Spark plasma sintering of novel ZrB2–SiC–TiSi2 composites with better mechanical properties

We report here the development of ultrafine grained ZrB2–SiC composites using TiSi2 as the sintering aid and spark plasma sintering (SPS) as the processing technique. It was observed that the presence of TiSi2 improved the sinterability of the composites, such that near theoretical densification (99.9%) could be achieved for ZrB2–18 wt.% SiC–5 wt.% TiSi2 composites after SPS at 1600 °C for 10 min at 50 MPa. Use of innovative multi stage sintering (MSS) route, which involved holding the samples at lower (intermediate) temperatures for some time before holding at the final temperature, while keeping the net holding time to 10 min, allowed attainment of full densification of ZrB2–18 wt.% SiC–2.5 wt.% TiSi2 at a still lower final temperature of 1500 °C at 30 MPa. TEM observations, which revealed the presence of anisotropic ZrB2 grains with faceted grain boundaries and TiSi2 at the intergranular regions, suggested the occurrence of liquid phase sintering in the presence of TiSi2. No additional phase was detected in XRD as well as TEM, which confirmed the absence of any sintering reaction. The as developed composites possessed an excellent combination of Vickers hardness and indentation toughness, both of which increased with increase in TiSi2 content, such that the ZrB2–18 wt.% SiC–5 wt.% TiSi2 (SPS processed at 1600 °C) possessed hardness of ∼20 GPa and indentation toughness of ∼5 MPa m1/2. The use of MSS SPS at 1500 °C for ZrB2–18 wt.% SiC–2.5 wt.% TiSi2 composite resulted in improvement in hardness of up to ∼27 GPa and attainment of high flexural strength of ∼455 MPa.