New attachment for controlling gas flow in the HVOF process

During the decade, the high-velocity oxyfuel (HVOF) process proved to be a technological alternative to the many conventional thermal spray processes. It would be very advantageous to design a nozzle that provides improved performance in the areas of deposition efficiency, particle in-flight oxidation, and flexibility to allow deposition of ceramic coatings. Based on a numerical analysis, a new attachment to a standard HVOF torch was modeled, designed, tested, and used to produce thermal spray coatings according to the industrial needs mentioned above. Performance of the attachment was investigated by spraying several coating materials including metal and ceramic powders. Particle conditions and spatial distribution, as well as gas phase composition, corresponding to the new attachment and the standard HVOF gun, were compared. The attachment provides better particle spatial distribution, combined with higher particle velocity and temperature. The original version of this article was published as part of the ASM Proceedings, Thermal Spray 2003: Advancing the Science and Applying the Technology, International Thermal Spray Conference (Orlando, FL), 5–8 May, 2003, Basil R. Marple and Christian Moreau, Ed., ASM International, 2003.     

Mathematical modeling of the gas and powder flow in HVOF systems

A mathematical model was developed to describe the gas dynamics and heat-transfer mechanism in the gas/particle flow of high- velocity oxyfuel (HVOF) systems. A numerical solution was carried out using a PC- based computer program. One- dimensional predictions of the temperature and velocity profiles of gas and particles along the axis of flow were obtained to conduct cost- effective parametric studies and quality optimization of thermal spray coatings produced by HVOF systems. The numerical computer model allows for the variation of the HVOF system parameters, such as air/fuel ratio and flow rates, cooling water inlet temperature and flow rate, barrel length, standoff distance, particle size, and gun geometry. Because of the negligible volume of the powder relative to the gas, the gaseous phase was modeled as continuous nonadiabatic, and friction flow with variable specific heats and changing cross- sectional areas of flow. The generalized continuity, momentum, and energy equations with the influence parameters were used to model the gaseous flow regime and predict its thermodynamic properties. Empirical formulas for the mean axial decay of both velocity and temperature in the supersonic jet plume region were generated from published measurements of these parameters using laser Doppler velocimeter and Ray leigh scattering techniques, respectively. The particle drag and heat- transfer coefficients were calculated by empirical formulas in terms of Reynolds, Nusselt, and Prandtl numbers to evaluate both the momentum and heat transferred between the combustion gases and the powder particles. The model predictions showed good agreement with the particle and gas temperature and velocity measurements that are available in the literature.     

超音速火焰喷涂纳米结构涂层研究进展

超音速火焰(High Velocity Oxy -Fuel,简称HVOF)喷涂具有高速和相对较低的温度两个重要特征,能够获得比普通火焰喷涂或等离子喷涂(Plasma Spray,简称PS)结合强度更高的致密涂层.纳米材料具有独特的表面效应、体积效应及量子尺寸效应,其电学、力学、磁学、光学和热学等性能产生了惊人的变化.随着材料科学技术的深入发展, 在实际生产和生活中运用性能优良的纳米材料倍受人们关注,其中,采用热喷涂技术制备纳米结构涂层是构筑纳米结构材料的最具前途的方法之一.从目前国内外的情况来看,HVOF喷涂纳米结构涂层技术的研究取得了较大的进展.综合国内外文献,总结了HVOF喷涂制备纳米结构涂层的研究现状,着重阐述了热喷涂纳米涂层的基本过程和结合机理,指出了利用HVOF喷涂纳米结构涂层存在的问题,并对热喷涂纳米结构涂层的发展前景作了展望.     

