High Velocity Oxy-fuel Gun Research Articles

A Comparison of Wear Due to Abrasion of WC–Ni Coatings on Aluminium Alloy Sprayed by HVOF and Detonation Gun

In this investigation, aluminum alloy is coated with WC–Ni layer deposited by detonation spraying and high velocity oxy fuel (HVOF) spraying. The important features of microstructure and hardness of the coating are characterized extensively. The wear rates of the coating due to abrasion are determined for different applied loads. The worn coatings are examined under scanning electron microscopy. A correlation is established between the wear rates of the coatings due to abrasion and hardness. The results show that abrasion action takes place by both particle rolling and particle sliding. Fracture of particles during abrasion increases wear rate. In general, the wear rate due to abrasion is higher for the coating deposited by detonation spraying than that of coating deposited by HVOF spraying.

Structural and photocatalytic characteristics of TiO2 coatings produced by various thermal spray techniques

Titanium dioxide (TiO2) was elaborated by four different thermal spray techniques—(i) plasma spraying using a water-stabilized torch, (ii) plasma spraying using a gas-stabilized torch, (iii) high velocity oxy-fuel gun, and (iv) oxy-acetylene flame. The porosity of the coatings was studied by optical microscopy, nano-structural features by scanning electron microscopy (SEM), phase composition by X-ray diffraction (XRD); the microhardness, surface roughness and wear resistance were evaluated. The diffuse reflectance was measured by ultra-violet/visible/near-infrared (UV/Vis/NIR) scanning spectrophotometer. The kinetics of photocatalytic degradation of gaseous acetone was measured under a UV lamp with 365 nm wavelength. After all the applied spray processes, the transformation of anatase phase from the initial powders to rutile phase in the coatings occurred. In spite of this transformation, all the coatings exhibited certain photocatalytic activity, which correlated well with their band gap energy calculated from reflectivity. All the coatings offer relatively good mechanical properties and can serve as robust photocatalysts.

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-Velocity Oxy-Fuel (HVOF) Suspension Spraying of Mullite Coatings

Mullite coatings (3Al2O3 · 2SiO2) were deposited by suspension thermal spraying of micron-sized (D50 = 1.8 μm) feedstock powders, using a high-velocity oxy-fuel gun (HVOF) operated on propylene (DJ-2700) and hydrogen fuels (DJ-2600). The liquid carrier employed in this approach allows for controlled injection of much finer particles than in conventional thermal spraying, leading to coatings with low porosity and fine and homogeneous porosity distribution, making this process potentially suitable for creating thin layers with low gas permeability. In-flight particle states were measured for a number of spray conditions of varying fuel-to-oxygen ratios and standoff distances and related to the resulting microstructure, stoichiometry, phase composition (EDS, SEM, XRD), and hardness (VHN 300g•f) of the coatings. In an attempt to retain the crystalline phase in the coatings, HVOF operating conditions were varied to limit in-flight particle melting. However, fully crystalline coatings were only obtained by gradually heating the coating during deposition to temperatures above 400 °C.

Design and optimisation of a multi-powder feed system for the HVOF deposition process

Currently no specific method exists for the deposition of High Velocity Oxy-Fuel (HVOF) thermal spray functionally graded coatings. This paper investigates the design and optimisation of a multi-powder HVOF thermal spray device in order to deposit aluminium/tool-steel functionally graded coatings. A multi-powder feed device concept was developed. The concept was based on a stand alone two powder chamber device which integrates with common hopper systems to allow the mixing of two powders during thermal spray deposition. This concept was verified by simulation the design of this device for multi-powder flow using Finite Element Analysis (FEA) to arrive at the optimum dual feed design. The FEA model predicted the mixing and flow of two powders of various ratios' of mass flow rate and velocity based on an optimum designed shape and pressure ratios' of nitrogen gas in the chamber to pick-up shaft of 2.25:1. This yielded the best results in terms of carrying the powders from the mixing zone into the nitrogen gas flow path, inside the pick-up shaft and on towards the HVOF gun. Post finite element analysis the device was manufactured for the utilisation within the HVOF process. Optimisation tests of the device included; powder flow bench tests and HVOF thermal spraying of graded deposits. The results revealed a calibration graph for the two powders in question and the compositional variation across the deposit during functionally graded deposition. The composition of the graded deposits were close to that anticipated hence this showed the suitability of the newly designed multi-powder deposition system in mixing two powders for the purpose of producing HVOF graded coatings.

