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IR transparent hot pressed Mg-α/β-Sialon:Ba2þ ceramics

Bhupendra Joshia, Bin Lia, Yuwaraj K. Kshetria, Hao Wangb, Soo Wohn Leea,n

aResearch Center for Eco Multi-Functional Nano Materials, Sun Moon University, Chungnam 336-708, Republic of KoreabState Key Lab of Advanced Technology for Material Synthesis and Processing,Wuhan University of Technology, Wuhan 430070, China

Received 30 March 2014; accepted 1 May 2014

Abstract

IR transparent Mg-α/β-Sialon:Ba2þ ceramics were fabricated by the hot press sintering method. The influence of BaCO3 addition to Si3N4–

MgO–AlN system on transparency, microstructures and mechanical properties were investigated. Reactions between the mixed powders wereanalyzed from differential thermogravimetric (DTG) curve and formation of Celsian (BaAl2Si2O8) was observed at 1175 1C. The α-Sialon phasein sintered bodies was increased with BaCO3 addition due to the excess liquid phase that allows Mg2þ diffusion in α-Si3N4 lattice to give higherα-Sialon phase as revealed from XRD patterns. To minimize the light scattering by mismatch of refractive indices between the Sialon phase andglassy phase, barium ion was introduced to match the refractive index of glassy phase to Sialon phase. The study of electron spin resonance(ESR) spectra shows that unpaired electrons affect the IR transmittance in Sialon ceramics. Weak ESR spectrum was shown for the IR transparentBaCO3 added Sialon ceramics. The highest light transmission was observed around 78% in IR region (2500 nm), and had better transparencythan previously reported Sialon ceramics. The fabricated Sialon ceramics had high hardness of 21.4 GPa and fracture toughness of 5.4 MPa m1/2.& 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Keywords: A. Hot pressing; B. Microstructure-final; C. Mechanical properties; C. Optical properties; D. Sialon

1. Introduction

Sialon ceramics are light-weight structural material, exhibitexcellent mechanical and thermal properties. Sialon ceramicscan operate in extreme conditions where other oxide ceramicsfailed. Therefore, Sialon ceramics are used as a cutting tooland in engine parts [1]. If Sialon ceramics could be used asoptical materials, this would broaden their range of applica-tions. Previously, transparent single crystal materials have beenused as transparent material for advanced applications, however,the fabrication method was complicated and also expensive.To overcome this limitation, polycrystalline transparent oxideceramics were introduced and they replaced transparent singlecrystal materials in many applications [2].

Recently, there is high demand for advanced materials forapplications in which in addition to high mechanical perfor-mance and partial or total transparency in a given spectral

10.1016/j.ceramint.2014.05.00114 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

g author. Tel.: þ82 41 530 2882; fax: þ82 41 530 2840.ss: swlee@sunmoon.ac.kr (S. Wohn Lee).

article as: B. Joshi, et al., IR transparent hot pressed Mg-α/β-Sialo05.001

range is required. Although materials with a hexagonal crystalstructure typically scatter more light than those with a cubiccrystal structure, hexagonal Sialon ceramics still have bettermechanical and thermal properties than transparent cubicmaterials such as YAG and Y2O3 [3–5]. Translucent Sialonceramics have been studied for more than a decade but therehas been little progress in improving their transparency [6–12].To date, a transmittance of more than 70% has not beenobserved in the IR spectral regions for Sialon ceramics.Sialon ceramics have a complicated structure of different

Sialon phases and also with intergranular glassy or crystallinephases. Little amount of additives can change the properties ofthe Sialon ceramics [13]. Knowing this fact, many researcherstried to fabricate different ratio of α to β phase Sialon ceramicsfor different application purposes. Main problem associatedwith fabrication of Sialon ceramics is the intergranular glassyphase that degrades the properties of the ceramics at hightemperature. Also, the mismatch of refractive indices betweenthe intergranular phase and α-Sialon grain degrades the opticalproperties of the material [14]. Most of the Sialon ceramics

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were fabricated with addition of higher wt% of metal oxideceramics that give rise to glassy grain boundary phase and alsomore wetting of grains causes increase in grain size.

