Fracture toughness in some hetero-modulus composite carbides: carbon inclusions and voids

O Popov,V Vishnyakov

doi:10.1080/17436753.2016.1208470

Abstract

ABSTRACTStructure and mechanical characteristics of dense ceramic composites synthesised by reactive hot pressing of TiC–B4C powder mixtures at 1800–1950°C under 30 MPa were investigated by X-ray diffraction, scanning electron microscopy and energy dispersive X-ray spectroscopy (SEM and EDX). The results show that during hot pressing solid-phase chemical reaction 2TiC + B4C = 2TiB2 + 3C has occurred with final products like TiB2–TiC–C, TiB2–C or TiB2–B4C–C hetero-modulus composite formation with around one micrometer size carbon precipitates. The fracture toughness depends on the amount of graphite precipitation and has a distinct maximum K1C = 10 MPa m1/2 at nearly 7 vol.-% of carbon precipitate. The fracture toughness behaviour is explained by the developed model of crack propagation. Within the model, it is shown that pores (voids) and low-modulus carbon inclusions blunt the cracks and can increase ceramic toughness in some cases.

Highlights

Carbide-based ceramics are known and used for more than 100 years as they possess excellent properties such as high melting temperature, high hardness and high electrical conductivity
Structure and mechanical characteristics of dense ceramic composites synthesised by reactive hot pressing of TiC–B4C powder mixtures at 1800–1950°C under 30 MPa were investigated by X-ray diffraction, scanning electron microscopy and energy dispersive X-ray spectroscopy (SEM and EDX)
The results show that during hot pressing solid-phase chemical reaction 2TiC + B4C = 2TiB2 + 3C has occurred with final products like TiB2–TiC–C, TiB2–C or TiB2–B4C–C heteromodulus composite formation with around one micrometer size carbon precipitates

Summary

Introduction

Carbide-based ceramics are known and used for more than 100 years as they possess excellent properties such as high melting temperature, high hardness and high electrical conductivity (see for instance review works edited by Matkovich and references within.). First in an intrinsic fracture toughness enhancement based on frontal process zone expansion which increases the fracture energy consumed in the damaged zone ahead of a crack tip This includes microcracking due to localised internal stresses around the second phase inclusions as well as so called transformation toughening.. The new way of brittle material toughening was developed for so called hetero-modulus ceramics (HMC) It is provided by combination of high Young’s modulus (400–700 GPa) matrix with low Young’s modulus (15–50 GPa) graphite or graphite-like boron nitride inclusions.. One of the early works has evaluated porous ceramic fracture toughness where spherical pores were shown as being able to improve matrix mechanical characteristics by blunting of the crack tip. The paper presents an investigation of the almost spherical low-modulus inclusion influence on fracture toughness of reactive hot-pressed TiC–TiB2–B4C–C HMC and demonstrates, with developed crack propagation model how ‘soft’ inclusions can toughen ceramics. Fracture toughness was estimated by measuring the crack lengths generated by Vickers indenter with a load of 147 N.28 The heat effect and adiabatic temperature were calculated using thermochemistry data from the NIST Chemistry WebBook.

Results and discussion

XRD spectra of sintered samples

The crack front is sagging between two spherical pores

The crack front is ‘cutting’ over-pore material

Conclusions