High entropy alloys, as a novel alloy system, demonstrated excellent mechanical performance. However, despite its excellent mechanical performance, the strength-ductility trade-off effect still limit its performance. In recent decades, it has been found that heterogenous or gradient microstructure can efficiently solve the conflict. Cold spray is a promising method to create heterogenous microstructure with high efficiency and low cost. In this work, equiatomic FeCoNiCrMn HEA was deposited by cold spray and the microstructure was systematically investigated by transmission electron microscopy (TEM) and transmission Kikuchi diffraction (TKD). In cold spray, a gradient microstructure was formed and segregated Ni and Mn in starting particle were also redistributed. Moreover, twinning in ultra-fine nanograins were detected in the region close to the impact interface. Compared with severe deformation of other low SFE metals, for FeCoNiCrMn HEA, twinning in nanograins also highly related to the grain size.

This study assesses the viability of using nitrogen instead of helium to cold spray NiCoCrAlTaY coatings onto single-crystal superalloy substrates. The process, though feasible, has a low deposition efficiency, leading to a high level of deformation that affects the microstructure of both the coating and substrate. SEM and TEM analysis revealed metallurgical and mechanical bonding at the interface and grain refinement in the coating. A fine grain structure that developed in the substrate after deposition was also observed possibly caused by dynamic recrystallization during the deposition process. Evidence of element segregation in the substrate, identifiable as zones with a deformed γ/γ’ structure, was found as well.

In the current work a nano-crystalline Ni-20Cr coating was developed on a boiler tube material by cold-spray process. The coating powder was synthesized in-house by mechanical milling using a planetary ball mill. The average crystallite size of the developed powder was calculated to be 10 nm. The crystallite size of the coating was found to be 18 nm, and hence the retention of nano-crystallinity in the coating was established. However the grain growth occurred to some extent, which may be due to temperature rise involved in the cold spray process. A detailed TEM analysis of coated samples before and after exposure to high temperature oxidation was carried out.

Detailed investigations of thermally grown oxides (TGO) zone and areas adjoining to it were presented in this article. The thermal barrier coating (TBC) based on gadolinium zirconate (Gd 2 Zr 2 O 7 , GZ) and deposited by air plasma spraying (APS) on AMS 5599 superalloy with NiCrAlY type of bond-coat was the analysed material. Analyses were made on TBC system after spraying process and after time to 500 hours of oxidation at temperature 1100°C in atmosphere of laboratory air. Ultrahigh resolution scanning-transmission (S/TEM) electron microscope TITAN 80-300 was used to conduct the tests. Quanta 200i Dual Beam apparatus by FEI for precise and localized focused Ga+ ion beam (FIB) sample preparation was used as well.The basic aim of presented investigations was related with detailed characterization of interface zone between ceramic top-coat (TC) of Gd 2 Zr 2 O 7 type and NiCrAlY bond-coat after spraying process and after 500 hours of deposition in oxidized atmosphere where the TGO zone was formed with the thickness ca. 5-10 µm. The special emphasis was placed on research of this zone and its morphology due to very strong influence of formed thermally grown oxides on overall life-time and destruction processes which took place on the interface between TGO and ceramic top-coat. Obtained data revealed that the most important factors influenced on destruction processes are related with the thickness of TGO zone (consisted mainly Al 2 O 3 , NiAl 2 O 4 and eventually NiO) rather than formation of undesirable phases such as GdAlO 3 complex oxide.

Epitaxial grain growth during the rapid solidification of molten TiO 2 in plasma spraying was studied. The crystallographic structure of the TiO 2 splats deposited on rutile and α-Al 2 O 3 substrates at 150, 300 and 500 °C was characterized by high resolution transmission electron microscopy and electron back scattering diffraction. The results reveal that homoepitaxial and hetero-epitaxial TiO 2 splats can be formed at the deposition temperature of 500 °C. Epitaxial growth is significantly influenced by the crystal orientation. It is easier to form an epitaxial TiO 2 splat with a <001> orientation in the direction perpendicular to the substrate surface. In order to explain the formation of epitaxial splat during plasma spraying, a competition mechanism between heterogeneous nucleation and epitaxial growth was proposed. It was indicated that the face (001) of rutile crystal exhibits the largest growth velocity, which is conducive to form an epitaxial splat for the melt with a largest undercooling degree. In addition, the effect of deposition temperature and crystalline orientation on the epitaxy was simulated. The simulation results are in agreement with the experimental observations.

