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The Effects of Spark-Plasma Sintering (SPS) on the Microstructure and Mechanical Properties of BaTiO3/3Y-TZP Composites Jing Li 1 , Bencang Cui 1 , Huining Wang 2 , Yuanhua Lin 1, *, Xuliang Deng 3 , Ming Li 1 and Cewen Nan 1 1

2 3

*

State Key Laboratory of New Ceramics and Fine Processing, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China; [email protected] (J.L.); [email protected] (B.C.); [email protected] (M.L.); [email protected] (C.N.) Department of Periodontics, Hospital of Stomatology Wenzhou Medical University, Wenzhou 325027, China; [email protected] Department of Geriatric Dentistry, School & Hospital of Stomatology, Peking University, Beijing 100081, China; [email protected] Correspondence: [email protected]; Tel.: +86-10-6277-1160

Academic Editor: Eugene A. Olevsky Received: 5 April 2016; Accepted: 25 April 2016; Published: 28 April 2016

Abstract: Composite ceramics BaTiO3 /3Y-TZP containing 0 mol %, 3 mol %, 5 mol %, 7 mol %, and 10 mol % BaTiO3 have been prepared by conventional sintering and spark-plasma sintering (SPS), respectively. Analysis of the XRD patterns and Raman spectra reveal that the phase composition of t-ZrO2 , m-ZrO2 , and BaTiO3 has been obtained. Our results indicate that SPS can be effective for the decrease in grain size and porosity compared with conventional sintering, which results in a lower concentration of m-ZrO2 and residual stress. Therefore, the fracture toughness is enhanced by the BaTiO3 phase through the SPS technique, while the behavior was impaired by the piezoelectric second phase through conventional sintering. Keywords: spark-plasma sintering (SPS); BaTiO3 /3Y-TZP; fracture toughness

1. Introduction As a field-assisted sintering technique, spark-plasma sintering (SPS) has attracted much attention since its advent in the late 1970s [1–3]. The starting powders in graphite die are sintered directly instead of being pre-pressed prior to sintering using the conventional processing technique. After graphite die is placed in the furnace, two pistons acting as electrodes load pressure on the upper and bottom surfaces. Due to the good electrical and thermal conductivity of the graphite die, adequate Joule heat is efficiently and quickly transferred to the starting powder under a relatively low voltages. Moreover, the heating rate can be as high as 1000 ˝ C/min, resulting from the adjustable current pulses (milliseconds) [1–3]. Both the compressive press and high heating rate work to obtain dense bulks with nano-size grains under a lower sintering temperature, leading to the extensive application of SPS in the dielectric, piezoelectric, and thermoelectric fields, among others [4–8]. Li et al. [9] prepared lead-free piezoelectric ceramics Na0.5 K0.5 NbO3 with 99% relative density at 920 ˝ C by using the SPS technique. It was fairly difficult to obtain when the ceramic sintered in a conventional furnace. Deng et al. [10] synthesized nanostructured bulk BaTiO3 with a grain size of 20 nm and a relative density of 97% via SPS, while the grain size of BaTiO3 was in micrometer range when sintered in a conventional furnace. Furthermore, a series of transparent ceramics [11], such as alumina [12], zirconia [13], and yttrium-aluminum-garnet [14], have been processed with aid of the SPS technique. Materials 2016, 9, 320; doi:10.3390/ma9050320

