Effect of sintering time on the microstructure and mechanical properties of (Ti,Ta)(C,N)-based cermets

Abstract

Complete solid-solution cermets based on titanium-tantalum carbonitride using a starting nominal composition with 80 wt.% of (Ti0.8Ta 0.2)(C0.5N0.5) and 20 wt.% of Co were performed by pressure-less sintering at 1550 C for different times (from 0 to 180 min) in an inert atmosphere. Chemical and phase analyses were conducted using X-ray diffraction (XRD), elemental analysis and energy dispersive X-ray spectrometry (EDX). The binder mean free path and the contiguity of the carbonitride particles were used to rationalise the microstructural effects of the mechanical behaviour. Mechanical characterisation included determining the Vickers hardness, the fracture toughness (conventional indentation microfractures, IM), the dynamic Young's modulus (ultrasonic technique), the biaxial strength (ball on three ball) and a detailed fractographic examination. Finally, the experimental findings were combined with a theoretical fracture mechanics analysis to estimate the critical processing flaw sizes. Binder-less carbonitride clusters, pores and coarse carbonitride grains were the main defects observed and were responsible for the fractures.