In cold spray (CS) additive manufacturing process, micrometer scale particles accelerated through a supersonic nozzle are targeted on a surface with velocities in the rage of 300-1500 m/s in solid state. The impact energy of the particles leads them to deform plastically with high shear energy near the impact interface and adhere to the surface metallurgically, mechanically, and chemically. Using CS, deposition of metals, metal matrix composites, and polymers are achieved with high adhesive/cohesive strength and low porosity. Sensitivity of the CS additive manufacturing process to the variabilities in the process parameters are still being understood. Among the process parameters, particle morphology can have significant implications on drag forces, and therefore, on the particle impact velocity. This in turn affects the deposition efficiency (DE) and the quality of products. In this work, a new approach is introduced for computing DE by incorporating particle sphericity and its variation into one-dimensional numerical models. Size, sphericity, and the variability of size and sphericity of aluminum, copper, titanium, and tantalum particles are measured from static optical microscope images. The data is used for predicting impact velocity, temperature, and DE. The model results are then compared with particle velocity measurements.

Resonant ultrasound spectroscopy was applied to analyze the elastic anisotropy of thick copper, aluminum, titanium, and nickel coatings prepared by cold spraying and to determine the respective elastic moduli. The results show that the coatings exhibit only weak deviations from perfect isotropy, and the obtained elastic moduli are comparable with those of the corresponding polycrystalline bulks. The increased internal friction observed in some of the studied coatings may indicate grain refinement and consequent grain boundary sliding.

High quality coatings of titanium can be obtained by cold spraying using high process gas temperatures and pressures. However, the performance of cold sprayed coatings is determined not only by the respective material properties and the impact conditions, but also by the temperature and properties of the substrate—including the already deposited— material. In the present study, cold spray of spherical titanium grade II powders was performed on titanium grade II, copper, and stainless steel substrates, using two sets of parameters and three different substrate temperatures. Single impacts and respective particle adhesion were investigated using wipe tests followed by a modified cavitation test. Higher bond strengths were achieved for substrates that were held at higher temperatures during spraying. Moreover, the electrical conductivity of coating, taken as a measure of particle-particle bonding quality within the coating, improved and the porosity decreased for increased substrate temperatures. The findings are discussed in view of the thermal conditions, as well as the mechanical response of the uppermost layer of the substrate/deposit set.

This paper reports on the influence of the He to N 2 ratio on the properties of low pressure cold sprayed titanium coatings and on the characteristics of the generated supersonic two-phase flow. Experiments were carried out varying the He to N 2 concentration ranging from pure He to pure N 2 . Samples were characterized by their microstructural properties (i.e. microhardness and porosity). Deposition rate was evaluated and particle velocities were measured for all conditions. Deposition efficiency, coating density, and microhardness were found to be a function of particle impact velocity. Velocity data were used to validate a computational fluid dynamic model. The numerical solution of the flow inside the nozzle was obtained from the Euler equations for the various He to N 2 concentrations. Particle tracking was carried out by using the computed distribution of density, Mach number, temperature, viscosity, and a second order Runge-Kutta scheme. In addition, mean particle velocities at the exit of the nozzle were determined. Computed velocities were found to be in good agreement with measured ones. The model was then used to calculate nozzle dimensions that would maximize particle velocity. Optimized dimensions are proposed.

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.

Biomedical Ti-coatings were deposited on Ti-substrates by the microplasma wire spraying method. The influence of different factors of the wire spraying process on sizes of the particles produced as a result of atomisation of the wire melt by the microplasma jet, as well as the deposition efficiency were determined by using multifactorial experimental design. Linear regression models were developed, showing the effect of the most significant process parameters (current, plasma gas flow rate, wire feed speed) on formation of a jet of the spraying Ti-particles, structure and properties of the Ti-coatings. Establishing the possibility of controlling macroporosity of the Ti-coatings in microplasma wire spraying and correspondence of the Ti-coatings bond strength in tensile and shear tests to the ISO 13779-2 and ASTM C633 requirements, as well as data of the Ti-coatings “in-vivo” tests proved the efficiency of using them for manufacture of various-purpose endoprostheses (hip joint, dental, etc.).