HVOF Combustion spraying of inconel powder

A major trend in the thermal spray industry has been to increase the gas jet velocity to obtain better coating attributes. One emerging technology now used in industry is the high-velocity oxygen fuel process (HVOF). High-velocity spray guns combine oxygen and a fuel gas to generate heat and extremely high particle velocities. In this study, Inconel 718 powder was deposited on steel substrates. The primary coating function was electrical resistivity for a heater application. Experiments were conducted using a Taguchi L8 statistical fractional/factorial design parametric study. The Taguchi experiment evaluated the effect of six HVOF processing variables on the measured responses. The parameters were oxygen flow, fuel flow, air envelope gas flow, powder feed rate, spray distance, and nozzle configuration. The coatings were characterized by hardness tests, surface profilometry, optical metallography, and image analysis. This article investigates coating hardness, porosity, surface roughness, deposition efficiency, and microstructure with respect to the influence of the processing parameters. Analytical studies were conducted to investigate gas, particle, and coating dynamics for two of the HVOF thermal spray experiments.     

Analysis and optimization of the HVOF process by combined experimental and numerical approaches

Thermal spraying with the HVOF technology is a well known approach to dense metallic, ceramic and cermets coatings with good mechanical properties. Any attempt for improving HVOF coating properties requires a fundamental understanding of the mechanisms that occur during HVOF spraying. Thermal spray processes are not only optimized by empirical testing and by correlation analysis between process parameters and coating properties but also with numerical approaches. Recent attempts to understand the momentum and heat transfer mechanisms between flame and particles, and thus improve the control of the thermokinetic deposition process by analysis of fundamental thermophysical and fluid mechanical processes, have led to computational modeling of the spraying process and verification of simulation results by in-flight particle analysis.This paper focuses on modeling (tracking) of the particle properties during HVOF spraying using alumina powder. The particle properties are sensitive to a large number of process parameters (e.g., gas temperature, gas expansion velocity, pressure, spraying distance, spray powder particle diameter, nozzle geometry, etc.). Variation of the operating parameters of the HVOF process (gas flow rates, stoichiometric oxy/fuel ratio, nozzle design, etc.) is performed during modeling and simulation. The SprayWatch® system for particle in-flight measurement is used for verification of the numerical analysis result.     

FeAl and Mo–Si–B intermetallic coatings prepared by thermal spraying

FeAl and Mo–Si–B intermetallic coatings for elevated temperature environmental resistance were prepared using high-velocity oxy-fuel (HVOF) and air plasma spray (APS) techniques. For both coating types, the effect of coating parameters (spray particle velocity and temperature) on the microstructure and physical properties of the coatings was assessed. Fe–24Al (wt%) coatings were prepared using HVOF thermal spraying at spray particle velocities varying from 540 to 700 m/s. Mo–13.4Si–2.6B coatings were prepared using APS at particle velocities of 180 and 350 m/s. Residual stresses in the HVOF FeAl coatings were compressive, while stresses in the APS Mo–Si–B coatings were tensile. In both cases, residual stresses became more compressive with increasing spray particle velocity due to increased peening imparted by the spray particles. The hardness and elastic moduli of FeAl coatings also increased with increasing particle velocity. For Mo–Si–B coatings, plasma spraying at 180 m/s resulted in significant oxidation of the spray particles and conversion of the T1 phase into amorphous silica and -Mo. The T1 phase was retained after spraying at 350 m/s.     

Improved Protection Properties by Using Nanostructured Ceramic Powders for HVOF Coatings

The potential of the high-velocity oxy-fuel (HVOF) thermal spray process for reduced porosity in coatings compared to those produced by other ambient thermal spray processes is well known. The ability to produce high-density ceramic coatings offers potential in high-performance applications in the field of wear, corrosion resistance, and dielectric coatings. However, due to operational limit of the HVOF process to effectively melt the ceramic particles, the process—structure relationship must be well optimized. It has been also demonstrated that benefits from HVOF ceramic coatings can be obtained only if particles are melted enough and good lamella adhesion is produced. One strategy to improve melting of ceramic particles in relative low-flame temperatures of HVOF process is to modify particle crystal structure and composition. In this paper the effect of the powder manufacturing method and the composition on deposition efficiency of spray process as well as on the mechanical properties of the HVOF sprayed are studied. Effect of fuel gas, hydrogen vs. propane, was also demonstrated. Studied materials were alumina-, chromia-, and titania-based agglomerated powders. Coating properties such as microstructure, hardness, abrasive wear resistance, and relative fracture toughness were compared to the coating manufactured by using conventional fused and crushed powders. It can be concluded that powder size distribution and microstructure should be optimized to fulfill process requirements very carefully to produce coatings with high deposition efficiency, dense structure, improved fracture toughness, and adhesion.     

