THE FORMATION OF NAMG2AL15O25 IN AN ALPHA-AL2O3 MATRIX AND ITS EFFECT ON THE MECHANICAL-PROPERTIES OF ALUMINA

An in-situ sintering reaction was designed to produce lath-like beta'''-alumina in an alpha-alumina matrix in order to make alumina ceramics stronger and tougher. The reaction sequence to produce beta'''-alumina (NaMg2Al15O25) requires the formation of beta-alumina (NaAl11O17) and spinel (MgAl2O4) at around 1100-degrees-C followed by a solid-state reaction of these two phases to give beta'''-alumina at elevated temperatures; this reaction is complete at around 1600-degrees-C. The in-situ sintering reaction produces near-theoretically dense alumina ceramics in which lath-like beta'''-aluminas are homogeneously distributed. The bending strength and fracture toughness increase to 620 M Pa and 5 M Pa m1/2, respectively; these increases are thought to be due to the suppression of grain growth as well as the crack deflection and bridging associated with lath-like beta'''-alumina.