High Velocity Oxygen-fuel Flame Research Articles

Characterization of the Structure and Wear Resistance of Ni65-Based Coatings via HVOF Flame Spraying and Its Application to Potato Digging Shovels

To improve the tribological properties of a potato digging shovel (PDS), Ni65-based coatings with rare earth oxides additions were fabricated on 65Mn# steel via High-Velocity Oxygen-Fuel (HVOF) flame spraying, the effect of macroscopic surface shape of PDSs on their wear resistance of PDSs was examined, and finally a kind of PDS with a specific macroscopic surface shape and satisfied wear resistance was obtained. The addition of CeO2 and Y2O3 decreased the defects in coatings, refined the microstructure, made hard phases distributed more uniformly and ultimately improved coating properties. According to the XRD analysis results, the Ni65-based coatings were composed of the matrix phase γ-Ni and hard phases formed by Cr, Fe and Ni with B, C and Si. More Cr7C3 phases were detected in coating B than in coating A, but the phases related to Y and Ce were also not detected because of the low content in both EDS and XRD analyses. Heat treatment and HVOF flame sprayed coatings both increased the hardness of specimens, and coating A (621HV1.0) provided a hardness nearly equivalent to that of the heat treatment specimens (617HV1.0), while coating B provided the highest hardness (664HV1.0). For all specimens, laser surface texturing (LST) structures weakened their corrosion resistance. However, the Ni65-based self-fluxing alloy coatings significantly improved the corrosion resistance of specimens, and coating B provided the best corrosion resistance. SEM images show that the main wear mechanism for worn specimens was abrasive wear, and less wear signs were observed on the surface of coating B. Abrasive wear examinations indicate that specimen BJ had the best wear resistance and, compared with specimen W, the mass loss of specimen BJ decreased by 28.56% and 20.83% at relative sliding speeds of 2.35 m/s and 3.02 m/s, respectively. However, considering the negative effect of LST structures on the corrosion resistance of specimens, the processing techniques of specimen A and specimen B are more applicable to PDSs. The macroscopic surface shapes affected the wear resistance of PDSs and ZF had the lowest mass loss but the highest draught force; comparatively, YS had a better balance on the draught force reduction and wear resistance. Finally, YS with coating B, which decreased the mass loss by more than 27.17%, is recommended in this paper. On the whole, the conclusions in this paper provide a reference for the design of potato digging shovels with lower draught force and better tribological properties.

Corrosion Resistance of VC–FeCr and VC–FeCrСо Coatings Obtained by Supersonic Gas-Flame Spraying

We study coatings of the VC–FeCr system with a thickness of 100 μm on a substrate of D16 aluminum alloy deposited by the method of supersonic gas-flame spraying (High Velocity Oxygen Fuel Flame Spraying Process; propane–oxygen fuel). For comparison, we apply the method of plasma spraying of coatings in a dynamic vacuum guaranteeing the highest quality of gas-thermal coatings. The powders for spraying were produced by the method of mechanical alloying with the use of a planetary mill. As initial components, we used powders of vanadium carbide, ferrochrome, and a cobalt–nickel alloy. We evaluate the corrosion-electrochemical properties of the coatings in a 3% NaCl solution at a temperature of 20 ± 0.2°С and show that they are characterized by high corrosion resistances, which correlates with their porosity. As a result of long-term holding of samples with coatings in a 3% NaCl solution, we observe the penetration of the corrosive medium to the coating–substrate interface, which may lead to underfilm corrosion and exfoliation of the coating. It is shown that a VC–FeCrCo coating whose porosity does not exceed 0.5% obtained by the plasma method in a dynamic vacuum has the lowest corrosion resistance. Its corrosion currents are twice weaker than for the coating obtained by the HVOF method.

