More about the ceramic toughening mechanism in ZTA:Transformation toughening is a breakthrough in achieving high-strength ceramic materials with very high fracture toughness. ZTA exhibits an internal mechanism for actually inhibiting crack propagation. A crack in a normal ceramic travels all the way through the ceramic with little inhibition, resulting in immediate and brittle fracture and catastrophic failure. Transformation Toughened Zirconia (TTZ) exhibits a fracture toughness (or resistance to crack propagation) which is 3-6 times higher than normal zirconia and most other ceramics. TTZ is so tough that it can be struck with a hammer or even fabricated into a hammer for driving nails. The stress intensity factor values for window glass (silica), transformation toughened alumina, and a typical iron/carbon steel range from 1 to 20 to 50 respectively.

Components manufactured from Zirconia Toughened Alumina (ZTA) show considerable improvement in strength and toughness over alpha alumina engineering ceramics. The increase in strength and toughness in ZTA is attributable to the stress induced transformation toughening mechanism which is introduced with the addition of optimized amounts of fine zirconia particles dispersed thoughout the alumina body.

Typical zirconia content is between 10% and 20%. As a crack grows through the ceramic, the crystal structure of the zirconia particles in the region of the crack changes from the metastable tetragonal phase to the stable monoclinic phase. The change increases the volume of the particles by about 3-4% and generates compressive stresses in the alumina matrix. This process closes the crack; although it does not ‘heal’ the damage, it does act as an energy barrier to further crack growth. The addition of zirconia to the alumina matrix increases fracture toughness by two times and can be improved by as high as four times, while compressive & flexural strength can be more than doubled.