High Velocity Oxygen Fuel Process Research Articles

Characteristics of Ni-Ti-Zr-Si-Sn Bulk Amorphous HVOF Coating

This study was conducted to observe the influence of oxygen to fuel ratio[O2/H2 ratio] of HVOF(High Velocity Oxygen Fuel) process on the amorphous formation and coating properties of NiTiZrSiSn bulk amorphous alloy. The bulk amorphous feedstock was prepared by inert gas atomization. Amorphous contents were followed by differential scanning calorimetry(DSC). Microstructural characterization was done by scanning electron microscope(SEM) and x-ray diffraction(XRD). Mechanical properties were obtained by Vickers microhardness testing and pin on disk wear testing. A maximum 72% amorphous phase could be obtained with an optimum O2/H2 ratio. High amorphous content resulted in a low friction coefficient and weight loss than high oxide content coating and fully crystallized coating.

Corrosion Behavior of HVOF Coated Sheets

High velocity oxygen-fuel (HVOF) thermal spray coating finds application in industry due to its superior resistance to corrosion and thermal loading. In the HVOF process, the metallic powders at elevated temperature are sprayed at supersonic speed onto a substrate material. The powder granules sprayed impact onto each other, forming a mechanical bonding across the coating layer. In most of the cases, the distances among the particles (powder granules sprayed) are not the same, which in turn results in inhomogeneous structure across the coating layer. Moreover, the rate of oxidation of the powder granules during the spraying process varies. Consequently, the electrochemical response of the coating layer surfaces next to the base material and free to atmosphere differs. In the current study, the electrochemical response of a coating sheet formed during HVOF thermal spraying was investigated. NiCrMoNb alloy (similar to Inconel 625) wass used for the powder granules. Thermal spraying was carried out onto a smooth surface of stainless steel workpiece (without grid blasting), and later the coating layer was removed from the surface to obtain the coating sheet for the electrochemical tests. It was found that the corrosion rate of the smooth surface (surface next to the stainless steel surface before its removal) is considerably larger than that corresponding to the rough surface (free surface) of the coating sheet, and no specific patterns were observed for the pit sites.

Modeling and analysis of HVOF thermal spray process accounting for powder size distribution

This work focuses on the high velocity oxygen–fuel (HVOF) thermal spray processing of coatings and presents a fundamental model for the process which explicitly accounts for the effect of powder size distribution. The model describes the evolution of the gas thermal and velocity fields, as well as the motion and temperature of agglomerate particles of different sizes. In addition to providing useful insight into the in-flight behavior of particles with different sizes, the model is used to make a control-relevant parametric analysis of the HVOF thermal spray process. This analysis allows us to systematically characterize the influence of controllable process variables such as combustion chamber pressure, oxygen/fuel ratio, as well as the effect of powder size distribution, on the values of particle velocity and temperature at the point of impact on substrate. Specifically, the study shows that particle velocity is primarily influenced by the combustion chamber pressure, and particle temperature is strongly dependent on the fuel/oxygen ratio. Furthermore, it shows that the particle velocity and temperature at the point of impact depend strongly on particle size. These findings are consistent with available experimental observations and set the basis for the formulation of the control problem for the HVOF process.

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.

Simulation and Application of a HVOF Process for MCrAlY Thermal Spraying

This work deals with numerical simulation and application of a high velocity oxygen fuel (HVOF) process for MCrAlY thermal spraying. The main objective of this study was to observe the correlations between coating oxidation and oxygen content in combustion products or flame temperature. Spray parameters were selected on the basis of the numerical simulation of combustion and particle behavior in the flame. The results of experiments revealed that the oxygen content is not the main key factor concerning the oxidation rate of MCrAlY coatings. On the contrary, the flame temperature has a decisive influence on oxidation. Combustion conditions corresponding to stoichiometric factors between 0.82 and 1.2 appeared not to be favorable for MCrAlY thermal spraying. Outside this range, it appeared preferable to use a combustion system on the fuel-rich side.

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.

