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Effect of changes in sintering parameters on monolithic translucent zirconia Kamal Ebeid a,b,∗ , Sebastian Wille a , Amina Hamdy b , Tarek Salah b , Amr El-Etreby b , Matthias Kern a a
Department of Prosthodontics, Propaedeutics and Dental Materials, School of Dentistry, Christian-Albrechts University at Kiel, Germany b Department of Fixed Prosthodontics, Faculty of Dentistry, Ain Shams University, Egypt
a r t i c l e
i n f o
a b s t r a c t
Article history:
Objectives. Aim of this study was to evaluate the effect of different sintering parameters on
Received 6 February 2014
color reproduction, translucency and biaxial flexural strength of monolithic zirconia.
Received in revised form
Methods. Translucent zirconia discs having 15 mm diameter, 1 mm thickness, and shade
15 June 2014
A3 were milled and divided according to the sintering temperatures (1460 ◦ C, 1530 ◦ C, and
Accepted 5 September 2014
1600 ◦ C) into three groups (n = 30). Each group was later divided into three subgroups (n = 10)
Available online xxx
according to the sintering holding time (1, 2, and 4 h). Easyshade spectrophotometer (Vita,
Keywords:
A3. Mean E values below 3.0 were considered “clinically imperceptible”, E values between
Bad Säckingen, Germany) was used to obtain the E between the specimens and the shade Dental materials
3.0 and 5.0 were considered “clinically acceptable” and E values above 5.0 were considered
Ceramics
“clinically unacceptable”. Contrast ratio (CR) was obtained after comparing the reflectance
Zirconia
of light through the specimens over black and white background. Biaxial flexural strength
Biaxial flexural strength
was tested using the piston-on-three balls technique in a universal testing machine.
Translucency
Results. Mean E results ranged from 4.4 to 2.2. Statistically significant decrease in the Delta E was observed as the sintering time and temperature increased. CR decreased from 0.75 to 0.68 as the sintering time and temperature increased. No significant change in the biaxial flexural strength was observed. Significance. Sintering zirconia using long cycles and high temperatures will result in reduction of E and CR. Biaxial flexural strength is not affected by changes in the evaluated sintering parameters. © 2014 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
All ceramic restorations have proved to be a promising alternative to metal-ceramic restorations mainly due to their excellent esthetics, chemical stability, and biocompatibility
[1]. Zirconia restorations became very popular due to their unique mechanical properties which made it possible to use them in long span restorations [2,3]. Zirconia is polymorphic in nature and exists in three forms: cubic, tetragonal, and monoclinic. At room temperature zirconia is present in its monoclinic form and is stable up to 1170 ◦ C. Above this
∗
Corresponding author at: Department of Prosthodontics, Propaedeutics and Dental Materials, School of Dentistry, Christian-Albrechts University at Kiel, Germany. Tel.: +20 1006084044. E-mail address: [email protected] (K. Ebeid) . http://dx.doi.org/10.1016/j.dental.2014.09.003 0109-5641/© 2014 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
Please cite this article in press as: Ebeid K, et al. Effect of changes in sintering parameters on monolithic translucent zirconia. Dent Mater (2014), http://dx.doi.org/10.1016/j.dental.2014.09.003
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temperature a transformation occurs to the tetragonal phase that is stable up to 2370 ◦ C. Beyond this temperature, zirconia assumes its cubic form [4]. Zirconia restorations are fabricated either in a partially sintered state by soft machining followed by a final sintering cycle, or they are fabricated in a fully sintered state by hard machining [5,6]. Hard machining may induce tetragonalmonoclinic transformation, introduce cracks, and wears the milling hardware at a higher rate. Soft machining however is much easier, but may produce less accurate frameworks due to the sintering shrinkage accompanied with the final sintering process [7]. The phenomenon of light scattering largely affects the translucency of dental ceramics. If the majority of light passing through a ceramic is scattered, the material will appear opaque. However, if most of the light passing is transmitted through the ceramic it will appear translucent [8]. The amount of light that is absorbed, transmitted, and reflected mainly depends on the microstructure of the ceramic itself [9,10]. Differences in perceived color (E) can be determined using the CIELAB coordinates. The CIELAB system has provided a quantitative representation of color and it has been extensively applied in dentistry to study esthetic materials, shade guides, and color reproductions [11–13]. The perceptibility and acceptability thresholds of the E vary widely in literature mainly due to the diversity of observers, objectives, and methodologies among the studies [14,15]. Clinically the tooth, restorations available, surrounding, and blending effect tend to expand the clinically acceptable range previously reported [16,17]. The mean E values as “clinically imperceptible” (E < 3), “clinically acceptable” (E between 3 and 5) and “clinically unacceptable” (E > 5) seem to be consistent with the clinical practice considering a non-color expert, which usually is the patient’s condition [11,18,19]. In order to overcome the main disadvantage of zirconia which is its opacity, the zirconia core is veneered with veneering porcelain to enhance its esthetics. However, the most common mode of failure that faced clinicians was the chipping of this veneering porcelain while the zirconia core remained unaffected [20,21]. The differences in sintering parameters of zirconia can directly affect its microstructure and properties [22]. The extent of this effect have become of interest in the field of dental research especially after the introduction of short sintering cycles by manufacturers. Several authors have studied the effect of the changes in sintering time and temperature on the translucency, grain size, and biaxial flexural strength of zirconia core ceramics; however the effect of these changes on the properties of monolithic nanozirconia remains in question [10,23–25]. The aim of this study was to evaluate the effect of using different sintering times and temperatures on the color reproduction, translucency, surface roughness, biaxial flexural strength, and the surface hardness of monolithic zirconia ceramic.
