Transformation Toughening of Ceramics

The results of a three-year study, aimed at developing a fundamental understanding of transformation toughening in ceramics and using that understanding to design and fabricate new microstructures with improved properties, are described. A new class of materials with dual scale laminar microstructures and greatly enhanced fracture toughness have been developed and tested Increases In fracture toughness from 5 to 18 MPAmto the l2 power were measured. These new microstructures were designed by combining micromechanics modeling of the effects of zone shape on toughness with direct measurements of transformation zone characteristics which provided critical testing of the theoretical modeling and measurements of transformation thermodynamics. To fabricate these microstructures a now colloidal processing method, which allowed construction of multilayered structures with layer thicknesses as small as 10 microns Lm, was developed. The direct transformation zone measurements included Raman microprobe spectroscopy to measure the distribution of transformed phases within the zones, and moire interferometry and high resolution strain mapping by digital image correlation to measure directly the transformation strains that are responsible for toughening. Calculations of crack tip shielding indicated modifications to transformation zone shapes that would increase the toughening.