Fabrication And Properties Of Nanodiamonds/Titanium Nanocomposites With Network Architecture

Discontinuous reinforced titanium matrix composites?DRTMCs?have broad application prospects in aerospace,weaponry,biomedical and other fields.Although the uniformly distributed reinforcing phase can bring a certain reinforcing effect to the base alloy,it will lead to a large plasticity reduction of the composites,and even exhibits severe room temperature brittleness.In this paper,nanodiamonds?NDs?with high hardness,high strength,good wear resistance and no cytotoxicity are selected as reinforcements.Spherical Ti particles are modified with polyvinyl alcohol and crosslink with the NDs,the NDs can be distributed uniformly on the surface of the spherical Ti particles.Then nanodiamond reinforced titanium matrix composites with network architecture are successfully prepared by spark plasma sintering technique.By using XRD,optical microscopy,electron microscope,microhardness tester,universal testing machine,laser thermal conductivity tester and other testing methods,the microstructures,mechanical and thermal properties of NDs/Ti composites with different NDs additions?0.05².0 wt.%?and different network sizes?15-53?m,45-105?m,106-180?m and 150-250?m?are systematically studied.The PVA in the mixed powders can be removed and consolidation can be achieved during SPS sintering process.The network-structured NDs/Ti composites with relative density above 99%are prepared,nano-and submicro-TiC crystals are in-situ formed due to the reaction between the Ti and NDs,and meanwhile some NDs remain.The reinforcements have a three-dimensional network distribution in the Ti matix.With increasing the additive amount of NDs or the size of network when the NDs content is fixed,the content of local reinforcements,in the network boundary region,increases.The reinforcements connect up gradually,from the discrete distribution along the network to the formation of a closed wall structure.Besides,semi-closed or quasi-continuous distribution is the optimal distribution of reinforcements in the composites.The results of hardness,compressive and tensile tests show that with the increasing of NDs content,the average microhardness of the as-sintered nanocomposites increases by 42%compared with pure Ti.The compressive yield strength increase from 346 MPa?pure Ti?to 519 MPa?1.5 wt.%NDs/Ti?,with increment of 50%.All the samples are fractured with large compressive strains,even when the NDs amount is as high as 1.5wt.%??,57.8%?.In terms of tensile results,the 0.25 wt.%NDs/Ti nanocomposites have excellent integrated mechanical properties,where the tensile strength increased by 39%,while the tensile ductility still maintained at a high level of 13.4%.When the content of reinforcements is fixed,the influences of network size on the average microhardness,yield strength and tensile strength of composites are not obvious,but for the plasticity of composites,the influence is great.The elongation of composites reduces by 74.7%,from 33.2%of sample 15-53?m to 8.4%of sample 150-250?m.The friction and wear tests show that the friction coefficient of the NDs/Ti composites decreases as increasing the NDs content,the maximum decrement is 23.4%,from 0.654 of pure Ti to 0.501 of1.5 wt.%NDs/Ti.When the content of NDs added is fixed to 0.25 wt.%,the small network size range is beneficial to the improvement of the wear resistance of the composites.Compared with pure Ti,the friction coefficient of the sample 15-53?m is reduced by 23%.The analysis of wear surface of the composites shows that in the dry grinding process,the main wear mechanism of the composites at room temperature is abrasive wear,accompanied by delamination and oxidation wear.The results of thermal performance tests show that the thermal conductivity of the NDs/Ti composites decreases with increasing NDs content,and decreases by 16.4%compared with pure Ti at room temperature.With the network size range decreases,it decreases from 20.56 W·m^-1·K^-1 of the sample 150-250?m to 18.15 W·m^-1·K^-1 of the sample 15-53?m,which reduces by 11.7%.The thermal expansion coefficient of NDs/Ti composites in the range of RT-250°C decreases with the increase of NDs content,and it tends to increase first and then decrease with the increase of network size.The study of strengthening and toughening mechanism indicates that the cracks occur at the interface and propagate along the network-structured boundary.The reinforcements in the network-structured boundary have a good load capacity,acting as“reinforcing beams”,which is the main reason for the improvement of both the hardness and strength of the composites.While the internal Ti matrix inside of the network containing a large quantitative of Ti grains still maintains the low hardness and high ductility of the pure Ti,exhibiting high strain bearing abilities,which can hinder the crack propagation,so that good ductility can be preserved to some extent in the composites.Eventually,the conflicts between strength and ductility of the Ti matrix composites are reconciled effectively by the controllable spatial network distribution of the nano-reinforcements.