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Effect of sintering and aging processes on the mechanical and optical properties of translucent zirconia

Effect of sintering and aging processes on the mechanical and optical properties of translucent zirconia



Effect of sintering and aging processes on the mechanical and optical properties of translucent zirconia




Journal of Prosthetic Dentistry, 2021-07-01, Volume 126, Issue 1, Pages 129.e1-129.e7, Copyright © 2021 Editorial Council for the Journal of Prosthetic Dentistry


Abstract

Statement of problem

Sintering holding times and aging conditions may affect the optical, mechanical, and structural properties of polychromatic multitranslucent zirconia. However, a consensus on the ideal sintering condition for this material is lacking.

Purpose

The purpose of this in vitro study was to investigate the effect of different sintering holding and autoclave aging times on the flexural strength (FS), grain size, and translucency parameter (TP 00 ) of a translucent zirconia.

Material and methods

Sixty bar-shaped and 60 rectangular-shaped specimens were prepared from computer-aided design and computer-aided manufacturing (CAD-CAM) zirconia blocks and split into 2 groups to be sintered at 1550 °C for different holding times (regular: 2 hours, prolonged: 5 hours). The specimens were then divided into 3 subgroups and subjected to aging processes (control, aging for 60 minutes, aging for 120 minutes). FS values were obtained by using a 3-point bend test and Weibull analysis was conducted. Grain size evaluations were performed by using scanning electron microscope (SEM) observations. TP 00 was measured with a spectrophotometer. Statistical evaluation included the Kolmogorov-Smirnov, 2-way ANOVA, and Tukey HSD tests (α=.05).

Results

The sintering procedure, aging, and their interaction significantly influenced the TP 00 ( P <.05). FS values and grain sizes were affected by aging only ( P <.001). Both aging times resulted in reduced FS and increased grain sizes. Prolonged sintering in combination with 120 minutes of aging negatively affect the TP 00 scores ( P <.05).

Conclusions

FS values were similar under regular and prolonged sintering. Prolonged sintering led to a decrease in translucency after 120 minutes of aging. Steam autoclave aging can affect the optical and mechanical properties of translucent zirconia.

Clinical Implications

Regular sintering may be a better choice than prolonged sintering for translucent zirconia in terms of optical properties. The aging process can affect the optical, mechanical, and structural properties and thus the clinical outcome of translucent zirconia.

Yttria-stabilized tetragonal zirconia polycrystal (Y-TZP) is among the most versatile metal-free materials available for digital dentistry, , with favorable biological and mechanical properties. The traditional approach to the production of zirconia restorations has been to use a highly esthetic veneering ceramic on a substructure with superior mechanical properties. Although zirconia as a core material has been successful, , its opacity may lead to poor esthetics. , Moreover, chipping of the veneering porcelain is a common complication of 2-layered zirconia ceramics; thus, monolithic zirconia materials with an improved light transmission property have been developed. , In addition, polychromatic (multilayered) zirconia materials have been introduced. These can achieve favorable esthetics to match the optical characteristics of natural teeth, including shade, surface quality, and translucency with a multilayered structure. ,

Multilayered materials enable the reproduction of lifelike esthetics without requiring further characterization or staining. Unlike other polychromatic zirconia ceramics in which shading could be changed only through the material with pigments, polychromatic multitranslucent blocks and disks offer a new shading technology. They provide an evolution of material composition and demonstrate a progression of chroma and translucency from dentin to enamel that is formulated to match natural tooth structure. , This type of material includes 5 mol% Y-TZP, a translucent, cubic zirconia that is placed in the incisal area to ensure a high level of translucency, and the more opaque 4 mol% Y-TZP, which is placed in the dentinal region for higher strength. The progression of shade and translucency within the material consists of a dentinal zone, transition zones (transition zones 1 and 2), and an incisal zone.

