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Flexural strength and Weibull analysis of Y-TZP fabricated by stereolithographic additive manufacturing and subtractive manufacturing
Journal of the European Ceramic Society xxx (xxxx) xxx–xxx
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Original Article
Flexural strength and Weibull analysis of Y-TZP fabricated by stereolithographic additive manufacturing and subtractive manufacturing Yuqing Lu1, Ziyu Mei1, Junjing Zhang, Shanshan Gao, Xingqiang Yang, Bo Dong, Li Yue, Haiyang Yu* State Key Laboratory of Oral Diseases, West China Hospital of Stomatology, Sichuan University, Chengdu 610041, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Digital light processing Flexural strength Weibull modulus Zirconia
Digital light processing (DLP) is a relatively mature technology of ceramic additive manufacturing and is promising for fabricating zirconia-based dental restorations. It allows for manufacturing ceramic components with nearly unlimited geometries compared to traditional subtractive manufacturing technology. In order to explore its potential for fabricating dental prosthesis and determine its clinical indications, it is essential to investigate its microstructural characteristics and mechanical behavior. In this study, yttria-stabilized tetragonal zirconia polycrystal (Y-TZP) fabricated by stereolithographic additive manufacturing, namely DLP acquired favorable flexural strength close to that of conventional subtractive-manufactured Y-TZP as indicated by uniaxial (threepoint bending) and biaxial (ring on ring) tests, though the Weibull modulus of DLP-manufactured zirconia was lower than that of subtractive-manufactured zirconia. The strength predicting approach that uses effective area calculations was found to be applicable for both DLP-manufactured zirconia and subtractive-manufactured zirconia. Though both materials showed similar microstructures considering grain size and phase composition, significant differences in critical defects were observed.
1. Introduction Yttria-stabilized tetragonal zirconia polycrystal (Y-TZP) is increasingly applied nowadays in clinical practice of restorative and implant dentistry, as prosthetic material because of its high biocompatibility, excellent mechanical properties, and desirable aesthetics. In general, zirconia restorations are produced by subtractive manufacturing (SM) technology usually based on milling dental work pieces from pre-sintered or fully sintered blocks [1–4]; nonetheless, several problems and challenges are associated with this technique. First, a compensation step of the computer aided design (CAD) software is needed to ensure that the cutter tool with exact diameter can reach the pre-designed surface without sacrificing necessary segment, which makes an impact on precision [5]. Second, subtractive milling progress causes wastage of an amount of material including powdered waste as well as non-machined parts [6]. With the rapid development of additive manufacturing, three-dimensional printing (3DP) methods have been applied for fabricating ceramic parts including zirconia dental restorations [7–9]. These technologies allow for manufacturing ceramic components with nearly
unlimited geometries compared to SM technology and no compensation is required. Another major advantage of this technique is the significant reduction in the wastage of raw material. Among all the 3DP methods, digital light processing (DLP) is regarded as a promising and relatively mature technology for fabrication of ceramic parts that require both high accuracy and surface quality [10,11]. This technique applies a DLP stereolithography printer to achieve layer-by-layer “printing” of 3D objects by the light-curing reaction of resin. Moreover, posttreatments including debinding (binder burnout) and sintering step are needed to achieve a dense part with acceptable mechanical properties after green parts are built [10,12]. However, there are still concerns about the mechanical properties of such additive-manufactured ceramic [7,13,14]. For DLP-fabricated zirconia, studies on the macro-scale fracture strength, reliability, and characterization of defects are still limited. Flexural strength of dental ceramics is one of the essential parameters used to determine clinical indications of dental prosthesis. Few studies have investigated the fracture mechanics difference between YTZPs fabricated by stereolithography-based additive manufacturing and conventional SM, which is regarded as the golden standard for CAD/
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Corresponding author at: 14 S Renmin Rd. 3rd Sec., Chengdu, Sichuan. 610041, PR China. E-mail addresses: [email protected] (L. Yue), [email protected] (H. Yu). 1 These authors have contributed equally to this work. https://doi.org/10.1016/j.jeurceramsoc.2019.10.058 Received 1 August 2019; Received in revised form 9 October 2019; Accepted 29 October 2019 0955-2219/ © 2019 Elsevier Ltd. All rights reserved.
Please cite this article as: Yuqing Lu, et al., Journal of the European Ceramic Society, https://doi.org/10.1016/j.jeurceramsoc.2019.10.058
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9 μm to a final stage of 1 μm using a polishing machine (Struers, Copenhagen, Denmark). The final dimension of parallelepipedic samples used for the 3PB tests was 1.4 mm × 4 mm × 22 mm (0.1 mm chamfer width) according to ISO 6872 [18], while the disc-shaped samples for the ROR bending tests had a diameter of 21.6 mm and thickness of 1.2 mm according to ASTM C1499 [19].
Table 1 Group names, ceramic compositions, and manufacturers. Material
Ceramic component
Manufacturer
Zirconia Slurry
Zirconium oxide (ZrO2+HfO2+Y2O3) 99.72% Yttrium oxide (Y2O3) 5.22% Aluminum oxide (Al2O3) 0.24% Remaining 0.04% Zirconium oxide (ZrO2+HfO2+Y2O3) ≥99.0% Yttrium oxide (Y2O3)＞4.5 ≤ 6.0% Aluminum oxide (Al2O3) ≤1.0% Remaining ≤5.0%
QuickDemos Company (Jiangsu, China)
Zenostar MO
2.2. Microstructural characterization Ivoclar Vivadent AG (Schaan, Liechtenstein)
The average density of as-sintered DLP-fabricated and subtractivefabricated Y-TZP specimens was determined by Archimedes’ method. The relative density of DLP-printed zirconia ceramic was calculated, whereas the zirconia powder density was taken as 6.08 g/cm3[20]. The micrographs of both materials were obtained by scanning electron microscopy (SEM, INSPECT F). The average grain size of the specimens was measured based on the SEM images by using the image analysis software (Image J 1.52a, National Institute of Health, USA) [21]. Crystalline phase structures of both materials were analyzed by X-ray diffraction (XRD, EMPYREAN) with Cu Kα radiation. The 2θ scan range was from 10° to 70° with a step size of 0.02° at a counting time of 1 s.
computer aided manufacturing (CAM) zirconia ceramic. The main objective of this study was to compare the flexural strength of Y-TZP fabricated by DLP technology with that of commercial dental Y-TZP fabricated by SM. In order to evaluate the strength of the material, different measurement approaches such as uniaxial and biaxial bending tests were used, which represent effective volumes/areas as well as different stress states [15–17]. In this study, flexural strength and Weibull statistics of DLP-fabricated Y-TZP were determined by both uniaxial (three-point bending, 3PB) and biaxial (ring on ring, ROR) bending tests, and comprehensively compared with subtractive-manufactured dental zirconia with similar components.
2.3. Mechanical testing procedure Both biaxial and uniaxial bending tests were conducted using an ElectroForce3330 mechanical test instrument. For each method of different groups, 30 specimens were used, respectively. The load was applied using a 3000 N compressive load cell, while the applied force and the displacement of the specimens were recorded using sensors. The maximum load was used to calculate the flexural strength of each specimen. For uniaxial flexural strength tests, the 3PB test was performed to measure the flexural strength of Y-TZP in this study. The specimens were positioned in the sample holder with a span distance of 16 mm and then loaded at a constant crosshead speed of 1 mm min−1 at room temperature. The uniaxial flexural strength values (σu ) were calculated based on ISO 6872 [18] by using the following equation:
2. Experimental procedure 2.1. Specimen preparation Both DLP-manufactured and conventional subtractive-manufactured 3Y-TZPs were used in this study. The group names, ceramic compositions, and manufacturers are listed in Table 1. The 3D objects of rectangular and disc shaped specimens were designed for uniaxial (3PB) and biaxial (ROR) tests respectively, and enlarged based on the shrinkage ratio provided by the manufacturers by using CAD software. All DLP-manufactured specimens were prepared using a DLP stereolithography printing machine from QuickDemos Company (Jiangsu, China). The zirconia slurry bought from QuickDemos Company (Jiangsu, China) in this study comprised a homogeneous mixture of photocurable monomers in which the zirconia powder with a concentration of 58 vol.% was dispersed. The 3D objects of the samples were put into the printer’s CAM software to obtain “sliced” 2D layers. The green bodies were printed with single layer thickness of 25 μm under light intensity of 90 mW/cm2. Both rectangular and disc shaped specimens were fabricated horizontally on the build platform, which is regarded as the most favorable building orientation [13]. After the layer-by-layer fabrication, debinding process was carried out by treating the parts at elevated temperatures to remove the organic resins. Primary layer was removed to avoid the negative influence of delamination on flexural strength. Subtractive-manufactured specimens were produced from commercial CAD/CAM blocks of partially-sintered 3 mol % yttria-stabilized zirconia (Zenostar, Ivoclar Vivadent, Liechtenstein) by soft machining. The specimens were milled using a CAD/CAM machine (Wieland Zenostar mini, Ivoclar Vivadent, Liechtenstein) recommended by the manufacturer. All the support structures were carefully removed before subsequent treatments. The specimens fabricated by DLP and SM were sintered at 1510 °C for 2 h. The heating rate and the cooling rate during the process of sintering were set at 300 °C/ h. After manufacturing and postprocessing, all the specimens were visually checked for any defects. Then all the sintered Y-TZP samples free of visual defects were ground to the final dimension using SiC abrasive papers (Struers, Copenhagen, Denmark) with grit sizes from P400 to P1200, and then polished with diamond paste descending from
σu =
3Pl 2bh2
(1)
where P is the maximum load, l is the distance between the two supports, b represents the specimen width, and h is the specimen thickness. ROR bending test for biaxial flexural strength was conducted in this study. Test fixtures included a loading ring with diameter of 16.8 mm and a supporting ring with diameter of 3.8 mm. The sample was placed on the loading/supporting fixtures and loaded with a crosshead speed of 1 mm min(1. The biaxial flexural strength values (σb ) were calculated from the breaking load at failure according to ASTM C1499[19]:
σb =
3P ⎡ r 1−ν ⎛ rs 2 − rl 2 ⎞ ⎤ (1+ν)ln ⎛ s ⎞ + ⎢ 2πt ⎣ 2 ⎝ rh 2 ⎠ ⎥ ⎝ rl ⎠ ⎦ ⎜
