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Sintering effects of mullite-doping on mechanical properties of bovine hydroxyapatite
Materials Science and Engineering C 77 (2017) 470–475
Contents lists available at ScienceDirect
Materials Science and Engineering C journal homepage: www.elsevier.com/locate/msec
Sintering effects of mullite-doping on mechanical properties of bovine hydroxyapatite M. Yetmez a,⁎, Z.E. Erkmen b, C. Kalkandelen c,d, A. Ficai e, F.N. Oktar f,g a
Department of Mechanical Engineering, Bulent Ecevit University, 67100 Zonguldak, Turkey Department of Metallurgical and Materials Engineering, Marmara University, 34722 Istanbul, Turkey Biomedical Engineering Program, Graduate School of Natural and Applied Sciences, Istanbul University, 34320 Istanbul, Turkey d Vocational School of Technical Sciences, Biomedical Devices Technology Department, Istanbul University, 34320 Istanbul, Turkey e Faculty of Applied Chemistry and Material Science, Politechnica University of Bucharest, Bucharest, Romania f Department of Bioengineering, Faculty of Engineering, Marmara University, 34722 Istanbul, Turkey g Advanced Nanomaterials Research Laboratory, Marmara University, 34722 Istanbul, Turkey b c
a r t i c l e
i n f o
Article history: Received 30 June 2016 Received in revised form 16 November 2016 Accepted 30 March 2017 Available online 02 April 2017 Keywords: Sintering Mechanical properties Bovine derived hydroxyapatite Mullite
a b s t r a c t In this study, sintering effects on microstructural behavior of bovine derived hydroxyapatite doped with powder mullite are considered in the temperature range between 1000 °C and 1300 °C. Results show that maximum values of both compressive strength and microhardness are achieved in the samples sintered at 1200 °C for all mullite additions of 5, 7.5, 10 and 12.5 wt%. Moreover, above 1000 °C, decomposition of HA and new phase formations such as whitlockite and gehlenite play a major role in both compressive strength and microhardness properties which increase up to 10 wt% mullite reinforcement. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Calcium phosphate ceramics (CPCs) are the most popular bioceramics because of promoting bone growth around the implant material [1]. Hydroxyapatite (HA, Ca5(PO4)3(OH)2) is one of the most popular CPCs, which is widely accepted for nearly five decades, and clinically used as a preferred biomaterial for skeletal, dental, cosmetic restorations and their treatments [2]. HA materials are produced synthetically or naturally. For the natural productions, biological HA is obtained from biological sources, such as sheep bone [3], fish waste [4] and marine shells [5–10]. Biological HA accommodates several substitu– tional trace elements at the Ca2+, PO3− 4 , and OH sites of its lattice with high importance in biological performance of HA after implantation [2]. Especially, bovine derived HA (BHA) is one of the most preferred HA types. BHA is usually produced by using diluted HCI acid which enables demineralization and freeze-drying of the tissues. Even strict regulations are applied, some deadly high priority diseases (bovine spongiform encephalopathy) can survive even after all controlled processes. And, high temperature calcination performed at 850 °C, prevents those diseases to occur. Actually, this production method is very economic when compared with traditional HA production methods [11–13]. ⁎ Corresponding author. E-mail address: [email protected] (M. Yetmez).
http://dx.doi.org/10.1016/j.msec.2017.03.290 0928-4931/© 2017 Elsevier B.V. All rights reserved.
