Synthesis of Silica/Hydroxyapatite Nanocomposite by Mechanochemical Method

Biocompatibility of Silica makes it a suitable material for biomedical applications. The main components of teeth and bones are calcium phosphate (CP). Hydroxyapatite (HA) is expected to be used in different elds not only in biomedical applications but also in agriculture as a fertilizer and pollution treatment. Various substitutions in the apatite lattice play a signicant role in its properties. In the present work, Silica-50, 40, 30 and 25 mol.% Hydroxyapatite nanocomposites were prepared successfully by mechanochemical processing method. X-ray diffraction (XRD) and Fourier Transform Infrared (FT-IR) indicated that silica stimulates the HA decomposition to β-tricalcium phosphate (β-TCP). XRD of heat-treated compacted sample at 1200 o C conrmed β-TCP and calcium phosphate silicate phases formation. Mechanical properties decreased with decreasing HA content. Electrochemical impedance spectroscopy (EIS) results revealed that pure HA possess the highest resistivity to corrosion, while the silica/HA samples showed lower corrosion resistance. The polarization resistance increases with HA content.


Introduction
Biocompatible silica nanoparticles recently have intrinsic biologic activity to the skeleton and to improve bone forming makes it a suitable material for biomedical applications [1][2][3][4]. Inorganic calcium phosphate based bio-ceramics materials such as hydroxyapatite Ca 10 (PO 4 ) 6 (OH) 2 has received considerable attention for bone repair applications and dental implants [5]. Also, hydroxyapatite is a promising candidate of air, water and soil pollution treatment due to its great adsorption capacities and ionexchange capability [6][7][8]. Hydroxyapatite nanowires composite effectively improve the electrochemical performance of the lithium-sulfur batteries [9]. Several methods have been investigated to prepare CP nanostructures [10][11][12]. Regardless of the researches, creative designed CP nanostructures have not been supplemented for the substantial applications [13,14]. The low dissolution of HA in the human body after implantation and fast release of calcium and phosphate ions from β-tricalcium phosphate when exposed to physiological uids limit its use in clinical applications [15]. Signi cant research has been carried out for both cationic and anionic hydroxyapatite substitutions [16][17][18][19].
Hydroxyapatite and silica have been used as llers for dental applications [20][21][22]12,17,23]. Even hydroxyapatite based composite resins are bioinert with hardness like natural teeth, its exhibit lower strength and higher failure rates. Silica is used as a reinforcing ller for dental applications. It improves the shrinkage of restorative composite resins [21]. Lower wear resistance and biocompatibility are the major shortcomings of composite resin. Hydroxyapatite whiskers and silica nanoparticles were bioactive [12,21]. Moreover, the silica addition as a silicon source to HA bio-ceramic materials is found to be essential for normal bone and connective tissue development. Addition of silica could reduce the HA grain size and stimulate the decomposition of HA to tricalcium phosphate (TCP) α-and β-TCP phases. [24,25]. HA-SiO 2 composites directly prepared by sintering the powders of HA mixed with silica [26].
Addition of silica to Polyvinyl alcohol/HA matrix presented an improved capacity for both loading and sustained release of vancomycin hydrochloride [27]. Nano-llers have superior mechanical properties and enhanced bioactivity in comparison to conventional cement [11,10,12]. The two approaches of manufacturing nano-size particles are top-down and bottomup. The top-down involves the production of nano materials from bulk material while bottom-up involves production of nano materials atom by atom [28]. Mechanochemical synthesis as a top-down method is a solid-state reaction process. The main advantages of mechanochemical synthesis method of ceramic powders are low cost, simplicity, lower temperature and produce large amounts of powder [29]. Silicahydroxyapatite nanocomposite was synthesized with different methods; sol-gel technique [30,12], plasma sprayed [31], coprecipitation method [32].
Depending on the above considerations, the purpose of this study was to investigate the effect of mechanochemical method for incorporation of bioactive hydroxyapatite with high content silica. Also, to study the mechanical and electrochemical properties of the formed composite materials.

