Hydrothermal preparation of cobalt, nickel, copper ferrite nanoparticles and study of microhardness of aluminum-based nanocomposites

INTRODUCTION
There are various methods for making nanoparticles, one of them is hydrothermal, which is widely used today due to its mild operation parameters [1] and better controllable particle size  [2, 3]. In addition to hydrothermal, the synthesis is one-step without the need for sintering and calcination  [4]. Also low cost, able to control the shape, size, and phase, and low energy consumption are other features of this method  [5-8] This method is an easy and eco-friendly method that is used to prepare various nanostructures such as nanospheres, nanowires and nanorods  [9-12]. Kurian et al. [13] synthesized ZnFe2O4, MgFe2O4, CuFe2O4 using the hydrothermal method. They observed a cubical and porous spherical morphology for the hydrothermal method. The two main absorption bands in the FTIR spectrum confirmed the spinel structure of the prepared ferrites. Also porous spherical MgFe2O4, ZnFe2O4 nanoparticles showed more ferromagnetic properties than cubical particles. Rathinavel et al. [14-19 synthesized magnesium ferrite using the hydrothermal method, and their results confirmed the spinel structure and crystal size of about 29 nm. Ansari et al [20] investigated the magnetic properties of cobalt ferrite synthesized by hydrothermal method as an eco-friendly method. Their results showed that the hydrothermal method can be used as an eco-friendly and economical method for the synthesis of magnetic and super magnetic nanostructured materials at relatively low temperatures. Using the hydrothermal method, Zhang et al. [21]synthesized cobalt ferrite and ZnS nanoparticles to absorb electromagnetic waves and observed particle sizes between 10 to 20 nm with a uniform structure. Also, the wave absorption properties of ferrite composite were improved compared to cobalt ferrite. Malinowska et al [22] synthesized cobalt ferrite nanoparticles and studied their morphology and magnetic properties. The CoFe2O4 particles from metal sulfate precursors showed the highest saturation magnetization and the lowest coercivity. Also, the average particle size increased from 46 nm to 54 nm with increasing metal concentration and ion resistance. Kurian et al. [23] synthesized CoFe2O4 nanoparticles using the hydrothermal method. They observed that the time and temperature of the hydrothermal reaction had little effect on the structural and magnetic parameters of the material, although pH played an important role in the physical properties of the nanoparticles. Prabhu et al [24] FSP the 6082 aluminum sheet and investigated its mechanical properties and found that after three FSP passes, the microhardness of the composites improved. Mazaheri et al. [25] produced A356/Al2O3 nanocomposite using FSP. Their results showed that with uniform distribution of alumina particles, the micro-hardness of the samples is improved. Zayed et al. [26] made the composite using the FSP technique and 5083 aluminum alloy. They mixed Sic and Al2O3 particles together and added them to the AA5083. The micro-hardness of the samples increased significantly, and their wear properties also improved. 
In this paper first, CoFe2O4, NiFe2O4, and CuFe2O4 nanoparticles were made using the hydrothermal method, and then the surface nanocomposite was produced with AA7075 aluminum alloy as base metal using FSP and its surface properties were investigated.

MATERIALS AND METHODS
To produce nanoparticles, 0.3 g of NiSO4 and 0.92 g of iron nitrate in 150 ml of distilled water were mixed for 30 minutes with a stirrer, and then NaOH 1 molar solution was slowly added to the base solution to bring the pH to 11. Then, the solution is poured into a special hydrothermal vessel and kept at 200 degrees for 4 hours. After this time, the solution is centrifuged and placed in an oven to dry. The same process is repeated to make other nanoparticles, except that 0.97 g of iron nitrate is added for 0.3 g of CoSO4 and 1.2 g of iron nitrate is added for 0.3 g of CuSO4.  Fig. 1 shows a schematic of the hydrothermal method.
For FSP, 7075 aluminum sheet was used. After the sheet was cut to the appropriate dimensions and prepared and placed inside the fixture, a groove was made on the surface of the sheet with a depth of 2.5 mm and a width of 0.5 mm, and the inside of the groove was filled with the produced nanoparticles. In the next step, the FSP tool which is made of H13 and has a pin with a diameter of 3 mm and a height of 3 mm and a shoulder with a diameter of 14 mm, performed the FSP operation and surface nanocomposites were created. Rotational and travel speeds of 1200 rpm and 32mm / min were selected, respectively. Then the samples were cut and prepared for micro-hardness testing. Microhardness tests were taken from the cross-section of the samples according to standard ASTM E92. The schematic of the friction stir processing is shown in Fig. 2.

