This contribution gives an overview concerning basic principles of cold spraying (CS) and current trends in respective applications. As powder spray technique dealing with solid impacts, cold spraying results in coatings of high purity and unique properties, not attainable by other spray methods. Particularly within the last two decades, cold spraying developed from laboratory scale to a reliable industrial process. The presentation summarizes current models and key parameters in order to achieve and to improve bonding and coating qualities, and gives examples for applications in electronics, mechanical part repair and additive manufacturing.

Graphite is useful in high-temperature applications in many engineering fields, such as heat-treating, brazing, and sintering industries. As the operation becomes severe, carbon experiences degradation leading to failure. In this study, a protective coating of W and Mo as the intermediate layer by air plasma spraying on graphite substrate was investigated to find a better intermediate layer. Their performance was explored as a bonding layer in a protective alumina-YSZ ceramic topcoat. X-ray diffraction and scanning electron microscope were used to observe the cross-section of coatings and the difference in the bonding characteristics between W and Mo, respectively. W was found inferior to Mo as a bonding performance over 1450 °C in view of carbide formation against the thermodynamic data. It seems to be related to the formation of a barrier layer as oxide during air plasma spraying.

In this work, amorphous Zr-based bulk metallic glass deposit was manufactured by cold spray. The bonding mechanism of metallic glass particles was systematically investigated through studying the deformation behavior of individual particles after deposition or rebound. We revealed two collective particle bonding mechanisms that contributed to the formation of metallic glass deposit, i.e., high-velocity impact induced localized metallurgical bonding at the fringe of interface, and high gas-temperature induced partial melting of particles and resultant annular metallurgical bonding band. Moreover, the dynamic evolution mechanism of amorphous phase into nanocrystal structures at severely deformed interfacial regions during cold spray was carefully investigated. For the first time, we observed different amorphous/nanocrystal structures in cold sprayed metallic glass particles, which can represent different evolution stages in nanocrystallization process. Based on the observation, it is inferred that the nanocrystallization process can be divided into following three stages: composition segregation, the formation of ordered 1D and 2D transition structures, and 3D nanocrystals. The current study provides new insights into bonding mechanisms and the mechanistic nanocrystallization origins in cold sprayed metallic glass.

In this work, the bonding mechanism between Cu particle and substrates of Mg, Cu and stainless Steel (SS) was investigated by the direct observation of bonding interface on detached particle and substrate crater. In the cases of Cu/Cu and Cu/SS, dimple-like fractures were found on the detached Cu particle and substrate crater for the first time. Accompanying with EDS line scan and mapping results, such dimple fractures can be considered as the signs of strong metallurgical bonding. However, the bonding interface in case of Cu/Mg is smooth without signs of metallurgical bonding. Finite element analysis results revealed a ring of high contact pressure zone on the surface of particle and substrate, which is exactly the place where metallurgical bonding was observed. It can suggest that the high contact pressure zone is the dominant factor for the formation of metallurgical bonding on the oxide-free interface. The evolution of maximum contact pressure in different cases shows that the substrate hardness plays an important role during the single particle bonding. The present study provides a profound insight into the bonding mechanism of a single cold sprayed particle, which can give the guidance to the full deposition of cold sprayed coating.

A contact model that accounts for interfacial cohesion and thermal conduction is developed to investigate the influence of bonding on the final residual stresses build-up in cold spray. The residual stress evolution in the cold sprayed Al-6061 coating on an Al-6061 substrate is investigated via three-dimensional (3D) single-particle and multi-particle impact simulations. It is shown that the interface bonding mainly affects the local residual stress distribution near the interfaces. The residual stresses are largely due to the kinetic peening and bonding effects. The thermal cooling has negligible influence. In general, the peening effect introduces a compressive stress while the bonding effect results in a relaxation to this compressive stress. This work suggests that the interface bonding should be considered as one of the essential factors in numerical modeling of the residual stresses evolution in cold spray.