2.
Materials and methods
Ninety translucent shaded zirconia ceramic discs (Bruxzir, Glidewell, Frankfurt, Germany) with a diameter of 15 mm, a
thickness of 1 mm, and shade A3 were milled and divided into three groups (n = 30) according to the sintering holding time (1, 2, and 4 h). Each group was later divided into three subgroups (n = 10) according to the sintering temperature (1460 ◦ C, 1530 ◦ C, and 1600 ◦ C). All specimens were sintered as milled in the manufacturers sintering furnace (Bruxzir FastFire, Glidewell, Frankfurt, Germany) at a heating and cooling rate of 10 ◦ C per minute. The temperature was controlled using the furnace’s internal thermometer.
2.1.
Color evaluation
Specimens were placed over a neutral gray background (CIE L* = 62.1, a* = 1.3, b* = −0.02) and the CIELAB coordinates were measured for each specimen using a spectrophotometer (Easyshade compact, Vita Zahnfabrik, Bad Säckingen, Germany). The Easyshade was set to the restoration mode and the shade A3 was selected. In this mode the color difference is determined by comparing the selected shade and the measured shade. For each specimen three measurements were taken at the center and their average was recorded. After each specimen was measured the Easyshade was recalibrated. Mean E values below 3.0 were considered “clinically imperceptible”, E values between 3.0 and 5.0 were considered “clinically acceptable” and E values above 5.0 were considered “clinically unacceptable”.
2.2.
Translucency evaluation
A quantitative measurement of translucency was obtained by measuring the CIELAB coordinates of the specimens after backing with a white (CIE L* = 96.7, a* = 0.1, b* = 0.2) and black (CIE L* = 10.4, a* = 0.4, b* = 0.6) background using the spectrophotometer. For each specimen three measurements were taken and their average was recorded. The contrast ratio (CR) for each specimens was calculated according to the following equation: CR = Yb /Yw where Y = [(L + 16)/116]3 × 100 and Yb is the reflectance over a black background and Yw is the reflectance over a white background [8,11]. In all calculations “0” is considered the most transparent and “1” is considered the most opaque.
2.3.
Surface roughness evaluation
All specimens were cleaned ultrasonically in 99% isopropanol solution for 3 min and then dried with air. The average surface roughness (Ra) for the specimens was measured using a 3D laser scanning microscope (Keyence VK-X100, Keyence GmbH, Neu-Isenbuerg, Germany). The wavelength of the laser was 658 nm. Three separate areas were measured on each specimen, the measured area was 500 m × 750 m and the distance between the separate scans was over 3 m. The mean Ra for each specimen was later recorded.
2.4.
Microstructure analysis
Three specimens were selected randomly from each subgroup for X-ray diffraction (XRD) surface analysis to detect the amount of tetragonal and monoclinic phases available. The specimens were placed in the holder of a diffractometer
Please cite this article in press as: Ebeid K, et al. Effect of changes in sintering parameters on monolithic translucent zirconia. Dent Mater (2014), http://dx.doi.org/10.1016/j.dental.2014.09.003
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Fig. 1 – Piston-on-three balls biaxial flexural strength test.
(Seifert XRD 3000 PTS, GE GmbH, Munich, Germany) and subjected to Cu K␣ radiation. The spectrum was recorded within the range of 20–40◦ , a step size of 0.02◦ and a scan time per step of 10 s. Voltage and current were set to 40 kV and 40 mA. One specimen from each subgroup was cleaned, dried, gold sputter coated (Bal-Tec SCD 050, Leica Mircosystems, Wetzlar, Germany) and examined under 10,000× magnification using a scanning electron microscope (XL 30 CP, Philips, Surrey, England) to calculate the average grain size using a base of at least 150 grains.
2.5.
Biaxial flexural strength testing
Specimens were tested for biaxial flexural strength using piston-on-three ball technique in a universal testing machine (Zwick Z010, Zwick, Ulm, Germany). A 10 mm diameter metallic platform was constructed, above which rested three 3.2 mm diameter steel balls that were equidistant from each other. Each disk was placed centrally on the steal balls and load was applied by a piston of 1.4 mm diameter and 0.5 mm/min crosshead speed using the universal testing machine (Fig. 1). The fracture load for each specimen was recorded and the biaxial flexural strength was calculated using the following equation: S=
−0.2387P(X − Y) d2
where: S, biaxial flexural strength (MPa); P, fracture load (N); d, specimen disk thickness at fracture origin (mm). X and Y were determined as follows: X : (1 + ) ln
r2 2 r3
Y : (1 + ) 1 + ln
+
1 − r2 2 2
r1 2 r3
r3
r1 2
+ (1 − )
r3
is Poisson’s ratio (0.25), r1 is the radius of the support circle, r2 is the radius of the loaded area, and r3 is the radius of the specimen.
