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High Velocity Processes
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Proceedings Papers
ITSC1997, Thermal Spray 1997: Proceedings from the United Thermal Spray Conference, 875-876, September 15–18, 1997,
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In thermal spraying, Fe-based alloys are often applied for relatively thick and inexpensive coatings. The main advantage of the Fe-based alloy coatings is their high ductility as compared to ceramic and hardmetal coatings. Other advantages such as high toughness, easy machineability and satisfactory corrosion resistance are characteristic of Fe-based alloys. The wear resistance is not outstanding, but nevertheless acceptable for a large number of applications. A further improvement of the wear resistance can be achieved by reinforcing the Fe-based alloy coatings, e.g. by addition of nitrogen to the spraying powder.
Proceedings Papers
ITSC1997, Thermal Spray 1997: Proceedings from the United Thermal Spray Conference, 877-883, September 15–18, 1997,
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Ceramic/polymer nanocomposites promise to be a new class of materials that will have wide application either for surface protection, providing low friction and inert corrosion barriers, or where tailored electrical and magnetic properties with increased abrasion and wear resistance are required. The high velocity oxy-fuel (HVOF) combustion spray process has been used to successfully process polymer-ceramic nanocomposites at 5 - 20 volume % of reinforcement. The latest results of process-structure- property relationship studies in silica and carbon black reinforced nylon 11 coatings are presented. It was found that the improvement in mechanical properties depends on the distribution and surface chemistry of the particulates and on the increase in matrix crystallinity due to the particulates.
Proceedings Papers
ITSC1997, Thermal Spray 1997: Proceedings from the United Thermal Spray Conference, 885-893, September 15–18, 1997,
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It is widely held that most of the oxidation in thermally sprayed coatings occurs on the surface of the droplet after it has flattened. The evidence in this paper suggests that, for the conditions studied here, oxidation of the top surface of flattened droplets is not the dominant oxidation mechanism. In this study, a mild steel wire (AISI 1025) was sprayed using a high-velocity oxy-fuel (HVOF) torch onto copper and aluminum substrates. Ion milling and Auger spectroscopy were used to examine the distribution of oxides within individual splats. Conventional metallographic analysis was also used to study oxide distributions within coatings that were sprayed under the same conditions. An analytical model for oxidation of the exposed surface of a splat is presented. Based on literature data, the model assumes that diffusion of iron through a solid FeO layer is the rate limiting factor in forming the oxide on the top surface of a splat. An FeO layer only a few thousandths of a micron thick is predicted to form on the splat surface as it cools. However, the experimental evidence shows that the oxide layers are typically 100x thicker than the predicted value. These thick, oxide layers are not always observed on the top surface of a splat. Indeed, in some instances the oxide layer is on the bottom, and the metal is on the top. The observed oxide distributions are more consistently explained if most of the oxide formed before the droplets impact the substrate.
Proceedings Papers
ITSC1997, Thermal Spray 1997: Proceedings from the United Thermal Spray Conference, 895-900, September 15–18, 1997,
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New near-net-shape structures of alloy Inconel 718 processed by HVOF spraying require optimum mechanical properties. Dominant factors defining the material quality are the particle properties velocity and temperature adjusted by the HVOF process parameters. Based on theoretical analysis of the HVOF process, experiments were performed with a defined variation of primary process parameters, producing coating samples of alloy 718 and measuring the particle velocities. Microstructural and X-ray analysis shows that in coatings with a high fraction of molten phase and high velocity, mainly divalent and spinell-type oxides are formed during particle impact on the substrate. Due to severe oxidation of the y'/y''- forming elements Ti, Al and Nb, precipitation-hardening effects of In 718 coatings are low. This leads to merely mediocre mechanical properties. The reduction of the molten phase to nearly zero leads to a drastic decrease of the oxide formation. The hardening γ'/γ'' phases are precipitated homogeneously in the Ni-base matrix. Strength values comparable to cast and wrought alloy In718 are attained by spraying with a low molten-phase fraction and high particle velocity. However, extensive intergranular 8-phase precipitation due to too high an Nb content of the powder causes only mediocre fracture elongation. Coatings up to 10 mm thick have been sprayed. The construction effort and hence the costs and weight of combustion chambers for hypersonic propulsion systems are to be reduced through direct thermal spraying of the loadbearing metallic pressure jacket onto the tubular cooling system. As a semifinished product, the selected Inconel 718 alloy exhibits good mechanical properties in the cryogenic temperature range as well as under higher thermal loads, and is commercially available in powder form. Aging serves to increase the strength up to the range of 1,200 N/mm2. For the sprayed In718 version, coating thicknesses in the centimeter range, a porosity < 1% and mechanical properties comparable with those of the cast version are required. The objective of the research work is to optimize spray-process control so that the resultant structural thick layers meet the design as well as the material requirements with respect to combustion-chamber technology. This necessitates elaborating the dominant microstructural parameters influencing the mechanical properties and the effect on them of the spraying process, and correlating them with the particle-condition parameters and the process parameters [1].
