MATERIALS 兴 HIENGE& ENGIEERING ELSEVIER Materials Science and Engineering A 458(2007)11-16 www.elseviercom/locate/msea Relationship between microstructure and mechanical properties of fibrous HAp-(t-ZrO2)/Al2O3-(m-ZrO2) composites Byong-Taek Lee Chi-Woo Lee a, Min-Ho Youn, Ho-Yeon Song a School of Advanced Materials Engineering, Kongju National University, 182 Shinkwan-dong, Kongju Ciry, Chungnam 314-701, South Korea Department of Microbiology, School of Medicine, Soonchunhyang University, 366-1 Ssangyoung-dong. Cheonan Ciry, Chungnam 330-090, South Korea Received 22 September 2006: received in revised form 29 November 2006: accepted 29 November 2006 Abstract The microstructure and mechanical properties of core/shell, fibrous HAp-(t-ZrO2NAl2O3-(m-ZrO2)composites, which were fabricated by the multi-pass extrusion process, were investigated depending on the sintering temperature. In the composite sintered at 1200C, many pores were observed in both HAp-(If-ZrO2)core and Al2O3-(m-ZrO2)shell regions. However, at 1500C, the shell regions showed dense and anisotropic grain growth and the core region were transformed to a-TCP and B-TCP phase. Also, as a minor reaction phase, the CaAl1O1g peaks detected. The values of relative density, hardness, bending strength and fracture toughness increased as the sintering temperature increased, and their maximum values were at 1500C about 890 Hv, 280 MPa and 4. 1 MPam", respectively. The fracture morphology was appeared with homogeneously rough surface, and indentation cracks showed short length due to the crack deflection and microcracking toughening mechanisms 2006 Elsevier B v. All rights reserved. Keywords: Hydroxyapatite; Composite; Microstructure; Mechanical properties 1. ntroduction On the other hand, for the application of bioimplant, Al2O3, ZrOz and their composite ceramics have been considered as a Because of the excellent biocompatibility and bioactive matrix as well as reinforcement phases due to their excellent characteristics, calcium phosphate based ceramics have been oxidation resistance, good biocompatibility and wear resistance received attention for the application of bone substitutes, scaf- [7-14. Shen et al. reported that the bending strength and the olds for tissue engineering and drug delivery system [1-3]. fracture toughness of HAp-50 vol %o ZrOz composite had high The most widely used calcium phosphate based bioceramics with 440 MPa and 2.5 MPam", respectively [15]. Kim et al are hydroxyapatite(HAp, Ca3(PO4)6(OH)2)and p-tricalcium made HAp-ZrO2 composite by pressureless sintering using cal phosphate(B-TCP, Ca3(PO4)2). Especially, the composition cium fluoride( CaF2)sintering additive and the values of bending of HAp is similar to the inorganic part of the natural bone strength and fracture toughness were achieved about 180 MPa and stable in body fluid. However, for the application of load and 2.3 MPam, respectively [16]. In general, the HAp crys- bearing part, it has been limited due to its inherent low mechan- talline phase can be easily transformed to B-TCP when the ical properties. Unfortunately, the fracture toughness of the HAp compact body was sintered at over 1200C. However, monolithic HAp ceramic does not exceed I MPam 2 that is no found the detailed reports on the microstructure change and compared with 2-12 MPam for human bone[4]. However, to fracture behavior properties of HAp and B-TCP-Al2O3-ZrO2 improve the mechanical properties of HAp ceramic, there were sintered bodies. Recently, the fibrous composites have been eas many approaches on the microstructure control of HAp sintered ily controlled using the novel fibrous monolithic process, which body [5]. Using the MgO-P2Os sintering additives, the fracture is frequently called multi-pass extrusion process, and also the toughness was not improved although the relative density was mechanical properties such as fracture toughness and strength remarkably increased 6] were remarkably improved due to the multi toughening mecha In this work, the fibrous HAp-(t-ZrO2)/Al2O3-(m-ZrO2 Corresponding author. Tel: +82 41 850 8677: fax: +82 41 858 2939 composite was fabricated to improve the mechanical proper E-mail address: Ibt @kongju ac kr(B.-T. Lee) ties using the multi-pass extrusion process. In addition, we 0921-5093/S-see front matter 2006 Elsevier B v. All rights reserved
