Preparation Of Coated Powders And Properties Of Ce-ZrO₂/CePO₄ Machinable Ceramics

In this article, two different routes were used to synthesize CePO₄, research on CePO₄ coated ZrO2 was made by the heterogeneous nucleation method and the sol-gel method, and the effect of Ce3+concentration, pH values, bathing temperatures and calcining temperatures on the coating results were studied. The morphology and phase form of reaction products and coating results were investigated by TEM, SEM and XRD. The powers were first dry pressed in mould under pressure of 190MPa, and then the green bodies were sintered separately at 1450℃,1500℃and 1550℃. At last, the mechanical and machinable properties were measured. The experimental results showed that the morphology of preliminary products was needle-like synthesized by H₃PO₄ and CeCl₃ and colloidal by Na3PO4 and CeCl₃ at room temperature. The colloidal product was the precursor of CePO₄, which existed mainly in amorphous state. After calcinations at 1000℃, the product synthesized by two routes was monoclinic CePO₄, which was confirmed by the XRD results. Using heterogeneous nucleation method, the hexagonal CePO₄·nH2O fibers could be coated on ZrO2 powders by controlling H₃PO₄ concentration when Ce3+ was 0.001mol/l. After calcinations at 600℃, the core-shell structure was formed. Using sol-gel method, it could be obtained coating or semi-coating structure. After calcination at 600℃, the amorphous CePO₄ transformed to monoclinic phase and the coating structure was formed .By the comparison of the mechanical and machinable properties before and after coated, we could conclude that the Ce-ZrO2/ CePO₄ ceramic after coated has much better mechanical and machinable properties except for higher relative density. The material has good machinability as well as maintaining outstanding mechanical properties. By analyzing the microstructure and mechanical property of Ce-ZrO2/ CePO4 ceramic, we studied the machinable mechanism of materials. From the microstructure of fracture surface we could find that the ZrO2 coated by CePO4 cracked mainly along grain boundary under outside stress as well as layered CePO4 cracked inside. With the content of CePO4 increased,more layered crack appeared. Machinable mechanism for Ce-ZrO2/ CePO4 ceramic mainly was brought about from two aspects: the layered crack of CePO4 and the weak combination between CePO4 and ZrO2 crystal grain. Because of these reasons, Ce-ZrO2/ CePO4 ceramic could be easily machined using traditional method.