This study investigates the influence of powder feed rate on the deposition efficiency of HVOF sprayed materials, including NiCr, FeNiCrMoCSi, 316L stainless, Cr 3 C 2 /NiCr, WC-Co/Cr, and WC-Cr 3 C 2 /Ni. A liquid-fuel HVOF spray gun is used in combination with a modified injector block that increases the number of powder injection ports from two to four. In the experiments, powder feed rates were incremented from a baseline of 100 g/min to 400 g/min for the metal powders and up to 500 g/min for the cermets, while measuring deposition efficiency for each run. Coating samples were also produced for metallographic analysis, hardness testing, and the evaluation of porosity and roughness. All results are presented and discussed along with potential implications on coating costs.

This work introduces a hybrid spray-and-fuse process and a modified CoCrMoC (Stellite) alloy that significantly expand the manufacturing window for wear-resistant coatings. The Co-based alloy was produced by adding Ni, B, and Si to Stellite 720 to lower its melting points and expand its melting range thereby improving the sprayability and fusibility of the material. The modified alloy was deposited on Inconel 718 balls and 1 in. diameter coupons by HVOF spraying and coating samples were sinter fused at high temperatures followed by furnace cooling. The processes used are described and test results are presented, showing that thick, metallurgically bonded coatings were achieved with high hardness and excellent wear and corrosion resistance.

Although detonation sprayed coatings are harder and better adhered than those achieved by plasma and even HVOF spraying, the method is far less used due to productivity and equipment challenges associated with mechanical valving and intermittent powder flows. This study shows how the use of propane as a fuel eliminates the need for mechanical valving and allows for continuous powder feeding through a high-frequency detonation spray gun. To demonstrate the capabilities of valveless detonation spraying, WC-CoCr powders were deposited under different conditions (oxygen-to-fuel ratio, flow rate, shot frequency) and the resulting coatings were assessed based on porosity, microhardness, deposition efficiency, and phase composition.

In the present work, hollow sphere YSZ powders were deposited by dc plasma spraying using a mixture of CO 2 and CH 4 gases. The plasma plume was monitored with an IR camera and in-flight particle velocity and temperature distributions were recorded at impact. Single splats were collected on mirror polished stainless steel and, along with coating samples, were characterized and compared to splats and coatings deposited by conventional argon spraying. In the case of CO 2 -CH 4 spraying, particle temperatures were at least 100 °C higher than the YSZ melting point and almost all splats were completely melted. Coating surfaces were also found to be smoother, indicating less unmelted or partially melted splats.

An air-oxygen controlled high velocity combustion spraying process has been developed that uses a special HVOF gun and a broad range of fuel-oxidant ratios. Extremely low flame temperatures can be achieved while maintaining a supersonic flow of combustion products, thus allowing the solid state deposition of almost all industrially relevant alloys. This work deals with the development of superhard cermet coatings using conventional and fine WC-Co(Cr) powders, optimized spray parameters, and different nozzle geometries. Results are compared based on coating microhardness, toughness, and sliding wear resistance.

A cryogenic cooling system has been developed that regulates part temperatures during thermal spraying, eliminating the need for inter-pass cooling breaks associated with traditional air cooling. A mixture of liquid nitrogen and cold nitrogen gas is directed through a nozzle onto the workpiece, with both flows being controlled based on noncontact temperature measurements. In spray trials, coatings were deposited in half the time with greater deposition efficiency, lower porosity, less oxidation and carbon loss, and minimized residual stresses.

This paper describes the basic design and operation of a multi-chamber detonation sprayer (MCDS) and explains how it differs from conventional detonation sprayers and other spraying methods. It presents and analyzes ceramic, metal, and cermet coatings produced with the new sprayer, highlighting its ability to deposit low-melting point and refractory materials. It compares the efficiency and operating costs of the MCDS with some of the more common HVOF torches and reviews the coating properties that have been achieved with different powders and spraying parameters.

This paper presents a new method for producing high-energy pulsed plasma flows with a dc air torch and demonstrates its use in surface hardening and thermal spraying applications. The method employs electromagnetic plasmadynamics and is capable of generating high-frequency pulsed-periodic plasmas at atmospheric and high pressure. Plasma flow velocities of 3-5 x 103 m/s at temperatures of 15 x 103 K have been achieved.

This work shows that with computer-controlled detonation spraying, the phase composition of coatings can be changed relative to that of the feedstock powders. New phases can appear in substantial quantities due to chemical reactions of reduction, oxidation, and nitridation as well as interfacial interactions between phases in composite powders. The key advantage of computer control is that it precisely regulates the quantity and stoichiometry of explosive gas mixtures. It has thereby been found that TiO 2 experiences partial reduction to titanium suboxides and that chemical reactions with nitrogen are also possible. It has also been found that when nitrogen is present, titanium aluminides, Ti 3 Al and TiAl, are likely to form nitrides in the sprayed coatings. Interfacial reactions between the phases of a composite have been studied, and in the case of the Ti 3 SiC 2 -Cu system, it has been found that deintercalation of Si can be prevented by maintaining relatively cold spraying conditions. At higher temperatures, coatings of an unusual phase composition form in which carbon-deficient TiCx inclusions are distributed in the Cu matrix as modified by the dissolution of silicon. The formation of new phases affects coating microstructure development and results in new microstructural features.

