## 3 Results and Discussion
### Ignition Response  
The temperature at which a sudden increase in the sample mass occurs was considered as the ignition temperature [27]. In the current study, the mass gain occurred from 575 \({}^{\circ}\)C to 750 \({}^{\circ}\)C due to considerable oxidation followed by ignition of the sample. The ignition temperature of the pure Mg increased by increasing the addition of Si\({}_{3}\)N\({}_{4}\) nanoparticles. Adding 0.5 vol% of Si\({}_{3}\)N\({}_{4}\) improved the ignition temperature to 596 \({}^{\circ}\)C, \(\sim\)3% greater than pure Mg (580 \({}^{\circ}\)C). The further addition of 1 and 1.5 vol% Si\({}_{3}\)N\({}_{4}\) improved the ignition temperature to 603 \({}^{\circ}\)C and 614 \({}^{\circ}\)C, which is \(\sim\)4% and \(\sim\)6% more than pure Mg. Figure 5 shows that the Mg/Si\({}_{3}\)N\({}_{4}\) nanocomposites have greater ignition resistance than AZ-series (aluminium and zinc are constituent elements), ZK-series (zinc and zirconium are constituent elements), and AM-series (aluminium is the major constituent element) magnesium alloys. The ignition temperature of the Mg-1.5 Si\({}_{3}\)N\({}_{4}\) nanocomposite is higher than all the traditionally used magnesium alloys. The significant improvement in ignition temperature of the Mg/Si\({}_{3}\)N\({}_{4}\) nanocomposites can be attributed to the following phenomenon. The ignition phenomenon of pure Mg primarily depends on oxidation kinetics [8]. At temperatures below 450 \({}^{\circ}\)C, the oxide layer developed on the surface of Mg is reasonably protective and restricts subsequent oxidation. However, at temperatures more than 450 \({}^{\circ}\)C, the Gibbs free energy of exothermic oxide formation causes a rapid increase in local heat, leading to the metal melting beneath the oxide layer and generating cracks in the layer. Further heat raises the local temperature, resulting in vapour pressure in the entrapped liquid. This pressurized fluid explodes through the cracks and reacts with oxygen, resulting in the ignition of Mg [8]. In a sense, ignition only occurs when the protective oxide layer breaks and loses its ability to inhibit the reaction with the oxygen. The Pilling-Bedworth ratio (PBR) denotes the oxide layer's quality formed on a metal surface. The oxide layer with a PBR value of less than 1 and more than 2 will provide poor protection. As MgO exhibits a PBR value of 0.81, the ignition is relatively easy to start. The addition of reactive elements, such as alloying elements and thermally stable nanoparticles, can aid in the development of a more stable and dense surface oxide layer, thereby improving Mg ignition resistance [18,28]. Figure 4: Density and porosity results of the samples. In the current work, the increased ignition resistance of the nanocomposites is attributed to the formation of a denser and more stable oxide layer below the thin surface layer of MgO. As the temperature increases, the thin Mg oxide layer breaks away, leaving behind a thick oxide layer, which further protects by restricting the reaction between Mg vapour and oxygen, thereby increasing the ignition resistance [30,31]. Additionally, the low thermal conductive Si\({}_{3}\)N\({}_{4}\) nanoparticles act as insulating sites in the pure Mg matrix, which improves its thermal stability and ignition resistance. A correlation between the thermal conductivity and nanocomposite ignition temperature was established to better understand the influence of nanoparticles on ignition behaviour. Using the rule of mixtures, the theoretical thermal conductivity (K\({}_{\rm c}\)) of the nanocomposites was determined (refer to Equation (1)) by considering the thermal conductivity of base matrix (K\({}_{\rm m}\)) pure Mg and reinforcement (K\({}_{\rm r}\)) Si\({}_{3}\)N\({}_{4}\) as 135 and 30 (W/m.K), respectively, for each volume percentage (V\({}_{\rm c}\)) of Si\({}_{3}\)N\({}_{4}\) nanoparticles. \[\rm K_{c}=K_{r}V_{c}+(1-V_{c})K_{m} \tag{1}\]  
The calculated thermal conductivity values are plotted (Figure 6) against the respective ignition temperatures of the samples.