In mechanical point of view, the applications of HA bioceramics are limited to non-load bearing implants because of its poor mechanical properties and poor reliability [14]. It has been investigated that load resistant HA bioceramics reinforced with other second-phase ceramic materials with moderate tolerable compressive strengths powders (for instance, glass, zirconia, titania, magnesium, alumina or titanium) [14–16]. Mullite is a rare silicate mineral of post-clay genesis. It can form two stoichiometric forms 3Al2O32SiO2 or 2Al2O3SiO2 [17]. It is a solid solution phase of alumina and silica commonly found in ceramics. Being the only stable intermediate phase in the Al2O3 − SiO2 system at atmospheric pressure, mullite is one of the most important ceramic materials. Mullite's temperature stability and refractory nature are superior to corundum's in certain high-temperature structural applications [18]. There are some limited HA-mullite composite applications in recent studies. Nath et al. consider synthetically derived HA with 10-2030 wt% mullite with respect to nanoindentation technique. Their results reveal lower hardness of 3–4 GPa than expected for HA–mullite composites not conforming with mullite content; whereas pure HA and mullite exhibit higher hardness values of ~4.5 GPa and ~9 GPa, respectively. Considerable decrease in modulus of elasticity (E) with mullite addition is also noticed. The composites have E of ~ 80 GPa, whereas higher values of ~ 125 GPa and 230 GPa are recorded for pure HA and pure mullite, respectively [19]. Nath et al. also conduct some in-vivo studies with 20 wt% mullite added and then sintered HA-mullite
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samples. They observe presence of new bone formation without any noticeable inflammation and minimal collagen at the interface, confirming the suitability of the HA-mullite [20]. In more recent study, Nath et al. reach maximum compressive strength of ~ 400 MPa (30 wt% mullite mixed in HA and sintered at 1350 °C for 2 h without providing any details on the compressive strength measurements) [21]. It is also reported that the best blend of HA and mullite is 10 wt% [22]. Nevertheless, recent studies say that there may be very few information about the mechanical properties of BHA-mullite composites for orthopaedic and dental graft applications. The aim of this study is to investigate the microstructural and mechanical properties of BHA-mullite composites by adding 5, 7.5, 10 and 12.5 wt% mullite into the BHA structure respectively. Fig. 1. Density results of mullite reinforced composites for 5, 7.5, 10 and 12.5 wt%.
Fig. 2. Microhardness results of mullite reinforced composites for 5, 7.5, 10 and 12.5 wt%.
2. Materials and methods BHA is prepared from bovine bones calcined at 850 °C [23]. The calcinated BHA powder is grinded and sieved to a scale of 100 μm particles size. After sieving, the BHA fine powder is mixed with 5, 7.5, 10 and 12.5 wt% mullite powder and ground using conventional ball milling for ~ 4 h. Then, the mixture is dried and pressed forming pellets with a uniaxial cold press at 350 MPa. The dimensions of composite pellets are 6 mm in diameter and 12 mm in height according to the British Standard of BS7253 [24]. Finally, the samples are sintered at temperatures 1000, 1100, 1200 and 1300 °C for 4 h with a ramp rate 5 °C/min [13]. Regarding to testing procedures of density, microhardness and compression, six samples of each group are taken for measurements. Density of the sintered samples is determined by the Archimedes method. A universal tensile testing machine (Devotrans FU 50kN, Turkey) is used for compression tests under a loading rate of 3 mm/min. Vickers microhardness measurements (HV) are conducted under 200 g load and 20 s of dwell time (Shimadzu HMV-2, Japan). Ten HV measurements are obtained from each of the samples. X-ray diffraction studies are performed using a vertical diffractometer (Bruker Instruments, Darmstadt, Germany). Micro-image analysis is carried out using a scanning electron microscope (JSM 7000F, JEOL Ltd., Japan). 3. Results 3.1. Mechanical analysis
Fig. 3. Compressive strength of mullite reinforced composites for 5, 7.5, 10 and 12.5 wt%.
Fig. 1 indicates that (i) the highest density value is determined as 2.766 g/cm3 at 1300 °C for 5 wt% mullite addition into BHA, (ii) the lowest value is 1.942 g/cm3 with 12.5 wt% mullite addition for 1000 °C sintering. The Vickers microhardness results are given in Fig. 2. Maximum value for HV is obtained as 1369.4 HV for pellets containing 10 wt% mullite sintered at 1300 °C whereas minimum
Fig. 4. SEM images of BHA-mullite 5 wt% sintered at A: 1000 °C and B: 1300 °C.
