Achieving uniform microstructure and superior mechanical properties in ultrafine grained TiB2–TiSi2 composites using innovative multi stage spark plasma sintering

We report here the development of ultrafine grained TiB2–TiSi2 composites, adopting multi stage Spark Plasma Sintering (SPS) route. As opposed to the commonly used Single Stage SPS (SSS), the samples are held for a desired timescale at intermediate temperatures, before reaching the final sintering temperatures (1400 °C, 1450 °C and 1500 °C). In our innovative sintering schedule, the powder compacts are held for a comparatively lesser period at intermediate stages, in order to maintain a net holding time of 10 min altogether at different holding stages during Two Stage Sintering (TSS) and Multi Stage SPS (MSS). TSS and MSS allowed the achievement of relatively higher sinter densities with respect to SSS, when the final sintering temperature remains same. The development of finer microstructural scale, with the presence of tabular TiB2 grains (1–1.5 μm) and TiSi2 grains (200–300 nm) is exhibited by the ceramics sintered via TSS/MSS, with respect to those developed via SSS. MSS resulted in sintering reactions between the TiB2 and TiSi2 components, leading to the formation of reaction layer of Ti5Si3 of 50–100 nm thickness. However, such sintering reactions were relatively suppressed in the case of SSS. Finer scale microstructure allowed much higher hardness (∼27–31 GPa) to be achieved on sintering via MSS. Also, more uniform microstructural development in the case of MSS led to an improvement in three-point flexural strength (∼500 MPa), compared to composites developed via conventional hot pressing or SSS route using SPS.

Research highlights▶ First report demonstrating that close to theoretical densification can be achieved through the innovative Multi Stage Spark Plasma Sintering (MSS) route for electrically conducting ceramics like TiB2 at a temperature of 1500 °C, at which single stage Spark Plasma Sintering (SSS) enables much less densification. ▶ MSS allows development of finer scale microstructure in TiB2.-based ceramics. ▶ MSS route provides impetus to sintering reactions (during holding at intermediate temperatures), which was beneficial for the TiB2-TiSi2 composites. ▶ MSS enables materials with better hardness and strength properties than single stage Spark Plasma Sintering (SSS) and two stage Spark Plasma Sintering (TSS).