Effects of Sintering Behaviors on Dimensional Accuracy, Surface Quality, and Mechanical Properties of Stereolithography-printed 3Y-ZrO2 Ceramics

Sintering process is essential to acquire the final components by stereolithography (SLA), which is a promising additive manufacturing technology for the fabrication of complex, custom-designed dental implants. 3Y-ZrO 2 ceramics at different sintering behaviors in air atmosphere were successfully obtained in this study. Firstly, the curing properties of homemade pastes were studied, and the penetration depth and critical exposure of the pastes were calculated as 17.2 μm and 4.80 mJ/cm 2 , respectively. The green ceramic parts were performed at 154 mW laser power and 6000 mm/s scanning speed. Then, the dimensional accuracy, surface quality, and mechanical properties of 3Y-ZrO 2 ceramics were investigated. The shrinkages of length, width, and height were 26%~27 %, 30%~31 %, and 27%~33 % in sintered ceramics, respectively. The Ra values of XOY, YOZ, and XOZ surfaces showed an anisotropic feature, and they were smallest as 0.52 μm, 2.40 μm, and 2.46 μm, respectively. Meanwhile, the mechanical properties presented a similar trend that they grew first and then dropped at various sintering behaviors. The optimal parameters were 1500 ℃, 60 min, and 4 ℃/min, and the maximum relative density of 96.18 %, Vickers hardness of 12.45 GPa, and fracture toughness of 6.35 MPa·m 1/2 were achieved. Finally, the X-ray diffraction (XRD) and energy-dispersive spectroscopy (EDS) analysis demonstrated that no change was observed in crystal transformation and phase composition, and the organic was completely removed in sintered ceramics. This research is expected to provide a technical guide for the fabrication of ceramics for dental implants using SLA technique.


Introduction
Zirconia (ZrO2) ceramics are considered as good potentiality for dental restorations and hip replacements owing to their high fracture strength, excellent wear resistance, and biocompatibility [1,2]. As a dental implant material, these properties of ZrO2 ceramics make it hard to fabricate with complex shapes. Traditional methods such as hot-press and spark plasma sintering have been adopted to manufacture ZrO2 ceramics for biological applications [3,4]. Rapid developments in computer-aided design and computer-aided manufacturing (CAD/CAM) technique have facilitated the fabrication of ZrO2 oral restorations by machining [5,6], and milled ZrO2 ceramics have shown satisfactory osseointegration and good tissue biocompatibility after a sufficient healing period [7]. However, these forming technologies need to use molds or cutting tools, which lead to the waste of raw materials and extend the production cycles. Meanwhile, the machining process would produce microcracks and defects on the surface. Thus, a method for direct fabrication of custom-designed teeth without the demand for mechanical machining would provide a huge improvement in dental implants.
Compared with traditional machining methods, additive manufacturing has recently provided a new resolution for these problems [8,9]. As reported, the 3D printing technologies applicable to ZrO2 ceramics mainly include stereolithography (SLA) [10], direct ink writing (DIW) [11], digital light processing (DLP) [12], fused deposition molding (FDM) [13], and selective laser sintering/melting (SLS/SLM) [14,15]. The SLA technology provides not only a smoother surface finish, but also an improved precision feature in production, which is suitable for fabricating custom-designed dental implants.
Sintering process is an essential solution to acquire excellent properties after the green bodies are formed, which have an obvious influence on the final parts fabricated by SLA [16,17]. Li  and 60 min, respectively [22]. These investigations focused on the mechanical properties of composite ceramics at different sintering temperatures alone. However, the sintering behaviors including sintering temperature, hold time, and sintering rate could not be studied systematically. Meanwhile, the surface quality and precision of the 3D-printed samples would not be taken into account.
In this study, the curing properties of homemade 3Y-ZrO2 pastes were firstly discussed in terms of cure depth and cure width, which would offer a suitable laser power and scanning speed for the fabrication of the green ceramic parts. Then, the green ceramic parts were heated to remove the organic from the green ceramic bodies and prepare for subsequent sintering. Based on the thermal analysis, the effects of sintering temperature (1400 ℃, 1450 ℃, 1500 ℃, 1550 ℃), holding time (0 min, 30 min, 60 min, 90 min), and sintering rate (2 ℃/min, 4 ℃/min, 6 ℃/min, 8 ℃/min) on the dimensional accuracy, surface quality, and mechanical properties of 3Y-ZrO2 ceramics were analyzed. Finally, the crystal transformation and phase composition were identified by the X-ray diffraction (XRD) and energy-dispersive spectroscopy (EDS).
The results of this study can reveal the optimal sintering behaviors for manufacturing 3Y-ZrO2 ceramic parts and provide an important reference for dental implants.

