Fabrication, microstructural characterization, and deformation of superplastic silicon aluminum oxygen nitrogen ceramics.

Abstract

Superplastic SiAlON materials, including hot-pressed $\alpha\sp\prime$-, $\alpha\sp\prime+\beta\sp\prime$-, and $\beta\sp\prime$-SiAlON, have been developed for the first time. These ceramics have submicron grain sizes, excellent biaxial formability, and novel flow characteristics. The investigation of kinetics and reaction pathways has established the special roles of ternary eutectic SiO$\sb2$-Y$\sb2$O$\sb3$-Al$\sb2$O$\sb3$ liquid and AlN in fabricating these materials at low temperature reaction hot-pressing. Formability, which is especially enhanced by the inclusion of $\beta\sp\prime$-SiAlON, has been demonstrated by punch-stretch testing at 1550$\sp\circ$C to large biaxial strains. The general deformation mechanism is found to be Newtonian viscous flow at lower flow stresses, and changes to shear thickening flow at a characteristic stress. This novel shear thickening transition can be rationalized by transient grain interlocking under compressive stress conditions. Extensive TEM work has also been conducted on these superplastic SiAlONs to examine microstructural development at various stages of processing and deformation. Epitaxial growth of $\alpha\sp\prime$-SiAlON on $\alpha$-Si$\sb3$N$\sb4$ and epitaxial growth of $\beta\sp\prime$-SiAlON on $\beta$-Si$\sb3$N$\sb4$, on supersaturated $\beta\sp\prime$-SiAlON, and even on $\alpha$-Si$\sb3$N$\sb4$ have all been identified. In addition, $\beta\sp\prime$-SiAlON grains were found to rotate and align themselves with the biaxial stretching directions. No evidence of cavitation was detectable after biaxial stretching in any of these materials. These observations have provided additional insight to the formability and alloy design of superplastic ceramics. They also have broader implications on the microstructural control of tough SiAlONs.