Introduction

Zirconia FDPs are successfully used to replace posterior teeth. This success is due to the high flexural strength and fracture toughness of zirconia applied as a framework material [1–3]. Fractures of zirconia frameworks have rarely been reported [4–8]. In contrast, chipping of the veneering ceramic is a frequent complication [4–8]. From a clinical point of view, the stability of the system is of importance consisting of both, the zirconia framework and the veneering ceramic.

In order to decrease the costs, and at the same time to overcome the chipping problem, it has become possible to produce monolithic zirconia FDPs without veneering ceramic. Such zirconia FDPs are esthetically unsuitable due to their high opacity. In ceramics, the translucency is affected by the thickness of the framework and by the crystalline content [9–12]. Sintering parameters have an effect on the crystalline content. It has been shown that the holding time during sintering causes grain growth in the material [13], possibly affecting translucency.

The monoclinic phase is stable up to 1,170°C; above this temperature, it transforms into the tetragonal phase and remains stable up to 2,370°C. The cubic phase of zirconia on the other hand, exists up to the melting point of 2,680°C [14, 15]. The tetragonal form for metastable zirconia could be achieved at room temperature by alloying zirconia with other oxides (stabilizers), such as CaO [16], MgO [17], Y2O3 [18, 19] and CeO2 [20]. Y2O3 is the most widely used stabilizer for dental zirconia [15]. In response to tensile stresses at the crack-tips, the stabilized tetragonal zirconia transforms to the more stable monoclinic phase with a local increase in volume of approximately 4–5% [20]. The toughening mechanism is based on crack-tip shielding under compressive stresses associated with transformation. Cracks with angle of 120° were reported to decrease the fracture toughness [1, 21]. When the microcracked material has a modulus that is different from the bulk ceramic, additional crack-tips may form. In fact, as the cracks grow, to some extent, the toughness does not increase [22]. This phenomenon is determined by crack-wake and crack-tip toughening mechanism [23, 24]. It is this transformation-toughening process which gives zirconia its strength and toughness, exceeding all currently available glass-based ceramics [20]. On the other hand, the size of the transformation zone changes as a function of temperature [25].

CAD/CAM technologies enable milling of zirconia into reconstructions with complex geometries. Two types of zirconia milling processes are currently available: soft-milling (“partially sintered state”) and hard-milling (“full sintered”). Soft-milled frameworks are subsequently sintered to full density. Different sintering parameters may show a strong influence on the properties of the zirconia frameworks.

The aim of this study was to investigate the effect of different sintering temperatures on flexural strength, contrast ratio, and grain size of Y-TZP ceramic. The tested hypotheses were that (a) the increase in final sintering temperature would not decrease the flexural strength and (b) the contrast ratio and the grain size would increase with the increased sintering temperature.

Materials and methods

All zirconia (Ceramill ZI, Amann Girrbach, Koblach, Austria, Lot No: FL08-04119) specimens were cut in the partially sintered state using a low-speed diamond saw (Well 3241, Well Diamantdrahtsägen, Mannheim, Germany) and ground to the final dimensions using SiC discs P220, P500, P1200, P2400, and P4000 (ScanDia, Hagen, Germany) in sequence. Specimens were sintered (LHT 02/16, Nabertherm GmbH, Lilienthal/Bremen, Germany) at a heat rate of 8°C/min to the one of the following final sintering temperatures: 1,300°C (group a), 1,350°C (group b), 1,400°C (group c), 1,450°C (group d), 1,500°C (group e), 1,550°C (group f), 1,600°C (group g), 1,650°C (group h), 1,700°C (group i) with 120 min holding time.

Three-point flexural strength

Three-point flexural strength (N = 198; n = 22 per group) was measured according to ISO 6872: 2008 [26]. After sintering procedures, the final dimensions of all specimens were 1.2 mm × 4 mm × 25 mm.

Before the flexural strength test, the dimensions of the specimens were measured with a digital micrometer (Mitutoyo, Andover, England) to an accuracy of 0.01 mm. The specimens were then placed in the appropriate sample holder and loaded in a Universal Testing Machine (Z010, Zwick, Ulm, Germany) at a crosshead speed of 1 mm/min until failure. The specimens were tested dry at room temperature. The flexural strength was calculated according to the following formula:

$$ \sigma = {\text{3Nl}}/{\text{2b}}{{\text{d}}^{{2}}} $$

where σ: flexural strength, N: fracture load (N), l: distance between supports (mm), b: width of the specimen (mm), and d: thickness of the specimen (mm).

Contrast ratio

For contrast ratio measurements, the specimens (N = 90; n = 10) with dimensions of 20 mm × 20 mm × 0.7 mm were produced. After sintering, the specimens had an average thickness of 0.5 ± 0.05 mm. The contrast ratio was measured using a spectrophotometer (CM-2600 d, Konica Minolta, Hannover, Germany) according to ISO 2471: 2008 [27] under the light source of CIE illuminant D65 with color temperature of 6,504 K. The measurement was performed three times in flashing mode for 0.1 s with an interval of 3 s. Subsequently, the software calculated the mean values, where contrast ratios were measured from the luminous reflectance (Y) of the specimens with a black (Y B) and a white background (Y W). In all calculations, “0” value was considered as transparent and “1” as opaque.

Zirconia grain size

After sintering, the surface of all specimens (N = 9, n = 1 per group) was polished up to 1 μm with a diamond suspension (Struers, Ballerup, Denmark) and ultrasonically cleaned in isopropanol. Specimens were then gold-sputtered and surface topography was evaluated under a scanning electron microscope (Carl Zeiss Supra 50 VP FESEM, Carl Zeiss, Oberkochen, Germany) operating at 5 kV with a working distance of 5.5–6.0 mm.

