Original Research PaperFormation mechanism of nano-hardystonite powder prepared by mechanochemical synthesis
Graphical Abstract
Introduction
Ceramic is a non-metallic, inorganic material that have a wide range of applications in aerospace, refinery, chemical industries, electronics, optics and healthcare [1], [2]. Amongst these applications, much interest has been devoted to the use of ceramics and glasses in biomedical applications including soft and hard tissue replacement and regeneration [3]. The great potential applications of bio-ceramics are due to their high biocompatibility, bioactivity, easy fabrication and high stiffness [4]. However, regardless of their suitable biological properties, the inherent brittle nature of this class of material limits their application in medicine [5]. Often, bio-ceramics are used with polymeric matrix to produce bio-composites [6] or are coated on the surface of implants to enhance tissue-material interactions in vivo [7].
Hydroxyapatite is an example of a bio-ceramic that has been in use for the past few decades as bone substituted material [8]. However, the weak mechanical properties (unsuitable elastic modulus, low fracture toughness and brittle behavior) and the low degradation rate of hydroxyapatite in biological environment limits their use in tissue engineering applications [9]. Recently, Calcium silicates (e.g. CaSiO3) have received great attention as an alternative bio-ceramic for hard tissue engineering due to their enhanced mechanical properties compared to hydroxyapatite [10]. However, calcium silicates are known to have a high degradation rate in physiological environment, leading to an increase in the local pH of the surrounding tissue, hence resulting in tissue damage [11].
The addition of elements such as Zn, Mg and Zr as network modifier into calcium silicate network can improve the dissolution and degradation rate [12]. Previous studies on hardystonite (Ca2ZnSi2O7) have demonstrated good mechanical [13] and biological properties. The mechanical properties of hardystonite, such as bending strength (136 MPa), fracture toughness (1.24 MPa m1/2) and Young’s modulus (37 GPa) are close to that of bone [13]. This is considered advantageous since for bone tissue replacement and regeneration the stress-shielding phenomenon, which may occur due to the difference mechanical properties of replaced material with natural tissue, can be negligible [14]. In addition, hardystonite possess enhanced chemical stability, in physiological environment, compared to calcium silicates [15]. The release of Zn from hardystonite network has shown to have anti-inflammatory and anti-bacterial properties [16].
To broaden the medical applications of hardystonite, Wang et al. [17] reported the fabrication of hardystonite-wollastonite scaffolds with a higher compressive strength and lower degradation rate compared to pure wollastonite scaffolds. In another study, hardystonite was successfully coated on Ti-6Al-4V substrate by plasma spraying method and an improved cell attachment, proliferation and differentiation were observed [18]. Zeriqat et al. [19] reported that the incorporation of Zn and Sr in calcium silicate ceramics could improve bone tissue regeneration and integration when tested in rat tibia in vivo.
Wu et al. [13] produced hardystonite powder through sol-gel synthesis route and applied a high sintering temperature (1200 °C) during the processing, which resulted in irregular, agglomerated and large (5–40 μm) particles. Therefore, a reduction in sintering temperature is a step to achieve nanostructures with enhanced surface properties. Recently, mechanochemical synthesis has proven to be an effective and economical method for nanostructure and nano-crystalline ceramic fabrications. In this method, surface of the reactants are mechanically activated and so a lower processing temperature is often required [20]. The main objective of this study was to obtain pure nano-hardystonite by mechanochemical synthesis and subsequent sintering. Phase evaluation during processing and the chemical reactions of nano-hardystonite formation are also studied. The effects of ball milling time and sintering temperature on the formation mechanism of nano-hardystonite are also evaluated. In addition, the two-step sintering method is applied for the first time to prepare bulk nano-hardystonite ceramics.
Section snippets
Powder preparation by MA method
In this research, hardystonite powder was prepared by mechanochemical synthesis. zinc oxide (ZnO, 99% purity, Merck), calcium carbonate (CaCO3, 98% purity, Merck) and silicate oxide (SiO2, 99%purity, Aldrich) powders, with a molar ratio of 1:2:2 respectively, were mixed in a planetary ball mill (Retsch, PM 100) in zirconia vial containing five zirconia balls of 20 mm in diameter. The ball/powder mass ratio was 10:1 and the rotational speed of the disc and vial was set at 250 and 500 rpm,
X-ray diffraction evaluation and reaction mechanism
Fig. 1 represents the XRD pattern of H1–H5 samples. Fig. 1 (H1) shows the XRD patterns for the raw materials, which are consistent with the standards for CaCO3 (XRD data file No. 5-0586), SiO2 (XRD data file No. 46-1045) and ZnO (XRD data file No. 1-075-0576) compiled by the Joint Committee on Powder Diffraction and Standards (JCPDS). Milling for 20 h (H5) has led to a broadening of the XRD peaks and a significant decrease in their intensities because of the internal strain and the formation of
Mechanical properties
To investigate the mechanical properties, nano-powders were milled for 20 h and pressed at 600 MPa. All samples were then sintered according to Fig. 9, which represents the regime of optimised two-step sintering process for the preparation the nano-hardystonite green bodies.
The two-step sintering is an effective way of reducing the grain growth in the final stage of sintering. At first, the sample is heated to a high temperature (T1) and it is kept for a short time and then the temperature is
Conclusion
In this study, the application of mechanochemical synthesis followed by sintering resulted in the formation of pure nano-hardystonite powder. Optimum processing conditions of the prepared hardystonite nano powder was found to be at a milling time of 20 h followed by sintering at 900 °C. The XRD patterns were utilized to study the chemical reactions of hardystonite formation. The crystallite and agglomerate particle size for the prepared powders under 20 h milling and subsequent sintering at 900 °C
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