International Journal of Lightweight Materials and Manufacture 1 (2018) 239e245

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International Journal of Lightweight Materials and Manufacture

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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: Additive manufacturing (AM) technology is showing great potential in dental restorations. In this paper,
Received 1 June 2018 3Y-TZP ceramics which are widely used in the fabrication of dental restorations were fabricated by se-
Received in revised form lective laser sintering (SLS) combined with cold isostatic pressing (CIP) technology, and the effect of
13 September 2018
sintering temperature on phase composition, microstructure and mechanical properties of 3Y-TZP ce-
Accepted 14 September 2018
Available online 20 September 2018
ramics 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
Keywords:
Selective laser sintering
layer thickness ¼ 0.09 mm). Then they were densified by CIP (280 MPa, 5 min) process and sintered to
3Y-TZP dental ceramics obtain 3Y-TZP ceramics. It was found that the sample had the highest flexural strength of
Cold isostatic pressing 279.50 ± 10.50 MPa and the maximum relative density of 86.65 ± 0.20% when sintered at 1500  C due to
Microstructure the appropriate grain size and phase composition. Finally, some all-ceramic dental restorations were
Mechanical properties successfully fabricated by this technology. This work provides a new way for the manufacture of indi-
vidualized 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/by-
nc-nd/4.0/).

1. Introduction compress these micro-crack defects in case of further propagation.

Consequently, the flexural strength is improved [5,6].
As a new type of fine ceramics, ZrO2 has good mechanical The application of all-ceramic restorations fabricated by sub-
properties, biocompatibility and stability [1,2]. These years, it has tractive methods has been developed [7,8]. However, these
drawn considerable interest in the biomedical fields, where methods usually waste excessive materials during processing. The
strength and esthetics are considered quite important for all- discarded materials are hard to reuse, which will lead to economic
ceramic restorations [3]. It is very common for zirconia to be losses and tooling cost. Besides, undercuts and locations that are
added with 3 mol% yttria (3Y-TZP) to maintain the tetragonal phase inaccessible cannot be machined by subtractive methods [9].
at room temperature in biomedicine [4]. Loads and stresses can Therefore, all-ceramic restorations are expensive and difficult to
lead to the formation of micro-crack and later generate tensile manufacture to date.
stress, resulting in the transformation from t-ZrO2 to m-ZrO2. In Selective Laser Sintering (SLS) technology is one kind of Additive
this transformation, a local volume increase occurs, which can 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
* Corresponding author. SLS. In direct SLS process, the scanning time of the laser is so short
** Corresponding author. Fax: þ86 27 87558155. that huge thermal stress is induced by large temperature gradient,
E-mail addresses: jiaminwu@hust.edu.cn (J.-M. Wu), shiyusheng@hust.edu.cn
as a result, cracks could easily form [11]. Instead, a low-melting
(Y.-S. Shi).
Peer review under responsibility of Editorial Board of International Journal of
sacrificial binder phase is usually added in indirect SLS process,
Lightweight Materials and Manufacture. 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/).
240 F. Chen et al. / International Journal of Lightweight Materials and Manufacture 1 (2018) 239e245

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 2. Material and methods
technology does not consume excessive materials [15]. Most
importantly, it shows advantages of high design flexibility and 2.1. Raw materials
short product development cycle without moulds [16], which is
significant for customizing all-ceramic restorations for patients. Commercially available yttria-stabilized zirconia granulating
However, it is not easy to densify the ceramic parts prepared by SLS powder doped with 3 mol% Y2O3 (Xuan Cheng Jing Rui New Ma-
technology during the sintering process because of low packing terial Co., Ltd, China) and epoxy resin E12 powder (Guangzhou
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 technol-
ogy can greatly promote the densification of ceramics. In this paper,
SLS/CIP technology was used to fabricate 3Y-TZP ceramics for all-
ceramic dental restorations.
According to previous studies, the mechanical properties of 3Y-
TZP 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 tem-
perature on the phase composition, microstructure and mechanical
properties of the final 3Y-TZP ceramics was investigated in this
paper. Fig. 3. The TG curve of the binder epoxy resin E12 powder.
 F. Chen et al. / International Journal of Lightweight Materials and Manufacture 1 (2018) 239e245 241

Table 1
The formability of SLSed samples with different parameters.