Materials Science and Engineering A 458 (2007) 11–16 Relationship between microstructure and mechanical properties of fibrous HAp-(t-ZrO2)/Al2O3-(m-ZrO2) composites Byong-Taek Lee a,∗, Chi-Woo Lee a, Min-Ho Youn a, Ho-Yeon Song b a School of Advanced Materials Engineering, Kongju National University, 182 Shinkwan-dong, Kongju City, Chungnam 314-701, South Korea b Department of Microbiology, School of Medicine, Soonchunhyang University, 366-1 Ssangyoung-dong, Cheonan City, Chungnam 330-090, South Korea Received 22 September 2006; received in revised form 29 November 2006; accepted 29 November 2006 Abstract The microstructure and mechanical properties of core/shell, fibrous HAp-(t-ZrO2)/Al2O3-(m-ZrO2) composites, which were fabricated by the multi-pass extrusion process, were investigated depending on the sintering temperature. In the composite sintered at 1200 ◦C, many pores were observed in both HAp-(t-ZrO2) core and Al2O3-(m-ZrO2) shell regions. However, at 1500 ◦C, the shell regions showed dense and anisotropic grain growth and the core region were transformed to -TCP and -TCP phase. Also, as a minor reaction phase, the CaAl12O19 peaks detected. The values of relative density, hardness, bending strength and fracture toughness increased as the sintering temperature increased, and their maximum values were at 1500 ◦C about 890 Hv, 280 MPa and 4.1 MPa m1/2, respectively. The fracture morphology was appeared with homogeneously rough surface, and indentation cracks showed short length due to the crack deflection and microcracking toughening mechanisms. © 2006 Elsevier B.V. All rights reserved. Keywords: Hydroxyapatite; Composite; Microstructure; Mechanical properties 1. Introduction Because of the excellent biocompatibility and bioactive characteristics, calcium phosphate based ceramics have been received attention for the application of bone substitutes, scaffolds for tissue engineering and drug delivery system [1–3]. The most widely used calcium phosphate based bioceramics are hydroxyapatite (HAp, Ca3(PO4)6(OH)2) and -tricalcium phosphate (-TCP, Ca3(PO4)2). Especially, the composition of HAp is similar to the inorganic part of the natural bone and stable in body fluid. However, for the application of load bearing part, it has been limited due to its inherent low mechanical properties. Unfortunately, the fracture toughness of the monolithic HAp ceramic does not exceed 1 MPa m1/2 that is compared with 2–12 MPa m1/2 for human bone [4]. However, to improve the mechanical properties of HAp ceramic, there were many approaches on the microstructure control of HAp sintered body [5]. Using the MgO–P2O5 sintering additives, the fracture toughness was not improved although the relative density was remarkably increased [6]. ∗ Corresponding author. Tel.: +82 41 850 8677; fax: +82 41 858 2939. E-mail address: lbt@kongju.ac.kr (B.-T. Lee). On the other hand, for the application of bioimplant, Al2O3, ZrO2 and their composite ceramics have been considered as a matrix as well as reinforcement phases due to their excellent oxidation resistance, good biocompatibility and wear resistance [7–14]. Shen et al. reported that the bending strength and the fracture toughness of HAp-50 vol.% ZrO2 composite had high with 440 MPa and 2.5 MPa m1/2, respectively [15]. Kim et al. made HAp-ZrO2 composite by pressureless sintering using calcium fluoride (CaF2) sintering additive and the values of bending strength and fracture toughness were achieved about 180 MPa and 2.3 MPa m1/2, respectively [16]. In general, the HAp crystalline phase can be easily transformed to -TCP when the HAp compact body was sintered at over 1200 ◦C. However, no found the detailed reports on the microstructure change and fracture behavior properties of HAp and -TCP-Al2O3–ZrO2 sintered bodies. Recently, the fibrous composites have been easily controlled using the novel fibrous monolithic process, which is frequently called multi-pass extrusion process, and also the mechanical properties such as fracture toughness and strength were remarkably improved due to the multi toughening mechanisms [17,18]. In this work, the fibrous HAp-(t-ZrO2)/Al2O3-(m-ZrO2) composite was fabricated to improve the mechanical properties using the multi-pass extrusion process. In addition, we 0921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.11.155
B -T Lee et al. / Materials Science and Engineering A 458(2007)11-16 are focused on the control of core/shell structure of filaments; by X-ray diffraction(XRD, D/MAX-250, Rigaku, Japan)usin he core structure which contained 20 vol. of t- Cu Ko of 0. 