Thermal Spray System Research Articles

Microstructure and Selected Properties of WC-Co-Cr Coatings Deposited by High Velocity Thermal Spray Processes

Selected information about high velocity thermal spraying method were presented in this article. Three generations of thermal spraying processes based on oxygen with propane mixture, liquid fuel with oxygen and propane with air were characterized. A powder of WC-Co-Cr 86-10-4 was used for coatings deposition on a steel substrate. Four coatings were deposited by different thermal spraying systems such as Diamond Jet, JP 5000, Micro HVOF and HVAF.

Solid particle erosion behavior of HVOF/HVAF sprayed WC-Co-Cr coatings

The erosion behavior of three WC-Co-Cr coatings obtained by high velocity oxy-fuel/air-fuel (HVOF/HVAF) thermal spray systems has been investigated using Al2O3 as erodent at four impingement angles (15°, 45°, 75°, 90°) and 90 m/s impact velocity. The results indicate that the micro-cutting and coatings spallation are main erosion mechanisms for coatings, and the proportion of the two mechanisms changes depending on the impingement angle. The ranking of the coatings in terms of erosion resistance is very much dependent on the microstructure of coatings. The behavior of formation and propagation of cracks also depends on the microstructure of coatings, and has been understood in terms of schematic diagrams.

Recent development of experimental and numerical analysis of adhesion strength of thermal spray coatings

Some important developments of adhesion strength research for thermal spray coating systems have been reviewed in the present paper. The following topics were included: the influence of spraying pre-treatment on adhesion strength, sources and mechanisms of adhesion strength in coatings, some algorithms and calculation of numerical models based on the energy release rate in fracture mechanics, and the effects and relative discussions of residual stress in thermal spray coatings on the adhesion strength. These topics would provide some important insights into life prediction of thermal spray systems based on the adhesion strength.

Effect of Particle and Injection Parameters on the Performance of a Dual-Stage High-Velocity Oxygen Fuel Thermal Spray System

For temperature-sensitive material (such as titanium) coatings, recently developed high-velocity oxygen fuel dual-stage thermal spray systems offer better control of particle oxidation and production of various coating structures. These advantages of the dual-stage thermal system are significantly influenced by the state of the coating particles being injected. Hence, the objective of the present study is to investigate the effects of particle size, shape, injection velocity, and injection angle on a dual-stage thermal spray system by employing a comprehensive mathematical model. The results demonstrate that the particle size, shape, injection velocity, and injection angle affect the particle velocity and temperature, which in turn may affect the coating quality. The results show that smaller particles have higher temperatures and velocities owing to decrease in particle thermal and mass inertia. Spherical particles have higher temperature and lower velocity than the non-spherical particles because of lower drag. The particle velocity and temperature also increase with the increase of the injection velocity. Similarly, the particles injection angle also plays an important role. Higher particle temperatures and velocities outside of the barrel are obtained if the particles are injected at oblique angles to the main gaseous flow.

Effect of Plasma Gases on the Structure and Properties of WC-CrC-Ni Coatings

The effect of plasma gases (argon, helium) on the structure and properties of the WC-CrC-Ni coatings deposited at the graphite substrate has been investigated. The coatings were deposited by plasma spraying method in equipment Cham Pro –Thermal Spray Systems produced by Sulzer company. The microstructure of coatings were investigated by light microscopy (MO) and scanning electron microscopy (SEM). The performed investigations have been shown that with the increasing of amounts of argon and reducing volumes of helium and nitrogen, the microhardnees increased. The plasma gases also influenced on the porosity of coatings. Keywords: WC-CrC-Ni coatings, plasma spraying, microstructure, porosity Acknowledgement: The work was supported by project No INNOTECH – K2/IN2/9/181851/NCBR/13

Bond Coat Engineering Influence on the Evolution of the Microstructure, Bond Strength, and Failure of TBCs Subjected to Thermal Cycling

Different types of thermal spray systems, including HVOF (JP5000 and DJ2600-hybrid), APS (F4-MB and Axial III), and LPPS (Oerlikon Metco system) were employed to spray CoNiCrAlY bond coats (BCs) onto Inconel 625 substrates. The chemical composition of the BC powder was the same in all cases; however, the particle size distribution of the powder employed with each torch was that specifically recommended for the torch. For optimization purposes, these BCs were screened based on initial evaluations of roughness, porosity, residual stress, relative oxidation, and isothermal TGO growth. A single type of standard YSZ top coat was deposited via APS (F4MB) on all the optimized BCs. The TBCs were thermally cycled by employing a furnace cycle test (FCT) (1080 °C-1 h—followed by forced air cooling). Samples were submitted to 10, 100, 400, and 1400 cycles as well as being cycled to failure. The behavior of the microstructures, bond strength values (ASTM 633), and the TGO evolution of these TBCs, were investigated for the as-sprayed and thermally cycled samples. During FCT, the TBCs found to be both the best and poorest performing and had their BCs deposited via HVOF. The results showed that engineering low-oxidized BCs does not necessarily lead to an optimal TBC performance. Moreover, the bond strength values decrease significantly only when the TBC is about to fail (top coat spall off) and the as-sprayed bond strength values cannot be used as an indicator of TBC performance.

