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1.
In thermal spray processes, the coating structure is the result of flattening and cooling of molten droplets on the substrate. The study of the cooling time and evolution of the splat size during impact is then of the highest importance to understand the influence of the spray parameters and substrate characteristics on the coating structure. Measurement of particle temperature during impact requires the use of a high-speed two-color pyrometer to collect the thermal emission of the particle during flattening. Simultaneous measurement of the splat size with this pyrometer is difficult since the size of the particle can change as it cools down. To measure the splat size independently, a new measurement technique has been developed. In this technique, the splat size is measured from the attenuation of the radiation of a laser beam illuminating the particle during impact. Results are presented for plasma-sprayed molybdenum particles impacting on a glass substrate at room temperature. It is shown that the molybdenum splat reaches its maximum extent about 2 μs after the impact. In this work, we show that this increase of the splat surface is followed by a phase during which the splat size decreases significantly during 2 to 3 μs.  相似文献   

2.
The filtered cathodic vacuum-arc (FCVA) technique is a supplementary and alternative technique with respect to convendtional physical and chemical vapour deposition which can remove macro-particles effectively and make the deposition process at ambient temperature. In this work, high quality TiN thin films were deposited on silicon substrates at low temperature using the improved filtered cathodic arc plasma (FCAP) technique. AFM, XRD, TEM were employed to characterize the TiN thin films. The effects of the negative substrate bias on the grain size, preferred crystalline orientation, surface roughness of TiN thin films were discussed.  相似文献   

3.
Recently a new kind stent,porous drug-eluting stent,has been developed to overcome the problems of bare metal stent(BMS)and drug-eluting stent(DES),and the clinic results reveal that it combined the advantage of BMS and DES.In this paper,a new method to fabricate surface pores on 316L stainless steel stent using anodic oxidation of aluminum mask film deposited by magnetron sputtering on 316L substrate was reported.The effect of experimental parameters,such as anodization time,anodization voltage,anodization solution and anodization temperature,on the pores’size distribution and density are investigated using SEM.It is found that the pores characterizations strongly depend on anodization time and pores expanding rate through AAO films and on stainless steel surface.  相似文献   

4.
Full-bridge Zero-Voltage-Switch PWM converter combines advantages of the PWM control technique and resonant technique,However,Full ZVS is achieved only under large load current because resonant tank of this circuit is made up of the parasitic capacitance of the power semiconductors and the leakage inductor of the transformer primary.In this paper two saturable inductors as magnetic switches are added to secondary,so output inductor is always reflected to primary and assists resonant transition.Full ZVS is achieved under lower load current.The above-mentioned investigated results are validated by the computerized simulation and hardware circuit experiment.  相似文献   

5.
Common thermal-spray techniques use the strong acceleration of powder particles to produce dense ceramic coatings with high bond strength. The residence time of the powder particles within the plasma jet is correspondingly low, and only relatively small particles can be molten. In this work, on the contrary, an inductively coupled radio-frequency (RF) inductively coupled plasma (ICP) torch was used to spray large oxide-ceramic powder particles under atmospheric conditions. The slow plasma flow of a RF plasma leads to large residence times of the powder particles, so that the powder size of the feedstock can be 100 μm and more. It was observed that these particles will not be strongly accelerated in the plasma and that their velocity at the moment of impact is in the range of 10 to 20 m/s. Ceramic coatings were ICP sprayed with a low porosity and a high bond strength, similar to direct current (DC) or high-velocity-oxygen-fuel (HVOF) sprayed coatings. The morphology of ICP-sprayed particles on smooth steel surfaces, as a function of the surface temperature, is described and compared with DC plasma-sprayed splats. Furthermore, the degree of deformation was measured and determined by different models, and the pronounced contact zones formed between the pancake and the substrate were investigated. The ICP-sprayed ceramic coatings show some special properties, such as the absence of metastable crystalline phases, which are common in other spray technologies.  相似文献   

6.
Thermoelectric technologies based on Seebeck and Peltier effects,as energy techniques able to directly convert heat into electricity and vice versa, hold promise for addressing the global energy and environmental problems. The development of efficient and low-cost thermoelectric modules is the key to their large-scale commercial applications. In this paper, using a non-equilibrium laser3 D printing technique, we focus an attention on the fabrication of mid-temperature p-type SnTe thermoelectric materials. The influence of laser power, scanning speed and layer thickness on the macro-defects, chemical and phase composition, micro structure and thermoelectric performance was systematically investigated. First and foremost,the processing parameter window for printing a highquality layer is determined. This is followed by the finite element method used to simulate and verify the influence of the laser-induced molten pool temperature distribution on the final composition and microstructure. Finally, the high-performance SnTe layer with 10 mm × 10 mm in area is produced within seconds with room temperature Seebeck coefficient close to that of SnTe manufactured by the traditional methods. Consequently,this work lays a solid foundation for the future fabrication of thermoelectric modules using laser non-equilibrium printing techniques.  相似文献   