A model of splats deposition state and wear resistance of WC-10Co4Cr coating

In this research both low temperature high velocity oxygen-fuel flame (LT-HVOF) and high velocity oxygen-fuel flame (HVOF) techniques were employed to prepare WC-10Co4Cr splats and coatings. In situ cutting of WC-10Co-4Cr splats was carried out with focused ion beam (FIB), and a model was proposed to describe how the wear resistance of WC-10Co4Cr coatings was correlated with its residual stress state and the splats deposition state. It was observed that in LT-HVOF spraying process, WC-10Co4Cr splats were slightly melted showing "hill" shape, while in HVOF spraying process, the splats were half melted having the appearance of "concavity". The residual stress of WC-10Co4Cr coatings is determined by the size, melting state, flight speed and temperature gradient of splats. In this paper, the quantitative function formula involving heating temperature and the flight speed of the powder is put forward for the first time to predict the wear resistance of the WC-10Co4Cr coatings. This will provide a theoretical basis for engineering practice and an effective way to save costs.

Manufacturing of nickel based cermet coatings by the HVOF process

Advanced particle diagnostic technology has been applied to establish process parameters to deposit high quality nickel based carbide cermet coatings for marine hydraulic applications. The cermet coatings are produced via the kerosene fuelled high velocity oxygen fuel (HVOF) spray process, which uses a hypersonic flame jet to melt and accelerate feedstock particles onto the component surfaces. The traditional ‘trial and error’ procedure is not technically robust, as well as being costly and time consuming. Instead, a superior method is implemented in the current study that performs real time monitoring of the process parameters associated with the HVOF flame jets. Subsequently, coatings can be produced with the knowledge of the inflight particle size, temperature and velocity profiles. The analytical results allow identification of suitable coating process parameters, which translate to coatings of lower porosity and enhanced mechanical performance.

Self-lubricating, low-friction, wear-resistant Al-based quasicrystalline coatings

After gas atomization, a quasicrystalline powder based on aluminium was used to prepare a thick coating by high-velocity oxygen-fuel flame torch spraying. This layer was deposited on top of a bond-coat layer on a steel plate. A post-spraying annealing treatment turned the two layers to their stable state, a γ-brass crystal and an icosahedral quasicrystal, respectively. The projection parameters were selected in such a way that the coating behaved like a self-lubricating material, which offered very good wear resistance (duration of pin-on-disk tests superior to 5 km with negligible material loss) and low friction (µ ≤ 6% against sintered tungsten carbide), in contrast to the state of the art. This property was achieved thanks to, on the one hand, excellent bonding to the substrate via the bound coat, and on the other hand, presence at the boundaries between quasicrystalline flakes of a mixture of both threefold and fourfold coordinated carbon originating from spray processing. Application to hard materials used in mechanical devices is appealing, especially because soft, lubricating additives may not be needed, thus considerably increasing the lifetime of the devices and reducing waste of materials.

Development of Processing Windows for HVOF Carbide-Based Coatings

Optimized processing windows for spraying high-quality metal carbide-based coatings are developed using particle diagnostic technology. The cermet coatings were produced via the high-velocity oxygen fuel (HVOF) spray process and are proposed for service applications such as marine hydraulics. The traditional “trial and error” method for developing coating process parameters is not technically robust, as well as being costly and time consuming. Instead, this contribution investigated the use of real-time monitoring of parameters associated with the HVOF flame jets and particles using in-flight particle diagnostics. Subsequently, coatings can be produced with knowledge concerning the molten particle size, temperature, and velocity profile. The analytical results allow identification of optimized coating process windows, which translate to coatings of lower porosity and improved mechanical performance.

Application of high velocity oxygen fuel flame (HVOF) spraying to fabrication of La0.8Sr0.2Ga0.8Mg0.2O3 electrolyte for solid oxide fuel cells

La0.8Sr0.2Ga0.8Mg0.2O3 (LSGM) is considered a promising electrolyte for intermediate-temperature solid oxide fuel cells (IT-SOFCs) due to its high ionic conductivity and stability under fuel cell operating conditions. Here we report our findings in investigating the feasibility of using a high velocity oxygen fuel flame (HVOF) spraying process for cost-effective fabrication of dense LSGM electrolyte membranes. The flame and in-flight particle behavior were simulated numerically to optimize the microstructure and phase compositions of the LSGM deposits. The measured gas leakage rate of an LSGM deposit is ∼7 × 10−7 cm4gf−1 s−1. The single cell assembled with 50–55 μm HVOF-sprayed LSGM electrolyte shows open circuit voltage (OCV) of 1.08 V at 800 °C, suggesting that the as-sprayed LSGM deposit is dense enough for direct application as SOFC electrolyte. At 800 °C, the ionic conductivity of the sprayed LSGM deposit is ∼0.04 S cm−1, indicating that the HVOF spraying is a promising process for low-temperature fabrication of dense LSGM electrolyte membranes for IT-SOFCs.

