The high-velocity air-fuel process (HVAF) is an emerging technology used in the thermal spray industry. The Praxair HVAF process combines air and a liquid fuel (e.g., kerosene, diesel) to generate an energy source with extremely high gas velocities. Analytical studies were conducted to investigate gas and particle dynamics in the Praxair HVAF process for coating with WC-l2Co and stainless steel powders. The mass, momentum, and energy conservation equations were first solved, using the TORCH computer program. Typical output from the model includes temperature and velocity profiles as a function of radial and axial position. The PROCESS gas/particle computer program was then used to calculate from these temperature and velocity profiles the dynamics of particles injected into the gas plume. The primary result of the gas/particle code is a description of the injected particle temperature and velocity as a function of position in the plume. A thorough understanding of the process was obtained using this modeling technique. The results of the modeling were confirmed with process diagnostics. Particle temperature measurements for the WC-Co powder system were obtained with a two-color pyrometer; particle velocity measurements were obtained using particle imaging velocimetry. The coatings produced in the study exhibit superior quality, with high-density, high-hardness, low-oxide content, and high-bond strength.

The High-Velocity Combustion Wire Process is a new high-velocity combustion process now being used in the thermal spray industry. This process combines air, oxygen, and a fuel gas to generate a high-temperature, high-velocity plume that is optimum for producing metallic coatings. Analytical studies were conducted to investigate gas and droplet dynamics for the spraying of three different materials: aluminum, stainless steel, and molybdenum. With the relatively low flame temperatures of the process, the feedstock wire is melted by convective heat transfer with no superheating or vaporization of the droplets. When the droplets strike the substrate, their temperature peaks as the high kinetic energy of the droplet is transformed into thermal energy. The conservation equations were solved using the TORCH computer model, yielding the temperature and velocity profiles as a function of location. The PROCESS gas/droplet computer program was then employed to calculate the dynamics of the molten droplets. The results of this modeling was confirmed with process diagnostics. Experimentation included droplet temperature measurements using a two-color pyrometer and droplet velocity measurements using particle imaging velocimetry for the stainless steel material system. The coatings produced in the study exhibit superior quality with high density, high hardness, low oxide content, and high bond strength.

The mechanical properties of EMAA copolymer are dependent upon the thermal spray processing parameters. The parameters determine coating temperatures which, in turn, affects the microstructure. If the deposition temperature is too low, (104 °C for PFl 13 and 160 °C for PFl 11) coatings have low strengths and low energy to break values. Increased coating temperatures allow the particles to fully coalesce resulting in maximized strength and elongation to break. However, at 271 °C, PFl 11 had visible porosity which decreased both strength and elastic modulus. Pigment acts as reinforcement in the sense that the modulus increased but the elongation to break decreased, thus reducing the energy to break. Water quenching reduces the elastic modulus and yield strength, but increases the elongation to break for both EMAA formulations. The mechanical properties of post consumer commingled plastic and PCCP / EMMA blends improved if the recycled plastic was pre-processed by melt-compounding. Melt compounding increased the strength and toughness by improving the compatibility among the various polymer constituents. The addition of PCCP increases the modulus and yield strength of ethylene methaciylic acid copolymer.

The High Velocity Continuous Combustion (HVCC) Spray System was invented in 1993. The HVCC process uses wire as a consumable feedstock. Atomization takes place in a supersonic air jet, which is produced using a converging-diverging nozzle arrangement. The arc point is located within this supersonic air stream, which produces extremely finely atomized particles averaging 30 µm in diameter. The morphology of the HVCC produced coatings are homogeneous and consist of flat splat platelets with thin, adherent oxides. Permeation by gases and liquids through coating structures is significantly lower than that typically seen for twin wire arc spray coatings. Wear and erosion resistance of materials are significantly better than the same material applied with twin wire arc spray. The HVCC process has been successfully deployed over the past 7 years. It has been successfully used in confined space, in-situ applications, where traditional High Velocity Oxy-Fuel (HVOF) processes cannot be used for reasons of safety and practicality. These include applications in the petrochemical, paper and pulp, utility and independent power generating industries. Many other more specialized workshop and in-situ applications have also been performed with success.

Though wire flame spraying is a relatively old thermal spray process, modern equipment permits production of high quality coatings featuring outstanding homogeneity, high density and low roughness due to increased particle velocities as a result of increased combustion gas velocity. Typically spray particles are accelerated to velocities exceeding 250 m/s, if the wires are atomized adequately. In order to make a wide spectrum of coating materials available for wire flame spraying use of cored wires needs to be considered. A high speed camera is used to determine the particle velocity depending on process conditions for massive, grooved cored and tube cored AISI 316L wires. Thereby the influence of the wire design without simultaneous influence by the chemical composition is studied. Additionally nickel based carbide reinforced coatings are sprayed and characterized concerning their microstructure and properties in use.

