Core/shell volume effect on the microstructure and mechanical properties of fibrous Al 2O 3–(m-ZrO 2)/t-ZrO 2 composite

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Core/shell volume effect on the microstructure and mechanical properties of fibrous Al 2O 3–(m-ZrO 2)/t-ZrO 2 composite

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  Materials Science and Engineering A 432 (2006) 317–323 Core/shell volume effect on the microstructure and mechanicalproperties of fibrous Al 2 O 3 –(m-ZrO 2 )/t-ZrO 2  composite Byong Taek Lee a , ∗ , Swapan Kumar Sarkar a , Asit Kumar Gain a ,Soo-Jae Yim b , Ho Yeon Song b a School of Advanced Materials Engineering, Engineering College, Kongju National University, 182 Shinkwan-dong,Kongju City, Chungnam 314-701, South Korea b School of Medicine, Soonchunhyang University, 366-1 Ssangyoun-dong, Cheonan City, Chungnam 330-090, South Korea Received 3 March 2006; accepted 10 June 2006 Abstract Al 2 O 3 –(monoclinic-ZrO 2 )/tetragonal-ZrO 2  composites (Al 2 O 3 –(m-ZrO 2 )/t-ZrO 2 ) were fabricated by the multi-pass extrusion process to intro-duce continuously fibrous textures with different core/shell volume fractions of Al 2 O 3 –(m-ZrO 2 ) and t-ZrO 2  (60/40, 50/50, 40/60, 30/70 and20/80). The values of the bending strength and fracture toughness increased as the t-ZrO 2  shell volume fraction increased, with maximum values of 1031MPa and 8.1MPam 1/2 in the 20 (core)/80 (shell) composite, respectively. However, the Vickers hardness value decreased as the core volumefraction decreased due to the increase of t-ZrO 2  content.© 2006 Elsevier B.V. All rights reserved. Keywords:  Core/shell volume effect; Alumina–zirconia; Fibrous microstructure 1. Introduction Al 2 O 3 –ZrO 2  composites have been considered excellentmaterials for use in many industrial components requiring highdegrees of strength, wear resistance, and corrosion resistance,as well as good oxidation and thermal stability at high temper-atures, due to their excellent mechanical, thermal and chemicalproperties [1]. The biocompatibility of the Al 2 O 3 –ZrO 2  com-posites also makes them appropriate for use in bio-implants,such as total hip and joint prostheses and dental implants [2–4].However, their inherently low fracture toughness limits theirpotential for biomedical applications. In order to enhance thefracture toughness, many researchers have focused on the fab-rication of composites by dispersion of the second phase, eitherceramic or metal [5]. The approaches have been based on the incorporation of fibers, whiskers or particles as reinforcementusingballmilling[6],electrolessdeposition[7],sol–gelprocess [8],etc.Ithasbeenreportedthatthewhiskerandfiberreinforcedceramic matrix composites have been focused on during the lastdecade because of their remarkable fracture toughening due to ∗ Corresponding author. Tel.: +82 41 8508677; fax: +82 41 8582939.  E-mail address:  lbt@kongju.ac.kr (B.T. Lee). crack bridging and fibrous pull-out mechanisms. However, toobtain the composites using them, many problems occurred inthe fabrication process and it was quite costly [9].Recently, an interesting approach, based on macroscaledmicrostructure control, using the multi-pass extrusion process,has been recognized as an effective way to improve the fracturetoughness of brittle ceramics. Extensive works and the in-depthinvestigation have been reported by the work of Halloran andothers on Si 3 N 4  /BN fibrous monoliths [10–12]. Furthermore, usingthisprocess,thefibrousmicrostructure,aswellasthecon-tinuously porous bodies, can be easily controlled [9,13,14]. In previous work, well controlled, fine, homogeneous Al 2 O 3 –(m-ZrO 2 )/t-ZrO 2  fibrouscompositeof50/50core/shellcompositionwith excellent mechanical properties was obtained [14]. The fracture toughness was remarkably increased (9.6MPam 1/2 )due to toughening mechanisms, such as crack bridging, microc-racksandphasetransformation.Althoughitwasrecognizedthatthe material properties of Al 2 O 3  /ZrO 2  composites are stronglydependent on the volume fraction of constituent phases, therewas no report on the core/shell volume effect on the microstruc-ture and mechanical properties.In this work, the fibrous Al 2 O 3 –(m-ZrO 2 )/t-ZrO 2  compos-ites with different core/shell volume fractions were fabricatedusing the multi-pass extrusion process. The basic concept of the 0921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.msea.2006.06.058  318  B.T. Lee et al. / Materials Science and Engineering A 432 (2006) 317–323 microstructure design was to introduce different volume ratiosof core/shell structure; i.e. (1) the core comprised with Al 2 O 3 and 25vol.