Effect of Substrate Temperature on Deposition Behavior of Copper Particles on Substrate Surfaces in the Cold Spray Process

The deposition behavior of sprayed individual metallic particles on the substrate surface in the cold spray process was fundamentally investigated. As a preliminary experiment, pure copper (Cu) particles were sprayed on mirror-polished stainless steel and aluminum (Al) alloy substrate surfaces. Process parameters that changed systematically were particle diameter, working gas, gas pressure, gas temperature, and substrate temperature, and the effect of these parameters on the flattening or adhesive behavior of an individual particle was precisely investigated. Deposition ratio on the substrate surface was also evaluated using these parameters. From the results obtained, it was quite noticeable that the higher substrate temperature brought about a higher deposition rate of Cu particles, even under the condition where particles were kept at room temperature. This tendency was promoted more effectively using helium instead of air or nitrogen as a working gas. Both higher velocity and temperature of the particles sprayed are the necessary conditions for the higher deposition ratio in the cold spraying. However, instead of particle heating, substrate heating may bring about the equivalent effect for particle deposition. 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.     

Effect of Ambient Pressure on Flattening Behavior of Thermal Sprayed Particles

Cu splats were thermally sprayed onto the mirror polished SUS304 substrate surface at various ambient pressures ranging from 6.66 to 101.33 kPa. The effect of ambient pressure on the flattening behavior of the particle was systematically investigated. It was observed that only around 10% or less of disk-shaped splats deposited at atmospheric pressure. The splat shape on the flat substrate had a transitional changing tendency from a splash splat to a disk one with a decrease of the ambient pressure. The tendency of splash splat change with increasing the ambient pressure agreed with the BET curve, which indicates that adsorption/desorption of the adsorbed gas/condensation plays an important role on the flattening behavior of thermal sprayed particles. Moreover, a part of substrates were polished to a mirror finish and heated to 573 K for 10 min, then elapsed to air atmosphere for different duration of up to 1 h. The fundamental static wetting behavior of the once heated substrate surface by a water droplet was investigated. The contact angle measurement results agreed well with the splat morphologies. No chemical change and surface topography change took place with the elapse time increasing. Hence, the occurrence of desorption caused by reducing the ambient pressure or by substrate preheating provided good wetting. Wetting of substrate surface by molten particles may dominate the flattening behavior of thermal sprayed particles.     

Deposition Behavior of Copper Fine Particles onto Flat Substrate Surface in Cold Spraying

Cold spray is a promising process to fabricate high-quality metallic coatings. However, it is necessary to improve some properties, especially the adhesive strength of the coating to the substrate to clarify deposition mechanism of the solid particles onto substrate surface. In this study, deposition behavior of the cold sprayed copper fine particles was observed precisely and the adhesive strength of the coating was evaluated. The deposition behavior of the sprayed individual copper particles on mirror polished stainless steel substrate was fundamentally investigated. The interface microstructure between sprayed particle and substrate revealed that an amorphous-like band region was recognized at interface during coating fabrication at high power conditions. For the deposition mechanism of the cold sprayed particles onto substrate surface, it was indicated that the deformation of the particles initially induce the destruction of its surface oxide and an appearance of the active fresh surface of the material may enhance the bonding between particles and substrate. On the other hand, in coating fabrication at high power condition, bonding between particle and substrate may be possibly formed via oxygen-rich amorphous-like layer at interface.     

Impacting Behavior of Large Oxidized Copper Particles in Cold Spraying

In a previous study, it has been experimentally demonstrated that surface oxide films of metallic particles have significant influence on the properties of cold-sprayed coatings. To clearly reveal the underlying mechanism, this study focused further on the effect of particle oxidation on the deposition behavior of oxidized Cu powder. Results show that the presence of the oxide films on the particles’ surface can inhibit the plastic deformation of the particles. In addition, results concerning the morphologies and oxygen content of the rebounded particles show that the particles have experienced large plastic deformation that results in the break-up of the oxide films during the impacting process. Correspondingly, the hardness of the coating deposited with the oxidized powder is a little lower than that with the annealed powder because of the inferior plastic deformation and strain-hardening effect.     

