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Effect of graphene reinforcement on the mechanical properties of Ti2AlC ceramic fabricated by spark plasma sintering
Author’s Accepted Manuscript Effect of Graphene Reinforcement on the Mechanical Properties of Ti 2AlC Ceramic Fabricated by Spark Plasma Sintering Tony Thomas, Cheng Zhang, Ashutosh Sahu, Pranjal Nautiyal, Archana Loganathan, Tapas Laha, Benjamin Boel, Arvind Agarwal www.elsevier.com/locate/msea
PII: DOI: Reference:
S0921-5093(18)30645-2 https://doi.org/10.1016/j.msea.2018.05.006 MSA36444
To appear in: Materials Science & Engineering A Received date: 20 February 2018 Revised date: 10 April 2018 Accepted date: 3 May 2018 Cite this article as: Tony Thomas, Cheng Zhang, Ashutosh Sahu, Pranjal Nautiyal, Archana Loganathan, Tapas Laha, Benjamin Boel and Arvind Agarwal, Effect of Graphene Reinforcement on the Mechanical Properties of Ti 2AlC Ceramic Fabricated by Spark Plasma Sintering, Materials Science & Engineering A, https://doi.org/10.1016/j.msea.2018.05.006 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Effect of Graphene Reinforcement on the Mechanical Properties of Ti2AlC Ceramic Fabricated by Spark Plasma Sintering Tony Thomasa, Cheng Zhanga, Ashutosh Sahub, Pranjal Nautiyala, Archana Loganathana, Tapas Lahab, Benjamin Boela , Arvind Agarwala,* a
Plasma Forming Laboratory, Department of Mechanical and Materials Engineering, Florida International University, Miami, FL 33174, USA b
*
Department of Metallurgical and Materials Engineering, Indian Institute of Technology, Kharagpur, India
Corresponding author: [email protected]
Abstract Composite of Ti2AlC and 2 vol.% Graphene nanoplatelets (GNP) was successfully fabricated by spark plasma sintering (SPS) technique to augment the fracture strength of Ti2AlC. Microscopic analysis revealed the reinforcement of GNP within the Ti2AlC ceramic matrix. As the consequence of positive reinforcement, there was a 335% improvement in the fracture energy of the GNP/Ti2AlC composite, 195% increase in the flexural stress and 41% increase in the flexural strain. From nano-scratch analysis at low load (7000 µN) and high temperature (300 °C), it was found that the GNP/Ti2AlC composites wear volume loss minimized by 75% as compared to pure Ti2AlC. Post microscopic analysis of the fractured surface gave insight into the improvement in fracture strength of the material by sheet sliding, crack deflection but mainly by the pull out mechanism of graphene from the Ti2AlC matrix. It was also found that GNP was sintered to Ti2AlC platelets thus improving the overall mechanical properties of the GNP/Ti2AlC composite.
Keywords: Ti2AlC; Graphene; Spark Plasma Sintering; Nano-scratch; Fracture Energy
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1 INTRODUCTION Ti2AlC is a class of ductile MAX-phase ceramic material group with the general molecular formula Mn+1AXn (n = 1, 2, 3….), where M is an early transition element, A is an element from the group ‘A’ of the periodic table and X is either nitride or carbide. Some of the important characteristics of MAX-phase ceramics are low density, high stiffness, machinability, excellent thermal and electrical conductivity and they even exhibit some plasticity at elevated temperature [1-4]. Ti2AlC has a low density of 4.1 g/cm3 and a high electrical conductivity of 3x106 Ω-1m-1. The presence of kink bands in the basal plane restricts the propagation of cracks along that plane, making these materials more fracture resistant with a fracture toughness of 6.5 MPa.m1/2 [5-8]. All these unique material properties of Ti2AlC qualifies this engineering ceramic for antiballistic body armor, oxidation, and corrosion resistant protective coating application. Various efforts have been made to synthesis phase pure Ti2AlC by combustion and reactive sintering [9-11]. There is also a report on synthesizing Ti2AlC using various carbon source like carbon black, graphite and even carbon fibers [12]. Most of the efforts to improve the mechanical properties of Ti2AlC was compositing it with an intermetallic compound like TiAl due to its high melting point, low density, oxidation and corrosion resistance. Ramseshan et al. [13] conducted one of the earliest research to produce TiAl/Ti2AlC composite by reactive sintering. TiAl/Ti2AlC composite exhibited high strength of 800 and 400 MPa at ambient temperature and 1173 K respectively. Ramseshan et al. also reported that the composite’s fracture toughness value reached to 17.8 MPa m1/2. Recently Nel [14] reported a unique carbon fiber reinforced Ti2AlC prepared by SPS. The as-fabricated ceramic composite did not exhibit any improvement in the mechanical properties due to the presence of pores from poor infiltration of Ti2AlC into carbon fiber weave. There have also been reports of carbon nanotubes (CNT) used as the carbon source in the synthesis of Ti2AlC/intermetallic composites with enhanced mechanical properties. Yang et al. [15] fabricated TiAl reinforced Ti2AlC composite with ultrafine microstructure by SPS. They used carbon nanotubes (CNT) as the carbon source in the preparation of Ti2AlC/TiAl composite which had a compressive strength of 2058 MPa and a hardness of 6.12 GPa. Kulkarni et al. [16] synthesized Ti2AlC by SPS of TiAl and CNTs as a carbon source and observed the improvement in the density of the SPS sample as well as an increase in the hardness of up to 5.08 GPa. CNTs as a material is used as a reinforcement for over a decade in a variety of ceramic matrices to 2
enhance the properties like strength, stiffness, and toughness [17, 18]. Like CNT, Graphene a 2D material has similar material properties but has an advantage of the higher specific surface area and easier dispersion into the matrix. Graphene has been used to reinforce ceramic matrices [19]. Walker et al. [20] synthesized Si3N4 matrix with 1.5 vol.% graphene and saw an improvement of 235% in fracture toughness. Liu et al. [21] synthesized zirconia toughened alumina (ZTA) nanocomposites reinforced with graphene nanoplatelets (GNPs) using SPS and reported 40% increase in the fracture toughness of the material for as little as 0.8 vol.% graphene loading. One of the most common phenomena observed was enhanced toughening by mechanisms such as bridging, pullout and crack deflection on the fractured surfaces with graphene present in between the grain boundaries of the ceramic. It is evident that graphene can efficiently enhance the fracture strength when reinforced into a ceramic matrix, that too at a very low material loading rate. Research on graphene ceramic composites has shown promising results, but is still unexploited, in the improvement of MAX-phase material properties [19]. Taking this as the motivation, in this study, graphene nanoplatelets (GNP)/Ti2AlC composite was fabricated by SPS, and the effect of GNP reinforcement on the Ti2AlC’s mechanical properties was analyzed. GNP/Ti2AlC composite fabricated for this study is a unique ceramic composite and is the first time any researcher has attempted to enhance the fracture toughness of Ti2AlC compositing with carbonbased nano particle. There have been reports on synthesizing Ti2AlC using CNTs as a carbon source [20], but CNTs or GNPs were never used as a reinforcement material to enhance the fracture toughness of Ti2AlC. In the current study, an effort has been made to enhance mechanical properties of Ti2AlC by reinforcing with 2 vol. % of GNP into the ceramic matrix. Mechanical properties like hardness and elastic modulus were characterized by nanoindentation, wear by nano-scratch, and fracture energy by three-point bending tests. The fracture mechanism of the GNP/Ti2AlC composite fabricated by SPS was studied by analyzing the micrographs of the fracture surface and compared with Ti2AlC.
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2 EXPERIMENTAL PROCEDURE 2.1 SAMPLE PREPARATION The Ti2AlC/GNPs composite used in this study composed of Ti2AlC powder (Maxthal 211, Sandvik Materials Technology) and 2 vol.% GNP powder (XG Science, Lansing, MI, USA with an average particle size of 15 µm and thickness of 6 – 14 nm). The composite powder mixture was prepared by first ultrasonicating 2 vol.% of GNP powder in 500 ml of acetone (PHARMCO-AAPER, UV Grade) for 90 mins followed by adding as-received Ti2AlC powder and ultrasonicating the entire mixture for 30 minutes. The solution mixture was dried overnight at 75oC. The lumps of Ti2AlC/GNPs composite mixture was broken down into fine particles using mortar and pestle. Both Ti2AlC and GNP/Ti2AlC composite test samples were fabricated by SPS technique. The powder mixture was filled into a graphite die sandwiched between two graphite punches. The powder was wrapped in a graphite foil for easy extraction of SPS samples and improved current flow. SPS process was performed in vacuum condition (Pressure < 10 Pa). The powder in the graphite die was initially heated to 300oC and held there for 10 mins. After preheating stage, the powder was sintered at 1200oC for 7 minutes at a heating rate of 100oC/min at 60 MPa of pressure.