1542 nm ZrOz particles in HAp matrix, can lead to enhance the fracture toughness by phase transformation mechanism, (2) the shell 3. Result and discussion structure, which comprises a dispersion of 25 vol. m-ZrO2 particles in the Al2O3 matrix which can lead to the microc- Fig. I shows the SEM cross-sectional micrographs of (a) acking and crack deflection mechanism and(3)the fibrous third passed fibrous HAp-(t-ZrO2) Al2O3-(m-ZrO2)composite microstructure control to acquire the crack bridging due to which was sintered at 1200C. In the low magnification image the fiber pull-out mechanisms. Furthermore, the relationship (a),(b)the core/shell microstructure having 4 um in diameter between microstructure and material pre operties of fibrous HAp- and I um in thickness was clearly observed with hexagonal unit (t-ZrO2/Al2O3-(m-ZrO2)composite depending on the sintering The hexagonal units were formed during extrusion of the third mperature were investigated using XRD, SEM and TEm passed filaments from arrangement of the second passed fila techniques. ments. From this observation. it is confirmed that the core/shell structure in which was respectively appeared with bright and 2. Experiment procedure dark contrasts was well controlled by the multi-pass extrusion process. The enlarged images(c)and(d)were taken from HAp- o make the fibrous HAp-(20 vol %o t-ZrO2)Al2O3- (t-ZrO2)core and Al2O3-(m-ZrO2)shell regions, respectively (25 vol %o m-ZrO2) composites using multi-pass extrusion From this observation, it is confirmed that the core and shell process, commercial HAp-(t-ZrO2)and Al2O3-(m-ZrO2)mix- regions are porous structure due to the low densification. ture powders were respectively mixed with binder and lubricant Fig. 2 shows the SEM cross-sectional micrographs of(a) using a shear mixer(Shina Platec Co., Korea), in which the third passed fibrous HAp-(t-ZrO2) Al2O3-(m-ZrO2)composite HAp-(20 vol %o t-ZrO2)and Al2O3-(25 vol %o m-ZrO2) mix- which was sintered at 1500C, and(b )are enlarged images of ture were prepared by ball milling process. The commercial HAp-(t-ZrO2)core and Al2O3-(m-ZrO2)shell regions. In the (1)HAp(about 10 um, Strem chemical, USA), Al2O3(about low magnification image(a), there was no significant change 0.3 um, AKP-50, Sumitomo, Japan), t-ZrO2(about 80 nm, of microstructure with Fig. 1(a). Also, there were no found Tosho, Japan), and m-ZrO2(about 80 nm, Tosho, Japan) pow- processing defects such as cracks or large sized shrinkage ders, (2) ethylene vinyl acetate(EVA)(ELVAX 210 and 250, cavities between the core and shell. In the enlarged images Dupont, USA)and(3)stearic acid(CH3(CH2)16COOH, Dae-(b), Al2O3-(m-ZrO2) shell regions were appeared with dense jung Chemicals Metals Co., Korea)were used as raw powders, microstructure due to the high temperature sintering, and Al2O3 binder and lubricant, respectively. The volume percentage of the and m-ZrO2 phases were seen with dark and white contrasts, mixture was 45/40/10 for the HAp-(t-ZrO2)polymer/lubricant respectively. Especially, one of interesting observation is and 50/40/10 for the Al2O3/m-ZrO2)polymer/lubricant. The that the oriented, anisotropic grain growth was observed shear mixed HAp-(t-ZrO2)polymer/lubricant and Al2 O3/m- at the Al2O3-(m-ZrO2)shell regions. On the other hand,a ZrO2)/polymer/lubricant were used to make the rod type core few amounts of pores ound at the HAp-(t-ZrO2)core (22 mm in diameter) and tube type shell(2 mm in thickness), regions due to the decomposition of HAp phase although a few respectively. Then, the core and shell, which consisted the vol- regions were observed with dense microstructure as marked ume ratio with 60/40, were assembled to make feed roll and with"P"region. In the Hap-(t-ZrO2) core, the fine t-zrO2 extruded in heated die to make the first passed filaments about particles were observed with spherical shape as indicated with 3.5 mm in diameter. The first passed filaments were cut and arrowheads loaded in a steel die and extruded to make the second passed fil- Fig. 3 shows the longitudinal SEM micrographs of (a) aments. The third passed filaments were produced by the same third passed fibrous HAp-(t-ZrO2)/Al2O3-(m-ZrO2)composite ay using the second passed filaments. To remove the eva which was sintered at 1500C. In the low magnification images binder, the burning-out process was carried out at 700C for 2h (a), the continuous fibers were clearly observed with alternate under N2 atmosphere. Finally, the pressureless sintering pro- layers. In the enlarged image(b), the HAp-(t-ZrO2)core and cess was carried out in range from 1200 to 1500C for 2 h in air Al2O3-(m-ZrO2) shell regions were appeared with porous and atmosphere dense microstructure, respectively. In the HAp-(t-ZrO2)core Density was measured by the Archimedes method with an region, fine sized t-ZrO2 particles were found as well as a few immersion medium of water. The average Vickers hardness was amounts of pores. measured by indentation with a load of 5 kg(10 points/sample). Fig 4 shows the Xrd profiles of third passed fibrous HAp- To prepare the bending test samples, 3. 