Zirconia restorations can be obtained by milling a fully sintered blank that requires no further sintering but results in excessive wear of the milling bur or by milling a partially sintered blank, which is easily machinable but needs additional sintering after production. The sintering procedure of monolithic zirconia includes heating, sintering, and cooling phases and is generally performed at 1350 °C to 1600 °C. , Multiple factors may influence the mechanical and optical properties of monolithic zirconia, including the sintering technique, , , , heating and cooling rates, holding time, , and aging procedure. Furthermore, the microstructure and the low-temperature degradation properties of the zirconia could be affected by the sintering and aging conditions. ,

Recent studies have reported on the strength, , , , , grain sizes, , , , , and translucency , , , , , of translucent zirconia. However, most of these studies focused on shortening the sintering time of zirconia, , , , and studies that compared prolonged sintering with conventional sintering are sparse, as are data about polychromatic multitranslucent monolithic zirconia. Since these types of materials are relatively new, further information is required. Therefore, the objective of the present in vitro study was to investigate the effect of different sintering holding and autoclave aging times on the flexural strength (FS), grain size, and translucency parameter (TP 00 ) of a polychromatic multitranslucent zirconia. The null hypotheses were that no differences would be found between the FS values and average grain sizes of tested zirconia ceramic after different sintering holding and aging times and that no differences would be found between the TP 00 scores of tested zirconia ceramic after different sintering holding and aging times.


Material and methods

The effect of different sintering holding and steam autoclave aging times on the FS values, grain sizes, and TP 00 scores of a monolithic zirconia was investigated. Sixty bar-shaped (5×19×1.5 ±0.02 mm) and 60 rectangular-shaped (10×15×1.25 ±0.02 mm) specimens (N=120) were prepared from shade A3 zirconia block (ZC, IPS e.max Zir CAD MT Multi; Ivoclar Vivadent AG) with a precision sectioning saw (IsoMet 1000 Precision Cutter; Buehler) and a disk-shaped blade. , The sample size was calculated according to previous studies. , , It was determined that 10 specimens per group provided a power of .95 at a significance level of .05. The material and groups used in this study are presented in Table 1 .

Table 1
Material and groups (n=10) used
Material Lot Sintering Procedure Autoclave Aging Group Name
IPS e.max ZirCAD MT Multi (ZC, Ivoclar Vivadent AG) Y27844 2 h of holding time=regular sintering (RS) Control=no aging (C) RS-C
Aging for 60 min (SA) RS-SA
Aging for 120 min (LA) RS-LA
IPS e.max ZirCAD MT Multi (ZC, Ivoclar Vivadent AG) Y27844 5 h of holding time=prolonged sintering (PS) Control=no aging (C) PS-C
Aging for 60 min (SA) PS-SA
Aging for 120 min (LA) PS-LA

The polychromatic specimens were obtained by slicing the blocks along their thickness direction, where layers with material in different shades were stacked. , Thus, all slices contained each of the layers with graded translucency along their length. The specimens were polished for 60 seconds with 600-, 800-, and 1200-grit silicon carbide papers by using a polishing device at 300 rpm to minimize surface irregularities and create a baseline roughness. The bar-shaped specimens were used to evaluate the FS, and the rectangular-shaped specimens were used to calculate the TP 00 scores and grain sizes.

The bar-shaped and rectangular-shaped specimens were divided into 2 groups by simple randomization according to the 2 different sintering holding times. Group RS received regular sintering for 2 hours of holding time, , and group PS received prolonged sintering for 5 hours of holding time. The sintering oven was heated to a final temperature of 1550 °C with a heating rate of 10 °C/min. After the required holding times, the oven was cooled to room temperature at a rate of 10 °C/min. The final dimensions of differently sintered specimens (4×15×1.2 ±0.01 mm for bar-shaped and 8×12×1 ±0.01 mm for rectangular-shaped specimens) were measured with a digital micrometer (Digimatic Indicator 0001-2°; Mitutoyo). , Any specimens with unsuitable sizes were excluded from the study, and new specimens were produced. The sintered specimens were then divided into 3 subgroups according to the aging times. Group C (control) was not subjected to aging, group SA aged for 60 minutes, and group LA aged for 120 minutes in a steam autoclave (CH-8583; Belimed Sauter). The aging procedures were performed at 134 °C and at 0.2 MPa. , The specimens were then cleaned ultrasonically with distilled water for 10 minutes and air dried.

