Abstract
Nano-carbon reinforced titanium matrix/hydroxyapatite (HA) biocomposites were successfully prepared by spark plasma sintering (SPS). The microstructure, mechanical properties, biocompatibility, and the relationship between microstructure and properties of biocomposites were systematically investigated. Results showed there are some new phases in sintered composites, such as β-Ti, TiO3, ZrO2, etc. Moreover, a small amount of Ti17P10, CaTiO3, Ca3(PO4)2 were also detected. The reaction that may occur during the preparation process is suppressed to some extent, which is because that the addition of second phases can prevent the direct contact of titanium with HA and reduce the contact areas. Transmission electron microscope (TEM) analysis proved the existence of elemental diffusion and chemical reactions in sintered composites. Compared with results of composites prepared by hot-pressed sintering before, mechanical properties (microhardness, compressive strength, and shear strength) of 0.5-GNFs composites prepared by SPS were increased by about 2.8, 4.8, and 4.1 times, respectively. The better mechanical properties of 0.5-GNFs composite in nano-carbon reinforced composites are mainly due to the lower degree of agglomeration of tubular carbon nanotubes (CNTs) compared to lamellar graphene nanoflakes (GNFs). Moreover, the strengthening and toughening mechanisms of nano-carbon reinforced titanium alloy/HA biocomposite prepared by spark plasma sintering (SPS) mainly included second phase strengthening, grain refinement strengthening, solution strengthening, graphene extraction, carbon nanotubes bridging, crack tail stripping, etc. In addition, in vitro bioactivity test revealed that the addition of nano-carbon was beneficial to promote the adhesion and proliferation of cells on the surface of titanium alloy/HA composite, because nano-carbon can enhance the formation of mineralized necks in the composites after transplantation, stimulate biomineralization and promote bone regeneration.
Highlights
The specific gravity of titanium alloy, which has excellent mechanical properties, is close to that of human bone, but it is mechanically fitted with bone tissue in the living body
Miranda et al successfully prepared Ti-6Al-4V/HA composites by powder metallurgy, and the results showed that HA can be evenly distributed in the titanium alloy matrix and tightly bonded with the matrix even after the shearing experiment, which avoids the problem of weak metal-ceramic interface bonding in the coating composites [5]
There was no significant difference in the XRD patterns obtained from different milled composite powders, because the added phases La, carbon nanotubes (CNTs) and graphene nanoflakes (GNFs) were not detected in the XRD patterns due to their low content
Summary
The specific gravity of titanium alloy, which has excellent mechanical properties, is close to that of human bone, but it is mechanically fitted with bone tissue in the living body. The composites composed of titanium alloys and HA are prepared by various processes for practical application. It has its intrinsic limitations and disadvantages in the prepared coating composites, such as high porosity, uneven thickness distribution, low crystallinity, weak bonding strength, etc. Singh et al found that the bond strength of HA-Al2O3, HA-ZrO2 coating with Ti-6Al-4V substrate reduce after heat treatment [9]. This was related to the diffusion of oxygen in the titanium alloy matrix, except the difference in thermal expansion coefficient between titanium alloy matrix and HA. There are the following problems in uncoated composites: (1) stress shielding phenomenon [5,7,10]; (2) Violent reactions between HA and Ti during high temperature sintering [11,12,13]; (3) The large difference in thermal expansion coefficient between HA and titanium alloy [14,15]
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