The effect of sintering conditions and concomitant microstructure of dental zirconia (ZrO 2 ) ceramics on their low-temperature degradation (LTD) behavior remains unclear.
Therefore, their effect on LTD of dental ZrO 2 ceramics was investigated.
Three commercial pre-sintered yttria-stabilized dental zirconia materials were sintered at three temperatures (1450 °C, 1550 °C and 1650 °C) applying three dwell times (1, 2 and 4 h). Grain size measurements and LTD tests were performed on polished sample surfaces. LTD tests were performed at 134 °C in an autoclave. The amount of monoclinic ZrO 2 on the exposed surface was measured by X-ray diffraction (XRD).
Higher sintering temperatures and elongated dwell times increased the ZrO 2 grain size. Simultaneously, a larger fraction of zirconia grains adopted a cubic crystal structure, resulting in a decreased yttria content in the remaining tetragonal grains. Both the larger grain sizes and the lower average stabilizer content made the tetragonal grains more susceptible to LTD. Overall, independent on the commercial dental zirconia grade tested, the specimens sintered at 1450 °C for 1 h combined good mechanical properties with the best resistance to LTD.
In general, increased sintering temperatures and times result in a higher sensitivity to low-temperature degradation of Y-TZP ceramics.
In recent years, all-ceramic restorations are more frequently employed for medium-to-large tooth reconstructions. Due to its superb biocompatibility and favorable mechanical properties, yttria-stabilized tetragonal zirconia polycrystalline (Y-TZP) ceramics can be used as alternative for conventional metal frameworks (as part of metal-ceramic fixed dental prostheses or FDP’s). Dental zirconia has more recently also been introduced for full-contour ‘all-zirconia’ crowns and bridges.
However, hundreds of zirconia THP-heads failed catastrophically between 1999 and 2001, having led to its withdrawal from the market soon after . Later in 2007, the problem of the catastrophic failure was attributed to low-temperature degradation (LTD), i.e. transformation of the metastable tetragonal to the monoclinic phase (at 20–250 °C), initiated and accelerated by water penetration . The cause of the failures was related to an accelerated tetragonal to monoclinic phase transformation of zirconia in a limited number of batches . Although the manufacturing process of those orthopedic zirconia femoral heads is significantly different from that of dental zirconia, recently, some papers have focused on LTD in the dental field . Two papers have reported the reduction of mechanical properties after LTD. One paper has evaluated novel porous zirconia dental implants and reported their aging sensitivity . Kim et al. have reported the influence of different surface treatments on LTD behavior of dental zirconia. Moreover, Chevalier mentioned that the issue of aging is still not discussed for dental implants . Denry et al. reported that some forms of zirconia are susceptible to aging and that processing conditions can play a critical role in the LTD of zirconia .
Several factors influence the LTD behavior of zirconia ceramics: the valency and size of the stabilizing cation, the stabilizer distribution, the phase composition, the grain size and the grain size distribution as well as the presence of secondary phases. The majority of the aforementioned parameters are interdependent and strongly influenced by the sintering condition, such as sintering time, temperature and atmosphere . The zirconia sintering condition is one of the predominating factors in obtaining a stable and durable dental zirconia ceramic. However, in the dental field, only few papers can be found regarding this topic . Moreover, the effect of sintering conditions on LTD is still not very well studied , most probably because the zirconia framework of FDP is covered by veneering ceramics or luting materials, and therefore is separated from the oral environment. However, some part of FDP might not be covered with veneering ceramics. Also, it has been shown that luting materials absorb water via dentin tubules . Moreover, in recent years, full-contour zirconia FDP’s are becoming more popular as well. Compared to conventional FDP’s, those full-contour FDP’s are directly exposed to saliva. Other examples are zirconia dental implants and abutments. They are also directly exposed to blood and saliva. Therefore, the zirconia itself might intra-orally be exposed to moisture that may lead to aging problems .
It is therefore most relevant to study and determine the effect of sintering conditions on mechanical properties, microstructure and LTD behavior of dental zirconia ceramics. The null hypothesis tested was that different sintering conditions do not affect (1) the mechanical properties, (2) the microstructure and (3) the LTD behavior of dental zirconia.
Materials and methods
The experimental design of our study is schematically explained in Fig. 1 .