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Stereolithographical fabrication of dense Si3N4 ceramics by slurry optimization and pressure sintering
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Stereolithographical fabrication of dense Si3N4 ceramics by slurry optimization and pressure sintering Yao Liua,b,c, Lina Zhanc, Yu Hec, Jun Zhangc, Jianjun Huc, Lijin Chengd, Qiumei Wua,b,∗, Shaojun Liua,b,∗∗ a
Shenzhen Research Institute, Central South University, Shenzhen, 510085, China State Key Laboratory for Powder Metallurgy, Central South University, Changsha, 410083, China c College of Mechanical and Electronic Engineering, Pingxiang College, Pingxiang, 337000, China d State Key Laboratory for Material Processing and Die & Mould Technology, School of Material Science and Engineering, Huazhong University of Science and Technology, Wuhan, 430074, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Stereolithography Silicon nitride Surface modification Rheological behavior
Photocurable gray-colored Si3N4 ceramic slurry with high solid loading, suitable viscosity and high curing depth is critical to fabricate dense ceramic parts with complex shape and high surface precision by stereolithography technology. In the present study, Si3N4 ceramic slurry with suitable viscosity, high solid loading (45 vol %) and curing depth of 50 μm was prepared successfully when surface modifier KH560 (1 wt%) and dispersant Darvan (1 wt%) were used. The slurry exhibits the shear thinning behavior. Based on the Beer-Lambert formula, Dp (the attenuation length) and Ec (the critical energy dose) of Si3N4 ceramic slurry with solid loading of 45 vol % were derived as 0.032 mm and 0.177 mJ/mm2, respectively. Si3N4 ceramic green parts with complex shape and high surface precision were successfully fabricated by stereolithography technology. After optimizing the debinding and sintering process for green parts, dense Si3N4 ceramics with 3.28 g/cm3 sintering density were fabricated. The microhardness and fracture toughness of as-sintered Si3N4 ceramics are ~14.63 GPa and ~5.82 MPa m1/2, respectively, which are comparable to those of the samples by traditional dry-pressed and pressureless sintering technology. These results show that ceramic stereolithography technology could be promising to fabricate high performance ceramics, especially for gray-colored monolithic Si3N4 ceramics.
1. Introduction
viscosities of the slurries and the precision of photocurable ceramic parts. Jang et al. [5] prepared high solid loading and low viscosity ferroelectric photocurable slurry by mixing barium titanate powder with hexanediol diacrylate (HDDA). Zhang et al. [6] modified Al2O3 powder with dibasic acid, and analyzed the adsorption of dicarboxylic acid on Al2O3 powder by high performance liquid chromatography and concluded that the viscosity and yield stress of Al2O3 slurry are the lowest when the dose of dicarboxylic acid reaches its maximum adsorption capacity. Re et al. [7] prepared the green parts after optimizing the photocuring and sintering parameters. When sintering temperature was set as 1400 °C, dense ZrO2 ceramic rods (~97 wt %) was prepared. The hardness and fracture toughness of as-sintered ZrO2 ceramics are ~13 GPa and ~6 MPa, respectively. However, reports on AM technologies that can generate dense monolithic ceramic bodies have been still very few [8,9]. Among them, ceramic stereolithography and related additive manufacture involving
Additive manufacturing (AM) technologies have been developed to become well-known technologies to fabricate polymeric or metallic prototypes or models [1]. However, it is still a big challenge to process monolithic ceramics with superior physico-chemical properties due to the introduction of feedstock, debinding and sintering processing. Dufaud et al. [2] studied systematically the photocurable Pb(Ti, Zr)O3 (PZT) piezoelectric ceramic slurry by identifying the effects of different kinds of dispersants, surfactants and temperatures on the rheological properties of PZT slurries, and prepared ceramic slurry with 47 vol % solid loading. Hinczewski et al. [3] mixed Al2O3 particles with monomer and analyzed the effects of monomers, diluents, dispersants, photo-initiators for the polymerization on the rheological properties of the slurries systematically. Badev et al. [4] showed that the particle sizes of SiO2 and Al2O3 powders are the main factors that affect the
∗
Corresponding author. State Key Laboratory for Powder Metallurgy, Central South University, Changsha, 410083, China. Corresponding author. Shenzhen Research Institute, Central South University, Shenzhen, 510085, China. E-mail addresses: [email protected] (Q. Wu), [email protected] (S. Liu).
∗∗
https://doi.org/10.1016/j.ceramint.2019.09.186 Received 5 August 2019; Received in revised form 18 September 2019; Accepted 19 September 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Yao Liu, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.09.186
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phosphine oxide (819). 1, 6-hexanediol diacrylate (HDDA) were used as diluent to reduce the viscosity of Si3N4 slurry. EA, HDDA, 184, 819 were purchased from BASF Shanghai, China. Polymethyl methacrylate (Darvan, molecular weight: MW = 520), tetramethylammonium hydroxide (TAHS, MW = 91.05), stearic acid (Phosphonic ester, MW = 284.48) (Nanjing Chemical, China) were used as dispersants.
photopolymerization of ceramic slurries have been a hot topic due to their high manufacturing precision, good surface quality and ability to prepare complex geometric parts [10–12]. To meet these requirements, photocurable slurries should present the following characteristics: 1) suitable viscosity that ensures uniform and flat coating of ceramic slurry on a previous layer; 2) sufficient curing depth to ensure excessive curing of the interface in the two cured layers that provides good cohesion; 3) a sufficiently high solid content to generate a low shrinkage rate. Especially, although photocuring molding technology to fabricate ceramics has been widely carried out [13–15], few researches have been addressed as referred to the gray-colored ceramics with high refraction index, for example SiC and Si3N4 ceramics [16]. Deng et.al [8] fabricated gray-colored SiC ceramic green parts with complex shapes successfully by stereolithography-based AM. Further investigation shows that the curing ability of SiC slurries is influenced profoundly by particle size, solid loading, stereolithography parameters, as well as photo-initiator and its concentration. However, no further investigation was addressed on the photocuring properties of nitrides, such as Si3N4, which might be partially contributed by high refractive index n = 2.4 of Si3N4 powder [17,18], resulting in a low curing depth, further making them difficult to curing [19]. It should be stressed that the reports on structure and properties of dense as-sintered SiC and Si3N4 ceramics by stereolithography based additive manufacturing technology still lack. It is known that Si3N4 ceramics is considered as one of the most promising advanced ceramics to fabricate components to be used at elevated temperatures or under harsh environments due to its hardness, heat resistance, wear resistance, good thermal shock resistance, as well as high temperature creep resistance and self-lubricating properties [20–22]. However, how to process Si3N4 ceramic components with high precision and complex geometries is a major concern in AM technology due to high refractive index that makes it difficult to cure. Additionally, the physicochemical properties of ceramic components are mainly determined by the composition and microstructure of ceramics that are largely dependent on the sintering processing. On the other hand, as for ceramic stereolithography, it is only considered as a ceramic net-near shaping step. The printed green ceramics are not finished parts since the stereolithography technology is similar to conventional ceramic injection molding and doctor-blade tape casting process which are binder rich ceramic forming technology. In the present paper, we demonstrates that surface modification of submicro- Si3N4 powder can be used to improve effectively the rheological behavior and the curing ability of Si3N4 ceramic slurry. Si3N4 slurry with high solid loading, suitable viscosity and high curing depth was synthesized successfully. Then, Si3N4 ceramic green parts with complex shapes were successfully fabricated by stereolithographybased AM. The slurry's rheological and photopolymerization properties, as well as the mechanical properties of the as-sintered Si3N4 parts were systemically investigated. In particular, the mechanical properties of the as-sintered Si3N4 ceramics with high sintering density were found to be comparable to the conventionally fabricated Si3N4 ceramics.
2.2. Preparation of Si3N4 ceramic slurry Raw Si3N4 powder was placed inside a drying oven at 75 oC for 12 h. The surface modified Si3N4 powder was prepared by mixing Si3N4 powder with surface modifier (KH560, 1 wt%) and experiencing highenergy ball milling for 6 h. Then, the mixture was subjected to second high-energy ball milling after the sintering additives (5 wt% Y2O3 and 3 wt% Al2O3 powders) were added into the surface modified Si3N4 powders. For the preparation of Si3N4 ceramic slurry, prepared Si3N4 powders with 5 wt% Y2O3 and 3 wt% Al2O3were further mixed with prepolymer liquid, which included prepolymer EA, diluent HDDA and initiators 819 and 184 with weight ratio of 65:33:1:1. They were simply prepared by stirring for 10 min at 1800 rpm in a vacuum mixer (HMV800, Shenzhen Hasi, China). The prepolymer liquid behaves as a Newtonian fluid with the viscosity of 0.8 Pa s-1. Finally, the dispersants (Darvan, THAS, Phosphonic ester) were added to the slurry that subsequently experienced another stirring for 5 min to obtain homogeneous photosensitive Si3N4 ceramic slurry.
