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This study investigated the impact behavior of ceramic particles in vacuum kinetic spray (VKS), also known as aerosol deposition method, for successful coating build-up using both experimental methods and numerical simulations. A coating trial and fluid dynamics simulation results showed that there existed a critical impact velocity at which successive coating build-up was possible. This indicated that particle–particle bonding could be achieved through sufficient kinetic energy of impacting particles. According to the results in this study, it is suggested that the critical velocity values for deposition of Al2O3 and SiO2 in VKS were 400 and 300 m/s, respectively. The AUTODYN simulation results revealed that the initial kinetic energy of particles at high impact velocity should be consumed by fracture and deformation of both impacting particles and the pre-deposited layer. Above critical impact velocity, it is expected that high von Mises stress, pressure, and temperature values contribute to dynamic fragmentation of impacting particles, additional size reduction of crystallites, and consolidation for uncontaminated intimate inter-crystallite faying surface bonding. It is expected that these material responses contributed to the dense and strong VKS coating fabrication. [ABSTRACT FROM AUTHOR]

Thermite materials, which are a pyrotechnic composition of metal and metal oxide, can create bursts of heat and high temperature thanks to an exothermic reduction–oxidation reaction. Fabrication methods of bulk thermite materials should preserve the reactivity of the materials and ensure a sufficient mixing of components. This work investigated the use of kinetic spraying and ball milling to overcome the limitations of existing preparation methods. Aluminum–molybdenum trioxide (Al-MoO3) was selected as thermite material. Ball milling and the fractions of metal and oxide were chosen as the variables of the study. The mechanical properties and reactivity of various powders and coatings were evaluated. The milled powders and coatings were successfully fabricated without reactivity loss by properly controlling the process parameters. The bond strength of the Al-MoO3 coating was however weaker than that of a pure Al coating because the MoO3 powder interrupted the intimate bonding between the Al splats. A reactivity analysis demonstrated that ball milling improved the reactivity of both the powders and the coatings by increasing the exothermic value, decreasing the reaction onset temperature, and decreasing the activation energy required for the thermite reaction. The mechanical properties and reactivity of coatings did not vary significantly with the MoO3 fraction. [ABSTRACT FROM AUTHOR]

Vacuum kinetic spraying (VKS) is a promising room-temperature process to fabricate dense ceramic films. However, unfortunately, the deposition mechanism is still not clearly understood. In this respect, the critical conditions for successful deposition were investigated. Based on simulation and microstructural analysis, it was found that as the particle velocity increased, fracture mode transition from tensile fracture to shear fracture occurred and particle did not bounce off anymore above a certain velocity. Simultaneously, particle underwent shock-induced plasticity and dynamic fragmentation. The plasticity assisted to prevent the fragments from rebounding by spending the excessive kinetic energy and fragmentation is essential for fragment bonding and film growth considering that the deposition rate increased as the fraction of fragmentation increased. Accordingly, plasticity and fragmentation take a crucial role for particle deposition. In this respect, the velocity that fracture mode transition occurs is newly defined as critical velocity. Consequently, for successful deposition, the particle should at least exceed the critical velocity and thus it is very crucial for film fabrication in VKS process at room temperature. [ABSTRACT FROM AUTHOR]

In the past, most studies into kinetic spraying technology focused on basic research, but a large portion of current research is devoted to industrial applications of the technology. To advance, however, studies about industrial applications of kinetic spraying require profound understanding of the scientific foundations of the kinetic spray process. Nevertheless, no one has yet provided a well-organized summary of the correlations among impact conditions, interface reactions, microstructural evolution, and mechanical properties across the whole field of kinetic spraying technology. This paper provides such an overview of these correlations for kinetic spraying of metals. For each correlation, the interactions between the given conditions and the material properties of the metal feedstock powder are the most influential. These interactions are so complicated that it is difficult to systematically classify all cases into certain types. Nonetheless, we try to explain and summarize the critical factors and their roles in each relationship. [ABSTRACT FROM AUTHOR]

The compressive deformation behavior in the thickness direction of a kinetic-sprayed pure-aluminum layer whose thickness is ~15 mm was investigated. The yield strength of coating material was 200 MPa, and a unique strain softening phenomenon occurred even at room temperature. Due to the initial severely deformed structure of the kinetic-sprayed coating material based on the results of microstructure analysis, the production of high-strength metal bulk materials using kinetic spray processes was deemed possible. [ABSTRACT FROM AUTHOR]

