The paper deals with an analysis of particle behavior in a HVOF-jet as a function of process parameters. The experiments were carried out using a commercial HVOF gun with hydrogen as fuel gas. A commercial 316 L stainless steel powder was used for spray material. The experiments were conducted by using on-line particle diagnostics in order to simultaneously detect particle velocity, temperature and diameter. The results were correlated with caught spray powder and splats. The experiments released that among many parameters the oxygen/fuel gas ratio is the most important parameter. It was found, that fragmentation of partially melted particles in a supersonic jet is not a neglectable phenomenon. It may cause excessive oxidation inflight and therefore is to be avoided in the production of oxygen-poor coatings. Furthermore, it was found that velocity or temperature measurements of particles have to be interpreted very carefully in order to characterize the process. Without taking a possible change of the particle size due to inflight fragmentation into consideration either measurement value will lead to very limited information benefit.

Hardmetal-like coatings of the TiC-Ni system are potential for use as wear, corrosion and heat resistant coatings in various operation conditions. Our previous works [1-12] have shown that these materials are well sprayable using different thermal spray processes such as plasma, D-Gun and HVOF spraying. Since HVOF spraying is today the most important process used to apply carbide based coatings, this study was carried out in order to evaluate more systematically the sprayability of these novel spray powders and the influence of HVOF spray parameters on some coating properties. Coating samples were prepared by using DJ Hybrid gun with propane as a fuel gas, and a CDS gun with hydrogen fuel gas. Oxygen flow rate was varied in both cases for changing the flame temperature. Microstructure, phase composition, hardness, and abrasion wear resistance of the coated samples were investigated. The results showed that both HVOF processes used give satisfactory coating properties and that the use of high oxygen flow rates is beneficial for improving the wear resistance of the coatings. Powders with fine particle size are beneficial in the DJ Hybrid process; the use of coarse powders results in coatings with somewhat higher wear rates. The optimum spray condition for the TiC-Ni system powders differs from that typically used for conventional WC-Co and Cr3C2-NiCr powders by a higher flame temperature.

In this paper, the production of NiCr-TiC powder by SHS, suitable for HVOF spraying, is discussed together with results on the microstructure and coating properties. Compacts for SHS were prepared by mixing elemental Ti and C with pre-alloyed Ni-20wt.% Cr powder to give an overall composition of 35wt.% NiCr and 65wt.% TiC. These were then ignited and a self-sustaining reaction proceeded to completion. Reacted compacts were crushed, sieved, and classified to give feedstock powders in size ranges of 10-45 µm and 45-75 µm. All powder was characterized prior to spraying based on particle size distribution, x-ray diffraction (XRD), and scanning electron microscopy (SEM/EDS). Thermal spraying was performed using both H2 and C3H6 as fuel gases in a UTP/Miller Thermal HVOF system. The resulting coatings were characterized by SEM and XRD analysis, and the microstructures correlated with powder size and spray conditions. Abrasive wear was determined by a modified 'dry sand rubber wheel' (DSRW) test and wear rates were measured. It has been found that wear rates comparable to those of HVOF sprayed WC-17wt% Co coatings can be achieved.

This investigation was conducted to clarify the effects of process parameters on the formation of the new amorphous coating using 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 65% depending on the spraying 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.

Shreenath has been engaged in the manufacturing and reclamation of engineering components by using the full range of thermal spraying since 1990. The analytical case studies of some of the tribological solutions provided to the core industries in Central India by HVOF spraying are presented in this paper. The case studies presented in this paper are the reclamation of the Vertical shaft of a bowl mill used for feeding crushed coal to the power plant, the engine spares of the earth moving machinery of the mining industries and the press rolls made up of chilled cast alloy steel, of defence industry. The surface characteristics needed for these applications, along with high hardness, bond strength and thermal conductivity could be achieved by the HVOF spraying using Topgun. In all the cases, the component life has been increased few folds and a direct cost saving of 30-70% was achieved. In addition, such an optimized reclamation reduces the down-time loss and improves the working efficiency.

High Velocity Oxygen Fuel (HVOF) spraying established itself as an effective method in addition to the conventional thermal spray processes within a very short period. Self fluxing nickel alloys, cermets (e.g. WC-Co / Cr3C2-NiCr) as well as oxide ceramic coatings have proved themselves suitable for wear protection applications. Weight reduction, the care of resources and the increase of efficiency for structural components leads to the substitution of customary hard particles. Titanium carbide (TiC) characterizes itself on account of the material features such as the high hardness, the high melting point, the high strength and the low density for the substitution of conventional carbides. The Self Propagating High Temperature Synthesis (SHS) is a suitable process for the production of composite powders. The powders produced by SHS show a high carbide content, which is finely distributed with an almost stoichiometric composition of the TiC inside the powder particles. The carbides are protected against dissociation and oxidation during the thermal spray process by a complete velum of matrix alloy. The current investigations deal with the wear resistance of TiC-composite coatings produced by HVOF compared to conventional wear resistant coatings. The investigations contain the analysis of the microstructure by optical and scanning electron microscope (SEM) and the measurement of the microhardness of the deposited coatings. Special attention is drawn to the interface between the hard particles and the matrix alloy. The optimized coatings are tested with different wear tests, such as Taber-Abraser test, sliding and oscillating wear test and are compared with common wear resistant coatings in order to underline the high potential for different wear applications. Moreover an additional corrosion test (salt fog test) is carried out with regard to the corrosion resistance of the different matrix alloys.

