Search Results for velocity

In wire arc spraying, atomizing gas velocity and particle velocity are important factors influencing coating quality. A nozzle with secondary gas injection has been developed to increase the gas velocity and to improve coating quality. In this study, wire arc spraying of stainless steel on aluminum substrates has been investigated with the objective of establishing correlations between atomizing gas velocities, particle velocities, particle sizes and coating bond strength. Cold gas velocity is measured with a Pitot tube. Particle velocities are determined from high speed images of particle streaks taken with a Kodak high speed vision system and evaluated using image analysis. Bond strength is measured with pull-off tensile test. Secondary gas atomization clearly leads to improved adhesion due to additional metallurgical bonding between the coating and the substrate achieved through higher particle temperatures at the moment of impact.

In this study, microstructural characterization is conducted on WC-17Co coatings produced via High Velocity Oxygen Fuel (HVOF), High Velocity Air Fuel (HVAF), and Cold Spraying (CS). All coatings prepared were observed to be of good quality and with relatively low porosity content. SEM study showed important microstructural features and grain morphologies of each coating. While composition of feedstock material was approximately similar, elemental composition using EDS showed higher Co content and lower WC in the CS deposited coating. XRD experiment identified formation of more complex oxides and tungsten phases in coatings deposited technologies involving melting of powders such as HVOF and HVAF. These phases consisted mainly of cobalt oxides and brittle phases such as W 3 Co 3 C or W 2 C caused by decarburization of the tungsten carbide particles. Hardness of all coating samples were examined and CS deposited coating exhibited considerably lower hardness compared to the other two coating samples instead of having significantly lower porosity content. It could be contributed to dissociation and physical loss of hard carbide phase during high velocity impact of particles in CS process. It is in good agreement with detection of higher amount of cobalt in CS deposited coating material. It is strongly believed that results obtained from this study can be used for future investigation in thermo-mechanical properties of WC-Co coatings.

In cold spray and thermal spray applications, one of the primary factors affecting coating deposition is the location where particles are injected into the gas jet. Therefore, a detailed knowledge of the gas flow distribution is required. Non-resonant laser scattering allows to spatially resolve the distribution of drift velocity and mass density within the flow, particularly at locations close to the injector. Based on laser scattering, this paper presents a new diagnostic that locally measures drift velocity, as well as a relative mass density distribution of a gas stream. Its application is mainly focused on cold gas flows, where velocity measurements in a supersonic nozzle, obtained by means of laser scattering, correlate well with theoretical calculations and particle image velocimetry (PIV) experimental results.

WC-Co cermet powders were deposited on aluminum substrates by cold gas spraying. XRD tests were run on the powder and coatings to reveal possible phase changes during spraying. Bonding strength, abrasive wear resistance, and corrosion resistance were also measured and are compared with values obtained from HVOF sprayed WC-Co coatings.

This study investigates particle velocities achieved by high-pressure warm spraying. Commercially pure titanium (CP-Ti) and Ti-6Al-4V powders were deposited on different substrates while varying spray parameters to determine their effect on particle velocity and coating quality. Particle image velocimetry was used to measure particle velocity, which peaked at 1,000 m/s. Coatings obtained under optimized conditions were characterized based on porosity, oxygen content, and hardness. The results show that the increased velocity of high-pressure warm spraying has significant beneficial effects in terms of improving density and controlling porosity and oxygen content.

The aim of this work is to obtain a better understanding of high-velocity suspension flame spraying though detailed modeling and analysis. A numerical model was developed and is used to analyze the effects of process parameters including droplet size, injection velocity, secondary droplet breakup, solution evaporation and combustion, mass flow rate, flame temperature, and the location of the injection point. The results show that initial injection parameters play an important role in controlling the process and should be specified to minimize cooling effects and promote droplet evaporation inside the combustion chamber. It was also found that the ideal location for the injection point is inside the chamber, which in practice, can be achieved by slightly modifying gun geometry.

Atmospheric plasma spray parameters were developed for a three-cathode torch with a high-velocity nozzle and MCrAlY powders of different particle size fractions. The main objectives of the work are to achieve bond coats with low oxygen content and porosity. Other goals are achieving sufficient surface roughness at high deposition rates and efficiencies. The oxidation behavior of the sprayed coatings was characterized by thermal gravimetric analyses and isothermal heat treatments.

In the present work, a coupled thermomechanical-Eulerian model was developed to calculate particle impact behavior during cold spraying. The model is based on temperature-displacement elements and is capable of nonlinear transient analysis of impact deformation and heat conduction. Simulation results show that heat conduction has a major effect on temperature distribution within the particle, but little influence on particle deformation. The FEA model is also used to calculate critical velocities of commonly sprayed materials and the results are evaluated based on the morphology of deformed particles and plastic strain analysis.