2.1 PHASE CHARACTERIZATION X-ray diffractometer (XRD. Siemens D500) technique was employed to analyze the phase purity of as-received Ti2AlC powder, sintered Ti2AlC and GNP/Ti2AlC pellets with Cu Kα (λ = 1.542 Å) x-ray source.
2.2 MICROSTRUCTURAL CHARACTERIZATION The microstructure of the as-received Ti2AlC powder, Ti2AlC/GNPs powder mixture, fractured Ti2AlC and GNP/Ti2AlC pellets were analyzed using JEOL JSM-6330F field emission scanning electron microscope (SEM). For SEM imaging, bulk samples were polished using alumina polishing media.
2.3 MECHANICAL CHARACTERIZATION The density of the SPS samples was measured using a gas pycnometer (AccuPyc ӀӀ 1340). Nanoindentation and nano-scratch tests were performed on the top surface and crosssection of the bulk, metallographically polished samples respectively using Hysitron 4
Triboindenter (TI 900 and TI 950, Hysitron Inc, Minneapolis, USA). The quasistatic nanoindentation was performed at room temperature (25oC) and high temperature (300oC) at a low load of 7000 µN. A standard Berkovich tip of 100 nm tip radius was used as the indenter throughout this study, with loading cycle of 10 – 3 – 10 seconds (loading – dwell – unloading). The RT quasistatic indentation tests were performed on a sample mounted in the epoxy resin. Epoxy mounting was avoided for high temperatures nanoindentation tests due to softening of the resin. High load (8N) nanoindentation test was also performed at RT to evaluate the elastic modulus and nanohardness at multi-length scale. A minimum of 30 indents was performed for both the samples. Nano-scratch test was performed at 300oC with a peak force of 7000 µN using a 60o conospherical diamond tip at constant loading with a scratch length of 10 µm. In-situ scanning probe microscopy (SPM) was used to image the indents and scratch. The images were processed in a 3-D processing image software (SPIPTM, version 5.1, Image Metrology, A/S, Denmark). Nano-scratch was also performed at a relatively high load of 8N at room temperature with a scratch length of 200 µN. SEM images of the scratch profile were analyzed to understand the wear behavior. Three-point flexure tests were performed on 15 x 1 x 0.8 mm slender rectangular specimens cut out of the SPS pellets using wire EDM as shown in Fig. 1. MTI SEM tester (Albany, USA) was used to conduct the three-point bending test. A 445 N capacity load cell was used for the tests. Test samples were loaded to the rate of 0.1 mm/min until the point of failure. 3 RESULT AND DISCUSSION 3.1 MORPHOLOGY STUDY 3.1.1 POWDER MORPHOLOGY Fig. 2 shows the morphology of the powder used for preparing SPS test samples in this study. From Fig. 2(a), the as-received Ti2AlC showed irregular particle shape and size ranging from 100 nm – 20 µm. Fig. 2(b) clearly shows nano sized particles of Ti2AlC (particles within yellow dash) smeared on micron sized Ti2AlC. Fig. 2(c) is the SEM of the as-received GNP powder. On an average, GNP has about 20 graphene sheets and each layer has an approximate thickness of ~0.35 nm. As mentioned in section 2.1, the average diameter of the GNP seen in Fig. 2(c) is 15 µm, with a relative surface area of 150 m2/g. A detailed physical and chemical analysis of the powder used in this work is presented elsewhere [22]. The distribution of GNP 5
flakes is evident in low magnification SEM image as shown in Fig. 2(d) (Represented by yellow dashed circle). As seen in Fig. 2(d), GNP flakes are smeared with nano-particles of Ti2AlC giving a perspective of GNP’s surface area that can provide a platform for efficient reinforcement in the matrix present.
3.1.2 MICROSTRUCTURE OF SINTERED Ti2AlC Fig. 3 shows the SEM images of the Ti2AlC test samples fabricated by SPS. The sample morphology consists of irregularly shaped and non-uniformly distributed grains which are densely packed with no signs of pores. Some amount of void observed in the micrograph can be attributed to the material removal during sample polishing. The bulk sample had a relative density of 99.5%. When Fig. 3(a) is closely observed, three distinct microstructures are seen. They comprise of elongated plate-like nano-laminated grains of Ti2AlC (12 – 32 µm) as seen in Fig. 3(b) with an average thickness of ~ 230 nm, fine and bright hexagonal crystal grains of TiC (1 – 3 µm) and flat plate-like non-uniformly distributed structures of Ti3AlC2 (5 – 8 µm). It should be noted that irregular particle shape and size is important to obtain a highly dense monolithic ceramic structure which directly influences its mechanical properties. The test sample of Ti2AlC had an average particle size of ~ 7 µm. There are other reports indicating similar grain morphology in bulk SPS Ti2AlC [23 – 24]. Fig. 4, is the SEM image of the sintered GNP/ Ti2AlC composite. The first thing to be noticed is, the majority of the particles exhibit elongated plate-like morphology with an average length of ~ 10 µm which is relatively smaller than Ti2AlC grains in Fig. 3(a). Addition of GNP has induced grain refinement by ~ 55%. The relative density of this highly dense bulk sample was more than 101.83% of theoretical density. This is due to the dissociation of Ti2AlC into TiAl and TiC as confirmed by the XRD pattern shown in Fig.5. The formation of excess TiC resulted in a density more than the theoretical density of Ti2AlC and close to the material density of TiC (4.93 g/cm3) [25]. Some thin GNP flakes can be seen across the microstructure (indicated by the yellow arrow in Fig. 4(a)). But the majority of the GNP is sandwiched between Ti2AlC nanolaminates as shown in Fig. 4(a) (indicated by the yellow boundary). This signifies successful reinforcement of GNP into the Ti2AlC ceramic matrix. In the inset shown in Fig. 4(b), delamination is seen between the layers of GNP flake and Ti2AlC laminates. This is due to the 6
mismatch in the thermal properties of the materials. Nevertheless, the presence of such distinctive artifact (dark region between Ti2AlC nano-laminates) is a clear indication of GNP reinforced into the material matrix. Fig. 5, shows the XRD analysis of the as-received Ti2AlC powder, sintered Ti2AlC, and GNP/Ti2AlC pellets. As-received Ti2AlC powder is a biphasic mixture of Ti2AlC (JCPDS 00029-0095) and TiC (JCPDS 03-065-0971). The exclusion of TiC is inevitable as Gibbs free energy for the formation (ΔG) of TiC (-300 kJ/mol) [26] is much lower than ΔG of Ti2AlC (-54.8 kJ/mol) [27]. As a result, TiC is easily formed and stays as a stable phase up to temperature as high as 1550oC. The XRD analysis of sintered Ti2AlC (Fig. 5) shows an additional inclusion of Ti3AlC2 (JCPDS 00-052-0875) apart from Ti2AlC and TiC. Similar studies on spark plasma sintering of Ti2AlC has demonstrated the formation of Ti3AlC2 due to the reaction between Ti2AlC and TiC at elevated sintering temperature [28]. For GNP/Ti2AlC composite, XRD analysis (Fig. 5) revealed the formation of TiAl (JCPDS 00-065-0428) in addition to Ti3AlC2. TiAl is an intermediate compound formed during the formation and dissociation of Ti2AlC [12, 29]. Agarwal et al. [30] in their work on SPS densification of GNP reinforced TaCx composite, reported that the composite achieved a density of 99% of the theoretical density of TaCx and also observed 60% reduction in the grain size. GNP acts as a sintering aid by reducing the melting temperature of the host material, and this synergetic effect of GNP is well reported [31-33]. Pietzka et al. [29] conducted a thermodynamic analysis of the formation mechanism of Ti2AlC and found that at a higher temperature, Ti2AlC dissociates into TiC and TiAl. The addition of GNP in the Ti2AlC may have decreased the decomposition temperature of Ti2AlC along with the melting temperature thus dissociating some amount of Ti2AlC, into TiC and TiAl.
