What would be the most appropriate parameters, namely, gas temperature and gas pressure, for cold spraying of a given feedstock material? This question is the focus of the present contribution. Initially, it is shown that main coating characteristics can be described as a unique function of a dimensionless parameter, defined as the ratio of particle velocity to critical velocity. Subsequently, these velocities and the respective ratio are worked out and expressed explicitly in terms of key process and material parameters, such as gas temperature and particle size. In this way, final properties of cold-sprayed deposits are linked directly to primary cold-spray parameters. Moreover, it is shown that the window of deposition, as well as the relationship between final properties of the deposit and the spraying conditions, can be incorporated conveniently into simple 2-D diagrams, showing contours of the velocity ratio, or the desired coating property, on the plane of the primary process parameters. Based on these diagrams, the process parameters related to a given coating property can be identified and selected, without a need to refer to intermediate variables such as particle velocity. The paper includes examples of the application of these maps for cold spraying of copper.

In cold spraying, the high strain rate plastic deformation during particle impact leads to a local temperature rise at the particle/substrate interface. This gives rise to thermal softening and thus further strain and heat generation, finally resulting in adiabatic shear instabilities, which are necessary to supply sufficient heat for successful bonding of the particles. These adiabatic shear instabilities can only occur, if a critical impact velocity is exceeded. A further increase of the impact velocity beyond this critical velocity continuously increases the fraction of well-bonded interfaces up to 95%, thus improving mechanical performance of the coatings. However, at far too high impact velocities, the efficiency again decreases and then changes to erosion due to hydrodynamic penetration. This erosion velocity is approximately two to three times higher than the critical velocity. The optimum velocity range between critical and erosion velocity is defined as “window of deposition”. Both critical and erosion velocity depend on the spray material properties, but also on particle impact temperature and particle size. Furthermore, they are also influenced by the powder purity. This study demonstrates the previously mentioned effects by calculations and experimental investigations. The presented link between fluid dynamics and impact dynamics enables to predict optimum spray parameters as well as the process effectiveness and resulting coating properties for certain cold spray conditions. Following this strategy, it was possible to increase the ultimate cohesive strength of cold-sprayed copper coatings from 80 MPa to more than 400 MPa, using nitrogen as process gas. In the annealed state, the ductility of these coatings corresponds to annealed bulk material. The overall optimization strategy is applicable to a wide variety of other spray materials. These developments should boost several new cold spray applications.

Cold gas spraying is a coating process by which coatings can be produced without significant heating of the sprayed powder. In contrast to the well-established thermal spray processes such as flame, arc and plasma spraying, in cold spraying there is no melting of particles prior to impact on the substrate. Bonding occurs when the impact velocities of the particle exceed a critical value. This critical velocity depends not only on the type of the spray material, but also on the powder quality, the particle size and the particle impact temperature. The present contribution summarizes general views and reports recent developments with respect to the understanding of the process and respective consequences for the optimization of the process. The presented optimization procedure covers principles to increase gas and particle velocities and rules to decrease the critical velocity for bonding. By consequently following such route for typical metallic spray materials, cold spraying as a quite new coating technique is already capable to provide coating qualities very similar to those of work hardened bulk material at powder feed rates similar to those of thermal spraying and deposition efficiencies of about 90 %.

In cold spraying, bonding is associated with shear instabilities caused by high strain rate deformation during the impact. It is well known, that bonding occurs, when the impact velocity of an impacting particle exceeds a critical value. This critical velocity depends not only on the type of spray material, but also on the powder quality, the particle size and the particle impact temperature. Up to now, optimization of cold spraying mainly focused on increasing the particle velocity. The new approach presented in this contribution demonstrates capabilities to reduce critical velocities by well-tuned powder sizes and particle impact temperatures. A newly designed temperature control unit was implemented to a conventional cold spray system and various spray experiments with different powder size cuts were performed to verify results from calculations. Microstructures and mechanical strength of coatings demonstrate that the coating quality can be significantly improved by using well-tuned powder sizes and higher process gas temperatures. The presented optimization strategy, using copper as an example, can be transferred to a variety of spray materials and thus, should boost the development of the cold spray technology with respect to the coating quality.

