Titanium Dioxide Coated Graphene Nanosheets as a Reinforcement in Aluminum Matrix Composites Based on Pressure Sintering Process


 Graphene nanoplatelets (GNPs) reinforced 7075 aluminum (Al) nanocomposites were successfully synthesized using the powder metallurgy method. A novel method for optimizing interfacial bonding by coating titanium dioxide (TiO 2 ) on the surface of GNPs was proposed in this manuscript. The effects of GNPs on mechanical properties and microstructure of the aluminum matrix nanocomposites, both with and without TiO 2 coating layers, have been investigated. Experimental results showed that the corresponding mechanical properties of the nanocomposites were further improved when the GNPs have TiO 2 coating layers, compared with the addition of pure GNPs. The yield strength, ultimate tensile strength, and microhardness of the nanocomposites reinforced with TiO 2 -coated GNPs increased by 22.9%, 25.9%, and 20.1%, respectively, in comparison to those of the matrix. The further improvement of the mechanical properties could be attributed to the existence of the coating layer, which optimizes the interface bonding between the reinforcement and the matrix, thereby improving the effectiveness of load transfer.


Introduction
Graphene, a two-dimensional material consisting six rings of sp 2 -hybridized carbon atoms, has been used as an important new nanosize reinforcement in the field of structural and functional composites since it was successfully separated in 2004 [1][2][3][4] . It has attracted much attention due to the extremely high mechanical strength and excellent thermal and electrical properties. Compared with carbon nanotubes (CNTs), graphene has a larger surface area and lower production costs. The sheet structure of graphene makes it easier to disperse in the matrix, which makes it an excellent alternative reinforcement for composites. Currently, Graphene has been extensively studied to enhance the mechanical and other properties of polymer or metal based composites [5,6].
Aluminum and aluminum-based composites are widely used in automotive and aerospace enterprises due to low weight and high corrosion resistance. With the development of industry, composites based on aluminum alloy are expected to demonstrate higher mechanical properties. Therefore, more and more materials scientists are trying to reinforce aluminum alloys with a strong, rigid, and lightweight phase [7]. Recently, various efforts have been made on graphene as a reinforcement of aluminum-based composites. Gang Li et al. [8] successfully fabricated a graphene nanosheets(GNSs)/Al nanocomposites by ball milling and vacuum hot pressing. The report showed that the ultimate tensile strength and yield strength of the pure Al could be improved by 56.19% and 38.27% due to the addition of GNSs. The microhardness of the nanocomposites improved significantly as the content of GNSs increased.
Haiping Zhang et al. [9] reports the similar results by adding 1wt. % GNPs to Al5083 alloy. They revealed that a high strengthening efficiency of the multi-layer GNPs may be mainly attributed to shear lag (load transfer) effect and grain refinement. And the nanosized Al4C3 formed could also act as a reinforcement. Moreover, the composite successfully prepared by Jingyue Wang et al. [10] exhibited a tensile strength improvement of 62% increment over unreinforced matrix with only 0.3 wt. % GNSs addition. However, despite some progress in the field of nanocarbon-reinforced aluminum-based composites, there are still some important issues remain to be resolved. The main problem is insufficient interface bonding between the nano-carbon material and the aluminum alloy, which results in that stress cannot be completely transferred from the matrix to the reinforcement during loading [11].
In order to optimize the wettability and interfacial bonding between nano-carbon materials and alloy matrix and further improve the final mechanical properties of the corresponding composite, surface modification techniques have been adopted in the field of nano-carbon reinforcement by materials scientists. CNTs coated SiC nano-layer were prepared by Yongha Park et al. [12] using solid heating reaction method between silicon powders and CNTs ( ). The report exhibited that the CNTs with SiC coating layer resulted in an improvement of the wettability and bonding strength in Mg-based composite. Carbon nanofibers (CNFs) coated with copper were also used to enhance the mechanical properties of magnesium alloys [13].
The result indicated that the enhanced of wettability between Al and CNFs as well as the free of Al4C3 are related to the Cu coating layer. There are a lot of reports about carbon nanotubes being coated with metal or nano-ceramic particles as a reinforcement to strengthen the properties of metal matrix composites, which reflect the beneficial effect of the coating on interfacial bonding and mechanical properties.
However, so far, the investigations on Al composites reinforced with coated graphene are still limited.
In this study, based on the 7075 aluminum alloy, the nanocomposites reinforced with TiO2 coated graphene nanoplatelets (TiO2@GNPs) and purified GNPs were prepared by means of Pressure Sintering. The effects of the TiO2@GNPs and the purified GNPs on the microstructure and the mechanical properties of the nanocomposites fabricated were characterized and investigated. Furthermore, the mechanisms for enhancing the mechanical properties of nanocomposites were also discussed in detail.

Materials
Commercial 7075 aluminum alloy powders (purity 99.7%, particle size 70~120 μm, from Shenyang Nonferrous Metal Research Institute, China) were used as the matrix material for this study. Table 1 shows the chemical composition of the aluminum powder. The starting reinforcement, Graphene nanoplatelets (provided by Nanchang Taiyang Nanocrystal Co., Ltd.) have a purity of 98.6 wt. %, an average thickness of 6-8 nm and a particle size of 12~15 µm, respectively.

