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Синтез и характеристики горячепрессованных
нанокомпозитов на основе магния и цинка с низким
содержанием оксида графена для имплантологии
A. Saberi1, H.R. Bakhsheshi-Rad1, E. Karamian1, M. Kasiri-Asgarani1, H. Ghomi1, M. Omidi1, S. Abazari2, A.F. Ismail3, S. Sharif3, F. Berto4
1 Исламский университет Азад, Неджефабад, Иран 2 Технологический университет им. Амира Кабира, Тегеран, 15875-4413, Иран 3 Технологический университет Малайзии, Джохор-Бару, 81310, Малайзия 4 Норвежский университет естественных и технических наук, Тронхейм, 7491, Норвегия
Магний и его сплавы обладают повышенной способностью к естественному разложению в организме, что делает их привлекательными для использования в имплантологии в качестве биоразлагаемых материалов. Однако недостатком чистого магния является его быстрое разложение в физиологической среде, что отрицательно влияет на механическую целостность кости до ее заживления. В статье методами полупорошковой металлургии и горячего прессования синтезирован биокомпозит Mg-6Zn/xGO (0.2 и 0.4 мас. %). Микроструктурные исследования выявили равномерное распределение нанолистов оксида графена в композите, а также присутствие зерен a-Mg в магниевой матрице. Согласно полученным результатам твердость и предел текучести при сжатии композита Mg-6Zn/xGO значительно превышают данные величины для чистого магния. Показано, что улучшение механических свойств происходит за счет механизмов закрытия и отклонения трещин, а также экранирования вершины трещины. Во время погружения образцов в модельную физиологическую жидкость наблюдалось заметное снижение скорости выделения H2 за счет равномерного распределения на-нолистов оксида графена в магниевой матрице. Электрохимические испытания показали меньшую плотность тока коррозии и более высокую коррозионную стойкость горячепрессованных композитов Mg-6Zn и Mg-6Zn/GO по сравнению с чистым магнием. В результате снижения скорости разрушения прессованный биокомпозит Mg-6Zn/GO имеет высокую цитосовместимость. Показана эффективность применения нанолистов графена оксида в качестве упрочняющего материала при изготовлении прессованного биокомпозита Mg-6Zn/GO, благодаря чему улучшаются его механические, коррозионные и биологические свойства.
Ключевые слова: магниевый композит, оксид графена, полупорошковая металлургия, процесс экструзии, механические свойства, коррозионное поведение
DOI 10.24412/1683-805X-2021-1-62-78
Synthesis and characterization of hot extruded magnesium-zinc nano-composites containing low content of graphene oxide for implant applications
A. Saberi1, H.R. Bakhsheshi-Rad1, E. Karamian1, M. Kasiri-Asgarani1, H. Ghomi1, M. Omidi1, S. Abazari2, A.F. Ismail3, S. Sharif 4, and F. Berto5
Advanced Materials Research Center, Department of Materials Engineering, Najafabad Branch, Islamic Azad University, Najafabad, Iran
2 Department of Materials and Metallurgical Engineering, Amirkabir University of Technology, Tehran, 15875-4413, Iran 3 Advanced Membrane Technology Research Center (AMTEC), Universiti Teknologi Malaysia, Johor Bahru, 81310, Malaysia 4 Faculty of Engineering, Universiti Teknologi Malaysia, Johor Bahru, 81310, Malaysia 5 Department of Mechanical and Industrial Engineering, Norwegian University of Science and Technology,
Trondheim, 7491, Norway
Magnesium and its alloys have great capability for degradation in the body in a natural way, so they are one of the main new candidates as biodegradable implant materials. Of course, one of the disadvantages of pure Mg is its rapid degradation in the physiological environment that prior to bone healing has a negative impact on its mechanical integrity. In the present paper, a semipowder metallurgy coupled with hot extrusion process was utilized to fabricate Mg-6Zn/xGO (0.2 and 0.4 wt %) biocomposite. According to the microstructural characterization, graphene oxide (GO) nanosheets had uniform distribution in the composite, and also partial and gradually wrapped a-Mg grains were observed inside the Mg matrix. Results showed that the hardness and compressive yield of the Mg-6Zn/xGO composite were notably higher in comparison to pure Mg. It was revealed that the mechanical properties were enhanced because of the mechanisms of crack bridging, crack deflection, and crack tip shielding. The H2 evolution throughout their immersion in simulated body fluid (SBF) was decreased remarkably because GO nanosheets were distributed uniformly in the Mg matrix. In addition, less corrosion current density and higher corrosion resistance of the extruded Mg-6Zn and Mg-6Zn/GO in comparison to pure Mg were shown by electrochemical tests. Since the rate of the degradation process was decreased, the extruded Mg-6Zn/GO biocomposite presented great cytocompatibility. The research results show that GO nanosheets are efficient reinforcement to fabricate the extruded Mg-6Zn/GO biocomposite, which leads to the improvement of mechanical, corrosion and biological properties.