% m-ZrO 2  that enhance the microcracks and crack deflection, (2) the shell of t-ZrO 2  phase that can be introducedwiththephasetransformationtougheningmechanismand(3)thefibrous microstructure control that leads to the crack deflectionand crack bridging phenomena. Furthermore, the relationshipbetween microstructure and material properties was investi-gated, depending on the volume ratio of core/shell. 2. Experimental procedure The multi-pass extrusion process was carried out by mixingthe ceramic powder with an organic binder, making a feed rollofpreferentialgeometryandrepeatedlyextrudingthesuccessivefilaments after grouping them in a predetermined arrangement.ThestartingAl 2 O 3  powder(AKP-50,Sumimoto,Japan),mono-clinicZrO 2  (TZ-0Y,Tosoh,Japan)andtetragonalZrO 2  powders(TZ-3Y,Tosoh,Japan)hadanaverageparticlesizeofabout300,80 and 80nm, respectively. To fabricate the core/shell structureof Al 2 O 3  –  (25% m-ZrO 2 )/t-ZrO 2  composites, the Al 2 O 3 –(25%m-ZrO 2 ) powders were mixed by ball milling for 24h. Thesemixtures of Al 2 O 3 –(m-ZrO 2 ) powders and tetragonal zirconia(t-ZrO 2 ) powder were mixed separately with a polymer binderusing a heated blender (Shina Platec. Co., Suwon, South Korea)at a temperature of 120 ◦ C. Ethylene vinyl acetate (EVA) (Elvax250 and Elvax 210, Dupont, USA) polymers were used asbinders of the ceramic powders. For lubrication during blend-ing, stearic acid (CH 3 (CH 2 ) 16 COOH, Daejung Chemicals &Metals Co. Ltd., Korea) was added. Mixtures of Al 2 O 3 –(m-ZrO 2 )/polymer (depicted as AP) was extruded as a cylindricalrod shape and ZrO 2  /polymer (depicted as ZP) was compactedinto the shell shape by warm pressing. After combining themtogether,theyweremadeintoafeedroll30mmindiameter.Dif-ferentvolumefractionsinthecore/shellweremadebychangingthecorediameterandshellthickness.Thefeedrollwasmountedon a heated extrusion die and then extruded with about a 73:1extrusion ratio. Compositions of the five fibrous composites aregiven in Table 1. The shell thickness and core diameter of the individual composition were made with standard die so that theoverallvolumefractionofeachphasematcheswiththoseshownin Table 1. The volume fractions of the composites described in this paper is thus stands for those of the green bodies.Thefeedrollforeachcompositionwasextrudedtoobtainthefirst passed filaments, which were 3.5mm in diameter. The firstpassed filaments were arranged in the same die and extrudedagain to obtain the second passed filaments with the same diam- Table 1Composition of the fibrous Al 2 O 3 –(m-ZrO 2 )/t-ZrO 2  compositesSample ID Al 2 O 3 –(m-ZrO 2 ) (vol%) t-ZrO 2  (vol%)60/40 60 4050/50 50 5040/60 40 6030/70 30 7020/80 20 SO eter. The process was repeated until the fourth passed filamentswere obtained. The extrusion temperature and rate were 120 ◦ Cand 8mm/min, respectively. The binder was removed from thegreen body during the binder burning-out process, which wascarried out at 700 ◦ C under a flowing nitrogen atmosphere. Thesamples were then sintered at 1450 ◦ C for 2h in air. The den-sity of the sintered bodies was measured using the ArchimedesMethod. The hardness of the sintered bodies was measuredusingtheVickersHardnessTester(HV-112,Akashi,Yokohama,Japan) under a load of 1kg for 10s. To investigate the trend of variation of the fracture toughness,  K  IC  was measured by theindentation method using Evan’s Equation. The equation is, K IC  = 0 . 16 H a 1 / 2 ( c/a ) − 3 / 2 where  H   is the Vickers hardness,  a  the half of indentation diag-onal and  c  is the half of crack length from indentation center.In order to measure the bending strength, a four point bend-ing test was carried out using a Universal Testing Machine(Unitech TM , R&B, Korea). The bend bars were cut 30mm inlength and the diameter was 2.65mm. No surface preparationwasmadebecausethesinteredspecimensweresmoothsurfacedand round shape (see Fig. 7). Microstructural variation of the fibrouscompositeswiththevolumefractionwasexaminedusingaFieldEmissionScanningElectronMicroscope(FE-SEM,JSM6335F, JEOL, Tokyo, Japan) and Transmission Electron Micro-scope (TEM, JEM2010, JEOL, Tokyo, Japan). 