bluuebunny/crossref_metadata_2025 huggingface dataset:
Effect of WC Addition in Ni-Cr Matrix on Performance as Coatings for Corrosion/Erosion Protection of Heat Exchanger Materials
Authors: Kumari S. , Khanna A.S.
Published: September 2022

Fireside corrosion and erosion of heat exchanger tubes is a serious problem. One of the methods to combat this is by applying corrosion and erosion resistant coatings. Nickel– chromium alloys have already been used as coatings to deal with oxidation environments at high temperature. The wear resistance of these coatings can be improved by adding different hard precipitates such as carbides of refractory metals and cemented carbides. In the present study, various compositions comprising of Ni-Cr with 15, 35, 60 and 100% wt% WC/Co were made using thermal spray grade powders. These were then coated on steel substrate by the HVOF method. After detailed characterization of the coatings, the performance of the coatings at high temperature was studied by exposing the coated samples in the temperature range of 600-700°C. Ni-alloy coatings with moderately (15%) added WC/Co showed better oxidation resistance than coatings with high percentage of WC/Co and also than the pure Ni-alloy coating in high temperature range of 600-800°C. From the characterization, it was found that Ni-Cr alloy with 15% WC/Co gave the optimum results.
Manufacturing of Corrosion Resistant Coatings Using Laser Assisted Thermal Spraying
Authors: Suutala J. , Tuominen J. , Vuoristo P.
Published: September 2022

In the present study, surface engineering related to simultaneously thermally sprayed and laser melted corrosion resistant coatings has been carried out. A 6 kW high power diode laser or a Nd:YAG laser and a flame spray gun were used to construct the laser assisted thermal spraying system. The main aims of the study were to find out the optimal processing parameters and materials to create dense corrosion resistant coatings on steels. The corrosion resistance of the manufactured coatings was measured and the microstructure characterized using optical and scanning electron microscopy.
Corrosion Behaviour of Ni-5Al/WC-12Co HVOF Coating Systems
Authors: Lekatou A. , Gavrilakis N. , Grimanelis D.
Published: September 2022

WC-12Co coatings (with and without Ni-5Al bond coats) as well as Ni-5Al coatings were deposited on Al 7075 by HVOF spraying. The corrosion behaviour of the coatings was investigated by salt spray testing for 40 days, followed by potentiodynamic tests in a 3.5% aqueous NaCl solution, at 25, 35 and 45°C.The thicker duplex coatings (500µm) were not susceptible to pit corrosion, at any temperature. A small fraction of the thinner duplex coatings (260µm) underwent localized corrosion, only at 45°C. Ni-5Al and a small fraction of WC-12Co single coatings showed low susceptibility to pit corrosion which increased with temperature increasing and thickness decreasing. The coating system Ni-5Al/WC-12Co exhibited the highest resistance to general corrosion, which was independent of the thickness at the two thicknesses tested.
Hot Corrosion Behaviour of HVOF Sprayed Coatings on ASTM SA213-T11 Steel
Authors: Sidhu H.S. , Sidhu B.S. , Prakash S.
Published: September 2022

Cr3C2-NiCr, NiCr, WC-Co and Stellite-6 alloy coatings were sprayed on ASTM SA213-T11 steel using the HVOF process. Liquid petroleum gas (LPG) was used as the fuel gas. Hot corrosion studies were conducted on the uncoated as well as HVOF sprayed specimens after exposure to molten salt at 9000C under cyclic conditions. The thermo-gravimetric technique was used to establish the kinetics of corrosion. XRD, SEM/EDAX and EPMA techniques were used to analyse the corrosion products. All these overlay coatings showed a better resistance to hot corrosion as compared to that of uncoated steel. NiCr Coating was found to be most protective followed by the Cr3C2-NiCr coating. WC-Co coating was least effective to protect the substrate steel. It is concluded that the formation of Cr2O3, NiO, NiCr2O4, and CoO in the coatings may contribute to the development of a better hot corrosion resistance. The uncoated steel suffered corrosion in the form of intense spalling and peeling of the scale, which may be due to the formation of unprotective Fe2O3 oxide scale.
Cracking and Spalling Behavior of WC-17% Co Cermet Coatings
Authors: Legoux J.-G. , Bouaricha S. , Sauer J.P.
Published: September 2022

Thermal spray WC based powders are now frequently used as chrome replacement alternatives for a wide range of industrial and aeronautical applications. In numerous cases, the carbide materials outperform the hard chrome in many property evaluations. However, their usage on highly stressed parts, especially in fatigue loading, can be limited by spalling resistance of the coating. While HVOF is being used on many flight critical parts, stringent applications like the landing gear components of carrier-based aircraft are still under investigation. This work, on WC-17%Co, relates coating bend test performance and fatigue/cyclic step loading behavior to the processing history using different HVOF systems. Initially, twelve (12) different coatings were monitored using a DPV- 2000 for temperature/velocity profiles. The mechanical properties were then assessed using an instrumented four-point bend test as well as uniaxial cyclic loading. After mechanical testing, the coating microstructures were characterized using X-Ray diffraction and electron microscopy in order to investigate the phase content and nature. In particular, the cracks generated during the bend test were measured using SEM on sample cross sections to understand characteristics such as spacing and crack penetration to the substrate. The interactions of processing parameters with the cracking/spalling resistance of the various coating deposits will be discussed and a potential criteria for the control of cracking phenomena will be presented.
Fatigue Fracture Mechanism of HVAF Sprayed Low Alloy Steel
Authors: Liu M. , Deng C. , Zhou K. , Song J. , Wu C.
Published: September 2022