Fabrication process
To take the shrinkage into consideration, the original dimensions were set by the CAD file to be of 30 mm×4 mm×3 mm (Length×Width×Height). Subsequently, the data were imported into the SLA machine (Ceramaker 300 system, 3D CERAM, France). 3Y-ZrO2 ceramic suspensions were uniformly paved on the working platform by a scraper, and an ultraviolet laser (355 nm in wavelength, 40 μm in spot diameter) was scanned the suspensions to create horizontal planes in a layer-by-layer fashion. A standard alternating x/y raster scanning mode was adopted in each layer for the reason that the pattern could reduce stress concentration compared with a unique x or y scanning mode [19]. Then, the platform moved down to submerge the solidified layer by new suspensions after forming a layer. These steps were repeated until the entire green parts were eventually obtained.
Sintering process was an extremely easy defect stage in forming the ceramic parts, and the unsuitability of the process caused some cracks and warpages of the ceramics [24,25]. Thus, selecting suitable sintering behavior was an essential approach to control the forming performances of the final ceramic parts. Before this, the thermal treatments were taken to exhaust the photosensitive resins after printing. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were recorded with a TGA/DSC-QMS analyzer (TGA/DSC1/1100LF, Mettler Toledo, Switzerland) to investigate the pyrolysis of photosensitive resins in debinding process aiming at judging whether the toxic substances could be completely eliminated. In this evaluation, 24.1422 mg of the printed 3Y-ZrO2 ceramic samples were heated from room temperature to 1000 ℃ with a heating rate of 5 ℃/min in air atmosphere. The experimental results were analyzed using a TA Analyzer software. The stages of weight loss, the onset temperature of thermal decomposition, the maximum thermal decomposition temperature, and the final thermal decomposition temperature of the organic can be determined from the TG-DSC curves [26]. The green bodies underwent debinding and sintering in a muffle furnace (KSL-1700X-A3, Hefei Ke Jing Materials Technology Co., Ltd, China). Firstly, the green parts were heated to 600 ℃ with a heating rate of 0.2 ℃/min and held for 10 h, then heated to 1000 ℃ with a heating rate of 1 ℃/min and held for 3 h to burnout the binder in air atmosphere. Then, the samples were heated with different target sintering temperatures (1400 ℃, 1450 ℃, 1500 ℃, 1550 ℃), and target holding time (0 min, 30 min, 60 min, 90 min), and target sintering rates (2 ℃/min, 4 ℃/min, 6 ℃/min, 8 ℃/min). Finally, the samples were cooled to 500 ℃ with a heating rate of 5 ℃/min and subsequently subjected to furnace cooling to obtain the final ceramic components.

Characterization
The cure depth and cure width of the green bodies were measured using a digital micrometer thickness gauge with a range of 0~10 mm and accuracy of 0.001 mm (547-400, Mitutoyo, Japan) and a 3D laser microscope (VK-X200K, KEYENCE, Japan), respectively. The surface roughness of three directions (Length×Width×Height) were also measured by a 3D laser microscope. The density of the sintered samples was measured by the Archimedes method using distilled water, and the relative density was calculated by the ratio of the actual density to the theoretical density (the theoretical density of zirconia ceramic is 6.10 g/cm 3 Where P was applied load (196N was adopted in this study), 2a was diagonal length of Vickers indenters observed by the 3D laser microscope (mm).
The fracture toughness KIC (MPa·m 1/2 ) of the sintered ceramics was calculated based on the hardness test, which was satisfied in Equation (2): Where c was the half of crack length (mm), was the half of diagonal length of Vickers indenters (mm), and was the Vickers hardness (MPa).
The fracture surfaces of the sintered 3Y-ZrO2 ceramics were coated with a thin layer of gold for 120 s to achieve higher resolution, and the morphologies and microstructure were observed and analyzed by a scanning electron microscopy (SEM, ZEISS, OXFORD Instruments) and energy-dispersive spectroscopy (EDS, PV9900, Philips, Netherlands). The possible crystal transformations and phase compositions of the 3Y-ZrO2 ceramic powders, printed, debinded, and sintered samples were characterized by the X-ray diffraction (XRD, Hitachi, PAX-10A-X, Japan). The XRD analysis were conducted in a 2θ range between 10 ° and 80 ° with a scanning speed of 10 °/min using Cu Kα radiation.

