- Email: [email protected]
Effect of substitutional Strontium on Mechanical Properties of Akermanite Ceramic Prepared by Solid-State Sintering
Available online at www.sciencedirect.com
ScienceDirect Materials Today: Proceedings 17 (2019) 929–936
www.materialstoday.com/proceedings
RAMM 2018
Effect of substitutional Strontium on Mechanical Properties of Akermanite Ceramic Prepared by Solid-State Sintering H. Mohammadia, Y.M.B. Ismaila, K.A. Shariffa, A-F.M. Noora* School of Materials and Mineral Resources Engineering, Universiti Sains Malaysia, Engineering Campus, 14300, Nibong Tebal, Penang, Malaysia
Abstract Akermanite (Ca2MgSi2O7) has attracted attention in the biomaterial research due to its excellent in vitro and in vivo bioactivity. Mechanical properties such as microhardness and fracture toughness are of great importance for bioactive materials to predict their performance before failure and thus determine the potential biomedical application. In previous work, sintered akermanite prepared by solid-state sintering at 1200°C showed in vitro apatite formation in simulated body fluid (SBF) solution, but having high porosity (more than 20%). In the present study, bulk akermanite bioceramics was fabricated and evaluated the mechanical properties i.e. microhardness and fracture toughness. The akermanite Sr-doped akermanite were synthesised by solid-state sintering at 1225°C for 4 h. Results indicated that the substitution of Sr enhanced the microhardness and fracture toughness which is attributed to the controlled grain size, improved density as well as better sinterability. However, by further increase in amount of dopant beyond 5 mol%, both hardness and fracture toughness decreased due to the larger grain size. akermanite at 1225°C led to a microhardness and fracture toughness of 2.42±0.24 GPa and 1.48±0.03 MPa.m1/2, respectively which makes it a suitable material for non-load bearing application such as, bone filler material. © 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the 6th International Conference on Recent Advances in Materials, Minerals & Environment (RAMM) 2018. Keywords: Solid-state sintering; fracture toughness; Hardness; Diametral tensile strength ____________________________________________________________________________________________________________________
* Corresponding author. Tel.: +604-5996174; fax: +604-594101. E-mail address: [email protected] 2214-7853 © 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the 6th International Conference on Recent Advances in Materials, Minerals & Environment (RAMM) 2018.
930
H. Mohammadi et al. / Materials Today: Proceedings 17 (2019) 929–936
1. Introduction The regeneration and repair of large bone defect is still a challenge issue in the biomedical field. Although the calcium phosphates such as hydroxyapatite (HA) or tricalcium phosphate (TCP) has been investigated over there decades, more recent calcium silicate ceramics (CaSiO3) have attracted great attentions in the biomaterial community [1]. This material has reported to have good bioactivity [2] and improved cell proliferation compared to phosphate-based ceramics [3]. However, the major drawbacks of silicates are higher dissolution rate which leads to high environmental pH value[4] that would be detrimental to cells [5] as well as low bending strength and fracture toughness [6]. The vital role of bioinorganics such as calcium (Ca2+), Si (Si4+) and magnesium (Mg2+) in bone development has been well documented in the literature [7]. Furthermore, the incorporation of bioinorganics such as magnesium (Mg2+) into the structure results in enhanced chemical stability and mechanical properties [8]. In recent decade, akermanite (Ca2MgSi2O7), a magnesium modified calcium silicate has shown higher in vitro and in vivo cell proliferation compared to β-TCP [9]. There are different ways to synthesize akermanite such as sol-gel[10], mechanical activation [11], and precermic polymer [12]. Apart from in vitro and in vivo performance of bioceramic, the mechanical evaluations is also of great importance to get an understanding of potential field of application. In previous study, akermanite has been synthesised via by solid-state sintering route at 1200°C and demonstrated its proper mechanical properties as well as in vitro apatite forming ability [13]. Nevertheless, akermanite was still porous at this sintering temperature, which may compromise the mechanical properties. However, at 1250°C, there was partial melting and 1300°C samples melted. Thus, this paper specifically reports the results obtained at 1225°C as it provides optimal results. The main purpose of this study is also to partially substitute strontium (Sr2+) into the akermanite to obtain highly densified ceramic with improved mechanical properties including diametral tensile strength, hardness and fracture toughness by solid-state sintering route. 2. Experimental procedure 2.1. Preparation and sintering of akermanite and Sr-substituted ceramic Akermanite powder was synthesized using a wet high-energy planetary ball milling. Briefly, calcium oxide (CaO, Merck, 98%), magnesium oxide (MgO, Merck, 98%), silicon dioxide (SiO2, Sigma-Aldrich, 99.9%), were weighed based on the stoichiometric ratio of akermanite and then were ball milled for 3 h in the planetary ball milling (PM 400-Reutch). The ball to powder weight ratio was 10:1, while the powder to deionized water ratio was 1:3. The speed of vial was 500 rpm. The deionized water the liquid media was used to increase the efficiency and prevent any dead zone. After milling, the mixture was dried in oven for 24 h at 100°C. Then, Ca2-xSrxMgSi2O7 (Sr/Sr+Ca to control the partial substitution of Sr with Ca) samples were prepared with the different amount of strontium oxide (SrO, Sigma-Aldrich, 99.9%, Germany), where the compositional parameter x represented 0.05, 0.10 and 0.15 mol% of SrO. Akermanite nanopowder was synthesised using a high-energy and high-speed planetary ball milling under wet condition. Then, the dried powders were ground using agate mortar pestle and sieved through 200 µm. Finally, sieved powders were pressed into pellets with the dimension of 13 mm (diameter) using uniaxial hydraulic press (24T Laboratory hydraulic Press, MTI Cooperation) under the pressure of 200 MPa. The pellets were then sintered 1225°C for 4 h with a heating rate of 5°C/min.
