d e n t a l m a t e r i a l s 3 0 ( 2 0 1 4 ) 669–678
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Influence of sintering conditions on low-temperature degradation of dental zirconia Masanao Inokoshi a , Fei Zhang b , Jan De Munck a , Shunsuke Minakuchi c , Ignace Naert a , Jozef Vleugels b , Bart Van Meerbeek a , Kim Vanmeensel b,∗ a
KU Leuven BIOMAT, Department of Oral Health Sciences, KU Leuven (University of Leuven) & Dentistry, University Hospitals Leuven, Kapucijnenvoer 7, blok a bus 7001, B-3000 Leuven, Belgium b Department of Metallurgy and Materials Engineering (MTM), KU Leuven (University of Leuven), Kasteelpark Arenberg 44, B-3001 Heverlee (Leuven), Belgium c Gerodontology and Oral Rehabilitation, Graduate School of Medical and Dental Sciences, Tokyo Medical and Dental University, 1-5-45 Yushima, Bunkyo, Tokyo 113-8549, Japan
a r t i c l e
i n f o
a b s t r a c t
Article history:
The effect of sintering conditions and concomitant microstructure of dental zirconia (ZrO2 )
Received 24 June 2013
ceramics on their low-temperature degradation (LTD) behavior remains unclear.
Received in revised form
Objectives. Therefore, their effect on LTD of dental ZrO2 ceramics was investigated.
25 February 2014
Methods. Three commercial pre-sintered yttria-stabilized dental zirconia materials were sin-
Accepted 7 March 2014
tered 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 ZrO2
Keywords:
on the exposed surface was measured by X-ray diffraction (XRD).
Zirconia
Results. Higher sintering temperatures and elongated dwell times increased the ZrO2 grain
Low-temperature degradation
size. Simultaneously, a larger fraction of zirconia grains adopted a cubic crystal structure,
Sintering
resulting in a decreased yttria content in the remaining tetragonal grains. Both the larger
Aging
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. Significance. In general, increased sintering temperatures and times result in a higher sensitivity to low-temperature degradation of Y-TZP ceramics. © 2014 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
∗ Corresponding author at: Department of Metallurgy and Materials Engineering (MTM), KU Leuven (University of Leuven), Kasteelpark Arenberg 44, B-3001 Heverlee (Leuven), Belgium. Tel.: +32 16 32 11 92; fax: +32 16 32 19 92. E-mail address: [email protected] (K. Vanmeensel).
http://dx.doi.org/10.1016/j.dental.2014.03.005 0109-5641/© 2014 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
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1.
d e n t a l m a t e r i a l s 3 0 ( 2 0 1 4 ) 669–678
Introduction
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 [1–3]. 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 [4–6]. The cause of the failures was related to an accelerated tetragonal to monoclinic phase transformation of zirconia in a limited number of batches [5]. 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 [7–12]. Two papers [7,11] have reported the reduction of mechanical properties after LTD. One paper has evaluated novel porous zirconia dental implants and reported their aging sensitivity [8]. Kim et al. [10] 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 [13]. 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 [14]. 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 [15]. 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 [16]. Moreover, the effect of sintering conditions on LTD is still not very well studied [9,17], 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 [18]. 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 [8]. 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.
2.
Materials and methods
The experimental design of our study is schematically explained in Fig. 1.
2.1.
Pilot study experiments
A pilot test, prior to the actual study, was performed in order to determine the amount of samples required to generate statistically relevant data. Pre-sintered yttria-stabilized zirconia samples of three commercial dental zirconia ceramics (Table 1: Aadva, GC, Tokyo, Japan; IPS e.max ZirCAD, Ivoclar-Vivadent, Schaan, Liechtenstein; In-CeramYZ, Vita Zahnfabrik, Bad Säckingen, Germany) were sliced into three samples with an overall volume of approximately 12.5 mm × 6 mm × 3.75 mm. Those were pressureless sintered in air at 1450 ◦ C for 2 h using a computer programmed furnace (Nabertherm, Germany). A heating rate of 20 ◦ C/min was applied up to 1000 ◦ C and subsequent heating at 10 ◦ C/min up to the sintering temperature. All specimens were mirror polished on both top and bottom surfaces. As such, 3 samples of each dental zirconia grade were sintered under identical conditions. Furthermore, since all 9 samples, 3 × 3 samples belonging to each of the different commercial grades, were sintered during the same thermal cycle, it can be guaranteed that all samples underwent exactly the same heat treatment. All sample densities were measured using the Archimedes principle. Cu K␣ (40 kV, 40 mA) X-ray diffraction (XRD, Seifert 3003 T/T, Seifert, Ahrensburg, Germany) analysis was used for phase identification and measuring the amount of m-ZrO2 on the sample surfaces exposed to water vapor during LTD. Both top and bottom surfaces of each specimen were analyzed from 27 to 33◦ 2 with a step size of 0.02◦ for 2 s. The volume fraction of m-ZrO2 was calculated according to the method of Toraya et al. [19] LTD tests were performed following ISO 13356: at 134 ◦ C for 6 h under a standard H2 O pressure of 2 bar.
2.2. Specimen preparation and characterization of the actual experiment Pre-sintered yttria-stabilized zirconia samples of three commercial dental zirconia ceramics (Table 1: Aadva, GC; IPS e.max ZirCAD, Ivoclar-Vivadent; In-CeramYZ, Vita Zahnfabrik) were each sliced into nine samples with an overall volume of approximately 12.5 mm × 12.5 mm × 3.75 mm. Those 27 samples were pressureless sintered in air at three different temperatures (1450, 1550 and 1650 ◦ C) for three different dwell times (1, 2 and 4 h) using a computer programmed furnace (Nabertherm, Germany). A heating rate of 20 ◦ C/min was applied up to 1000 ◦ C and subsequent heating at 10 ◦ C/min up to the sintering temperature. Then, all specimens were sectioned into two halves, of which one was used for grainsize measurement, while the other half was subjected to a
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Fig. 1 – Flow chart of the experimental study set-up.
hydrothermal degradation test. The obtained specimen size was 10.0 mm × 5.0 mm × 3.0 mm.
2.3.
Density measurement
The density of the specimens was determined according to the Archimedes principle. Although the generally reported density for 3 mol% yttria stabilized tetragonal zirconia (3YTZP) is 6.05 g/cm3 [20], the relative density was calculated using theoretical density values originating from Rietveld refinements, enabling the calculation of phase fractions (monoclinic, tetragonal, cubic) and yttria stabilizer content in each of the composing phases.
2.4.
Vickers hardness and fracture toughness
Vickers hardness and fracture toughness were determined using the indentation technique (Model FV-700, Future-Tech, Tokyo, Japan) applying the Anstis model for fracture toughness calculation [21]. An indentation load of 10 kg was applied.
2.5.
Microstructural investigation
Thermal etching was performed at 1350 ◦ C for 20 min to reveal the grain boundary network. Microstructural investigation was performed by scanning electron microscopy (SEM, XL30-FEG, FEI, The Netherlands). The average grain size of the phases was measured according to the linear intercept method [22]. The reported values are the average of at least 400 grains measured by means of image analysis on SEM photomicrographs using Image-pro-plus software (version 6.0, Media Cybernetics, Silver Spring, MD).
2.6.