Fig. 2 – Biaxial flexural strength of subgroups sintered using different sintering times and temperatures.
2.6.
Vickers hardness evaluation
A piece from each broken specimen from each subgroup was randomly selected and tested for Vickers hardness using an indentation tester (ZHV10, Zwick, Ulm, Germany). An indentation was made on each specimen under a loading mass of 5 kg and dwell time of 10 s. The diagonal length was measured using an optical microscope (20×) and the hardness was calculated from the following equation [26,27]: VHN = 1.8544 ×
F d2
where VHN is the Vickers hardness number, F is the applied load expressed in kg, and d is the mean length of the diagonals of the indentation (mm).
2.7.
Statistical analysis
The data collected was checked for normal distribution and analyzed using two-way analysis of variance (ANOVA), followed by Tukey’s HSD test (SPSS v20, Chicago, IL, USA) at a significance level of P ≤ 0.05 to determine the effect of the changes in sintering time and temperature on each of the variables tested.
3.
Results
Regarding the color evaluation, two-way ANOVA revealed significant differences in the E between the subgroups when sintered using different sintering temperatures and times (P ≤ 0.05). Tukey’s HSD tests revealed that there was a significant decrease in the E as the sintering temperature increased. It also revealed that there was no significant difference in the E between the 2 and 4 h sintering holding times however, both showed a significant decrease when compared to the 1 h sintering holding time (Table 1). None of
Please cite this article in press as: Ebeid K, et al. Effect of changes in sintering parameters on monolithic translucent zirconia. Dent Mater (2014), http://dx.doi.org/10.1016/j.dental.2014.09.003
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Table 1 – Mean E and standard deviation () of test groups sintered using different sintering times and temperatures.
Table 4 – Mean grain size in m and standard deviation () of test groups sintered using different sintering times and temperatures.
Sintering temperature/◦ C
Sintering temperature/◦ C
Sintering holding time/h 1
2 a
1460 1530 1600
4 d
4.4 (0.3) 3.1 (0.1)b 2.4 (0.1)c
4.0 (0.1) 2.8 (0.3)e 2.2 (0.1)f
3.8 (0.1)d 2.9 (0.1)e 2.2 (0.1)f
Means with different superscript letters are significantly different at the 95% confidence level.
the subgroups showed any unacceptable color results, with the sintering temperatures 1460 ◦ C and 1530 ◦ C subgroups showing clinically acceptable results and the sintering temperature 1600 ◦ C showing clinically imperceptible color results. Concerning translucency, two-way ANOVA revealed significant differences in CR between the subgroups when sintered with different sintering temperatures and times (P ≤ 0.05). Tukey’s HSD tests revealed that there was a significant decrease in CR when comparing the sintering temperatures 1460 ◦ C to the 1530 ◦ C and 1600 ◦ C. It also revealed that there was a significant decrease in CR when comparing the 1 and 2 h sintering holding times to the 4 h (Table 2). As for the surface roughness measurements, the results showed that there was a decrease in the mean Ra as the sintering temperature and time increased, however this decrease was not statistically significant. The highest mean Ra measured was with specimens sintered with a holding time of 1 h at a temperature of 1460 ◦ C (1.030 m) while the lowest mean Ra measured was with the specimens sintered with a holding time of 4 h at a temperature of 1600 ◦ C (0.826 m) (Table 3).
Table 2 – Mean CR and standard deviation () of test groups sintered using different sintering times and temperatures. Sintering temperature/◦ C
Sintering holding time/h 1
1460 1530 1600
2 a
0.75 (0.02) 0.72 (0.01)b 0.71 (0.01)b
4 a
0.75 (0.03) 0.71 (0.01)b 0.70 (0.01)b
0.71 (0.01)c 0.69 (0.01)d 0.68 (0.01)e
Means with different superscript letters are significantly different at the 95% confidence level.
Table 3 – Mean Ra and standard deviation () of test groups sintered using different sintering times and temperatures. Sintering holding time/h
Sintering temperature/◦ C 1 1460 1530 1600
1.030 (0.190)a 1.026 (0.304)a 0.898 (0.191)a
2 1.023 (0.164)a 0.878 (0.215)a 0.957 (0.242)a
4 0.991 (0.198)a 1.069 (0.086)a 0.826 (0.161)a
Means with different superscript letters are significantly different at the 95% confidence level.
Sintering holding time/h 1
2 a
1460 1530 1600
0.55 (0.10) 0.65 (0.13)b 0.89 (0.19)c
4 d
0.64 (0.11) 0.77 (0.13)e 1.0 (0.11)f
0.79 (0.15)d 0.92 (0.25)e 0.92 (0.14)f
Means with different superscript letters are significantly different at the 95% confidence level.
Table 5 – Mean VHN and standard deviation () of test groups sintered using different sintering times and temperatures. Sintering holding time/h
Sintering temperature/◦ C 1 1460 1530 1600
1456 (212)a 1553 (111)a 1461 (108)a
2 1504 (123)a 1548 (77)a 1507 (71)a
4 1497 (138)a 1437 (74)a 1415 (60)a
Means with different superscript letters are significantly different at the 95% confidence level.