Activated Combustion HVAF Spraying (AC-HVAF) involves a jet of air-fuel combustion products to deposit coatings of metallic and carbide powders. In the process, spray particles are heated below their melting temperature while accelerated to velocity typically 700-850 m/s, forming a coating upon impact with a substrate. Extremely low oxygen content and high density are distinguished features of the AC-HVAF coatings, resulting in their excellent performance under conditions of severe wear and corrosion. Besides new level of coating quality, the AC-HVAF process demonstrates outstanding technological efficiency and spray rates 5-10 times exceeding those of the HVOF counterparts. The paper presents results on characterization of selected metallic and carbide coatings and describes their applications.

The aim of this work is to develop an analytical methodology for the analysis and prediction of high-velocity suspension flame spraying (HVSFS) under various operating conditions, to determine the effect of individual parameters on the process, and to aid and promote the design of HVSFS torches. A key aspect of the work is the development of a model that accounts for fuel gas combustion, evaporation of organic solvents, and heat, momentum, and mass transfer between the flame and suspension droplets.

In order to select the effective sprayed coating for giving the thermal barrier to the Al alloy components of internal-combustion engine. The properties of three thermal sprayed coatings (Coating : Top coat; Al2O3, Bond coat; Ni-Cr, Coating②: Top coat; YSZ, Bond coat; CoNiCrAlY, Coating ③ : SUS316) were compared. The results are as follows. (1)After thermal cycle test, there was not any crack and delamination in the coating ③ , and a partial cracks and delamination were observed at the top coating in coating① and ②. (2)The coating①, ②and③ have sufficient adhesion strength. The adhesion strength of coating② was specially high. (3)The thermal barrier property of the coating② was better than that of coating① and coating③. (4) In the result of comprehensive evaluation, the coating② had good thermal barrier property and the coating③ was a reasonable material because of low cost as the thermal sprayed coating applied to the Al alloy components of internal-combustion engine.

Higher operating temperatures coupled with biomass-derived fuels can lead to aggressive corrosion damage to the superheater/reheater tubes in power plants. In this study, a HVOF sprayed NiCr coating was deposited onto a 9 % Cr substrate, which were exposed in simulated coal-biomass combustion gases with a screening deposit containing Na2SO4, K2SO4 and Fe2O3 at 700-750°C for 1000 h. The tests were carried out using the “deposit-recoat” test method and pre and post-exposure dimensional metrology was used to quantify the coating damage in terms of metal loss distributions. The exposed samples were also examined in a SEM/ EDX. The coatings developed a protective Cr2O3 layer at the coatings/ deposit interface and a Cr depleted zone was observed underneath the oxide layer. NiCr coating provided suitable corrosion protection with a median metal loss of ~35μm in 1000h.

This paper investigates the microstructure and corrosion resistance of Hastelloy C-276 and 316L coatings produced by various thermal spray methods, including arc spraying, flame spraying, HVOF spraying, and a recently developed method called high-velocity combustion wire spraying. The microstructures of the coatings were examined before and after corrosion testing in order to gain information on corrosion mechanisms. Several corrosion tests were performed on each sample and various coating properties were measured including thickness, hardness, oxygen content, porosity, and adhesion strength. Test results for sealed coatings and detached layers are also presented in the paper, giving additional insight into corrosion behavior. Paper includes a German-language abstract.

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.

Atmospheric Rheo-Spraying (ARS) using the HVOF process enables injection-molded structures and thick-film coatings from steels and from high-temperature Ni-based alloys with layer thicknesses down to the centimeter. The ARS process control is based on the thermal spraying of particles in the solid state at a maximum average speed of more than 600 m/s. The coating consolidation to porosity values below 1% occurs through the particle impact with high kinetic energy. Because of the low particle oxidation, the mechanical properties of the heat-treated injection-molded structures are comparable to those of forged alloys. In this paper, ARS injection molding is successfully implemented in combination with an innovative manufacturing technique in rocket engine technology to produce a model composite combustion chamber with a thermally sprayed internal pressure jacket. Paper includes a German-language abstract.

This paper investigates the stability of molybdenum base silicides, which are located in combustion chambers and in an endothermic environment, for use in radiant tubes for heat treatment. The subject matter was plasma-sprayed molybdenum disilicide, pentamolybdenum trisilicide, hot-pressed molybdenum disilicide, and molybdenum disilicide composites containing SiC and silicon nitride reinforcements. Results of the investigation show that the oxidation resistance of plasma sprayed molybdenum disilicide can be detrimentally effected due to the silicon loss that occurs during the high temperature plasma spray process. Paper includes a German-language abstract.