Number P (W) l (mm) v (mm/s) e (J/mm2) Formability

1 7 0.15 2400 0.019 Unable to form

2 7 0.15 2600 0.018 Well
3 7 0.15 2800 0.017 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 Table 2
Shrinkage in X, Y and Z directions in different process.
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 Process Shrinkage % (X/Y) Shrinkage % (Z)
determined by three factors (laser power, scanning speed and hatch CIP process 17.17 (±0.80) 25.34 (±0.31)
spacing), it can be calculated from the following formula (as shown Sintering process (1500  C) 19.00 (±0.59) 19.66 (±0.25)
in Eq. (1)): CIP/Sintering process (1500  C) 34.26 (±0.33) 38.76 (±0.13)

P
e¼ (1)
l$v

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 pa-
rameters. 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 decom-
pose 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.
242 F. Chen et al. / International Journal of Lightweight Materials and Manufacture 1 (2018) 239e245

green samples were heated at 2  C/min to 325  C, and at 0.5  C/min diffraction (XRD-7000s, Shimadzu, Japan). The TG curve of the
to 575  C for 1 h to remove the organic phase. Then the samples binder epoxy resin E12 was measured by a differential scanning
were heated to 800  C at 2  C/min and held for 1 h. Thirdly, the calorimeter (DSC, Diamond, PerkinElmer Instruments Inc.,
temperature was increased at 5  C/min to the expected tempera- Shanghai, China). The densities of the sintered samples were
ture (1350 Ce1550  C) and held for 3 h. Finally the temperature determined using the Archimedes method. The values of relative
dropped to room temperature at 5  C/min. density were calculated from the following formula (as shown in
Eq. (2)):

2.3. Characterization r1
R¼  100% (2)
r2
Particle size distribution of 3Y-TZP powder was obtained using a
laser diffraction-based particle size analyzer (Mastersizer 3000, where R is the relative density, r1 and r2 are the density (g/cm3) and
Worcestershire, United Kingdom), whereas the morphology of the theoretical density (g/cm3), respectively. The linear shrinkage of
samples was studied by scanning electron microscopy (SEM, JSM- 3Y-TZP samples was determined by the following equation (as
7600 F, JEOL Ltd., Japan). The XRD patterns were identified by X-ray 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.
 F. Chen et al. / International Journal of Lightweight Materials and Manufacture 1 (2018) 239e245 243