Development of coatings using the TriplexPro 200 plasma gun has provided an ideal means for implementing process maps due to the large operating window in terms of particle velocity and particle temperature, as well as the flexibility to use multiple plasma gasses to tailor the coating process. Process mapping enables tracking of coating characteristics, such as hardness, and relating those characteristics to the conditions of the particle that are induced upon the particle by the process parameters. Work performed to date has provided new insights into conditions of the powder particle that result in specific characteristics in the coating. An example is the ability to determine the critical particle energy state that affects coating stress. This work affords an understanding of general theory behind coating characteristics that result from the conditions of the particle. This paper describes the parameter impact in controlling coating stresses and determining optimum particle conditions to produce a desired, or set of desired, coating characteristics.

In this paper, the effect of velocity on the characteristics of atmospheric plasma sprayed yttria stabilized zirconia was investigated through adjusting auxiliary helium flow. The temperature and velocity of in-flight particles were measured with DPV2000 analyzer. The results showed that helium flow significantly influenced particle velocity and less distinctly influenced particle temperature. The microstructure of the coatings was characterized by scanning electron microscopy and X-ray diffraction analyzer. The ionic conductivity of the deposits through thickness direction was measured by a potentiostat/galvanostat based on three-electrode assembly approach in a temperature range of 500-1000 °C. The specific gas permeability was estimated. The results showed that the gas permeability was improved by increasing the in-flight particle velocity. However, the in-flight particle velocity has little effect on the ionic conductivity of specimens.

Process mapping is an ideal method for tracking coating characteristics in the thermal spray process. With the increased utilization of in-flight particle diagnostic tools in recent years it is now possible to quickly and effectively characterize inflight powder particle properties. With industries' increasing understanding of the relationship of these properties and coating characteristics, it is now possible to rapidly understand the implications of in-process changes with respect to coating performance. This paper is an exploratory exercise that describes the utilization of process mapping of in-flight particle velocity and temperature characteristics to optimize tungsten carbide (WC) coatings sprayed with a High Velocity Plasma torch (HVP). Key performance factors of WC coatings include high inherent hardness, low porosity and neutral to compressive stress conditions. The combination of these factors all contribute to the coatings' overall success in it's intended application and elude to its toughness, wear resistance, corrosion resistance and general ability to protect the required components. Presently, the High Velocity Oxygen Fuel (HVOF) and High Velocity Liquid Fuel (HVLF) combustion processes are the favored method of applying dependable and commercially viable WC coatings that meet all of these criteria.

In this paper, submicron α-Fe/nylon-12 microwave absorbing composite coatings were deposited by a Low Temperature High Velocity Air Fuel (LTHVAF) spraying technique. The microstructure and the electromagnetic parameters of coatings and powders were tested. The coatings are dense and have low porosity. The microwave reflectivity coefficient of the coatings was calculated with permeability and permittivity of the powders. It shows that there is a relationship between the mass fraction of composite powders and microwave absorption ability of coatings. At the threshold value, the composite coatings can absorb microwave strongly. When the coatings thickness increases, the minimal reflectivity coefficient moves to the low microwave frequency. There exists an appropriate coatings thickness in order to optimize the absorption of the microwave energy. The mass fraction and the thickness can affect the performance of composite absorber coatings.

Wear at the electrode surfaces of a one-cathode plasma torch changes the characteristic fluctuation pattern of the plasma jet. This affects the trajectory of the particles injected into the plasma jet in a non-controllable way, which degrades the reproducibility of the process. Time-based voltage measurements and Fourier analysis were carried out on a one-cathode F4 torch at different wear conditions to determine the evolution of wear-dependent characteristics. A significant correlation is observed between increasing torch wear and decreasing voltage roughness and high frequency noise. Furthermore, by means of particle diagnostic systems, the change in the particle velocity and temperature has been measured. The variations of the particle characteristics are significant and thus an influence on the sprayed coating microstructure is to be expected.

The structural and mechanical properties, in terms of surface roughness, hardness and wear resistance, of WC/12wt%Co coatings are investigated as a function of powder properties, such as powder size and WC grain size, as well as high velocity oxygen-fuel (HVOF) spray conditions. It has been found that the WC/12wt%Co coatings with relatively smoother surface, about 2 µm in average surface roughness (Ra), retaining high wear resistance can be formed by optimizing the powder and spray parameters. Further, we have found that powder size can control the surface roughness over a wide range; from about 2 to 7 µm (about Δ5 µm) in Ra. On the other hand, Ra changed only about Δ1 µm or more when changing spray conditions, such as barrel length and spray distance.