H. Mohammadi et al. / Materials Today: Proceedings 17 (2019) 929–936
931
2.2. Characterization of akermanite and Sr-substituted akermanite ceramic 2.2.1. Phase analysis The X-ray diffraction (XRD, D8 Bruker Advance Diffractometer, England) conducted with CuKα1 radiation (λ=1.541 Å at 20 kV and 30 mA) with step size of 0.03, step scan time of 33 s in diffraction angles (2θ°) between 10° and 90° to determine phase, lattice parameter and crystallite size of the synthesised powders. The data was analysed by PANalytical X’Pert High Score Plus (ver 2.2). The X-ray patterns were matched to the International Center for Diffraction Data (ICDD) reference files. Furthermore, the modified Scherrer equation was used to calculate the crystallite size of the powders based on the XRD patterns for the three highest intensity peaks [14]:
ln ln(
k 1 ) ln( ) L cos
(1)
The lattice parameters a and c was calculated by the standard tetragonal unit cell relationship between the interplanar spacing (dhkl) in the set of Miller indices (hkl) and the lattice parameters as follows [15]:
1
a 1 (h 2 k 2 l 2 ( )2 ) 2 (d hkl ) c a
(2)
2
2.2.2. Microstructural analysis The morphology and microstructure of surface and fracture parts of sintered pellet at 1225°C was observed by field emission scanning electron microscopy (FESEM) with energy dispersive spectroscopy (Zeiss SupraTM35VP, Germany). Prior to SEM, the specimens were then coated with a thin layer of gold (Au) by sputtering (EMITECH K450X, England) to increase the conductivity of sample. 2.2.3. Porosity and density The apparent density ( a ), relative density ( Re ) open porosity (PO) is calculated based on the ASTM B96217[16] by using the following formula [16]in which theoretical density ( ) was calculated from XRD data (unit cell volume as well as lattice constants) [15, 17] :
a (%)
w1 100 w 1 w 2
Re (%)
PO (%)
a 100
w 3 w 1 100 w 3 w 2
(3)
(4)
(5)
932
H. Mohammadi et al. / Materials Today: Proceedings 17 (2019) 929–936
PT (%) 100
a 100
PC (%) PT PO
(6) (7)
Whereby, w 1 is the weight of the dry sample in air, w 2 is the weight of the sample suspended in deionized water and w 3 is the weight of the sample after being saturated in deionized water. 2.2.4. Microhardness and fracture toughness The Vickers microhardness test was performed on the samples by using the Vickers microhardness tester (Future Tech, Japan). The sintered pellets were cold mounted using a mixture of epoxy resin and hardener, then mechanically ground using SiC papers (Buehler Ltd., USA) with 240–1000 grades, and subsequently polished with a 0.3 micron alumina suspension to get a smooth flat surface. Four measurements were performed on each sample using a diamond indenter at 5 kgf for 10 s and average values were recorded. The ASTM-E384-10 was used to measure the Vickers hardness of samples as follows [18]:
HV
0.18544 P d2
(8)
Where, HV (GPa) is the Vickers hardness, P is the load applied (N) and d is the average of two diagonal lengths of indentation (mm). The fracture toughness K IC MPa. m was calculated from the Vickers microhardness (HV) using Niihara formula[19]: 1
K IC 0.203 HV (ain ) 2 (
c in 32 ) ain
(9)
Where, KIC (MPa.m1/2) is the fracture toughness ain is half of the indent diagonal (µm) and c in is the indentation crack length (µm) from the center of the indentation. 2.2.1. Diametral tensile strength The Brasilian test was performed on the sintered pellet (13 mm diameter and 11 mm height) by utilising a universal experimental instrument (INSTRON 3367) at a speed rate of 0.5 mm/min. The ASTM-D3967 was used to calculate the tensile strength of the samples by diametral line as follows [20]:
Tensile
2 P D h
(10)
Where, Tensile (MPa) is tensile strength, D is the diameter (mm), h is the sample thickness (mm) and P is the applied force (N).
H. Mohammadi et al. / Materials Today: Proceedings 17 (2019) 929–936
933
3. Statistics The results of experiments are represented as mean and arithmetic standard deviation (±S.D.) for four samples. Analysis of results was carried out using one-way ANOVA test with a significance level of (p*<0.05) using GraphPad Prism software (Ver 6.07). 4. Results and discussion The XRD patterns of the akermanite sample sintered at 1225°C is shown in Fig.1a. The diffraction patterns showed that the main phase formed is akermanite (ICDD No. 035-0592). The main akermanite peak was located at 31.43°. The main characteristic peaks of (121) for 2θ=31.09, (201) for 2θ=28.87, (312) for 2θ=51.82 and (122) for 2θ=44.40 indicated the formation of akermanite phase. The lattice parameters of akermanite and Sr-substituted akermanite samples are summarized in Table 1. The enlargement of peak with high intensity revealed slight shift to the lower angle (Fig.1b), that is attributed to the substitution of strontium (Sr2+:1.13Å) with higher ionic radius compared to calcium (Ca2+:0.99Å) [21]. Additionally, the lattice parameter was also increased due to the substitution of Sr (Table 1).
Fig. 1. The XRD pattern of akermanite and Sr-substituted akermanite sintered at 1225°C
Table 1. Lattice parameters of akermanite and Sr-substituted akermanite sintered at 1225°C Lattice constants Sample
Ak
0.05Sr
0.10Sr
0.15Sr
a(Å)
7.828
7.844
7.855
7.862
c(Å)
5.006
5.016
5.023
5.029
V(Å3)
307.755
308.626
309.924
310.847
The FESEM micrographs of surface and fracture parts of sintered akermanite are depicted in Fig. 2. The results suggest a good densification after sintering at 1225°C. The FESEM of the surface showed a relatively uniform rain microstructure while the fractured part showed the typical ceramic fractures (i.e. transgranular and intergranular fractures). The sintered akermanite at 1225°C showed 78.04 ± 1.66% density and 21.36 ± 1.63% porosity which is in agreement with SEM micrograph (Fig.2a). However, the density of akermanite was increased by the substitution of Sr ions (Fig.2b-d).
