Mechanical properties of gas pressure sintered Si3N4/SiC nanocomposites

The mechanical properties of gas pressure sintered Si3N4/SiC nanocomposite materials were studied at room temperature and in the range of 1400–1600 °C. Fracture strength at room temperature was found to be between 640 and 840 MPa and thus within the range of data reported in the literature for other gas pressure sintered silicon nitride ceramics. At elevated temperature the creep and stress rupture behaviour were determined in four-point bending. The creep behaviour is characterised by primary and pronounced secondary creep, very high activation energies for creep in the range of 1050–1450 kJ/mol, and two different stress dependencies. At stresses below a characteristic transition stress in the range 170–200 MPa the stress dependency is described by a stress exponent of around two. Above the transition stress, stress rupture was observed to be controlled by subcritical crack growth with stress rupture exponents of 30–40. Annealing of the as-sintered materials promotes crystallisation of the intergranular glassy phase resulting in an increase in creep strength and resistance against subcritical crack growth. An extensive analysis of the available literature data revealed that the materials tested in this study belong to the most creep resistant silicon nitride ceramics prepared by gas pressure sintering.