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Effect of sintering temperature on microstructure, flexural strength, and optical properties of a fully stabilized monolithic zirconia
RESEARCH AND EDUCATION
Effect of sintering temperature on microstructure, flexural strength, and optical properties of a fully stabilized monolithic zirconia Kátia Vieira Cardoso, DDS, MSc,a Gelson Luis Adabo, DDS, MSc, PhD,b Eduardo Mariscal-Muñoz, DDS, MSc, PhD,c Selma Gutierres Antonio, Chem Eng, MSc, PhD,d and João Neudenir Arioli Filho, DDS, MSc, PhDe The pursuit of esthetic excelABSTRACT lence in fixed dental prostheStatement of problem. Fully stabilized monolithic zirconia (FSZ) has been developed as an ses (FDPs) has been guided by alternative to zirconia veneered with porcelain. However, how sintering conditions might affect the evolution of dental ceits microstructure and optical and mechanical properties is unclear. ramics. An ideal ceramic Purpose. The purpose of this in vitro study was to determine the effect of different sintering should cost low and have temperatures on the microstructure and optical and mechanical properties of FSZ. biocompatibility, optimal meMaterial and methods. Bar-shaped FSZ specimens were prepared and divided into 2 groups chanical properties, and es(n=15) according to final sintering temperatures (1450 C and 1600 C). The average reflectance, thetics that replicate those of opacity, translucency parameter, and sum of light absorption-scattering values were obtained by natural teeth. Yttria-stabilized using a spectrophotometer, and DE00 was calculated. The 3-point bend test was performed in a tetragonal zirconia polycrystal universal testing machine. Scanning electron microscopy (SEM) was conducted for microstructure (Y-TZP) is a widely used analysis. Crystalline phase quantification was obtained by X-ray diffraction (XRD). Data were ceramic biomaterial for FDPs analyzed by using D’Agostino-Pearson and Student t tests (a=.05). because of its excellent meResults. A significant difference was detected in the reflectance and sum of light absorptionchanical performance.1 Howscattering values between the 2 groups. The translucency parameter, opacity, and flexural ever, owing to its high opacity, strength showed no statistical differences. DE00 was 0.98. XRD indicated cubic (47.41% for 1450 conventional zirconia was C; 46.04% for 1600 C) and tetragonal content (52.59% for 1450 C; 53.96% for 1600 C). No monoclinic content was found. SEM images showed more definite grain boundaries in the 1600 veneered with feldspathic C group. Mean grain size was 0.49 mm for the 1450 C group and 1.99 mm for the 1600 C group. porcelain,2 although chipping has been a problem with these Conclusions. Higher sintering temperatures increased the grain size but did not change the crystal phase concentration. A significant difference was found in the reflectance and sum of light FDPs.2-4 absorption-scattering, but no differences were found among the translucency parameter, opacity, To solve the problem of or flexural strength. (J Prosthet Dent 2019;-:---) chipping, more translucent zirconias were developed for can be lost through refraction and scattering (diffuse use in anatomic contour prostheses. The opacity and reflection), through birefringence, and through pores and translucence of zirconia are influenced by size, crystal impurities in the material.7,8 Y-TZP is partially stabilized isotropy,5 and thickness.6 When light reaches a solid such in the tetragonal phase at an ambient temperature by as zirconia, after initial reflection and some absorption, it a
Doctoral student, Department of Dental Materials and Prosthodontics, Araraquara School of Dentistry, São Paulo State University (UNESP), Araraquara, Brazil. Professor of Dental Materials, Department of Dental Materials and Prosthodontics, Araraquara School of Dentistry, São Paulo State University (UNESP), Araraquara, Brazil. Associate Professor, Department of Dental Clinics, Centro Universitario de Ciencias de la Salud, Universidad de Guadalajara, Guadalajara, Mexico. d Postdoctoral research associate, Department of Physical Chemistry, Araraquara Institute of Chemistry, São Paulo State University (UNESP), Araraquara, Brazil. e Adjunct Professor of Complete Prosthodontics, Department of Dental Materials and Prosthodontics, Araraquara School of Dentistry, São Paulo State University (UNESP), Araraquara, Brazil. b c
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Clinical Implications Processing fully stabilized monolithic zirconia with an increased final sintering that does not reduce its mechanical properties may improve the esthetics of the material.
Optical and mechanical properties Translucency
Average reflectance
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Table 1. Mean ±standard deviation and coefficient of variation for optical and mechanical properties of ceramic sintered at 1450 C and 1600 C
Opacity
metal oxides (especially yttria).9 Tetragonal crystals improve the mechanical resistance of Y-TZP because of phase transformation toughening9,10 when a tetragonal to monoclinic transformation is induced mechanically around a crack. However, tetragonal crystals are optically anisotropic and reduce its translucency because of differences in the refractive index (birefringence).11,12 According to the Rayleigh scattering model described by Apetz and van Bruggen,6 when light rays are incident on a birefringent material, the smaller the particle dispersed for a given wavelength, the larger the light transmission at the grain boundaries. Tetragonal grain sizes range from 0.2 mm to 0.8 mm,13 which is greater than the wavelength range of visible light (400 nm to 700 nm). Therefore, the use of nanometric tetragonal crystals should minimize the birefringence effect and improve light transmission.14 In contrast, FSZ exhibits higher translucence with increasing yttria concentration, which stabilizes higher cubic phase content.8 Cubic grains have an isotropic orientation, which means that there is less interference with light transmission among the grains.6 Another advantage is that cubic grains are larger than tetragonal grains, and this reduces the number of grain boundaries, which are sources of light scattering.6,15 However, increased cubic phase may reduce mechanical properties as the transformation-toughening effect is diminished because cubic grains do not undergo the tetragonal to monoclinic phase transformation.13 The sintering temperature of zirconia affects the ceramic’s mean grain size,16-18 which results in a change in its optical properties. Furthermore, pore diameter, microstructure, mechanical properties, and lowtemperature degradation behavior could be affected by the sintering conditions.8,15,16,19 When it was first introduced, the zirconia used in this study had a final manufacturer-recommended sintering temperature of 1600 C. However, the sintering protocol has since changed to a final sintering temperature of 1450 C. The purpose of this study was to determine the effect of different sintering temperatures on the microstructure and optical and mechanical properties of a fully stabilized zirconia. The null hypothesis was that the microstructure, flexural strength, and optical properties would not be affected by changes in sintering temperature.