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Fig. 5. SEM images of BHA-mullite 7.5 wt% sintered at A: 1000 °C and B: 1300 °C.
one is 85.98 HV for 10 wt% mullite addition sintered at 1000 °C. Comparing microhardness data, it can be claimed that the HV values of 867.1, 947, 1369.4 and 994.9 for 5, 7.5, 10, and 12.5 wt% mullite addition at 1300 °C respectively are better than the recent HV data of HA composites [25]. Compression test results are presented in Fig. 3. It is seen that (i) the highest compression strength is determined as 118 MPa for 12.5 wt% mullite addition into BHA sintered at 1200 °C whereas the lowest one is 29.6 MPa for 12.5 wt% mullite addition into BHA sintered at 1000 °C, (ii) the results at 1300 °C sintering temperature for 5, 7.5 and 10 wt%
mullite addition are found very stable as 102, 92 and 101 MPa respectively.
3.2. SEM analysis Among many micrographs of SEM, the most significant ones are given in Figs. 4–7. In these figures, 5, 7.5, 10, and 12.5 wt% mullite reinforced composites sintered at 1000 and 1300 °C are presented at low magnification (×1000).
Fig. 6. SEM images of BHA-mullite 10 wt% sintered at A: 1000 °C and B: 1300 °C.
Fig. 7. SEM images of BHA-mullite 12.5 wt% sintered at A: 1000 °C and B: 1300 °C.
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Fig. 8. Micrograph of BHA-mullite 7.5 wt% sintered at A: 1000 °C and B: 1300 °C.
When the mullite content increases for a fixed temperature such as either 1000 °C or 1300 °C, a more perturbed (dimple like) structure is observed (vertical descent). One may say that this observation is due to the crystallization of new phases. BSE (Backscattered Electron Analysis) does not produce significant differences in distinguishing phases. To reveal the phase distribution of samples, XRD and FTIR analyses are performed in the next step. When comparing A with B in Fig. 7 for a fixed composition, it is clear that 1300 °C sintering causes merging and enlargement of grains with sharp edges possibly due to diffusion effects and new phase formations including glassy phase. Fig. 8 indicates the sintering of grains at 1000 and 1300 °C at high magnification (×3000) for 7.5 wt%
mullite reinforcement respectively. It is obviously seen that grains of nearly 1 μm size grow nearly to 10 μm and they are merged leaving very few pores behind as expected increasing temperature from 1000 to 1300 °C. 3.3. XRD analysis Figs. 9–11 provide the corresponding XRD analysis of composites reinforced 5, 7.5, 10 and 12.5 wt% mullite sintered at 1000 and 1300 °C as previously. Fig. 9 shows that intensities of HA decrease nearly of the order of 25–50 for 2θ N 40o, whereas the major diffraction peak of mullite
Fig. 9. XRD Analysis of 5 wt% mullite reinforced sintered at A: 1000 °C B: 1300 °C.
Fig. 10. XRD Analysis of 7.5 wt% mullite reinforced sintered at A: 1000 °C B: 1300 °C.
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Fig. 11. XRD Analysis of 12.5 wt% mullite reinforced sintered at A: 1000 °C B: 1300 °C.
phase increases from 300 to 400 when sintering temperature increased from 1000 to 1300 °C. Since other diffraction peaks of mullite phase remain the same, this increase in the major peak could be attributed to an extraneous effect. Comparing Fig. 10 with Fig. 9, one can easily notice that two new phases appear as sintering temperature increases from 1000 to 1300 °C: Gehlenite (2CaO·Al 2 O 3 ·SiO 2 ) and whitlockite (Ca3 (PO4 )2). This is possibly due to the decomposition and subsequent reaction of HA with mullite phase after which the latter is completely consumed. Fig. 11 presents the XRD patterns of 12.5 wt% mullite reinforced composite after 1000 and 1300 °C sintering. Similarly, when comparing Fig. 11 with Fig. 10, it is clearly seen that at 1300 °C sintering, HA phase completely transforms to Ca3(PO4)2 (W). Mullite phase is also partly consumed in reacting with HA to form gehlenite but still exists in the microstructure. Table 1 summarizes the
Table 1 Phase distribution for different batch compositions and sintering temperatures. Composition
1000 °C
1300 °C
5 wt% Mullite + BHA HA, mullite (few) HA, mullite (high) 7.5 wt% Mullite + BHA HA, mullite HA, gehlenite, whitlockite 12.5 wt% Mullite + BHA HA, mullite Mullite (low), gehlenite, whitlockite
phase distribution after XRD for different batch compositions and sintering temperatures.