The influence of the high velocity air fuel (HVAF) sprayed coating on the fatigue behavior of the low alloy steel was studied at different stress levels. It was observed that only one single main crack initiator existed in the substrate after fatigue at low stress levels, but there were multi-cracks on the substrate surface at high stress level. Detailed investigations showed that the cracks in the HVAF coatings sinuously extended to the interface and deflected thereat along the interface. Consequently, free-standing coating was formed due to its limited bond strength to the substrate and the lower elastic modulus than that of the substrate. The gap between the free-standing coating and the substrate surface was found to be correlated with the stress level. The high stress can greatly degrade the adhesion to the substrate causing the delamination of the coating. The cracks in the HVAF coating had no significant effect on the fatigue life of the substrate.
Processing and Properties of Titanium Coating Produced by Warm Spraying
Authors: Wu T. , Kuroda S. , Kawakita J. , Katanoda H. , Reed R.
Published: September 2022

Thermal spraying of dense titanium coatings in the air atmosphere was achieved by using a two-stage high velocity oxy-fuel process (HVOF) called the Warm Spray Process. In the process nitrogen gas is mixed with the combustion gas to lower the gas temperature. Gas dynamics modeling of the flow field of the gas in the spray apparatus as well as the acceleration and heating of titanium powder injected from the powder feed ports were conducted. Based on the obtained temperature history of a titanium powder particle, its oxidation during flight was also predicted by using a Wagner-type oxidation model. These results were compared with measured velocity and temperature of sprayed particles by DPV2000 and the properties of deposited coatings. Significant discrepancy in the temperature of sprayed particles was found between the calculation and measurement whereas the measured velocity was closer to the model calculation. The model prediction of oxygen content was in a good agreement with the analysis of actual coatings. Furthermore, properties of the sprayed coatings such as porosity, oxygen content were correlated with the particle velocities and temperatures. Nitrogen gas was highly effective in lowering the oxygen content, but excessive nitrogen addition caused the coating porosity to increase due to insufficient particle temperatures.
Analysis of Potential Improvements of HVOF Based Processes
Authors: Voronetski A. , Belashchenko V.
Published: September 2022

The paper presents an analysis of possible improvements in HVOF torches and associated processes. It is shown that increasing the efficiency of combustion modules, widening the available range of operating pressures as well as optimizing the powder injection and nozzle expansion lead to significant improvements in comparison with the currently utilized HVOF processes using liquid fuel and radial powder injection. The analysis is confirmed by experimental data obtained using an advanced HVOF torch designed to use ethyl alcohol as a fuel. The new torch operates at a higher thermal efficiency of a combustion module, able to generate products of combustion with better capability to transfer heat to particles and more dissociated molecules. High thermal efficiency also results in a wider operating window and better control of a desirable balance between particles’ kinetic and thermal energy, which depends on the spraying material and a particular coating’s requirements. Shock wave generators inside the nozzle/barrel are also included in the design to allow an improvement in powder injection and an increase in the heat exchange between products of combustion and particles. These advantages have allowed for the minimization of the throat diameter providing throat cross-sectional surface area approximately 2 times less in comparison with other liquid fuel HVOF torches presently used in the industry. The reduction in throat diameter results in significantly less consumption of fuel and oxygen, and also results in an overall decrease of the operating costs. Coating quality, deposition efficiency and deposition rate in this case are the same or better in comparison with other currently utilized liquid fuel HVOF processes.
Coating Properties of HVOF Sprayed Carbide-based Ultrafine Powders
Authors: Matthäus G. , Bobzin K. , Lugscheider E. , Zwick J.
Published: September 2022

HVOF spraying of carbides is a well established method to produce high performance wear and corrosion protection coatings for various industrial applications. Using ultrafine feedstock powders with particle sizes well below 25 µm can offer many advantages with respect to the resulting coating properties, such as improved coating density and homogeneity and high dimensional accuracy. In this study, fine powder grades of WC 10Co 4Cr, WC 17Co and Cr3C2 25(Ni 20Cr) in a particle size range of –15 +5 µm, -10 +3 µm and –5 µm have been sprayed with suitable HVOF systems on outer surfaces as well as on internal diameters. Resulting coatings have been analysed with respect to coating structure and microhardness. The coatings featured a favourable microstructure and very good hardness values. Furthermore, the Cr3C2 25(Ni 20Cr) material was applied as wear protection coating on piston rings. The coating performance was evaluated in engine tests and proved to be significantly better than a conventionally graded standard material system for this application.
The Effects of Ceria Addition to HVOF Thermal Sprayed NiAl Intermetallic Coatings
Authors: Wang Y. , Liu G. , Tian W.
Published: September 2022