The excellent corrosion resistance and biocompatibility of titanium make of it the material to choose for biomedical applications. Cold spraying, as a new coating technique, can be used to deposit protective Ti coatings onto less performing materials such as stainless steel and Co-Cr alloys, commonly used for biomedical implants. In addition, Cold Spray has the advantage, in comparison with conventional thermal spray techniques, to permit the deposition of oxygen-sensitive materials. In this study, Cold Sprayed Ti coatings were prepared on Co-Cr alloy substrates by using different spray process conditions. The microstructure of coatings was observed by SEM and the inner porosity was estimated by image analysis. Oxygen and nitrogen contents were investigated on a set of free standing deposits obtained using different process parameters. In the same way, the roughness and microhardness of deposits, such as the adhesion strength with the substrate, were measured. Finally, the corrosion performance of the coatings was evaluated by mean of open circuit potential measurement (OCP) and potentiodynamic polarizations scans. The electrochemical response was therefore discussed and compared to the corrosion behaviour of the Co-Cr alloy substrate and the bulk Titanium.

This research aims at introducing new biodegradable/non-biodegradable materials (biopolymers) to the existing Hydroxyapatite (HA)-titanium combination or as a single coating in order to overcome some of the limitations of HA coatings. Biopolymers can act as drug carriers for a localised drug release following implantation; they can also have a structural role by improving the mechanical performance of implants at the bone –implant interface. The proposed materials consisted of biodegradable and non-biodegradable polymers widely used as drug delivery systems: polymethylmethacrylate and polyhydroxybutyrate 98%/ polyhydroxyvalerate 2%. The method used to apply the polymeric powders was oxygen/acetylene flame spraying, due to its superior mechanical advantages over other techniques. Screening tests were used to determine the suitable range of spraying parameters, followed by optimisation to understand of the effects of spraying parameters on coating characteristics (thickness, roughness, adhesion, wettability), in order to obtain an optimal coating design. The polymers were sprayed onto bare titanium substrates. FTIR results showed that the coatings underwent little chemical degradation. Biocompatibility tests showed that cells proliferated well on flame sprayed polymer coatings, which confirms that the coating technique used did not affect the biological performance of the material.

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.

Fracture toughness and phase stability are crucial properties of thermal barrier coatings (TBC) during highly loaded thermomechanical operations in gas turbines. While several alternative TBC materials have exhibited excellent thermal resistance, their potential applicability has been limited due to poor endurance to cyclic stresses. The addition of TiO 2 to the non-transformable tetragonal t´-YSZ has been found to effectively enhance the fracture toughness and phase stability of YSZ at high temperature exposures. Thermal cycling tests in a burner rig were conducted on TBCs prepared from atmospheric plasma sprayed titania-doped YSZ to verify this phenomena. Exposure temperature was 1400°C at the surface and thermal gradient across the sample was provided by simultaneous back-cooling treatment. Cycling tests reveal that the slight increase in the tetragonality of the deposited coatings with increasing amount of dopant did not cause a significant effect to the lifetime of the TBCs. Moreover, increasing amount of Ti-substitution did not influence the fracture toughness of the bulk YSZ.

Characterization of coatings made with the help of Computer Controlled Detonation Spraying (CCDS) was performed. The applied coatings include hard alloys (WC/Co -75/25, WC/Co - 88/12, WC/Co/Cr - 86/10/4, and Cr 2 C 3 / NiCr), aluminum oxide, nickel-chromium self-fluxing alloy, titanium, bronze, and stainless steel. Tribological investigations of coatings were provided using abrasion test (ASTM standard G65), erosion test (ASTM standard G76), and hydro-abrasive test. To make hydro-abrasive tests special device and method were elaborated based on the interaction of water jet saturated with corundum particles with a coating surface.