Dimensional accuracy
Cure depth (Cd) and cure width (Cw) were important parameters which determine the accuracy and speed in SLA process. Thus, the curing properties of homemade pastes were investigated before the fabrication of the green 3Y-ZrO2 ceramic parts. In an ideal state, the absorption of laser light by liquid photosensitive resins generally confirmed to the Beer-Lambert, which was presented in Equation (3) [27]. Exposure energy E was related to laser power P, scanning speed v, and laser spot diameter w0 in SLA process [28,29], and these parameters were satisfied in Equation (4).
Where Cd was cure depth; was penetration depth, E was exposure energy; was critical exposure; P was laser power; v was scanning speed, and 0 was laser spot diameter.
The honeycomb structure was reduced in size until the internal grid structure was composed of only a single scanning line in this study. Fig. 1 shows the cure depth and cure width of the 3Y-ZrO2 suspensions at different laser powers and scanning speeds.
In Fig. 1  and cure width is not a linear growth. In Fig. 1 (b and 87.10 μm, respectively. The linear fit of the Cd and the lnE was presented in Fig. 1 (c). Based on Equation (3) into three stages in the process. The first stage (Ⅰ period) was associated to the removal of physically adsorbed water from room temperature to 310 ℃, with 1.56% weight loss.
The second stage (Ⅱ period) was involved with the thermal decomposition of the organic with the continuous heat between 310 ℃ and 620 ℃, and the weight loss was 23.27%. In combination with the DTG and DSC curves, the greatest weight loss happened at 375 ℃ and 430 ℃, and the endothermic peak took place at 428 ℃ and 621 ℃ in this stage. When the temperature continued to rise, the TG curve almost remained unchanged, and the weight loss was 0.27% in the third stage (Ⅲ period). This indicated that the organic was almost eliminated [30,31]. According to the abovementioned analysis, the sintering process was designed.
For proper fabrication of 3Y-ZrO2 ceramics by SLA, the anisotropy of shrinkage that occurred were also studied in the sintering process [32]. Fig. 3 Fig. 3 (b,c)  Inset Fig. 3 Here.

Surface quality
To form ceramic structures by SLA successfully, the distance between two adjacent scanning lines Hs was smaller than the Cw. Meanwhile, the effective thickness of the cured layer T was less than the Cd because of the arc-sharped cross section of the curing line. Xing et al. optimized the scanning space Hs and moving distance T, and proposed that they should satisfy the relationships Hs=1/2Cw and T=3/5Cd, respectively [33].
As a result, the Hs and T values were set as 50 μm and 25 μm in this study, respectively. Fig. 4 (a) shows a scanning path and layer-and-layer manufacturing character of SLA process effect on the surface quality. As presented in Fig. 4 (b-d), the surfaces of the SLA-printed 3Y-ZrO2 ceramic bodies presented a clearly anisotropic feature, and the surface roughness of XOY, YOZ, and XOZ surfaces were Ra=0.51 μm, Ra=1.79 μm, and Ra=2.10 μm, respectively. The low surface roughness values proved that the quality fabricated by SLA showed a better potential than that of other 3D printing technologies [34,35]. Meanwhile, there was an obvious layer-and-layer process in the YOZ and XOZ surfaces, which demonstrated that the surface roughness on the YOZ and XOZ were quite severe than the XOY surface.
Inset Fig. 4 Here. When the green bodies were sintered, the organic was burned out from those surfaces, which led to an increase of Ra value compared with that of the green bodies. In accordance, the Ra values of YOZ and XOZ surfaces were larger than that of XOY surface under the same conditions. Fig. 5  Inset Fig. 5 Here.  Table 1 in detail. In Fig. 6 (a), the relative density grew first and then decreased at different sintering temperatures with the same holding time of 30 min and sintering rate of 0.2 ℃/min, and it was observed that the maximum value of 95.49 ± 0.97 % took place at 1500 ℃. The organic in the ceramic bodies was removed through the previous debinding process, and the 3Y-ZrO2 ceramics continued to undergo a solid-phase state in the subsequent sintering process. The rise of sintering temperature was accompanied with the increase of sintering energy and driving force, which led to powder densification. However, the relative density dropped to 94.29 ± 0.37 % at 1550 ℃ where the migration rate of the pore was much lower than that of the grain boundary. Related to the relative density, the Hv and fracture toughness had the same trend that they increased first and then decreased. The maximum values of 12.11 ± 0.23 GPa and 6.04 ± 0.12 MPa·m 1/2 also occurred at 1500 ℃, respectively.