7.
ZH1 technique was used to form a corrosion resistant layer on LC4 alloy. The composition of the layer was studied by X-ray photoelectron spectroscopy (XPS). It is found that the layer is composed of oxides of metals on the substrate, such as A12O3, ZnO, MgO2, CuO, and MnO2. The electrochemical impedance spectrums (EIS) of LC4 aluminum alloy specimen were measured in NaCl solutions to study the mechanism of the corrosion resistance of the alloy treated with ZHI technique. The results show that in NaCl solutions the capacitance of the Helmholtz layer and space charge layer of the oxide coating formed on the surface of LC4 alloy is less than that of a normal metal electrode while its ohmic resistance is relatively greater. At the same time, the thickness of the Helmholtz layer is 1-3 exponentially greater than that of a normal metal electrode. Compared with a normal metal electrode, all these characteristics make it more difficult for charges to transfer between the solution and the surface of the electrode. That is why the polarized current density of LC4 alloy treated with ZHI technique kept small in 3.5% NaCl solution within a wide range of potential, and why the polarized curves of LC4 alloy treated with ZH1 technique changed a little in 3.5% NaCl solution of different pH values.Moreover, according to the capacitance of the space charge layer (Csc) obtained at different potentials in 3.5% NaCI solution, I/Csc^2-E curve was laid out. It is found that there does not exist a simple linear relation between I/Csc^2 and the potential. Therefore, the oxide coating formed on LC4 alloy with ZH1 technique is not a semiconductor at room temperature.  相似文献   

8.
杨世彦  于志  刘喆 《中国焊接》2003,12(2):112-115
With the background of the control of additional mechanical force droplet transfer in MIG/MAG welding, regarding the AC servo motor as core, a novel oscillating wire feeding system has been developed with excellent performances of control and dynamic acceleration which is tested. System constitution and operation principle are introduced in this paper. Influences of parameters on dynamic acceleration performance are analyzed and discussed emphatically, such as oscillating frequency, oscillating amplitude and draw-back speed. Experimental result indicates that according to the technique of welding control, the novel wire feeding system responds rapidly to various kinds of control orders of wire feeding and draw-back, and realizes flexible control of welding wire axial movement, including dynamic shifting ,oscillating and so on.  相似文献   

9.
Titanium nitride (TIN) films with nanostructure were prepared at ambient temperature on a (111) silicon substrate by the filtered cathodic arc plasma (FCAP) technology with an in-plane "S" filter. The effects of deposition parameters on the grain size, texture and nano-hardness of the films were systematically investigated. The grain size was obtained through calculation using the Scherrer formula and observed by TEM. The results of X-ray diffraction and electron diffraction indicated that increasing either negative substrate bias or argon flow promoted the formation of (111) preferred orientation. High argon flow leads to biaxial texture. The micro-hardness of the TIN films as a function of grain size showed a behavior according to the Hall-Petch relation under high argon flow.  相似文献   

10.
In the last years a variety of plasma sources have been developed for film deposition by plasma activated deposition techniques. In addition to RF- and DC-sources, pulsed plasma sources are gaining increased attention. This interest is driven by the wish of depositing coatings with superior properties as compared to those deposited by conven-tional techniques. One prominent example of coatings that are significantly enhanced by the usage of pulsed plasmas is alumina. Although crystalline a-alumina can be de-posited by thermal CVD at temperatures above 1000℃ for two decades, no process for the deposition of crystalline alumina on heat sensitive substrates like tempered steels at low temperatures is commercially available up to now. In this paper, the deposition of alumina films from gaseous mixtures of AlCl3-N2-H2-Ar in a bipolar pulsed glow discharge at about 500℃ is reported. Special attention was paid on the correlation be-tween plasma characteristics and film properties. The measurements revealed th  相似文献   

