Effect of substrate hardness on the deformation behavior of subsequently incident particles in cold spraying

A systematic finite element analysis (FEA) on the subsequently incident particles which refer to the particles depositing after the formation of the first layer coating is conducted in this study to clarify the bonding mechanism inside the cold sprayed coating. A simplified particle impact model is proposed and the simulated results based on this model demonstrate that substrate hardness exerts some effects on the deformation behavior of the subsequently incident particles. Hard substrate makes these particles deform intensively but using soft substrate enables them to achieve a slight deformation. At the same time, it is also found that substrate hardness plays its best role only when the formed coating is thin, as the development of the coating, the particle deformation behavior becomes more and more insensitive to the substrate hardness. The multi-particle impact simulation based on Eulerian method is also performed and reaches the same conclusion, confirming the accuracy of the simplified model. Besides, it is also found that when the velocity is increased to a hypervelocity, excessive deformation occurs in the formed coatings due to the impact of the subsequently incident particles.     

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.     

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.     

Some aspects on 3D numerical modeling of high velocity impact of particles in cold spraying by explicit finite element analysis

Three-dimensional modeling of particle impacting behavior in cold spraying by using ABAQUS/Explicit was conducted for copper and other materials. Various combinations of calculation settings concerning material damage, Arbitrary Lagrangian Eulerian adaptive meshing, distortion control and contact interaction were examined. The effects of meshing size and particle size on the impact behavior were analyzed compared to the previous results. The results show that the simulations with material damage cope well with the element excessive distortion and the resultant output is more reasonable than that obtained without material damage. In addition, the meshing size has less effect on the output with the material damage than without material damage. Although particle size has little effect on the morphologies of the deformed particles, it has some effect on the failure of elements at contact interfaces. The critical velocity for particle deposition could be estimated given the appropriate material properties.     

Characterization of cold-sprayed nanostructured Fe-based alloy

The ball-milled Fe-Si alloy was used as feedstock for deposition of nanocrystalline Fe-Si by cold spraying process. The microstructure of the as-sprayed nanostructured Fe-Si was characterized by using optical microscopy, scanning electron microscopy and transmission electron microscopy. The grain sizes of the feedstock and as-sprayed deposit were estimated based on X-ray diffraction analysis. The microhardness and coercivity of the deposited Fe-Si alloy were characterized. The results showed that the as-sprayed deposit presented a dense microstructure. The mean grain size of the as-deposited Fe-Si was several tens nanometers and comparable to that of the corresponding milled feedstock. The temperature of driving gas presented little effect on the microstructure of cold-sprayed nanostructured Fe-Si deposit. The mechanical alloying induced oxygen contents up to 8 wt% in the feedstocks and subsequent deposits. The microhardness of the deposit reached about 400 Hv. The deposit achieved a high coercivity up to 190 kA/m indicating the potential possibility for applications to recording materials.     

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.     

Significant influences of metal reactivity and oxide films at particle surfaces on coating microstructure in cold spraying

Based on large amount of experimental observations, the effects of metal reactivity and oxide films at particle surfaces on coating deposition behavior in cold spraying were presented and discussed. The oxygen contents in as-sprayed Ti, Ti-6Al-4V and Al coatings were higher than those in the corresponding starting powders. The obvious flashing jets outside nozzle exit during deposition of Ti and Ti-6Al-4V were caused by the reaction of the particles with oxygen in the entrained or the adopted air. For Ti and Ti-6Al-4V coatings, their porous structures are predominantly attributed to the surface reactivity (defined as reactivity with oxygen). This surface reaction could be helpful for formation of a metallurgical bonding between the deposited particles. For Al, even though it is more reactive than Ti, the oxide films at Al particle surfaces suppress the surface activity.     

On high velocity impact of micro-sized metallic particles in cold spraying

In this study, a systematic examination of particle deformation behaviour in cold spraying was conducted for Cu particle using both the Lagrangian and Arbitrary Lagrangian Eulerian (ALE) methods. It is found that the meshing size in modelling by Largrangian method influences significantly the localized shear instability at interface areas. With refining the meshing size the onset velocity for interface shear instability decreases. The extrapolation of these results yields a reasonable critical velocity comparable to the actual one in cold spray practice. The results indicate that both the flattening ratio and compression ratio of the deformed particles increase with the increase in particle velocity, which are in good agreement with the experiment results. The ALE method provides a suitable way to examine the particle deformation in cold spraying. Moreover, the numerical results also show that there exists the similarity for the deformation of particles of different diameters.     

