Possible Alternative Transformation Tougheners to Zirconia: Crystallographic Aspects

Several new potential alternative transformation tougheners to zirconia have been identified on the basis of crystallographic considerations and experimental observations. Examples can be found in ceramics, minerals, and components of glass and of cement. The displacive transformations exhibit martensitic characteristics of dusting or self‐disintegration, and a critical particle size effect operates. Large positive volume changes (< 12%) accompany the transformations on cooling, in the lanthanide sesquioxides (Ln 2 O 3 ), dicalcium silicate (Ca 2 SiO 4 ), and nickel sulfide (NiS). High M s temperatures (<2000°C) in the Ln 2 O 3 compounds also suggest the possibility of high‐temperature transformation toughening. Detrimental negative volume changes are seen in composites with enstatite (MgO · SiO 2 ) and the lanthanide borates (LnBO 3 ). The relative effects of volume change, X‐ray unit‐cell shape change, and the (martensitic) macroscopic shape change on the mechanical properties of a composite ceramic can thus be investigated.