High-velocity Oxygen-fuel Spraying Research Articles

Microstructures and sliding wear resistances of 0.2% carbon steel coatings deposited by HVOF and PTWA thermal spray processes

Microstructures, and sliding wear resistances of low carbon (0.2% C) steel coatings produced using a plasma transfer wire arc (PTWA) process, and a high velocity oxygen fuel (HVOF) thermal spray process were compared. The wire feed stock used for both coatings was low carbon steel with a nominal 1020 composition. Results are also presented for HVOF coatings that had a 2.5 at.% Al addition to the steel wire feed stock. All thermal spray coatings had a layered microstructure consisting of steel splats and oxide veins between them. Detailed microstructural analyses showed that important differences existed in the microstructures and local compositions of coatings. Wear tests were performed using a pin-on-disc type wear tester within a load range of 10–75 N and a sliding speed range of 0.2–2.5 m/s against tool steel pin in a dry air atmosphere (10% RH). Severe deformation of the steel splat tips on the contact surface, and their fragmentation was responsible for high wear rates at low speeds and high loads. Wear rates of PTWA 1020 and HVOF 1020—2.5% Al coatings decreased with increasing the sliding speed. However, the wear rates of HVOF 1020 coatings increased considerably at high sliding speeds, and this was associated with formation of thick oxide layers on its sliding surfaces, which reached 10 μm at 75 N and 2 m/s. The wear mechanisms responsible for the differences in the sliding wear behaviour of the coatings are discussed in terms of differences in the microstructures of the coatings.

Effect of sprayed powder particle size on the oxidation behavior of MCrAlY materials during high velocity oxygen-fuel deposition

The NiCrAlY powder particles of different sizes were used for high velocity oxygen-fuel (HVOF) spraying. The oxidation behavior at processing resulting in the oxide content inside sprayed coatings was investigated. Moreover, the powder passing through the HVOF flame was collected for the examination of the in-flight oxidation. The oxygen contents in coatings and collected powders were estimated by chemical analysis. It was found that the oxygen contents in the collected powders and coatings depended on the particle size. With decreasing in particle size, the oxygen content in the collected powders increased exponentially. The oxygen content inside the coating was approximately equal to that in the collected powders for the mean particle size of <50 μm. For large particles (>50 μm), there was a clear difference in the oxygen contents inside the coating and in the collected powders. This difference can be attributed to the post-impact oxidation of a splat. The results suggested that there are two essential oxidation mechanisms: an in-flight oxidation and a post-impact one. When the small particles are used, the in-flight oxidation determines the oxygen content in the coating. When large particles are used, the post-impact oxidation controls the oxygen content in the coating.

Noise Emissions in Thermal Spray Operations

Acoustic noise generation is an accompanying effect produced during thermal spraying. This type of noise is found both during the preparatory stages, such as grit blasting and compressed air cleaning, and during thermal spraying. A real-time noise meter was used to measure the noise level at frequencies between 63 and 8000 Hz during the operation of powder flame, wire flame, wire arc, air plasma, and high velocity oxygen fuel (HVOF) spraying processes. Noise was reported as either an A-weighted noise spectrum or an equivalent sound pressure level. The effect of different parameters, such as secondary plasma gas type, modes of wire flame torch operation, and use of compressed air cooling were investigated. The results indicated that the turbulence of the gas departing from the torch gives rise to jet noise. High gas flows mainly contributed to the lower frequencies, whereas combustion and plasma generation contributed to the higher frequencies. Noise level was the highest (123 dB(A)) with HVOF spraying and air plasma spraying with the use of a small-diameter nozzle and hydrogen as a secondary plasma gas. All manual operators of thermal spray equipment require hearing protection. The use of different hearing protection devices is discussed and the attenuation provided by each device is reported.

Microstructural Evolution in Nanocrystalline Ni Coatings

Gas-atomized Ni powders were mechanically milled under liquid nitrogen and subsequently sprayed using a high-velocity oxygen-fuel thermal spraying system. The resultant coatings were evaluated using various analytical methods, including scanning electron microscopy (SEM), transmission electron microscopy, and atom probe field ion microscopy (APFIM). The results indicated that the majority of the grains in nanocrystalline Ni coating were equiaxed with an average grain size of 50 ± 23 nm. SEM elemental dot mapping analysis suggested that oxide particles were distributed along the prior droplet boundaries and interpass boundaries in the coatings. The APFIM analysis indicated that the distribution of oxygen was very inhomogeneous in the nanocrystalline coating samples studied. Mostly small NiO precipitates with a size range of 1–7 nm were found throughout the grains, and the average size of these precipitates was approximately 4 nm. However, large oxide precipitates with a size up to 55 nm were also observed. In addition, increased amounts of oxygen were found at the grain boundaries and at structural defects.