3.3 NANO-MECHANICAL CHARACTERIZATION 3.3.1 QUASISTATIC NANOINDENTATION Nanoindentation studies were conducted at low load of 7000 µN (to establish the nanoscale length mechanical properties) and high load of 8 N (to establish macro scale length mechanical properties) on the polished cross-section of the test samples. Low load nanoindentation tests were conducted at room temperature (RT) and high temperature of 300oC. Fig. 6(a) and 6(b), shows a generic load-displacement curves of Ti2AlC and GNP/Ti2AlC at RT and 300oC respectively, when subjected to a peak load of 7000 µN. 7
The load-displacement curves of Ti2AlC at room temperature (Fig. 6(a)) show signs of kickback due to strain burst during the loading cycle. This occurs upon shearing of inter planar nanolaminates of Ti2AlC, thus deviating the crack propagation perpendicular to the applied load and limiting further penetration, which is an inherent property of MAX-phase materials (selfhealing and self-lubricating). At 300oC, the load-displacement curve is smooth for Ti2AlC as seen in Fig. 6(b). This is the result of surface oxidation of Ti to TiO2 at low temperature [34]. For GNP/Ti2AlC, frequent kickbacks in the loading curve are observed during the loading cycle at both RT and 300oC. This signifies two scientific events upon deformation: i) enhanced kinking due to the presence of uniformly distributed GNP, and ii) controlled strain burst due to the presence of delaminated nano-laminated structure between GNP/Ti2AlC interface (inset in Fig. 4(b)). From the frequency of the sinusoidal pattern of these kickbacks (highlighted with the red dotted oval in Fig. 6(a)) during the loading cycle, it was determined that at ~ 3 nm, GNP are present within the Ti2AlC nanolaminates, represented by the vertical lines as shown in the inset in Fig. 6(a). This GNP introduce a preset defect in the composite (inset in Fig. 4(b)). The effect of this delamination is reflected on materials true elastic modulus (Et) at RT. As seen in Table 1, the Et for Ti2AlC is higher than GNP/Ti2AlC composite with similar nanohardness of 29.9 2.46 GPa at RT. But at 300oC, GNP/Ti2AlC composite exhibit a higher Et than Ti2AlC with a higher nanohardness as seen in Table 1. Though in both the materials, a surface oxidation layer of TiO2 is developed at 300oC as expected, in the case of GNP/Ti2AlC, an additional oxide species of graphene oxide can be formed at low temperature [35]. Fig. 6 also gives an insight into the materials elastic deformation characteristics at room temperature and 300oC. At room temperature Ti2AlC exhibit 22.5% more elastic deformation than the composite but at 300oC, GNP/Ti2AlC exhibit 67.12% more elastic deformation than Ti2AlC for an applied load of 7000 µN. From the results of low load, high-temperature nanoindentation, it is evident that the GNP/Ti2AlC composite provides effective oxidation and mechanical shock protection coating than Ti2AlC under similar conditions. Fig. 7 is the load-displacement curves of Ti2AlC and GNP/Ti2AlC at RT subjected to a peak load of 8 N. The information obtained here can be related to the bulk property of the material. As expected, for the applied peak load, the deformation is elastic for both the materials with an Et of 176.26 176.26
9.52 GPa for Ti2AlC and 108.32
3.48 GPa for GNP/Ti2AlC. The Et of
9.52 GPa for Ti2AlC can be compared to the theoretical bulk modulus of 166 GPa for 8
Ti2AlC [36, 37]. The presence of delaminated nanolayers again holds good to explain the reduction in Et when Ti2AlC is composited with GNP. At an applied peak load of 8 N, Ti2AlC had a nanohardness of 6.06
0.84 GPa, whereas GNP/Ti2AlC recorded a nanohardness of 5.53
0.44 GPa, which is almost comparable. Fig. 8, shows the SPM images of the indent on the sample surface. From both the images (Left) and (Right), there are no signs of crack formation on the indent boundary proving that fracture strength of Ti2AlC was not affected due to the presence of delaminated nanolayers in the GNP/Ti2AlC composite. 3.3.2 NANO-SCRATCH WEAR The polished cross-section surfaces of the sintered GNP/Ti2AlC and Ti2AlC samples were subjected to a nano-scratch test at a constant load of 7000 µN at 300oC, to quantify the improvement in the wear behavior. Fig. 9 shows the coefficient of friction (COF) vs. scratch length for both the materials tested at a constant load of 7000 µN at 300oC. Pure Ti2AlC has a low COF that varied between 0.18 – 0.23. GNP/Ti2AlC had a higher COF than pure Ti2AlC, and it varied between 0.2 – 0.35. From the nanoindentation test at similar conditions, it was revealed that compositing GNP into Ti2AlC matrix improved the hardness of the material (Table 1) due to the formation of low-temperature thin oxide layers of TiO2 and graphene oxide. Also, from the XRD analysis, it was found that GNP/Ti2AlC composite had a secondary phase of TiAl, which is an intermetallic compound formed when Ti2AlC dissociated into TiAl and TiC. The presence of excess TiC due to dissociation of Ti2AlC also increased the density of the material, confirmed by pycnometer. Hence the formation of TiO2 and graphene oxide layer at a surface level along with the presence of excess TiC in the bulk of the material improved the hardness of the novel composite resulting in higher COF. Moreover, most of the GNPs were encountered by the stylus in cross-section and not along the basal plane. GNPs along their thickness does not aid in reducing friction. The large variation in COF for both the materials along the scratch length is due to the stylus passing over peaks and valleys formed by the oxide layers as well as irregularities in the surface of the test samples. But these variations are comparatively higher in the composite and can be attributed to the stylus passing over GNP (dotted line on the inset SEM image in Fig. 9) as well as delaminated nano laminates formed due to a mismatch in the CTE of GNP and Ti2AlC.
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The wear volume loss (V) was calculated for both materials based on the theory of mechanical wear explained by Bakshi et al. [38]. Mathematical relation used to calculate the wear volume loss is shown below: ⁄
∫ ⁄
Where V is the true wear volume, l is the sliding distance during nano-scratch (10 µm), h is the instantaneous depth penetrated by the indenter and C is the cross section area factor of the Berkovich tip which varies with the value of θ and Φ as determined by Bakshi et al. [38]. By examining the starting scratch edge from the SPM image, the angles can be calculated. As seen in Fig. 10, at 300oC and for a constant load of 7000 µN, the addition of 2 vol. % GNP, resulted in the reduction of wear volume loss by 75% due to the improvement in the hardness of the material as found by indentation test. The inset in Fig. 10 is the SPM image of the scratch from which the actual scratch length (l) and scratch width were measured to calculate the wear volume loss (length and width indicated by white cross arrows). High load scratch test was also undertaken at room temperature with a constant load of 8 N for a scratch length of 200 µm. This was done in order to understand and compare the macro scale wear behavior at RT. For both the test samples, SEM of the scratched area was taken and the length and width of the scratch at their widest contour was measured as seen in Fig. 11. It is somewhat intuitive to understand that, more depth the indenter penetrates, wider the scratch width is (due to the conospherical shape of the indenter). For the applied constant load of 8 N at RT, on Ti2AlC surface, the indenter scratched the specified length of 200 µm with a maximum scratch width of 95 µm as seen in Fig. 11(a). However, a similar test on the surface of GNP/Ti2AlC showed the actual scratch length to be 160 µm with a maximum scratch width of 85 µm (Fig. 11(b)), which is 10 µm lesser than the scratch width measured for Ti2AlC. This is because GNP/Ti2AlC composite offered more resistance to the movement of stylus, indicating improved fracture strength even across the bulk when 2 vol.% GNP was composited with Ti2AlC. When damaged features were closely observed through SEM (Fig. 11(a) and 11(b)), both the materials exhibited a brittle tensile cracking behavior with chevron type cracks formed along the scratch direction. There were no signs of chipping on the edge of the scratch groove 10
indicating a more ductile type plastic deformation (inset in Fig. 11(a) and 11(b)), due to localized heating caused by the dissociation of energy upon material removal. Three-point bending test was performed to measure the improvement in the fracture energy with GNP addition. 3.3.3 FLEXURE STRENGTH AND FRACTURE ENERGY Fig. 12, is the flexural stress-strain curve obtained from the three-point bend test. It is evident that the introduction of GNP enhances Ti2AlC’s fracture strength as there was 195% improvement in the critical flexural stress and 41% improvement in the critical flexural strain. The energy absorbed by the materials before failure was calculated by integrating the area under the stress-strain curve in Fig. 12. It was found that fracture energy of Ti2AlC improved by 335 % with just 2 vol. % GNP addition. Table 2, summarizes the critical stress-strain and energy values for the materials under study. Fig. 13 shows the SEM images of the fractured cross-section obtained after the threepoint bend test. Fig. 13(a) is the cross-section of fractured Ti2AlC after the three-point bending test. Multiple bent nano laminates can be observed in the microstructure indicating the kinking mechanism, which is a well-established fracture mechanism in Ti2AlC [1, 6, 8]. In the case of GNP/Ti2AlC, the fracture mechanism also includes the GNP sliding, bending and pull-out type, as several transparent graphene flakes can be seen protruding from the fracture surface as observed in the Fig. 13(b) micrograph. The graphene flakes are highlighted within the circle as seen in Fig. 13(b). A high magnification view of the protruding graphene flakes is observed in Fig. 13(c). It was found that these flakes are sintered to the ceramic matrix (a yellow line indicating the sintered region), signifying a good bonding, thus an improvement in the fracture strength. Yang et al. [39] have reported that Ti2AlC can exhibit three different forms of end planes namely Ti2C, Al and Ti6Al [40]. Ab initio studies [39] suggests that work of adhesion for Ti-C plane is 4.55 J/m2, which is 50% higher than the surface energy of the Ti6Al ((0001) basal plane as shown in Fig. 14) [40]. Hence it is more likely that graphene (0001) basal plane is sintered to low surface energy Ti6Al basal plane of Ti2AlC. In the schematic shown (Fig.14), the hatched region is the most probable site for graphene layer to bond with Ti2AlC with minimal crystal distortion due to the similarity in the crystal structure (both are HCP). All these tests lead to the same conclusion that addition of 2 vol.% GNP to fabricate novel Ti2AlC composite by SPS 11
results in a material with improved high-temperature hardness and substantial improvement in the fracture strength over pure Ti2AlC. The novel composite material can also be a used as a protective coating, as scratch test indicated low material loss under mechanical wear at both RT and 300oC.