⎟
⎜
⎟
(2)
where P is the breaking load, t is the sample thickness, rh is the radius of specimen, rl is the radius of the loading ring, rs is the radius of the supporting ring, and ν is the Poisson’s ratio. The Poisson’s ratio of 0.3 was selected hereinchosen. 2.4. Weibull statistics Weibull analysis is applied to characterize the reliability of the two material. If surface flaws predominate, the description of the twoparameter Weibull distribution is given by [22]:
F= 1 − exp[−Se (σmax /σ0) m]
(3)
Where where σ0 is characteristic strength; σmax is the reference maximum stress in the body; m denotes the Weibull modulus; F represents the failure probability; Seff is the effective stress surface area given by [23]: 2
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Se =
∫ (σ/σmax)mdS
3.2. Fracture strength and Weibull analysis
(4)
For DLP-manufactured and subtractive-manufactured specimens, both uniaxial and biaxial bending strengths were measured by 3PB and ROR bending tests. The load-displacement curves of DLP and SM specimens by different testing methods were similar, which can be attributed to the same density [29]. The values including the specimen number, the mean flexural strengths, the coefficients of variation, unbiased values and 90% confidence intervals of the Weibull parameters, are listed in Table 2. For subtractive-fabricated specimens, the mean uniaxial flexural strength obtained by 3PB test was 1157.6 MPa (SD = 114.4 MPa) and the mean biaxial flexural strength by ROR bending test was 984.0 MPa (SD = 94.7 MPa). For DLP-fabricated specimens, the average uniaxial flexural strength was 1012.7 MPa (SD = 125.5 MPa), while the average biaxial flexural strength was 737.4 MPa (SD = 99.5 MPa). Student’s t-test was performed to analyze the difference between the two materials, and the results showed that subtractive-fabricated specimens exhibited a statistically significant higher strength than DLP-fabricated specimens for both uniaxial (P < 0.001) and biaxial (P < 0.001) test. Weibull analysis is used to estimate the variability of the materials. The Weibull probability plots of the bending strength for each group are shown in Fig. 2, from which the Weibull modulus and characteristic strength presented in Table 1 were extracted from the straight line fitted by linear regression according to the slope and the intercept. The Weibull modulus value of DLP specimen by both the uniaxial and biaxial tests is ∼9 and that of SM specimen by both tests is about 12. In both uniaxial and biaxial tests, the Weibull modulus of subtractivefabricated specimens is significantly higher than that of DLP-printed specimens. Moreover for DLP and SM specimens respectively, the Weibull modulus obtained from the biaxial bending tests is similar to that obtained from the uniaxial bending tests, which indicates that the flaw population of both materials is uniformly distributed in the different loading configuration. The effective area calculations were applicable for the DLP-manufactured specimen as well as for subtractivemanufactured zirconia ceramics, and the flexural strength measured by one test can be predicted from the values measured by the other based on [Eqs. (6)–(8)]. The predicted fracture strengths from the 3PB tests were plotted in Fig. 3 according to the measured fracture strengths obtained by the ROR bending tests. The ratios of measured flexural strength values and the predicted uniaxial strength values obtained by the other using Eq. (6) are listed in Table 3. The predicted ratios of flexural strengths measured by the two testing methods are close to the ratios obtained from the measured values for both materials. The deviation of predicted biaxial values from measured biaxial values is 5% for SM specimen and 8% for DLP specimen, which is acceptable [17].
where σ represents the stress as a function of position in the stressed volume dv . Thus, the Weibull distribution can be simplified as follows [24]:
1 ⎞ = mlnσmax − mlnσ0 lnln ⎛ ⎝ 1−F ⎠
(5)
where σ0 is taken as σ63.21% (the specific characteristic strength of the sample at F = 0.6321). According to the above mentioned function, a straight line can be drawn and the slope can be calculated as the value of m. The Unbiased values and 90 % confidence intervals of the Weibull parameters were calculated according to [25]. 3PB test and ROR test represent different stress states as well as different effective volumes/areas; therefore, the values of strength obtained from the two methods cannot be compared directly. The Weibull distribution model (Eq. 3) leads to a strength dependency on sample size, it is thus possible to predict the flexural strength of one method from that of the other measurement and determine whether the difference is only an influence of loading geometry. The average strengths in the two bending tests can be related by the effective surface area (S u ) by using the following relation [26]: 1
σu S m = ⎛ b⎞ σb ⎝ Su ⎠ ⎜
⎟
(6)
For the 3PB test, the effective S u can be calculated as follows [26]:
Su =
l[h+b(m+1)] ( m+ 1)2
(7)
Moreover, for the ROR bending test, the effective surface area (Vb ) is given by [27]:
Sb = 2πrl 2m0.45
(8)
2.5. Fractographic analysis Fracture surfaces were analyzed according to the fractographic principles to verify the failure origin and the spatial distribution. The analysis was carried out using a stereomicroscope and a FEI scanning electron microscope. Before fractographic analysis, the fracture surfaces of the specimens were carefully cleaned in an ultrasonic bath for 15 min and then immersed in 92.8% ethanol for another 15 min. The location of the fracture origin was first determined by examination under a stereomicroscope. Then, the samples were gold-coated for the SEM analysis. Qualitative analyses of the fracture surfaces were completed to measure the critical defect size based on the fractographic investigation [28].
3.3. Fractographic analysis After the 3PB test and ROR bending test, the fracture surfaces of zirconia ceramics belonging to the DLP group and SM group were respectively investigated. Morphological characteristics of fracture surface of both materials show predominant intergranular mode as presented in Fig. 4. Fractographic features left on the fracture surfaces were observed to indicate the fracture origin. For both materials, the surface around the fracture origin is relatively smooth and it appears to become rougher outwards. The type, location, and approximate size of the critical defects of DLP-manufactured and subtractive-manufactured specimens are listed in Table 4. For both materials, following three types of defects responsible for fracture initiation were observed: pores, agglomerates, and edge damage. For subtractive-manufactured specimens, the strength was controlled by surface flaws except by a few edge flaws, which is consistent with the finding of Ramos et al. [30]. For DLP-manufactured specimens, few volume defects that initiated fracture events were found, though dominating defects were still located at or near the tensile surface. Agglomerates and pores were identified as
3. Results 3.1. Microstructural analysis No significant difference was observed in the density (P > 0.05) of both the samples. The densities of DLP-manufactured and subtractivemanufactured specimens are 6.02 ± 0.02 and 6.04 ± 0.01 g/cm3, respectively. The relative density of the DLP specimens was about 99%. SEM images of microstructures of both the specimens are shown in Fig. 1. Mean grain size was calculated based on the SEM images. Both materials show similar mean grain sizes (P > 0.05). DLP-manufactured specimen exhibits an average grain size of 0.60 ± 0.03 μm, while the mean grain size of subtractive-manufactured zirconia specimen is 0.59 ± 0.03 μm. The XRD diffraction patterns of both Y-TZP specimens were the same and revealed only the tetragonal zirconia phase. 3
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Fig. 1. SEM images of the microstructure of (a) DLP-manufactured and (b) subtractive-manufactured zirconia, showing similar mean grain sizes.
Table 2 Results of uniaxial test and biaxial test. Testing Method
Material
Specimen number
Mean flexural strength (MPa)
Standard deviation
Coefficient of variation (%)
Weibull Modulus m [90% CI]
Characteristic Strength σ0 (MPa) [90% CI]
Uniaxial Tests
DLP SM DLP SM
30 30 30 30
1012.7 1157.6 737.4 984.0
125.5 114.4 99.5 94.7
12.0 9.9 13.5 9.6
9.3[6.7–12.3] 12.0[8.7–15.8] 8.7[6.3–11.5] 12.3[8.9–16.2]
1065.8[1030.8–1100.8] 1206.1[1173.4–1238.9] 778.3[751.3–805.3] 1024.2[996.8–1051.6]
Biaxial Tests
DLP: digital light processing; SM: subtractive manufacturing.
obtained by the uniaxial tests are higher than those determined by the biaxial tests. Similar results have also been described in previous studies [17,23]. This phenomenon can be attributed to the difference in the effective area or volume of the material subjected to maximum stress between these two testing approaches based on the size–strength relationship described by Weibull theory [24], considering that the similar Weibull modulus values obtained by both methods indicate uniform flaw distribution in both DLP and SM specimens. In general, larger specimens are weaker because they have greater possibility to contain critical flaws. In this study, for example, DLP disc shaped specimens during biaxial test were subjected to a significantly larger effective area (62.3 mm2) than effective area (6.4 mm2) of the 3PB specimens, which resulted in a smaller flexural strength in the biaxial test according to Eq. (3). Furthermore, the flexural strength determined by either test can be predicted from the strength measured by the other by using Eqs. (6)–(8). The ratios of flexural strengths determined by these two testing methods for both DLP specimen and SM specimen are also similar to those measured from the predictions, which indicates that the surfacedistributed predicted approach applies in this case and for both types of zirconia ceramic materials the dominating flaws are surface-distributed, which is consistent with the main fracture-initiating flaws observed by fractography. In this study, nearly all the values obtained from 3PB strength of DLP specimens reach above 800 MPa (the required flexural strength value of ISO standard 6827 [18] for dental ceramic), which shows the prospect of DLP technology for the processing of zirconia restorations. For DLP-fabricated specimens, after the 3PB test, average flexural strength was 1012.7 MPa and the characteristic strength reached 1067.0 MPa, which indicates that DLP-fabricated Y-TZP is able to obtain a desirable flexural strength that is close to the strength of conventional subtractive-fabricated dental zirconia ceramic (for subtractive Y-TZP, the value is previously reported as 800–1200 MPa) [31]. Table 2 summarizes that the Weibull modulus value of DLP-fabricated zirconia is in the range of 10.6–11.2 (for subtractive Y-TZP it is previously reported as 5.1–16.5) [32], but comparatively lower than that of the control group, which reveals a relatively large scattering of fracture strengths than the conventional subtractive-manufactured
Fig. 2. Weibull plots of the uniaxial and biaxial strength of Y-TZP fabricated by DLP and conventional subtractive-manufacturing.
