Alumina-based hetero-modulus ceramic composites with extreme dynamic strength - phase transformation of Si^sub 3^N^sub 4^ during high speed collisions with metallic bodies

摘要

Till today several kinds of ceramics and ceramic matrix composites are developed for extreme environmental conditions. Most of these ceramics have microstructures with relatively "big" crystals, having high rigidness and strong inclination to nick, pitting and rigid fractures, so they are not usable for collision with metallic or other bodies under high speeds like 800 m/sec or more. On the basis of several years experiments in development and testing of ceramic materials and corundum matrix composites the authors successfully developed new alumina-matrix composite materials reinforced with Si^sub 2^ON^sub 2^, SiAION, AIN and Si^sub 3^N^sub 4^. These new alumina based ceramic matrix composites were tested under collisions with different metallic bodies having high densities and speeds higher than 800 m/sec. During the collisions the kinetic energy of flying metallic objects distributing to fracture energies, heatings and recrystallizations both of ceramic and metallic bodies. In the centres of collisions, where oxygen was absent, the authors have found new, high density "diamond-like Si^sub 3^N^sub 4^" materials with cubic crystals, where nitrogen atoms distributed in the centres of the cubes. These new crystal structures of Si^sub 3^N^sub 4^ in the alumina matrix have extreme dynamic strength and hardness, like diamond. Having surplus of oxygene in the centres of collisions this new "diamond-like Si^sub 3^N^sub 4^" was not observed, when a very strong oxydation of metallic bodies was taken place.