Experimental procedures
The acid-treated 0.2g of GNPs were first placed in 20ml ethanol and sonicated for 10 minutes (90W). 0.5 mL of Tetrabutyl titanate(TBT)and 10 mL of glycerol were added to the resulting solution and sonicated for 5 minutes again. The mixed solution was subsequently transferred to an autoclave and kept in muffle furnace at 160~200 ℃ for 12 hours. After that, the obtained solution was centrifuged to obtain powder precipitates and then the precipitates were washed with pure ethanol (99.7pct purity) dried at 70 °C for 24hours. The resulting powders were then calcined in argon at 460 ~ 480 ° C for 3 hours. Scanning electron microscopy and transmission electron microscopy were used to study the microstructure of the TiO2 coated GNPs (TiO2@GNPs) after this coating process. Figure 1 illustrates the synthesis procedure of TiO2 coated GNPs-7075Al composites. As shown in the figure, 0.5 wt. % of TiO2@GNPs and 99.5 wt. % of 7075Al powder were put into corundum mixing tanks containing corundum grinding balls with diameters of 5~20 mm. The initial ball powder weight ratio was 10: 1. The mixing tanks were stirred using a planetary ball mill at 380 rpm for 24 hours in an argon atmosphere. 1 wt. % stearic acid needs to be added as a process control agent to prevent excessive sticking and aggregation of the powders (cold welding) during ball milling. After that, the as-prepared nanocomposite powders were placed in a high-temperature graphite die and subjected to hot-press sintering at 595 ℃ for 2h in a vacuum of 10 -3 torr under a uniaxial pressure of 28 MPa. The sintering parameters are shown in Figure 1. For comparison, the alloy without GNPs was also fabricated by the same process.

Characterization
Nikon Eclipse MA200 Optical microscope (OM) and NOVA NanoSEM 450 scanning electron microscope (SEM) were used to characterize the microstructures and the fracture surfaces of the GNPs/7075Al nanocomposites. The nanostructures and interfaces of GNPs/7075Al were also investigated using a transmission electron microscope (TEM, JEM-2100

Microstructure
Microstructure of the purified GNPs has been shown in figure 2(a).  [14] , and (1 0 )TiO2 planes (figure 2(f)), respectively. Therefore, it could be confirmed that titanium oxide has been successfully generated and coated on the surface of GNPs. In addition, it was observed that TiO2 nanoparticles did not fall off the surface of GNPs even after sonication during TEM sample preparation. This phenomenon indicates that a strong interfacial bond has been formed between nanoparticles and GNPs [15] .   indicated that the GNPs addition had an effect of refining the matrix grains due to the fact that GNPs distributed at the grain boundaries could hinder the grain growth [9].

3( ) -
In addition, some micropores were revealed on the surface of the alloy and GNPs/7075Al samples, presumably due to the uneven grain refinement in the matrix.
It can also be seen from figure 4(b) that agglomerations of GNPs appeared at the grain boundaries. It could be attributed to the poor dispersibility and wettability of the GNPs in the matrix [17]. As shown in figure 4 (c), no obvious reinforcement agglomeration occurred at the grain boundaries of the composite with added TiO2@GNPs. This phenomenon is mainly results from the optimization of the wettability of GNPs with the matrix and thus promotes the combination of GNPs and alloy, which leads to a reduction in structural defects during the process.  2) and (32  ) planes of TiO2. Therefore, it can be confirmed that the TiO2 nanoparticles were still present and in close contact with the GNPs surface.  [15]. The generation of MgO could be attributed to a chemical reaction between GNPs and TiO2 nanoparticles. The interfacial reaction can be described [19] as in Eq. (1):

2( )
(1) When the free energy calculated is lower than zero, the reaction proceed in according to the Eq. (1). Hereinafter, the highest sintering temperature provided by the equipment in the current process is 595 ° C (868K). The calculation results showed that the free energy of reaction is less than zero. It implied the possibility of this reaction.

Mechanical Properties
The mechanical properties of 7075Al and the nanocomposites reinforced by purified GNPs or TiO2@GNPs are given in figure 6. As shown in figure  presence of the TiO2 coating layer [19,20].
In the Eq. (2), is the shear modulus of Al , is the change in temperature, is defined as the CET difference between GNPs and the matrix. and are the volume fraction and average diameter of GNPs, respectively. is burger vector of matrix (0.286 nm).
On the other hand, the addition of purified GNPs or TiO2@GNPs resulted in a strengthening mechanisms, the grains refinement has a very small proportion of the improvement effect of composites strength [15]. Therefore, the strengthening of nanocomposites by grain refinement was ignored in this manuscript.
Furthermore, the most important strengthening mechanism model among the three strengthening mechanisms is the shear lag model, which explains the load transfer from the matrix to the reinforcement. In short, the applied load can be transferred from the matrix to the reinforcement at the interfacial shear stress.
Increase in YS of composites can be calculated by Kelly-Tyson model [22] as follows: In Eq. (3), represents the volume fraction in also. is the YS of the matrix.
According to the calculation results of Eq.  [21]. Second, the theoretical prediction of YS is based on the assumption that all the added GNPs are evenly dispersed in the matrix. These simple assumptions reduce the prediction accuracy of the effective contribution to yield strength [23].
In addition, stress transfer is considered to be a critical enhancement mechanism as mentioned before. It provides the greatest contribution to nanocomposite strengthening. Therefore, the strong bonding interface and the wettability directly   25.9%, and 20.1% higher than those of the matrix alloy, respectively. The improvement in mechanical properties can be attributed to grain refinement, coefficient of thermal expansion and load transfer mechanism. Among them, the load transfer is believed to provide the maximum contribution to composite strengthening.
The existence of TiO2 coating layers on the surface of GNPs strengthen the interfacial bonding, which results in that the stress be effectively transferred from the matrix to GNPs during loading. These findings indicate that the addition of GNPs can significantly enhance the mechanical properties of Al nanocomposites, while the TiO2 particle layer coated on GNPs have a positive effect on this.

Availability of data and materials
The datasets analyzed supporting the conclusions are included in this manuscript.