Numerical Modeling of Suspension HVOF Spray

A three-dimensional two-way coupled Eulerian-Lagrangian scheme is used to simulate suspension high-velocity oxy-fuel spraying process. The mass, momentum, energy, and species equations are solved together with the realizable k-ε turbulence model to simulate the gas phase. Suspension is assumed to be a mixture of solid particles [mullite powder (3Al₂O₃·2SiO₂)], ethanol, and ethylene glycol. The process involves premixed combustion of oxygen-propylene, and non-premixed combustion of oxygen-ethanol and oxygen-ethylene glycol. One-step global reaction is used for each mentioned reaction together with eddy dissipation model to compute the reaction rate. To simulate the droplet breakup, Taylor Analogy Breakup model is applied. After the completion of droplet breakup, and solvent evaporation/combustion, the solid suspended particles are tracked through the domain to determine the characteristics of the coating particles. Numerical simulations are validated against the experimental results in the literature for the same operating conditions. Seven or possibly eight shock diamonds are captured outside the nozzle. In addition, a good agreement between the predicted particle temperature, velocity, and diameter, and the experiment is obtained. It is shown that as the standoff distance increases, the particle temperature and velocity reduce. Furthermore, a correlation is proposed to determine the spray cross-sectional diameter and estimate the particle trajectories as a function of standoff distance.     

Simulation of particle-shock interaction in a high velocity oxygen fuel process

High velocity oxygen fuel process (HVOF) involves supersonic two-phase flow of gas-solid particles. Two kinds of shocks are formed in a typical high velocity oxygen fuel process. Adjustment of the overexpanded flow to the atmospheric pressure at the exit of the nozzle results in formation of shock diamonds while high speed flow impingement on a substrate creates bow shock. The latter is found to be responsible for deviation of the injected particles from their trajectories near the substrate, which significantly reduces the chance of some particles landing on the substrate. An attempt is made to study the behavior of particle trajectory as it interacts with the bow shock formed near the substrate. The strength and location of bow shock was found to vary for different substrate geometries and standoff distances. In this work, various particle sizes impinging on substrates with various configurations (flat, concave, and convex) are simulated and the effect of shock diamonds and bow shock on particle trajectory is studied. This article was originally published inBuilding on 100 Years of Success, Proceedings of the 2006 International Thermal Spray Conference (Seattle, WA), May 15–18, 2006, B.R. Marple, M.M. Hyland, Y.-Ch. Lau, R.S. Lima, and J. Voyer, Ed., ASM International, Materials Park, OH, 2006.     

Slurry based multilayer environmental barrier coatings for silicon carbide and silicon nitride ceramics — I. Processing

Multilayer mullite/gadolinium silicate (Gd₂SiO₅) environmental barrier coatings (EBCs) were deposited on α-SiC (Hexaloy) and Si₃N₄ (SN282) substrates through cost-effective slurry based dip-coat processing. Coatings applied by two approaches, alcohol and sol-based slurries, were examined and optimized in terms of their recipes and air sintering temperatures. A significant increase in densification rates was found for the sol-based EBCs applied on both SiC and SN282 substrates due to the fine mullite particles formed during reaction sintering of well-mixed silica and alumina sols. Mechanical alloying of the starting powder mixtures instead of their simple rotary-blending was found to be beneficial in terms of enhanced coat sintering kinetics. Dense thick coatings that were well-bonded to the substrate were obtained.     

A Three-Dimensional Analysis of the Cold Spray Process: The Effects of Substrate Location and Shape

A three-dimensional model of a Cold Gas Dynamic Spray system with a peripheral nonaxisymmetric powder feeder is studied in this work. It is found that the stagnation pressure alternates for different substrate standoff distances due to the nature of the supersonic flow interaction with the substrate. One can find the optimum substrate location for any given operating condition, which results in minimum pressure buildup on the substrate. The three-dimensional analysis sheds more light on the complex gas and particle flow fields generated due to the three-dimensional particle injection process. In addition, the three-dimensional model allows us to further investigate the effect of practical substrate shapes (such as convex and concave) on the flow field and consequently to determine the optimum conditions to deposit coating particles. This article is an invited paper selected from presentations at the 2007 International Thermal Spray Conference and has been expanded from the original presentation. It is simultaneously published in Global Coating Solutions, Proceedings of the 2007 International Thermal Spray Conference, Beijing, China, May 14-16, 2007, Basil R. Marple, Margaret M. Hyland, Yuk-Chiu Lau, Chang-Jiu Li, Rogerio S. Lima, and Ghislain Montavon, Ed., ASM International, Materials Park, OH, 2007.     