4 CONCLUSION A novel composite materials of 2 vol. % GNP and Ti2AlC was fabricated by SPS technique. It showed a significant improvement in the Ti2AlC’s mechanical properties at both surface level and through the bulk. GNP/Ti2AlC composite showed 335% improvement in the fracture energy over pure Ti2AlC due to GNP reinforcement. The GNP flakes were sandwiched between Ti2AlC nanolaminates, resulting in a composite with improved fracture strength. The toughening mechanism was mainly being GNP pull-out from the Ti2AlC along with the distinct kinking and sliding mechanism. At the high temperature of 300oC, the novel composite material showed ~ 12% increase in its elastic modulus with 66.4% increase in the hardness. GNP/Ti2AlC composites displayed 75% lower wear volume loss as compared to pure Ti2AlC. The improvement in the mechanical properties at high temperature is attributed to the formation of TiO2 and graphene oxide on the surface of the novel composite. The current study provides baseline results and confirmation that GNP improves fracture strength in Ti2AlC.
ACKNOWLEDGEMENT Authors would like to acknowledge the characterization facility used in Advanced Materials Engineering Research Institute (AMERI) at FIU.
DATA AVAILABILITY The raw/processed data required to reproduce these findings cannot be shared at this time due to technical or time limitations.
The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study.
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13. R. Ramaseshana A. Kakitsujib S.K. Seshadri, N.G. Nair, H. Mabuchi, H. Tsuda, T. Matsui, K. Morii, Microstructure and some properties of TiAl-Ti2AlC composites produced by reactive processing, Intermetallics, 7 (1999) 571-577. 14. P.H. Nel, Development and characterization of a carbon fibre reinforced MAX phase composite material. PhD Thesis. Stellenbosch University, South Africa, 2017. 15. F. Yang, F.T. Kong, Y.Y. Chen, S.L. Xiao, Effect of spark plasma sintering temperature on the microstructure and mechanical properties of Ti2AlC/TiAl composite, J. Alloys Compd. 496 (2010) 462-466. 16. S.R. Kulkarni, A.V.D.K.H. Wu, Synthesis of Ti2AlC by spark plasma sintering of TiAlcarbon nanotube powder mixture, J. Alloys Compd. 490 (2010) 155-159. 17. S. Iijima, Helical microtubules of graphitic carbon, Nature, 354 (1991) 56-58. 18. N.A. Koratkar, J. Suhr, Characterizing energy dissipation in single-walled carbon nanotube polycarbonate composites, Appl. Phys. Lett. 87 (2005) 063102. 19. A. Nieto, A. Bisht, D. Lahiri, C. Zhang, A. Agarwal, Graphene reinforced metal and ceramic matrix composites: A review, Intl. Mater. Rev. 65 (2017) 241-302. 20. L.S. Walker, V.R. Marotto, M.A Rafiee, N. Koratkar, E.L Corral, Toughening in graphene ceramic composites, ACS Nano, 5 (2011) 3182-3190. 21. J. Liu, H.X. Yan, M.J. Reece, K. Jiang, Toughening of zirconia/alumina composites by the addition of graphene platelets, J. Eur. Ceram Soc. 32 (2012) 4185-4193. 22. A. Nieto, Debrupa Lahiri, A. Agarwal, Synthesis and properties of bulk graphene nanoplatelets consolidated by spark plasma sintering, Carbon 50 (2012) 4065-4077. 23. A.G. Zhou, M.W. Barsoum, S. Basu, S.R. Kalidindi, T. El-Raghy, Incipient and regular kink bands in fully dense and 10 vol.% porous Ti2AlC, Acta Mater. 54 (2006) 1631-1639. 24. W.B Zhou, B.C. Mei, J.Q. Zhu, X.L. Hong, Rapid synthesis of Ti2AlC by spark plasma sintering technique, Mater Lett. 59 (2005) 131-134. 25. Y. Zou, Z.M. Sun, S. Tada, H. Hashimoto, Synthesis reactions for Ti3AlC2 through pulse discharge sintering Ti/Al4C3/TiC powder mixture, Scripta Mater. 55 (2006) 767-770. 14
26. M. Bingchu, Y. Ming, Z. Joaquin, Z. Weibing, Preparation of TiAl/Ti2AlC composites with Ti/Al/C powders by in-situ hot pressingJournal of Wuhan University of Technology - Mater. Sci. Ed. 21 (2006) 1-3. (Title of the paper not found) 27. M,W. Barsoum, D. Brodkin, T. El-Raghy, Layered machinable ceramics for hightemperature applications, Scripta Mater. 36 (1997) 535-541. 28. B. Mei, Z. Weibing, J. Zhu, X. Hong, Synthesis of high-purity Ti2AlC by spark plasma sintering (SPS) of the elemental powders, J. Mater. Sci. 39 (2004) 1471-1472. 29. M.A. Pietzka, J.C. Schuster, Summary of constitutional data on the aluminum-carbontitanium system, J. Phase Equilib. 15 (1994) 392-400. 30. A. Nieto, D. Lahiri, A. Agarwal, Graphene nanoplatelets reinforced tantalum carbide consolidated by spark plasma sintering, Mater. Sci. Eng. A, 58 (2013) 338-346. 31. W. Zhai, X. Shi, M. Wang, Z. Xu, J. Yao, S. Song, Y. Wang, Grain refinement: A mechanism for graphene nanoplatelets to reduce friction and wear of Ni3Al matrix selflubricating composites, Wear 310 (2014) 33–40 32. M. Rashad, F. Pan, M. Asif, Exploring mechanical behavior of Mg–6Zn alloy reinforced with graphene nanoplatelets, Mater. Sci. Eng. A 649 (2016) 263–269 33. M. Rashad, F. Pan, D. Lin, M. Asif, High-temperature mechanical behavior of AZ61 magnesium alloy reinforced with graphene nanoplatelets, Mater. Des. 89 (2016) 1242-1250. 34. Z. Zhang, J. Chai, H. Jin, J. Pan, L. M. Wong, S. H. Lim, M. B. Sullivan, S. J. Wang, Oxidation of Single Crystalline Ti2AlN Thin Films between 300 and 900 °C: A Perspective from Surface Analysis, J. Phys. Chem. C. 120 (2016) 18520−18528. 35. L. Liu, S. Ryu, M. R. Tomasik, E. Stolyarova, N. Jung, M. S. Hybertsen, M. L. Steigerwald, L. E. Brus, G. W. Flynn, Graphene Oxidation: Thickness-Dependent Etching and Strong Chemical Doping Nano Lett. 8[7] (2008) 1965-1970. 36. Z. Sun, R. Ahuja, S. Li, J.M. Schneider, Structure and bulk modulus of M2AlC (M=Ti, V, and Cr), Appl. Phys. Lett. 83 (2003) 899-871.