X-ray diffraction analysis
Cu K␣ (40 kV, 40 mA) XRD (Seifert 3003 T/T, Seifert) was used for phase identification and calculation of the relative phase content of monoclinic and tetragonal ZrO2 . Both top and bottom surfaces of each specimen were analyzed from 27 to 33◦ 2 with a step size of 0.02◦ for 2 s. The transformed zirconia fraction was defined as the difference in monoclinic zirconia (m-ZrO2 ) content between a (partially) degraded and a polished
Table 1 – Overview of the experimental groups. Material
Type of material
Aadva IPS e.max ZirCAD In-Ceram YZ
Y-TZP Y-TZP Y-TZP
a
Manufacturer GC, Tokyo, Japan Ivoclar-Vivadent, Schaan, Liechtenstein Vita Zahnfabrik, Bad Säckingen, Germany
Information as provided by the manufacturers.
Batch number 1003291 P55259 35080
Compositiona ZrO2 , Y2 O3 , HfO2 , Al2 O3 ZrO2 , Y2 O3 , HfO2 , Al2 O3 ZrO2 , Y2 O3 , HfO2 , Al2 O3 , SiO2
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Table 2 – Pilot study results showing the absolute density after sintering at 1450 ◦ C for 2 h and the m-ZrO2 fraction after LTD at 134 ◦ C for 6 ha . Density (g/cm3 ) Mean Aadva IPS e.max ZirCAD In-Ceram YZ a
6.02 6.03 6.04
SD 0.007 0.002 0.008
m-ZrO2 fraction (%) Mean 14.5 15.5 15.5
SD 0.9 1.1 1.3
For each dental zirconia grade, 3 specimens or 6 exposed sample surfaces were tested.
surface. The volume fraction of m-ZrO2 is calculated according to the method of Toraya et al. [19]. The aging kinetics were described using the Mehl–Avrami–Johnson (MAJ) equation:
f = 1 − exp −(bt)
n
where f is the transformation fraction, t is the time, and b and n are constants [4]. Especially, b is a parameter dependent both on nucleation rate and growth velocity of monoclinic nuclei, and n is related to the spatial characteristics of the crystallization process [23]. The kinematic parameter b, and the Avrami exponent n were determined by means of leastsquares estimates using the Golub-Pereyra algorithm for partially linear least-squares models (nls function, R3.01, R foundation for Statistical Computing, Vienna, Austria). All tests were performed at a significance level of ˛ = 0.05 using the abovementioned software package. Rietveld analysis was used to calculate the unit cell dimensions of the commercial Y-ZrO2 grades that were pressureless sintered. Therefore, XRD scans were taken from polished sample surfaces in the 20–80◦ 2 range, using a step size of 0.02◦ for 2 s. Based on the calculated unit cell parameters, the Y2 O3 content of the tetragonal and cubic phases in the different commercial grades after sintering can be calculated using the formulas described by Krogstad et al. [24].
2.7.
LTD tests
LTD tests were performed following ISO 13356: at 134 ◦ C under a standard H2 O vapor pressure of 2 bar. The autoclave was placed in an oil bath to establish an internal temperature of 134 ◦ C, as monitored by a thermocouple in the autoclave. The amount of m-ZrO2 on the exposed sample surface was measured by XRD (Seifert 3003 T/T, Seifert).
3.
Results
3.1.
Pilot study
The results of the pilot study are summarized in Table 2. For each dental zirconia grade, three specimens were sintered and all specimens were afterwards subjected to a LTD treatment at 134 ◦ C for 6 h. Regarding the density measurement, all specimens exhibited the same absolute density of 6.08 g/cm3 , with very small standard deviations. After 6 h LTD testing, the average monoclinic volume fraction found on the exposed surfaces of the Aadva grade was 14.5 ± 0.9%, while that of the IPS e.max ZirCAD and In-Ceram YZ grades was 15.5 ± 1.1%
and 15.5 ± 1.3%, respectively. Again, the differences between the different zirconia grades were very small and, even more importantly, the observed standard deviations within one sample group were limited. Based on the identical absolute density values and the limited variation on the monoclinic fraction after 6 h LTD within one sample group, it was decided that one sample per dental zirconia grade was representative to study their behavior during LTD testing in the actual experiments. Thus, one sample, each containing two exposed sample surfaces, was used to study the LTD behavior of three dental zirconia grades following three sintering temperatures and three sintering times, totaling to 27 samples in the actual comparative experimental testing matrix.
3.2.
Actual experiments
3.2.1.
Density measurement
High densities were obtained for all samples, since no open porosity was observed after sintering. An overview of the densities of the different samples, obtained after pressureless sintering at different conditions is shown in Fig. 2a. The theoretical density of Y-ZrO2 depends on its phase constitution as well as its stabilizer content. Using the results from X-ray diffraction experiments, combined with Rietveld refinements, an exact theoretical density can be calculated. The calculated theoretical density of all ceramics varied between 6.05 and 6.07 g/cm3 , depending on the exact phase composition and the yttria stabilizer content in the different phases, as indicated in Table 3. All the pressureless sintered samples exhibit a relative density of 99.0% or more.
3.2.2.
Vickers hardness and fracture toughness
An overview of the Vickers hardness and fracture toughness data is shown in Fig. 2b and c, respectively. When the sintering temperature was increased, the Vickers hardness decreased. On the other hand, when the sintering temperature was increased, the fracture toughness was almost constant up to 1550 ◦ C and slightly increased when sintered at 1650 ◦ C.
3.2.3.
Microstructural investigation
An overview of the grain size measurements performed on the different ceramics, obtained after pressureless sintering at different conditions is shown in Fig. 3. Fig. 4 shows examples of secondary electron (SE) images of the Aadva (GC) specimens sintered at different conditions. The average grain size of the specimen sintered at 1450 ◦ C for 1 h was 0.26 m. On the other hand, the average grain size of the specimen from the same grade sintered at 1650 ◦ C for 4 h was 0.69 m. In general, higher sintering temperatures increased the ZrO2 grain size. At 1450 ◦ C and 1550 ◦ C, a variation of sintering dwell times did not result in significant coarsening of the microstructure. However, at 1650 ◦ C, elongated dwell times clearly increased the grain size.
3.2.4.
X-ray diffraction
The XRD patterns recorded on polished ceramic surfaces that were pressureless sintered at 1450 ◦ C for 1 h, indicated that all three grades consisted of mainly tetragonal zirconia. However, when the sintering time or sintering temperature was increased, diffraction peaks originating from the cubic
d e n t a l m a t e r i a l s 3 0 ( 2 0 1 4 ) 669–678
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Fig. 2 – Summary of density measurements and mechanical properties. (a) Results of the density measurements. All the sintered specimens showed relative density of 99.0% or more. At 1650 ◦ C, the measured density was relatively lower than the other specimens. (b) Results of the Vickers hardness measurements. When the sintering temperature was increased, the Vickers hardness value was decreased. (c) Results of the toughness measurements. When the sintering temperature was increased, the indentation toughness was also increased.
zirconia phase could be detected. At the highest sintering temperature and time, even monoclinic phase peaks could be detected due to spontaneous transformation of the metastable tetragonal zirconia phase into monoclinic zirconia. Therefore, 3 different ZrO2 crystal structures were taken into account to perform the Rietveld refinement: a cubic ZrO2 phase with fluorite structure belonging to the Fm-3m (225) space group, a tetragonal phase belonging to the P42/nmc (137) space group and a monoclinic phase belonging to the P21/c (14) space group. The calculated weight percentages of the different ZrO2 phases that were detected in the In-Ceram YZ (Vita
Zhanfabrik), sintered at different temperatures and times, are summarized in Table 3. A similar trend was observed for the two other zirconia grades. As the cubic zirconia content in the sintered ceramic increased, the yttria content of the tetragonal phase decreased.
