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 %.

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.

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.

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.

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.