Measurement of particle velocity and characterization of deposition in aluminum alloy kinetic spraying process

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.     

Influence of substrate hardness transition on built-up of nanostructured WC-12Co by cold spraying

The ability of cold spray process to retain the feedstock microstructure into coating makes it possible to deposit nanostructured WC-Co coatings. In the present study, the deposition behavior of nanostructured WC-12Co coating was examined through the surface morphology and cross-sectional structure of the deposited single WC-12Co particle impacting on the substrates with different hardness using a nanostructured WC-12Co powder. Substrates included stainless steel, nickel-based self-fluxing alloy coatings with different hardness. It was observed from the top surface and cross-section of individual WC-12Co particles that the penetration leading to particle deposition depends on substrate hardness. When the substrate surface is covered by WC-12Co particles, the hardness of the newly formed substrate, i.e. the coating, increases greatly. The significant increase of the surface hardness leads to the rebounding off of impacting particles and erosion of the deposited particles, which prohibits effective built-up of coating. However, it was found that with spray jet fixed, a deposit with a thickness up to over 700 μm can be built-up. A model involving in substrate hardness transition during deposition is proposed to explain such phenomenon, which can be employed to optimize the conditions to build up a uniform nanostructured WC-12Co coating.     

Effect of gas temperature on critical velocity and deposition characteristics in kinetic spraying

This study evaluated the effects of particle temperature and substrate material on critical velocity and deposition efficiency in kinetic spraying. A wide range of process gas pressures and temperatures was used in this experiment to vary both particle velocity and temperature. A bronze (Cu-Sn alloy) feedstock was deposited onto aluminum, mild steel and bronze substrates. The experimental results showed that the critical velocity was strongly dependent on the combinations of particle/substrate and particle temperature. The decreasing critical velocity could be obtained at the same particle velocity, due to an increasing particle temperature. In our experiments, the critical velocity decreased by 50 m/s when the process gas temperature increased by 100 °C. When process conditions are optimized to have good bond strength and deposition efficiency, two critical velocities must be considered; one is that of the particle deposition onto the substrate (Vcr1) and the other is that of particle-particle bonding (Vcr2).     

Relationship between particle size and deformation in the cold spray process

The flattening of aluminum and copper particles cold sprayed onto ceramic surfaces has been described by an ellipsoidal function. The accuracy of this approximation was verified from particle dissections performed in a focused ion beam/scanning electron microscope (FIB/SEM). Flattening data were collected from SEM measurements. It was shown that aluminum particle deformation was limited below ∼5 μm, and copper deformation below ∼2 μm. Deceleration of the particles through the bow shock, strain rate hardening and thermal conduction were factors that contributed to the differences in deformation behaviour.     

The influence of process parameters on deposition characteristics of a soft/hard composite coating in kinetic spray process

In kinetic spray processes, the non-uniformity of resultant composite coatings is generally caused by the difference in critical velocity and deposition efficiency between the components of a mixed feedstock. In the present paper, the effects of process parameters, such as feed rate, spray distance, and particle velocity, on the compositional variation between the mixed feedstock and resultant composite coating have been investigated. The results showed that the high diamond fraction in the coating can be achieved using a low feed rate, intermediate spray distance, and high impact particle velocity. The possibility of impact between hard brittle diamond particles is the main factor affecting the diamond fraction in the coating. Although the deposition efficiency, diamond fraction, and bond strength of the coating increase with particle velocity, a slight decrease of cohesive strength between diamond particle and bronze base was also observed.     

Significant influence of particle surface oxidation on deposition efficiency, interface microstructure and adhesive strength of cold-sprayed copper coatings

The critical velocity for particle deposition in cold spraying is a key parameter, which depends not only on the material type, but also the particle temperature and oxidation condition. The dependency of deposition efficiency of cold spray Cu particles on the particle temperature and surface oxidation was examined. The effect of particle surface oxide scales on the interfacial microstructure and adhesive strength of the cold-sprayed Cu coatings was investigated. The results show that the deposition efficiency significantly increases with increasing the gas temperature but decreases with augmenting the oxygen content of the starting powder. The oxide inclusions at the interfaces between the deposited particles inhibit the effective bonding of fresh metals and remarkably lower the bond strength of the deposited Cu coatings on steel.     