Effect of Operating Parameters on a Dual-Stage High Velocity Oxygen Fuel Thermal Spray System

High velocity oxygen fuel (HVOF) thermal spray systems are being used to apply coatings to prevent surface degradation. The coatings of temperature sensitive materials such as titanium and copper, which have very low melting points, cannot be applied using a single-stage HVOF system. Therefore, a dual-stage HVOF system has been introduced and modeled computationally. The dual-spray system provides an easy control of particle oxidation by introducing a mixing chamber. In addition to the materials being sprayed, the thermal spray coating quality depends to a large extent on flow behavior of reacting gases and the particle dynamics. The present study investigates the influence of various operating parameters on the performance of a dual-stage thermal spray gun. The objective is to develop a predictive understanding of various parameters. The gas flow field and the free jet are modeled by considering the conservation of mass, momentum, and energy with the turbulence and the equilibrium combustion sub models. The particle phase is decoupled from the gas phase due to very low particle volume fractions. The results demonstrate the advantage of a dual-stage system over a single-stage system especially for the deposition of temperature sensitive materials.

Investigation of a dual-stage high velocity oxygen fuel thermal spray system

The high velocity oxygen fuel (HVOF) thermal spray coatings are used to protect the surfaces from deterioration. The base material surface properties can be modified to achieve the longevity of the product. Besides spraying material, the coating quality depends greatly on the gas and particle dynamics. The coating quality is also affected by the particle temperature, particularly for the materials which are temperature sensitive such as titanium and copper. As the gas phase temperature is high, the material gets melted and oxidized before it reaches the substrate. To avoid this problem, a dual-stage thermal spray system, has been developed for coating temperature sensitive materials. This process involves making coatings by high velocity impact of powder particles heated to temperatures below their melting point. The advantages of a dual-stage thermal spray process include an easy control of particle oxidation and production of various coating structures by controlling the particle velocity and temperature. The particle temperature can be controlled by varying the coolant flow rate in the mixing chamber.The present study investigates the effect of various geometric parameters of a dual-stage thermal spray system by developing a comprehensive mathematical model. The objective is to develop a predictive understanding of various design parameters. Conservation of mass, momentum and energy of reacting gases were taken into account in developing the model. Due to low particle loading, the particle phase was decoupled from the gas phase. The results demonstrate the advantage of dual stage system over single stage system especially for the deposition of temperature sensitive materials.

Fabrication of Wire Mesh Heat Exchangers for Waste Heat Recovery Using Wire-Arc Spraying

Waste heat can be recovered from hot combustion gases using water-cooled heat exchangers. Adding fins to the external surfaces of the water pipes inserted into the hot gases increases their surface area and enhances heat transfer, increasing the efficiency of heat recovery. A method of increasing the heat transfer surface area has been developed using a twin wire-arc thermal spray system to generate a dense, high-strength coating that bonds wire mesh to the outside surfaces of stainless steel pipes through which water passes. At the optimum spray distance of 150 mm, the oxide content, coating porosity, and the adhesion strength of the coating were measured to be 7%, 2%, and 24 MPa, respectively. Experiments were done in which heat exchangers were placed inside a high-temperature oven with temperature varying from 300 to 900 °C. Several different heat exchanger designs were tested to estimate the total heat transfer in each case. The efficiency of heat transfer was found to depend strongly on the quality of the bond between the wire meshes and pipes and the size of openings in the wire mesh.

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.

Influence of high velocity oxygen-fuel spraying parameters on the wear resistance of Al–SiC composite coatings deposited on ZE41A magnesium alloy

High velocity oxygen-fuel (HVOF) has been used as fabrication technique to deposit aluminium coatings reinforced with silicon carbide particles on Mg–Zn substrates. The aim of the investigation is to improve the tribological performance of the ZE41A magnesium alloy. The parameters of the thermal spraying system have been optimized in order to maximize the SiC particles incorporation in the aluminium matrix of the coating and to minimize the mechanical deterioration of the light alloy substrate. Pin-on-disc tests were developed to characterize the tribological behavior of the different specimens. Composite coatings with thicknesses of 120μm, reinforced with ∼10wt% and with high adhesion to the substrate were achieved. The wear resistance of the substrates was increased and the wear rate decreased in two orders of magnitude respect to that of the bare Mg-alloy after the optimization of the spraying parameters.

