Zirconia Phase Transformation Kinetic and Clinical Relevance

Objective:

Three commercial available dental zirconia materials were hydrothermally treated up to extreme long durations of 150h. Changes in surface crystal phases, fracture strength and hardness were measured.

Method:

LAVA Frame Zirconia(LF), LAVA Plus High Translucency Zirconia(LP) (3M ESPE) and BruxZirHT2.0(BZ) (Glidewell) were cut, sintered according recommended sintering cycles, ground and polished to slices (XRD) and bars of 3x4x45mm (strength and hardness). Samples were treated up to 150h in an autoclave at 134°C and 2 bar water vapor. At each treatment stage crystal phase content was measured by XRD (n=3, Bruker D8-Discover). Four-point flexural strength (20mm/40mm span, 1mm/s, n=15) and Vickers Hardness via Micro Hardness Testing System (Leco) were measured initially and at comparable monoclinic phase contents. All fracture strength tests have been analyzed and compared by Weibull statistics.

Result:

Table 1: Crystal phase content, fracture strength and hardness depending on hydrothermal treatment time

Conclusion:

The crystal phase transformation kinetics of LF and LP are slower compared to BZ. BZ transformed to a maximum monoclinic content above 60wt% after 30h, whereas LF and LP reached approximately 5wt% monoclinic after the same time. After hydrothermal treatment of 15h, BZ shows a higher monoclinic content and a lower strength (42wt%, 1096MPa, m=20) compared to LF (3wt%, 1285MPa, m=13) and LP (3wt%, 1270MPa, m=13).

At comparable monoclinic surface phase content of about 40wt%, Weibull strength was higher than 1000MPa for all tested materials, however this phase transformation was reached after different treatment times (LF 157h, 1027MPa, m=36 / LP=117h, 1007MPa, m=15 / BZ=15h, 1096MPa, m=16). Surface hardness decreased with increasing hydrothermal treatment time for all three materials.

Different dental zirconia show different transformation kinetics, however monoclinic surface phase did not show a considerable impact on strength as one of the most important clinical relevant material properties.