Low temperature deposition and characterization of TiO2 photocatalytic film through cold spray

Cold spray was employed as a novel low temperature approach to deposit titanium dioxide (TiO₂) photocatalytic film. The film microstructure was characterized using X-ray diffraction and scanning electron microscopy. The photocatalytic performance was examined through acetaldehyde degradation under ultraviolet illumination. Results showed that TiO₂ film was successfully deposited on substrate surface through cold spray. The film thickness reached up to 15 μm. The film presented a rough surface and porous structure. Owing to the low temperature of spray powder, no phase and particle size changes occurred to TiO₂ during deposition. It was found that the cold-sprayed TiO₂ film was active for photodegradation of acetaldehyde.     

Alloying of cold-sprayed Al-Ni composite coatings by post-annealing

A new cold spray coating technique for thick Al coating with finely dispersed Al-Ni intermetallic compounds was tested. For easy powder preparation and high yield, rather than using of Al/compound mixture feed stock, the spraying of pure Al and Ni powders mixture followed by post-annealing was suggested. The powder composition of Al and Ni was 75:25, and 90:10 (wt.%) to expect full consumption of pure Ni into intermetallic compounds. After Al-Ni composite coatings, the Ni particles were finely dispersed and embedded in the Al matrix with a good coating yield. Above 450 °C of post-annealing temperature, the Al₃Ni and Al₃Ni₂ phases were observed in the cold-sprayed Al-Ni coatings. The Ni particles in the Al matrix were fully consumed via compounding reaction with Al at 550 °C of the annealing temperature.     

Microstructure, microhardness and dry friction behavior of cold-sprayed tin bronze coatings

In this paper, two types of tin bronze coatings (Cu-6 wt.% Sn and Cu-8 wt.% Sn) were prepared by cold spray process. The as-sprayed coatings were subjected to a vacuum heat treatment at 600 °C for 3 h. The coating microstructure, microhardness and tribological performance were characterized. The effects of the tin content and the vacuum heat treatment on the microstructure, microhardness and tribological behavior of the coatings were investigated. It is found that the as-sprayed CuSn6 (As6) and CuSn8 (As8) coatings exhibit practically an identical porosity. Meanwhile, As8 presents a higher microhardness than As6. In addition, the increase of the tin content in the powder feedstock leads to a lower wear rate. After a heat treatment, coating porosities are significantly reduced. However, the coating hardness is significantly decreased and the coating presents a much decreased wear resistance. For the as-sprayed coatings, such factors as ploughing and particle delamination could determine the sliding process. The heat treatment results in a distinct modification of the tribological behavior. For the annealed coatings, the adhesion, between the coating and the counterpart, could play a dominant role in the sliding process.     

Simultaneous measurement of particle size, velocity, andtemperature in thermal plasmas

The in-flight measurement of particle parameters (size, velocity, temperature, and local number density) can prove insight into the plasma processing of solid materials. A measurement technique for simultaneously obtaining the size, velocity, and temperature of particles entrained in high-temperature flow fields is described. Particle size and velocity are obtained from a combination laser-particle-sizing system and laser Doppler velocimeter. The particle temperature is determined by a two-color pyrometry technique and the data rate is a measure of relative particle number density. Typical measured temperatures and velocities for the 5-100 μm particles used in plasma spraying are 1600-3500 K and 100-300 m/s, respectively. Since particle size, velocity, and temperature are measured simultaneously, cold particles (<1600 K) are identified and their relative number density can be quantified. Data from two plasma spray systems, a metal one (Ni-Al) and a metal oxide one (Al₂O₃), are presented and their application to understanding the plasma spray-coating process is illustrated     

The bond strength of Al-Si coating on mild steel by kinetic spraying deposition

Kinetic spraying (or cold gas dynamic spraying) works by accelerating small solid particles to supersonic velocities, and then impacting them onto a substrate. These high impact velocities, and low particle temperatures are the principal attributes of kinetic spraying technology. However, only recently has this technology's interfacial behavior, due to particle/substrate impaction, become well understood. In order to investigate the particle/substrate bond behavior, Al-Si feedstock was deposited onto mild steel, over a range of particle velocities; next, their respective coating bond strengths were measured by the stud pull coating adherence test. The effects of the particle velocity and the substrate surface roughness on the coating bond strength were presented, and a model of the particle/substrate bond generation was discussed in an effort to estimate the bond strength.     

