Numerical investigations of the effect of oblique impact on particle deformation in cold spraying by the SPH method

In this study, a systematic examination of the oblique impacting of copper particles in cold spraying was conducted by using the smoothed particle hydrodynamics (SPH) method compared to the Lagrangian method. 3D models were employed owing to the asymmetric characteristic of the oblique impacting. It is found that in the oblique impact, the additional tangential component of particle velocity along the substrate surface could create a tensile force and decrease the total contact area and bonding strength between the particle and the substrate. The simulation results compare fairly well to the experiment results. Meanwhile, the asymmetric deformation can result in the focus of the shear friction on a small contact zone at one side, which may rise the interfacial temperature and thus facilitate the occurrence of the possible shear instability. Therefore, there probably exists an angle range, where the deposition efficiency may be promoted rather than the normal angle. Moreover, the particle deformation behavior simulated by the SPH method is well comparable to that simulated by the Lagrangian method and the experimental results, which indicates the applicability of the SPH method for simulating the impact process in cold spraying besides the previously used Arbitrary Lagrangian Eulerian (ALE) method.     

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.     

Numerical simulation of deformation behavior of Al particles impacting on Al substrate and effect of surface oxide films on interfacial bonding in cold spraying

In this study, a comprehensive examination of the deformation behavior of Al particles impacting on Al substrate was conducted by using the Arbitrary Lagrangian Eulerian (ALE) method to clarify the deposition characteristics of Al powder and the effect of surface oxide films in cold spraying. It was found that the deformation behavior of Al particles is different from that of Cu particles under the same impact conditions owing to its lower density and thus less kinetic energy upon impact. The results indicated that a higher velocity was required for Al particles to reach the same compression ratio as that of Cu particles. On the other hand, the numerical results showed that the oxide films at particle surfaces influenced the deformation and bonding condition between the particle and substrate. The inclusions of the crushed oxide films at the interfaces between the depostied particles inhibit the deformation.     

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).     

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.     

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.     

An energy approach to the modelling of particle removal by pulsed laser irradiation

An energy model to explain particle removal mechanism has been developed. This model is based on a detailed investigation of contact deformation of a particle on a solid surface, as well as particle motion during the process of substrate surface expansion under uniform laser irradiation. Calculation results show that small particles mainly gain kinetic energy during pulsed laser irradiation, whereas large particles mainly gain elastic deforming potential energy. The particle removal condition is derived from the viewpoint of energy. The relationship of particle removal efficiency with laser fluence and particle size is discussed. Theoretical results are compared with experimental results. Received: 30 July 1998 / Accepted: 14 December 1998 / Published online: 17 March 1999     

Nanotribological behavior of ZnO films prepared by atomic layer deposition

We used atomic layer deposition to form ZnO thin-film coatings on Si substrates and then evaluate the effect of pile-up using the nanoscratch technique under a ramped mode. The wear volume decreased with increasing annealing temperature from room temperature to 400 °C for a given load. Elastic-to-plastic deformation occurred during sliding scratch processing between the groove and film for loading penetration of 30 nm. The onset of non-elastic behavior and greater contact pressure were evident for loading penetration of 150 nm; thus, full plastic deformation occurred as a result of a substrate effect. We suspect that elastic–plastic failure events were related to edge bulging between the groove and film, with elastic–plastic deformation attributable to adhesion discontinuities and/or cohesion failure of the ZnO films.     

Loss of adhesion strength of PVD Cu films on carbon substrates after heat treatment and correlated effects on the thermal interface properties

The study of coating-substrate systems consisting of a thin copper film on a flat carbon substrate is of great interest in order to get information on the interfacial behaviour of such systems. This work is focused on the mechanical adhesion strength and the correlated interfacial thermal contact resistance which are influenced by heat treatment. Using plasma-assisted pre-treatment of the carbon substrate prior to the deposition of copper coatings via physical vapor deposition (PVD), the adhesion strength between copper coatings and substrate has increased significantly, while the thermal contact resistance decreased in the as deposited state. After heat treatment at 800 °C for 1 h, considerably decreased adhesion strengths have been observed, accompanied by increased values of the thermal contact resistance.     

Comparison of experiments and computations for cold gas spraying through a mask. Part 2

This paper presents experimental and simulation results of cold spray coating deposition using the mask placed above the plane substrate at different distances. Velocities of aluminum (mean size ~ 30 μm) and copper (mean size ~ 60 μm) particles in the vicinity of the mask are determined. It was found that particle velocities have angular distribution in flow with a representative standard deviation of 1.5–2 degrees. Modeling of coating formation behind the mask with account for this distribution was developed. The results of model agree with experimental data confirming the importance of particle angular distribution for coating deposition process in the masked area.     

