| Dielectric Barrier Discharge Plasma (DBDP) assisted ball milling is a high-energy, efficient external field assisted milling method,which is based on the binding of high-energy ball milling and DBDP technology. It shows a unique advantage in fabrication of nano-composite WC-Co powders. In this dissertation, the history and fabrication process of nano-meter WC-Co hardmetal were reviewed, as well as the history and machanism of DBDP. Then the reasons and effecting factors of the electrode bar breakdown were analyzed. Some methods such as interference fitting, double insulation and sleeve isolation were used to increasing the service life of electrode bars. After that we focused on the effect of ball–powder-ratio and mill-time on the DBDP milling of W-C-10Co mixed powders . Considering the lower of carbonization temperature, the formation ofη-phase and the morphology of WC , we obtained the optimal DBDP milling process and the optimal carbonization process. This article further analyzed the phase transition during DBDP milling and carbonizing, as well as the role of pute plasma treating on W-C-10Co powders. Finally the mechanism about fabrication of nano-composite WC-10Co powders by DBDP milling was discussed.It has been found that the breakdown of electrodes bars is mainly electrochemical breakdown during DBDP milling. The interference fitting of core and polytetrafluoroethylene (PTFE) coat can eliminate the gap of electrode bars, and avoid the breakdown of electrode bars caused by corona discharge in the gap; Double insulation—inside layer is oxidation of Al2O3 and the outer layer is PTFE , can increase the heat capacity of electrode bars, but the loose and porous surface of oxidation layer are still susceptible to cause partial discharge; Sleeve isolation device can effectively isolate electrode bars from balls, eliminate the damage and mechanical stress of balls on the electrode bars, and improve their service life.DBDP milling W-C-10Co powders can quickly reduce the grain size of W. After milling for 0.5h, W grain size reduces to about 43nm. After milling for 1h, the reducing of grain size is not change significantly with increasing milling time and ball powder ratio. After milling for 1h, powder particles deform to lamellars. With increasing milling time, lamellar powder follows the cycle of superposition-break - then superimposed - and break again. W and Co are more homogeneously distributed. With increasing milling time and ball powder ratio, the carbon lack of powder is more serious during heating, but the temperature of carbonation reaction reduces. Heating at 1000℃for 60min in low pressure sintering furnace, noη-phase existed in nano-composite WC-10Co powders with different ball powder ratio.With decreasing temperature and holding time, the moreη-phases appear. Among them, the nano-composite WC-10Co powders with the ball powder ratio of 60:1 and milling time of 3h have smaller particle size. The morphology of WC is lamellar and fine granular. The distribution of Co-phase is homogeneous.Therefore, optimal carbonization process-"heating at 1000℃, holding for 1h" and optimal DBDP milling process- "ball powder ratio is 60:1, milling time is 3h" are obtained.Allotropic transformation fromα-Co with fcc structure toε-Co with hcp structure occurs during DBDP milling W-C-10Co mixed powders for 0.5h (BPR=50:1). After DBDP milling W-C-10Co powders for 3h (BPR=50:1) , the range of carbonization temperature is about 831 ~ 875℃; Heating in DSC, Co6W6C(η-phase) is formed because of the mobile of atmosphere, while heating in low pressure sintering furnace, nano-composite WC-10Co powders withoutη-phase can be prepared. The morphology of WC is lamellar, Co-phase is homogeneously distributed. Pute DBDP treating W-C-10Co powders for 3h (without milling), allotropic transformation fromα-Co toε-Co also can occurs, the cleavage of W particles and graphites can be observed ,and W , C formed a unique powder-sphere becuse of partial melting. |
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