This literature survey provides an introduction to functionally graded materials, WC-Co composites and estimations of residual stresses in such material systems.
Functionally graded materials feature gradual transitions in microstructure and/or composition. Gradual transitions are preferred over abrupt transitions which introduce local stress concentrations. The various manufacturing methods for functionally graded materials are divided into two classes. Constructive processes (e.g., powder processes) use an appropriate distribution of constituents, often as a precursor to the component, to create the gradient. Transport based processes (e.g., infiltration) rely on natural transport phenomena such as fluid flow, diffusion of atomic species or conduction of heat.
Liquid phase sintering of mixed and pressed powder is used to produce WC-Co functionally graded composites. The mechanical properties of WC-Cocomposites and their constituent phases are reviewed. A wide range of overall composite behavior can be achieved when the Co content is varied. For example, the hardness is reported to be 1650 HV for WC with 6 wt.% Co and 780 HV for WC with 25 wt.% Co. The corresponding fracture toughness values are 8.5 and 14.5 MPa m1/2, respectively. These values suggest, that WC contributes to the hardness of the composite and Co contributes to the toughness. Therefore, the possibility of tailoring material performance by varying composition in the component arises.
During cooling from the sintering temperature, thermal residual stresses develop because of the differences between the WC and Co thermal expansion coefficients. It is possible to use X-ray diffraction methods to measure residual stresses in composites. Results from measurements, and finite element (FE) analysis of the residual stresses in WC-Co composites are also reviewed.