-
S/A Flexural strength
Temperature ( C)
Mean ±SD (MPa)
CV (%)
1450
15.45a ±0.18
1.21
1600
15.58a ±0.37
2.42
1450
64.68a ±0.44
0.68
1600
64.20a ±0.81
1.28
1450
70.96a ±0.86
1.22
1600
68.25b ±0.80
1.18
1450
98.14a ±0.13
0.13
1600
97.68b ±0.13
1450
542.9a ±112.2
20.7
1600
577.5a ±99.3
17.2
0.13
CV, coefficient of variation; SD, standard deviation; S/A, absorption-scattering sum of light; MPa, mega pascals. Different superscript letters indicate statistically significant difference (P<.05).
MATERIAL AND METHODS Bars (25×5×2.1 mm) were cut from presintered fully stabilized zirconia blocks (Prettau Anterior; Zirkonzahn) with a diamond wheel (Diamond Cup Grinding Wheel; Buehler) and ground by using abrasive papers (P1200, P1500, and P2500; Imperial Wetordry; 3M) to a thickness of 1.5 mm. According to the manufacturer information, the composition of the zirconia was 8% to 12% Y2O3, 0% to 1% Al2O3, 0.02% SiO2, 0.01% Fe2O3, and 0.04% Na2O. The bars were divided into 2 groups (n=15) by simple randomization and sintered at 2 different temperatures: 1450 C and 1600 C. The sintering protocol was started at room temperature, with an 8 C per minute heating rate up to the maximum temperature (1450 C or 1600 C). After a 2-hour step time, the temperature was decreased to room temperature at an 8 C per minute cooling rate. After sintering, the bars had dimensions of approximately 20×4×1.2 mm ±0.01 mm, according to the ISO6872: 2015 standard.20 To assess the optical properties, spectral reflectance was measured at wavelengths of 400 nm to 740 nm at 10nm intervals by using a computerized spectrophotometer (CM 2600d; Konica Minolta Sensing Inc). The conditions included ambient room temperature (24 ±3 C), standard primary illuminant D65 (daylight, 6504 K), UV secondary illuminant, observer at 2 degrees, and the Commission Internationale de l’Eclairage L*a*b* (CIELab) system. Measurements were made at the center of the specimens, limiting the surface area to a diameter of 3 mm by means of a black mask (CM-A147; Konica Minolta Sensing Inc) coupled to the spectrophotometer optical port. The instrument was calibrated with a white plate (CM-A145; Konica Minolta Sensing Inc). The reflectance data acquired were processed by means of a computer program (Spectra-Magic version 3.61; Konica Minolta Sensing Inc) that provided lightness (L*) and chromaticity (a* and b*) values. The optical properties of average Cardoso et al
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O = RRwhbl × 100, where Rbl is the average reflectance at each wavelength over blackdCIE L*=25.6976, a*=-0.09, and b*=-0.81dand Rwh is the average reflectance at each wavelength over white backgroundsdCIE L*=99.45, a*=-0.11, and b*=-0.13. The translucency parameter was obtained by using the following equation: TP = ð½Lwh −Lbl 2 +½awh −abl 2 +½bwh −bbl 2 Þ1=2 , where bl is measurements against a black backgrounddCIE L*=25.6976, a*=-0.09, and b*=-0.81dand wh is measurements against a white backgrounddCIE L*=99.45, a*=-0.11, and b*=-0.13. The percentage of absorption-scattering sum of light (S/A) was obtained by subtracting the sum of scattered/ absorbed light in each wavelength divided by 100. This parameter was automatically calculated by the spectrophotometer program with the data collection. Additionally, the color difference calculated according to CIEDE2000 (DE00)22,23 is based on the equation 0 " 2 2 B 0 0 D D L C 24 B described by Ghinea et al : DE = @ KL SL + KC SC +
0
DH K H SH
2
+RT
0
DC K C SC
0
DH K H SH
#12
1 C C, where DL, DC, and DH A
represent the differences in lightness, chroma, and hue in CIEDE2000; RT is a function that accounts for the interaction between chroma and hue differences in the blue region. SL, SC, and SH are weighting functions that adjust the total color difference for variation in the location of color difference in L0 , a0 , b0 coordinates. KL, KC, and KH are parametric factors to correct terms for experimental conditions. In the present study, the parametric factors of the CIEDE2000 color difference formula were set to 1. X-ray diffraction analysis was performed (DRX-Rint 2000; Rigaku) (n=1) to identify the crystalline phases present after each sintering protocol. Scanning conditions were as follows: Cu-Ka radiation (40 kV, 70 mA) and scan range from 20 degrees to 90 degrees (2q) with a step size of 0.02 degrees for 3 seconds. In addition, the percentage crystalline content was analyzed by the Rietveld refinements.25 Scanning electron microscopy was conducted (MIRA3; TESCAN) to observe microstructural changes Cardoso et al
1450 ˚C 1600 ˚C
Intensity (a.u.)
reflectance, opacity, translucency parameter, and absorption-scattering sum of light were calculated according to the equations of Shiraishi et al21: Average reflectance: average of reflectance percentage in each wavelength. Opacity percentage was calculated by using the following equation:
20
30
40
50
60
70
2θ Figure 1. Representative X-ray diffraction patterns of fully stabilized monolithic zirconia sintered at different temperatures.