934
H. Mohammadi et al. / Materials Today: Proceedings 17 (2019) 929–936
Fig. 2. The FESEM micrographs of (a-d) surface and (e-f) fractured of akermanite and Sr-substituted akermanite sintered at 1225°C Table 2. Porosity and density of akermanite and Sr-substituted akermanite sintered at 1225°C Porosity and density Sample
Re
Ak
0.05Sr
78.04±1.66
86.72±1.82
88.89±2.74
0.10Sr
92.17±1.84
0.15Sr
PO (%)
21.36±1.63
6.56±1.20
6.43±1.22
4.39±1.32
PT (%)
21.96±1.66
13.28±1.82
11.11±2.74
7.83±1.95
PC (%)
0.61±0.31
6.72±2.27
4.68±1.61
3.44±0.63
Table 3. Mechanical properties of akermanite and Sr-substituted akermanite sintered at 1225°C Mechanical properties Sample
Ak
0.05Sr
0.10Sr
0.15Sr
HV(GPa)
2.42±0.24
4.91±0.35
3.93±0.11
4.20±0.16
KIC
1.48±0.03
1.96±0.03
1.36±0.04
1.42±0.07
DTS
10.19±21
12.93±1.17
11.32±2.87
11.28±2.36
Pores can be categorized as isolated within the solid (closed) and as interconnected channels threading through the solid from the surface (open). The entire porosity in ceramics is almost open pore prior to sintering process but upon heat treatment, many of the open pore become closed pores due to the decrease in the volume fraction of porosity [22]. In the present study, the Sr substitution increased the closed porosity compared to akermanite sample which in line with density results (Table 2). Furthermore, in conventional sintering, the material surface is exposed to heat first when the temperature is increased [23], then the closed porosity could be increased due to the increased flow of ions and subsequent closure of surface porosity. The pores are known to act as flaw or stress raiser in ceramics and there is an inverse relationship between the mechanical properties of material and its open porosity. Based on the minimum solid area models [24], the strength porosity dependence could be estimated by the following expression:
0 exp(bp )
(11)
Where, is the strength of porous structure at porosity p, 0 is the strength corresponding to nonporous structure and b is the empirical constant, which is determined by the pore characteristics. Therefore, there is an exponential increase in mechanical strength by decrease in porosity. The results of porosity-density measurements in the present
H. Mohammadi et al. / Materials Today: Proceedings 17 (2019) 929–936
935
study showed that the open porosity was decreased by the Sr substitution. In addition, it has shown that the hardness and fracture toughness of non-transforming ceramics are highly affected by grain size and the porosity [25]. According to Table 2, by increasing the amounts of Sr into the akermanite structure, the densification increased which is further confirmed by FESEM micrographs in Fig.2. This decrease in open porosity increased the mechanical properties. It is also generally accepted that the strength of ceramics decreases with an increase in grain size and porosity [26]. Furthermore, an interesting characteristic of ceramic materials is their grain size refinement ability that could simultaneously enhance the fracture toughness and hardness of ceramic due to the change of cracking mode from transgranular to intergranular as well as the prevention of propagating cracks [27]. Additionally, according to the Hall-Petch equation, the decrease in grain size increase the mechanical properties particularly hardness as follows [28]:
HV K O (K H G
1 2
)
(12)
The porosity of akermanite rapidly decreased by sintering at 1225°C compared to previous study [13]. As the sintering temperature incased, the presence of strontium dopant enhanced the sinterability of akermanite ceramic and stimulated the densification, which was the reason for improvement in mechanical properties. However, by further increasing of Sr substitution beyond 5 mol%, the mechanical properties decreased which could be attributed to the grain growth in the microstructure. The fracture toughness of sintered akermanite and Sr-substituted akermanite in the present study was higher than that CaSiO3 synthesized by chemical precipitation and spark plasma sintering (0.5 ± 0.2 MPa.m1/2 ) [29] and close to human cortical bone (2–12 MPa.m1/2), particularly 0.05Sr [30]. Furthermore, the fracture toughness of human dentin was reported to vary in the range of 1–2 MPa.m1/2 [31, 32]. The fracture toughness of 0.05Sr and 0.10Sr was significantly different compared to akermanite (p*<0.05) (Table 3). The hardness value of akermanite was improved by Sr substitution and reached to highest value for 0.05Sr. The hardness values obtained in this study is close to that of enamel in the range of 3–6 GPa [33]. Besides, the hardness values obtained in the present study was close to that of dentin in the range of 0.3–4 GPa [31, 32]. The hardness of Sr-substituted samples was significantly different compared to akermanite (p*<0.05). The DTS values obtained in this study was in the range of 1.5–38 MPa for cancellous bone [34]. Further, no significant difference was observed for DTS values after Sr substitution (p*>0.05). 5. Conclusion The akermanite and Sr-substituted akermanite bioceramic were successfully synthesised by solid-state sintering route. The XRD pattern showed the phase formation of akermanite. The FESEM micrographs showed uniform grain size while the sample was still porous. In this study, the substitution of SrO had enhanced the densification of akermanite bioceramic. Furthermore, the SrO addition improved mechanical properties, including fracture toughness, hardness, and diametral tensile strength. This enhancement of the mechanical properties may be due to the synergistic effect of enhanced sinterability and densification. All the results suggest that akermanite and Srsubstituted could be used bone and dental biomaterial. Acknowledgements The authors sincerely acknowledge the Fundamental Research Grant Scheme, FRGS (6071376) of the Ministry of Higher Education (Malaysia) for supporting this work.
