High pressure sintering of TiB2 ceramics at different temperatures

In this paper the effect of high-pressure sintering (HPS) temperature on the microstructure, physical and mechanical properties of TiB2 ceramics has been investigated. Initial TiB2 powder with the average particle size of 5 μm was sintered in a modifi ed high-pressure anvil-type apparatus under static pressure of 4 GPa in the temperature range of 1400–1800°C. It is shown that HPS allows preparing full-dense TiB2 ceramics with fi ne-grained structure. The density of samples rises with increasing the sintering temperature up to 1800°C while the maximal microhardness is observed on samples prepared in the temperature range of 1500–1600°C. XRD analysis has shown that this fact is connected with an increase of the level of internal stresses in these samples.


I. Introduction
Because of its properties (high melting point, high hardness, good corrosion resistance and excellent ele ctric conductivity) TiB 2 ceramics is suitable for many applications including cutting tools, wearresistant parts, armor materials and cathode materials for Hall-Heroult cells [1][2][3].Bulk TiB 2 ceramics without additives is usually produced by hot pressing [4,5].However, up to now, the use of TiB 2 ceramics is rather limited due to the diffi culties that exist in preparing a full-dense material.The high-pressure sintering processing (HPS) has been considered as an effective method for producing full-dense TiB 2 ceramics without any additives.The main features of HPS are a low sintering temperature and high sintering rate.
High pressure sintering kinetics of TiB 2 powder with a mean particle diameter of 60 μm was studied earlier [6].It was shown that the densifi cation process consists of two stages which proceed with different rates.On the fi rst stage, densifi cation rate is fast and the activation energy is low.It was assumed that the features of the fi rst stage are caused by fragmentation and creep.On the second stage, densifi cation is provided by the afterfl ow and substitutional diffusion in the metallic sublattice.
Structural transformations of titanium diboride under high pressures and temperatures were studied by Olejnik et al. [7].It was shown that the plastic deformation of TiB 2 under the pressure lower than 7.7 GPa and the temperature lower than 2200°C occurs by creep and intensive fragmentation of grains.The processes of primary and collective recrystallization play an important role during the formation of the grain structure of TiB 2 ceramics under high pressures and temperatures.
The aim of this study was to investigate the effect of the HPS temperature on microstructure and physical and mechanical properties of TiB 2 ceramics.The results about features of the sintering process, microstructure and mechanical properties are reported.

II. Experimental
TiB 2 powder produced by the "Donetsk Factory of Chemical Reagents" with the average particle size of 5 μm was used as a starting material in the present in-vestigation.Green pellets with a diameter of 11 mm and thickness of 3 mm were formed in a steel die under the pressure of 1 GPa.A modifi ed high pressure anvil-type apparatus [8] was used for high-pressure sintering.Such type of apparatus is simple to manufacture and widely used for both industrial synthesis of super-hard materi-als and research.Hexagonal BN covering was used to avoid reaction between contact material and TiB 2 pellets.The samples were subjected to a pressure of up to 4 GPa and temperature from 1400 to 1800°C during 2 min.Control of the sintering temperature and pressure was carried out using a computerized system [9].The density of the sintered samples was measured using Archimedes' method in carbon tetrachloride at room temperature.The microhardness measurements were carried out at a load of 1 N and the indentation time of 10 s on the samples polished with diamond powders to a smooth mirror surface.The X-ray diffraction (XRD) analysis was used to confi rm the phase composition and determine change of the (211) peak broadening versus sintering temperature.X-Ray diffractometer DRON-2 with CuK α radiation (step 0.1° and exposition in a point -20 sec) was used.Microstructural observations were carried out using LEICA DM IRM inverted microscope for materials research.Polishing of the samples was carried out using diamond pastes.

III. Results and Discussion
Fig. 1 displays the evolution of the microstructure of TiB 2 ceramics versus the high-pressure sintering temperature.Microstructure analysis has shown that the HPS process allows preparing TiB 2 ceramics with full-dense fi ne-grained structure.The mean grain size appeared to be less than 10 μm.Insignificant grain growth is observed with increasing the sintering temperature while the structural inhomogeneity is reduced.The beginning of the formation of the grain structure is observed in the temperature range of 1400-1500°C.At higher temperatures recrystallization processes take place.
Fig. 2 presents the evolution of the relative density and microhardness of TiB 2 ceramics versus the highpressure sintering temperature.It is shown that the relative density of TiB 2 samples rises with increasing the sintering temperature and reaches the maximal value of 99.3 % at 1700-1800°C.At the same time, the highest microhardness (about 33 GPa) was achieved in the temperature range 1500-1600°C.This fact can be explained using the data obtained by XRD analysis (Fig. 3).The evolution of the (211) peak broadening with raising the sintering temperature demonstrates that the increase in microhardness is accompanied by the lattice deformation, which is caused by an increase of the level of internal stresses in the samples.Then, the reduction in microhardness is related to the beginning of the recrystallization process and relaxation of internal stresses.

VI. Conclusions
The effect of the HPS temperature at the pressure of 4 GPa on microstructure and physical and mechanical properties of TiB 2 ceramics was investigated.It was shown that HPS permits preparing full-dense TiB 2 ceramics with fi ne-grained structure.Insignifi cant grain growth is observed with increasing the sintering temperature while the structural inhomogeneity is reduced.The beginning of the formation of grain structure is observed in the temperature range of 1400-1500°C.At higher temperatures recrystallization processes take place.The density of samples rises with increasing the sintering temperature up to 1800°C while the maximal microhardness is observed in samples sintered in the range 1500-1600°C.The XRD analysis has shown that this fact can be ascribed to an increase of the level of internal stresses in these samples.
Acknowledgements: This research was supported by the Belarussian State Programme "Nanomaterials and Nanotechnologies" under grant # 2.02.

Figure 2 .Figure 3 .
Figure 2. Relative density and microhardness of TiB 2 ceramics sintered under the pressure of 4 GPa versus the sintering temperature