Tungsten Carbide-Cobalt with Nano-Crystalline Tungsten Carbide Platelets ---Synthesis, Processing, Sintering and Properties.

摘要

Tungsten Carbide --- Cobalt (WC-Co) is one of the most important ceramic-metal composites, which has been used in variety areas of different industries. Future applications of WC-Co in more areas of industries require higher hardness and fracture toughness. Current technology of synthesis, processing and sintering of WC-Co barely enhances the hardness and fracture toughness of WC-Co simultaneously (including nanostructured WC-Co). Within this project, a new approach of effectively controlling the structure of WC to lead to the simultaneous enhancement of hardness and fracture toughness has been investigated. The nano-WC platelet is proposed to be promising for the simultaneous enhancement. Several different aspects related to the formation of nano-WC platelets are studied. A new and effective method to produce nanostructured WC-Co powder is established in this study. This method is a combination of high energy milling and carbothermic reduction of the mixture of tungsten oxide (WO 3) and cobalt oxide (Co3O4). The condition to remove the free carbon without decarburization of WC has been achieved by thermodynamic analysis and coupled experiments. By the utilization of density functional theory simulations and Wulff construction, the equilibrium shape of WC crystals inside WC-Co is predicted to be bulky structure instead of platelet structure. The growth mechanisms for the WC in commercial coarse grained WC-Co under different heating conditions are observed. It is found that the heating temperature is a critical factor affecting the structure of WC inside WC-Co. Higher temperature will lead to the layer-by-layer structure formation due to higher thermodynamic driving force. The in situ formation of nano-WC platelet is achieved using different W containing sources. The formation mechanism has been studied. Bulk WC-Co samples with nano-WC platelets are obtained through sintering of WC-Co nano-powder at relatively low sintering temperatures. The mechanical properties of the nano-WC platelet containing samples show simultaneous enhancement of hardness and fracture toughness.