EFFECT OF AGING ON THE FLEXURAL STRENGTH OF TRANSLUCENT MONOLITHIC YTTRIA STABILIZED ZIRCONIA

Aim: the purpose of the present study was to investigate the effect of thermal cycling and mechanical loading on the flexural strength of: Zeno Star (T) a translucent monolithic Yttria stabilized zirconia (Y-TZP).
Materials and Methods: a total number of 40 fully sintered bar – shaped specimens (18mm x 4mm x 1.2mm) were fabricated. The specimens were divided into 4 groups (n=10) according to the aging time: Group I: is the control group with no aging procedures and was tested after 24 hours. Group II: was subjected to thermal cycling (600 cycles) and load cycling 120.000 cycles equivalent to 6 months of clinical service. Group III: subjected to thermal cycling (1200 cycles) and load cycling 240.000 cycles equivalent to 1 year of clinical service. Group IV: subjected to thermal cycling 6000 thermal cycles and load cycles (1.200.000 cycles) equivalent to 5 years of clinical service. For thermal cycling, all specimens were transferred in hot and cold water baths between 5 and 55°C in one minute cycle for each bath with dwell time of 30 seconds. The flexural strength was determined by three points bending test in a universal testing machine at a cross head speed of 1mm/min. Tertragonal to monoclinic transformations due to thermal cycling and mechanical loading (LTD) was identified with X-ray diffraction (XRD).
Results: Comparing the flexural strength of the tested specimens, the mean flexural strength of zirconia in group II (representing six months of clinical service) was significantly higher (1563.7±103.8 MPa ) than that of the control (group I) (1009.7±107.2 MPa) Thermal cycling and mechanical loading equivalent to one year of clinical service (group III) led to significant decrease in the flexural strength (1381±295 MPa) The lowest flexural strength was determined in group IV (833.7±15.2 MPa) where zirconia was subjected to thermal cycling and mechanical loading equivalent to 5 years of clinical service. The relative XRD peak intensity showed that there were some monoclinic transformations at the end of the aging period which is equivalent to 5 years of clinical service.
Conclusions: Thermal cycling and mechanical loading caused a reduction in the mean flexural strength at the period equivalent to one year of clinical service. Further decrease of the flexural strength occurred following aging at the period simulating 5 years of clinical service with some phase transformation from tetragonal to monoclinic.