11.
涂层成形过程中的缺陷含量、残余应力、沉积效率、组织结构及力学性能等指标均会随着工艺参数与基体预处理状态的不同而发生显著变化,因而需要从更加微观的角度深入理解等离子喷涂涂层的微观构筑过程,即单个熔滴的铺展凝固现象。本研究分别从熔滴凝固的类型与机理、不同因素对熔滴凝固过程的影响及凝固斑点形态的定量表征方法 3个方面详细综述了等离子喷涂熔滴撞击基体快速凝固过程的研究现状。结果表明,熔滴的铺展形态主要可以分为5类,包括圆盘型、破碎型、放射型、花瓣型及气泡型,影响铺展过程的因素主要包括熔滴特性(速度、温度、粒径、材料属性、熔化状态等)与基体状态(表面粗糙形貌、表面化学状态、吸附物及冷凝物、界面润湿性及接触热阻等)2大类,综合采用一系列参数对熔滴铺展几何形态进行表征,可实现熔滴沉积质量的定量评价。  相似文献   

12.
This paper presents a number of numerical models that illustrate the experimental details of the key physical phenomena affecting the operation of novel materials deposition processes for electronics assembly. These processes include both laser and electric arc based droplet deposition processes. These are being developed in the electronics industry to allow the use of new lead-free alloys as replacements for high temperature lead based alloys, such as Sn5Pb95. The paper presents models of physical processes with respect to the desired process metrics of droplet size and deposition accuracy and reviews the potential limiting product - process interactions such as excessive thermal excursions and their effects on the target materials being joined.  相似文献   

13.
Solution precursor plasma spraying (SPPS) is a novel technology with great potential for depositing finely structured ceramic coatings with nano- and sub-micrometric features. The solution is injected into the plasma jet either as a liquid stream or gas atomized droplets. Solution droplets or the stream interact with the plasma jet and break up into fine droplets. The solvent vaporizes very fast as the droplets travel downstream. Solid particles are finally formed, and the particle are heated up and accelerated to the substrate to generate the coating. The deposition process and the properties of coatings obtained are extremely sensitive to the process parameters, such as torch operating conditions, injection modes, injection parameters, and substrate temperatures. This article numerically investigates the effect of injection modes, a liquid stream injection and a gas-blast injection, on the size distribution of injected droplets. The particle/droplet size, temperature, and position distributions on the substrate are predicted for different injection modes.  相似文献   

14.
A pneumatic drop-on-demand (DOD) system has been applied to solder complex and dense interconnects of modern micro electronic devices. Initial parameters of uniform droplets were first measured. Then influences of experiment parameters, such as the crucible temperature, the substrate temperature and the droplet velocity on spread of solder droplets, were investigated experimentally and theoretically. The results showed that effects of impact velocity on the spreading could be negligible and the solder spreading process was driven by capillary forces because of the low Weber (We) number of depositing droplet. The influence of initial droplet temperature on the droplet spreading was not regular. The contact diameter of solder droplets on copper substrate increased when the substrate temperature increased from 443 K to 493 K. At last, copper cables were successfully soldered to pins of a flexible circuit using the direct solder deposition and re-melting process. The soldering results showed the feasibility of the data-driven soldering technique using the pneumatic DOD deposition technology.  相似文献   

15.
This paper presents an investigation of the influence of plasma spray process conditions on the in-flight particle behavior and their cumulative deposition to form a coating on the substrate. Three-dimensional computational fluid dynamics (CFD) analyses were performed to model the in-flight particle behavior in the plasma-spray process and their deposition on the substrate. The plasma spray was modeled as a jet issuing from the torch nozzle through the electrical heating of the arc gas. In the model, particles were injected into the plasma jet where they acquired heat and momentum from the plasma, some got melted and droplets were formed. By means of a droplet splatting model, the particle in-flight data generated by the CFD analyses were further processed to build up an imaginary three-dimensional deposition profile on a flat stationary substrate. It is found that the powder carrier gas flow rate influences the particle distribution on the substrate by imparting an injection momentum to the particles that were directed radially into the plasma jet in a direction perpendicular to the plasma jet. The larger sized particles will acquire higher injection momentum compared with the smaller sized particles. This causes particle distribution at the substrate surface that is elliptical in shape with the major axis of ellipse parallel to the particle injection port axis as illustrated in Fig. 1. Larger particles tend to congregate at the lower part of the ellipse, due to their greater momentum. The distribution of particle size, temperature, velocity, and count distribution at the substrate was analyzed. Further, based on the size and the computed particle temperature, velocity histories, and the impact sites on the substrate, the data were processed to build up a deposition profile with the Pasandideh-Fard model. The shapes of deposition profiles were found to be strongly driven by the segregation effect.  相似文献   