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Materials 2016, 9, 320

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Fracture toughness is a highly essential behavior for dental materials [15]. It is believed that the Materials 2016, 9, 320 2 of 9 piezoelectric addition could enhance the toughness of ceramics based on the piezoelectric secondary phase toughening mechanism [16–18]. Under load, the piezoelectric effect lead to domain Fracture toughness is a highly essential behavior for dental materials [15]. would It is believed that the addition could enhance of ceramics based the piezoelectric wallpiezoelectric motion and dissipate energy in the thetoughness tips of cracks. Yang et al.on[19] studied the secondary effect of the phase toughening mechanism load, the piezoelectric effect lead to domain wall piezoelectric second phase, Nd2 Ti[16–18]. the fracture toughness of Al and the toughness increased 2 O7 , onUnder 2 O3 ,would 1/2 motion and dissipate energy in the tips of cracks. Yang et al. [19] studied the effect of the piezoelectric to 6.7 MPa¨ m . Chen et al. [20] prepared a Sr2 Nb2 O7 /3Y-TZP composite and found that the fracture second was phase, Nd2Ti2Ohigher 7, on the fracture toughness of Al2O3, and the toughness significant than that of 3Y-TZP, as high as 13 MPa¨ m1/2toughness . However,increased Yang et al.to[21] 6.7 MPa·m1/2. Chen et al. [20] prepared a Sr2Nb2O7/3Y-TZP composite and found that the fracture found that the addition of BaTiO3 suppressed the effect of the transformation toughening of 3Y-TZP, toughness was significant higher than that of 3Y-TZP, as high as 13 MPa·m1/2. However, Yang et al. and the fracture toughness decreased instead of increased. Moreover, it has been proven that the [21] found that the addition of BaTiO3 suppressed the effect of the transformation toughening of 3Yelectrical charges have a positive effect on the growth and differentiation of osteoblast cells, resulting TZP, and the fracture toughness decreased instead of increased. Moreover, it has been proven that from preferential adsorption of ions and proteins onto the polarized surfaces [22,23]. As piezoelectric the electrical charges have a positive effect on the growth and differentiation of osteoblast cells, materials could surface charges under load, theproteins piezoelectric addition might induce improved resulting fromvary preferential adsorption of ions and onto the polarized surfaces [22,23]. As bone formation around restorations. piezoelectric materials could vary surface charges under load, the piezoelectric addition might induce The aim bone of this study was to restorations. investigate effects of the SPS technique on the microstructure, improved formation around and mechanical properties of BaTiO composites as atechnique function on of the BaTiO The aim of this study was to investigate of the SPS microstructure, andnull 3 /3Y-TZPeffects 3 content. The hypotheses of this study of were that SPS would help synthesize dense bulks with mechanical properties BaTiO 3/3Y-TZP composites as a function of BaTiO BaTiO33 /3Y-TZP content. The null hypotheses ofand thiswould study were thatthe SPSmechanical would helpbehaviors. synthesize dense BaTiO3/3Y-TZP bulks with nano-size grains improve nano-size grains and would improve the mechanical behaviors.

2. Results and Discussion 2. Results and Discussion

2.1. Phase Structure Analysis 2.1. Phase Structure Analysis

The X-ray diffraction (XRD) patterns of BaTiO3 /3Y-TZP specimens prepared by different sintering Theare X-ray diffraction (XRD) of BaTiO 3/3Y-TZP prepared by different techniques shown in Figure 1. Inpatterns this study, CS stands for aspecimens conventionally sintered specimen sintering techniques are shown in Figure 1. In this study, CS stands for a conventionally sintered that is sintered in a conventional air furnace. All of the XRD patterns present a crystalline phase specimen that is sintered in a conventional air furnace. All of the XRD patterns present a crystalline of tetragonal ZrO2 , and the characteristic peaks can be indexed to PDF card #50-1089. With respect phase of tetragonal ZrO2, and the characteristic peaks can be indexed to PDF card #50-1089. With to specimens sintered via the SPS technique, diffraction peaks due to BaTiO3 were detected as the respect to specimens sintered via the SPS technique, diffraction peaks due to BaTiO3 were detected contents of BaTiO3 increased to 7 mol % and 10 mol %. No m-ZrO2 (monoclinic ZrO2 ) phase was as the contents of BaTiO3 increased to 7 mol % and 10 mol %. No m-ZrO2 (monoclinic ZrO2) phase observed. However, as far as conventionally sintered specimens are concerned, peaks can be attributed was observed. However, as far as conventionally sintered specimens are concerned, peaks can be to BaTiO detected with increasing content of BaTiO3 . Peaks at 39˝ can be indexed to 3 , and attributed tom-ZrO BaTiO23, was and m-ZrO 2 was detected with increasing content of BaTiO3. Peaks at 39° can be the indexed m-ZrO2 to phase and BaTiO According to the relative intensity of peaks of each phase, it is 3 phase. the m-ZrO 2 phase and BaTiO 3 phase. According to the relative intensity of peaks of each ˝ mainly originates from the m-ZrO phase. Content reasonable that thetointensity a peak at 39of 2 the m-ZrO2 phase, ittoisfind reasonable find thatofthe intensity a peak at 39° mainly originates from of the m-ZrO phase was significantly lower in the spark-plasma-sintered samples; therefore, peaks at phase. Content of the m-ZrO2 phase was significantly lower in the spark-plasma-sintered samples; 2 ˝ 39 therefore, seem to bepeaks absent theseem spark-plasma-sintered Moreover, thesamples. dominant ZrO2 structure at in 39° to be absent in thesamples. spark-plasma-sintered Moreover, the dominant 2 structure seems to be a monoclinic phase rather than tetragonal the to seems to be a ZrO monoclinic phase rather than a tetragonal phase when theacontents of phase BaTiOwhen 3 increase contents increase to 7 mol %aand 10 mol %, demonstrating a quite different phase structure. 7 mol % andof10BaTiO mol 3%, demonstrating quite different phase structure.

