The densification kinetics and structure of TiB2-TiC-C, TiB2-C and TiB2-B4C-C hetero-modulus ceramics produced via reaction hot-pressing of B4C and TiС precursors are investigated. The reaction begins at 1100°C with boron carbide decomposition and progresses in two main stages which can be predominantly determined by the boron atoms to TiC grains diffusion mechanisms. The solid phase grain boundary diffusion starts at 1100°C and effective gas phase transport finalises the reaction at temperatures above 1400°C. Two distinctive waves of the charge consolidation allow densifying investigated refractory materials at 1900°C and 30MPa during 16 minutes. The reaction is shown to define the features of the composite structure: submicron TiB2 particles and faceted voids in B4C matrix, flake-like graphite and TiB2 inclusions in TiC matrix. High concentration of carbon atoms (~ 10 at.%) in synthesized diboride titanium grains have been observed. IntroductionAnoxic ceramics possess excellent properties such as high melting points, hardness and Young's modulus [1] which make it possible to use them as structural materials under extreme conditions of ultrahigh temperature [2,3]. The main issue for all materials based on covalent bonding structure is their brittleness which results in low toughness and low thermal stress resistance [4]. This also adds to poor machinability [5] and limit practical use of such materials. Hasselman with coworkers managed to improve thermal shock durability of alumina [6] and zirconium carbide [7] by adding h-BN (hexagonal boron nitride) and experimentally confirmed [4] that the thermal stress resistance of ceramics with high values of Young's modulus can be increased considerably by the adding "soft" dispersed phase particles. Similarly, low-E (where E is Young's modulus) inclusions of graphite or graphite-like boron nitride can also improve machinability and oxidation stability of such superhard materials as silicon nitride [5,8], boron carbide [9], transitional metals carbides [10 -12]. The term "Hetero-Modulus Ceramics" (HMC) is slowly established in the ceramic community. It is worth to note that mentioned above HM gained machinability and thermal shock resistance, inevitably lost hardness but their high temperature characteristics have not degraded as both boron nitride and graphite melting points are higher than 3000°C [13].It is possible to anticipate that the toughness and strength would decrease with "soft" phase fraction inclusion which indeed has been shown experimentally for h-BN-containing HMC with various matrixes such as B4C [9], ZrC [7], Al2O3 [6] and Si3N4 [5]. Nevertheless, there are some data in the similar composites showing strength [8] or toughness [14] rise at 5 -10 vol. % of soft phase. The only substantial difference of the latter set of ceramics was that the nanoscale sizes of boron nitride inclusions they contained. On this basis it is possible to predict that one, by the providing proper composite dispersion, can improve both machinability and toughness.Ceramic sinterin...

Reactive hot pressing of TiC-B4C precursors was undertaken at 1800 ℃ to produce TiB2 with carbon inclusions. Atomic mechanisms of titanium diboride nucleation, as well as spongelike carbon inclusions and submicron platelets of graphite precipitation has been investigated. Precursor grain size, green body composition and synthesis time were varied to analyse phases transformation. Boron from B4C grain sublimation is shown to result in carbon-based foam formation. Ab-initio calculations confirm that the boron atoms accumulation on (111) TiC plains leads to tensile stress. The developed stress cleaves TiC grains and enhances further reaction. Most of carbon expelled from TiC during its transformation into TiB2 forms graphite platelets.

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