Phenomenological Relation between Final-Stage Densification and Creep Rate in Hot-Isostatically Pressed Si3N4-Based Ceramics

It is shown that final-stage densification and creep behavior of hot-isostatically pressed (HIP) Si3N4 materials are phenomenologically related by a simple law independently of the particular mechanism mainly affecting the behaviors. Data from fifteen Si3N4-based (HIPed) materials were examined taking as reference material a Si3N4 polycrystal densified without external addition of additives. Three main mechanisms were identified and discussed leading to either softening or hardening effects on Si3N4. They were grain-boundary softening, solid-solution softening and dispersion hardening as produced by adding to the reference system various additives with or without solubility in Si3N4 and hard ceramic dispersoids, respectively. It is phenomenologically recognized and quantitatively established that a common driving force acts in opposite directions during densification and creep processes.