In cold gas spraying, successful bonding occurs when particle impact velocities exceed the critical velocity. The critical velocity formula depends on material properties and temperature upon impact, relying mainly on tabulated data of bulk material. However, rapid solidification of powder particles during gas atomization can result in strengths up to twice that of bulk materials, causing an underestimation of the critical velocity. Thus, a re-adjustment of the semi-empirical calibration constants could supply a more accurate prediction of the requested spray conditions for bonding. Using copper and aluminum as examples, experimentally determined particle strengths for various particle sizes were 43% and 81% higher than those of the corresponding soft bulk materials. Cold gas spraying was performed over a wide range of parameter sets, achieving deposition efficiencies ranging from 2% to 98%. Deposition efficiencies were plotted as functions of particle impact velocities and temperatures, as calculated by a fluid dynamic approach. By using deposition efficiencies of 50%, the critical velocities of the different powders and the corresponding semi-empirical constants were determined. Based on particle strengths, the results reveal slight material-dependent differences in the mechanical pre-factor. This allows for a more precise description of individual influences by particle strengths on critical velocities and thus coating properties. Nevertheless, the general description of the critical velocity based on bulk data with generalized empirical constants still proves to be a good approximation for predicting required parameter sets or interpreting achieved coating properties.

Thick deposits were produced from pure Al powder of three different sieve sizes using cold spraying at the same process parameters. The in-plane mechanical and fracture properties of the deposits were investigated using bending of small specimens in four specimen orientations. It was shown that increasing the Al particle size by approximately 50% and 100% leads to small, but statistically significant differences of yield strength. Further, the increase in the powder particle size led to higher fracture toughness K IC but lower fatigue crack growth threshold ΔK thr . This can be attributed to two different fracture mechanisms in the cold sprayed deposits. A trans-particular fracture in the near-threshold fatigue regime is controlled by the microstructure and work hardening of the particles. At higher cyclic loads and in quasi-static regime, the particle decohesion and the resulting crack path determine the fracture behavior instead. However, the observed effect of particle size was rather small, much smaller than the effect of spray process parameters observed in the previous research.

In cold spray, optimum process conditions to accelerate particles vary with different densities and melting temperatures of the materials. Therefore, material-specific nozzle designs are required. In the present study, a nozzle geometry optimization concept based on 3D-CFD simulations was developed to provide a specific nozzle design for a given material. Al6061 and pure copper with mean particle diameters of 40 μm were taken as examples. Together with a design of experiments (DoE) approach, the model seeks for the optimal nozzle geometry. In order to reach the highest particle velocity prior to impact upon the substrate, different geometry parameters were varied, such as the nozzle throat cross section, the aspect ratio, and the nozzle divergent section length. The process gas was nitrogen with set stagnation pressure and temperature of 50 bars and 500 °C. For both materials, the simulation identified nozzle divergent section length as the most influential parameter, followed by the throat cross-section. The aspect ratio must be tuned to avoid over expansion of the gas in the nozzle.

Cold spraying (CS) of high strength materials, e.g., Inconel 625 is still challenging due to the limited material deformability and thus high critical velocities. Further fine tuning and optimization of cold spray process parameters is required, to reach higher particle impact velocities as well as temperatures, while avoiding nozzle clogging. Only then, sufficiently high amounts of well-bonded particle-substrate and particle-particle interfaces can be achieved, assuring high cohesive strength and minimum amounts of porosities. In this study, Inconel 625 powder was cold sprayed on carbon steel substrates using N 2 as propellant gas under different refined spray parameter sets and powder sizes for a systematic evaluation. Coating microstructure, porosity, electrical conductivity, hardness, cohesive strength and residual stress were characterized in as-sprayed condition. Increasing the process gas temperature or pressure leads to low coating porosity of less than 1 % and higher electrical conductivity. The as-sprayed coatings show microstructures with highly deformed particles and well bonded internal boundaries. X-ray diffraction reveals that powder and deposits are present as γ- solid-solution phase without any precipitations. By work hardening and peening effects, the deposits show high microhardness and compressive residual stresses. With close to bulk material properties, the optimized deposits should fulfill criteria for industrial applications.

Due to its suitable semiconductor band gap energies and associated visible light absorption, bismuth vanadate offers high photon efficiencies in solar photo-anodes, enabling green hydrogen generation in photoelectrochemical water splitting cells. Respective bismuth vanadate films have to ensure high efficiencies in electron / hole pair generation, and sufficiently high rates of charge transfer, for both, electrons to the conducting substrate, as well as holes to the electrolyte. Thus, tuning of coating properties has to aim for high phase purity and good layer integrity. So far, respective films are mainly produced by thin film techniques, but at rather high costs and low deposition rates. Less costly processing routes are opened by thermal spraying or sol-gel techniques, however, these cannot guarantee the required phase purity or absence of remnants from the binder. As solid state and binderless alternative, Aerosol Deposition (AD) offers several advantages: comparative low costs, high deposition rates, no undesired phase transformations, and no impurities or residues that could reduce the photoelectrochemical activity. Under the scope of this research on photo-electrochemically active bismuth vanadate films, powder sizes were tailored by milling, and spray parameter sets like the process gas pressure were varied, in order to elucidate their influence on microstructure and application properties. Covering a wide parameter range in aerosol deposition allowed for the development of a window of deposition. Most promising combinations for layer build-up were derived. The results on stainless steel substrates were transferred to FTO-coated glass substrates, as needed in backlit cell layouts. For fine tuning of maximum photocurrents, layer thickness and conductivity were then systematically adjusted. Homogeneous large-scale prototypes demonstrate that aerosol deposition is suitable for processing layers for solar energy harvesting.

