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Ceramics International

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Effect of La2O3 addition and sintering mode on the mechanical propertiesand microstructural evolution on an 8YSZ ceramic alloy

A. Muthuchamya, Nidhi Nagarajua, Dinesh K. Agrawalb, A. Raja Annamalaic,⁎

a School of Mechanical Engineering, VIT, Vellore, IndiabMaterial Research Institute, Pennsylvania State University, USAc Centre for Innovative Manufacturing Research, VIT, Vellore, India

A R T I C L E I N F O

Keywords:Lanthanum oxideMicrowave sintering8YSZPyrochlore phase

A B S T R A C T

In this research, the influence of La2O3 addition on the microstructure, phase stability and mechanical propertiesof 8mol% yttria stabilized zirconia (8YSZ) was studied. 8YSZ with La2O3 (9, 12 and 15 wt%) ceramics werefabricated by microwave and conventional sintering at 1400 °C/ 20min and 1400 °C/ 5 h, respectively.Irrespective of the sintering technique, the relative sintered density was found to decrease with increasingamount of La2O3. The grain growth of 8YSZ was enhanced significantly by the addition of La2O3. The XRDresults demonstrated that addition of La2O3 up to 15wt% did not disrupt the cubic 8YSZ phase regardless ofsintering technique; additionally evolution of pyrochlore phase, La2Zr2O7 was observed in all sintered speci-mens. Vickers hardness of 8YSZ ceramic compacts were also found to decrease with increasing amount of La2O3.

1. Introduction

Yttria-stabilized zirconia (YSZ) based ceramics have excellentproperties such as high hardness, low thermal conductivity, good che-mical inertness, high strength and high fracture toughness. Theseproperties make zirconia ceramics as suitable candidates for thermalinsulation barriers in SCWR components [1–3]. Pure zirconia is apolymorphic material, which exhibits a monoclinic crystal structure atlower temperatures, tetragonal structure in the range 1170–2680 °C andcubic structure at temperatures above 2680 °C. During cooling, thephase transformation from tetragonal to monoclinic is associated with alattice expansion of 5%. This lattice strain results in the development ofcracks within the material [4,5]. In order to avoid the undesirabletetragonal to monoclinic phase transformation and also the develop-ment of micro cracks, some lower-valence oxides such as Y2O3 [6] andLn2O3 [7,8] are added to ZrO2. The composites of YSZ + rare-earthoxides partially react to form pyrochlore phase with a general formulaM2Zr2O7 (where M is the rare-earth element). The pyrochlore phaseformed due to the addition of rare-earth elements results in improve-ment of toughness, shock resistance and also the stability of crystalstructure of the ceramic [9]. In general, it is very difficult to obtain highdensities of nanostructured materials due to its strong tendency to ag-glomerate and spontaneous grain growth during sintering [10,11].Therefore, adopting suitable sintering method will play major role inimproving the sinterability of YSZ ceramics. Conventional sintering of

ceramic materials requires very high temperatures to obtain high den-sity of sintered compacts. This normally results in excessive graingrowth and hence it is difficult to produce a fine grain microstructure.

In order to overcome excessive grain growth, microwave sintering(MW) method has been used in this study to produce finer and homo-genous microstructure. In microwave heating of ceramics, heat is gen-erated inside the material due to interaction of electromagnetic waveswith the molecules via dielectric loss mechanism. Advantages of mi-crowave sintering include higher heating rates, selective heating, lowersintering temperatures and shorter soaking time. Higher densities areobtained through microwave sintering than those achieved by con-ventional sintering [12,13]. In general, MW consumes 10–90% lessenergy than in conventional heating [14]. Previous studies [15,16] onLa2O3 +YSZ composites, have reported that La2O3 addition resulted inlowering the thermal conductivity substantially, and increasing thethermal stability of the its coating. However, most of the work[5,17–23] related to fabrication of La2O3 +8YSZ and La2O3 +ZrO2

composites and analyzing their mechanical properties restricted toconventional sintering only. Thermal conductivity studies on coatingsof La2O3 +YSZ composites on metallic substrates were studied by a fewresearchers [24–26]. An earlier study on spark plasma sintering ofLa2O3 +YSZ composites by Arelleno et al. [27] reported high relativedensities and better mechanical properties. The microwave effect onsinterability of zirconia based ceramics, and their mechanical proper-ties were well explained by various researchers [28–31]. Wroe and

https://doi.org/10.1016/j.ceramint.2018.11.028Received 25 June 2018; Received in revised form 12 October 2018; Accepted 5 November 2018

⁎ Corresponding author.E-mail address: raja.annamalai@vit.ac.in (A.R. Annamalai).

