Flame Spray Technique Research Articles

Pool boiling of iso-butane and quasi azeotropic refrigerant mixture on coated surfaces

An experimental study was carried out to investigate saturated pool boiling of iso-butane and quasi-azeotropic mixture (R-600a and R-410A) for four horizontal copper surfaces fabricated by flame spraying technique. The copper powder was used as a coating material applied to the outer surface of copper tube. Boiling experiments were performed on 25.4mm diameter copper tubes with the active length of 116mm. Data were taken over a heat flux range of 5–50kWm−2 and saturation temperature of 10°C. The effects of heat flux and coating parameters (coating thickness, pore diameter and porosity) on nucleate boiling heat transfer coefficient of both refrigerants are thoroughly compared. The boiling heat transfer coefficient of copper coated surface was enhanced approximately 1.1–2 times that of plain surface. The experimental heat transfer coefficients were also compared with some existing correlations and the results of other authors published data. An empirical correlation was also developed to predict the heat transfer coefficient of both refrigerants during pool boiling over coated surfaces.

High-Temperature Decomposition of Fe- and Cr-Doped NiO Produced by a Novel Flame Spray Technique

A novel thick film technology has been developed for the manufacture of electric heating elements from partially oxidised Ni–Cr–Fe alloy powder. The sprayed films were a mixture of metallic alloys and Ni-based oxides. Electron microscopy and analysis were used to monitor the transformations in the films during subsequent heat treatment. In particular, it was found that homogeneous grains of Cr- and Fe-doped Ni-based oxides transformed into NiO grains containing numerous Ni-, Cr- and Fe-rich spinel particles with a typical size of 40 nm.

Testing Results of Plasma Spraying Ceramics Coatings by Pulse Plasma Modulation Technology

The work below was done mainly in the USSR (Tajikistan) and partly continued in Israel. This technology provides substantially better coating performance compared with the respective individual processes (plasma, Cold Spraying, arc or oxy-fuel, etc.). The coatings of large areas can be accomplished at much higher quality and deposition rates than conventional plasma, arc or flame spray techniques and at a far lower cost. These benefits have been accomplished through a specially designed torch head and power supply modulation unit. The system does not require expensive argon-helium based plasma of hydrogen, nitrogen or helium mixtures but rather uses air plasma alone or in combination with hydrocarbon fuel. The plasma chemical technology overcomes a previously fundamental limitation of air plasma systems - oxidation of the coating. In addition, the proprietary design of the torch head and supply modulation unit produces both superior coating and substantially reduces the erosion rate of expensive torch electrodes thereby significantly increasing the resource of operating time of a plasma generator. Detailed design specifications and process validation experiments have been of successfully completed, and the design, technology and market of a pre-production prototype has passed through an evaluation by potential strategic partners with the impressive estimation.

Thermal behavior and mechanical properties of Y2SiO5 environmental barrier coatings after isothermal heat treatment

Y2SiO5 coatings are deposited by a flame-spray technique as protection layer on SiC substrates to prevent oxidation and steam corrosion. In this research, Y2SiO5 coatings were isothermally heat treated at different temperatures and different exposure times in a laboratory environment. The thermal behaviors such as phase transformation, microstructural change and thermally grown oxide (TGO) formation have been examined by XRD, SEM, and EDS analysis. Different modes of TGO growth behavior were found at different temperatures. In addition, the mechanical properties were evaluated by a Martens hardness tester. The results show that the change of microstructure and composition is not too critical, but higher temperatures and longer heating times do lead to the formation of Y2SiO5 crystalline phases and a β-Y2O3 phase. Thus, the isothermal heat treatment improves the hardness and elastic modulus of Y2SiO5 coatings.

Experimental study of nucleate boiling heat transfer of R-134a and R-600a on thermal spray coating surfaces

This paper presents the results of experimental investigations carried out with refrigerants R-134a and R-600a during pool boiling on horizontal coated tubes of outside diameter 25.4 mm at saturation temperature of 10 °C. The coated tubes were prepared by powder flame spraying technique. This technique using copper as a coating material was applied to the outside of the copper tube with an effective length of 116 mm. The experimental data were taken in the range of heat flux from 5000 to 50000 Wm−2 with an interval of 5000 Wm−2. The effect of coating parameters on heat transfer coefficients and boiling characteristics of both refrigerants were presented. Enhancement of heat transfer coefficients due to coating were found for both boiling refrigerants. The boiling heat transfer coefficient of copper coated tube was enhanced approximately 0.99–2.1 times that of uncoated tubes. The experimental heat transfer coefficients were also compared with the existing correlations and the results of other authors published data. Further, thermal performance of copper coated tubes were correlated in terms of coating parameters (thickness, pore size and porosity) to examine the consistency of experimental data conducted on coated tubes of different characteristics.