2.3. Ceramics stereolithography The as-prepared photosensitive Si3N4 slurry was placed on a digital light processing (DLP) 3D printer with 20–50 μm resolution. The diagrammatic sketch of the DLP procedure is showed in Fig. 1. The principal of DLP 3D printer is based on the liquid photosensitive resin with UV radiation curing characteristics. When a single-layer was solidified, the work platform moved up so that the slurry was re-coated on the solidified surface. The same process was repeated until final printed ceramic green parts with the shape designed by PROE software were obtained. The layer thickness was set as 20 μm, 30 μm, and 50 μm to investigate symmetrically the effect of layer thickness on the photocuring properties of Si3N4 ceramic slurry, respectively. The exposure time was set as 15 s. The prepared green parts were washed with ethanol solution to remove the residual slurry.
2. Materials and methods 2.1. Raw materials Si3N4 powder with particle size of ~800 nm was used (purity > 99.6 wt%, Aladdin, China) as raw materials. Silane coupling agent KH560 (Nanjing Chemical, China) were used as surface modifiers of Si3N4 powder. Y2O3 and Al2O3 powders (Aladdin, China) were used as sintering additives. Both the purities and average particle sizes of Y2O3 and Al2O3 powders were 99.99 wt% and 200–300 nm, respectively. As for stereolithography, epoxy acrylate (EA) was used for the polymerization initiated by radical photo-initiators, such as hydroxycyclohexyl phenyl ketone (184) and 2,4,6-trimethylbenzoyl-
Fig. 1. The diagrammatic sketch of the DLP based additive manufacturing procedure. 2
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2.4. Debinding and sintering The cured sample was placed in a tube furnace on GSL-1100X-XX-S (Kejing, China) at a certain heating rate from room temperature to 500 °C to debind the binder. Then, the debound samples were sintered at 1750 °C for 2 h inside the sintering furnace (FPW12.5SP, FCT, Germany) with nitrogen atmosphere. The heating rate was set as 10 °C/ min. In order to study the effect of pressure during the sintering on the properties of as-sintered parts, pressureless sintering and pressure sintering with 5 MPa were carried out, respectively. 2.5. Materials characterization Element analysis of surface modified and raw Si3N4 powders was performed by X-ray photoelectron spectrometer (XPS) (Phi-5000 Versaprobe, Ulvca, USA). Functional groups of surface modified powder were analyzed by infrared spectrometer (Nicolet IS 50, Seymerfeld, USA). The rheometer was measured by a transformer (AR2000EX, TA, UK). Thermal analysis was performed on Dupont 2000 (Dupont, USA) in N2 with a heating rate of 0.5 °C/min. Crystal structure of as-sintered ceramics was observed by X-ray diffractometer using Cu Kα (wavelength 1.5418 Å) (Ultima IV, Rigaku, Japan). Density of as-sintered samples was measured using the Archimedes method. Morphology and microstructure of sintered ceramics were observed by field emission scanning electron microscope (Quanta 250 FEG, FEI, USA). Hardness and fracture toughness of the sample were tested by indentation using Vickers hardness tester (FV-800, FT, Japan) with a crosshead speed of 0.1 mm/min. At least five specimens were tested to obtain average values.
Fig. 3. Particle sizes and size distributions of Si3N4 powder: (a) as-received raw powder and (b) with KH560 surface modifier.
3. Results and discussion 3.1. Rheological properties of Si3N4 photopolymerizable slurry It is well known that the rheological properties of ceramic slurry could be influenced by particle size and its distribution, particle shape, the solid loading, range and magnitude of interparticle forces [23]. Several methods can be used to adjust the rheological properties of ceramic slurry. For example, the particle shape, particle size and size distribution may be adjusted by high-energy ball milling technology [24]. Additionally, the range and magnitude of interparticle forces can be tuned by surfactants and/or dispersants [25]. In the present study, the silane coupling agent KH560 modified Si3N4 ceramic slurry with different dispersants were prepared with the prepolymer liquid as dispersion medium. The content of silane coupling agent in the slurry are 1.0 wt% and the dispersants content is controlled in a range from 0.5 wt % to 2.5 wt%. Fig. 2 shows the morphology of raw and Si3N4 powder with KH560 surface modifier, respectively. The article sizes and size distributions of the raw Si3N4 powder and the powder with surface modifier are shown in Fig. 3. It can be observed that the raw Si3N4 powder is easily agglomerated with the particle size about 1.18 μm. Additionally, it is clear that KH560 surface modifier can effectively
Fig. 4. Infrared spectra of Si3N4 powder: (a) as-received and (b) with KH560 surface modifier.
reduce the particle size of the agglomerated Si3N4 powder from ~1.18 μm to ~750 nm. Fig. 4 shows the infrared absorption spectra for Si3N4 powders with and without KH560 surface modifier. As shown, although no obvious new peaks can be observed after surface modification, it is noticeable that the intensities of two characteristic peaks centered at ~1385 cm-1 and ~3428 cm-1 increase significantly, which can be attributed to the -OH and N–H absorption of the coupling agent KH560 and KH560 hydrolysis formation of -Si(OH)3 absorption, respectively [26]. Meanwhile, the dehydration condensation reaction between -Si(OH)3 on
Fig. 2. Morphology of Si3N4 powder: (a) as-received and (b) with KH560 surface modifier. 3
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Fig. 5. XPS full-scan spectra of Si3N4 powder: (a) as-received and (b) with KH560 surface modifier.
KH560 and active group -OH on Si3N4 surface may also take place to form new chemical bonds, such as ether bond -O- [27,28]. XPS spectra for the Si3N4 powder before and after the KH560 modification are shown in Fig. 5. It can be seen that the binding energy of N1s on the surface of Si3N4 powder decreases from 398.05 eV to 397.71 eV after surface modification, which is due to the conversion of -Si-NH- on the surface of Si3N4 to -Si-NH-CO- after the alkoxy group -OCH3 adsorbed on the surface [29]. As for the Si2p, the binding energy decreases from 102.38 eV to 101.75 eV after surface modification. It should be attributed to the Si–OH formation when submicron sized Si3N4 powder was placed in air before the surface modification. It is well known that oxygen is more electronegative, resulting in the increase of the binding energy of Si2p electron [30]. After surface modification by the KH560, -Si-OH- bonds were connected to the surface of Si3N4 powders through the chemical reaction to form the -Si-O-C- bonds that weaken the electronegativity of oxygen elements. The influences of different dispersants, such as phosphonic ester, TAHS, and Darvan on the rheological properties of the Si3N4 ceramic slurries with 10 vol% solid loading are shown in Fig. 6(a). As shown, the slurry with phosphonic ester and TAHS as dispersants exhibit more obvious shear thinning behavior than the slurry with the Darvan as dispersant. It is well known that shear thinning behavior is attributed to the perturbation of the slurry by shear. At low shear rates, the structure of the slurry is close to equilibrium due to the thermal motion dominating over the viscous forces. In contrast, at higher shear rates, shear thinning takes place due to the effect of the viscous forces on the structure of the slurry. At much higher shear rates, the viscous forces dominate and the plateau in the viscosity curve manifest the resistance to the flow of the slurry with a completely hydrodynamically controlled structure [31]. On the other hand, during the investigated range of the shear rate, the viscosity of the slurry with the Darvan as the dispersant is the lowest and does not show a strong dependence on the shear rate. In contrast, the viscosities of the slurries with phosphonic ester and TAHS as the dispersants are comparable, but they are much higher than the viscosities of the slurries with Darvan as the dispersant when the shear rate is lower than 10 s-1. It is well known that the dispersant is some kind of interfacial activators, which contains amphiphilic groups, i.e. hydrophilic and hydrophobic groups, which promotes the particles to disperse uniformly in the medium to form stable suspension. Clearly, Darvan could behave as better interfacial activator compared with the other two dispersant for Si3N4 ceramic slurry. In order to optimize Darvan content in the slurry, the rheological properties of HK560 modified Si3N4 ceramic slurry with Darvan contents in a range from 0.5 wt% to 2.5 wt% were measured. The results are shown in Fig. 6(b). It is obvious that all the slurries still exhibit shear thinning behavior. As shown, the viscosity of the ceramic slurry decreases from 0.75 Pa s-1 to 0.45 Pa s-1 with 100 s-1 shear rate when Darvan content increases from 0 wt% to 1.0 wt%. When Darvan content further increases to 2.5 wt%, the viscosity of the ceramic slurry increases to 0.65 Pa s-1. It is known that the dispersant molecules can be adsorbed on the surface of Si3N4 powder in the slurry. It is further
Fig. 6. (a) Rheological properties of KH560 modified Si3N4 ceramic slurry with different dispersant and (b) the dependence of rheological properties of Si3N4 ceramic slurry on Darvan content.