In this paper, the effects of ceria addition to HVOF thermal sprayed NiAl intermetallic coatings were investigated through micro-indentation, thermal shock testing, and microstructural analysis techniques including SEM with EDX, and XRD analysis. It has been found that the addition of ceria to NiAl coatings reduces the tendency of brittle peeling during thermal spraying. This reduction in peeling is presumably due to the improved wetting of the substrate by the molten coating material, which ultimately leads to better coating adhesion. The addition of ceria also resulted in higher coating hardness and elastic modulus as determined by micro-indentation. The coatings containing ceria also exhibited significant increases in thermal shock resistance when compared with that of the pure NiAl coatings. The possible mechanisms responsible for the improvement of the above mentioned properties upon the addition of ceria are also addressed in this paper.
HVAF Sprayed Composite Coating with Ni-base Self-Fluxing Alloy and WC on a Copper Substrate
Authors: Wu C.Q. , Zhou K.S. , Deng C.G. , Deng C.M.
Published: September 2022

In order to improve the wear resistance and service life of the copper, the composite coating consisting of a Ni-base self-fluxing alloy (NiCrWB+50%Al2O3) and WC (WC-12%Co) alloy were sprayed on a copper substrate using High Velocity Air Fuel(HVAF). The coating could meet the operating requirements including high hardness, good wear resistance and low cost. The Ni-base composite coating was analyzed by means of optical microscopy, scanning electron microscopy (SEM), electron probe microanalysis (EPMA) and X-ray diffraction (XRD). The results indicated that the structure of coating was composed of melted particles and partly unmelted round particles of Ni-base alloy, and WC particle. Only a small proportion of the Al2O3 particles were retained in the coating. The phases in the coating consisted of γ-Ni, WC and a little Ni3B. Amorphous structures appeared and some Al2O3 phase existed. The adhesion strength between coating and copper substrate was more than 50MPa. Wear results showed that the Ni-base composite coating exhibited better wear resistance than the coating with no WC particles.
Study of Dynamic Diffusion Bonding of 90MnCrV8 High Strength Steel Coated with WC-Co Obtained by HVOF Thermal Spray
Authors: Gómez de Salazar J.M. , Barrena M.I. , Matesanz L. , Guilemany J.M. , Dosta S. , Miguel J.R.
Published: September 2022

The results for joints obtained by dynamic diffusion bonding of a 90MnCrV8 high strength steel coated with WC-Co are shown in the present work. This high strength steel substrate was coated with WC-Co, sprayed by HVOF technique (Diamond Jet Hybrid DJH-2700) using propylene as fuel gas at different conditions. The dynamic diffusion bonding was carried out in a high frequency furnace, all joints were made in air. Before doing the joints, the steel was coated with Ni and Cu by electrochemical processes in order to obtain a soft 20 m interlayer of Ni30Cu alloy. Microstructure and reacted zones in the joints were investigated by means of Scanning Electron Microscopy (SEM) and Dispersive X Ray Spectrometry (EDX). In all joints different reacted zones can be distinguished, caused by diffusion processes which take place during the joint tests. The mechanical properties of the joints were quantified in a tensile machine, using a constant load of 0.1 MPa·s-1. All joints broke by the WC-Co coating zone by delamination processes. The fracture surface was studied by SEM-EDX in order to know the fracture mechanism of the joints. The maximum tensile strength obtained confirm a very promising technology for industrial applications.
A New High Velocity Oxy-Fuel Process for Making Finely Structured and Highly Bonded Alloy Layers from Liquid Precursors
Authors: Ma X.Q. , Roth J. , Gandy D.W. , Frederick G.J.
Published: September 2022

High velocity oxygen-fuel (HVOF) thermal spray processes are used in applications requiring the highest density and adhesion strength, which is not achievable in most other thermal spray processes. Like other thermal spray processes, however, a normal HVOF process is not able to apply fine powders less than 10 µm via a conventional powder feeder. The advantages of using smaller and even nano-sized particles in a HVOF process include uniform coating, less defective microstructure, higher cohesion and adhesion, full density, lower internal stress and higher deposition efficiency. A new process has been developed to realize HVOF forming of fine-grained alloy layers by using liquid precursors containing fine metallic particles. Process investigations have shown the benefits for making single and duplex layered coatings with full density and high bond strength attributing to the very high kinetic energy of particles striking on the substrate surface and the better melting of the small particles. One of the targeted applications is for the water walls of a fossil-fired boiler that operate in a high temperature and corrosive environment. The new coating system is based on material selection, structure design, process innovation and diagnostics, microstructure, and property evaluation. It is promising to provide better protection of the boilers against various types of degradations like corrosion, oxidation, erosion and interfacial failure.
Suspension Plasma Spraying of Nanostructured WC-12 Co Coatings
Authors: Oberste Berghaus J. , Marple B. , Moreau C.
Published: September 2022

Nanostructured WC-12%Co coatings were deposited by suspension plasma spraying of submicron feedstock powders, using an internal injection plasma torch. The liquid carrier employed in this approach allows for controlled injection of much finer particles than in conventional thermal spraying, leading to thin coatings with a fine surface finish. A PEI (polyethylene-imine) dispersant was used to stabilize the colloidal suspension in an ethanol carrier. In-flight particle states were measured for a number of operating conditions of varying plasma gas flow rates, feed rates and standoff distances, and related to the resulting microstructure, phase composition (EDS, SEM, XRD) and Vickers hardness. High in-flight particle velocities, in excess of 800 m/sec, were generated, leading to dense coatings. It was observed that the coating quality was generally compromised by the high temperature and reactivity of the small particles. To compensate for this shortcoming, the suspension feed rate was adjusted, thereby varying the thermal load on the plasma. Results showed that a slightly larger agglomerate size, in conjunction with low particle jet temperatures, could somewhat limit the decomposition of WC into brittle W2C/W3C and amorphous cobalt containing binder phases.
Microstructure and Electronic Emission of Plasma Sprayed TiO2 Functional Coating with Gradient of Crystal Grain Size
Authors: Tomaszek R. , Pawlowski L. , Gengembre L. , Laureyns J. , Znamirowski Z. , Zdanowski J.
Published: September 2022