Regarding microstructure analysis, all specimens analyzed using XRD showed only tetragonal ZrO2 phase characteristics peaks. None of the samples showed any tetragonalmonoclinic transformation. However, samples showed a significant increase in the average grain size as the sintering temperature increased. Samples sintered at 2 and 4 h sintering holding time also showed a statistically significant increase in the average grain size when compared to samples sintered at a 1 h sintering holding time (Table 4). The mean biaxial flexural strength of subgroups ranged from 906 MPa to 1000 MPa. However, there was no statistically significant difference between subgroups (Fig. 2). The mean Vickers hardness of subgroups ranged from 1437 to 1553 VHN. Again, there was no statistically significant difference between the subgroups concerning their surface hardness (Table 5).
4.
Discussion
The use of the Vita Easyshade spectrophotometer to measure color and translucency by assessing the CIELAB coordinates of specimens is widely used in the field of dental research [12,14]. Spectral reflectance which is used in this study together with direct transmission and total transmission are the three ways used to measure translucency [28]. We calculated the contrast ratio which was used by several authors to obtain a quantitative measure of translucency [8,29,30]. Our results showed that there was a decrease in the E from 4.4 to 2.2 and the contrast ratio from 0.75 to 0.68 as the sintering temperature and time increased. This could be attributed to the assumption that the sintering process reduces the pores between the grains and increases the final density of zirconia, thus producing less light scattering and more light transmission leading
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to better translucency and optical characteristics [31]. This assumption was consistent with our results as increasing the sintering temperature and time led to an increase in the average grain size of zirconia. Also the increase in the sintered density of the zirconia may lead to a more uniform crystalline arrangement thus promoting better specular reflection, light transmission and a better perception of color. Concerning the contrast ratio, our results were similar to a study performed by Jiang et al. [24], where sintering temperatures 1350 ◦ C, 1400 ◦ C, 1450 ◦ C, and 1500 ◦ C were used and their effect on the translucency of zirconia core discs was measured. They concluded that as the sintering temperature increased the translucency of the discs increased. Our results were also in agreement with Stawarczyk et al. [25] as they concluded that there was a decrease in the CR of regular zirconia core material specimens having 0.5 mm thickness from 0.85 to 0.70 as the sintering temperature increased from 1300 to 1700 ◦ C. A non-contact laser profilometer was used in this study as there were questions regarding the accuracy of contact profilometry with assumptions saying that they underestimate the surface roughness thus present a smoother surface [32]. Since we had no significant differences between the subgroups regarding the surface roughness, we can assume the fact that the reduction in the pores between the grains is not enough to produce a significant difference, we can also assume the fact that the changes in the contrast ratio and E are mainly due to changes in the microstructure rather than changes in surface morphology. Simulating pure bending and preventing edge loss can be achieved best using the piston-on-three ball technique for testing the biaxial flexural strength, as the specimens in this technique are resting on the stainless steel ball which form a smaller diameter than the specimen itself [33,34]. In our study there was no effect on the biaxial flexural strength as the sintering parameters changed. Our results are in agreement with the results of a study conducted by Hjerppe et al. [35], who used different rising and holding times for sintering zirconia. Stawarczyck et al. [25] also found no significant differences in the biaxial flexural strength of zirconia core material when the sintering temperature was raised from 1400 ◦ C to 1550 ◦ C, however the flexural strength significantly decreased above and below these temperatures. In a similar study by Jiang et al. [24], they concluded that raising the sintering temperature above 1550 ◦ C will lead to migration of the yttrium to the grain boundaries thus lowering the biaxial flexural strength of the zirconia. However, this was not in agreement with our results as even when our sintering temperature was raised to 1600 ◦ C the biaxial flexural strength was not affected nor the average grain size increased beyond 1 m. It has to be mentioned that all of our specimens were sintered as milled and they were not polished, thus comparing our biaxial flexural strength values to values from other studies with polished specimens is questionable. In our study, there was no difference between the groups regarding Vickers surface hardness. This coincide with not detecting any monoclinic phase in our specimens since theoretically monoclinic phase transition at surface would cause compressive stress to outer layer of zirconia, which could increase surface hardness and biaxial flexural strength.
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A limitation of this study is that only one brand of zirconia was investigated. The results may not apply for other brands of monolithic nanozirconia thus other brands may require further investigations. In addition, the effect of aging was not investigated in our study and therefore also requires further investigation.
5.
Conclusions
On the basis of the results and condition of this study, the following conclusions can be drawn:
1. Increasing the sintering temperature and time will lead to enhanced color reproduction and translucency of shaded monolithic nanozirconia ceramic. 2. There is a direct relation between zirconia grain size and the sintering temperature and time. 3. Changing the sintering parameters within the selected range will not cause any tetragonal-monoclinic phase transformation of zirconia ceramic. 4. Changing the sintering parameters within the range selected will have no effect on the surface roughness, biaxial flexural strength, and surface hardness of monolithic nanozirconia ceramic.
Acknowledgments The authors wish to express appreciation for technical assistance and laboratory support of Frank Lehmann, Detlev Gostomsky, and Rüdiger Möller, of Department of Prosthodontics, Propaedeutics and Dental Materials, School of Dentistry, Christian-Albrechts University at Kiel and also Prof. Klaus Rätzke from the Institute of Materials Science, ChristianAlbrechts University at Kiel. This study was supported by Glidewell, Frankfurt, Germany which provided the materials used.