A Numerical Study of Suspension Injection in Plasma-Spraying Process

Suspension feedstock in plasma spraying opened a new chapter in coating process with enhanced characteristics. The suspension carrying sub-micron up to few micron-sized particles is radially injected into an atmospheric plasma plume. Understanding the trajectory, velocity, and temperature of these small particles upon impacting on the substrate is a key factor to produce repeatable and controllable coatings. A three dimensional two-way coupled Eulerian-Lagrangian scheme is utilized to simulate the flow field of the plasma plume as well as the interactions between the evaporative suspension droplets with the gas phase. To model the breakup of droplets, Kelvin-Helmholtz Rayleigh-Taylor breakup model is used. After the breakup and evaporation of suspension is complete, the solid suspended particles are tracked through the domain to determine the characteristics of the coating particles. The numerical results are validated against experiments using high-speed imaging.     

Carbon nanotube synthesis using magnetic fluids on various substrates

A new carbon nanotube synthesis method using a magnetic fluid is developed. The catalyst particles can be formed by simple spin coating of the magnetic fluid of Fe₃O₄ nano particles mixed with polyvinyl alcohol on substrates and subsequent heat treatment. A quantitative analysis of the catalyst particles distribution on Si substrate is carried out. The mixing of the magnetic fluid in polyvinyl alcohol gives viscosity to the solution, secures uniform particle distribution without agglomeration, and controls the catalyst particle density on the substrate. The distribution of the catalyst particles is mainly controlled by the NH₄OH concentration in the magnetic fluid solutions. Carbon nanotubes grown on various substrates of Si, alumina, and various metal plates revealed different morphologies due to the difference in the wetting of the catalyst particles on the substrate in the various particle-substrate systems.     

Influence of Impact Conditions on Feedstock Deposition Behavior of Cold-Sprayed Fe-Based Metallic Glass

Cold spray is a promising method by which to deposit dense Fe-based metallic glass coatings on conventional metal substrates. Relatively low process temperatures offer the potential to prevent the crystallization of amorphous feedstock powders while still providing adequate particle softening for bonding and coating formation. In this study, Fe₄₈Mo₁₄Cr₁₅Y₂C₁₅B₆ powder was sprayed onto a mild steel substrate, using a variety of process conditions, to investigate the feasibility of forming well-bonded amorphous Fe-based coatings. Particle splat adhesion was examined relative to impact conditions, and the limiting values of temperature and velocity associated with successful softening and adhesion were empirically established. Variability of particle sizes, impact temperatures, and impact velocities resulted in splat morphologies ranging from well-adhered deformed particles to substrate craters formed by rebounded particles and a variety of particle/substrate interface conditions. Transmission electron microscopy studies revealed the presence of a thin oxide layer between well-adhered particles and the substrate, suggesting that bonding is feasible even with an increased oxygen content at the interface. Results indicate that the proper optimization of cold spray process parameters supports the formation of Fe-based metallic glass coatings that successfully retain their amorphous structure, as well as the superior corrosion and wear-resistant properties of the feedstock powder.     

Mullite and Mullite/ZrO2-7wt.%Y2O3 Powders for Thermal Spraying of Environmental Barrier Coatings

Mullite and mullite/ZrO₂-7wt.%Y₂O₃ coatings could be thought among the main protective layers for environment barrier coatings (EBCs) to protect Si-based substrates in future gas turbine engines. Considering that feedstock of the compound powder is not commercially available, two powder processing routes Spray Drying (SD) and Flame Spheroidization (FS) were implemented for both types of powders. For each method the particle size, the morphology, and microstructure of the powder particles was determined. In addition, the effect of the heat treatment on the powder crystallinity and microstructure of FS powders was also investigated. To evaluate their suitability as feedstock materials, the powders were plasma sprayed and their in-flight particle characteristics monitored for coatings production. The powder morphology was correlated to the in-flight particle characteristics and splat morphology to gain insight about into the influence of powder characteristics on the coating formation.     