It is usually difficult to deposit dense ceramic coatings with splats well bonded by plasma spraying at room temperature. Following the recent research progress on the splat interface bonding formation, it was found that there is a well defined relationship between the critical bonding temperature and materials melting point. Thus, it was proposed to control the lamellae bonding through the deposition temperature. In this study, to examine the feasibility of the bonding theory, a novel approach for the development of coating microstructure through materials design is proposed. Accordingly typical ceramic materials were selected of relative low melting point for plasma spraying of dense coating with well bonded splats. The experiment was conducted by using K 2 Ti 6 O 13 for splat deposition at ~110°C cooling down from a higher temperature to avoid substrate adsorbates and coating was deposited at room temperature in ambient temperature without substrate preheating. Results show that the splat is fully bonded with a ceramic substrate, while the coatings present a dense microstructure with a similar fracture morphology to sintered bulk ceramics. Moreover, the erosion test at 90° further confirmed the formation of a ceramic coating with lamellae fully bonded.

Molten droplet temperature is a dominant factor influencing the bonding formation at the interface between splats during plasma spraying. In this study, the effect of molten droplet temperature on the interface bonding formation between nickel-base alloy splats was investigated. One novel shellcore- structured Mo-clad NiCr powder and conventional NiCr powder were used as the feedstock. Stainless steel with surface polishing was used as the substrate. The NiCrMo particle with a temperature over 2647 °C was generated by plasma spraying in comparison with that about 2167 °C for NiCr droplet. The molten droplet temperature when it impacted on the substrate surface was changed continuously by changing spray distance. The particle temperature was measured by DPV2000 and the interface bonding state was characterized by the cross-section samples of splat-substrate interface prepared by FIB. Results show that the interface bonding ratio can be increased from ~40% to 80% when the average molten droplet temperature is increased from ~2167 to 2647 °C.

In cold spraying, oxide-free interface is an important factor for fresh metal bonding between particles and substrate, which determines the bonding strength and final coating quality. In this study, a well-designed experiment was performed to examine the deformation behaviour of the oxide film on copper alloy particle surface after deposition. The experiment results show that partial oxide film could be disrupted during the high-speed impact. However, most of the oxide films were found to remain intact after particle deposition, which limited the exposure of oxide free interface. The presence of oxide film at the interfaces between deposited particles and substrate seriously affected the metallurgical bonding. Besides, substrate material is found to have a strong influence on the deformation behaviour and final state of the oxide film. The study also demonstrated that the bonding mode between deposited particle and substrate strongly depends on the type of substrate.

The bonding at the interface in cold sprayed coating is considered to be a very crucial factor determining its mechanical properties, physical and chemical behavior such as tensile strength, hardness, electrical and thermal conductivities, as well as corrosion resistance. Therefore, in this study the deformation behavior of the particle initial surface is investigated in order to reveal the evolution of free-oxide interface during the high-velocity particle impact in cold spray. The variation of the stress at the interface during the impact is also examined to evaluate the bonding between particle and substrate, and further to predict the bonding strength for the experiments. Results show that the area ratio of the free-oxide interface and the whole interface are 0.52, 0.7 and 0.76, respectively, for the case of copper particle impact at 500 m/s, 800 m/s and 1100 m/s. Moreover, the free-oxide interface in case of 800 m/s is about 3 times as much as that in case of 500 m/s while the free-oxide interface of 1100 m/s is approximately 5 times as much as that of 500 m/s. The compressive stress in the normal direction at the position where free-oxide interface occurs is higher than the yield strength of the material and during the whole impact, the tensile stress is no more than the tensile strength of the material.

In order to clarify the bonding mechanism and to control the quality of cold-sprayed coatings, it is necessary to accurately measure the in-flight velocity and impact velocity of a projectile. In this study, the in-flight velocity of an aluminum alloy (A2017) 1 mm sphere shot from a small two-stage light gas gun was measured as being 1 km/s using a laser-cut velocity measurement technique. So as to estimate the impact velocity of the projectile, the projectile was caused to impact targets made of aluminum (A1050), copper (C1012), mild steel (SPCC), and stainless steel (SUS304). After the impact tests, the impact crater shapes of the targets was measured using scanning electron microscopy(SEM), energy dispersive X-ray spectroscopy (EDS), and laser microscopy. The impact velocity of a projectile was estimated from obtained crater depth of the targets. In addition, microstructures of the interface between projectile and target were analyzed by EDS, electron backscattered diffraction (EBSD) and transmission electron microscopy (TEM).