4 mm in diameter, the bulk (t-ZrO2)Al2O3-(m-ZrO2) composites which were sintered at samples were cut 35 mm in length and measured by a 4-point 1200'C (a) and 1500C( b). At the low temperature sintering bending method with a crosshead speed of 0. 1 mm/min, using a of 1200C, it is confirmed that the peaks of HAp, t-ZrO2, Al2O3 universal testing machine(UnitechTM, R&B, Korea). The frac- and m-ZrO2 phases were clearly detected without the formation ture toughness was measured by the indentation method using of any reaction compounds. However, at 1500C, the HAp peaks a load of 10kg. Microstructures were examined using back- were not detected due to the phase transformation of HAP to a scattered electron scanning electron microscope(BSE-SEM, and B-TCP phases, and the CaAl12O19 peaks were also detected EOL-JSM 5410)technique. The crystal phases were analyzed as a minor reaction phase
12 B.-T. Lee et al. / Materials Science and Engineering A 458 (2007) 11–16 are focused on the control of core/shell structure of filaments; i.e., (1) the core structure, which contained 20 vol.% of tZrO2 particles in HAp matrix, can lead to enhance the fracture toughness by phase transformation mechanism, (2) the shell structure, which comprises a dispersion of 25 vol.% m-ZrO2 particles in the Al2O3 matrix which can lead to the microcracking and crack deflection mechanism and (3) the fibrous microstructure control to acquire the crack bridging due to the fiber pull-out mechanisms. Furthermore, the relationship between microstructure and material properties of fibrous HAp- (t-ZrO2)/Al2O3-(m-ZrO2) composite depending on the sintering temperature were investigated using XRD, SEM and TEM techniques. 2. Experiment procedure To make the fibrous HAp-(20 vol.% t-ZrO2)/Al2O3- (25 vol.% m-ZrO2) composites using multi-pass extrusion process, commercial HAp-(t-ZrO2) and Al2O3-(m-ZrO2) mixture powders were respectively mixed with binder and lubricant using a shear mixer (Shina Platec. Co., Korea), in which the HAp-(20 vol.% t-ZrO2) and Al2O3-(25 vol.% m-ZrO2) mixture were prepared by ball milling process. The commercial (1) HAp (about 10 m, Strem chemical, USA), Al2O3 (about 0.3m, AKP-50, Sumitomo, Japan), t-ZrO2 (about 80 nm, Tosho, Japan), and m-ZrO2 (about 80 nm, Tosho, Japan) powders, (2) ethylene vinyl acetate (EVA) (ELVAX 210 and 250, Dupont, USA) and (3) stearic acid (CH3(CH2)16COOH, Daejung Chemicals & Metals Co., Korea) were used as raw powders, binder and lubricant, respectively. The volume percentage of the mixture was 45/40/10 for the HAp-(t-ZrO2)/polymer/lubricant and 50/40/10 for the Al2O3/m-ZrO2)/polymer/lubricant. The shear mixed HAp-(t-ZrO2)/polymer/lubricant and Al2O3/mZrO2)/polymer/lubricant were used to make the rod type core (22 mm in diameter) and tube type shell (2 mm in thickness), respectively. Then, the core and shell, which consisted the volume ratio with 60/40, were assembled to make feed roll and extruded in heated die to make the first passed filaments about 3.5 mm in diameter. The first passed filaments were cut and loaded in a steel die and extruded to make the second passed filaments. The third passed filaments were produced by the same way using the second passed filaments. To remove the EVA binder, the burning-out process was carried out at 700 ◦C for 2 h under N2 atmosphere. Finally, the pressureless sintering process was carried out in range from 1200 to 1500 ◦C for 2 h in air atmosphere. Density was measured by the Archimedes method with an immersion medium of water. The average Vickers hardness was measured by indentation with a load of 5 kg (10 points/sample). To prepare the bending test samples, 3.4 mm in diameter, the bulk samples were cut 35 mm in length and measured by a 4-point bending method with a crosshead speed of 0.1 mm/min, using a universal testing machine (UnitechTM, R&B, Korea). The fracture toughness was measured by the indentation method using a load of 10 kg. Microstructures were examined using backscattered electron scanning electron microscope (BSE-SEM, JEOL-JSM 5410) technique. The crystal phases were analyzed by X-ray diffraction (XRD, D/MAX-250, Rigaku, Japan) using Cu K of 0.1542 nm. 3. Result and discussion Fig. 1 shows the SEM cross-sectional micrographs of (a) third passed fibrous HAp-(t-ZrO2)/ Al2O3-(m-ZrO2) composite which was sintered at 1200 ◦C. In the low magnification image (a), (b) the core/shell microstructure having 4m in diameter and 1m in thickness was clearly observed with hexagonal unit. The hexagonal units were formed during extrusion of the third passed filaments from arrangement of the second passed filaments. From this observation, it is confirmed that the core/shell structure in which was respectively appeared with bright and dark contrasts was well controlled by the multi-pass