Characterization of Ni-Al Solar Absorber Prepared by Flame Spray Technique

In this research, a Ni-Al solar absorber was successfully prepared by the flame spray technique with Ni-5 wt.%Al particles as a starting material. The Ni-5 wt.%Al particles were melted and sprayed onto the outer surface of a stainless steel 316L tube in order to form a Ni-Al composite coating. The phase, morphology and reflectance (R) spectrum of the Ni-Al solar absorber were characterized by X-ray Diffraction (XRD), a Scanning electron microscope (SEM) equipped with an energy dispersive X-ray (EDX) analyzer and an Ultraviolet-visible-near infrared spectrophotometer at the wavelength 300-2500 nm. The results revealed that the surface of the Ni-Al solar absorber was rough, and its cross section was overlapped layer by layer. The Ni-Al solar absorber was composed of Nickel (Ni) and aluminum (Al) phases. NiO and Al2O3 phases were also found on the surface. The chemical composition of the Ni-Al solar absorber was Ni (72.94 wt.%), Al (11.76 wt.%) and O (15.29 wt.%). The solar absorptance (α)of this solar absorber was 0.77. This demonstrates that a Ni-Al composite coating can be applied as a solar absorber material for solar collector at high operating temperatures.

Efecto de la temperatura del sustrato sobre la resistencia al desgaste de recubrimientos de Nitec depositados utilizando la técnica de proyección térmica con llama

En este trabajo se presentan los resultados obtenidos en el depósito de recubrimientos a base de níquel sobre sustratos de hierro, con preparación de superficie mediante granallado por arena, utilizando la técnica de proyección térmica por llama. Básicamente, se estudió la influencia que tiene la temperatura de depósito del sustrato sobre las propiedades estructurales, morfológicas y en la resistencia al desgaste. Los recubrimientos fueron caracterizados estructuralmente mediante difracción de rayos X (DRX); la morfología de la superficie se caracterizó mediante microscopía electrónica de barrido (MEB) y microscopia laser confocal (MLC). La composición química se determinó mediante fluorescencia de rayos X (FRX) y la respuesta al desgaste se evaluó mediante pruebas de desgaste abrasivo a tres cuerpos, bola sobre disco y rayado. Los resultados de DRX permitieron establecer que los recubrimientos son policristalinos y tienen una estructura cristalográfica cubica centrada en el cuerpo (BCC) para todas las temperaturas de depósito; el análisis de MEB determino que las superficies de las películas disminuyen su porosidad a medida que la temperatura de depósito aumenta y las pruebas de desgaste mostraron que los recubrimientos son más resistentes a ésta prueba que el sustrato.

Hydrophobisation of wood surfaces by combining liquid flame spray (LFS) and plasma treatment: dynamic wetting properties

Abstract The hydrophilic nature of wood surfaces is a major cause for water uptake and subsequent biological degradation and dimensional changes. In the present paper, a thin transparent superhydrophobic layer on pine veneer surfaces has been created for controlling surface wettability and water repellency. This effect was achieved by means of the liquid flame spray (LFS) technique, in the course of which the nanoparticulate titanium dioxide (TiO2) was brought to the surface, followed by plasma polymerisation. Plasma polymerised perfluorohexane (PFH) or hexamethyldisiloxane (HMDSO) were then deposited onto the LFS-treated wood surfaces. The same treatment systems were applied to silicon wafers so as to have well-defined reference surfaces. The dynamic wettability was studied by the multicycle Wilhelmy plate (mWP) method, resulting in advancing and receding contact angles as well as sorption behavior of the samples during repeated wetting cycles in water. Atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS) were employed to characterise the topography and surface chemical compositions and to elucidate the question how the morphology of the nanoparticles and plasma affect the wetting behavior. A multi-scale roughness (micro-nano roughness) was found and this enhanced the forced wetting durability via a superhydrophobic effect on the surface, which was stable even after repeated wetting cycles. The hydrophobic effect of this approach was higher compared to that of plasma modified surfaces with their micro-scale modification.

