Fracture toughness of dental zirconia before and after autoclaving

Abstract

The purpose of this study was to evaluate the low temperature aging degradation (LTAD) of two types of zirconia through the determination of fracture toughness before and after autoclaving. The fracture toughness of yttria stabilized tetragonal zirconia polycrystals (Y-TZP) and zirconia/alumina nanocomposite stabilized with cerium oxide (Ce-TZP/Al 2 O 3 nanocomposite, so called NANOZR) were determined by Single Edge V-Notch Beam (SEVNB) method before and after autoclaving at 134°C for 5 h. The fracture surfaces were characterized by micro-X-ray diffractometry and micro-Raman spectroscopy. Mean fracture toughness of Y-TZP slightly decreased from 6.31 to 5.79 MPa·m 1/2 with the autoclaving ( p < 0.01). On the other hand, mean fracture toughness of NANOZR showed no change as 11.03 MPa·m 1/2 ( p > 0.1) and was 1.751.91 times larger than that of Y-TZP. It seems to be caused by that NANOZR is susceptible to stress-induced transformation from tetragonal to monoclinic, but quite stable against LTAD.

Phase transformation of zirconia ceramics by hydrothermal degradation

“…For example, a flexural strength of 900-1,200 MPa was recorded for Y-TZP in vitro, which is higher than the flexural strength for other dental alloys 7) . Despite the success of zirconia and its variegated applications, it has now become apparent that zirconia ceramics also have a drawback, in that they have a propensity to undergo low-temperature degradation (LTD) in the presence of moisture [8][9][10][11][12][13] . This is a kinetic phenomenon in which the polycrystalline tetragonal material slowly transforms into monoclinic zirconia over a rather narrow but important temperature range, typically between room temperature and approximately 400°C, depending on the stabilizer used, its concentration, and the grain size of the ceramic.…”

“…This is a kinetic phenomenon in which the polycrystalline tetragonal material slowly transforms into monoclinic zirconia over a rather narrow but important temperature range, typically between room temperature and approximately 400°C, depending on the stabilizer used, its concentration, and the grain size of the ceramic. This phase transformation is followed by microcracking and loss of strength [11][12][13] . In the application of zirconia to dental restorations, there is concern about its degradation when constantly used in a humid atmosphere at body temperature over a long period of time and when it is autoclaved for sterilization.…”

“…Further, the addition of alumina helped suppress the propagation of phase transformation into the bulk of the material and increased the hydrothermal stability of the tetragonal phase 14,16) . Ban et al suggested that there were no changes in fracture toughness of ATZ after autoclaving, but the conventional Y-TZP showed a slight decreasing after autoclaving 11) . When the degradation of Y-TZP and ATZ was compared in a hydrothermal environment, ATZ exhibited a lower phase transformation rate compared with Y-TZP and a decelerated aging progress in a hydrothermal environment at body temperature after 50 years in vivo 9,16) .…”

Effect of thickness of zirconia-ceramic crown frameworks on strength and fracture pattern

“…These superior mechanical properties exhibited by zirconia ceramics are attributed to the stress-induced transformation from the tetragonal to monoclinic phase and its release during crack propagation 10) . Ce-TZP/A are more susceptible to stress-induced transformation than Y-TZP 11,12) . Y-TZP have a few problems concerning the low temperature aging degradation (LTAD) caused by phase transformation 10,13) , whereas Ce-TZP/A have complete resistance to LTAD 11,12,14) .…”

HRTEM observation of bonding interface between Ce-TZP/Al₂O₃ nanocomposite and porcelain

“…The toughness, strength, and hardness of this nanocomposite are two to three times those of conventional Y-TZP 5) . Ban et al have shown its general superiority as a dental restorative material over myriad commercial dental ceramics [6][7][8][9] . For use in crowns and bridges, the surface of zirconia is generally veneered with feldspathic porcelain to compensate for its insufficient translucency.…”