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Effect of changes in sintering parameters on monolithic translucent zirconia Kamal Ebeid a,b,∗ , Sebastian Wille a , Amina Hamdy b , Tarek Salah b , Amr El-Etreby b , Matthias Kern a a
Department of Prosthodontics, Propaedeutics and Dental Materials, School of Dentistry, Christian-Albrechts University at Kiel, Germany b Department of Fixed Prosthodontics, Faculty of Dentistry, Ain Shams University, Egypt
a r t i c l e
i n f o
a b s t r a c t
Article history:
Objectives. Aim of this study was to evaluate the effect of different sintering parameters on
Received 6 February 2014
color reproduction, translucency and biaxial flexural strength of monolithic zirconia.
Received in revised form
Methods. Translucent zirconia discs having 15 mm diameter, 1 mm thickness, and shade
15 June 2014
A3 were milled and divided according to the sintering temperatures (1460 ◦ C, 1530 ◦ C, and
Accepted 5 September 2014
1600 ◦ C) into three groups (n = 30). Each group was later divided into three subgroups (n = 10)
Available online xxx
according to the sintering holding time (1, 2, and 4 h). Easyshade spectrophotometer (Vita,
Keywords:
A3. Mean E values below 3.0 were considered “clinically imperceptible”, E values between
Bad Säckingen, Germany) was used to obtain the E between the specimens and the shade Dental materials
3.0 and 5.0 were considered “clinically acceptable” and E values above 5.0 were considered
Ceramics
“clinically unacceptable”. Contrast ratio (CR) was obtained after comparing the reflectance
Zirconia
of light through the specimens over black and white background. Biaxial flexural strength
Biaxial flexural strength
was tested using the piston-on-three balls technique in a universal testing machine.
Translucency
Results. Mean E results ranged from 4.4 to 2.2. Statistically significant decrease in the Delta E was observed as the sintering time and temperature increased. CR decreased from 0.75 to 0.68 as the sintering time and temperature increased. No significant change in the biaxial flexural strength was observed. Significance. Sintering zirconia using long cycles and high temperatures will result in reduction of E and CR. Biaxial flexural strength is not affected by changes in the evaluated sintering parameters. © 2014 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
All ceramic restorations have proved to be a promising alternative to metal-ceramic restorations mainly due to their excellent esthetics, chemical stability, and biocompatibility
[1]. Zirconia restorations became very popular due to their unique mechanical properties which made it possible to use them in long span restorations [2,3]. Zirconia is polymorphic in nature and exists in three forms: cubic, tetragonal, and monoclinic. At room temperature zirconia is present in its monoclinic form and is stable up to 1170 ◦ C. Above this
∗
Corresponding author at: Department of Prosthodontics, Propaedeutics and Dental Materials, School of Dentistry, Christian-Albrechts University at Kiel, Germany. Tel.: +20 1006084044. E-mail address: [email protected] (K. Ebeid) . http://dx.doi.org/10.1016/j.dental.2014.09.003 0109-5641/© 2014 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
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temperature a transformation occurs to the tetragonal phase that is stable up to 2370 ◦ C. Beyond this temperature, zirconia assumes its cubic form [4]. Zirconia restorations are fabricated either in a partially sintered state by soft machining followed by a final sintering cycle, or they are fabricated in a fully sintered state by hard machining [5,6]. Hard machining may induce tetragonalmonoclinic transformation, introduce cracks, and wears the milling hardware at a higher rate. Soft machining however is much easier, but may produce less accurate frameworks due to the sintering shrinkage accompanied with the final sintering process [7]. The phenomenon of light scattering largely affects the translucency of dental ceramics. If the majority of light passing through a ceramic is scattered, the material will appear opaque. However, if most of the light passing is transmitted through the ceramic it will appear translucent [8]. The amount of light that is absorbed, transmitted, and reflected mainly depends on the microstructure of the ceramic itself [9,10]. Differences in perceived color (E) can be determined using the CIELAB coordinates. The CIELAB system has provided a quantitative representation of color and it has been extensively applied in dentistry to study esthetic materials, shade guides, and color reproductions [11–13]. The perceptibility and acceptability thresholds of the E vary widely in literature mainly due to the diversity of observers, objectives, and methodologies among the studies [14,15]. Clinically the tooth, restorations available, surrounding, and blending effect tend to expand the clinically acceptable range previously reported [16,17]. The mean E values as “clinically imperceptible” (E < 3), “clinically acceptable” (E between 3 and 5) and “clinically unacceptable” (E > 5) seem to be consistent with the clinical practice considering a non-color expert, which usually is the patient’s condition [11,18,19]. In order to overcome the main disadvantage of zirconia which is its opacity, the zirconia core is veneered with veneering porcelain to enhance its esthetics. However, the most common mode of failure that faced clinicians was the chipping of this veneering porcelain while the zirconia core remained unaffected [20,21]. The differences in sintering parameters of zirconia can directly affect its microstructure and properties [22]. The extent of this effect have become of interest in the field of dental research especially after the introduction of short sintering cycles by manufacturers. Several authors have studied the effect of the changes in sintering time and temperature on the translucency, grain size, and biaxial flexural strength of zirconia core ceramics; however the effect of these changes on the properties of monolithic nanozirconia remains in question [10,23–25]. The aim of this study was to evaluate the effect of using different sintering times and temperatures on the color reproduction, translucency, surface roughness, biaxial flexural strength, and the surface hardness of monolithic zirconia ceramic.
