|14-043||Pavol Sajgalik||Sajgalik, P.(Institute of Inorganic Chemistry SAS);||
Silicon carbide and silicon nitride ceramic composites were prepared under this study. Freeze-granulated and afterwards under infrared lamp annealed silicon carbide powder was densified to the full density without any sintering aids by hot-pressing/ultra-rapid hot-pressing at 1850 °C. This densification temperature is at least 150-200 °C lower compared to the up to now known solid state sintered silicon carbide powders. Presented silicon carbide hot-pressed ceramics have excellent mechanical properties. Vickers hardness is 29 GPa and indentation fracture toughness is 5.25 MPa.m1/2. Samples densified by ultra-rapid hot-pressing have also full density and hardness of 27.4 GPa and fracture toughness of 5.3 MPa.m1/2. Creep rate of ultra-rapid hot-pressed samples at 1450 °C and 100 MPa load is 3.8 x 10-9 s-1 and at 1400 °C and the same load conditions is 9.9 x 10-10 s-1. Partial phase transformation ?/?- SiC was observed in the granulated and hot-pressed/ultra-rapid hot-pressed samples. The oxidation behavior of this way prepared SiC ceramics at 1350-1450°C/0-204h was investigated. This way prepared SiC ceramics is characterized by an excellent oxidation resistance (4.91x10-5 mg2/cm4h at 1450°C).
Dense Si3N4/SiC micro/nano composites were densified by hot-pressing with the addition of Lu2O3. Formation of SiC nano–inclusions within the silicon nitride matrix were prepared by in situ carbothermal reduction of added SiO2 by C during the sintering process. These composites have high hardness (> 17 GPa) and reasonably high fracture toughness (> 5.5 MPa.m1/2) and strength (<700 MPa). Besides these RT properties the composite containing the SiC nanoinclusions has excellent creep resistance (10-8 s-1 at 1400 °C and 100 MPa load) and oxidation rate constant of this material at 1400 °C for 200 hours was approx. 10-6 mg2/cm4.h.