Materials And Samples
Silica powders and 50, 40, 30 and 25 mol.% commercial hydroxyapatite (purchased from Sigma-Aldrich -21223) were placed into high energy vibratory ball milling. The powder treated with hard steel balls for one hour, with the optimum balls to powder ratio equals to 20:1. As-prepared samples were uniaxial compacted at 5 ton/square inch into circular discs samples with. Compacted samples, 13 mm diameter and 3 mm height, were heat treated at different temperatures between 500 and 1200°C for one hour holding time.
The compositions and formed phases of the prepared and heat-treated samples were analyzed by X-Ray powder diffractometer, D5000 using Cu K α radiation (wavelength l = 0.15406 nm) using with a nickel lter. The diffractometer was operated at 40 kV and 30 mA within range of 20° < 2q < 60° with 0.05 degree/sec. The obtained phases were determined by the comparison with the standard ICDD (JCPDS). (Ca ++ 4mEq/L) at 35 ± 1 °C. PEIS test was performed from 10 mHz to 100 kHz . The parameters of corrosion were calculated by using EC-Lab software V11.12 directly by curve tting of the Tafel curve method. The specimen area exposed to the Ringer's solution was 0.785 cm 2 .
Results And Discussion Figure 1 shows XRD patterns of silica and HA without additives and that obtained after one-hour mechanical treatment by high energy ball. The intensity of the HA peaks decreases sharply after mechanical treatment while silica peaks slightly affected. With increasing silica content, the HA degree of crystallinity decreases. Same peaks related to rhombohedral beta tricalcium phosphate β-Ca 3 (PO 4 ) 2 (JCPDS # 00-009-0169) phase appeared at 2θ equal 25.80, 27.77, 31.03 and 34.37 o . The contribution of beta tricalcium phosphate (β-TCP) phase nearly the same with different HA content. The broadening of silica peaks increases as a result of induced severe plastic deformation and the particle size accordingly decreases. Through the milling process of this brittle-ductile system, ne hard silica powder dispersed in a ductile HA and the whole system particles becomes work hardened after mechanical treatment forming ultra ne powder. During the mechanical treatment intense impulsive forces induce severe plastic deformation energy transferred to HA which then transformed into β-TCP phase.
According to Roh et al., [33] tricalcium phosphate, have become preferred bone graft materials of choice. After heat treatment of the as-milled sample, the intensity of HA diffraction peak gradually decreased until it disappeared, and the β-TCP intensity increased. With increasing HA content, β-TCP phase gradually increases. The accumulation of defects into the crystalline lattice of silica/HA system give rise to locally distorted structures which enhance the chemical reactivity of the system. Beside the silica and β-TCP phase (decomposition product of HA with mechanical and heat treatment), silica could continue to react with HA forming monoclinic calcium silicate CaSiO 3 (JCPDS # 01-075-1396) and orthorhombic calcium phosphate silicate Ca 5 (PO 4 ) 2 SiO 4 (JCPDS # 00-040-0393). With increasing sintering temperature from 900°C to 1200°C, all the formed phases still found with higher quality.
In ionic materials solid-state reactions take place by the diffusion of ions across the interface. Pure HA partially transformed to β-TCP when sintered at 1100°C according to the following reaction, equation (1) From the XRD patterns, Fig. 3 CaO phase was not detected sample with HA additives which means that all the CaO formed reacted with SiO 2 to form calcium silicate.
Crystallite size is a critical feature of nanomaterials that have a highly crystalline structure. Mechanical milling/alloying and mechanochemical treatment involves repeated fracturing and cold welding of the treated powder particle. Through the process crystallite size of the deformed powder decrease while accumulated strain, and dislocations increases. From the XRD analysis of line broadening, the estimated average crystallite size (D) of heat-treated samples can be calculated from the Scherrer's equation [37]: where, β is the X-ray diffraction line broadening, θ is the Bragg's diffraction angle, k is the shape factor (0.9), and λ is the wavelength of Cu-k α radiation (0.154 nm). Table 1 shows average crystallite size determined from Scherrer's equation of heat-treated Silica/HA samples at different temperatures.  Table 2 shows the mechanical properties of silica/HA system after heat treatment at 1200°C by using Vickers hardness number which calculated from equation (4) and (5)  where P is the applied force (in N) and d the diagonal length (in mm) of the indentation. Yield stress (σ) can be approximated from the Hv values by [40]; σ = Hv/0.3 (5) Density and micro hardness increased with higher content of HA. According to [41], the maximum hardness of sintered β-TCP obtained at 1100 °C was 240 HV for green pressed sample at 3 ton /square inch while [42] the maximum hardness of 407 HV appeared at 900 °C, then decreased with increasing sintering temperatures for green pressed sample at 1.5 ton /square inch. According to [43], hardness of 860 HV appeared at 1150 °C of green pressed sample at ~ 4 ton/square inch. Hardness of pure silica is 8 GPa [44].
Polarization curves and electrochemical impedance spectra (EIS) measurements were veri ed in Ringer's simulated physiological solution. According to Tafel's law, in an electrochemical reaction, the logarithm of the current density varies linearly with the electrode potential. Fig. 4 reveals anodic and cathodic Tafel polarization curves of HA and Silica/HA samples. As shown large anodic passive regions can be observed on the anodic branches of potentiodynamic curves of lower HA content samples. The cathodic processes with small slopes of the cathodic branches were under kinetic control while anodic processes can be considered mixed kinetic and diffusion controlled due to their large slope. Similar results have been reported in [45]. From the intersection of the anodic and cathodic lines extrapolation, the corrosion potential (E corr ) and corrosion current (i corr ) were calculated. The values of corrosion potential, corrosion current and corrosion rate are listed in Table 3.  Fig. 5b. Fig. 5c shows Nyquist graph where the real and the imaginary parts of complex impedance are plotted. Nyquist plots shows a semicircle with high frequency range for HA and Silica/HA samples. the largest semicircle veri es low ion release rate. Higher HA content samples have a lower degradation rate in Ringer's solution. The polarization resistance (R p ) increases with HA content.  FT-IR spectrum of silica/HA after mechanical treatment.  Complex impedance (Nyquist diagram) of heat-treated HA and Silica/HA samples at 1200°C