On the probabilistic particle simulation of an arcjet flow expansion

We apply a particle code (Direct Simulation Monte Carlo, DSMC) to a nozzle flow expansion set-up which is typical for thermal plasma spray systems. Although those systems tend to have a pressure level which is too high to be treated with DSMC we obtain good results with respect to Pitot pressure measurement based Mach number estimations at different axial distances from the nozzle exit plane. Precisely, we compare the Mach numbers at 0, 30, 60, and 90 mm distance from the nozzle exit plane which was chosen as inflow boundary. The relative deviation of the simulated Mach numbers (Ma) from the measured is mostly about 10%. An influence of the flow field upstream due to a static boundary condition downstream is also observed. The vital code extensions which allowed this simulation with DSMC are briefly discussed. Further code extensions for future research activities are outlined.

A study of liquid droplet disintegration for the development of nanostructured coatings

AbstractThermal spray coatings produced from a liquid feedstock are receiving an increasing level of interest due to the advanced, nanostructured coatings which are obtainable by these processes. In this article, a high‐velocity oxy‐fuel (HVOF) thermal spray system is computationally investigated to make a scientific assessment of the liquid droplet behavior on injection. An existing liquid‐fuelled HVOF thermal spray gun is simulated using the computational fluid dynamic approach. The steady‐state gas‐phase dynamics are initialized by the introduction of liquid kerosene and oxygen which react within the combustion chamber producing a realistic compressible, turbulent jet. Discrete‐phase water droplets are injected at the powder injection port. On injection, the water droplets breakup and vaporize, while being entrained through the acceleration barrel of the HVOF system. The results obtained give an insight to the mechanism which control the water droplet sizes and disintegration process, and serve as a fundamental reference for future development of liquid feedstock devices. © 2012 American Institute of Chemical Engineers AIChE J, 2012

Microstructure and Properties of HVOF Spraying WC-10Co-4Cr Coatings

Three WC-10Co-4Cr coatings were deposited with various spraying parameters (6 and 6.5 GPH kerosene flux, 1850 and 1950 SCFH oxygen flux, and 330 and 380mm spray distance) by using Parxair JP-8000 HVOF thermal spray system. The coating properties such as hardness, crystalline phase, fracture toughness, thickness per pass and abrasive wear resistance have been investigated. The results indicates that kerosene flux, oxygen flux, and spray distance have remarkable effects on coatings microstructure and performance, but only slightly effect on phase of coatings. The hardness of WC-10Co-4Cr coatings increases with the kerosene flux and oxygen flux. Both the wear loss and the fracture toughness decrease with the increasing of coating hardness. Also, the WC-10Co-4Cr coatings exhibit excellent wear resistance compared to hardening and tempering 45 steel.

Studies of Rapid Tooling Spraying Technology Using Low Melting Metal Alloy

To manufacture rapid metal tools, an innovative method based on rapid prototyping and high pressure spraying technologies is proposed in this study. The Sn-Bi metal alloy with low melting point and small thermal shrink was used as the spraying materials. Applying an electric heated device and high pressure boost pump, the melting Sn-Bi Alloy went through a small spray nozzle and atomized into semi-melting minute particles. The atomized particles are then deposited and integrated on the surface of intermediate silicone rubber mold, which is made from the rapid prototyping (RP) system and are successfully used to manufacture rapid wax injection mould. Using the thermal spraying system to manufacture rapid metal mold, the surface quality and the dimensional accuracy of the spray mold is excellent and the fabricated cycle time is short.

Analysis of WC/Ni-Based Coatings Deposited by Controlled Short-Circuit MIG Welding

This study investigates the recently developed controlled short-circuit metal inert gas (CSC-MIG) welding system for depositing WC/Ni-based claddings on carbon steel substrates. WC/Ni-based coatings deposited by CSC-MIG were analyzed by optical light microscopy and scanning electron microscopy (SEM) equipped with energy dispersive spectroscopy (EDS) and electron backscatter diffraction (EBSD) capabilities. X-ray diffraction (XRD) and hardness measurements of depositions are also reported. The CSC-MIG welding system provides a significant amount of user control over the current waveform during welding and has lower heat input when compared with traditional MIG welding. Heat input for the analyzed coatings ranged from 10.1 to 108.7 J/mm. Metallurgically bonded coatings free from spatter and with 0.75% average porosity were produced. It was found that the detrimental decarburization of the WC particles seen in thermal spray systems does not occur when welding with the CSC-MIG. Precipitation of a reaction layer around the reinforcing phase was identified as WC; the average thickness of which increases from 3.8 to 7.2 μm for the low and high heat input condition, respectively. Precipitation of newly formed WC particles was observed; their size distribution increased from D50 of 2.4 μm in the low heat input weldment to 6.75 μm in the high heat input weldment. The level of dilution of the reinforcing phase increases significantly with heat input. The hardness of the deposited coatings decreases from 587 HV10 to 410 HV10 when the energy input was increased from 10.1 to 108.7 J/mm.

