Microstructure and mechanical properties of 3Y-TZP dental ceramics fabricated by selective laser sintering combined with cold isostatic pressing

Microstructure and mechanical properties of 3Y-TZP dental ceramics fabricated by selective laser sintering combined with cold isostatic pressing

International Journal of Lightweight Materials and Manufacture xxx (2018) 1e7 Contents lists available at ScienceDirect International Journal of Lig...

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International Journal of Lightweight Materials and Manufacture xxx (2018) 1e7

Contents lists available at ScienceDirect

International Journal of Lightweight Materials and Manufacture journal homepage: https://www.sciencedirect.com/journal/ international-journal-of-lightweight-materials-and-manufacture

Original Article

Microstructure and mechanical properties of 3Y-TZP dental ceramics fabricated by selective laser sintering combined with cold isostatic pressing Fen Chen, Jia-Min Wu*, Huan-Qi Wu, Ying Chen, Chen-Hui Li, Yu-Sheng Shi** State Key Laboratory of Materials Processing and Die & Mould Technology, School of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 1 June 2018 Received in revised form 13 September 2018 Accepted 14 September 2018 Available online xxx

Additive manufacturing (AM) technology is showing great potential in dental restorations. In this paper, 3Y-TZP ceramics which are widely used in the fabrication of dental restorations were fabricated by selective laser sintering (SLS) combined with cold isostatic pressing (CIP) technology, and the effect of sintering temperature on phase composition, microstructure and mechanical properties of 3Y-TZP ceramics was investigated. 3Y-TZP/MgO/Epoxy resin E12 composite powder with good flowability and homogeneity was prepared by mechanical mixing method. The SLSed samples were obtained with optimum parameters (laser power ¼ 7 W, scanning speed ¼ 2600 mm/s, hatch spacing ¼ 0.15 mm and layer thickness ¼ 0.09 mm). Then they were densified by CIP (280 MPa, 5 min) process and sintered to obtain 3Y-TZP ceramics. It was found that the sample had the highest flexural strength of 279.50 ± 10.50 MPa and the maximum relative density of 86.65 ± 0.20% when sintered at 1500  C due to the appropriate grain size and phase composition. Finally, some all-ceramic dental restorations were successfully fabricated by this technology. This work provides a new way for the manufacture of individualized all-ceramic dental restorations. © 2018 The Authors. Production and hosting by Elsevier B.V. on behalf of KeAi Communications Co., Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/bync-nd/4.0/).

Keywords: Selective laser sintering 3Y-TZP dental ceramics Cold isostatic pressing Microstructure Mechanical properties

1. Introduction As a new type of fine ceramics, ZrO2 has good mechanical properties, biocompatibility and stability [1,2]. These years, it has drawn considerable interest in the biomedical fields, where strength and esthetics are considered quite important for allceramic restorations [3]. It is very common for zirconia to be added with 3 mol% yttria (3Y-TZP) to maintain the tetragonal phase at room temperature in biomedicine [4]. Loads and stresses can lead to the formation of micro-crack and later generate tensile stress, resulting in the transformation from t-ZrO2 to m-ZrO2. In this transformation, a local volume increase occurs, which can

* Corresponding author. ** Corresponding author. Fax: þ86 27 87558155. E-mail addresses: [email protected] (J.-M. Wu), [email protected] (Y.-S. Shi). Peer review under responsibility of Editorial Board of International Journal of Lightweight Materials and Manufacture.

compress these micro-crack defects in case of further propagation. Consequently, the flexural strength is improved [5,6]. The application of all-ceramic restorations fabricated by subtractive methods has been developed [7,8]. However, these methods usually waste excessive materials during processing. The discarded materials are hard to reuse, which will lead to economic losses and tooling cost. Besides, undercuts and locations that are inaccessible cannot be machined by subtractive methods [9]. Therefore, all-ceramic restorations are expensive and difficult to manufacture to date. Selective Laser Sintering (SLS) technology is one kind of Additive Manufacturing (AM) technologies. The manufacture of the parts is completed by selective scanning of the laser beam layer by layer in the SLS process [10]. In general, the SLS technology for ceramics could be divided into two categories, namely, direct and indirect SLS. In direct SLS process, the scanning time of the laser is so short that huge thermal stress is induced by large temperature gradient, as a result, cracks could easily form [11]. Instead, a low-melting sacrificial binder phase is usually added in indirect SLS process, which can be easily fused by laser to obtain green parts [11e14].

