## 3 Results and Discussion
Samples without the additive had a fracture toughness between 3.4 MPa\(\cdot\)m\({}^{1/2}\) and 3.7 MPa\(\cdot\)m\({}^{1/2}\) for TC3030 and \(\alpha\)-\(\beta\)0T, while with the addition of TiN, fracture toughness increased from 4.1 MPa\(\cdot\)m\({}^{1/2}\) to 4.7 MPa\(\cdot\)m\({}^{1/2}\) for \(\alpha\)-\(\beta\)10T and \(\alpha\)-\(\beta\)20T, respectively. Moreover, TiN particles had a higher (\(\alpha_{0\text{-}1000}\)\({}^{\circ}\)C = 9.4 \(\times\) 10\({}^{-6}\)/\({}^{\circ}\)C) [42] coefficient of thermal expansion than SiAlON (\(\alpha_{0\text{-}1000}\)\({}^{\circ}\)C = 3.1 \(\times\) 10\({}^{-6}\)/\({}^{\circ}\)C). This difference caused compressive stresses in the matrix that could also increase fracture toughness due to the crack deflection mechanism (Figure 3C). Results of strength parameters are presented in Table 3. As can be seen, the addition of TiN to the ceramic matrix reduced the strength of \(\alpha\)/\(\beta\)-SIAlON-based composites. We assume that this may have been due to the shrinkage difference between TiN particles and ceramic matrix during cooling, which can lead to tensile residual stresses and consequently to changes in strength values. The fracture surfaces of all studied composites are presented in Figure 5. In all cases, brittle fractures were observed. Some pits on the surface of the samples, especially the \(\alpha\)-\(\beta\)0T composite, were found because some of the grains fell. On the other hand, the occurrence of microcracks or pores was not observed. Electrical resistivity values of the nanocomposites as a function of TiN loading are presented in Table 3. Simultaneously, together with the addition of TiN, the resistivity value of SPS composites dropped to 10.2 \(\times\) 10\({}^{1}\)\(\Omega\cdot\)m and 1.6 \(\times\) 10\({}^{-4}\)\(\Omega\cdot\)m for \(\alpha\)-\(\beta\)10T and \(\alpha\)-\(\beta\)20T, respectively. This number is lower than the EDM machinable limit of 1-3 \(\Omega\cdot\)m. Consequently, this makes the material suitable for nontraditional machining processes [9; 11; 17]. Figure 6 shows the SEM cross-sectional view after WEDM for \(\alpha\)-\(\beta\)20T ceramic. Multi-component ceramic compositions with a high melting point, in which there is a thermal expansion mismatch between constituents, are more susceptible to spalling due to increased stress concentrations at the matrix-particle interphase. Thus, the mechanism of erosion is thermal spalling, in contrast to metals, where the mechanism of material removal is melting and evaporation. It should be pointed out that the width of the heat-affected zone was 120-125 \(\upmu\)m, although some cracks were observed in the parent material (Figure 6B). Clearly, the presence of this type of damage in the material directly affects the flexural strength of composites. Topography of the machined surface of \(\alpha\)-\(\beta 20\)T ceramic is presented in Figure 7.  
We assume that such porous and foamy structure was formed due to expansion of nitrogen (N\({}_{2}\)) and hydrogen (H\({}_{2}\)) gases from the surface as a result of oxidation and/or decomposition of Si\({}_{3}\)N\({}_{4}\) and TiN induced during the WEDM process, as illustrated by the following equations:  
As can be seen, various oxidation reactions can occur. For example, Equations (2) and (3) show simultaneously oxidation of both Si\({}_{3}\)N\({}_{4}\) and TiN, while Equations (4) and (5) occur in dielectric liquid. Equation (6) demonstrates decomposition of silicon nitride into nitrogen and silicon, which, in turn, oxidizes further in the deionized water (Equations (7) and (8)). This leads to a large volume of the gases and, consequently, to the formation of pores on the WEDMed surface.