Due to its oxidation resistance, HVOF thermal sprayed Cr 3 C 2 -NiCr coatings have been widely used for wear resistance at elevated temperatures up to 850ºC. During HVOF spraying deposition, compositional degradation occurs through dissolution of the carbide phase into the metal matrix. The occurrence of carbide dissolution and the high working temperatures affects on the final properties of the coating. The aim of this work is to study the effect on the structure and wear properties at elevated temperatures of Cr 3 C 2 - NiCr coatings using hydrogen as a fuel gas against propylene. The structural characterization was done by SEM-EDS, XRD, and Scanning White Light Interferometry (SWLI). Wear properties at room and high temperatures have been evaluated by Ball-on-Disk (ASTM G99-90). Oxidation resistance of all coatings was carried out using a calorimetric technique Abstract only; no full-text paper available.

The high velocity oxy-fuel (HVOF) combustion spray process has been demonstrated to be a suitable technique for the deposition of nano-reinforced polymer matrix composite coatings by controlling both the particle dwell time and the substrate temperature. HVOF-sprayed polymer matrix composites incorporating reinforcements with size scales ranging from 7 nm to 100 µm are being studied to bridge between the nano and conventional scale regimes. Microstructural characterization has been used to characterize the dispersion and distribution of the ceramic reinforcements within the polymer matrix. The effect of particle size distribution on reinforcement dispersion and distribution has been studied, and the influence of substrate temperature on coating adhesion has also been investigated. Changes in crystallinity, as determined by Differential Scanning Calorimetry (DSC), are being correlated to coating microstructure, reinforcement loading and process parameter variations. Results of optical and scanning electron microscopy, scratch testing and DSC characterization of the feedstock materials and sprayed coatings are presented. Coatings of nominal 60 µm Nylon 11 with 10 vol. % of nano and micron size hydrophilic silica reinforcements exhibited a ~22 % improvements in scratch resistance compared to pure Nylon 11 coatings. An ~15 % improvement in scratch resistance was obtained for coatings containing 10 vol. % nano scale hydrophilic silica reinforcement.

Hard chromium, electrochemical deposited, is a widely used and established coating solution for surface protection against wear and corrosion as well as for decorative applications. Due to the possibility to coat bulk goods also, this technique is well suitable to protect small pieces and mass-produced goods. However, the most disadvantage of that technique is the appearance of highly carcinogenic hexa-valence chromium. Because of human health and environmental aspects the replacement of this coating technique is more and more required. At least for bigger parts with larger surfaces H(igh) V(elocity) O(xy) F(uel) -spraying can already produce competitive coatings to galvanic hard chromium with equal or even better properties. Especially thermal sprayed carbide coatings with in terms of corrosion resistance tailored metallic matrices offering the best potential to replace hard chromium in various applications. Beside of the material properties also the spraying conditions have essential influence on the corrosion resistance of those coatings. The present paper will give an overview of different carbide containing materials for HVOF-spraying and their properties with special respect to the replacement of galvanic hard chromium. The closest attention will be on the corrosion resistance of HVOF-coatings of those materials in different aqueous solutions compared with hard chromium. For a liquid fuel high velocity spraying system also the trends will be shown in which way the spraying parameters are influencing the general corrosion behaviour of such coatings. Those tendencies by conviction of the authors are also transferable to other HVOF systems.

It is widely known that during high velocity oxy-fuel (HVOF) spraying of tungsten carbide – cobalt (WC-Co) coatings, decomposition occurs resulting in the formation of W2C and a relatively brittle amorphous binder phase (along with other carbides and even metallic tungsten). Decomposition has generally been seen to be deleterious to the wear resistance of these coatings and, as such, there have been moves to reduce it. Since decomposition during spraying initiates with WC dissolution into the molten binder phase, strategies for its minimization have been based on reduction of particle temperatures and exposure times during spraying. Moves in spraying from gas-fuelled systems to liquid-fuelled systems have contributed towards these goals. This paper examines microstructural features and wear behaviour of WC-Co coatings deposited with both a liquid-fuelled and a gas-fuelled system. Contrary to expectation, it was found that the wear rate of the liquid-fuel sprayed coating was five to ten times higher than that of the gas-fuel sprayed coating. It was shown that whilst the degree of decomposition was limited during spraying with a liquid-fuelled system, the solid core of WC-Co suffers significant mechanical damage on impact as it is deposited, resulting in carbide fracture and size reduction and thus to the low observed wear resistance.

In the current study, the oxidation of hardmetal coatings (WC-12%Co, WC-17%Co, WC-10%Co4%Cr, WC-20%“CrC”-7%Ni, Cr 3 C 2 -25%NiCr, (Ti,Mo)(C,N)-29%Ni and (Ti,Mo)(C,N)-29%Co) in the temperature range 350- 900°C was studied for test durations ranging from 2 h to 128 h. The formation of oxide scales was investigated by X-ray diffraction, as well as by optical microscopy and SEM (including EDX) of coating cross sections. For coatings obtained by spraying with DJH 2700 and TopGun HVOF systems, the phase composition had only a moderate influence on high-temperature oxidation behavior in atmospheric conditions. The first oxides detectable by X-ray diffraction appeared on the coating surfaces after oxidation at 350°C for 128 h for all coatings. Pronounced oxidation (formation of oxide scales with thicknesses of greater than 10 µm) started at 600°C. Oxide scale growth differed significantly above this temperature among the hardmetal compositions studied here. WC-20%"CrC"-7%Ni and Cr 3 C 2 -NiCr had the highest oxidation resistance, with the oxide scale thicknesses lying below 10 ìm after oxidation at 800°C and 900°C for the two materials, respectively.