Applications in thermal and kinetic spraying increasingly aim for coating of parts with complex geometries. So far, respective robot programming for the required path during deposition is usually adjusted individually in time-consuming procedures. Thus, it is essential to develop methods that allow a fast adaptation to part geometries and production conditions as well as possible quality control. To tackle these problems, this work addresses novel strategies for robot programming and post-spray analyses. The design of the method and workflow follows routes of smart manufacturing and should enable fast and accurate implementation into spray procedures. Here, the developed application can handle complex parts of arbitrary geometry in the form of CAD files. Supported features include (i) cutting the objects according to the object boundary, (ii) creating self-intersecting curves, (iii) generating a set of index-sequence-based spatial discrete points and (iv) reordering the discrete points to generate adaptive paths. Robot offline programming allows for process simulation, analysis and optimization of the robot kinematics. By optical scanning profilometry, the layer-by-layer deposit build-up could be monitored for quality control, as well as for the determination of the final overall coating thickness. The entire procedure was tested by cold spraying onto a complex workpiece, validating the capability of the proposed strategy. Based on the universal layout of the applied methods, the strategies can also be applied for thermal spraying in general, considering individual boundary conditions. With respect to cold spraying, the implementation framework of this study provides a good basis for part repair and additive manufacturing.

The present study compares needed prerequisites for the application of cavitation resistant bronzes by applying different coating techniques, such as cold spraying, HVOF spraying, warm spraying and arc spraying. By optimization to optimum cavitation resistance, the deposited coatings can increase the service life of ship rudders significantly and even serve as repair processes for ship propellers. The given overview aims to support the selection of processes when specifying the target properties to be set with regard to cavitation protection. By using high-pressure warm spraying and cold spraying, properties similar to those of cast nickel aluminum bronze were achieved, however at relatively high costs. In contrast, coatings produced by using HVOF and arc spraying have erosion rates that are only about four respectively three times higher as compared to cast nickel aluminum bronze, while far outperforming bulk shipbuilding steel. Hence, their properties should be sufficient for acceptable service life or docking intervals for ship rudder applications. Propeller repair might demand for better coating properties as obtained by cold spraying. With respect to costs, HVOF and arc spraying in summary might represent a good compromise to reach coating properties needed in application.

In state-of-the-art manufacturing of sliding bearings, brass components are soldered to respective parts, which is costly and energy-intensive. Furthermore, up to now most bearings still contain lead, which by EU regulations for new part has to be omitted due to associated health risks. Cold spraying can be employed as additive manufacturing technique and opens the perspective to deposit the requested bearings in desired leadfree layout where needed. Aside cohesion and tribological behaviour, sufficient adhesion of the coating is essential for applications. The present study aims to systematically elucidate the influence of surface roughness on adhesion. The surface roughness was adjusted by varying the grit blasting material, grit size, blast pressure, blast distance and substrate material with the aim to study influences by impact conditions, surface topography on particle deformation and bonding in cold spraying. The results show that the adhesion strength reaches a maximum for a certain roughness. The ideal surface roughness to ensure good adhesion of cold-sprayed coatings apparently depends on specific impact conditions related to the powder material strength but also on the substrate material strength and particle size distribution. By systematic tuning of blasting conditions, coating adhesion can be increased by about a factor of two, thus meeting the requirements for new lead-free bearings.

In current process techniques to manufacture sliding bearings, bronze components are soldered to the respective parts, which is cost and energy intensive. Apart from that, so far most bearing materials still contain lead, which in new applications is omitted by EU law to avoid associated health risks. The present study aims to offer solutions for both by using cold gas spraying as additive manufacturing technique for processing bearings directly onto steel parts and by applying that to new lead-free bronze alloys. A lead-free bronze alloy was processed as powders by gas atomization and classified to optimum sizes for cold spraying. During cold spraying, the process gas pressures and temperatures as well as the substrate temperature were varied with the aim to study influences by impact conditions and effective surface temperature on particle deformation and bonding. Respective coatings show low porosity, high hardness and high electrical conductivities. With properties similar to that of bulk cast material respectively manufactured parts should meet the requirements for new bearing applications.