Numerical simulation on supersonic flow in High-Velocity Oxy-Fuel thermal spray gun

This paper analyzes the behaviour of coating particles as well as the gas flow both inside and outside of the High-Velocity Oxy-Fuel (HVOF) thermal spray gun by using a quasi-one-dimensional analysis and a numerical simulation. The HVOF gun in the present analysis is an axially symmetric convergent-divergent nozzle with the design Mach number of 2.0. From the present analysis, the distributions of velocity and temperature of the coating particles flying inside and outside of the HVOF gun are predicted. The velocity and temperature of the coating particles at the exit of the gun calculated by the present method agree well with the previous experimental results. Therefore, the present method of calculation is considered to be useful for predicting the HVOF gas and particle flows.

Quasi-One-Dimensional Analysis of the Effects of Pipe Friction, Cooling and Nozzle Geometry on Gas/Particle Flows in HVOF Thermal Spray Gun

The High Velocity Oxy-Fuel (HVOF) thermal spraying can make highly tough and dense coatings on a solid surface. It is experimentally recognized that the quality of the coating largely depends on the particle velocity and temperature, which depend on the gas flow in the HVOF gun. The gas flow in the gun is affected by two factors; pipe friction and cooling along the inner wall of the long gun. This paper investigates the gas/particle flow in the HVOF gun by using quasi-one-dimensional analysis including the effects of pipe friction, cooling and nozzle geometry. The standard values of the parameters, to include the effects of pipe friction and cooling are selected based on the experimental result. The nozzle length is varied in the range of 110-330 mm. The calculated results show that (1) the pipe friction decreases the particle velocity and increases the particle temperature, (2) the cooling increases the particle velocity and decreases the particle temperature, (3) the maximum particle velocity is obtained when the length of the diverging part of the nozzle is approximately equal to the length of the following non-diverging straight part, (4) the longer nozzle results in larger particle velocity at the nozzle exit. These numerical results help understand the gasdynamics occurring in the HVOF gun, and assist in the design of the gun.

Gasdynamic Study on Flow Characteristics of High Velocity Oxy-Fuel Thermal Spray Gun (1st Report, The Influence of Chamber Pressure and Barrel Length)

The High Velocity Oxy-Fuel (HVOF) thermal spraying is a technique to make a coating on a solid surface. The gun consists of a supersonic nozzle followed by a straight passage called barrel. In this study, the internal flow of HVOF gun was analyzed by the theory of Fanno flow and the flow patterns are classified depending on barrel length and chamber pressure. The analytical results were evaluated by the experimental results including pressure distributions in the gun and schlieren photographs of jet. The analytical results show good agreement with the experimental results except for the condition that shock train stays inside the barrel.

Aerodynamic study on supersonic flows in high-velocity oxy-fuel thermal spray process

To clarify the characteristics of gas flow in high velocity oxy-fuel (HVOF) thermal spray gun, aerodynamic research is performed using a special gun. The gun has rectangular cross-sectional area and sidewalls of optical glass to visualize the internal flow. The gun consists of a supersonic nozzle with the design Mach number of 2.0 followed by a straight passage called barrel. Compressed dry air up to 0.78 MPa is used as a process gas instead of combustion gas which is used in a commercial HVOF gun. The high-speed gas flows with shock waves in the gun and jets are visualized by schlieren technique. Complicated internal and external flow-fields containing various types of shock wave as well as expansion wave are visualized.