observed that within a certain content range, the adsorption capacity increases with increasing dispersant content increasing. And the stability effect of dispersant on the particles increases. This increases repulsion force between particles and decreases the viscosity of the Si3N4 ceramic slurry. It is stressed that when the viscosity of the slurry reaches the lowest value, the non-adsorbed dispersant content in the slurry is low. Therefore, its effect on the slurry viscosity is not obvious. However, when the dispersant content further increases, the dispersant itself is easy to form micelles in the solution, which results in the reduction of the dynamic stability of Si3N4 ceramic slurry. It is clear that there is an optimum dispersant content that can be effectively reduce the viscosity of Si3N4 ceramic slurry [32]. In the present investigation, the optimized Darvan content is determined to be 1.0 wt%. 4
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Fig. 7. (a) Rheological properties of Si3N4 ceramic slurry with Darvan as the dispersant with and without KH560 modifier and (b) dependence of the viscosity on solid loading for the slurries with or without KH560 modification at 10 s-1 shear rate.
Fig. 8. (a) Dependence of the curing depth on the exposure energy for ceramic slurries with different Si3N4 solid loading, 1 wt% KH560 and 1 wt% Darvan and (b) the curve of the curing depth (Cd) against the logarithm of the energy dose (LnE) for the Si3N4 slurry with 45 vol% solid loading.
It was reported by Chartier et al. [33] that high solid loading and fine particle size are necessary to prepare dense ceramics. However, the submicron sized Si3N4 particles tend to agglomerate easily due to –OH groups on the powder surface. This might result in low solid loading and agglomeration of particles in the prepolymer. The rheological behavior of ceramic slurry with Si3N4 powder with and without KH560 modifier is further shown in Fig. 7. The solid loading of Si3N4 and the Darvan content in the slurries are 30 vol% and 1.0 wt%, respectively. It is observed clearly in Fig. 7(a) that the viscosity of the slurry with unmodified Si3N4 powder is much higher than that of the modified one before the stable viscosity platform appears. When the shear rate is 100 s-1, the viscosity of the slurry with unmodified Si3N4 powder is 5.6 Pa s1 , which is 14 times higher than the viscosity of the slurry with surface modified Si3N4 (0.4 Pa s-1). These results indicate the importance of the use of surface modifier in reducing the viscosity of photopolymerizable ceramic slurry. Usually the silane coupling agent KH560 has two kinds of functional group in the molecular level, i.e., silicon alkoxy -Si(OCH3)3 and organic group –CH(O)CH–. Bifunctional groups could promote the Si3N4 dispersion in the prepolymer liquid because silicon alkoxy could contribute to the formation of ether bonds -O- between Si3N4 and KH560. The other organic group could connect the prepolymer liquid [34]. Additionally, surface modified Si3N4 powder might be helpful to reduce the difference of the refractive index (Δn) between the Si3N4 powder and the dispersion medium. Subsequently, this results in the reduction of the size of refraction coefficient Q and
the increase of the curing depth (Cd) [18]. Fig. 7(b) shows the dependence of the viscosity of the ceramic slurry on the solid loading of Si3N4 slurries with and without KH560 modifier at 10 s-1 shear rate. The maximum solid loading can be derived when the viscosity become infinite in accordance with the Krieger-Dougherty equation [35,36]. As shown in Fig. 7(b), maximum solid loading of the slurry with raw Si3N4 powder is 35 vol%. In contrast, it is significantly improved to be 45vol% for the slurry with KH560 surface modified Si3N4 using Darvan as dispersant. These results show that Si3N4 photopolymerizable slurry with high critical maximum solids volume fraction (45 vol%) and good rheological behavior can be prepared by combining submicro Si3N4 powder (~750 nm) modified by KH560 and Darvan. Therefore, Si3N4 slurries discussed in the following sections are limited to Si3N4 ceramic slurry with 45 vol% solid loading, in which 1 wt% KH560 and 1 wt% Darvan are used as surface modifier and dispersant, respectively. 3.2. Photopolymerization of the slurries We would like to mention that the factor that has to be considered is that Si3N4 photopolymerizable slurry has a low curing depth due to its high refractive index n = 2.4 of the gray-colored Si3N4 powder. During the photopolymerization process, photo-initiators which usually create free radicals and initiate the polymerization of the monomers are 5
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Fig. 9. Side view of green parts by stereolithography with slice thickness of (a) 20 μm, (b) 30 μm and (c) 50 μm and 10 s exposure time; (d)–(e) front view of Si3N4 green parts by stereolithography with 20 μm slice thickness of and 10 s exposure time. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
interface of the cross section of the blank begins to be blurred, the “texture” appears in the blank, and the upper part of the blank has an upward contraction. In contrast, when the thickness further increases to 40 μm, the interface of the cross section of the blank is blurred. In addition to serious “texture” phenomena, the upward contraction aggravates as well. It is believed it is due to low transmission depth of ultraviolet light in the slurry, only ~0.032 mm as derived above. When the slice thickness is greater than the transmission depth, ultraviolet light cannot penetrate the slurry. This results in only partial curing of the slurry and easy formation of the texture phenomena. Clearly, the smaller the layer thickness, the smaller the step generated on the solid surface, and the higher the surface precision and quality. It is stressed that during the formation of a new solidified layer in the photopolymerization, the warpage deformation could stem from the curing shrinkage and the laminar stress could also be generated by the solidified layer during photocuring. Therefore, in order to reduce the interlayer stress, the curing depth of the single layer should be reduced as thin as possible to lower the solidification volume. In the present case, Si3N4 green parts with complex shape have been printed successfully by optimizing the slice thickness and exposure time as 20 μm and 10s-1, as shown in Fig. 9(d)–(e).