石墨颗粒表面化学镀铜研究

为了充分利用Cu的导电性能，碳石墨的润滑性能，改善Cu/C的润湿性，用化学镀铜的方法成功地对石墨颗粒表面进行镀覆，详细地研究了镀铜液组分及工艺与石墨颗粒表面镀铜层厚度、沉积速率的关系，并得出较好的组分工艺方案，采用优化工艺可以得到平均厚度约7．1μm的镀铜层。同时应用X射线、金相显微镜、扫描电镜及电子探针对镀铜层的厚度、表面形貌、镀铜层与基体的界面进行了全面观察。分析表明，Cu/C界面存在过渡层，界面成锯齿状，机械冶金结合的特征十分明显，改善了铜碳界面的相溶性。     

Effect of Substrate Preheating on Bonding Strength of Cold-Sprayed Mg Coatings

A poor bonding strength between the magnesium coating and substrate limits magnesium coating’s application. This paper aims at improving the bonding strength between the coating and substrate using substrate preheating. Aluminum substrates were heated to 100, 200 and 300 °C by a flame nozzle prior to cold spraying. The effects of substrate preheating on the microstructure and bonding strength were studied. The deformation behavior of single particles was simulated by a commercial software ABAQUS, and observed using SEM. The results show that substrate preheating can increase the bonding strength significantly. The bonding strength increased from 3.3 ± 0.8 to 11.6 ± 0.5 MPa when the substrate temperature increased from room-temperature to 200 °C. The fracture analyses show that the coating fracture occurred within the coating when the substrate was preheated at 200 °C.     

Effects of Substrate Hardness and Spray Angle on the Deposition Behavior of Cold-Sprayed Ti Particles

In this study, finite element analysis combined with experimental observation was conducted to clarify the effects of substrate hardness and spray angle on the deposition behavior of cold-sprayed Ti particles. It is found that metallurgical bonding is highly possible to occur between the Ti particle and Cu substrate due to the intensive metal jet at the rim of the interface which helps to remove the cracked oxides. Because metallurgical bonding and large interfacial contact area can guarantee high adhesion strength, the thick Ti coating is achieved after deposition on the Cu substrate. As for the soft Al substrate, the first layer Ti particles are embedded in and then trapped by the soft substrate material, which results in the occurrence of mechanical interlock at the interface. As a consequence, the final coating thickness is also relatively large. When using hard stainless steel as the substrate, the essential conditions for forming the mechanical interlock are lacked due to the high hardness of the substrate material. In addition, the metal jet at rim of the interface is less prominent and also the interfacial contact area is smaller in comparison with the Ti-Cu case. Therefore, the particle-substrate bonding strength and the consequent coating thickness are relatively low. Besides, it is also found that the particle deformation and coating quality are significantly affected by the spray angle. The deformation of the particle localizes at only one side due to the additional tangential momentum. Also, such localized deformation becomes increasingly intensive with decreasing the spray angle. Moreover, the coating thickness is found to reduce with the decrease in spay angle, but the coating porosity shows a reverse trend.     

Combination Effect of Dry-Ice Blasting and Substrate Preheating on Plasma-Sprayed CoNiCrAlY Splats

CoNiCrAlY splats were plasma-sprayed on the stainless steel substrate which was pretreated by dry-ice blasting. Only impact marks were distinguished on the glycerol-polluted substrate, while halo donut splats formed on the pretreated substrate because of the cleaning effect of dry-ice blasting on this organic substance. The proportions of different splat types vary as a function of the treatment time of dry-ice blasting. The condensation phenomenon was also detected on the substrate surface accompanying the cleaning effect after the pretreatment of dry-ice blasting. In this study, dry-ice blasting was investigated to be coupled with substrate preheating to control the substrate temperature. It was found that a regular disk-like CoNiCrAlY splat can be obtained as the substrate temperature is higher than dew point temperature.     