Figure 1. X-ray diffraction(XRD) (XRD)patterns patterns of of spark-plasma-sintered spark-plasma-sintered and sintered Figure 1. X-ray diffraction andconventionally conventionally sintered BaTiO 3 /3Y-TZP ceramics. BaTiO3 /3Y-TZP ceramics.

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Furthermore, the zirconia is investigated by one Furthermore, the T-M T-Mphase phasetransformation transformationwithin within zirconia is investigated by of onetheofmost the effective techniques: Raman spectroscopy [24].[24]. Figure 2a 2a shows most effective techniques: Raman spectroscopy Figure showsthe theRaman Ramanspectrum spectrum of of aa −1, ´ 1 , 464 ´1 , representative specimen. specimen. Characteristic cmcm 464 cm−1cm , and representative Characteristic peaks peaks at at wavenumbers wavenumbers 147 147 cm cm−1´, 1265 , 265 −1 −1 −1 ´ 1 ´ 1 ´ 1 642 cm represent t-ZrO2t-ZrO , and 2peaks wavenumbers 181 cm 181 andcm 190 cm the presence of and 642 cm represent , and at peaks at wavenumbers and reveal 190 cm reveal the m-ZrO2 [25]. Figure 2b shows a volume fraction of m-ZrO 2m-ZrO calculated according to Tabares and presence of m-ZrO [25]. Figure 2b shows a volume fraction of calculated according to Tabares 2 2 Anglada [26].[26]. TheThe content of of m-ZrO 2 increases with 3 concentration, with and Anglada content m-ZrO withBaTiO BaTiO which is is consistent with 2 increases 3 concentration,which the XRD XRD patterns. Compared with conventionally spark-plasma-sintered the patterns. Compared with conventionally sintered sintered ceramics, ceramics, spark-plasma-sintered specimens specimens have much lower concentrations of m-ZrO 2 , ranging between 4% and 29.4%, whereas the have much lower concentrations of m-ZrO2 , ranging between 4% and 29.4%, whereas the m-ZrO 2 m-ZrO2 of content of conventionally composites varies 8% to 71.2%. Thediscrepancy great discrepancy content conventionally sinteredsintered composites varies from 8%from to 71.2%. The great of the of the structure phase structure between specimens may originate from different stresscaused states caused phase between specimens may originate from different residualresidual stress states by the by the addition of BaTiO 3 . Less m-ZrO 2 content is better for zirconia ceramics based on the welladdition of BaTiO3 . Less m-ZrO2 content is better for zirconia ceramics based on the well-known phase known phase transformation toughening mechanism therefore, the low of concentration of m-ZrO transformation toughening mechanism [27]; therefore,[27]; the low concentration m-ZrO2 might have a2 might have a positive effect on the fracture toughness of spark-plasma-sintered positive effect on the fracture toughness of spark-plasma-sintered composites. composites.

Figure 2. 2. (a) spectrum of BaTiO 3/3Y-TZP (with 7 mol % BaTiO3) prepared via the SPS method; Figure (a)Raman Raman spectrum of BaTiO 3 /3Y-TZP (with 7 mol % BaTiO3 ) prepared via the SPS 2 content of spark-plasma-sintered sintered (b) Volume fraction of m-ZrO method; (b) Volume fraction of m-ZrO2 content of spark-plasma-sinteredand andconventionally conventionally sintered BaTiO33/3Y-TZP /3Y-TZP ceramics. BaTiO ceramics.