Dense α-Sialon phase with small size grains, less inter-granular phase and similar refractive index of intergranularphase to that of Sialon grains can enhance the light transmis-sion in Sialon ceramics. Considering the above fact, we tried adifferent approach to sinter a Sialon ceramics. Hereby, wechoose BaCO3 as an additive that melts and decomposesat lower temperature. The melting causes wetting and rearran-gement of α-Sialon ceramics at low temperature. At hightemperature barium containing silicate glass changes to oxyni-tride glass in nitrogen environment. The barium containingoxynitride glassy phase had high refractive index as comparedto magnesium containing oxynitride glass [15]. In Sialonceramics, the loss in light transmittance is due to the lightscattering by grain boundary phase. Moreover, the mismatchbetween the refractive index between the α-Sialon phase andgrain boundary phase causes more scattering. Therefore, highrefractive index grain boundary phases are required to mini-mize the scattering of light by grain boundary phase. With thisidea we have improved the light transmittance of the materialand detail study was made on the effect of BaCO3 addition onthe Sialon ceramics.

2. Experimental procedure

The composition was made according to the phase diagramgiven by Kuang et al. [16]. The values of m¼0.9 and n¼0.45were taken to fabricate Sialon composite, where m and ncorrespond to level of substitution of Si–N bond by (Al–N) and(Al–O) in Si3N4 crystal structure, respectively. The starting powder

Fig. 1. TEM image of SAMB05 (0.5 wt% BaCO3 doped Sialon ceramics): (a) grainspectra showing different elements at points 1, 2, and 3, respectively.

Please cite this article as: B. Joshi, et al., IR transparent hot pressed Mg-α/β-Sialoceramint.2014.05.001

used for the preparation of the composition was α-Si3N4 (SN-E10,UBE Co., Japan), AlN (Grade F, Tokuyama Corp., Japan), andMgO (High purity chemicals Co. Ltd., Japan). In the abovecomposition, 0–1 wt% of BaCO3 (Daejung chemicals and metalsCo., Ltd., Korea) was added as an additive. Different samples werenamed as SN, SAMB025, SAMB05, SAMB075 and SAMB1 forwithout BaCO3 addition, 0.25 wt% BaCO3 addition, 0.5 wt%BaCO3 addition, 0.75 wt% BaCO3 addition and 1 wt% BaCO3

addition, respectively. The powders were mixed in ethanol withhigh purity silicon nitride balls and were ball milled for 24 h. Afterwet ball milling, the powders were dried and again dry milled for12 h. Then the mixed powders were sieved through 150 μmaperture. The powder mixtures were heated up to 1050 1C invacuum around 20 Pa for 2 h for removal of CO2 and then hotpress sintered under a uniaxial compression of 30 MPa pressure in0.1 MPa of N2 environment at 1850 1C for 1 h.Differential thermogravimetric (DTG) analysis was obtained

by using a NETZSCH STA 409 PC/PG thermal analyzer,which operated at a ramp of 10 1C/min from room temperatureto 1100 1C. A sample of 28 mg of mixed powder was placed inan Al2O3 crucible under an N2 environment. In addition, thedifferential thermogravimetric (DTG) curve was derived fromthe TG curve.After sintering, the crystalline phase was examined by X-ray

diffractometer (XRD, Cu Kα, RIGAKU D/MAX2200HRdiffractometer, Japan). To calculate the lattice parameter Siwas used as internal standard. The 2θ angle was measuredfrom 101 to 901 at the rate of 11/min. Densities of the sinteredsamples were measured by the Archimedes immersion methodusing water. The grain morphology and chemical compositionwas analyzed by 400 kV TEM (JEM-4010, JEOL, Co., Ltd.,Japan).

morphology based on EDS analysis of different regions; (b), (c), and (d) EDS

n:Ba2þ ceramics, Ceramics International (2014), http://dx.doi.org/10.1016/j.

Fig. 2. DSC, TG, and DTG curves of 1 wt% BaCO3 added Mg–α-Sialonceramics. In the DTG curve, reaction peaks were observed above 750 1C and at1175 1C for the formation of barium orthosilicate and barium aluminumsilicates, respectively.

Table 1Phase compositions, lattice parameters and densities of different sintered Sialonceramics.

Sample Phase composition (vol%) α-Sialon Density (g/cm3)

α β a (Å) c (Å)

SN 64.50 35.50 7.786 5.6499 3.180SAMB025 81.43 18.57 7.788 5.6505 3.182SAMB05 96.94 03.06 7.769 5.641 3.184SAMB075 94.53 05.47 7.768 5.6423 3.186SAMB1 94.54 05.46 7.764 5.634 3.187

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The electron spin resonance (ESR) spectra were measuredwith an ESR spectrometer (JES-TE300, JEOL) at roomtemperature and in 25 mT magnetic field.