https://doi.org/10.1016/j.ijlmm.2018.09.002 2588-8404/© 2018 The Authors. Production and hosting by Elsevier B.V. on behalf of KeAi Communications Co., Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Please cite this article in press as: F. Chen, et al., Microstructure and mechanical properties of 3Y-TZP dental ceramics fabricated by selective laser sintering combined with cold isostatic pressing, International Journal of Lightweight Materials and Manufacture (2018), https://doi.org/10.1016/ j.ijlmm.2018.09.002

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F. Chen et al. / International Journal of Lightweight Materials and Manufacture xxx (2018) 1e7

Fig. 1. SEM morphology (a) and particle diameter distribution diagram (b) of the 3Y-TZP granulating powder.

Fig. 2. Schematic diagram of the CIP process.

Compared with the traditional manufacturing technology, SLS technology does not consume excessive materials [15]. Most importantly, it shows advantages of high design flexibility and short product development cycle without moulds [16], which is significant for customizing all-ceramic restorations for patients. However, it is not easy to densify the ceramic parts prepared by SLS technology during the sintering process because of low packing density of SLSed green parts, so that the strength is generally too low for actual applications [17,18]. In 2010, Liu et al. [19] combined SLS technology with cold isostatic pressing (CIP) technology, and the final alumina parts had a density of 92% theory density, which is relatively high for ceramic parts prepared by SLS technology. Deckers et al. [20] obtained alumina parts with a relative density of 85.5e88.0% theory density using SLS/CIP technology. CIP technology can greatly promote the densification of ceramics. In this paper, SLS/CIP technology was used to fabricate 3Y-TZP ceramics for allceramic dental restorations. According to previous studies, the mechanical properties of 3YTZP are highly related to its grain size [21e23]. Consequently, the sintering temperatures greatly affect the mechanical properties of the final parts since they can greatly affect the grain size [24]. Mechanical mixing method was used to prepare 3Y-TZP/Y2O3/ Epoxy resin E12 composite powder suitable for SLS process in this paper. Then green parts were prepared by SLS/CIP technology. Based on the previous research [25], the effect of sintering temperature on the phase composition, microstructure and mechanical properties of the final 3Y-TZP ceramics was investigated in this paper.

2. Material and methods 2.1. Raw materials Commercially available yttria-stabilized zirconia granulating powder doped with 3 mol% Y2O3 (Xuan Cheng Jing Rui New Material Co., Ltd, China) and epoxy resin E12 powder (Guangzhou

Fig. 3. The TG curve of the binder epoxy resin E12 powder.

Please cite this article in press as: F. Chen, et al., Microstructure and mechanical properties of 3Y-TZP dental ceramics fabricated by selective laser sintering combined with cold isostatic pressing, International Journal of Lightweight Materials and Manufacture (2018), https://doi.org/10.1016/ j.ijlmm.2018.09.002

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Table 1 The formability of SLSed samples with different parameters. Number

P (W)

l (mm)

v (mm/s)

e (J/mm2)

Formability

1 2 3

7 7 7

0.15 0.15 0.15

2400 2600 2800

0.019 0.018 0.017

Unable to form Well Bad surface quality

Shinshi Chemical Co., Ltd., China) was used as the raw materials. MgO powder (Sinopharm Chemical Reagent Co., Ltd., China) was used as sintering aids. The 3Y-TZP powder was mechanically mixed with 0.5 wt% MgO powder and 6.0 wt% epoxy resin E12 powder at a rotational speed of 150 r/min in a horizontal ball mill for 6 h to ensure that all the components were mixed uniformly. SEM morphology and particle diameter distribution diagram of 3Y-TZP are shown in Fig. 1. The 3Y-TZP particles are spherical and possess good flowability, which is beneficial for the powder spreading. Besides, the 3Y-TZP powder shows a particle diameter distribution with a relatively small median particle diameter (D50) of 38.8 mm (shown in Fig. 1(b)).

Fig. 5. Relative densities of the 3Y-TZP ceramics at different sintering temperatures.