activated in the irradiated regions. The depth of the exposed feature usually is determined by energy dose and photo-properties of Si3N4 ceramic photopolymerizable slurry. Therefore, it is important to identify the dependence of the curing depth on the energy dose for Si3N4 ceramic slurry. The Beer-Lambert formula below describes the relationship between the curing depth Cd and the applied energy dose E [37–39]: Cd = DpLnE - DpLnEc
(1)
Where Cd is the curing depth, E is the applied energy dose, Dp and Ec are the attenuation length and the critical energy dose to reach the gel point for the slurry, respectively. Fig. 8(a) shows the curing depth Cd as a function of the applied energy dose lnE for the photopolymerizable slurries with different solid loading and 1 wt% KH560 and 1 wt% Darvan. As expected, the curing depth gradually increases as the solid loading decreases and the applied energy dose increases. However, it is clear that Dp and Ec have a strong dependence on the component and content of the Si3N4 ceramic slurry. Especially, the attenuation length (Dp) and critical energy dose (LnEc) of Si3N4 ceramic slurry can be derived from the slope and intercept of the curve according to the BeerLambert formula, respectively. Fig. 8 (b) shows the plot of Cd against LnE for the Si3N4 ceramic slurry with 45 vol% solid loading. The attenuation length (Dp) and critical energy dose (LnEc) are derived as 0.032 mm and 0.177 mJ/mm2, respectively. Fig. 9(a)-(c) shows the optical microscopy images of the side view of the photopolymerized green parts by stereolithography with different slice thickness and 10s-1 exposure time. As shown in Fig. 9(a), when the layer thickness of the photopolymerized slice is 20 μm, the interface between the layers in the green parts can be observed clearly. However, no large pores, cracks and texture structure are detected obviously in 3D printed Si3N4 green parts. When the slice thickness is 30 μm, the
3.3. Debinding and sintering of Si3N4 green parts After the photopolymerization, as-printed ceramic green parts are still binder-rich which have to be debound to remove the binder before sintering. However, thermal debinding of the binders has to be controlled carefully since the explosive gaseous products can be generated due to the decomposition of the polymer binder, which can induce defects inside the green parts. The thermogravimetric/differential thermal analysis curves of the photocured green parts prepared by the 6
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curing is high. However, it is possible that the decomposition of polymerized macromolecules might take place in this stage, but it is believed that the decomposition products still remain in the green parts. In contrast, in the BC stage (300–450 °C), the weight loss become obvious. This might be attributed to the evaporation of small molecules that are the decomposition products of the polymerized macromolecules. As expected, in the CD stage (> 450 °C), the weight loss becomes stable, indicating most of organic polymers in the green parts have been decomposed and evaporate from the green parts. These results are in a good consistent with previous reports [40–42]. Based on the results of the thermal analysis, the debinding temperature was set above 450 °C in the present investigation. In order to avoid the introduction of defects inside the green parts, the heating steps during the thermal debinding process have been designed carefully. A reasonable heating curve of the debinding processing is shown in Fig. 10(b). As shown, the heating rate is 0.5 °C/min from room temperature to 150 °C, then the heating rate is controlled at 0.2 °C/min from 150 °C to 350 °C, with two temperature holding platforms at 250 °C and 350 °C for 2 h, respectively. Based on TG/DSC curves, 2 h holding time is long enough to make sure that the small molecules and the decomposed products of polymerized macromolecules can escape from the green parts and no defects are induced inside the green parts. Then, a heating rate of 0.5 °C/min is controlled between 350 °C and 500 °C. When the debinding temperature reaches 500 °C, it holds for 2 h. Fig. 11(a)-(d) shows the photographs of debound green parts with different heating rate from the temperature range of 150 oC–350 oC. As shown, there are no visible warpage and collapse defects in the debound parts with the heating rate of 0.2 °C/ min. As the heating rate increases, macro- and micro-defects become obvious. It is clear in order to minimize the induced defects during the thermal debinding, heating rate must be controlled very carefully. A critical step to achieve the desirable mechanical properties of Si3N4 ceramics could happen in the sintering and post-sintering processing after the debound green part without induced defects has been obtained. However, we would like to mention that detailed relationship between the structure and properties of the Si3N4 ceramics prepared by stereolithography will be reported elsewhere. In contrast, more attention has been paid to the preparation of Si3N4 ceramic slurry with high solid loading, suitable viscosity and high curing depth in the present investigation. The stereolithographical and debound Si3N4 ceramics were sintered at 1750 °C for 2 h in nitrogen with 5 MP applied gas pressure and 10 °C/min heating rate from room temperature to 1750 oC. The pressureless sintering was also carried out for the samples with the same sintering temperature, holding time, and heating rate. Fig. 12 shows the XRD patterns of the as-sintered Si3N4 ceramics prepared by stereolithography and followed by debinding. As shown, main crystal phase of as-sintered Si3N4 ceramics by both pressureless
Fig. 10. (a) Thermogravimetry curves of as-printed green part by stereolithography using slurry with 45 vol% Si3N4 solid loading, 1 wt% KH560 and 1 wt% Darvan and (b) heating curve for the debinding of as-printed Si3N4 green parts. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
Si3N4 slurry with 45 vol% Si3N4 loading, 1 wt% KH560 and 1 wt% Darvan are shown in Fig. 10(a). From the DSC curve, two endothermic peaks centered at ~165 °C and ~365 °C can be observed. Combined with the TG curve, the initial weight loss of the sample in the stage of AB (100–300 °C) may mainly result from the evaporation of small organic molecules, which contain surface modifier KH560, dispersant Darvan and unpolymerized prepolymer. It can be observed that the loss percentage in this stage is only ~10 wt%, indicating the degree of light
Fig. 11. Photographs of the debound green parts with (a) 0.2 °C/min, (b) 1 °C/min, (c) 2 °C/min, and (d) 5 °C/min heating rate for 150 °C–350 °C. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.) 7
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pores still exist in the sample and the sintering density is only ~2.95 g/ cm3. In the present investigation, the 3.45 g/cm3 density was used for the evaluation of the relative sintering density. The microhardness and fracture toughness of the sample are measured as 13.43 ± 0.11 GPa and 5.75 ± 0.16 MPa m1/2, respectively. In contrast, Fig. 13(b) displays the SEM images of as-sintered Si3N4 ceramics by pressure sintering with 5 MP applied nitrogen pressure. It is observed that the morphology of as-sintered Si3N4 ceramics is also rod-shaped, but the pore amount and size decrease significantly. The sintering density is improved to ~3.28 g/cm3 (~95% relative density), much higher than that of as-sintered Si3N4 ceramics by pressureless sintering (~2.95 g/ cm3). The microhardness and fracture toughness of the sample are measured as 14.63 ± 0.45 GPa and 5.82 ± 0.42 MPa m1/2, respectively, which are comparable to the reported microhardness (16.93 ± 0.25 GPa) and fracture toughness (6.85 ± 0.26 MPa m1/2) values of as-sintered Si3N4 ceramics prepared by conventional ceramics fabrication technology [21]. It is well-known that the achievable mechanical properties of Si3N4 ceramics can critically depend on their microstructure that is intrinsically related to materials design, sintering processing and sintering additives as well [1]. Therefore, we believe that the mechanical properties of as-sintered Si3N4 ceramics fabricated by stereolithography-based AM method can be further improved by optimizing the debinding and high-temperature sintering process.
Fig. 12. XRD patterns of as-sintered Si3N4 ceramics prepared by pressureless sintering and gas pressure sintering with 5 MPa applied pressure.
4. Conclusions Si3N4 ceramic slurry with 45 vol% Si3N4 loading and 50 μm curing depth was prepared successfully using KH560 and Darvan as the surface modifier and the dispersant, respectively. Dp and LnEc values of Si3N4 ceramic slurry were derived as 0.032 mm and 0.177 mJ/mm2 based on the Beer-Lambert formula, respectively. By optimizing the exposure time and energy dose, Si3N4 ceramic greens part with complex shape and high surface precision were successfully printed by stereolithography-based AM. The microhardness and fracture toughness of the as-sintered dense Si3N4 ceramics (~3.28 g/cm3) are 14.63 ± 0.45 GPa and 5.82 ± 0.42 MPa m1/2, respectively, which are comparable to the reported values of as-sintered Si3N4 ceramics prepared by the traditional ceramics fabrication technology. The stereolithography based ceramic AM would be a promising technology to fabricate gray-colored dense Si3N4 ceramics with complex shape, high surface precision and good mechanical properties. Acknowledgement This work is funded by the Shenzhen Science and Technology Innovation Commission Technology Research Project (Grant Nos 20170410221235842), State Key Laboratory for Powder Metallurgy Foundation (Grant Nos 621021826) and Graduate innovation program of Central South University (2017zzts102). The use of facilities in the Institute for Materials Microstructure and the State Key Laboratory for Powder Metallurgy at Central South University is acknowledged. References [1] A. Zocca, P. Colombo, C.M. Gones, J. Gunster, Additive manufacturing of ceramic: issue, potentialities, and opportunities, J. Am. Ceram. Soc. 98 (2015) 1983–2001. [2] O. Dufaud, P. Marchal, S. Corbel, Rheological properties of PZT suspensions for stereolithography, J. Eur. Ceram. Soc. 22 (2002) 2081–2092. [3] C. Hinczewski, S. Corbel, T. Chartier, Ceramic suspensions suitable for stereolithography, J. Eur. Ceram. Soc. 18 (1998) 583–590. [4] A. Badev, Y. Abouliatim, T. Chartier, L. Lecamp, P. Lebaudy, C. Chaput, C. Delage, Photopolymerization kinetics of a polyether acrylate in the presence of ceramic fillers used in stereolithography, J. Photochem. Photobiol., A 222 (2011) 117–122. [5] J.H. Jang, S. Wang, S.M. Pilgrim, W.A. Schulze, Preparation and characterization of barium titanate suspension for stereolithography, J. Am. Ceram. Soc. 83 (2000) 1804–1807. [6] S. Zhang, N. Sha, Z. Zhao, Surface modification of α-Al2O3 with dicarboxylic acids for the preparation of UV-curable ceramic suspensions, J. Eur. Ceram. Soc. 37 (2017) 1607-616.
