冷喷涂固态颗粒沉积中颗粒间结合形成机制研究进展

就目前主流的冷喷涂颗粒结合形成机理进行了系统总结和评述,为冷喷涂沉积体性能的调控和后续研究提供借鉴。分别就经典的颗粒界面绝热剪切失稳结合机理,颗粒界面应力波释放诱导材料射流形成结合机理,以及高速碰撞诱导颗粒表面氧化膜破碎、新鲜金属接触结合机理的基本概念、原理、特点进行了概括总结。通过大量系统文献的调研,指出现有理论目前存在的相悖和不足之处,并简要分析了现有颗粒间结合形成理论对冷喷涂沉积体质量调控方面的指导意义。最后基于现有研究的不足,对冷喷涂颗粒界面结合机制方面的研究进行了展望。     

冷喷涂技术的研究进展

冷喷涂是基于空气动力学原理的一种新型喷涂技术.它是利用低温(一般低于600℃)超音速气体射流加速喷涂粒子,使粒子不熔化,以固态形式与基体发生塑性碰撞而实现涂层沉积.相比于热喷涂技术,冷喷涂可以避免材料在喷涂过程中发生过热、氧化、晶粒长大等现象,适用于非晶、纳米晶涂层的制备.介绍了冷喷涂技术的原理、特点、沉积机制和工艺参数,总结了冷喷涂纳米涂层的研究进展.     

On the Bonding Mechanism in Cold Spray of Deformable hex-BN-Ni Clusters

Bond strength and the lubrication potential of coatings made of 7 µm Hexagonal Boron Nitride particles encapsulated with nickel (hBN-Ni), and deposited onto aluminum 6061 substrates via cold spray were examined; for all tests, N₂ was used as the carrier gas at a temperature of 480 °C and pressure of 2.4 MPa. Results showed significant improvement in both wear resistance and reduced surface friction. Coated samples also demonstrated unexpected high bond strength, which was much greater than pure nickel cold sprayed onto aluminum. However, while the results were truly promising, the primary reason for the observed high bond strength could not be explained using existing cold spray theories which were primarily developed for pure metal particles. Based on the present findings compared to cold-sprayed layers of composite nickel-nickel (nickel particles encapsulated with nickel), a mechanism for bonding of hBN-Ni particles to aluminum based on the level of plastic deformation and hardenability is proposed. Indeed, the high bond strength between the coating and substrate is related to the relatively high initial ductility of the nickel encapsulation, compliance of the hBN, as well as the ensuing significant plastic deformation of the composite particles during cold spray deposition.     

Gas Flow,Particle Acceleration,and Heat Transfer in Cold Spray: A review

Cold spraying is increasingly attracting attentions from both scientific and industrial communities due to its unique ‘low-temperature’ coating build-up process and its potential applications in the additive manufacturing across a variety of industries. The existing studies mainly focused on the following subjects: particle acceleration and heating, coating build-up, coating formation mechanism, coating properties, and coating applications, among which particle acceleration and heating can be regarded as the premise of the other subjects because it directly determines whether particles have sufficient energy to deposit and form the coating. Investigations on particle acceleration and heating behavior in cold spraying have been widely conducted both numerically and experimentally over decades, where many valuable conclusions were drawn. However, existing literature on this topic is vast; a systematical summery and review work is still lack so far. Besides, some curtail issues involved in modeling and experiments are still not quite clear, which needs to be further clarified. Hence, a comprehensive summary and review of the literature are very necessary. In this paper, the gas flow, particle acceleration, and heat transfer behavior in the cold spray process are systematically reviewed. Firstly, a brief introduction is given to introduce the early analytical models for predicting the gas flow and particle velocity in cold spraying. Subsequently, special attention is directed towards the application of computational fluid dynamics technique for cold spray modeling. Finally, the experimental observations and measurements in cold spraying are summarized.     