Microstructure and texture formation of an Al coating during kinetic spraying (or cold gas dynamic spraying) and heat treatment were investigated. Coating formation by kinetic spraying is based on super-sonic collision of in-flight micron-sized particles and their severe interfacial plastic deformation under ultra-high strain rates (1.0 × 10-0.5 × 10 s), which induces adiabatic shear instability. Shear texture, 45°-rotated Cube {001} <110>, and static recovered microstructure were formed at the interface of Al during kinetic spraying because Al has the equivalent slip system {111} <110> of a face-centered cubic having high stacking fault energy (SFE). During heat treatment, discontinuous recrystallization and grain growth led to transformation of the shear texture into a strong Cube texture, {001} <100>, and misorientation angle transition. Given the mechanical and physical properties of the Al, metallurgical mechanisms of microstructure and texture formation of kinetic-sprayed and heat-treated Al coatings were suggested based on transmission electron microscopy and electron backscatter diffraction analysis. [ABSTRACT FROM AUTHOR]

Abstract: Impacting behavior and deposition formation of Ni–Ti–Zr–Si–Sn bulk metallic glass during kinetic spraying were investigated in this study. Bulk metallic glass particle deposition was primarily dependent on the impacting particle velocity. Secondly, the impacting particle temperature could transfer the deposition mode from fracture-induced local melting to severe viscous flow, especially for the splat on previously deposited splat. It is suggested that the deposition behavior of bulk metallic glass particles is determined by the competition between fracture and deformation. Fracture and deformation are controlled by adiabatic shear instability at the faying interface during impacting, with causal factors including temperature-dependent superplasticity of the bulk metallic glass and an abnormally high strain rate of the individual particle impact during kinetic spraying. [Copyright &y& Elsevier]

Abstract: In this study, vacuum plasma spraying (VPS) was chosen for making Ni–Ti–Zr–Si–Sn bulk metallic glass (BMG) coating and the process parameters were optimized in view of phase composition of as-sprayed BMG overlay. When it comes to the phase evolution of BMG particle in thermal spraying process, the crystallization does occur by chemical instability and/or thermal instability. For the VPS process, the chemical instability such as in-flight particle oxidation can be neglected and thus the crystallization of BMG feedstock can be considered to result from the thermal instability. Actually, the crystalline phase fraction of the as-sprayed coatings was largely affected by the plasma gas composition which determines the plasma jet characteristics and also in-flight particle melting state. As the gas enthalpy is increased with the increase of hydrogen gas flow rate, number density of unmelted particle within the coating and crystalline phase fraction were consistently decreased. It implied that the phase composition of VPS BMG coating was largely dependent on the melting state of the impacting BMG particle. [Copyright &y& Elsevier]

Numerous efforts have been focused on the fundamental research regarding kinetic spraying processes, but previous studies were biased to only few kinds of metallic materials. In particular, for Ta with body-centered cubic crystal structure, systematic studies are currently lacking in the literature. In this study, the characteristic deposition behaviors, microstructures, and mechanical properties of kinetic sprayed Ta under various impact conditions were systematically investigated. Generally, unusual features were usually related to strain-rate-dependent response (i.e. Peierls-Nabarro short range barriers) of Ta at high-temperature. This made thermal softening effect to be saturated above a certain degree of temperature, contributing to inferior deposition efficiency for the pre-heating at 600 °C than those of 400 and 500 °C by creating synergy with powder oxidation. In terms of microstructural evolutions, the dislocation density was abnormally increased at a certain powder preheating temperature (i.e., 600 °C), owing to the strain-rate-dependent response of Ta at high-temperature. On the other hand, grain refinement generated by rotational dynamic recrystallization was mainly observed under the higher kinetic energy condition (He carrier gas) at RT. Especially, the dislocation density and volume-average domain size evaluated via high-resolution X-ray diffraction peak profile analysis were corresponding to the results of the microstructural observations. Additionally, the nanomechanical properties (i.e., nanohardness and elastic modulus) evaluated via nanoindentation tests were in good agreement with the results from transmission electron microscopy investigations. [ABSTRACT FROM AUTHOR]

In this study, optimization of substrate roughness for enhancing interface bonding state in cold spray for different material combinations is performed. Material combinations are classified into four different particle/substrate impact pairs (i.e., soft/soft, soft/hard, hard/soft and hard/hard) based on their thermos-mechanical and physical properties. Finite Element Analysis (FEA) was employed to simulate the models in relation with constant powder size (25 μm), for estimating impact interface parameters. Experimentally, particles were deposited on different roughened substrates in order to compare the simulation results. Microstructures, deposition efficiencies and bond strengths were obtained for the coatings. On the basis of simulation and experimental results, optimum substrate roughnesses for different cases are suggested for better bonding. The optimum roughness for different material combinations was obtained and found to be the fraction of particle size based on the thermo-mechanical behavior of materials upon impact. [ABSTRACT FROM AUTHOR]

A correction to the article "J Therm Spray Tech DOI 10.1007/s11666-010-9559-z" that was published in the previous issue is presented.

