High Velocity Oxy-fuel Thermal Spraying Research Articles

The effect of cryomilling on the morphology and particle size distribution of the NiCoCrAlYSi powders with and without nano-sized alumina

Abstract Particle size is a crucial parameter in the High Velocity Oxy-Fuel thermal spray processing that directly affects coating microstructure and porosity, and eventually, coating strength, and hardness. In this work, commercially available NiCoCrAlYSi powder with and without 2 wt.% nano alumina powder particles were cryomilled for 8, 12 and 16 h. The microstructure and morphology of these powders were characterized by X-ray diffraction, scanning electron microscope and energy dispersive spectroscopy and particle size distribution obtained with Image Analyzer and Particle Size Analyzer. The average agglomerate size increased with milling time for NiCoCrAlYSi powder, but adding Al2O3 particles caused a reduction in average particle size after cryomilling. After 12 h cryomilling, the Al2O3 particles were distributed homogeneously inside NiCoCrAlYSi particles. Cryomilling of NiCoCrAlYSi powder led to the formation of flake-shaped agglomerates, but cryomilling of NiCoCrAlYSi powder mixed with nano-sized alumina after 12 h led to the formation of highly irregular agglomerates and particle size distribution. The broadening of characteristic Ni,Cr-rich (γ phase) and Ni3Al (γ′ phase) peaks with milling time and the absence of the β peak on the milled powder were observed.

터보불로워 용 회전체 주축 소재의 마찰, 마모 및 부식 저항 향상을 위한 WC-metal 분말의 초고속화염용사코팅

High velocity oxy-fuel (HVOF) spray coating of WC-metal powder (powder) was carried out to improve the resistances of friction, wear and corrosion of magnetic bearing shaft material Inconel718 (In718) of turbo blower. A micron sized WC-metal powder (86.5% WC, 9.5% Co 4% Cr) was coated onto In718 surface using HVOF thermal spraying. During the spraying, the binder metals and alloy such as Co, Cr and Co-Cr alloy were molten and a small portion of WC particles were partially decomposed to <TEX>$W_2C$</TEX> and free carbon at above its decomposition temperature of <TEX>$1250^{\circ}C$</TEX>. The free carbon and excessively sprayed oxygen formed carbon oxide gases, resulting a porous coating of porosity of <TEX>$2.2{\pm}0.3%$</TEX>. The surface hardness of substrate increased approximately three times from 400 Hv of In718 to <TEX>$1260{\pm}30Hv$</TEX> of the coating The friction coefficients of the coating were approximately <TEX>$0.33{\pm}0.03$</TEX> at <TEX>$25^{\circ}C$</TEX> and <TEX>$0.26{\pm}0.03$</TEX> at <TEX>$450^{\circ}C$</TEX>. These values were smaller than those of In718 substrate at both temperatures due to the lubrication from the free carbon and the cobalt oxide debris. The corrosion resistance of the coating was higher than that of In718 both in salt water of 3.5% NaCl and acid of 1 M HCl solutions, on the contrary, it was lower in base solution of 1 M NaOH. According to this study, the HVOF WC-metal powder coating is recommended for the durability improvement of magnetic bearing shaft of turbo blower.

Fabrication and Characterization of Aluminium Matrix Composites by High Velocity Oxy-Fuel Thermal Spraying

The High Velocity Oxy-fuel (HVOF) spray technique has been used by many researchers to deposit composite coatings, but there is less available literature discussing the fabrication of bulk metal matrix composites by this technique. In the current study, aluminum matrix composites with dispersions of alumina (Al2O3) and silicon carbide (SiC) particulates are fabricated by HVOF spray technique. The study comprises the steps of providing the substrate, preparing a mixed powder comprising a first metal powder as a matrix and a second metal powder comprising a ceramic and selected as reinforcement. The detailed procedures to prepare the bulk by using the same are reported. An in-depth characterization of the composite formed has been performed by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). Hardness of the composite formed is measured on Vickers microhardness tester. This study showed the possibility of fabricating bulk metal matrix composites of larger size by HVOF spray technique.

