Ceramics and their composites are widely used in many industries. These materials are synthesized in a variety of ways, including chemical precipitation, sol-gel, and hydrothermal process [1-3]. In the hydrothermal method, ceramics are synthesized at high temperatures and pressures and no calcination is required. One of the important applications of ceramics is medical application. Among the ceramics, the calcium phosphate family has received much attention. These materials have unique biomaterial properties such as biocompatibility and osteoconductive properties. HA is a member of this group that is very similar to the chemical structure of bone [4-13]. This ceramic is synthesized in different ways, such as combustion preparation, solid-state reaction, electrochemical deposition, sol-gel, hydrolysis, precipitation, sputtering, multiple emulsion, biomimetic deposition, solvothermal method, and hydrothermal process, and has different morphologies including rods, wires, ribbons, and tubes. It therefore has good potential for bone replacement as an implant. But despite its excellent biological properties, it is inherently brittle, has low wear resistance and, most importantly, low fracture toughness [14-32]. One of the strategies researchers have used to overcome this mechanical weakness is the use of reinforcing materials. The most popular reinforcing materials are carbon nanomaterials (carbon nanotube and graphene). And recently, graphene has outperformed.
Graphene has a honeycomb structure, has excellent mechanical properties and high specific surface area which enhances its reinforcing properties. It is biocompatible and has been widely used in research with HA. One of the attractive ways to synthesize graphene-HA nanocomposites is the hydrothermal process that results in situ synthesis of graphene-HA hybrid powders. Graphene oxide (GO) is the precursor used in this method for graphene. Anchors at the graphene oxide surface cause Van der Waals bonds with synthesized HA particles and these oxidizing agents are partially reduced by the hydrothermal process. Recently, injections of hydrogen and nitrogen gases into the autoclave have been used to increase the degree of GO reduction. The results showed that increasing the hydrothermal pressure increased the graphene oxide reduction rate and improved the HA properties [33-40].
In this study, hydrogen gas was injected into a hydrothermal autoclave with varying initial pressures. The powders obtained from this process were consolidated by spark plasma sintering method. Before examining the mechanical properties of the sintered samples, the synthesized powders were first evaluated by X-ray diffraction (XRD), Raman spectroscopy, Fourier transform infrared spectroscopy (FTIR), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDS), and elemental mapping methods. The sintered samples were subjected to mechanical analysis by Vickers indentation technique. For this purpose, a load of 1 N was used. It is expected that the increased pressure applied in this study will increase the mechanical properties of the nanocomposites.
Table 1. Specifications of the devices and the methods used for characterization
Device and specification
X’ Pert Pro, Panalytical Co., Co Kα radiation (λ=1.78901 Å)
Renishaw inVia spectrometer, Wavelength of 532 nm, green laser (recording 5 times for 10 seconds of each accumulation)
VERTEX 70, Bruker Corp., applying 200 MPa pressures (1mm thickness)
Hitachi S4700 equipped with energy dispersive X-ray spectroscopy, Au coated by sputtering
The materials used in this study and the method of powder synthesis have been reported previously . The hydrothermal time for the synthesis of these powders was three hours. Three samples were synthesized where the applied gas pressure for each sample was 10 (I), 20 (II), and 30 (III) bar. The synthesized powders were consolidated according to the previously reported SPS methods . The amount of graphene in these nanocomposites was considered to be 1.5% by weight.
Figure 1. (a) Time-load diagram, (b) Vickers indenter affected zone
Figure 2. (a) XRD patterns of synthesized powders, (b) HA crystal structure
The sintered samples were subjected to mechanical evaluation by Vickers indentation technique. The applied load was 10 N and the rest of the details were as reported previously . Figure 1 shows the loading diagram of the mechanical evaluation along with the Vickers indenter affected zone.
Table 1 shows the specifications of the devices used for characterization. Origin pro 2016, Diamond 3.2, and ImageJ softwares were used in this study.