Review on the Oxidation of Metallic Thermal Sprayed Coatings: A Case Study with Reference to Rare-Earth Permanent Magnetic Coatings

Thermal spray fabrication of rare-earth permanent magnetic coatings (PMCs) presents potential manufacturing routes for micro-magnetic devices. Despite this potential, thermal spray of PMCs is still not widely explored due to oxidation concerns. It was established that oxidation leads to the loss of ferromagnetic phases in these materials and results in deterioration of magnetic performance. Although this review focuses on a specific class of material, i.e., magnetic materials, there is significant technical crossover to all classes of feedstocks that are employed in thermal spray processing. The oxidation mechanisms and the associated influencing factors are explored in this work to implement effective processing techniques during the deposition process. This paper reviews the various stages and mechanisms of oxidation in thermal spray processes. The factors that influence the extent of oxidation depend on the type of oxidation that is dominant and rely on the type of spray system, powder injection position, and the particle size of feedstock. Among the aspects that are reviewed include the oxygen-fuel ratio for high velocity oxygen-fuel (HVOF), current intensity, gas flow rate, particle size, spray distance, and substrate temperature. Protection strategies to minimize oxidation in thermal spray processes, such as gas shrouding and shielding, are presented.     

Numerical analysis of a high-velocity oxygen-fuel thermal spray system

The fluid and particle flow field characteristics of a high-velocity oxygen-fuel (HVOF) thermal spray (TS) system are analyzed using a two-phase flow model and simulated using computational fluid dynamics (CFD) techniques. The model consists of a conservation equation and constitutive relations for both gas and particle phases. Compressible, turbulent flow is modeled by ak-ɛ turbulent model. A Lagrangian formulation is used to model particle trajectory, and heat and momentum transfer. The fluid velocity fluctuations resulting from gas turbulence are simulated by a stochastic model and the particle motion in the turbulent flow is calculated in a Lagrangian Stochastic-Deterministic (LSD) method. Details of gas flow field, particle temperature and particle velocity histories, and particle temperature and velocity profiles in the system are presented. For the validation of the numerical analysis, the computed results are compared with available experimental measurement. Excellent agreement between simulations and measurements is obtained for both gas and particle flow fields. A parametric study is also conducted for different particle sizes and different nozzle barrel lengths. The flow phenomena for different flow parameters are analyzed and explained as the result of gas dynamics and heat and momentum transfer between the two phases. The developed methodology provides a means to analyze, design, and optimize the TS process. The numerical analysis presents a first comprehensive, fundamental quantitative analysis for the HVOF TS system.     

HVOF thermal spray deposited Y<Subscript>2</Subscript>O<Subscript>3</Subscript>-stabilized ZrO<Subscript>2</Subscript> coatings for thermal barrier applications

High velocity oxy-fuel (HVOF) thermal spray has been successfully used to deposit yttria-stabilized zirconia (YSZ) for thermal barrier coating (TBC) applications. Adherent coatings were obtained within a limited range of spray conditions using hydrogen as fuel gas. Spray parameters such as hydrogen-to-oxygen ratio, spray distance, and substrate cooling were investigated. Spray distance was found to have a pronounced effect on coating quality; adherent coatings were obtained for spray distances between 75 and 125 mm from the gun exit for the hydrogen-to-oxygen ratios explored. Compared to air plasma spray (APS) deposited YSZ coatings, the HVOF deposited coatings were more fully stabilized in the tetragonal phase, and of similar density, surface roughness, and cross-sectional microhardness. Notably, fracture surfaces of the HVOF coatings revealed a more homogeneous structure. Many theoretical models predict that it should not be possible to melt YSZ in an HVOF flame, and therefore it should not be possible to deposit viable YSZ coatings by this process. The experimental results in the present work clearly contradict those expectations. The present results can be explained by taking into account the effect of partial melting and sintering on particle cohesion, as follows. Combustion chamber pressures (P _o) of ∼3.9 bar (58.8 psi) realized during HVOF gun operation allows adiabatic flame temperature values that are above the zirconia melting temperature. Under these conditions, the Ranz-Marshall heat transfer model predicts HVOF sprayed particle surface temperatures T _p that are high enough for partial melting of small (∼10 μm) zirconia particles, T _p=(1.10−0.95)T _m. Further analysis shows that for larger particles (38 μm), adherent coatings are produced when the particle temperature, T _p=0.59−0.60 T _m, suggesting that sintering may have a role in zirconia particle deposition during HVOF spray. These results suggest two different bonding mechanisms for powders having a broad particle size distribution.     