Keywords: magnesium composite, graphene oxide, semi powder metallurgy, extrusion process, mechanical properties, corrosion behavior
© Saberi A., Bakhsheshi-Rad H.R., Karamian E., Kasiri-Asgarani M., Ghomi H., Omidi M., Abazari S., Ismail A.F., Sharif S., Berto F., 2021
1. Introduction
In the past few years, since magnesium-based fracture fixation devices are suitable for the natural bone due to their high specific strength, proper mechanical properties including Young's modulus, and compres-sive yield strength, they have drawn the attention of researchers significantly [1-5]. Magnesium is the fourth most plentiful cation in the human body, and most of the magnesium is stored in bone and the excess amount excreted via the urine [6]. Magnesium influences human metabolism greatly and has stimulatory effects on the growth of the new bone [7, 8]. Moreover, a soluble, nontoxic oxide is created by degrading magnesium-based implants in a physiological solution that could act as a barrier film and protect from underlayer [9].
Magnesium-based fixation devices could be utilized as degradable materials for nonload-bearing applications like needles, plates, pins, and screws. It has commonly been absorbed within the time of new bone tissue growth, these devices will supply enough mechanical strength at the fracture area, and finally, natural tissue take their places [10, 11]. Although, they have some drawbacks such as high corrosion rate and low fracture strain and strength of Mg at room temperature because of the HCP structure [12]. Related to these drawbacks, literature shows that the magnesiumoxide layer, which naturally formed on its surface, is generally porous and is not able to protect effectively. So, Mg alloys can get easily dissolved in an environment comprising chlorine or carbon dioxide, which leads to a reduction of their extensive application [13]. Various methods, like composite preparation with suitable bioactive reinforcements, alloying, or surface modifications, have been proposed so far in order to fix these problems. Among them, composite preparation is drawing more attention these days [11].
In comparison to conventional engineering materials, metal matrix composites (MMC) materials are used more due to their high strength, low density, strengthened mechanical and thermal properties, and easy forming [14]. GO uniform distribution in the metal is complicated utilizing traditional methods (i.e., ball milling). Since they have a large surface area resulting in a high agglomeration tendency, they do not disperse uniformly. A lot of new processing routes have addressed the nonuniform distribution issue [15]. In this perspective, some researchers used the semi-powder metallurgy (SPM) method to fabricate Mg-GO composite for biomedical applications. According to their observations, GO's encapsulation into the Mg matrix significantly escalated the mechanical per-
formance of the composite. Turan et al. [16] analyzed the impacts of adding carbon-based materials on the microstructure and properties of Mg-based composites. The uniform distribution of graphene oxide nanosheets in the magnesium matrix, which was reported, proves that it is an efficient reinforcing filler material for deformation prevention [14, 17].
Inhabitation of dislocations at the interface causes the strength enhancement in the Mg/graphene composite. The surface area of the reinforcing particles influences the dislocation density in the composite
[18]. This composite strength was increased by limiting the dislocation movement caused by the proper reinforcement rate in Mg/graphene composite and the optimum level of the distance between reinforcements
[19]. Using Orowan looping, the movement within the structure is prohibited by nanosized graphenes resulting in bending between dislocations and graphene [15, 19]. Therefore, the resulted inverse stress leads to yield strength increase and dislocation movement inhabitation [14]. Similarly, it was reported that GO is a proper candidate as a reinforcement for metal matrix composites because of its extraordinarily high strength and modulus of around 130 GPa and >0.5-1 TPa, respectively [13, 20] along with significant physical and cellular behavior [13, 21]. However, there are few research investigating the formation mechanisms, corrosion performances, especially the mechanical property of GO incorporated on the matrix of Mg alloy and also and their potentials to improve corrosion resistance and mechanical and biological performance and there is a need for further investigation [13, 22, 23]. Therefore, in the present paper, the extruded Mg-Zn/GO biocomposites were fabricated using the semi-powder metallurgy method coupled with hot extrusion process with high mechanical properties, suitable anticorrosion, and cell response.
2. Materials and methods
Mg (99.5%) and Zn (99.9%) powders provided through Merck Co. were mechanically alloyed under inert medium to create Mg-6 wt % Zn alloy in a planetary ball mill with details parameters of the alloying as listed in Table 1.
In this paper, the magnesium matrix reinforced by GO nanosheets was fabricated using semipowder metallurgy (SPM) as shown in Fig. 1. To achieve this goal, GO nanosheets and ethanol solution were put in an ultrasonic device, which leads to the failure of the molecular bond; after that, the magnesium alloy was incorporated into the solution and mixed by magnetic stirring for 2 h at 300 rpm, following the dry proce-
Table 1. Mechanical alloying procedure parameters used pertaining to milling the Mg-6Zn powders
Parameter Value
Rotation speed, rpm 250
Ball-powder weight ratio 20 : 1
Vial and ball material Steel
Mass of powder, g 32
Milling time, h 15
Capacity of the vial, ml 250
Diameter of the balls, mm 10 and 20
dure in the oven for 24 h. After that, stainless steel mold was employed to prepare billets with specific sizes from composite powder under the pressure of 300 MPa, and subsequently, the sintering was conducted under inert medium at 600°C for 1 h. The hot extruded was performed using a hydraulic press and steel mold at 300°C with an extrusion ratio of 5 : 1 (the exit channel diameter of 10 mm), and a ram speed was set at 1 m/min.