3.2.5.
LTD tests
The results of the LTD tests are summarized in Fig. 5. All ceramics experienced a partial tetragonal to monoclinic phase transformation after steam exposure at 134 ◦ C. The samples sintered at 1450 ◦ C for 1 h exhibited a saturation monoclinic
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Fig. 3 – Summary of grain size measurement. Higher sintering temperature showed larger grain size. At 1450 ◦ C and 1550 ◦ C, the elongated dwelling time did not affect the grain size. At 1650 ◦ C, however, the elongated dwelling time clearly increased the grain size.
Fig. 4 – SEM images of sintered and thermally etched Aadva (GC) specimens: (a) Specimen sintered at 1450 ◦ C, 1 h with an average grain size of 0.26 m. (b) Specimens sintered at 1450 ◦ C for 2 h with an average grain size of 0.30 m. (c) Specimens sintered at 1450 ◦ C for 4 h with an average grain size of 0.36 m. (d) Specimens sintered at 1550 ◦ C for 1 h with an average grain size of 0.43 m. (e) Specimens sintered at 1550 ◦ C for 2 h with an average grain size of 0.54 m. (f) Specimens sintered at 1550 ◦ C for 4 h with an average grain size of 0.62 m. (g) Specimens sintered at 1650 ◦ C for 1 h with an average grain size of 0.81 m. (h) Specimens sintered at 1650 ◦ C for 2 h with an average grain size of 1.03 m. (i) Specimen sintered at 1650 ◦ C, 4 h with an average grain size of 1.69 m.
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Fig. 5 – Representative results of LTD tests. The monoclinic volume fraction is plotted as function of LTD time (h). The saturation monoclinic volume fraction reached a value of 0.4–0.6 in the specimens pressureless sintered at 1450 ◦ C for 1 h, after 30 h LTD testing. At elongated sintering times and elevated sintering temperatures, a clear increase in LTD sensitivity is observed, resulting in larger monoclinic phase saturation levels ∼0.8. At a sintering temperature of 1650 ◦ C, the saturation monoclinic volume fraction decreases to 0.65–0.7, because of the presence of a larger fraction of non-transformable cubic zirconia grains.
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Table 3 – Representative data of the Rietveld Analysis (In-Ceram YZ). wt% of different ZrO2 polymorphs
Y2 O3 content in tetragonal phase (mol%)
Theoretical density (g/cm3 )
Tetragonal
Cubic
Monoclinic
1450 ◦ C – 1 h 1450 ◦ C – 2 h 1450 ◦ C – 4 h
97.8 85.5 85.2
0.6 11.6 12.9
1.6 2.9 1.9
3.0 2.7 2.6
6.07 6.06 6.06
1550 ◦ C – 1 h 1550 ◦ C – 2 h 1550 ◦ C – 4 h
83.4 82.7 81.1
15.4 16.5 17.6
1.2 0.8 1.3
2.6 2.5 2.5
6.07 6.06 6.06
1650 ◦ C – 1 h 1650 ◦ C – 2 h 1650 ◦ C – 4 h
82.4 81.1 78.2
16.7 17.7 17.1
0.9 1.2 4.7
2.4 2.5 2.5
6.07 6.07 6.05
volume fraction between 0.4 and 0.6, after 30 h LTD testing. When the sintering time was elongated, much faster degradation and higher saturation monoclinic fractions were obtained. After 30 h LTD testing, the saturation monoclinic volume fraction reached a value of 0.8 when sintered at 1450 ◦ C for 2 h or 4 h. An even faster degradation was observed when sintered at 1550 and 1650 ◦ C. At 1650 ◦ C, however, the saturation monoclinic volume fraction decreased to 0.65–0.7. A summary of the kinematic parameter b, the Avrami exponent n and estimated saturation level for the different LTD test samples are represented in Table 4. These results clearly indicate that the slowest degrading zirconia ceramics were sintered at 1450 ◦ C for 1 h. Aadva (GC) ceramics showed a clearly higher b value, indicating a faster degrading than the other commercial zirconia.
4.
Discussion
Higher sintering temperatures and elongated dwell times influenced the density and phase composition of dental ZrO2 ceramics. The observed density variations could mainly be attributed to differences in phase composition, rather than to differences in remnant porosity. Furthermore, the microstructure, mechanical properties and the LTD behavior of the zirconia ceramics were affected by the sintering conditions. Therefore, the null hypothesis, stating that the sintering conditions do not affect the microstructural evolution, mechanical properties and LTD behavior of dental zirconia ceramics, should be rejected. In the present study, at first, a pilot test was performed to verify whether the density of one sintered sample as well as its LTD behavior, taking into account the transformation behavior of two exposed surfaces, were representative to study the influence of sintering time and temperature on the LTD behavior of dental zirconia ceramics. According to the results of the pilot test, all three dental zirconia grades tested showed a sufficiently small standard deviation, both with respect to density and transformed monoclinic phase fraction. Therefore, in the actual test, when analyzing the influence of sintering time and temperature on the density, microstructural development and LTD degradation behavior, only one specimen per sintering condition and per dental zirconia grade was analyzed. Regarding density measurements, all the pressureless sintered ceramics showed relative density values of 99.0% or more. However, a higher sintering temperature (1650 ◦ C)
decreased the density of the dental zirconia ceramics. This could be related to ‘de’-sintering behavior, accommodated by coarsening of the microstructure [25] and to partial transformation of the metastable t-ZrO2 phase into less dense c-ZrO2 and m-ZrO2 [26]. According to the Rietveld analysis that was performed on the XRD spectra of the polished specimens, the ceramics contained an increasing amount of cubic phase when the sintering time was elongated or the sintering temperature was increased above 1450 ◦ C. Moreover, ceramics sintered at 1650 ◦ C for 4 h showed a small amount of monoclinic phase. Although a theoretical density of 6.05 g/cm3 for t-ZrO2 is mentioned in literature, the theoretical density of our specimens was calculated taking into account the results from the Rietveld analysis, i.e. the fractions of monoclinic, tetragonal and cubic zirconia phases as well as the calculated yttria contents in the latter two. The calculated theoretical density values ranged between 6.05 and 6.07 g/cm3 (Table 3), with a maximum deviation of 0.35% compared to the literature value of 6.05 g/cm3 . Trunec mentioned that the density of sintered Y-TZP bodies was constant up to 1650 ◦ C and at 1650 ◦ C, the density of Y-TZP ceramics decreased with increased sintering time [27]. Our findings were in