| W-Cu composite is one of the most extensively studied metallic matrix composite materials with the excellent property of complementary. Cu has good electrical conductivity, thermal conductivity and corrosion resistance, but it has low strength; W has high hardness, strength, good wear resistance and thermal stability. The combination of the advantages of Cu and W has made the W-Cu composite become a potential candidate material for a variety of high-tech application such as electron sealing, integrated circuit, national defense military and aerospace. However, with the use condition becoming harsher, such as in the high-temperature environment, the homogeneous composite material would not be able to meet the need as with its thermal and mechanical property. To solve the problem, people proposed a kind of W-Cu composite material with a variety of excellent performances named W-Cu functionally graded material (FGM). Many studies have been focused on the design, preparation and performance of the W-Cu functionally graded material for different service conditions. According to the previous reported results, it’s hard to prepare the material with the design requirement structure and the high comprehensive performance, as many problems exist, such as complex molding process, sintering difficulty and hard control of the preparation process.So this work concentrate on the W-Cu functionally graded material. To solve the problems mentioned above, this work use the electroless plating process to plate a uniform copper layer on the surface of the tungsten particals without limiting by the shape or size of the tungsten particles. By changing the state of the tungsten particles surface, the wetting performance between the tungsten and copper was enhanced. Use Plasma Activated Sintering technique to activate the sintering powder, reduce the sintering temperature, increase the density of the sintering body. Use the aqueous tape-casting technique to solve the component controling and molding problem. The paper studied the key preparation process, including the raw material processing, molding and sintering of the W-Cu functionally graded material theoretically and experimentally in detail. And then characterize the structures and the performances of W-Cu homogeneous composites and W-Cu functionally graded composite. The main conclusions are as follow:Firstly, glyoxic acid was used to replace the formaldehyde as the reducing agent. A novel electroless plating method—copper salt dripping method has been proposed for the first time. Designing of experiment (DOE) technique is used for optimize the data processing. Data analysis show that the optimal conditions are as the followings: flow velocity-5.01ml/min, pH-12.25, Temperature-45.35℃The effects of the pH value, reaction temperature, flow rate of copper salt and reducing agent were investigated in this work. The deposits obtained in this work was found to be polycrystalline fcc copper, in which no monovalent or divalent Cu ions were detected. In addition, these powders displayed good dispersion property. It means that we’ve already successfully prepared the Cu@W core-shell structure composite powders through copper salt dripping method in glyoxylic acid-water system, which will provide the high purity, quantitative coated powder raw materials for the preparation of the W-Cu functionally graded material.On this basis, the thermodynamic and the dynamics processes of the dripping method for direct electroless copper plating on W particle were analysis. The study showed that the heterogeneous nucleation play a decisive role in the dripping method for direct electroless copper plating on W particle process. Dripping method can help keeping the concentration of the copper ions to a low level, a main factor of obtaining a good coating effect, under such condition the pictogram nuclear would be avoid and the main driving force of the coating process-over-saturation would be reduced. According to analysis the reaction mechanism of dripping method for direct electroless copper plating on W particle, the model of the formation of the Cu plating on W particles was established. The model showed that firstly a large amount of Cu2+ions gathered on the tungsten particles surfaces, forming a positively charged entity. In this case the surface of the copper ions entity has the same effect with the surface of the metallic copper-autocatalator, and become the catalytically active centers, which trigger the initial oxidation-reduction reaction between the glyoxylate and copper ions to form a copper plating layer. The metallic copper is deposit on the surface of it to form a dense shell structure. Then the deposited Cu becomes the catalytic centers for the further electroless Cu plating. Subsequently, the deposited copper layer continue to serve as the activation center to induced the oxidation-reduction electroless plating proceed.Secondly, W-Cu composite cast sheet with the uniform thickness, high solid content were prepared by the powder synthetized above and the aqueous tape-casting technique, which provide the raw material for the sintering of the W-Cu functionally graded material. In this paper, the Computational Fluid Dynamics (CFD) and the Bingham model were chose to help the numerical simulation of the whole tape casting process. The influences of the blade gap, the casting speed and the slurry’s rheological property to the W-Cu cast sheet were studied. The critical process parameters and slurry properties for the stable cast molding were achieved: Speed>2mm/s, Viscosity>3P By experiment the composition of the tape casting slurry and the technological condition of the tape casting, optimal recipe for the aqueous slurry was got: Y=0.0141+3.39E-4·X, R=0.988Where X is the volume of the solid powder, Y is the mass ratio of the binder (PVA-2488) with the solid phase powder. The mass ratio of the plasticizer (glycerin) and a binder is0.6. Through optimizing the solid content of the casting slurry, the binder and the plasticizer, the tungsten copper aqueous tape-casting film with high solid content, uniform structure, great performance were successfully produced.Thirdly, this work successfully prepared the homogeneous W-Cu composite with the ideal structure, high density and excellent performance by the PAS sintering technology. The densification and various performance of the W-Cu homogeneous composite were studied in details. The influence of the W volume fraction to the density, coefficient of thermal expansion, thermal conductivity and electrical conductivity of the composite were researched. The result showed that using the powder with the coating structure greatly reduced the difficulty of the densification. Under the low temperature sintering condition of950degrees-100MPa-lh, different component W-Cu composites all achieved the high density of more than98%. The density, thermal expansion coefficient, thermal conductivity, electrical conductivity decreased and the hardness increased with the increasing of the W. We also got the prediction model formula about the performance and the component of the W-Cu composition, which can provide the predictive design models and the sintering condition for W-Cu functionally graded material.Finally, this work completes the functional design of the W-Cu functionally graded materials directing at the W-Cu composite applications. For The requirements of the high thermal conductivity and low thermal expansion coefficient for the electronic packaging field, high hardness and high electrical conductivity for the electrical contacts or electrodes industry, as well as the high temperature resistance and high thermal conductivity for the plasma-oriented wall material. This work prepared two kinds of W-Cu functionally graded material. Three layers with the copper content in the range of35～87vol.%. Six layers with the copper content in the range of35～94.1vol.%and nine layers with the copper content in the range of35～96.2vol.%. By using optimized tape casting and plasma activated sintering (PAS) processing above, The whole sample is dense. The intermediate layer combines tightly. And the volume fraction of the W varied gradiently. The properties of the W-Cu functionally gradient material were then studied in detail. The results show that W-Cu functionally graded material achieved good structure and distributed compactly. The thermal conductivity along the thickness direction of the three layers W-Cu functionally gradient material with the distribution index of1was301.8W/(m·K), and the electrical conductivity was0.366×108S/m. The thermal conductivity along the thickness direction of the six layers W-Cu functionally gradient material with the distribution index of1was309.2W/(m·K), and the electrical conductivity was0.378×108S/m. The thermal conductivity along the thickness direction of the nine layers W-Cu functionally gradient material with the distribution index of1was310.1W/(m·K), and the electrical conductivity was0.381×108S/m. With the layers increase, the performance of the W-Cu FGM was improved. This work successfully developed the application aspect for the W-Cu functionally gradient material in the high-tech areas of electronic packaging, integrated circuit, defense industry and aerospace. |
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