The cylindrical composites (10 mm 0 x 15 mm) were employed to determine compressive strength (ASTM-E9 standard) with a speed and a load of 2 mm/min and 10 kN at ambient temperature, respectively. The specimen microhardness was determined through Vickers hardness (LECO M-400) instrument under 300g force.
To evaluate the corrosion property, the potentio-metric polarization test with an EC-Lab machine was
applied at a voltage range of -250 to +250 mVSCE, an open circuit potential (OCP) at a rate of 0.5 mV/s in the solution. In this technique, a graphite electrode, saturated calomel electrode, and specimens were employed as the counter, reference, and working electrodes, respectively, where a specific surface of 1 cm2 was subjected to solutions. Electrochemical impedance spectrometry (EIS) was employed based on the ASTM G106 standard at OCP at a range of 105 to 10-2 Hz, utilizing a sine signal with a potential amplitude of 10 mV. The immersion tests of the composite were performed as reported by the ASTM G31-72 standard [24]. The solution pH value, mass loss, and H2 evolution were assessed based on [24].
All samples were sterilized employing ultraviolet radiation for a minimum of 2 h prior to the cell test. The cell viability of all samples was evaluated (n = 3), making use of the MTT approach, as shown in [22]. The ALP activity assay was performed on the various days to assess the impact of GO on the commencing osteogenic differentiation of M-G63 cells according to [24]. Optical microscopy, scanning electron microscopy (SEM, JEOL JSM-6380LA), and transmission electron microscopy (TEM, HT7700 Hitachi, Japan) were applied to examine the specimen microstructures. An X-ray diffractometer (Siemens D5000) was used to disclose the phase composition applying Cu-Ka radiation (XCuKa = 0.154 nm) over 26 range of 0° to 90° at a scanning speed of 4°/min. Fourier transform infrared spectroscopy (FTIR), which was assessed over a range of 4000-450 cm-1, was employed to depict the surface functional group GO nanosheets.
Extrusion Sintering under Ar gas Raw billets Compaction
Fig. 1. A schematic illustration of the fabrication process of the nanocomposite (color online)
I
3551 0-H
4000 3500 3000 2500 2000 1500 1000
Wave number, cm-1
Fig. 2. TEM images related of Mg-6Zn (a), graphene oxide nanosheets (b), Mg-6Zn/0.4GO composite (c), EDS of Mg-6Zn/0.4GO composite (d) and FTIR absorption spectra of graphene oxide (GO) nanosheets (e) (color online)
3. Results and discussion
3.1. Morphology characterization
The TEM of the synthesized Mg-Zn and GO nanosheets are shown in Figs. 2, a and b. As it is shown in the mentioned figures, the Mg-Zn particles presented spherical shape, while GO nanosheets possess a wrinkled-sheet-like shape with a thickness of about 4 nm. It was revealed that after treating graphene by ultra-
sonic irradiation, it was oxidized, which leads to the successful exfoliation of the multilayered graphene oxide that is also evident by the transparency of the GO nanosheets in the TEM image. However, Mg-6Zn/GO exhibited a sheet-like shape with a thickness of ~1 nm containing uniform distribution of matrix particle (Mg alloy) within the sheet with great adhesion as shown in Fig. 2, c. Figure 2, d show the EDS
Fig. 3. SEM micrographs of Mg powders powders (g, j)
Zn powders (c, d), graphene oxide nanosheets (e, f ), Mg-6Zn/0.4GO composite
analysis indicating the existence of GO in composite structures where verified the characteristic carbon Ka peak at 0.277 eV [25]. The FTIR spectrum of GO nanosheets presented oxygen-containing groups at the band of 1053, 1252, 1408, and 1721 cm-1 is assigned to C-O stretching, C-OH stretching, C-O-H deformation, and carboxyl groups (-COOH) as shown in Fig. 2, e. Besides, the band at 1625 cm-1 corresponds
to the C-C stretching in the graphitic domains of GO nanosheets, and the typical peak of OH (3384 cm-1) was also detected in the spectrum [26, 27].
The morphology of Mg, Mg-6Zn, and Mg-6Zn/xGO biocomposites was explored thoroughly using SEM (Figs. 3, a, b). To have the other alloying element in Mg uniformly distributed, the initial powder particle size of Mg and Zn were chosen to be too different.
