Glazes are attractive materials as they can be applied onto metallic or ceramic substrates to confer on them specific properties. They find numerous applications, from art ornamenting to protection against corrosion. Conventional process (vitreous glazing) requires a high temperature treatment (up to 1400 °C in some cases) to fuse glazes after their application on the surface to be covered. This treatment cannot be hence applied onto heat-sensitive substrates without severe degradation. Previous studies showed that manufacturing glaze layers by flame spraying prevents the substrate from thermal degradation. The coating formation mechanisms are different from the ones encountered with crystallized ceramic materials: the high surface tension of glazes prevents the particles from being totally spread (i.e., "dewetting" phenomena). Effects of glaze powder characteristics (chemical composition, particles morphology) on coatings structures were also studied. Furthermore, chemical analyses proved that flame spraying did not modify glaze compositions. The most adapted powder to flame spraying has been hence selected. This contribution describes the coating formation mechanism and discusses the influence of the feedstock powder physical properties on coating characteristics. It also estimates effects of spraying parameters on coatings morphology.

High velocity oxygen fuel (HVOF) sprayed WC coatings have been found wide applications as substitutions of chrome plating in the airplane industry. The wear of HVOF WC coated landing gear against Al-Ni-Bronze alloy leads to the wear or spalling of WC particles, and finally results in the degradation of sealing property for landing gear system. WC-Co coating with nano-size WC particles is expected to exhibit good wear resistance, and the weight loss of Al-Ni-Bronze alloy can also be reduced, which is beneficial to the sealing property of landing gear system. Three types of coatings are prepared using agglomerated, sintered nano-scaled WC-12Co, conventional WC- 12Co and WC-10Co4C powders, respectively. The wear behaviors of the as-sprayed coatings against Al-Ni- Bronze alloy are investigated at the different loads and linear velocities. The study is expected to provide the basis for the application of HVO/AF nano-scaled WC coating in landing gear.

Flame sprayed Al-12Si coatings were produced onto the surface of composite castings parts in order to enhance the adhesion of such castings. Due to the high surface roughness and the presence of pores in the coatings combined with the formation of an intermetallic phase at the interface, the adhesion of flame sprayed composite castings could be enhanced by a factor of 2 in comparison to blank castings and by a factor of 1.3 when compared to sand-blasted castings. However, results also show that gaps are mostly present at the interface between the Al profiles and the flame sprayed coatings and these gaps have a negative effect on the adhesion values of the composite casting parts. Therefore, an optimization of the adhesion of the coating on the Al profiles through an optimization of both the sand-blasting and the flame spraying parameters would be beneficial for further enhancement of the adhesion of composite casting parts.

Mullite coatings (3Al 2 O 3 ·2SiO 2 ) 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 300gf) 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.

A study was carried out to determine the role of HVOF-spray Ni-20Cr coatings to enhance high temperature oxidation resistance of T22 and 347H boiler steels. Ni-20Cr coatings were deposited on the boiler steels. Subsequently high temperature oxidation behavior of these coatings was investigated by laboratory tests at an elevated temperature of 9000C under cyclic conditions. These coatings were also subjected to a boiler environment at an elevated temperature of 700ºC under cyclic conditions to ascertain their erosion-corrosion behaviour. Thermogravimetric data was taken after each cycle to formulate the oxidation, as well as, erosion-corrosion kinetics. The specimens were also physically examined after each cycle to notice any macrographic physical changes, spallation tendency of their oxide scales etc. The exposed specimens were examined using scanning electron microscopy and Energy Dispersive Spectroscopy (SEM/EDS) analysis. It was observed that oxidation rate was subsequently low in the coated substrates as compare to uncoated steels. It was observed that Ni-20Cr coating on 347H boiler substrate showed the maximum oxidation resistance. Moreover, both the steels showed better resistance to erosion-corrosion in actual boiler conditions after the application of Ni-20Cr coating.

Nano structured coatings applied by supersonic flame spray processes show a better bonding mechanism, superior hardness and better wear resistance compared to coatings with micron scale structure. However, handling and particle feeding of smaller scale (< 20µm) spray powders is difficult due to their large surface area and easy agglomeration, but also health risks. Therefore, nano structured oxide ceramic powders are mixed with organic solvents in order to form liquid suspensions that are suitable to improve the particle feeding properties. Recent attempts to understand the momentum and heat transfer mechanisms between flame and particles in HVOF flame spraying led to measurement of the in-flight particle properties and computational modeling of the processes. In this work, modeling and simulation of the HVOF spraying process as a two phase model is applied in order to analyze thermal and mass flow processes for an optimization of the spray particle properties and the final properties of the coatings themselves. Simulation results are given for particle tracking during the spray process. Thereby, particle properties are sensitive to a large number of process parameters as well as the particle diameter. Numerical results are validated by experimental diagnosis of particle properties with the SprayWatch system and by the analysis of experimental coatings.

A significant group of steel-making process parts is exposed to high contact pressure, shock abrasive wear and elevated temperature. High productivity repair techniques are necessary because of the large size of the parts. Analysis of coating metallographic investigations, wear and corrosion test results, full-scale tests shows that restoration of base share of these parts is possible by High Velocity Oxygen Fuel / High Velocity Air Fuel (HVOF/HVAF) process. Comparison of manufacture's data has showed that HVAF excels HVOF alternatives noticeably at productivity. At the same time production costs are 2-2.5 times less. With regard to typical steel-making process parts some investigations results and examples of HVAF restoration at Joint Stock Company "Mashprom" are represented.

The Cr 3 C 2 -NiCr coatings were sprayed on an AA 7050-T7 alloy by HVOF Thermal Spray Process using AMPERIT 586.054 powder. The substrate was refrigerated during the sample preparation using liquid nitrogen in a device specially designed for this purpose. The spray conditions were also performed increasing the carrier gas flow or the oxygen flow. The XRD and SEM studies showed well-bounded coating/substrate interface, pores, metallic matrix, chromium oxides, carbides, and carbides dissolution into the matrix. The structure comparison between the samples showed a denser structure due to the increase in the carrier gas velocity and the highest carbide dissolution due to the high temperature achieved during the spray process. All samples showed similar wear abrasive behavior, and all of them showed wear rates 7 times higher than the aluminum alloy. All samples also showed corrosion resistance much higher than the AA7050 alloy in NaCl solution.

Thermal fluxes transferred during HVOF thermal spray have an important influence on the thermal history of the work-piece. Thus, there is an interest in quantifying these fluxes. Today, different studies based on the use of calorimeters were proposed in the literature and CFD calculations may also be used to confirm the measures. In the present study, a new method based on the measurement of the transient increase of the work-piece temperature was used in order to estimate the thermal exchanges between an HVOF jet impinging on different diameter cylinders. The results indicate that fluxes of several MW/m 2 may be transferred depending on the standoff distance, spray parameters and gun.