The paper aims at the development of coating having a gradient of crystal grain size. Thick, inner layer was plasma sprayed using coarse TiO2 powder. This layer has the thickness ranging from 30 to 50 µm. Thin, outer layer of thickness smaller than 10 µm, was plasma sprayed using different aqueous suspensions of fine powders of TiO2. The morphology of coarse and fine powders was characterized using scanning electron microscope (SEM) X-ray diffraction (XRD). Electronic emission was tested using home made setup. X-ray diffraction enabled to find out an interesting result which is formation of a mixture of rutile and anatase in suspension sprayed coatings. This was also confirmed by Raman spectroscopy investigations. The technology of suspensions plasma spraying was optimized to obtain homogeneous and dense deposits. The sizes were in the range of tenth to one hundred nanometers in initial powders and get clearly smaller in the coatings sprayed using coarse powder but remained quite similar in suspension sprayed films. X-ray photoelectric spectroscopy (XPS) was used to analyze quantitatively TiO2 powders and coatings. Electronic emission was correlated with phase composition of the coating and their grain size.
Atmospheric Plasma Spraying of High-Energy Ball-Milled Al2O3-TiO2 Powders
Authors: Coste S. , Bertrand G. , Coddet C. , Gaffet E. , Hahn H. , Sieger H. , Meunier C. , Vives S.
Published: September 2022

Al2O3-TiO2 materials are very attractive materials for the manufacturing of wear resistant ceramics coatings. Moreover, nanostructured coatings show superior properties (higher hardness and enhanced fracture toughness) as compared to their counterparts containing microscale grains. The aim of this study is to produce nanostructured Al2O3-TiO2 coatings through the plasma spraying technique that can lead to metastable transformations. The starting powders containing 13wt% and 44wt% of titania, have been mechanically activated by high energy ball milling with a planetary mill, (Model P4 by Fritsch, Germany) using steel vials and balls in such conditions to obtained the high pressure TiO2 II phase. The milled powders were then plasma sprayed with an Ar:H2 gas mixture, by varying the plasma parameters (gas flow rates, nozzle diameter, aggregates size). The effect of the milling and the spraying conditions on the crystalline phases and the microhardness of the coatings are evaluated.
Processing Strategies for Tailoring Ceramic-Based Nanostructured Thermal Spray Coatings
Authors: Marple B.R. , Lima R.S.
Published: September 2022

New and more demanding applications and higher performance requirements are creating the need for a greater degree of sophistication in engineering coating structures. The use of nanostructured feedstocks provides the possibility of tailoring the structure of thermal spray ceramic-based coatings at the nanoscale. In the present study, it has been found that such an approach can produce coatings with enhanced mechanical, thermal and bioperformance characteristics. It has been shown that the internal structure and external size of agglomerates as well as the spray conditions employed for deposition play a key role in determining the nature and extent of zones of nanostructured material produced in coatings. The characteristics (such as porosity and bonding) of these zones can have an important effect on the coating performance. This approach has been used to tailor Al2O3-TiO2, ZrO2-Y2O3, WC-Co, TiO2, and hydroxyapatite coatings targeted for use as abradables, for TBCs, against wear and on orthopedic implants.
Liquid Plasma Sprayed Coatings of Yttria Stabilized Zirconia for SOFC Electrolytes
Authors: Rampon R. , Bertrand G. , Toma F.-L. , Coddet C.
Published: September 2022

In order to achieve SOFC with reduced costs, the atmospheric plasma spray process (APS) could be an attractive technique. To obtain dense layer as electrolyte, the route has been adapted by providing a suspension as feedstock material. Suspensions of yttria stabilized zirconia powders in methanol have been prepared with various solid loadings and states of dispersion. An external injection system was used to ensure the atomisation and radial injection of the suspension into the Ar-H2 plasma under atmospheric conditions. The coatings were characterized by SEM, Archimedes porosity and differences in the microstructure of the deposits were observed. The aim of this work has thus been to study the influence of various parameters (of the suspensions, plasma, torch, injection) on the layer morphology. For this purpose, the atomization has been investigated and efforts have been made to understand relationships between suspension properties, atomization and microstructure.
Proven and Promising Applications of Thermal Sprayed Nanostructured Coatings
Authors: Kim G.E.
Published: September 2022

Since its inception, the Office of Naval Research (ONR) program entitled, “Thermal Spray Processing of Nanostructured Coatings”, has produced numerous successes relating to military and industrial applications. Commercially available thermal spray processes have been used to apply ceramic coatings from nanostructured powders, producing dramatically improved wear performance over coatings from conventional non-nanostructured powder. The nanostructured alumina-titania coating has been instrumental in extending the life of a critical Navy need, the protection of the main propulsion shaft of mine countermeasures ships. The advancement in nanostructured oxide coatings has led to the development of the first industrial application of a thermal sprayed nanostructured coating. A nanostructured titanium oxide coating has been developed and successfully incorporated into severe-service ball valves used in hydrometallurgical extraction of gold, nickel, and cobalt. This paper will also present promising results from current development work relating to nanostructured coatings for thermal barrier, biomaterial, and reconditioning applications.
Fabrication of Nanostructured TiO2 Fibers on TiO2 Coatings Produced from a Nanostructured Feedstock
Authors: Lima R.S. , Marple B.R. , Lee H. , Akbar S.A.
Published: September 2022