In a vacuum kinetic spraying (VKS) system, submicron ceramic particles impact the substrate and undergo dynamic fragmentation within nano-seconds, resulting in a nano grain film. In this report, a hypothesized mechanism for dynamic fragmentation generated during Al 2 O 3 particle deposition is suggested and tested via Auto-dyn simulation linked to an analysis of distinct fragment microstructures and debris size calculations. Based on this data, shock-induced dynamic fragmentation, one of the crucial elements of clarifying the deposition mechanism, was investigated. [ABSTRACT FROM AUTHOR]

Abstract: Nanoscale twins parallel to the {111} planes were observed to be formed by deformation twinning in a nano-sized grain during kinetic spraying of nickel (>99.5% purity) at ultrahigh strain rates (>107 s−1), high strain levels (>100%) and high temperatures (>873K). An interfacial region dominated by strain hardening over thermal softening upon impact, as shown by finite element simulations, is considered to facilitate twinning with the aid of the effects of small amounts of solute atoms and nanocrystalline/amorphous nickel oxides trapped at local interfaces. [Copyright &y& Elsevier]

Abstract: Owing to the specific high-strain-rate thermomechanical characteristics of Ni particle impact in kinetic spraying, the rebound phenomenon of the impacting particles hinders the formation of the first layer and impedes successful build-up of the coating. Even at higher impact velocities, the deposition efficiency of the coating is quite low because of excessive kinetic energy, which induces the rebound and/or erosion of the highly flattened particles. This paper reports noticeably improved bonding and deposition characteristics of Ni particles resulting from suppressed equivalent (von Mises) flow stress and enhanced interface heat-up as a result of powder preheating. Experimental observations coupled with finite-element modeling (FEM) corroborate the fact that the thermally softened Ni particle is very effective for enhanced adhesive and cohesive bonding. Based on the FEM results, the thermal boost-up zone, increased by thermally accelerated adiabatic shear instability, is proposed as a crucial factor for enhancing bonding between the particles, which is essential in producing better coating properties. Moreover, nanocrystal formation (<100nm) in the coating was more pronounced than cases previously reported in the literature, mainly because of the enhanced thermal activation and straining of the severely deformed particles, which was verified by transmission electron microscopy investigations and nanoindentation tests. [Copyright &y& Elsevier]

Abstract: The conventional manufacturing process of the automotive brazed heat exchanger includes complex preparation processes before brazing: aluminum brazing filler alloy is pre-claded on both sides of a fin by an extrusion method, and holed aluminum tubes are coated on both sides with Zn for corrosion protection by a wire arc spraying process. The intent of this study is to simplify the preparation process by kinetic spraying using all of the components, including Al–12%Si (for the brazing filler metal), Zn (for corrosion protection), and KAlF4 (flux powder). Four kinds of blended powders were evaluated with and without flux. The bond properties and composition distribution of composition at the braze joint area were evaluated by scanning electron microscope (SEM) and electron probe micro analyzer (EPMA). In this study, kinetic spray condition was optimized, in order to fabricate the heat exchanger. It was observed that the joints of the brazed specimens at each side were sounder than that achieved by the conventional methods. It is necessary to control the Zn content to get the high corrosion resistance and good brazeability of the aluminum heat exchanger. Further, the kinetic sprayed heat exchanger showed acceptable corrosion protection. [Copyright &y& Elsevier]

Nanocrystalline (nc) Al matrix composites reinforced with multi-walled carbon nanotubes (MWCNTs) (1.0 and 3.0 vol.%) were fabricated by mechanical attrition and consolidated using a kinetic spraying process, followed by heat-treatment at a homologous temperature of 0.67 T/T M for 1–4 h. Microstructural features including thermal stability of the nc structures, the interface between MWCNTs and the Al matrix, and the MWCNT structures of as-sprayed and heat-treated MWCNT/nc Al composite coatings were characterized and related to their corresponding mechanical and physical properties. The relationship between microstructural features and properties of the composite coatings was investigated. [ABSTRACT FROM AUTHOR]