Influence of high velocity oxy-fuel parameters on the corrosion resistance of NiCr coatings

In the present study, the corrosion behavior of Nickel-base coatings on carbon steel as a substrate was analyzed. The samples were coated using a high velocity oxy-fuel thermal spray technique. In order to investigate the effects of spray variables on coating quality as compared with corrosion behavior of NiCr alloy in NaCl 3.5% solution, six design experiments were considered. The NiCr powder alloy contained 20% Cr. The sprays variables were fuel: oxygen, 1.07 and 1.25 both in reduction condition and also the powder feed rate 18, 27 and 36 g/min were selected. The high velocity oxy-fuel coated samples were corrosion tested using the potentiostatic polarization tester. Analyzing the polarization graph allowed the selection of the optimum operating conditions to achieve the desired specifications of the high velocity oxy-fuel coatings for their best corrosion resistance in the chosen environment. The samples structures were characterized by a scanning electron microscopy, energy-dispersive X-ray spectroscopy analysis and X-ray diffraction. The analysis of the results indicated that the fuel/oxygen and the powder feed rate have a significant effect on the porosity and corrosion resistance of these coatings. Corrosion behavior of the sample coated by fuel: oxygen 1.25 and powder feed rate 18 g/min was the best.

Effect of heat-treatment of spray-dried powder on properties of ultrafine-structured WC–Co coating

The ultrafine WC–Co composite powder was firstly synthesized by in situ reactions, and was then spray-dried and heat-treated to prepare the feedstock powder for the high velocity oxy-fuel thermal spraying. The effects of the heat-treatment of the spray-dried powder on the properties of the final ultrafine-structured WC–Co coating were studied in detail. The results show that increasing the heating temperature leads to a significant increase in the density and the flowability of the feedstock powder, together with the coarsening of the WC particles. The decarburization during the formation of the coating is effectively inhibited. As a result, the fracture toughness and abrasive wear resistance of the ultrafine-structured coating are enhanced due to the increased bonding strength of WC and Co inside the splats and also that between the neighboring splats. The present study suggests that controlling the heat-treatment process for the spray-dried powder is an effective approach to improve the properties of the ultrafine-structured WC–Co coatings.

High-Temperature Behavior of a High-Velocity Oxy-Fuel Sprayed Cr3C2-NiCr Coating

High-velocity oxy-fuel (HVOF) sprayed coatings have the potential to enhance the high-temperature oxidation, corrosion, and erosion-corrosion resistance of boiler steels. In the current work, 75 pct chromium carbide-25 pct (nickel-20 pct chromium) [Cr3C2-NiCr] coating was deposited on ASTM SA213-T22 boiler steel using the HVOF thermal spray process. High-temperature oxidation, hot corrosion, and erosion-corrosion behavior of the coated and bare steel was evaluated in the air, molten salt [Na2SO4-82 pct Fe2(SO4)3], and actual boiler environments under cyclic conditions. Weight-change measurements were taken at the end of each cycle. Efforts were made to formulate the kinetics of the oxidation, corrosion, and erosion-corrosion. X-ray diffraction (XRD) and field-emission scanning electron microscopy (FE-SEM)/energy dispersive spectroscopy (EDS) techniques were used to analyze the oxidation products. The coating was found to be intact and spallation free in all the environments of the study in general, whereas the bare steel suffered extensive spallation and a relatively higher rate of degradation. The coating was found to be useful to enhance the high-temperature resistance of the steel in all the three environments in this study.