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.     

Study on impact fusion at particle interfaces and its effect on coating microstructure in cold spraying

This paper deals with the impact melting phenomenon at the interfaces between the deposited particles in cold-sprayed coatings and its effect on coating microstructure and particle bonding mechanism. Al-12Si, Al2319, Ti, Ti-6Al-4V, Ni and NiCoCrAlTaY powders were selected as feedstocks, which have various thermal and mechanical properties. The analytical results showed that most of the used materials possibly experienced the local melting at the contact interfaces of particles under certain impact conditions. Low melting point, relatively high gas temperature and chemical reaction with the atmosphere are the main factors contributing to the impact fusion during cold spraying. The results also indicated that the local melting would benefit the formation of a metallurgical bonding between the deposited particles and enhance the coating cohesion.     

The optimization of microbial influenced corrosion resistance of HVOF sprayed nanostructured WC-10Co-4Cr coatings by ultrasound-assisted sealing

In this study, high-velocity oxygen-fuel (HVOF) sprayed nanostructured WC-10Co-4Cr coatings were subjected to seawater with sulfate-reducing bacteria (SRB) for different time. The effect of ultrasound-assisted sealing with aluminum phosphate on the microstructural features and microbial influenced corrosion (MIC) behavior was evaluated using scanning electron microscopy (SEM), potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). The results showed that the ultrasound-assisted sealing promoted the infiltration of the sealant into as-sprayed coating, enhanced the resistance values of about one order of magnitude, and reduced the corrosion current density. During the whole immersion period, the MIC process of both unsealed and sealed coatings can be divided into two different stages, and the ultrasound-assisted sealing treatment significantly delayed the MIC process, suggesting that the ultrasound-assisted sealing with aluminum phosphate is an effective way for controlling the MIC of SRB on the WC-10Co-4Cr cermet coatings in marine environment.     

Influence of annealing treatment on the microstructure and mechanical performance of cold sprayed 304 stainless steel coating

In this study, 304 stainless steel coatings were deposited on interstitial-free steel substrate by cold spraying method. The effect of annealing treatment on microstructure, microhardness, ultimate tensile strength and fracture performance of the coatings were studied. The results showed that annealing treatment had made a dominant contribution to heal up the incomplete interfaces between the deposited particles. Both of the microstructure and the mechanical properties have been obviously optimized by annealing treatment. In addition, the coating microhardness decreased from 345 HV_0.2 for the as-sprayed coating to 201 HV_0.2 for the annealed coating. The coating ultimate tensile strength increased from 65 MPa for the as-sprayed coating to 357 MPa for the annealed coating, which resulted from the increase of the metallurgically bonded areas in the coating induced by annealing treatment. Fracture morphology of the coatings also revealed that annealing treatment changed the fracture character of the cold sprayed 304 stainless steel coating from brittle type to plastic type.     

Co元素对硬质合金基底金刚石涂层膜基界面结合强度的影响

采用基于密度泛函理论的第一性原理平面波赝势方法, 研究了硬质合金刀具基底黏结相Co元素对金刚石涂层膜基界面结合强度的影响机理. 借助Materials Studio软件建立了WC/Diamond膜基界面模型和WC-Co/Diamond膜基界面模型, 采用CASTEP仿真软件计算了WC/Diamond膜基界面模型和WC-Co/Diamond膜基界面模型的最优稳定结构. 通过仿真计算, 获得了WC/Diamond膜基界面模型和WC-Co/Diamond膜基界面模型的界面结合能、电荷密度图及Mulliken重叠布居数. 经对比分析后发现, 硬质合金基底中磁性元素Co的存在能转移金刚石涂层膜基界面处W元素及C元素的电荷, 从而使膜基界面处的原子因失电荷而相斥, 这直接导致了金刚石涂层膜基界面间距变大, 使得金刚石涂层膜基界面结合能降低.     

