine the cytocompatibility of wollastonite coatings. The tensile bonding strength of the coating was measured with ASTM C-633 methods. The results obtained indicated that the plasma sprayed wollastonite coating had a rough surface and lamellar structure including some pores and microcracks. The primary crystalline phase of the coating was triclinic wollastonite. Glassy phase with nanocrystals was also found in the coating. The bonding strength of wollastonite coating reaches to about 39 MPa. This was explained in term of that the thermal expansion coefficient of wollastonite coatings is close to the Ti-6Al-4V substrate. The apatite layer was formed on the surface of plasma sprayed wollastonite coating soaked in SBF solution, which indicates wollastonite coating had a good bioactivity. Osteoblasts survive and proliferate on the surface of wollastonite coating, which indicates plasma sprayed wollastonite coatings possess excellent cytocompatibility.
Microcrystallographic Study of Plasma Sprayed Alumina Deposit by Transmission Electron Microscopy
Authors: Zahari Nur Ismarrubie , Sugano M. , Satake T. , Ohmori A.
Published: September 2022

To clarify the formation micromechanism of plasma spray deposits, internal microstructures have been examined by transmission electron microscopy on the thin films prepared from different depths of the plasma sprayed Al2O3 deposits. Microstructures in the top layer have consisted of both the coarse grains of 1 µm and the fine ones of 0.2 µm in average diameter. Point defects were contained mostly in γ phase grains. In the mid-layer, columnar structures often developed in parallel or in radial directions which adjoined with γ phase grains. From the SAD analyses, columnar grains were identified as α phase and lay in a twin crystal orientation relationship to each other between the joined grains. The bottom layer was composed of a mixture of the coarse grains of 0.2 µm and the fine ones of 0.02 µmin average grain diameter. It was found that the spray deposit was composed of different microstructures in crystallization, depending on depth to the thickness direction of the spray deposit.
The Use of Acoustic Emission Techniques for Characterizing Failure Mechanisms of Thermal Barrier Coatings Under Thermal Cycling Conditions
Authors: Robin Philippe C. , Gitzhofer François , Boulos Maher I.
Published: September 2022

Thermal barrier coatings are mainly used to protect underlying alloys from heat and chemical aggressions, especially in jet turbines and diesel engines. The challenge in TBCs’ applications is to use them to protect the upper moving turbine’s blades where their surface temperature can reach 1200°C. The limiting effect is the reliability of these coatings. The work described in this paper is a continuation of our earlier research work, which is focused on the use of Acoustic Emission Technique to assess the long-term behavior of thermal barrier coatings under thermal cycling conditions. Emphasis is placed in this presentation on the comparison of different signal processing techniques and the evaluation of their potential usefulness for the prediction of the coating behavior and failure modes. The work is carried out in parallel with a finite element modeling study of the thermal and stress distribution in the coating, which provides a valuable insight in the coating stress distribution prior to failure.
Analysis of Pore Connectivity Modification and Structure Densification of Y-PSZ Coatings After In Situ Laser Remelting Implementing Electrochemical Test
Authors: Antou G. , Hlawka F. , Cornet A. , Montavon G. , Coddet C. , Staerck J. , Fréneaux O.
Published: September 2022

The morphologies of Y-PSZ coatings remelted after (i.e., post-treated) or simultaneously (i.e., in situ) with their deposition were observed by optical and scanning electron microscopy in order to study the behavior of the coatings during laser irradiation. A change in the microstructure, from lamellar to dendritic, was observed in both cases. Moreover, cracks and delaminations are less emphasized for the coatings treated during deposition than for those treated after deposition. Finally, the pore connectivity was evaluated implementing an electrochemical test. Results clearly indicate that the coatings obtained by in situ laser remelting are significantly more impervious than as-sprayed coatings.
A Raman Study on Plasma-Sprayed Thermal Barrier Coatings During Thermal Cycling
Authors: Niccolai F. , Bardi U. , Muniz Miranda M. , Giannozzi M.
Published: September 2022