RESULTS AND DISCUSSION
Fig. 3 shows the XRD pattern of cobalt ferrite nanoparticles (CoFe2O4), this spectrum has appropriate accordance with pure cobalt ferrite standard using JCPDS: 22-1086 and miller indexes: (111), (220), (311), (222), (400), (422), (511) and (440).
Fig. 4 illustrates the X-ray diffraction pattern (XRD) of nickel ferrite (NiFe2O4) nanoparticles, the pattern has a suitable agreement with pure material standard using JCPDS: 86-2267 and Miller indexes include (111), (220), (311), (222), (400), (422), (511), (440), (533), (622), (444) are observed in the pattern and approve purity of the phase in this reaction.
Fig. 5 depicts the XRD pattern of copper ferrite nanoparticle (CuFe2O4) with JCPDS: 34-0425 and by miller indexes: (220), (313), (320), (400), (422), (511), (440).
The Crystallite size of all three fabricated ferrites nanoparticles is measured using the Debye-Sherrer equation, the results show that cobalt, nickel, and copper ferrites nanoparticles have diameter values of 10, 12, and 15 nm, respectively.
Fig. 6 show scanning electron microscope images, as can be seen in the images, the nanoparticles are well distributed and the particle size is 50-80 nm for cobalt ferrite nanoparticles.
SEM images for nickel ferrite nanoparticles are shown in Fig. 7, mono-disperse and uniform structure with a grain size between 40-70 nm is obtained after the synthesis of nanoparticles.
Fig. 8 shows SEM images of copper ferrite nanoparticles and the size of the nanoparticles are between 40-70 nanometers. As all samples show despite suitable magnetic property all nanoparticles were prepared at uniform and monodisperse size. 
Figs. 9, 10 illustrate vibrating sample magnetometer (VSM) curves of the three magnetic ferrites consist of cobalt, nickel, and copper ferrites, respectively. VSM loops depict nearly ferromagnetic effects for all three samples of cobalt, nickel, and copper ferrites have saturation magnetization values around 25, 30, and 11 emu/g, respectively. According to these figures, magnetization remanence values of cobalt, nickel, and copper ferrites are about 5, 6, and 3 emu/g, respectively. Also, coercivities values are around 130, 200, and 400 Oersted for cobalt, nickel, and copper ferrites nanoparticles, respectively.
Fig. 11 shows the results of the micro-hardness tests. As can be seen, the microhardness values increase as it approaches the center of the stir zone, the higher the microhardness due to the presence of more nanoparticles and their uniform distribution. However, by moving away from the center part of the stir zone, the microhardness values decrease due to the reduction in the volume of nanoparticles. 
It is clear that the maximum microhardness value of FSPed AA7075 and AA7075/CuFe2O4, AA7075/NiFe2O4, and AA7075/CoFe2O4 surface nanocomposites have the highest values as in ascending order.

CONCLUSION
In this paper, different ferrite nanoparticles were first synthesized in an easy and cost-effective method and their properties were investigated. Then surface nanocomposites were fabricated with these nanoparticles using FSP and their microhardness properties were investigated. Based on the results of this paper, hydrothermal was a suitable method for the production of nanoparticles with a uniform and fine-grained structure, and the fabricated nanoparticles comply with standard peaks. These nanoparticles also have magnetic properties that can be used for radar wave absorption applications. In addition, the fabricated nanoparticles can also improve the mechanical property of the hardness of the surface nanocomposites.

Conflict of Interest
The authors declare that there is no conflict of interests regarding the publication of this manuscript.