extrusion process. The enlarged images (c) and (d) were taken from HAp- (t-ZrO2) core and Al2O3-(m-ZrO2) shell regions, respectively. From this observation, it is confirmed that the core and shell regions are porous structure due to the low densification. Fig. 2 shows the SEM cross-sectional micrographs of (a) third passed fibrous HAp-(t-ZrO2)/ Al2O3-(m-ZrO2) composite which was sintered at 1500 ◦C, and (b) are enlarged images of HAp-(t-ZrO2) core and Al2O3-(m-ZrO2) shell regions. In the low magnification image (a), there was no significant change of microstructure with Fig. 1(a). Also, there were no found processing defects such as cracks or large sized shrinkage cavities between the core and shell. In the enlarged images (b), Al2O3-(m-ZrO2) shell regions were appeared with dense microstructure due to the high temperature sintering, and Al2O3 and m-ZrO2 phases were seen with dark and white contrasts, respectively. Especially, one of interesting observation is that the oriented, anisotropic grain growth was observed at the Al2O3-(m-ZrO2) shell regions. On the other hand, a few amounts of pores were found at the HAp-(t-ZrO2) core regions due to the decomposition of HAp phase although a few regions were observed with dense microstructure as marked with “P” region. In the HAp-(t-ZrO2) core, the fine t-ZrO2 particles were observed with spherical shape as indicated with arrowheads. Fig. 3 shows the longitudinal SEM micrographs of (a) third passed fibrous HAp-(t-ZrO2)/Al2O3-(m-ZrO2) composite which was sintered at 1500 ◦C. In the low magnification images (a), the continuous fibers were clearly observed with alternate layers. In the enlarged image (b), the HAp-(t-ZrO2) core and Al2O3-(m-ZrO2) shell regions were appeared with porous and dense microstructure, respectively. In the HAp-(t-ZrO2) core region, fine sized t-ZrO2 particles were found as well as a few amounts of pores. Fig. 4 shows the XRD profiles of third passed fibrous HAp- (t-ZrO2)/Al2O3-(m-ZrO2) composites which were sintered at 1200 ◦C (a) and 1500 ◦C (b). At the low temperature sintering of 1200 ◦C, it is confirmed that the peaks of HAp, t-ZrO2, Al2O3 and m-ZrO2 phases were clearly detected without the formation of any reaction compounds. However, at 1500 ◦C, the HAp peaks were not detected due to the phase transformation of HAP to α and -CP phases, and the CaAl12O19 peaks were also detected as a minor reaction phase.
B.-T. Lee et al. Materials Science and Engineering A 458(2007)11-16 0 Fig. 1. SEM micrographs of(a),(b) third passed cross-sectional HAp-(I-ZrO2)Al2O3-(m-ZrO2)bodies sintered at 1200C and(c). (d), enlarged images of core and Fig 5 shows the relative density and vickers hardness of the Fig. 6 shows the bending strength and fracture toughness third passed fibrous HAp-(t-ZrO2)Al2O3-(m-ZrO2)composites of third passed HAp composites depending on the sintering depending on the the sintering temperature. In the sample sintered temperature. In the sample sintered at 1200oC, their value at 1200C, the values of the relative density and hardness werewere low about 75 MPa and 1.8 MPam, respectively comparatively low about 83% and 170 MPa, respectively, due to However, as the sintering temperature increased, the values the low densification as mentioned in Fig. 1. However, as increas- of bending strength and fracture toughness increased due to ing the sintering temperature, the values of relative density and the enhancing of densification. Thus, at 1500C, their values hardness increased due to the enhancing of the densification. were about 280 MPa and 4. 1 MPam", respectively. In general, Thus, the maximum relative density and hardness values were using the conventional processes, the fracture toughness of the obtained at 1500C, and the values were 96% and 890 Hv, HAp-(50 vol %o ZrO2) composites did not exceed the value of respectively. The hardness value also remarkably increased as 2.5 MPa m[16]. However, in this work, using the multi-pass the sintering temperature increased due to the high densification. extrusion process, the fracture toughness of the third passed Shell Fig. 2. SEM ohs of (a)third passed cross-sectional HAp-(t-ZrO2MAlO3-(m-ZrO2) bodies sintered at 1500C and(b), enlarged images of core and shell