Surface nanocrystallization and its properties of a rare earth magnesium alloy induced by HVOF–SMB

The nanocrystalline microstructure in the surface of a rare earth magnesium alloy induced by a new process named HVOF–SMB (high velocity oxygen-fuel flame supersonic microparticles bombarding) has been characterized by means of X-ray diffractometry (XRD) and high-resolution transmission electron microscopy (HRTEM). The effects of HVOF–SMB on mechanical properties and corrosion resistance of the magnesium alloy have been investigated by microhardness distribution and polarization curves, and the cross-sectional observation of the treated Mg alloy sample has been characterized by optical microscopy (OM). Results showed that HVOF–SMB can induce surface nanocrystallization with a grain size level less than 20 nm at topmost surface layer, and the microhardness at the top surface layer increases up to about 170 HV 0.025 which is twice more than that of the substrate. The grain size increases as well as the microhardness decreases gradually with the depth varying from surface to substrate. The potentiodynamic polarization curves indicate that the HVOF–SMB treated surfaces show lower corrosion resistance in comparison with the untreated magnesium alloy in 3.5% NaCl solution. The underlying mechanism of the electrochemical properties on the nanocrystalline Mg-based alloy’s surface induced by severe plastic deformation in corrosion resistance is discussed.

Surface nanocrystallization mechanism of a rare earth magnesium alloy induced by HVOF supersonic microparticles bombarding

A nanostructured surface layer with a thickness up to 60 μm was produced on a rare earth Mg–Gd–Y magnesium alloy using a new process named HVOF-SMB (high velocity oxygen-fuel flame supersonic microparticles bombarding). The microstructural features of the treated surface at various depth of the deformed layer were characterized by optical microscopy (OM), transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) with an aim to reveal the formation mechanism. Results showed that three steps during grain refinement process were found, i.e., twinning dominates the plastic deformation and divides the coarse grains into finer twin platelets at the initial stage, stacking faults are generated and a number of dislocation slip systems are activated leading to the cross slips with increasing strain and strain rate, eventually high-density dislocation networks, dislocation cells and dislocation arrays are formed, which further subdivides the twin platelets and residual microbands into sub-microstructures. As a result, homogeneous nanostructure with a grain size of about 10–20 nm is formed through dynamic recrystallization in the topmost surface layer. Based on the experimental observations, a grain refinement mechanism induced by plastic deformation with higher strain rate during the HVOF-SMB treatment in the rare earth Mg–Gd–Y alloy was proposed.

Microstructure and photocatalytic properties of nanostructured TiO 2 and TiO 2–Al coatings elaborated by HVOF spraying for the nitrogen oxides removal

This paper deals with the study of the photocatalytic behaviours of TiO 2 and TiO 2–10 wt.% Al coatings elaborated by high-velocity oxygen fuel (HVOF) spraying using spray-dried nanosized feedstock powders. In the HVOF flame, the powders were injected by two methods: internal injection, i.e. as in conventional HVOF process and external injection, i.e. outside the torch nozzle. Scanning electron microscopy and X-ray diffraction were employed to characterize the microstructure and the crystalline structure of the TiO 2-based coatings. The photocatalytic activities were evaluated by the nitrogen oxides removal and compared with that of the initial powders. It was found that the amount of anatase in the coatings depends on the nature of the powder and also on the type of injection method. The TiO 2 coatings, as well as, the TiO 2–Al coatings elaborated using the external powder injection presented a better photocatalytic activity (the nitrogen oxides conversion rates were about 25–42% for NO and 14–18% for NO x ) than those obtained by the conventional HVOF process (<5%). Besides, TiO 2 coatings containing aluminium particles had an enhanced photocatalytic activity than those of un-doped titania deposits.