Yttria Partially Stabilized Zirconia (YPSZ) coatings are used as Thermal Barrier Coatings (TBCs) because of their capability to improve gas turbine performance by allowing higher turbine inlet temperature and reduced cooling air flow. Usually, YPSZ layers are applied by plasma spray on Ni-based superalloys pre-coated with a bond coat layer of MCrAlY, where M is Co, Ni, or both. In this work we used Raman spectroscopy to study the performance of TBCs during thermal cycling at two different temperature (1273K and 1373K). Raman scattering is capable of detecting monoclinic zirconia and to analyze residual stresses of the coatings with an higher sensitivity than traditional X-ray diffraction measurements. Raman spectra were analyzed by deconvolution methods in order to study the evolution of the relative intensities and position of different bands. We found that the position of the 635 cm-1 band shifts to higher values of wave number during thermal cycling and by comparing this data with those reported in literature, it is possible to retrieve the value of residual stresses in zirconia coating. We also observed a relation between the intensities of the bands at 602 cm-1 and 635 cm-1 band and the number of thermal cycles performed.
Functionally Graded HVOF Sprayed NiCr-Al2O3 Coatings for Demanding Applications
Authors: Turunen E. , Varis T. , Ruusuvuori K. , Gustafsson T.E. , Hannula S.-P. , Gasik M. , Antila E.
Published: September 2022

Functionally graded (FG) coatings were manufactured by High Velocity Oxy-Fuel (HVOF) thermal spraying and tested aiming for the high temperature applications. Single layers were manufactured and their elastic modulus measured by using Impulse Excitation Technique (IET). Obtained data was used for modeling of optimal gradient structure. Dual feeding hose for HVOF gun was developed. Calibration procedure for the concurrent use of two powder feeders was performed. NiCr-Al2O3 coatings with coating thickness of 600 µm and 1000 µm were manufactured and tested. Promising results were obtained from high temperature corrosion tests.
The Effect of a High Thermal Gradient on Sintering and Stiffening in the Top Coat of a Thermal Barrier Coating (TBC) System
Authors: Tsipas S.A. , Golosnoy I.O. , Clyne T.W.
Published: September 2022

Superalloy substrates coated with plasma sprayed CoNiCrAlY bond coats and yttria-stabilized zirconia top coats have been subjected to a high heat flux in a controlled atmosphere chamber. The sintering exhibited by the top coat under these conditions has been studied and compared with the behavior observed during isothermal heating, both when attached to the substrate and when detached. Sintering has been characterized by (a) microstructural examinations, (b) dilatometry, in both in-plane and through-thickness directions, and (c) stiffness measurements, using both cantilever bending and nanoindentation. A numerical heat flow model has been used to explore the stress state under isothermal and thermal gradient conditions. Sintering proceeds faster at higher temperature, but is retarded by the presence of tensile stresses (from differential thermal expansion between coating and substrate) within the top coat. Sintering occurs preferentially near the free surface of the top coat under gradient conditions, not only because of the higher temperature, but also because the in-plane stress is more compressive in that region.
The Influence of Thermal Barrier Top Coating on The Initiation and Growth of Thermally Grown Oxide
Authors: Ogawa K. , Gotoh N. , Shoji T.
Published: September 2022