B.-T. Lee et al. / Materials Science and Engineering A 458 (2007) 11–16 13 Fig. 1. SEM micrographs of (a), (b) third passed cross-sectional HAp-(t-ZrO2)/Al2O3-(m-ZrO2) bodies sintered at 1200 ◦C and (c), (d), enlarged images of core and shell regions. Fig. 5 shows the relative density and Vickers hardness of the third passed fibrous HAp-(t-ZrO2)/Al2O3-(m-ZrO2) composites depending on the sintering temperature. In the sample sintered at 1200 ◦C, the values of the relative density and hardness were comparatively low about 83% and 170 MPa, respectively, due to the low densification as mentioned in Fig. 1. However, as increasing the sintering temperature, the values of relative density and hardness increased due to the enhancing of the densification. Thus, the maximum relative density and hardness values were obtained at 1500 ◦C, and the values were 96% and 890 Hv, respectively. The hardness value also remarkably increased as the sintering temperature increased due to the high densification. Fig. 6 shows the bending strength and fracture toughness of third passed HAp composites depending on the sintering temperature. In the sample sintered at 1200 ◦C, their values were low about 75 MPa and 1.8 MPa m1/2, respectively. However, as the sintering temperature increased, the values of bending strength and fracture toughness increased due to the enhancing of densification. Thus, at 1500 ◦C, their values were about 280 MPa and 4.1 MPa m1/2, respectively. In general, using the conventional processes, the fracture toughness of the HAp-(50 vol.% ZrO2) composites did not exceed the value of 2.5 MPa m1/2 [16]. However, in this work, using the multi-pass extrusion process, the fracture toughness of the third passed Fig. 2. SEM micrographs of (a) third passed cross-sectional HAp-(t-ZrO2)/Al2O3-(m-ZrO2) bodies sintered at 1500 ◦C and (b), enlarged images of core and shell regions
B -T Lee et al. / Materials Science and Engineering A 458(2007)11-16 SEM micrographs of (a) third passed longitudinal section HAp-(t-ZrO2)Al2O3-(m-ZrO2) bodies sintered at 1500C and (b)enlarged images of core and regions. fibrous HAp composites dramatically increased. To investigate +a-TCP..p-TCP-ZrO, CaAln O the main reason for showing the high fracture toughness the fracture surfaces and crack propagation, which were made by 4-point bending test and Vickers indentation, respectively, were observed by SEM and TEM techniques. Fig. 7 shows the SEM fracture surfaces of (a) third passed fibrous HAp composite sintered at 1500.C and enlarged image (b). In the low magnification image(a), the fracture surface was appeared with homogeneous structure although the pull outing phenomenon off In the enlarged image(b), the dense Al2O3-(m-zrO2) shell region showed mixed fracture mode with intergranular and trans- granular fracture corresponding with rough and flat surfaces respectively, and also, most of shell regions showed locally Fig.4. XRD profiles of HAp-(I-ZrO2MAl2O3-(m-ZrO2)bodies depending on oriented fracture surface due to the existence of anisotropic the sintering temperatures: (a)1200"C, (b)1500C. grain growth as shown in Fig. 2(b). On the other hand, the HAp-(t-ZrO2)core regions were appeared with severely rough surface although some pores also remained. Especially, many 4.5 1200 P Relative density 豆 Bending strength 4.0 o Vickers hardness 250. Fracture tou 1000 1200 Sintering temperature(C) Sintering temperature(C) 5. Relative density and Vickers hardness of third passed HAp-(I- Fig. 6. Bending strength and fracture toughness of third passed HAp-(
14 B.-T. Lee et al. / Materials Science and Engineering A 458 (2007) 11–16 Fig. 3. SEM micrographs of (a) third passed longitudinal section HAp-(t-ZrO2)/Al2O3-(m-ZrO2) bodies sintered at 1500 ◦C and (b) enlarged images of core and shell regions. Fig. 4. XRD profiles of HAp-(t-ZrO2)/Al2O3-(m-ZrO2) bodies depending on the sintering temperatures; (a) 1200 ◦C, (b) 1500 ◦C. Fig. 5. Relative density and Vickers hardness of third passed HAp-(tZrO2)/Al2O3-(m-ZrO2) composites depending on sintering temperature. fibrous HAp composites dramatically increased. To investigate the main reason for showing the high fracture toughness, the fracture surfaces and crack propagation, which were made by 4-point bending test and Vickers indentation, respectively, were observed by SEM and TEM techniques. Fig. 7 shows the SEM fracture surfaces of (a) third passed fibrous HAp composite sintered at 1500 ◦C and enlarged image (b). In the low magnification image (a), the fracture surface was appeared with homogeneous structure although the pullouting phenomenon of fibrous microstructure was not observed. In the enlarged image (b), the dense Al2O3-(m-ZrO2) shell region showed mixed fracture mode with intergranular and transgranular fracture corresponding with rough and flat surfaces, respectively, and also, most of shell regions showed locally oriented fracture surface due to the existence of anisotropic grain growth as shown in Fig. 2(b). On the other hand, the HAp-(t-ZrO2) core regions were appeared with severely rough surface although some pores also remained. Especially, many Fig. 6. Bending strength and fracture toughness of third passed HAp-(tZrO2)/Al2O3-(m-ZrO2) composites depending on sintering temperature
