Effects of Rare Earth (RE) Intergranular Adsorption on the Phase Transformation and Microstructure Evolution in Silicon Nitride with RE2O3 + MgO Additives: Fracture Behavior

Journal Article · Tue Jul 01 00:00:00 EDT 2008 · Journal of the American Ceramic Society

DOI:https://doi.org/10.1111/j.1551-2916.2008.02448.x· OSTI ID:951081

Becher, Paul F ^[1]; Painter, Gayle S ^[1]; Shibata, Naoya ^[2]; Waters, Shirley B ^[1]; Lin, Hua-Tay ^[1]

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1. ORNL
2. University of Tokyo, Japan

Silicon nitride powders consist primarily of the alpha phase, which transforms to the beta phase during the densification and microstructural evolution of Si3N4 ceramics. The temperature at which the transformation initiates in the presence of a combination of MgO and RE2O3 densification additives is found to decrease with increasing atomic number of the rare earth (RE). This trend coincides with the predicted and observed decrease in the affinity of the rare earth to segregate to and absorb on the prism planes of hexagonal prism shaped beta grains with increase in the atomic number of the RE. When RE adsorption is diminished, Si (and N) attachment on the smooth prism planes is enhanced, which increases diametrical growth rates, normally reaction-rate limited by an attachment mechanism. Combined with the typically fast [0001] growth, it is this augmented grain growth that contributes towards the initiation of the alpha-beta transformation at lower temperatures. With the enhanced transformation, observations reveal an increase in the number of beta grains growing in the early stages of densification. On the other hand, increased RE adsorption leads to greater growth anisotropy resulting in the formation of higher aspect ratio grains. Thus, Lu2O3 generates larger diameter, yet elongated, reinforcing grains, while La2O3 results in reinforcing grains of higher aspect ratio. The Gd2O3 additive transformation and microstructual characteristics lie intermediate to those of the lanthanide end member elements. Despite these differences, a substantial fraction of large reinforcing grains were found for each additive composition. As a result, the mechanical properties of the resultant ceramics are similar with flexure strengths in excess of 1 GPa, fracture toughness values greater than 10 MPa m1/2 at room temperature and excellent strength retention (>800 MPa) at 1200 C.

• Research Organization: Oak Ridge National Laboratory (ORNL); Shared Research Equipment Collaborative Research Center
• Sponsoring Organization: SC USDOE - Office of Science (SC)
• DOE Contract Number: AC05-00OR22725
• OSTI ID: 951081
• Journal Information: Journal of the American Ceramic Society, Journal Name: Journal of the American Ceramic Society Journal Issue: 7 Vol. 91; ISSN 0002-7820; ISSN JACTAW
• Country of Publication: United States
• Language: English