Novel Routes for Sintering of Ultra-high Temperature Ceramics and their Properties.

摘要

The objective is to obtain new knowledge about mechanism of activated sintering of eutectic systems based on refractory compounds as well as regularities of structure formation of UHTCs. The suggested approach is to find out methods of elevation of diffusion mobility of surface atoms of grains being sintered. The effective way to increase this mobility is introducing additives in charge that together with the main phases form eutectic systems. The primary results obtained: 1. Mechanisms of eutectic interactions and processes ongoing in eutectic systems at high temperatures were investigated, namely: -A) at the boundary of grains of eutectic alloys, diffusion zones are formed where the processes of eutectic melting develop as well as processes responsible for sintering; -B) it is showed that Debay temperature of Al in composite is 288 that is about by 25% lower than that in monophase sample. Thus, at high temperatures the thermal vibration amplitude of aluminum atoms in grains of eutectic alloys is by 25% higher than in monophase condition and it defines the elevated diffusion activity at sintering. 2. In the ZrB2-SiC-5% Cr3C2 system, hot pressing allows to obtain porous free ceramics at temperatures 1500-1600 deg , and in the mode of free sintering at temperatures 1800-1900 , meanwhile the ceramic ZrB2-SiC has a porosity about 0-5% at hot pressing at temperatures 2100-2200 . Sintering of ZrB2-SiC-5% Cr3C2 ceramic occurs in conditions of eutectic interactions with disappearing liquid phase and formation of secondary high temperature phases (on the base of ZrC and CrB) in diffusion zones. 3. A result of eutectic interaction of sintered compositions ZrB2-SiC-5% Cr3C2 is also the elevated creep rate of those that is one of the factors of intensification of their sintering. 4. As a result of investigations performed, being based on the proposed approach, new compositions of perspective UHTCs were developed.