It is generally believed that residual stresses in cold spray coatings are caused by particle impact and deformation and are hence predominantly compressive, as opposed to thermal spraying where thermal effects, particularly solidification, play the lead role. This study aims to provide a preliminary review of this notion through finite element analysis, distortion measurements on copper-coated steel, and the consideration of thermal history. The dimensions and elastic properties of the coating and substrate are key factors in the study along with heat input. Based on the review, a simple model is proposed to predict the final state of residual stress and distortion in flat surfaces coated by cold spraying.

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.

As compared to thermal spray techniques, cold spraying allows to retain metastable phases of the feedstock material like amorphous structures, due to lower process gas temperatures. Compared to crystalline metals, metallic glasses are brittle at ambient temperature but viscous at higher temperatures. Therefore, cold spray parameters must be optimized for conditions that allow softening of the amorphous spray material for successfully producing coatings. For this study, a FeCoCrMoBC metallic glass was used that in comparison to others offers advantages with respect to higher hardness, less costly feedstock powder and minimum reactivity with the environment. Necessary impact conditions were investigated to meet the window of deposition. According to calculations and cold spray experiments, neither the glass transition temperature Tg nor the melting temperature Tm can describe required conditions for bonding. Thus, a so called softening temperature between the glass temperature and the melting temperature had to be defined to calculate the critical velocity of metallic glasses. With respect to the bonding mechanism, impact morphologies could prove that a transition to viscous flow gets more prominent for harsher spray conditions. By sufficiently exceeding critical condition for bonding, coatings with rather dense microstructures can be processed at deposition efficiencies of about 70 %. The coatings have a hardness of 1100 HV 0.3, but the results also demonstrate that further work is still needed to explore the full potential for bulk metallic glasses.

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 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.