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 cavitation erosion behavior of WC–Co–Cr coating on 1Cr18Ni9Ti stainless steel by HVOF thermal spraying

A WC–Co–Cr coating was deposited by a high velocity oxy-fuel thermal spray (HVOF) onto a 1Cr18Ni9Ti stainless steel substrate to increase its cavitation erosion resistance. After the HVOF process, it was revealed that the amorphous phase, nanocrystalline grains (Co–Cr) and several kinds of carbides, including Co3W3C, Co6W6C, WC, Cr23C6, and Cr3C2 were present in the coating. The hardness of the coating was improved to be 11.3GPa, about 6 times higher than that of the stainless steel substrate, 1.8GPa. Due to the presence of those new phases in the as-sprayed coating and its higher hardness, the cavitation erosion mass loss eroded for 30h was only 64% that of the stainless steel substrate. The microstructural analysis of the coating after the cavitation erosion tests indicated that most of the corruptions took place at the interface between the un-melted or half-melted particles and the matrix (Co–Cr), the edge of the pores in the coating, and the boundary of the twin and the grain in the stainless steel 1Cr18Ni9Ti.

G050044 超音速流を用いる高速フレーム溶射のOpenFOAMによる数値シミュレーション

G050044 超音速流を用いる高速フレーム溶射のOpenFOAMによる数値シミュレーション

An overview of slurry erosion control by the application of high velocity oxy fuel sprayed coatings

Slurry erosion of turbine components is a serious problem in most of the hydraulic power plants all around the world. Although some counter measures such as catchment area treatment to reduce the amount of sand particles going through the turbine components and modifying the hydraulic shape of turbine to reduce slurry erosion have been taken, still slurry erosion is responsible for heavy economic losses. Research is also in progress for improving the surface properties of the turbine components by the application of thermal sprayed coatings. Among various thermal spray processes, high-velocity oxy fuel (HVOF) spray process is a relatively industrially established and economical process. In this paper, an attempt has been made to describe HVOF thermal spray technology. Further, this paper presents a tutorial introduction, comprehensive history, and review of research work done on the use of this technique to provide protection against slurry erosion of some metals and alloys with a special emphasis on the hydraulic turbine steels.

Optimization and Characterization of High Velocity Oxy-fuel Sprayed Coatings: Techniques, Materials, and Applications

In this work High Velocity Oxy-fuel (HVOF) thermal spray techniques, spraying process optimization, and characterization of coatings are reviewed. Different variants of the technology are described and the main differences in spray conditions in terms of particle kinetics and thermal energy are rationalized. Methods and tools for controlling the spray process are presented as well as their use in optimizing the coating process. It will be shown how the differences from the starting powder to the final coating formation affect the coating microstructure and performance. Typical properties of HVOF sprayed coatings and coating performance is described. Also development of testing methods used for the evaluation of coating properties and current status of standardization is presented. Short discussion of typical applications is done.

Corrosion and Corrosion Fatigue Characteristics of Cast NAB Coated with NAB by HVOF Thermal Spray

Cast Ni-Al bronze (NAB) alloy, which is a most common material of ship propeller operating in sea water, is coated with the same NAB alloy by high-velocity oxy-fuel (HVOF) thermal spray process. Application of a HVOF NAB coating to cast NAB can be considered to enhance the resistance to corrosion fatigue failure of a NAB-based ship propeller and repair its damaged location at low cost. Linear sweep voltammetries, potentiostatic corrosion tests, and 80-day immersion corrosion tests are conducted in a 3.5% NaCl solution to investigate the corroding behavior of HVOF-NAB-coated cast NAB. Corrosion pits and microcracks through which the NaCl solution can easily permeate are developed from many pores included in the HVOF NAB coating. More serious corrosion compared to uncoated cast NAB occurs with producing corrosion products of Cu2O, Al2O3, and Fe2O3. Corrosion fatigue tests of both HVOF-NAB-coated cast NAB and uncoated cast NAB for reference are also conducted in the NaCl solution. Corrosion fatigue strength of the HVOF-NAB-coated cast NAB is measured as 140 MPa, which is much higher than 90 MPa of the uncoated cast NAB.

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.

