Speed-sintering and the mechanical properties of 3–5 mol% Y₂O₃-stabilized zirconias

The clinical success of classical, conventionally sintered zirconia, namely, 3 mol% Y₂O₃-stabilized zirconia (3YSZ) is striking, as per the clinical evidence showing low fracture rates compared to glass–ceramics [1, 2]. The excellent clinical performance of 3YSZ is credited to the high fracture toughness induced by the high amount of the tetragonal (t) phase that is readily transformable to the monoclinic (m) symmetry [3] upon local stress, which induces a volumetric expansion ahead of propagating cracks [4]. Today, more translucent zirconias with reduced amount of stabilizer (i.e., 4YSZ and 5YSZ), and thus reduced amount of t-phase, find increasing clinical applicability, especially in the monolithic form, a trend ignited by the well-known chipping problem in veneered-zirconia systems [5, 6].

Conventionally, dental YZS ceramics are sintered for purpose of densification and grain growth in a process that takes up to 6–8 h from heating up to about 1500 °C, with a usual dwell time of 2 h and a relative slow cooling down phase. Such long sintering programs are conducted in conventional furnaces built using tubular heating elements that generate heat through resistive heating of ceramic conductive wires, such as silicon carbide (SiC, up to ~ 1625 °C) or molybdenum disilicide (MoSr₂, up to ~ 1850 °C). This form of heat generation is slow compared to other technologies such as induction heating used in newer generations of sintering furnaces, which make use of an electromagnetic field induced to a copper coil in alternate current. That magnetic field creates eddy currents in the material that feels resistance toward flow, heating up by Joule heating. Induction furnaces are thus more efficient and can operate in high heating rates, and have recently entered the dental market for purpose of sintering zirconia ceramics, thus optimizing workflow in dental laboratories and enabling zirconia to be used also in chairside applications. With induction furnaces, sintering of dental zirconias following the so-called speed- and super-speed-sintering became approaches of highest interest, despite backed by limited experimental evidences. Initial evaluations seem to show that such protocols including fast heating and short dwell times can be used safely to sinter dental zirconias, with negligible adverse effects to density, translucency, and mechanical properties [7, 8]. Although the mechanical stability of speed-sintered zirconias has been covered in terms of force-at-fracture experiments and more standardized strength testing [9‐12], the property of fracture toughness—of highest relevance than strength—has yet not been addressed in the context of speed-sintering.

Although induction furnaces constitute the state-of-the-art in sintering technology for dental zirconias, they also represent a significant financial investment that cannot be currently taken to be widespread, making the use of conventional furnaces still the standard practice. The purpose of this study was therefore to evaluate the feasibility of speed-sintering programs using a conventional furnace regarding the physical and mechanical properties, including Young’s modulus, biaxial flexural strength, and fracture toughness.

The Young’s modulus, density, and Poisson’s ratio are listed in Table 2 and showed no statistical different between sintering programs. The results of the biaxial flexural strength testing, treated using two-parameter Weibull statistics, are detailed in Table 3 in terms of the scale σ₀ and shape (m) parameters of the distribution. The results of the fracture toughness are also included in Table 3. In Fig. 3, the results of the characteristic strength and fracture toughness are summarized side-by-side.

Although there were significant differences in characteristic strength for some of the materials between the two sintering programs, speed-sintering did not always lead to a reduction of the strength, with the material IPS e.max ZirCAD MT showing a significant increase in strength after speed-sintering. Also, differences were not restricted to only one material type (amount of stabilizer), but affected 3, 4, and 5 mol% Y₂O₃-stabilized materials. Regarding fracture toughness, statistically significant differences were only observed for the two Cercon materials, where speed-sintering reduced the fracture toughness. Otherwise, for all other materials, the fracture toughness was statistically comparable between the sintering programs.