Development of Particle Interface Bonding in Thermal Spray Coatings: A Review

Thermal spray ceramic coatings deposited following the conventional routine exhibit a typical lamellar structure with a limited interface bonding ratio. The bonding between particles in the coating dominates coating properties and performance. In this review paper, the bonding formation at the interface between thin lamellae in the coating is examined. The effect of spray parameters on the bonding ratio is presented to reveal the main droplet parameters controlling bonding formation, which reveals that the temperature of the spray particle rather than its velocity dominates the bonding formation. The limitation to increase significantly the ceramic particle temperature inherent to the thermal spray process leads to the observation of a maximum bonding ratio of about 32%, while through controlling the surface temperature of the coating prior to molten droplet impact, the bonding at the lamellar interface can be significantly increased. Consequently, it is shown that with the proper selection of deposition conditions and control of the deposition temperature, the bonding ratio of ceramic deposits can be altered from a maximum of 32% for a conventional deposit to a maximum of 100%. Such wide adjustability of the lamellar bonding opens new possibilities for using thermal spray coatings in various applications requiring different microstructures and properties. The examination of recent studies shows that the bonding control makes it possible to fabricate porous deposits through surface-molten particles. Such an approach could be applied for the fabrication of porous materials, the deposition of high temperature abradable ceramic coatings, and for forming functional structured surfaces, such as a surface with super-hydrophobicity or a solid oxide fuel cell cathode interface with high specific surface area and high catalytic performance. Furthermore, complete interface bonding leads to crystalline structure control of individual splats through epitaxial grain growth.     

用冷喷涂法制备PTC陶瓷的Al电极

研究了冷气动力喷涂方法(CGDS)制备PTC陶瓷欧姆接触Al电极的喷涂工艺,用扫描电子显微镜分析了电极的显微结构.结果表明,该方法制备的PTC陶瓷欧姆接触Al电极性能较优.     

Theoretical and Experimental Particle Velocity in Cold Spray

In an effort to corroborate theoretical and experimental techniques used for cold spray particle velocity analysis, two theoretical and one experimental methods were used to analyze the operation of a nozzle accelerating aluminum particles in nitrogen gas. Two-dimensional (2D) axi-symmetric computations of the flow through the nozzle were performed using the Reynolds averaged Navier-Stokes code in a computational fluid dynamics platform. 1D, isentropic, gas-dynamic equations were solved for the same nozzle geometry and initial conditions. Finally, the velocities of particles exiting a nozzle of the same geometry and operated at the same initial conditions were measured by a dual-slit velocimeter. Exit plume particle velocities as determined by the three methods compared reasonably well, and differences could be attributed to frictional and particle distribution effects.     

Numerical and Experimental Investigation of Cold Spray Gas Dynamic Effects for Polymer Coating

Low melting temperature materials such as polymers are known to be difficult to deposit using traditional cold spray techniques. Computational fluid dynamics (CFD) models were created for various nozzle geometries and flow conditions. A schlieren optical system was used to visualize the density gradients and flow characteristics in the free jet impingement region. Based on the CFD models, it was determined that a diffuser placed into the carrier gas flow near the nozzle exit not only leads to lower particle impact velocity required for polymer deposition, but also provides for appropriate application of compression heating of the particles to produce the conditions necessary at impact for successful coating adhesion of these materials. Experiments subsequently confirmed the successful deposition of polyethylene powder onto a 7075-T6 aluminum substrate. Using air as the carrier gas, polyethylene particles of 53-75???m diameter and 0.94?g/cm³ density_, were cold spray deposited onto the aluminum substrate, with a critical impact velocity of 191?m/s. No apparent melting of the polymer particles was observed. Refinements to these concepts are currently under investigation and a patent disclosure for the idea is pending.     

Development of Holistic Three-Dimensional Models for Cold Spray Supersonic Jet

A three-dimensional, computational fluid dynamics (CFD) model is developed to estimate cold spray gas conditions. This model is calibrated and validated with respect to thermal history of a substrate exposed to the cold spray supersonic jet. The proposed holistic model is important to track state of gas and particles from injection point to the substrate surface with significant benefits for optimization of very rapid "nanoseconds" cold spray deposition. The three-dimensional model is developed with careful attention with respect to computation time to benefit broader cold spray industry with limited access to supercomputers. The k-ε-type CFD model is evaluated using measured temperature for a titanium substrate exposed to cold spray nitrogen at 800 °C and 3 MPa. The model important parameters are detailed including domain meshing method with turbulence, and dissipation coefficients during spraying. Heat transfer and radiation are considered for the de Laval nozzle used in experiments. The calibrated holistic model successfully estimated state of the gas for chosen high temperature and high pressure cold spray parameters used in this study. Further to this, the holistic model predictions with respect to the substrate maximum temperature had a good agreement with earlier findings in the literature.     