Scanning electrochemical microscopy at thermal sprayed anti-corrosion coatings: Effect of thermal spraying on heterogeneous electron transfer kinetics

The effect of thermal spraying on the electrochemical activity of an anti-corrosion superalloy was studied quantitatively using scanning electrochemical microscopy (SECM). The superalloy used was Inconel 625 (a Ni base superalloy) and thin coatings of the alloy were formed on mild steel using high velocity oxy-fuel (HVOF) thermal spraying. The kinetics of electron transfer (ET) across the Inconel 625 coating/electrolyte interface were studied using SECM using ferrocenemethanol as the redox mediator. For comparison, the kinetics of ET across stainless steel/electrolyte and bulk wrought Inconel 625/electrolyte interfaces were also studied using SECM. The standard heterogeneous ET rate constant, k°, for ferrocenemethanol reduction at stainless steel was 1.0 ± 0.5 × 10 −3 cm s −1, compared to 2.6 ± 1.8 × 10 −2 cm s −1 at the wrought Inconel 625 surface. However, at the HVOF-sprayed Inconel 625 surface, the kinetics of ET varied across the surface and k° for ferrocenemethanol reduction ranged between ∼2.2 × 10 −4 cm s −1 and ∼2.6 × 10 −3 cm s −1. These results clearly demonstrate that SECM can be used to quantify the effect of thermal spraying on the electrochemical properties of Inconel 625 and that thermal spraying results in an electrochemically-heterogeneous surface.

Erosion–corrosion resistance of microcrystalline and near-nanocrystalline WC–17Co high velocity oxy-fuel thermal spray coatings

Cermet based coatings are being increasingly used to combat erosion–corrosion in oil sands pipelines and pumps where the degradation is caused by a slurry mix of sand particles and aqueous solution. This research assesses the erosion–corrosion resistance of cermet composite coatings obtained by HVOF thermal spraying of microcrystalline and ‘duplex cobalt coated’ near-nanocrystalline WC–17Co feedstock powders. Electrochemical measurements, surface characterization, and the extent of weight loss were studied through an impingement jet system. Results suggest that the erosion–corrosion mechanism in the coatings was dominated by pure erosion in the microcrystalline coating and corrosion-enhanced erosion in the near-nanocrystalline coating.

Experimental and numerical evaluation of the performance of supersonic two-stage high-velocity oxy-fuel thermal spray (Warm Spray) gun

The water-cooled supersonic two-stage high-velocity oxy-fuel (HVOF) thermal spray gun was developed to make a coating of temperature-sensitive material, such as titanium, on a substrate. The gun has a combustion chamber (CC) followed by a mixing chamber (MC), in which the combustion gas is mixed with the nitrogen gas at room temperature. The mixed gas is accelerated to supersonic speed through a converging-diverging (C-D) nozzle followed by a straight passage called the barrel. This paper proposes an experimental procedure to estimate the cooling rate of CC, MC and barrel separately. Then, the mathematical model is presented to predict the pressure and temperature in the MC for the specific mass flow rates of fuel, oxygen and nitrogen by assuming chemical equilibrium with water-cooling in the CC and MC, and frozen flow with constant specific heat from stagnant condition to the throat in the CC and MC. Finally, the present mathematical model was validated by comparing the calculated and measured stagnant pressures of the CC of the two-stage HVOF gun.

High temperature corrosion behaviour of HVOF-sprayed Ni-20Cr coating on boiler steel in molten salt environment at 900

Ni-20Cr coating was deposited on a commonly used boiler steel ASTM A213 347H by high-velocity oxy-fuel (HVOF) thermal spray process to enhance its high-temperature corrosion resistance. High temperature corrosion behaviour was investigated for the uncoated and HVOF sprayed Ni-20Cr coated boiler steel in a simulated boiler environment (Na2SO4-60%V2O5) at 900°C for 50 cycles. The information regarding this system of coating-substrate steel-coating process with regard to its high temperature corrosion is not available in the open literature. Each cycle consisted of one hour heating in the silicon carbide tube furnace followed by 20 min. cooling in air. Mass change technique was used to approximate the kinetics of corrosion. Techniques like X-ray diffraction (XRD) and scanning electron microscopy/energy dispersive (SEM/EDS) were used to characterise the corrosion products. It was observed that the uncoated steel suffered corrosion in the form of spalled powder and peeling up of its oxide scale. On the contrary, the Ni-20Cr coating provided considerable resistance to the corrosion. The Ni-20Cr coating was successful in reducing the mass gain of the steel by 77% which may be due to the formation of oxides of nickel and chromium in its oxide scale.