B.-T. Lee et al. Materials Science and Engineering A 458(2007)11-16 Fig. 7. SEM fracture surfaces of (a) third passed HAp-(t-ZrO2MAl2O3-(m-ZrO2)bodies sintered at 1500C temperature and(b),(c)enlarged images of core and shell regions. fine t-ZrO2 particles were observed on the fracture surface Some crack bridging and branching were observed due to the of core region, and this observation is a typical evidence of anisotropic grain growth, when a crack propagated perpendicu microcracking lar into Al2O3-(m-Zro2shell region(c). At the HAp-(t-ZrO2) e. Fig. 8 shows the crack propagation made by Vickers inden- core region, the crack was propagated with deflection due to on in the third passed fibrous HAp composite sintered at the homogeneously dispersion of t-ZrO2 particles On the other 1500C. In the low magnification SEM image(a), the marked hand, Fig. 8(d) shows TEM image of crack tip zone which r indicates an indentation site and cracks were shortly prop- was taken from the HAp-(t-ZrO2)region. The t-zrO2 and HAp agated from four corner of an indentation site. In the enlarged phases were appeared with dark and gray contrasts, respectivel images(b, c), which were respectively taken from the longitudi- The main crack was straightly propagated into HAp matrix with nal and perpendicular direction of the fibrous microstructure, the transgranular fracture mode. However, an important observation rack propagation was clearly observed as indicated with arrow- was that when the crack met the t-ZrO2 particles, a remarkable heads. In the longitudinal direction(b), the crack was propagated crack deflection behavior was observed at the interfaces between along the AlO3(m-zrO2)shell region with heavy deflection. HAp and t-ZrO2, which may be formed due to the mismatching To pm 200nm Crack propagation of third passed HAp-(t-ZrO2)AlzO3-(m-ZrOzcomposites: (a) low SEM magnification,(b),(c)enlarged images of (a) and (d), TEM ore region
B.-T. Lee et al. / Materials Science and Engineering A 458 (2007) 11–16 15 Fig. 7. SEM fracture surfaces of (a) third passed HAp-(t-ZrO2)/Al2O3-(m-ZrO2) bodies sintered at 1500 ◦C temperature and (b), (c) enlarged images of core and shell regions. fine t-ZrO2 particles were observed on the fracture surface of core region, and this observation is a typical evidence of microcracking. Fig. 8 shows the crack propagation made by Vickers indentation in the third passed fibrous HAp composite sintered at 1500 ◦C. In the low magnification SEM image (a), the marked “I” indicates an indentation site and cracks were shortly propagated from four corner of an indentation site. In the enlarged images (b, c), which were respectively taken from the longitudinal and perpendicular direction of the fibrous microstructure, the crack propagation was clearly observed as indicated with arrowheads. In the longitudinal direction (b), the crack was propagated along the Al2O3-(m-ZrO2) shell region with heavy deflection. Some crack bridging and branching were observed due to the anisotropic grain growth, when a crack propagated perpendicular into Al2O3-(m-ZrO2) shell region (c). At the HAp-(t-ZrO2) core region, the crack was propagated with deflection due to the homogeneously dispersion of t-ZrO2 particles. On the other hand, Fig. 8(d) shows TEM image of crack tip zone which was taken from the HAp-(t-ZrO2) region. The t-ZrO2 and HAp phases were appeared with dark and gray contrasts, respectively. The main crack was straightly propagated into HAp matrix with transgranular fracture mode. However, an important observation was that when the crack met the t-ZrO2 particles, a remarkable crack deflection behavior was observed at the interfaces between HAp and t-ZrO2, which may be formed due to the mismatching Fig. 8. Crack propagation of third passed HAp-(t-ZrO2)/Al2O3-(m-ZrO2) composites; (a) low SEM magnification, (b), (c) enlarged images of (a) and (d), TEM images of core regions