Ceramic coatings for a corrosion-resistant nuclear waste container evaluated in simulated ground water at 90 °C

Ceramic materials provide an innovative opportunity for corrosion-resistant coatings for nuclear waste containers. Their suitability can be derived from the fully oxidized state for selected metal oxides. Ceramic coatings applied to plain carbon steel substrates by several thermal spray techniques have been exposed to 90 °C simulated ground water (at 10 times typical concentration) for nearly 6 years. Thermal spray processes examined in this work included plasma spray, high-velocity oxy fuel (HVOF), and detonation gun. Some thermal spray coatings have demonstrated superior corrosion protection for the plain carbon steel substrate. In particular, the HVOF and detonation gun thermal spray processes produced coatings with low connected porosity, which limited the growth rate of corrosion products. It was also demonstrated that these coatings resisted spallation of the coating even when an intentional flaw (which allowed for corrosion of the carbon steel substrate underneath the ceramic coating) was placed in the coating. An approach for a theoretical basis for prediction of the corrosion protection provided by ceramic coatings is also presented. The theoretical development includes the effect of the morphology and amount of the porosity within the thermal spray coating and provides a prediction of the exposure time needed to produce a crack in the ceramic coating.

New Attachment for Controlling Gas Flow in the HVOF Process

During the decade, the high-velocity oxyfuel (HVOF) process proved to be a technological alternative to the many conventional thermal spray processes. It would be very advantageous to design a nozzle that provides improved performance in the areas of deposition efficiency, particle in-flight oxidation, and flexibility to allow deposition of ceramic coatings. Based on a numerical analysis, a new attachment to a standard HVOF torch was modeled, designed, tested, and used to produce thermal spray coatings according to the industrial needs mentioned above. Performance of the attachment was investigated by spraying several coating materials including metal and ceramic powders. Particle conditions and spatial distribution, as well as gas phase composition, corresponding to the new attachment and the standard HVOF gun, were compared. The attachment provides better particle spatial distribution, combined with higher particle velocity and temperature.

Analysis of particle behavior in High-Velocity Oxy-Fuel thermal spraying process

This paper analyzes the behavior of coating particle as well as the gas flow both of inside and outside the High-Velocity Oxy-Fuel (HVOF) thermal spraying gun by using quasi-one-dimensional analysis and numerical simulation. The HVOF gun in the present analysis is an axisymmetric convergent-divergent nozzle with the design Mach number of 2.0 followed by a straight passage called barrel. In the present analysis it is assumed that the influence of the particles injected in the gas flow is neglected, and the interaction between the particles is also neglected. The gas flow in the gun is assumed to be quasi-one-dimensional adiabatic flow. The velocity, temperature and density of gas in the jet discharged from the barrel exit are predicted by solving Navier-Stokes equations numerically. The particle equation of motion is numerically integrated using three-step Runge-Kutta method. The drag coefficient of the particle is calculated by linear interpolation of the experimental data obtained in the past. Particle mean temperature is calculated by using Ranz and Marchalls’ correlation for spherical particles. From the present analysis, the distributions of velocity and temperature of the coating particles flying inside and outside the HVOF gun are predicted.

HVOF Thermal Spray Deposited Y<SUB>2</SUB>O<SUB>3</SUB>-Stabilized ZrO<SUB>2</SUB> Coatings for Thermal Barrier Applications

High velocity oxy-fuel (HVOF) thermal spray has been successfully used to deposit yttria-stabilized zirconia (YSZ) for thermal barrier coating (TBC) applications. Adherent coatings were obtained within a limited range of spray conditions using hydrogen as fuel gas. Spray parameters such as hydrogen-to-oxygen ratio, spray distance, and substrate cooling were investigated. Spray distance was found to have a pronounced effect on coating quality; adherent coatings were obtained for spray distances between 75 and 125 mm from the gun exit for the hydrogen-to-oxygen ratios explored. Compared to air plasma spray (APS) deposited YSZ coatings, the HVOF deposited coatings were more fully stabilized in the tetragonal phase, and of similar density, surface roughness, and cross-sectional microhardness. Notably, fracture surfaces of the HVOF coatings revealed a more homogeneous structure. Many theoretical models predict that it should not be possible to melt YSZ in an HVOF flame, and therefore it should not be possible to deposit viable YSZ coatings by this process. The experimental results in the present work clearly contradict those expectations. The present results can be explained by taking into account the effect of partial melting and sintering on particle cohesion, as follows. Combustion chamber pressures (P o) of ∼3.9 bar (58.8 psi) realized during HVOF gun operation allows adiabatic flame temperature values that are above the zirconia melting temperature. Under these conditions, the Ranz-Marshall heat transfer model predicts HVOF sprayed particle surface temperatures T p that are high enough for partial melting of small (∼10 µm) zirconia particles, T p=(1.10−0.95)T m. Further analysis shows that for larger particles (38 µm), adherent coatings are produced when the particle temperature, T p=0.59−0.60 T m, suggesting that sintering may have a role in zirconia particle deposition during HVOF spray. These results suggest two different bonding mechanisms for powders having a broad particle size distribution.