936
H. Mohammadi et al. / Materials Today: Proceedings 17 (2019) 929–936
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34]
L.A. Adams, E.R. Essien, E.E. Kaufmann, A new route to sol-gel crystalline wollastonite bioceramic, Journal of Asian Ceramic Societies, 6(2), 2018, pp. 132-138. S. Ni, J. Chang, L. Chou, A novel bioactive porous CaSiO3 scaffold for bone tissue engineering, Journal of Biomedical Materials Research Part A,76(1), 2006,pp. 196-205. S. Ni, J. Chang, L. Chou, W. Zhai, Comparison of osteoblast-like cell responses to calcium silicate and tricalcium phosphate ceramics in vitro, Journal of biomedical materials research. Part B, Applied biomaterials,80(1), 2007,pp. 174-83. Y. Iimori, Y. Kameshima, K. Okada, S. Hayashi, Comparative study of apatite formation on CaSiO 3 ceramics in simulated body fluids with different carbonate concentrations, Journal of materials science: materials in medicine,16(1), 2005,pp. 73-79. A. El-Ghannam, P. Ducheyne, I. Shapiro, Formation of surface reaction products on bioactive glass and their effects on the expression of the osteoblastic phenotype and the deposition of mineralized extracellular matrix, Biomaterials,18(4), 1997,pp. 295-303. L. Long, L. Chen, S. Bai, J. Chang, K. Lin, Preparation of dense β-CaSiO3 ceramic with high mechanical strength and HAp formation ability in simulated body fluid, Journal of the European Ceramic Society,26(9), 2006,pp. 1701-1706. P. Habibovic, J. Barralet, Bioinorganics and biomaterials: bone repair, Acta biomaterialia,7(8), 2011,pp. 3013-3026. C. Wu, J. Chang, A review of bioactive silicate ceramics, Biomedical materials,8(3), 2013,pp. 032001. Y. Huang, X. Jin, X. Zhang, H. Sun, J. Tu, T. Tang, J. Chang, K. Dai, In vitro and in vivo evaluation of akermanite bioceramics for bone regeneration, Biomaterials,30(28), 2009,pp. 5041-5048. C. Wu, J. Chang, A novel akermanite bioceramic: preparation and characteristics, Journal of biomaterials applications,21(2), 2006,pp. 119129. A.K. Sharafabadi, M. Abdellahi, A. Kazemi, A. Khandan, N. Ozada, A novel and economical route for synthesizing akermanite (Ca2MgSi2O7) nano-bioceramic, Materials Science and Engineering: C,71, 2017,pp. 1072-1078. L. Fiocco, S. Li, M.M. Stevens, E. Bernardo, J.R. Jones, Biocompatibility and bioactivity of porous polymer-derived Ca-Mg silicate ceramics, Acta Biomaterialia,50, 2017,pp. 56-67. H. Mohammadi, Y.M.B. Ismail, K.A.B. Shariff, A.-F.M. Noor, Synthesis and Characterization of Akermanite by Mechanical Milling and Subsequent Heat Treatment, Journal of Physics: Conference Series, IOP Publishing, 2018, p. 012021. A. Monshi, M.R. Foroughi, M.R. Monshi, Modified Scherrer equation to estimate more accurately nano-crystallite size using XRD, World Journal of Nano Science and Engineering,2(3), 2012,pp. 154-160. B.D. Cullity, Elements of X-ray Diffraction, Addison-Wesley Publishing Company1978. ASTM, Standard Test Methods for Density of Compacted or Sintered Powder Metallurgy (PM) Products Using Archimedes’ Principle, ASTM B962-17, 2017. J.N. Lalena, D.A. Cleary, Principles of inorganic materials design, John Wiley & Sons2010. ASTM, Standard test method for Knoop and Vickers hardness of materials, ASTM E384-10, 2010. K. Niihara, R. Morena, D. Hasselman, Evaluation ofK Ic of brittle solids by the indentation method with low crack-to-indent ratios, Journal of materials science letters,1(1), 1982,pp. 13-16. ASTM, Standard test method for splitting tensile strength of intact rock core specimens, ASTM D3967-08, 2008. R.D. Shannon, Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides, Acta crystallographica section A: crystal physics, diffraction, theoretical and general crystallography,32(5), 1976,pp. 751-767. W.E. Lee, M. Rainforth, Ceramic Microstructures: Property control by processing, Springer Netherlands1994. S. Ramesh, N. Zulkifli, C.Y. Tan, Y.H. Wong, F. Tarlochan, S. Ramesh, W.D. Teng, I. Sopyan, L.T. Bang, A.A.D. Sarhan, Comparison between microwave and conventional sintering on the properties and microstructural evolution of tetragonal zirconia, Ceramics International,44(8), 2018,pp. 8922-8927. R. Rice, Comparison of stress concentration versus minimum solid area based mechanical property-porosity relations, Journal of materials science,28(8), 1993,pp. 2187-2190. D. Veljović, B. Jokić, R. Petrović, E. Palcevskis, A. Dindune, I. Mihailescu, D. Janaćković, Processing of dense nanostructured HAP ceramics by sintering and hot pressing, Ceramics International,35(4), 2009,pp. 1407-1413. F. Knudsen, Dependence of mechanical strength of brittle polycrystalline specimens on porosity and grain size, Journal of the American Ceramic Society,42(8), 1959,pp. 376-387. D. Lahiri, S. Ghosh, A. Agarwal, Carbon nanotube reinforced hydroxyapatite composite for orthopedic application: a review, Materials Science and Engineering: C,32(7), 2012,pp. 1727-1758. H. Ryou, J.W. Drazin, K.J. Wahl, S.B. Qadri, E.P. Gorzkowski, B.N. Feigelson, J.A. Wollmershauser, Below the Hall–Petch Limit in Nanocrystalline Ceramics, ACS nano,12(4), 2018,pp. 3083-3094. L.H. Long, L.D. Chen, S.Q. Bai, J. Chang, K.L. Lin, Preparation of dense β-CaSiO3 ceramic with high mechanical strength and HAp formation ability in simulated body fluid, Journal of the European Ceramic Society,26(9), 2006,pp. 1701-1706. L.L. Hench, Bioceramics: from concept to clinic, Journal of the american ceramic society,74(7), 1991,pp. 1487-1510. V. Imbeni, J.J. Kruzic, G.W. Marshall, S.J. Marshall, R.O. Ritchie, The dentin-enamel junction and the fracture of human teeth, Nature materials,4(3), 2005,pp. 229-32. Y.R. Zhang, W. Du, X.D. Zhou, H.Y. Yu, Review of research on the mechanical properties of the human tooth, International journal of oral science,6(2), 2014,pp. 61-9. Y.-Q. Wu, J.A. Arsecularatne, M. Hoffman, Attrition-corrosion of human dental enamel: A review, Biosurface and Biotribology,3(4), 2017,pp. 196-210. F. Witte, N. Hort, C. Vogt, S. Cohen, K.U. Kainer, R. Willumeit, F. Feyerabend, Degradable biomaterials based on magnesium corrosion, Current opinion in solid state and materials science,12(5-6), 2008,pp. 63-72.