and characterize the surface. The average grain size was calculated directly from the SEM images according to the linear intercept method26 based on at least 90 grains by using a computer software program (Motic Images Plus 2.0 ML; Motic). The specimens were given a gold coating (40 seconds, 18.7 mA) to improve electron scattering. The 3-point flexural strength test was conducted according to the ISO 6872: 2015 standard.20 Fifteen specimens were tested. The test was conducted under dry conditions and performed in a universal testing machine (MTS 810; Materials Testing System) at a crosshead speed of 1 mm/min until failure. Flexural strength was calculated from the following equation: s=3Nl/(2bd2), where s is the flexural strength, N is fracture load, l is distance between supports (mm), b is width of the specimen (mm), and d is thickness of the specimen (mm). Optical properties and the 3-point flexural strength data were tested for distribution (D’Agostino & Pearson Omnibus, a=.05) and showed a normal distribution. For comparison between groups, an unpaired Student t test was used (a=.05). RESULTS The mean, standard deviation, and coefficient of variation values of the optical properties for the 1450 C and 1600 C groups are shown in Table 1. A statistically significant difference was found between the groups for average reflectance (P<.001) and absorption-scattering sum of light (P<.001). Translucency parameters (P=.67) and opacity (P=.07) showed no statistical difference. The color difference between the 2 groups was DE00=0.98. Rietveld analyses for crystalline phase quantification indicated a cubic phase of 47.41% for the 1450 C and 46.04% for the 1600 C group and a tetragonal phase of 52.59% for the 1450 C group and 53.96% for the 1600 C group. Figure 1 illustrates the diffraction patterns for
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Figure 2. Scanning electron microscopy images after different sintering conditions. A, Final sintering temperature of 1450 C. B, Final sintering temperature of 1600 C. Original magnification ×5000.
the different sintering temperatures. No monoclinic content was found for either temperature group. SEM micrographs were obtained at ×5000 magnification; Figure 2 shows microstructural differences between specimens based on variation of sintering temperature. Grain size analysis by the linear intercept method27 showed significant differences in the mean grain size of the ceramic after sintering at 1450 C (0.49 mm) and 1600 C (1.99 mm). In addition, a more definite and sharper limit at grain boundaries was observed in the ceramic sintered at 1600 C. Descriptive statistics (mean, standard deviation, and coefficient of variation) of the flexural strength testing are summarized in Table 1. No statistically significant difference (Student t test, P=.378) was found for the FSZ bars sintered at 1600 C. DISCUSSION This study was motivated by a revision of the manufacturer’s instructions for Prettau Anterior that altered the sintering cycle, by reducing the top temperature from 1600 C to 1450 C. The reason for this alteration was not specified, but presumably it was to optimize the mechanical properties without compromising the optical characteristics. The null hypothesis was accepted because no difference was found between the ceramic sintered at the different temperatures in terms of 3-point flexural strength, translucency, or opacity. However, a statistically significant difference in reflectance and absorption scattering was observed. Analysis of microstructure revealed different grain sizes, and crystallographic analysis showed that the concentration of tetragonal and cubic phases was maintained. Sintering temperature and holding time can affect the microstructure of monolithic zirconia because of the partial tetragonal phase transformation into cubic phase and a grain size increase.27 In this study, temperature
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variations led to different grain sizes. SEM images showed that specimens sintered at 1450 C had 4 times smaller grain sizes than those sintered at 1600 C (Fig. 2). Grain size may play a crucial role in mechanical properties.6,8,17,19 The FSZ Prettau Anterior has up to 12% yttria content to stabilize the cubic phase, with a reduction in the mechanical properties. However, in the present study, the flexural strength was not significantly affected. Sen et al,27 who studied Prettau Anterior and other ceramics at the same sintering temperature, reported similar results. However, the Rietveld analysis revealed a negligible difference in cubic phase concentration in the specimens heated at 1450 C (47.41%) and those heated at 1600 C (46.04%). Thus, considering that no significant change was observed in cubic phase concentration, it may be assumed that the similar percentage cubic phase led to the similar 3-point flexural strengths, while the grain size had a little effect. Microstructure and composition can also influence optical properties. In a material with smaller grain sizes, the increased intercept areas at grain boundaries significantly affect light scattering.28 Despite the enlarged grains obtained in the 1600 C group (Fig. 2), no significant difference was found in translucency or opacity, although the absorption-scattering sum of light and reflectance was significantly different for the average reflectance (Table 1). Sen et al27 also reported similar translucency at the same sintering temperatures (1450 C and 1600 C), but grain size was not measured. Apparently, the crystal phase predominates in the optical properties. The isotropic cubic phase reduces light scattering at the grain boundaries,28 but there was similar cubic phase concentration between the ceramic sintered at different temperatures. The change in sintering temperature led to a DE00=0.98, which indicates that color difference is perceptible but acceptable, considering the 50:50% values
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of the perceptibility threshold (DE00=0.81) and acceptability threshold (DE00=1.77) based on Paravina et al.29 The present study has limitations. Only a single ceramic (Prettau Anterior) was tested, which limits the results to this specific material. Future studies should include a more complete comparison of different monolithic zirconias and include other sintering cycles with different temperatures and holding times. Moreover, fracture toughness and fatigue testing could also add important information with regard to clinical performance. CONCLUSIONS Within the limitation of this in vitro study, the following conclusions were drawn: 1. Higher sintering temperatures led to changes in the microstructure with larger grain size but did not change the composition and concentration of crystal phases. 2. Temperature sintering did not affect the flexural strength, translucency parameter, or opacity but did affect other optical properties. 3. The color change was acceptable, and the perceptibility was close to thresholds described in the literature. REFERENCES 1. Piconi C, Maccauro G. Zirconia as a ceramic biomaterial. Biomaterials 1999;20:1-25. 2. Conrad HJ, Seong WJ, Pesun IJ. Current ceramic materials and systems with clinical recommendations: a systematic review. J Prosthet Dent 2007;98: 389-404. 3. Zarone F, Russo S, Sorrentino R. From porcelain-fused-to-metal to zirconia: clinical and experimental considerations. Dent Mater 2011;27:83-96. 4. Sailer I, Makarov NA, Thoma DS, Zwahlen M, Pjetursson BE. All-ceramic or metal-ceramic tooth-supported fixed dental prostheses (FDPs)? A systematic review of the survival and complication rates. Part I: Single crowns (SCs). Dent Mater 2015;31:603-23. 5. Pecho OE, Ghinea R, Ionescu AM, Cardona Jde L, Paravina RD, Perez Mdel M. Color and translucency of zirconia ceramics, human dentine and bovine dentine. J Dent 2012;40 Suppl 2:e34-40. 6. Apetz R, van Bruggen MPB. Transparent alumina: a light-scattering model. J Am Ceram Soc 2003;86:480-6. 7. Chen YM, Smales RJ, Yip KH, Sung WJ. Translucency and biaxial flexural strength of four ceramic core materials. Dent Mater 2008;24:1506-11. 8. Shahmiri R, Standard OC, Hart JN, Sorrell CC. Optical properties of zirconia ceramics for esthetic dental restorations: a systematic review. J Prosthet Dent 2018;119:36-46. 9. 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. 10. Studart AR, Filser F, Kocher P, Gauckler LJ. Fatigue of zirconia under cycling loading in water and its implications for the design of dental bridges. Dent Mater 2007;23:106-14.