16.
Droplet based additive manufacturing is a branch of novel processes to build full dense metal parts by adding material droplet by droplet on a build platform. As each droplet solidifies individually upon contact, the quality of bonding to the existing material is determined by the adjacent surface temperatures and the temperature of the arriving droplet. To design a manufacturing process that ensures good bonds between all droplets, it is necessary to understand the relations between process parameters, the part’s geometry and thermal conditions for each arriving droplet. This paper presents a thermal simulation model that is based on Flow 3D software. By adding a user routine to the solver, it is possible to simulate the building process of a part consisting of several thousand droplets with an acceptable effort. This simulation is used to study the effect of production parameters (substrate temperature, droplet temperature and deposition frequency) as well as the parts geometry (layer size and height) on the resultant temperature field. The model was successfully validated with experimental data and can deliver valuable information during further development of this additive manufacturing process.  相似文献   

17.
Micro metal droplet is the basic building block of three-dimensional metal parts fabricated by micro droplet deposition manufacturing (MDDM) technique. In this paper, the effect of wetting behavior between liquid metal and spray nozzle on the generation of micro aluminum droplets produced by pneumatic drop-on-demand (DOD) technique was investigated by simulation and experiment. A finite element model of liquid–gas flow was established based on the improved level set method (LSM). Then the generation of micro liquid aluminum droplets under different wetting conditions was simulated. A series of spraying experiments were also performed on micro droplet deposition experiment platform. The results show that the generation and flight of micro aluminum droplets are influenced by wetting condition between liquid metal and the nozzle surface significantly. Additionally, the effect of wetting behavior on the droplet size was analyzed to achieve the smallest building block. It was found that the droplet radius decreased with the increase of contact angle exponentially, which agreed with the numerical calculation and experiment results. On this basis, a wettability criterion was proposed for selecting nozzle materials. These works would be helpful for the processing optimization and equipment improvement of MDDM technique.  相似文献   

18.
High temperature oxidation is one of the main failure modes of the hot‐section components in gas turbines, boilers, waste incinerations, diesel engines, coal gasification plants, chemical plants and other energy generation systems. In such applications the use of Fe‐, Ni‐ and Co‐based alloys, especially of superalloys is well known. The superior mechanical strength and good corrosion resistance of the superalloys at high temperature make them favorites for such applications. However, the presence of combustion gases constitutes an extreme environment and hot corrosion is inevitable when superalloys are used at high temperatures for long durations of time. Therefore these alloys need to be protected against this type of oxidation. Several countermeasures have been suggested in the literature to combat the same. One such countermeasure against hot corrosion and oxidation constitutes the deposition of protective coatings on these alloys. Among the various techniques used for deposition of coatings, plasma spraying is a versatile technology that has been successful as a reliable cost‐effective solution for many industrial problems. It allows the spraying of a wide range of high performance materials from superalloys and refractory intermetallic compounds to ceramics with continuously increasing commercial applications. Furthermore it does not cause deterioration of the substrate alloys, and relatively thick coatings can be formed with high deposition rates. In this paper the technique of plasma spraying has been detailed and the role of plasma sprayed coatings to arrest high temperature oxidation has been discussed with the help of a comprehensive literature survey. The main focus of this investigation is the studies related to plasma sprayed NiCrAlY, Ni‐Cr, Nickel aluminide and Co‐based coatings.  相似文献   

19.
喷射沉积过程中半固态流变组织演变   总被引:1,自引:1,他引:0  
分析了喷射沉积过程中半固态流变组织演变过程,提出喷射沉积合金细小的等轴晶的凝固组织是飞行阶段快速冷却和沉积阶段沉积体表面糊状凝固层中半固态流变组织演变的结果,沉积体凝固过程是一种微米级流变铸造过程:不同大小、不同状态、不同温度、不同的运动速度的雾滴以高速高频的雾滴撞击沉积体表面,产生足够的剪切应力和剪切速率,使沉积体半固态糊状凝固层具有流变特性,飞行阶段形成快速凝固组织在沉积阶段沉积体表面糊状凝固层中发生变温粗化,最终形成细小的等轴晶。  相似文献   

20.
Thermo-physical processes in liquid ceramic precursor droplets in plasma were modeled. Models include aerodynamic droplet break-up, droplet transport, as well as heat and mass transfer within individual droplets. Droplet size, solute concentration, and plasma temperature effects are studied. Results are discussed with the perspective of selecting processing conditions and injection parameters to obtain certain types of coating microstructures. Small droplets (<5 microns) are found to undergo volumetric precipitation and coating deposition with small unpyrolized material. Droplets can be made to undergo shear break-up by reducing surface tension and small droplets promote volumetric precipitation. Small particles reach substrate as molten splats resulting in denser coatings. Model predicts that larger droplets (>5 microns) tend to surface precipitate-forming shells with liquid core. They may be subjected to internal pressurization leading to shattering of shells and secondary atomization of liquid within. They arrive at the substrate as broken shells and unpyrolized material.  相似文献   

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