Topic: GB chemistry of silicon nitride based ceramics – implications to the ceramics properties
Speaker: Prof. Pavol Sajgalik
Slovak Academy of Sciences
Abstract：
Six different sintering aids (Lu2O3, Yb2O3, Y2O3, Sm2O3, Nd2O3 and La2O3) were used for the processing of dense Si3N4/SiC micro/nano composites. Formation of SiC nano–inclusions was achieved by in situ carbothermal reduction of SiO2 by C during the sintering process. Room temperature fracture toughness, hardness and strength showed increasing tendency when the used rare-earth element in the oxide additive changes from a large to a small rare-earth cation (i.e. from La3+ to Lu3+). Besides the highest hardness and reasonably high fracture toughness and strength of composite material with Lu2O3 sintering additive. The first principle calculations were used for the explanation of the fracture behaviour of the composites depending on the rare earth additive. The energy of fracture was calculated with respect to the chemical composition of GB. The experimental and theoretical results will be discussed.
Besides mechanical properties composites the thermal shock resistance, wear resistance and oxidation resistance are discussed.
Thermal shock resistance: A critical temperature difference increased with an increasing ionic radius of RE3+ for both the composites and the monoliths.
Wear behavior: The friction coefficient as well as the specific wear rate decreased with a decreasing ionic radius of rare-earth elements in both the monoliths and the composites. High bonding strength and the high fracture toughness are the reasons why the ceramics doped by Lu exhibited the best wear resistance.
Oxidation resistance: Composites exhibited predominately parabolic oxidation behaviour indicated diffusion as the rate limiting mechanism. Exception was only the Si3N4-SiC composite sintered with Lu2O3. In this case diffusion of cation has been strongly suppressed because of the beneficial effect of stable grain boundary phase and the presence of the SiC particles predominately located at the grain boundaries of Si3N4.
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