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Fig. 1. Sintered Ti2AlC and Ti2AlC-GNP samples used for three-point bending test.
16
(a)
(b)
17
(c)
(d) Fig. 2. SEM images of powders: (a) As-received Ti2AlC, (b) Nano particles observed on the asreceived Ti2AlC, (c) Pure GNP and (d) Ti2AlC/GNP.
18
(a)
(b) Fig. 3. SEM images of sintered Ti2AlC: (a) Random grain geometry observed in the Ti2AlC, (b) Elongated plate-like nano-laminates of Ti2AlC. 19
(a)
(b) Fig. 4. SEM images of GNP/Ti2AlC: (a) Elongated plate-like nano-laminated structure with a large flake of wrinkled and transparent GNP, (b) GNP flakes sandwiched between the Ti2AlC nano-laminates. Inset is showing the magnified highlighted area.
20
Fig. 5. X-ray diffraction pattern of as-received Ti2AlC powder, sintered Ti2AlC, and Ti2AlCGNP.
21
(a)
(b) Fig. 6. Load vs. displacement curves from nanoindentation performed at (a) RT and (b) 300 C. 22
Fig. 7. Load vs. displacement curves from high load (8 N) nanoindentation performed at RT.
23
Fig. 8. SPM images of the high load (8 N) indent. (Left) Ti2AlC, (Right) GNP/Ti2AlC.
24
Fig. 9. COF vs. lateral displacement curves obtained by a nano-scratch test performed at 300 C with a load of 7000 µN.
25
Fig. 10. Wear volume loss after low load (7000 µN) high temperature (300 C) scratch test. (Inset) SPM image of the scratch profile used to calculate the wear volume loss.
26
(a)
(b) Fig. 11. SEM images of the scratch obtained at RT with an 8 N load (a) Ti2AlC, (b) GNP/Ti2AlC. 27
400
Flexural Stress (MPa)
350
GNP/Ti2AlC
300 250 200 150 100
Ti2AlC
50 0 0.000
0.001
0.002
0.003
0.004
0.005
0.006
0.007
Flexural Strain
Fig. 12. Flexural stress-strain curves obtained from three-point bending test.
28
(a)
(b) 29
(c) Fig. 13. SEM images of the fractures surface after three point bending test: (a) Ti2AlC, (b) GNP/ Ti2AlC, and (c) SEM image showing GNP sintered with Ti2AlC nano-laminates.
30
Fig. 14. Schematic is representing the bonding site of 2D graphene with 3D Ti2AlC.
Table 1. Nano mechanical properties of Ti2AlC and GNP/Ti2AlC True Elastic
Material
Load
Temperature (C)
Ti2AlC
7000 µN
25
553.27
88.4
29.9
2.46
GNP/ Ti2AlC
7000 µN
25
431.48
59.04
29.35
7.51
Ti2AlC
7000 µN
300
213.78
48.67
8.47 3.02
GNP/ Ti2AlC
7000 µN
300
240.14
19.37
Ti2AlC
8N
25
176.26
9.52
14.09 1.94 6.06 0.84
GNP/ Ti2AlC
8N
25
108.32
3.5
5.53
31
Modulus (Et, GPa)
Hardness (H, GPa)
0.44
Table 2. Bulk mechanical properties obtained from three-point bending test Material
Flexural stress
Flexural Strain
(MPa)
Energy adsorbed (MJ/m3)
Ti2AlC
127.67
0.0045
0.17
GNP/ Ti2AlC
376.78
0.0063
0.74
32
PII: DOI: Reference:
S0921-5093(18)30645-2 https://doi.org/10.1016/j.msea.2018.05.006 MSA36444
To appear in: Materials Science & Engineering A Received date: 20 February 2018 Revised date: 10 April 2018 Accepted date: 3 May 2018 Cite this article as: Tony Thomas, Cheng Zhang, Ashutosh Sahu, Pranjal Nautiyal, Archana Loganathan, Tapas Laha, Benjamin Boel and Arvind Agarwal, Effect of Graphene Reinforcement on the Mechanical Properties of Ti 2AlC Ceramic Fabricated by Spark Plasma Sintering, Materials Science & Engineering A, https://doi.org/10.1016/j.msea.2018.05.006 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Effect of Graphene Reinforcement on the Mechanical Properties of Ti2AlC Ceramic Fabricated by Spark Plasma Sintering Tony Thomasa, Cheng Zhanga, Ashutosh Sahub, Pranjal Nautiyala, Archana Loganathana, Tapas Lahab, Benjamin Boela , Arvind Agarwala,* a
Plasma Forming Laboratory, Department of Mechanical and Materials Engineering, Florida International University, Miami, FL 33174, USA b
*
Department of Metallurgical and Materials Engineering, Indian Institute of Technology, Kharagpur, India
Corresponding author: [email protected]
Abstract Composite of Ti2AlC and 2 vol.% Graphene nanoplatelets (GNP) was successfully fabricated by spark plasma sintering (SPS) technique to augment the fracture strength of Ti2AlC. Microscopic analysis revealed the reinforcement of GNP within the Ti2AlC ceramic matrix. As the consequence of positive reinforcement, there was a 335% improvement in the fracture energy of the GNP/Ti2AlC composite, 195% increase in the flexural stress and 41% increase in the flexural strain. From nano-scratch analysis at low load (7000 µN) and high temperature (300 °C), it was found that the GNP/Ti2AlC composites wear volume loss minimized by 75% as compared to pure Ti2AlC. Post microscopic analysis of the fractured surface gave insight into the improvement in fracture strength of the material by sheet sliding, crack deflection but mainly by the pull out mechanism of graphene from the Ti2AlC matrix. It was also found that GNP was sintered to Ti2AlC platelets thus improving the overall mechanical properties of the GNP/Ti2AlC composite.