ScienceDirect Materials Today: Proceedings 17 (2019) 929–936
www.materialstoday.com/proceedings
RAMM 2018
Effect of substitutional Strontium on Mechanical Properties of Akermanite Ceramic Prepared by Solid-State Sintering H. Mohammadia, Y.M.B. Ismaila, K.A. Shariffa, A-F.M. Noora* School of Materials and Mineral Resources Engineering, Universiti Sains Malaysia, Engineering Campus, 14300, Nibong Tebal, Penang, Malaysia
Abstract Akermanite (Ca2MgSi2O7) has attracted attention in the biomaterial research due to its excellent in vitro and in vivo bioactivity. Mechanical properties such as microhardness and fracture toughness are of great importance for bioactive materials to predict their performance before failure and thus determine the potential biomedical application. In previous work, sintered akermanite prepared by solid-state sintering at 1200°C showed in vitro apatite formation in simulated body fluid (SBF) solution, but having high porosity (more than 20%). In the present study, bulk akermanite bioceramics was fabricated and evaluated the mechanical properties i.e. microhardness and fracture toughness. The akermanite Sr-doped akermanite were synthesised by solid-state sintering at 1225°C for 4 h. Results indicated that the substitution of Sr enhanced the microhardness and fracture toughness which is attributed to the controlled grain size, improved density as well as better sinterability. However, by further increase in amount of dopant beyond 5 mol%, both hardness and fracture toughness decreased due to the larger grain size. akermanite at 1225°C led to a microhardness and fracture toughness of 2.42±0.24 GPa and 1.48±0.03 MPa.m1/2, respectively which makes it a suitable material for non-load bearing application such as, bone filler material. © 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the 6th International Conference on Recent Advances in Materials, Minerals & Environment (RAMM) 2018. Keywords: Solid-state sintering; fracture toughness; Hardness; Diametral tensile strength ____________________________________________________________________________________________________________________
* Corresponding author. Tel.: +604-5996174; fax: +604-594101. E-mail address: [email protected] 2214-7853 © 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the 6th International Conference on Recent Advances in Materials, Minerals & Environment (RAMM) 2018.
930
H. Mohammadi et al. / Materials Today: Proceedings 17 (2019) 929–936
1. Introduction The regeneration and repair of large bone defect is still a challenge issue in the biomedical field. Although the calcium phosphates such as hydroxyapatite (HA) or tricalcium phosphate (TCP) has been investigated over there decades, more recent calcium silicate ceramics (CaSiO3) have attracted great attentions in the biomaterial community [1]. This material has reported to have good bioactivity [2] and improved cell proliferation compared to phosphate-based ceramics [3]. However, the major drawbacks of silicates are higher dissolution rate which leads to high environmental pH value[4] that would be detrimental to cells [5] as well as low bending strength and fracture toughness [6]. The vital role of bioinorganics such as calcium (Ca2+), Si (Si4+) and magnesium (Mg2+) in bone development has been well documented in the literature [7]. Furthermore, the incorporation of bioinorganics such as magnesium (Mg2+) into the structure results in enhanced chemical stability and mechanical properties [8]. In recent decade, akermanite (Ca2MgSi2O7), a magnesium modified calcium silicate has shown higher in vitro and in vivo cell proliferation compared to β-TCP [9]. There are different ways to synthesize akermanite such as sol-gel[10], mechanical activation [11], and precermic polymer [12]. Apart from in vitro and in vivo performance of bioceramic, the mechanical evaluations is also of great importance to get an understanding of potential field of application. In previous study, akermanite has been synthesised via by solid-state sintering route at 1200°C and demonstrated its proper mechanical properties as well as in vitro apatite forming ability [13]. Nevertheless, akermanite was still porous at this sintering temperature, which may compromise the mechanical properties. However, at 1250°C, there was partial melting and 1300°C samples melted. Thus, this paper specifically reports the results obtained at 1225°C as it provides optimal results. The main purpose of this study is also to partially substitute strontium (Sr2+) into the akermanite to obtain highly densified ceramic with improved mechanical properties including diametral tensile strength, hardness and fracture toughness by solid-state sintering route. 2. Experimental procedure 2.1. Preparation and sintering of akermanite and Sr-substituted ceramic Akermanite powder was synthesized using a wet high-energy planetary ball milling. Briefly, calcium oxide (CaO, Merck, 98%), magnesium oxide (MgO, Merck, 98%), silicon dioxide (SiO2, Sigma-Aldrich, 99.9%), were weighed based on the stoichiometric ratio of akermanite and then were ball milled for 3 h in the planetary ball milling (PM 400-Reutch). The ball to powder weight ratio was 10:1, while the powder to deionized water ratio was 1:3. The speed of vial was 500 rpm. The deionized water the liquid media was used to increase the efficiency and prevent any dead zone. After milling, the mixture was dried in oven for 24 h at 100°C. Then, Ca2-xSrxMgSi2O7 (Sr/Sr+Ca to control the partial substitution of Sr with Ca) samples were prepared with the different amount of strontium oxide (SrO, Sigma-Aldrich, 99.9%, Germany), where the compositional parameter x represented 0.05, 0.10 and 0.15 mol% of SrO. Akermanite nanopowder was synthesised using a high-energy and high-speed planetary ball milling under wet condition. Then, the dried powders were ground using agate mortar pestle and sieved through 200 µm. Finally, sieved powders were pressed into pellets with the dimension of 13 mm (diameter) using uniaxial hydraulic press (24T Laboratory hydraulic Press, MTI Cooperation) under the pressure of 200 MPa. The pellets were then sintered 1225°C for 4 h with a heating rate of 5°C/min.