Interaction of particles with carrier gas in HVOF spraying systems

Several designs of high-velocity oxygen fuel (HVOF) thermal spray systems have been created during the last decade. The most advanced systems are now producing coatings comparable in quality to detonation (D-gun) coatings. This paper presents numerical analysis of the interaction of dispersive particles with the carrying gas flow for three different HVOF systems, along with a method to calculate the parameters of sprayed particles that highlights the advantages and limitations of each design. The method includes gas dynamical calculations of the gas flow in an accelerating channel and calculations of the injected par-motion and thermal state (temperature and melted mass fraction). The calculations were performed for particles of tungsten carbide, aluminum oxide, and zirconium oxide with size distributions of 10 to 80 μm. Two conventional types of HVOF systems were considered: those with a supersonic accelerating channel and those with a subsonic accelerating channel (without a de Laval nozzle). A novel design is pro-posed that contains a combined gas dynamical path with functionally separated regions of heating and acceleration. The regularities and distinctions in the behavior of the metallic and ceramic oxide particles are discussed for different jet configurations. The results obtained indicate that it is possible to signifi-cantly affect particle parameters by using the new configuration solutions without creating construction complications.     

Numerical Investigation of a Hybrid HVOF-Plasma Spraying Process

This study deals with the numerical investigation of a hybrid thermal spray process that combines HVOF and thermal plasma technologies. In this process, a thermal plasma is used to assist the combustion process that proceeds in a quasi-conventional HVOF system. It is expected that this coupling makes the HVOF system more flexible in terms of working parameters and sprayed materials. Also, a rather low fuel gas consumption and high deposition rate compared to that of most of the conventional HVOF guns are sought. Modeling this process can help to understand the phenomena that control the operation of the spray system and, therefore, help to optimize it. The model involves the plasma formation, combustion process, and expansion of the supersonic jet in the ambient atmosphere. In this study, the system uses argon as plasma-forming gas and methane as fuel gas. Fuel and oxidant are not premixed before entering the combustion chamber. In the model, methane oxidation is represented by a single-step global reaction considering only a few chemical species (fuel, oxidant, and product species); the turbulent non-premixed combustion is modeled by a fast-chemistry combustion model that assumes that the rate of chemical reaction is controlled by turbulence. The model equations are solved using the CFD software Fluent 6.3. The main gas flow characteristics (velocity, temperature, and pressure) in presence and absence of the plasma source are compared and discussed, and the benefits of the plasma source are discussed in the light of predictions and fuel combustion mechanisms.     

Current Status and Future Prospects of Warm Spray Technology

A modification of high-velocity oxy-fuel (HVOF) thermal spray process named as warm spray (WS) has been developed. By injecting room temperature inert gas into the combustion gas jet of HVOF, the temperature of the propellant gas can be controlled in a range approximately from 2300 to 1000 K so that many powder materials can be deposited in thermally softened state at high impact velocity. In this review, the characteristics of WS process were analyzed by using gas dynamic simulation of the flow field and heating/acceleration of powder particles in comparison with HVOF, cold spray (CS), and high-velocity air-fuel (HVAF) spray. Transmission electron microscopy of WS and CS titanium splats revealed marked differences in the microstructures stemming from the different impact temperatures. Mechanical properties of several metallic coatings formed under different WS and CS conditions were compared. Characteristics of WC-Co coatings made by WS were demonstrated for wear resistant applications.     