Since the smaller size of Mg, the more surface reactivity, it was difficult to use them. SEM images show the Zn powder big particles of Zn with spherical shape due to high surface energy (Figs. 3, c, d). The GO nanosheets exhibited an extremely thin layer with a few wrinkles and folds (Figs. 3, e, f. As shown in the figure, the gray areas were wrapped or half-wrapped by black areas. As evident, the GO nanosheets half-wrapped Mg alloy matrix; however, some white particles were clustered together with a random distribution in the matrix. It should be noted which to have homogeneous reinforcement of GO nanosheets; the semipowder metallurgy process was utilized. As it is shown in the figures, the GO nanosheets are incorporated in the Mg alloy matrix effectively (Figs. 3, g, j).
The XRD pattern of Mg-6Zn/xGO biocomposite revealed a peak of a-Mg (JCPDS # 07-7908) and MgZn2 (JCPDS # 15-8247) (Fig. 4) [28]. There were peaks at 29 of 32°, 34° and 37°, related to various planes (100), (002) and (101), respectively. There were peaks just related to Mg and MgZn2 phases. There were not any other intermetallic compounds indicating not oxidizing composites throughout semi-
powder metallurgy. Even though GO nanosheet peaks have to be at 29 of ~12° of the (002) plane, because of the reasonably low mass fraction of GO nanosheets, the XRD patterns did not have any GO peaks [29]. In this regard, it was exhibited that the graphite shows a sharpened peak at 29: 26.64° that is related to the (002) reflection, and also using Bragg law, the inter-layer distance of graphite was calculated as 0.334 nm. Although, at 29: 10.35°, a wider peak with less intensity was observed that corresponds to the (001) reflection. Increasing interlayer distance of GO nanosheets to 0.85 nm is shown by a significant peak change to lower angles. This may happen because of the existence of oxygen-containing functional groups on the edges and surface area of the GO nanosheets [26, 27]. The XRD pattern of the pure Zn and pure Mg powders depicted the existence of all features peaks of Mg and Zn without the presence of an impurity in the powders.
The grain size features of fabricated Mg-6Zn and Mg-6Zn/xGO composites are presented in Fig. 5. The average grain size of fabricated composites (20 ± 29 ^.m) is smaller compared to the Mg-6Zn alloy (30 ± 2 ^.m) because of the existence of GO encapsula-
Fig. 4. XRD pattern of pure Mg, pure Zn, graphene oxide (GO) nanosheets, Mg-6Zn and Mg-6Zn/xGO (x = 0.2 and 0.4 wt %) composite (color online)
Fig. 5. Optical microscopic images of as extruded Mg-6Zn (a), Mg-6Zn/0.2GO (b), and Mg-6Zn/0.4GO composites (c)
tion. The grain size slightly lessens with escalating in the GO amounts incorporations from 0.2 to 0.4 wt %. These findings suggested that GO influences refining the microstructure of Mg-6Zn/xGO composites, which has a significant effect on the mechanical properties of the composite containing GO according to the Hall-Petch relationship [1, 14] as the main strengthening mechanism for Mg-based matrix composite.
3.2. Mechanical behavior
Based on the fracture morphology (Fig. 6, a-d) of the extruded Mg-6Zn/0.4GO composite, the strengthening of GO nanosheets in the Mg-based composite matrix is linked to primary mechanisms such as load transfer through the matrix to GO nanosheets. When compressive stress is applied, the matrix is compressed, and the load is transferred from the compressed matrix to the additive phase via shear stresses present at the matrix-additive phase interface. The additive phase (GO) is therefore immune to fracture.
In this perspective, the performance of rupture additive phase relies on their matrix distribution, weight percentage, interfacial permeability, and their direction. Compressive and hardness tests were conducted to investigate the impacts of the reinforcement phase on the mechanical properties of the biocomposites (Figs. 6, e-g). The Mg matrice microhardness were increased by adding Zn and GO nanosheets. The hardness of pure Mg, Mg-6Zn, and Mg-6Zn/0.4GO was 41 ± 3.1 HV, 68 ± 3.8 HV, and 89.4 ± 5 HV, respectively. Generally, mechanical properties were enhanced by adding GO nanosheets to the Mg matrices through synergetic strengthening mechanisms that would be addressed in the following (Fig. 6, e). The improvement of the compressive strength was gradually achieving a peak value of 367 ± 5.3 MPa for the Mg-6Zn/0.4GO biocomposite (Figs. 6,f g). Another reason to improvemechanical properties was grain refinement. Besides, the high-stress region load transfer to GO nanosheets was increased because of the adequate interface intensity between GO nanosheets and Mg matrix since it leads to the desired affinity and large specific surface area [12, 28, 30]. The major mechanisms to improve yield strength of composites are: (i) the load transfer strengthening mechanism, (ii) grain refinement, and (iii) difference in coefficient of thermal expansion CTE) [12, 30]. Since graphene has efficient load transfer and grain refinement calculated through the Hall-Petch relationship, it increases strength [12, 14, 30]:
Aagr = K(d;1/2 - da"12). (1)
There is no doubt about the significance of the load transfer strengthening mechanism. Proper interaction with the matrix alloy, which could also be related to the wrinkled GO nanosheets with a high aspect ratio, leads to efficient load transfer. Moreover, the mechanical properties of the composites could be affected by the distribution state of GO nanosheets because of the large surface area of two-dimensions wrinkled GO nanosheets. The GO distributed along with stress direction is a response for load transfer [12]. Yield strength corresponding to calculated value can be decreased by the random distribution of reinforcement and agglomeration of some GO nanosheets [12]. As reported, there is a homogeneous distribution of GO nano-sheets in the thermally expanded Mg matrix. GO na-nosheets can be wrinkled by this distinction in thermal expansion between the Mg matrix and GO nanosheets. This could oppose dislocation motion in the Mg matrix during plastic deformation [31]. Correspondingly,
Fig. 6. Cross section of the Mg-6Zn/0.4GO composite in different magnifications (a-d), microhardness value (e), compressive yield stress test f), and ultimate compressive strength of pure Mg, Mg-6Zn and Mg-6Zn/xGO (x = 0.2 and 0.4 wt %) composite (g), schematic figure demonstrate stress distribution mechanisms by graphene oxide in Mg/GO composite (h) (color online)
it was reported which the difference in the CTE values prevents dislocation motion among the matrix and GO nanosheets interfaces [6, 32-34]. Therefore, because of the strain hardening, dislocation density increase leads to the composite strength increment. One of the main mechanisms to increase strength in such metal matrix composites is Orowan looping. Residual dislocation loops are formed around the reinforcing phases by the GO nanosheets in the Mg alloy matrix. Dislocations start the movement under the load influence,
and their movements are hindered by GO nanosheets, and dislocations are bent. Back stress is created by this bend and results in the increment of the flow strength. However, a number of dislocations could cross GO nanosheets through creating bow motion. At this stage, the loops referred to as Orowan looping are created. These loops result in substantial work hardening [14, 35]. In the present paper, the mentioned mechanism was used to increase the strength in GO na-nosheets reinforced Mg-6Zn alloys. Since the semi-
70 Saberi A., Bakhsheshi-RadH.R., Karamian E. et al. / Физическая мезомеханика 24 1 (2021) 62-78 Table 2. Comparison of mechanical properties of Mg-6Zn/xGO composite with previous studies
Samples Processing route Compressive yield strength, MPa Ultimate compressive strength, MPa Hardness HV Ref.
Pure Mg SPM+HTE 123±3 268±3.5 41 ±3.1 This work
Mg-6Zn SPM+HTE 126±3.1 323 ±2.2 68±3.8 This work
Mg-6Zn/0.2GO SPM+HTE 160±6 363±3.5 86±3.6 This work
Mg-6Zn/0.4GO SPM+HTE 161 ±4.5 367±5.3 89.4±5 This work
AZ61 SLM - 162±2 90±2 [36]
AZ61-1rGO/MgO SPM ± SLM - 200±2 97±2 [36]
AZ61 -2rGO/MgO SPM ± SLM - 230±2 103±2 [36]
AZ61-0.2GO SPM +SLM ~177.5 - ~90 [2]
AZ61-0.4GO SPM +SLM ~188.5 - ~95 [2]
AZ61-0.6GO SPM +SLM ~202.5 - ~97 [2]
ZK60 SPM+HTE+ SC + HTE 126±3.0 364 ±2.8 68±2.8 [37]
ZK60-0.05GNPs SPM+HTE+ SC + HTE 249±4.0 473 ±6.2 78±2.0 [37]
ZK60-0.1GNPs SPM+HTE+ SC + HTE 279 ±3.4 463±5.0 75±2.5 [37]
AZ31 alloy PM+Sintering + Extrusion 160±6 363±3.5 58±3 [38]
AZ31-0.3GNPs PM+Sintering + Extrusion 161 ±4.5 397±5.3 71 ±2.1 [38]
Mg-6Zn DMD + Sintering + HTE 109±4.5 426±6.1 ~62.5 [39]
Mg-6Zn-0.5GNPs DMD + Sintering + HTE 131 ±5.1 435±5.4 ~75 [39]
Mg-6Zn-1.5GNPs DMD + Sintering + HTE 171 ±2.4 448 ±4.7 ~70 [39]
AZ31-1.5GNPs SC 121 ±4.7 415 ±3.4 ~55 [40]
AZ31-3GNPs SC 120±2.8 406±4.1 ~59 [40]
SPM—semipowder metallurgy, GNPs—graphene nanoplatelets, rGO—reduced graphene oxide, GO—graphene oxide, PM—powder metallurgy, HTE—hot extrusion, SLM—selective laser melting, SC—stir casting, HEBM—high energy ball milling, DMD—disintegrated melt deposition.
powder metallurgy method was used to produce the lowing: crack tip shielding, since there is not enough
Mg-GO composite in the present paper, the strength energy to debond interface, the crack tip is limited in
values attained might indicate some differences in the vicinity of graphene [35]. At the time that the
comparison to the literature [14, 35]. The summary of crack encounters GO nanosheets, it diverts into an-
mechanisms for graphene-based materials are as fol- other plane, which leads to a flexuous path, and more
energy is wasted to propagate the crack. According to the results, GO nanosheets can effectively suppress crack propagation in Mg alloy matrix. Compressive strength and fracture toughness were enhanced by the mechanisms of graphene crack bridging, crack deflection, and crack tip shielding [26]. When a matrix crack is started and spreads, since the elastic modulus is different, the load moves from the matrix to gra-phene. The wrinkled surface texture of graphene provides effective mechanical interlocking and load transfer with the matrix.