line with his report. Some authors reported the presence of large cubic grains in Y-TZP sintered at higher temperature [28–30]. Taking into account the different fractions and theoretical density of the cubic, tetragonal and monoclinic phase, it can be concluded that all ceramics densified under the aforementioned conditions contained less than 1 vol% pores. For all three zirconia grades, near full density was obtained after pressureless sintering at 1450 ◦ C for 1 h. Generally, lower absolute density values were measured when the temperature was increased to 1650 ◦ C. The mechanical properties indicated that as the sintering temperature was increased, the Vickers hardness decreased, while the indentation toughness was almost constant up to 1550 ◦ C and slightly increased with increasing dwell time when sintering at 1650 ◦ C. The Rietveld analysis indicated that the 3Y-TZPs sintered at 1450 ◦ C for 2 or 4 h and above 1550 ◦ C showed a lower tetragonal phase fraction. Cottom et al. reported that the Vickers hardness of ZrO2 ceramics was independent of grain size [31], whereas Trunec reported that increased sintering temperatures decreased the Vickers hardness of Y-TZP [27]. Our findings confirmed the latter report. Trunec also reported that the fracture toughness was almost constant up to a grain size of 0.4 m, in his case corresponding to a sintering temperature of around 1650 ◦ C. Our experimental data were in line with this report. In conclusion,
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Table 4 – Summary of parameters of MAJ equation. Temperature (◦ C) Time (h)
b (h−1 )
n
Estimated saturation level
Aadva IPS e.max ZirCAD
In-Ceram YZ Aadva IPS e.max ZirCAD
In-Ceram YZ
Aadva IPS e.max ZirCAD
In-Ceram YZ
1450
1 2 4
0.008 0.08 0.3
0.007 0.08 0.13
0.005 0.03 0.2
1.54 1.21 0.7
1.4 0.89 0.99
1.55 1.35 0.95
0.79 0.83 0.85
0.87 0.92 0.71
0.69 0.75 0.79
1550
1 2 4
0.31 0.53 1.65
0.21 0.33 0.36
0.21 0.4 0.67
1.26 1.12 0.47
0.99 0.74 0.66
1.43 0.88 0.65
0.8 0.79 0.76
0.77 0.82 0.76
0.8 0.79 0.74
1650
1 2 4
1.34 1.09 0.71
0.6 0.44 0.53
0.42 1.15 0.72
0.64 0.99 1.24
1.04 1.2 0.76
1.21 0.7 0.78
0.7 0.67 0.64
0.71 0.67 0.64
0.72 0.66 0.64
the measured mechanical properties indicated that in order to obtain higher hardness, a lower sintering temperature of 1450 ◦ C is essential. While on the other hand, higher sintering temperatures (∼1650 ◦ C) and increased dwell times (2–4 h) should be selected to increase the fracture toughness. Addressing the microstructural analysis, extremely large grains were observed in the 3Y-TZPs sintered at 1650 ◦ C. Some authors reported the presence of large cubic grains in TZPs sintered at higher temperature [28–30]. The performed Rietveld analysis confirmed that the ceramics sintered at 1450 ◦ C for 2 or 4 h and all materials sintered at 1550 ◦ C or 1650 ◦ C contained a significant fraction of cubic phase. Therefore, it was suggested that the larger grains had a cubic crystal symmetry. The grain size distributions of the Aadva (GC) ceramics sintered at different conditions are shown as insets in Fig. 4. As the sintering temperature increased, a broader and more bimodal grain size distribution was observed. It is believed that the fraction of large grains were those that adopted a cubic crystal structure, especially when sintering at 1650 ◦ C. When correlating the microstructural development and LTD sensitivity, longer sintering dwell times only affected the grain size in 3Y-TZP sintered at 1650 ◦ C. In general, larger grain sizes made the investigated dental zirconia ceramics more sensitive to LTD [16]. Most sensitive were the zirconia ceramics sintered at 1650 ◦ C for 4 h. Besides the higher fraction of cubic grains in the material sintered at elevated times and temperatures, the lower average yttria content in the remaining tetragonal fraction, made them more susceptible to LTD [32]. In all ceramics sintered at 1450 ◦ C and 1550 ◦ C, the saturation monoclinic volume fraction reached 0.8 after LTD testing. The remaining 20% were cubic or untransformable t -phase grains [33,34]. In the material sintered at 1650 ◦ C, the saturation monoclinic volume fraction was only 0.65–0.7, due to the larger fraction of cubic grains. Moreover, in the ceramics sintered at 1650 ◦ C for 4 h, a significant amount of monoclinic phase could be detected, indicating that longer dwell times at the highest sintering temperature could already induce monoclinic phase formation in the as-sintered condition. In the present study, we applied a partially linear model to fit the MAJ equation, in order to estimate the saturation level, corresponding to the amount of tetragonal phase that was susceptible to hydrothermal degradation and subsequent t→m transformation (Table 4). In all cases, the saturation level differed from 1, as cubic, monoclinic and non-transformable tetragonal grains pre-existed in the samples. It can be observed from Table 4
that the estimated saturation level in all ceramic ZrO2 grades decreased significantly when the sintering time was increased from 1550 to 1650 ◦ C. The difference in estimated saturation levels when sintering was performed at 1450 ◦ C can mainly be attributed to the fact that the fitting procedure was applied to hydrothermal degradation data that had not yet reached the saturation level. From the LTD test results, combined with the earlier observed mechanical property trends, it could be concluded that the ideal sintering condition for 3Y-TZP was 1450 ◦ C for 1 h.
5.
Conclusion
Different sintering conditions affected the ZrO2 grain size and the ZrO2 phase composition. More elevated sintering temperatures and times resulted in an increased ZrO2 grain size accompanied by a larger fraction of cubic grains and a lowered average stabilizer content in the remnant tetragonal grains. Both the increased grain size and lowered stabilizer content made the dental zirconia ceramics more susceptible to low-temperature degradation. Therefore, the ideal sintering condition for a 3Y-TZP ceramic was 1450 ◦ C for 1 h.
Acknowledgements This study was supported by a Flemish scholarship for Japanese students to study in Flanders, which was granted to M.I., and by the research project ‘G.0431.10’ funded by the Research Foundation - Flanders (FWO-Flanders) and the project ‘OT/10/052’ funded by KU Leuven (University of Leuven). K. Vanmeensel acknowledges FWO-Flanders for granting a postdoctoral fellowship.