Particle image velocimetry in thermal spraying

Abstract In thermal spraying processes, the particle velocity is an important influence parameter with respect to the coating properties. Additionally, the analysis of the particle velocities in many cases enables a better understanding of the process. Consequently, suitable diagnostic methods for the measurement of particle velocities meeting the requirements in the area of thermal spraying processes are needed. One diagnostic method being field-tested is the laser-optical particle image velocimetry (PIV) method. In this paper, the concept of the PIV system and analysis methods for PIV images are presented. Furthermore, short descriptions of the two common thermal spraying processes high velocity oxygen-fuel flame spraying (HVOF) and atmospheric plasma spraying (APS) are given. The influences on the kinetics of an HVOF- and an APS-process obtained by means of PIV are described. In this context, it has to be pointed out that PIV is qualified for measurements of particle velocities in thermal spraying processes.

Corrosion of coating materials in oxidizing and hydrogen chloride containing atmospheres

AbstractThe boiler tubes in waste incinerator plants and power plants are mainly corrosive strained. This paper presents the corrosion behaviour of various coating materials in two corrosive atmospheres at 500°C. In a first test, Ni‐based materials applied by High Velocity Oxygen Fuel Flame (HVOF)‐spraying and Atmospheric Plasma Spraying (APS) as well as Cr‐based materials obtained by diffusion coating process are tested in oxidizing atmosphere with water saturation. The results of the corrosion test in this H2O‐O2‐N2‐atmosphere show that NiCr applied by HVOF is less oxidized than that applied by APS‐coating, whereas NiCrBSi coating by APS shows higher resistance than that applied by HVOF‐spraying. The diffusion coating of Cr/Si is better than that of pure Cr. In a second test, metallic Mo and W‐based materials applied by thermal spraying are tested to verify their corrosion resistance in HCl‐H2O‐O2‐N2‐atmosphere. It is shown that pure Mo applied by APS on 13 CrMo 44 and WCrNi sprayed on 15 Mo 3 by HVOF as well as on 13 CrMo 44 show relatively high corrosion resistance, whereas Mo sprayed by APS on 15 Mo 3 is not resistant. Both tungsten‐cobalt‐chromium mixtures show extremely high oxidation rates. Base materials in both tests were the standard boiler tube materials 15 Mo 3 and 13 CrMo 44. But even the low corrosion rates of the best materials in these tests are higher compared to those of materials already presented by Cha et al. in an earlier paper.

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.

Corrosion Behavior of Thermally Sprayed WC Coating in Na&lt;SUB&gt;2&lt;/SUB&gt;SO&lt;SUB&gt;4&lt;/SUB&gt; Aqueous Solution

WC-Co thermally sprayed coating, formed by high velocity oxygen-fuel flame spraying, has had the problem of low corrosion resistance in aqueous solutions. Furthermore, much remains unknown regarding corrosion reactions on coating surfaces and the formation of passivation films. This study focused on changes of the corrosion surfaces of two thermally sprayed coatings, WC-12Co and WC-10Co-4Cr, with immersion time. The chemical composition of the corrosion surfaces was analyzed by electron spectroscopy, and the relation with passivation film stability is discussed. For WC-12Co, the thermally sprayed coating was immersed in Na 2 SO 4 solution. The W and Co components were dissolved by extended anodic reaction. Oxides of W and Co were then formed on the surface of the thermally sprayed coating. However, these oxides appeared to have little effect on the corrosion. Passivation film, which mainly consists of Cr 2 O 3 , was formed on the surface of WC-10Co-4Cr thermally sprayed coating immediately after immersion, and limited the dissolving of coating components into the aqueous solution. However, it is suspected that this film did not completely cover the entire coating surface. It is probably a thin porous film formed as islands.

132 転がり-滑り接触下における WC サーメット溶射皮膜の耐久性に及ぼす下地処理の影響

Using a two-roller testing machine, the authors have examined the surface durability of thermally sprayed WC-Cr-Ni cermet coating in lubricated rolling or rolling with sliding contact conditions. In the present investigation, the coating of 20 to 200μm in thickness was formed onto the blasted or ground roller specimens made of a thermally refined carbon steel, an induction hardened carbon steel and a carburized alloy steel by means of the high energy type flame spraying (Hi-HVOF) as well as the conventional high velocity oxygen-fuel flame spraying (HVOF). The WC cermet coated roller finished to a mirror-like condition was mated with the carburized steel roller without coating and a maximum Hertzian stress of PH=1.2 or 1.4GPa was applied in line contact. As a result, it was found that the coating on the blasted substrate roller generally shows a higher durability compared with that on the ground substrate roller and this difference appears more distinctly as the coating thickness decreases. In addition, it was also confirmed that the life to flaking of coated roller is prolonged as the hardness of substrate is raised and the coating thickness increases.