Proceedings Papers
ITSC1997, Thermal Spray 1997: Proceedings from the United Thermal Spray Conference, 901-908, September 15–18, 1997,
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Tungsten carbide-12 wt.% cobalt (WC-12Co) coatings and chromium plating are used to provide wear resistant surfaces in gas turbine applications. These treatments provide surfaces with hardnesses greater than 60 Rockwell C. In addition, a surface finish better than 8 microinches RMS is required for optimum performance. To achieve this surface finish, diamond grinding is required. The diamond grinding step adds considerable cost to the product and economical benefits could be achieved if more conventional grinding techniques were incorporated. A program was initiated to develop an alternative thermal spray coating, with a target hardness lower than 60 Rockwell C, but high enough to provide the wear resistance required. Spray development was conducted on five commercially available materials using the Diamond Jet 2600 high velocity oxy-fuel process. Laboratory evaluation included coating microstructure, macro- and microhardness, bond strength, salt spray corrosion, and cyclic compression tests.
Proceedings Papers
ITSC1997, Thermal Spray 1997: Proceedings from the United Thermal Spray Conference, 909-915, September 15–18, 1997,
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Agglomerated and sintered TiC-Ni based powders were sprayed by detonation gun spray (DGS) and high-velocity oxy-fuel (HVOF) spray processes. Influence of the binder content (20 and 27 vol.-%) and some alloying elements, such as Mo, Co and N on the coating properties were investigated. The coating structures and properties were investigated by optical microscopy, hardness measurements, X-ray diffraction analysis and by rubber-wheel abrasion wear test. It was found that alloying the hard phase with Mo and N leads to an improvement of the coating properties. Alloying of the binder phase with Co did not affect the coating properties. Porosity in the powder granules was found to beneficial in order to melt more efficiently the particles in the DGS process and especially in the HVOF process. HVOF spraying of powders with the higher binder content of 27 vol.-% was found to be advantageous for the preparation of coatings with dense microstructures and good wear resistances.
Proceedings Papers
ITSC1997, Thermal Spray 1997: Proceedings from the United Thermal Spray Conference, 917-923, September 15–18, 1997,
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Two boron-rich Fe/Cr based gas atomised powders (Armacor M and Armacor C) have been thermally sprayed using the HVOF process and the resultant deposits subsequently characterised, using X-ray diffraction, scanning electron microscopy (SEM), plan view transmission electron microscopy (TEM), and microhardness measurements. The wear and corrosion characteristics of the two alloy coatings have also been investigated by three body abrasive wear (utilising cross-sectional TEM to examine the worn surfaces) and potentiodynamic corrosion testing respectively. Results from microstructural analysis of the as-sprayed deposits revealed the presence of small chromium-iron boride precipitates within a predominantly amorphous matrix in the Fe-based Armacor M coating. The Fe-Cr-based Armacor C coating, however, consisted mainly of regions of nano- and microcrystalline material interspersed with chromium boride precipitation. Iron-chromium oxides have been observed within both of the alloy coatings studied. Both of these alloys exhibit good abrasive wear resistance when compared with other metallic based HVOF sprayed coatings. Both Armacor M and Armacor C also exhibit extensive passivation on exposure to an acidic solution. The wear and corrosion test results are related to the microstructural observations.
