Driven by the search for an optimum combination of particle velocity and process temperature to achieve dense hard metal coatings at high deposition efficiencies and powder feed rates, the high velocity air-fuel spraying process (HVAF) was developed. In terms of achievable particle velocities and temperatures, this process can be classified between high velocity oxy-fuel spraying (HVOF) and cold gas spraying (CGS). The particular advantages of HVAF regarding moderate process temperatures, high particle velocities as well as high productivity and efficiency suggest that the application of HVAF should be also investigated for the manufacture of MCrAlY (M = Co and/or Ni) bond coats (BCs) in thermal barrier coating (TBC) systems. In this work, corresponding HVAF spray parameters were developed based on detailed process analyses. Different diagnostics were carried out to characterize the working gas jet and the particles in flight. The coatings were investigated with respect to their microstructure, surface roughness and oxygen content. The spray process was assessed for its effectiveness. Process diagnostics as well as calculations of the gas flow in the jet and the particle acceleration and heating were applied to explain the governing mechanisms on the coating characteristics. The results show that HVAF is a promising alternative manufacturing process.

Atmospheric plasma sprayed (APS) CuNiIn coatings have been widely used for fretting wear protection in many important areas such as aircraft engines for decades. The oxides in CuNiIn coating prepared by APS hinder splat bonding formation and thus degrade the coating fretting performance. In this study, CuNiIn powders of different boron contents were designed to realize the self-oxide-cleaning effect for in-flight molten droplets and thus deposit the dense CuNiIn coating with high fretting performance. Scanning electron microscope was used to characterize the microstructure. The oxygen content in the coating was measured by the inert gas fusion technique. Fretting test was performed for three coatings under different loadings. The results show that CuNiIn2B and CuNiIn4B coatings presented the oxide content of 0.40wt% and 0.38wt%, which are lower than 1.6wt% of the CuNiIn coating. The oxygen content in the CuNiIn4B coating decreased with the increase of spray distance while the oxygen content in CuNiIn coating increased with the increase of the spray distance. Such results clearly reveal the boron in-situ deoxidizing effect of inflight molten droplets. As a result, the dense CuNiIn2B and CuNiIn4B coatings were deposited with oxide-free molten droplets. The test results showed that the fretting wear performance of B-alloyed CuNiIn coatings were increased by a factor over three comparing with conventional CuNiIn coating.

This paper presents the results of two metals coatings, molybdenum and tantalum, prepared by Controlled Atmosphere Plasma Spray (CAPS) onto Al 6061 substrates that were thermal cycled to calculate the effective coating modulus. Traditional uniaxial tensile testing samples were prepared from thicker duplicate coatings for comparison, as well as to measure thermal expansion properties and oxygen and nitrogen content. The molybdenum samples cut from thicker coatings were un-able to be tensile tested due to their fragility. Thermal cycle testing of molybdenum on an Al 6061 substrate was found to have a modulus approximately 18 to 19% of literature values for bulk molybdenum using the bi-layer beam thermal cycling method. Additionally, non-linear modulus behaviour was observed in the molybdenum sample when enough thermal strain was induced to shift the coating from a compressive to tensile stress state. The tantalum coating was found to have a modulus approximately 42 to 46% of literature values for bulk tantalum using the bi-layer thermal cycling method. Traditional tensile testing measured a modulus approximately 44 to 46% of bulk, which shows good agreement between the two methods and supports that the bi-layer thermal cycling method is valid for plasma sprayed refractory metal coatings.

Thermal spray coatings are widely used to protect materials from corrosion, wear, and oxidation, but they have yet to reach their full potential because of porosity limitations and the detrimental effects of oxidation on interlamellar bonding. This paper investigates an atmospheric plasma spraying process that deposits oxide-free dense metallic coatings with well bonded lamellae. The process produces ultrahigh temperature metallic droplets, up to 2650 °C, using specially designed powders that are deoxidized in-flight through the evaporation or gasification of oxides. The impact of these oxide-free ultrahigh temperature droplets has a spreading-fusing, self-metallurgical bonding effect resulting in fully dense bulk-like metallic coatings. Various coating materials, including NiCrMo, 304SS-Mo, NiCrBSi, and Al, are investigated, demonstrating the versatility of the new technique.

