Liquid phase sintering of functionally graded tungsten carbide-cobalt composites.

摘要

Functionally graded WC-Co composites offer solutions to the tradeoff between wear resistance and fracture toughness in WC-Co composites. Although liquid phase sintering is the most economical and viable method for producing WC-Co parts, the main challenge in fabricating a functionally graded WC-Co composite using liquid phase sintering is that the cobalt content homogenizes across the layers resulting in a sintered part with a uniform cobalt content. The homogenization process has been attributed to liquid phase migration. Liquid phase migration (LPM) is an interfacial-energy-driven flow phenomenon that takes place in a solid-liquid two-phase system. Liquid phase migration is known to be a function of solid particle size and liquid volume fraction. Understanding and controlling LPM is crucial to liquid phase sintering of functionally graded materials. This thesis establishes a dependence of WC particle size and liquid volume fraction of the WC-Co composite on the liquid migration pressure. The unique dependence of liquid migration pressure on solid particle size and liquid volume fraction has been exploited to create an optimization chart that provides guidelines for suitable selection of WC grain size and cobalt content differences for the design of functionally graded WC-Co composites.; Carbon is a critical factor that can be used to produce a gradient of cobalt content in WC-Co composites. A cobalt gradient is established after sintering by introducing an initial gradient in carbon within a WC-Co bi layer prior to sintering. The graded microstructure is a function of sintering time and temperature as well as other factors including the volume fraction of Co3W3C (eta) phase, liquid migration pressure and carbon content. A study of the kinetics of the process is therefore necessary to fully understand and control the process and achieve the desired graded microstructure. This thesis describes a kinetic model that can be used to optimize the process. The kinetic model explains the effect of important parameters such as the volume fraction of eta phase, carbon content, and liquid migration pressure on the process kinetics.