The mechanical properties and microstructure of SiC ceramics, hot pressed by simultaneously adding nano-SiC and oxides (MgO+Al2O3+Y2O3) or nitrate salts (Mg(NO3)2+Al(NO3)3+Y(NO3)3) as additives, were evaluated. The oxide additives system slightly influenced the mechanical properties of the ceramics, while the addition of nano-SiC lead to finer microstructure, and 5 vol.% nao-SiC changed the fracture mode from intergranular type to transgranular type. The ceramics with nitrate salts had fine, equiaxed grains with an average grain size larger than that of the system added oxides, thus inducing lower Viker’s hardness and flexural strength, while the presence of crystalline YAG phase improved the fracture toughness by 54.7%. Also, an observed increase in grain growth—with decreasing weight fraction of liquid and the grounded grain morphology in this system—confirmed a diffusion-controlled growth mechanism. Although the sample with the least amount of additives has the lowest relative density and largest grain size, its flexural strength did not drastically decrease. The influence of nano-SiC on the fracture toughness in the nitrate additive system was negligible.

Coexisting LiTaO3/Al2O3 ceramic composite was hot pressed at 1573 K. The highest values of flexural strength and fracture toughness reached 492.9 MPa and 5.3 MPam1/2, respectively, for the composite containing 15 vol.% LiTaO3. Domain switching which was clearly discerned near the crack tip in LiTaO3 grains is suggested as a new toughening mechanism in structural ceramics.

LiTaO3/Al2O3 ceramic composites were fabricated by hot pressing and pressureless sintering, respectively, to investigate the effect of piezoelectric secondary phase on the properties of structural ceramics. Phase constitution, microstructure and mechanical properties of the ceramic composites were studied. LiTaO3 could stably coexist with Al2O3 during sintering. LiTaO3 phase was located at the grain boundaries of Al2O3 matrix fabricated by different processing routes. Domain structures were clearly seen in LiTaO3, confirming that LiTaO3 remained hexagonal ferroelectric phase. About 5 vol.%LiTaO3/Al2O3 ceramic composite prepared by pressureless sintering at 1300 ℃ showed an increase in fracture toughness. Energy dissipation due to domain switching was suggested as a toughening mechanism. With the increasing of LiTaO3 content, the domain switching toughening mechanism was decompensated by the decreased relative density of LiTaO3/Al2O3 ceramic composites. For LiTaO3/Al2O3 ceramic composites prepared by hot pressing at 1500 ℃, the bending strength decreased because of the increased grain size of Al2O3 matrix. The toughening effect due to domain switching was also decompensated by the coarsening of Al2O3 matrix grains, the weaker net-like LiTaO3 phase and the weaker bonding strength between LiTaO3 phase and Al2O3 matrix.

The ambient and elevated temperature mechanical properties of two kinds of hot-pressed fused silica matrix composites, SiO2+5vol.% Si3N4 and SiO2+5 vol.% Si3N4+ 10 vol.% Cf, were investigated. Si3N4 additions greatly enhanced the ambient strength and fracture toughness, while, further incorporation of chopped carbon fibers only but sharply increased the fracture toughness value from 1.22 to 2.4 MPa m1/2. The strength of the two composites synchronously exhibited anomalous gains at certain elevated temperature range especially from 1000 to 1200 ℃, and reached their maximum values at 1000℃, 168.9 and 130.6 MPa, which were 77.0 and 77.4% higher than their ambient strength, respectively. The two composites exhibited catastrophic fracture even at 1000 ℃, but manifested prominent plastic deformation at 1200 ℃ and usually no fracture occurred during the strength test. Vickers’ indentation crack propagation behavior, combined with fractographs studies, suggested that toughening from carbon fiber was attributed primarily to the fiber bridging, pull-out and crack deflection.