5.

Critical loads P_R for intaglio radial cracking in monolithic Y-TZP (Prozyr Y-TZP, Norton, East Granby, CT) on a dentin substrate as function of number of contacts with a hard sphere [52]. Data shown for as-polished, sandblasted, and indented (Vickers, 10 N) zirconia undersurfaces. Linear fits to polished and sandblasted cases are in accordance with strength loss from moisture-assisted slow crack growth. Curve through data for indented surfaces is empirical fit, revealing enhanced degradation from mechanical fatigue for flaws within quasiplastic contact zone. Arrows indicate 'runouts', i.e. no failure after 10⁷ cycles.