According to Fig. 6, h, at first, a crack spreads in the Mg-6Zn matrix and, then it diverts into a different plane encountering graphene. As the crack plane is not perpendicular to the axis of stress anymore, more energy is needed to propagate the crack. A tortuous path is also created for crack propagation by the deflection process that leads to too much energy loss. Crack deflection might be very efficient for graphene-based materials in virtue of the large specific surface area. As shown in the schematic, because of the inadequate energy needed to debond interface, the crack tip is limited in the vicinity of GO nanosheets. Briefly, GO nanosheets are investigated as a potential reinforcement to Mg-6Zn biocomposite to enhance the present paper's mechanical properties.
A comparison of the mechanical characteristic of present study research with prior research is exhibited in Table 2. Guler et al. [25] depicted that compared to AZ31 and ZK60 alloys strengthened with a greater amount of GNPs, the microhardness of the extruded Mg-Zn/0.4GO composite's fabricated in this study is considered to be higher. In comparison with pure Mg and its alloys, the strength of the Mg-6Zn/0.4GO composite is also to be found greater. The present result depicted that a minimal GO amount is extremely beneficial due to the low amount of nanosize additive phase that will not considerably agglomerated and affect magnesium mechanical, corrosion and biological characteristics. This assessment displays that an appropriate combination of processing approach and components are utilized in the present study. Hence, the fabricated materials via combination of semipow-der metallurgy and hot extrusion could be presented as a novel class of composites for the function of biomedical purposes.
3.3. Corrosion behavior
As shown in Figs. 7 and 8, the electrochemical test, pH test, weight loss, and Mg ion concentration test were used to investigate the degradation properties of Mg, Mg-6Zn, and Mg-6Zn/GO biocomposites. The po-
larization curves of Mg-based specimens as shown in Fig. 7, a exhibited higher corrosion potentials (Ecorr) and lower corrosion current densities (icorr) for composite sample encapsulated with GO compared to the pure magnesium. It is well known that the rapid decrease of the icorr of Mg by adding Zn. In general, less icorr in potentiodynamic polarization testing shows more corrosion resistance. Moreover, GO nanosheets had fine corrosion inhibition ability in the Mg matrices because the lowest value (32 ± 2 |A cm-2) was attained Mg-6Zn/0.4GO biocomposite based on the icorr.
Figure 7, b shows the Nyquist curves of the Mg-based composite and Mg-6Zn/GO composites immersed in the SBF solution, showing single and two loops in Mg-6Zn and Mg-6Zn/GO composite of the curves. The first one that corresponds to the oxide layer is at a low frequency (left-hand side), and the second one relating to the charge transfer at the interface between the metal and the electrolyte is at a high frequency (right-hand side). Increasing the corrosion resistance of the system increases the diameter of these two loops. So, based on the Nyquist curves shown by Fig. 7, b and also the comparison between the Mg, and Mg-6Zn/xGO (x = 0.0, 0.2 and 0.4), the main influence of GO nanosheets on charge transfer resistance, Rct can be expressed sequentially as Mg < Mg-6Zn < Mg-6Zn/0.2GO < Mg-6Zn/0.4GO.