高圧高速フレーム溶射ＷＣ‐Ｃｏ皮膜のはく離強度に及ぼす繰返し面圧の影響

Substrate specimens of mild steel (JIS: SS400) and tool steel (JIS: SKD5) were coated with WC-Co cermet by High-Pressure High-Velocity Oxygen Fuel (HP-HVOF) spraying. Repeating pressure was applied on the surface of the specimens, and tensile tests and edge-indent tests were carried out to evaluate the tensile strength and the delamination strength of the coating.The tensile strength σC of WC-Co coating increases first and then decreases with increasing surface pressure cycles N when the maximum repeating pressure σmax is 240MPa, while σC increases with increasing N for σmax=480MPa. Both the interfacial fracture toughness Gc1/2 and the delamination strength Ed for SS400 specimens decreases with increasing N. On the other hand, Ed for SKD5 specimens does not change with N in each σmax. The SEM micrographs of cross section of coating show the existence of cracks parallel to the interface for SS400 specimens and they are developed with increasing pressure cycles, while the crack extension is small for SKD5 specimens.

Effect of the Increase in the Entrance Convergent Section Length of the Gun Nozzle on the High-Velocity Oxygen Fuel and Cold Spray Process

Nozzle geometry, which influences combustion gas dynamics and, therefore, sprayed particle behavior, is one of the most important parameters in the high-velocity oxygen-fuel (HVOF) thermal spray process. The nozzle geometry is also important in the cold spray method. The gas flows in the entrance convergent section of the nozzle exhibit a relatively higher temperature and are subsonic; thus, this region is most suitable for heating spray particles. In this study, numerical simulation and experiments investigated the effect of the entrance geometry of the gun nozzle on the HVOF process. The process changes inside the nozzle, as obtained by numerical simulation studies, were related to the coating properties. An Al2O3-40 mass% TiO2 powder was used for the experimental studies. The change in entrance convergent section length (rather than barrel part length or total length) of the gun nozzle had a significant effect on the deposition efficiency, microstructure, and hardness. The deposition efficiency and hardness increased as this geometry increased. On the other hand, the calculated and measured particle velocity showed a slight decrease. This effect on the HVOF process will also be applied to the nozzle design for the cold spray method.

Amorphous phase formation of Zr-based alloy coating by HVOF spraying process

This investigation was conducted to clarify the effects of process parameters on the formation of the new amorphous coating using a Zr-based alloy, which is known as bulk metallic glass forming alloy, by a HVOF (High Velocity Oxygen Fuel) spraying process. Powders used for spraying was prepared by vacuum gas atomization and then crushed by a centrifugal mill. HVOF spraying experiments were carried out using a Tafa JP-5000 spraying gun. DTA (Differential Thermal Analysis) measurements have shown that the amorphous content of the coatings was measured up to about 62% depending on the spraying process parameters. The amorphous fraction of the coatings is decreased with increasing the spray distance and the fuel flow rate. Microstructural observations and X-ray diffraction analysis of the spray coated layers reveal that the amorphization behavior during the spraying is attributed to the degree of the solidification of droplets and the oxide (ZrO2) formation in spray coated layers. Therefore, flame temperature and spray distance that can control the carrier gas temperature and undercooling effects of the droplets are the most crucial factors for the evolution of the amorphous phase using this bulk metallic glass forming alloy.

Development of Coating Structure and Adhesion During High Velocity Oxygen-Fuel Spraying of WC-Co Powder on a Copper Substrate

This paper deals with the mathematical modeling of the development of the WC-Co coating structure and adhesion on a copper substrate during high velocity oxygen-fuel (HVOF) spraying. Two types of substrates are considered: smooth (polished) and rough (grit blasted). Variations of solidification time, solidification velocity, thermal gradient, and cooling velocity in the coating and substrate interfacial region are studied. Development of the amorphous and crystalline structures in the coating and of the crystalline structure in the substrate interfacial region is discussed. Behavior of the crystal size and intercrystalline distance with respect to the thermal spray parameters and morphology of the substrate surface is analyzed. Optimal conditions for the formation of fine and dense crystalline structure are determined. Structural changes in the solid state of the substrate occurring because of heating and rapid cooling are considered. Mechanical and thermal mechanisms of development of the substrate-coating adhesion are discussed. Results obtained agree well with experimental data.

