Spark Plasma Sintering : A Promising New Technique and its Mechanism

Abstract

The sintering of ceramic powder compacts is usually carried out by raising the temperature to a point below the melting point of the main component in the compact. Mass transportation, such as diffusion and/or vapor transportation, can take place during sintering. Though the time at the sintering temperature is usually around 1 h, the time needed to heat up and to cool down the sintering furnace is nevertheless long. Sintering is thus a time-consuming process. Furthermore, the sintering profile has to be controlled carefully in order to achieve high density. An external pressure can enhance the densification rate of a ceramic powder compact. With the help of the external pressure, the maximum temperature needed to achieve full density can be slightly lower than that needed for pressureless sintering. However, the time to heat up and cool down the hot-press furnace is still considerable. The use of an electric field or current to influence material processing has been attempted for many years. The presence of an electric field can affect the defect migration, crystal growth, evaporation, phase transformation and diffusion, etc. Recent studies demonstrated that applying both external pressure and an electric field simultaneously could significantly enhance the densification of metallic or ceramic powder compacts. Coarsening is not affected by the pressure. The previous studies also showed that a pulsed electric current is more beneficial than a steady one in terms of densification enhancement. Spark plasma sintering (SPS), also known as pulse electric current sintering, is a novel processing technology for materials synthesis and processing. Figure 8.1 is a schematic sketch of an SPS facility. The SPS has the capability to densify ceramics, metals and composites at a relatively low temperature in a very short time. However, the sintering mechanism for SPS is not well established yet.