Investigations of subcritical crack propagation of the Empress 2 all-ceramic system

Introduction The mechanical properties and slow crack propapagation of the all-porcelain system Empress® 2 (Ivoclar Vivadent, Schaan, Liechtenstein) with its framework compound Empress® 2 and the veneering compounds “Empress® 2 and Eris® were examined. Methods For all materials, the fracture strength, Weibull parameter and elastic moduli were experimentally determined in a four-point-bending test. For the components of the Empress® 2 system, the fracture toughness KIC was determined, and the crack propagation parameters n and A were determined in a dynamic fatigue method. Using these data, life data analysis was performed and lifetime diagrams were produced. The development of strength under static fatigue conditions was calculated for a period of 5 years. Results The newly developed veneering ceramic Eris® showed a higher fracture strength (σ0 = 66.1 MPa) at a failure probability of PF = 63.2%, and crack growth parameters (n = 12.9) compared to the veneering ceramic Empress® 2 (σ0 = 60.3 MPa). For Empress® 2 veneer the crack propagation parameter n could only be estimated (n = 9.5). This is reflected in the prognosis of long-term resistance presented in the SPT diagrams. For all materials investigated, the Weibull parameter m values (Empress® 2 framework m = 4.6; Empress® 2 veneer m = 7.9; Eris® m = 6.9) were much lower than the minimum demanded by the literature (m = 15). Significance The initial fracture strength value alone is not sufficient to characterize the mechanical resistance of ceramic materials, since their stressability is time-dependent. Knowledge about the crack propagation parameters n and A are of great importance when preclinically predicting the clinical suitability of dental ceramic materials. The use of SPT diagrams for lifetime calculation of ceramic materials is a valuable method for comparing different ceramics.