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

Abstract

Structure and mechanical characteristics of dense ceramic composites synthesised by reactive hot pressing of TiC-B 4 C powder mixtures at 1800-1950°C under 30 MPa were investigated by Xray 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 + B 4 C = 2TiB 2 + 3C has occurred with final products like TiB 2 -TiC-C, TiB 2 -C or TiB 2 -B 4 C-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 m 1/2 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. ARTICLE HISTORY

Reactive sintering of TiB2-SiC-CNT ceramics

“…This is in agreement with reaction (1). As shown in [36], the high temperature interaction between titanium and boron carbides starts with the nucleation of TiB2 on the TiC surface, which causes the first densification step (See Fig.1), similar to that presented in [19] for a 2TiC-B4C composition. Silicon binds some carbon emerging from the reacting phases, but the process stops as the solid reaction product, i.e.…”

Mechanisms of TiB2 and graphite nucleation during TiC–B4C high temperature interaction

“…Achieving maximum of the interface area at the given and minimal low soft phase volume is therefore one of the most important directions of HMC structure optimization. As was shown in [18] the optimal interface/volume ratio can increase material toughness at least two fold. Moreover, effective optimization can be done with a specific architecture of ultra-fine, supposedly plate-like, soft inclusions evenly distributed inside the hard matrix.…”

“…The process bases on quick (for 1 min at 1800 °C [19]) precursor disappearance accompanied with TiB2 and graphite solid phase synthesizing. During the process, titanium diboride forms the material hard matrix, while graphite can be observed as (i) 2 -4 μm sponge-like areas or (ii) submicron platelets [18]. As is shown in [20], other transitional metal carbides can also interact with B4C in similar way.…”

“…Theoretical and experimental research, presented in all the mentioned studies, have showed that the influence of low-modulus phase on the material characteristics is closely related to sizes, shapes, and the distribution of the soft inclusions within the solid matrix. As it can be concluded from numerous works [6,9,11,17,18], soft particles can not only "weaken" material but also improve its fracture resistance. This is achieved when cracks are arrested by blunting crack tips at low-Ehigh-E material interfaces.…”

“…One of the most valuable ways to create the mentioned ceramics is the reactive hot pressing of TiC-B4C precursors [18]. The process bases on quick (for 1 min at 1800 °C [19]) precursor disappearance accompanied with TiB2 and graphite solid phase synthesizing.…”