Keywords: Ti2AlC; Graphene; Spark Plasma Sintering; Nano-scratch; Fracture Energy
1
1 INTRODUCTION Ti2AlC is a class of ductile MAX-phase ceramic material group with the general molecular formula Mn+1AXn (n = 1, 2, 3….), where M is an early transition element, A is an element from the group ‘A’ of the periodic table and X is either nitride or carbide. Some of the important characteristics of MAX-phase ceramics are low density, high stiffness, machinability, excellent thermal and electrical conductivity and they even exhibit some plasticity at elevated temperature [1-4]. Ti2AlC has a low density of 4.1 g/cm3 and a high electrical conductivity of 3x106 Ω-1m-1. The presence of kink bands in the basal plane restricts the propagation of cracks along that plane, making these materials more fracture resistant with a fracture toughness of 6.5 MPa.m1/2 [5-8]. All these unique material properties of Ti2AlC qualifies this engineering ceramic for antiballistic body armor, oxidation, and corrosion resistant protective coating application. Various efforts have been made to synthesis phase pure Ti2AlC by combustion and reactive sintering [9-11]. There is also a report on synthesizing Ti2AlC using various carbon source like carbon black, graphite and even carbon fibers [12]. Most of the efforts to improve the mechanical properties of Ti2AlC was compositing it with an intermetallic compound like TiAl due to its high melting point, low density, oxidation and corrosion resistance. Ramseshan et al. [13] conducted one of the earliest research to produce TiAl/Ti2AlC composite by reactive sintering. TiAl/Ti2AlC composite exhibited high strength of 800 and 400 MPa at ambient temperature and 1173 K respectively. Ramseshan et al. also reported that the composite’s fracture toughness value reached to 17.8 MPa m1/2. Recently Nel [14] reported a unique carbon fiber reinforced Ti2AlC prepared by SPS. The as-fabricated ceramic composite did not exhibit any improvement in the mechanical properties due to the presence of pores from poor infiltration of Ti2AlC into carbon fiber weave. There have also been reports of carbon nanotubes (CNT) used as the carbon source in the synthesis of Ti2AlC/intermetallic composites with enhanced mechanical properties. Yang et al. [15] fabricated TiAl reinforced Ti2AlC composite with ultrafine microstructure by SPS. They used carbon nanotubes (CNT) as the carbon source in the preparation of Ti2AlC/TiAl composite which had a compressive strength of 2058 MPa and a hardness of 6.12 GPa. Kulkarni et al. [16] synthesized Ti2AlC by SPS of TiAl and CNTs as a carbon source and observed the improvement in the density of the SPS sample as well as an increase in the hardness of up to 5.08 GPa. CNTs as a material is used as a reinforcement for over a decade in a variety of ceramic matrices to 2
enhance the properties like strength, stiffness, and toughness [17, 18]. Like CNT, Graphene a 2D material has similar material properties but has an advantage of the higher specific surface area and easier dispersion into the matrix. Graphene has been used to reinforce ceramic matrices [19]. Walker et al. [20] synthesized Si3N4 matrix with 1.5 vol.% graphene and saw an improvement of 235% in fracture toughness. Liu et al. [21] synthesized zirconia toughened alumina (ZTA) nanocomposites reinforced with graphene nanoplatelets (GNPs) using SPS and reported 40% increase in the fracture toughness of the material for as little as 0.8 vol.% graphene loading. One of the most common phenomena observed was enhanced toughening by mechanisms such as bridging, pullout and crack deflection on the fractured surfaces with graphene present in between the grain boundaries of the ceramic. It is evident that graphene can efficiently enhance the fracture strength when reinforced into a ceramic matrix, that too at a very low material loading rate. Research on graphene ceramic composites has shown promising results, but is still unexploited, in the improvement of MAX-phase material properties [19]. Taking this as the motivation, in this study, graphene nanoplatelets (GNP)/Ti2AlC composite was fabricated by SPS, and the effect of GNP reinforcement on the Ti2AlC’s mechanical properties was analyzed. GNP/Ti2AlC composite fabricated for this study is a unique ceramic composite and is the first time any researcher has attempted to enhance the fracture toughness of Ti2AlC compositing with carbonbased nano particle. There have been reports on synthesizing Ti2AlC using CNTs as a carbon source [20], but CNTs or GNPs were never used as a reinforcement material to enhance the fracture toughness of Ti2AlC. In the current study, an effort has been made to enhance mechanical properties of Ti2AlC by reinforcing with 2 vol. % of GNP into the ceramic matrix. Mechanical properties like hardness and elastic modulus were characterized by nanoindentation, wear by nano-scratch, and fracture energy by three-point bending tests. The fracture mechanism of the GNP/Ti2AlC composite fabricated by SPS was studied by analyzing the micrographs of the fracture surface and compared with Ti2AlC.
3
2 EXPERIMENTAL PROCEDURE 2.1 SAMPLE PREPARATION The Ti2AlC/GNPs composite used in this study composed of Ti2AlC powder (Maxthal 211, Sandvik Materials Technology) and 2 vol.% GNP powder (XG Science, Lansing, MI, USA with an average particle size of 15 µm and thickness of 6 – 14 nm). The composite powder mixture was prepared by first ultrasonicating 2 vol.% of GNP powder in 500 ml of acetone (PHARMCO-AAPER, UV Grade) for 90 mins followed by adding as-received Ti2AlC powder and ultrasonicating the entire mixture for 30 minutes. The solution mixture was dried overnight at 75oC. The lumps of Ti2AlC/GNPs composite mixture was broken down into fine particles using mortar and pestle. Both Ti2AlC and GNP/Ti2AlC composite test samples were fabricated by SPS technique. The powder mixture was filled into a graphite die sandwiched between two graphite punches. The powder was wrapped in a graphite foil for easy extraction of SPS samples and improved current flow. SPS process was performed in vacuum condition (Pressure < 10 Pa). The powder in the graphite die was initially heated to 300oC and held there for 10 mins. After preheating stage, the powder was sintered at 1200oC for 7 minutes at a heating rate of 100oC/min at 60 MPa of pressure.
2.1 PHASE CHARACTERIZATION X-ray diffractometer (XRD. Siemens D500) technique was employed to analyze the phase purity of as-received Ti2AlC powder, sintered Ti2AlC and GNP/Ti2AlC pellets with Cu Kα (λ = 1.542 Å) x-ray source.
2.2 MICROSTRUCTURAL CHARACTERIZATION The microstructure of the as-received Ti2AlC powder, Ti2AlC/GNPs powder mixture, fractured Ti2AlC and GNP/Ti2AlC pellets were analyzed using JEOL JSM-6330F field emission scanning electron microscope (SEM). For SEM imaging, bulk samples were polished using alumina polishing media.
2.3 MECHANICAL CHARACTERIZATION The density of the SPS samples was measured using a gas pycnometer (AccuPyc ӀӀ 1340). Nanoindentation and nano-scratch tests were performed on the top surface and crosssection of the bulk, metallographically polished samples respectively using Hysitron 4
Triboindenter (TI 900 and TI 950, Hysitron Inc, Minneapolis, USA). The quasistatic nanoindentation was performed at room temperature (25oC) and high temperature (300oC) at a low load of 7000 µN. A standard Berkovich tip of 100 nm tip radius was used as the indenter throughout this study, with loading cycle of 10 – 3 – 10 seconds (loading – dwell – unloading). The RT quasistatic indentation tests were performed on a sample mounted in the epoxy resin. Epoxy mounting was avoided for high temperatures nanoindentation tests due to softening of the resin. High load (8N) nanoindentation test was also performed at RT to evaluate the elastic modulus and nanohardness at multi-length scale. A minimum of 30 indents was performed for both the samples. Nano-scratch test was performed at 300oC with a peak force of 7000 µN using a 60o conospherical diamond tip at constant loading with a scratch length of 10 µm. In-situ scanning probe microscopy (SPM) was used to image the indents and scratch. The images were processed in a 3-D processing image software (SPIPTM, version 5.1, Image Metrology, A/S, Denmark). Nano-scratch was also performed at a relatively high load of 8N at room temperature with a scratch length of 200 µN. SEM images of the scratch profile were analyzed to understand the wear behavior. Three-point flexure tests were performed on 15 x 1 x 0.8 mm slender rectangular specimens cut out of the SPS pellets using wire EDM as shown in Fig. 1. MTI SEM tester (Albany, USA) was used to conduct the three-point bending test. A 445 N capacity load cell was used for the tests. Test samples were loaded to the rate of 0.1 mm/min until the point of failure. 3 RESULT AND DISCUSSION 3.1 MORPHOLOGY STUDY 3.1.1 POWDER MORPHOLOGY Fig. 2 shows the morphology of the powder used for preparing SPS test samples in this study. From Fig. 2(a), the as-received Ti2AlC showed irregular particle shape and size ranging from 100 nm – 20 µm. Fig. 2(b) clearly shows nano sized particles of Ti2AlC (particles within yellow dash) smeared on micron sized Ti2AlC. Fig. 2(c) is the SEM of the as-received GNP powder. On an average, GNP has about 20 graphene sheets and each layer has an approximate thickness of ~0.35 nm. As mentioned in section 2.1, the average diameter of the GNP seen in Fig. 2(c) is 15 µm, with a relative surface area of 150 m2/g. A detailed physical and chemical analysis of the powder used in this work is presented elsewhere [22]. The distribution of GNP 5
flakes is evident in low magnification SEM image as shown in Fig. 2(d) (Represented by yellow dashed circle). As seen in Fig. 2(d), GNP flakes are smeared with nano-particles of Ti2AlC giving a perspective of GNP’s surface area that can provide a platform for efficient reinforcement in the matrix present.