B -T Lee et al. / Materials Science and Engineering A 458(2007)11-16 of their thermal expansion coefficients. Also, the strong strain References field contrast was observed at HAp matrix. However, at the crack process and tip zones, no found remarkable twin defects in the t- [1 W Suchanek, M. Yoshimura, J Mater Res 13(1998)94-117 ZrO2 grains, which are expected due to the stress-induced phase (21 P Ducheyne, Q Qiu, Biomaterials 20(1999)2287-2303 transformation [3]LL. Hench,J,Am. Ceran.Soc.81(7)(1998)1705-1728 [4 W. Suchanek, M. Yashima, M. Kakihana, M. Yashima, Biomaterials 18 (1997)923-933 4. Conclusion [5 B T Lee, N.Y. Shin, JK Han, H.Y. Song, Mater. Sci Eng. A 429(2006) 348-352. The relationship between microstructure and material prop- [6] S.J. Kalita, D. Rokusek, S. Bose, H.L. Hosick, A. Bandyopadhyay, J erties of the fibrous HAp-(1-ZrO2)Al203-(m-ZrO2)composites (7B.T. Lee, L.C. Kang. I. Am. Ceram Soc. 88(2005)2262-2266 were investigated depending on the sintering temperature. In the [8]BT.Lee, S.K. Sarkar, A.K. Gain, S JYim, H.Y. Song, Mater. Sci Eng.A ample sintered at 1200C, HAp-(t-ZrO2)core and Al2O3-(m- 432(2006)317-323 ZrO2)shell showed the porous structure. However at 1500C, 191A K Gain, B.T. Lee, Mater. Sci Eng- A 419(2006)269-275 the shell was appeared with dense and anisotropic grain grow [10] B.T. Lee, D H Jang, l.C. Kang, C.w. Lee, J. Am. Ceram Soc. 88(2005) 2874-2878 and the HAp core regions were transformed to a and p-ICP [11 M.C. Heisel. M.M. Silva, M T P. Schmalzried, J Bone Joint Surg. 85(2003) phases. The maximum values of hardness, bending strength and 1366-1379 fracture toughness were obtained at 1500C, and their values [12]SG. Huang, J. Vleugels, L Li, O.V. Biest, PL. Wang, J.Eur. Ceram. Soc. were about 890 Hv, 280 MPa and 4. I MPam, respectively 25(2005)3109-3115 The increasing of fracture toughness was basically due to the [13 M. Uo, G. Sjoren, A Sundh, E. Watari, M. Bergman, U. Lerner, Dent crack deflection toughening mechanism, which was occurred [14] P. Christel, A. Meunier, M. Heller, J. P. Torre, B. Cales, C.N. Peille. J at the Al2O3-(m-ZrO2)shell and HAp-(t-ZrO2) core regions, Biomed Mater Res. 23(1989)45-61 [15] Z Shen, E. Dolfasson, M. Nygren, L Gao, H Kawaoka, K Niihara, Ad Mater.1302001)214-216 Acknowledgement [16]H W. Kim, Y.J. Noh, Y.H. Koh, H.E. Kim, H.M. Kim, Biomaterials 23 (2002)4113-4121 [17] B.T. Lee, K.H. Kim, J K. Han, J Mater Res 19(2004)3234-3241 This work was supported by NRL research program of the [18]B T. Lee, C W. Lee. A K Gain, H.Y. Song, J. Eur. Ceram Soc. 27(200 orean Ministry of Science and Technology 157-163
16 B.-T. Lee et al. / Materials Science and Engineering A 458 (2007) 11–16 of their thermal expansion coefficients. Also, the strong strain field contrast was observed at HAp matrix. However, at the crack process and tip zones, no found remarkable twin defects in the tZrO2 grains, which are expected due to the stress-induced phase transformation. 4. Conclusion The relationship between microstructure and material properties of the fibrous HAp-(t-ZrO2)/Al2O3-(m-ZrO2) composites were investigated depending on the sintering temperature. In the sample sintered at 1200 ◦C, HAp-(t-ZrO2) core and Al2O3-(mZrO2) shell showed the porous structure. However at 1500 ◦C, the shell was appeared with dense and anisotropic grain growth and the HAp core regions were transformed to α and -TCP phases. The maximum values of hardness, bending strength and fracture toughness were obtained at 1500 ◦C, and their values were about 890 Hv, 280 MPa and 4.1 MPa m1/2, respectively. The increasing of fracture toughness was basically due to the crack deflection toughening mechanism, which was occurred at the Al2O3-(m-ZrO2) shell and HAp-(t-ZrO2) core regions, respectively. Acknowledgement This work was supported by NRL research program of the Korean Ministry of Science and Technology. References [1] W. Suchanek, M. Yoshimura, J. Mater. Res. 13 (1998) 94–117. [2] P. Ducheyne, Q. Qiu, Biomaterials 20 (1999) 2287–2303. [3] L.L. Hench, J. Am. Ceram. Soc. 81 (7) (1998) 1705–1728. [4] W. Suchanek, M. Yashima, M. Kakihana, M. Yashima, Biomaterials 18 (1997) 923–933. [5] B.T. Lee, N.Y. Shin, J.K. Han, H.Y. Song, Mater. Sci. Eng. A 429 (2006) 348–352. [6] S.J. Kalita, D. Rokusek, S. Bose, H.L. Hosick, A. Bandyopadhyay, J. Biomed. Mater. Res. 71 (2004) 35–44. [7] B.T. Lee, I.C. Kang, J. Am. Ceram. Soc. 88 (2005) 2262–2266. [8] B.T. Lee, S.K. Sarkar, A.K. Gain, S.J. Yim, H.Y. Song, Mater. Sci. Eng. A. 432 (2006) 317–323. [9] A.K. Gain, B.T. Lee, Mater. Sci. Eng. A. 419 (2006) 269–275. [10] B.T. Lee, D.H. Jang, I.C. Kang, C.W. Lee, J. Am. Ceram. Soc. 88 (2005) 2874–2878. [11] M.C. Heisel, M.M. Silva, M.T.P. Schmalzried, J. Bone Joint Surg. 85 (2003) 1366–1379. [12] S.G. Huang, J. Vleugels, L. Li, O.V. Biest, P.L. Wang, J. Eur. Ceram. Soc. 25 (2005) 3109–3115. [13] M. Uo, G. Sjoren, A. Sundh, F. Watari, M. Bergman, U. Lerner, Dent. Mater. 19 (2003) 487–492. [14] P. Christel, A. Meunier, M. Heller, J.P. Torre, B. Cales, C.N. Peille, J. Biomed. Mater. Res. 23 (1989) 45–61. [15] Z. Shen, E. Dolfasson, M. Nygren, L. Gao, H. Kawaoka, K. Niihara, Adv. Mater. 13 (2001) 214–216. [16] H.W. Kim, Y.J. Noh, Y.H. Koh, H.E. Kim, H.M. Kim, Biomaterials 23 (2002) 4113–4121. [17] B.T. Lee, K.H. Kim, J.K. Han, J. Mater. Res. 19 (2004) 3234–3241. [18] B.T. Lee, C.W. Lee, A.K. Gain, H.Y. Song, J. Eur. Ceram. Soc. 27 (2007) 157–163.