Carbon influence on fracture toughness of niobium carbides

Abstract

This paper explores the fracture behavior of niobium carbides of varying compositions between NbC1.0 and NbC0.5. The surface crack in flexure (SCF) method was used to evaluate the fracture toughness as a function of carbon concentration. Additionally, hardness measurements were conducted with a Knoop indenter, and X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to identify the phase content and microstructures. As the carbon content decreased, the hardness increased from 8 GPa for NbC1.0 to 12 GPa for NbC0.5 and the fracture toughness decreased from 2.5 MPa m to 0.44 MPa m. Notably, the NbC0.67 sample exhibited a secondary precipitate lath-like microstructure with the laths indexed to β-Nb2C and a KIC near 2 MPa m. Though similar lath like structures in tantalum carbides have been reported to yield a KIC of approximately 15 MPa m, the laths in these two materials have fundamentally different structures where bonding in the former is comprised of β-Nb2C and the latter of ζ-Ta4C3-x. This results in the observed different fracture properties, which can be explained through concepts of microstructural toughening.