Photocatalytic Iron Oxide Coatings Produced by Thermal Spraying Process

Recently, hematite coatings with semiconductor properties have received attention for photocatalytic applications. In this study, plasma and flame spraying techniques were used for hematite deposition on 316 stainless steel plates. X-ray diffraction was used for phase composition analysis, and methylene blue was used as an organic pollutant to evaluate the photocatalytic activity of thermally sprayed coatings. The results showed that all these coatings could act under visible-light irradiation but the one deposited by flame spraying at 20 cm stand-off distance showed the highest photocatalytic activity. The results showed that wavelength of the light source and pH of the solution affected the photocatalytic activity significantly. It was also shown that thermally sprayed iron oxide coatings could have a high photo-absorption ability, which could positively affect the photocatalytic activity.

A Comprehensive Review on Fluid Dynamics and Transport of Suspension/Liquid Droplets and Particles in High-Velocity Oxygen-Fuel (HVOF) Thermal Spray

In thermal spraying processes, molten, semi-molten, or solid particles, which are sufficiently fast in a stream of gas, are deposited on a substrate. These particles can plastically deform while impacting on the substrate, which results in the formation of well-adhered and dense coatings. Clearly, particles in flight conditions, such as velocity, trajectory, temperature, and melting state, have enormous influence on the coating properties and should be well understood to control and improve the coating quality. The focus of this study is on the high velocity oxygen fuel (HVOF) spraying and high velocity suspension flame spraying (HVSFS) techniques, which are widely used in academia and industry to generate different types of coatings. Extensive numerical and experimental studies were carried out and are still in progress to estimate the particle in-flight behavior in thermal spray processes. In this review paper, the fundamental phenomena involved in the mentioned thermal spray techniques, such as shock diamonds, combustion, primary atomization, secondary atomization, etc., are discussed comprehensively. In addition, the basic aspects and emerging trends in simulation of thermal spray processes are reviewed. The numerical approaches such as Eulerian-Lagrangian and volume of fluid along with their advantages and disadvantages are explained in detail. Furthermore, this article provides a detailed review on simulation studies published to date.

Microstructural and Tribological Properties of Al2O3-13pctTiO2 Thermal Spray Coatings Deposited by Flame Spraying

THe present investigation has been conducted to study the tribological properties of Al2O3-13pctTiO2 (AT-13) ceramic coatings deposited on a low carbon steel type E335 by using a thermal flame spray technique. The microstructure and phase composition of wire and coatings were analyzed by scanning electron microscope, energy dispersive spectroscopy (EDS), and X-ray diffraction (XRD). Measurements of micro hardness were also performed on the surface of the coatings. The tribological tests were carried out using a pin-on-disk tribometer at different loads. All tests were performed using two disks as counter body, namely Al2O3-ZrO2 (AZ-25) and Al2O3-TiO2 (AT-3) which formed couple 1 and couple 2, respectively, in order to work out the wear rate and friction coefficient. Roughness profiles were also evaluated before and after each test. The SEM showed that the dense microstructure of Al2O3-TiO2 (AT-13) coatings have a homogenous lamellar morphology and complex of several phases with the presence of porosities and unmelted particles. The XRD analysis of the wire before the spray showed a majority phase of α-Al2O3 rhombohedral structure and a secondary phase of Al2TiO5 orthorhombic structure with little traces of TiO2 (rutile) tetragonal structure, whereas the XRD of the coating revealed the disappearance of TiO2 replaced by the formation of a new metastable phase γ-Al2O3 cubic structure. The tribological results showed that the applied contact pressure affects the variation of the friction coefficient with time and that it decreases with the rise of the normal force of contact. It was found also that the couple 2 with nearly chemical compositions of spray-coated (AT-13) and disk (AT-3) exhibited much higher wear resistance than the couple 1 although they have sliding coefficient of friction nearly.

High-Temperature Oxidation and Hot Corrosion Studies on NiCrAlY Coatings Deposited by Flame-Spray Technique

The NiCrAlY coatings deposited by flame-spray technique on the superalloy substrate were oxidized in the presence of air and Na2SO4 + V2O5 salt at 900 °C for 100 cycles. The kinetics of oxidation showed that the coatings deposited by flame-spray technique possess better oxidation resistance compared with coatings deposited by high-velocity oxy fuel (HVOF)-sprayed technique. The oxidized coatings were further characterized by XRD, FESEM/EDS, and x-ray mapping techniques. The mechanisms of the oxidation and hot corrosion were substantiated by analyzing the results obtained from the various characterization techniques.