Titania (TiO2) coatings were produced using the high velocity oxy-fuel (HVOF) technique on Ti-6Al-4V substrates. The titania feedstock powder exhibited nanostructured morphology, formed by the agglomeration of individual nanostructured titania particles (spray-drying) smaller than 100 nm. The resulting coatings were dense (porosity <1%) and exhibited rutile and anatase as phases with percentages of ~75% and ~25%, respectively. These coatings were heat-treated in a H2/N2 environment at 700oC for 8 h. During the heat-treatment, nanostructured titania fibers were formed on specific surface regions of the coatings. The nanofibers formed by this “chemical or reaction-based texturing” exhibited diameters of 50-400 nm and lengths in the order to 1-5 µm. It is thought that engineering these surfaces at nano and microscales may lead to interesting applications of titania coatings related to cell attachment/growth (for biomedical applications).
Effects of Powder Injection on Deposit Efficiency in Plasma Spraying
Authors: Abukawa S. , Takabatake T. , Tani K.
Published: September 2022

In plasma spraying, generally, powders are radially injected into plasma jet. However, percentage of particles which have enough enthalpy to melt is not high in radial injection, possibly leading to low deposit efficiency especially for high melting point powders. In this research, powder port for radial injection was attached to an axial injection plasma torch in order to compare the plasma flame in both cases of axial and radial powder injections. The powder trajectory and the shape of the plasma plume were observed, and particle flux density, temperature, velocity, and deposit efficiency of alumina powders were measured and the results were compared between the two powder injection methods.
Instabilities in Plasma Spray Jets
Authors: Outcalt D. , Hallberg M. , Yang G. , Heberlein J. , Pfender E. , Strykowski P.
Published: September 2022

Instabilities in plasma spray jets can result in coatings with inconsistent properties. The arc root fluctuation and shear layer instability due to strong gradients are of foremost concern. The shear layer instabilities result from shear between the high velocity, low density hot core gas, the intermediate density and velocity boundary layer, and the high density quiescent environment. A cold-flow facility with density gradients similar to a plasma torch has been used for implementation of traditional fluid dynamics measurements such as hot-wire anemometry. Methods to control these instabilities are developed and tested using both the plasma torch and the cold flow facility. Through nozzle design modifications the instabilities resulting from arc root fluctuations and high density gradients have been reduced. The effectiveness of the control on the plasma jet is determined using in-flight particle characterization along with high speed imaging and photodiode measurements of the jet.
Effect of Powder Gas on Low Pressure Plasma Sprayed Yttria Coatings
Authors: Young S. , Hall A.C. , Huskisson D. , Hirschfeld D.A. , Muehlberger E. , Barbash D.
Published: September 2022

The morphology and chemistry of low pressure plasma sprayed (LPPS) yttria coatings was investigated. White yttria powder used as the coating source material was 99.99% pure and contained particles from 1 to 5 microns in size. This yttria powder was plasma sprayed onto 8.9 cm diameter, 0.635 cm thick aluminum substrates using Ar-He as the arc gas. The powder gas was varied from Ar, to air to O2 between samples. The sample sprayed using Ar as the powder gas used a chamber pressure of 2.5 kPa and resulted in a very dark colored coating. Subsequent samples were sprayed using air and then oxygen as the powder gas, both using a chamber pressure of 4.0 kPa. The resulting coating colors were medium gray and very light gray, respectively. A duplicate spraying using oxygen as the powder gas was performed to demonstrate repeatability. Porosity, splat shape, and the presence of amorphous phases were determined using secondary and backscattered electron microscopy imaging and related to processing conditions. Energy dispersive spectrometry was used to examine the chemistry of the coating and substrate. X-ray photoelectron spectroscopy (XPS) was used to determine the oxide stoichiometry of each of the coatings and this was correlated to coating color.
Vacuum Plasma Spraying of Thick W Coatings onto Surface Sculptures for Fusion Armour Applications
Authors: Thomas G. , Grant P.S. , Coad P. , Matthews G. , Castro R.
Published: September 2022

Vacuum plasma sprayed (VPS) tungsten (W) coatings hold great promise for plasma facing components in future fusion devices. However, the large coefficient of thermal expansion (CTE) mismatch between W and underlying structural steels poses a significant problem for manufacturing and service life because of the evolution of large thermally induced stresses leading to failure. In this paper both the concept of functionally graded material (FGM) W/steel interlayers and the use of steel substrate surfaces with regular surface sculptures of millimetre scale created by e-beam surface manipulation, termed surfi-sculpt and developed by TWI of the UK are investigated. The objective of these approaches is to enhance coating adhesion and to engineer macroscopic variations in the effective CTE through the thickness of the subsequently VPS deposited W coating. The effects of surface geometry on coating adhesion and microstructure have been investigated, and preliminary conclusions on the key surface sculpture geometrical features required for high adhesion dense W coatings have been identified.
Mechanical Properties of Plasma Sprayed Titania Coatings
Authors: Ctibor P. , Neufuss K. , Chraska P.
Published: September 2022