冷动力喷涂法制备钨和钨合金涂层 及其涂层的计算模拟

采用冷动力喷涂法以纯钨和钨-镍-铁合金为原料在铜合金基体上制备了钨涂层和钨-镍-铁涂层. 研究了冷喷涂过程中钨粉粒径、喷涂距离等因素对涂层性能的影响. 用扫描电子显微镜分析了涂层的表面、断面微观结构, 并用原子力显微镜测量了涂层的粗糙度. 此外, 计算了冷喷涂过程中粉末颗粒的实际速度, 并采用有限元分析软件ANSYS/LS-DYNA模拟了冷喷涂过程中颗粒撞击基体时的变形情况.     

Flame spray deposition of nanocrystalline dense Ce0.8Gd0.2O2 − δ thin films: Deposition mechanism and microstructural characterization

Nanocrystalline dense sub-micron thin films of Ce_0.8Gd_0.2O_2 − δ have been successfully deposited by flame spray deposition. The deposition mechanism has been identified as droplet deposition. The deposition temperature of the substrate was as low as 200 °C and the deposition rate ∼ 30 nm/min. Under these conditions the droplets have a very limited residence time in the hot zone of the flame and are deposited as liquid, forming smooth films free of particles. They exhibited a dense and crack-free microstructure. Grain growth, lattice constant, crystallographic density and surface roughness have been investigated as a function of annealing temperature. The good quality of the films, in association with the high deposition rates at low deposition temperatures shows the preeminence of the flame spray method for depositing thin films for micro-solid oxide fuel cells.     

Very low pressure plasma sprayed alumina and yttria-stabilized zirconia thin dense coatings using a modified transferred arc plasma torch

As a novel thermal spray process, very low pressure plasma spray (VLPPS) process has been significantly used to deposit thin, dense and homogenous ceramic coating materials for special application needs in recent years. In this study, in order to enhance low-energy plasma jet under very low pressure ambience, a home-made transferred arc nozzle was made and mounted on a low-power F100 plasma torch to fully melt or evaporate powder feedstock. As a result, thin and dense alumina (Al₂O₃) and yttria-stabilized zirconia (YSZ) ceramic coatings with an average thickness of 30-40 μm were successfully elaborated by the VLPPS process below 1 mbar. An optical emission spectroscopy (OES) was used to analyze the plasma jet properties. The microstructures of the coatings were observed by means of a scanning electron microscopy (SEM). It was found that the YSZ coatings displayed a bimodal microstructure which was composed of splats formed by melted particles and a little amount of vapor condensation from evaporated particles. However, vapor condensation could not be observed in the Al₂O₃ coatings, and only lamellar splats were found. The mechanical properties of both coatings were also evaluated.     

Correlation between Al2O3 particles and interface of Al-Al2O3 coatings by cold spray

Al-Al₂O₃ composite coatings with different Al₂O₃ particle shapes were prepared on Si and Al substrate by cold spray. The powder compositions of metal (Al) and ceramic (Al₂O₃) having different sizes and agglomerations were varied into ratios of 10:1 wt% and 1:1 wt%. Al₂O₃ particles were successfully incorporated into the soft metal matrix of Al. It was found that crater formation between the coatings and substrate, which is typical characteristic signature of cold spray could be affected by initial starting Al₂O₃ particles. In addition, when the large hard particles of fused Al₂O₃ were employed, the deep and big craters were generated at the interface between coatings and hard substrates. In the case of pure soft metal coating such as Al on hard substrate, it is very hard to get proper adhesion due to lack of crater formation. Therefore, the composite coating would have certain advantages.     