2.
Materials and methods
Ninety translucent shaded zirconia ceramic discs (Bruxzir, Glidewell, Frankfurt, Germany) with a diameter of 15 mm, a
thickness of 1 mm, and shade A3 were milled and divided into three groups (n = 30) according to the sintering holding time (1, 2, and 4 h). Each group was later divided into three subgroups (n = 10) according to the sintering temperature (1460 ◦ C, 1530 ◦ C, and 1600 ◦ C). All specimens were sintered as milled in the manufacturers sintering furnace (Bruxzir FastFire, Glidewell, Frankfurt, Germany) at a heating and cooling rate of 10 ◦ C per minute. The temperature was controlled using the furnace’s internal thermometer.
2.1.
Color evaluation
Specimens were placed over a neutral gray background (CIE L* = 62.1, a* = 1.3, b* = −0.02) and the CIELAB coordinates were measured for each specimen using a spectrophotometer (Easyshade compact, Vita Zahnfabrik, Bad Säckingen, Germany). The Easyshade was set to the restoration mode and the shade A3 was selected. In this mode the color difference is determined by comparing the selected shade and the measured shade. For each specimen three measurements were taken at the center and their average was recorded. After each specimen was measured the Easyshade was recalibrated. Mean E values below 3.0 were considered “clinically imperceptible”, E values between 3.0 and 5.0 were considered “clinically acceptable” and E values above 5.0 were considered “clinically unacceptable”.
2.2.
Translucency evaluation
A quantitative measurement of translucency was obtained by measuring the CIELAB coordinates of the specimens after backing with a white (CIE L* = 96.7, a* = 0.1, b* = 0.2) and black (CIE L* = 10.4, a* = 0.4, b* = 0.6) background using the spectrophotometer. For each specimen three measurements were taken and their average was recorded. The contrast ratio (CR) for each specimens was calculated according to the following equation: CR = Yb /Yw where Y = [(L + 16)/116]3 × 100 and Yb is the reflectance over a black background and Yw is the reflectance over a white background [8,11]. In all calculations “0” is considered the most transparent and “1” is considered the most opaque.
2.3.
Surface roughness evaluation
All specimens were cleaned ultrasonically in 99% isopropanol solution for 3 min and then dried with air. The average surface roughness (Ra) for the specimens was measured using a 3D laser scanning microscope (Keyence VK-X100, Keyence GmbH, Neu-Isenbuerg, Germany). The wavelength of the laser was 658 nm. Three separate areas were measured on each specimen, the measured area was 500 m × 750 m and the distance between the separate scans was over 3 m. The mean Ra for each specimen was later recorded.
2.4.
Microstructure analysis
Three specimens were selected randomly from each subgroup for X-ray diffraction (XRD) surface analysis to detect the amount of tetragonal and monoclinic phases available. The specimens were placed in the holder of a diffractometer
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Fig. 1 – Piston-on-three balls biaxial flexural strength test.
(Seifert XRD 3000 PTS, GE GmbH, Munich, Germany) and subjected to Cu K␣ radiation. The spectrum was recorded within the range of 20–40◦ , a step size of 0.02◦ and a scan time per step of 10 s. Voltage and current were set to 40 kV and 40 mA. One specimen from each subgroup was cleaned, dried, gold sputter coated (Bal-Tec SCD 050, Leica Mircosystems, Wetzlar, Germany) and examined under 10,000× magnification using a scanning electron microscope (XL 30 CP, Philips, Surrey, England) to calculate the average grain size using a base of at least 150 grains.
2.5.
Biaxial flexural strength testing
Specimens were tested for biaxial flexural strength using piston-on-three ball technique in a universal testing machine (Zwick Z010, Zwick, Ulm, Germany). A 10 mm diameter metallic platform was constructed, above which rested three 3.2 mm diameter steel balls that were equidistant from each other. Each disk was placed centrally on the steal balls and load was applied by a piston of 1.4 mm diameter and 0.5 mm/min crosshead speed using the universal testing machine (Fig. 1). The fracture load for each specimen was recorded and the biaxial flexural strength was calculated using the following equation: S=
−0.2387P(X − Y) d2
where: S, biaxial flexural strength (MPa); P, fracture load (N); d, specimen disk thickness at fracture origin (mm). X and Y were determined as follows: X : (1 + ) ln
r2 2 r3
Y : (1 + ) 1 + ln
+
1 − r2 2 2
r1 2 r3
r3
r1 2
+ (1 − )
r3
is Poisson’s ratio (0.25), r1 is the radius of the support circle, r2 is the radius of the loaded area, and r3 is the radius of the specimen.
Fig. 2 – Biaxial flexural strength of subgroups sintered using different sintering times and temperatures.
2.6.