The effect of substrate template effect on the crystalline structure of plasma sprayed 8YSZ (8mol%Y2O3) splats was investigated by high resolution transmission electron microscopy (HR-TEM) examination of FIB-processed splat samples. 8YSZ splats were deposited by the atmospheric plasma spraying (APS) on the polished sintered tetragonal structure substrate (3YSZ) and cubic structure substrate (8YSZ) at different preheating temperatures. The focused ion beam (FIB) was utilized to prepared TEM cross-sectional sample of splats. The crystalline structures of both the splat and the underlying substrate were examined by HRTEM. Results showed that the 8YSZ splats deposited on the polished sintered cubic structure 8YSZ substrate at a substrate surface temperature of 900°C exhibited cubic structure and the epitaxial grain growth was confirmed between the crystalline of splat grain and immediately underlying cubic crystalline substrate grain. Moreover, epitaxial grain growth was confirmed between the crystalline of splat grain and the tetragonal structure substrate when substrate surface temperature was increased to 1200°C. The present results suggest that the crystalline structure formation of 8YSZ splats produced by plasma spraying was affected by the substrate template effect.

In this work, YSZ coatings were deposited on air-cooled substrates by atmospheric plasma spraying. High-resolution transmission electron microscopy (HR-TEM) was used to examine local microstructure near the interface of bonded splats. The coatings mainly consist of typical columnar grain microstructure with metastable tetragonal phase. At the bonded zones, the top surfaces of previously deposited splats act as heterogeneous nucleation sites for the next splat. The examinations also revealed large columnar grains perpendicular to the bonded interface along with high defect density.

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.

Alumina splats were deposited on the polished single crystal alumina substrates with two different crystalline facet orientations of [001] and [110] by atmospheric plasma spraying (APS) at a substrate preheating temperature of 900°C to examine the epitaxy during splat cooling. The cross-sectional samples for high resolution transmission electron microscopy examination was prepared by focused ion beam assisted scanning electron microscopy (FIB-SEM). The results show that the whole splats with a thickness ranging from ~600 to ~1000nm exhibited the same crystalline structure as the substrate. Moreover, the splat deposited on the single crystalline alumina substrates exhibited exactly the same orientation as the substrate. The results evidently indicate that the epitaxial grain growth occurs after alumina droplets impact on single crystal alumina substrate. The present results suggest that the crystalline structure of alumina deposit formed by plasma spraying can be possibly controlled by the substrate preheating temperature.

Oxidation is a serious issue for thermally sprayed alloy coatings, especially in atmospheric plasma spray (APS) process, by which it is believed to be impossible to obtain alloy coatings with low oxide content. Oxidation behavior of Ni20Cr alloy and NiCrSiBC alloy in APS were compared by evaluating collected in-flight particles and sprayed coatings in our previous study. The results clearly show that the oxide content in the in-flight particles and coating of NiCrSiBC alloy was significantly lower than that of Ni20Cr alloy. In the present study, further study of the surface oxide layer of NiCrSiBC coating was carried out by TEM, which showed 10 nm thick oxygen enriched layer. The effects of particle temperature and velocity on the oxidation of the NiCrSiBC alloy were investigated. It was found that increasing particle temperature and velocity did not affect the oxidation of inflight particle of the NiCrSiBC alloy but significantly increased the oxidation after particle impacting substrate. The results are discussed in conjunction with effectiveness of preferential oxidation of Si, B, C and simultaneous vaporization of the formed oxides on suppressing in-flight oxidation and influence of a large amount of sub-particles produced upon impacting substrate on oxidation characteristics when increasing plasma power.