origins of the fracture for DLP specimens, while the major fracture initiation sites for SM specimens were agglomerates, as presented in Table 4. Besides, pores were detected as origins for fracture more frequently in DLP specimens than in SM specimens. Moreover, DLP specimens exhibited higher mean value and wider range of critical defect size than SM specimens, especially for the initiating pores. The specimens were chamfered; therefore, only several edge defects were detected as fracture origin for each group. 4. Discussion Two testing approaches were used to estimate the flexural strength of DLP-manufactured and subtractive-manufactured Y-TZPs ceramics. For all the specimens including DLP-fabricated and subtractive-fabricated Y-TZP, as presented in Table 2, the values of flexural strength 4
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Fig. 3. Weibull probability plot obtained by uniaxial bending tests and predicted from biaxial bending tests for (a) SM specimens and (b) DLP specimens.
phase components of both as-sintered DLP and SM Y-TZP are similar. Besides, both materials exhibited similar average grain sizes of 0.62 μm for DLP-fabricated zirconia and 0.6 μm for subtractive-fabricated zirconia. The sintering condition was controlled in this study, and this may be attributed to the similar particle size of raw material. Similar chemical components, phase components, as well as grain sizes mentioned above may indicate similarity in microstructures of both materials. Thus, in order to explain the discrepancy of flexural strength found between the two types of Y-TZPs, defects initiating the fracture events were carefully studied. The defects in Y-TZP can be identified by the fractographic analysis. Notably, the types of defects initiating fracture event in DLP specimen and SM specimen were found to be similar. Two main types of characteristic defects (Figs. 5–8.) including micron-sized pores and agglomerates were observed other than several edge damages. These flaws can act as stress concentrators and lead to the failure of the specimens when loaded. However, differences were found in the fracture initiations of DLP and SM specimens, which are mainly reflected in the proportion, location, and size of fracture initiating pores. Statistics presented in Table 4 demonstrates that the pores of DLP specimens have wider range of critical defect size, which can explain the relatively low measured strength value and Weibull modulus of DLP-manufactured specimens. For both materials, surface defects (at surface or below surface) were detected as the main source for facture initiation, which is consistent with the results of the measured flexural strength and predicted strength by effective surface area calculations. This
Table 3 Results of measured strength values from both methods and the predicted biaxial strength. Material
DLP SM
Measured values
Predicted values
Ratio σROR/σ3PB
σROR (Predicted) /σ3BP
0.728 0.850
0.785 0.804
σ3PB
σROR
σROR
1012.7 1157.6
737.4 984.0
794.6 931.4
(Predicted)
DLP: digital light processing; SM: subtractive manufacturing; 3PB: three-point bending; ROR: ring-on-ring.
specimen test in this experiment. Differences between two materials were also found when using prediction calculations. The predicted biaxial strength of SM specimen showed a positive deviation with relatively low value; however, the predicted biaxial strength of DLP specimen displayed a negative deviation with higher value. In this study, efforts were made to avoid the large processing defects for both DLP-manufactured and subtractive-manufactured specimens. The above mentioned results reveal possible existence of microstructural differences that are responsible for the mechanical behavior of zirconia fabricated by these two different methods. According to the ceramic components provided by manufacturers, both materials are 3Y-TZPs with similar chemical components. XRD analysis of samples before surface treatment shows that tetragonal phase is found as the only zirconia phase, which indicates that the
Fig. 4. Fracture surfaces of (a) DLP-fabricated and (b) subtractive-fabricated Y-TZP showing transgranular mode. 5
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Table 4 The type, location, and size of the critical defects of DLP-manufactured and subtractive-manufactured uniaxial specimens by fractography. Flaw type
Agglomerate Pore Edge damage
DLP (n = 30)
SM (n = 30)
Number
Location
Size* (μm)
Number
Location
Size* (μm)
13 (43.3 %) 16 (53.3 %) 1 (10 %)
surface Surface and volume edge
11–28 9–58 25
20 (67 %) 6 (23 %) 3 (10 %)
surface surface edge
9–23 6–17 15–27
DLP: digital light processing; SM: subtractive manufacturing. * The critical defect size refers to the depth for a surface origin or half minor axis length for a volume origin based on ASTM1322 [27].
relationship [34]:
phenomenon observed in both DLP-manufactured and subtractivemanufactured zirconia may be attributed to the higher stress concentrated on the tensile surface than in volume when loading. Saâdaoui et al. [33] detected flaws such as pores and agglomerates throughout the entire volume of zirconia specimens by another stereolithographybased additive manufacturing technology, namely LCM using X-Ray tomography. However, volume flaws hardly appeared as fracture initiation because they were not so highly tensile loaded compared to surface flaws [12]. The volume pores in DLP-manufactured specimens were occasionally found as source for fracture initiation. Typical pores at different locations are shown in Fig. 6 and the observed volume defect indeed shows larger size than defects at the tensile surface or near the surface, which indicates that the pores found in DLP zirconia initiate fracture when they are huge enough. This could explain why the predicted biaxial strength was lower than the measured value, and the deviation was higher than SM specimen in which only surface flaws were found to initiate fracture. For SM specimens, the measured biaxial strength was comparatively higher than predicted strength perhaps because biaxial test was not influenced by edge defect [32]. For DLP specimens; however, strength was mainly controlled by surface distributed agglomerates and volume distributed pores. Compared to uniaxial specimens, the increase of effective volume in biaxial specimens was significantly higher than the increase of effective surface area. Volume calculations were also applied to predict biaxial strength of DLP zirconia from measured uniaxial strength by using the following
1
σu V m = ⎛ b⎞ σb ⎝ Vu ⎠ ⎜
⎟
(9)
For the 3PB test, the effective volume (Vu ) can be calculated as follows [26]:
Vu =
hbl 2( m+ 1)2
(10)
where b is the specimen width; h is the thickness; and l is the span distance. Moreover, for the ROR bending test, the effective volume (Vb ) is given by [35]:
Vb = 2πrl 2m0.45
(11)
where rh is the radius of the loading ring. The measured biaxial strength was expected to be a bit higher than the predicted value. Instead, it shifted to the lower side, as shown in Fig. 9. However, the predicted line failed to fit the measured data and surface area calculation seemed more favorable, which indicated that the probability of volume defects initiating fracture was very low and surface flaws still dominated, although volume flaws as fracture initiation indeed occurred. First, probability of occurrence of such large pores might be quite low, as presented in Table 5. Saâdaoui et al. [33] also detected similar pore sizes ranging from 7 to 54 μm in LCM sample; nonetheless, the
Fig. 5. Fracture origin of DLP-fabricated Y-TZP: agglomerates below the surface. 6
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Fig. 6. Fracture origins of DLP-fabricated Y-TZP: Most pores are (a) at the surface or (b) near the surface, (c) and only a few are large volume pores.
dominant pores exhibited a range of 7–20 μm with a cumulative frequency of more than 90%. Second, they might not cause fracture if they are not highly stressed enough, considering their location. The microstructural differences between the two materials tested in this study can fundamentally be attributed to their processing technologies. Subtractive-manufactured Y-TZP specimens used in this study were milled from pre-sintered block made by cold isostatic pressing and partial sintering [36,37]. This technique is widely used in producing dental zirconia restorations. The similarity between these two processing technologies is that it is vital for both of them to achieve dense and homogenous ceramic compact that is well-shaped and enlarged to compensate for shrinkage before subsequent sintering. According to previous studies, the agglomerates and pores observed in SM specimens might be related to the starting powder [30,38]. However, for stereolithographic additive manufacturing, the entire process including slurry preparation, printing process, debinding, and sintering, is relatively complicated. A number of details ought to be aware of in order to prevent the ceramic component from flaws such as pores, delamination, cracks, machine defects, and so on. Previous literature studies [7,12,13] reported the existence of two types of pore structures including small pores ranging from 200 to 400 nm and large micron-scale ones. The former is regarded as the residual porosity caused by the insufficient densification because of the presence of a high amount of binder. The
latter one is micron-sized pore considered as air bubbles formed during printing process due to the high viscosity of ceramic slurry. The size of these pores is too large to be removed by normal debinding or sintering procedure [7,12]. Some of them travel to the surface, but are trapped by surface tension; while some of them remain inside the body. Moreover, it was also observed that some bubbles retained the spherical or elliptical shape, and some might be deformed by the pressure around them during the heat treatment (Fig. 6(c)). Existence of agglomerates was another defect that controlled the strength of DLP specimens. They might be the remnants of the powder processing or produced during powder-binder preparation [12,33]. Other microscopic and macroscopic defects of Y-TZP fabricated by stereolithographic additive manufacturing mentioned in previous literatures include cleaning defects [12], machining damage [12], cracks [39,40], and delamination [12,31,40]. In this study, the surface of zirconia specimen was well prepared so that the apparent surface defects occurring during processing were not significant in the fracture initiation. The phenomena of delamination and cracks was not visible within the printed samples, indicating that the volume fraction of zirconia suspensions adhesion between layers of the green parts during the printing progress was sufficient, and debinding and sintering processes were found to be appropriate for the printed samples. Fracture toughness can also be estimated from the values of uniaxial
Fig. 7. Fracture origin of subtractive-fabricated Y-TZP: agglomerates below the surface. 7
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Fig. 8. Fracture origin of subtractive-fabricated Y-TZP: pores at the surface.
