| Cemented tungsten carbide is a two-phase composite with a hard, brittle WC phase embedded in the tough, ductile Co metal matrix. Through decades of research and development, the processing, microstructure, and mechanical properties have been optimized; correlations between microstructure parameters and properties (hardness, transverse rupture strength, and fracture toughness) have been extensively studied. However, the performance of the product in the real application is such a complex function of material properties that none of current characterized material property correlates well with the behavior. Therefore a complete mechanistic basis for their superior mechanical performance is yet to be developed.;The Hertzian indentation technique was utilized to study the mechanical behavior of both homogeneous and functionally graded WC-Co materials. Under indentation load, the WC-Co material undergoes initial elasticity, quasi-plasticity and final fracture. Indentation stress-strain curve demonstrates that the material exhibits quasi-plasticity and apparent strain hardening behavior during the quasi-plastic deformation process; the mechanism of the quasi-plastic deformation of WC-Co composites involves the deformation of the cobalt binder phase, the slip within the WC crystals, and the formation of microcracks. The formation of microcrack strongly depends on the microstructure of the material, with the fixed value of hardness and fracture toughness, large WC grains are prone to microcracking. When the indentation load exceeds a certain value, a ring crack or radial crack forms. The propensity of the WC-Co material to either form ring cracks or undergo quasi-plastic deformation can be evaluated based on its brittleness index. The brittleness index is a function of not only the material properties but also the radius of the indenter with which the indentation load is applied.;The mechanical behavior of bilayer WC-Co was also explored using the Hertzian indentation method. Indentation stress-strain curve and damage accumulation below the contact circle reveal that bilayer WC-Co has the merits of wear resistance and toughness. However, the study of dependence of critical load for ring crack on the top layer thickness shows that more research work needs to be done before it is ready to use the Hertzian indentation technique to characterize the bilayer WC-Co material. |