The Hitachi MaxJet II is a unique multi-purpose combustion gun. It can be used for spraying wire but it also has the capability to spray multiple materials via-powder and powder cord form. The equipment operates using commonly available gases: compressed air, Oxygen and Propane or Mapp gas. It requires no major facility cost since it uses only a 35 cfm air compressor and 115 volt power supply. Safety is assured with a safety purge system, separation of electrical and gases systems and flash back arresters. The small compact system weighs less than two hundred pounds, which makes it easily movable for on site work. It’s low capital investment and high quality coatings with low porosity and excellent bond strengths. The electronic pusher type wire feed provides consistent feed rates for a large variety of wires and wire sizes (1/16”, 1/8”, and 3/16”). The spray gun weighs only three pounds and can be mounted on a robot or used for hand spray applications. It functions well in a shop environment or onsite spraying of bridge components.

Improved HVOF spraying with a gas shroud has been developed to fabricate environmental barrier coatings of corrosion resistant alloys such as HastelloyC. For such coatings, control of oxidation of the powder material during spraying is very important and the gas shroud has been effective to lower oxygen content to 0.19mass%. In the present study, further reduction of oxygen content to 0.063mass% was achieved by changing the composition of combustion gas by introducing nitrogen into the combustion chamber. This value is almost comparable to the oxygen content 0.042mass% of the feedstock powder but the porosity of the coating increased. Introduction of nitrogen to the combustion chamber lowered the temperature of the spray particles in flight while maintaining their high velocity. Another coating with 0.14mass% was obtained with open porosity below 0.1vol% by changing the mixing ratio of nitrogen, which exhibited improved environmental barrier property in artificial seawater.

In Activated Combustion HVAF process, coatings are formed of powder particles, heated and accelerated by high-velocity jet of air and gaseous fuel combustion products. A distinguished feature of the process is that spray particles are heated below their melting point while accelerated to velocity well above 700 m/s. Such spray scheme appeared beneficial for deposition of cemented carbides, in particular, WC-based composites. Dense, practically non-oxidized neither heat-deteriorated coatings were formed. Spray rates from 1 to 25 kg/hr were achieved without decline of coating quality or deposition efficiency. Specific coating structure resulted in noticeably improved resistance to fatigue at high level of stresses.

To improve wear resistance of the atmospheric thermal plasma sprayed molybdenum coating, diamond deposition on the molybdenum plate and the atmospheric plasma sprayed molybdenum coating by the combustion flame chemical vapor deposition (CVD) was carried out. Diamond has excellent properties such as low surface energy, hardness, chemical corrosion resistance ability and so on. Besides, since the combustion flame CVD is the process carried out in the air, diamond/ molybdenum complex coating can be deposited without any vacuum facilities by using this technique if molybdenum coating is deposited by atmospheric thermal spray. In this study, acetylene welding torch was used as diamond synthesis apparatus and mass flow ratio C 2 H 2 /O 2 was varied from 0.9 to 1.3. Consequently, many diamond particles which were 10 micrometer in diameter respectively were deposited on the molybdenum plate by only 20 minutes combustion flame irradiation in the case of 1.2 in mass flow ratio of C 2 H 2 /O 2 . Especially, the molybdenum coating was covered with diamond films consists of 10 micrometer diameter particles in the case of over 1373K in deposition temperature. Besides, according to the results of wear testing, wear mass loss of diamond deposited coatings were much lower than that of original thermal sprayed molybdenum coatings. From these results, this process was found to have a high potential in order to improve wear resistance of thermal sprayed coating.

Thermal spray processes using wires as feedstock are widely used to produce wear and corrosion protective coatings of nickel, cobalt or iron based alloys. In general, these coatings are processed by flame or arc spraying. In view of using massive wires as spraying material, the hardness and wear resistance of layers is limited by the possibility to produce the corresponding wires of such materials. In addition, the performance of wire sprayed coatings can be restrained by the amount of defects in the microstructure, like pores, oxides and cracks, which are particularly evident in the cases of flame and arc spraying. New High Velocity Combustion Wire (HVCW) systems open the opportunity to reduce the amount and size of the defects by an increased particle velocity. Also, improvements on wear resistance may be achieved by using cored wires. The paper gives an overview on recent developments in HVCW spraying using massive and cored wires.

In this paper, a coating system for a thermal barrier, consisting of a CoNiCrAlY adhesive layer (HVOF) and a zirconium dioxide top layer (APS), is optimized with the help of particle diagnostics. The paper describes the optimization approach for a thermal barrier coating system with the aid of particle diagnostics and considering the results of the hot tests carried out within the scope of TEKAN on the DLR technology test stand P8 in Lampoldshausen. The objective is to engineer a thermal protective coating in a duplex setup for a cryogenic rocket engine and its heat flow densities of approximately 80-90 MW/meter square. Paper includes a German-language abstract.

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.