Cold spraying has attracted serious attention since unique coating properties can be obtained by that process which are not achievable by conventional thermal spraying. This is due to the fact that coating deposition takes place without exposing the spray or substrate material to high temperatures and, in particular without melting of spray particles. Thus oxidation and other undesired reactions can be avoided. Spray particles adhere to the substrate only due to their high kinetic energy upon impact. For successful bonding powder particles have to exceed a critical velocity upon impact, which is dependent on properties of the particular spray material. This requires new concepts for the description of coating formation but also indicates applications beyond the market for typical thermal spray coatings. The present contribution is aimed to summarize the current ‘state of the art‘ in cold spraying and to demonstrate concepts for further process optimization. That concerns the management of impact velocities and temperatures as well as the development of powders tailored to the process. So far, a wide variety of different spray materials ranging from different metals or alloys to even metal matrix composites, has successfully been tested as promising coating material. All together, the advantages of cold spraying can enhance new applications in surface technologies.

In Cold Spraying, bonding occurs when the impact velocities of particles exceed a critical value. This critical velocity depends not only on the type of spray material, but also on the powder quality, particle size and the particle impact temperature. For metallic materials, the critical velocity is in the range of 200 – 1200 m/s. In analogy with explosive welding, bonding in Cold Spraying is associated with adiabatic shear instabilities caused by high strain rate deformation during impact. Numerical and experimental methods are developed to investigate the influence of impact conditions and related phenomena on the coating quality. For a deeper understanding of impact phenomena and coating formation, the particle impact was modelled by using the finite element software ABAQUS/Explicit. The numerical analyses indicate shear instabilities localized to the particle surfaces, and thus provide a basis for the calculation of critical velocity in terms of materials properties and process parameters. In addition, modelling is used to obtain information about the effect of process parameters on the bonding quality. For most materials, high-strain-rate data are not available. For a quantitative analysis, therefore, the respective materials behaviour was investigated through individual spraying experiments, which were complemented by additional relevant experiments such as impact tests or explosive powder compaction. In this way, impact dynamics, bonding mechanism and critical velocities could be linked. This type of analysis was proved as a powerful tool to reduce the number of experiments for the optimisation of coating quality in Cold Spraying and also to provide a broader overview of the process.

In cold spraying, for successful bonding powder particles have to exceed a critical velocity upon impact, which is dependent on properties of the particular material. So far a detailed physical explanation of the underlying mechanisms of bonding is still lacking. In the present study, computational materials science and high resolution microscopical methods are used to investigate the microstructural development at the particle-substrate interface. The modelling can show that the critical velocity is related to a transition in the flow-behaviour on the particle or substrate surfaces. The presence of microstructural features predicted by modelling in detail could be confirmed by SEM and TEM analyses of internal interfaces of cold sprayed coatings. By describing the mechanisms of bonding, the calculations could also demonstrate the influence of material properties or microstructural defects on the conditions for successful impacts. With respect to particular powder properties, the results should promote the development of optimum process parameters in cold spraying.

In cold gas spraying, the powder is not molten before impact on the substrate. The bonding of the coating only depends on powder characteristics and impact conditions. To optimize coating microstructure and properties, spray conditions have to be tuned for a particular powder. The optimization procedure usually requires a systematic variation of spray conditions and an analysis of the sprayed coatings which is time consuming and costly. Therefore, alternative test methods which are less expensive and operate with similar load mechanisms on powder particles have to be developed. High strain rate deformation can be easily studied by explosive powder compaction. In this method, the powder is loaded by a shock wave and deformed under high strain rates. The bonding conditions of powder particles should be similar to those obtained in cold spraying. By a special design, shock loading in explosive powder compaction can cover a wide energy range in one single experiment. Therefore, the method appears feasible to determine the energy input required for successful bonding of particles. To evaluate the capability of the method, microstructural features of particle/particle interfaces are investigated and compared to those of cold sprayed coatings. In addition, the results can supply more information concerning the bonding mechanisms in cold gas spraying.

By cold spraying, coatings are produced without significant heating of the spray powder and the substrate material. The powder particles are accelerated in a preheated gas stream to velocities of more than 500 m/s without melting and form a dense and tightly bonded coating. Bonding occurs only due to plastic deformation and the heat created thereby. Due to the absence of melting of the powder particles during cold spraying, several negative phenomenon, such as oxidation and phase transformations associated with thermal spray processes, such as HVOF, arc, flame and plasma spray can be minimized or avoided. This paper presents an overview of cold spray process developments and of coating characteristics. A variety of metallic powders having a low or high melting temperature including Cu, Al, Ti as well as alloys were sprayed onto different substrate materials, and the microstructure and properties of the coatings were evaluated.