In this study, an experimental approach to investigate basic dependencies on impact and bonding of agglomerated ceramic particles in cold spraying is presented. Single impact morphologies of ceramic particles obtained from wipe tests are correlated with data obtained from powder compression experiments with a modified nanoindenter. Different feedstock powders of agglomerated TiO 2 -nanoparticles were used and also partially heat treated. The powder shapes and sizes prior and after the compression tests were analyzed by confocal microscopy. The single particle impacts were characterized by SEM. Besides the expected influence of substrate material, substrate temperature, and spray conditions, the deformation and bonding of ceramic particles to metal substrates critically depend on the powder properties. To which degree particles fracture or contribute to layer formation upon the high-energy impact is highly correlated to their individual deformation behaviour in quasi-static compression tests.

Bronze materials such as Ni-Al-bronze show exceptional performance against corrosion, erosive wear, and cavitation erosion due to their high fatigue strength and resistance to plastic deformation, and are thus used for ship propellers and in turbines, pumps, and other equipment where alternating stresses occur. Usually, the respective parts are cast, but in this study, a number of opportunities are evaluated to apply bronze as a coating to critical part surfaces. Initial experiments with cold gas spraying were promising enough to assess the use of warm spraying, a nitrogen-cooled HVOF process that provides similar particle impact velocities but higher particle temperatures, while still minimizing the effects of oxidation. The formation and performance of warm sprayed Ni-Al-bronze coatings was systematically investigated for different combustion pressures and nitrogen flow rates. Substrate preheating was also used to improve coating adhesion. The coatings obtained show low porosities, high strengths, and in some cases, cavitation resistance similar to that of the bulk material.

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.

The aim of this work is to determine whether local disordering plays a role in the bonding of FeAl intermetallic coatings produced by cold spraying. XRD analysis of the powder and coatings revealed superlattice peaks, indicative of an ordered intermetallic structure. Nevertheless, locally disordered structures were detected in the deposits by TEM imaging. This may be related to a deformation induced order-to-disorder transformation due to the high strains involved in cold spraying, which is supported by comparing the magnetic properties of the deposits with those of HVOF sprayed coatings produced using disordered ball-milled feedstock.

In the present study, spherical Ti-6Al-4V powders were cold sprayed on titanium, aluminum, and magnesium alloy substrates to investigate influences over a wide range of damping conditions and respective deceleration of impacting particles. Single impacts were produced via wipe tests and bonding was evaluated by cavitation testing followed by SEM examination of impact and fracture morphologies. The results show that better bonding is achieved for material combinations with similar properties due to high adiabatic shear instabilities that result in microfusion at the particle-substrate interface. In the case of dissimilar materials, the conditions for bonding can be reached in an intermediate stage, but bonded areas may later separate due to particle movement around the interface.

The present study investigates the microstructure and properties of cold spray coatings produced from gas-atomized CuAl10Fe5Ni5 powders. To obtain information relevant to ship rudder cavitation-erosion performance, GL-A shipbuilding steel, equivalent to S235, was chosen as the substrate material. Thick Cu-Al-bronze coatings were deposited on grit-blasted plates using a wide range of parameter sets with different powder treatments, nozzle geometries, gas and substrate temperatures, and particle impact conditions. Coating samples were examined via SEM and XRD analysis, cavitation tests were performed, and bond strengths were measured. Powder and single impact morphologies were also investigated and, along with coating properties and structures, are correlated with spraying conditions. The results indicate that cold sprayed bronze coatings have good potential for ship rudder protection.

In this investigation, cold spraying is used to deposit a simple binary amorphous alloy with technical purity. Cu 50 Zr 50 was chosen as the model system due to its glass-forming ability and insensitivity to changes in composition. Critical velocities for coating formation were experimentally determined by systematic variation of spray parameter sets. These values were then used to tune existing bonding models to cold spraying of amorphous Cu 50 Zr 50 powder. It is shown that under suitable conditions, well adhering coatings with the amorphous structure of the powder can be obtained by cold spraying with nitrogen as the process gas.

In this study, Eulerian finite element analysis is used to explore the deformation and bonding behavior of composite particles deposited by cold spraying. The objective is to account for shear instability and bonding as well as the geometry of the deforming material. An accurate description of the geometry is essential when the amount of deforming material is limited as in composite particles. Another goal is to provide a framework for modeling the impact of agglomerates. In this case, deposition is influenced by bonding and fragmentation or detachment due to plastic rebound. To account for the latter effect, thin layers of nonbonding material are added to particle and substrate surfaces in the model. Simulations of metal-clad ceramic particles show that there is a critical shell thickness beyond which maximum stress in the hard phase abruptly increases.

The present study deals with the production and characterization of Ti-6Al-4V coatings produced by cold spraying. All experiments were performed using nitrogen as the process gas. By systematic variation of spray parameters up to a nitrogen gas temperature of 1000 °C and pressure of 5 MPa, the coatings could be tuned for optimum mechanical properties. Attainable coating properties are described in terms of a newly introduced coating quality parameter based on the ratio of particle velocity to critical velocity. The new parameter, η, is used both to interpret and predict coating properties.