where KIc is the fracture toughness, σ is the fracture stress at the origin location, Y is the geometric factor of stress intensity related to the defect geometry, and a is the measured origin size. Y was obtained according to ASTM1322-15 standard [28]. The calculated fracture toughness of DLP-manufactured Y-TZP is 5.399 ± 0.538 MPa m1/2 and it is consistent with previously reported values (4.70–6.03 MPa m1/2) [12,41] for Y-TZP obtained by stereolithographic additive manufacturing. Building orientation influences the flexural strength values as well. Osman et al. [13] tested DLP-printed zirconia discs manufactured vertically at 0/45/90-degree angle on the building platform, and found 0degree vertically-built specimens obtained the highest flexural strength and 45-degree specimens were observed to have the lowest value. This result was attributed to more structural defects of printed disc built at 45°. Harrer et al. [12] investigated the four-point bending strength of bars built upright and horizontal by LCM-technology and observed that different building orientations resulted in various processing damage despite microscopic flaws of the material. Saaˆdaoui et al. [33] also detected similar defects as detected by Harrer et al. [12] in LCMmanufactured zirconia specimens built uprightly and horizontally by XCT analysis. These previous findings indicate that when applying effective area/volume calculations to compare the flexural strength of 3D-printed zirconia, building orientation should be considered. Undeniably, a lot more systematic explorations are demanded to look into the structural defects and their distributions in different building orientations and understand how they finally affect the fracture strength of as-sintered component with specific shape when applied to the true situations which will be pursued in future study. Among the limitations of the present study, all the specimens were built parallel to the platform, so both uniaxial and biaxial tests were only conducted in the best orientation. In addition, although the two YTZPs exhibited same grain size, density, and tetragonal content, they might differ in yttrium content, yttrium repartition as well as additional oxide components since they were not produced from the same raw powder, which could somewhat play a role on the difference between the two populations of strengths. Furthermore, only 30 samples were
Fig. 9. Weibull probability plot obtained by biaxial bending tests and predicted from uniaxial bending tests for DLP specimens by effective surface area/volume calculations. Table 5 The number and size of pores at different locations in DLP-manufactured specimen. Location
At surface
Near surface
Volume
Defect size (μm) Number
9–29 8(50 %)
10–26 5(31 %)
45–58 3(19 %)
DLP: digital light processing.
flexural strength and critical defect size based on following relationship [28]:
KIC = Y c ∙σ
(12) 8
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tested for each modality, which has an influence on the confidence limit of Weibull parameters and the strength populations. Besides, only a selection of samples was shown for the fractography.
[10] G. Mitteramskogler, R. Gmeiner, R. Felzmann, S. Gruber, C. Hofstetter, J. Stampfl, J. Laubersheimer, Light curing strategies for lithography-based additive manufacturing of customized ceramics, Addit. Manuf. 1 (2014) 110–118. [11] D. Anssari Moin, B. Hassan, D. Wismeijer, A novel approach for custom three-dimensional printing of a zirconia root analogue implant by digital light processing, Clin. Oral Implants Res. 28 (2017) 668–670. [12] W. Harrer, M. Schwentenwein, T. Lube, R. Danzer, Fractography of zirconia-specimens made using additive manufacturing (LCM) technology, J. Eur. Ceram. Soc. 37 (2017) 4331–4338. [13] R.B. Osman, A.J. van der Veen, D. Huiberts, D. Wismeijer, N. Alharbi, 3D-printing zirconia implants; a dream or a reality? An in-vitro study evaluating the dimensional accuracy, surface topography and mechanical properties of printed zirconia implant and discs, J. Mech. Behav. Biomed. Mater. 75 (2017) 521–528. [14] A. Zocca, P. Colombo, C.M. Gomes, J. Gunster, Additive manufacturing of ceramics: issues, potentialities, and opportunities, J. Am. Ceram. Soc. 98 (2015) 1983–2001. [15] M.N. Giovan, G. Sines, Biaxial and uniaxial data for statistical comparisons of a ceramic’s strength, J. Am. Ceram. Soc. 62 (1979) 510–515. [16] B. Pick, J.B.C. Meira, L. Driemeier, R.R. Braga, A critical view on biaxial and shortbeam uniaxial flexural strength tests applied to resin composites using Weibull, fractographic and finite element analyses, Dent. Mater. 26 (2010) 83–90. [17] L. Wang, R. Dou, G. Wang, Y. Li, M. Bai, D. Hall, Y. Chen, Fracture strength and Weibull analysis of Ba0. 5Sr0. 5Co0. 8Fe0. 2O3− δ oxygen transport membranes evaluated by biaxial and uniaxial bending tests, Mater. Sci. Eng.: A 670 (2016) 292–299. [18] International Organization for Standardization, ISO 6872: Dentistry-Dental Ceramics, (2015). [19] ASTM C1499-19, Standard Test Method for Monotonic Equibiaxial Flexural Strength of Advanced Ceramics at Ambient Temperature. ASTM Int, 2019. [20] X. Bin, L. Duan, Y. Xie, et al., ZrO2 Nanopowders Prepared by Low-Temperature Vapor-Phase Hydrodysis, J. Am. Ceram. Soc. 83 (5) (2000) 1077–1080. [21] ASTM E112-13, Standard test methods for determining average grain size, ASTM Int (2013). [22] D.J. Green, An Introduction to the Mechanical Properties of Ceramics[M], Cambridge University Press, 1998. [23] K. Breder, T. Andersson, K. Schölin, Fracture strength of α‐and β‐SiAION measured by biaxial and four‐point bending, J. Am. Ceram. Soc. 73 (1990) 2128–2130. [24] N.A. Weil, I.M. Daniel, Analysis of fracture probabilities in nonuniformly stressed brittle materials, J. Am. Ceram. Soc. 47 (1964) 268–274. [25] I.J. Davies, Confidence limits for Weibull parameters estimated using linear least squares analysis, J. Eur. Ceram. Soc. 37 (2017) 5057–5064. [26] G.D. Quinn, Weibull strength scaling for standardized rectangular flexure specimens, J. Am. Ceram. Soc. 86 (2003) 508–510. [27] K. Breder, T. Andersson, K. Schölin, Fracture strength of α‐and β‐SiAION measured by biaxial and four‐point bending, J. Am. Ceram. Soc. 73 (1990) 2128–2130. [28] ASTM C1322-15, Standard practice for fractography and characterisation of fracture origins in advanced ceramics, ASTM Int (2015). [29] R.W. Rice. Porosity of Ceramics: Properties and Applications[M], CRC Press, 2017. [30] C.M. Ramos, P.F. Cesar, E.A. Bonfante, J.H. Rubo, L. Wang, A.F. Borges, Fractographic principles applied to Y-TZP mechanical behavior analysis, J Mech Behav Biomed Mater. 57 (2016) 215–223. [31] E.E. Daou, The zirconia ceramic: strengths and weaknesses, Open Dent J. 18 (2014) 33–42. [32] C. Schatz, M. Strickstrock, M. Roos, D. Edelhoff, M. Eichberger, I.M. Zylla, B. Stawarczyk, Influence of specimen preparation and test methods on the flexural strength results of monolithic zirconia materials, Materials (Basel) 9 (2016) 180. [33] M. Saâdaoui, F. Khaldoun, J. Adrien, X-ray tomography of additive-manufactured zirconia: processing defects–strength relations, J. Eur. Ceram. Soc. (2019). [34] D.K. Shetty, A.R. Rosenfield, W.H. Duchworth, P.R. Held, A biaxial-flexure test for evaluating ceramic strengths, J. Am. Ceram. Soc. 66 (1983) 36–42. [35] S. Batdorf, Some approximate treatments of fracture statistics for polyaxial tension, Int. J. Fract. 13 (1977) 5–11. [36] E. Siarampi, E. Kontonasaki, L. Papadopoulou, N. Kantiranis, T. Zorba, K.M. Paraskevopoulos, P. Koidis, Flexural strength and the probability of failure of cold isostatic pressed zirconia core ceramics, J Prosthet Dent. 108 (2012) 84–95. [37] B. Ji, A.A. Alrayes, J. Zhao, Y. Feng, Z. Shen, Grinding and polishing efficiency of a novel self-glazed zirconia versus the conventional dry-pressed and sintered zirconia ceramics, Adv. Appl. Ceram. 118 (2019) 46–55. [38] K. Noguchi, Y. Matsuda, M. Oishi, T. Masaki, S. Nakayama, M. Mizushina, Strength analysis of yttria‐stabilized tetragonal zirconia polycrystals, J. Am. Ceram. Soc. 73 (1990) 2667–2676. [39] K.J. Jang, J.H. Kang, J.G. Fisher, S.W. Park, Effect of the volume fraction of zirconia suspensions on the microstructure and physical properties of products produced by additive manufacturing, Dent. Mater 35 (2019) 97–106. [40] M. Zhou, W. Liu, H. Wu, X. Song, Y. Chen, L. Cheng, Preparation of a defect-free alumina cutting tool via additive manufacturing based on stereolithography–Optimization of the drying and debinding processes, Ceram. Int. 42 (2016) 11598–11602. [41] R. He, W. Liu, Z. Wu, Fabrication of complex-shaped zirconia ceramic parts via a DLP-stereolithography-based 3D printing method, Ceram. Int. 44 (2018) 3412–3416.