Figure 3: (a, b) FESEM images of synthesized powders (III), (c) TEM image of HA nanorods (III)
Figure 4: (a) Raman spectroscopy, (b) FTIR analysis for the synthesized powders
3. Results and discussion
Figure 2 shows the XRD patterns of synthesized powders along with the schematic image of HA crystal. As with previous published research, the patterns of all the synthesized powders are consistent with the pure HA pattern. Given the presence of GO in the precursors, and the absence of its corresponding peak at 2theta≈ 10, it can be concluded that GO has been reduced in the hydrothermal process. On the other hand, the characteristic reduced graphene oxide peak is covered with the HA (002) peak. In HA crystals, (002), (211), and (300) planes play the most role in the growth of crystals. But under the hydrothermal conditions of this research (002) planes are growing faster and causing the particle morphology to become nanorods. Increasing hydrogen gas pressure has increased the peaks intensity. This increase in pressure also affects particle size, crystallinity, and crystallite size [37, 41, 42].
Figure 3 shows the FESEM images of III powders along with the TEM image of HA nanorods in III powders. These images confirm the presence of graphene sheets in the powders. In these powders, the HA particles with nanorod morphology of about 50 nm in diameter and variable length cover the surface of the graphene sheets. Previous researches have shown that nanorods have grown in the C direction [36, 37].
Figure 5: (a) EDS analysis (III), (b-e) elemental maps (III)
Figure 6: (a) Load-displacement diagram, (b) mechanical properties of the sintered samples
Figure 7: (a) schematic image of contact depth, (b) time-displacement diagram of the sintered samples
Figure 4 shows Raman spectroscopy and the FTIR analysis for the synthesized powders. Considering the Raman spectroscopy results (Figure 4a), it is clear that all three samples contain graphene sheets and HA. The increase in gas pressure increased the peak intensity of HA (960 cm-1) due to the increase in crystallinity. Also the ID/IG ratio has increased with increasing pressure, which indicates a greater degree of reduction in GO. Considering the results of FTIR analysis (Figure 4b), all three samples are similar and only the intensity of the peaks has changed slightly. As can be seen, the characteristic peaks of graphene oxide in all three samples decreased or disappeared. These findings show that in all three samples there are two phases of reduced graphene oxide and HA. With increasing pressure, the degree of reduction is increased [41, 42].
Figure 5 shows the EDS analysis and elemental maps for III powders. This analysis confirms the presence of trace elements and the homogeneity of their distribution. Previous studies have also shown that the ratio of calcium to phosphate in these types of powders is 1.67 .
Figure 6 shows the load-displacement diagrams and mechanical properties of the sintered samples. As the gas pressure increases, these diagrams shift to the left and the contact depth decreases. Reducing the contact depth reduces the contact area and increases the hardness according to Olive-Pharr method . Also, moving the graphs to the left increases the slope of the elastic zone and increases the elastic modulus. The results for the hardness and elastic modulus are shown in Figure 6b.
Figure 7 shows a schematic image of contact depth and the time-displacement diagram of the sintered samples. As described in the previous graphs, increasing gas pressure has increased mechanical properties and decreased contact depth. The findings of this research should be considered from two perspectives. First, the increase in gas pressure increases the HA crystallinity and increases the mechanical properties of this phase. Second, increasing gas pressure reduces the GO further and increases the mechanical properties of the recovered sheets. The findings of this study, along with other published researches, will be useful in the development of tissue engineering [44-50].
The findings of this study showed that increasing the pressure of hydrogen gas increased crystallinity, crystallite size and particle size in HA phase. Also, increasing the pressure of hydrogen gas increased the rate of GO reduction. Increasing the gas pressure in the powder synthesis section reduced the contact depth in Vickers analysis, increased the hardness and elastic modulus of the nanocomposites. The results of this study could be useful for the synthesis and applications of graphene-HA nanocomposites.
Conflict of Interests
The authors certify that they have no affiliations with or involvement in any organization or entity with any financial interest, or non-financial interest in the subject matter or materials discussed in this manuscript.