Study on Process Optimization of Cold Gas Spraying

Cold gas dynamic spraying is a relatively new spray coating technique capable of depositing a variety of materials without extensive heating. As a result the inherent degradation of the powder particles found during traditional thermal spraying can be avoided. The simplicity of this technique is its most salient feature. High pressure gas is accelerated through a convergent-divergent nozzle up to supersonic velocity. The powder particles are carried to the substrate by the gas and on impact the particles deform at temperatures below their melting point. Computational modeling of thermal spray systems can provide thorough descriptions of the complex, compressible, particle-laden flow, and therefore can be utilized to strengthen understanding and allow technological progress to be made in a more systematic fashion. The computational fluid dynamic approach is adopted in this study to examine the effects of changing the nozzle cross-section shape, particle size and process gas type on the gas flow characteristics through a cold spray nozzle, as well as the spray distribution and particle velocity variation at the exit.

Computational simulation of liquid-fuelled HVOF thermal spraying

Liquid-fuelled high-velocity oxygen–fuel (HVOF) thermal spraying systems are gaining more attentions due to their advantage of producing denser coatings in comparison to their gas-fuelled counterparts. The flow through a HVOF gun is characterized by a complex array of thermodynamic phenomena involving combustion, turbulence and compressible flow. Advanced computational models have been developed to gain insight to the thermochemical processes of thermal spraying, however little work has been reported for the liquid-fuelled systems. This investigation employs a commercial finite volume CFD code to simulate the flow field through the most widely used liquid-fuel HVOF gun, JP5000 (Praxair, US). By combining numerical combustion and discrete phase models the turbulent spray flame is captured and the development of supersonic gas flow is revealed. The flow field is thoroughly examined by adjusting the nozzle throat diameter and combustion chamber size. The influence of fuel droplet size on the flame shame shape and combusting gas flow is also examined.

A new method for thermal spraying of Zn–Al coatings

This paper presents the development of the thermal spraying system built on the principles of the high velocity air flame (HVAF) process. HVAF sprayed coatings showed considerably higher bond strength than coatings obtained by the conventional methods, indicating the advantage of this method in areas where the adhesion strength is critically important. The highly dense structure of the coating obtained with HVAF eliminates a need for a top paint coat, which is typically applied on metal sprayed coatings to extend service life. The thermal sprayed coatings were characterized by the standard techniques, such as light microscopy, scanning electron microscopy with energy-dispersive spectroscopy, X-ray diffraction, salt spray and bond strength tests. The results show that thermal sprayed coatings have a dense structure, low presence of oxides and high resistance to corrosion. High spray rate and good coating quality make the HVAF thermal spray method a viable alternative to the conventional thermal spraying technologies, such as Wire Flame and Twin-Wire Arc.

Efficient stochastic simulation of thermal spray processes

Thermal spray processes allow the production of coatings with a wide variety of useful properties, and therefore are used in a lot of different industrial applications. However, in specific applications, it can be a challenging and time consuming task to tune the parameters of a thermal spraying system so that required surface properties, such as wear- , corrosion- or heat resistance, can be achieved. Computer-aided stochastic simulations of thermal spray processes try to overcome these problems by modelling the coating build-up on a per-droplet basis, and by deriving the properties of interest from the generated structure afterwards. Because of the large number of droplets that need to be treated during the simulation to generate a sufficiently large virtual coating, the use of efficient algorithms is essential. On the other hand more complex algorithms are needed to increase the accuracy of the simulation. In this report we present novel extensions to a simulation model by Ghafouri et al. concerning plasma spraying. The extensions include a physically inspired splat spreading approach, a method to generate splat fingers, and a filtering technique that allows for the calculation of pores below overlapping splats. Furthermore, features characterizing coatings beyond the commonly used porosity or surface roughness are presented and analyzed with respect to their appropriateness. Features are considered appropriate if they are depending in a unique way on the parameters of the simulation model. This kind of features might be used in an automatic adaptation of the simulation model to real processes. Finally, several contributions are made in order to increasing the computational efficiency of the simulation. Most important, dexel models are introduced to represent the coating.