Hypereutectoid steel coatings obtained by thermal flame spraying — Effect of annealing on microstructure, tribological properties and adhesion energy

Metallic coatings were processed on a 35CrMo4 substrate using a thermal flame spray technique. The deposited metal was a high carbon steel, low alloyed with Cr, Mn and Si. In those coatings FeO and Fe3O4 oxides were found, as well as stable α-ferrite phase and some γ-austenite phase. γ-Austenite phase is metastable and transforms into stable α-ferrite during annealing treatment at 300°C. This γ to α-phase transformation, as well as the evolution of the oxides during the annealing post-treatment, has some influence on the mechanical properties of the coating: hardness, tribological properties, Young's modulus and adhesion on the 35CrMo4 substrate.

Paper-based microfluidics: fabrication technique and dynamics of capillary-driven surface flow.

Paper-based devices provide an alternative technology for simple, low-cost, portable, and disposable diagnostic tools for many applications, including clinical diagnosis, food quality control, and environmental monitoring. In this study we report a two-step fabrication process for creating two-dimensional microfluidic channels to move liquids on a hydrophobized paper surface. A highly hydrophobic surface was created on paper by TiO2 nanoparticle coating using a high-speed, roll-to-roll liquid flame spray technique. The hydrophilic pattern was then generated by UV irradiation through a photomask utilizing the photocatalytic property of TiO2. The flow dynamics of five model liquids with differing surface tensions 48-72 mN·m(-1) and viscosities 1-15 mN·m(-2) was studied. The results show that the liquid front (l) in a channel advances in time (t) according to the power law l=Zt0.5 (Z is an empirical constant which depend on the liquid properties and channel dimensions). The flow dynamics of the liquids with low viscosity show a dependence on the channel width and the droplet volume, while the flow of liquids with high viscosity is mainly controlled by the viscous forces.

Comparison of the Mechanical and Electrochemical Properties of WC-25Co Coatings Obtained by High Velocity Oxy-Fuel and Cold Gas Spraying

Cold gas spray (CGS) coatings were previously produced by spraying WC-25Co cermet powders onto Al7075-T6 and low-carbon steel substrates. Unlike conventional flame spray techniques (e.g., high-velocity oxy-fuel; HVOF), no melting of the powder occurs; the particles are deformed and bond together after being sprayed by a supersonic jet of compressed gas, thereby building up several layers and forming a coating. WC-Co cermets are used in wear-resistant parts, because of their combination of mechanical, physical, and chemical properties. XRD tests were previously run on the initial powder and the coatings to determine possible phase changes during spraying. The bonding strength of the coatings was measured by adhesion tests. Here, WC-25Co coatings were also deposited on the same substrates by HVOF spraying. The wear resistance and fracture toughness of the coatings obtained previously by CGS and the HVOF coatings obtained here were studied. Their corrosion resistance was determined by electrochemical measurements. It was possible to achieve thick, dense, and hard CGS coatings on Al7075-T6 and low-carbon steel substrates, with better or the same mechanical and electrochemical properties as those of the HVOF coatings; making the former a highly competitive method for producing WC-25Co coatings.

Thermally sprayed finestructured WC-12Co coatings finished by ball burnishing and grinding as an innovative approach to protect forming tools against wear

The forming of high-strength steel sheets offers novel possibilities to produce lightweight structural parts with a high stiffness for the aircraft and automotive industries. However, the employment of such sheet materials leads to intense wear and thus reduces the lifetime of forming tools. At the same time, the requirements concerning their performance, their geometrical complexity, and their shape accuracy have been significantly increased. To counteract this problem, the tools are either treated by different thermo-chemical processes (e.g. hardening, nitriding) or are coated using thin film techniques such as PVD or CVD.In this study, thermal spraying is presented as a cost efficient and more flexible approach to protect the surface of forming tools against wear. For this purpose, planar samples as well as cylindrical deep drawing dies were coated by means of the high velocity oxy-fuel (HVOF) flame spraying technique, utilizing a fine WC-12Co powder (agglomerate size 2–10μm) with a carbide size of 400nm. Prior to the coating operation, a comprehensive parameter optimization was performed based on the statistical design of experiments (DOE) to achieve coatings with improved mechanical and tribological properties. The planar samples were used to ascertain the sliding and rolling wear behavior within two standardized test methods (Ball-on-Disc and Taber Abraser tests). The coated dies were smoothened by ball burnishing as well as grinding and afterwards evaluated within the deep drawing of high strength (HC380LA) steel sheets. In addition, the results were compared to those achieved with an uncoated conventional cold work steel die, which is commonly employed for this operation. In contrast to the cold work steel, both coated dies, the ball burnished as well as the ground die, showed a significantly better wear performance after the forming of 10,000 parts.