Thermally sprayed coatings exhibit a lamellar structure with a bonding ratio less than 32% when a coating is deposited at ambient temperature. The lamellar bonding is one of the most important factors controlling the mechanical, thermal and electrical properties of the coatings. However, it is not clear why only limited lamellar bonding exists in a thermal spray coating even though many studies have been focused on the formation of bonding. In our previous study, it was found that there exists a critical deposition temperature for depositing ceramic splats to form the bonding with the underlying identical substrate, i.e., critical bonding temperature. Moreover, the critical bonding temperature is related to the interface temperature prior splat solidification which is determined by the glass transition temperature of splat material. In the present study, the critical bonding temperature and its relationship with interface temperature are used to understand the limited lamellar bonding ratio in a coating. A numerical simulation model involving heat transfer among depositing splat was proposed to establish the sufficient condition for liquid splat to form the bonding with the underlaying splats. The non-uniform splat thickness model was established to calculate theoretically the interface bonding formation. The calculation based on the model yielded a bonding ratio of 38.5% which agrees reasonably with the observed maximum interface bonding ratio.

The standard technique for applying zirconium (Zr) to uranium (U) is roll bonding, where a thin foil of Zr is placed on each side of a U plate which is then encased in steel and rolled at high temperatures. This study evaluates an alternative approach in which Zr layers are plasma sprayed on U and then clad with aluminum (Al) by hot isostatic pressing. The interface region between the Zr and Al is examined by SEM, revealing a reaction layer consisting of Al, Zr, and Si. SEM images show good conformance between the Al sheet and Zr surface along with the presence of Al in the porous Zr. Initial test results indicate that increased interface roughness and Al penetration into the plasma-sprayed Zr have the potential to improve bond strength by impeding crack propagation in the reaction layer.

Thermally sprayed coatings are formed through the successive impact of molten droplets and/or semi-molten particles followed by flattening, rapid cooling, and solidification. Individual droplets flatten to form splats of several micrometers in thickness upon impact and result in the formation of a coating that has a lamellar structure with limited interface bonding. The inclusion of semi-molten particles in the coating modifies its microstructure. The bonding between particles dominates coating properties and performance. This review paper examines the bonding formation at the interface between thin lamellae in the coating. The effect of spray parameters on the bonding ratio is presented to reveal the main droplet parameters controlling bonding formation. It is shown that spray particle temperature dominates the bonding formation more than particle velocity. Significant increases in ceramic particle temperature are not possible due to the inherent characteristics of thermal spray processing; therefore, the bonding ratio is limited to a maximum of about 32%. On the other hand, it was found that through controlling the surface temperature of coating prior to molten droplet impact, the bonding at the lamellar interface can be significantly increased. Consequently, with the proper selection of deposition conditions and control of surface temperature, the bonding ratio of ceramic deposits can be altered from a maximum of 32% for a conventional deposit to the maximum of 100%. Such wide adjustability of lamellar bonding extends the applicability of plasma spray coatings to applications requiring different microstructures and properties. Moreover, this bonding control makes it possible to fabricate porous surfaces and structures through the deposit of surface-molten particles, to deposit high temperature abradable ceramic coatings, and to form super-hydrophobic surfaces. Furthermore, the ability to deposit coatings with complete interface bonding allows crystalline structure control of individual splats through epitaxial grain growth.