H. Mohammadi et al. / Materials Today: Proceedings 17 (2019) 929–936
931
2.2. Characterization of akermanite and Sr-substituted akermanite ceramic 2.2.1. Phase analysis The X-ray diffraction (XRD, D8 Bruker Advance Diffractometer, England) conducted with CuKα1 radiation (λ=1.541 Å at 20 kV and 30 mA) with step size of 0.03, step scan time of 33 s in diffraction angles (2θ°) between 10° and 90° to determine phase, lattice parameter and crystallite size of the synthesised powders. The data was analysed by PANalytical X’Pert High Score Plus (ver 2.2). The X-ray patterns were matched to the International Center for Diffraction Data (ICDD) reference files. Furthermore, the modified Scherrer equation was used to calculate the crystallite size of the powders based on the XRD patterns for the three highest intensity peaks [14]:
ln ln(
k 1 ) ln( ) L cos
(1)
The lattice parameters a and c was calculated by the standard tetragonal unit cell relationship between the interplanar spacing (dhkl) in the set of Miller indices (hkl) and the lattice parameters as follows [15]:
1
a 1 (h 2 k 2 l 2 ( )2 ) 2 (d hkl ) c a
(2)
2
2.2.2. Microstructural analysis The morphology and microstructure of surface and fracture parts of sintered pellet at 1225°C was observed by field emission scanning electron microscopy (FESEM) with energy dispersive spectroscopy (Zeiss SupraTM35VP, Germany). Prior to SEM, the specimens were then coated with a thin layer of gold (Au) by sputtering (EMITECH K450X, England) to increase the conductivity of sample. 2.2.3. Porosity and density The apparent density ( a ), relative density ( Re ) open porosity (PO) is calculated based on the ASTM B96217[16] by using the following formula [16]in which theoretical density ( ) was calculated from XRD data (unit cell volume as well as lattice constants) [15, 17] :
a (%)
w1 100 w 1 w 2
Re (%)
PO (%)
a 100
w 3 w 1 100 w 3 w 2
(3)
(4)
(5)
932
H. Mohammadi et al. / Materials Today: Proceedings 17 (2019) 929–936
PT (%) 100
a 100
PC (%) PT PO
(6) (7)
Whereby, w 1 is the weight of the dry sample in air, w 2 is the weight of the sample suspended in deionized water and w 3 is the weight of the sample after being saturated in deionized water. 2.2.4. Microhardness and fracture toughness The Vickers microhardness test was performed on the samples by using the Vickers microhardness tester (Future Tech, Japan). The sintered pellets were cold mounted using a mixture of epoxy resin and hardener, then mechanically ground using SiC papers (Buehler Ltd., USA) with 240–1000 grades, and subsequently polished with a 0.3 micron alumina suspension to get a smooth flat surface. Four measurements were performed on each sample using a diamond indenter at 5 kgf for 10 s and average values were recorded. The ASTM-E384-10 was used to measure the Vickers hardness of samples as follows [18]:
HV
0.18544 P d2
(8)
Where, HV (GPa) is the Vickers hardness, P is the load applied (N) and d is the average of two diagonal lengths of indentation (mm). The fracture toughness K IC MPa. m was calculated from the Vickers microhardness (HV) using Niihara formula[19]: 1
K IC 0.203 HV (ain ) 2 (
c in 32 ) ain
(9)
Where, KIC (MPa.m1/2) is the fracture toughness ain is half of the indent diagonal (µm) and c in is the indentation crack length (µm) from the center of the indentation. 2.2.1. Diametral tensile strength The Brasilian test was performed on the sintered pellet (13 mm diameter and 11 mm height) by utilising a universal experimental instrument (INSTRON 3367) at a speed rate of 0.5 mm/min. The ASTM-D3967 was used to calculate the tensile strength of the samples by diametral line as follows [20]:
Tensile
2 P D h
(10)
Where, Tensile (MPa) is tensile strength, D is the diameter (mm), h is the sample thickness (mm) and P is the applied force (N).
H. Mohammadi et al. / Materials Today: Proceedings 17 (2019) 929–936
933
3. Statistics The results of experiments are represented as mean and arithmetic standard deviation (±S.D.) for four samples. Analysis of results was carried out using one-way ANOVA test with a significance level of (p*<0.05) using GraphPad Prism software (Ver 6.07). 4. Results and discussion The XRD patterns of the akermanite sample sintered at 1225°C is shown in Fig.1a. The diffraction patterns showed that the main phase formed is akermanite (ICDD No. 035-0592). The main akermanite peak was located at 31.43°. The main characteristic peaks of (121) for 2θ=31.09, (201) for 2θ=28.87, (312) for 2θ=51.82 and (122) for 2θ=44.40 indicated the formation of akermanite phase. The lattice parameters of akermanite and Sr-substituted akermanite samples are summarized in Table 1. The enlargement of peak with high intensity revealed slight shift to the lower angle (Fig.1b), that is attributed to the substitution of strontium (Sr2+:1.13Å) with higher ionic radius compared to calcium (Ca2+:0.99Å) [21]. Additionally, the lattice parameter was also increased due to the substitution of Sr (Table 1).
Fig. 1. The XRD pattern of akermanite and Sr-substituted akermanite sintered at 1225°C
Table 1. Lattice parameters of akermanite and Sr-substituted akermanite sintered at 1225°C Lattice constants Sample
Ak
0.05Sr
0.10Sr
0.15Sr
a(Å)
7.828
7.844
7.855
7.862
c(Å)
5.006
5.016
5.023
5.029
V(Å3)
307.755
308.626
309.924
310.847
The FESEM micrographs of surface and fracture parts of sintered akermanite are depicted in Fig. 2. The results suggest a good densification after sintering at 1225°C. The FESEM of the surface showed a relatively uniform rain microstructure while the fractured part showed the typical ceramic fractures (i.e. transgranular and intergranular fractures). The sintered akermanite at 1225°C showed 78.04 ± 1.66% density and 21.36 ± 1.63% porosity which is in agreement with SEM micrograph (Fig.2a). However, the density of akermanite was increased by the substitution of Sr ions (Fig.2b-d).