Microstructure and Properties of Tungsten Carbide Coatings Sprayed with Various High-Velocity Oxygen Fuel Spray Systems

This article reports on a series of experiments with various high-velocity oxygen fuel spray systems (Jet Kote, Top Gun, Diamond Jet (DJ) Standard, DJ 2600 and 2700, JP-5000, Top Gun-K) using different WC-Co and WC-Co-Cr powders. The microstructure and phase composition of powders and coatings were analyzed by optical and scanning electron microscopy and x-ray diffraction. Carbon and oxygen content of the coatings were determined to study the decarburization and oxidation of the material during the spray process. Coatings were also characterized by their hardness, bond strength, abrasive wear, and corrosion resistance. The results demonstrate that the powders exhibit various degrees of phase transformation during the spray process depending on type of powder, spray system, and spray parameters. Within a relatively wide range, the extent of phase transformation has only little effect on coating properties. Therefore, coatings of high hardness and wear resistance can be produced with all HVOF spray systems when the proper spray powder and process parameters are chosen.

Characteristics of MCrAlY Coatings Sprayed by High Velocity Oxygen-Fuel Spraying System

High velocity oxygen-fuel (HVOF) spraying system in open air has been established for producing the coatings that are extremely clean and dense. It is thought that the HVOF sprayed MCrAlY (M is Fe, Ni and/or Co) coatings can be applied to provide resistance against oxidation and corrosion to the hot parts of gas turbines. Also, it is well known that the thicker coatings can be sprayed in comparison with any other thermal spraying systems due to improved residual stresses. However, thermal and mechanical properties of HVOF coatings have not been clarified. Especially, the characteristics of residual stress, that are the most important property from the view point of production technique, have not been made clear. In this paper, the mechanical properties of HVOF sprayed MCrAlY coatings were measured in both the case of as-sprayed and heat-treated coatings in comparison with a vacuum plasma sprayed MCrAlY coatings. It was confirmed that the mechanical properties of HVOF sprayed MCrAlY coatings could be improved by a diffusion heat treatment to equate the vacuum plasma sprayed MCrAlY coatings. Also, the residual stress characteristics were analyzed using a deflection measurement technique and a X-ray technique. The residual stress of HVOF coating was reduced by the shot-peening effect comparable to that of a plasma spray system in open air. This phenomena could be explained by the reason that the HVOF sprayed MCrAlY coating was built up by poorly melted particles. [S0742-4795(00)00701-8]

腐食防食 ＷＣ系サーメット溶射皮膜の腐食速度

The corrosion rates of thermal sprayed WC cermet coatings in 0.05mol/dm3 sodium sulfate solutions at pH 1.0, 6.5 and 12.0 have been measured using AC impedance method and quantitative analysis of the solutions. The cermet coating films were made of WC-Co, WC-Co-Cr and WC-Cr-Ni powder matrials by the high velocity oxygen-fuel spraying method. The WC-Co coating film has shown the lowest corrosion rate in the acidic solution. The WC-Co-Cr and WC-Cr-Ni coating films have shown better corrosion resistance in the neutral and the basic solutions. It was found that corrosion current measured by AC impedance corresponds to that calculated by the quantitative yield of dissolved metal ions.

Thermal Processes in HVOF Sprayed WC-Co Coating on a Copper Substrate

Mathematical modelling of the heat transfer between a WC-Co coating and a copper substrate during high-velocity oxygen-fuel (HVOF) spraying was undertaken. The modeling included the investigation of temperature variation, coating solidification, melting and solidification in the substrate interfacial region, and specific features of the substrate-coating thermal interaction.

ＷＣ系サーメット溶射皮膜の分極特性に及ぼすクロムの効果

Thermal-spray WC cermet coatings corrosion by high velocity oxygen-fuel spraying was studied using potentiodynamic and potentiostatic polarization in an aerated sodium sulfate solution. The cermet coating was the WC-Co, WC-Co-Cr, and WC-Cr-Ni matrials on the carbon steel substrate. The anodic polarization curves of WC cermet coating were passive, transpassive, and reqired oxygen evolution. Cathodic polarization curves show a limited diffusion current of oxygen and hydrogen evolution. Coatings tended to decrease passive current density with increasing pH of solution. The presence of Cr in the matrix alloy decreases passive current and controll the dissolution of WC and Co. The Ni-Cr alloy matrix without Co showed no dissolved WC.

Novel Design and Fabrication of Thermal Battery Cathodes Using Thermal Spray

ABSTRACTLi-Alloy/FeS2 thermal batteries are the predominant thermal battery chemistry today. Conventional electrodes are fabricated by cold pressing of powders. A better means of providing thin electrodes would dramatically increase volumetric and gravimetric energy densities and cost efficiency of thermal batteries. In this study, experiments were conducted on fabricating the cathode via high-velocity oxygen-fuel (HVOF) and dc-arc plasma thermal spray technique. The deposited films were characterized by cross-section examination using Scanning Electron Microscopy (SEM) and X-ray Diffraction. The thermal decomposition of pyrite was suppressed by a proprietary additive. The electrochemical test results showed that pyrite cathodes prepared by dc-arc plasma spraying with additives demonstrated better performance compared traditional pressed-powder electrodes.