samples have a shrinkage of ~17.17% and ~25.34% in the X/Y-

la  lb direction and the Z-direction (the building direction), respec-
S¼  100% (3)
la tively. It is very common for SLSed samples that the shrinkage in
the X/Y direction is different from the Z direction after CIP treat-
where S is the shrinkage, la and lb are the diameter of samples ment [27]. The binder between layers cannot be fused as
before and after a certain process (mm), respectively. The flexural completely as that in horizontal direction due to the energy
strength measurement of sintered 3Y-TZP samples was measured gradient of the laser, resulting in the low bonding force between
by three-point bending test on mechanical testing machine (AG- layers. As a consequence, it is easy to deform in the Z-direction
100KN, Zwick/Roll, Germany) using a loading rate of 0.05 mm/min when subjected to force. In the sintering process at 1500  C, the
and a span length of 15 mm. At least three samples were tested at SLSed/CIPed samples have a shrinkage of ~19.00% and ~19.66% in
every sintering temperature. The flexural strength was calculated the X/Y-direction and the Z-direction, respectively. They have little
by the following expression (as shown in Eq. (4)): difference because the CIP process has initially densified the green
parts. T he total shrinkage of ~34.26% and ~38.76% in the X/Y-
3Pl
s¼ (4) direction and the Z-direction is measured in CIP/Sintering process
2ub2
at 1500  C.
where s is the value of flexural strength (MPa), P is the bending load Fig. 6 shows the XRD patterns of 3Y-TZP ceramics sintered at
(N), l is the span length (mm), u and b is the width and the thick- different temperatures, and the main composition is ZrO2. How-
ness of the sample respectively (mm). ever, there are phase transitions at different sintering tempera-
tures. The diffraction peak intensity of m-ZrO2 increases when the
sintering temperature increases to 1500  C, indicating the gradual
3. Results and discussion transformation from t-ZrO2 to m-ZrO2 since the tetragonal grains
grow larger than the critical size [28]. When the sintering
The result of the formability of SLSed samples with different
parameters is shown in Table 1. Samples in the first group are un-
able 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 main-
tains 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
Fig. 8. Average grain size of the 3Y-TZP ceramics sintered at different temperatures.
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% respec-
tively, indicating that the contact area among different 3Y-TZP
ceramic particles increases. Fig. 4(c) shows the 3Y-TZP green sam-
ples 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 tem-
perature is 1550  C, the relative density of 3Y-TZP samples de-
creases 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 Fig. 9. Effect of sintering temperature on the flexural strength of the sintered samples.
244 F. Chen et al. / International Journal of Lightweight Materials and Manufacture 1 (2018) 239e245

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 speed ¼ 2600 mm/s, hatch spacing ¼ 0.15 mm and layer
continues to intensify, which could lead to a loss of the thickness ¼ 0.09 mm. The SLSed samples were densified by CIP
transformation-toughening mechanism and a serious deterioration technology and then sintered to obtain 3Y-TZP ceramics with a
of mechanical properties. relative density of 86.65 ± 0.10%. The optimum sintering temper-
The ceramic materials were characterized by SEM to further ature was proved to be 1500  C. The sample sintered at 1500  C had
study the microstructure of 3Y-TZP ceramics sintered at different the highest flexural strength of 279.50 ± 10.50 MPa and the
temperatures. As shown in Fig. 7, (a), (c), (e) and (g) are SEM maximum densification of 86.65 ± 0.20% due to the dense grains
morphology of surfaces, while (b), (d), (f) and (h) are SEM and appropriate phase composition. In summary, this work laid
morphology of fracture surfaces. The microstructures vary greatly foundations for the manufacture of 3Y-TZP all-ceramic dental res-
as the sintering temperature increases. In Fig. 7(b) and (d), the torations using SLS/CIP technology.
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 Acknowledgment
compact, and apparent grain boundary forms (shown in Fig. 7(f)
and (h)). Moreover, sintering temperature supplies the driving Our research work presented in this paper was supported by
energy for grain growth and strongly affects grain size [29]. In National Natural Science Foundation of China (51605177), National
Fig. 7(a), (c), (e) and (h), it can be seen that 3Y-TZP ceramics have Science and Technology Major Project (2013ZX02104-001-002),
larger grain size with sintering temperature. Fig. 8 shows the China Postdoctoral Science Foundation (2017T100550,
average grain sizes of the 3Y-TZP samples, and the largest average 2015M572136), Hubei Chenguang Talented Youth Development
grain size is 0.66 mm at 1550  C. Excessive grain growth is detri- Foundation and the Fundamental Research Funds for the Central
mental because it can lead to the transformation from t-ZrO2 to m- University (2018KFYYXJJ030). The authors are grateful for the State
ZrO2 and severely reduce its mechanical properties. Key Laboratory of Materials Processing and Die & Mould Technol-
Fig. 9 shows the effect of sintering temperatures on the flexural ogy as well as the Analysis and Testing Center of Huazhong Uni-
strength at room temperature. Based on the above analysis, the versity of Science and Technology for mechanical property, XRD
densification, phase composition and microstructure are all and SEM tests.
important parameters of mechanical properties for 3Y-TZP ce-
ramics. It can be seen that the highest flexural strength is
279.50 MPa at the sintering temperature of 1500  C. The flexural References
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