Silicon nitride (Si3N4) is one of the most promising candidates for application in gas turbine and piston engines, catalyst supports, diesel particulate filters and interpenetrating composites mainly because it possesses good thermal and chemical stability, high hardness, and excellent corrosion and wear resistance under harsh conditions. Porous Si3N4 ceramics with high porosity have previously been developed by a combination of reaction bonding of Si powder, sacrificial templates and gel-casting. To modify the microstructural and mechanical properties, this technique maybe followed by a post heat-treatment in which samples that contain various oxide aids are heated at elevated temperatures and embedded in a Si3N4-based powder bed. Despite many studies on the fabrication and characterization of porous Si3N4, there is still a lack of understanding of a) the role of additives on the nitriding mechanism and formation of a specific Si3N4 grain morphology, b) the relationship between microstructure and properties of porous ceramics and c) the influence of the powder bed composition on the microstructure of heat-treated Si3N4 ceramics. 
The main objective of this study was to investigate the microstructure of porous reaction bonded Si3N4 and its properties in the presence of MgO, CaO and Al2O3. Samples with high porosity were prepared by a process that utilizes a sacrificial template technique, along with gel-casting and nitriding. Mechanisms and thermodynamics of reactions, microstructure of phases present, morphologies of grains and pores, pore size distribution, pore interconnectivity and accessibility, porosity and density, linear shrinkage and weight loss, as well as mechanical strength of the fabricated foams are discussed. For this purpose, microstructural transformations of foams both during in situ nitridation at T≤ 1425℃ and post heat-treatment at T≤ 1700℃ have been considered. Throughout the text, ‘RBSN’ refers to reaction bonded Si3N4 samples with no additive, while in the presence of the above-mentioned oxides, samples have been labelled as MgO-RBSN, CaO-RBSN and Al2O3-RBSN, respectively. 
The results revealed that in an atmosphere of ultra-pure nitrogen and during the in situ nitridation of Si, whiskers grew in globular agglomerates into the large pores and decreased the pore interconnectivity and size. However, upon adding MgO, the grain morphology was altered and eventually at 12 wt.% MgO, whisker formation was completely eliminated, and the microstructure was modified to a combination of Si3N4 α-matte grains and β-rods. Based on the results of XRD analyses along with thermodynamic studies, some intermediate phases formed as a result of reactions between volatile MgO and silica, which is naturally present on the surface of Si particles. This possibly decreased the content of SiO (g), as the predominant reactant of α-whisker formation, and altered the ratio of α/β-Si3N4 phases. Subsequently, in this microstructure, the formation of fully interconnected pore channels with large pores (< 40 μm) and clean cavities was confirmed by 3D and 2D visualization images. 
Subsequently, the microstructural transition of Si3N4 grains heat-treated in the presence and absence of MgO in the Si3N4-based powder bed (i.e., labelled as SN-MgO and SN powder beds, respectively) was investigated. This illustrated that the grain morphology and microstructures of samples were strongly affected by the presence of volatile MgO in the powder bed. During liquid phase sintering at 1700℃, α-matte dissolved, grains precipitated as coarse β-rods and a Mg-Si-O-N glassy phase was developed. Similarly, α-whiskers gradually disappeared and recrystallized as 100% fine β-grains. During heat-treatment, weight losses and α/β phase ratio were substantially decreased while linear shrinkage and grain coarsening were increased. Unlike α-matte grains, α-whiskers extensively densified upon heat-treatment. This led to a high densification and a sharp decrease in porosity level. Consequently, a macro/microstructure with less interconnectivity, clean spherical cavities and large pore size resulted. The observed microstructural changes occurred due to the high volatility of magnesia, by vapor phase transport of species from the MgO-SN powder bed to the porous structure, and through a subsequent solution-precipitation mechanism. No significant morphology change, shrinkage, densification or phase transformation were observed in the samples heat-treated in SN powder bed. 
Subsequently, morphology and microstructures of samples were correlated to the structure of pores, pore size distribution, level of porosity and density. Pores were classified based on their shape (spherical or irregular), size, interconnectivity (open or isolated) and their fabrication mechanism. Pores with large and whisker-free spherical cavities and unimodal pore size distributions were developed in samples composed of either α-matte grains or β-rods. In contrast, complex, fine and irregular pores were fabricated in the presence of elongated Si3N4 whiskers; therefore, a bimodal pore size distribution resulted. This study showed that the obtained foams had tailorable properties allowing mean pore sizes ((d)) ̅ ranging from 1.3 to 13.8 μm and 30 to 76 vol.% porosity.
In addition, the interaction of CaO, MgO and Al2O3 with silicon and nitrogen were examined comparatively. In the presence of CaO and through a reaction between Si (g) and molecular nitrogen, the dominant microstructure was α-matte grains (α/β-Si3N4 ratio≈ 3.9) with interconnected, spherical and open cavities. Formation of intermediate phases due to the reactions with surface silica could have possibly affected the content of SiO vapor and thus hindered the formation of whiskers. By substituting CaO (15 mol%≈  6 wt.%) with the same mol.% of MgO (15 mol%≈ 4.3 wt.%), the content of intermediate phases in the microstructure was reduced, a low number of α-whiskers formed, and the α/β-Si3N4 ratio was decreased by almost 55%. Yet, the highest quantity of whiskers and the lowest α/β-Si3N4 ratio were developed in the presence of Al2O3 (15 mol%≈ 11 wt.%). High thermal and chemical stability of Al2O3 possibly decreased the content of oxygen generated and hindered the formation of intermediate phases. This in turn could increase the likelihood of SiO (g), a reactant required for α-whiskers formation, and atomic nitrogen, a reactant needed for β-phase formation, being present in the atmosphere.  
Both compressive strength and flexural strength were increased in the presence of interlocked whiskers with fine inter-particle pores or in structures with low porosity levels, whereas a combination of high porosity and large pores limited the strength of the resultant foams. In addition to the microstructure of pores, interconnectivity and accessibility of pore channels, the amount of shrinkage and densification, weight loss and density were strongly dependent on the presence of different oxide additives. Hence by designing the composition of the starting powder mixture and the surrounding powder bed, tailored microstructures with different mechanical strengths can be easily obtained. This research should be of interest to scientists and engineers concerned with the processing and use of porous ceramics and, in particular, this information and findings can be used in the microstructural design of Si3N4 and related materials.