Kinetic and thermal spray processes were used to fabricate the multi-walled carbon nanotube (MWCNT) reinforced Cu composite coatings with 0–1.0 wt% MWCNTs. The microstructures of the composite coatings were analyzed and electrical and mechanical properties such as the electrical resistivity, micro-hardness, wear rate, and friction coefficient were evaluated. Since the deposition and microstructural evolution mechanisms of kinetic and thermal spraying are entirely different, the characteristics of the spray processes dominated the microstructure and interface of the Cu matrix, MWCNT structure, and the nature of the interface between MWCNTs and the Cu matrix, which are correlated with the properties. In this study, the relationship among electrical and mechanical properties with (MWCNT) state and the microstructural factors of the MWCNT/Cu composite coatings were investigated via microstructural and physical metallurgical analyses. [ABSTRACT FROM AUTHOR]

In a vacuum kinetic spraying system, plasticity and fragmentation have been considered as the main deposition mechanisms because the deposited layer exhibits a flattened morphology with nanosized grains. In this research, Al 2 O 3 film was fabricated, and the microstructure was analyzed using Auto-dyn simulation to investigate the deposition mechanism. Plasticity was generated in a ceramic particle by shock loading, and the particle impact intensity affected the fragmentation behavior and consolidation of a previously deposited layer. [ABSTRACT FROM AUTHOR]

Abstract: Multi-walled carbon nanotube (MWCNT) reinforced Al composite powders were deposited using a high velocity oxygen fuel (HVOF) spraying process to form coatings. High thermal energy and physical contact with atmospheric oxygen were supplied as the MWCNT/Al composite particles were exposed to a gas flow field of high temperature (~3.0×103 K) during HVOF spraying. As a result, the particles underwent full or partial melting and rapid solidification during deposition. Large fraction of lamellar structure was formed in the HVOF sprayed coatings due to the low melting point of Al. The high temperature environment and the exposure to oxygen induced the interfacial reaction of MWCNTs within the splats. The electrical and mechanical properties (elastic modulus and micro-hardness) of the MWCNT/Al composite coatings were evaluated. The variations of measured properties of the MWCNT/Al composite coatings were related to the contribution of the remaining MWCNT and the typical lamellar structure. In this study, the relationship among the properties, structure and the interaction of the MWCNTs with the Al matrix is experimentally and theoretically discussed. [Copyright &y& Elsevier]

Abstract: This study aims to examine the effect of particle size on the microstructure and corresponding properties in kinetic sprayed coatings of commercially pure nickel (CP–Ni) in conjunction with finite element modeling (FEM). Prior to the experiments, the adhesion factors (interface temperature, contact time and contact area), rebound factor (relative recovery energy) and the resultant critical velocities of CP–Ni for different particle sizes and temperatures were estimated by FEM. Based on the simulations, three different sized CP–Ni powders were successfully deposited onto mild steel substrates using a powder preheating system. Here we suggest optimized windows of operation for particle sizes of CP–Ni based on the microstructure and properties of the coatings (i.e. deposition efficiency, bond strength and micro-hardness) which are in good correspondence with the simulation results. [Copyright &y& Elsevier]

Abstract: This study aims to explore the bonding features and mechanisms present in kinetic spray coatings of commercially pure titanium (CP-Ti) by linking experimental observations with finite element modeling for the purpose of optimizing the deposition process. The numerically predicted deformation features and interface thermomechanical responses of the particles adequately elucidate the deposition characteristics for different impact conditions. The formation of a porous coating by local bonding at relatively low velocity and of a very dense coating accompanied by interfacial melting at higher velocity resulted from deformation localization at high strain rates due to the relatively higher adiabacity as compared with other materials, which is one of the unique bonding mechanisms in CP-Ti. The enhanced thermal boost-up zone due to thermally accelerated adiabatic shear instability by powder preheating resulted in improved coating properties, and is essential for the optimization of economical coating processes. [Copyright &y& Elsevier]

