Test results showed that monolithic lithium disilicate outperformed veneered zirconia restorations. Through a 1,000N load and 1,000,000 cycles, none of the lithium disilicate crowns fractured. Only superficial damage in the form of wear facets similar those of natural teeth can be observed.

The Results — Layered Porcelain on Zirconia
Example of shear failure of porcelain on a zirconia restorations at 300N and approximately 50k cycles.
The Results — Monolithic Lithium Disilicate
No fractures, only superficial damage in the form of wear facets.
Material Survival with Cyclic Fatigue Testing