Graphene-reinforced mechanical properties of calcium silicate scaffolds by laser sintering

Abstract

Graphene may have great potential as the reinforced phase in bioceramics by virtue of its extraordinary mechanical properties and intrinsic biocompatibility. Calcium silicate (CaSiO₃) bioceramics have been proposed as promising biomaterials but suffer from poor mechanical properties. In this study we report for the first time the use of graphene to improve the strength and toughness of CaSiO₃ bioceramics for bone scaffolds. Graphene–CaSiO₃ composite scaffolds were fabricated via selective laser sintering technology, in which the sintering time was reduced to seconds or even microseconds through the rapid heating and cooling process of a laser. The fracture morphology, chemical composition and mechanical properties of the composite scaffolds were investigated and analyzed. The results showed that graphene was octopus-like with tall and straight tentacles embedded in the bioceramic matrix, indicating a toughening mechanism of pull-out. The remaining graphene in the composite scaffolds increased logarithmically with the graphene addition. The strength and toughness firstly increased with graphene content (0–0.5 wt% in this study) which was attributed to the load transfer from the ceramic matrix to graphene when fractured, while they decreased as the graphene content further increased to 1.0 or above due to the occurrence of graphene agglomeration and holes induced by excessive graphene. There were optimal improvements of fracture toughness by 46% and compressive strength by 142%.

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1 Introduction

In the past decades, there has been an ever-increasing demand for artificial bone to repair massive bone defects.¹ Bone scaffolds play a key role in bone repair and regeneration by providing structural supports for defect sites and a microenvironment to facilitate cell adhesion, proliferation and function.^2,3 And the scaffolds are expected to degrade and be adsorbed in vivo until completely replaced by new bone tissue.⁴ These require the scaffolds to be biocompatible and biodegradable, and possess enough mechanical strength and interconnected porous structure.⁵ Silicon is an essential element in human nutrition and capable of promoting bone calcification and new bone formation.⁶ As one kind of silicon-containing bioactive materials, calcium silicate (CaSiO₃) bioceramics have been proposed as potential biomaterials, since they exhibit excellent ability for improving bone-like apatite formation, osteoblast deposition and bone-related gene expression.⁷ However, the poor mechanical properties of CaSiO₃ bioceramics limit their applications in bone regeneration.⁸ Therefore, it has become a research hotspot to improve the strength and toughness of CaSiO₃ bioceramic scaffolds.

Graphene is a new two-dimensional carbon nano-material with Young's modulus ∼0.5–1 TPa, tensile strength ∼130 GPa and specific surface area ∼2630 m² g⁻¹.^9,10 The extraordinary physicochemical and mechanical properties indicate the potentials as reinforcement for ceramic composites. Simultaneously, considering its intrinsic biocompatibility,¹¹ graphene is particularly promising to toughen and strengthen CaSiO₃ bioceramics for bone generation. So far, graphene-reinforced ceramic composites have been investigated mainly for functional applications, and few studies have taken advantage of the excellent mechanical properties of graphene in ceramic composites for structural applications.¹² This was mainly attributed to the gradual loss of graphene during long time sintering under high temperature, which made it difficult to be incorporated into ceramic composites after dozens hours of high temperature processing in the conventional sintering methods.^13,14

Recent studies focused on spark plasma sintering (SPS) to fabricate graphene–ceramic composites by reducing the processing time from hours to minutes. L. S. Walker et al.¹⁵ reported the use of graphene platelets (GPL) as reinforcement in bulk silicon nitride (Si₃N₄) ceramics at 1650 °C by SPS. It was found that the fracture toughness of Si₃N₄ ceramic increased to 235% with 1.5% GPL volume fraction. K. Wang et al.¹⁶ prepared graphene nanosheet–alumina (Al₂O₃) composite with SPS method. Results showed that graphene–Al₂O₃ composite (2.0 wt% graphene) had a 53% improvement in fracture toughness over pure Al₂O₃. Nevertheless, SPS method has limitations in fabricating customized shape and interconnected porous structure, especially lack of control over pore size, number and connectivity¹⁷ which plays an important role in cell adhesion and tissue growth. Thus, there is an urgent need to develop a processing route for fabricating graphene-reinforced bioceramic scaffolds with controllable porous structure.

