Mechanical Properties of Two Zirconia Ceramics after Heat-Treatment and Aging

The temperature variations during the veneering firing cycles of a zirconia dental ceramic can negatively affect its mechanical properties. A possible synergistic effect of both heat-treatment and aging after exposure to the oral environment could result to premature failure. The aim of the present study was to investigate the effect of heat treatment followed during veneering and in vitroaging on the mechanical and microstructural properties of two zirconia dental ceramics.

Method:

Three bar-shaped specimens (28x4x2mm) from each of two zirconia blocks (Ivoclar IPS e.max ZirCAD (IV) and Wieland ZENO Zr (WI)) were cut by CAD/CAM technology, fully sintered and polished. Each one was cut in four equal pieces. One piece was used as control (C), one was heat-treated (H), one was aged (A) (134^oC, 2 bars, 10 hours) and one was heat-treated and subsequently aged (HA). The mechanical properties (nano-hardness (H) and elastic modulus (E*)) were investigated by nanoindentation tests while the surface characterization was carried out with X-Ray diffraction analysis (XRD), Fourier transform Infra-red spectroscopy (FTIR) and Scanning Electron Microscopy (SEM).

Result: Different treatments on IV and WI samples resulted in a statistical significant reduction of both H and E* values (p<0.001). The combination of treatments impose a cumulative effect enhancing the influence of both H and E* values. This reduction in mechanical properties was followed by an increase of monoclinic content, up to 9% w.t. Greater variations in both H and E*values were recorded for WI samples.

Conclusion:

Both nano-hardness and elastic modulus were significantly reduced after heat-treatment and in vitro aging, while the consequential aging after heat-treatment resulted in the worst result. The clinical performance of zirconia dental ceramics may negatively be affected during firing and aging, resulting in unavoidable failure.