Statistical analysis

The data were analyzed using a statistical software (SPSS Version 19, SPSS INC, Chicago, IL, USA). Initially, the descriptive statistics were computed. One-way ANOVA was used followed by Scheffé post hoc test (α = 0.05) for the analysis of flexural strength and contrast ratio with respect to sintering temperatures. The Pearson correlation coefficient test evaluated the effect of the sintering temperatures on flexural strength and contrast ratio. Furthermore, for the calculation of the Weibull statistics, the least square estimates of the modulus and characteristic flexural strength were computed according to the mean rank plotting. In all tests, p values smaller than 5% were considered statistically significant.

Results

The results of the descriptive statistics (mean, SD, and 95% CI) for the flexural strength test and contrast ratio measurements for each group are presented in Table 1.

Table 1 Mean, standard deviation (SD) values, and Weibull statistics of flexural strength and contrast ratio with 95% confidence intervals (95% CI) of all tested groups
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Three-point flexural strength

The lowest mean flexural strength was observed in group i, where the sintering temperature was 1,700°C (p < 0.05). Significantly higher flexural strength values (p < 0.05) were observed in groups sintered between 1,400 and 1,550°C (Table 1, Fig. 1). The highest Weibull modulus was obtained with zirconia sintered at 1,400°C and the lowest one at 1,700°C.

Fig. 1
figure 1

Mean flexural strength of zirconia after different sintering temperatures

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Contrast ratio

The contrast ratio of zirconia decreased with the increase in sintering temperature (Table 1, Fig. 2). Group a (1,300°C) showed the lowest translucency (p < 0.05), whereas the highest one was observed in group i (1,700°C) (p < 0.05).

Fig. 2
figure 2

Contrast ratios of zirconia after different sintering temperatures

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Zirconia grain size

The grain size of zirconia increased with higher sintering temperatures above 1,300°C and with the highest results at 1,700°C (Fig. 3a–i). The specimens with a final sintering temperature above 1,600°C were accompanied by hollow opening in the zirconia microstructure (Fig. 4a–f).

Fig. 3
figure 3

a–i Zirconia grain size after different sintering temperatures (×50,000), a 1,300°C, b 1,350°C, c 1,400°C, d 1,450°C, e 1,500°C, f) 1,550°C, g) 1,600°C, h) 1,650°C, i) 1,700°C

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Fig. 4
figure 4

Surface topography of sintered zirconia at a–c) 1,650°C (1st row) and d–f 1,700°C (2st row)

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Sintering temperature showed a significant negative correlation with flexural strength (r 2 = −0.313, p < 0.001) and the contrast ratio (r 2 = −0.96, p < 0.001).

Discussion

The highest fracture strength was observed for zirconia sintered between1,400°C and 1,550°C. However, above 1,600°C, the flexural strength decreased significantly, yielding to the rejection of the first hypothesis. Sintering temperatures at 1,300°C and 1,350°C showed the lowest mean flexural strength. The increase in sintering temperature above 1,300°C enlarged grain size and increased contrast ratio. Therefore, the second hypothesis was accepted.

It has frequently been recommended to sinter with higher final sintering temperature for achieving decreased contrast ratio. In this study, the flexural strength of zirconia decreased when sintered above 1,600°C. It has previously been reported that ceramics with lower flexural strength were generally more translucent than those with higher flexural strength [11]. In this study, the grain size of zirconia increased with increasing sintering temperature.

The increased grain size may result in enhanced crack formation [28] The transformation from tetragonal to monoclinic zirconia decreases with tensile stress [14]. Higher sintering temperatures as well as longer sintering time yield larger grain size [18, 29–31]. Today, the available zirconia is generally sintered between 1,350 and 1,600°C. Higher sintering temperatures were found to migrate yttrium to the grain boundaries [30]. The phase diagram shows cubic zirconia at the grain boundaries and depletion within the grain [30]. Uneven distribution of the yttrium-stabilizing ions caused cubic phases which are not desirable [31].

No clinical data are available reporting on the performance of translucent monolithic zirconia. Nevertheless, according to the results of this study, when a compromise needs to be made for the optical and mechanical properties, the sintering temperature should not exceed 1,550°C. With this settings, clinical failures should be avoided.

In this study, a three-point flexural strength test was used to evaluate the mechanical properties of zirconia sintered at different temperatures. The flexural strength data were supported with Weibull distribution in which failure probability can be predicted at any level of stress. Using statistical analyzing program (SPSS 19), only the absolute estimates could be obtained, but information on the 95% CI and the post hoc test for the Weibull parameter was not possible to calculate. Therefore, a statistical comparison between the tested groups was not possible. Sintering temperatures at 1,400°C and 1,550°C presented the highest Weibull modulus, whereas at 1,700°C, the lowest Weibull modulus and the highest translucency were observed.

A limitation of this study was that only one zirconia brand was used. The results may not apply for other zirconia materials with different grain sizes.

Conclusions

  1. 1.

    Zirconia ceramic tested showed the highest flexural strength at final sintering temperatures between 1,400°C and 1,550°C.

  2. 2.

    Contrast ratio of the tested zirconia increased with the increase in final sintering temperatures above 1,300°C.

  3. 3.

    Enlarged grains of the zirconia microstructure were observed with the increase in sintering temperatures above 1,300°C.

  4. 4.

    Sintering temperatures above 1,600°C resulted in grain growth and hollow holes in the zirconia microstructure.