FS values were determined by using a 3-point bend test with a universal testing machine (Autograph AGS-X; Shimadzu). Each bar-shaped specimen was placed on a metal fixture with a 12-mm support distance and a centrally located loading rod. , The load was applied vertically to the long axis with a 1 mm/min crosshead speed until failure, and maximum load was recorded. The FS ( σ ), failure probability (P f ), Weibull modulus (m), and the characteristic strength (σ 0 ) were calculated based on the following formula:





σ


=




3



F


f



L




2


b



h


2





;



P


f



=




i





0.5



N



;



P


f



=


1






e


x


p



[








(



σ



σ


0




)



m




]



,


where
F
f is the fracture load,
L is the distance between the center of the supports (mm),
b is the width of the specimen (mm), and
h is the thickness of the specimen (mm). The
i denotes the rank in strength from least to greatest.
N is the total number of specimens in each group.

A rectangular-shaped specimen of each group was processed for scanning electron microscope (SEM) observations (JSM-7900F; Jeol) to determine grain sizes. These were performed at 3 different areas in the dentinal zone of each specimen under ×5000 magnification. Representative SEM images are shown in Figure 1 . Twenty measurements were carried out for each group with a dedicated software program (ImageJ; National Institutes of Health). The average grain sizes were calculated in accordance with the linear intercept method.

Representative scanning electron micrographs (original magnification ×5000). A, Group RS-C. B, Group RS-SA. C, Group RS-LA. D, Group PS-C. E, Group PS-SA. F, Group PS-LA. C, control; LA, 120 minutes of aging; PS, 5 hours of sintering holding time; RS, 2 hours of sintering holding time; SA, 60 minutes of aging.
Figure 1
Representative scanning electron micrographs (original magnification ×5000). A, Group RS-C. B, Group RS-SA. C, Group RS-LA. D, Group PS-C. E, Group PS-SA. F, Group PS-LA. C, control; LA, 120 minutes of aging; PS, 5 hours of sintering holding time; RS, 2 hours of sintering holding time; SA, 60 minutes of aging.

TP 00 evaluations were carried out on black and white backgrounds by using a custom-made mold ( Fig. 2 ) and a spectrophotometer (Vita Easyshade Advance 4.0; Vita Zahnfabrik). The mold featured a moving platform with a hole (Ø6 mm) matched to the probe tip. Since the tested material consisted of 4 distinctive layers with progressively darker shades, 4 measurements were performed in 4 relevant areas of each specimen. The platform of the mold was displaced for 1.65 mm after each measurement; thus, the color readings were conducted at the equal intervals. All these measuring areas covered the whole surface of each specimen. The probe tip was placed vertically to the center of the measuring area to provide full contact between the tip and the analyzed surface. The device was calibrated according to the manufacturer’s instructions before the measurements. For each measuring area, 3 readings were made and the means calculated. The TP 00 values were determined using the CIEDE2000 formula. , The parametric factors of the formula were set to 1.





T



P


00



=






(




L



'


b






L



'


w






k


L




S


L





)



2



+




(




C



'


b






C



'


w






k


c




S


c





)



2



+




(




H



'


b






H



'


w






k


H




S


H





)



2



+


R


T



(




C



'


b






C



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w






k


c




S


c





)




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H



'


b






H



'


w






k


H




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H





)





,


where subscript
b refers to the color values over the black background and
w refers to values over the white background.
RT describes the interaction between hue and chroma in the blue region ;
S
L ,
S
C , and
S
H are the weighting factors for lightness, chroma, and hue;
k
L ,
k
C , and
k
H are the parametric weighting factors for variations in experimental conditions.
,

Representative images. A, Rectangular-shaped specimen. B, White background. C, Black background inside custom-made mold.
Figure 2
Representative images. A, Rectangular-shaped specimen. B, White background. C, Black background inside custom-made mold.

Statistical analyses were conducted with a statistical software program (SPSS Statistics for Windows v17.0; SPSS Inc). The normality of the data was analyzed by using the Kolmogorov-Smirnov test. Since the variables were normally distributed, 2-way ANOVA and the Tukey HSD test were used for data assessment (α=.05). Weibull distribution values, including the characteristic strength, Weibull modulus, and upper-lower bounds for the 95% confidence interval (CI) were calculated by using a relevant program (Minitab Version 14; Minitab Inc).


Results

Table 2 presents the means, standard deviations (SDs), and statistical significance of the FS. FS was only affected by aging ( P ˂.001). Group PS-C showed the highest FS value (506.92 ±98.31 MPa), while group RS-SA showed the lowest (354.15 ±83.26 MPa). Both aging times negatively affected the FS values ( P =.001 for Groups C and SA; P =.003 for Groups C and LA). Mean σ 0 and m values ( Table 3 ) of group PS were higher than those of group RS for all aging times.

