Dense HfC with High Fracture Toughness through Microstructure Design using a Novel SiCN-C Sintering System

Reference	Presenter	Authors (Institution)	Abstract
14-085	Na Ni	Ni, N.(Shanghai Jiao Tong University);	Hafnium carbide (HfC) typically exhibits high melting point, high hardness and modulus, as well as excellent chemical and physical stability at ultra-high temperatures. However, associated with the intrinsic rigid covalent bonds, HfC show low toughness and plasticity, which limits its application as high-temperature structural materials. Here we report a processing route of dense HfC with high toughness by incorporating a two-component sintering aid composed of amorphous SiCN and amorphous carbon in the spark plasma sintering process. The addition of the SiCN-C not only enhances the final density (up to a relative density of ~97%), but more importantly, results in an increase of the fracture toughness from 2.94 MPa m1/2 to 5.50 MPa m1/2 largely with only a 10 % reduction in the bending strength. The improvement has been attributed to the unique microstructure of the obtained samples, which exhibits several important characteristics: 1) homogeneously dispersed SiC and C secondary phases in the HfC matrix; 2) HfC grains enriched with different levels of Si, O and N; and 3) enrichment of C and O at grain boundaries. Based on the detailed microstructure and chemical analysis using Raman, XRD and multi-scale electron microscopy down to the atomic scale, the fundamental mechanisms for the observed enhancement on the density and fracture toughness are discussed, which shed light on rational microstructure design for optimum mechanical properties in high-temperature ceramics.
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