High-velocity air-fuel (HVAF) is a combustion process that allows solid-state deposition of metallic particles with minimum oxidation and decomposition. Although HVAF and cold spray are similar in terms of solid-state particle deposition, slightly higher temperature of HVAF may allow further particle softening and in turn more particle deformation upon impact. The present study aims to produce dense Ti-6Al-4V coatings by utilizing an inner-diameter (ID) HVAF gun. The ID gun is considered a scaled-down version of the standard HVAF with a narrower jet, beneficial for near-net-shape manufacturing. To explore the potential of the ID gun in the solid-state deposition process, an investigation was made into the effect of spraying parameters (i.e., spraying distance, fuel pressure, and nozzle length) on the characteristics of in-flight particles and the attributes of the as-fabricated coating such as porosity, oxygen content, and hardness. Using online diagnostics to monitor temperature and velocity of in-flight Ti-6Al-4V particles is challenging due to exothermic oxidation reaction of fine particles, while larger particles are too cold to be detected from their thermal emission. However, DPV diagnostic system was successfully employed to differentiate the non-emitting solid particles from the burning ones. It was found that increasing air and fuel pressure of the ID-HVAF jet led to an increase of the velocity of the in-flight particles, and resulted in improved density and hardness of the as-sprayed samples. However, increasing the spraying distance had a negative effect on the density and hardness of the deposits. It was also observed that the phases of the Ti-6Al-4V deposits were altered by producing vanadium oxide due to the high temperature of the spray jet.

Thermal spraying of fine and ultrafine powders is realised by a novel method based on highly filled filaments as feedstock material for high velocity oxy fuel flame spraying (HVOF). Hereby, the desired coating material is supplied as finely dispersed powder within a polymer filament. Thus, the polymer works only as a transport medium and is fully decomposed when entering the combustion zone instantly releasing the solid dispersion, similar to the liquid dispersion medium in a suspension. It can work as an appropriate method to process fine and ultrafine powders. The solid nature of the dispersion medium poses several benefits compared to liquids, especially from the manufacturing point of view, since the process is geared to a wire flame spraying method. This work focusses on the challenges and benefits of this novel approach. First experimental results of spraying different filaments are presented.

The coating stresses induced by thermal spray using a High Velocity Gas Fuel (HVOF) and Liquid Fuel (HVLF) gun and a High Velocity Plasma (HVP) gun with the high velocity nozzle are compared using a curvature based in-situ coating stress analysis approach that measures the deflection of a beam while a coating is applied to it. This novel diagnostic tool provides new insights into the internal stresses generated in a coating system during the actual application of the coating. Coatings were sprayed with three process guns and the same material feed stock that result in similar coating structures and properties. HVOF, HVLF and HVP processes induce similar particle energy states at high velocity regimes as measured with particle diagnostic tools during spraying but due to the differences in particle history are expected to result in different coating stresses. In some cases the actual measured stress conditions using the in-situ coating stress method were dramatically different. Analysis is presented to explain the reason for these surprising results. The understanding of these differences will lead to an improved methodology for mapping coating processes from one another along with a more in depth understanding of coating stresses buildup.

High Velocity Suspension Flame Spraying (HVSFS) has been developed to thermally spray suspensions containing micron, sub micron and nanoparticles with hypersonic speed. For this purpose, the suspension is introduced directly into the combustion chamber of a modified HVOF torch. The aim in mind is to achieve dense coatings with a refined microstructure. Especially from nanostructured coatings superior physical properties are expected for many potential applications. Direct spraying of suspensions offers flexibility in combining and processing different materials. It is a cost saving process and allows the allocation of entirely new application fields. The paper gives an overview of the HVSFS spray method and will present some actual results that have been achieved by spraying the nanooxide ceramic materials Al 2 O 3 , TiO 2 , 3YSZ and Cr 2 O 3 .

The High-Velocity Suspension Flame Spraying (HVSFS) technique, a recently-developed modification to the standard HVOF process enabling the use of suspension feedstock, was employed in order to deposit Al 2 O 3 coatings from a nanopowder suspension. These coatings were compared to conventional APS and HVOF-sprayed ones. HVSFS coatings possess lower overall porosity and lower pore interconnectivity degree. Indeed, most of the nanoparticles were fully melted by the gas jet, thus forming very thin, well-flattened lamellae, having smaller columnar crystals than conventional coatings. Accordingly, HVSFS coatings possess higher hardness and elastic modulus, as determined from nanoindentation tests. Ball-on-disk tribological tests also indicate that HVSFS coatings possess much better sliding wear resistance than conventional ones, because they are capable of forming denser and more protective surface tribofilms during dry sliding.

Cold gas dynamic spraying, namely cold spray, is an innovative coating process in which powder particles are injected in a supersonic gas flow to be accelerated above a certain critical velocity. Even though particles adhesion onto the substrate has not be yet elucidated, it appears clearly that it is influenced by particle impact velocity, which results from spraying conditions, diameter of particles and their positions from the center of the particle jet. Particle velocity can change dramatically depending on particle position from the core to the rim of the jet. In the present work, an original experimental set-up was designed to discriminate the particles as a function of the levels of velocity to investigate the influence of this parameter on adhesion. Particles at given positions in the jet could therefore be observed using SEM (Scanning Electron Microscopy), which showed different morphologies and microstructures as a function of impact velocity. High pressure and tangential velocity at the interface during impact were calculated from numerical simulations using ABAQUS. TEM (Transmission Electron Microscopy) analyses of thin foils were carried out to investigate into resulting local interface phenomena. These were correlated to particle impact velocity and corresponding adhesion strength which was obtained from LASAT testing (LAser Shock Adhesion Test).