The effect of GNPs on wear and corrosion behaviors of pure magnesium
Introduction
Magnesium is the most eight abundant on the earth and third structural metal after aluminum and iron [1]. Magnesium based materials are lighter than aluminum (about %35) and iron [2], [3]. It has low density (1.74 g/cm3), good machinability, damping and high specific strength and specific stiffness [3], [4]. So magnesium and alloys have been attractive subject especially in automotive and aircraft industry due to demanding lightweight and high specific strength materials to improve energy efficiency [5]. However magnesium has low ductility due to its HCP structure and also it has poor wear and corrosion resistance [6], [7]. Researchers have been trying to overcome this problem by developing magnesium composites with particulate reinforcements [8], [9], [10]. Micro-size particles such as SiC, B4C and Al2O3 are commonly used as a reinforcement material. However, these reinforcements enhance hardness and modulus while they lead to limit ductility [11]. Nano-size reinforcements are being studied to improve mechanical properties due to their excellent properties such as large surface area to volume ratio [12].
During past decades, carbon based nanoparticles are preferred using as a reinforcement for metal matrix. Researchers have made several successful studies about carbon based nano-size reinforcements to improve mechanical properties and wear performance of metal matrix composite. Generally, carbon nanotube which is an allotrope of carbon was used as a nanoparticle reinforcement [13], [14], [15], [16].
Carbon nanotubes are formed by rolling a sheet of carbon atoms. This material has 1–2 nm diameters and excellent mechanical properties [12]. For example, carbon nanotubes have up to 0.9 TPa elastic modulus and up to 150 GPa fracture strength so they are becoming popular as nano size reinforcement material in magnesium matrix. Previous studies showed that using carbon nanotubes in magnesium alloys increases tensile strength and elastic modulus [17], [18]. C.S. Goh et al. [19] investigated effect of carbon nanotubes on mechanical properties of pure magnesium. Up to 1.3% carbon nanotube addition leads to increasing mechanical properties of magnesium. However, carbon nanotube may cause an agglomeration because of Van der Waals attractions between carbon atoms. This is a result of poor dispersion of CNTs. So it is still serious problem for practical applications [20]. Thus, graphene has begun to be used as an alternative reinforcement instead of carbon nanotube for magnesium matrix metal composites.
Graphene is another common form of carbon [21]. It has wide range of applications, especially in electronics area. Graphene is attractive with unique mechanical, thermal and electrical properties for nanotechnology. Elastic modulus is 1 TPa and fracture strength is 125 GPa for graphene [12], [21]. There are weak van der Waals bonding among the graphite layers but carbon atoms are bounded to each other very strongly [22].
Effect of graphene on magnesium and its alloys have not been investigated intensively compared with carbon nanotube. According to Z. Hu et al. [23], The first study was about Mg/Graphene composites reported by Chen et al., in 2012. Chen et al. [24] fabricated graphene reinforced magnesium matrix composites. Uniform dispersion was achieved so mechanical properties were enhanced significantly [24]. In 2013, Rashad et al. [11] reported a study about production of Mg/Graphene composites. In this study, graphene reinforced Magnesium/Titanium alloy was synthesized via semi-powder metallurgy. In another study about an addition of graphene on magnesium was investigated by M. Rashad et al. [21]. Graphene content was kept as 0.18 wt% and its effects on mechanical properties were studied [21]. In another research, M. Rashad et al. also observed that presence of 0.3 wt% graphene in magnesium matrix improves elastic modulus, yield strength, ultimate strength and Vickers hardness [25]. Recently, M. Rashad et al. studied the effects of GNPs on microstructural and mechanical properties of Mg-6Zn alloy. Composites were fabricated using the disintegrated melt deposition method (DMD). Results showed that addition of GNPs leads to increase in microhardness, tensile and compression strength [26]. In a similar study, M. Rashad et al. synthesized AZ61 magnesium alloy reinforced GNPs using by same method (DMD). Improvement of mechanical properties was achieved due to refining grain size and changing in basal textures [27].
As mentioned above, effects of graphene addition on tensile properties and microstructure of magnesium alloys were investigated. However, there are limited studies about tribological effect and corrosion performance of graphene nanoparticles on pure magnesium or magnesium alloys. Tabandeh-Khorshid et al. studied about Tribological performance of aluminum matrix by the use of graphene as a reinforcement material. According to them, Graphene may act as a self-lubricating material and this allows the improvement of wear performance in metal matrix nanocomposites [28]. But the roles of graphene nanoparticles on wear behaviors for pure magnesium have not been investigated. On the other hand, there are little studies about the effect of graphene on corrosion behavior of magnesium and alloys. M. Selvam et al. were investigated the corrosion behavior of mg/graphene. Thin layer graphene coating was applied to the magnesium and corrosion performance was evaluated by the use of different aqueous electrolyte solutions [16].
In this study, besides pure magnesium, three types of composites which have different graphene contents were fabricated by semi powder metallurgy. Tribological performances of samples were investigated to understand effect of graphene content on wear behaviors of pure magnesium. Furthermore, electrochemical corrosion tests were performed for all samples. To sum up, the role of graphene was investigated for wear and corrosion performances on pure magnesium with present study.
Section snippets
Materials and processing
Magnesium powder with 99.7% purity and 100 μm size range was purchased from Nanografi Co. Ltd. Turkey. Graphene that has 5–8 nm thickness and 750 m2/gr surface area was supplied by same company (Nanografi). X-ray diffraction (XRD) patterns of graphene are shown in Fig. 2.
Ball milling is unsuitable technique for magnesium and its alloys for powder metallurgy because magnesium produces heat so it can cause to be burnt [11]. Therefore, semi powder method was used to fabricate magnesium matrix
X-ray diffraction results
The X-ray diffraction pattern of pure magnesium and graphene reinforced composites is shown in Fig. 2. It is clear from the figure that peaks for magnesium are present at 2θ equal to 32.230, 34.400, 36.620, 47.530, 57.370, 63.060, 67.310, 68.630, 70.000 and 72.490.
Note that in Fig. 2 graphene (002) peak is not clearly visible in the full continuous scan. This is partly because of much lower concentration of graphene in the samples. Yet, it should be mostly attributed to the separation of
Conclusions
In this study, magnesium matrix reinforced by graphene nanoparticles was fabricated by semi powder metallurgy. Wear behavior and corrosion performance were investigated of pure magnesium and synthesized composites. The experimental findings are summarized as follows:
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Mg+0.5 wt% GNP has the highest hardness compared than pure magnesium and other composites. Furthermore, there is a direct proportion between graphene concentration and hardness values.
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Graphene is distributed homogenously into the
Acknowledgement
The authors gratefully acknowledge Karabuk University (KBUBAP-17-DR-048) for financial support and KBU Iron and Steel Institute due to supplying mechanical analyzes.
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