Tetragonal zirconia polycrystals (TZP)

Question 1

What’s the relationship between fracture toughness & strength and conc. of metastable tetragonal SS?

Answer 1

~ fracture toughness and strength increase with conc. of metastable tetragonal SS

Question 2

How do you get a microstructure of nearly entirely metastable tetragonal SS?

Answer 2

~ requires that the metastable tetragonal SS grains are BELOW the critical size

~ this has been realized with 2-3mol% Y2O3 additions and sub-micron starting powders

Question 3

What happens at 1400-1500C?

Answer 3

~ both the single phase tetragonal SS is stabilize and the particle compact is sintered (the much lower sintering temp than Mg-PSZ or Ca-PSZ attenuates grain growth), forming 0.2-2um grains of tetragonal SS. Some cubic SS may also be present

Question 4

How does densification occur?

Answer 4

~ via liquid phase sintering with a Al2O3-SiO2 (from impurity)-Y2O3 eutectic liquid, becoming grain boundary glass at room temp

Question 5

What happens at increased temps?

Answer 5

~ TZP suffers fracture toughness degradation with increasing temp similar to PSZ

~ it is especially weakened in the presence of water vapor in the 200-300C range

Question 6

Compare TZP and PSZ.

Answer 6

~ PSZ:
~ coarse crystalline particulates
~ tempered with regard to strength: fine structure, mainly tetragonal
~ tempered with regard to resistance to temp changes: fine structure with high monoclinic share
~ lower bending strength
~ medium to high cost

~ TZP:
~ fine crystalline particulates
~ normal sintered: density 96-98% of theory
~ hot isostatic pressed: density > 99% theory
~ higher bending strength
~ very expensive