Agglomerated titania nanopowder and a “classical” titania were sprayed by the high throughput WSP and thoroughly compared. Optical microscopy with image analysis as well as mercury intrusion porosimetry were utilized for quantification of porosity. Results indicate that the “nano” coatings in general exhibit finer pores than coatings of the “conventional” micron-sized powders. Mechanical properties like Vickers microhardness and slurry abrasion response were measured and linked to the structural investigation. Impact of the variation in the slurry composition on wear resistance of tested coatings and on character of the wear damage is discussed. The over-all results however suggest that the “nano” coatings properties are better only for carefully selected sets of spraying parameters, which seem to have a very important impact.
Improving Plasma Sprayed YSZ Coatings for SOFC Electrolytes
Authors: Syed A.A. , Ilhan Z. , Arnold J. , Schiller G. , Weckmann H.
Published: September 2022

Using a D-optimal design of experiments (DOE), influence of feedstock powder and plasma gases were examined on deposition efficiency, gas tightness and electrochemical behavior of vacuum plasma sprayed YSZ for SOFC electrolytes. In-flight particle temperature and velocity, measured by on-line particle diagnostics, were correlated with plasma and deposit properties. Electrochemical testing of cells was performed to determine the influence of gas tightness and microstructure of electrolyte deposit on cell behavior.
Improvement of Plasma Spraying Conditions for SOFC Applications: Effect of Nozzle Geometry and Gas Shrouds
Authors: Kavka T. , Henne R. , Arnold J.
Published: September 2022

Thermal plasma jets interact intensively with the surrounding atmosphere. This interaction leads to strong changes of the plasma jet properties affecting the resulting products. Modification of the nozzle parameters and conditions at the exit of the torch helps to vary and better control the process of plasma jet and ambient air interaction. In the present study, the DC arc plasma torch was equipped with a modified anode nozzle (M2.5) and a surrounding shroud at the jet exit. The process of air entrainment was investigated when shroud gas was supplied producing a protecting envelope to reduce the air entrainment. Schlieren photography and the enthalpy probe with the mass spectrometer were applied to study the influence on plasma jet behavior. The effect of shroud nozzle geometry as well as the effect of the shroud gas flow rate was investigated. Likewise, influence of gas shrouding on the resulting coatings was studied.
High Velocity DC-VPS for Diffusion and Protecting Barrier Layers in SOFCs
Authors: Henne R.H. , Franco T. , Ruckdäschel R.
Published: September 2022

High temperature fuel cells of SOFC type as direct converter of chemical into electrical energy show a high potential for reducing considerably the specific energy consumption in different application fields. Of particular interest are advanced light-weight planar cells for electricity supply systems in cars and other mobile systems. Such cells, in one current design, consist mainly of metallic parts, e.g. of ferrite steels. These cells shall operate in the temperature range of 700 to 800 °C where oxidation and diffusion processes can be of detrimental effect on cell performance for long-term operation. Problems arise in particular by diffusion of Cr-species from the inter-connect or the casing into the electrolyte/cathode interface forming insulating phases and the mutual diffusion of substrate and anode material, e.g. iron and chromium from the ferrite into the anode and nickel from the anode into the ferrite which in both cases reduces performance and system lifetime. Protecting intermediate layers, can reduce such effects considerably if they are dense, stable and of high electronic conductivity. Perovskite-type layers (e.g. doped LaCrO3) applied with high-velocity Direct Current-Vacuum Plasma Spraying (DCVPS) promise to solve reliably such problems.
Effect of Copper Ion Doping on Photocatalytic Performance of Liquid Flame Sprayed TiO2 Coatings
Authors: Yang G.-J. , Li C.-J. , Li C.-X. , Wang Y.-Y. , Huang X.-C.
Published: September 2022

Copper ion was added in liquid feedstock to deposit ion doping TiO2 photocatalytic coatings through liquid flame spraying. The coating microstructure was characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). The photocatalytic performance of coatings was examined by photodegradation of acetaldehyde. XRD analysis shows that the crystalline structure of coatings is not significantly influenced by the metal ion doping. The photocatalytic activity of the TiO2 coatings is enhanced by the copper ion doping. It is found that a high concentration of ion doping decreases the activity. XPS analysis shows that the adsorbed oxygen concentration is increased with the increase of Cu2+ dopant concentration and decreases with the further increase of dopant concentration. The enhancement of photocatalytic activity can be attributed to the adsorption ability of oxygen and other reactants on the surface of doping TiO2 coatings.
Spraying of Agglomerated TiO2 Nanopowder by Water-Stabilized Plasma
Authors: Ctibor P. , Neufuss K. , Dubsky J. , Kolman B. , Rohan P. , Chraska P.
Published: September 2022

TiO2 nanopowder was used as a feedstock for spraying with the water stabilized plasma (WSP®) in search for superior mechanical properties and wear resistance of titania coatings. It has been proved that good quality coatings can be made even with the high throughput WSP®. Single splats evaluation and the free flight particles were used for the spray optimization. Phase compositions, stoichiometry and selected properties, such as density and elastic modulus, were then studied at the as-sprayed coatings. As for the phase composition of coatings, mainly rutile with possible traces of Magneli phases have been found. A comparison between tension and compression loading shows that values of the Elastic modulus for compression are slightly higher than these for tension, as it is usual in plasma sprayed coatings.
Influence of Feedstock Powder in Reactive Plasma Spraying for Fabrication of Aluminum Nitride Coatings
Authors: Yamada M. , Lee C.C. , Kouzaki Y. , Yasui T. , Fukumoto M. , Takahashi K.
Published: September 2022