Titanium grade 2 as well as steel 446 and 316L powders are applied for production of X-HVOF coatings on mild steel substrates. Deposition efficiency is determined by process parameters. Microstructural investigations are carried out by means of optical microscopy, SEM and XRD. In addition comparisons of oxygen and nitrogen content in titanium coatings and powder feedstock are drawn. Corrosion protection capability of produced coatings is studied by current density-potential measurements and by salt fog tests. Depending on the process parameters increase of oxygen and nitrogen content can be restricted to factor of 2 compared to the powder feedstock. Coatings showing nearly theoretical density in metallographical inspections are possible. In direct comparison to wrought titanium grade 2 material the corrosion behavior of the titanium coatings is very promising. Keeping in mind that coatings have been produced under atmospheric conditions the observed increase of the corrosion current density by factor four is regarded an excellent result. During the corrosion tests no damage, neither to the surface nor the substrate - indicated by rust precipitates on the specimen surface, is observed. So penetration of corrosive medium to the substrate is securely avoided.

A new WC-WB-Co feedstock material was sprayed with two HVOF torches. During spraying, the in-flight temperature and velocity history were monitored using the DPV2000 tool. Coatings sprayed with both guns had a relatively high microhardness and good abrasion properties, even with a higher level of porosity for some coatings. This was explained by the nature of the matrix which was composed of an amorphous/nanocrystalline structure and W-Co-B phases. It is suggested that the matrix is harder than conventional binders such as cobalt, for instance, and exhibits a better cohesion with the WC hard phase.

The HVOF spraying process has recently been considered for the deposition of dense alumina coatings for dielectric coatings in semiconductor applications and as a turbine blade tip coating in aeroengines. However, due to the lower flame temperature of the HVOF process compared to the plasma spray process it is necessary to have good control over the key process parameters to achieve the correct coating characteristics. The work reported presents the results of a design of experiment study carried out on a TopGun HVOF system used to prepare coatings of alumina. The influence of several key parameters on coating characteristics such as porosity, alumina phase type, microhardness, surface roughness and adhesion have been determined. The parameters varied included oxygen and hydrogen fuel gas flow rates, and spray distance. Based on the results of these investigations recommendations are made on the control of key parameters and the range of coating characteristics that can be expected.

In the present paper, both CFD computations and experiments were carried out in order to quantify thermal fluxes from an impinging HVOF jet. The case of a CDS gun fed by natural gas is considered. Standoff distances ranging from 50 mm up to 300 mm were investigated. The CFD model implements a 2-layer extension to the Chen-Kim k-? turbulence model for a better quantification of thermal exchanges with the front body surface. Experiments were performed using two different methods. The first device incorporates two square calorimeters positioned side by side. It was elaborated in order to attempt defining the radial profile of the transferred thermal flux. Additionally, an infrared thermal camera was used to measure the front part surface temperature.

HVOF thermal spraying has been developed to deposit dense Al 2 O 3 -coatings with improved protective properties for various applications. Nanocrystalline coatings have been found to offer better thermal shock resistance, lower thermal conductivity and better wear resistance than their conventional counterparts. In this paper we describe the development of nanocrystalline Al 2 O 3 and Al 2 O 3 -Ni -coatings, where the grain size of Al 2 O 3 has been decreased and a few percents of nickel has been added in order to toughen the coating. Coatings were manufactured by HV- 2000 HVOF using spray parameters determined based on the on-line spray diagnostics. Parameters were selected aiming at different melting stages of the powder. The resulting microstructure of the coatings and its influence on the coating properties is discussed

Thermal spray has traditionally been used for depositing metallic, carbide and ceramic coatings, however, it has recently been found that the high kinetic energy of the High Velocity Oxy-Fuel (HVOF) thermal spray process also enables the solventless processing of high melt viscosity polymers, eliminating the need for harmful, volatile organic solvents. A primarily goal of this work was to develop a knowledge base and improved qualitative understanding of the impact behavior of polymeric particles sprayed by the HVOF combustion spray process. Numerical models of particle acceleration, heating and impact deformation during HVOF spraying of polymer particles have been developed. A Volume-of-Fluid (VoF) computational fluid mechanics package, Flow3D®, was used to model the fluid mechanics and heat transfer during particle impacts with a steel substrate. The radial temperature profiles predicted using particle acceleration and heat transfer models were used as initial conditions in Flow3D® together with a temperature-dependent viscosity model to simulate polymer particles with a low temperature, high viscosity core and high temperature, lower viscosity surface. This approach predicted deformed particles exhibiting a large, nearly hemispherical, core within a thin disk, and was consistent with experimental observations of thermally sprayed splats made using an optical microscope.

The mechanical deformation and microstructure formation at the interface of a high velocity oxygen fuel (HVOF) sprayed WC-Co splat on a carbon steel (0.45%C) have been studied by the focused ion beam (FIB) cross-sectioning and examined by scanning electron microscopy (SEM). Several types of WC-12wt.%Co powders with different WC particle sizes, 0.2 ~ 7.0µm were used for spraying to investigate the size effect of these WC particles onto the splat morphology and microstructure around the interface. The interface fracture toughness of the coatings made of these powders was evaluated by the pre-notched four-point bending test and the correlation between the splat microstructure and the toughness variation was investigated. The interface fracture toughness of these WC-12%Co coating / carbon steel was evaluated as about 600~1700J/m 2 . It was revealed that the WC particle size in the original powders has large effects onto the interface adhesion.