Lifetime prediction of CAD/CAM dental ceramics

The dynamic fatigue method was used to obtain subcritical crack growth parameters n and A for a commercial feldspathic dental porcelain and for a lanthanum-glass-infiltrated alumina glass ceramic. Five stress rates d sigma/dt ranging from 50 to 0.01 MPa s(-1) were applied. The inert strength values were calculated with the use of Weibull statistics and maximum-likelihood approaches for the Weibull parameter m. Strength-probability-time (SPT) diagrams were derived for both materials. The alumina glass composite showed a high fracture strength sigma(0) (442 MPa) at a failure probability of P(F) = 63.2% and a high resistance against subcritical crack growth (n = 36.5). The development of strength under fatigue conditions was calculated for exemplary 1 year. The strength of the alumina glass material dropped to 228 MPa within this period. This fact is due to a low content of infiltrated lanthanum glass phase in the composite material (25 wt%). In contrast, for the high-silica-glass-containing porcelain a distinct decrease of strength sigma(0) from initial 133 to 47 MPa after 1 year was predicted. This, mainly because of a low crack growth resistance (n = 16.8) of the feldspathic porcelain. Much lower strength values were calculated, assuming a failure probability of P(F) = 5%. The decrease is mainly caused by the sensitivity of high-glass-containing ceramics against water corrosion.