2.2. Microstructure Microstructure Analysis Analysis 2.2. Figure 33 shows shows scanning scanning election election microscopy microscopy (SEM) (SEM) images images of of spark-plasma-sintered spark-plasma-sintered and and Figure conventionally sintered BaTiO 3/3Y-TZP ceramics as a function of BaTiO3 content. The As for conventionally sintered BaTiO3 /3Y-TZP ceramics as a function of BaTiO3 content. The As for conventionally sintered sintered composites, composites, two two different different kinds kinds of of grains, grains, ZrO ZrO2 and and BaTiO BaTiO3 are are clearly clearly conventionally 2 3 observed. With increasing amounts amounts of of BaTiO BaTiO3,, the the grain grain size size of of ZrO ZrO2 is is about about 200 200 nm, nm, remaining remaining observed. With increasing 3 2 substantially unchanged, while grain size of BaTiO 3 increases from 1 to 3.5 μm with the BaTiO3 substantially unchanged, while grain size of BaTiO3 increases from 1 to 3.5 µm with the BaTiO3 content. content. Since size ofis BaTiO 3 is 5 to 17.5 times larger than that of ZrO2, the mismatch of grain Since grain sizegrain of BaTiO 5 to 17.5 times larger than that of ZrO2 , the mismatch of grain size leads 3 size leads to pores in ceramics and might introduce stress between resulting the stress-induced to pores in ceramics and might introduce stress between grains, grains, resulting in the in stress-induced T-M T-M phase transformation. Hence, the content of m-ZrO 2 increases, as shown in Figure 2. Compared phase transformation. Hence, the content of m-ZrO2 increases, as shown in Figure 2. Compared with with conventionally composites, spark-plasma-sintered ceramics exhibit smaller significantly conventionally sinteredsintered composites, spark-plasma-sintered ceramics exhibit significantly grain smaller grain sizes, especially for BaTiO 3 grains, which can be attributed to the compressive press sizes, especially for BaTiO3 grains, which can be attributed to the compressive press and the high and the rate. high heating rate.with Moreover, with close radii and the same valence, it isTi likely Ti4+ to 4+ to for heating Moreover, close ion radii andion the same valence, it is likely for partially 4+ partially substitute Zr a, solid forming a solid solution, (Zr,Ti)O2 [21]. Thus, spark-plasma-sintered substitute Zr4+ , forming solution, (Zr,Ti)O 2 [21]. Thus, spark-plasma-sintered ceramics show ceramics show traces of liquid-phase sintering with increasing of BaTiO 3, resulting in more traces of liquid-phase sintering with increasing content of BaTiO3content , resulting in more dense composite dense composite bulks [28]. The larger grain size for higher BaTiO 3 content in conventionally sintered bulks [28]. The larger grain size for higher BaTiO3 content in conventionally sintered specimens may specimens support this conclusion. also supportmay thisalso conclusion.


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Figure 3. 3. Scanning Scanning election election microscopy microscopy (SEM) (SEM) images images of of fracture fracture surfaces surfaces of of spark-plasma-sintered spark-plasma-sintered /3Y-TZP ceramics. (a–d) Specimens are prepared by conventional conventionally sintered sintered BaTiO BaTiO33/3Y-TZP and conventionally sintering method; method;(e–h) (e–h)Specimens Specimensare areprepared preparedvia viathe the SPS method. (a,e) 3 mol BaTiO 3; (b,f) 5 mol sintering SPS method. (a,e) 3 mol %% BaTiO 5 mol % 3 ; (b,f) 3 ; (c,g) 7 mol % BaTiO 3 ; (d,h) 10 mol % BaTiO 3 . The red arrow denotes the BaTiO 3 phase and % BaTiO BaTiO ; (c,g) 7 mol % BaTiO ; (d,h) 10 mol % BaTiO . The red arrow denotes the BaTiO phase and the 3 3 3 3 the yellow arrow denotes the 3Y-TZP phase. yellow arrow denotes the 3Y-TZP phase.