Proceedings Papers
ITSC1997, Thermal Spray 1997: Proceedings from the United Thermal Spray Conference, 925-928, September 15–18, 1997,
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X ray diffraction and transmission electron microscopy studies and measurements of hardness and void content were carried out for WC-20% Co coatings produced by detonation flame spraying at various oxygen/acetylene ratios in the detonating gas mixture. It was demonstrated that successive transition from (WC+Co) to (W 2 C+Co 3 W 3 C) to (W+CO 7 W 6 ) occurs as the oxygen content in the mixture is increased, and that amorphous-nanocrystalline structures form in the coating. Two types of these hybrid structures were revealed, one including an amorphous metallic matrix containing precipitates of intermetallic nanocrystals, the other having an amorphous oxide matrix and nanocrystalline precipitates of CO 3 O 4 and WO 3 . The hybrid structures were shown to improve coating density and hardness.
Proceedings Papers
ITSC1997, Thermal Spray 1997: Proceedings from the United Thermal Spray Conference, 929-933, September 15–18, 1997,
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The feasibility of using the HVOF process for the thermal spray-forming of free-standing components has been investigated. HVOF spray forming offers a number of potential advantages compared to the established procedure of plasma forming, including increases in component density, and reduction in material decomposition during spraying. Using blends of carbide and superalloy powders in various proportions, HVOF spraying has been successfully used to form free-standing cylinders and cones of various lengths and thicknesses. Microstructural examination of the spray-formed material has shown a homogeneous distribution of carbides in the superalloy matrix, with very low levels of porosity. Refinement of the procedure has allowed reduction of the matrix content, and the forming of fragile materials.
Proceedings Papers
ITSC1997, Thermal Spray 1997: Proceedings from the United Thermal Spray Conference, 935-942, September 15–18, 1997,
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The technology of thermal spraying is approaching maturity, and in the quest to reduce production costs whilst maintaining coating quality, attention is turning increasingly to more cost-effective routes for the manufacture of the starting powders. One such route is self-propagating high-temperature synthesis (SHS), which reduces the required energy input for powder production. In this work, TiC-Ti+Ni and (Ti, W)C-Ni powders produced by the SHS process have been studied in the as-received and as-sprayed states, to evaluate the suitability of SHS powders for the production of wear-resistant coatings. The starting powders and the coatings produced by atmospheric-plasma and HVOF spraying have been characterised using analytical (XRD, EDS) and microscopical techniques (optical, SEM). The technological properties of the as-sprayed coatings have also been characterised, including hardness, wear resistance (using a Rubber-Wheel test (ASTM G-65)) and corrosion resistance (in marine water environment).
Proceedings Papers
ITSC1997, Thermal Spray 1997: Proceedings from the United Thermal Spray Conference, 943-948, September 15–18, 1997,
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Mathematical modelling of the formation of the WC-Co coating structure and adhesion on copper substrate during high velocity oxygen-fuel (HVOF) spraying is provided. Smooth (polished) and rough (grit blasted) substrates are considered. Variations of solidification time, solidification velocity, thermal gradient, and cooling velocity in the coating and substrate interfacial region are studied. Formation of the amorphous and crystalline structures in the coating and of the crystalline structure in the substrate interfacial region is investigated. Behaviour of the crystal size and intercrystalline distance with respect to the thermal spray parameters and morphology of the substrate surface is analysed. Optimal conditions for the development of fine and dense crystalline structure are determined. Mechanical and thermal mechanisms of development of the substrate-coating adhesion are discussed. Results obtained agree well with experimental data.