Vickers hardness evaluation
A piece from each broken specimen from each subgroup was randomly selected and tested for Vickers hardness using an indentation tester (ZHV10, Zwick, Ulm, Germany). An indentation was made on each specimen under a loading mass of 5 kg and dwell time of 10 s. The diagonal length was measured using an optical microscope (20×) and the hardness was calculated from the following equation [26,27]: VHN = 1.8544 ×
F d2
where VHN is the Vickers hardness number, F is the applied load expressed in kg, and d is the mean length of the diagonals of the indentation (mm).
2.7.
Statistical analysis
The data collected was checked for normal distribution and analyzed using two-way analysis of variance (ANOVA), followed by Tukey’s HSD test (SPSS v20, Chicago, IL, USA) at a significance level of P ≤ 0.05 to determine the effect of the changes in sintering time and temperature on each of the variables tested.
3.
Results
Regarding the color evaluation, two-way ANOVA revealed significant differences in the E between the subgroups when sintered using different sintering temperatures and times (P ≤ 0.05). Tukey’s HSD tests revealed that there was a significant decrease in the E as the sintering temperature increased. It also revealed that there was no significant difference in the E between the 2 and 4 h sintering holding times however, both showed a significant decrease when compared to the 1 h sintering holding time (Table 1). None of
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Table 1 – Mean E and standard deviation () of test groups sintered using different sintering times and temperatures.
Table 4 – Mean grain size in m and standard deviation () of test groups sintered using different sintering times and temperatures.
Sintering temperature/◦ C
Sintering temperature/◦ C
Sintering holding time/h 1
2 a
1460 1530 1600
4 d
4.4 (0.3) 3.1 (0.1)b 2.4 (0.1)c
4.0 (0.1) 2.8 (0.3)e 2.2 (0.1)f
3.8 (0.1)d 2.9 (0.1)e 2.2 (0.1)f
Means with different superscript letters are significantly different at the 95% confidence level.
the subgroups showed any unacceptable color results, with the sintering temperatures 1460 ◦ C and 1530 ◦ C subgroups showing clinically acceptable results and the sintering temperature 1600 ◦ C showing clinically imperceptible color results. Concerning translucency, two-way ANOVA revealed significant differences in CR between the subgroups when sintered with different sintering temperatures and times (P ≤ 0.05). Tukey’s HSD tests revealed that there was a significant decrease in CR when comparing the sintering temperatures 1460 ◦ C to the 1530 ◦ C and 1600 ◦ C. It also revealed that there was a significant decrease in CR when comparing the 1 and 2 h sintering holding times to the 4 h (Table 2). As for the surface roughness measurements, the results showed that there was a decrease in the mean Ra as the sintering temperature and time increased, however this decrease was not statistically significant. The highest mean Ra measured was with specimens sintered with a holding time of 1 h at a temperature of 1460 ◦ C (1.030 m) while the lowest mean Ra measured was with the specimens sintered with a holding time of 4 h at a temperature of 1600 ◦ C (0.826 m) (Table 3).
Table 2 – Mean CR and standard deviation () of test groups sintered using different sintering times and temperatures. Sintering temperature/◦ C
Sintering holding time/h 1
1460 1530 1600
2 a
0.75 (0.02) 0.72 (0.01)b 0.71 (0.01)b
4 a
0.75 (0.03) 0.71 (0.01)b 0.70 (0.01)b
0.71 (0.01)c 0.69 (0.01)d 0.68 (0.01)e
Means with different superscript letters are significantly different at the 95% confidence level.
Table 3 – Mean Ra and standard deviation () of test groups sintered using different sintering times and temperatures. Sintering holding time/h
Sintering temperature/◦ C 1 1460 1530 1600
1.030 (0.190)a 1.026 (0.304)a 0.898 (0.191)a
2 1.023 (0.164)a 0.878 (0.215)a 0.957 (0.242)a
4 0.991 (0.198)a 1.069 (0.086)a 0.826 (0.161)a
Means with different superscript letters are significantly different at the 95% confidence level.
Sintering holding time/h 1
2 a
1460 1530 1600
0.55 (0.10) 0.65 (0.13)b 0.89 (0.19)c
4 d
0.64 (0.11) 0.77 (0.13)e 1.0 (0.11)f
0.79 (0.15)d 0.92 (0.25)e 0.92 (0.14)f
Means with different superscript letters are significantly different at the 95% confidence level.
Table 5 – Mean VHN and standard deviation () of test groups sintered using different sintering times and temperatures. Sintering holding time/h
Sintering temperature/◦ C 1 1460 1530 1600
1456 (212)a 1553 (111)a 1461 (108)a
2 1504 (123)a 1548 (77)a 1507 (71)a
4 1497 (138)a 1437 (74)a 1415 (60)a
Means with different superscript letters are significantly different at the 95% confidence level.
Regarding microstructure analysis, all specimens analyzed using XRD showed only tetragonal ZrO2 phase characteristics peaks. None of the samples showed any tetragonalmonoclinic transformation. However, samples showed a significant increase in the average grain size as the sintering temperature increased. Samples sintered at 2 and 4 h sintering holding time also showed a statistically significant increase in the average grain size when compared to samples sintered at a 1 h sintering holding time (Table 4). The mean biaxial flexural strength of subgroups ranged from 906 MPa to 1000 MPa. However, there was no statistically significant difference between subgroups (Fig. 2). The mean Vickers hardness of subgroups ranged from 1437 to 1553 VHN. Again, there was no statistically significant difference between the subgroups concerning their surface hardness (Table 5).