Composite coatings of titanium powder were deposited on aluminium samples by using the cumulative-detonation equipment. Consumption of fuel mixture components (propane-oxygen-air) was up to 4.3 m 3 per kilo of a coating. The coatings were examined by scanning electron microscopy (SEM), transmission electron microscopy (TEM) with diffraction, X-ray phase analysis, hardness measurements, as well as plasticity and adhesion/cohesion resistance scratch tests. It is shown that the coatings are characterised by the presence of nanodispersed ceramic formations, and feature high values of plasticity, hardness and adhesion strength.

The adhesion mechanisms involved in the cold spray coatings are not still well elucidated. The quality of the deposit does depend mainly on particles and dynamic characteristics (which result from nozzle type, nozzle-substrate distance, etc.). The present work is based on the study of particle-substrate and particle-particle interfaces in the tantalum-copper coating-substrate system. The content focuses on the influence of the oxygen content in the starting powder on interface features, consequently on coating properties. Tantalum powders with different oxygen levels were studied using SEM (Scanning Electron Microscopy) and EPMA (Electron Probe Microanalysis). Laser shock spallation of cold-sprayed Ta coatings was developed as a reliable and flexible process to achieve Ta spalls to be deposited at a high-velocity onto Cu targets. The velocity due to the laser shock could be controlled to be similar to that of particles in conventional cold spray. This results in Ta-Cu interfaces, the study of which was carried out to go into interface phenomena involved in cold spray, using TEM (Transmission Electron Microscopy) in particular. Results were compared to those obtained from laser shock spallation of Ta bulk specimens (i.e. made of a conventional Ta sheet). The role of powder oxidation on interface soundness was exhibited. Adhesion was shown to be all the lower as powder oxygen content was higher, using LASAT (“ Laser Shock Adhesion Test”) in addition to direct observation of interfaces. Results were exploited to discuss properties of the corresponding Ta coatings onto Cu, i.e. which were cold sprayed using powders with different oxygen contents.

The processes of microstructural evolution in titanium during deposition by cold spray were investigated. Titanium particles underwent plastic deformation at very high strain rates, resulting in (a) extensive refinement and (b) large inhomogeneities in microstructure. There were large nanostructured areas (several micrometres in extent) with high dislocation densities and subgrain sizes less than 100 nm. Dense dislocation tangles and elongated subgrains were observed in other places. It is hoped that by further understanding the microstructural changes that occur during cold spray, improvements in mechanical properties may be realised.

In cold spraying, the required heat for bonding is provided by plastic deformation of the impacting ductile particles. Therefore, cold spraying is a well-established method for metal on metal coatings. However, few authors have investigated the impact phenomena and layer formation process for impacting brittle ceramic particles on ductile metal surfaces. For this study, titanium dioxide (TiO 2 ) on metal surfaces was chosen as a model system, and layer formation on aluminum, copper, titanium and steel substrates was investigated by SEM, TEM, XRD and Raman spectroscopy. The results show that the deposition efficiency depends on spray temperature, powder properties, and in particular on substrate ductility, even for an impact of ceramic particles during a second pass over already coated areas. High-resolution TEM images revealed no crystal growth or phase transitions at the ceramic/metal interfaces. Nevertheless, a clear dependence of the photocatalytic activity on spray parameters and substrate material could be observed. Cold-sprayed TiO 2 -coatings have potential applications in biomedical implants or as photo-catalytic functional systems

In this study, we investigated microstructures of thermal sprayed coatings and single deposited splats using two types of ion beam milling: one is argon ion beam for the cross-sectioning of thermal sprayed coatings in a cross section polisher, the other is gallium focused ion beam for the cross-sectioning and TEM sample preparation of single deposited splats. The cross section of WC-Co coatings fabricated by the polisher showed that it created a mirrored surface with minimizing artifacts such as pull-outs of ceramic particles or smearing of pores during conventional metallographic preparations. A thin and locally re-thinned membrane of single warm-sprayed nickel splat was feasible to observe the internal interface of particle/substrate in high resolution electron images. The substrate was heavily deformed by the impact of nickel particle with high kinetic and thermal energies. The particle and the substrate were intimately bonded without any voids or gaps.