Low-temperature HVOF spraying finds its potential applications in the preparation of high-performance metallic coatings with low oxygen content and porosity. In this study, the spraying method is used to deposit fine WC-10Co4Cr powders on stainless steel substrates at different spraying distances and the effects on microstructure, microhardness, fracture toughness, splat morphology, and surface roughness are investigated and discussed.

In Warm Spraying (WS), the temperature of the combustion flame is reduced and controlled by injecting nitrogen gas into the combustion flame before the injection of spray powders. Thus, temperatures of spray particles are kept under their melting points with moderately heated and thermally softened states. As compared to HVOF-spraying, the oxidation of particles can be significantly suppressed due to lower deposition temperatures, whereas, as compared to cold spraying, the degree of particle deformation upon impact can be enhanced by attaining higher particle temperatures. In the present study, Ti, Cu, and Al coatings were fabricated by WS under various nitrogen flow rates. The mechanical properties of the coatings were evaluated by tubular coating tensile (TCT) and micro flat tensile (MFT) tests. For the lower impact temperature regime, the coatings became denser and the ultimate strengths of Ti or Cu coatings increased reaching a maximum by decreasing the nitrogen flow rates. A further decrease of nitrogen flow rates and reaching the upper temperature regime reduced the coating strength. These results clearly demonstrate how particle temperatures affect the microstructures and mechanical properties of WS coatings and that optimum spray conditions have to be balanced between softening and oxidation by adjusting particle temperatures.

A conventional GTV K2 kerosene fuel HVOF spraying system has been modified with the aim to achieve process conditions comparable to cold gas spraying concerning the average particle velocities and surface temperatures in the spray distance. The employed measurements include the use of expansion nozzles that have been optimized for supersonic conditions up to a Mach number of 2.5 and the use of combustion chambers with reduced critical diameters that provide increased combustion chamber pressures up to 1600 kPa. Copper powders with different size fractions and oxygen content are sprayed with the novel HVOF technology. Coatings are analysed concerning their microstructure, oxygen content and electrical conductivity. In-flight particle velocities and surface temperatures are determined by the GTV NIR Sensor. Results are compared with those obtained for cold gas spraying using identical powders. The new HVOF technology permits the production of copper coatings that show similar levels of porosity, oxygen content and electrical conductivity like cold gas sprayed coatings. Also aluminium powder has been sprayed successfully with the novel technology. In-flight particle velocities can be almost as high as in modern cold gas spraying systems. Coatings are analysed and show a microstructure comparable to cold gas sprayed coatings.

Superior characteristics of the cold sprayed coating have led to many high-tech applications. Until recently, all these applications were carried out using ‘stationary’ systems only, while some applications such as in-situ repair of aircraft body/engine parts require a portable system. Recently a ‘Portable High Pressure Cold Spray System’ called KINETIKS 2000-2 has been developed. This system is capable of 400 C/20 bars nitrogen/helium jet, and produces dense coatings with clean interfaces of many materials. In order to establish the suitability of this process for producing aluminum alloy coatings for aerospace and other high tech industries, various aluminum alloys (CP-Al, HP-Al, 6061 Al, 7005 Al) coatings were produced over many substrate materials (2024 Al, 7005 Al, 4041 Steel, ZE41A Mg). Coatings were characterized using microstructure, bond strength, bend test, corrosion studies, etc. Microstructural study showed that dense coatings with about 2-4% porosity values were produced with clean and well bonded interfaces. Bond strength of these coatings varied between 20 to 35 MPa, Bend test results showed that the coatings have adequate strengths and could withstand severe strain conditions. Salt fog corrosion studies (ASTM B 117) showed that the coatings impart corrosion resistance to the substrates.

Solution precursor plasma spray (SPPS) is a thermal spray process in which deposits are formed by injecting solutions with the appropriate chemistry directly into the plasma. The deposits consist of grains or particles as small as ~20nm, and may be very porous or nearly dense, depending on the solution and deposition parameters. Recently, the potential of SPPS to deposit fine particle, porous coatings suitable for use as electrochemical electrodes for fuel cells and gas sensors has been demonstrated. This paper describes the efforts to deposit LiFePO 4 coatings which may be of interest for Li ion battery electrodes with SPPS. In this case, along with the porosity, surface area, and microstructure of the deposited coatings, crystal structure also plays an important role in determining the performance of the LiFePO 4 electrodes. Solution precursors with different solution chemistries containing lithium, iron and phosphorus ions are injected into hydrocarbon plasma issuing from a DC-arc torch. The effects of solution chemistries on coating morphologies and crystal structure were investigated. The results indicate that the porosity and crystal structure of the coatings can be tailored by selecting different additives.

In this work suspension-HVOF spraying (S-HVOF) was used to prepare dense and mechanical stable Al 2 O 3 sprayed coatings with high contents of α-Al 2 O 3 phase. Aqueous suspensions with various contents of powder (from 25 wt.% up to 50 wt.%) have been developed starting from two commercial α-Al 2 O 3 powders, characterized by different purity and particle sizes. The suspensions have been internally injected in a modified combustion chamber of a HVOF TopGun-torch. Coating microstructures, phase compositions and mechanical properties resulting from the interaction between suspension characteristics and spray parameters are presented. Use of suspensions with high solid contents allowed the production of thick, dense and mechanically stable coatings. The α-Al 2 O 3 was the main phase in the coatings produced through the injection of suspensions containing powders with very high purity.