Flash sintering in metallic ceramics: finite element analysis of thermal runaway in tungsten carbide green bodies

Flash sintering is a powerful tool for the ultrarapid consolidation of green ceramic compacts, although its activation mechanisms in electrically conductive PTC (Positive Temperature Coefficient for resistivity) materials' is poorly understood. It was argued that a flash event could be initiated and sustained for a transitory period in certain PTC ceramics because of an initial negative dependence of the green material resistivity with temperature. The thermal runaway phenomenon and its activation conditions on binderless tungsten carbide (WC) green bodies are investigated in the present work by numerical simulations using finite element methods (FEM). The flash event is recreated and studied within the COMSOL Multiphysics software at the macroscale, i.e., considering the flash as an electrical power surge driven by an increasing sample's conductivity. During the flash, very high temperatures in the range of 1800–2000 °C can be reached in the WC green sample in a few seconds. The accurate numerical simulation of such event results in heating rates exceeding 1000 °C/s, a condition that theoretically brings a powder compact at temperatures high enough to accelerate and prioritize sintering densifying mechanisms over non-densifying ones. Therefore, the sample's regions where the maximum sintering temperature is reached more slowly because of thermal contacts with the electrodes remain highly porous at the end of the process

Role of surface carbon nanolayer on the activation of flash sintering in tungsten carbide

Flash sintering has been recently successfully activated also in conductive ceramics like tungsten carbide (WC). The present work aims at understanding how the WC particles surface chemistry can influence the electrical properties of the material and play a fundamental role in the flash sintering phenomenon. An electrical contact resistance (ECR) model was developed to understand the role of resistive surface layers on the electrical behaviour of WC green compacts under different applied pressures during the initial stages of the processes. It is established that the large resistivity measured on green compacts can be attributed to the sole presence of an ultrathin carbon layer on the particles' surface. A carbon nanolayer with a thickness of about 1–2 nm, as detected by XPS and TEM analyses, is found to be responsible for the high resistance reached at the particles' contact points while evolving during the flash event. Flash sintering conditions can be achieved during the electrical resistance flash sintering (ERFS) process in WC nanoparticles covered by such carbon layer and independently of the presence of W oxides