Deposition characteristics of Al-12Si alloy coating fabricated by cold spraying with relatively large powder particles

In this paper, the microstructure, microhardness and adhesive strength of Al-12Si coating produced by cold spraying were investigated. It is found that a thick, dense and well bonded Al-12Si coating could be produced by cold spraying with a relatively large powder through the control of spray conditions. The critical velocity for large Al-12Si particles was lower than that of small Al-12Si particles. The as-deposited Al-12Si coating had the same crystal structure as Al-12Si powder. The localized interface melting occurred resulting from both the adiabatic shearing upon impact and the thermal effect of hot gas. Some fine Si particles precipitated in α-Al matrix because of the thermal effect of hot gas during coating deposition. The dispersed Si particles in Al-12Si coating improved the coating microhardness.     

Effect of Ag doping on the photoluminescence properties of ZnO films

ZnO:Ag films were grown on Si (1 0 0) substrates by ultrasonic spray pyrolysis at various substrate temperatures. The effect of deposition temperature on the structural and the room temperature photoluminescence (RT-PL) properties of ZnO:Ag films was studied. With the deposition temperature rising to 550 °C, the intensity of the near-band edge (NBE) emission at 378 nm decreased and a new emission peak at 399 nm was observed. On the basis of the X-ray diffraction pattern (XRD), the X-ray photoelectron (XPS) spectra of ZnO:Ag films, and the effects of annealing on the PL, we suggest that the 399 nm emission should be attributed to the electron transition from the conduction band to Ag_Zn-related complexes defects radiative centers above the valence band.     

Columnar-grown porous films of lithium manganese oxide spinel (LiMn2O4) prepared by ultrasonic spray deposition

Using LiNO₃ and Mn(Ac)₂ as raw materials, ultrasonic spray deposition (USD) technique was used to fabricate LiMn₂O₄ films on platinum substrate at different substrate temperatures from 310 to 390 °C. The prepared thick films were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Electrochemical performance of the USD-derived films was also evaluated with LiMn₂O₄/Li cells. It is found that all of the LiMn₂O₄ films are porous and composed of orderly oriented columnar particles. The substrate temperature affects the fine microstructure of the columnar particles. The film prepared at 360 °C substrate temperature give rise to best electrochemical behavior.     

Effect of Sn particle size on the intermetallic compound formations of cold sprayed Sn-Ni coatings

Comparative studies on the intermetallic compound (IMC) formations of small (aggregated) and large Sn (irregular) with Ni and Cu in cold gas dynamic sprayed coatings were carried out. The Sn with high purity were selected and prepared as raw materials mixture in order to be sprayed onto Ni and Cu plate-shape substrates. The small particles of Sn (<1 μm) were successfully coated under conventional coating parameters when they are mixed with larger powder materials. And microstructural observation regards to compound formation similarly worked out for both small and large Sn mixture. However, the intermetallic formation behavior was turned out to be different. After post-annealing, the larger Sn particles in the composite coating formed larger amount of IMC with Ni than small Sn although, owing to larger interfacial area, more intensive reactivities were expected. Also, there were significant differences in the size and distribution of eutectic pores as well.     

Influence of metal powder shape on drag coefficient in a spray jet

In plasma spraying, particle shape, size, distribution and density are the important factors to be considered in order to ensure high spray efficiency and better coating properties. In the present work, nickel, iron and aluminium irregular powders in the size range from 50 to 63 μm were spheroidized using thermal plasma processing. The spheroidization experiments have been carried out at different gas flow rates and plasma torch power levels. The sphericity was analyzed using shape factor. Drag coefficients of the powders were estimated using Reynolds number and sphericity of the powders in plasma. For irregular particles, the drag coefficient is higher than that of the spherical because of its large area of contact with plasma. The temperature dependent on drag coefficient is also discussed. Increasing temperature increases the drag coefficient of the powder particles injected in to the plasma jet. Increasing plasma jet temperature changes the density and viscosity of the plasma which affects the particle’s drag coefficient in the plasma. The results are reported and discussed.