3. Results and discussion Fig. 1 shows the cross-sectional SEM images of thefourth passed Al 2 O 3 –(m-ZrO 2 )/t-ZrO 2  composites, where thecore/shell was: (a) 60/40, (b) 40/60 and (c) 20/80. The black and white contrasts are Al 2 O 3 -(m-ZrO 2 ) core and t-ZrO 2  shell,respectively. Thus, it is confirmed that a t-ZrO 2  phase was seenwith a network-type microstructure. The white contrast parti-cles (m-ZrO 2 ) in the black cores were homogenously dispersedin the Al 2 O 3  matrix. A hexagonal, cell-like microstructure, asindicated with dotted lines, was observed in all composites,independent of volume fraction of core/shell. These cells werecomprised with 61 core/shell structures and they were the indi-vidual second passed filaments. Although the second passedfilaments were circular-shaped in the cross section, during therepeated extrusions, they changed to a hexagon-like shape, toform a stable frame structure. The Al 2 O 3 -(m-ZrO 2 ) core diam-eter decreased as the core volume fraction decreased from 60 to20vol% as shown in Fig. 1(a–c). However, the shell thickness increased. The microstructure of the composites gives clear evi-denceofthemicrostructuralvariationretainingthehomogeneity.However,themorphologyofafewAl 2 O 3 –(m-ZrO 2 )coresdevi-ated at the boundary regions which were previously mentionedas individual second passed cells. Similar change in geometryin the green body with extrusion pass was reported in the work Halloran and his group [10,15]. However, in this work we did more one pass extrusion and the geometrical symmetry was stillretained in some extent. In general, during the extrusion pro-cess to obtain the second passed filaments, the circumferentialfirst passed filaments received severe friction near the extru-   B.T. Lee et al. / Materials Science and Engineering A 432 (2006) 317–323  319Fig. 1. Cross-sectional SEM images of fourth passed Al 2 O 3 –(m-ZrO 2 )/t-ZrO 2  composites depending on volume fractions of core/shell: (a) 60/40, (b) 40/60 and (c)20/80.Fig. 2. Longitudinal sectional SEM images of fourth passed Al 2 O 3 –(m-ZrO 2 )/t-ZrO 2  composites depending on volume fractions of core/shell: (a and b) 60/40, and(c and d) 20/80.  320  B.T. Lee et al. / Materials Science and Engineering A 432 (2006) 317–323 sion die wall. Thus, an irregular flow of the mixtures (AP andZP) occurred and the srcinal circular shape was deformed. Thegeometric shape of the cores was nearly circular in cross sec-tion and was uniformly dispersed within the t-ZrO 2  phase withquite similar thickness all over the composite. However, a smalldeviation of geometry compared with the as received feed rollwasobserved.Thiswasbecausetherheologicalpropertyofcore(AP) and shell (ZP) was not exactly the same and the slight mis-match makes a difference in the linear flow behavior of the core(AP)andshell(ZP)materials.Moreover,toobtainthesuccessivepassed filaments, the cylindrical filaments were pre-compactedin the extrusion die before carrying out every extrusion. Thiscaused a lateral flow of the green materials to cover the gaps inbetween the individual filaments, thereby contributing a slightdeformation. But the deviation was nominal and homogeneousas well as very fine scaled microstructure was obtained.Fig. 2 shows the longitudinal section SEM images of theAl 2 O 3 –(m-ZrO 2 )/t-ZrO 2  composites which are comprised with(a) 60/40 and (c) 20/80 composites. In the low magnificationimages (a, c), continuous fibers were clearly observed. Thefibrous monoliths of the core and shell phases were prolongedthrough the entire length. The uni-directional alignment of thefibrousmonolithswaswell-maintained,makingahomogeneousmicrostructure.Intheenlargedimages(b,d),itwasclearthattheshell thickness remarkably increased as the volume fraction of theshellincreased.Theaveragevaluesoft-ZrO 2  shellthicknessand the Al 2 O 3 –(m-ZrO 2 ) core diameter for all the compositesare shown in Table 2.Fig. 3(a) shows the enlarged cross-sectional SEM images of 60/40 composite. Sound core/shell microstructure was clearly Table 2Core diameter and shell thickness of the Al 2 O 3 –(m-ZrO 2 )/t-ZrO 2  compositesSample ID Al 2 O 3 –(m-ZrO 2 ) corediameter (  m)t-ZrO 2  shellthickness (  m)60/40 3.8 1.050/50 3.6 1.340/60 3.3 1.830/70 2.9 2.520/80 2.2 3.4 observed without any processing defects such as large cracksor shrinkage cavities. In the shell region (b), taken from theP mark in Fig. 3(a), it is confirmed that the average grain size