In this study, NiCr alloy coatings were deposited by arc spraying using different combinations and mixtures of pressurizing gases and other process modifications. Coating properties were examined mainly by SEM, EDS, and conductivity measurements. The results show significantly reduced oxygen contents and improved coating morphologies compared to reference coatings produced using current plasma processes. Improved microstructure is shown to have a positive effect on surface quality and specific resistivity, making it possible to texture arc-sprayed coatings just as successfully as the plasma-sprayed reference layers. Moreover, the temperature coefficients and resistivities of arc-sprayed NiCr were found to be superior to those of conventionally manufactured coatings.

In the course of this investigation, thermal spraying with different fuel and shroud gas combinations was investigated in terms of its effect on the in-flight particle properties (temperature, velocity) and on the final coating properties (coating thickness, porosity, oxygen content and corrosion behaviour). Independent on the shroud gas, the particle in-flight temperature and velocity were highest when using ethylene as fuel gas and lowest when using propane. Methylene resulted in intermediate properties. The change in the shroud gas from air to nitrogen generally resulted in lower in-flight particle temperatures and also lower velocity. The coating properties in terms of porosity and oxygen content directly correlated to the particle in-flight properties. With decreasing velocity and increasing temperature, the porosity and the oxygen content increased, respectively. The corrosion behaviour of the nickel coatings was studied in 0.5 M sulfuric acid media by means of potentiodynamic polarization curves. Good corrosion properties were observed when methylene and air served as fuel gas and shroud gas, respectively.

In this investigation, different atomizing gases, arc wire spray guns, and wire sizes were used to deposit ZnAl coatings on high-strength steel substrates. Sample sets corresponding to different gas mixtures and pressures as well as other parameters were produced and the coatings obtained were evaluated based on morphology, porosity, composition, phase distribution, and oxide content. The results are presented and discussed, particularly with regard to corrosion lifetime and performance.

This work focuses on the properties of Cu-Ag alloys deposited by cold spraying. Helium was used as the carrier gas, accelerating particles to 823 m/sec, which is in the middle of the deposition window for Cu alloys. To avoid oxygen contamination, the gun was placed in a helium-filled chamber and a closed-loop circulating system was used to minimize helium loss. Deposition parameters were varied during spraying and their effect on hardness, tensile properties, residual stress, and porosity was assessed in as-sprayed and heat-treated samples. Ultimate tensile strengths of 450 MPa and yield strengths of about 420 MPa were obtained for the as-sprayed samples and it was shown that strength and ductility can be tailored by heat treating, reaching elongation values higher than 45%. An increase in deposition rate from 55 to 142 g/min was also achieved without a significant decrease in mechanical properties.

In this present work, investigators determine how particle temperature, combustion pressure, and heat treatment affect the porosity, oxide content, and tensile properties of warm-sprayed titanium. Coatings were deposited with nitrogen flow rates ranging from 0.5 to 1.5 m 3 /min and combustion pressures of 1 and 4 MPa. Optimal coating properties were found for specimens formed at a nitrogen flow rate of 0.75 m 3 /min and a combustion pressure of 4 MPa. Post-spray heat treatment was found to improve bonding between deposited particles, significantly increasing the strength and ductility of the titanium coatings.

In this study, twin wire arc spraying is used to bond wire mesh to the outside surfaces of stainless steel pipes in order to increase heat transfer surface area. At the optimum spray distance, the oxide content, porosity, and adhesion strength of the coatings are shown to be 6.6%, 2.1%, and 24 MPa, respectively. Pipes with different wire mesh configurations were placed in an oven and heated to temperatures from 300 °C to 900 °C. Water temperatures were measured at the inlet and outlet of the pipe for flow rates between 0.2 and 0.5 gpm. A maximum water temperature rise of 13 °C was achieved, corresponding to a total heat flux of 57 kW/m2. Heat transfer efficiency is shown to depend strongly on the quality of the bonds between the wire mesh and pipe and the spacing of wires in the mesh.

