# SHS Produced TiB\({}_{2}\)-Si Powders for Selective Laser Melting of Ceramic-Based Composite
## 3 Results and Discussion  
### Combustion Dynthesis in the Ti-B-Si System  
The SHS process is a self-sustained combustion where the heat is generated by the direct synthesis reaction to support combustion wave propagation. The combustion synthesis was performed based on thermodynamic calculations taking into consideration a temperature of TiB\({}_{2}\)/Si formation (\(\sim\)1430 \({}^{\circ}\)C) [15]. The combustion limit was achieved for the mixture with n (Si) = 2.75 regardless to the ambient gas pressure (0.1-2 MPa) and the relative density of the initial specimen (30-60%). Combustion temperature was 100-500 \({}^{\circ}\)C lower than the adiabatic temperature, which is conditioned by large heat losses through radiation and the geometrical set-up of the apparatus. According to the thermodynamic model of the Ti-B-Si system, the formation of TiB\({}_{2}\) is a spontaneous process due to negative Gibbs free energy (\(\Delta G^{0}\)T) at the calculated temperature interval (1500-2000 K). At temperatures exceeding 1700 K, the evolution of titanium monoboride (TiB) and titanium silicides (TiSi) may be considered as the competitive processes to TiB\({}_{2}\) formation. Therefore, the combustion synthesis was performed in the Ti-2B-\(\pi\)Si system at a temperature around 1700 K to avoid by-products and to provide formation of TiB\({}_{2}\)-Si fine particles. An amount of silicon was varied from 1 up to 3 moles to ensure a sufficient quantity of liquid silicon fuse to cover titanium diboride particulates. Parameters of the combustion process in the Ti-B-\(\pi\)Si mixtures are highly affected by the amount of silicon. At a silicon content of n = 2.75 moles, Si occupies \(\sim\)63 vol.% of the Ti-B-Si mixture. Considering the packing geometry of Ti, B, Si particles, a heat transfer at the Ti-B interface mostly occurs through silicon particles during the combustion reaction. Low thermal conductivity of boron (0. 26 W-cm\({}^{-1}\cdot\)K\({}^{-1}\) at 300 K) and silicon (1.42 W-cm\({}^{-1}\cdot\)K\({}^{-1}\) at 300 K and 0.287 W-cm\({}^{-1}\cdot\)K\({}^{-1}\) at 1400 K) may result in an inhomogeneous heat transfer and consequent interruption of the CS reaction. Both combustion temperature and velocity decrease with an increase in Si amount in the initial mixture. As illustrated in Figure 1a, a combustion limit is achieved below the melting point of silicon. The combustion wave propagation in the Ti-B-Si system is also governed by the size of the specimen. Pilot batches of \(30\times 40\times 120\) mm\({}^{3}\) provided a complete conversion of reagents attributed to decreased radial heat losses; therefore, the combustion temperature was equal to the adiabatic temperature. At a lower than stoichiometric amount of the boron source, the formation of titanium silicides (TiSi, Ti\({}_{5}\)Si\({}_{3}\)) and titanium monoboride (TiB) becomes favorable. In terms of preparation of the TiB\({}_{2}\)-Si composite powder, where nano- or ultra-fine scaled TiB\({}_{2}\) particles are held together by molten silicon, the stoichiometric amounts of titanium and boron are relevant at the silicon amount of 2 moles at the combustion temperature over the melting point of silicon. The interaction of the solid Ti particles with the melt of Si or eutectics in the combustion wave was reported in the temperature interval 1330 \({}^{\circ}\)C \(<\) T \(<\) 1670 \({}^{\circ}\)C [23; 24] accompanied by the formation of TiSi\({}_{2}\) and TiSi. In these works, it was  
Figure 1: Combustion law for Ti + 2B + \(\pi\)Si system (**a**), and combustion thermograms of Ti + 2B + 2 Si mixture (**b**) at P\({}_{\rm(Argon)}\) = 1 MPa. shown that the interaction between the reagents at a temperature of 1330 \({}^{\circ}\)C, which is below all reactant melting points, occurs through the eutectic reaction, L = TiSi\({}_{2}\) + Si (L-liquid). At higher temperatures, the fusion of both silicon and titanium appears to proceed almost simultaneously and results in the formation of Ti\({}_{5}\)Si\({}_{3}\).