3.4. EDS mapping The Energy Dispersive Spectroscopic mapping for 12.5 wt% mullite reinforced HA composites is given in Fig. 12. The localized segregations (i.e. denser parts) are probably due to nonhomogeneous density distribution. All four elements in the three phases (HA, W, M) are concentrated in these segregations.
4. Conclusion In this study, it is concluded that mullite addition to BHA increases enormously the HV values up to 1369 HV with increasing the sintering temperatures from 1000 to 1300 °C for 10 wt% mullite reinforced composites. This is most probably due to liquid phase sintering and glassy phase formation following cooling of the samples (see Fig. 6). Identically, density and compression strength values increase up to 2.62 g/cm3 and 101 MPa respectively for the same conditions. Above 1000 °C, decomposition of HA and new phase formations such as whitlockite and gehlenite play also a major role in both HV and strength while increasing up to 10 wt% mullite reinforcement.
Fig. 12. EDS mapping of 12.5 wt% mullite reinforced HA sample heat treated at A: 1000 °C B: 1300 °C.
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References [1] F.N. Oktar, M. Yetmez, S. Agathopoulos, T.M. Lopez Goerne, G. Goller, I. Peker, J.M.F. Ferreira, J. Mater. Sci. Mater. Med. 17 (11) (2006) 1161–1171. [2] O. Gunduz, E.M. Erkan, S. Daglilar, S. Salman, S. Agathopoulos, F.N. Oktar, J. Mater. Sci. 43 (8) (2008) 2536–2540. [3] N. Demirkol, O. Meydanoglu, E.S. Kayali, F.N. Oktar, Acta Phys. Pol. A 121 (1) (2012) 274–276. [4] T.M. Coelho, E.S. Nogueira, A. Steimacher, A.N. Medina, W.R. Weinand, V.M. Lima, M.L. Baesso, A.C. Bento, J. Appl. Phys. 100 (9) (2006), 094312. [5] M.L. Tamasan, L.S. Ozyegin, F.N. Oktar, V. Simon, Mater. Sci. Eng. C 33 (2013) 2569–2577. [6] D. Agaogulları, D. Kel, H. Gokce, I. Duman, M.L. Öveçoğlu, A.T. Akarsubasi, D. Bilgic, F.N. Oktar, Acta Phys. Pol. A 121 (1) (2012) 23–26. [7] I.J. Macha, L.S. Ozyegin, J. Chou, R. Samur, F.N. Oktar, B. Ben-Nissan, J. Aust. Ceram. Soc. 49 (2013) 122–128. [8] J.H.G. Rocha, A.F. Lemos, S. Agathopoulos, P. Valério, S. Kannan, F.N. Oktar, J.M.F. Ferreira, Bone 37 (6) (2005) 850–857. [9] D. Kel, H. Gökçe, D. Bilgiç, D. Ağaoğulları, I. Duman, M.L. Öveçoğlu, E.S. Kayalı, I.A. Kiyici, S. Agathopoulos, F.N. Oktar, Key Eng. Mater. 493–494 (2012) 287–292. [10] D. Kel, U. Karacayli, M. Yetmez, L.S. Ozyegin, E.S. Kayali, F.N. Oktar, Int. J. Artif. Organs 34 (8) (2011) 699. [11] O. Gunduz, Y.M. Sahin, S. Agathopoulos, D. Ağaoğulları, H. Gökçe, E.S. Kayali, C. Aktas, B. Ben-Nissan, F.N. Oktar, Key Eng. Mater. 587 (2014) 80–85.