This study investigates the effects of gas composition on cold-sprayed titanium coatings deposited under nine different spray conditions. Experiments show that higher levels of gas purity translate to higher particle velocities and measurable improvements in bending strength. The influence of gas temperature, pressure, and chemical composition is considered in the study along with interactions between carrier gases and sprayed particles. In addition to bending strength, the resulting coatings are assessed in terms of porosity and oxygen content.

This study investigates particle velocities achieved by high-pressure warm spraying. Commercially pure titanium (CP-Ti) and Ti-6Al-4V powders were deposited on different substrates while varying spray parameters to determine their effect on particle velocity and coating quality. Particle image velocimetry was used to measure particle velocity, which peaked at 1,000 m/s. Coatings obtained under optimized conditions were characterized based on porosity, oxygen content, and hardness. The results show that the increased velocity of high-pressure warm spraying has significant beneficial effects in terms of improving density and controlling porosity and oxygen content.

Lightweight gamma titanium aluminide (γ-TiAl) intermetallic alloys have recently found application in low-pressure turbine blades in the aviation industry, but their use is currently limited to around 700 °C due to oxidation. This study evaluates the potential of various multilayer coating systems to increase the operating temperature range of γ-TiAl. The coating systems tested are based on a CoNiCrAlY topcoat for oxidation protection and a YSZ diffusion barrier, both applied by atmospheric plasma spraying using a three-cathode torch. Two bond coats, NiCrBSi and CoNiCrAlY, were also tested. Test specimens with bond coats withstood 1000 h of exposure at 900 °C without delamination and no detectable oxygen at the coating-substrate interface. Samples produced with varying feed rates showed that graded coatings can be achieved using the APS process.

Atmospheric plasma spray parameters were developed for a three-cathode torch with a high-velocity nozzle and MCrAlY powders of different particle size fractions. The main objectives of the work are to achieve bond coats with low oxygen content and porosity. Other goals are achieving sufficient surface roughness at high deposition rates and efficiencies. The oxidation behavior of the sprayed coatings was characterized by thermal gravimetric analyses and isothermal heat treatments.

This study investigates the effect of dry-ice blasting distance on the deposition of CoNiCrAlY coatings obtained by plasma spraying. Dry-ice blasting was used before, during, and after spraying and its effect on coating quality was measured. The results show how blasting distance influences the deposition efficiency as well as the microstructure, porosity, adhesion strength, hardness, and oxide content of plasma-sprayed coatings. The optimal dry-ice blasting distance was proposed as 25 mm.

Recent developments of High-Velocity Air-Fuel (HVAF) spraying and blasting focused on a substantial increase of spray particles velocity. The efforts further improved coating quality, allowing deposition of metallic and carbide-base coatings non-permeable to gas at thickness as low as 40-50 micron. The coatings demonstrate low dissolved oxygen content, a favorable combination of high hardness and toughness. Coupled with the enhanced technological efficiency of modern HVAF equipment, this initiated not only the acceptance of HVAF technologies in established thermal spray markets in the oil and gas industry, but also the development and successful implementation of new coating applications. The examples are wear and corrosion resistant tungsten carbide-based coatings on hydraulics rods of dock cranes, corrosion resistant Ni-Cr-Mo-type coatings on vessels of sulfur removal equipment, tungsten carbide coatings on restriction grid plates and slide gates of catalyst towers, high-temperature erosion resistant chromium carbide- based coatings on thermowells and valve stems, wear and cavitation resistant Co-Cr-W-C-type and carbide coatings on housing wear rings and impeller hubs of high-temperature pumps.