Silver nanocombs and branched nanowires formation in aqueous binary surfactants solution

The field emission characteristics of patterned carbon nanotubes (CNTs) the average diameter of which is 16?nm cathodes on substrates with different surface treatments were investigated. The surface treatments of the substrate were performed by nickel electroless plating and palladium coating, which is an activation procedure of electroless plating. CNTs were patterned on the surface-treated substrate with radius of 200???m through conventional photolithography process. Two deposition methods, electrophoresis deposition and spray deposition, were used to investigate the effects of deposition methods on field emission characteristics of the cathodes. It was revealed that the two deposition methods showed similar turn-on field trends, which means that the different surface morphologies of the substrates have more influence on the field emission characteristics than the different deposition methods performed in this study. Through the surface treatments, the roughness of the surface increased and cathodes with a high roughness factor showed better field emission characteristics compared to non-treated ones.     

The effects of powder properties on in-flight particle velocity and deposition process during low pressure cold spray process

In cold spray process, impacting velocity and critical velocity of particles dominate the deposition process and coating properties for given materials. The impacting velocity and critical velocity of particles depend on the powder properties and cold spray conditions. In the present study, the in-flight particle velocity of copper powder in low pressure cold spraying was measured using an imaging technique. The effects of particle size and particle morphology on in-flight particle velocity and deposition efficiency were investigated. The critical velocity of copper powder was estimated by combining the in-flight particle velocity and deposition efficiency. The effect of annealing of feedstock powder on deposition and critical velocity was also investigated. The results showed that the irregular shape particle presents higher in-flight velocity than the spherical shape particle under the same condition. For irregular shape particles, the in-flight velocity decreased from 390 to 282 m/s as the particle size increases from 20 to 60 μm. Critical velocities of about 425 m/s and more than 550 m/s were estimated for the feedstock copper powder with spherical and irregular shape morphology, respectively. For the irregular shape particles, the critical velocity decreased from more than 550 to 460 m/s after preheating at 390 °C for 1 h. It was also found that the larger size powder presents a lower critical velocity in this study.     

Electroless deposition of copper and fabrication of copper micropatterns on CVD diamond film surfaces

Electroless deposition of copper on as-grown and amino-modification diamond substrates was investigated. The compact and uniform copper films were successfully electrolessly deposited on as-grown and amino-modification diamond substrates after activation by Pd/Sn colloid nanoparticles. The adhesion interaction between copper films and diamond substrates was roughly estimated by the ultrasonic treatment. The results showed the higher adhesion interaction between copper films and amino-modification diamond substrates than that between the copper films and as-grown diamond substrates due to the greater attractive force between the Pd/Sn colloid nanoparticles and amino-modified diamond surface. The favorable copper micropatterns were successfully constructed on diamond film surfaces by means of the catalyst lift-off method and the copper lift-off method. Furthermore, the electrochemical behavior of copper-modified boron-doped diamond (BDD) was studied for glucose oxidation in 0.2 M sodium hydroxide solution by using cyclic voltammetry, and the result indicated that copper-modified BDD exhibited high catalytic activity to electrochemical oxidation of glucose in alkaline media.     

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.     

Wetting and superhydrophobic properties of PECVD grown hydrocarbon and fluorinated-hydrocarbon coatings

Wetting characteristics of micro-nanorough substrates of aluminum and smooth silicon substrates have been studied and compared by depositing hydrocarbon and fluorinated-hydrocarbon coatings via plasma enhanced chemical vapor deposition (PECVD) technique using a mixture of Ar, CH₄ and C₂F₆ gases. The water contact angles on the hydrocarbon and fluorinated-hydrocarbon coatings deposited on silicon substrates were found to be 72° and 105°, respectively. However, the micro-nanorough aluminum substrates demonstrated superhydrophobic properties upon coatings with fluorinated-hydrocarbon providing a water contact angle of ∼165° and contact angle hysteresis below 2° with water drops rolling off from those surfaces while the same substrates showed contact angle of 135° with water drops sticking on those surfaces. The superhydrophobic properties is due to the high fluorine content in the fluorinated-hydrocarbon coatings of ∼36 at.%, as investigated by X-ray photoelectron spectroscopy (XPS), by lowering the surface energy of the micro-nanorough aluminum substrates.     