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[12] F.N. Oktar, S. Agathopoulos, L.S. Ozyegin, I.G. Turner, O. Gunduz, N. Demirkol, S. Brück, B. Ben-Nissan, R. Samur, E.S. Kayali, C. Aktas, Key Eng. Mater. 529–530 (2013) 609–614. [13] G. Goller, F.N. Oktar, Mater. Lett. 56 (2002) 142–147. [14] Z.E. Erkmen, Y. Genc, F.N. Oktar, J. Am. Ceram. Soc. 90 (9) (2007) 2885–2892. [15] P. Valerio, F.N. Oktar, G. Goller, A.M. Goes, M.F. Leite, Key Eng. Mater. 254 (2004) 777–780. [16] B.V. Sampaio, G. Goller, F.N. Oktar, P. Valerio, A.M. Goes, M.F. Leite, Key Eng. Mater. 284–286 (2005) 639–642. [17] http://en.wikipedia.org/wiki/Mullite (last accessed: September 19, 2016). [18] D.J. Duval, S.H. Risbud, J.F. Shackelford, Ceramic and Glass Materials Structure, Properties and Processing, Springer, 2008. [19] S. Nath, A.K. Mukhopadhyay, A. Dey, B. Basu, Mater. Sci. Eng. A 513–514 (2009) 197–201. [20] S. Nath, B. Basu, M. Mohanty, P.V. Mohanan, J. Biomed. Mater. Res. B Appl. Biomater. 90B (2009) 547–557. [21] S. Nath, K. Biswas, K. Wang, R.K. Bordia, B. Basu, J. Am. Ceram. Soc. 93 (6) (2010) 1639–1649. [22] S. Nath, K. Biswas, B. Basu, Scr. Mater. 58 (2008) 1054–1057. [23] F.N. Oktar, M.R. Demirer, O. Gunduz, Y. Genc, S. Agathopoulos, I. Peker, L.S. Ozyegin, S. Salman, Key Eng. Mater. 309–311 (2006) 49–52. [24] BS 7253, British Standard: Non-metallic Materials for Surgical Implants, 1990. [25] F.N. Oktar, S. Agathopoulos, L.S. Ozyegin, O. Gunduz, N. Demirkol, Y. Bozkurt, S. Salman, J. Mater. Sci. Mater. Med. 18 (11) (2007) 2137–2143.
Contents lists available at ScienceDirect
Materials Science and Engineering C journal homepage: www.elsevier.com/locate/msec
Sintering effects of mullite-doping on mechanical properties of bovine hydroxyapatite M. Yetmez a,⁎, Z.E. Erkmen b, C. Kalkandelen c,d, A. Ficai e, F.N. Oktar f,g a
Department of Mechanical Engineering, Bulent Ecevit University, 67100 Zonguldak, Turkey Department of Metallurgical and Materials Engineering, Marmara University, 34722 Istanbul, Turkey Biomedical Engineering Program, Graduate School of Natural and Applied Sciences, Istanbul University, 34320 Istanbul, Turkey d Vocational School of Technical Sciences, Biomedical Devices Technology Department, Istanbul University, 34320 Istanbul, Turkey e Faculty of Applied Chemistry and Material Science, Politechnica University of Bucharest, Bucharest, Romania f Department of Bioengineering, Faculty of Engineering, Marmara University, 34722 Istanbul, Turkey g Advanced Nanomaterials Research Laboratory, Marmara University, 34722 Istanbul, Turkey b c
a r t i c l e
i n f o
Article history: Received 30 June 2016 Received in revised form 16 November 2016 Accepted 30 March 2017 Available online 02 April 2017 Keywords: Sintering Mechanical properties Bovine derived hydroxyapatite Mullite
a b s t r a c t In this study, sintering effects on microstructural behavior of bovine derived hydroxyapatite doped with powder mullite are considered in the temperature range between 1000 °C and 1300 °C. Results show that maximum values of both compressive strength and microhardness are achieved in the samples sintered at 1200 °C for all mullite additions of 5, 7.5, 10 and 12.5 wt%. Moreover, above 1000 °C, decomposition of HA and new phase formations such as whitlockite and gehlenite play a major role in both compressive strength and microhardness properties which increase up to 10 wt% mullite reinforcement. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Calcium phosphate ceramics (CPCs) are the most popular bioceramics because of promoting bone growth around the implant material [1]. Hydroxyapatite (HA, Ca5(PO4)3(OH)2) is one of the most popular CPCs, which is widely accepted for nearly five decades, and clinically used as a preferred biomaterial for skeletal, dental, cosmetic restorations and their treatments [2]. HA materials are produced synthetically or naturally. For the natural productions, biological HA is obtained from biological sources, such as sheep bone [3], fish waste [4] and marine shells [5–10]. Biological HA accommodates several substitu– tional trace elements at the Ca2+, PO3− 4 , and OH sites of its lattice with high importance in biological performance of HA after implantation [2]. Especially, bovine derived HA (BHA) is one of the most preferred HA types. BHA is usually produced by using diluted HCI acid which enables demineralization and freeze-drying of the tissues. Even strict regulations are applied, some deadly high priority diseases (bovine spongiform encephalopathy) can survive even after all controlled processes. And, high temperature calcination performed at 850 °C, prevents those diseases to occur. Actually, this production method is very economic when compared with traditional HA production methods [11–13]. ⁎ Corresponding author. E-mail address: [email protected] (M. Yetmez).
http://dx.doi.org/10.1016/j.msec.2017.03.290 0928-4931/© 2017 Elsevier B.V. All rights reserved.
In mechanical point of view, the applications of HA bioceramics are limited to non-load bearing implants because of its poor mechanical properties and poor reliability [14]. It has been investigated that load resistant HA bioceramics reinforced with other second-phase ceramic materials with moderate tolerable compressive strengths powders (for instance, glass, zirconia, titania, magnesium, alumina or titanium) [14–16]. Mullite is a rare silicate mineral of post-clay genesis. It can form two stoichiometric forms 3Al2O32SiO2 or 2Al2O3SiO2 [17]. It is a solid solution phase of alumina and silica commonly found in ceramics. Being the only stable intermediate phase in the Al2O3 − SiO2 system at atmospheric pressure, mullite is one of the most important ceramic materials. Mullite's temperature stability and refractory nature are superior to corundum's in certain high-temperature structural applications [18]. There are some limited HA-mullite composite applications in recent studies. Nath et al. consider synthetically derived HA with 10-2030 wt% mullite with respect to nanoindentation technique. Their results reveal lower hardness of 3–4 GPa than expected for HA–mullite composites not conforming with mullite content; whereas pure HA and mullite exhibit higher hardness values of ~4.5 GPa and ~9 GPa, respectively. Considerable decrease in modulus of elasticity (E) with mullite addition is also noticed. The composites have E of ~ 80 GPa, whereas higher values of ~ 125 GPa and 230 GPa are recorded for pure HA and pure mullite, respectively [19]. Nath et al. also conduct some in-vivo studies with 20 wt% mullite added and then sintered HA-mullite
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samples. They observe presence of new bone formation without any noticeable inflammation and minimal collagen at the interface, confirming the suitability of the HA-mullite [20]. In more recent study, Nath et al. reach maximum compressive strength of ~ 400 MPa (30 wt% mullite mixed in HA and sintered at 1350 °C for 2 h without providing any details on the compressive strength measurements) [21]. It is also reported that the best blend of HA and mullite is 10 wt% [22]. Nevertheless, recent studies say that there may be very few information about the mechanical properties of BHA-mullite composites for orthopaedic and dental graft applications. The aim of this study is to investigate the microstructural and mechanical properties of BHA-mullite composites by adding 5, 7.5, 10 and 12.5 wt% mullite into the BHA structure respectively. Fig. 1. Density results of mullite reinforced composites for 5, 7.5, 10 and 12.5 wt%.