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11. Jiang L, Liao Y, Wan Q, Li W. Effects of sintering temperature and particle size on the translucency of zirconium dioxide dental ceramic. J Mater Sci Mater Med 2011;22:2429-35. 12. Krell A, Klimke J, Hutzler T. Transparent compact ceramics: inherent physical issues. Opt Mater 2009;31:1144-50. 13. Muñoz EM, Longhini D, Antonio SG, Adabo G. The effects of mechanical and hydrothermal aging on microstructure and biaxial flexural strength of an anterior and a posterior monolithic zirconia. J Dent 2017;63:94-102. 14. Klimke J, Trunecy M, Krell A. Transparent tetragonal yttria-stabilized zirconia ceramics: influence of scattering caused by birefringence. J Am Ceram Soc 2011;94:1850-8. 15. Boulesteix R, Maître A, Baumard JF, Rabinovitch Y, Reynaud F. Light scattering by pores in transparent Nd:YAG ceramics for lasers: correlations between microstructure and optical properties. Opt Express 2010;18: 14992-5002. 16. Sulaiman TA, Abdulmajeed AA, Shahramian K, Lassila L. Effect of different treatments on the flexural strength of fully versus partially stabilized monolithic zirconia. J Prosthet Dent 2017;118:216-20. 17. Stawarczyk B, Ozcan M, Hallmann L, Ender A, Mehl A, Hämmerlet CH. The effect of zirconia sintering temperature on flexural strength, grain size, and contrast ratio. Clin Oral Investig 2013;17:269-74. 18. Chevalier J, Deville S, Munch E, Jullian R, Lair F. Critical effect of cubic phase on aging in 3mol% yttria-stabilized zirconia ceramics for hip replacement prosthesis. Biomaterials 2004;25:5539-45. 19. Inokoshi M, Zhang F, De Munck J, Minakuchi S, Naert I, Vleugels J, et al. Influence of sintering conditions on low-temperature degradation of dental zirconia. Dent Mater 2014;30:669-78. 20. International Organization for Standardization. ISO 6872, dentistry ceramic materials. 4th ed. Geneva: International Organization for Standardization; 2015. Available at: https://www.iso.org/standard/59936.html. 21. Shiraishi T, Wood DJ, Shinozaki N, van Noort R. Optical properties of base dentin ceramics for all-ceramic restorations. Dent Mater 2011;27: 165-72. 22. Sharma G, Wu W, Dalal EN. The CIEDE2000 color-difference formula: implementation notes, supplementary test data, and mathematical observations. Col Res Appl 2005;30:21-30. 23. International Commission on Illumination. CIE: Technical report. Colorimetry. 3rd ed. Washington: CIE; 2004. Available at: https://law.resource.org/ pub/us/cfr/ibr/003/cie.15.2004.pdf. 24. Ghinea R, Pérez MM, Herrera LJ, Rivas MJ, Yebra A, Paravina RD. Color difference thresholds in dental ceramics. J Dent 2010;38 Suppl 2: e57-64. 25. Rietveld HM. A profile refinement method for nuclear and magnetic structures. J Appl Crystallogr 1969;2:65-71. 26. Wurst JC, Nelson JA. Linear intercept technique for measuring grain size in two-phase polycrystalline ceramics. J Am Ceram Soc 1972;55:109. 27. Sen N, Sermet IB, Cinar S. Effect of coloring and sintering on the translucency and biaxial strength of monolithic zirconia. J Prosthet Dent 2018;119: 308.e1-7. 28. Zhang Y. Making yttria-stabilized tetragonal zirconia translucent. Dent Mater 2014;30:1195-203. 29. Paravina RD, Ghinea R, Herrera LJ, Bona AD, Igiel C, Linninger M, et al. Color difference thresholds in dentistry. J Esthet Restor Dent 2015;27 Suppl 1: S1-9. Corresponding author: Dr Kátia Vieira Cardoso Department of Dental Materials and Prosthodontics Araraquara Dental School Univ Estadual Paulista UNESP 1680, Humaitá Street, Araraquara, SP 14801-903 BRAZIL Email: [email protected] Copyright © 2019 by the Editorial Council for The Journal of Prosthetic Dentistry. https://doi.org/10.1016/j.prosdent.2019.08.007
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Effect of sintering temperature on microstructure, flexural strength, and optical properties of a fully stabilized monolithic zirconia Kátia Vieira Cardoso, DDS, MSc,a Gelson Luis Adabo, DDS, MSc, PhD,b Eduardo Mariscal-Muñoz, DDS, MSc, PhD,c Selma Gutierres Antonio, Chem Eng, MSc, PhD,d and João Neudenir Arioli Filho, DDS, MSc, PhDe The pursuit of esthetic excelABSTRACT lence in fixed dental prostheStatement of problem. Fully stabilized monolithic zirconia (FSZ) has been developed as an ses (FDPs) has been guided by alternative to zirconia veneered with porcelain. However, how sintering conditions might affect the evolution of dental ceits microstructure and optical and mechanical properties is unclear. ramics. An ideal ceramic Purpose. The purpose of this in vitro study was to determine the effect of different sintering should cost low and have temperatures on the microstructure and optical and mechanical properties of FSZ. biocompatibility, optimal meMaterial and methods. Bar-shaped FSZ specimens were prepared and divided into 2 groups chanical properties, and es(n=15) according to final sintering temperatures (1450 C and 1600 C). The average reflectance, thetics that replicate those of opacity, translucency parameter, and sum of light absorption-scattering values were obtained by natural teeth. Yttria-stabilized using a spectrophotometer, and DE00 was calculated. The 3-point bend test was performed in a tetragonal zirconia polycrystal universal testing machine. Scanning electron microscopy (SEM) was conducted for microstructure (Y-TZP) is a widely used analysis. Crystalline phase quantification was obtained by X-ray diffraction (XRD). Data were ceramic biomaterial