热喷涂过程中压强对粒子扁平化的影响

为了了解热喷涂过程中压强对粒子扁平固化过程的作用,利用减压等离子喷涂装置.在不锈钢SUS304基材表面喷涂了Ni、Cu以及Al粉末,用光学显微镜和扫猫电镜对沉积物的正面、底面以及断面形态进行了观察.结果显示,压强影响等离子焰的长度,从而影响到粒子的熔化状态,同时还影响到基材表面吸附物质的吸附量:熔点低的粒子在与基材碰撞后能较长时间维持在液态,易于在基材表面铺展,因而低压条件下更易于形成盘状沉积.     

铜合金表面Fe基铸渗层的热疲劳性能研究

用负压铸渗法制备了铜合金表面铁基渗层并研究了其热疲劳性能.结果表明:热循环次数小于20次时,渗层表面出现一定程度的氧化.无微裂纹产生:当热循环次数超过60次时,渗层与基体的结合界面处以及渗层表面会产生微裂纹;渗层表面及渗层与基体的结合界面处第一次出现微裂纹的时间随渗层厚度增加而不同程度的提前.     

基体预热温度对热障涂层热残余应力影响的研究

研究了用等离子喷涂工艺制备ZrO2-NiCrAl-LD-8基体的热障涂层过程中,基体预热温度对热障涂层热残余应力的影响.通过有限元计算分析,表明选择合适的基体温度,能大大降低热障涂层的热残余应力,为喷涂制备过程中基体预热温度的选择提供了参考.     

Investigation of Impact Behavior of Cold-Sprayed Large Annealed Copper Particles and Characterization of Coatings

In this research, the large gas-atomized copper powder was selected as the feedstock. Some powder was annealed in a vacuum circumstance to avoid the effect of grain boundaries on the high velocity impact behavior of particles during cold spraying. The annealed Cu powder was deposited by cold spraying with respect to the single impacts and coating deposition under certain gas condition. In addition, the rebounded copper particles were collected for morphology analysis compared to the adhered particles. The results show that the average size of the rebounded particles is apparently increased compared with the starting powder because of the rebounding of larger particles and intensive plastic deformation of the rebounded particles. For the deposited particles, obvious plastic deformation causes a higher hardness to the coating. It is found that the rebounded particles have also experienced large deformation and shear instability at the impact interfaces.     

Effects of Binder Phase Content and Substrate Hardness on Deposition Behavior of Cermet Particles in Cold Spraying Process

In this paper,three kinds of cermet particles with different binder phase and content,namely WC-12Co,WC-17Co and Cr3C2-25NiCr were deposited on the Q235 low carbon steel and WC-12Co coating substrates to investigate the effect of binder phase content and substrate hardness on deposition behavior of cermet particle during cold spraying.The morphology of the deposited particles was examined by scanning electron microscopy(SEM)from the particle surface and cross-sections.It was found that with increasing substrate hardness,the deposition behavior of cermet particles transformed from completely embedding into substrate to partially flattening and even rebounding.Varying the Co content from 12 wt%to 17 wt% influenced significantly the deposition deformation behavior of the WC-Co particles,the higher Co content brought about a fuller flattening of WC-Co particles.However,Cr3C2-25NiCr particle exhibited the relatively lower flattening ratio and significant rebounding of Cr3C2 even with the highest binder phase content,which may be attributed to the bigger Cr3C2 size and weak bonding between Cr3C2 and NiCr.     