Zwick 3212 hardness tester was used for the measurementof hardness and fracture toughness. The indentation wasperformed for 15 s at 98 N loads for hardness and fracturetoughness using Vickers diamond indenter. Olympus Ultra-sonic Transducer was used to measure Poisson ratio andYoung's modulus.

The sintered samples were ground to 0.5 mm thickness andboth sides were polished on a diamond plate, and then lighttransmittance was measured by UV/vis/NIR Jasco 570 withinthe range of 200–2500 nm wavelengths.

3. Results and discussion

3.1. DTG, XRD and microstructure analysis

IR-transparent Sialon ceramics were fabricated by addingdifferent amounts of BaCO3 (in wt%) to the α-Si3N4–MgO–AlN system. The addition of BaCO3 produces interestingresults in the Sialon ceramics. EDS analysis (Fig. 1) showsthat the addition of BaCO3 resulted in the formation of lowoxygen content Sialon grains while oxygen was concentratedat the intergranular phase with barium. During the sinteringprocess, BaCO3 reacts with the silica present on the surface ofα-Si3N4. The BaO formed after the decomposition of BaCO3

had higher basicity than MgO, the silica on the surface of α-Si3N4 are known to be hard acid and BaO as hard basepreferred much more over to MgO for the reaction with silica.The barium orthosilicate (BaSiO4) is considered a product ofthe solid-state reaction between BaCO3 and SiO2 at 730–750 1C. Then, above 1150 1C, the formed barium silicatereacts with alumina on the surface of AlN to form meltedbarium aluminum silicate and thus, at low hot press tempera-ture densification process starts. The formation of bariumaluminum silicates was observed as an exothermic peak inDTG curve around 1175 1C as shown in Fig. 2. A similarreaction was reported by Basu et al. [17] in the formation ofcelsian (BaAl2Si2O8). At higher temperature above 1700 1C,celsian are not stable and formed oxynitride glass in nitrogenenvironment. The (Ba, Mg)–Si–Al–O–N and Ba–Mg–Si–O–Nglassy phases, and other crystalline phases most likely toappeared in intergranular phase as end product. Other types ofglasses, such as magnesium silicate and barium aluminate, mayalso appear in the intergranular glassy phase.

Silica and alumina diffused to produce oxygen-enrichedSialon ceramics, but when they react with BaO at lowertemperature, the substitution of Si–N to Al–O in α-Si3N4 islowered, which yields higher nitrogen containing Sialongrains. The possibility of AlN diffusion in silicon nitride ishigher than that of other components and was confirmedby EDS.

Ba2þ ion cannot stabilize the α-Sialon phase due to its largecationic size. Therefore, MgO was added as a sinteringadditive as well as stabilizer to the α-Sialon phase with theMg2þ cation. The α-Si3N4–AlN–MgO system was chosen to

Please cite this article as: B. Joshi, et al., IR transparent hot pressed Mg-α/β-Sialoceramint.2014.05.001

produce Sialon ceramics, where a smaller amount of additivesis needed, excluding alumina. This system produces α/β-Sialoncomposite phase because the liquid-phase formation decreasesduring sintering and the Mg2þ diffusion inside the α-Si3N4

lattice is slow. With the addition of BaCO3, the Mg2þ

diffusion also increases, resulting in a higher percentageof the α-Sialon phase. Although only a small amount of theβ-Sialon phase is present, the Sialon ceramics formed in thismanner are referred to as composite α/β-Sialon ceramics.Different Sialon phases were determined with XRD patterns.

From peak intensity of different Sialon phases, volume fraction(%) of α- and β-phases was calculated and given in Table 1 foreach sintered body [18]. The XRD patterns for differentcompositions are given in Fig. 3. Only addition of MgO asan additive, single α-phase could not be obtained [16]. Increasein the amount of BaCO3 up to 1 wt% increased the vol% ofα-Sialon phase. Interestingly, BaCO3 addition increased theα-Sialon phase, however, Ba2þ ion was not found in theα-Sialon phase due to its large cationic radius as shown byEDS in Fig. 1.XRD patterns reveal the formation of the α-phase, and the

lattice parameters decreased with increasing BaCO3 content.The lattice parameters calculated for different samples are

n:Ba2þ ceramics, Ceramics International (2014), http://dx.doi.org/10.1016/j.