Fig. 13. SEM images of as-sintered Si3N4 ceramics at 1750 °C for 2 h in nitrogen and with 10 °C/min heating rate from room temperature to 1750 °C by (a) pressureless sintering and (b) gas pressure sintering with 5 MPa applied pressure.
and pressure sintering is β-Si3N4. Fig. 13(a) exhibits the SEM images of as-sintered Si3N4 ceramics by pressureless sintering. As shown, the morphology of as-sintered Si3N4 ceramics is rod-shaped β-Si3N4, indicating typical liquid sintering took place during the sintering of Si3N4 ceramics with sintering additives [1]. However, a certain amount of 8
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Stereolithographical fabrication of dense Si3N4 ceramics by slurry optimization and pressure sintering Yao Liua,b,c, Lina Zhanc, Yu Hec, Jun Zhangc, Jianjun Huc, Lijin Chengd, Qiumei Wua,b,∗, Shaojun Liua,b,∗∗ a
Shenzhen Research Institute, Central South University, Shenzhen, 510085, China State Key Laboratory for Powder Metallurgy, Central South University, Changsha, 410083, China c College of Mechanical and Electronic Engineering, Pingxiang College, Pingxiang, 337000, China d State Key Laboratory for Material Processing and Die & Mould Technology, School of Material Science and Engineering, Huazhong University of Science and Technology, Wuhan, 430074, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Stereolithography Silicon nitride Surface modification Rheological behavior
Photocurable gray-colored Si3N4 ceramic slurry with high solid loading, suitable viscosity and high curing depth is critical to fabricate dense ceramic parts with complex shape and high surface precision by stereolithography technology. In the present study, Si3N4 ceramic slurry with suitable viscosity, high solid loading (45 vol %) and curing depth of 50 μm was prepared successfully when surface modifier KH560 (1 wt%) and dispersant Darvan (1 wt%) were used. The slurry exhibits the shear thinning behavior. Based on the Beer-Lambert formula, Dp (the attenuation length) and Ec (the critical energy dose) of Si3N4 ceramic slurry with solid loading of 45 vol % were derived as 0.032 mm and 0.177 mJ/mm2, respectively. Si3N4 ceramic green parts with complex shape and high surface precision were successfully fabricated by stereolithography technology. After optimizing the debinding and sintering process for green parts, dense Si3N4 ceramics with 3.28 g/cm3 sintering density were fabricated. The microhardness and fracture toughness of as-sintered Si3N4 ceramics are ~14.63 GPa and ~5.82 MPa m1/2, respectively, which are comparable to those of the samples by traditional dry-pressed and pressureless sintering technology. These results show that ceramic stereolithography technology could be promising to fabricate high performance ceramics, especially for gray-colored monolithic Si3N4 ceramics.
1. Introduction
viscosities of the slurries and the precision of photocurable ceramic parts. Jang et al. [5] prepared high solid loading and low viscosity ferroelectric photocurable slurry by mixing barium titanate powder with hexanediol diacrylate (HDDA). Zhang et al. [6] modified Al2O3 powder with dibasic acid, and analyzed the adsorption of dicarboxylic acid on Al2O3 powder by high performance liquid chromatography and concluded that the viscosity and yield stress of Al2O3 slurry are the lowest when the dose of dicarboxylic acid reaches its maximum adsorption capacity. Re et al. [7] prepared the green parts after optimizing the photocuring and sintering parameters. When sintering temperature was set as 1400 °C, dense ZrO2 ceramic rods (~97 wt %) was prepared. The hardness and fracture toughness of as-sintered ZrO2 ceramics are ~13 GPa and ~6 MPa, respectively. However, reports on AM technologies that can generate dense monolithic ceramic bodies have been still very few [8,9]. Among them, ceramic stereolithography and related additive manufacture involving
Additive manufacturing (AM) technologies have been developed to become well-known technologies to fabricate polymeric or metallic prototypes or models [1]. However, it is still a big challenge to process monolithic ceramics with superior physico-chemical properties due to the introduction of feedstock, debinding and sintering processing. Dufaud et al. [2] studied systematically the photocurable Pb(Ti, Zr)O3 (PZT) piezoelectric ceramic slurry by identifying the effects of different kinds of dispersants, surfactants and temperatures on the rheological properties of PZT slurries, and prepared ceramic slurry with 47 vol % solid loading. Hinczewski et al. [3] mixed Al2O3 particles with monomer and analyzed the effects of monomers, diluents, dispersants, photo-initiators for the polymerization on the rheological properties of the slurries systematically. Badev et al. [4] showed that the particle sizes of SiO2 and Al2O3 powders are the main factors that affect the
∗
Corresponding author. State Key Laboratory for Powder Metallurgy, Central South University, Changsha, 410083, China. Corresponding author. Shenzhen Research Institute, Central South University, Shenzhen, 510085, China. E-mail addresses: [email protected] (Q. Wu), [email protected] (S. Liu).
∗∗
https://doi.org/10.1016/j.ceramint.2019.09.186 Received 5 August 2019; Received in revised form 18 September 2019; Accepted 19 September 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Yao Liu, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.09.186
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phosphine oxide (819). 1, 6-hexanediol diacrylate (HDDA) were used as diluent to reduce the viscosity of Si3N4 slurry. EA, HDDA, 184, 819 were purchased from BASF Shanghai, China. Polymethyl methacrylate (Darvan, molecular weight: MW = 520), tetramethylammonium hydroxide (TAHS, MW = 91.05), stearic acid (Phosphonic ester, MW = 284.48) (Nanjing Chemical, China) were used as dispersants.
photopolymerization of ceramic slurries have been a hot topic due to their high manufacturing precision, good surface quality and ability to prepare complex geometric parts [10–12]. To meet these requirements, photocurable slurries should present the following characteristics: 1) suitable viscosity that ensures uniform and flat coating of ceramic slurry on a previous layer; 2) sufficient curing depth to ensure excessive curing of the interface in the two cured layers that provides good cohesion; 3) a sufficiently high solid content to generate a low shrinkage rate. Especially, although photocuring molding technology to fabricate ceramics has been widely carried out [13–15], few researches have been addressed as referred to the gray-colored ceramics with high refraction index, for example SiC and Si3N4 ceramics [16]. Deng et.al [8] fabricated gray-colored SiC ceramic green parts with complex shapes successfully by stereolithography-based AM. Further investigation shows that the curing ability of SiC slurries is influenced profoundly by particle size, solid loading, stereolithography parameters, as well as photo-initiator and its concentration. However, no further investigation was addressed on the photocuring properties of nitrides, such as Si3N4, which might be partially contributed by high refractive index n = 2.4 of Si3N4 powder [17,18], resulting in a low curing depth, further making them difficult to curing [19]. It should be stressed that the reports on structure and properties of dense as-sintered SiC and Si3N4 ceramics by stereolithography based additive manufacturing technology still lack. It is known that Si3N4 ceramics is considered as one of the most promising advanced ceramics to fabricate components to be used at elevated temperatures or under harsh environments due to its hardness, heat resistance, wear resistance, good thermal shock resistance, as well as high temperature creep resistance and self-lubricating properties [20–22]. However, how to process Si3N4 ceramic components with high precision and complex geometries is a major concern in AM technology due to high refractive index that makes it difficult to cure. Additionally, the physicochemical properties of ceramic components are mainly determined by the composition and microstructure of ceramics that are largely dependent on the sintering processing. On the other hand, as for ceramic stereolithography, it is only considered as a ceramic net-near shaping step. The printed green ceramics are not finished parts since the stereolithography technology is similar to conventional ceramic injection molding and doctor-blade tape casting process which are binder rich ceramic forming technology. In the present paper, we demonstrates that surface modification of submicro- Si3N4 powder can be used to improve effectively the rheological behavior and the curing ability of Si3N4 ceramic slurry. Si3N4 slurry with high solid loading, suitable viscosity and high curing depth was synthesized successfully. Then, Si3N4 ceramic green parts with complex shapes were successfully fabricated by stereolithographybased AM. The slurry's rheological and photopolymerization properties, as well as the mechanical properties of the as-sintered Si3N4 parts were systemically investigated. In particular, the mechanical properties of the as-sintered Si3N4 ceramics with high sintering density were found to be comparable to the conventionally fabricated Si3N4 ceramics.
2.2. Preparation of Si3N4 ceramic slurry Raw Si3N4 powder was placed inside a drying oven at 75 oC for 12 h. The surface modified Si3N4 powder was prepared by mixing Si3N4 powder with surface modifier (KH560, 1 wt%) and experiencing highenergy ball milling for 6 h. Then, the mixture was subjected to second high-energy ball milling after the sintering additives (5 wt% Y2O3 and 3 wt% Al2O3 powders) were added into the surface modified Si3N4 powders. For the preparation of Si3N4 ceramic slurry, prepared Si3N4 powders with 5 wt% Y2O3 and 3 wt% Al2O3were further mixed with prepolymer liquid, which included prepolymer EA, diluent HDDA and initiators 819 and 184 with weight ratio of 65:33:1:1. They were simply prepared by stirring for 10 min at 1800 rpm in a vacuum mixer (HMV800, Shenzhen Hasi, China). The prepolymer liquid behaves as a Newtonian fluid with the viscosity of 0.8 Pa s-1. Finally, the dispersants (Darvan, THAS, Phosphonic ester) were added to the slurry that subsequently experienced another stirring for 5 min to obtain homogeneous photosensitive Si3N4 ceramic slurry.