Pure Al coatings were fabricated on Cu substrates via kinetic spraying for double-layered Cu liner. The coatings need to endure the high strain rate severe plastic deformation during explosion, in this study, the process optimization of Al deposition was initiated with a definition of “critical velocity” of Al particle in kinetic spraying on a basis of numerical modeling and computations using ABAQUS finite element codes. The simulation results revealed that the critical velocity of Al particle at room temperature (RT) was 780 m s -1 , and the critical velocity decreased as particle temperature increased. On the basis of simulation results, mechanical properties such as bond strength of the coatings formed under various process conditions were evaluated and compared. These properties were discussed in terms of the processing-structure-property relationships.

It has been widely demonstrated that the bonding between particles in cold-sprayed coatings results from impact-induced extensive deformation in the interfacial area. However, the mechanism of bonding remains obscure. This present work provides theoretical and experimental evidence of localized interatomic bonds between particles. The chemical bonding energy differences between Me-O (bonding energy of metallic and oxygen atoms) and Me-Me (bonding energy of metallic atoms) indicate a preferential trend of breaking down of Me-Me bonds and therefore a new interatomic bond was established. This hypothesis is addressed in terms of dynamics based on data generated by numerical modeling. In addition, interfacial regions of cold-sprayed nanocrystalline composite coatings were observed by TEM. The results revealed that whether or not recrystallization occurred in these places was determined by development of metallic bonding between particles.

The new kinetic spray coating technique, vacuum kinetic spray (aerosol deposition), utilizes the pressure gap between powder hopper and coating chamber which is vacuumed. In this study, to investigate the deposition mechanisms present in the vacuum kinetic spray coatings, α-Al 2 O 3 and glass were chosen as the powder and substrate materials, respectively, and these were considered as the reference materials to examine the effect of free surfaces after particle fractures. Based on the finite-element modeling (using an AUTODYN-2D 12.1), single particle impacts were simulated, and the results elucidated the material shape, temperature variation and mass change of particle due to its fracture during impact. The plots of total mass change as a function of particle impact velocity demonstrate the deposition-optimized velocity zone (DOVZ) for successful deposition. Compared to as-received powders, from the transmission electron microscope (TEM) images, the defects such as dislocations of the ball-milled powders might increase the tendency of the powder particles to fracture upon impact. The cross-section images of the coating showed that the particle sizes of the coating were drastically decreased compared to those of initial powders. During coating, fractured particles enlarged the thermodynamically unstable free surface area and have a tendency of formation of bonding.

The effect of chemical interaction compared to mechanical influences on the bonding mechanism in cold-gas spraying is a matter of great interest. In this study, combinations of different metals (Al, Cu, steel) sprayed onto galvanized surfaces (Cr, Ni) will be used for a first approach to gain information about substrate-particle combinations with very different chemical affinities and hardnesses. Single impact morphologies and coating cross-sections are compared with respect to mechanical deformation and bonding features. The results show that a strong mechanical interaction is required to build up the contact area between spray particle and substrate. Only if the intimate contact between the materials is given, chemical interaction can contribute to bonding.

Supersonic plasma spraying has been used to deposit WC-12Co coatings on polished stainless steel and aluminum substrate surface. The microstructure was observed by SEM and the bond mechanism was analyzed. It can be seen that individual WC particles can embed the substrate and make the substrate surface rough. The substrate materials differ, the depth that WC embed is also different. Bond among coatings mainly belonged to mechanical bonding. Bond between coatings and substrate mainly belonged to mechanical bonding, and partially belonged to metallurgy bonding, physical bonding, diffusion and micro-forging.

Carbon fibre reinforced carbon (CFC) composites have been more and more used in different industrial areas as high temperature materials. Some application examples are CFC components in modern furnaces for heat-treatment and brazing. Because CFC components can react with metallic materials when they contact each other, diffusion barrier coatings are essentially important for such CFC components. The aim of the project IGF 14.880 N “Thermally sprayed diffusion barrier coatings for CFC components in high temperature applications” is to develop diffusion barrier coatings by thermal spraying technology. In the project, different coating systems have been developed and investigated regarding the coating build-up, coating microstructure, bonding, thermal shock resistance and diffusion barrier function. The research results reveal that some developed coating systems are suitable for applications in furnaces. In the present paper, some research results of this project are reported.

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 %.