Novel Method of Aluminum to Copper Bonding by Cold Spray

Cold spray bonding (CSB) has been proposed as a new method for joining aluminum and copper. At high speeds, solid Al particles impacted the groove between the two substrates to form a bond between Al and Cu. Compared to traditional welding technologies, CSB does not form distinct intermetallic compounds. Large stainless steel particles were introduced into the spray powders as in situ shot peen particles to create a dense Al deposit and to improve the bond strength of joints. It was discovered that introducing shot peen particles significantly improved the flattening ratio of the deposited Al particles. Increasing the proportion of shot peen particles from 0 to 70 vol.% decreased the porosity of the deposits from 12.4 to 0.2%, while the shear strength of joints significantly increased. The tensile test results of the Al-Cu joints demonstrated that cracks were initiated at the interface between the Al and the deposit. The average tensile strength was 71.4 MPa and could reach 81% of the tensile strength of pure Al.     

A Three-Dimensional Analysis of the Cold Spray Process: The Effects of Substrate Location and Shape

A three-dimensional model of a Cold Gas Dynamic Spray system with a peripheral nonaxisymmetric powder feeder is studied in this work. It is found that the stagnation pressure alternates for different substrate standoff distances due to the nature of the supersonic flow interaction with the substrate. One can find the optimum substrate location for any given operating condition, which results in minimum pressure buildup on the substrate. The three-dimensional analysis sheds more light on the complex gas and particle flow fields generated due to the three-dimensional particle injection process. In addition, the three-dimensional model allows us to further investigate the effect of practical substrate shapes (such as convex and concave) on the flow field and consequently to determine the optimum conditions to deposit coating particles. This article is an invited paper selected from presentations at the 2007 International Thermal Spray Conference and has been expanded from the original presentation. It is simultaneously published in Global Coating Solutions, Proceedings of the 2007 International Thermal Spray Conference, Beijing, China, May 14-16, 2007, Basil R. Marple, Margaret M. Hyland, Yuk-Chiu Lau, Chang-Jiu Li, Rogerio S. Lima, and Ghislain Montavon, Ed., ASM International, Materials Park, OH, 2007.     

A Morphological Approach to the Modeling of the Cold Spray Process

A coating buildup model was developed, the aim of which was simulating the microstructure of a tantalum coating cold sprayed onto a copper substrate. To do so, first was operated a fine characterization of the irregular tantalum powder in 3D, using x-ray microtomography and developing specific image analysis algorithms. Particles were grouped by shape in seven classes. Afterward, 3D finite element simulations of the impact of the previously observed particles were realized. To finish, a coating buildup model was developed, based on the results of finite element simulations of particle impact. In its first version, this model is limited to 2D.     

Particle In-Flight Velocity and Dispersion Measurements at Increasing Particle Feed Rates in Cold Spray

Cold spray (CS) is attracting interest of research and industry due to its rapid, solid-state particle deposition process and respective advantages over conventional deposition technologies. The acceleration of the particles is critical to the efficiency of CS, and previous investigations rarely consider the particle feed rate. However, because higher particle loadings are typically used in the process, the effect of this cannot be assumed negligible. This study therefore investigates the particle velocities in the supersonic jet of an advanced CS system at low- and high pressure levels and varying particle feed rates using particle image velocimetry. The particle dispersion and velocity evolution along the jet axis were investigated for several feedstock materials. It was found that the average particle velocity noticeably decreases with increasing particulate loading in all cases. The velocity distribution and particle dispersion were also observed to be influenced by the feed rate. Effects are driven by both mass loading and volume fraction, depending on the feedstock’s particle velocity parameter. Increased particle feed rates hence affect the magnitude and distribution of impact velocity and consequently the efficiency of CS. In particular, numerical models neglecting this interconnection are required to be further improved, based on these experimental studies.     

Optical Diagnostics Study of Gas Particle Transport Phenomena in Cold Gas Dynamic Spraying and Comparison with Model Predictions

Cold gas dynamic spraying (CGDS), a relatively new thermal spraying technique has drawn a lot of attention due to its inherent capability to deposit a wide range of materials at relatively low-operating temperatures. A De Laval nozzle, used to accelerate the powder particles, is the key component of the coating equipment. Knowledge concerning the nozzle design and effect of process parameters is essential to understand the coating process and to enable selection of appropriate parameters for enhanced coating properties. The present work employs a one-dimensional isentropic gas flow model in conjunction with a particle acceleration model to calculate particle velocities. A laser illumination-based optical diagnostic system is used for validation studies to determine the particle velocity at the nozzle exit for a wide range of process and feedstock parameters such as stagnation temperature, stagnation pressure, powder feed rate, particle size and density. The relative influence of process and feedstock parameters on particle velocity is presented in this work.     