According to to the the relative relative density, density, bulk bulk porosities porosities of of composites composites (Table (Table 1) 1) are are calculated calculated by by using using According the following equation: P = (1 − ρ) × 100%, where P is the bulk porosity, and ρ is the relative density. the following equation: P = (1 ´ ρ) ˆ 100%, where P is the bulk porosity, and ρ is the relative density. With increasing 3, porosity trends between conventionally sintered specimens, and With increasing amounts amountsofofBaTiO BaTiO 3 , porosity trends between conventionally sintered specimens, spark-plasma-sintered composites vary greatly. Porosity increases the and spark-plasma-sintered composites vary greatly. Porosity increaseswith withBaTiO BaTiO33content, content, due due to to the mismatch of grain size in conventionally sintered ceramics. By contrast, both of the slight mismatches mismatch of grain size in conventionally sintered ceramics. By contrast, both of the slight mismatches in grain grain size size and in and the the liquid-phase liquid-phase sintering sintering produce produce effects effects on on the the spark-plasma-sintered spark-plasma-sintered composites; composites; therefore, porosity values float slightly. However, it is reasonable to find that spark-plasma-sintered therefore, porosity values float slightly. However, it is reasonable to find that spark-plasma-sintered composites are denser than the conventionally sintered ones, which are in consistency with the composites are denser than the conventionally sintered ones, which are in consistency with the observed SEM images. observed SEM images. Table1. 1. The The porosity porosity of of spark-plasma-sintered spark-plasma-sinteredand and conventionally conventionallysintered sinteredBaTiO BaTiO3/3Y-TZP /3Y-TZP ceramics. ceramics. Table 3 0 mol % BaTiO3 Content BaTiO 3 Content Porosity (%) (CS) 0 mol 2.7% Porosity(%) (%)(CS) (SPS) 0.5 Porosity 2.7 Porosity (%) (SPS) 0.5

3 mol % 3 mol 5.9 %

5 mol % 5 mol 9.6 %

7 mol % 7 mol 12.5 %

4.8 5.9 4.8

3.3 9.6 3.3

1.5 12.5 1.5

10 mol % 1014.8 mol % 3.2 14.8 3.2

2.3. Mechanical Properties 2.3. Mechanical Properties The residual stress state of the composites could be effective for not only the phase transformation alsostate the of fracture toughness. Asbeaneffective attemptfortonot depict residual stress state, The residualbut stress the composites could only the the phase transformation 2) were recorded, and the data were analyzed by MATLAB Raman maps of each specimen (5 × 5 μm but also the fracture toughness. As an attempt to depict the residual stress state, Raman maps of each software. (5 Specimens without BaTiOand 3 serve thewere control group, the mean wavenumber is specimen ˆ 5 µm2 ) were recorded, the as data analyzed by and MATLAB software. Specimens −1 ´1 . of 145.5 cmBaTiO . A shift of the peak toward higher wavelength number indicates the presence residual without serve as the control group, and the mean wavenumber is 145.5 cm A shift of 3 compressive stress, which helps crack closure. By contrast, a peak shift towardcompressive lower wavelength the peak toward higher wavelength number indicates the presence of residual stress, number reveals theclosure. presenceByofcontrast, residual atensile stress.toward Moreover, a larger peak shift means a higher which helps crack peak shift lower wavelength number reveals the residual stress. As seen in Figure 4, a mixture of tensile and compressive stress is recorded in presence of residual tensile stress. Moreover, a larger peak shift means a higher residual stress. As seen spark-plasma-sintered composites, but significantly more tensile stress is found in the conventionally in Figure 4, a mixture of tensile and compressive stress is recorded in spark-plasma-sintered composites, sintered specimens. Moreover, the peak shift in is the larger for conventionally sintered specimens with but significantly more tensile stress is found conventionally sintered specimens. Moreover, more BaTiO 3 content . the peak shift is larger for conventionally sintered specimens with more BaTiO3 content.


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Figure 4. Quantitative Raman maps of spark-plasma-sintered and conventionally sintered Figure 4. Quantitative Raman maps of spark-plasma-sintered and conventionally sintered BaTiO3/3Y-TZP ceramics. (a–d) Specimens are prepared by conventional sintering method; (e–h) BaTiO3 /3Y-TZP ceramics. (a–d) Specimens are prepared by conventional sintering method; Specimens are prepared via the SPS method. (a,e) 3 mol % BaTiO3; (b,f) 5 mol % BaTiO3; (c,g) 7 mol (e–h) Specimens are prepared via the SPS method. (a,e) 3 mol % BaTiO3 ; (b,f) 5 mol % BaTiO3 ; % BaTiO3; (d,h) 10 mol % BaTiO3. (c,g) 7 mol % BaTiO3 ; (d,h) 10 mol % BaTiO3 .