Numerical modelling of in-flight particle dynamics of non-spherical powder

High velocity oxygen fuel (HVOF) is an important thermal spraying technology in depositing high quality coatings. Its ability to produce high particle velocities and relatively low particle temperatures is its most salient feature. Several computational fluid dynamic (CFD) models have been developed to study the in-flight particle behavior during thermal spraying. These models are limited to spherical particles, which are only appropriate for modelling gas atomised powders. On the other hand, hardmetal powders such as WC-Co are created using high energy ball milling and are not normally spherical. To examine the effect of particle morphology on particle dynamics, mathematical models are developed in the present paper to predict the in-flight particle behavior in a liquid fuelled HVOF thermal spray gun. The particle transport equations are coupled with the three-dimensional, chemically reacting, turbulent gas flow, and solved in a Lagrangian manner. The melting and solidification within the particles as a result of heat exchange with the surrounding gas flow are solved numerically. The results demonstrate that non-spherical particles gain more momentum and less heat during the HVOF process than spherical particles. Non-spherical particles are also predicted to stay closer to the center of the gas jet than spherical particles.     

Oxidation of stainless steel in the high velocity oxy-fuel process

The high velocity oxy-fuel (HVOF) spray process has been primarily used for the application of wear-resistant coatings and, with the introduction of new, more powerful systems, is being increasingly considered for producing corrosion-resistant coatings. In this study, the influence of various spray parameters for the JP-5000 and Diamond Jet (DJ) Hybrid systems on the oxidation of stainless steel 316L is characterized. Experimental results reveal that coating oxygen contents of less than 1 wt.% can be more easily attained with the JP-5000 than the DJ Hybrid systems because of the former’s design. In both cases, however, the low particle temperatures necessary for low oxygen content coatings may impair bond and cohesive strength. Heat treating the coatings after processing reduces hardness, metallurgically enhances bond strength, and enables the spheroidization of oxide layers surrounding unmelted particles. An empirical model describing oxidation in the thermal spray process was expanded to explain the oxidation in the HVOF spraying of stainless steel. It was concluded that for these oxygen-sensitive materials, maintaining a relatively low particle temperature throughout the spray process minimizes oxygen pickup by preventing an autocatalytic oxidation process and particle fragmentation upon impact. For the DJ Hybrid systems, understoichiometric fuel settings are selected, whereas for the JP-5000, oxygen-rich mixtures are preferred.     

Particle In-Flight and Coating Properties of Fe-Based Feedstock Materials Sprayed with Modern Thermal Spray Systems

New developments in the field of thermal spraying systems (increased particle velocities, enhanced process stability) are leading to improved coatings. Innovations in the field of feedstock materials are supporting this trend. The combination of both has led to a renaissance of Fe-based feedstocks. Using modern APS or HVOF systems, it is now possible to compete with classical materials for wear and corrosion applications like Ni-basis or metal-matrix composites. This study intends to give an analysis of the in-flight particle and spray jet properties achievable with two different modern thermal spraying systems using Fe-based powders. The velocity fields are measured with the Laser Doppler Anemometry. Resulting coatings are analyzed and a correlation with the particle in-flight properties is given. The experiments are accompanied by computational fluid dynamics simulations of spray jet and particle velocities, leading to a comprehensive analysis of the achievable particle properties with state-of-the-art HVOF and APS systems.     

Development of Low-Oxide MCrAlY Coatings for Gas Turbine Applications

Advanced high-energy plasma systems are being used to achieve the benefits of the high-velocity oxy-fuel (HVOF) system without losing the inherent advantages of plasma for coating of gas turbine parts. MCrAlY coatings play a very important role in the performance and reliability of gas turbine components. One of the important considerations for next generation of gas turbines, which have more demanding conditions and need to withstand ever increasing operating temperatures, is that they should possess very low oxygen content levels in the coating. Low oxygen content coatings are applied by the expensive low-pressure plasma spray (LPPS)/vacuum plasma spray (VPS) technique for critical components in aero- and land-based gas turbines. This work deals with the development of low-cost LPPS equivalent coatings (having low oxygen content) using the high-energy high-velocity plasma spray (HEHVPS) gun and inert gas shroud. A comparison has also been made with CoNiCrAlY coatings by HVOF.     