Deposition of Metal Oxide Films from Metal–EDTA Complexes by Flame Spray Technique

R2O3 (R = Y, Eu, Er) metal oxides were synthesized from metal–ethylenediaminetetraacetic acid (EDTA) complexes using a flame spray technique. As this technique enables high deposition rates, films with thickness of several tens of micrometers were obtained. Films of yttria, europia, and erbia phase were synthesized on stainless-steel substrates with reaction assistance by H2–O2 combustion gas. The oxide films consisted of the desired crystalline phase with micropores. The porosity of the films was in the range of 6–15%, varying with the metal used. These results suggest that the true density of the metal oxide obtained from metal–EDTA powder through the thermal reaction process plays an important role in achieving film with the desired porosity.

Effect of Agglomerate Powders Particle Size on Preparation of Hollow Multiphase Ceramic Microspheres by Self-Reactive Quenching Method

Adopting self-reaction quenching technology based on flame spraying technique, self-propagating high-temperature synthesis and quick chilling technique, hollow multiphase ceramic microspheres are prepared. With Al+Cr2O3+Fe+Fe2O3+MnO2 as reactive system, agglomerate powders are obtained and then distinguished as thick powders and fine powders according to the different particle size. The relationship between agglomerate powders initial particle size and hollow ceramic microspheres is analyzed by the means of SEM, EDS, XRD and particle-size analyzer. The result shows that average particle diameter of thick powder is 24.49μm and that of fine powder is 17.15μm. Particle diameters of thick powder quenching product distribute within the range of 20~50μm, and those of fine powder distribute within the range of 5~30μm. The thick powder quenching product phase composition consists of Mn2AlO4,Fe2O3,Fe3O4,FeCr2O4,Cr2O3,Al2O3The fine powder quenching product phase composition consists of Mn2AlO4,Fe2O3,FeCr2O4,Fe3O4.

Luminescent rare-earth ions doped Al2O3–Y2O3–SiO2 glass microspheres prepared by flame synthesis

A flame-spraying technique was applied for preparation of undoped and 1mol% Nd- and Er-doped Al2O3–Y2O3–SiO2 (YAS) glasses with 5, 10, 15, and 20mol% of SiO2. The glasses were prepared in the form of microspheres by melting precursor powders synthesized by the Pechini method in CH4–O2 flame, and by quenching micro-droplets of the melt by spraying them with deionized water. Prepared transparent glass microspheres were characterized by optical microscopy, SEM, XRD, DTA, UV–vis–NIR, and fluorescence spectroscopy. The undoped microspheres were amorphous, as confirmed by the X-ray powder diffraction. Addition of silica has negligible effect on glass transition temperature, and the temperature of crystallization, due to high structural similarity of all prepared glasses. UV–vis–NIR/fluorescence spectra showed typical absorption/emission/excitation bands due to the presence of optically active Nd3+ and Er3+ ions in the host glass. The Er3+ doped glasses exhibited the strongest excitation at 378nm, which corresponds to 4I15/2→4G11/2 transition of the Er3+ ion. They exhibited strong green emission at 548nm (4S3/2→4I15/2) and a weak green emission at 525nm (2H11/2→4I15/2). The intensities of emission bands decreased with increasing SiO2 content. The Nd3+ doped glasses exhibit three major emission bands at 905, 1067 and 1340nm, corresponding to the 4F3/2→4I9/2, 4F3/2→4I11/2 and 4F3/2→4I13/2 transitions, respectively, of the Nd3+ activator. The emission intensities were not influenced by the content of SiO2. The wavelength of emitted light was not influenced by the structure of glass.

Sliding and Rolling Wear Behavior of HVOF-Sprayed Coatings Derived from Conventional, Fine and Nanostructured WC-12Co Powders

Fine structured and nanostructured materials represent a promising class of feedstock for future applications, which has also attracted increasing interest in the thermal spray technology. Within the field of wear protection, the application of fine structured or nanostructured WC-Co powders in the High Velocity Oxy-Fuel flame spraying technique (HVOF) provides novel possibilities for the manufacturing of cermet coatings with improved mechanical and tribological characteristics. In this study the tribological behavior of HVOF sprayed coatings derived from conventional, fine and nanostructured WC-12Co powders under sliding and rolling wear are investigated and the results are compared to C45 steel (Mat.-No. 1.0503). In addition, sliding and rolling wear effects on a microscopic level are scrutinized. It has been shown that under optimized spray conditions the corresponding fine and nanostructured WC-12Co coatings are able to obtain higher wear resistances and lower friction coefficients than the conventional coatings. This can be attributed to several scaling effects of the microstructure and to the phase evolution of the coating, which are discussed.