for FDPs analyzed by using D’Agostino-Pearson and Student t tests (a=.05). because of its excellent meResults. A significant difference was detected in the reflectance and sum of light absorptionchanical performance.1 Howscattering values between the 2 groups. The translucency parameter, opacity, and flexural ever, owing to its high opacity, strength showed no statistical differences. DE00 was 0.98. XRD indicated cubic (47.41% for 1450 conventional zirconia was C; 46.04% for 1600 C) and tetragonal content (52.59% for 1450 C; 53.96% for 1600 C). No monoclinic content was found. SEM images showed more definite grain boundaries in the 1600 veneered with feldspathic C group. Mean grain size was 0.49 mm for the 1450 C group and 1.99 mm for the 1600 C group. porcelain,2 although chipping has been a problem with these Conclusions. Higher sintering temperatures increased the grain size but did not change the crystal phase concentration. A significant difference was found in the reflectance and sum of light FDPs.2-4 absorption-scattering, but no differences were found among the translucency parameter, opacity, To solve the problem of or flexural strength. (J Prosthet Dent 2019;-:---) chipping, more translucent zirconias were developed for can be lost through refraction and scattering (diffuse use in anatomic contour prostheses. The opacity and reflection), through birefringence, and through pores and translucence of zirconia are influenced by size, crystal impurities in the material.7,8 Y-TZP is partially stabilized isotropy,5 and thickness.6 When light reaches a solid such in the tetragonal phase at an ambient temperature by as zirconia, after initial reflection and some absorption, it a
Doctoral student, Department of Dental Materials and Prosthodontics, Araraquara School of Dentistry, São Paulo State University (UNESP), Araraquara, Brazil. Professor of Dental Materials, Department of Dental Materials and Prosthodontics, Araraquara School of Dentistry, São Paulo State University (UNESP), Araraquara, Brazil. Associate Professor, Department of Dental Clinics, Centro Universitario de Ciencias de la Salud, Universidad de Guadalajara, Guadalajara, Mexico. d Postdoctoral research associate, Department of Physical Chemistry, Araraquara Institute of Chemistry, São Paulo State University (UNESP), Araraquara, Brazil. e Adjunct Professor of Complete Prosthodontics, Department of Dental Materials and Prosthodontics, Araraquara School of Dentistry, São Paulo State University (UNESP), Araraquara, Brazil. b c
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Clinical Implications Processing fully stabilized monolithic zirconia with an increased final sintering that does not reduce its mechanical properties may improve the esthetics of the material.
Optical and mechanical properties Translucency
Average reflectance
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Table 1. Mean ±standard deviation and coefficient of variation for optical and mechanical properties of ceramic sintered at 1450 C and 1600 C
Opacity
metal oxides (especially yttria).9 Tetragonal crystals improve the mechanical resistance of Y-TZP because of phase transformation toughening9,10 when a tetragonal to monoclinic transformation is induced mechanically around a crack. However, tetragonal crystals are optically anisotropic and reduce its translucency because of differences in the refractive index (birefringence).11,12 According to the Rayleigh scattering model described by Apetz and van Bruggen,6 when light rays are incident on a birefringent material, the smaller the particle dispersed for a given wavelength, the larger the light transmission at the grain boundaries. Tetragonal grain sizes range from 0.2 mm to 0.8 mm,13 which is greater than the wavelength range of visible light (400 nm to 700 nm). Therefore, the use of nanometric tetragonal crystals should minimize the birefringence effect and improve light transmission.14 In contrast, FSZ exhibits higher translucence with increasing yttria concentration, which stabilizes higher cubic phase content.8 Cubic grains have an isotropic orientation, which means that there is less interference with light transmission among the grains.6 Another advantage is that cubic grains are larger than tetragonal grains, and this reduces the number of grain boundaries, which are sources of light scattering.6,15 However, increased cubic phase may reduce mechanical properties as the transformation-toughening effect is diminished because cubic grains do not undergo the tetragonal to monoclinic phase transformation.13 The sintering temperature of zirconia affects the ceramic’s mean grain size,16-18 which results in a change in its optical properties. Furthermore, pore diameter, microstructure, mechanical properties, and lowtemperature degradation behavior could be affected by the sintering conditions.8,15,16,19 When it was first introduced, the zirconia used in this study had a final manufacturer-recommended sintering temperature of 1600 C. However, the sintering protocol has since changed to a final sintering temperature of 1450 C. The purpose of this study was to determine the effect of different sintering temperatures on the microstructure and optical and mechanical properties of a fully stabilized zirconia. The null hypothesis was that the microstructure, flexural strength, and optical properties would not be affected by changes in sintering temperature.