负压铸渗法制备铜合金表面Ni60A渗层的摩擦学性能

用MM-200型摩擦实验机,对偶采用GCr15钢环,对比考察了ZQAl9-4和其表面Ni60A渗层在干滑动摩擦状态下的摩擦磨损性能以及摩擦热对对偶钢的影响。研究表明:渗层的宏观硬度呈明显的梯度变化;在相同条件下,Ni60A渗层的磨损率比ZQAl9-4低两个数量级,这主要由于渗层的耐热性能优于ZQAl9-4;表面渗层的磨损受氧化、转移和粘着磨损机制控制;随着PV值的增加,摩擦热使得摩擦表面温度呈线性升高,从而引起了钢的回火处理效应,使钢的硬度降低。     

不锈钢基体上化学镀铜工艺研究

不锈钢基体上化学镀铜易造成镀层鼓泡,这不仅影响了镀层与基体的结合力,而且直接影响到外观质量.为此,将镀前酸处理过的不锈钢片放在烘箱中加热,以除去酸洗时渗入到基体的氢.确定了热处理温度和时间,采用此方法解决了镀层起泡问题,得到所需要的镀层.     

金属熔滴在基片上方和基片下方的润湿性研究

为了研究重力对润湿过程的影响,设计了2组润湿实验方案,即Zr41.2Ti13.8Cu12.5Ni10.0Be22.5合金熔体在SiC基片上方和Zr41.2Ti13.8Cu12.5Ni10.0Be22.5合金熔体在SiC基片下方.结果表明:当合金熔滴在SiC基片下方时,在重力的作用下,熔滴的高度随温度的升高先增大后减小;合金熔滴在SiC基片上方时,熔滴的高度只是随温度的升高而减小.无论是在连续升温情况下还是在等温润湿过程中,Zr41.2Ti13.8Cu12.5Ni10.0Be22.5合金熔体在SiC基片上方时的润湿角都要更小一些.说明在地面重力场环境中,熔滴与基片的不同空间方位对液态Zr41.2Ti13.8Cu12.5Ni10.0Be22.5合金与固态SiC基片之间的润湿角有很大影响.     

Electrochemical Corrosion Behavior of Thermal-Sprayed Stainless Steel-Coated Q235 Steel in Simulated Soil Solutions

The corrosion behavior of a thermal-sprayed stainless steel (SS)-coated Q235 steel has been investigated in simulated soil solutions using electrochemical measurements, x-ray photoelectron spectroscopy analysis, and scanning electron microscope. The as-received Q235 steel and galvanized steel for grounding grids were also examined for the purpose of comparison. The effects of pH value of testing solutions have been examined. The thermal-sprayed SS-coated steel showed the best corrosion resistance among the three kinds of materials. With increasing pH value, the corrosion resistance of SS-coated Q235 steel increased. In weak alkaline solutions, the SS-coated Q235 steel showed the largest polarization resistance (3.2 × 10⁵ Ω cm²), the lowest anodic current density (1.4 × 10^?2 μA/cm²), and the largest film resistance (4.5 × 10⁶ Ω cm²), suggesting that the coated steel has the best corrosion resistance in weak alkaline environment. Related corrosion mechanisms are also discussed.     

Study on Grain Boundary Structure and Corrosion Behavior of Copper Alloys

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Modeling of Rapid Solidification with Undercooling Effect During Droplet Flattening on a Substrate in Coating Formation

The coating deposit on the substrate in thermal spray coating process develops by solidification of individual molten particle which impacts, flattens and solidifies on the surface of the substrate. Droplet flattening and solidification typically involves rapid cooling. In this paper, a model for non-equilibrium rapid solidification of a molten droplet spreading onto a substrate is presented. Transient flow during droplet impact and its subsequent spreading is considered using the volume of fluid surface tracking method which was fully coupled with the rapid solidification model. The rapid solidification model includes undercooling, nucleation, interface tracking, non-equilibrium solidification kinetics and combined heat transfer and fluid flow as required to treat a non-stagnant splat formed from droplet flattening. The model is validated with the literature results on stagnant splats. Subsequently, using the model the characteristics of the rapidly solidifying interface for non-stagnant splat, such as interface velocity and interface temperature, are described and the effect of undercooling and interfacial heat transfer coefficient are highlighted. In contrast to the stagnant splat, the non-stagnant splat considered in this study displays interesting features in the rapidly solidifying interface. These are attributed to droplet thinning and droplet recoiling that occur during the droplet spreading process.