Table 2
Means ±standard deviations and statistical significance of FS (MPa) for each experimental group
Experimental Groups RS PS P
C 480.76 ±111.01 aA 506.92 ±98.31 aA .584
SA 354.15 ±83.26 bB 391.86 ±42.33 bB .218
LA 392.91 ±111.51 bB 393.06 ±79.55 bB .997
P .030 .003
C, control; FS, flexural strength; LA, 120 minutes of aging; PS, 5 hours of sintering holding time; RS, 2 hours of sintering holding time; SA, 60 minutes of aging.
Values in each column with different lowercase superscript letters significantly different (
P ˂.05). Values in each row with different uppercase superscript letters significantly different (
P ˂.05).

Table 3
Means (95% CI) of characteristic strength (MPa) and Weibull modulus (CI 95%) for each experimental group
Experimental Groups Characteristic Strength Weibull Modulus
RS PS RS PS
C 523.98 (451.27-608.41) 544.04 (496.86-595.69) 4.41 (2.86-6.80) 7.14 (4.21-12.11)
SA 386.30 (336.08-444.02) 408.64 (389.04-429.22) 4.72 (2.97-7.50) 13.19 (7.90-22.00)
LA 433.85 (369.27-509.73) 424.68 (380.22-474.34) 4.07 (2.51-6.61) 5.92 (3.61-9.73)
C, control; CI, confidence interval; LA, 120 minutes of aging; PS, 5 hours of sintering holding time; RS, 2 hours of sintering holding time; SA, 60 minutes of aging.

The means, SDs, and statistical significance of the grain sizes and TP 00 scores are given in Tables 4 and 5 . The results of ANOVA revealed that aging influenced the grain sizes ( P ˂.001) but not the sintering holding times ( P =.072). Significant differences were found among the average grain sizes of the control groups and aged groups for both prolonged ( P =.005 for groups C and SA; P =.007 for groups C and LA) and regularly ( P ˂.001 for groups C and SA; P =.001 for groups C and LA) sintered specimens. For group PS and RS, grain sizes were ordered as group C˂group SA=group LA ( P ˂.05). The aging processes generated moderate irregularities on the surface according to the SEM observations.

Table 4
The means ±standard deviations and statistically significance of grain sizes (μm)
Experimental Groups RS PS P
C 0.60 ±0.07 aA 0.55 ±0.08 aA .051
SA 0.71 ±0.09 bB 0.68 ±0.07 bB .169
LA 0.71 ±0.15 bB 0.70 ±0.08 bB .826
P .002 <.001
C, control; FS, flexural strength; LA, 120 minutes of aging; PS, 5 hours of sintering holding time; RS, 2 hours of sintering holding time; SA, 60 minutes of aging.
Values in each column with different lowercase superscript letters significantly different (
P ˂.05). Values in each row with different uppercase superscript letters significantly different (
P ˂.05).

Table 5
Means ±standard deviations and statistically significance of TP
00 scores
Experimental Groups RS PS P
C 8.08 ±0.26 aA 7.97 ±0.87 aA .709
SA 7.95 ±0.77 aA 7.93 ±0.26 aA .916
LA 7.83 ±0.39 aA 6.64 ±0.99 bB .002
P .577 .001
C, control; LA, 120 minutes of aging; PS, 5 hours of sintering holding time; RS, 2 hours of sintering holding time; SA, 60 minutes of aging; TP
00 , translucency parameter.
Values in each column with different lowercase superscript letters significantly different (
P ˂.05). Values in each row with different uppercase superscript letters significantly different (
P ˂.05).

The TP 00 scores were significantly influenced by the sintering procedure ( P =.016), aging ( P =.001), and the interaction between the 2 ( P =.009). The lowest mean TP 00 score was observed in group LA ( P =.001for groups C and LA; P =.004 for groups SA and LA). The group of prolonged sintering and 120 minutes of aging resulted in a significantly lower TP 00 score compared with the other subgroups. For the 120 minutes of aging group, a significant difference was found between the TP 00 scores of regularly sintered and prolonged sintered specimens ( P =.002).


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