Fig. 3. XRD patterns of the different wt% BaCO3 added Sialon ceramics,which clearly showing α-Sialon peaks.

Fig. 4. Comparison of different peaks of different wt% BaCO3 doped Sialonceramics and the undoped BaCO3 Sialon ceramic (SN). The peaks are shiftedtoward the right side with increasing amount of BaCO3, suggesting a decreasein lattice parameters with higher content of BaCO3.

Fig. 5. TEM micrograph showing grain morphology of 0.5 wt% BaCO3 dopedSialon ceramics (SAME05).

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shown in Table 1. The XRD patterns show that the α-Sialonpeak shifts to the right with increasing BaCO3 concentration,which suggests that the lattice parameters are in decreasingorder as shown in Fig. 4. The Sialon ceramics formed in thiscase are closer to α-Si3N4 than α-Sialon.

EDS observation in Fig. 1(b–d) for different regions inFig. 1(a) also shows that the Ba2þ ion was not found in grainsbut was allocated in intergaranular phase. The grain morphol-ogy as observed by TEM shows equiaxed α-Sialon with tightgrain boundaries and β-phase with large grain and also withoutpores suggesting almost complete densification took placeduring sintering. The point 3 in Fig. 1(a) suggested largegrain β-phase having no magnesium ion as confirmed by EDS

Please cite this article as: B. Joshi, et al., IR transparent hot pressed Mg-α/β-Sialoceramint.2014.05.001

analysis of point 3. Also, in previous work, no metal cationwas found in β-phase [19]. Metal cations are found in α-Sialonphase to stabilize its crystal structure but β-Sialon phase isstable without metal cations.In present work, with the addition of BaCO3, small sized

grains with higher percentage of α-Sialon phase were observed.However, only with MgO, large grain sized β-Sialon phase wasobserved [20]. BaCO3 addition forms fitted polyhedral smallgrains were observed with less grain boundary phase as shownin Fig. 5. Polyhedral grains are formed in very less amount ofliquid phase in Sialon ceramics. As in previous work higher wt% of Y2O3 doped Sialon shows granular shaped grains thanlower wt% [19]. The EDS analysis shows that Mg cation wasfound in α-Sialon structure but β-Sialon had no Mg or Ba cationin it, rather has some aluminum diffusion.The precursor α-silicon nitride powder contained SiO2 about

2–2.5 wt% and barium cation is a strong SiO2-networkmodifier and is expected to segregate at SiO2-glass interface,thus systematically lowering their viscosity. Thus, the tightgrain structures were observed with thin grain boundary.Pezzotti and Nishumura [21] reported that with addition oflittle BaO in SiC did not have any effect on grain size. Also, inour system, there was no effect on grain size observed byTEM, and a tight grain structure with no pores, suggesting analmost complete densification shown in Fig. 5. The averagegrain size was below 0.5 μm, as observed from the micro-structure. The thin grain boundaries with fitted polyhedralgrain structures increased the light transmittance by decreasingthe light scattering centers (grain boundary phase and pores).

3.2. Densification

Densification of the silicon nitride based ceramics isgenerally affected by the additives, sintering temperature andpressure. The liquid phase obtained with MgO has highermelting point and viscosity, which results the lower density.The addition of BaCO3 lowers the hot press temperature byforming and melting of Celsian around 1200 1C having lowviscosity that enhanced the densification process. Almost pore

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free densified Sialon ceramics were sintered with BaCO3

addition. The densities with different wt% of BaCO3 wereobserved and are listed in Table 1. From the microstructure,observed by TEM shows tight grain structure with no poressuggested that almost complete densification took place asshown in Fig. 5. Increasing the amount of BaCO3 increases thedensity of the Sialon ceramics. The formation of BaO afterdecomposition of BaCO3, have low melting point than that ofMgO and formation of liquid phase at lower temperatureadjusted the viscosity of grain boundary phase for particlerearrangement. Thus, densification can be improved withBaCO3 addition. The properties of Sialon ceramics rely onthe densification.