4.
Discussion
The use of the Vita Easyshade spectrophotometer to measure color and translucency by assessing the CIELAB coordinates of specimens is widely used in the field of dental research [12,14]. Spectral reflectance which is used in this study together with direct transmission and total transmission are the three ways used to measure translucency [28]. We calculated the contrast ratio which was used by several authors to obtain a quantitative measure of translucency [8,29,30]. Our results showed that there was a decrease in the E from 4.4 to 2.2 and the contrast ratio from 0.75 to 0.68 as the sintering temperature and time increased. This could be attributed to the assumption that the sintering process reduces the pores between the grains and increases the final density of zirconia, thus producing less light scattering and more light transmission leading
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to better translucency and optical characteristics [31]. This assumption was consistent with our results as increasing the sintering temperature and time led to an increase in the average grain size of zirconia. Also the increase in the sintered density of the zirconia may lead to a more uniform crystalline arrangement thus promoting better specular reflection, light transmission and a better perception of color. Concerning the contrast ratio, our results were similar to a study performed by Jiang et al. [24], where sintering temperatures 1350 ◦ C, 1400 ◦ C, 1450 ◦ C, and 1500 ◦ C were used and their effect on the translucency of zirconia core discs was measured. They concluded that as the sintering temperature increased the translucency of the discs increased. Our results were also in agreement with Stawarczyk et al. [25] as they concluded that there was a decrease in the CR of regular zirconia core material specimens having 0.5 mm thickness from 0.85 to 0.70 as the sintering temperature increased from 1300 to 1700 ◦ C. A non-contact laser profilometer was used in this study as there were questions regarding the accuracy of contact profilometry with assumptions saying that they underestimate the surface roughness thus present a smoother surface [32]. Since we had no significant differences between the subgroups regarding the surface roughness, we can assume the fact that the reduction in the pores between the grains is not enough to produce a significant difference, we can also assume the fact that the changes in the contrast ratio and E are mainly due to changes in the microstructure rather than changes in surface morphology. Simulating pure bending and preventing edge loss can be achieved best using the piston-on-three ball technique for testing the biaxial flexural strength, as the specimens in this technique are resting on the stainless steel ball which form a smaller diameter than the specimen itself [33,34]. In our study there was no effect on the biaxial flexural strength as the sintering parameters changed. Our results are in agreement with the results of a study conducted by Hjerppe et al. [35], who used different rising and holding times for sintering zirconia. Stawarczyck et al. [25] also found no significant differences in the biaxial flexural strength of zirconia core material when the sintering temperature was raised from 1400 ◦ C to 1550 ◦ C, however the flexural strength significantly decreased above and below these temperatures. In a similar study by Jiang et al. [24], they concluded that raising the sintering temperature above 1550 ◦ C will lead to migration of the yttrium to the grain boundaries thus lowering the biaxial flexural strength of the zirconia. However, this was not in agreement with our results as even when our sintering temperature was raised to 1600 ◦ C the biaxial flexural strength was not affected nor the average grain size increased beyond 1 m. It has to be mentioned that all of our specimens were sintered as milled and they were not polished, thus comparing our biaxial flexural strength values to values from other studies with polished specimens is questionable. In our study, there was no difference between the groups regarding Vickers surface hardness. This coincide with not detecting any monoclinic phase in our specimens since theoretically monoclinic phase transition at surface would cause compressive stress to outer layer of zirconia, which could increase surface hardness and biaxial flexural strength.
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A limitation of this study is that only one brand of zirconia was investigated. The results may not apply for other brands of monolithic nanozirconia thus other brands may require further investigations. In addition, the effect of aging was not investigated in our study and therefore also requires further investigation.
5.
Conclusions
On the basis of the results and condition of this study, the following conclusions can be drawn:
1. Increasing the sintering temperature and time will lead to enhanced color reproduction and translucency of shaded monolithic nanozirconia ceramic. 2. There is a direct relation between zirconia grain size and the sintering temperature and time. 3. Changing the sintering parameters within the selected range will not cause any tetragonal-monoclinic phase transformation of zirconia ceramic. 4. Changing the sintering parameters within the range selected will have no effect on the surface roughness, biaxial flexural strength, and surface hardness of monolithic nanozirconia ceramic.
Acknowledgments The authors wish to express appreciation for technical assistance and laboratory support of Frank Lehmann, Detlev Gostomsky, and Rüdiger Möller, of Department of Prosthodontics, Propaedeutics and Dental Materials, School of Dentistry, Christian-Albrechts University at Kiel and also Prof. Klaus Rätzke from the Institute of Materials Science, ChristianAlbrechts University at Kiel. This study was supported by Glidewell, Frankfurt, Germany which provided the materials used.
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Please cite this article in press as: Ebeid K, et al. Effect of changes in sintering parameters on monolithic translucent zirconia. Dent Mater (2014), http://dx.doi.org/10.1016/j.dental.2014.09.003
