Synthesis of titanium carbide and titanium diboride for metal processing and ceramics production

OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 23 No. 4 2021 Ta b l e 4 Results of sedimentation analysis of titanium diboride samples Sample Average size of particles / aggregates, μ m Standard deviation, μ m Sample Average size of particles / aggregates, μ m Standard deviation, μ m 2-1 7.4 2.41 2-3 11.1 2.22 2-2 8.0 2.33 2.4 11.2 2.28 They are mainly aggregated. The results of sedimentation analysis of titanium diboride samples are given in Table 4. The analysis was conducted according to the technique described in [22]. As follows from the obtained results, the sizes of aggregated particles tend to increase with the rise in temperature from 1,600 to 1,700 о С . All powder samples are polydispersive. The analysis of carbide and boron formation process presupposes that the pressure of carbon va- pors is signi fi cantly lower than the pressure of metal and boron oxide vapors at the synthesis tem- peratures. Hence, the pressure of carbon vapors reaches 3·10 –6 ; 4·10 –5 ; 2.6·10 –3 ; 8.5·10 –2 Pa at 1,630; 1,730; 1,930; 2,130 о С respectively [23]. The vapor pressure above titanium oxide at a temperature of 2,030 °C (almost corresponds to the optimum temperature for the synthesis of titanium carbide) is 1 Pa, and at a temperature of 1,730 о С (almost corresponds to the optimum temperature for the synthesis of titanium diboride) is 0.01 Pa. The vapor contains Ti + and TiO + ions, TiO and TiO 2 molecules, and Ti atoms above this oxide [24]. It is known [25] that pressure of boron vapor above boron-carbon carbide system at 1730 о С (that nearly corresponds to optimum temperature of titanium diboride synthesis) is equal to 1 Pa. Furthermore, pressure of other gaseous components is two orders ( ВС 2 ) and three or- ders ( В 2 С ) lower. Therefore, pressure of oxides/boron vapors signi fi cantly (by several orders) exceeds carbon vapor pressure at optimum temperatures under the synthesis of these refractory compounds. Consequently, with a high degree of probability, it can be argued that these processes are carried out by transferring vapors of higher and lower oxides to the surface of solid carbon (synthesis of titanium carbide) and transferring vapors of higher and lower oxides along with boron vapor to the surface of solid carbon (synthesis of titanium diboride). These are conventional adsorption processes. It appears to be an indirect proof of the carbide formation completion. An indirect proof of this is the relatively short synthesis times of the considered refractory compounds. Diffusion processes start on completion of chemical reactions. How- ever, diffusion processes can really take place at the imme- diate contact between hard reagents. Undoubtedly, a posi- tive role in these relatively fast processes is played by the developed surface of the NFC . It clearly reduces the time of diffusion processes, which are completed by the complete conversion of reagents into target compounds. To produce ceramics from boron carbide with the modifying additive from titanium diboride, the charge can be prepared by the reaction (2) with excess boron carbide. Complete transfor- mation of reagents into composite powder B 4 C-TiB 2 occurs in the temperature range 1,560…2,200 о С [26]. Being prepared by the reaction (2), the charge was compacted. SEM image of a sintered sample section is given in Fig. 5. Light inclusions (TiB 2 particles) are evenly distributed in the boron carbide matrix. The inclusions could be the size up to several tens of microns. Pores are absent. Fig. 5 SEM image of the surface of a sample of ceramic B 4 C-TiB 2