The present study investigates the influence of the content of Y2O3-Al2O3 sintering additive on the sintering behavior and microstructure of Si3N4 ceramics. The Y2O3:Al2O3 ratio was fixed at 5:2, and sintering was conducted at temperatures of 1300°-1900℃. Increased sintering-additive content enhanced densification via particle rearrangement; however, phase transformation and grain growth were unaffected by additive content. After phase transformation was almost complete, a substantial decrease in density was identified, which resulted from the impingement of rodlike b-Si3N4 grain growth. Phase transformation and grain growth were concluded to occur through a solution-reprecipitation mechanism that was controlled by the interfacial reaction.

Using boron carbide (B4C) and boron nitride (here refers to hexagonal BN) as reactants, a series of boride-containing ceramic composites, mainly in the present work zirconium diboride-containing composites including zirconium diboride-zirconium carbide (ZrB2-ZrC), zirconium diboride-zirconium nitride (ZrB2-ZrN), zirconium diboride-silicon carbide (ZrB2-SiC) and zirconium diboride-aluminum nitride (ZrB2-AlN) were prepared by in situ reactive hot pressing. The features and the development mechanisms of the composite microstructures were characterized and modeled. The obtained zirconium diboride-containing composites demonstrated high bending strength. In addition, some general problems such as transformation between B4C and BN and thermodynamics of using B4C and BN as reactants were also briefly discussed.

Three different series of porous silicon nitride ceramics with volume fraction porosities in the range 0–0.5 were fabricated using different preparation routes: (i) partial sintering, (ii) the addition of fugitive inclusions, and (iii) partial hot pressing. The use of different sintering additives and firing conditions, depending on the preparation route, gives rise to different materials within a certain porosity range with variations in terms of microstructure and grain boundary phase. Mechanical properties, elastic moduli, and strength have been evaluated separately for each series of materials. Porosity dependences of Young's modulus, shear modulus, Poisson's ratio, and fracture strength have been assessed and a comparison of the different materials is made and discussed in relation to their microstructural features.

Porous Si3N4 ceramics were fabricated by liquid-phase sintering with a Yb2O3 sintering additive, and the microstructure and mechanical properties of the ceramics were investigated, as a function of porosity. Low densification was achieved using a lower Yb2O3 additive content. Fibrous β-Si3N4 grains developed in the porous microstructure, and the grain morphology and size were affected by different sintering conditions. A high porosity, ～40-60%, with b-Si3N4 grain development, was obtained by adjusting the additive content. Superior mechanical properties, as well as strain tolerance, were obtained for porous ceramics with a microstructure of fine, fibrous grains of a bimodal size distribution.

Successful net-shape sintering offers a significant advantage for producing large or complicated products. Porous Si3N4 ceramics with very low shrinkage were developed, in the present investigation, by the addition of a small amount of carbon. Carbon powders (1-5 vol%) of two types, with different mean particle sizes (13nm and 5mm), were added to a-Si3N4-5 wt% Y2O3 powders. SiC nanoparticles formed through reaction of the added carbon with SiO2 on the Si3N4 surface or with the Si3N4 particles themselves. Such reactionformed SiC nanoparticles apparently had an effective reinforcing effect, as in nanocomposites. Sintered Si3N4 porous ceramics with a high porosity of 50%-60%, a very small linear shrinkage of ～2%-3%, and a strength of ～100MPa were obtained.