5. Conclusion Yttria-stabilized tetragonal zirconia polycrystal (Y-TZP) fabricated by stereolithographic additive manufacturing, namely digital light processing (DLP) can acquire favorable flexural strength close to that of Y-TZP fabricated by conventional subtractive method. The measured uniaxial strengths of Y-TZP fabricated by DLP and subtractive-manufacturing are 1004.4 and 1171.2 MPa, and the biaxial strengths of DLP and SM Y-TZP are 741.8 and 984.4 MPa, respectively. The predicted approach by effective area calculations applies for both DLP-manufactured zirconia and subtractive-manufactured zirconia. Both materials exhibit similar microstructures considering grain size, phase composition, and main defect types (pores and agglomerates). For subtractive-manufactured Y-TZP, the critical flaws initiating the fracture events are surface-distributed. For DLP-manufactured Y-TZP, surface flaws are dominant but a few volume pores also exist. The DLPmanufactured Y-TZP shows relatively great variability in defect size. Funding This study is supported by National Natural Science foundation of China (No. 81571006). Ethical approval This article does not contain any studies with human participants or animals, performed by any of the authors. Declaration of Competing Interest None. Acknowledgement The authors gratefully acknowledge QuickDemos company (Jiangsu, China) supplying the ceramic 3D printing machine and ceramic slurry. References [1] Y. Zhang, B.R. Lawn, Evaluating dental zirconia, Dent. Mater. 35 (2019) 15–23. [2] S. Zarkovic Gjurin, C. Özcan, M. Oblak, Zirconia ceramic fixed partial dentures after cyclic fatigue tests and clinical evaluation: a systematic review, Adv. Appl. Ceram. 118 (2019) 62–69. [3] C. da Silva Rodrigues, I.L. Aurélio, M. da Rosa Kaizer, Y. Zhang, L.G. May, Do thermal treatments affect the mechanical behavior of porcelain-veneered zirconia? A systematic review and meta-analysis, Dent Mater. 35 (2019) 807–817. [4] J. Chevalier, L. Gremillard, 1.6 zirconia as a biomaterial, Compr. Biomater. II (2017) 122–144. [5] A. Örtorp, D. Jönsson, A. Mouhsen, P. Vult von Steyern, The fit of cobalt–chromium three-unit fixed dental prostheses fabricated with four different techniques: a comparative in vitro study, Dent Mater. 27 (2011) 356–363. [6] P.F. Gouveia, L.M. Schabbach, J.C.M. Souza, New perspectives for recycling dental zirconia waste resulting from CAD/CAM manufacturing process, J. Cleaner Prod. 152 (2017) 454–463. [7] H. Li, L. Song, J. Sun, J. Ma, Z. Shen, Dental ceramic prostheses by stereolithography-based additive manufacturing: potentials and challenges, Adv.. Appl. Ceram. 118 (2019) 30–36. [8] M. Dehurtevent, L. Robberecht, J.C. Hornez, et al., Stereolithography: a new method for processing dental ceramics by additive computer-aided manufacturing [J], Dent. Mater. 33 (5) (2017) 477–485. [9] L. Ferrage, G. Bertrand, P. Lenormand, D. Grossin, B. Ben-Nissan, A review of the additive manufacturing (3DP) of bioceramics: alumina, zirconia (PSZ) and hydroxyapatite, J. Aust. Ceram. Soc. 53 (2017) 11–20.
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Contents lists available at ScienceDirect
Journal of the European Ceramic Society journal homepage: www.elsevier.com/locate/jeurceramsoc
Original Article
Flexural strength and Weibull analysis of Y-TZP fabricated by stereolithographic additive manufacturing and subtractive manufacturing Yuqing Lu1, Ziyu Mei1, Junjing Zhang, Shanshan Gao, Xingqiang Yang, Bo Dong, Li Yue, Haiyang Yu* State Key Laboratory of Oral Diseases, West China Hospital of Stomatology, Sichuan University, Chengdu 610041, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Digital light processing Flexural strength Weibull modulus Zirconia
Digital light processing (DLP) is a relatively mature technology of ceramic additive manufacturing and is promising for fabricating zirconia-based dental restorations. It allows for manufacturing ceramic components with nearly unlimited geometries compared to traditional subtractive manufacturing technology. In order to explore its potential for fabricating dental prosthesis and determine its clinical indications, it is essential to investigate its microstructural characteristics and mechanical behavior. In this study, yttria-stabilized tetragonal zirconia polycrystal (Y-TZP) fabricated by stereolithographic additive manufacturing, namely DLP acquired favorable flexural strength close to that of conventional subtractive-manufactured Y-TZP as indicated by uniaxial (threepoint bending) and biaxial (ring on ring) tests, though the Weibull modulus of DLP-manufactured zirconia was lower than that of subtractive-manufactured zirconia. The strength predicting approach that uses effective area calculations was found to be applicable for both DLP-manufactured zirconia and subtractive-manufactured zirconia. Though both materials showed similar microstructures considering grain size and phase composition, significant differences in critical defects were observed.
1. Introduction Yttria-stabilized tetragonal zirconia polycrystal (Y-TZP) is increasingly applied nowadays in clinical practice of restorative and implant dentistry, as prosthetic material because of its high biocompatibility, excellent mechanical properties, and desirable aesthetics. In general, zirconia restorations are produced by subtractive manufacturing (SM) technology usually based on milling dental work pieces from pre-sintered or fully sintered blocks [1–4]; nonetheless, several problems and challenges are associated with this technique. First, a compensation step of the computer aided design (CAD) software is needed to ensure that the cutter tool with exact diameter can reach the pre-designed surface without sacrificing necessary segment, which makes an impact on precision [5]. Second, subtractive milling progress causes wastage of an amount of material including powdered waste as well as non-machined parts [6]. With the rapid development of additive manufacturing, three-dimensional printing (3DP) methods have been applied for fabricating ceramic parts including zirconia dental restorations [7–9]. These technologies allow for manufacturing ceramic components with nearly
unlimited geometries compared to SM technology and no compensation is required. Another major advantage of this technique is the significant reduction in the wastage of raw material. Among all the 3DP methods, digital light processing (DLP) is regarded as a promising and relatively mature technology for fabrication of ceramic parts that require both high accuracy and surface quality [10,11]. This technique applies a DLP stereolithography printer to achieve layer-by-layer “printing” of 3D objects by the light-curing reaction of resin. Moreover, posttreatments including debinding (binder burnout) and sintering step are needed to achieve a dense part with acceptable mechanical properties after green parts are built [10,12]. However, there are still concerns about the mechanical properties of such additive-manufactured ceramic [7,13,14]. For DLP-fabricated zirconia, studies on the macro-scale fracture strength, reliability, and characterization of defects are still limited. Flexural strength of dental ceramics is one of the essential parameters used to determine clinical indications of dental prosthesis. Few studies have investigated the fracture mechanics difference between YTZPs fabricated by stereolithography-based additive manufacturing and conventional SM, which is regarded as the golden standard for CAD/
⁎
Corresponding author at: 14 S Renmin Rd. 3rd Sec., Chengdu, Sichuan. 610041, PR China. E-mail addresses: [email protected] (L. Yue), [email protected] (H. Yu). 1 These authors have contributed equally to this work. https://doi.org/10.1016/j.jeurceramsoc.2019.10.058 Received 1 August 2019; Received in revised form 9 October 2019; Accepted 29 October 2019 0955-2219/ © 2019 Elsevier Ltd. All rights reserved.
Please cite this article as: Yuqing Lu, et al., Journal of the European Ceramic Society, https://doi.org/10.1016/j.jeurceramsoc.2019.10.058
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9 μm to a final stage of 1 μm using a polishing machine (Struers, Copenhagen, Denmark). The final dimension of parallelepipedic samples used for the 3PB tests was 1.4 mm × 4 mm × 22 mm (0.1 mm chamfer width) according to ISO 6872 [18], while the disc-shaped samples for the ROR bending tests had a diameter of 21.6 mm and thickness of 1.2 mm according to ASTM C1499 [19].
Table 1 Group names, ceramic compositions, and manufacturers. Material
Ceramic component
Manufacturer
Zirconia Slurry
Zirconium oxide (ZrO2+HfO2+Y2O3) 99.72% Yttrium oxide (Y2O3) 5.22% Aluminum oxide (Al2O3) 0.24% Remaining 0.04% Zirconium oxide (ZrO2+HfO2+Y2O3) ≥99.0% Yttrium oxide (Y2O3)＞4.5 ≤ 6.0% Aluminum oxide (Al2O3) ≤1.0% Remaining ≤5.0%
QuickDemos Company (Jiangsu, China)
Zenostar MO
2.2. Microstructural characterization Ivoclar Vivadent AG (Schaan, Liechtenstein)
The average density of as-sintered DLP-fabricated and subtractivefabricated Y-TZP specimens was determined by Archimedes’ method. The relative density of DLP-printed zirconia ceramic was calculated, whereas the zirconia powder density was taken as 6.08 g/cm3[20]. The micrographs of both materials were obtained by scanning electron microscopy (SEM, INSPECT F). The average grain size of the specimens was measured based on the SEM images by using the image analysis software (Image J 1.52a, National Institute of Health, USA) [21]. Crystalline phase structures of both materials were analyzed by X-ray diffraction (XRD, EMPYREAN) with Cu Kα radiation. The 2θ scan range was from 10° to 70° with a step size of 0.02° at a counting time of 1 s.
computer aided manufacturing (CAM) zirconia ceramic. The main objective of this study was to compare the flexural strength of Y-TZP fabricated by DLP technology with that of commercial dental Y-TZP fabricated by SM. In order to evaluate the strength of the material, different measurement approaches such as uniaxial and biaxial bending tests were used, which represent effective volumes/areas as well as different stress states [15–17]. In this study, flexural strength and Weibull statistics of DLP-fabricated Y-TZP were determined by both uniaxial (three-point bending, 3PB) and biaxial (ring on ring, ROR) bending tests, and comprehensively compared with subtractive-manufactured dental zirconia with similar components.