Figure 5 shows the fracture toughness, Vickers hardness, and elastic modulus values as a Figure 5ofshows fracture Even toughness, Vickers hardness,ofand elastic modulus valuesthe as afracture function function BaTiOthe 3 content. though the addition BaTiO 3 would enhance of toughness BaTiO3 content. Even though the addition of BaTiO would enhance the fracture toughness through 3 accompanied high porosity and the m-ZrO through the piezoelectric effect, both the 2 thecontent piezoelectric effect, both the on accompanied high porosity and the m-ZrO negative have negative effects the behavior of specimens prepared by conventional sintering. 2 content have

compressive press and the high heating rate of the SPS technique, the grain size of BaTiO3 is quite smaller, and ceramics show traces of liquid-phase sintering, resulting in more dense composites and a low concentration of m-ZrO2. Thus, the expected coupling effects of the piezoelectric secondary phase toughening mechanism and phase transformation toughening mechanism lead to a high toughness of the specimens prepared via the SPS method. As for composites with 3 mol % BaTiO3, Materials 2016, 9, 320 6 of 9 the fracture toughness of the conventionally sintered specimen is significantly higher than that of the spark-plasma-sintered specimen, which may be attributed to the difference in the effect of the effects on thesecondary behavior phase of specimens prepared by conventional sintering. Therefore, the piezoelectric toughening mechanism, resulting from different contents of thefracture BaTiO3 toughness greatly trends downward aftershow increasing slightly. Owing to theimages compressive and the phase. XRD patterns of both specimens no BaTiO 3 phase, but SEM revealpress the existence high heating rateinofthe theconventionally SPS technique,sintered the grain size of BaTiO quite show3 of BaTiO 3 grains specimen (Figure 3a). Thesmaller, additionand of 3ceramics mol % BaTiO 3 is traces liquid-phase sintering,specimen resulting in more dense and arather low concentration of m-ZrO2 . in theofspark-plasma-sintered may serve as composites a doping agent than a polycrystalline Thus, expected coupling effects toughness. of the piezoelectric secondary phase toughening mechanism and phase,the which destroys the fracture phaseBoth transformation toughening lead atosimilar a hightrend toughness of the specimens prepared via3. the elastic modulus andmechanism hardness show with increasing amounts of BaTiO the method. As for composites with 3 mol % BaTiO , the fracture toughness of the conventionally TheSPS as-prepared BaTiO 3 has a lower elastic modulus (~72.8 GPa) and hardness (~1.4 GPa) than does 3 sintered higher thaninthat the spark-plasma-sintered specimen, 3Y-TZP,specimen but thereisissignificantly no expected decline theofcomposites. After decreasing at first,which these may two be attributed to the difference in the effect ofwhich the piezoelectric secondaryto phase mechanism, behaviors increase with BaTiO 3 content, may be attributed the toughening formation of the solid solution, from (Zr,Ti)O 2. Ascontents spark-plasma-sintered ceramics suffer aofhigh press during resulting different of the BaTiO3 phase. XRD patterns both compressive specimens show no BaTiO 3 sintering, more traces liquid-phase sintering, suggesting more amounts of solidspecimen solution phase, butthey SEMshow images reveal theofexistence of BaTiO in the conventionally sintered 3 grains [29]. Therefore, specimens a higher elastic modulus and hardness, compared withas the (Figure 3a). The these addition of 3 molreveal % BaTiO spark-plasma-sintered specimen may serve a 3 in the conventionally sintered doping agent rather thanspecimens. a polycrystalline phase, which destroys the fracture toughness.

Figure 5.5. The mechanical properties of of specimens specimens with with different different BaTiO BaTiO3 3 contents. contents. (a) Fracture Fracture Figure toughness; (b) (b) elastic elastic modulus; modulus; (c) (c) hardness. hardness. toughness;

Both the elastic modulus and hardness show a similar trend with increasing amounts of BaTiO3 . The as-prepared BaTiO3 has a lower elastic modulus (~72.8 GPa) and hardness (~1.4 GPa) than does 3Y-TZP, but there is no expected decline in the composites. After decreasing at first, these two behaviors increase with BaTiO3 content, which may be attributed to the formation of the solid solution, (Zr,Ti)O2 . As spark-plasma-sintered ceramics suffer a high compressive press during sintering, they show more traces of liquid-phase sintering, suggesting more amounts of solid solution [29]. Therefore, these specimens reveal a higher elastic modulus and hardness, compared with the conventionally sintered specimens.