-
S/A Flexural strength
Temperature ( C)
Mean ±SD (MPa)
CV (%)
1450
15.45a ±0.18
1.21
1600
15.58a ±0.37
2.42
1450
64.68a ±0.44
0.68
1600
64.20a ±0.81
1.28
1450
70.96a ±0.86
1.22
1600
68.25b ±0.80
1.18
1450
98.14a ±0.13
0.13
1600
97.68b ±0.13
1450
542.9a ±112.2
20.7
1600
577.5a ±99.3
17.2
0.13
CV, coefficient of variation; SD, standard deviation; S/A, absorption-scattering sum of light; MPa, mega pascals. Different superscript letters indicate statistically significant difference (P<.05).
MATERIAL AND METHODS Bars (25×5×2.1 mm) were cut from presintered fully stabilized zirconia blocks (Prettau Anterior; Zirkonzahn) with a diamond wheel (Diamond Cup Grinding Wheel; Buehler) and ground by using abrasive papers (P1200, P1500, and P2500; Imperial Wetordry; 3M) to a thickness of 1.5 mm. According to the manufacturer information, the composition of the zirconia was 8% to 12% Y2O3, 0% to 1% Al2O3, 0.02% SiO2, 0.01% Fe2O3, and 0.04% Na2O. The bars were divided into 2 groups (n=15) by simple randomization and sintered at 2 different temperatures: 1450 C and 1600 C. The sintering protocol was started at room temperature, with an 8 C per minute heating rate up to the maximum temperature (1450 C or 1600 C). After a 2-hour step time, the temperature was decreased to room temperature at an 8 C per minute cooling rate. After sintering, the bars had dimensions of approximately 20×4×1.2 mm ±0.01 mm, according to the ISO6872: 2015 standard.20 To assess the optical properties, spectral reflectance was measured at wavelengths of 400 nm to 740 nm at 10nm intervals by using a computerized spectrophotometer (CM 2600d; Konica Minolta Sensing Inc). The conditions included ambient room temperature (24 ±3 C), standard primary illuminant D65 (daylight, 6504 K), UV secondary illuminant, observer at 2 degrees, and the Commission Internationale de l’Eclairage L*a*b* (CIELab) system. Measurements were made at the center of the specimens, limiting the surface area to a diameter of 3 mm by means of a black mask (CM-A147; Konica Minolta Sensing Inc) coupled to the spectrophotometer optical port. The instrument was calibrated with a white plate (CM-A145; Konica Minolta Sensing Inc). The reflectance data acquired were processed by means of a computer program (Spectra-Magic version 3.61; Konica Minolta Sensing Inc) that provided lightness (L*) and chromaticity (a* and b*) values. The optical properties of average Cardoso et al
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O = RRwhbl × 100, where Rbl is the average reflectance at each wavelength over blackdCIE L*=25.6976, a*=-0.09, and b*=-0.81dand Rwh is the average reflectance at each wavelength over white backgroundsdCIE L*=99.45, a*=-0.11, and b*=-0.13. The translucency parameter was obtained by using the following equation: TP = ð½Lwh −Lbl 2 +½awh −abl 2 +½bwh −bbl 2 Þ1=2 , where bl is measurements against a black backgrounddCIE L*=25.6976, a*=-0.09, and b*=-0.81dand wh is measurements against a white backgrounddCIE L*=99.45, a*=-0.11, and b*=-0.13. The percentage of absorption-scattering sum of light (S/A) was obtained by subtracting the sum of scattered/ absorbed light in each wavelength divided by 100. This parameter was automatically calculated by the spectrophotometer program with the data collection. Additionally, the color difference calculated according to CIEDE2000 (DE00)22,23 is based on the equation 0 " 2 2 B 0 0 D D L C 24 B described by Ghinea et al : DE = @ KL SL + KC SC +
0
DH K H SH
2
+RT
0
DC K C SC
0
DH K H SH
#12
1 C C, where DL, DC, and DH A
represent the differences in lightness, chroma, and hue in CIEDE2000; RT is a function that accounts for the interaction between chroma and hue differences in the blue region. SL, SC, and SH are weighting functions that adjust the total color difference for variation in the location of color difference in L0 , a0 , b0 coordinates. KL, KC, and KH are parametric factors to correct terms for experimental conditions. In the present study, the parametric factors of the CIEDE2000 color difference formula were set to 1. X-ray diffraction analysis was performed (DRX-Rint 2000; Rigaku) (n=1) to identify the crystalline phases present after each sintering protocol. Scanning conditions were as follows: Cu-Ka radiation (40 kV, 70 mA) and scan range from 20 degrees to 90 degrees (2q) with a step size of 0.02 degrees for 3 seconds. In addition, the percentage crystalline content was analyzed by the Rietveld refinements.25 Scanning electron microscopy was conducted (MIRA3; TESCAN) to observe microstructural changes Cardoso et al
1450 ˚C 1600 ˚C
Intensity (a.u.)
reflectance, opacity, translucency parameter, and absorption-scattering sum of light were calculated according to the equations of Shiraishi et al21: Average reflectance: average of reflectance percentage in each wavelength. Opacity percentage was calculated by using the following equation:
20
30
40
50
60
70
2θ Figure 1. Representative X-ray diffraction patterns of fully stabilized monolithic zirconia sintered at different temperatures.