It is well known that thermally sprayed aluminum and aluminum alloys can be used to protect low-alloyed steel against marine corrosion in offshore applications. The efficiency and service life of this protection can be, however, severely limited by the amount and distribution of defects, which are usually present in coating microstructures. In thermal spraying, microstructures and properties are strongly influenced by the type of spray system used for the production of coatings. To investigate the influence of defects like pores, oxides and cracks on the corrosion performance, coatings were processed by conventional thermal spray techniques, such as Flame Spraying (FS) and Arc Spraying (AS). In addition, the more recently introduced High Velocity Combustion Wire (HVCW) spraying technique was used, which, due to higher particle velocities, results in lower porosity and finer coating microstructures as compared to conventional processes. The influence of spray conditions and related microstructures on the performance in corrosion tests was investigated for protective coatings of Al99.5, AlMg5 and Al - 30 wt. % W2C. The resistance against corrosion was analyzed by different electrochemical methods, such as corrosion potential monitoring, polarization resistance and potentiodynamic anodic polarization measurements. Additionally, the microstructures of the coatings were examined before and after the electrochemical tests. The results from these tests are correlated and attributed to the different microstructures obtained by the various spray techniques and different compositions of the feedstock material.

This paper investigates the microstructure and properties of TiC-based nanocrystalline coatings produced by HVOF and VPS processes. It assesses the extent to which nanocrystalline cermet powders can be tailored for specific applications using high-energy milling and the commercial viability of HVOF sprayed coatings made from such powders. Paper includes a German-language abstract.

Various metals such as aluminum, copper, zinc, steel, nickel, titanium, and niobium have been deposited on a wide range of substrate materials via cold spraying. This paper provides a detailed overview of the cold spray process and the coatings typically produced. It discusses the powders and gases used, the dynamics of gas-particle flow in spray nozzles, the effect of temperature and pressure, and the concept of critical velocity. It also presents examples of the properties and microstructures recently achieved in cold sprayed aluminum, zinc, NiCr, MCrAlY, and Cu-Al coatings. Paper includes a German-language abstract.

In cold spraying, in contrast to thermal spraying the coating material is not melted prior to the impingement onto a substrate. The powder particles are accelerated to high velocities by a supersonic gas jet. Even though the particles are in a solid state, they form a dense and solid bonded coating upon impact. In order to form a dense coating with sufficient adhesion to the substrate, the particles have to reach a certain velocity before hitting the substrate. This velocity is characteristic of the coating material and also depends on the particle temperature. A variety of experiments have been carried out with copper as spay material in order to determine the critical velocity for solid bonding of particles onto the substrate. To investigate the effect of spray parameters and nozzle geometry on the velocity and temperature of the particles, computational fluid dynamics was performed. The calculations allow a direct correlation between experimentally obtained deposition efficiencies and process parameters. Finite element modeling of the particle impact could relate successful bonding to high strain rate phenomena at the particle interface. In view of the above criteria an optimization strategy for cold spray process can be developed.

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 cold spraying, coatings are formed by a high velocity impact of solid particles. The particles are accelerated in a supersonic gas jet at temperatures of only a few hundred degrees centigrade. In contrast to thermal spray processes no melting of the particles and negligible heating of the substrate occurs. A series of spray experiments with copper powders of different particle size ranges were performed to study the effect of various process parameters on microstructure and properties of the coatings. The coatings have been evaluated for their microstructure, density, oxygen content, hardness and bond strength. With nitrogen as process gas and a -25 +5µm powder, dense coatings were obtained within a broad range of gas inlet pressure and gas inlet temperature.