Numerical Studies of the Application of Shock Tube Technology for Cold Gas Dynamic Spray Process

A new method for a combustion-free spraying is studied fundamentally by modeling and simulation in comparison with first experiments. The article focuses on the numerical simulation of the gas-particle nozzle flow, which is generated by the shock reflection at the end wall section of a shock tube. To study the physical fundamentals of this process, at present only a single shot operation is considered. The particles are injected downstream of the nozzle throat into a supersonic nozzle flow. The measurements of the particle velocity made by a laser Doppler anemometry (LDA) set up show that the maximum velocity amounts to 1220 m/s for stainless steel particles of 15 μm diameter. The CFD-Code (Fluent) is first verified by a comparison with available numerical and experimental data for gas and gas-particle flow fields in a long Laval-nozzle. The good agreement implied the great potential of the new dynamic process concept for cold-gas coating applications. Then the flow fields in the short Laval nozzle designed and realized by the Shock Wave Laboratory (SWL) are investigated. The gas flow for experimentally obtained stagnation conditions is simulated. The gas-particle flow without and with the influence of the particles on the gas flow is calculated by the Surface Engineering Institute (IOT) and compared with experiments. The influence of the injection parameters on the particle velocities is investigated, as well. This article is an invited paper selected from presentations at the 2007 International Thermal Spray Conference and has been expanded from the original presentation. It is simultaneously published in Global Coating Solutions, Proceedings of the 2007 International Thermal Spray Conference, Beijing, China, May 14-16, 2007, Basil R. Marple, Margaret M. Hyland, Yuk-Chiu Lau, Chang-Jiu Li, Rogerio S. Lima, and Ghislain Montavon, Ed., ASM International, Materials Park, OH, 2007.     

Numerical Study of In-flight Particle Parameters in Low-Pressure Cold Spray Process

A 2-D model of the low-pressure cold spray with a radial powder feeding was established using CFD software in this study. The flow field was simulated for both propellant gases of nitrogen and helium. To predict the in-flight particle velocity and temperature, discrete phase model was introduced to simulate the interaction of particle and the supersonic gas jet. The experimental velocity of copper powder with different sizes was used to validate the calculated one for low-pressure cold spray process. The results show that the computational model can provide a satisfactory prediction of the supersonic gas flow, which is consistent with the experimental Schlieren photos. It was found that similar velocity was obtained with the drag coefficient formula of Henderson and with that of Morsi and Alexander. As the shape factor was estimated, the reasonable prediction of velocity for non-spherical particle can be obtained, to compare with the experimental results.     

冷喷涂单颗粒铜在铝基体上的显微结构研究

采用冷喷涂法在铝(Al)基体上沉积单颗粒铜(Cu),利用聚焦离子束/电子束(FIB/SEM)系统精确定位并原位制备了完整单个颗粒Cu沉积在Al基体上的透射样品,分析其显微结构及形成原因。实验结果表明,撞击过程中温度与应力分布不均匀,导致沉积Cu颗粒不均匀形变。Cu/Al界面受影响较大:颗粒动能转化为形变能和热能,打破了界面处氧化膜,使界面附近温度迅速升高,发生动态再结晶,生成金属间化合物Cu_9Al_4;Cu颗粒内距界面越远的区域,受温度和应力的影响越小,其变形主要是通过晶体内位错增殖和移动;沉积颗粒顶部,远离Cu/Al界面,几乎不受应力和温度影响,保持原始显微结构。     

Critical Deposition Condition of CoNiCrAlY Cold Spray Based on Particle Deformation Behavior

Previous research has demonstrated deposition of MCrAlY coating via the cold spray process; however, the deposition mechanism of cold spraying has not been clearly explained—only empirically described by impact velocity. The purpose of this study was to elucidate the critical deposit condition. Microscale experimental measurements of individual particle deposit dimensions were incorporated with numerical simulation to investigate particle deformation behavior. Dimensional parameters were determined from scanning electron microscopy analysis of focused ion beam-fabricated cross sections of deposited particles to describe the deposition threshold. From Johnson-Cook finite element method simulation results, there is a direct correlation between the dimensional parameters and the impact velocity. Therefore, the critical velocity can describe the deposition threshold. Moreover, the maximum equivalent plastic strain is also strongly dependent on the impact velocity. Thus, the threshold condition required for particle deposition can instead be represented by the equivalent plastic strain of the particle and substrate. For particle-substrate combinations of similar materials, the substrate is more difficult to deform. Thus, this study establishes that the dominant factor of particle deposition in the cold spray process is the maximum equivalent plastic strain of the substrate, which occurs during impact and deformation.