. Explain the following toughness values for Mg–PSZ measured at temperatures above room temperature:
Temperature, 1C 200 400 600 800
Toughness, MPam1/2 8.5 6.5 5.5 5
. Transformation-toughening Al2O3 has a higher toughness and strength than pure polycrystalline Al2O3 : EAl2O3 ¼ 380 GPa and EZrO2 ¼ 230 GPa, nAl2O3 ¼ 0:23, and nZrO2 ¼ 0:31.
a. Offer a thermodynamic explanation why the toughness of 15% pure ZrO2 in Al2O3 is greater than that of 15% (ZrO2/3% Y2O3) in Al2O3.
b. Suppose that grinding the surface of ZrO2–Al2O3 converts tetragonal ZrO2 to monoclinic. Calculate the stresses surrounding the particle. Assume an isolated particle and no cracking occurs. Is the latter a good assumption?
c. The net stress from all the particles at the surface (part b) is compression. Will this stress increase the fracture strength?
d. Transformation toughening is a low-temperature toughening mechanism; that is, at 500–10001C it is not too effective. Why?
e. Why might the compressive surface layer (part c) be effective to a higher temperature? To what temperature?