Thermal spraying of nanocrystalline inconel 718

Nanocrystalline Inconel 718 was thermal sprayed utilizing a HVOF (High Velocity Oxygen Fuel) thermal spraying facility. First, nanocrystalline powder was produced by high energy ball milling of Inconel 718 (average particle size, 10μm) under methanol and gaseous nitrogen. The ball milled powder was then processed by HVOF to produce a coating 250 μm thick. The nanocrystalline Inconel 718 coating exhibited a significant increase in hardness (approximately 60%) over that of the Inconel 718 control sample after the thermal spray process. The grain sizes of the as-received, as-ball milled, and HVOF processed materials were 10μm, 25nm, and 32nm, respectively, as determined by X-ray diffraction. A scanning electron microscopy (SEM) analysis of the powders and coating exhibited typical morphologies of as-received and as-ball milled materials. After HVOF spraying, some of the particles appear to have gone through the processing without complete melting.

Residual Stress Characteristics of High-Velocity Oxygen-Fuel Coatings.

A high-velocity oxygen-fuel (HVOF) system in air has been established for producing coatings that are extremely clean and dense. It is thought that the HVOF-sprayed MCrAlY (M is Ni, Co or both) coatings can resist oxidation and corrosion in the hot section of gas turbines. Also, it is known that in order to improve the residual stress coatings thicker than those from other thermal spraying systems can be sprayed. However, residual stress characteristics of HVOF coatings remain to be clarified. In this paper, the residual stress characteristics were analyzed using the deflection measurement technique and the X-ray technique. It was confirmed that the residual stress of HVOF coatings was reduced by the shot-peening effect. The generation mechanism of residual stress was investigated from the measurement results of deflection of cantilever specimens during HVOF spraying.

Analysis of a high-velocity oxygen-fuel (HVOF) thermal spray torch part 2: Computational results

The fluid and particle dynamics of a high-velocity oxygen-fuel (HVOF) torch are analyzed using computational fluid dynamic (CFD) techniques. The thermal spray device analyzed is similar to a Metco Diamond Jet torch with powder injection. The details of the CFD simulation are given in a companion paper. This paper describes the general gas dynamic features of HVOF spraying and then discusses in detail the computational predictions of the present analysis. The gas velocity, temperature, pressure, and Mach number distributions are presented for various locations inside and outside the torch. The two-dimensional numerical simulations show large variations in gas velocity and temperature both inside and outside the torch due to flow features such as mixing layers, shock waves, and expansion waves. Characteristics of the metal spray particle velocity, temperature, trajectory, and phase state (solid or liquid) are also presented and discussed. Particle velocities and temperatures are shown to be lower for this type of torch than previously believed.

Heat transfer between WC-Co coating and aluminum alloy substrate during high-velocity oxygen-fuel (HVOF) spraying

Mathematical simulation of the heat transfer between a WC-Co coating and an aluminum alloy (Al-4%Cu) substrate during HVOF spraying is provided. This simulation includes the investigation of tem-perature evolution, coating solidification, fusion and solidification in the substrate interfacial region, and particular features of the substrate-coating thermal interaction. Optimal thermal conditions for forming the coating structure are estimated. The results obtained are used in another paper (Ref 15), “Formation of Structure of WC-Co Coating on Aluminum Alloy Substrate During High-Velocity Oxygen-Fuel (HVOF) Spraying,” to predict the structural parameters, which agree well with the experimental data.

Formation of structure of WC-Co coatings on aluminum alloy substrate during high-velocity oxygen-fuel (HVOF) spraying

This paper is based on Ref 1 and concerns the mathematical simulation of the structure formation of the WC-Co coating on aluminum alloy (Al-4% Cu) substrate during high-velocity oxygen-fuel (HVOF) spraying. Variations of the solidification velocity, thermal gradient, and cooling velocity in the coating and substrate interfacial region are studied. Formation of the amorphous and crystalline structures in the coating and of the crystalline structure in the substrate interfacial region is discussed. Behavior of the crystal size and intercrystalline distance with respect to the thermal spray parameters is analyzed. Opti-mal conditions for the development of fine and dense crystalline structure are established. Results agree well with experimental data.

The “sonarc” process: Combining the advantages of arc and HVOF spraying

Electric arc spraying has been successfully used for many industrial applications for more than 30 years. High-velocity oxygen fuel (HVOF) spraying is another well-established technology. Coatings produced by the HVOF process, especially carbide-containing coatings, exhibit excellent quality in terms of density and hardness. One approach to obtaining both high deposition rates and dense coatings is to combine electric arc spraying and HVOF spraying in a technique known as the “Sonarc” process. This process al-lows many feed combinations through the simultaneous use of wire and powder. This paper presents the development of a prototype. Examples of composite materials and structures, their properties, and potential applications are given.