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Available online 06 November 20180272-8842/ © 2018 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

T

Rowley et al., [32] for instance, reported that microwave sinteringcauses enhancement of the diffusion mechanisms in YSZ. Nightingaleet al., [33] and Binner et al., [34] reported on microwave sintering of3mol% yttria stabilized zirconia, high densities and finer grain sizes ofsintered samples than those obtained through conventional sintering.However, to the best of our knowledge, so far no study has been re-ported on microwave sintering of La2O3 +8YSZ. The current studyaims at investigating the microwave heating effect on the consolidationof 8YSZ with lanthanum oxide additions and characterization of theirmechanical properties and microstructural evolution. A comparativeanalysis of the mechanical properties of microwave and conventionalsintered samples was also carried out.

2. Materials and methods

2.1. Sample fabrication and property measurements

Starting precursor powders used in this research were 8mol% ofhigh-purity (> 99%) yttria-stabilized cubic zirconia (8YSZ), particlesize of 15–40 µm (Sigma Aldrich India Pvt Ltd, Bangalore, India) andhigh-purity (> 99.9%) lanthanum oxide (La2O3), particle size of20–50 µm (Sigma Aldrich India Pvt Ltd, Bangalore, India). 8YSZ powderwas mixed with 9, 12, 15 wt% of La2O3 powders. The mixtures wereball milled for about 15min at 300 revmin−1 to ensure uniformmixing. Green compacts of cylindrical shape (8mm radius and 2–3mmheight) were produced by pouring the powder mixtures in a tungstencarbide die and compacted using a hydraulic press with 3 ton load. Inorder to reduce friction between the die and punch, zinc stearatepowder was used as a die-wall lubricant. The density of green compactsmeasured was approximately 2.8 g/cm3, that is, 50% of theoreticaldensity (6.10 g/cm3). Three sets of green compacts were prepared forsintering in conventional and microwave furnaces, respectively.Conventional sintering of samples was performed at 1400 °C with adwell time of 5 h and a heating rate of 5 °C per min. For microwavesintering, a multimode microwave furnace with maximum 6 kW poweroperating at 2.45 GHz was used. Though the sintering temperature wasthe same as for conventional sintering, the heating rate used was 50 °Cper min, and the soaking time was 20min. Since zirconia based cera-mics have poor dielectric properties at low temperatures, SiC was usedas a susceptor. The sample temperature was measured using an IR py-rometer focused on the sample surface through a circular hole locatedon the top of the microwave cavity. An important parameter that de-termines the interaction of microwaves with materials is the dielectricloss factor, according to which materials can be classified as transparent(low loss materials), opaque (bulk metals) or absorbing (high loss ma-terials). At room temperature, SiC shows a high loss factor and there-fore couples well with the microwaves at room temperature, whereasZrO2 has a low loss factor. At higher temperatures, the dielectric lossfactor increases resulting into volumetric/bulk heating of ZrO2 and thedesired sintering temperature of 1400 °C is achieved.

The sintered densities of the samples were measured usingArchimedes’ principle. Vickers hardness was assessed by indentationmethod. Prior to hardness measurements all the sintered samples weremounted and well-polished in a disc polisher using diamond paste inorder to identify the indentation diagonals clearly. Vickers microhardness of the samples was measured by using a semi-automaticVickers micro hardness tester (Chennai Metco Private Limited, Chennai,India) at 500 kgf load with the diamond pyramid indenter for 10 s ofholding time. The length of the diagonals of indentation marks weremeasured using an optical microscope. Ten such indents were madethroughout the sample surface and an average hardness value is re-ported. Cylindrical sintered compacts with 5mm diameter and 10mmheight were used for compression tests.

