Effect of Aging on the Flexural Strength of Translucent Monolithic Yttria Stabilized Zirconia

Abstract

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. (1540)

Fracture resistance and fracture modes in endodontically treated maxillary premolars restored using different CAD-CAM onlays

“…For the IPS e.max CAD, the current data show that water storage and thermocycling do not significantly decrease the mechanical properties of the materials in terms of fracture toughness, flexural strength, and hardness [26,56,58,59]. By contrast, thermocycling and cyclic loading induce low-temperature degradation (LTD) and degrade the mechanical properties of zirconia [60,61]. LTD is the spontaneous phase transformation from tetragonal to monoclinic when zirconia is exposed to moisture at low temperatures for a long time [61].…”

“…LTD is the spontaneous phase transformation from tetragonal to monoclinic when zirconia is exposed to moisture at low temperatures for a long time [61]. However, the chemical and crystalline compositions of zirconia determine its susceptibility to LTD; the predominant tetragonal crystalline structure is critically susceptible to LTD, whereas cubic-containing zirconia is generally more stable [60][61][62]. Katana Zirconia UTML contains up to 75 wt% cubic phase and is resistant to LTD after aging [62]; therefore, thermocycling and mechanical loading may have little effect.…”