Regarding the statistical treatment of strength data, m is a representation of the distribution of critical defect sizes responsible for triggering the fracture in the sampled effective surface/volume, which is dependent on the specimen size and loading configuration. In the context of sintering, significant changes in m suggest a different dynamic in the densification of the white-body, related not to the heat distribution once both heating rates were very similar, but to the dwell time. Belli and Lohbauer [18] clearly showed that in dental zirconias, the defect distribution in the fully sintered piece is inherited from the defect distribution in the white-body. Passed down to the sintered analogs, the morphology of critical defects in the white-body constitute junction vertices between spherical spray-dried granulates that failed to reach intimate contact during pressing. It is conjectured that during uniaxial compaction, the stiff binder at the outer granule shell becomes trapped at the vertices, assuming polyhedral shapes that further oppose compaction [28]; the burn-out of the binder concomitant to the shrinkage of the granules toward their center of mass leaves behind empty spaces at the junctions that cannot be filled by mass transport [29]. The morphology of such defects is a three-dimensional “crowfoot” with “spikes” that are invariably unfavorably oriented in relation to the direction of tension. The parameter m was reduced significantly for three materials, namely Cercon ht, Prettau, and IPS e.max ZirCAD MT, while increasing for Lava Esthetic. In the speed-sintered specimens, apart from IPS e.max ZirCAD MT, all material showed a Weibull modulus between 7 and 13, showing less variability than under conventional sintering conditions (m between 6 and 19). Another sign of the sintering strategy having an effect on how sintering defects develop was a change in σ₀, which represents the strength at a 63.2% failure probability as a reflection of the scale of the sizes of critical defects. That was seen in Prettau, Prettau Anterior, and Cercon xt leading to a significant reduction of σ₀, with the opposite effect in IPS e.max ZirCAD MT. A decrease in σ₀ might imply that defects remained larger compared to when the dwell time is increased from 35 min to 2 h. Possibly, an increased dwell time favors sintering to extend and close existing voids in the white-body, especially at the edges of defects, where the faces between pressed granules are in closer contact. The effect, however, does not seem to be systematic or dependent on stabilizer content, but be rather material dependent. Comparisons to the literature are not always straightforward in respect to speed-sintering, since time, temperatures, furnaces, and heating principles (e.g., induction, plasma, microwave, etc.) are particular for each study. Nonetheless, some patterns can be recognized, namely, most strength experiments report no significant differences to the conventional sintering [7, 8, 11, 30, 31], with few observing even some improvement [32]. Kaizer et al. [33] though showed that speed and super-speed-sintering lead to higher amounts of pitting during contact sliding wear compared to conventional sintering, pointing to densification issues. Other studies describe that speed-sintering led to a change in grain size distribution [7, 8], with speed-sintering inducing larger fraction of fine grains, and a decrease in medium grains [8]. In terms of phase fractions, sintering protocol seems to result in changes in the proportion of tetragonal and cubic grains, but this phenomenon does not seem to follow a certain pattern, but also be material dependent [7, 8].

Microstructural aspects, specifically grain size, have shown to affect the distribution of the stabilizer within the grains, in turn determining the tetragonal to monoclinic transformability, the property which confers the highest gain in toughening in yttria-stabilized zirconias [3]. Actually, using the phase content of the materials evaluated here sintered conventionally, including a 2 mol% Y₂O₃-stabilized zirconia, we established a power-law relationship between K_Ic and the volume content of the t-phase [13]. That led to an inverse power-law relationship between K_Ic and yttria content of the form K_Ic(xY₂O₃) = K_I,0 e^−λx + K_Ic,cubic, with λ = -1.102 mol%⁻¹, challenging the classic relationship of Lange [34] and agreeing rather to the trend seen by Masaki [35]. With speed-sintering, that relationship is not disturbed, suggesting that the main factors affecting the fracture toughness, namely grain size distribution and t-phase content, remained mostly preserved with the decrease in dwell time. Two exceptions must be noted here, namely, the fracture toughness of the materials Cercon ht and Cercon xt, both from the manufacturer Dentsply-Sirona, were the only ones that showed a significant decrease (10% and 16%, respectively) resulting from speed-sintering. Interestingly, the speed-sintering program utilized here corresponded exactly to the heating rate, maximum temperature, and dwell time recommended by Dentsply-Sirona. For both products though, the conventional sintering program had the longest dwell time of all products, which might have actually contributed to an increased K_Ic-value for the conventionally sintered specimens. When compared to the materials having the same composition (Lava Plus and Prettau; IPS e.max ZirCAD MO has a Al₂O₃ of 0.3 mol%, significantly higher than all other materials), Cercon ht shows the highest K_Ic-value when conventionally sintered, and still statistically similar to Lava Plus despite its drop after speed-sintering. For Cercon xt, its K_Ic-value despite now statistically lower than Prettau Anterior remained statistically similar to Katana STML. In that view, the drop in K_Ic-values for both materials after speed-sintering might be relativized, once still in the range of other materials for comparable compositions.

The comparison of conventional vs. speed-sintering protocols was here performed in the same furnace based on electrical resistive heating elements, and could be supplemented by future research evaluating the use of induction furnaces. Additionally, even though values of strength and fracture toughness can be indicative of the phase composition in dental zirconias, as shown in Refs. [13] and [18], further evaluations on microstructural parameters (grain size and distribution) and phase content using scanning electron microscopy and X-ray diffraction, respectively, would be warranted for additional insights on more specific effects of speed-sintering relative to conventional sintering programs.

From the results of the present study, we can conclude that sintering dental zirconias (3-5YSZ) using a speed-sintering protocol in a conventional furnace does not significantly compromise their mechanical properties and could be adopted safely, in detriment of the longer, more energy-consuming conventional sintering protocol.