Aluminum nitride (AlN) is one of the attractive ceramics with respect to its excellent mechanical and electrical properties. In this study, AlN coatings were fabricated and the influence of feedstock powders was investigated by reactive RF (Radio Frequency) plasma spraying. Two different particle sizes of commercial aluminum (Al) powders and Al/AlN mixed powders were used as the feedstock powder. The feedstock powder was injected into a RF plasma, and sprayed particles were deposited onto carbon steel or quartz substrates. As a result, it was possible to fabricate thick and dense AlN coating using smaller particle size of Al powders and quartz substrate. However, many agglomerates were formed in the coatings. On the other hand, 50 wt% or above of AlN addition in the feedstock powders was effective to prevent the formation of the agglomerates. Therefore, Al/AlN mixed powder with smaller particle size was useful for fabrication of AlN coatings by reactive RF plasma spraying.
Plasma Spray Synthesis from Precursors: Progress, Issues and Considerations
Authors: Ravi B.G. , Sampath S. , Gambino R. , Devi P.S. , Parise J.B.
Published: September 2022

Precursor plasma spray synthesis is an innovative and rapid method to make functional oxide ceramic coatings by starting from solution precursors and directly producing inorganic films. This emerging method, utilizes molecularly mixed precursor liquids, which essentially avoids the handling and selection of powders, opening up new avenues for developing compositionally complex functional oxide coatings. Precursor plasma spray also offers excellent opportunities in exploring the non-equilibrium phase evolution during plasma spraying of multi-component oxides from inorganic precursors. Although there have been efforts in this area since the 1980s and early 1990s with the goal of synthesizing nanoparticles, only recently has the work progressed in the area of functional systems. At the Center for Thermal Spray Research an integrated investigative strategy has been conducted to explore the benefits and limits of this synthesis strategy. Water and alcohol based sol/solution precursors derived from various chemical synthesis methods were used as feedstocks to deposit thin/thick films of spherical and nanostructured coatings of yttrium aluminum garnet (YAG), yttrium iron garnet (YIG), lanthanum strontium manganite (LSM) and Zr-substituted yttrium titanates, compositions of Y2O3-Al2O3 and their microstructural space centered around stochiometric YAG. A detailed discussion of the salient features of RF induction plasma spraying (RFPPS) approach, results obtained in the investigations to develop various functional oxide coatings and process issues and challenges are presented.
Examination of Substrate Surface Melting-Induced Splashing During Splat Formation in Plasma Spraying
Authors: Li C.-J. , Li C.-X. , Yang G.-J. , Wang Y.-Y.
Published: September 2022

Impacting of a molten droplet with melting point much higher than substrate results in melting of substrate around the impact area. The melting of the substrate surface to certain depth alters the flow direction of droplet fluid. The significant change of fluid flow direction leads to detaching of fluid from contact with the substrate. Consequently, splashing occurs during droplet spreading process. In the present study, Mo splats were formed on stainless steel substrate under different plasma spraying conditions. For comparison, Mo splats were also deposited on Mo surface. The substrate surface was polished prior to deposition. The powders used have a narrow particle size distribution. The results show that the morphology of splats depends significantly on the thermal interaction between the molten particle and the substrate. The splat observed was only a central part of an ideal disk-like complete splat. The typical pattern of Mo splats was the split type presenting a small split structure on stainless steel substrate surface. With Mo particles, the preheating of steel substrate has no effect on splat morphology. On the other hand, disk-like type Mo splat with a reduced diameter of a dimple-like structure at the central area of the splat was formed on Mo substrate and splashing can be suppressed through substrate preheating. Based on the experimental results, a surface-melting- induced splashing model was proposed to explain the formation mechanism of Mo splat on steel surface.
Photographing Impact of Plasma-Sprayed Particles on Metal Substrates
Authors: McDonald A. , Chandra S. , Lamontagne M. , Moreau C.
Published: September 2022

Plasma-sprayed, molten molybdenum particles (~40 µm diameter) were photographed during impact (with velocity ~110 m/s) on Inconel surfaces that were preheated or maintained at room temperature or 400oC. A droplet approaching the surface was sensed using a photodetector and after a known delay, a fast CCD camera was triggered to capture images of the spreading splat from the substrate front surface. A rapid two-color pyrometer was used to collect the thermal radiation from the impacting particles to follow the evolution of their temperature and size after impact. Molten molybdenum particles impacting on surfaces at room temperature disintegrated and splashed, after achieving a maximum diameter larger than 400 µm. Impact on preheated and heated Inconel produced splats with maximum diameters between 200 µm and 300 µm and with less splashing. The cooling rate of splats on the preheated Inconel was larger than that of splats on non-heated Inconel, suggesting that the splat-substrate contact was improved.
Fragmentation of Plasma-Sprayed Molybdenum Particles on Glass Surfaces
Authors: McDonald A. , Raessi M. , Chandra S. , Mostaghimi J. , Moreau C.
Published: September 2022