2.3. Mechanical testing procedure Both biaxial and uniaxial bending tests were conducted using an ElectroForce3330 mechanical test instrument. For each method of different groups, 30 specimens were used, respectively. The load was applied using a 3000 N compressive load cell, while the applied force and the displacement of the specimens were recorded using sensors. The maximum load was used to calculate the flexural strength of each specimen. For uniaxial flexural strength tests, the 3PB test was performed to measure the flexural strength of Y-TZP in this study. The specimens were positioned in the sample holder with a span distance of 16 mm and then loaded at a constant crosshead speed of 1 mm min−1 at room temperature. The uniaxial flexural strength values (σu ) were calculated based on ISO 6872 [18] by using the following equation:
2. Experimental procedure 2.1. Specimen preparation Both DLP-manufactured and conventional subtractive-manufactured 3Y-TZPs were used in this study. The group names, ceramic compositions, and manufacturers are listed in Table 1. The 3D objects of rectangular and disc shaped specimens were designed for uniaxial (3PB) and biaxial (ROR) tests respectively, and enlarged based on the shrinkage ratio provided by the manufacturers by using CAD software. All DLP-manufactured specimens were prepared using a DLP stereolithography printing machine from QuickDemos Company (Jiangsu, China). The zirconia slurry bought from QuickDemos Company (Jiangsu, China) in this study comprised a homogeneous mixture of photocurable monomers in which the zirconia powder with a concentration of 58 vol.% was dispersed. The 3D objects of the samples were put into the printer’s CAM software to obtain “sliced” 2D layers. The green bodies were printed with single layer thickness of 25 μm under light intensity of 90 mW/cm2. Both rectangular and disc shaped specimens were fabricated horizontally on the build platform, which is regarded as the most favorable building orientation [13]. After the layer-by-layer fabrication, debinding process was carried out by treating the parts at elevated temperatures to remove the organic resins. Primary layer was removed to avoid the negative influence of delamination on flexural strength. Subtractive-manufactured specimens were produced from commercial CAD/CAM blocks of partially-sintered 3 mol % yttria-stabilized zirconia (Zenostar, Ivoclar Vivadent, Liechtenstein) by soft machining. The specimens were milled using a CAD/CAM machine (Wieland Zenostar mini, Ivoclar Vivadent, Liechtenstein) recommended by the manufacturer. All the support structures were carefully removed before subsequent treatments. The specimens fabricated by DLP and SM were sintered at 1510 °C for 2 h. The heating rate and the cooling rate during the process of sintering were set at 300 °C/ h. After manufacturing and postprocessing, all the specimens were visually checked for any defects. Then all the sintered Y-TZP samples free of visual defects were ground to the final dimension using SiC abrasive papers (Struers, Copenhagen, Denmark) with grit sizes from P400 to P1200, and then polished with diamond paste descending from
σu =
3Pl 2bh2
(1)
where P is the maximum load, l is the distance between the two supports, b represents the specimen width, and h is the specimen thickness. ROR bending test for biaxial flexural strength was conducted in this study. Test fixtures included a loading ring with diameter of 16.8 mm and a supporting ring with diameter of 3.8 mm. The sample was placed on the loading/supporting fixtures and loaded with a crosshead speed of 1 mm min(1. The biaxial flexural strength values (σb ) were calculated from the breaking load at failure according to ASTM C1499[19]:
σb =
3P ⎡ r 1−ν ⎛ rs 2 − rl 2 ⎞ ⎤ (1+ν)ln ⎛ s ⎞ + ⎢ 2πt ⎣ 2 ⎝ rh 2 ⎠ ⎥ ⎝ rl ⎠ ⎦ ⎜
⎟
⎜
⎟
(2)
where P is the breaking load, t is the sample thickness, rh is the radius of specimen, rl is the radius of the loading ring, rs is the radius of the supporting ring, and ν is the Poisson’s ratio. The Poisson’s ratio of 0.3 was selected hereinchosen. 2.4. Weibull statistics Weibull analysis is applied to characterize the reliability of the two material. If surface flaws predominate, the description of the twoparameter Weibull distribution is given by [22]:
F= 1 − exp[−Se (σmax /σ0) m]
(3)
Where where σ0 is characteristic strength; σmax is the reference maximum stress in the body; m denotes the Weibull modulus; F represents the failure probability; Seff is the effective stress surface area given by [23]: 2
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Se =
∫ (σ/σmax)mdS
3.2. Fracture strength and Weibull analysis
(4)
For DLP-manufactured and subtractive-manufactured specimens, both uniaxial and biaxial bending strengths were measured by 3PB and ROR bending tests. The load-displacement curves of DLP and SM specimens by different testing methods were similar, which can be attributed to the same density [29]. The values including the specimen number, the mean flexural strengths, the coefficients of variation, unbiased values and 90% confidence intervals of the Weibull parameters, are listed in Table 2. For subtractive-fabricated specimens, the mean uniaxial flexural strength obtained by 3PB test was 1157.6 MPa (SD = 114.4 MPa) and the mean biaxial flexural strength by ROR bending test was 984.0 MPa (SD = 94.7 MPa). For DLP-fabricated specimens, the average uniaxial flexural strength was 1012.7 MPa (SD = 125.5 MPa), while the average biaxial flexural strength was 737.4 MPa (SD = 99.5 MPa). Student’s t-test was performed to analyze the difference between the two materials, and the results showed that subtractive-fabricated specimens exhibited a statistically significant higher strength than DLP-fabricated specimens for both uniaxial (P < 0.001) and biaxial (P < 0.001) test. Weibull analysis is used to estimate the variability of the materials. The Weibull probability plots of the bending strength for each group are shown in Fig. 2, from which the Weibull modulus and characteristic strength presented in Table 1 were extracted from the straight line fitted by linear regression according to the slope and the intercept. The Weibull modulus value of DLP specimen by both the uniaxial and biaxial tests is ∼9 and that of SM specimen by both tests is about 12. In both uniaxial and biaxial tests, the Weibull modulus of subtractivefabricated specimens is significantly higher than that of DLP-printed specimens. Moreover for DLP and SM specimens respectively, the Weibull modulus obtained from the biaxial bending tests is similar to that obtained from the uniaxial bending tests, which indicates that the flaw population of both materials is uniformly distributed in the different loading configuration. The effective area calculations were applicable for the DLP-manufactured specimen as well as for subtractivemanufactured zirconia ceramics, and the flexural strength measured by one test can be predicted from the values measured by the other based on [Eqs. (6)–(8)]. The predicted fracture strengths from the 3PB tests were plotted in Fig. 3 according to the measured fracture strengths obtained by the ROR bending tests. The ratios of measured flexural strength values and the predicted uniaxial strength values obtained by the other using Eq. (6) are listed in Table 3. The predicted ratios of flexural strengths measured by the two testing methods are close to the ratios obtained from the measured values for both materials. The deviation of predicted biaxial values from measured biaxial values is 5% for SM specimen and 8% for DLP specimen, which is acceptable [17].
where σ represents the stress as a function of position in the stressed volume dv . Thus, the Weibull distribution can be simplified as follows [24]:
1 ⎞ = mlnσmax − mlnσ0 lnln ⎛ ⎝ 1−F ⎠
(5)
where σ0 is taken as σ63.21% (the specific characteristic strength of the sample at F = 0.6321). According to the above mentioned function, a straight line can be drawn and the slope can be calculated as the value of m. The Unbiased values and 90 % confidence intervals of the Weibull parameters were calculated according to [25]. 3PB test and ROR test represent different stress states as well as different effective volumes/areas; therefore, the values of strength obtained from the two methods cannot be compared directly. The Weibull distribution model (Eq. 3) leads to a strength dependency on sample size, it is thus possible to predict the flexural strength of one method from that of the other measurement and determine whether the difference is only an influence of loading geometry. The average strengths in the two bending tests can be related by the effective surface area (S u ) by using the following relation [26]: 1
σu S m = ⎛ b⎞ σb ⎝ Su ⎠ ⎜
⎟
(6)
For the 3PB test, the effective S u can be calculated as follows [26]:
Su =
l[h+b(m+1)] ( m+ 1)2
(7)
Moreover, for the ROR bending test, the effective surface area (Vb ) is given by [27]:
Sb = 2πrl 2m0.45
(8)
2.5. Fractographic analysis Fracture surfaces were analyzed according to the fractographic principles to verify the failure origin and the spatial distribution. The analysis was carried out using a stereomicroscope and a FEI scanning electron microscope. Before fractographic analysis, the fracture surfaces of the specimens were carefully cleaned in an ultrasonic bath for 15 min and then immersed in 92.8% ethanol for another 15 min. The location of the fracture origin was first determined by examination under a stereomicroscope. Then, the samples were gold-coated for the SEM analysis. Qualitative analyses of the fracture surfaces were completed to measure the critical defect size based on the fractographic investigation [28].
3.3. Fractographic analysis After the 3PB test and ROR bending test, the fracture surfaces of zirconia ceramics belonging to the DLP group and SM group were respectively investigated. Morphological characteristics of fracture surface of both materials show predominant intergranular mode as presented in Fig. 4. Fractographic features left on the fracture surfaces were observed to indicate the fracture origin. For both materials, the surface around the fracture origin is relatively smooth and it appears to become rougher outwards. The type, location, and approximate size of the critical defects of DLP-manufactured and subtractive-manufactured specimens are listed in Table 4. For both materials, following three types of defects responsible for fracture initiation were observed: pores, agglomerates, and edge damage. For subtractive-manufactured specimens, the strength was controlled by surface flaws except by a few edge flaws, which is consistent with the finding of Ramos et al. [30]. For DLP-manufactured specimens, few volume defects that initiated fracture events were found, though dominating defects were still located at or near the tensile surface. Agglomerates and pores were identified as
3. Results 3.1. Microstructural analysis No significant difference was observed in the density (P > 0.05) of both the samples. The densities of DLP-manufactured and subtractivemanufactured specimens are 6.02 ± 0.02 and 6.04 ± 0.01 g/cm3, respectively. The relative density of the DLP specimens was about 99%. SEM images of microstructures of both the specimens are shown in Fig. 1. Mean grain size was calculated based on the SEM images. Both materials show similar mean grain sizes (P > 0.05). DLP-manufactured specimen exhibits an average grain size of 0.60 ± 0.03 μm, while the mean grain size of subtractive-manufactured zirconia specimen is 0.59 ± 0.03 μm. The XRD diffraction patterns of both Y-TZP specimens were the same and revealed only the tetragonal zirconia phase. 3
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Fig. 1. SEM images of the microstructure of (a) DLP-manufactured and (b) subtractive-manufactured zirconia, showing similar mean grain sizes.