High-velocity air fuel (HVAF) spraying was selected for spray trials of a Cr3C2-NiCr powder. To determine the effect of spray parameters on coating characteristics, particularly porosity and phase degradation, a statistical design of experiments was implemented. A wide range of statistical designs have been applied to the optimization of thermal spray coatings with a great deal of success. In this instance, a lack of prior knowledge and the need to assess many process-variable interactions efficiently led to the selection of a two-level full factorial design. High and low settings for each variable, including spray distance, traverse speed, and powder feedrate, were chosen based on the ranges typically used to spray similar materials. The resulting coatings were assessed for microhardness, porosity, residual stress, deposition efficiency, and phase transformation, after which several follow-up runs were conducted to explore trends brought to light by the initial factorial design.

Nozzle geometry has a profound effect on HVOF spraying, influencing combustion gas dynamics as well as particle behavior. Nozzle dimensions are also important in cold gas-dynamic spraying (CGDS), particularly the length of the nozzle which affects gas flow temperature and speed. In this study, numerical simulations and experiments were conducted to determine how the length of the entrance convergent section of gun nozzles affects HVOF spraying. Process changes that occur inside the nozzle (as predicted by simulation) were correlated with coating properties. An Al2O3-TiO2 powder was used for the experimental studies. Changes in nozzle length had a significant impact on deposition efficiency, microstructure, hardness, and particle velocity. These relationships (as measured and calculated) were then applied to the nozzle design for the CGDS method.

This paper compares the coating characteristics of two HVOF processes: air-cooled converging-nozzle Diamond Jet (DJ) spraying and Hybrid 2600 air/water-cooled converging/diverging technology. WC-Co coatings were deposited on steel substrates using different combinations of spray parameters, gas flow ratios, and cooling gas types. The coatings were then examined and tested to determine the extent to which microstructure, hardness, surface roughness, wear resistance, and deposition efficiency can be controlled. In addition to investigating process relationships, the paper also addresses the issues of practicality and cost.

Four high velocity thermal spray guns were evaluated in the production of 10%Co-4%Cr tungsten carbide cermets. Three HVOF guns (the JP-5000, JP-5000ST and DJ-2700) and one plasma gun, (the Mettech Axial III) were used to spray the same angular, agglomerated and crushed WC-10Co-4Cr powder. The DPV-2000 was used to monitor the in-flight velocity and temperature of the WC cermet sprayed particles. From those values, spray conditions were selected to produce coatings that were evaluated in terms of porosity, hardness and deposition efficiency. Results show that the plasma Axial III provides the highest particle temperature, between 2000°C and 2600°C, depending on the spray conditions. The JP-5000 imparts the highest velocity to the particles, between 550 m/s and 700 m/s, depending on the spray conditions. The ST version of the JP-5000 provides the same velocity as the standard version but with lower particle temperature. The DJ-2700 sprays particles with temperature and velocity between those of the JP5000 and the Mettech Axial III. Minimum porosity values of 2.1%, 3.7% and 5.3%) were obtained for the JP-5000, the DJ-2700 and the Axial III guns respectively. The porosity and carbide degradation are found to mostly depend on the particle velocity and temperature respectively. The values for the Vickers microhardness number (200g) ranged from 950 to 1250. Measurements of the deposition efficiency indicated a variation between 10 and 80%o, depending on the spray conditions and the gun used.

Owing to high particle velocity upon impact, and consequently low porosity and high bond strength of so-obtained coatings, HVOF spraying process is widely used to improve components life in service. However, many parameters can affect metallic coatings properties, especially un-melted particles and oxidation level. Flame parameters, such as calorific power, combustion ratio and temperature, are of prime importance. The aim of this work was focused on the influence of these parameters on stainless steel coatings characteristics. For different substrate temperatures, maintained through CO2 cooling nozzles, those parameters varied independently. Flame characteristics were computed using a simple model for propylene as fuel gas. Microstructure investigation as well as oxide content measurements and microhardness were obtained. It appeared that combustion temperature, in the range studied (2600-2750K) was not a critical factor. However, combustion ratio and calorific power greatly influenced coating properties: an increase of oxide content, and consequently a higher microhardness, was observed when combustion ratio decreased as well as when calorific power increased.

BaTiO3 has been successfully sprayed by HVOF to produce dense 25-150 µm thick deposits for use as dielectric and capacitive layers within prototype multilayer conformal electronics. Parameter optimization has been shown to play a critical role in the effective spraying of these materials as thin structurally homogeneous deposits. The affect of standoff distance and combustion chamber size on the phase structure of the coatings have been studied and related to the dielectric properties of the layer. The proportion of crystalline to amorphous phase was found to be critically dependent upon the degree of melting of the particles in the flame and the rate of cooling of the deposits. The crystalline/amorphous ratio is directly related to the dielectric properties of the layer with greater crystallinity giving higher values of dielectric constant. Microcracks and splat/splat interfaces are also believed to adversely affect the dielectric properties. The maximum dielectric constant (K) values achieved using the HVOF method for deposition have been in the range 70-115.