2.3. Ceramics stereolithography The as-prepared photosensitive Si3N4 slurry was placed on a digital light processing (DLP) 3D printer with 20–50 μm resolution. The diagrammatic sketch of the DLP procedure is showed in Fig. 1. The principal of DLP 3D printer is based on the liquid photosensitive resin with UV radiation curing characteristics. When a single-layer was solidified, the work platform moved up so that the slurry was re-coated on the solidified surface. The same process was repeated until final printed ceramic green parts with the shape designed by PROE software were obtained. The layer thickness was set as 20 μm, 30 μm, and 50 μm to investigate symmetrically the effect of layer thickness on the photocuring properties of Si3N4 ceramic slurry, respectively. The exposure time was set as 15 s. The prepared green parts were washed with ethanol solution to remove the residual slurry.
2. Materials and methods 2.1. Raw materials Si3N4 powder with particle size of ~800 nm was used (purity > 99.6 wt%, Aladdin, China) as raw materials. Silane coupling agent KH560 (Nanjing Chemical, China) were used as surface modifiers of Si3N4 powder. Y2O3 and Al2O3 powders (Aladdin, China) were used as sintering additives. Both the purities and average particle sizes of Y2O3 and Al2O3 powders were 99.99 wt% and 200–300 nm, respectively. As for stereolithography, epoxy acrylate (EA) was used for the polymerization initiated by radical photo-initiators, such as hydroxycyclohexyl phenyl ketone (184) and 2,4,6-trimethylbenzoyl-
Fig. 1. The diagrammatic sketch of the DLP based additive manufacturing procedure. 2
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2.4. Debinding and sintering The cured sample was placed in a tube furnace on GSL-1100X-XX-S (Kejing, China) at a certain heating rate from room temperature to 500 °C to debind the binder. Then, the debound samples were sintered at 1750 °C for 2 h inside the sintering furnace (FPW12.5SP, FCT, Germany) with nitrogen atmosphere. The heating rate was set as 10 °C/ min. In order to study the effect of pressure during the sintering on the properties of as-sintered parts, pressureless sintering and pressure sintering with 5 MPa were carried out, respectively. 2.5. Materials characterization Element analysis of surface modified and raw Si3N4 powders was performed by X-ray photoelectron spectrometer (XPS) (Phi-5000 Versaprobe, Ulvca, USA). Functional groups of surface modified powder were analyzed by infrared spectrometer (Nicolet IS 50, Seymerfeld, USA). The rheometer was measured by a transformer (AR2000EX, TA, UK). Thermal analysis was performed on Dupont 2000 (Dupont, USA) in N2 with a heating rate of 0.5 °C/min. Crystal structure of as-sintered ceramics was observed by X-ray diffractometer using Cu Kα (wavelength 1.5418 Å) (Ultima IV, Rigaku, Japan). Density of as-sintered samples was measured using the Archimedes method. Morphology and microstructure of sintered ceramics were observed by field emission scanning electron microscope (Quanta 250 FEG, FEI, USA). Hardness and fracture toughness of the sample were tested by indentation using Vickers hardness tester (FV-800, FT, Japan) with a crosshead speed of 0.1 mm/min. At least five specimens were tested to obtain average values.
Fig. 3. Particle sizes and size distributions of Si3N4 powder: (a) as-received raw powder and (b) with KH560 surface modifier.
3. Results and discussion 3.1. Rheological properties of Si3N4 photopolymerizable slurry It is well known that the rheological properties of ceramic slurry could be influenced by particle size and its distribution, particle shape, the solid loading, range and magnitude of interparticle forces [23]. Several methods can be used to adjust the rheological properties of ceramic slurry. For example, the particle shape, particle size and size distribution may be adjusted by high-energy ball milling technology [24]. Additionally, the range and magnitude of interparticle forces can be tuned by surfactants and/or dispersants [25]. In the present study, the silane coupling agent KH560 modified Si3N4 ceramic slurry with different dispersants were prepared with the prepolymer liquid as dispersion medium. The content of silane coupling agent in the slurry are 1.0 wt% and the dispersants content is controlled in a range from 0.5 wt % to 2.5 wt%. Fig. 2 shows the morphology of raw and Si3N4 powder with KH560 surface modifier, respectively. The article sizes and size distributions of the raw Si3N4 powder and the powder with surface modifier are shown in Fig. 3. It can be observed that the raw Si3N4 powder is easily agglomerated with the particle size about 1.18 μm. Additionally, it is clear that KH560 surface modifier can effectively
Fig. 4. Infrared spectra of Si3N4 powder: (a) as-received and (b) with KH560 surface modifier.
reduce the particle size of the agglomerated Si3N4 powder from ~1.18 μm to ~750 nm. Fig. 4 shows the infrared absorption spectra for Si3N4 powders with and without KH560 surface modifier. As shown, although no obvious new peaks can be observed after surface modification, it is noticeable that the intensities of two characteristic peaks centered at ~1385 cm-1 and ~3428 cm-1 increase significantly, which can be attributed to the -OH and N–H absorption of the coupling agent KH560 and KH560 hydrolysis formation of -Si(OH)3 absorption, respectively [26]. Meanwhile, the dehydration condensation reaction between -Si(OH)3 on
Fig. 2. Morphology of Si3N4 powder: (a) as-received and (b) with KH560 surface modifier. 3
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Fig. 5. XPS full-scan spectra of Si3N4 powder: (a) as-received and (b) with KH560 surface modifier.
KH560 and active group -OH on Si3N4 surface may also take place to form new chemical bonds, such as ether bond -O- [27,28]. XPS spectra for the Si3N4 powder before and after the KH560 modification are shown in Fig. 5. It can be seen that the binding energy of N1s on the surface of Si3N4 powder decreases from 398.05 eV to 397.71 eV after surface modification, which is due to the conversion of -Si-NH- on the surface of Si3N4 to -Si-NH-CO- after the alkoxy group -OCH3 adsorbed on the surface [29]. As for the Si2p, the binding energy decreases from 102.38 eV to 101.75 eV after surface modification. It should be attributed to the Si–OH formation when submicron sized Si3N4 powder was placed in air before the surface modification. It is well known that oxygen is more electronegative, resulting in the increase of the binding energy of Si2p electron [30]. After surface modification by the KH560, -Si-OH- bonds were connected to the surface of Si3N4 powders through the chemical reaction to form the -Si-O-C- bonds that weaken the electronegativity of oxygen elements. The influences of different dispersants, such as phosphonic ester, TAHS, and Darvan on the rheological properties of the Si3N4 ceramic slurries with 10 vol% solid loading are shown in Fig. 6(a). As shown, the slurry with phosphonic ester and TAHS as dispersants exhibit more obvious shear thinning behavior than the slurry with the Darvan as dispersant. It is well known that shear thinning behavior is attributed to the perturbation of the slurry by shear. At low shear rates, the structure of the slurry is close to equilibrium due to the thermal motion dominating over the viscous forces. In contrast, at higher shear rates, shear thinning takes place due to the effect of the viscous forces on the structure of the slurry. At much higher shear rates, the viscous forces dominate and the plateau in the viscosity curve manifest the resistance to the flow of the slurry with a completely hydrodynamically controlled structure [31]. On the other hand, during the investigated range of the shear rate, the viscosity of the slurry with the Darvan as the dispersant is the lowest and does not show a strong dependence on the shear rate. In contrast, the viscosities of the slurries with phosphonic ester and TAHS as the dispersants are comparable, but they are much higher than the viscosities of the slurries with Darvan as the dispersant when the shear rate is lower than 10 s-1. It is well known that the dispersant is some kind of interfacial activators, which contains amphiphilic groups, i.e. hydrophilic and hydrophobic groups, which promotes the particles to disperse uniformly in the medium to form stable suspension. Clearly, Darvan could behave as better interfacial activator compared with the other two dispersant for Si3N4 ceramic slurry. In order to optimize Darvan content in the slurry, the rheological properties of HK560 modified Si3N4 ceramic slurry with Darvan contents in a range from 0.5 wt% to 2.5 wt% were measured. The results are shown in Fig. 6(b). It is obvious that all the slurries still exhibit shear thinning behavior. As shown, the viscosity of the ceramic slurry decreases from 0.75 Pa s-1 to 0.45 Pa s-1 with 100 s-1 shear rate when Darvan content increases from 0 wt% to 1.0 wt%. When Darvan content further increases to 2.5 wt%, the viscosity of the ceramic slurry increases to 0.65 Pa s-1. It is known that the dispersant molecules can be adsorbed on the surface of Si3N4 powder in the slurry. It is further
Fig. 6. (a) Rheological properties of KH560 modified Si3N4 ceramic slurry with different dispersant and (b) the dependence of rheological properties of Si3N4 ceramic slurry on Darvan content.