Zirconium (Zr) metal is of interest for chemical corrosion protection and nuclear reactor core applications. Inert chamber plasma spraying has been used to produce thin Zr coatings on stainless steel (SS) substrates. The coatings were deposited while using transferred arc (TA) cleaning/heating at 5 different current levels. In order to better understand thermal diffusion governed processes, the coating porosity, grain size and interdiffusion with the substrate were measured as a function of TA current. Low porosity (3.5% to < 0.5%), recrystallization with fine equiaxed grain size (3-8 µm diameter) and varying elemental diffusion distance (0-50 µm) from the coating substrate interface were observed. In addition, the coatings were low in oxygen content compared to the wrought SS substrates. The Zr coatings sprayed under these conditions look promising for highly demanding applications.

In the past years a number of publications reported about Titanium coatings cold sprayed with a nominal power input between 17 to 47 KW (e. g Kinetiks 4000) reaching gas temperatures of maximum 850 °C and gas pressure of maximum 4 MPa. In a recent study at Helmut-Schmidt University (HSU), a Kinetiks 8000 prototype was used to spray titanium, employing a nominal power of about 92 KW to increase the gas temperature up to 1000°C at a pressure of 4 MPa. Under these parameters, a high tensile strength of over 480 MPa and a deposition efficiency (DE) close to 100% were achieved. The present study focuses on further enhanced gas and particle velocities by optimized nozzle designs. The increased particle velocities in comparison to that obtained by using commercial nozzles (types 24, 51) result in better coating performance and allow deviations from ideal (90°) impact angle without significantly reducing coating strength. The influences of process conditions are evaluated and discussed on the basis of coating strengths by Micro Flat Tensile and Tubular Coating Tensile tests, as well as electrical conductivities, nitrogen and oxygen contents.

Thick titanium coatings were prepared by warm spraying (WS) and cold spraying (CS) process to investigate the oxidation and microstructure of the coating layers. Prior to the coating formations, the temperature and velocity of in-flight titanium powder particle were numerically calculated. Significant oxidation occurred in WS process using higher gas temperature conditions with low nitrogen flow rate, which is mixed to the flame jet of an HVOF spray gun in order to control the temperature of the propellant gas. Oxidation, however, decreased strikingly as the nitrogen flow rate increased. In CS process using nitrogen or helium as a propellant gas, little oxidation was observed. Although most of the cross-sections of the coating layers prepared by conventional mechanical polishing looked dense, coating cross sections prepared by an ion-milling method revealed the actual microstructures containing small pores and unbounded interfaces between deposited particles. Even when scanning electron microscopy or x-ray diffraction method did not detect oxides in the coating layers by WS using high nitrogen flow rate or CS using helium, the inert gas fusion method revealed minor increase of oxygen content below 0.3 wt%.

The adhesion mechanisms involved in the cold spray coatings are not still well elucidated. The quality of the deposit does depend mainly on particles and dynamic characteristics (which result from nozzle type, nozzle-substrate distance, etc.). The present work is based on the study of particle-substrate and particle-particle interfaces in the tantalum-copper coating-substrate system. The content focuses on the influence of the oxygen content in the starting powder on interface features, consequently on coating properties. Tantalum powders with different oxygen levels were studied using SEM (Scanning Electron Microscopy) and EPMA (Electron Probe Microanalysis). Laser shock spallation of cold-sprayed Ta coatings was developed as a reliable and flexible process to achieve Ta spalls to be deposited at a high-velocity onto Cu targets. The velocity due to the laser shock could be controlled to be similar to that of particles in conventional cold spray. This results in Ta-Cu interfaces, the study of which was carried out to go into interface phenomena involved in cold spray, using TEM (Transmission Electron Microscopy) in particular. Results were compared to those obtained from laser shock spallation of Ta bulk specimens (i.e. made of a conventional Ta sheet). The role of powder oxidation on interface soundness was exhibited. Adhesion was shown to be all the lower as powder oxygen content was higher, using LASAT (“ Laser Shock Adhesion Test”) in addition to direct observation of interfaces. Results were exploited to discuss properties of the corresponding Ta coatings onto Cu, i.e. which were cold sprayed using powders with different oxygen contents.