and characterize the surface. The average grain size was calculated directly from the SEM images according to the linear intercept method26 based on at least 90 grains by using a computer software program (Motic Images Plus 2.0 ML; Motic). The specimens were given a gold coating (40 seconds, 18.7 mA) to improve electron scattering. The 3-point flexural strength test was conducted according to the ISO 6872: 2015 standard.20 Fifteen specimens were tested. The test was conducted under dry conditions and performed in a universal testing machine (MTS 810; Materials Testing System) at a crosshead speed of 1 mm/min until failure. Flexural strength was calculated from the following equation: s=3Nl/(2bd2), where s is the flexural strength, N is fracture load, l is distance between supports (mm), b is width of the specimen (mm), and d is thickness of the specimen (mm). Optical properties and the 3-point flexural strength data were tested for distribution (D’Agostino & Pearson Omnibus, a=.05) and showed a normal distribution. For comparison between groups, an unpaired Student t test was used (a=.05). RESULTS The mean, standard deviation, and coefficient of variation values of the optical properties for the 1450 C and 1600 C groups are shown in Table 1. A statistically significant difference was found between the groups for average reflectance (P<.001) and absorption-scattering sum of light (P<.001). Translucency parameters (P=.67) and opacity (P=.07) showed no statistical difference. The color difference between the 2 groups was DE00=0.98. Rietveld analyses for crystalline phase quantification indicated a cubic phase of 47.41% for the 1450 C and 46.04% for the 1600 C group and a tetragonal phase of 52.59% for the 1450 C group and 53.96% for the 1600 C group. Figure 1 illustrates the diffraction patterns for
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Figure 2. Scanning electron microscopy images after different sintering conditions. A, Final sintering temperature of 1450 C. B, Final sintering temperature of 1600 C. Original magnification ×5000.
the different sintering temperatures. No monoclinic content was found for either temperature group. SEM micrographs were obtained at ×5000 magnification; Figure 2 shows microstructural differences between specimens based on variation of sintering temperature. Grain size analysis by the linear intercept method27 showed significant differences in the mean grain size of the ceramic after sintering at 1450 C (0.49 mm) and 1600 C (1.99 mm). In addition, a more definite and sharper limit at grain boundaries was observed in the ceramic sintered at 1600 C. Descriptive statistics (mean, standard deviation, and coefficient of variation) of the flexural strength testing are summarized in Table 1. No statistically significant difference (Student t test, P=.378) was found for the FSZ bars sintered at 1600 C. DISCUSSION This study was motivated by a revision of the manufacturer’s instructions for Prettau Anterior that altered the sintering cycle, by reducing the top temperature from 1600 C to 1450 C. The reason for this alteration was not specified, but presumably it was to optimize the mechanical properties without compromising the optical characteristics. The null hypothesis was accepted because no difference was found between the ceramic sintered at the different temperatures in terms of 3-point flexural strength, translucency, or opacity. However, a statistically significant difference in reflectance and absorption scattering was observed. Analysis of microstructure revealed different grain sizes, and crystallographic analysis showed that the concentration of tetragonal and cubic phases was maintained. Sintering temperature and holding time can affect the microstructure of monolithic zirconia because of the partial tetragonal phase transformation into cubic phase and a grain size increase.27 In this study, temperature
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variations led to different grain sizes. SEM images showed that specimens sintered at 1450 C had 4 times smaller grain sizes than those sintered at 1600 C (Fig. 2). Grain size may play a crucial role in mechanical properties.6,8,17,19 The FSZ Prettau Anterior has up to 12% yttria content to stabilize the cubic phase, with a reduction in the mechanical properties. However, in the present study, the flexural strength was not significantly affected. Sen et al,27 who studied Prettau Anterior and other ceramics at the same sintering temperature, reported similar results. However, the Rietveld analysis revealed a negligible difference in cubic phase concentration in the specimens heated at 1450 C (47.41%) and those heated at 1600 C (46.04%). Thus, considering that no significant change was observed in cubic phase concentration, it may be assumed that the similar percentage cubic phase led to the similar 3-point flexural strengths, while the grain size had a little effect. Microstructure and composition can also influence optical properties. In a material with smaller grain sizes, the increased intercept areas at grain boundaries significantly affect light scattering.28 Despite the enlarged grains obtained in the 1600 C group (Fig. 2), no significant difference was found in translucency or opacity, although the absorption-scattering sum of light and reflectance was significantly different for the average reflectance (Table 1). Sen et al27 also reported similar translucency at the same sintering temperatures (1450 C and 1600 C), but grain size was not measured. Apparently, the crystal phase predominates in the optical properties. The isotropic cubic phase reduces light scattering at the grain boundaries,28 but there was similar cubic phase concentration between the ceramic sintered at different temperatures. The change in sintering temperature led to a DE00=0.98, which indicates that color difference is perceptible but acceptable, considering the 50:50% values
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of the perceptibility threshold (DE00=0.81) and acceptability threshold (DE00=1.77) based on Paravina et al.29 The present study has limitations. Only a single ceramic (Prettau Anterior) was tested, which limits the results to this specific material. Future studies should include a more complete comparison of different monolithic zirconias and include other sintering cycles with different temperatures and holding times. Moreover, fracture toughness and fatigue testing could also add important information with regard to clinical performance. CONCLUSIONS Within the limitation of this in vitro study, the following conclusions were drawn: 1. Higher sintering temperatures led to changes in the microstructure with larger grain size but did not change the composition and concentration of crystal phases. 2. Temperature sintering did not affect the flexural strength, translucency parameter, or opacity but did affect other optical properties. 3. The color change was acceptable, and the perceptibility was close to thresholds described in the literature. REFERENCES 1. Piconi C, Maccauro G. Zirconia as a ceramic biomaterial. Biomaterials 1999;20:1-25. 2. Conrad HJ, Seong WJ, Pesun IJ. Current ceramic materials and systems with clinical recommendations: a systematic review. J Prosthet Dent 2007;98: 389-404. 3. Zarone F, Russo S, Sorrentino R. From porcelain-fused-to-metal to zirconia: clinical and experimental considerations. Dent Mater 2011;27:83-96. 4. Sailer I, Makarov NA, Thoma DS, Zwahlen M, Pjetursson BE. All-ceramic or metal-ceramic tooth-supported fixed dental prostheses (FDPs)? A systematic review of the survival and complication rates. Part I: Single crowns (SCs). Dent Mater 2015;31:603-23. 5. Pecho OE, Ghinea R, Ionescu AM, Cardona Jde L, Paravina RD, Perez Mdel M. Color and translucency of zirconia ceramics, human dentine and bovine dentine. J Dent 2012;40 Suppl 2:e34-40. 6. Apetz R, van Bruggen MPB. Transparent alumina: a light-scattering model. J Am Ceram Soc 2003;86:480-6. 7. Chen YM, Smales RJ, Yip KH, Sung WJ. Translucency and biaxial flexural strength of four ceramic core materials. Dent Mater 2008;24:1506-11. 8. Shahmiri R, Standard OC, Hart JN, Sorrell CC. Optical properties of zirconia ceramics for esthetic dental restorations: a systematic review. J Prosthet Dent 2018;119:36-46. 9. 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. 10. Studart AR, Filser F, Kocher P, Gauckler LJ. Fatigue of zirconia under cycling loading in water and its implications for the design of dental bridges. Dent Mater 2007;23:106-14.