Standoff distance and bow shock phenomena in the Cold Spray process

Cold Spray involves the deposition of metallic powder particles using a supersonic gas jet. When the nozzle standoff distance is small, a bow shock is formed at the impingement zone between the supersonic jet and the substrate. It has long been thought that this bow shock is detrimental to process performance as it can reduce particle impact velocities. By using computational fluid dynamics, Particle Image Velocimetry and Schlieren imaging it was possible to show that the bow shock has a negative influence on deposition efficiency as a result of a reduction in particle velocity. Furthermore, the existence of the bow shock was shown to be dependent on the length of the nozzle's supersonic potential core. Experiments were carried out with aluminium, copper and titanium powders using a custom-made helium nozzle, operating at 2.0 MPa and 20 °C, and a commercial nitrogen nozzle operating at 3.0 MPa and 300 °C. In all cases, it was found that there is a direct relationship between standoff distance and deposition efficiency. At standoff distances less than 60 mm, the bow shock reduced deposition efficiencies by as much as 40%.     

Hydrothermal synthesis of titanium-supported nanoporous palladium–copper electrocatalysts for formic acid oxidation and oxygen reduction reaction

Nanoporous Pd and binary Pd-Cu particles were prepared by a hydrothermal method using ethylene glycol as a reduction agent and they were directly immobilized on Ti substrates named as Ti-supported Pd-based catalysts. Their electrocatalytic activity for formic acid oxidation and oxygen reduction reaction (ORR) in alkaline media was examined by voltammetric techniques. Among the as-prepared catalysts, nanoPd₈₁Cu₁₉/Ti catalyst presents the highest current density of 39.8 mA/cm² at ?0.5 V or 66.4 mA/cm² at ?0.3 V for formic acid oxidation. The onset potential of ORR on the nanoPd₈₁Cu₁₉/Ti catalyst presents an about 70 mV positive shift compared to that on the nanoPd/Ti, and the current density of ORR at ?0.3 V is 2.12 mA/cm², which is 3.7 times larger than that on the nanoPd/Ti.     

Three-Dimensional Modeling of Suspension Plasma Spraying with Arc Voltage Fluctuations

In this study, a three-dimensional DC plasma torch is modeled using Joule effect method to simulate the plasma jet and its voltage fluctuations. The plasma gas is a mixture of argon/hydrogen, and the arc voltage fluctuation is used as an input data in the model. Reynolds stress model is used for time-dependent simulation of the oscillating flow of the plasma gas interacting with the ambient air. The results are used to investigate the plasma oscillation effects on the trajectory, temperature, and velocity of suspension droplets. Suspensions are formed of ethanol and yttria-stabilized zirconia submicron particles and modeled as multicomponent droplets. To track the droplets/particles, a two-way coupled Eulerian–Lagrangian method is employed. In addition, in order to simulate the droplet breakup, Kelvin–Helmholtz/Rayleigh–Taylor (KH–RT) breakup model is used. After the completion of suspension breakup and evaporation, the sprayed particles are tracked to obtain the in-flight particle conditions including trajectory, size, velocity, and temperature. The arc voltage fluctuations were found to cause more than two times wider particle trajectories resulting in wider particle temperature, velocity, and size distributions compared with the case of constant voltage.     

Transmission electron microscopy study of interfacial microstructure formed by reacting Al–Mg alloy with mullite at high temperature

Transmission electron microscopy (TEM) has been used to study the interfacial microstructure formed by reacting Al–Mg alloy with mullite (Al₆Si₂O₁₃) at high temperature (>900°C). The TEM study was used in order to understand the strong effect of Mg addition on the nature of the reaction between Al and mullite used to form Al/Al₂O₃ composite. After reaction at 1050°C, the formation of a layered structure between the Al–1% Mg alloy and mullite was observed. An alloy layer with a much higher concentration of Mg than the starting alloy was found present next to the initial mullite surface. Between the alloy layer and mullite, a dense and continuous layer made of small MgAl₂O₄ (spinel) and Si particles was present. The layer apparently stopped further reaction between Al–Mg alloy and mullite by preventing transport of the metals to the reaction front and the Si reaction product away from the reaction front. The microstructure resulting from the initial reaction indicated the reaction proceeded by replacing Si atoms with Al and Mg atoms on mullite {210} lattice planes and forming MgAl₂O₄ {311} lattice planes simultaneously.     