3.1.2 MICROSTRUCTURE OF SINTERED Ti2AlC Fig. 3 shows the SEM images of the Ti2AlC test samples fabricated by SPS. The sample morphology consists of irregularly shaped and non-uniformly distributed grains which are densely packed with no signs of pores. Some amount of void observed in the micrograph can be attributed to the material removal during sample polishing. The bulk sample had a relative density of 99.5%. When Fig. 3(a) is closely observed, three distinct microstructures are seen. They comprise of elongated plate-like nano-laminated grains of Ti2AlC (12 – 32 µm) as seen in Fig. 3(b) with an average thickness of ~ 230 nm, fine and bright hexagonal crystal grains of TiC (1 – 3 µm) and flat plate-like non-uniformly distributed structures of Ti3AlC2 (5 – 8 µm). It should be noted that irregular particle shape and size is important to obtain a highly dense monolithic ceramic structure which directly influences its mechanical properties. The test sample of Ti2AlC had an average particle size of ~ 7 µm. There are other reports indicating similar grain morphology in bulk SPS Ti2AlC [23 – 24]. Fig. 4, is the SEM image of the sintered GNP/ Ti2AlC composite. The first thing to be noticed is, the majority of the particles exhibit elongated plate-like morphology with an average length of ~ 10 µm which is relatively smaller than Ti2AlC grains in Fig. 3(a). Addition of GNP has induced grain refinement by ~ 55%. The relative density of this highly dense bulk sample was more than 101.83% of theoretical density. This is due to the dissociation of Ti2AlC into TiAl and TiC as confirmed by the XRD pattern shown in Fig.5. The formation of excess TiC resulted in a density more than the theoretical density of Ti2AlC and close to the material density of TiC (4.93 g/cm3) [25]. Some thin GNP flakes can be seen across the microstructure (indicated by the yellow arrow in Fig. 4(a)). But the majority of the GNP is sandwiched between Ti2AlC nanolaminates as shown in Fig. 4(a) (indicated by the yellow boundary). This signifies successful reinforcement of GNP into the Ti2AlC ceramic matrix. In the inset shown in Fig. 4(b), delamination is seen between the layers of GNP flake and Ti2AlC laminates. This is due to the 6
mismatch in the thermal properties of the materials. Nevertheless, the presence of such distinctive artifact (dark region between Ti2AlC nano-laminates) is a clear indication of GNP reinforced into the material matrix. Fig. 5, shows the XRD analysis of the as-received Ti2AlC powder, sintered Ti2AlC, and GNP/Ti2AlC pellets. As-received Ti2AlC powder is a biphasic mixture of Ti2AlC (JCPDS 00029-0095) and TiC (JCPDS 03-065-0971). The exclusion of TiC is inevitable as Gibbs free energy for the formation (ΔG) of TiC (-300 kJ/mol) [26] is much lower than ΔG of Ti2AlC (-54.8 kJ/mol) [27]. As a result, TiC is easily formed and stays as a stable phase up to temperature as high as 1550oC. The XRD analysis of sintered Ti2AlC (Fig. 5) shows an additional inclusion of Ti3AlC2 (JCPDS 00-052-0875) apart from Ti2AlC and TiC. Similar studies on spark plasma sintering of Ti2AlC has demonstrated the formation of Ti3AlC2 due to the reaction between Ti2AlC and TiC at elevated sintering temperature [28]. For GNP/Ti2AlC composite, XRD analysis (Fig. 5) revealed the formation of TiAl (JCPDS 00-065-0428) in addition to Ti3AlC2. TiAl is an intermediate compound formed during the formation and dissociation of Ti2AlC [12, 29]. Agarwal et al. [30] in their work on SPS densification of GNP reinforced TaCx composite, reported that the composite achieved a density of 99% of the theoretical density of TaCx and also observed 60% reduction in the grain size. GNP acts as a sintering aid by reducing the melting temperature of the host material, and this synergetic effect of GNP is well reported [31-33]. Pietzka et al. [29] conducted a thermodynamic analysis of the formation mechanism of Ti2AlC and found that at a higher temperature, Ti2AlC dissociates into TiC and TiAl. The addition of GNP in the Ti2AlC may have decreased the decomposition temperature of Ti2AlC along with the melting temperature thus dissociating some amount of Ti2AlC, into TiC and TiAl.
3.3 NANO-MECHANICAL CHARACTERIZATION 3.3.1 QUASISTATIC NANOINDENTATION Nanoindentation studies were conducted at low load of 7000 µN (to establish the nanoscale length mechanical properties) and high load of 8 N (to establish macro scale length mechanical properties) on the polished cross-section of the test samples. Low load nanoindentation tests were conducted at room temperature (RT) and high temperature of 300oC. Fig. 6(a) and 6(b), shows a generic load-displacement curves of Ti2AlC and GNP/Ti2AlC at RT and 300oC respectively, when subjected to a peak load of 7000 µN. 7
The load-displacement curves of Ti2AlC at room temperature (Fig. 6(a)) show signs of kickback due to strain burst during the loading cycle. This occurs upon shearing of inter planar nanolaminates of Ti2AlC, thus deviating the crack propagation perpendicular to the applied load and limiting further penetration, which is an inherent property of MAX-phase materials (selfhealing and self-lubricating). At 300oC, the load-displacement curve is smooth for Ti2AlC as seen in Fig. 6(b). This is the result of surface oxidation of Ti to TiO2 at low temperature [34]. For GNP/Ti2AlC, frequent kickbacks in the loading curve are observed during the loading cycle at both RT and 300oC. This signifies two scientific events upon deformation: i) enhanced kinking due to the presence of uniformly distributed GNP, and ii) controlled strain burst due to the presence of delaminated nano-laminated structure between GNP/Ti2AlC interface (inset in Fig. 4(b)). From the frequency of the sinusoidal pattern of these kickbacks (highlighted with the red dotted oval in Fig. 6(a)) during the loading cycle, it was determined that at ~ 3 nm, GNP are present within the Ti2AlC nanolaminates, represented by the vertical lines as shown in the inset in Fig. 6(a). This GNP introduce a preset defect in the composite (inset in Fig. 4(b)). The effect of this delamination is reflected on materials true elastic modulus (Et) at RT. As seen in Table 1, the Et for Ti2AlC is higher than GNP/Ti2AlC composite with similar nanohardness of 29.9 2.46 GPa at RT. But at 300oC, GNP/Ti2AlC composite exhibit a higher Et than Ti2AlC with a higher nanohardness as seen in Table 1. Though in both the materials, a surface oxidation layer of TiO2 is developed at 300oC as expected, in the case of GNP/Ti2AlC, an additional oxide species of graphene oxide can be formed at low temperature [35]. Fig. 6 also gives an insight into the materials elastic deformation characteristics at room temperature and 300oC. At room temperature Ti2AlC exhibit 22.5% more elastic deformation than the composite but at 300oC, GNP/Ti2AlC exhibit 67.12% more elastic deformation than Ti2AlC for an applied load of 7000 µN. From the results of low load, high-temperature nanoindentation, it is evident that the GNP/Ti2AlC composite provides effective oxidation and mechanical shock protection coating than Ti2AlC under similar conditions. Fig. 7 is the load-displacement curves of Ti2AlC and GNP/Ti2AlC at RT subjected to a peak load of 8 N. The information obtained here can be related to the bulk property of the material. As expected, for the applied peak load, the deformation is elastic for both the materials with an Et of 176.26 176.26
9.52 GPa for Ti2AlC and 108.32
3.48 GPa for GNP/Ti2AlC. The Et of
9.52 GPa for Ti2AlC can be compared to the theoretical bulk modulus of 166 GPa for 8
Ti2AlC [36, 37]. The presence of delaminated nanolayers again holds good to explain the reduction in Et when Ti2AlC is composited with GNP. At an applied peak load of 8 N, Ti2AlC had a nanohardness of 6.06
0.84 GPa, whereas GNP/Ti2AlC recorded a nanohardness of 5.53
0.44 GPa, which is almost comparable. Fig. 8, shows the SPM images of the indent on the sample surface. From both the images (Left) and (Right), there are no signs of crack formation on the indent boundary proving that fracture strength of Ti2AlC was not affected due to the presence of delaminated nanolayers in the GNP/Ti2AlC composite. 3.3.2 NANO-SCRATCH WEAR The polished cross-section surfaces of the sintered GNP/Ti2AlC and Ti2AlC samples were subjected to a nano-scratch test at a constant load of 7000 µN at 300oC, to quantify the improvement in the wear behavior. Fig. 9 shows the coefficient of friction (COF) vs. scratch length for both the materials tested at a constant load of 7000 µN at 300oC. Pure Ti2AlC has a low COF that varied between 0.18 – 0.23. GNP/Ti2AlC had a higher COF than pure Ti2AlC, and it varied between 0.2 – 0.35. From the nanoindentation test at similar conditions, it was revealed that compositing GNP into Ti2AlC matrix improved the hardness of the material (Table 1) due to the formation of low-temperature thin oxide layers of TiO2 and graphene oxide. Also, from the XRD analysis, it was found that GNP/Ti2AlC composite had a secondary phase of TiAl, which is an intermetallic compound formed when Ti2AlC dissociated into TiAl and TiC. The presence of excess TiC due to dissociation of Ti2AlC also increased the density of the material, confirmed by pycnometer. Hence the formation of TiO2 and graphene oxide layer at a surface level along with the presence of excess TiC in the bulk of the material improved the hardness of the novel composite resulting in higher COF. Moreover, most of the GNPs were encountered by the stylus in cross-section and not along the basal plane. GNPs along their thickness does not aid in reducing friction. The large variation in COF for both the materials along the scratch length is due to the stylus passing over peaks and valleys formed by the oxide layers as well as irregularities in the surface of the test samples. But these variations are comparatively higher in the composite and can be attributed to the stylus passing over GNP (dotted line on the inset SEM image in Fig. 9) as well as delaminated nano laminates formed due to a mismatch in the CTE of GNP and Ti2AlC.