As one kind of rapid prototyping techniques, selective laser sintering (SLS) is capable to fabricate parts with complex geometry and internal structure.¹⁸ This allows accurate control of the shape and porous structure of bone scaffolds to be easily developed by selective sintering process. Moreover, the rapid heating and cooling process of laser sintering can reduce the sintering time further to seconds or milliseconds over other sintering methods, which would limit the thermal effect of high sintering temperature on graphene. To our knowledge, few studies have been reported on the applications of graphene in CaSiO₃ bioceramic scaffolds by SLS method.

In this study, graphene was incorporated into CaSiO₃ bioceramics to improve the fracture toughness and mechanical strength. Graphene–CaSiO₃ bioceramic scaffolds were fabricated by SLS in order to reduce the damage to graphene and realize accurate control of porous structure. The morphology and composition of the composite scaffolds were analyzed with scanning electron microscope (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD) and high frequency infrared carbon & sulfur analyzer. The fracture toughness, compressive strength and corresponding elastic modulus were also tested. The reinforced mechanism of graphene on CaSiO₃ bioceramics was discussed based on the above test results.

2 Materials and methods

2.1 Materials

Graphene (thickness: 0.7–1.2 nm, diameter: 0.8–3 μm, single layer ratio: 99.8%, and purity: 99.8%) was purchased from Nanjing JCNANO Tech Co., Ltd. It was prepared by modified Hummers's method and had a content of metal impurity below 10 ppm without any surfactant. CaSiO₃ powders were obtained from Kunshan Huaqiao New Materials Co., Ltd. The particle size was 0.2–2 μm with a total metal content (mg kg⁻¹) ≤50, thereinto As ≤ 3, Cd ≤ 5, Hg ≤ 5 and Pb ≤ 30. The two powders were dispersed in separate containers with deionized water at room temperature. The solutions were mixed together and subjected to sonication for 30 min with a sonicator, following by ball milling at low rotational speed (600 rpm for 30 min). These parameters were determined to avoid any structural damage to graphene. Graphene–CaSiO₃ composite powders with different weight percents (0, 0.1, 0.25, 0.5, 1.0 and 2.0 wt%) were obtained after dried with a rotary evaporator.

2.2 The composite scaffolds fabricated by SLS

Graphene–CaSiO₃ porous scaffolds were fabricated on the homemade SLS system^19,20 with laser power of 8.5 W, spot diameter of 1 mm, scanning speed of 100 mm min⁻¹ and layer thickness of 0.1–0.2 mm. The composite powders were first placed on the sintering platform. The powders were sintered selectively by a CO₂ laser under computer control. The sintered powders fused and became dense, while unsintered powders remained loose. When finished, the sintering platform dropped by a layer's thickness and a new layer of powder was placed and sintered on the former layer. The three dimensional (3D) structure was realized by repeating the above process. The graphene–CaSiO₃ porous scaffold was obtained after the unsintered powders were removed, as shown in Fig. 1. The scaffold was step-like with size of 17 mm × 17 mm × 12 mm (L × W × H). The pores were fully interconnected and distributed throughout the scaffold with size of about 800 μm. Besides, good connections between pore walls were observed.

Fig. 1 Graphene–CaSiO₃ porous scaffold. (a) Top view. (b) Side view. (c) Magnified view.