Recently, thermal barrier coatings (TBCs) have been used in advanced gas turbine plants for improved performance. Usually, TBCs consist of an inner layer of metallic bond coating (MCrAlY) and an outer layer of ceramic top coating. According to several studies, the failure of the TBC is induced by thermal stresses due to the formation of thermally grown oxide (TGO) at the interface between the TBC and MCrAlY. Therefore, it is important to investigate the high temperature oxidation behavior of the interface. In this work, the TGO is characterized in detail. In particular, in order to clarify the role of the TBC top coating in regard to initiation and growth of TGO at the interface, a specimen with TBC and one without TBC were compared. In both specimens, the TGO had two different contrasting layers. One was alumina, and the other was a combination of chromium oxide, nickel oxide, cobalt oxide, and spinels (hereafter call mixed oxide). The TGO thickness of the specimen with TBC was thicker than that obtained without TBC. These specimens had different oxidation behaviors. It is thought that the reason for the difference in TGO thicknesses of both specimens is due to a difference in oxygen potential, as the oxide compositions in the mixed oxides were different. In case of the specimen with TBC, the mixed oxide consists of chromium oxide, nickel oxide, and cobalt oxide, separately. On the other hand, in case of the specimen without TBC, the mixed oxide consists mainly of spinels such as (Ni, Co)(Cr, Al)2O4.
Correlation Between Spraying Conditions and Micro Crack Density and Their Influence on Thermal Cycling Life of Thermal Barrier Coatings
Authors: Vaßen Robert , Traeger Franziska , Stöver Detlev
Published: September 2022

It is generally known that the porosity of thermal barrier coatings is essential to guarantee a sufficiently high strain tolerance of the coating during thermal cycling. However, much less is known about the influence of the specific kind of porosity such as micro cracks and typically larger pores on the performance of the coatings. Both features are usually formed during plasma spraying of yttria stabilized zirconia (YSZ) thermal barrier coatings (TBCs). In this investigation the influence of micro cracks on the thermal cycling behavior was studied. The amount of micro cracks within YSZ thermal barrier coatings was changed by changing the powder feed rate. However, only small changes of the total porosity were observed by changing powder feed rate. Mercury porosimetry served as a tool to investigate both the amount of micro cracks and pores in the coating. Additionally, micro crack densities were determined from metallographical investigations. A linear dependence between the amount of fine pores determined by Hg porosimetry and the crack density was obtained for one set of coatings. Thermal cycling specimens with thermal barrier coatings having different micro crack densities were produced and tested in a gas burner test facility. At high surface temperatures above 1300°C failure occurred in the ceramic close to the surface. Under these conditions the samples with increased horizontal micro crack densities showed a significant increase of thermal cycling life.
Behavior and Characterization of Two 7-8 wt% Yttria-Stabilized Zirconia Powders and Coatings Produced Using Plasma Spray Deposition
Authors: Bisson J.F. , Moreau C. , Dorfman M. , Dambra C. , Mallon J.
Published: September 2022

Two 7-8 wt% yttria-stabilized zirconia powders of similar size and chemistry but having different microstructure properties and manufacturing routes were studied. One significant difference was the density and internal porosity of the starting powders. Deposition efficiency (DE) of the low density (LD) powder was found to be higher and less sensitive to changes in the spray process parameters than the high density (HD) powder. Probing the in-flight particle characteristics with the DPV- 2000 made it possible to link the observed DE values with the in-flight particle temperature. For each powder, DE was found to depend mainly on a single variable, the in-flight particle temperature. DE was found to vary strongly with particle temperature for temperatures under 2700°C, whereas the dependence with particle temperature was much less important above 2700°C. Variations in DE seemed to evolve according to variations of the melted fraction of the sprayed material. Since the LD powder was found to achieve higher particle temperatures at given spray conditions, DE was found to be higher for the LD material and the range of variations in DE was found to be much less than that observed with the HD material. Examination of the coating microstructures revealed that a coating produced with the LD powder had slightly higher porosity than that produced with the HD powder at similar inflight parameters. Spraying at higher in-flight particle temperature or velocity, which resulted in higher and more robust DE values, tended to yield coatings with lower porosity, resulting in coating density exceeding the tolerance range specified by some end-users. Increasing the powder feed rate and using conditions that produced a higher in-flight particle temperature were found to increase the porosity up to an acceptable level without significantly degrading DE. Therefore, a solution was found that not only reduced the sensitivity of DE to changes in the spraying conditions but also increased the rate of production by reducing the time required to spray the part.