Theoretical Calculation of Pressure Characteristics in Two-Stage High-Velocity Oxy-Fuel Thermal Spray Gun Employing Supersonic Flow (Validation by Cold Flow and Actual Spray Gun Tests)

The theoretical calculation of a two-stage high-velocity oxy-fuel thermal spray gun was conducted for the purpose of predicting the stagnant pressure in the mixing chamber (MC). The theoretical results were compared with the experimental ones obtained by the cold flow and actual spray gun tests. The effect of the diffuser shape installed between the combustion chamber (CC) and the MC was also investigated in the cold flow test. It was clarified in this study that 1) the theoretical calculation shows that in order to obtain a larger pressure in the MC the diameter rations of d3t/d1t and d1e/d1t should be chosen to satisfy Eq.(11), to keep the diffuser throat unchoked, 2) the theoretical results of p03/p01 agree with the results of cold flow test within an accuracy of 6%, 3) the theoretical results of p01 agree with the results of actual two-stage HVOF gun test within an accuracy of 4%, demonstrating the validity of the present theoretical calculation.

Influence of high velocity oxy-fuel parameters on properties of nanostructured TiO2 coatings

A liquid fuel high velocity oxy-fuel (HVOF) thermal spray process has been used to deposit TiO2 nanostructured coatings utilizing a commercially available nanopowder as the feedstock. The coatings were characterized by means of X-ray diffraction analysis (XRD), scanning electron microscopy (SEM) and transmission electron microscope (TEM), respectively. Photocatalytic activity was evaluated as a rate constant of decomposition reaction of methylene blue (MB) determined from the changes of relative concentration of MB with UV irradiation time. The results indicate that the sprayed TiO2 coatings were composed of both TiO2 phases viz. anatase and rutile, with different phase contents and crystallite sizes. A high anatase content of 80% by volume was achieved at 0·00015, fuel-to-oxygen ratio with nanostructure coating by grain size smaller than feedstock powder. Photocatalytic activity evaluation results indicated that all the TiO2 coatings are effective to degradation MB under UV radiation and their activities differ in different spray conditions. It is found that fuel flow rate strongly influenced on phase transformation of anatase to rutile and by optimizing the rate which can promote structural transformation and grain coarsening in coating and improving photocatalytic activity.

PELAPISAN PERMUKAAN DALAM NOSEL ROKET RKX100 DENGAN Cr2C3-NiCr HVOF: OPTIMASI KEKUATAN LEKAT MELALUI VARIASI KEKASARAN PERMUKAAN

Nozzle of RKX100 rocket contributes 30 percent to the total weight of the structure, so that allowing further research on weight reduction. An alternative for this is by substitution of massive graphite, which is currently used as thermal protector in the nozzle, with thin layer of HVOF (High Velocity Oxy-Fuel) thermal spray layer. A series of study on the characteristics of various type of HVOF coating material have been being conducted. This paper presented the investigation on the HVOF Cr2C3-NiCr thermal spray coating, in particular, the optimization of bonding strength by varying surface roughness of substrates. Characterization included bonding strength test, micro hardness measurement and micro structural observation with optical microscope and scanning electron micriscope (SEM). The results showed that grit blasting pressure increass the surface roughness from 4,54 um to 5.72 um at the pressure of 6 bar. Average micro hardness of the coating was 631 VHN 300. Coating applied to the surface with rougness of 5.42 um possessed the highest bonding strength, 44 MPa. Microstructural observation by using optical microscope and scanning electron microscope (SEM) confirmed dense lamellae structure with variable composition. High coating adherence was found to be due to mechanical interlocking.