2.2. Fabrication process Green parts were fabricated using a HK C250 (Wuhan Huake 3D Technology Co., Ltd., China) equipped with a CO2 laser beam that has a power of 100 W and a wavelength of 10.6 mm. Laser energy density, as the most important parameter of SLS technology, was determined by three factors (laser power, scanning speed and hatch spacing), it can be calculated from the following formula (as shown in Eq. (1)):



P l$v

Table 2 Shrinkage in X, Y and Z directions in different process. Process

Shrinkage % (X/Y)

Shrinkage % (Z)

CIP process Sintering process (1500  C) CIP/Sintering process (1500  C)

17.17 (±0.80) 19.00 (±0.59) 34.26 (±0.33)

25.34 (±0.31) 19.66 (±0.25) 38.76 (±0.13)

(1)

where e is the laser energy density (J/mm2), P is the laser power (W), l is the hatch spacing (mm) and v is the scanning speed (mm/ s). In this study, laser power (7 W), scanning speed (2400e2800 mm/s) and hatch spacing (0.15 mm) were selected at a constant deposited layer thickness of 0.09 mm. Multilayer parts (50  10  5 mm) were fabricated with the above-mentioned parameters. After the parts were processed, the remaining powder would be recycled and reused. CIP technology was applied to increase the density and improve the performance of ceramic parts. The SLSed parts are vacuum packed in elastic rubber bags, sealed and placed in a high pressure cylinder. Then they are isostatically pressed at 280 MPa for 5 min at room temperature through liquid medium (kerosene) according to the previous research [19]. Fig. 2 shows the CIP process. Fig. 3 shows the thermal gravity (TG) curve of the binder epoxy resin E12 powder. The epoxy resin E12 powder begins to decompose when the temperature increases to 325  C and only leaves about 4 wt% carbon residue at 575  C. Therefore, the SLSed/CIPed

Fig. 6. X-ray diffraction patterns of samples sintered at different temperatures.

Fig. 4. SEM morphology of (a) SLSed green sample and (b) SLSed/CIPed green sample at 280 MPa; (c) the photograph of 3Y-TZP green samples fabricated by SLS/CIP technology.

Please cite this article in press as: F. Chen, et al., Microstructure and mechanical properties of 3Y-TZP dental ceramics fabricated by selective laser sintering combined with cold isostatic pressing, International Journal of Lightweight Materials and Manufacture (2018), https://doi.org/10.1016/ j.ijlmm.2018.09.002

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green samples were heated at 2  C/min to 325  C, and at 0.5  C/min to 575  C for 1 h to remove the organic phase. Then the samples were heated to 800  C at 2  C/min and held for 1 h. Thirdly, the temperature was increased at 5  C/min to the expected temperature (1350 Ce1550  C) and held for 3 h. Finally the temperature dropped to room temperature at 5  C/min.

diffraction (XRD-7000s, Shimadzu, Japan). The TG curve of the binder epoxy resin E12 was measured by a differential scanning calorimeter (DSC, Diamond, PerkinElmer Instruments Inc., Shanghai, China). The densities of the sintered samples were determined using the Archimedes method. The values of relative density were calculated from the following formula (as shown in Eq. (2)):

2.3. Characterization



Particle size distribution of 3Y-TZP powder was obtained using a laser diffraction-based particle size analyzer (Mastersizer 3000, Worcestershire, United Kingdom), whereas the morphology of samples was studied by scanning electron microscopy (SEM, JSM7600 F, JEOL Ltd., Japan). The XRD patterns were identified by X-ray

r1  100% r2

(2)

where R is the relative density, r1 and r2 are the density (g/cm3) and the theoretical density (g/cm3), respectively. The linear shrinkage of 3Y-TZP samples was determined by the following equation (as shown in Eq. (3)):

Fig. 7. SEM morphology of surfaces and fracture surfaces of the 3Y-TZP ceramics: (a) (b) 1400  C; (c) (d) 1450  C; (e) (f) 1500  C; (g) (h) 1550  C.