934
H. Mohammadi et al. / Materials Today: Proceedings 17 (2019) 929–936
Fig. 2. The FESEM micrographs of (a-d) surface and (e-f) fractured of akermanite and Sr-substituted akermanite sintered at 1225°C Table 2. Porosity and density of akermanite and Sr-substituted akermanite sintered at 1225°C Porosity and density Sample
Re
Ak
0.05Sr
78.04±1.66
86.72±1.82
88.89±2.74
0.10Sr
92.17±1.84
0.15Sr
PO (%)
21.36±1.63
6.56±1.20
6.43±1.22
4.39±1.32
PT (%)
21.96±1.66
13.28±1.82
11.11±2.74
7.83±1.95
PC (%)
0.61±0.31
6.72±2.27
4.68±1.61
3.44±0.63
Table 3. Mechanical properties of akermanite and Sr-substituted akermanite sintered at 1225°C Mechanical properties Sample
Ak
0.05Sr
0.10Sr
0.15Sr
HV(GPa)
2.42±0.24
4.91±0.35
3.93±0.11
4.20±0.16
KIC
1.48±0.03
1.96±0.03
1.36±0.04
1.42±0.07
DTS
10.19±21
12.93±1.17
11.32±2.87
11.28±2.36
Pores can be categorized as isolated within the solid (closed) and as interconnected channels threading through the solid from the surface (open). The entire porosity in ceramics is almost open pore prior to sintering process but upon heat treatment, many of the open pore become closed pores due to the decrease in the volume fraction of porosity [22]. In the present study, the Sr substitution increased the closed porosity compared to akermanite sample which in line with density results (Table 2). Furthermore, in conventional sintering, the material surface is exposed to heat first when the temperature is increased [23], then the closed porosity could be increased due to the increased flow of ions and subsequent closure of surface porosity. The pores are known to act as flaw or stress raiser in ceramics and there is an inverse relationship between the mechanical properties of material and its open porosity. Based on the minimum solid area models [24], the strength porosity dependence could be estimated by the following expression:
0 exp(bp )
(11)
Where, is the strength of porous structure at porosity p, 0 is the strength corresponding to nonporous structure and b is the empirical constant, which is determined by the pore characteristics. Therefore, there is an exponential increase in mechanical strength by decrease in porosity. The results of porosity-density measurements in the present
H. Mohammadi et al. / Materials Today: Proceedings 17 (2019) 929–936
935
study showed that the open porosity was decreased by the Sr substitution. In addition, it has shown that the hardness and fracture toughness of non-transforming ceramics are highly affected by grain size and the porosity [25]. According to Table 2, by increasing the amounts of Sr into the akermanite structure, the densification increased which is further confirmed by FESEM micrographs in Fig.2. This decrease in open porosity increased the mechanical properties. It is also generally accepted that the strength of ceramics decreases with an increase in grain size and porosity [26]. Furthermore, an interesting characteristic of ceramic materials is their grain size refinement ability that could simultaneously enhance the fracture toughness and hardness of ceramic due to the change of cracking mode from transgranular to intergranular as well as the prevention of propagating cracks [27]. Additionally, according to the Hall-Petch equation, the decrease in grain size increase the mechanical properties particularly hardness as follows [28]:
HV K O (K H G
1 2
)
(12)
The porosity of akermanite rapidly decreased by sintering at 1225°C compared to previous study [13]. As the sintering temperature incased, the presence of strontium dopant enhanced the sinterability of akermanite ceramic and stimulated the densification, which was the reason for improvement in mechanical properties. However, by further increasing of Sr substitution beyond 5 mol%, the mechanical properties decreased which could be attributed to the grain growth in the microstructure. The fracture toughness of sintered akermanite and Sr-substituted akermanite in the present study was higher than that CaSiO3 synthesized by chemical precipitation and spark plasma sintering (0.5 ± 0.2 MPa.m1/2 ) [29] and close to human cortical bone (2–12 MPa.m1/2), particularly 0.05Sr [30]. Furthermore, the fracture toughness of human dentin was reported to vary in the range of 1–2 MPa.m1/2 [31, 32]. The fracture toughness of 0.05Sr and 0.10Sr was significantly different compared to akermanite (p*<0.05) (Table 3). The hardness value of akermanite was improved by Sr substitution and reached to highest value for 0.05Sr. The hardness values obtained in this study is close to that of enamel in the range of 3–6 GPa [33]. Besides, the hardness values obtained in the present study was close to that of dentin in the range of 0.3–4 GPa [31, 32]. The hardness of Sr-substituted samples was significantly different compared to akermanite (p*<0.05). The DTS values obtained in this study was in the range of 1.5–38 MPa for cancellous bone [34]. Further, no significant difference was observed for DTS values after Sr substitution (p*>0.05). 5. Conclusion The akermanite and Sr-substituted akermanite bioceramic were successfully synthesised by solid-state sintering route. The XRD pattern showed the phase formation of akermanite. The FESEM micrographs showed uniform grain size while the sample was still porous. In this study, the substitution of SrO had enhanced the densification of akermanite bioceramic. Furthermore, the SrO addition improved mechanical properties, including fracture toughness, hardness, and diametral tensile strength. This enhancement of the mechanical properties may be due to the synergistic effect of enhanced sinterability and densification. All the results suggest that akermanite and Srsubstituted could be used bone and dental biomaterial. Acknowledgements The authors sincerely acknowledge the Fundamental Research Grant Scheme, FRGS (6071376) of the Ministry of Higher Education (Malaysia) for supporting this work.