In cutting tool technologies WC-Co based materials are increasingly replaced by composites containing TiC as hard phase and Ni or Co based metallic binders to improve life time and the performance at higher temperatures. These new light-weight materials are also promising for wear resistant coatings. However, while the production of WC-Co coatings by thermal spray techniques, especially high velocity oxy-fuel flame (HVOF) spraying, is well-established, thermal spraying of TiC-based powders did not lead to satisfactory results so far. This could be attributed to the oxidation during the spray process and the insufficient bond between hard phases and the metallic binder. Strategies to improve the properties of TiC-based coatings aim for microstructural modifications, especially by alloying additives into the thermal spray powder. By HVOF and vacuum plasma spraying (VPS), modified TiC-based coatings are produced, which globally show similar microstructures but significantly differ in their oxide contents. Investigations of mechano-technological properties and wear mechanisms demonstrate that alloying Mo into the hard phases or the metallic binder of thermal spray powders can improve the adhesion between hard phases and metallic binder of the coatings. In addition, properties of the metallic matrix can be tuned up for specific applications by solution hardening. In case of HVOF-coatings these effects are partially compensated by high oxygen contents. The overall better performance of coatings produced by VPS demonstrates that the high potential to improve properties of TiC-based composites by alloying additives can only be attained by minimizing the oxidation during the spray process.

The development and introduction of more advanced spray systems continues to drive the thermal spray market. Most of these developments benefit from an increase in particle velocities. The latest generation of HVOF systems can spray cermet coatings of higher density and higher hardness, wear resistance and corrosion resistance than HVOF systems introduced previously, due to an 30 to 50 % increase in particle velocity. HVAF systems are offered as an cost-effective alternative to HVOF systems. A further increase in particle velocity and the introduction of cold gas spraying can be seen as a transition from thermal to kinetic spraying and may open a wide field of new applications for coatings and structures of oxidation sensitive materials. Advances are also reported for wire spraying. New arc spray systems are capable to increase the density of metal and alloy coatings considerably and to reduce the oxide content of these coatings. Similar improvements but at lower spray rates may be achieved with newly developed HVOF wire systems. Paper text in German.

The paper reports a series of experiments with various HVOF spray systems (Jet Kote, Top Gun, Diamond Jet (DJ) Standard, DJ 2600 and 2700, JP-5000) using different types of WC-Co and WC-Co-Cr powders. The microstructure and phase composition of powders and coatings were analyzed by optical and scanning electron microscopy and X-ray diffraction. Carbon and oxygen content of the coatings were determined in order to study the decarburization and oxidation of the material during the spray process. Coatings were also characterized by their hardness, bond strength, abrasive wear and corrosion resistance. The results demonstrate that the powders exhibit various degrees of phase transformation during the spray process depending on the type of powder, the spray system and the spray parameters. Within a relatively wide range, the extend of phase transformations has only little effect on coating properties. Therefore coatings of high hardness and wear resistance can be produced with all HVOF spray systems when the proper spray powder and process parameters are chosen.

The cavitation erosion of various hardfacing coatings was investigated by using a vibratory cavitation apparatus according to ASTM G 32. Coatings of austenitic stainless steel containing 10 % cobalt were applied by arc welding. High velocity oxy-fuel spraying (HVOF) was employed to produce coatings of various kinds of cermets and metallic alloys. For each coating, the steady state erosion rate was determined and the effect of process parameters and alloy composition on the microstructure and erosion rate was investigated. The morphology and microstructure of the coatings before and after cavitation testing were analysed by metallographic methods in order to study the erosion mechanism. It is demonstrated that the high resistance to cavitation erosion of the cobalt-alloyed steel can even further increase when the fluxed core arc welding process and an improved pulsed power source are used to produce the coatings. The erosion resistance of the HVOF coatings, however, was limited by pores, microcracks and oxides and did not significantly exceed the level typical for bulk stainless steel 316 L.

During Thermal Spraying material is partially or totally melted in between milliseconds, accelerated to high velocities and propelled onto the surface to be coated. Different temperatures, velocities and cooling times of the particles arise from different spraying techniques and conditions. The microstructure can be widely varied by tuning the spraying parameters. To optimize the coating properties with respect to a specific function one has to know i) the influence of the spraying conditions on the microstructure and ii) the correlation between microstructure and coating properties. Therefore analyzing methods are needed to determine the microstructure and to characterize mechanical, physical and chemical properties of the coatings. The proposed paper summarizes methods to characterize the microstructure including metallographic techniques, electron microscopy and X-ray analysis. Methods to determine the properties of the coatings including various adhesion tests, residual stress measurement, tribological and corrosion tests will be described in more detail. The increasing importance of Quality Management in all industrial sectors call heavily for reliable, destructive and especially non-destructive characterization techniques of coatings. An overview of common characterization techniques as well as new trends will be given. Recent development of International standardized tests will also be reported.