Table 2 Results of uniaxial test and biaxial test. Testing Method
Material
Specimen number
Mean flexural strength (MPa)
Standard deviation
Coefficient of variation (%)
Weibull Modulus m [90% CI]
Characteristic Strength σ0 (MPa) [90% CI]
Uniaxial Tests
DLP SM DLP SM
30 30 30 30
1012.7 1157.6 737.4 984.0
125.5 114.4 99.5 94.7
12.0 9.9 13.5 9.6
9.3[6.7–12.3] 12.0[8.7–15.8] 8.7[6.3–11.5] 12.3[8.9–16.2]
1065.8[1030.8–1100.8] 1206.1[1173.4–1238.9] 778.3[751.3–805.3] 1024.2[996.8–1051.6]
Biaxial Tests
DLP: digital light processing; SM: subtractive manufacturing.
obtained by the uniaxial tests are higher than those determined by the biaxial tests. Similar results have also been described in previous studies [17,23]. This phenomenon can be attributed to the difference in the effective area or volume of the material subjected to maximum stress between these two testing approaches based on the size–strength relationship described by Weibull theory [24], considering that the similar Weibull modulus values obtained by both methods indicate uniform flaw distribution in both DLP and SM specimens. In general, larger specimens are weaker because they have greater possibility to contain critical flaws. In this study, for example, DLP disc shaped specimens during biaxial test were subjected to a significantly larger effective area (62.3 mm2) than effective area (6.4 mm2) of the 3PB specimens, which resulted in a smaller flexural strength in the biaxial test according to Eq. (3). Furthermore, the flexural strength determined by either test can be predicted from the strength measured by the other by using Eqs. (6)–(8). The ratios of flexural strengths determined by these two testing methods for both DLP specimen and SM specimen are also similar to those measured from the predictions, which indicates that the surfacedistributed predicted approach applies in this case and for both types of zirconia ceramic materials the dominating flaws are surface-distributed, which is consistent with the main fracture-initiating flaws observed by fractography. In this study, nearly all the values obtained from 3PB strength of DLP specimens reach above 800 MPa (the required flexural strength value of ISO standard 6827 [18] for dental ceramic), which shows the prospect of DLP technology for the processing of zirconia restorations. For DLP-fabricated specimens, after the 3PB test, average flexural strength was 1012.7 MPa and the characteristic strength reached 1067.0 MPa, which indicates that DLP-fabricated Y-TZP is able to obtain a desirable flexural strength that is close to the strength of conventional subtractive-fabricated dental zirconia ceramic (for subtractive Y-TZP, the value is previously reported as 800–1200 MPa) [31]. Table 2 summarizes that the Weibull modulus value of DLP-fabricated zirconia is in the range of 10.6–11.2 (for subtractive Y-TZP it is previously reported as 5.1–16.5) [32], but comparatively lower than that of the control group, which reveals a relatively large scattering of fracture strengths than the conventional subtractive-manufactured
Fig. 2. Weibull plots of the uniaxial and biaxial strength of Y-TZP fabricated by DLP and conventional subtractive-manufacturing.
origins of the fracture for DLP specimens, while the major fracture initiation sites for SM specimens were agglomerates, as presented in Table 4. Besides, pores were detected as origins for fracture more frequently in DLP specimens than in SM specimens. Moreover, DLP specimens exhibited higher mean value and wider range of critical defect size than SM specimens, especially for the initiating pores. The specimens were chamfered; therefore, only several edge defects were detected as fracture origin for each group. 4. Discussion Two testing approaches were used to estimate the flexural strength of DLP-manufactured and subtractive-manufactured Y-TZPs ceramics. For all the specimens including DLP-fabricated and subtractive-fabricated Y-TZP, as presented in Table 2, the values of flexural strength 4
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Fig. 3. Weibull probability plot obtained by uniaxial bending tests and predicted from biaxial bending tests for (a) SM specimens and (b) DLP specimens.
phase components of both as-sintered DLP and SM Y-TZP are similar. Besides, both materials exhibited similar average grain sizes of 0.62 μm for DLP-fabricated zirconia and 0.6 μm for subtractive-fabricated zirconia. The sintering condition was controlled in this study, and this may be attributed to the similar particle size of raw material. Similar chemical components, phase components, as well as grain sizes mentioned above may indicate similarity in microstructures of both materials. Thus, in order to explain the discrepancy of flexural strength found between the two types of Y-TZPs, defects initiating the fracture events were carefully studied. The defects in Y-TZP can be identified by the fractographic analysis. Notably, the types of defects initiating fracture event in DLP specimen and SM specimen were found to be similar. Two main types of characteristic defects (Figs. 5–8.) including micron-sized pores and agglomerates were observed other than several edge damages. These flaws can act as stress concentrators and lead to the failure of the specimens when loaded. However, differences were found in the fracture initiations of DLP and SM specimens, which are mainly reflected in the proportion, location, and size of fracture initiating pores. Statistics presented in Table 4 demonstrates that the pores of DLP specimens have wider range of critical defect size, which can explain the relatively low measured strength value and Weibull modulus of DLP-manufactured specimens. For both materials, surface defects (at surface or below surface) were detected as the main source for facture initiation, which is consistent with the results of the measured flexural strength and predicted strength by effective surface area calculations. This
Table 3 Results of measured strength values from both methods and the predicted biaxial strength. Material
DLP SM
Measured values
Predicted values
Ratio σROR/σ3PB
σROR (Predicted) /σ3BP
0.728 0.850
0.785 0.804
σ3PB
σROR
σROR
1012.7 1157.6
737.4 984.0
794.6 931.4
(Predicted)
DLP: digital light processing; SM: subtractive manufacturing; 3PB: three-point bending; ROR: ring-on-ring.
specimen test in this experiment. Differences between two materials were also found when using prediction calculations. The predicted biaxial strength of SM specimen showed a positive deviation with relatively low value; however, the predicted biaxial strength of DLP specimen displayed a negative deviation with higher value. In this study, efforts were made to avoid the large processing defects for both DLP-manufactured and subtractive-manufactured specimens. The above mentioned results reveal possible existence of microstructural differences that are responsible for the mechanical behavior of zirconia fabricated by these two different methods. According to the ceramic components provided by manufacturers, both materials are 3Y-TZPs with similar chemical components. XRD analysis of samples before surface treatment shows that tetragonal phase is found as the only zirconia phase, which indicates that the
Fig. 4. Fracture surfaces of (a) DLP-fabricated and (b) subtractive-fabricated Y-TZP showing transgranular mode. 5
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Table 4 The type, location, and size of the critical defects of DLP-manufactured and subtractive-manufactured uniaxial specimens by fractography. Flaw type
Agglomerate Pore Edge damage
DLP (n = 30)
SM (n = 30)
Number
Location
Size* (μm)
Number
Location
Size* (μm)
13 (43.3 %) 16 (53.3 %) 1 (10 %)
surface Surface and volume edge
11–28 9–58 25
20 (67 %) 6 (23 %) 3 (10 %)
surface surface edge
9–23 6–17 15–27
DLP: digital light processing; SM: subtractive manufacturing. * The critical defect size refers to the depth for a surface origin or half minor axis length for a volume origin based on ASTM1322 [27].
relationship [34]:
phenomenon observed in both DLP-manufactured and subtractivemanufactured zirconia may be attributed to the higher stress concentrated on the tensile surface than in volume when loading. Saâdaoui et al. [33] detected flaws such as pores and agglomerates throughout the entire volume of zirconia specimens by another stereolithographybased additive manufacturing technology, namely LCM using X-Ray tomography. However, volume flaws hardly appeared as fracture initiation because they were not so highly tensile loaded compared to surface flaws [12]. The volume pores in DLP-manufactured specimens were occasionally found as source for fracture initiation. Typical pores at different locations are shown in Fig. 6 and the observed volume defect indeed shows larger size than defects at the tensile surface or near the surface, which indicates that the pores found in DLP zirconia initiate fracture when they are huge enough. This could explain why the predicted biaxial strength was lower than the measured value, and the deviation was higher than SM specimen in which only surface flaws were found to initiate fracture. For SM specimens, the measured biaxial strength was comparatively higher than predicted strength perhaps because biaxial test was not influenced by edge defect [32]. For DLP specimens; however, strength was mainly controlled by surface distributed agglomerates and volume distributed pores. Compared to uniaxial specimens, the increase of effective volume in biaxial specimens was significantly higher than the increase of effective surface area. Volume calculations were also applied to predict biaxial strength of DLP zirconia from measured uniaxial strength by using the following
1
σu V m = ⎛ b⎞ σb ⎝ Vu ⎠ ⎜
⎟
(9)
For the 3PB test, the effective volume (Vu ) can be calculated as follows [26]:
Vu =
hbl 2( m+ 1)2
(10)
where b is the specimen width; h is the thickness; and l is the span distance. Moreover, for the ROR bending test, the effective volume (Vb ) is given by [35]:
Vb = 2πrl 2m0.45
(11)
where rh is the radius of the loading ring. The measured biaxial strength was expected to be a bit higher than the predicted value. Instead, it shifted to the lower side, as shown in Fig. 9. However, the predicted line failed to fit the measured data and surface area calculation seemed more favorable, which indicated that the probability of volume defects initiating fracture was very low and surface flaws still dominated, although volume flaws as fracture initiation indeed occurred. First, probability of occurrence of such large pores might be quite low, as presented in Table 5. Saâdaoui et al. [33] also detected similar pore sizes ranging from 7 to 54 μm in LCM sample; nonetheless, the
Fig. 5. Fracture origin of DLP-fabricated Y-TZP: agglomerates below the surface. 6
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Fig. 6. Fracture origins of DLP-fabricated Y-TZP: Most pores are (a) at the surface or (b) near the surface, (c) and only a few are large volume pores.
