Effect of heat treatment and in vitro aging on the microstructure and mechanical properties of cold isostatic-pressed zirconia ceramics for dental restorations

Abstract Objectives 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 while exposed to the oral environment could result to catastrophic failure. The aim of the present study was to investigate the effect of heat treatment followed during veneering and in vitro aging on the mechanical and microstructural properties of zirconia dental ceramics. Methods Three specimens 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 parts. One part was used as control (C), one was heat-treated (H), one was aged (A) (134 °C, 2 bar, 10 h) and one was heat-treated and subsequently aged (HA). The mechanical properties (nano-hardness ( H ) and elastic modulus ( E* )) were investigated by nano-indentation tests while the surface characterization was carried out with XRD, FTIR and SEM. Results Different treatments on IV and WI samples resulted in a reduction of both H and E* values, however the differences were not statistically significant ( p > 0.05). The combination of treatments imposes an overall effect ( p < 0.001), enhancing the influence on both H and E* values. This reduction in mechanical properties was followed by an increase of monoclinic content. Greater variations in both H and E* values were recorded for WI samples. Significance The clinical performance of zirconia dental ceramics may be affected during firing and aging resulting in increased probability of failure.