MODIFICATION OF ENGINEERING SILICON-NITRIDE CERAMICS BY ENERGETIC-IONBOMBARDMENT

The microstructural and mechanical properties of hot-pressed Si3N4 cer amics after Si+ ion bombardent and annealing in N2 atmosphere have bee n investigated as a function of the ion fluence and the annealing temp erature. The irradiations were carried out at target temperatures of a bout 80 K and 450 K with ion energies of 0.5 MeV and 1.0 MeV. In all c ases the fluence range was subdivided into two regimes: a low-fluence regime with improved microhardness and fracture toughness, and a high- fluence regime with an absolute degradation of these properties. The t ransition fluence was found to strongly depend on the ion energy and i mplantation temperature. This property transition coincides with a mic rostructural transition from a highly damaged, but still crystalline m aterial, to the formation of a buried amorphous layer. The amorphizati on results in a strong volume swelling which causes a closure of surfa ce flaws. The latter process significantly enhances the fracture stren gth of the implanted material. Thermal relaxation of the modified mech anical properties was found to occur at temperatures above 800-degrees -C. The relationships between the ion-induced changes of the mechanica l properties and the microstructural modifications will be discussed.