dominant pores exhibited a range of 7–20 μm with a cumulative frequency of more than 90%. Second, they might not cause fracture if they are not highly stressed enough, considering their location. The microstructural differences between the two materials tested in this study can fundamentally be attributed to their processing technologies. Subtractive-manufactured Y-TZP specimens used in this study were milled from pre-sintered block made by cold isostatic pressing and partial sintering [36,37]. This technique is widely used in producing dental zirconia restorations. The similarity between these two processing technologies is that it is vital for both of them to achieve dense and homogenous ceramic compact that is well-shaped and enlarged to compensate for shrinkage before subsequent sintering. According to previous studies, the agglomerates and pores observed in SM specimens might be related to the starting powder [30,38]. However, for stereolithographic additive manufacturing, the entire process including slurry preparation, printing process, debinding, and sintering, is relatively complicated. A number of details ought to be aware of in order to prevent the ceramic component from flaws such as pores, delamination, cracks, machine defects, and so on. Previous literature studies [7,12,13] reported the existence of two types of pore structures including small pores ranging from 200 to 400 nm and large micron-scale ones. The former is regarded as the residual porosity caused by the insufficient densification because of the presence of a high amount of binder. The
latter one is micron-sized pore considered as air bubbles formed during printing process due to the high viscosity of ceramic slurry. The size of these pores is too large to be removed by normal debinding or sintering procedure [7,12]. Some of them travel to the surface, but are trapped by surface tension; while some of them remain inside the body. Moreover, it was also observed that some bubbles retained the spherical or elliptical shape, and some might be deformed by the pressure around them during the heat treatment (Fig. 6(c)). Existence of agglomerates was another defect that controlled the strength of DLP specimens. They might be the remnants of the powder processing or produced during powder-binder preparation [12,33]. Other microscopic and macroscopic defects of Y-TZP fabricated by stereolithographic additive manufacturing mentioned in previous literatures include cleaning defects [12], machining damage [12], cracks [39,40], and delamination [12,31,40]. In this study, the surface of zirconia specimen was well prepared so that the apparent surface defects occurring during processing were not significant in the fracture initiation. The phenomena of delamination and cracks was not visible within the printed samples, indicating that the volume fraction of zirconia suspensions adhesion between layers of the green parts during the printing progress was sufficient, and debinding and sintering processes were found to be appropriate for the printed samples. Fracture toughness can also be estimated from the values of uniaxial
Fig. 7. Fracture origin of subtractive-fabricated Y-TZP: agglomerates below the surface. 7
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Fig. 8. Fracture origin of subtractive-fabricated Y-TZP: pores at the surface.
where KIc is the fracture toughness, σ is the fracture stress at the origin location, Y is the geometric factor of stress intensity related to the defect geometry, and a is the measured origin size. Y was obtained according to ASTM1322-15 standard [28]. The calculated fracture toughness of DLP-manufactured Y-TZP is 5.399 ± 0.538 MPa m1/2 and it is consistent with previously reported values (4.70–6.03 MPa m1/2) [12,41] for Y-TZP obtained by stereolithographic additive manufacturing. Building orientation influences the flexural strength values as well. Osman et al. [13] tested DLP-printed zirconia discs manufactured vertically at 0/45/90-degree angle on the building platform, and found 0degree vertically-built specimens obtained the highest flexural strength and 45-degree specimens were observed to have the lowest value. This result was attributed to more structural defects of printed disc built at 45°. Harrer et al. [12] investigated the four-point bending strength of bars built upright and horizontal by LCM-technology and observed that different building orientations resulted in various processing damage despite microscopic flaws of the material. Saaˆdaoui et al. [33] also detected similar defects as detected by Harrer et al. [12] in LCMmanufactured zirconia specimens built uprightly and horizontally by XCT analysis. These previous findings indicate that when applying effective area/volume calculations to compare the flexural strength of 3D-printed zirconia, building orientation should be considered. Undeniably, a lot more systematic explorations are demanded to look into the structural defects and their distributions in different building orientations and understand how they finally affect the fracture strength of as-sintered component with specific shape when applied to the true situations which will be pursued in future study. Among the limitations of the present study, all the specimens were built parallel to the platform, so both uniaxial and biaxial tests were only conducted in the best orientation. In addition, although the two YTZPs exhibited same grain size, density, and tetragonal content, they might differ in yttrium content, yttrium repartition as well as additional oxide components since they were not produced from the same raw powder, which could somewhat play a role on the difference between the two populations of strengths. Furthermore, only 30 samples were
Fig. 9. Weibull probability plot obtained by biaxial bending tests and predicted from uniaxial bending tests for DLP specimens by effective surface area/volume calculations. Table 5 The number and size of pores at different locations in DLP-manufactured specimen. Location
At surface
Near surface
Volume
Defect size (μm) Number
9–29 8(50 %)
10–26 5(31 %)
45–58 3(19 %)
DLP: digital light processing.
flexural strength and critical defect size based on following relationship [28]:
KIC = Y c ∙σ
(12) 8
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tested for each modality, which has an influence on the confidence limit of Weibull parameters and the strength populations. Besides, only a selection of samples was shown for the fractography.
[10] G. Mitteramskogler, R. Gmeiner, R. Felzmann, S. Gruber, C. Hofstetter, J. Stampfl, J. Laubersheimer, Light curing strategies for lithography-based additive manufacturing of customized ceramics, Addit. Manuf. 1 (2014) 110–118. [11] D. Anssari Moin, B. Hassan, D. Wismeijer, A novel approach for custom three-dimensional printing of a zirconia root analogue implant by digital light processing, Clin. Oral Implants Res. 28 (2017) 668–670. [12] W. Harrer, M. Schwentenwein, T. Lube, R. Danzer, Fractography of zirconia-specimens made using additive manufacturing (LCM) technology, J. Eur. Ceram. Soc. 37 (2017) 4331–4338. [13] R.B. Osman, A.J. van der Veen, D. Huiberts, D. Wismeijer, N. Alharbi, 3D-printing zirconia implants; a dream or a reality? An in-vitro study evaluating the dimensional accuracy, surface topography and mechanical properties of printed zirconia implant and discs, J. Mech. Behav. Biomed. Mater. 75 (2017) 521–528. [14] A. Zocca, P. Colombo, C.M. Gomes, J. 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5. Conclusion Yttria-stabilized tetragonal zirconia polycrystal (Y-TZP) fabricated by stereolithographic additive manufacturing, namely digital light processing (DLP) can acquire favorable flexural strength close to that of Y-TZP fabricated by conventional subtractive method. The measured uniaxial strengths of Y-TZP fabricated by DLP and subtractive-manufacturing are 1004.4 and 1171.2 MPa, and the biaxial strengths of DLP and SM Y-TZP are 741.8 and 984.4 MPa, respectively. The predicted approach by effective area calculations applies for both DLP-manufactured zirconia and subtractive-manufactured zirconia. Both materials exhibit similar microstructures considering grain size, phase composition, and main defect types (pores and agglomerates). For subtractive-manufactured Y-TZP, the critical flaws initiating the fracture events are surface-distributed. For DLP-manufactured Y-TZP, surface flaws are dominant but a few volume pores also exist. The DLPmanufactured Y-TZP shows relatively great variability in defect size. Funding This study is supported by National Natural Science foundation of China (No. 81571006). Ethical approval This article does not contain any studies with human participants or animals, performed by any of the authors. Declaration of Competing Interest None. Acknowledgement The authors gratefully acknowledge QuickDemos company (Jiangsu, China) supplying the ceramic 3D printing machine and ceramic slurry. References [1] Y. Zhang, B.R. Lawn, Evaluating dental zirconia, Dent. Mater. 35 (2019) 15–23. [2] S. Zarkovic Gjurin, C. Özcan, M. Oblak, Zirconia ceramic fixed partial dentures after cyclic fatigue tests and clinical evaluation: a systematic review, Adv. Appl. Ceram. 118 (2019) 62–69. [3] C. da Silva Rodrigues, I.L. Aurélio, M. da Rosa Kaizer, Y. Zhang, L.G. May, Do thermal treatments affect the mechanical behavior of porcelain-veneered zirconia? A systematic review and meta-analysis, Dent Mater. 35 (2019) 807–817. [4] J. Chevalier, L. Gremillard, 1.6 zirconia as a biomaterial, Compr. Biomater. II (2017) 122–144. [5] A. Örtorp, D. Jönsson, A. Mouhsen, P. Vult von Steyern, The fit of cobalt–chromium three-unit fixed dental prostheses fabricated with four different techniques: a comparative in vitro study, Dent Mater. 27 (2011) 356–363. [6] P.F. Gouveia, L.M. Schabbach, J.C.M. Souza, New perspectives for recycling dental zirconia waste resulting from CAD/CAM manufacturing process, J. Cleaner Prod. 152 (2017) 454–463. [7] H. Li, L. Song, J. Sun, J. Ma, Z. Shen, Dental ceramic prostheses by stereolithography-based additive manufacturing: potentials and challenges, Adv.. Appl. Ceram. 118 (2019) 30–36. [8] M. Dehurtevent, L. Robberecht, J.C. Hornez, et al., Stereolithography: a new method for processing dental ceramics by additive computer-aided manufacturing [J], Dent. Mater. 33 (5) (2017) 477–485. [9] L. Ferrage, G. Bertrand, P. Lenormand, D. Grossin, B. Ben-Nissan, A review of the additive manufacturing (3DP) of bioceramics: alumina, zirconia (PSZ) and hydroxyapatite, J. Aust. Ceram. Soc. 53 (2017) 11–20.
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