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la  lb  100% la

(3)

where S is the shrinkage, la and lb are the diameter of samples before and after a certain process (mm), respectively. The flexural strength measurement of sintered 3Y-TZP samples was measured by three-point bending test on mechanical testing machine (AG100KN, Zwick/Roll, Germany) using a loading rate of 0.05 mm/min and a span length of 15 mm. At least three samples were tested at every sintering temperature. The flexural strength was calculated by the following expression (as shown in Eq. (4)):



3Pl 2ub2

(4)

where s is the value of flexural strength (MPa), P is the bending load (N), l is the span length (mm), u and b is the width and the thickness of the sample respectively (mm).

3. Results and discussion The result of the formability of SLSed samples with different parameters is shown in Table 1. Samples in the first group are unable to form because of the excessive burning loss of epoxy resin E12 caused by the excessively high energy density. Samples in the third group have bad surface quality because the energy density is too low that there is unfused epoxy resin E12 powder in the green parts. The second group has the best forming quality due to the appropriate energy density. Therefore, laser power (7 W), scanning speed (2600 mm/s) and hatch spacing (0.15 mm) were selected. Fig. 4(a) shows the SEM morphology of the SLSed samples using optimum parameters. It can be seen that the shape of ceramic particles is almost unchanged after laser scanning and still maintains spherical. Meanwhile, there are many bonding necks between the particles, which are formed by the fusing and solidification of epoxy resin E12 after laser scanning. However, there are still a large quantity of pores could be observed in the SLSed sample, because the interspaces between stacking 3Y-TZP particles are large and there are gaps left after epoxy resin E12 is burnt out. Therefore, it is necessary to use CIP technology to increase the density of green samples. Fig. 4(b) shows the SEM morphology of the CIPed samples at 280 MPa for 5 min. The microstructures change remarkably compared to the SLSed samples. The relative densities of SLSed samples and SLSed/CIPed samples are 26.74% and 61.21% respectively, indicating that the contact area among different 3Y-TZP ceramic particles increases. Fig. 4(c) shows the 3Y-TZP green samples fabricated by SLS/CIP technology. Fig. 5 shows the relative densities of 3Y-TZP ceramics at different sintering temperatures. With the increase of sintering temperature, the density of samples increases first and then decreases. The relative density increases from 85.09% to 86.65% with the sintering temperature increasing from 1350  C to 1500  C. When the temperature is 1550  C, the relative density of 3Y-TZP samples decreases to 85.72%. The relative density increases in the range of 1350e1500  C because of the solid-state diffusion in sintering densification process. However, oversintering might lead to excessive grain growth. If the grain grows too fast, the gas will be wrapped in the grains before it can discharge, which can explain that the relative density decreases at 1550  C [26]. Therefore, the 3Y-TZP ceramics fabricated by SLS/CIP technology have a highest relative density when sintered at 1500  C. Table 2 shows the direction and shrinkage of sintered 3Y-TZP samples in CIP and sintering processes. In the CIP process, the SLSed

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samples have a shrinkage of ~17.17% and ~25.34% in the X/Ydirection and the Z-direction (the building direction), respectively. It is very common for SLSed samples that the shrinkage in the X/Y direction is different from the Z direction after CIP treatment [27]. The binder between layers cannot be fused as completely as that in horizontal direction due to the energy gradient of the laser, resulting in the low bonding force between layers. As a consequence, it is easy to deform in the Z-direction when subjected to force. In the sintering process at 1500  C, the SLSed/CIPed samples have a shrinkage of ~19.00% and ~19.66% in the X/Y-direction and the Z-direction, respectively. They have little difference because the CIP process has initially densified the green parts. T he total shrinkage of ~34.26% and ~38.76% in the X/Ydirection and the Z-direction is measured in CIP/Sintering process at 1500  C. Fig. 6 shows the XRD patterns of 3Y-TZP ceramics sintered at different temperatures, and the main composition is ZrO2. However, there are phase transitions at different sintering temperatures. The diffraction peak intensity of m-ZrO2 increases when the sintering temperature increases to 1500  C, indicating the gradual transformation from t-ZrO2 to m-ZrO2 since the tetragonal grains grow larger than the critical size [28]. When the sintering

Fig. 8. Average grain size of the 3Y-TZP ceramics sintered at different temperatures.

Fig. 9. Effect of sintering temperature on the flexural strength of the sintered samples.

