Nucleation control for hot-working of silicon oxynitride based ceramics

An attempt was made to develop an engineering ceramic plastically deformabl e at high temperatures with low flow stress at high strain rates, and witho ut strain hardening. Dense ceramic preforms were fabricated by pressureless sintering Si3N4 + SiO2 mixed powders with an addition of MgAl2O3 at 1500 d egrees C. A transient liquid, which occurs during the reaction sintering of Si2N2O, was utilized for subsequent net-shape forming, The ceramic (6 phi x 6 mm column) was deformed without any cracks and cavities in compression tests at high strain rates (10(-2)-10(-3) s(-1)) at 1500 degrees C, but thi s was not achieved in a test at lower strain rates for a long time. because of the growth of elongated Si2N2O grains during the test. Potassium fluori de (KF) was used as a dopant for retardation of nucleation of Si2N2O during sintering and hot-working, The KF-doped preforms were successfully plastic ally deformed even in the test for a long time.