Fig. 7. Potentiodynamic polarization curves (a) and Nyquist plot curves (b) of pure Mg, Mg-6Zn and Mg-6Zn/xGO (x = 0.2 and 0.4 wt %) composite (color online)
Fig. 8. pH value (a), Mg + ion concentration (b), CRWL (measured from weight loss) (c) and CRH2 (measured from H2 evolution) of pure Mg, Mg-6Zn and Mg-6Zn/xGO (d) (x = 0.2 and 0.4 wt %) composite after soaking in SBF (color online)
The pH values obtained in the test had a fast increment throughout the immersion process in the first time interval for 48 h. Although pH had a fairly mild increasing trend (Fig. 8, a). Most preeminently, the pH curves showed the least pH value of 8.41 ± 0.1 for the Mg-6Zn/0.4GO biocomposite compared to 9.52 ± 0.3 for Mg and 8.57 ± 0.3 for Mg-6Zn biocomposites. While, the immersion test identified the alteration of Mg ion concentration (Fig. 8, b) and Mg ion concentration reduced from 7.28 ± 0.4 mg L-1 for the Mg-6Zn biocomposite to a comparatively lower value of 6.51 ± 0.3 mg L-1 for the Mg-6Zn/0.4GO biocomposite, indicating the same phenomenon with the degradation rates. The weight loss test was also used to determine the degradation rates carefully (Fig. 8, c). By adding Zn into Mg matrix (3.70 ± 0.2 mm/y), the degradation rate of pure Mg (5.81 ± 0.3 mm/y) was reduced to some degree, as adding 0.4 wt % GO into Mg-6Zn (2.47 ± 0.2 mm/y) reduced it evidently, which indicates a somewhat lower degradation rate. This is because of this that GO nanosheets have a unique 2D structure and high specific surface area and have the ability to act as a physical obstacle for preventing the penetration of corroding factors to the specimens [27]. Figure 8, d shows the H2 evolution versus immersion time curves
for the sintered samples of pure Mg, Mg-6Zn, and Mg-6Zn/GO in SBF. H2 evolution had the least value for the Mg-6Zn/GO in comparison to pure Mg and Mg-6Zn composites. It is well known that H2 evolution was used to measure the corrosion rate (CR) of the samples of pure Mg, Mg-6Zn, and Mg-6Zn/GO. In terms of the CRh2 for the pure Mg, Mg-6Zn, and Mg-6Zn/GO, the H2 evolution tests showed 18 ± 0.9, 12.4 ± 0.7, 9.9 ± 0.6 and 8.37 ± 0.5 mm/y, respectively. According to the results of the CR from H2 evolution, there was a considerable increase in CR in the Mg-6Zn/GO in comparison to the pure Mg and Mg-6Zn. In terms of the corrosion inhibition ability of GO nanosheets, it was reported [31] in the literature which an atomic-scale obstacle is created by a single atomic layer of GO nanosheets consisting of sp2-C bonds, which is impermeable to gas molecules and no gas can cross it.
Furthermore, SEM and EDS were used to investigate the distinct degradation morphology surface of Mg, Mg-6Zn, and Mg-6Zn/GO the following soaking for 7 days (Fig. 9). It was clear that a corroded surface completely covering an exfoliated layer of corrosion products was provided by the Mg alloy/GO biocomposite. A relatively low degree of corrosion was seen for the Mg-6Zn/GO biocomposite since the degrada-
Fig. 9. SEM images and EDS analysis of the samples after immersion in SBF solution for 7 days, Mg (a), Mg-6Zn (b), Mg-6Zn/0.2GO (c) and Mg-6Zn/0.4GO composite (d) (color online)
Fig. 10. Schematic illustration of corrosion behaviors of Mg composite in SBF solution (color online)
tion morphology plainly indicated an integrated surface with a few cracks. Moreover, the SEM image disclosed that low corrosion cracks and corrosion products on the corroded surface of Mg-6Zn/GO biocomposite, indicating which in comparison to Mg-6Zn and Mg-6Zn/GO biocomposites, the Mg-6Zn/GO biocomposite have the best degradation resistance. As shown by EDS, which examined the product layer on the composite surface, consisted of Mg, C, P, Ca, and O elements representing formation of Mg(OH)2 and HAp.
Figure 10 shows the schematic illustration of the corrosion behavior. After soaking the Mg-6Zn/GO composites in the SBF solution, because of the existence of the GO nanosheets into the composite, where Mg matrix acts as the anode and the GO nanosheets act as the cathode, microgalvanic corrosion happened. The Mg matrix was corroded and subsequently Mg ions were released by its anode reaction role, according to Eq. (2). In this respect cathodic reaction occurred based on the Eq. (3) when the electrons created from the dissolution of matrix moved from grains to second phases. Based on Eq. (4), precipitation caused the forming of the hydroxide layer. In this context, Mg2+ can cross the loosened Mg(OH)2 layer and create a new Mg(OH)2 layer on the exterior surface. The loosened surface is penetrated by the SBF solution components, which also react with the interior Mg substrate resulting in more degradation of the composite [29]:
Mg ^ Mg2+ + 2e~, anodic reaction, (2) 2H2O + 2e" ^ H2 + 2OH", cathodic reaction, (3)
Mg2++ 2OH- ^ Mg(OH)2.
(4)
With extension of incubation time, hydroxyapatite Ca10(PO4)6(OH)2 was created because of the reaction among phosphate ions (HPO42- or PO43-) and Ca2+ in the solution causing Ca/P compounds precipitating on the surface of the magnesium composite based on the Eq. (5):
10Ca2+ + 6PO4" + 2OH" ^ Ca10(Po4)6(OH)2. (5)
The corroded sample XRD patterns are shown in Fig. 11 after immersion in SBF for 7 days at 37°C. GO nanosheets have the corrosion inhibition ability in Mg matrices since the XRD pattern of the Mg-6Zn/GO showed more intensity of the Mg peak (101) at 36.62° in a same lattice plane in comparison to the pure Mg. Along with Mg peaks, XRD patterns of the corroded pure Mg, Mg-6Zn, and Mg-6Zn/GO composite showed added peaks related to Mg(OH)2 that was created as a corrosion product on the substrates. The reactions mentioned in Eqs. (4) and (5) control the general corrosion mechanism of Mg-based composite and forming corrosion products such as Mg(OH)2 and hydroxy-apatite. The peak intensity of Mg(OH)2 for all specimens was different, where Mg possesses the highest intensity, while Mg alloy/GO presented the lowest one. Since fewer corrosion products of Mg(OH)2 deposited on the surface results in a less intensity of Mg(OH)2 peaks, this less intensity of Mg(OH)2 in the Mg-6Zn/GO might be because of the more corrosion resistance of the GO-reinforced Mg-based matrix.