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11. Jiang L, Liao Y, Wan Q, Li W. Effects of sintering temperature and particle size on the translucency of zirconium dioxide dental ceramic. J Mater Sci Mater Med 2011;22:2429-35. 12. Krell A, Klimke J, Hutzler T. Transparent compact ceramics: inherent physical issues. Opt Mater 2009;31:1144-50. 13. Muñoz EM, Longhini D, Antonio SG, Adabo G. The effects of mechanical and hydrothermal aging on microstructure and biaxial flexural strength of an anterior and a posterior monolithic zirconia. J Dent 2017;63:94-102. 14. Klimke J, Trunecy M, Krell A. Transparent tetragonal yttria-stabilized zirconia ceramics: influence of scattering caused by birefringence. J Am Ceram Soc 2011;94:1850-8. 15. Boulesteix R, Maître A, Baumard JF, Rabinovitch Y, Reynaud F. Light scattering by pores in transparent Nd:YAG ceramics for lasers: correlations between microstructure and optical properties. Opt Express 2010;18: 14992-5002. 16. Sulaiman TA, Abdulmajeed AA, Shahramian K, Lassila L. Effect of different treatments on the flexural strength of fully versus partially stabilized monolithic zirconia. J Prosthet Dent 2017;118:216-20. 17. Stawarczyk B, Ozcan M, Hallmann L, Ender A, Mehl A, Hämmerlet CH. The effect of zirconia sintering temperature on flexural strength, grain size, and contrast ratio. Clin Oral Investig 2013;17:269-74. 18. Chevalier J, Deville S, Munch E, Jullian R, Lair F. Critical effect of cubic phase on aging in 3mol% yttria-stabilized zirconia ceramics for hip replacement prosthesis. Biomaterials 2004;25:5539-45. 19. Inokoshi M, Zhang F, De Munck J, Minakuchi S, Naert I, Vleugels J, et al. Influence of sintering conditions on low-temperature degradation of dental zirconia. Dent Mater 2014;30:669-78. 20. International Organization for Standardization. ISO 6872, dentistry ceramic materials. 4th ed. Geneva: International Organization for Standardization; 2015. Available at: https://www.iso.org/standard/59936.html. 21. Shiraishi T, Wood DJ, Shinozaki N, van Noort R. Optical properties of base dentin ceramics for all-ceramic restorations. Dent Mater 2011;27: 165-72. 22. Sharma G, Wu W, Dalal EN. The CIEDE2000 color-difference formula: implementation notes, supplementary test data, and mathematical observations. Col Res Appl 2005;30:21-30. 23. International Commission on Illumination. CIE: Technical report. Colorimetry. 3rd ed. Washington: CIE; 2004. Available at: https://law.resource.org/ pub/us/cfr/ibr/003/cie.15.2004.pdf. 24. Ghinea R, Pérez MM, Herrera LJ, Rivas MJ, Yebra A, Paravina RD. Color difference thresholds in dental ceramics. J Dent 2010;38 Suppl 2: e57-64. 25. Rietveld HM. A profile refinement method for nuclear and magnetic structures. J Appl Crystallogr 1969;2:65-71. 26. Wurst JC, Nelson JA. Linear intercept technique for measuring grain size in two-phase polycrystalline ceramics. J Am Ceram Soc 1972;55:109. 27. Sen N, Sermet IB, Cinar S. Effect of coloring and sintering on the translucency and biaxial strength of monolithic zirconia. J Prosthet Dent 2018;119: 308.e1-7. 28. Zhang Y. Making yttria-stabilized tetragonal zirconia translucent. Dent Mater 2014;30:1195-203. 29. Paravina RD, Ghinea R, Herrera LJ, Bona AD, Igiel C, Linninger M, et al. Color difference thresholds in dentistry. J Esthet Restor Dent 2015;27 Suppl 1: S1-9. Corresponding author: Dr Kátia Vieira Cardoso Department of Dental Materials and Prosthodontics Araraquara Dental School Univ Estadual Paulista UNESP 1680, Humaitá Street, Araraquara, SP 14801-903 BRAZIL Email: [email protected] Copyright © 2019 by the Editorial Council for The Journal of Prosthetic Dentistry. https://doi.org/10.1016/j.prosdent.2019.08.007
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