Fig. 2. Microhardness results of mullite reinforced composites for 5, 7.5, 10 and 12.5 wt%.
2. Materials and methods BHA is prepared from bovine bones calcined at 850 °C [23]. The calcinated BHA powder is grinded and sieved to a scale of 100 μm particles size. After sieving, the BHA fine powder is mixed with 5, 7.5, 10 and 12.5 wt% mullite powder and ground using conventional ball milling for ~ 4 h. Then, the mixture is dried and pressed forming pellets with a uniaxial cold press at 350 MPa. The dimensions of composite pellets are 6 mm in diameter and 12 mm in height according to the British Standard of BS7253 [24]. Finally, the samples are sintered at temperatures 1000, 1100, 1200 and 1300 °C for 4 h with a ramp rate 5 °C/min [13]. Regarding to testing procedures of density, microhardness and compression, six samples of each group are taken for measurements. Density of the sintered samples is determined by the Archimedes method. A universal tensile testing machine (Devotrans FU 50kN, Turkey) is used for compression tests under a loading rate of 3 mm/min. Vickers microhardness measurements (HV) are conducted under 200 g load and 20 s of dwell time (Shimadzu HMV-2, Japan). Ten HV measurements are obtained from each of the samples. X-ray diffraction studies are performed using a vertical diffractometer (Bruker Instruments, Darmstadt, Germany). Micro-image analysis is carried out using a scanning electron microscope (JSM 7000F, JEOL Ltd., Japan). 3. Results 3.1. Mechanical analysis
Fig. 3. Compressive strength of mullite reinforced composites for 5, 7.5, 10 and 12.5 wt%.
Fig. 1 indicates that (i) the highest density value is determined as 2.766 g/cm3 at 1300 °C for 5 wt% mullite addition into BHA, (ii) the lowest value is 1.942 g/cm3 with 12.5 wt% mullite addition for 1000 °C sintering. The Vickers microhardness results are given in Fig. 2. Maximum value for HV is obtained as 1369.4 HV for pellets containing 10 wt% mullite sintered at 1300 °C whereas minimum
Fig. 4. SEM images of BHA-mullite 5 wt% sintered at A: 1000 °C and B: 1300 °C.
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Fig. 5. SEM images of BHA-mullite 7.5 wt% sintered at A: 1000 °C and B: 1300 °C.
one is 85.98 HV for 10 wt% mullite addition sintered at 1000 °C. Comparing microhardness data, it can be claimed that the HV values of 867.1, 947, 1369.4 and 994.9 for 5, 7.5, 10, and 12.5 wt% mullite addition at 1300 °C respectively are better than the recent HV data of HA composites [25]. Compression test results are presented in Fig. 3. It is seen that (i) the highest compression strength is determined as 118 MPa for 12.5 wt% mullite addition into BHA sintered at 1200 °C whereas the lowest one is 29.6 MPa for 12.5 wt% mullite addition into BHA sintered at 1000 °C, (ii) the results at 1300 °C sintering temperature for 5, 7.5 and 10 wt%
mullite addition are found very stable as 102, 92 and 101 MPa respectively.
3.2. SEM analysis Among many micrographs of SEM, the most significant ones are given in Figs. 4–7. In these figures, 5, 7.5, 10, and 12.5 wt% mullite reinforced composites sintered at 1000 and 1300 °C are presented at low magnification (×1000).
Fig. 6. SEM images of BHA-mullite 10 wt% sintered at A: 1000 °C and B: 1300 °C.
Fig. 7. SEM images of BHA-mullite 12.5 wt% sintered at A: 1000 °C and B: 1300 °C.
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Fig. 8. Micrograph of BHA-mullite 7.5 wt% sintered at A: 1000 °C and B: 1300 °C.