2.3 Chemico-physical characterization

A JEOL SEM (JSM-6490LV, JEOL Ltd., Japan) operating at 20 kV was used to observe the morphology and microstructure of specimens. The element distribution was investigated to locate the position of graphene by EDS (Neptune XM4, EDAX Inc., USA) coupled to SEM. Prior to the observation, platinum was sputtered on the surfaces to avoid the accumulation of static electric fields. Quantification of survived graphene in the specimens was analyzed after combustion by high-frequency induction using a high frequency infrared carbon sulfur analyzer (CS844, LECO, USA). Phase compositions were characterized with XRD (D/MAX-2550V, Rigaku, Japan) with Cu Kα radiation. The diffraction angle ranged from 10 to 80° with a step increment of 0.02°.

2.4 Mechanical characterization

Compressive evaluations were performed on specimens loaded axially at a crosshead speed of 0.5 mm min⁻¹ using a screw driven load frame (WD-D1, Shanghai Zhuoji instruments Co. LTD, China). Before testing, all the specimens were polished with diamond paste in order to obtain regular surfaces, followed by cleaning in ethanol for 3 h with an ultrasonic cleaner (SK3300H) to remove any surface debris. The compressive strength and modulus were calculated from the compressive stress–strain curves. The final data were shown with X̄ ± SD which were calculated from six specimens for each group.

The fracture toughness of the specimens was investigated with a digital microhardness tester (HXD-1000TM/LCD, Shanghai Taiming Optical Instrument Co. Ltd). Prior to the test, the specimens were inlayed and ground using 1500 grit abrasive paper. The indentations were prepared at six locations for each specimen with an applied load of 4.903 N (500 gf) and dwelling time of 15 s. The fracture toughness was determined by the applied load and the diagonal crack lengths produced at the indentation corners, as shown in the following equation.²¹

where K_IC is the fracture toughness (Pa m^1/2), P is the applied load (N), and c is the diagonal crack length (m).

3 Results and discussion

The morphology of as-received CaSiO₃ and graphene powders were shown in Fig. 2a and b, respectively. CaSiO₃ particles were spherical-like with average particle size of 1 μm, and graphene was flake-like. After mixing and ball milling, graphene was decorated with individual CaSiO₃ particles in the composite powder, and CaSiO₃ particles were well-dispersed throughout the surface of graphene (Fig. 2c). The surface of all the specimens reveals to be smooth (Fig. 2d) after sintering process.

Fig. 2 SEM micrographs. (a) As-received CaSiO₃. (b) As-received graphene. (c) The composite powders (0.5 wt% graphene content). (d) Surface morphology after sintering (0.5 wt% graphene content).

In order to investigate the morphology and distribution of graphene in CaSiO₃ scaffolds, the microstructure and element distribution of the fracture surface of sintered specimens with different graphene contents were obtained by SEM/EDS (Fig. 3). Good compactness and some roughness were observed on the fracture surface of specimen sintered with 0 wt% graphene (Fig. 3a). There was a similar fracture morphology (the pull-out of graphene) between specimens sintered with 0.1, 0.25, 0.5, 1.0 and 2.0 wt% (Fig. 3b–f, respectively). Grapheme was octopus-like with tentacles embedded in the ceramic matrix, as marked with white arrows. The tall and straight tentacles indicated high interface bonding strength between graphene and CaSiO₃ bioceramics. The others remained flake-like shown in Fig. 3e. This may be attributed to the different morphological distribution of graphene in the 3D structure of CaSiO₃ ceramics. Besides, graphene agglomeration and small holes were observed on the fracture surfaces when graphene content further increased to 1.0 or 2.0 wt%, which would result in the decrease of sintering density. Element analyses were carried out on two typical region of fracture surfaces (marked with black triangle and rectangle in Fig. 3f), and the spectra were shown in Fig. 3g and h. The spectrum was dominated by the peaks of calcium, silicon and oxygen elements at energy of about 0.5, 1.8 and 3.8 keV (Fig. 3g).²² While a strong peak at energy of about 0.3 keV, referring to carbon element,²³ was observed in Fig. 3h. This further confirmed the existence of graphene in CaSiO₃ bioceramics after sintering. The peaks of Pt element appeared at energy of about 2.1 keV in all spectra, which represents the platinum sputtered on specimens prior to SEM/EDS analysis.²⁴

Fig. 3 Fracture morphology under SEM (a–f) and EDS spectra (g–h) of sintered specimens. (a) 0 wt%. (b) 0.1 wt%. (c) 0.25 wt%. (d) 0.5 wt%. (e) 1.0 wt%. (f–h) 2.0 wt% graphene.