references
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[3] Norton MR, Yarlagadda R, Anderson GH. Catastrophic failure of the Elite Plus total hip replacement, with a Hylamer acetabulum and Zirconia ceramic femoral head. J Bone Joint Surg Br 2002;84:631–5. [4] Chevalier J, Cales B, Drouin JM. Low-temperature aging of Y-TZP ceramics. J Am Ceram Soc 1999;82:2150–4. [5] Chevalier J, Gremillard L, Deville S. Low-temperature degradation of zirconia and implications for biomedical implants. Annu Rev Mater Res 2007;37:1–32. [6] Lawson S. Environmental degradation of zirconia ceramics. J Eur Ceram Soc 1995;15:485–502. [7] Cattani-Lorente M, Scherrer SS, Ammann P, Jobin M, Wiskott HW. Low temperature degradation of a Y-TZP dental ceramic. Acta Biomater 2011;7:858–65. [8] Chevalier J, Loh J, Gremillard L, Meille S, Adolfson E. Low-temperature degradation in zirconia with a porous surface. Acta Biomater 2011;7:2986–93. [9] Hallmann L, Mehl A, Ulmer P, Reusser E, Stadler J, Zenobi R, et al. The influence of grain size on low-temperature degradation of dental zirconia. J Biomed Mater Res B: Appl Biomater 2012;100B:447–56. [10] Kim JW, Covel NS, Guess PC, Rekow ED, Zhang Y. Concerns of hydrothermal degradation in CAD/CAM zirconia. J Dent Res 2010;89:91–5. [11] Kohorst P, Borchers L, Strempel J, Stiesch M, Hassel T, Bach FW, et al. Low-temperature degradation of different zirconia ceramics for dental applications. Acta Biomater 2012;8:1213–20. [12] Lughi V, Sergo V. Low temperature degradation – aging – of zirconia: a critical review of the relevant aspects in dentistry. Dent Mater 2010;26:807–20. [13] Chevalier J. What future for zirconia as a biomaterial? Biomaterials 2006;27:535–43. [14] Denry I, Kelly JR. State of the art of zirconia for dental applications. Dent Mater 2008;24:299–307. [15] Hallmann L, Ulmer P, Reusser E, Louvel M, Haemmerle CHF. Effect of dopants and sintering temperature on microstructure and low temperature degradation of dental Y-TZP-zirconia. J Eur Ceram Soc 2012;32:4091–104. [16] Kosmacˇ T, Kocjan A. Ageing of dental zirconia ceramics. J Eur Ceram Soc 2012;32:2613–22. [17] Stawarczyk B, Ozcan M, Hallmann L, Ender A, Mehl A, Hammerlet CH. The effect of zirconia sintering temperature on flexural strength, grain size, and contrast ratio. Clin Oral Investig 2013;17:269–74. [18] Jevnikar P, Sersa I, Sepe A, Jarh O, Funduk N. Effect of surface coating on water migration into resin-modified glass ionomer cements: a magnetic resonance micro-imaging study. Magn Reson Med 2000;44:686–91. [19] Toraya H, Yoshimura M, Somiya S. Calibration curve for quantitative analysis of the monoclinic-tetragonal ZrO2
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Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.intl.elsevierhealth.com/journals/dema
Influence of sintering conditions on low-temperature degradation of dental zirconia Masanao Inokoshi a , Fei Zhang b , Jan De Munck a , Shunsuke Minakuchi c , Ignace Naert a , Jozef Vleugels b , Bart Van Meerbeek a , Kim Vanmeensel b,∗ a
KU Leuven BIOMAT, Department of Oral Health Sciences, KU Leuven (University of Leuven) & Dentistry, University Hospitals Leuven, Kapucijnenvoer 7, blok a bus 7001, B-3000 Leuven, Belgium b Department of Metallurgy and Materials Engineering (MTM), KU Leuven (University of Leuven), Kasteelpark Arenberg 44, B-3001 Heverlee (Leuven), Belgium c Gerodontology and Oral Rehabilitation, Graduate School of Medical and Dental Sciences, Tokyo Medical and Dental University, 1-5-45 Yushima, Bunkyo, Tokyo 113-8549, Japan
a r t i c l e
i n f o
a b s t r a c t
Article history:
The effect of sintering conditions and concomitant microstructure of dental zirconia (ZrO2 )
Received 24 June 2013
ceramics on their low-temperature degradation (LTD) behavior remains unclear.
Received in revised form
Objectives. Therefore, their effect on LTD of dental ZrO2 ceramics was investigated.
25 February 2014
Methods. Three commercial pre-sintered yttria-stabilized dental zirconia materials were sin-
Accepted 7 March 2014
tered 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 ZrO2
Keywords:
on the exposed surface was measured by X-ray diffraction (XRD).
Zirconia
Results. Higher sintering temperatures and elongated dwell times increased the ZrO2 grain
Low-temperature degradation
size. Simultaneously, a larger fraction of zirconia grains adopted a cubic crystal structure,
Sintering
resulting in a decreased yttria content in the remaining tetragonal grains. Both the larger
Aging
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. Significance. In general, increased sintering temperatures and times result in a higher sensitivity to low-temperature degradation of Y-TZP ceramics. © 2014 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
∗ Corresponding author at: Department of Metallurgy and Materials Engineering (MTM), KU Leuven (University of Leuven), Kasteelpark Arenberg 44, B-3001 Heverlee (Leuven), Belgium. Tel.: +32 16 32 11 92; fax: +32 16 32 19 92. E-mail address: [email protected] (K. Vanmeensel).
http://dx.doi.org/10.1016/j.dental.2014.03.005 0109-5641/© 2014 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
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1.
d e n t a l m a t e r i a l s 3 0 ( 2 0 1 4 ) 669–678
Introduction
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 [1–3]. 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 [4–6]. The cause of the failures was related to an accelerated tetragonal to monoclinic phase transformation of zirconia in a limited number of batches [5]. 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 [7–12]. Two papers [7,11] have reported the reduction of mechanical properties after LTD. One paper has evaluated novel porous zirconia dental implants and reported their aging sensitivity [8]. Kim et al. [10] 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 [13]. 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 [14]. 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 [15]. 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 [16]. Moreover, the effect of sintering conditions on LTD is still not very well studied [9,17], 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 [18]. 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 [8]. 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.
2.
Materials and methods
The experimental design of our study is schematically explained in Fig. 1.
2.1.
Pilot study experiments
A pilot test, prior to the actual study, was performed in order to determine the amount of samples required to generate statistically relevant data. Pre-sintered yttria-stabilized zirconia samples of three commercial dental zirconia ceramics (Table 1: Aadva, GC, Tokyo, Japan; IPS e.max ZirCAD, Ivoclar-Vivadent, Schaan, Liechtenstein; In-CeramYZ, Vita Zahnfabrik, Bad Säckingen, Germany) were sliced into three samples with an overall volume of approximately 12.5 mm × 6 mm × 3.75 mm. Those were pressureless sintered in air at 1450 ◦ C for 2 h using a computer programmed furnace (Nabertherm, Germany). A heating rate of 20 ◦ C/min was applied up to 1000 ◦ C and subsequent heating at 10 ◦ C/min up to the sintering temperature. All specimens were mirror polished on both top and bottom surfaces. As such, 3 samples of each dental zirconia grade were sintered under identical conditions. Furthermore, since all 9 samples, 3 × 3 samples belonging to each of the different commercial grades, were sintered during the same thermal cycle, it can be guaranteed that all samples underwent exactly the same heat treatment. All sample densities were measured using the Archimedes principle. Cu K␣ (40 kV, 40 mA) X-ray diffraction (XRD, Seifert 3003 T/T, Seifert, Ahrensburg, Germany) analysis was used for phase identification and measuring the amount of m-ZrO2 on the sample surfaces exposed to water vapor during LTD. Both top and bottom surfaces of each specimen were analyzed from 27 to 33◦ 2 with a step size of 0.02◦ for 2 s. The volume fraction of m-ZrO2 was calculated according to the method of Toraya et al. [19] LTD tests were performed following ISO 13356: at 134 ◦ C for 6 h under a standard H2 O pressure of 2 bar.
2.2. Specimen preparation and characterization of the actual experiment Pre-sintered yttria-stabilized zirconia samples of three commercial dental zirconia ceramics (Table 1: Aadva, GC; IPS e.max ZirCAD, Ivoclar-Vivadent; In-CeramYZ, Vita Zahnfabrik) were each sliced into nine samples with an overall volume of approximately 12.5 mm × 12.5 mm × 3.75 mm. Those 27 samples were pressureless sintered in air at three different temperatures (1450, 1550 and 1650 ◦ C) for three different dwell times (1, 2 and 4 h) using a computer programmed furnace (Nabertherm, Germany). A heating rate of 20 ◦ C/min was applied up to 1000 ◦ C and subsequent heating at 10 ◦ C/min up to the sintering temperature. Then, all specimens were sectioned into two halves, of which one was used for grainsize measurement, while the other half was subjected to a
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d e n t a l m a t e r i a l s 3 0 ( 2 0 1 4 ) 669–678
Fig. 1 – Flow chart of the experimental study set-up.
hydrothermal degradation test. The obtained specimen size was 10.0 mm × 5.0 mm × 3.0 mm.
