| Polycrystalline Diamond(PCD) has good wear resistance, hardness, and impact resistance of hard alloy. In addition, the anisotropy is better than single crystal diamond’s,therefore, PCD is widely used in coal geological exploration, oil and gas exploration,mechanical processing and other fields,but the metal solvent cobalt from traditional PCD affects the PCD’s thermal stability seriously when the PCD is used in high temperature and high stress environment, therefore, studying the thermal stability of PCD has great theoretical significance and very good practical value.In order to improve the thermal stability of PCD, we used Co and W mixed powder as binder,and sintered PCD based on the research status of PCD thermal stability at home and abroad. On the one hand, these experiments hope metal catalyst cobalt can promote the formation of D-D type bonding among the diamonds under high temperature and high pressure conditions, and many D-D type bondings increase the strength and wear resistance of PCD, on the other hand, these trials hope W powdesr,Co powders and diamonds can be mixed and generate new solid solution, namely the WC-Co solid solution which can change the existence state of residual Co in PCD and improve the thermal stability of D-D bonding PCD.In addition, we also used lewis acid-Fe Cl3 as a reagent which can remove the internal residual metal phases from PCD and improve the thermal stability of the PCD. Experimental results show that:(1) The synthesis of PCD’s comprehensive performance is best when the synthesis pressure is 5.5 GPa, sintering temperature is 1450 ℃, and holding time is 4 minutes. Under this condition the diamond grains are bonded by D-D type, and the binder phase distributed uniformly among the diamond grains is a white main veins. Results of XRD show that, the phases in PCD sample are diamond phase, CoCx phase, WC phase and Co3C3 W phase, and the diffraction peaks of them are obvious, and there is no graphite diffraction peak in PCD sample.This shows that W powders, binder cobalt powder,and diamond powders generated new solid solution at the same time, namely the WC-Co phase,which changes the existence of residual Co in the PCD,and the heat resistance of increased.(2) Experimental results of leaching Cobalt show that:in the same temperature and time,the leaching depth of FeCl3 is at least 200 μm,and aqua regia is only about 130 μm,obviously,the effect of FeCl3 is better than aqua regia, in addition, FeCl3 cant’t remove the WC phase when it removed the cobalt in PCD, so WC phase is still contained when comparedwith aqua regia. The internal residual WC phases not only reduce the porosity in the PCD, and avoid the PCD’s intensity dropping to low due to completely remove the metal phase,but also overcome the shortcoming of bad thermal conductivity of PCD in some ways.The main reason for bad thermal conductivity is the air in the pores which are left after removing the metal phase Co. Obviously,the effect of FeCl3 is better than aqua regia.(3) Using single factor method analyses the factors affecting the effects of leaching cobalt in this paper, and the factors contain the temperature of reagent, leaching time, and adding amount of FeCl3. Results showed that: the best amount of the lewis acid is 10-15 g per100ml HCl,and the leaching depth of Cobalt increased with the increase of time removing Cobalt at the conditions of same time and reagent concentration of FeCl3,but the increase rate decreases slowly. The leaching depth was increasing with the temperature increasing of removing cobalt,but the scope of the temperature which used to remove the Cobalt in the experiment is only to 200℃ because of the performance of PTFE material.(4) The leaching depth has a great influence on the thermal stability of the PCD, and we found that the thermal stability of PCD was improved with the increase of leaching depth.When the Cobalt from the PCD was completely removed, the thermal stability of PCD in the protective atmosphere can be as high as 1200℃. In addition, PCD’s wear resistance has an significant increase after removing the residual cobalt in PCD. |
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