The cold gas dynamic spray process offers a unique advantage to form composite coatings by applying powder mixtures. The powder mixture constituents are supposed to interact with each other during impact. In this study, Al and Cu-based powder mixtures are used with the aim to define specific features of the coating formation. Composite coatings with different Al 2 O 3 , SiC, and Ti content are sprayed. Impact behavior of various powder mixtures is analyzed based on scanning electron microscopy images. The Al 2 O 3 and SiC phases of the initial powder are found to be fractured on impact and preserved in the coatings. Another advantage of the kinetic spray process is the ability to mix materials which would normally react with each other and form a composite coating. Some experimental data of such reactions are discussed. Within the composite coating, each constituent changes the initial properties of the sprayed powder material: for example, the soft matrix is strengthened, and hard particles are fractured. The fracture and deformation behavior of the particles and their reactions induced by the impact are determined by micromechanical tests and EDX analysis. Morphology, physical and mechanical properties of the sprayed coatings are discussed.

This research systematically examines the effect of heat treatment on the microstructural properties of cold sprayed titanium coatings. Heat treatments were performed on as-sprayed coatings at 200, 400, 600, and 800°C for four hours under argon atmosphere. Vickers microhardness, microstructural investigation using FEG-SEM, structural characterization using XRD, and porosity evaluation using SEM image analysis were performed on as-sprayed and heat treated coatings. Results demonstrated that static recovery and static recrystallization may have occurred for heat treated coatings at 600 and 800°C. In addition, for the heat treated coating at 800°C, significant oxidation occurred and a slight decrease in porosity took place. Furthermore, a thin metallic layer characteristic of a solid solution or an intermetallic compound, was found at the coating/substrate interface.

Cold spraying has a high potential for building up thick coatings or structures, like for rapid prototyping or production of free-standing parts. This method is particularly interesting for high strength materials that are difficult to process or to shape, e.g. Titanium for special applications in aviation industries. In this contribution, the two major challenges are addressed: (I) optimizing mechanical properties by systematic variation of process parameters, and (ii) evaluating the influence of the spray angle with respect to complex geometries as requested by aviation industry. While in the past, high quality titanium coatings were only achieved using Helium as process gas, Nitrogen is used in this study to reduce costs. High deposition efficiencies of more than 95 % can be obtained and the coatings show very low porosities as well as high tensile strength of over 480 MPa. The influence of process conditions on the mechanical properties is discussed on the basis of coating microstructures, Micro Flat Tensile (MFT) and Tubular Coating Tensile tests (TCT-test).

In cold spraying, the required heat for bonding is provided by plastic deformation of the impacting ductile particles. Therefore, cold spraying is a well-established method for metal on metal coatings. However, few authors have investigated the impact phenomena and layer formation process for impacting brittle ceramic particles on ductile metal surfaces. For this study, titanium dioxide (TiO 2 ) on metal surfaces was chosen as a model system, and layer formation on aluminum, copper, titanium and steel substrates was investigated by SEM, TEM, XRD and Raman spectroscopy. The results show that the deposition efficiency depends on spray temperature, powder properties, and in particular on substrate ductility, even for an impact of ceramic particles during a second pass over already coated areas. High-resolution TEM images revealed no crystal growth or phase transitions at the ceramic/metal interfaces. Nevertheless, a clear dependence of the photocatalytic activity on spray parameters and substrate material could be observed. Cold-sprayed TiO 2 -coatings have potential applications in biomedical implants or as photo-catalytic functional systems

Measurement of the particle temperature and velocity in detonation spraying is significantly complicated by the pulsed character of the process. In the present study, these parameters are measured for powders with strongly different nature and properties such as WC/Co, Inox and Ti. Experiments are performed using an original computer-controlled detonation spraying (CCDS) installation developed by the authors. The system is distinguished by the mode of powder feeding into the gun barrel which is pulsed in time and localized in space. Evolution of the particle-in-flight velocity and size is examined by an original CCD-camera-based diagnostic tool developed by the authors. A significant spatial separation of the particles along the detonation plume is observed during their acceleration: 15 μm fine particles overtake 45 μm coarse particles by more than 10 plume diameters. For this reason, distributed scanning over the plume length is applied in order to obtain adequate results. A previously developed mathematical model of the process is experimentally validated. Calculations are found to be in a qualitative agreement with the experimental results. As far as particle-in-flight velocity is concerned, the agreement is even quantitative.