2.3.
Density measurement
The density of the specimens was determined according to the Archimedes principle. Although the generally reported density for 3 mol% yttria stabilized tetragonal zirconia (3YTZP) is 6.05 g/cm3 [20], the relative density was calculated using theoretical density values originating from Rietveld refinements, enabling the calculation of phase fractions (monoclinic, tetragonal, cubic) and yttria stabilizer content in each of the composing phases.
2.4.
Vickers hardness and fracture toughness
Vickers hardness and fracture toughness were determined using the indentation technique (Model FV-700, Future-Tech, Tokyo, Japan) applying the Anstis model for fracture toughness calculation [21]. An indentation load of 10 kg was applied.
2.5.
Microstructural investigation
Thermal etching was performed at 1350 ◦ C for 20 min to reveal the grain boundary network. Microstructural investigation was performed by scanning electron microscopy (SEM, XL30-FEG, FEI, The Netherlands). The average grain size of the phases was measured according to the linear intercept method [22]. The reported values are the average of at least 400 grains measured by means of image analysis on SEM photomicrographs using Image-pro-plus software (version 6.0, Media Cybernetics, Silver Spring, MD).
2.6.
X-ray diffraction analysis
Cu K␣ (40 kV, 40 mA) XRD (Seifert 3003 T/T, Seifert) was used for phase identification and calculation of the relative phase content of monoclinic and tetragonal ZrO2 . Both top and bottom surfaces of each specimen were analyzed from 27 to 33◦ 2 with a step size of 0.02◦ for 2 s. The transformed zirconia fraction was defined as the difference in monoclinic zirconia (m-ZrO2 ) content between a (partially) degraded and a polished
Table 1 – Overview of the experimental groups. Material
Type of material
Aadva IPS e.max ZirCAD In-Ceram YZ
Y-TZP Y-TZP Y-TZP
a
Manufacturer GC, Tokyo, Japan Ivoclar-Vivadent, Schaan, Liechtenstein Vita Zahnfabrik, Bad Säckingen, Germany
Information as provided by the manufacturers.
Batch number 1003291 P55259 35080
Compositiona ZrO2 , Y2 O3 , HfO2 , Al2 O3 ZrO2 , Y2 O3 , HfO2 , Al2 O3 ZrO2 , Y2 O3 , HfO2 , Al2 O3 , SiO2
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Table 2 – Pilot study results showing the absolute density after sintering at 1450 ◦ C for 2 h and the m-ZrO2 fraction after LTD at 134 ◦ C for 6 ha . Density (g/cm3 ) Mean Aadva IPS e.max ZirCAD In-Ceram YZ a
6.02 6.03 6.04
SD 0.007 0.002 0.008
m-ZrO2 fraction (%) Mean 14.5 15.5 15.5
SD 0.9 1.1 1.3
For each dental zirconia grade, 3 specimens or 6 exposed sample surfaces were tested.
surface. The volume fraction of m-ZrO2 is calculated according to the method of Toraya et al. [19]. The aging kinetics were described using the Mehl–Avrami–Johnson (MAJ) equation:
f = 1 − exp −(bt)
n
where f is the transformation fraction, t is the time, and b and n are constants [4]. Especially, b is a parameter dependent both on nucleation rate and growth velocity of monoclinic nuclei, and n is related to the spatial characteristics of the crystallization process [23]. The kinematic parameter b, and the Avrami exponent n were determined by means of leastsquares estimates using the Golub-Pereyra algorithm for partially linear least-squares models (nls function, R3.01, R foundation for Statistical Computing, Vienna, Austria). All tests were performed at a significance level of ˛ = 0.05 using the abovementioned software package. Rietveld analysis was used to calculate the unit cell dimensions of the commercial Y-ZrO2 grades that were pressureless sintered. Therefore, XRD scans were taken from polished sample surfaces in the 20–80◦ 2 range, using a step size of 0.02◦ for 2 s. Based on the calculated unit cell parameters, the Y2 O3 content of the tetragonal and cubic phases in the different commercial grades after sintering can be calculated using the formulas described by Krogstad et al. [24].
2.7.
LTD tests
LTD tests were performed following ISO 13356: at 134 ◦ C under a standard H2 O vapor pressure of 2 bar. The autoclave was placed in an oil bath to establish an internal temperature of 134 ◦ C, as monitored by a thermocouple in the autoclave. The amount of m-ZrO2 on the exposed sample surface was measured by XRD (Seifert 3003 T/T, Seifert).
3.
Results
3.1.
Pilot study
The results of the pilot study are summarized in Table 2. For each dental zirconia grade, three specimens were sintered and all specimens were afterwards subjected to a LTD treatment at 134 ◦ C for 6 h. Regarding the density measurement, all specimens exhibited the same absolute density of 6.08 g/cm3 , with very small standard deviations. After 6 h LTD testing, the average monoclinic volume fraction found on the exposed surfaces of the Aadva grade was 14.5 ± 0.9%, while that of the IPS e.max ZirCAD and In-Ceram YZ grades was 15.5 ± 1.1%
and 15.5 ± 1.3%, respectively. Again, the differences between the different zirconia grades were very small and, even more importantly, the observed standard deviations within one sample group were limited. Based on the identical absolute density values and the limited variation on the monoclinic fraction after 6 h LTD within one sample group, it was decided that one sample per dental zirconia grade was representative to study their behavior during LTD testing in the actual experiments. Thus, one sample, each containing two exposed sample surfaces, was used to study the LTD behavior of three dental zirconia grades following three sintering temperatures and three sintering times, totaling to 27 samples in the actual comparative experimental testing matrix.
3.2.
Actual experiments
3.2.1.
Density measurement
High densities were obtained for all samples, since no open porosity was observed after sintering. An overview of the densities of the different samples, obtained after pressureless sintering at different conditions is shown in Fig. 2a. The theoretical density of Y-ZrO2 depends on its phase constitution as well as its stabilizer content. Using the results from X-ray diffraction experiments, combined with Rietveld refinements, an exact theoretical density can be calculated. The calculated theoretical density of all ceramics varied between 6.05 and 6.07 g/cm3 , depending on the exact phase composition and the yttria stabilizer content in the different phases, as indicated in Table 3. All the pressureless sintered samples exhibit a relative density of 99.0% or more.
3.2.2.
Vickers hardness and fracture toughness
An overview of the Vickers hardness and fracture toughness data is shown in Fig. 2b and c, respectively. When the sintering temperature was increased, the Vickers hardness decreased. On the other hand, when the sintering temperature was increased, the fracture toughness was almost constant up to 1550 ◦ C and slightly increased when sintered at 1650 ◦ C.
3.2.3.
Microstructural investigation
An overview of the grain size measurements performed on the different ceramics, obtained after pressureless sintering at different conditions is shown in Fig. 3. Fig. 4 shows examples of secondary electron (SE) images of the Aadva (GC) specimens sintered at different conditions. The average grain size of the specimen sintered at 1450 ◦ C for 1 h was 0.26 m. On the other hand, the average grain size of the specimen from the same grade sintered at 1650 ◦ C for 4 h was 0.69 m. In general, higher sintering temperatures increased the ZrO2 grain size. At 1450 ◦ C and 1550 ◦ C, a variation of sintering dwell times did not result in significant coarsening of the microstructure. However, at 1650 ◦ C, elongated dwell times clearly increased the grain size.