wasabout250nmindiameter.However,intheAl 2 O 3 –(m-ZrO 2 )coreregion(c)takenfromQmarkinFig.3(a),thedarkandbright contrastswereAl 2 O 3  andm-ZrO 2  phasesandtheiraveragegrainsizes were about 300 and 200nm, respectively.Fig. 4 shows the TEM images of the 60/40 composites. Inthe low magnification image (a), core/shell microstructure withbright and dark contrast was clearly observed. In the enlargedimageofthecoreregion(b),astrainfieldwasobservedduetothephase transformation of t–m ZrO 2  during the sintering process[16,17]. Some dislocations and twin defects were observed inthe ZrO 2  and Al 2 O 3  grains.The dependence of relative density and Vickers hardness onthe core/shell volume fraction of the Al 2 O 3 –(m-ZrO 2 )/t-ZrO 2 composites is shown in Fig. 5. To calculate the relative density, the initial volume fractions of the constituent phases tailoredin the green ceramic was used in case of sintered ceramic too.As the volume fraction of Al 2 O 3 –(m-ZrO 2 ) core decreased, the Fig. 3. SEM cross-sectional images of (a) fourth passed Al 2 O 3 –(m-ZrO 2 )/t-ZrO 2  composites (60/40). (b and c) Enlarged images of shell and core regions.   B.T. Lee et al. / Materials Science and Engineering A 432 (2006) 317–323  321Fig. 4. TEM images of fourth passed Al 2 O 3 –(m-ZrO 2 )/t-ZrO 2  composite. (a) 60/40 Composite and (b) enlarged image of Al 2 O 3 –(m-ZrO 2 ) core. relative density slightly decreased and their values were around98%. However, the Vickers hardness significantly decreased asthe shell thickness increased. Their values in 60/40 and 20/80composites were about 1443 and 1333 (Hv), respectively. Thereason for showing a decrease of hardness as the core volumefraction decreased may be due to the increase in ZrO 2  content,which has lower hardness than Al 2 O 3 .Fig. 6 shows the bending strength and fracture toughness of the fourth passed Al 2 O 3 –(m-ZrO 2 )/t-ZrO 2  composites depend-ing on the volume fraction of shell and core. Bending strengthincreased almost linearly from 847 to 1031MPa with increasedt-ZrO 2  phase. However, the bending strength of the fibrousmonolithic composites is higher compared to the monolithict-ZrO 2  (830MPa) or Al 2 O 3 –(m-ZrO 2 ) (622MPa) made by thesame processing rout using single pass extrusion .  The valuesare higher compared to those made by analogous process-ing route [18]. The result indicates that the core/shell fibrous microstructure also improves the bending strength. However,the observation shows opposite trend compared to the Si 3 N 4 -BN fibrous monoliths [19]. In this system, the weak inter-phase layer of the BN cause the degradation of the bending strength.But in our system, the two phases were very strongly bonded Fig. 5. Relative density and Vickers hardness of fourth passed Al 2 O 3 –(m-ZrO 2 )/t-ZrO 2  composites depending on volume fractions of core/shell. and intact. The higher bending strength was achieved probablybecause of the unique processing route and lack of large flawsand processing defects. The role of residual stress might havean effect as the thermal expansion coefficient (CTE) of Al 2 O 3 and ZrO 2  phases are different. However, further investigationis needed for the insight for this particular core/shell system of Al 2 O 3 –(m-ZrO 2 )/t-ZrO 2 .Thefracturetoughnessalsoimprovedas the t-ZrO 2  content of the composites increased. For the 60/40composite, the value of fracture toughness was 5.1MPam 1/2 ,whereas for the 20/80 composites, the value increased remark-ablyto8.1MPam 1/2 .Thehigherfracturetoughnessvalueisduetothehighervolumefractionoft-ZrO 2 ,whichundergoesastressinduced t–m phase transformation and absorbs the crack prop-agation energy. However the value is intermediate compared tothat reported elsewhere [9,13,18].During the bending strength measurement, all samples didnotshowtheflatfracturesurfacewhichwasfrequentlyobservedin the ceramic sintered bodies. Fig. 7 shows the fracture modeof the fourth passed (a) 60/40 and (b) 20/80 sintered bodies.In the 20/80 composite, the crack path was larger than that inthe 60/40 composite. Although the bending strength for the for-mer one increased remarkably, still the crack has considerabledeflection. The trend was observed in all the samples we have Fig. 6. Bending strength and fracture toughness of fourth passed of Al 2 O 3 –(m-ZrO 2 )/t-ZrO 2  composites depending on volume fractions of core/shell.
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