Optimal design of a convergent-barrel cold spray nozzle by numerical method

A convergent-barrel (CB) cold spray nozzle was designed through numerical simulation. It was found that the main factors influencing significantly particle velocity and temperature include the length and diameter of the barrel section, the nature of the accelerating gas and its pressure and temperature, and the particle size. Particles can achieve a relatively low velocity but a high temperature under the same gas pressure using a CB nozzle compared to a convergent-divergent (CD) nozzle. The experiment results with Cu powder using the designed CB nozzle confirmed that particle deposition can be realized under a lower gas pressure with a CB nozzle.     

The effect of substrate on magnetic properties of Co/Cu multilayer nanowire arrays

Ordered Co/Cu multilayer nanowire arrays have been fabricated into anodic aluminium oxide templates with Ag and Cu substrate by direct current electrodeposition. This paper studies the morphology, structure and magnetic properties by transmission electron microscopy, selective area electron diffraction, x-ray diffraction, and vibrating sample magnetometer. X-ray diffraction patterns reveal that both as-deposited nanowire arrays films exhibit face-centred cubic structure. Magnetic measurements indicate that the easy magnetization direction of Co/Cu multilayer nanowire arrays films on Ag substrate is perpendicular to the long axis of nanowire, whereas the easy magnetization direction of the sample with Cu substrate is parallel to the long axis of nanowire. The change of easy magnetization direction attributed to different substrates, and the magnetic properties of the nanowire arrays are discussed.     

过渡金属Fe，Co，Ni居里点附近热电势的实验研究

测量了纯金属Fe，Co，Ni的热电势发现，在居里点附近热电势随温度的变化关系曲线均呈现先凹后凸的反常现象.由曲线的转折处可确定三个居里温度，即铁磁态居里点T_f，居里点T_C和顺磁居里点θ_p.由曲线可见，金属由铁磁态到顺磁态的相转变，存在一定温度间隔的转变过程，居里温度是这一过程的中间温度.分析曲线表明，温度在T_f与T_C范围有空穴参与导电，说明磁性负载者是d带中的空穴.对于温度在T_C与θ_p范围可能存在短程有序进行了讨论. 关键词： Fe Co Ni 热电势 居里温度     

Interface behavior study of WC92–Co8 coating produced by electrospark deposition

WC92–Co8 coating produced by electrospark deposition effectively improves the surface performance of the substrate. The behavior of the interface between the WC92–Co8 coating and the substrate is studied in this paper. The high-melting-point WC92–Co8 was deposited onto the surface of Ti alloy, and the coating was usually more than 50 μm thick. The surface of the coating is mainly composed of TiC and W₂C besides a small amount of W, and its micro hardness reaches HV1129. The coating dramatically improves the performance of the substrate.     

Study on crystalline to amorphous structure transition of Cr coatings by magnetron sputtering

The influence of deposition rate on crystalline to amorphous microstructure transition of Cr coatings was studied through preparation of Cr coatings deposited onto silicon wafers using magnetron sputtering technique. The microstructure and morphology of Cr coatings were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD) and transmission electron microscopy (TEM). Results show that Cr coating prepared at 400 W exhibits dense columnar crystalline structure and the crystallite size and crystallization rate are increased expressly in the initial 5 min. When the deposition rate achieved to the maximum, Cr coating shows a case of infinite periodic renucleation where new crystals are assumed to be nucleated periodically on the surfaces of growing crystals and strong persistence of the columnar growth morphology is apparent. However, Cr coating exhibits overall microstructure of amorphous phase mixed with a few nano-crystal grains as the deposition rate decreases to the minimum.     

3d过渡金属元素掺杂Al12N12纳米线几何结构和电子性能的第一性原理研究

基于密度泛函理论的第一性原理计算方法,系统地研究了不同3d过渡金属元素(Sc、Ti、V、Cr、Mn、Fe、Co和Ni)掺杂Al₁₂N₁₂纳米线的几何结构、稳定性和电子结构.结果表明：所有掺杂体系均是热力学稳定的;掺杂Ni时体系保留了原有的非磁性间接带隙半导体特性;当掺杂其它原子(Sc、Ti、V、Cr、Mn、Fe、Co)时体系仍然保持为半导体,但带隙明显减小.掺杂过渡金属原子对于Al₁₂N₁₂纳米线的电子结构具有明显的调控作用,在能带调控和光电方面有潜在的应用前景.