2.2. XRD analysis and microstructural characterization

The phase composition of the sintered ceramics was identified andanalyzed using XRD (BRUKER D8 Advanced, Yokohama, Japan, Cu Kα,λ=1.5405 Å) operated at an exciting potential of 40 kV, and a currentof 30mA. XRD patterns were recorded for all specimens in 2Thetarange of 20–90° at a scan speed of 0.0487deg.per second; the observeddiffracted peaks were identified by using ICDD cards. The latticeparameters of each ceramic composition were evaluated from themeasured diffraction angles. The phase fractions were calculated usingMAUD program which is based on the RITA/RISTA method developedby Lutterotti and Ferrari [35]. The microstructures of the sinteredsamples were observed by scanning electron microscopy (Zeiss PentaFET precision, Model: 51‐ADD0048, Carl Zeiss Pvt Ltd, Bangalore,India). Prior to SEM analysis, the surfaces of the specimens were wellpolished in disc polisher using diamond paste, and were thermallyetched by holding in a furnace at 100 °C below the sintering tempera-ture for about 1 h. Energy dispersive X-ray Spectroscopy (EDS) tech-nique was used to estimate the elemental distribution across the sam-ples. The grain sizes of the samples were measured by the mean linearintercept method. The average grain sizes of the sintered compacts werecalculated using the following equation:

=D (L ) / (N .M)i i (1)

Where Li is the length of the line, Ni is the number of intersections of theline with grain boundaries, and M is the magnification corresponding tothe micrograph of the specimen.

3. Results and discussion

3.1. Phase composition

The X-ray diffractograms of La2O3-8YSZ samples sintered by bothconventional and microwave techniques are shown in Fig. 1. As can beseen from Fig. 1, addition of La2O3 up to 15wt% did not disrupt thecubic 8YSZ phase regardless of sintering technique. There is no evi-dence of any other phases other than the cubic 8YSZ phase andLa2Zr2O7 (pyrochlore) phase. No diffraction peaks corresponding toLa2O3 was identified in the samples. This is due to the dissolution ofLa2O3 in YSZ forming a solid solution phase at high temperatures andwhile cooling, it forms La2Zr2O7 [36] phase. The smaller La2Zr2O7

grains were observed along with 8YSZ grains in SEM analysis (Fig. 4).The intensities of La2Zr2O7 peaks were much weaker than YSZ peaks.The pyrochlore phase was found to be present in all La2O3 added YSZ

Fig. 1. X-ray diffractograms of (a) 8mol% YSZ Powder, (b) La2O3 powder and(c) Conventional (Conv) 8YSZ (d) Microwave (MW) 8YSZ (e) Conv 8YSZ+9wt% La2O3 (f) MW 8YSZ+9wt% La2O3 (g) Conv 8YSZ+12wt% La2O3 (h) MW8YSZ+12wt% La2O3 (i) Conv 8YSZ+15wt% La2O3 and (j) MW 8YSZ+15wt%La2O3.

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ceramic compacts. The presence of pyrochlore phase in specimensprepared by both the sintering temperatures is consistent with the ZrO2-La2O3 phase diagram [37] as well as the results are consistent with theprevious reports [5,19,22,23] carried out on YSZ+La2O3 system. Thepeaks shifts in X-ray diffractograms of MW sintered samples in com-parison with conventional counterparts are shown in Fig. 2. The phaseconstituents and their weight fractions obtained from X-ray diffracto-grams are shown in Table 1. Calculated lattice parameters from XRDpeaks of cubic phase are also listed in Table 1. The lattice parameter ofconventional and microwave sintered 8mol% YSZ was 5.018 A°, themean lattice parameter of 8YSZ increased with the addition of La2O3.The increment in the lattice constants of 8 YSZ by the addition of La2O3

is mainly due to replacement of Zr4+ ions by larger size La3+ ions incubic crystal lattice of 8YSZ [38]. Since La3+ has a large ionic radius,this results in the formation of a solid solution with an enlarged latticeand hinders Zr diffusion and grain growth. Diffusion of La3+ is more in8YSZ lattice during microwave sintering, as ionic diffusion is relativelyfaster in MW than in conventional sintering. This has resulted in anincrease in lattice parameter of microwave sintered ceramics. The lat-tice parameter values obtained in the present study are in closeagreement with the previous works [19,22].