Abstract: Soldering and erosion of casting moulds and their components are one of the most prominent and persistent problems occurring in the die-casting industry. In this study, MoB- and Co-based cermets and alloy, respectively, were deposited on bond coated hard substrates (SKD61) using kinetic spray coating technique. Coating of hard powder materials on hard substrate using kinetic spray technique has also been considered a difficult phenomenon. Microstructural and mechanical characterization of the coatings was carried out by SEM, XRD, micro-hardness, bond strength and wear resistance tests. The results showed that Co-based alloy displayed the highest deposition efficiency along with microstructural properties. However, MoB/NiCr demonstrated the highest mechanical properties. The coatings were tested for soldering resistance by immersion of coated SKD61 cylinders in molten aluminium alloy (ADC-12) bath. MoB/NiCr displayed superior soldering resistance among the coatings. Among the candidate coatings, MoB/NiCr is considered as the one with the highest durability in a die-casting operation. [Copyright &y& Elsevier]

Abstract: In this study, the effect of protective nickel film on diamond particles was studied with the goal of preventing fracture and obtaining a uniform diamond distribution in a bronze/diamond composite coating during kinetic spraying. Two types of bronze/diamond composite were deposited on aluminum substrate. For comparison with experimental results, the impact behavior between diamonds in the gas flow field was simulated by finite element analysis (using ABAQUS/Explicit 6.7-2). Size distribution and deposition efficiency of the diamond particles in the composite coatings were analyzed through scanning electron microscopy and image analysis methods. Diamond fracturing was avoided because the impact energy was mostly absorbed by the outer protective nickel film on the diamond particle during impact. The uniform distribution and deposition efficiency of diamond particles in the coating layer could also be achieved by tailoring the physical properties (density, size, etc.) of the feedstock. [Copyright &y& Elsevier]

Abstract: In this study, Al–Sn binary alloy coatings were prepared with Al–5wt.% Sn (Al–5Sn) and Al–10wt.% Sn (Al–10Sn) gas atomized powders by low pressure and high pressure cold spray process. The microstructure and microhardness of the coatings were characterized. To understand the coarsening of tin in the coating, the as-sprayed coatings were annealed at 150, 200, 250 and 300°C for 1h, respectively. The effect of annealing on microstructure and the bond strength of the coatings were investigated. The results show that Al–5Sn coating can be deposited by high pressure cold spray with nitrogen while Al–10Sn can only be deposited by low pressure cold spray with helium gas. Both Al–5Sn and Al–10Sn coatings present dense structures. The fraction of Sn in as-sprayed coatings is consistent with that in feed stock powders. The coarsening and/or migration of Sn phase in the coatings were observed when the annealing temperature exceeds 200°C. Furthermore, the microhardness of the coatings decreased significantly at the annealing temperature of 250°C. EDXA analysis shows that the heat treatment has no significant effect on fraction of Sn phase in Al–5Sn coatings. Bonding strength of as-sprayed Al–10Sn coating is slightly higher than that of Al–5Sn coating. Annealing at 200°C can increase the bonding strength of Al–5Sn coatings. [Copyright &y& Elsevier]

Abstract: Particle velocity is a very important parameter in kinetic spraying (or cold gas dynamic spraying). It is difficult to measure the velocity of a particle with supersonic speed at low temperature (lower than 500°C). Thus, in many investigations only estimated values are used for evaluating coating processes. In this paper, the modeling of particle acceleration was reviewed, and the measurement of in-flight particle velocity in a kinetic spraying process was performed. Particle velocity and flux distributions from different process gas temperatures and pressures were investigated. The influences of process gas temperature and pressure on particle velocity were discussed. Characteristic of Al–Si feedstock deposition onto a mild steel substrate was described by comparing coatings structures with the in-flight particle conditions. The deposition behavior showed two critical particle velocities for Al–Si powder deposition onto a substrate and for particle–particle bonding. [Copyright &y& Elsevier]

Abstract: An inert gas atomized NiTiZrSiSn powder was sprayed using vacuum plasma spraying, high velocity oxy-fuel (HVOF) spraying, and kinetic spraying. Through this study, the effects of thermal energy and kinetic energy of in-flight particles and oxidizing power of the working environment on the amorphous phase formation were evaluated. In view of the melting state of the impacting particle, the processes are divided into thermal spraying and kinetic spraying. On the other hand vacuum plasma and kinetic spraying can be considered to be inert processes while the HVOF process is regarded as an oxidation process. For the former classification, the peak temperature experienced by the in-flight particles during flight is the critical factor for amorphous formation. The preferential oxidation of Ti and Zr was observed during the oxidative process and this made the chemical composition of the feedstock deviate from the optimum composition. The more in-flight oxidation there was, the less amorphous phase fraction there was. [Copyright &y& Elsevier]