Mathematical modelling of Inconel 718 particles in HVOF thermal spraying

High velocity oxygen fuel (HVOF) thermal spray technology is able to produce very dense coating without over-heating powder particles. The quality of coating is directly related to the particle parameters such as velocity, temperature and state of melting or solidification. In order to obtain this particle data, mathematical models are developed to predict particle dynamic behaviour in a liquid fuelled high velocity oxy-fuel thermal spray gun. The particle transport equations are solved in a Lagrangian manner and coupled with the three-dimensional, chemically reacting, turbulent gas flow. The melting and solidification within particles as a result of heat exchange with the surrounding gas flow is solved numerically. The in-flight particle characteristics of Inconel 718 are studied and the effects of injection parameters on particle behavior are examined. The computational results show that the particles smaller than 10 μm undergo melting and solidification prior to impact while the particle larger than 20 μm never reach liquid state during the process.     

Design and optimisation of a multi-powder feed system for the HVOF deposition process

Currently no specific method exists for the deposition of High Velocity Oxy-Fuel (HVOF) thermal spray functionally graded coatings. This paper investigates the design and optimisation of a multi-powder HVOF thermal spray device in order to deposit aluminium/tool-steel functionally graded coatings. A multi-powder feed device concept was developed. The concept was based on a stand alone two powder chamber device which integrates with common hopper systems to allow the mixing of two powders during thermal spray deposition. This concept was verified by simulation the design of this device for multi-powder flow using Finite Element Analysis (FEA) to arrive at the optimum dual feed design. The FEA model predicted the mixing and flow of two powders of various ratios' of mass flow rate and velocity based on an optimum designed shape and pressure ratios' of nitrogen gas in the chamber to pick-up shaft of 2.25:1. This yielded the best results in terms of carrying the powders from the mixing zone into the nitrogen gas flow path, inside the pick-up shaft and on towards the HVOF gun. Post finite element analysis the device was manufactured for the utilisation within the HVOF process. Optimisation tests of the device included; powder flow bench tests and HVOF thermal spraying of graded deposits. The results revealed a calibration graph for the two powders in question and the compositional variation across the deposit during functionally graded deposition. The composition of the graded deposits were close to that anticipated hence this showed the suitability of the newly designed multi-powder deposition system in mixing two powders for the purpose of producing HVOF graded coatings.     

Numerical modeling of in-flight characteristics of inconel 625 particles during high-velocity oxy-fuel thermal spraying

A computational fluid dynamics (CFD) model is developed to predict particle dynamic behavior in a high-velocity oxyfuel (HVOF) thermal spray gun in which premixed oxygen and propylene are burnt in a combustion chamber linked to a long, parallel-sided nozzle. The particle transport equations are solved in a Lagrangian manner and coupled with the two-dimensional, axisymmetric, steady state, chemically reacting, turbulent gas flow. Within the particle transport model, the total flow of the particle phase is modeled by tracking a small number of particles through the continuum gas flow, and each of these individual particles is tracked independently through the continuous phase. Three different combustion chamber designs were modeled, and the in-flight particle characteristics of Inconel were 625 studied. Results are presented to show the effect of process parameters, such as particle injection speed and location, total gas flow rate, fuel-to-oxygen gas ratio, and particle size on the particle dynamic behavior for a parallel-sided, 12 mm long combustion chamber. The results indicate that the momentum and heat transfer to particles are primarily influenced by total gas flow. The 12 mm long chamber can achieve an optimum performance for Inconel 625 powder particles ranging in diameter from 20 to 40 μm. At a particular spraying distance, an optimal size of particles is observed with respect to particle temperature. The effect of different combustion chamber dimensions on particle dynamics was also investigated. The results obtained for both a 22 mm long chamber and also one with a conical, converging design are compared with the baseline data for the 12 mm chamber.