Microstructure-property relationship tailoring by additives, this is a dream for ceramists. It is often found that additives remain at two-grain junctions to form intergranular glassy film (IGF), especially in Si3N4 where IGFs are ubiquitously present with equilibrium width, which are independent to crystallographic orientations. Recently, such IGFs were correlated to abnormal grain growth in Al2O3 ceramics via transitions between “complexions”, or stable grain-boundary structures as interfacial “phases”, adding a strong thermodynamic nature for IGF.
However, the chemical analyses of IGF reveal rather different scenarios: IGF is sensitive not only to grain surface structures but also to the structure/chemistry of neighboring multi-grain intergranular junctions. In a low-CaO-doped high-purity Si3N4 system, basal and non-basal facets exhibit a bi-modal distribution for dopant segregation to IGF which is independent to doping level, leaving the extra CaO re-partitioned in the glassy pockets [1]. Abnormal grain growth follows up dopants re-redistributions [2]. In liquid-phase sintered SiC ceramics and in rare-earth-doped SiAlON ceramics, presence of IGF and variation in composition were found subject to the adjacent pocket phases either in crystalline or glassy forms, revealing purely kinetic limited cases [3,4]. In low-TiO2-doped Al2O3 ceramics with minor SiO2 impurity, not only the preferential segregation of SiO2 and TiO2 to basal and non-basal IGFs respectively was consistently observed [5,6], there was also several levels of SiO2 enrichment at basal facets that exposed further the “transient” role of IGF before initiating the anisotropic grain growth on the non-basal facets, which is dictated by a ternary melts to and from neighboring pockets [7].
In retrospect, we provide an alternative picture for IGF with a transient nature to initiate microstructure development in connection with the phase evolution from adjacent pockets, hence render IGF back as the kinetic limited structure. In meanwhile, the seemingly equilibrium IGF thickness reflects the structural nature of the crystalline-amorphous interface, which is not an thermodynamic issue.
- Gu, Cannon, Tanaka, Rühle, Mater. Sci. Engin. A 422 (2006), 51-65.
- Gu, Tanaka, Cannon, Pan, Rühle, Int. J. Mater. Res. 101 (2010), 66-74.
- Huang, Gu, Zhang, Jiang, Acta Mater. 53 (2005), 2521-29.
- Zhu, Gu, Holz, Hoffmann, Scripta Mater. 54 (2006), 1469-73.
- Chi, Gu, Interf. Sci. 12 (2004), 335-42.
- Qian, Gu, Aldinger, Int. J. Mater. Res. 99 (2008), 240-44.
- Qian, Gu, Chi, J. Am. Ceram. Soc. 93 (2010), 326-29.