3.3. Mechanical properties

Young's modulus and Poisson ratio are shown in Table 2.Addition of BaCO3 increased Young's modulus value due toincrease in α-phase and also barium cation is strong SiO2-network modifier that can give tighter bonding between thegrains. The small α-Sialon phase with lesser grain boundaryphase obviously increased Young's modulus.

Vickers hardness of a material depends upon the densifica-tion. However, in multiphase ceramic composites, the differentphases also determine the hardness of the material. In Sialonceramics, α-Sialon phase had higher hardness than β-Sialonphase due to their small grain size and have high atomicdensity in unit crystal structure. BaCO3 addition shows highhardness because of higher densification and higher α-Sialoncontent than without BaCO3 as in sample SN. High meltingpoint and high viscosity of MgO lowers the wetting andrearrangement of particles during sintering but addition ofBaCO3 enhances the densification process during sintering.Without proper wetting of the particle during sintering lowersthe possibility of Mg2þ ion diffusion in α-Si3N4 lattice to formα-Sialon, therefore, higher β-Sialon phase was observed in SNthan BaCO3 added Sialon ceramics. β-Sialon is large sizedgrains as compared to the α-Sialon that was observed in higherpercentage for sample SN and therefore it had lower hardness.The hardness values of different composites are given inTable 2.

Lawn and Fuller equation [22] was used to calculate thefracture toughness of Sialon ceramic composites. Fracturetoughness was examined on the basis of indentation cracklength measurement during Vickers hardness test. The addition

Table 2Mechanical properties of different sintered Sialon ceramics.

Samples Vickershardness(GPa)

Fracturetoughness,KIC (MPa m1/2)

Young'smodulus(GPa)

Poisson'sratio

SN 18.9670.36 5.2770.09 311 0.284SAMB025 20.6170.65 6.3270.11 326 0.271SAMB05 21.4170.77 5.3670.07 334 0.268SAMB075 20.9270.46 4.6670.09 338 0.265SAMB1 20.4670.44 4.6270.07 337 0.269

Please cite this article as: B. Joshi, et al., IR transparent hot pressed Mg-α/β-Sialoceramint.2014.05.001

of little amount (0.25 wt%) of BaCO3 shows high fracturetoughness but increasing the amount of BaCO3 had negativeeffect on fracture toughness due to increase in α-Sialon withreduced grain size. Fracture toughness values are shown inTable 2. Elongated β-Sialon phase have high fracture tough-ness than small equiaxed α-Sialon phase.

3.4. Transparency

Without BaCO3 addition, 35.5% of β-Sialon phase wasobserved. In previous work, although addition of little amountof different rare earth oxides to Si3N4–MgO–AlN enhance theformation of α-phase, the light transmittance was not improved[19,20]. The reason behind the poor transmittance is due to theincrease in intergranular phase having different refractiveindices to that of α-Sialon phase. The refractive index ofintergranular phase plays an important role in the transparencyof the silicon nitride based ceramics [14]. The mismatchbetween the refractive indices of two phases causes the morescattering and absorption of light. It is difficult to calculate therefractive index of grain boundary phase in sintered ceramicsbut can be assumed that large atoms like barium had higherpolarizability than magnesium. High polarizabiltiy of bariumion can increase the refractive index of intergranular phasecloser to α-Sialon. BaO formed had high refractive index(n¼1.98) [23] and is near to the refractive index of α-Sialon(n¼2.15) [24]. The refractive index of MgO (n¼1.74) [14] islower than that of BaO and therefore, addition of BaCO3

increased the refractive index of intergranular phase and lowerthe gap of difference between the grain and grain boundaryphase. However, BaO do not exist in grain boundary phase but(Ba, Mg) –Si–Al–O–N and (Ba, Mg) –Si–O–N glassy oxini-tride glassy phase must exist as intergranular phase. Theaddition of barium in Ca–Si–O–N glass increased the refrac-tive index than magnesium addition as reported by Sharafatet al. [15]. Thus, the light transmission was dramaticallyincreased with BaCO3 addition.The effect of polaron (electron trapped in an electrical field