3.2.4.
X-ray diffraction
The XRD patterns recorded on polished ceramic surfaces that were pressureless sintered at 1450 ◦ C for 1 h, indicated that all three grades consisted of mainly tetragonal zirconia. However, when the sintering time or sintering temperature was increased, diffraction peaks originating from the cubic
d e n t a l m a t e r i a l s 3 0 ( 2 0 1 4 ) 669–678
673
Fig. 2 – Summary of density measurements and mechanical properties. (a) Results of the density measurements. All the sintered specimens showed relative density of 99.0% or more. At 1650 ◦ C, the measured density was relatively lower than the other specimens. (b) Results of the Vickers hardness measurements. When the sintering temperature was increased, the Vickers hardness value was decreased. (c) Results of the toughness measurements. When the sintering temperature was increased, the indentation toughness was also increased.
zirconia phase could be detected. At the highest sintering temperature and time, even monoclinic phase peaks could be detected due to spontaneous transformation of the metastable tetragonal zirconia phase into monoclinic zirconia. Therefore, 3 different ZrO2 crystal structures were taken into account to perform the Rietveld refinement: a cubic ZrO2 phase with fluorite structure belonging to the Fm-3m (225) space group, a tetragonal phase belonging to the P42/nmc (137) space group and a monoclinic phase belonging to the P21/c (14) space group. The calculated weight percentages of the different ZrO2 phases that were detected in the In-Ceram YZ (Vita
Zhanfabrik), sintered at different temperatures and times, are summarized in Table 3. A similar trend was observed for the two other zirconia grades. As the cubic zirconia content in the sintered ceramic increased, the yttria content of the tetragonal phase decreased.
3.2.5.
LTD tests
The results of the LTD tests are summarized in Fig. 5. All ceramics experienced a partial tetragonal to monoclinic phase transformation after steam exposure at 134 ◦ C. The samples sintered at 1450 ◦ C for 1 h exhibited a saturation monoclinic
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d e n t a l m a t e r i a l s 3 0 ( 2 0 1 4 ) 669–678
Fig. 3 – Summary of grain size measurement. Higher sintering temperature showed larger grain size. At 1450 ◦ C and 1550 ◦ C, the elongated dwelling time did not affect the grain size. At 1650 ◦ C, however, the elongated dwelling time clearly increased the grain size.
Fig. 4 – SEM images of sintered and thermally etched Aadva (GC) specimens: (a) Specimen sintered at 1450 ◦ C, 1 h with an average grain size of 0.26 m. (b) Specimens sintered at 1450 ◦ C for 2 h with an average grain size of 0.30 m. (c) Specimens sintered at 1450 ◦ C for 4 h with an average grain size of 0.36 m. (d) Specimens sintered at 1550 ◦ C for 1 h with an average grain size of 0.43 m. (e) Specimens sintered at 1550 ◦ C for 2 h with an average grain size of 0.54 m. (f) Specimens sintered at 1550 ◦ C for 4 h with an average grain size of 0.62 m. (g) Specimens sintered at 1650 ◦ C for 1 h with an average grain size of 0.81 m. (h) Specimens sintered at 1650 ◦ C for 2 h with an average grain size of 1.03 m. (i) Specimen sintered at 1650 ◦ C, 4 h with an average grain size of 1.69 m.
d e n t a l m a t e r i a l s 3 0 ( 2 0 1 4 ) 669–678
675
Fig. 5 – Representative results of LTD tests. The monoclinic volume fraction is plotted as function of LTD time (h). The saturation monoclinic volume fraction reached a value of 0.4–0.6 in the specimens pressureless sintered at 1450 ◦ C for 1 h, after 30 h LTD testing. At elongated sintering times and elevated sintering temperatures, a clear increase in LTD sensitivity is observed, resulting in larger monoclinic phase saturation levels ∼0.8. At a sintering temperature of 1650 ◦ C, the saturation monoclinic volume fraction decreases to 0.65–0.7, because of the presence of a larger fraction of non-transformable cubic zirconia grains.
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Table 3 – Representative data of the Rietveld Analysis (In-Ceram YZ). wt% of different ZrO2 polymorphs
Y2 O3 content in tetragonal phase (mol%)
Theoretical density (g/cm3 )
Tetragonal
Cubic
Monoclinic
1450 ◦ C – 1 h 1450 ◦ C – 2 h 1450 ◦ C – 4 h
97.8 85.5 85.2
0.6 11.6 12.9
1.6 2.9 1.9
3.0 2.7 2.6
6.07 6.06 6.06
1550 ◦ C – 1 h 1550 ◦ C – 2 h 1550 ◦ C – 4 h
83.4 82.7 81.1
15.4 16.5 17.6
1.2 0.8 1.3
2.6 2.5 2.5
6.07 6.06 6.06
1650 ◦ C – 1 h 1650 ◦ C – 2 h 1650 ◦ C – 4 h
82.4 81.1 78.2
16.7 17.7 17.1
0.9 1.2 4.7
2.4 2.5 2.5
6.07 6.07 6.05
volume fraction between 0.4 and 0.6, after 30 h LTD testing. When the sintering time was elongated, much faster degradation and higher saturation monoclinic fractions were obtained. After 30 h LTD testing, the saturation monoclinic volume fraction reached a value of 0.8 when sintered at 1450 ◦ C for 2 h or 4 h. An even faster degradation was observed when sintered at 1550 and 1650 ◦ C. At 1650 ◦ C, however, the saturation monoclinic volume fraction decreased to 0.65–0.7. A summary of the kinematic parameter b, the Avrami exponent n and estimated saturation level for the different LTD test samples are represented in Table 4. These results clearly indicate that the slowest degrading zirconia ceramics were sintered at 1450 ◦ C for 1 h. Aadva (GC) ceramics showed a clearly higher b value, indicating a faster degrading than the other commercial zirconia.
4.