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3. Materials and Methods 3.1. Materials 3Y-TZP (TZ-3YSB-E, Tosoh Co., Tokyo, Japan) with an average particle size of 90 nm and BaTiO3 (Sinopharm Chemical Reagent Co., Shanghai, China) with an average particle size of 100 nm were used to prepare the BaTiO3 /3Y-TZP composite. 3.2. Preparation of Porous Zirconia Ceramic The starting materials, 3Y-TZP and BaTiO3 , at 0 mol %, 3 mol %, 5 mol %, 7 mol %, and 10 mol % were mixed together by alcohol-based ball milling for 12 h, respectively. The mixture powders were dried in oven for sintering. Some of the mixtures were pressed at a pressure of 4 MPa, followed by a cold isostatic pressing at 200 MPa, and the samples were then heated up to 1400 ˝ C at a rate of 100 ˝ C/h and kept for 2 h in a conventional air furnace. Some of the mixtures were sintered directly via SPS. The heating rate was 110 ˝ C/s, and the sintering temperature was 1175 ˝ C. 3.3. Characterization X-ray diffraction spectroscopy (Rigaku, D/MAX-2550V, Tokyo, Japan) was employed to analyze the phase composition. Morphologies of fracture surfaces were examined via SEM (Hitachi, S-2500N, Tokyo, Japan). The relative density was measured by the Archimedes method. The volume fraction of monoclinic ZrO2 was measured by a Raman spectrometer ((Hiroba, LabRAM HR Evolution, Tokyo, Japan). It was calculated based on the equation: Vm “

Im181 ` Im190 ˘ 0.32 It147 ` It265 ` Im181 ` Im190 `

(1)

where It and Im are the integrated intensities of the tetragonal and monoclinic peaks, respectively. A nano-indentation tester (MTS, Palo Alto, CA, USA) was applied to analysis of the Vickers hardness and elastic modulus. The hardness was calculated by the equation: HV “ 1.8544P{d2

(2)

where HV is the Vickers hardness; P is the load; and d is the diagonal of the indentation. The elastic modulus was further inferred by using the equation: E “ 0.45HV { pa{b ´ a{b1 q

(3)

where E is the elastic modulus; b is the length of the shorter diagonal; b1 is the length of the longer diagonal; and a is the length of the crack. A universal test instrument (Shimadzu, EZ-100, Tokyo, Japan) was employed to measure the fracture toughness of specimens by the single-edge-notched beam method with a loading rate of 0.05 mm/min. Bending bars (n = 12) per specimen were cut into 2 ˆ 4 ˆ 16 mm3 with a diamond blade, and the notch depth was approximately 2 mm. The fracture toughness was calculated using formula: K IC “

a P0 ˆ l fp q 3 { 2 W BW

(4)

where P0 is the load; l is the span; B is the height of bar; W is the width of the bar; and a is the depth of the notch.


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4. Conclusions A series of BaTiO3 /3Y-TZP ceramics have been prepared by conventional sintering and SPS, respectively. The phase structure, microstructure, and mechanical properties of the composites were investigated as a function of BaTiO3 content. Our results show that the SPS technique has a remarkably positive effect on the behaviors of BaTiO3 /3Y-TZP composites. Spark-plasma-sintered specimens are superior in fracture toughness due to the coupling effects of the piezoelectric secondary phase toughening mechanism and the phase transformation toughening mechanism. These results reveal that the piezoelectric secondary phase, BaTiO3 , could enhance the fracture toughness of zirconia through the SPS technique. Acknowledgments: This work was a result of collaboration between the Materials Science and Engineering School in Tsinghua University, Department of Periodontics in Hospital of Stomatology Wenzhou Medical University and the Outpatient Dental Center in Peking University. It was supported by the National Science and Technology Ministry of China (Grant no. 2012BAI07B00). Author Contributions: Huining Wang, Yuanhua Lin, Xuliang Deng, Ming Li, and Cewen Nan organized the research; Jing Li and Bencang Cui performed the experiments; Jing Li wrote the manuscript; all authors reviewed the manuscript. Conflicts of Interest: The authors declare no conflict of interest.

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