Please cite this article in press as: F. Chen, et al., Microstructure and mechanical properties of 3Y-TZP dental ceramics fabricated by selective laser sintering combined with cold isostatic pressing, International Journal of Lightweight Materials and Manufacture (2018), https://doi.org/10.1016/ j.ijlmm.2018.09.002

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Fig. 10. All-ceramic dental restorations fabricated by SLS/CIP technology: (a) digital tooth models; (b) ceramic products.

temperature increases to 1550  C, the diffraction peak of m-ZrO2 continues to intensify, which could lead to a loss of the transformation-toughening mechanism and a serious deterioration of mechanical properties. The ceramic materials were characterized by SEM to further study the microstructure of 3Y-TZP ceramics sintered at different temperatures. As shown in Fig. 7, (a), (c), (e) and (g) are SEM morphology of surfaces, while (b), (d), (f) and (h) are SEM morphology of fracture surfaces. The microstructures vary greatly as the sintering temperature increases. In Fig. 7(b) and (d), the structure of ceramic materials is relatively loose, indicating the sintering temperature is insufficient. As the sintering temperature increases, the ZrO2 crystal structure becomes more and more compact, and apparent grain boundary forms (shown in Fig. 7(f) and (h)). Moreover, sintering temperature supplies the driving energy for grain growth and strongly affects grain size [29]. In Fig. 7(a), (c), (e) and (h), it can be seen that 3Y-TZP ceramics have larger grain size with sintering temperature. Fig. 8 shows the average grain sizes of the 3Y-TZP samples, and the largest average grain size is 0.66 mm at 1550  C. Excessive grain growth is detrimental because it can lead to the transformation from t-ZrO2 to mZrO2 and severely reduce its mechanical properties. Fig. 9 shows the effect of sintering temperatures on the flexural strength at room temperature. Based on the above analysis, the densification, phase composition and microstructure are all important parameters of mechanical properties for 3Y-TZP ceramics. It can be seen that the highest flexural strength is 279.50 MPa at the sintering temperature of 1500  C. The flexural strength of 3Y-TZP ceramics increases first and then decreases with the increase of the sintering temperature, which is consistent with the change of the relative density. Therefore, the densification can affect the flexural strength to some extent. 3Y-TZP ceramics sintered at 1500  C have better microstructure and grain size, resulting in the better mechanical properties. When sintering temperature is above 1500  C, the formation of m-ZrO2 and excessive grain growth results in a decrease of flexural strength. Therefore, the optimum sintering temperature in this paper is 1500  C because of the highest flexural strength. Some digital tooth models provided by Wuhan Union Hospital and the sintered all-ceramic dental restorations fabricated by SLS/ CIP technology are presented in Fig. 10. 4. Conclusions In this paper, some all-ceramic dental restorations were successfully fabricated using SLS/CIP technology. 3Y-TZP/MgO/Epoxy resin E12 composite powder were prepared by mechanical mixing method, and then the SLSed samples were fabricated using optimum parameters of laser power ¼ 7 W, scanning

speed ¼ 2600 mm/s, hatch spacing ¼ 0.15 mm and layer thickness ¼ 0.09 mm. The SLSed samples were densified by CIP technology and then sintered to obtain 3Y-TZP ceramics with a relative density of 86.65 ± 0.10%. The optimum sintering temperature was proved to be 1500  C. The sample sintered at 1500  C had the highest flexural strength of 279.50 ± 10.50 MPa and the maximum densification of 86.65 ± 0.20% due to the dense grains and appropriate phase composition. In summary, this work laid foundations for the manufacture of 3Y-TZP all-ceramic dental restorations using SLS/CIP technology.

Acknowledgment Our research work presented in this paper was supported by National Natural Science Foundation of China (51605177), National Science and Technology Major Project (2013ZX02104-001-002), China Postdoctoral Science Foundation (2017T100550, 2015M572136), Hubei Chenguang Talented Youth Development Foundation and the Fundamental Research Funds for the Central University (2018KFYYXJJ030). The authors are grateful for the State Key Laboratory of Materials Processing and Die & Mould Technology as well as the Analysis and Testing Center of Huazhong University of Science and Technology for mechanical property, XRD and SEM tests.

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