Cold spray produces superior coatings with many unique properties, which have led to many new applications of sprayed coatings. In a few applications, such as in-situ repair of aircraft body/engine parts, etc., a portable system is required. Though some portable low pressure (5 – 10 bars) systems are available, these systems have many limitations on coating materials and coating qualities. Recently a ‘Portable high pressure cold spray system’, called Kinetiks 2000, has been developed. Kinetiks 2000 system can operate at 400 °C max nitrogen temperature and 20 bars max gas pressure. A touch panel on the console is used to input and control the process parameters. A hand held gun with a filament heater and a gun body, mounted directly onto the heater, is used for manual spray. This system operates with two different powder hoppers. Coatings of many materials, including aluminum, copper, titanium, zinc, etc have been produced. Microstructural investigations of sprayed coatings have shown that good, strong, dense coatings with clean interfaces and strong bonding to various substrates can be produced. Experiments are continuing to quantify the process characteristics and record the properties of sprayed coatings.

Ti and Ti alloys can be applied to the steels as a protective coating in view of its excellent resistance to corrosive environment. Cold spraying as a new surface engineering technique has potential advantages in Ti coating manufacturing in comparison with conventional thermal spraying techniques. Ti coatings were prepared on a carbon steel substrate by cold spraying via controlling the process condition variables concluding carrying gas, temperature and pressure. The microstructure of coatings was observed by SEM. Potentiodynamic polarization experiments were performed to understand the corrosion behavior of the coatings. The SEM examination showed that the coatings become more compact with increase of molecular weight, pressure and temperature of carrier gas. Potentiodynamic polarization technique was used to measure the corrosion and electrochemical property of coatings deposited under different process conditions and surface conditions. The polarization curves indicated that the coatings which had lower porosity had lower corrosion current. The polishing treatment peeled the rough outer layer including the small pores as well as decreasing of the actual surface area of the coating, leading to the considerable improvement of corrosion resistance.

The impact of plasma sprayed sieved Ni-5%Al particles on a titanium substrate was investigated. The particles had a narrow diameter range (-65 + 75 µm), and a speed and temperature just prior to impact of about 100m/s and 2400°C, respectively. These powder particles were sprayed on two sets of polished titanium alloy surfaces. One set was a non-oxidized surface and the other one was a previously oxidized surface at 600°C for two hours. Resulting splats were characterised experimentally by infrared pyrometry and scanning electron microscopy. The effects of the substrate’s oxide layer on the shape of the splat, also on the cooling rate and flattening speed during impact were studied and discussed. This problem is also being investigated numerically. A first step was devoted to the selection of relevant simulation parameters. Test cases to study qualitatively the effect of surface oxidation through parameters such as the contact angle and thermal contact resistance are in progress. They will be compared to the experimental results.

In the hypersonic plasma particle deposition process, vapor phase reactants are injected into a plasma and rapidly quenched in a supersonic nozzle, leading to nucleation of nanosize particles. These particles impact a substrate at high velocity, forming a coating with grain sizes of 10 to 40 nm. As previously reported, coatings of a variety of materials have been obtained, including silicon, silicon carbide, titanium carbide and nitride, and composites of these, all deposited at very high rates. Recent studies have shown that slight modifications of the process can result in nanosize structures consisting of single crystal silicon nanowires covered with nanoparticles. These nanowires are believed to grow in a vapor deposition process, catalyzed by the presence of titanium in the underlying nanoparticle film. However, simultaneously nanoparticles are nucleated in the nozzle and deposited on the nanowires, leading to structures that are the result of a plasma CVD process combined with a nanoparticle spray process. The combination of these two process paths opens new dimensions in nanophase materials processing.