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Macrochannelled tetragonal zirconia polycrystals (TZP) coated with a calcium phosphate layer were fabricated using a coextrusion process to produce strong and bioactive porous bioceramics. The initial feedrod, composed of three materials [TZP (shell), calcium phosphate (intermediate layer), and carbon black (core)], was coextruded through a 750-μm orifice at 120 °C, producing a continuous and flexible filament. Each sheet, which was composed of a unidirectional array of filaments, was stacked and then warm-pressed at 140 °C with 10 MPa. After binder burnout, the green billet was sintered between 1350 and 1600 °C for 1 h in air, leaving uniform macrochannels clad on the inside with bioactive calcium phosphate on a strong TZP body. The compressive strength of the specimen was much higher than that of calcium phosphate with a similar structure.

To improve the oxidation resistance of Si3N4 material, a dense and continuous layer, composed of small Y2Si2O7 crystallites and silicate glass, was formed on the surface. The surface layer was formed by exposing the specimens in a flowing H2 atmosphere containing 0.1% H2O at 1450 °C for 1 h. For the purpose of comparison, specimens of the same material were heat treated in air at 1450 °C for 1 h. Small equiaxed Y2Si2O7 crystallites were formed when the specimen was heat treated in the wet H2 atmosphere, while relatively large and elongated Y2Si2O7 crystallites developed when the specimens were heat treated in air. The oxidation resistance of Si3N4 material was improved remarkably by the heat treatment in the wet H2, while no improvement in oxidation resistance was observed from the specimen heat treated in air. The improvement was attributed to the retardation of the transport of oxidants through the dense and continuous layer formed on the surface.

The effect of ZrO2 addition on sintering behavior and mechanical properties of both hot-pressed and pressureless-sintered B4C was investigated. The addition of ZrO2 improved the densification behavior of B4C remarkably via a reaction with the B4C to form ZrB2 at elevated temperatures. When B4C was densified at 2000 °C by hot pressing, only a small amount (approximately 2.5 vol%) of ZrO2 was necessary to achieve a full densification. Excellent mechanical properties (hardness, elastic modulus, flexural strength, and fracture toughness) were observed in those specimens. As the amount of ZrO2 was increased further, the mechanical properties were reduced, except for the fracture toughness, apparently due to the formation of too much ZrB2 in the specimen. Without the applied pressure, larger amounts of ZrO2 should be added to obtain a body with high relative density. When the B4C was sintered at 2175 °C with addition of 10 vol% ZrO2, the specimen has a density higher than 95% of the theoretical, and hardness and flexural strength of 25 GPa and 400 MPa, respectively.

The oxidation behavior and the effect of oxidation on the flexural strength of Si3N4/SiC nanocomposites were investigated. Parabolic weight gains, with respect to the oxidation time, were observed for all the specimens containing different amounts of SiC. However, the parabolic rate constant decreased with increasing SiC content. The main oxidation product, being in equilibrium with SiO2, was Y2Si2O7. The oxidation product became denser and retained more crystalline phase as the amount of SiC increased. The flexural strength of the nanocomposites increased by up to 40%, while that of pure Si3N4 decreased after oxidation at 1300 or 1400 %C for 100 h. This excellent oxidation resistance of Si3N4 nanocomposite was attributed to the enhancement of the crystallization of the oxidation product and the grain boundary phase by the SiC nanoparticles.

The effects of SiC-nanoparticle and Si3N4-whisker additions on the microstructural evolution and mechanical properties of Si3N4 were investigated. The addition of SiC nanoparticles suppressed Si3N4 grain growth, leading to an improvement in the flexural strength. On the other hand, Si3N4 whiskers in the specimen promoted the formation of large elongated grains, which were found to be beneficial to the fracture toughness of the material. When both SiC nanoparticles and Si3N4 whiskers were added concurrently, large grains were formed in fine matrix grains. The microstructure of Si3N4 was controlled by adjusting the relative concentrations of SiC nanoparticles and the Si3N4 whiskers added. These compositional and microstructural variations of the Si3N4 had significant influence on the mechanical properties, such as strength, fracture toughness, R-curve behavior, and high-temperature strength.

Microstructural evolution and mechanical properties of gas-pressure-sintered Si3N4 with 4 wt% Yb2O3 as a sintering aid were investigated. The microstructure was not uniform throughout the specimen. Extremely large elongated grains were formed at the outer region near the surface, while relatively small elongated grains were formed at the inner region of the specimen. The outer region expanded inward with the sintering time. Mechanical properties, such as flexural strength, fracture toughness, and R-curve behavior of the specimens were strongly influenced by these variations in microstructure. The fracture toughness and the R-curve behavior of the outer region were higher than those of the inner region of the same specimen. On the other hand, the strength of the inner region was higher than that of the outer region. By controlling the relative thickness of each region, Si3N4 specimens having functionally graded microstructure were obtained. The Si3N4 with such microstructure exhibited high strength, high fracture toughness, and good flaw tolerance at the same time.