EFFECT OF HIGH-PRESSURE ON POLYTYPISM AND STACKING DISORDER IN SINTERED SiC. S. Stel'makh, S. Gierlotka, B. Palosz, Murali Mohan^*, C. Divakar^*, S.K.Bhaumik^* & A.K.Singh^*, High Pressure Research Center, UNIPRESS, Warsaw, POLAND, ^*National Aerospace Laboratories NAL, Bangalore, INDIA

Isostatic high pressure technique was used for sintering of SiC polycrystals. Belt apparatus (pressure up-to 8 GPa) and cubic anvil cell (3 GPa) in the temperature range up-to 2200^oC were used. The following SiC polycrystals were examined: a-SiC (STARCK), b-SiC powder obtained by Self-propagating High-temperature Synthesis (SHS), High Pressure self-Combustion Synthesis (HPCS) and also a series on nanocrystalline SiC powders. Comparative structural studies of these materials sintered without external pressure and with application of high isostatic pressure were performed. The structural analysis was performed with the use of a method of computer modelling of disordered structures and simulation of corresponding diffraction patterns. This method permitted us for identification of stacking faults that occur in starting materials and to follow the structural changes induced under high temperature and high pressure. The SiC polycrystals synthesized by SHS method under high pressure (3-6 GPa) contain only very little number of stacking faults. Very strong inhomegenous stresses applied to starting powders (compacted without liquid phase) generate microtwins and stacking faults at low temperature. At high temperature, the thermaly activated dislocation motions lead to pressure-induced structural changes in the stacking of the layers. These changes always reduce one-dimensional disorder present in starting polycrystals. They are not however inter-polytype transformations that originate from thermodynamic conditions and relative stability of cubic 3C and a-type polytypes: They obviously follow changes of the dislocation network forced by pressure and activated at high temperature.