高速フレーム溶射皮膜 (WC-27 mass%NiCr) 特性に及ぼす熱処理の影響

The effect of heat treatment on the characteristics of WC-27 mass%NiCr sprayed coatings on 13%Cr steel made by the High Velocity Oxygen-Fuel Flame Spraying (HVOF) was studied. Specimens were heated to 673 K and held at 5, 10, 20 and 30 h in air. Corrosion resistance was measured by a salt-spray test (CASS test) and strength was measured by a three-point bend test. The results obtained were as follows: (1) Corrosion resistance increased with heating time. This increase is dependent on the surface void ratio of the sprayed coatings which decreases as the temperature increases. (2) The three-point bending strength decreases at 5 h heating, and increases again with continued heating. The initial decrease at 5 h heating was due to release of compressive stress in the coatings, and the increase afterwards was due to the surface void ratio decreasing.

高速ガス炎溶射法におけるWC-Co溶射粒子の加速挙動

The representative factors that have been studied in the development of thermal spraying processes are the heating and the acceleration techniques of spray particles. As the simultaneous promotion of heating and acceleration of particles in thermal spraying, which is performed in an extremely short time, is contrary to each other, there is a limit for the attainment of both. In this study, from the point of acceleration of spraying particles, the velocity of WC cermet spraying particles in two types of high velocity oxygen-fuel flame spraying process was measured by using a CCD video camera system with a high-speed electrical shutter. The determined velocities of the WC-12 mass%Co powder in the size range of 45 to 5 μm and the WC-20 mass%Cr3C2-7 mass%Ni powder in the size range of 45 to 10 μm were 300∼600 m·s−1 and 300∼500 m·s−1, respectively. It is apparent that the particle velocity in the high velocity oxygen-fuel flame spraying process was much higher than that in the plasma spraying process. These results were consistent with those reported previously. The driving force for higher acceleration in the high velocity oxygen-fuel flame spraying process was discussed. As the result of the estimation of the drag force for particles in spraying fluid, it was derived that the main factors which dominated the higher acceleration were large in density and high in velocity of the combustion gas mixture as compared with Ar-He plasma jet.

WC-Co系溶射皮膜形成における粒子扁平化とその積層構造

Lamellar structure, splat formation of molten droplets, coating microstructure, transfiguration of carbide particles and deposition yield of the powders in high velocity oxygen-fuel flame spraying of WC-Co coatings using powders with different morphologies were examined. The ratio of the particle diameter to the thickness of splats determined in this experiment is 7 to 17. These values are in good agreement with the calculated values according to the splat formation model proposed by Jones et al.The formation of coating deposits is followed by the expanding flow of the molten binding phase accompanying solid WC particles. The smaller size of WC particles in the spray powder allows the fluidization and flattening together with the molten binding phase, on the contrary, the flattening behavior of particles are restrained by the size of WC particles in the case that the size of spray powder is large. Susceptivity to flattening of the spray powder tends to result in the compactness or the high density of the coating. Consequently, spray powders composed of the smaller size of WC particles allow the formation of the denser coatings. The selection of the spray powder composed of smaller-sized primary WC particles and the moderately larger size of cermet particles is effective for the higher deposition yield of WC-Co spraying.

A pragmatic analysis and comparison of HVOF processes

A number of high-velocity oxygen-fuel flame (HVOF) systems have evolved during the last 9 years. The most advanced is now challenging the coating qualities produced by the very successful detonation (D-Gun) process. The fundamentals of these various processes are described and compared. A mathematical analysis of an established HVOF gun is profiled. Gas and particle temperatures, velocities, pressures, and Mach numbers are calculated and plotted at various points within the gun and spray stream. Significantly, all measured values were in close agreement with calculated and predicted values. Flow patterns and shock-wave phenomena are also described and compared with actual observations.