Effect of TiC addition on structure and properties of spark plasma sintered ZrB2-SiC-TiC ultrahigh temperature ceramic composite

In this study, the role of 5 and 10 vol% TiC addition on densification, microstructure, phase evolution and mechanical properties of ZrB2-20 vol% SiC composite has been investigated maintaining the volume fraction of SiC constant. It is found that TiC addition reduces the spark plasma sintering (SPS) temperature from 1900 degrees C to 1700 degrees C producing in a nearly fully dense (similar to 99.6% theoretical density) material and forming (Zr,Ti)B-2, a mixed compound of Zr and Ti. The lattice parameters (c and a) of (Zr,Ti)B-2 are found to lie between that of pure ZrB2 and TiB2 with its c/a ratio varying inversely with the extent of TiC addition. Phase and microstructure analyses reveal that, while the entire amount of TiC dissolves in ZrB2 in the ZrB2-20SiC-5TiC (vol%), undissolved TiC exist in the ZrB2-20SiC-10TiC (vol%) composite after sintering indicating that the solubility limit of TiC in sintered ZrB2 lies in between 5 and 10 vol%. The elastic modulus values of the TiC added composites predicted by model based calculation agree well with the values experimentally determined by nano-indentation suggesting that these sintered composites are highly dense and nearly defect-free. The calculated residual stress of the matrix phase (using appropriate analytical models) was found to be tensile in nature but decreasing with increase in TiC addition. The residual stress of the reinforcement phase manifests similar trend but is compressive in nature. Compared to the conventional ZrB2-20SiC (vol%) composite, TiC added composites record better bulk and nano-mechanical properties (e.g., hardness, indentation fracture toughness, flexural strength, stiffness and elastic modulus) at ambient temperature indicating that the ZrB2-SiC-TiC system is a more promising ultrahigh temperature ceramics than the base composite. (C) 2020 Elsevier B.V. All rights reserved.