Abstract

In situ toughened TiB₂–TiC_x composites were fabricated using reaction synthesis of B₄C and Ti powders at high temperatures. The resulting materials possessed very high relative densities and well developed TiB₂ plate-like grains, leading to a rather high fracture toughness, up to 12.2 MPa⋅m^1/2. The microstructure was examined by means of XRD, SEM, TEM and EDAX. The reaction products mainly consisted of TiB₂ and TiC_x. No other phases, e.g. Ti₃B₄, TiB, Ti₂B₅ and free Ti, were observed regardless of whether the starting composition was Ti:B₄C=3:1 or 4.8:1, and whether the sintering temperature was 1700 or 1800°C. The microstructural morphology is characterised by TiB₂ plate-like grains distributed uniformly in the TiC_x matrix. Some inclusions and defects were found in TiB₂ grains. The very high reaction temperature was believed to be responsible for the formation of plate-like grains, which, in turn, is responsible for the much improved mechanical properties. The main toughening mechanisms were likely to be crack deflection, platelet pull-out and the micro-fracture of TiB₂ grains.

Abstract

TiC/TiB₂ ceramic–matrix composites were produced by self-propagating high temperature combustion syntheses combined with pseudo hot isostatic pressing using Ti, B₄C and carbon powder. The results of XRD and SEM analysis showed that the final product was pure and no unreacted B₄C, Ti or carbon appeared. The microstructure of synthesized products consisted of the clubbed TiB₂ grains and equiaxed or irregular TiC grains. The optimal ratio with Ti:B₄C=3:1 of the reactant powder was determined by experiments. The maximum relative density, hardness value, compression strength and bending strength was attained for TiC–2TiB₂ ceramic when the delay time is 5 s, keeping pressure time is 5 s and the pressure is 160 MPa, which is higher than TiC–TiB₂ and 2TiC–TiB₂ ceramics. The introduction of more clubbed TiB₂ grains into TiC/TiB₂ ceramics resulted in a drastic increase in the fracture toughness for TiC–2TiB₂ synthesized compacts.

Abstract

In this work, the fabrication of bulk TiC_0.7/TiB₂ nanostructured composites through metastable transformation processing is investigated by taking advantages of two non-conventional powder metallurgy methods. First, the highly metastable TiC_0.7/TiB₂ agglomerated powders are synthesized by the so-called self-propagating high-temperature synthesis (SHS), followed by rapid quenching. Then, the spark plasma sintering (SPS) method is adopted to consolidate the SHSed powders.

A bulk ceramic composite with nanocrystalline microstructure characterized by a high-relative density is then obtained. Dwell temperature of 1400 °C, heating time of 3 min, and total processing time equal to 5 min, while applying a mechanical pressure of 20 MPa, are found to be the optimal SPS experimental conditions in order to obtain near-fully densified samples.

The obtained TiC_0.7/TiB₂ samples exhibit hardness HV5 as high as 24 GPa, modulus of elasticity of about 400 GPa, fracture toughness of about 5.6 MPa m^1/2, and a compressive strength of about 2.9 GPa. A very low-wear rate (W_v = 3.8 × 10⁻⁶ mm³/(N m)) and a good thermal shock resistance (ΔT_c = 250 °C) are also displayed. In addition, a high-abrasive wear factor (AWF) equal to 1.84 is evaluated on the basis of the achieved mechanical properties. These results make the obtained TiC_0.7/TiB₂ composite suitable for wear resistant parts as well as cutting tool materials.