Characterization Of TiB₂and TiB₂-TiC Synthesized At High Pressure And High Temperature

Transition-metal compounds, such as titanium diboride (TiB2) and other composite materials composed of transition-metal compounds have been applied in many fields for the excellent properties including high temperature resistance, wear resistance, high hardness, good chemical inertness, conductivity and thermal conductivity, etc. In general, the high temperature condition is necessary to prepare TiB2-TiC and TiB2with the solid-phase method. Furthermore, it is difficult to prepare bulk TiB2and TiC with high density which is important to applying TiB2. In order to decrease the sintering temperature, some additives, such as Al, are usually added in the raw materials. But it is difficult to remove these additives in the as-prepared samples.As a kind of extreme conditions, high temperature and high pressure (HPHT) play an important role in the field of materials research. Some new phenomena including new structure or properties in materials which are induced by high pressure effect may be expected. In this work, TiB2and TiB2-TiC composites with enhanced properties have been prepared at HPHT.The raw materials were powdery titanium (200mesh,99.99%in purity) and amorphous boron powder (99.99%). Powder mixtures which contained Ti:B atomic ratios of1:2were mixed in an agate mortar for more than1hour. And then the mixed powders were cold-pressed into cylindrical samples with6mm in-diameter and2mm in-height. At last, the samples were carried out in cubic anvil HPHT apparatus (SPD-6×600) at temperature of1700K-2000K, and pressure of5.2GPa for a holding time of15～60min. The as-synthesized samples are TiB2which is characterized by X-ray diffraction. The grain size of TiB2is uniformity and about500nm characterized by scanning electronic microscopy (SEM). It is worth noting that the relative density of TiB2is more than99%of the theoretic density and TiB2is a superhard material under the loading press (F=0.49N). The Vickers hardness values are40and28.9GPa corresponding to at the loading press0.49and4.9N, respectively. The value of resistivity for TiB2,10-8Ω m, which is measured by the Vanderbilt method, is close to the value of TiB2single crystal samples.TiB2-TiC compounds with the grain size of about2.5μm have been prepared with Ti and B4C powder at HPHT. The value of the relative density for TiB2-TiC compound is more than98%of the theoretic value. The Vickers hardness of the as-synthesized compounds are36and25GPa at loading press0.49N and4.9N, respectively.The results of enhanced properties in TiB2and TiB2-TiC compounds indicate that high pressure and high temperature method is a useful tool to prepare high-melting compounds with enhanced properties.