Mechanical properties and microstructure
In accordance, when the sintering temperature rose to 1550 ℃, the Hv and fracture toughness values decreased to 10.14 ± 0.22 GPa and 5.24 ± 0.11 MPa·m 1/2 , respectively.
In Fig. 6 (b), similar to different sintering temperatures, the relative density, Hv, and fracture toughness increased first and then fell at different holding time with the same sintering temperature of 1500 ℃ and sintering rate of 0.2 ℃/min. The maximum values were 96.18 ± 0.83 %, 12.45 ± 0.12 GPa, and 6.31 ± 0.06 MPa·m 1/2 with 60 min, respectively, for the reason that the grain grew and the pore migrated gradually with the improvement of holding time. When the holding time exceeded 60 min, the relative density declined to 94.37 ± 0.26 % due to the abnormal grain growth and uneven grain distribution, which also affected the Hv and fracture toughness of the sintered 3Y-ZrO2 ceramics. In Fig. 6 (c), there was a similar trend in the mechanical properties at different sintering rates with the same sintering temperature of 1500 ℃ and holding time of 30 min. The maximum relative density, Hv, and fracture toughness were 95.87 ± 0.44 %, 13.30 ± 0.48 GPa, and 6.35 ± 0.23 MPa·m 1/2 with 4 ℃/min, respectively. The grain grew and the pore migrated steadily with a reasonable sintering rate, while the incomplete grain growth and pore migration would happen with a high sintering rate.
As a result, the relative density, Hv and fracture toughness decreased to 95.31 ± 0.31 %, 12.27 ± 0.28 GPa and 5.60 ± 0.13 MPa·m 1/2 with 8 ℃/min, respectively. In summary, a sintering temperature of 1500 ℃, a holding time of 60 min, and a sintering rate of 4 ℃/min was one of the optimal sintering behaviors to obtain better mechanical properties of the sintered 3Y-ZrO2 ceramics.
Inset Table 1 Here. Fig. 7 presents the fracture microstructure and EDS of the printed and debinded 3Y-ZrO2 ceramics. As presented in Fig. 7 (a), 3Y-ZrO2 ceramic powders were dispersed with the various resins in the printed samples, and EDS analysis proved a large amount of C elements which was accounted for 44.26 wt.%. Fig. 7 (b) showed that a great quantity 3Y-ZrO2 ceramic powders were agglomerated without no apparent resins. The C elements decreased to 2.59 wt.% indicated by EDS analysis, which demonstrated that the resins were almost exhausted in the debinding process.
Inset Fig. 7 Here.  Fig. 8 (a), the ceramic powders were densified to a solidphase state at 1400 ℃. However, there were a large amount of pores in the sintered samples, which lowered the relative density, Hv and fracture toughness. The grains grew and the grain boundary became smooth with the improvement of sintering temperature, accompanied with the increase in the mechanical properties of sintered samples.
However, there existed some pores in the sample at 1550 ℃ (Fig. 8 b) compared with the microstructure of the sintered sample at 1500 ℃ (Fig. 8 c). Thus, the relative density, Hv and fracture toughness of the sintered samples were enhanced, while these mechanical properties of the sample at 1500 ℃ were larger than those of the sample at 1450 ℃. As presented in Fig. 8 (d), the grain grew abnormally and the grain size became large, resulting to the worst mechanical properties of the sintered sample at 1550 ℃. Meanwhile, there were some transgranular fractures in the microstructure of the sample at 1550 ℃. Fig. 9 presents the fracture microstructure of the sintered 3Y-ZrO2 ceramics at different holding time with the same sintering temperature of 1500 ℃ and sintering rate of 0.2 ℃/min. There were no obvious pores in the microstructure of the sintered samples. In Fig. 9 (a), there was insufficient time for the grain to grow without holding time, and the grain size was too small to reduce the mechanical properties of the sintered sample. As shown in Fig. 9 (b,c), the grain continued to grow and the grain size became large at 30 min and 60 min. As a result, the relative density, Hv and fracture toughness of these samples improved. However, when the holding time exceeded 60 min, there were some abnormal grain growth and transgranular fractures in the microstructure, leading to the mechanical properties of the sintered sample at 90 min. Fig. 10 displays the fracture microstructure of the sintered 3Y-ZrO2 ceramics at different sintering rates with the same sintering temperature of 1500 ℃ and holding time of 30 min. As presented in Fig. 10 (a-c), there were no apparent difference in the microstructure of the sintered samples at 2 ℃/min, 4 ℃/min, and 6 ℃/min. In accordance, the relative density, Hv and fracture toughness of these samples presented no obvious discrepancy with a reasonable sintering rate. In Fig. 10 (d), there existed the incomplete grain growth and transgranular fractures in the microstructure at 8 ℃/min. Thus, the worst mechanical properties of the sintered samples took place due to the high sintering rate.
Inset Fig. 10 Here. (2) The surfaces of SLA-printed 3Y-ZrO2 ceramics presented a clearly anisotropic feature, and there was an obvious layer-and-layer process in the YOZ and XOZ surfaces.    TG-DTG and DSC curves for thermogravimetric and differential scanning calorimetry of the SLA-printed 3Y-ZrO2 green bodies.     Fracture microstructure and EDS of (a) the printed 3Y-ZrO2 ceramics; (b) the debinded 3Y-ZrO2 ceramics.    X-ray diffraction (XRD) patterns of the printed, debinded, and sintered samples at different sintering behaviors.