3.2. Microstructural development and grain size

3.2.1. Effect of sintering mode on microstructureFig. 3 displays SEM images of 8mol% YSZ and La2O3 starting

powders. The SEM micrographs of microwave and conventional

sintered specimens of 8mol%YSZ ceramics with 9, 12 and 15wt%La2O3 are shown in Fig. 4. Equi-axed grain morphology was observed inall the sintered samples. A minimal residual porosity was observedalong the grain boundaries. It is evident from the micrographs that bothmicrowave and conventional sintered specimens were sintered well.From the previous research studies [28,39,40], it is noted that micro-wave sintering provides fine microstructures even at high sinteringtemperatures [40,41]. The rapid heating and short soaking time duringmicrowave processing limits grain growth. In this work also, disparitiesin the grain sizes were observed between the CS and MW sinteredsamples. From SEM micrographs it is observed that MW sintering re-sults into homogenous microstructures with finer grain size (Table 1).This is mainly due to high ramp rate (50 °C/min) used in the microwavesintering. In conventional sintering, slower heating rate (5 °C/min) isemployed to prevent thermal fluctuations within the sample; hencethere is considerable increase in the sintering time at high temperatureleading to grain coarsening.

3.2.2. Effect of La2O3 content on microstructureIrrespective of the experimental conditions, La2O3 −8YSZ ceramics

show higher mean grain size than the 8YSZ. MW sintered YSZ has loweraverage grain size of 0.292 µm as compared with conventional sintered8YSZ of 3.5 µm. The increase in the grain growth of 8YSZ matrix withthe addition of La2O3 is mainly because of high diffusion coefficient anddissolution of La2O3 in YSZ matrix [5]. The bimodal microstructures ofconventional 12 wt% and microwave sintered 9 and 15 wt% La2O3

added YSZ ceramics are shown in Fig. 5. EDAX analysis of microwavesintered 8YSZ+9wt% La2O3 (Fig. 6) ceramics is confirmed by thepresence of Zr, O, Y, La elements. The evolution of new phase in theLa2O3 −8YSZ compacts can be observed in Fig. 4b, c and d. The de-veloped phase precipitated inside the grain and at grain boundaries of9, 12 and 15wt% of La2O3 −8YSZ ceramic and is identified as La2Zr2O7

on the basis of XRD analysis (Fig. 1). The amount of La2Zr2O7 phaseprecipitated from the 8YSZ solid solution increases with increasingLa2O3 content. In agreement with the microstructural features noticedin the present study, previous literature also reported the presence ofLa2Zr2O7 pyrochlore phase in micrographs of La2O3 with YSZ ceramics[17,27,37,42–44].

3.3. Sintering behavior of 8YSZ+La2O3 Ceramic compacts

3.3.1. Effect of sintering process and La2O3 addition on densityThe effect of microwave sintering and La2O3 addition on the sin-

terability of 8YSZ is graphically shown in Fig. 7. Microwave sinteredceramics showed higher sintered densities even at lower sintering time(20min) than the conventional sintered samples (5 h) while main-taining the same sintering temperature of 1400 °C in both cases. It wasobserved that the 8YSZ+La2O3 ceramics coupled well with microwavesand gets heated up rapidly. According to the heat transfer mechanism

Fig. 2. Peak shift in X-ray diffractograms of MW sintered samples in compar-ison with conventional counterparts.

Fig. 3. Scanning electron micrographs of as received (a) 8 mol% YSZ powder and (b) La2O3 powder.

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within samples, in CS, the thermal energy is transferred from the sur-face to the interior of the compact through thermal conductivity me-chanism. It is a sluggish process and hence takes relatively longer timeto get thermal equilibrium. This also often results in poor sintereddensities. The high sintered densities in microwave processing were

because of effective high rate of material diffusion due to an additionalnon-thermal effect. Irrespective of the sintering technique, the % sin-tered densities of ceramic samples decreased with increasing amount ofLa2O3 [19,45]; it is observed that La2O3 addition hinders the densifi-cation of the specimens. The relative density of conventional sintered

Fig. 4. Scanning electron micrographs of sintered 8YSZ compacts with (a) 0 wt% La2O3 (b) 9 wt% La2O3, (c) 12 wt% La2O3, (d) 15 wt% La2O3 [conventional (left)and microwave (right)].