936
H. Mohammadi et al. / Materials Today: Proceedings 17 (2019) 929–936
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34]
L.A. Adams, E.R. Essien, E.E. Kaufmann, A new route to sol-gel crystalline wollastonite bioceramic, Journal of Asian Ceramic Societies, 6(2), 2018, pp. 132-138. S. Ni, J. Chang, L. Chou, A novel bioactive porous CaSiO3 scaffold for bone tissue engineering, Journal of Biomedical Materials Research Part A,76(1), 2006,pp. 196-205. S. Ni, J. Chang, L. Chou, W. Zhai, Comparison of osteoblast-like cell responses to calcium silicate and tricalcium phosphate ceramics in vitro, Journal of biomedical materials research. Part B, Applied biomaterials,80(1), 2007,pp. 174-83. Y. Iimori, Y. Kameshima, K. Okada, S. Hayashi, Comparative study of apatite formation on CaSiO 3 ceramics in simulated body fluids with different carbonate concentrations, Journal of materials science: materials in medicine,16(1), 2005,pp. 73-79. A. El-Ghannam, P. Ducheyne, I. Shapiro, Formation of surface reaction products on bioactive glass and their effects on the expression of the osteoblastic phenotype and the deposition of mineralized extracellular matrix, Biomaterials,18(4), 1997,pp. 295-303. L. Long, L. Chen, S. Bai, J. Chang, K. Lin, Preparation of dense β-CaSiO3 ceramic with high mechanical strength and HAp formation ability in simulated body fluid, Journal of the European Ceramic Society,26(9), 2006,pp. 1701-1706. P. Habibovic, J. Barralet, Bioinorganics and biomaterials: bone repair, Acta biomaterialia,7(8), 2011,pp. 3013-3026. C. Wu, J. Chang, A review of bioactive silicate ceramics, Biomedical materials,8(3), 2013,pp. 032001. Y. Huang, X. Jin, X. Zhang, H. Sun, J. Tu, T. Tang, J. Chang, K. Dai, In vitro and in vivo evaluation of akermanite bioceramics for bone regeneration, Biomaterials,30(28), 2009,pp. 5041-5048. C. Wu, J. Chang, A novel akermanite bioceramic: preparation and characteristics, Journal of biomaterials applications,21(2), 2006,pp. 119129. A.K. Sharafabadi, M. Abdellahi, A. Kazemi, A. Khandan, N. Ozada, A novel and economical route for synthesizing akermanite (Ca2MgSi2O7) nano-bioceramic, Materials Science and Engineering: C,71, 2017,pp. 1072-1078. L. Fiocco, S. Li, M.M. Stevens, E. Bernardo, J.R. Jones, Biocompatibility and bioactivity of porous polymer-derived Ca-Mg silicate ceramics, Acta Biomaterialia,50, 2017,pp. 56-67. H. Mohammadi, Y.M.B. Ismail, K.A.B. Shariff, A.-F.M. Noor, Synthesis and Characterization of Akermanite by Mechanical Milling and Subsequent Heat Treatment, Journal of Physics: Conference Series, IOP Publishing, 2018, p. 012021. A. Monshi, M.R. Foroughi, M.R. Monshi, Modified Scherrer equation to estimate more accurately nano-crystallite size using XRD, World Journal of Nano Science and Engineering,2(3), 2012,pp. 154-160. B.D. Cullity, Elements of X-ray Diffraction, Addison-Wesley Publishing Company1978. ASTM, Standard Test Methods for Density of Compacted or Sintered Powder Metallurgy (PM) Products Using Archimedes’ Principle, ASTM B962-17, 2017. J.N. Lalena, D.A. Cleary, Principles of inorganic materials design, John Wiley & Sons2010. ASTM, Standard test method for Knoop and Vickers hardness of materials, ASTM E384-10, 2010. K. Niihara, R. Morena, D. Hasselman, Evaluation ofK Ic of brittle solids by the indentation method with low crack-to-indent ratios, Journal of materials science letters,1(1), 1982,pp. 13-16. ASTM, Standard test method for splitting tensile strength of intact rock core specimens, ASTM D3967-08, 2008. R.D. Shannon, Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides, Acta crystallographica section A: crystal physics, diffraction, theoretical and general crystallography,32(5), 1976,pp. 751-767. W.E. Lee, M. Rainforth, Ceramic Microstructures: Property control by processing, Springer Netherlands1994. S. Ramesh, N. Zulkifli, C.Y. Tan, Y.H. Wong, F. Tarlochan, S. Ramesh, W.D. Teng, I. Sopyan, L.T. Bang, A.A.D. Sarhan, Comparison between microwave and conventional sintering on the properties and microstructural evolution of tetragonal zirconia, Ceramics International,44(8), 2018,pp. 8922-8927. R. Rice, Comparison of stress concentration versus minimum solid area based mechanical property-porosity relations, Journal of materials science,28(8), 1993,pp. 2187-2190. D. Veljović, B. Jokić, R. Petrović, E. Palcevskis, A. Dindune, I. Mihailescu, D. Janaćković, Processing of dense nanostructured HAP ceramics by sintering and hot pressing, Ceramics International,35(4), 2009,pp. 1407-1413. F. Knudsen, Dependence of mechanical strength of brittle polycrystalline specimens on porosity and grain size, Journal of the American Ceramic Society,42(8), 1959,pp. 376-387. D. Lahiri, S. Ghosh, A. Agarwal, Carbon nanotube reinforced hydroxyapatite composite for orthopedic application: a review, Materials Science and Engineering: C,32(7), 2012,pp. 1727-1758. H. Ryou, J.W. Drazin, K.J. Wahl, S.B. Qadri, E.P. Gorzkowski, B.N. Feigelson, J.A. Wollmershauser, Below the Hall–Petch Limit in Nanocrystalline Ceramics, ACS nano,12(4), 2018,pp. 3083-3094. L.H. Long, L.D. Chen, S.Q. Bai, J. Chang, K.L. Lin, Preparation of dense β-CaSiO3 ceramic with high mechanical strength and HAp formation ability in simulated body fluid, Journal of the European Ceramic Society,26(9), 2006,pp. 1701-1706. L.L. Hench, Bioceramics: from concept to clinic, Journal of the american ceramic society,74(7), 1991,pp. 1487-1510. V. Imbeni, J.J. Kruzic, G.W. Marshall, S.J. Marshall, R.O. Ritchie, The dentin-enamel junction and the fracture of human teeth, Nature materials,4(3), 2005,pp. 229-32. Y.R. Zhang, W. Du, X.D. Zhou, H.Y. Yu, Review of research on the mechanical properties of the human tooth, International journal of oral science,6(2), 2014,pp. 61-9. Y.-Q. Wu, J.A. Arsecularatne, M. Hoffman, Attrition-corrosion of human dental enamel: A review, Biosurface and Biotribology,3(4), 2017,pp. 196-210. F. Witte, N. Hort, C. Vogt, S. Cohen, K.U. Kainer, R. Willumeit, F. Feyerabend, Degradable biomaterials based on magnesium corrosion, Current opinion in solid state and materials science,12(5-6), 2008,pp. 63-72.