of surrounding ions in insulators) on infrared absorption ontranslucent Si3N4 ceramics was observed by Yang et al. [25].Herein, we also observed the ESR spectra for hot presssintered Sialon ceramics and g values were larger thanprecursor silicon nitride powder as shown in Fig. 6. Siliconnitride powder has g value of 2.0037, which is similar to the gvalue given by Wang et al. [26]. And they also reported thatwith the compaction of powder, the g value was increased to2.005. These both g values are for Si dangling bonds but lowerg value (2.003) is for Si–N3 dangling bonds and higher g value(2.005) is for Si–Si3 dangling bonds [26]. In our sinteredSialon ceramics, g values for bulk SN and SAMB05 are 2.005had 2.0048, respectively. As shown in Fig. 6(a), the ESRspectrum obtained for silicon nitride powder has symmetricline and signal is for single silicon dangling bond Si–N3.Whereas, ESR spectra obtained for bulk SN and SAMB05 areof asymmetric lines and these asymmetric lines are arise due tosuperposition of the signal of silicon dangling bond and otherdangling bonds. During compaction of powder large numbers

n:Ba2þ ceramics, Ceramics International (2014), http://dx.doi.org/10.1016/j.

Fig. 6. ESR spectra showing peak height and width as well as g value for different samples: (a) silicon nitride powder (precursor), (b) SN (without BaCO3) bulkceramics, and (c) SAMB05 (0.5 wt% BaCO3) bulk ceramics.

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of dangling bonds comes closer and spin density increased insintered body than powder. However, these dangling bonds arein rigid lattice in sintered body and shows weak ESR signalthan in powder. The lower g value of SAMB05 than SN showsthat the spin density is lower than SN and infrared absorptionby polaron is weaker in SAMB05 than SN. The unpairedelectrons or polaron absorb light of higher wavelength butcannot follow the high frequency light of lower wavelength.Therefore, the effect of polaron can be seen for IR transparentmaterials. The fabricated Sialon ceramics are IR transparentand SAMB05 has higher transparency with weak ESR peakand less g value than SN. The result shows that there is someeffect of polaron on IR transmittance in fabricated Sialonceramics.

Highest transmission of light was observed for SAMB05(0.5 wt% of BaCO3) in NIR around 78%, for 0.5 mm thicknesssample as shown in Fig. 7. Reduction in intergranular phasealso improved the transmission in the material. Different phaseslike α, β, and intergranular phase have different refractive indices,

Please cite this article as: B. Joshi, et al., IR transparent hot pressed Mg-α/β-Sialoceramint.2014.05.001

thus single phase with intergranular phase having similar refrac-tive index with grains is better for higher transmission. Higherlight transmission for BaCO3 added Sialon ceramics was due tothe higher percentage of α-phase with similar refractive index ofintergranular phase. Addition of little amount of BaCO3 enhancedthe densification process by eliminating pores that are responsiblefor more light scattering, resulting the increase in α-Sialon phase.The increase in Ba cation content in intergranular phase givesgood bonding between the grains that simultaneously causes thegood transmittance.

4. Conclusions

Dense Sialon composite ceramics were fabricated with differentwt% of BaCO3 addition. The fabricated Sialon ceramics exhibitshigher light transmittance in IR region. Sintered Sialon bodiesshow higher α-Sialon phase with increasing the amount of BaCO3.α-Sialon phase is considered to have better hardness and opticalproperties than β-Sialon phase. However, the fracture toughness

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Fig. 7. Light transmission spectra of different added BaCO3 Sialon ceramics inthe visible and NIR regions for 0.5 mm-thick samples. In addition, an opticalimage of the 0.5 wt% BaCO3 Sialon ceramics (0.5 mm thickness) is alsoshown.

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was lower than that of Sialon ceramics without BaCO3 but showsbetter fracture toughness than other single phase α-Sialon ceramicsreported by other researchers. BaCO3 addition gives highrefractive index glassy phase than that of only MgO added glassyphase and nearly match with the refractive index of α-Sialon phaseshowing better light transmittance. Moreover, the ESR peak wasobserved for SAMB05 and was weaker than SN ESR peak andtherefore, the interference of polaron on IR radiation is lower onBaCO3 added Sialon ceramics. These fabricated translucent Sialonceramics exhibits good mechanical properties as well as good IRtransmittance. The material shows much better mechanical proper-ties than transparent YAG, AlON, Y2O3, spinel, etc. This materialcan be widely applied, where abrasion and corrosion-resistant IRtransparent materials are needed.

Acknowledgments

This research was supported by the Global Research Lab.Program of the National Foundation of Korea (NRF) fundedby the Ministry of Education, Science and Technology(MEST) of Korea (Grant number 2010-00339).

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