Electrospray of fine droplets of ceramic suspensions for thin-film preparation

An electrostatic atomization technique has been developed to generate ultra-fine spray of ZrO₂ and SiC ceramic suspensions in a range of 4–5 μm with a narrow size distribution (1–9 μm). The aim of this work is to generate fine spray of ceramic suspensions for the preparation of uniform thin films of these ceramic materials on substrates. Thin-film formation using electrostatic atomization process allows one to tightly control the process while meeting the economics in comparison with some other competing process technologies such as chemical vapour deposition, physical vapour deposition and plasma spray, etc. Preliminary results have shown that for low through put atomization, the cone-jet is the most suitable method to produce a fine charged aerosol with a narrow size distribution. It was found that the droplet size of the spray is in the range of a few micrometers with a narrow size distribution and that droplet size and spray current obey theoretical prediction of scaling law. As prepared ZrO₂ and SiC thin films were observed to be homogenous with a particle size of less than 10 μm.     

Hydrothermal growth of ZnO microstructures on Ar plasma treated graphite

We have investigated the effect of argon (Ar) plasma treatment on the surface of graphite and the hydrothermal growth of zinc oxide (ZnO) microstructures. With the plasma treatment, the growth behavior of ZnO microrods on the graphite substrates changed drastically. After the graphite surface was exposed to the Ar plasma, the number density of ZnO was one order of magnitude higher than that on the pristine graphite without plasma treatment. Raman spectroscopy revealed that Ar plasma treatment created the structural defects on the graphite surfaces and decreased the mean distance of defects. Surface characterization through atomic force microscopy and X-ray photoelectron spectroscopy showed that the graphite surface was roughened and that oxygen–carbon bonding was formed. The enhanced nucleation of ZnO can be explained by the generation of structural defects, surface roughness, and surface functional groups on the graphite substrate. Therefore, Ar plasma treatment can be used as a simple method to engineer the surface properties of graphite substrates and to control the crystal nucleation and growth of inorganic materials on their surface.     

Micropatterning of fluoropolymers

Fluoropolymer (PTFE and FEP) substrates have been patterned through micro-contact printing of an aminosilane. The silane pattern was activated with a palladium catalyst that allowed the electroless deposition of copper which was used to form micropatterned copper electrodes. Conducting polymer micropatterns were then fabricated by electrodeposition of polypyrrole (PPy) onto the copper. The resulting patterns of 80 μm and 10 μm grids and 2 μm and 5 μm checkerboards were characterized using imaging XPS, TOF-SIMS, AFM and SEM. The size and resolution of the smallest copper patterns were limited by the copper grain size created during electroless deposition. The polypyrrole patterns were also limited by the roughness of the electrolytically deposited polymer film.     

Enhanced diamond nucleation on copper substrates by graphite seeding and CO2 laser irradiation

Diamond nucleation on copper (Cu) substrates was investigated by graphite seeding and CO₂ laser irradiation at initial stages of the combustion-flame deposition. A graphite aerosol spray was used to generate a thin layer of graphite powders (less than 1 μm) on Cu substrates. The graphite-seeded Cu substrates were then heated by a continuous CO₂ laser to about 750 °C within 1 min. It was found that diamond nucleation density after this treatment was more than three times as much as that on the virgin Cu substrates. As a consequence, diamond films up to 4 μm were obtained in 5 min. The enhancement of diamond nucleation on the graphite-seeded Cu substrates was attributed to the formation of defects and edges during the etching of the seeding graphite layers by the OH radicals in the flame. The defects and edges served as nucleation sites for diamond formation. The function of the CO₂ laser was to rapidly heat the deposition areas to create a favorable temperature for diamond nucleation and growth.     

Mechanically stable and corrosion resistant superhydrophobic sol-gel coatings on copper substrate

Development of the anticorrosion coatings on metals having both passive matrix functionality and active response to changes in the aggressive environment has raised tremendous interest in material science. Using a sol-gel deposition method, superhydrophobic copper substrate could be obtained. The best hydrophobic coating sol was prepared with methyltriethoxysilane (MTES), methanol (MeOH), and water (as 7 M NH₄OH) at a molar ratio of 1:19.1:4.31 respectively. The surface morphological study showed the ball like silica particles distributed on the copper substrate with particle sizes ranging from 8 to 12 μm. The coatings showed the static water contact angle as high as 155° and the water sliding angle as low as 7°. The superhydrophobic nature was maintained even though the deposited copper substrate was soaked for 100 h in 50% of HCl solution. The coatings are stable against humidity and showed superhydrophobic behavior even after 90 days of exposure. The coatings are mechanically stable and water drops maintained the spherical shape on the bent copper substrate, which was bent more than 90°.