Homogenous mixtures of Ce 0.8 Gd 0.2 O 1.9 (GDC) and La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3 (LSCF6428) nanopowders were successfully synthesized using radio frequency (RF) induction plasma by axial injection of a solution. Two kinds of powders with different mass ratio of GDC/LSCF, such as 3/7 and 6/4, were obtained. The crystallinity and morphological features of the powders were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The particles are almost globular in shape with a diameter lower than 100nm and the BET specific areas around 20m 2 /g. In addition, suspensions, made with the composite nanopowders and ethanol, were used to deposit some cathode coatings using suspension plasma spray method. Several initial results of the coatings are also presented. The coatings are homogeneous and porous with cauliflower structures.

This study investigates microstructural alterations in copper powder during cold spraying. A highly nonhomogeneous structure was observed with average strains of almost 50% close to the interface. Large strains and high strain rates resulted in significant grain refinement and the formation of sub 100-nm grains. Deformation occurred by slip and twinning, the latter promoted by impurities in the copper feedstock. Thin foils from the deposits were studied by TEM analysis and cross-sections of deformed particles were made and examined using FIB-SEM techniques.

Titanium particles were deposited on a steel substrate by the impact of high velocity in warm spraying. In the process, nitrogen gas at various flow rates was mixed to control the temperature of a supersonic gas flow generated by combustion. TEM and other techniques were used to analyze the microstructure of the interface between the titanium coatings and the substrate. At the lower nitrogen flow rate, thick oxide double layers in the interface region were observed. The adhesive strength of the coating was high even at lower particles’ velocity possibly because the mechanical interlocking between the titanium particle and the substrate could be enhanced by the high deformability of heated particles. As the nitrogen flow rate increased, however, just a little oxide and a very thin oxide layer covering on the titanium splats were locally detected. The highly localized pressure and the resultant intensive shear stress generated within a titanium particle by the impact could reveal the fresh metal surface through break-up of the thin oxide films on the particle and the substrate. As a result, the metallic bonding between the deposited particle and the substrate was formed and increased the adhesive strength remarkably beyond a certain impact velocity.

Hypoeutectic AlSi engine blocks of modern passenger cars are generally equipped with cast iron liners in order to provide cylinder running surfaces that meet the tribological requirements. A very promising alternative to the use of cylinder liners lies within the application of thermally sprayed coatings onto the walls of cylinder bores as friction partners for the piston rings. This work describes the development of a novel iron based wire feedstock as well as its application by the Plasma Transferred Wire Arc internal diameter coating system. The material developed within the frame of this work leads to partially amorphous coatings with embedded nanoscale precipitations if processed by thermal spraying. The coatings were applied onto the inner diameters of test liners made of Aluminium EN AW 6060 and onto cylinder bore walls of in-line 4 cylinder engines. All substrates were mechanically roughened in order to obtain high bond strengths of the sprayed coatings. The coatings microstructure was analysed by light optical microscopy, hardness measuring by transmission electron microscopy. Furthermore the oil storage capacities of the honed surfaces were determined.

Thermal-sprayed MCrAlY coatings are widely used for land-based gas turbine applications. The cold spray may increase the coating density owing to the high-velocity particle impacts during spraying. Many researchers have considered critical velocity to be the most important factor of the deposition mechanism of cold-sprayed coatings. However, this dominant parameter of critical deposition condition has not been completely understood. In order to understand the mechanism, two approaches were used in this study. One is the transmission electron microscope (TEM) observation of the interface between the coating and the substrate, and the other is the cross-sectional observation of the deposited particle by using the focused ion beam (FIB) cutting technique. From the TEM observations, there are no evidences of melting at the interface, and it is found that the actual bonding occurred at the nascent surfaces. Generally, there is a native oxide on the surface of the particles and substrate. After the plastic deformation of the particles and substrate, the native oxide breaks down; subsequently, a nascent surface can be created and direct contact initiates deposition. From the results of these investigations, it is thought that the dominant factor for deposition is the plastic deformation of the particles and substrates.