Acoustic emission study on WC–Co thermal sprayed coatings

Thermally sprayed coatings contain residual stresses that are produced in the spraying process. These may reduce the coating lifetime. In order to determine the optimum spraying conditions with respect to the residual stress level present, the acoustic emission (AE) during four-point bend tests on tungsten carbide–cobalt coatings sprayed onto mild steel substrates was investigated. Samples tested at different levels of deformation were studied by means of scanning electron microscopy and AE in order to understand the cracking mechanisms. Relationships between the number and amplitude of AE events detected and the type of cracking processes occurring were established. It has been possible to compare the residual stresses caused by the effect of different spraying parameters, such as coating thickness, spraying distance and high velocity oxy-fuel gun.

Effect of grinding on the erosion behaviour of a WC–Co–Cr coating deposited by HVOF and detonation gun spray processes

There is a growing interest on the use of thermal spray coatings for a variety of wear resistance applications due to the several advantages it offers. WC–Co coating is widely used for wear resistance applications. It is known that addition of Cr to WC–Co improves binding of the metallic matrix with the WC grains and provides better wear resistant coating. Thus, WC–Co–Cr is considered to be a potential wear resistant coating material as compared to WC–Co coating. The thermally sprayed carbide coatings are in general surface finished by machining or grinding after the coating process. Therefore, it is imperative to understand the influence of the finishing operation on the characteristics and performance of carbide coatings. This work aims at investigating the effect of grinding on the erosion behaviour of a WC–Co–Cr coating. As a part of this work a comparison has also been brought out between two high velocity coating processes namely high velocity oxy-fuel (HVOF) and detonation gun spray process (DS). A WC–10Co–4Cr powder has been sprayed on a medium carbon steel using the above mentioned high velocity spray processes. Some of the coated specimens were further ground by a diamond wheel with controlled parameters. The coating in both ‘as-coated’ and ‘as-ground’ conditions has been tested for solid particle erosion behaviour. The erosion experiments were carried out using an air-jet erosion test rig with silica erodents at a velocity of 80 m/s. It has been found that surface grinding improved the erosion resistance. This work presents detailed characterisation of the WC–Co–Cr coating in both ‘as-coated’ and ‘as-ground’ form. A detailed analysis indicates that the increase in residual stress in the ground specimen is a possible cause for the improvement in erosion resistance.

高速フレーム溶射皮膜の残留応力 １ 高速フレーム溶射皮膜に発生する応力のその場測定

Generation of residual stresses in high velocity oxy-fuel (HVOF) thermal sprayed coatings was studied by measuring the curvature change of substrates in-situ during spraying. 316L stainless steel, Hastelloy C, and WC-12%Co powders were sprayed by a HVOF gun powered by the combustion of kerosene and oxygen. It was found that the substrate curvature changed through 3 stages : 1) abrupt change at the onset of spraying, 2) continuous change during spraying, and 3) continuous smooth change during cooling after spraying. They correspond to 1) generation of stress within the surface layer of the substrate, 2) within the coating, and 3) superposition of macroscopic thermal stress. The stress during spraying was compressive and could be as much as 400 MPa, depending on the powder material and spray conditions. SEM observation of splats collected on polished substrates revealed that the majority of HVOF particles were not fully molten before impact. Therefore, HVOF sprayed coatings consisted of much thicker and plastically deformed particles than plasma sprayed coatings. The microhardness of the surface layer of the substrates beneath the HVOF coatings was found to be increased significantly whereas no such change was observed for plasma sprayed coatings. These results indicate that the plastic deformation caused by impingement of poorly molten particles at high velocity is the cause of compressive stress during HVOF thermal spraying.