Plasma Sprayed Coating Using Mullite and Mixed Alumina/Silica Powders

Plasma sprayed ceramic coatings are widely used for thermal barrier coating applications. Commercially available mullite powder particles and a mixture of mechanically alloyed alumina and silica powder particles were used to deposit mullite ceramic coatings by plasma spraying. The coatings were deposited at three different substrate temperatures (room temperature, 300?°C, and 600?°C) on stainless steel substrates. Microstructure and morphology of both powder particles as well as coatings were investigated by using scanning electron microscopy. Phase formation and degree of crystallization of coatings were analyzed by x-ray diffraction. Differential thermal analysis (DTA) was used to study phase transformations in the coatings. Results indicated that the porosity level in the coatings deposited using mullite initial powder particles were lower than those deposited using the mixed initial powder particles. The degree of crystallization of the coatings deposited using the mixed powder particles was higher than that deposited using mullite powder particles at substrate temperatures of 25 and 300?°C. DTA curves of the coatings deposited using the mixed powders showed some transformation of the retained amorphous phase into mullite and alumina. The degree of crystallization of the as sprayed coatings using the mixed powder particles was significantly increased after post deposition heat treatments. The results indicated that the mechanically alloyed mixed powder can be used as initial powder particles for deposition of mullite coatings instead of using mullite powders.     

Mullite/molybdenum ceramic–metal composites

Dense (>98 th%) homogeneous mullite/Mo (32 vol.%) composites with two different Mo average grain sizes (1.4 and 3 μm) have been obtained at 1650°C in vacuum and in reducing condition. Depending on the Mo grain size and processing atmosphere, the K_IC ranges from 4 to 7 MPa m^1/2 and σ_f from 370 to 530 MPa. The MoO₂–2SiO₂·3Al₂O₃–Mo system was found to be compatible in solid state, and a solid solution of ≈4 wt% of MoO₂ in mullite at 1650°C was detected. A solid state dewetting of MoO₂ from the surface of the Mo particle takes place during sintering. It was found that the absence of MoO₂ in the mullite/Mo composites by processing in reducing conditions increases the strength of the metal/ceramic interface and the plasticity of the Mo metal particles, thus strengthening the composite by a crack bridging mechanism. As a result, the K_IC and the σ_f values of the ceramic–metal composite were found to be ≈4 times and ≈2 times higher than the ones corresponding to the mullite matrix.     

Relation between microstructure and adhesion of hot dip galvanized zinc coatings on dual phase steel

The microstructure of hot dip galvanized zinc coatings on dual phase steel was investigated by electron microscopy and the coating adhesion characterized by tensile testing. The zinc coating consists of a zinc layer and columnar ζ-FeZn₁₃ particles on top of a thin inhibition layer adjacent to the steel substrate. The inhibition layer is a thin compact and continuous layer that consists of η-Fe₂Al_5–xZn_x fine and coarse particles. The coarse faceted particles are on top and fine faceted particles are at the bottom. The steel surface is covered with small fraction manganese oxides, which may impair adhesion of the zinc coating. The adhesion at various interfaces that exist in zinc-coated steel was quantitatively estimated using a so-called “macroscopic atom” model. In addition, the adhesion at the interfaces in zinc-coated steel was qualitatively assessed by examining the fracture and delamination behavior upon tensile testing. In accordance with this model, fracture along zinc grain boundaries preceded fracture along the zinc layer/inhibition layer and ζ-FeZn₁₃ particle/inhibition layer interfaces.     

Wear and oxidation resistance of Al2O3 particle dispersed Al3Ti composite with a nanostructure prepared by pulsed electric current sintering of mechanically alloyed powders

Al₃Ti-matrix composite layers containing Al₂O₃ particles were formed on Ti substrate by pulsed electric current sintering (PECS) of mechanically alloyed (MA) powders to improve the wear and oxidation properties of the Ti substrate. Reducing the grain size of each element by MA makes the combustion synthesis of Al₃Ti possible at a lower temperature. The grain size formed by the combustion synthesis of Al–Ti–Al₂O₃ powder mechanically alloyed for 720 ks was about 10 nm and its growth during sintering was suppressed by the existence of Al₂O₃. The densification behavior of the powder was investigated quantitatively. The obtained Al₃Ti/Al₂O₃ composite layer showed better wear and oxidation resistance than the monolithic Al₃Ti layer.     