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Table 1phase composition and lattice parameter of 8YSZ-La2O3 ceramics.

Composition Sintering method Grain size (µm) wt% of phases Lattice parameters

YSZ La2Zr2O7

8YSZ CS 1400-5 h 3.5 ± 0.21 100 – 5.018 ± 0.09MW 1400-20 min 0.29 ± 0.01 100 – 5.018 ± 0.09

9La2O3 +8mol% YSZ CS 1400-5 h 4.48 ± 0.94 88.24 11.76 5.119 ± 0.018MW 1400-20 min 0.78 ± 0.01 90 10 5.086 ± 0.0067

12La2O3 + 8mol% YSZ CS 1400-5 h 6.34 ± 0.01 82.9 17.1 5.122 ± 0.0031MW 1400-20 min 0.92 ± 0.06 89 11 5.099 ± 0.0075

15La2O3 + 8mol% YSZ CS 1400-5 h 9.44 ± 1.17 72 28 5.128 ± 0.0124MW 1400-20 min 0.89 ± 0.061 79 21 5.1076 ± 0.0024

Fig. 5. Microstructures of La2O3-8YSZ composites (a) Conventional sintered 12wt%+8YSZ (b) Microwave Sintered 9La2O3 +8YSZ (c) Microwave Sintered 15 wt%La2O3 + 8YSZ showing pyrochlore phase.

Fig. 6. Elemental analysis of 9 wt% La2O3-8YSZ (microwave sintered).

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8YSZ is 90.5%, which is reduced to 87%, 85%, and 83.5% by the ad-dition of 9%, 12% and 15% La2O3, respectively. Similar densificationresponse observed in microwave sintering. It is apparent from the SEMimages (Fig. 4) of La2O3 added YSZ ceramics, the amount of La2Zr2O7

phase becomes richer in the matrix of 8YSZ with increasing La2O3

content. The precipitated La2Zr2O7 phase slows the diffusion rate ofatoms at grain boundaries of 8 YSZ, as a result there is an increase in thegrain boundary diffusion [19].

3.4. Vickers hardness

Vickers hardness of La2O3 +YSZ composites sintered by microwaveand conventional methods are presented in Fig. 8 as a function of wt%La2O3. It can be seen from Fig. 8, that microwave sintered 8YSZ com-posites showed the highest hardness among all samples. From the re-sults it is observed that the hardness of the 8YSZ+La2O3 compositesdecreases with increasing La2O3 content. Microwave sintered 8YSZsample achieved maximum hardness of 12.86 GPa which decreases by11%, 15% and 17% with respect to 9, 12 and 15wt% of La2O3 addition,respectively. Hardness values of conventionally sintered samples alsofollowed the decreasing trend. Previous studies [19,46] reported thesimilar effect of La2O3 on the Vickers hardness of YSZ+La2O3. Thedecrease in Vickers hardness with increasing La2O3 content was due tothe difference between hardness values of pyrochlore, La2Zr2O7 (9.9 ± 0.4 GPa) and 8YSZ(13 ± 1GPa). In summary, MW sinteredsamples showed high hardness values than those conventionally pro-cessed materials at same sintering temperature (1400 °C) with differ-ences in dwell times.

4. Conclusions

In the present research densification, microstructural features andVickers hardness of the 8YSZ based ceramics added with 9, 12 and15wt% of La2O3 prepared by microwave and conventional sinteringwere studied. The results can be concluded as the following.

1) La2O3 addition has no effect on destabilization of the cubic phase of8YSZ. With increasing La2O3 dopant concentration, grain growthobserved in 8YSZ ceramics and also precipitation of secondaryLa2Zr2O7 phase on grains and grain boundaries of 8YSZ.

2) Addition of La2O3 had adverse effects on sinterability of 8YSZ spe-cimens.

3) Formation of pyrochlore La2Zr2O7 phase throughout the micro-structures of the La2O3 with 8YSZ ceramics was accompanied by areduction in hardness and compressive strength of the sinteredcompacts.

4) Higher Vickers hardness value was obtained for the microwavesintered pure 8YSZ than in the conventionally sintered sample.

Acknowledgement

The authors would like to convey their gratitude toward theDepartment of Science and Technology (DST-SERB) for their financialsupport in the course of this project. (YSS/2016/001525).

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