When the mullite content increases for a fixed temperature such as either 1000 °C or 1300 °C, a more perturbed (dimple like) structure is observed (vertical descent). One may say that this observation is due to the crystallization of new phases. BSE (Backscattered Electron Analysis) does not produce significant differences in distinguishing phases. To reveal the phase distribution of samples, XRD and FTIR analyses are performed in the next step. When comparing A with B in Fig. 7 for a fixed composition, it is clear that 1300 °C sintering causes merging and enlargement of grains with sharp edges possibly due to diffusion effects and new phase formations including glassy phase. Fig. 8 indicates the sintering of grains at 1000 and 1300 °C at high magnification (×3000) for 7.5 wt%
mullite reinforcement respectively. It is obviously seen that grains of nearly 1 μm size grow nearly to 10 μm and they are merged leaving very few pores behind as expected increasing temperature from 1000 to 1300 °C. 3.3. XRD analysis Figs. 9–11 provide the corresponding XRD analysis of composites reinforced 5, 7.5, 10 and 12.5 wt% mullite sintered at 1000 and 1300 °C as previously. Fig. 9 shows that intensities of HA decrease nearly of the order of 25–50 for 2θ N 40o, whereas the major diffraction peak of mullite
Fig. 9. XRD Analysis of 5 wt% mullite reinforced sintered at A: 1000 °C B: 1300 °C.
Fig. 10. XRD Analysis of 7.5 wt% mullite reinforced sintered at A: 1000 °C B: 1300 °C.
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Fig. 11. XRD Analysis of 12.5 wt% mullite reinforced sintered at A: 1000 °C B: 1300 °C.
phase increases from 300 to 400 when sintering temperature increased from 1000 to 1300 °C. Since other diffraction peaks of mullite phase remain the same, this increase in the major peak could be attributed to an extraneous effect. Comparing Fig. 10 with Fig. 9, one can easily notice that two new phases appear as sintering temperature increases from 1000 to 1300 °C: Gehlenite (2CaO·Al 2 O 3 ·SiO 2 ) and whitlockite (Ca3 (PO4 )2). This is possibly due to the decomposition and subsequent reaction of HA with mullite phase after which the latter is completely consumed. Fig. 11 presents the XRD patterns of 12.5 wt% mullite reinforced composite after 1000 and 1300 °C sintering. Similarly, when comparing Fig. 11 with Fig. 10, it is clearly seen that at 1300 °C sintering, HA phase completely transforms to Ca3(PO4)2 (W). Mullite phase is also partly consumed in reacting with HA to form gehlenite but still exists in the microstructure. Table 1 summarizes the
Table 1 Phase distribution for different batch compositions and sintering temperatures. Composition
1000 °C
1300 °C
5 wt% Mullite + BHA HA, mullite (few) HA, mullite (high) 7.5 wt% Mullite + BHA HA, mullite HA, gehlenite, whitlockite 12.5 wt% Mullite + BHA HA, mullite Mullite (low), gehlenite, whitlockite
phase distribution after XRD for different batch compositions and sintering temperatures.
3.4. EDS mapping The Energy Dispersive Spectroscopic mapping for 12.5 wt% mullite reinforced HA composites is given in Fig. 12. The localized segregations (i.e. denser parts) are probably due to nonhomogeneous density distribution. All four elements in the three phases (HA, W, M) are concentrated in these segregations.
4. Conclusion In this study, it is concluded that mullite addition to BHA increases enormously the HV values up to 1369 HV with increasing the sintering temperatures from 1000 to 1300 °C for 10 wt% mullite reinforced composites. This is most probably due to liquid phase sintering and glassy phase formation following cooling of the samples (see Fig. 6). Identically, density and compression strength values increase up to 2.62 g/cm3 and 101 MPa respectively for the same conditions. Above 1000 °C, decomposition of HA and new phase formations such as whitlockite and gehlenite play also a major role in both HV and strength while increasing up to 10 wt% mullite reinforcement.
Fig. 12. EDS mapping of 12.5 wt% mullite reinforced HA sample heat treated at A: 1000 °C B: 1300 °C.
M. Yetmez et al. / Materials Science and Engineering C 77 (2017) 470–475
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