observed that within a certain content range, the adsorption capacity increases with increasing dispersant content increasing. And the stability effect of dispersant on the particles increases. This increases repulsion force between particles and decreases the viscosity of the Si3N4 ceramic slurry. It is stressed that when the viscosity of the slurry reaches the lowest value, the non-adsorbed dispersant content in the slurry is low. Therefore, its effect on the slurry viscosity is not obvious. However, when the dispersant content further increases, the dispersant itself is easy to form micelles in the solution, which results in the reduction of the dynamic stability of Si3N4 ceramic slurry. It is clear that there is an optimum dispersant content that can be effectively reduce the viscosity of Si3N4 ceramic slurry [32]. In the present investigation, the optimized Darvan content is determined to be 1.0 wt%. 4
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Fig. 7. (a) Rheological properties of Si3N4 ceramic slurry with Darvan as the dispersant with and without KH560 modifier and (b) dependence of the viscosity on solid loading for the slurries with or without KH560 modification at 10 s-1 shear rate.
Fig. 8. (a) Dependence of the curing depth on the exposure energy for ceramic slurries with different Si3N4 solid loading, 1 wt% KH560 and 1 wt% Darvan and (b) the curve of the curing depth (Cd) against the logarithm of the energy dose (LnE) for the Si3N4 slurry with 45 vol% solid loading.
It was reported by Chartier et al. [33] that high solid loading and fine particle size are necessary to prepare dense ceramics. However, the submicron sized Si3N4 particles tend to agglomerate easily due to –OH groups on the powder surface. This might result in low solid loading and agglomeration of particles in the prepolymer. The rheological behavior of ceramic slurry with Si3N4 powder with and without KH560 modifier is further shown in Fig. 7. The solid loading of Si3N4 and the Darvan content in the slurries are 30 vol% and 1.0 wt%, respectively. It is observed clearly in Fig. 7(a) that the viscosity of the slurry with unmodified Si3N4 powder is much higher than that of the modified one before the stable viscosity platform appears. When the shear rate is 100 s-1, the viscosity of the slurry with unmodified Si3N4 powder is 5.6 Pa s1 , which is 14 times higher than the viscosity of the slurry with surface modified Si3N4 (0.4 Pa s-1). These results indicate the importance of the use of surface modifier in reducing the viscosity of photopolymerizable ceramic slurry. Usually the silane coupling agent KH560 has two kinds of functional group in the molecular level, i.e., silicon alkoxy -Si(OCH3)3 and organic group –CH(O)CH–. Bifunctional groups could promote the Si3N4 dispersion in the prepolymer liquid because silicon alkoxy could contribute to the formation of ether bonds -O- between Si3N4 and KH560. The other organic group could connect the prepolymer liquid [34]. Additionally, surface modified Si3N4 powder might be helpful to reduce the difference of the refractive index (Δn) between the Si3N4 powder and the dispersion medium. Subsequently, this results in the reduction of the size of refraction coefficient Q and
the increase of the curing depth (Cd) [18]. Fig. 7(b) shows the dependence of the viscosity of the ceramic slurry on the solid loading of Si3N4 slurries with and without KH560 modifier at 10 s-1 shear rate. The maximum solid loading can be derived when the viscosity become infinite in accordance with the Krieger-Dougherty equation [35,36]. As shown in Fig. 7(b), maximum solid loading of the slurry with raw Si3N4 powder is 35 vol%. In contrast, it is significantly improved to be 45vol% for the slurry with KH560 surface modified Si3N4 using Darvan as dispersant. These results show that Si3N4 photopolymerizable slurry with high critical maximum solids volume fraction (45 vol%) and good rheological behavior can be prepared by combining submicro Si3N4 powder (~750 nm) modified by KH560 and Darvan. Therefore, Si3N4 slurries discussed in the following sections are limited to Si3N4 ceramic slurry with 45 vol% solid loading, in which 1 wt% KH560 and 1 wt% Darvan are used as surface modifier and dispersant, respectively. 3.2. Photopolymerization of the slurries We would like to mention that the factor that has to be considered is that Si3N4 photopolymerizable slurry has a low curing depth due to its high refractive index n = 2.4 of the gray-colored Si3N4 powder. During the photopolymerization process, photo-initiators which usually create free radicals and initiate the polymerization of the monomers are 5
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Fig. 9. Side view of green parts by stereolithography with slice thickness of (a) 20 μm, (b) 30 μm and (c) 50 μm and 10 s exposure time; (d)–(e) front view of Si3N4 green parts by stereolithography with 20 μm slice thickness of and 10 s exposure time. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
interface of the cross section of the blank begins to be blurred, the “texture” appears in the blank, and the upper part of the blank has an upward contraction. In contrast, when the thickness further increases to 40 μm, the interface of the cross section of the blank is blurred. In addition to serious “texture” phenomena, the upward contraction aggravates as well. It is believed it is due to low transmission depth of ultraviolet light in the slurry, only ~0.032 mm as derived above. When the slice thickness is greater than the transmission depth, ultraviolet light cannot penetrate the slurry. This results in only partial curing of the slurry and easy formation of the texture phenomena. Clearly, the smaller the layer thickness, the smaller the step generated on the solid surface, and the higher the surface precision and quality. It is stressed that during the formation of a new solidified layer in the photopolymerization, the warpage deformation could stem from the curing shrinkage and the laminar stress could also be generated by the solidified layer during photocuring. Therefore, in order to reduce the interlayer stress, the curing depth of the single layer should be reduced as thin as possible to lower the solidification volume. In the present case, Si3N4 green parts with complex shape have been printed successfully by optimizing the slice thickness and exposure time as 20 μm and 10s-1, as shown in Fig. 9(d)–(e).