The impact of plasma-sprayed molybdenum particles on glass surfaces held at 25 and 400°C was photographed. A two-color pyrometer was used to collect thermal radiation from the particles to follow their temperature evolution and to calculate the splat cooling rate. Significant fragmentation of the splat on the surface at 25°C was observed. A 3D model of droplet impact and solidification was used to estimate the thermal contact resistances between the splat and glass. It was found that the thermal contact resistance was approximately two orders of magnitude smaller on the surface at 400°C, indicating faster solidification, which reduced splashing. The larger thermal contact resistance between the non-heated glass and splat was attributed to the presence of a gas barrier at the surface.
Phase Transformation of Al-Cu-Cr Quasicrystalline Coatings in the Plasma Spray Processes
Authors: Fu Y.Q. , Gao Y. , Hei Z.K.
Published: September 2022

The effects of substrate temperature during the low-power plasma spraying (2.5~8.0kW) and subsequent annealing on the phase transformation of Al-Cu-Cr quasicrystalline coatings were investigated. Compared with the original powder mainly composed of icosahedral I-Al65Cu24Cr11 quasicrystalline phase and monoclinic θ-Al13Cr4, the coatings show an icosahedral quasicrystal I-Al65Cu24Cr11 and its body-centered cubic approximate phase α-Al69Cu18Cr13. XRD patterns indicated that, for identical spraying condition, a rise in substrate temperature resulted in an increase of the I/α peak intensity ratio in the coatings, and the crystalline phases of AlCu4 and Al86Cr14 were observed in the coatings after annealing at 1113K for half an hour.
Polyamide-11 Powder Coatings by Flame Spraying
Authors: McAndrew T.P. , Céré F.
Published: September 2022

Thermoplastic Polyamide-11 powder coatings serve many industries – such as water handling, automotive, and appliances. This utility is based on the ability to simultaneously provide exceptional resistance to: corrosion, impact/abrasion, and numerous chemicals. Typically application is by traditional methods – electrostatic spray or fluidized bed dipping. The present work demonstrates for the first time that the flame spray method can produce Polyamide- 11 powder coatings very close in performance to those produced by traditional methods. The keys are proper substrate pre-heating, and flame conditions that minimize polymer degradation. Coatings performance, impact resistance, and molecular weight data are presented.
Effect of Substrate Surface Topography and Temperature on Millimeter and Micrometer Sized Splat Formation and on Thermal Contact Resistance
Authors: Zhang G. , Liao H. , Cherigui M. , Coddet C.
Published: September 2022

Flame sprayed PEEK (poly-ether-ether-ketone) coatings, with an amorphous structure, were subjected to isothermal treatments with annealing temperatures from 180 to 300 °C and holding time from 1 to 30 min. The coating structures were studied by means of DSC (Differential Scanning Calorimetry) and XRD (X-Ray Diffraction) analyses. All annealed coatings exhibit semi-crystalline structures. Coexistence of thick and thin lamellae in the spherulites of annealed coatings can be deduced from the analysis results. The Knoop hardness and the interfacial adhesion of the coatings were examined. The annealed coatings exhibit higher hardness than the amorphous ones. The formation of the thick lamellae is the determinant factor for improving the coating hardness. However, the annealed coatings exhibit a weak adherence to the substrate. Some fissures or spherical porosities could be observed, in certain zones, on the coating/substrate interface. The formation of these fissures and porosities could be ascribed to the coating residual stress and the big volume contraction during the crystallisation that occurred under the annealing condition.
Wetting and Spreading of PEEK Single Splats on Aluminium
Authors: Withy B.P. , Hyland M.M. , James B.J.
Published: September 2022

PEEK was sprayed with a Browning Aerospray HVAF thermal spray gun to enable the study of the wetting and interaction of single splats with an aluminium 5005 substrate. Single splats were obtained by exposing the substrate to the spray flame for 0.02 s by dropping a steel shutter with a 25 mm aperture milled in the centre. The single splats were then analysed through SEM (scanning electron microscopy) and FIB microscopy (focussed ion beam). Splat shape was found to be dependant on nozzle length, with a 100 mm nozzle resulting in more splashing, and a 450 mm spray distance providing more disc splats. PEEK splats do not wet the aluminium oxide surface well. Porosity occurs independently of nozzle length, in the form of cracks and pores in the splats, some cracks completely segmenting a splat.
Effect of Substrate Roughness on Splatting Behavior of HVOF Sprayed Polymer Particles: Modeling and Experiments
Authors: Ivosevic M. , Gupta V. , Cairncross R.A. , Twardowski T.E. , Knight R. , Baldoni J.A.
Published: September 2022

A three-dimensional model of particle impact and deformation on rough surfaces has been developed for HVOF sprayed polymer particles. Fluid flow and particle deformation was predicted by the Volume of Fluid (VoF) method using Flow-3D® software. The effect of roughness on the mechanics of splatting and final splat shapes was explored through the use of several prototypical rough surfaces, e.g. steps and grooves. In addition, a numerical representation of a more realistic rough surface, generated by optical interferometry of an actual grit blasted steel surface, was also incorporated into the model. Predicted splat shapes were compared with SEM images of Nylon 11 splats deposited onto grit blasted steel substrates. Rough substrates led to the generation of fingers and other asymmetric three-dimensional instabilities that are seldom observed in simulations of splatting on smooth substrates.