Answer TRUE or FALSE regarding the optimum design criteria of 7TZP thermal barrier coatings for jet engine turbine blades? The yttria content of the top coat layer (columnar grains) should be maximized and should exceed at least 15 wt% in the ceramic.
The yttria content of the bonding layer should be maximized and should exceed at least x = 1.0 within a bonding layer of Ni-16-Cr-5Al-xY.
The coefficients of thermal expansion for the TBC sublayers should be matched as close as possible to prevent thermal stresses that could promote cracking and spalling of the TBC. [Select] The thermal conductivities of the TBC sublayers should be matched as close as possible to the underlying base metal (usually a Ni-based superalloy).
Air gaps between grains of top coat 7TZP should be minimized, since they act as stress concentrators for each of the as-grown grains.
Soda-lime glass is the preferred composition for the bonding layer of modern columnar TBCs.