The fracture morphology of the specimens after inlayed and ground with abrasive paper was shown in Fig. 4. A dense structure with smooth surface was presented in Fig. 4a on the fracture surface of specimen without graphene. Graphene was found to be small flake-like and distribute uniformly along different directions in CaSiO₃ ceramic matrix, as pointed out with white arrows in Fig. 4b–f. The fracture morphology illustrated the ability of graphene to prevent the crack propagation in plane and promote the crack deflection in three dimensions. The different morphology of graphene in Fig. 3 and 4 might be attributed to the grinding process, by which only the part embedded in the ceramic matrix were retained on the fracture surfaces. In addition, some holes and graphene agglomeration were observed on the ground surfaces of specimens with 1.0 and 2.0 wt% graphene, which was consistent with the findings in Fig. 3.

Fig. 4 Fracture morphology of the specimens with or without graphene after ground with abrasive paper. (a) 0 wt%. (b) 0.1 wt%. (c) 0.25 wt%. (d) 0.5 wt%. (e) 1.0 wt%. (f) 2.0 wt% graphene.

Carbon contents in the specimens were measured with a high frequency infrared carbon sulfur analyzer. The results showed a logarithmic increase of graphene content after sintering with graphene content before sintering (Fig. 5). There was a graphene content of 0.09 wt% left in the sintered specimen when graphene addition was 0.1 wt%. The amount of graphene after sintering quickly increased to 0.13 and 0.16 wt% when graphene content before sintering were 0.25 and 0.5 wt%, respectively. However, the increasing rate of graphene content after sintering slowed down as the graphene content before sintering further increased to 1.0 or 2.0 wt%. This indicated that more amount of graphene addition (>1.0 wt%) might not result in noticeable increase of graphene content in the sintered specimens.

Fig. 5 Graphene contents in the specimens.

The XRD patterns of the starting powders and sintered specimens were presented in Fig. 6. It can be clearly identified that CaSiO₃ was the only phase in the as-received powders (Fig. 6a).²⁵ The XRD pattern of graphene was found to be broad with a disperse peak at diffraction angle of about 25°,²⁶ shown in Fig. 6b. The pattern of composite powders before sintering was shown in Fig. 6c. It was dominated by the diffraction peaks of CaSiO₃, while graphene's peaks were absent from the pattern, suggesting its low content in the composite powders. There was a similar pattern between specimens sintered with graphene content of 0.1, 0.25, 0.5, 1.0 and 2.0 wt%, respectively. Therefore, one XRD pattern of sintered specimens was given in Fig. 6d. Compared with Fig. 6c, no other phases were found except for a certain degree of phase transformation from β-CaSiO₃ to α-CaSiO₃.²⁷

Fig. 6 XRD patterns. (a) As-received CaSiO₃. (b) As-received graphene. (c) Composite powders. (d) Specimens after sintering.

In order to explore the relationship between graphene content and the mechanical properties of sintered specimens, compressive properties were firstly measured and the results were shown as a function of graphene content (Fig. 7). The compressive strength of CaSiO₃ increased with the content of graphene going up to 0.5 wt%, thereafter decreased with further increasing of graphene content. For the CaSiO₃ bioceramics with addition of 0.5 wt% graphene, the compressive strength reached 42.45 ± 4.30 MPa which had an increase of 142% than that of pure CaSiO₃ bioceramics. For the CaSiO₃ bioceramics with additions of 1.0 and 2.0 wt% graphene, the compressive strength dropped to 34.89 ± 3.78 and 25.34 ± 1.66 MPa respectively, but still higher than that of pure CaSiO₃ bioceramics. The compressive modulus of the composite scaffolds increased from the initial 117.51 ± 17.80 MPa to the maximum value 160.71 ± 18.11 MPa (graphene content 0.5 wt%) and then decreased to 131.17 ± 15.44 MPa with addition of 2.0 wt% graphene, indicating a similar variation with the compressive strength.