Discussion
Higher sintering temperatures and elongated dwell times influenced the density and phase composition of dental ZrO2 ceramics. The observed density variations could mainly be attributed to differences in phase composition, rather than to differences in remnant porosity. Furthermore, the microstructure, mechanical properties and the LTD behavior of the zirconia ceramics were affected by the sintering conditions. Therefore, the null hypothesis, stating that the sintering conditions do not affect the microstructural evolution, mechanical properties and LTD behavior of dental zirconia ceramics, should be rejected. In the present study, at first, a pilot test was performed to verify whether the density of one sintered sample as well as its LTD behavior, taking into account the transformation behavior of two exposed surfaces, were representative to study the influence of sintering time and temperature on the LTD behavior of dental zirconia ceramics. According to the results of the pilot test, all three dental zirconia grades tested showed a sufficiently small standard deviation, both with respect to density and transformed monoclinic phase fraction. Therefore, in the actual test, when analyzing the influence of sintering time and temperature on the density, microstructural development and LTD degradation behavior, only one specimen per sintering condition and per dental zirconia grade was analyzed. Regarding density measurements, all the pressureless sintered ceramics showed relative density values of 99.0% or more. However, a higher sintering temperature (1650 ◦ C)
decreased the density of the dental zirconia ceramics. This could be related to ‘de’-sintering behavior, accommodated by coarsening of the microstructure [25] and to partial transformation of the metastable t-ZrO2 phase into less dense c-ZrO2 and m-ZrO2 [26]. According to the Rietveld analysis that was performed on the XRD spectra of the polished specimens, the ceramics contained an increasing amount of cubic phase when the sintering time was elongated or the sintering temperature was increased above 1450 ◦ C. Moreover, ceramics sintered at 1650 ◦ C for 4 h showed a small amount of monoclinic phase. Although a theoretical density of 6.05 g/cm3 for t-ZrO2 is mentioned in literature, the theoretical density of our specimens was calculated taking into account the results from the Rietveld analysis, i.e. the fractions of monoclinic, tetragonal and cubic zirconia phases as well as the calculated yttria contents in the latter two. The calculated theoretical density values ranged between 6.05 and 6.07 g/cm3 (Table 3), with a maximum deviation of 0.35% compared to the literature value of 6.05 g/cm3 . Trunec mentioned that the density of sintered Y-TZP bodies was constant up to 1650 ◦ C and at 1650 ◦ C, the density of Y-TZP ceramics decreased with increased sintering time [27]. Our findings were in line with his report. Some authors reported the presence of large cubic grains in Y-TZP sintered at higher temperature [28–30]. Taking into account the different fractions and theoretical density of the cubic, tetragonal and monoclinic phase, it can be concluded that all ceramics densified under the aforementioned conditions contained less than 1 vol% pores. For all three zirconia grades, near full density was obtained after pressureless sintering at 1450 ◦ C for 1 h. Generally, lower absolute density values were measured when the temperature was increased to 1650 ◦ C. The mechanical properties indicated that as the sintering temperature was increased, the Vickers hardness decreased, while the indentation toughness was almost constant up to 1550 ◦ C and slightly increased with increasing dwell time when sintering at 1650 ◦ C. The Rietveld analysis indicated that the 3Y-TZPs sintered at 1450 ◦ C for 2 or 4 h and above 1550 ◦ C showed a lower tetragonal phase fraction. Cottom et al. reported that the Vickers hardness of ZrO2 ceramics was independent of grain size [31], whereas Trunec reported that increased sintering temperatures decreased the Vickers hardness of Y-TZP [27]. Our findings confirmed the latter report. Trunec also reported that the fracture toughness was almost constant up to a grain size of 0.4 m, in his case corresponding to a sintering temperature of around 1650 ◦ C. Our experimental data were in line with this report. In conclusion,
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Table 4 – Summary of parameters of MAJ equation. Temperature (◦ C) Time (h)
b (h−1 )
n
Estimated saturation level
Aadva IPS e.max ZirCAD
In-Ceram YZ Aadva IPS e.max ZirCAD
In-Ceram YZ
Aadva IPS e.max ZirCAD
In-Ceram YZ
1450
1 2 4
0.008 0.08 0.3
0.007 0.08 0.13
0.005 0.03 0.2
1.54 1.21 0.7
1.4 0.89 0.99
1.55 1.35 0.95
0.79 0.83 0.85
0.87 0.92 0.71
0.69 0.75 0.79
1550
1 2 4
0.31 0.53 1.65
0.21 0.33 0.36
0.21 0.4 0.67
1.26 1.12 0.47
0.99 0.74 0.66
1.43 0.88 0.65
0.8 0.79 0.76
0.77 0.82 0.76
0.8 0.79 0.74
1650
1 2 4
1.34 1.09 0.71
0.6 0.44 0.53
0.42 1.15 0.72
0.64 0.99 1.24
1.04 1.2 0.76
1.21 0.7 0.78
0.7 0.67 0.64
0.71 0.67 0.64
0.72 0.66 0.64
the measured mechanical properties indicated that in order to obtain higher hardness, a lower sintering temperature of 1450 ◦ C is essential. While on the other hand, higher sintering temperatures (∼1650 ◦ C) and increased dwell times (2–4 h) should be selected to increase the fracture toughness. Addressing the microstructural analysis, extremely large grains were observed in the 3Y-TZPs sintered at 1650 ◦ C. Some authors reported the presence of large cubic grains in TZPs sintered at higher temperature [28–30]. The performed Rietveld analysis confirmed that the ceramics sintered at 1450 ◦ C for 2 or 4 h and all materials sintered at 1550 ◦ C or 1650 ◦ C contained a significant fraction of cubic phase. Therefore, it was suggested that the larger grains had a cubic crystal symmetry. The grain size distributions of the Aadva (GC) ceramics sintered at different conditions are shown as insets in Fig. 4. As the sintering temperature increased, a broader and more bimodal grain size distribution was observed. It is believed that the fraction of large grains were those that adopted a cubic crystal structure, especially when sintering at 1650 ◦ C. When correlating the microstructural development and LTD sensitivity, longer sintering dwell times only affected the grain size in 3Y-TZP sintered at 1650 ◦ C. In general, larger grain sizes made the investigated dental zirconia ceramics more sensitive to LTD [16]. Most sensitive were the zirconia ceramics sintered at 1650 ◦ C for 4 h. Besides the higher fraction of cubic grains in the material sintered at elevated times and temperatures, the lower average yttria content in the remaining tetragonal fraction, made them more susceptible to LTD [32]. In all ceramics sintered at 1450 ◦ C and 1550 ◦ C, the saturation monoclinic volume fraction reached 0.8 after LTD testing. The remaining 20% were cubic or untransformable t -phase grains [33,34]. In the material sintered at 1650 ◦ C, the saturation monoclinic volume fraction was only 0.65–0.7, due to the larger fraction of cubic grains. Moreover, in the ceramics sintered at 1650 ◦ C for 4 h, a significant amount of monoclinic phase could be detected, indicating that longer dwell times at the highest sintering temperature could already induce monoclinic phase formation in the as-sintered condition. In the present study, we applied a partially linear model to fit the MAJ equation, in order to estimate the saturation level, corresponding to the amount of tetragonal phase that was susceptible to hydrothermal degradation and subsequent t→m transformation (Table 4). In all cases, the saturation level differed from 1, as cubic, monoclinic and non-transformable tetragonal grains pre-existed in the samples. It can be observed from Table 4
that the estimated saturation level in all ceramic ZrO2 grades decreased significantly when the sintering time was increased from 1550 to 1650 ◦ C. The difference in estimated saturation levels when sintering was performed at 1450 ◦ C can mainly be attributed to the fact that the fitting procedure was applied to hydrothermal degradation data that had not yet reached the saturation level. From the LTD test results, combined with the earlier observed mechanical property trends, it could be concluded that the ideal sintering condition for 3Y-TZP was 1450 ◦ C for 1 h.
5.
Conclusion
Different sintering conditions affected the ZrO2 grain size and the ZrO2 phase composition. More elevated sintering temperatures and times resulted in an increased ZrO2 grain size accompanied by a larger fraction of cubic grains and a lowered average stabilizer content in the remnant tetragonal grains. Both the increased grain size and lowered stabilizer content made the dental zirconia ceramics more susceptible to low-temperature degradation. Therefore, the ideal sintering condition for a 3Y-TZP ceramic was 1450 ◦ C for 1 h.
Acknowledgements This study was supported by a Flemish scholarship for Japanese students to study in Flanders, which was granted to M.I., and by the research project ‘G.0431.10’ funded by the Research Foundation - Flanders (FWO-Flanders) and the project ‘OT/10/052’ funded by KU Leuven (University of Leuven). K. Vanmeensel acknowledges FWO-Flanders for granting a postdoctoral fellowship.
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