3.4. Cytocompatibility
In vitro cytotoxicity test and also proper cytocom-patibility is mandatory for biomedical implants. MTT assay was found in extracts for various times to ana-
Position 20
Fig. 11. XRD patterns of corroded samples in SBF solution (color online)
lyze the cell viability (Fig. 12, a). There was an increase of the cell activity with culture time, and the optical densities in the extracts of Mg, Mg-6Zn, and Mg-6Zn/GO biocomposites were more than that in control. Besides, less pH and ion concentration in the extract of Mg-6Zn/GO biocomposite might cause a more optical density in the Mg-6Zn/GO extract in comparison to the Mg-6Zn extract, for both 3 and 7 days cultivation.
The level of ALP activity, commonly utilized as an initial hallmark of osteoblast differentiation, was evaluated within the seventh day. The ALP expression for Mg-6Zn extract and, mainly, for Mg-6Zn/GO extract was incredibly enhanced compared to that for pure Mg extract, according to Fig. 12, b. ALP staining also proved these results. According to the ALP expression, the Mg-GO composite comprising GO nanosheets profits osteoblastic cell differentiation. As stated by
Fig. 12. Cell viability (a) and ALP activity of MG63 cells cultured for various times on the Mg, Mg-6Zn, and Mg-6Zn/GO biocomposites (b) (color online)
Han et al. [41], cytotoxicity of cells can be caused by damaging the membrane of the cell wall by direct contact interaction with the graphene oxide plates, and also delaying the cell cycles by graphene leads to the cell viability reduction. According to the obtained results, incorporating the GO nanosheets can efficiently improve the biocompatibility of Mg-6Zn biocomposite.
4. Conclusion
The present paper aims to fabricate the Mg-6Zn/ GO biocomposite at different GO nanosheets contents by the use of the semipowder metallurgy process coupled with hot extrusion process to extend their cell behavior, mechanical performance, and corrosion behavior. The successful incorporation of GO nano-sheets into the Mg-6Zn matrix was proved by the XRD and FTIR results and based on the microstructural evolution, and GO was successfully incorporated into the composite by semipowder metallurgy approach without the creation of any apparent structural damage. Adding graphene did not have any effect on the phase composition of Mg-6Zn biocomposite. Increasing GO content increased the hardness of Mg-6Zn/0.4GO biocomposite with a maximum amount of 89.4 ± 5 HV for Mg-6Zn/0.4GO biocomposite. Moreover, increasing GO nanosheets content (0.2-0.4 wt %) leads to an increase in the compressive strength of Mg-6Zn. The results also point out that the mechanical properties enhanced because of the crack deflection and crack tip shielding as a result of GO na-nosheet role inside the Mg alloy matrix. The corrosion layer was prevented from falling off the Mg alloy matrix by the presence of the GO nanosheets through the bridging effect. The apatite depositing was enhanced, and corrosive medium intruding was prevented by the oxygen-containing groups on GO nanosheets. The biological properties of the Mg-6Zn were enhanced by GO (0.2-0.4 wt %) through cell viability and differentiation promotion. The results of the present study showed the significant potentiality of 0.4 wt % GO nanosheets encapsulated Mg-6Zn composite to be utilized for implant applications.
Acknowledgments
The authors would like to thank the Norwegian University of Science and Technology, Islamic Azad University, Najafabad, and Universiti Teknologi Malaysia for providing the facilities for this research.
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Received 11.11.2020, revised 11.11.2020, accepted 18.11.2020
Сведения об авторах
A. Saberi, Najafabad Branch, Islamic Azad University, Iran
Hamid Reza Bakhsheshi-Rad, Najafabad Branch, Islamic Azad University, Iran, rezabakhsheshi@gmail.com, rezabakhsheshi@pmt.iaun.ac.ir
E. Karamian, Najafabad Branch, Islamic Azad University, Iran M. Kasiri-Asgarani, Najafabad Branch, Islamic Azad University, Iran H. Ghomi, Najafabad Branch, Islamic Azad University, Iran M. Omidi, Najafabad Branch, Islamic Azad University, Iran S. Abazari, Amirkabir University of Technology, Iran Ahmad Fauzi Ismail, Universiti Teknologi Malaysia, Malaysia Safian Sharif, Universiti Teknologi Malaysia, Malaysia
Filippo Berto, Prof., Norwegian University of Science and Technology, Norway, filippo.berto@ntnu.no