activated in the irradiated regions. The depth of the exposed feature usually is determined by energy dose and photo-properties of Si3N4 ceramic photopolymerizable slurry. Therefore, it is important to identify the dependence of the curing depth on the energy dose for Si3N4 ceramic slurry. The Beer-Lambert formula below describes the relationship between the curing depth Cd and the applied energy dose E [37–39]: Cd = DpLnE - DpLnEc
(1)
Where Cd is the curing depth, E is the applied energy dose, Dp and Ec are the attenuation length and the critical energy dose to reach the gel point for the slurry, respectively. Fig. 8(a) shows the curing depth Cd as a function of the applied energy dose lnE for the photopolymerizable slurries with different solid loading and 1 wt% KH560 and 1 wt% Darvan. As expected, the curing depth gradually increases as the solid loading decreases and the applied energy dose increases. However, it is clear that Dp and Ec have a strong dependence on the component and content of the Si3N4 ceramic slurry. Especially, the attenuation length (Dp) and critical energy dose (LnEc) of Si3N4 ceramic slurry can be derived from the slope and intercept of the curve according to the BeerLambert formula, respectively. Fig. 8 (b) shows the plot of Cd against LnE for the Si3N4 ceramic slurry with 45 vol% solid loading. The attenuation length (Dp) and critical energy dose (LnEc) are derived as 0.032 mm and 0.177 mJ/mm2, respectively. Fig. 9(a)-(c) shows the optical microscopy images of the side view of the photopolymerized green parts by stereolithography with different slice thickness and 10s-1 exposure time. As shown in Fig. 9(a), when the layer thickness of the photopolymerized slice is 20 μm, the interface between the layers in the green parts can be observed clearly. However, no large pores, cracks and texture structure are detected obviously in 3D printed Si3N4 green parts. When the slice thickness is 30 μm, the
3.3. Debinding and sintering of Si3N4 green parts After the photopolymerization, as-printed ceramic green parts are still binder-rich which have to be debound to remove the binder before sintering. However, thermal debinding of the binders has to be controlled carefully since the explosive gaseous products can be generated due to the decomposition of the polymer binder, which can induce defects inside the green parts. The thermogravimetric/differential thermal analysis curves of the photocured green parts prepared by the 6
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curing is high. However, it is possible that the decomposition of polymerized macromolecules might take place in this stage, but it is believed that the decomposition products still remain in the green parts. In contrast, in the BC stage (300–450 °C), the weight loss become obvious. This might be attributed to the evaporation of small molecules that are the decomposition products of the polymerized macromolecules. As expected, in the CD stage (> 450 °C), the weight loss becomes stable, indicating most of organic polymers in the green parts have been decomposed and evaporate from the green parts. These results are in a good consistent with previous reports [40–42]. Based on the results of the thermal analysis, the debinding temperature was set above 450 °C in the present investigation. In order to avoid the introduction of defects inside the green parts, the heating steps during the thermal debinding process have been designed carefully. A reasonable heating curve of the debinding processing is shown in Fig. 10(b). As shown, the heating rate is 0.5 °C/min from room temperature to 150 °C, then the heating rate is controlled at 0.2 °C/min from 150 °C to 350 °C, with two temperature holding platforms at 250 °C and 350 °C for 2 h, respectively. Based on TG/DSC curves, 2 h holding time is long enough to make sure that the small molecules and the decomposed products of polymerized macromolecules can escape from the green parts and no defects are induced inside the green parts. Then, a heating rate of 0.5 °C/min is controlled between 350 °C and 500 °C. When the debinding temperature reaches 500 °C, it holds for 2 h. Fig. 11(a)-(d) shows the photographs of debound green parts with different heating rate from the temperature range of 150 oC–350 oC. As shown, there are no visible warpage and collapse defects in the debound parts with the heating rate of 0.2 °C/ min. As the heating rate increases, macro- and micro-defects become obvious. It is clear in order to minimize the induced defects during the thermal debinding, heating rate must be controlled very carefully. A critical step to achieve the desirable mechanical properties of Si3N4 ceramics could happen in the sintering and post-sintering processing after the debound green part without induced defects has been obtained. However, we would like to mention that detailed relationship between the structure and properties of the Si3N4 ceramics prepared by stereolithography will be reported elsewhere. In contrast, more attention has been paid to the preparation of Si3N4 ceramic slurry with high solid loading, suitable viscosity and high curing depth in the present investigation. The stereolithographical and debound Si3N4 ceramics were sintered at 1750 °C for 2 h in nitrogen with 5 MP applied gas pressure and 10 °C/min heating rate from room temperature to 1750 oC. The pressureless sintering was also carried out for the samples with the same sintering temperature, holding time, and heating rate. Fig. 12 shows the XRD patterns of the as-sintered Si3N4 ceramics prepared by stereolithography and followed by debinding. As shown, main crystal phase of as-sintered Si3N4 ceramics by both pressureless
Fig. 10. (a) Thermogravimetry curves of as-printed green part by stereolithography using slurry with 45 vol% Si3N4 solid loading, 1 wt% KH560 and 1 wt% Darvan and (b) heating curve for the debinding of as-printed Si3N4 green parts. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
Si3N4 slurry with 45 vol% Si3N4 loading, 1 wt% KH560 and 1 wt% Darvan are shown in Fig. 10(a). From the DSC curve, two endothermic peaks centered at ~165 °C and ~365 °C can be observed. Combined with the TG curve, the initial weight loss of the sample in the stage of AB (100–300 °C) may mainly result from the evaporation of small organic molecules, which contain surface modifier KH560, dispersant Darvan and unpolymerized prepolymer. It can be observed that the loss percentage in this stage is only ~10 wt%, indicating the degree of light
Fig. 11. Photographs of the debound green parts with (a) 0.2 °C/min, (b) 1 °C/min, (c) 2 °C/min, and (d) 5 °C/min heating rate for 150 °C–350 °C. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.) 7
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pores still exist in the sample and the sintering density is only ~2.95 g/ cm3. In the present investigation, the 3.45 g/cm3 density was used for the evaluation of the relative sintering density. The microhardness and fracture toughness of the sample are measured as 13.43 ± 0.11 GPa and 5.75 ± 0.16 MPa m1/2, respectively. In contrast, Fig. 13(b) displays the SEM images of as-sintered Si3N4 ceramics by pressure sintering with 5 MP applied nitrogen pressure. It is observed that the morphology of as-sintered Si3N4 ceramics is also rod-shaped, but the pore amount and size decrease significantly. The sintering density is improved to ~3.28 g/cm3 (~95% relative density), much higher than that of as-sintered Si3N4 ceramics by pressureless sintering (~2.95 g/ cm3). The microhardness and fracture toughness of the sample are measured as 14.63 ± 0.45 GPa and 5.82 ± 0.42 MPa m1/2, respectively, which are comparable to the reported microhardness (16.93 ± 0.25 GPa) and fracture toughness (6.85 ± 0.26 MPa m1/2) values of as-sintered Si3N4 ceramics prepared by conventional ceramics fabrication technology [21]. It is well-known that the achievable mechanical properties of Si3N4 ceramics can critically depend on their microstructure that is intrinsically related to materials design, sintering processing and sintering additives as well [1]. Therefore, we believe that the mechanical properties of as-sintered Si3N4 ceramics fabricated by stereolithography-based AM method can be further improved by optimizing the debinding and high-temperature sintering process.
Fig. 12. XRD patterns of as-sintered Si3N4 ceramics prepared by pressureless sintering and gas pressure sintering with 5 MPa applied pressure.
4. Conclusions Si3N4 ceramic slurry with 45 vol% Si3N4 loading and 50 μm curing depth was prepared successfully using KH560 and Darvan as the surface modifier and the dispersant, respectively. Dp and LnEc values of Si3N4 ceramic slurry were derived as 0.032 mm and 0.177 mJ/mm2 based on the Beer-Lambert formula, respectively. By optimizing the exposure time and energy dose, Si3N4 ceramic greens part with complex shape and high surface precision were successfully printed by stereolithography-based AM. The microhardness and fracture toughness of the as-sintered dense Si3N4 ceramics (~3.28 g/cm3) are 14.63 ± 0.45 GPa and 5.82 ± 0.42 MPa m1/2, respectively, which are comparable to the reported values of as-sintered Si3N4 ceramics prepared by the traditional ceramics fabrication technology. The stereolithography based ceramic AM would be a promising technology to fabricate gray-colored dense Si3N4 ceramics with complex shape, high surface precision and good mechanical properties. Acknowledgement This work is funded by the Shenzhen Science and Technology Innovation Commission Technology Research Project (Grant Nos 20170410221235842), State Key Laboratory for Powder Metallurgy Foundation (Grant Nos 621021826) and Graduate innovation program of Central South University (2017zzts102). The use of facilities in the Institute for Materials Microstructure and the State Key Laboratory for Powder Metallurgy at Central South University is acknowledged. References [1] A. Zocca, P. Colombo, C.M. Gones, J. Gunster, Additive manufacturing of ceramic: issue, potentialities, and opportunities, J. Am. Ceram. Soc. 98 (2015) 1983–2001. [2] O. Dufaud, P. Marchal, S. Corbel, Rheological properties of PZT suspensions for stereolithography, J. Eur. Ceram. Soc. 22 (2002) 2081–2092. [3] C. Hinczewski, S. Corbel, T. Chartier, Ceramic suspensions suitable for stereolithography, J. Eur. Ceram. Soc. 18 (1998) 583–590. [4] A. Badev, Y. Abouliatim, T. Chartier, L. Lecamp, P. Lebaudy, C. Chaput, C. Delage, Photopolymerization kinetics of a polyether acrylate in the presence of ceramic fillers used in stereolithography, J. Photochem. Photobiol., A 222 (2011) 117–122. [5] J.H. Jang, S. Wang, S.M. Pilgrim, W.A. Schulze, Preparation and characterization of barium titanate suspension for stereolithography, J. Am. Ceram. Soc. 83 (2000) 1804–1807. [6] S. Zhang, N. Sha, Z. Zhao, Surface modification of α-Al2O3 with dicarboxylic acids for the preparation of UV-curable ceramic suspensions, J. Eur. Ceram. Soc. 37 (2017) 1607-616.
Fig. 13. SEM images of as-sintered Si3N4 ceramics at 1750 °C for 2 h in nitrogen and with 10 °C/min heating rate from room temperature to 1750 °C by (a) pressureless sintering and (b) gas pressure sintering with 5 MPa applied pressure.
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