Fig. 7 Compressive strength and modulus of sintered specimens with different graphene contents.

Afterwards, microindentation tests were carried out to evaluate the fracture toughness of graphene–CaSiO₃ scaffolds (Fig. 8). There was only a fracture toughness of 1.19 ± 0.09 MPa m^1/2 for CaSiO₃ scaffolds. The fracture toughness was gradually improved with increasing graphene content and reached 1.73 ± 0.14 MPa m^1/2 by the addition of 0.5 wt% graphene, which indicated an improvement by 46%. Nevertheless, the toughness decreased to 1.31 ± 0.13 and 1.39 ± 0.11 MPa m^1/2 as the graphene content further increased to 1.0 and 2.0 wt%, respectively.

Fig. 8 Fracture toughness of sintered specimens with different graphene contents.

The variation of mechanical properties is mainly associated with graphene content and its distribution in ceramic matrix. For the ceramics with low graphene content, the nanometer-level dispersion of graphene in CaSiO₃ bioceramics was favorable to load transfer from ceramic matrix to graphene, leading to the enhancement of fracture toughness and compressive properties.²⁸ However, the mechanical properties of ceramics decreased when graphene content increased above a critical value (0.5 wt%). This may be attributed to graphene agglomeration and the holes induced by excessive graphene. The pull-out of graphene indicated a strong resistance to crack propagation when the ceramics were fractured. The above fractography analysis was responsible for the enhancement of the fracture toughness of CaSiO₃ ceramics by 46% and compressive strength by 142%.

4 Conclusions

Graphene–CaSiO₃ scaffolds were successfully fabricated with SLS by reducing the sintering time to seconds or even microseconds. Graphene was found to remain in the composite scaffolds after the high temperature sintering process. The carbon content test indicated a logarithmic increase of remaining graphene after sintering with the addition of graphene. XRD analysis showed no other phases formed except for a certain degree of phase transformation from β-CaSiO₃ to α-CaSiO₃. The mechanical properties of CaSiO₃ ceramics increased with graphene content from 0 to 0.5 wt%, and then decreased with more graphene addition due to graphene agglomeration and the holes appeared around graphene. The fracture toughness was improved by 46% and compressive strength by 142% with the addition of 0.5 wt% graphene. Moreover, the pull-out of graphene was observed on the fracture surfaces of ceramics. Graphene were found to be octopus-like with tall and straight tentacles embedded in the ceramic matrix. This fracture morphology indicated a strong resistance to crack propagation. These findings may provide guidance for graphene as reinforcement to enhance the mechanical properties of ceramic scaffolds in the application of bone tissue engineering.

Acknowledgements

This work was supported by the following funds: (1) The Natural Science Foundation of China (51222506, 81000972); (2) Hunan Provincial Natural Science Foundation of China (14JJ1006); (3) Program for New Century Excellent Talents in University (NCET-12-0544); (4) The Fundamental Research Funds for the Central Universities (2011JQ005, 2012QNZT015); (5) Project supported by the Fok Ying-Tong Education Foundation, China (131050); (6) Shenzhen Strategic Emerging Industrial Development Funds (JCYJ20130401160614372); (7) The Open-End Fund for the Valuable and Precision Instruments of Central South University; (8) The faculty research grant of Central South University (2013JSJJ011, 2013JSJJ046); (9) Hunan Provincial Innovation Foundation For Postgraduate.

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