Wear performance of metamorphic alloy coatings

Two Fe–Cr–B based gas atomized powders, proposed as metamorphic alloys (Armacor M and Armacor 16), were thermally sprayed using the HVOF (High Velocity Oxygen Fuel) process. Armacor M was also weld-surfaced by PTA (Plasma Transferred Arc) process. The resultant deposits were subsequently characterized, using X-ray diffraction, scanning electron microscopy, and microhardness measurement. The wear performances of the different coatings were investigated by two-body abrasive and pin-on-disc dry sliding wear tests. The results from microstructural analysis of as-sprayed deposits reveal oxide and boride phases such as Fe 3O 4 and Cr 1.65Fe 0.35B 0.96 in α matrix for HVOF-sprayed Armacor 16 coating, and only the boride phases (Cr 1.65Fe 0.35B 0.96 and Cr 2B) in α matrix for HVOF-sprayed Armacor M coating. PTA weld-surfaced Armacor M coating contains needle-type long precipitates of large Cr 2B and Cr 1.65Fe 0.35B 0.96 in α matrix. Wear test results show that HVOF-sprayed Armacor 16 coating exhibits the lowest wear resistance. While HVOF-sprayed and PTA weld-surfaced Armacor M coatings have the same hardness, the latter shows better abrasive and sliding wear resistance because of the size and orientation of its boride phases. More broadening in XRD patterns and hardness increase after wear testing suggest that the transformation from crystalline to amorphous structure has occurred on the uppermost layer during the wear testing. However, the wear resistance of the metamorphic alloy coatings depends more on the density and microstructure of the coatings rather than on the formation of amorphous surface structure.

Characterization of Fe-Cr-B based coatings produced by HVOF and PTA processes

Two Fe-Cr-B based gas atomized powders, Armacor M and 16, were thermally sprayed on a low carbon steel substrate, using the HVOF (High Velocity Oxygen Fuel) process. Armacor M was also weld-surfaced with the PTA (Plasma Transferred Arc) process. The resultant deposits were subsequently characterized, using X-ray diffraction, scanning electron microscopy, and microhardness measurement. The effects of heat treatment were also studied for HVOF-sprayed coatings. The wear performance of the coatings was investigated by two-body abrasive wear tests. The results of microstructural analysis of as-sprayed deposits revealed oxide and boride phases such as Fe3O4 and Cr1.65F0.35B0.96 in an α matrix for the HVOF-sprayed Armacor 16 coating, and only the boride phases (Cr1.65F0.35B0.96 and Cr2B) in an α matrix for the HVOFsprayed Armacor M coating. PTA weld-surfaced Armacor M coating contains needle-type long precipitates of Cr2B) and Cr1.65F0.35B0.96, in the α matrix. The hardness of the HVOF-sprayed Armacor 16 coating after heat treatment was substantially less than that of the as-sprayed coating due to the phase transformation from α to γ phase. Heat treatments of the HVOF-sprayed Armacor M coating did not produce changes in phase and its hardness decreased as compared to that of the as-sprayed coating. While HVOF-sprayed and PTA weld-surfaced Armacor M coatings have the same hardness, the latter shows better abrasive wear resistance because of the size and orientation of its boride phases. The broadening of the XRD patterns and the increase in hardness after wear testing suggest that the transformation from the crystalline to the amorphous structure occurred on the uppermost layer during wear testing.

Mechanical properties of HVOF coatings

High-velocity oxygen fuel (HVOF) thermal-sprayed carbide coatings are distinguished by high hardness, low porosity, and good wear resistance compared to other thermal spray technologies. However, for many engineering applications the ductility and fatigue resistance are the most important material properties. In the use of HVOF systems, these properties are influenced by many boundary conditions. This paper presents the effects of different spraying parameters on the fatigue resistance of samples coated by the HVOF process.

Reciprocating Wear of WC-17 Co Coatings in aqueous Environments

Surface Engineering is briefly reveiwed and the deposition of hard coatings by a High Velocity Oxygen Fuel process is described. Features of the deposit microstructure are discussed in terms of production parameters and their effect on both strength and wear behaviour. An experimental programme to create controlled reciprocating wear of the coatings in the presence of air, aqueous and aqueous abrasive media is described. The resulting wear is assessed quantitatively in terms of volume los and change in surface roughness. Introduction of seawater to the wearing interface reduces wear by a factor of four and the introduction of alumina slurries results in wear rates, and scar roughness values intermediate between those in seawater and air. The reduced wear rates in aqueous media are attributed to the cushioning effects of the liquid and the relatively benign effects of the abrasive to the replacement of early adhesive wear (in air) by abrasive wear. The fine (0.1 μm) abrasive to the replacement of early adhesive wear (in air) by abrasive wear. The fine (0.1 μm) abrasive is more damaging than the coarse (7μm) one which is thought to be due to the easy embedding of the 0.1 μm alumina in the soft phase, which has a typical microstructural mean free path of about 0.5 μm

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.