Mathematical modelling and numerical optimization of particle heating process in copper flash furnace

A mathematical model of the particle heating process in the reaction shaft of flash smelting furnace was established and the calculation was performed. The results indicate that radiation plays a significant role in the heat transfer process within the first 0.6 m in the upper part of the reaction shaft, whilst the convection is dominant in the area below 0.6 m for the particle heating. In order to accelerate the particle ignition, it is necessary to enhance the convection, thus to speed up the particle heating. A high-speed preheated oxygen jet technology was then suggested to replace the nature gas combustion in the flash furnace, aiming to create a lateral disturbance in the gaseous phase around the particles, so as to achieve a slip velocity between the two phases and a high convective heat transfer coefficient. Numerical simulation was carried out for the cases with the high-speed oxygen jet and the normal nature gas burners. The results show that with the high-speed jet technology, particles are heated up more rapidly and ignited much earlier, especially within the area of the radial range of R=0.3–0.6 m. As a result, a more efficient smelting process can be achieved under the same operational condition.     

Preparation of Al2O3–ZrO2–SiO2 ceramic composites by high-gravity combustion synthesis

By a furnace-free technique of high-gravity combustion synthesis, Al₂O₃–ZrO₂–SiO₂ ceramic composites were prepared via melt solidification instead of conventional powder sintering. The solidification kinetics and microstructure evolution of the ceramic composites in high-gravity combustion synthesis were discussed. The phase assemblage of the ceramic composites depended on the chemical composition, where both (Al₂O₃ + ZrO₂) and (mullite + ZrO₂) composites were obtained. The ceramic composites consisted of ultrafine eutectics and sometimes also large primary crystals. In the (mullite + ZrO₂) composites, two different morphologies and orientations were observed for the primary mullite crystals, and the volume fraction of mullite increased with increasing SiO₂ content. The ceramic composites exhibited a hardness of 11.2–14.8 GPa, depending on the chemical composition and phase assemblage.     

Peening action and residual stresses in high-velocity oxygen fuel thermal spraying of 316L stainless steel

316L stainless steel powder was sprayed by a high-pressure high-velocity oxygen fuel (HVOF) process. Effects of powder size and the pressure in the combustion chamber on the velocity and temperature of sprayed particles were studied by using an optical instrument, first, at the substrate position. A strong negative correlation between the particle temperature and the diameter was found, whereas the correlation between the velocity and the diameter was not significant. The pressure in the combustion chamber affected the velocity of sprayed particles significantly, whereas the particle temperature remained largely unchanged. In-situ curvature measurement was employed in order to study the process of stress generation during HVOF spraying. From the measured curvature changes, the intensity of peening action and the resultant compressive stress by HVOF sprayed particles were found to increase with the kinetic energy of the sprayed particles. The results were further used to estimate the stress distribution within the coatings. X-ray stress measurement revealed that the residual stress on the surface of the HVOF coatings is low and often in tension, but the stress inside the coatings is in a high level of compression.     

Rapidly solidified thick cobalt-based alloy deposit with carbide particles produced by plasma spraying

Abstract

Thermal spraying can produce rapidly solidified thick layers, because the droplets that accumulate on the substrate experience cooling rates of 10⁵–10⁸ K s^–1. It is thus an attractive method to produce composite deposits containing fine particles formed in situ. In the present work, Co–1·5C, Co–1·5C–14Cr and Co–1·5C–29Cr (wt-%) alloy powders produced by argon atomisation were low-pressure plasma sprayed onto water cooled or uncooled substrates. The as sprayed Co–1·5C deposit formed on a water-cooled substrate consists of a metastable β-Co phase, supersaturated with carbon due to the high cooling rate. Heat treatment at >673 K led to the formation of α-Co and graphite. The deposit heat treated at 1073 K contained graphite particles ～0·8 μm in size. At higher treatment temperatures, coarsening of graphite and the elimination of carbon supersaturation result in lower hardness. As-sprayed Co–1·5C deposit on uncooled substrates consisted of β-Co, α-Co and graphite. As-sprayed deposit of Co–1·5C–14Cr and Co–1·5C–29Cr powders on uncooled substrates consisted of β-Co, α-Co, Cr₃C₂, Cr₇C₃ and graphite. The fine (0·5 μm) precipitates in the as-sprayed Co–1·5C–14Cr deposit on uncooled substrates were identified as chromium carbide. As-sprayed Co–1·5C–29Cr deposits on uncooled substrates contained many precipitates ～0·6 μm in size and showed the highest microhardness of ～600 HV (2·94 N).