9
The wear volume loss (V) was calculated for both materials based on the theory of mechanical wear explained by Bakshi et al. [38]. Mathematical relation used to calculate the wear volume loss is shown below: ⁄
∫ ⁄
Where V is the true wear volume, l is the sliding distance during nano-scratch (10 µm), h is the instantaneous depth penetrated by the indenter and C is the cross section area factor of the Berkovich tip which varies with the value of θ and Φ as determined by Bakshi et al. [38]. By examining the starting scratch edge from the SPM image, the angles can be calculated. As seen in Fig. 10, at 300oC and for a constant load of 7000 µN, the addition of 2 vol. % GNP, resulted in the reduction of wear volume loss by 75% due to the improvement in the hardness of the material as found by indentation test. The inset in Fig. 10 is the SPM image of the scratch from which the actual scratch length (l) and scratch width were measured to calculate the wear volume loss (length and width indicated by white cross arrows). High load scratch test was also undertaken at room temperature with a constant load of 8 N for a scratch length of 200 µm. This was done in order to understand and compare the macro scale wear behavior at RT. For both the test samples, SEM of the scratched area was taken and the length and width of the scratch at their widest contour was measured as seen in Fig. 11. It is somewhat intuitive to understand that, more depth the indenter penetrates, wider the scratch width is (due to the conospherical shape of the indenter). For the applied constant load of 8 N at RT, on Ti2AlC surface, the indenter scratched the specified length of 200 µm with a maximum scratch width of 95 µm as seen in Fig. 11(a). However, a similar test on the surface of GNP/Ti2AlC showed the actual scratch length to be 160 µm with a maximum scratch width of 85 µm (Fig. 11(b)), which is 10 µm lesser than the scratch width measured for Ti2AlC. This is because GNP/Ti2AlC composite offered more resistance to the movement of stylus, indicating improved fracture strength even across the bulk when 2 vol.% GNP was composited with Ti2AlC. When damaged features were closely observed through SEM (Fig. 11(a) and 11(b)), both the materials exhibited a brittle tensile cracking behavior with chevron type cracks formed along the scratch direction. There were no signs of chipping on the edge of the scratch groove 10
indicating a more ductile type plastic deformation (inset in Fig. 11(a) and 11(b)), due to localized heating caused by the dissociation of energy upon material removal. Three-point bending test was performed to measure the improvement in the fracture energy with GNP addition. 3.3.3 FLEXURE STRENGTH AND FRACTURE ENERGY Fig. 12, is the flexural stress-strain curve obtained from the three-point bend test. It is evident that the introduction of GNP enhances Ti2AlC’s fracture strength as there was 195% improvement in the critical flexural stress and 41% improvement in the critical flexural strain. The energy absorbed by the materials before failure was calculated by integrating the area under the stress-strain curve in Fig. 12. It was found that fracture energy of Ti2AlC improved by 335 % with just 2 vol. % GNP addition. Table 2, summarizes the critical stress-strain and energy values for the materials under study. Fig. 13 shows the SEM images of the fractured cross-section obtained after the threepoint bend test. Fig. 13(a) is the cross-section of fractured Ti2AlC after the three-point bending test. Multiple bent nano laminates can be observed in the microstructure indicating the kinking mechanism, which is a well-established fracture mechanism in Ti2AlC [1, 6, 8]. In the case of GNP/Ti2AlC, the fracture mechanism also includes the GNP sliding, bending and pull-out type, as several transparent graphene flakes can be seen protruding from the fracture surface as observed in the Fig. 13(b) micrograph. The graphene flakes are highlighted within the circle as seen in Fig. 13(b). A high magnification view of the protruding graphene flakes is observed in Fig. 13(c). It was found that these flakes are sintered to the ceramic matrix (a yellow line indicating the sintered region), signifying a good bonding, thus an improvement in the fracture strength. Yang et al. [39] have reported that Ti2AlC can exhibit three different forms of end planes namely Ti2C, Al and Ti6Al [40]. Ab initio studies [39] suggests that work of adhesion for Ti-C plane is 4.55 J/m2, which is 50% higher than the surface energy of the Ti6Al ((0001) basal plane as shown in Fig. 14) [40]. Hence it is more likely that graphene (0001) basal plane is sintered to low surface energy Ti6Al basal plane of Ti2AlC. In the schematic shown (Fig.14), the hatched region is the most probable site for graphene layer to bond with Ti2AlC with minimal crystal distortion due to the similarity in the crystal structure (both are HCP). All these tests lead to the same conclusion that addition of 2 vol.% GNP to fabricate novel Ti2AlC composite by SPS 11
results in a material with improved high-temperature hardness and substantial improvement in the fracture strength over pure Ti2AlC. The novel composite material can also be a used as a protective coating, as scratch test indicated low material loss under mechanical wear at both RT and 300oC.
4 CONCLUSION A novel composite materials of 2 vol. % GNP and Ti2AlC was fabricated by SPS technique. It showed a significant improvement in the Ti2AlC’s mechanical properties at both surface level and through the bulk. GNP/Ti2AlC composite showed 335% improvement in the fracture energy over pure Ti2AlC due to GNP reinforcement. The GNP flakes were sandwiched between Ti2AlC nanolaminates, resulting in a composite with improved fracture strength. The toughening mechanism was mainly being GNP pull-out from the Ti2AlC along with the distinct kinking and sliding mechanism. At the high temperature of 300oC, the novel composite material showed ~ 12% increase in its elastic modulus with 66.4% increase in the hardness. GNP/Ti2AlC composites displayed 75% lower wear volume loss as compared to pure Ti2AlC. The improvement in the mechanical properties at high temperature is attributed to the formation of TiO2 and graphene oxide on the surface of the novel composite. The current study provides baseline results and confirmation that GNP improves fracture strength in Ti2AlC.
ACKNOWLEDGEMENT Authors would like to acknowledge the characterization facility used in Advanced Materials Engineering Research Institute (AMERI) at FIU.
DATA AVAILABILITY The raw/processed data required to reproduce these findings cannot be shared at this time due to technical or time limitations.
The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study.
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Fig. 1. Sintered Ti2AlC and Ti2AlC-GNP samples used for three-point bending test.
16
(a)
(b)
17
(c)
(d) Fig. 2. SEM images of powders: (a) As-received Ti2AlC, (b) Nano particles observed on the asreceived Ti2AlC, (c) Pure GNP and (d) Ti2AlC/GNP.
18
(a)
(b) Fig. 3. SEM images of sintered Ti2AlC: (a) Random grain geometry observed in the Ti2AlC, (b) Elongated plate-like nano-laminates of Ti2AlC. 19
(a)
(b) Fig. 4. SEM images of GNP/Ti2AlC: (a) Elongated plate-like nano-laminated structure with a large flake of wrinkled and transparent GNP, (b) GNP flakes sandwiched between the Ti2AlC nano-laminates. Inset is showing the magnified highlighted area.
20
Fig. 5. X-ray diffraction pattern of as-received Ti2AlC powder, sintered Ti2AlC, and Ti2AlCGNP.
21
(a)
(b) Fig. 6. Load vs. displacement curves from nanoindentation performed at (a) RT and (b) 300 C. 22
Fig. 7. Load vs. displacement curves from high load (8 N) nanoindentation performed at RT.
23
Fig. 8. SPM images of the high load (8 N) indent. (Left) Ti2AlC, (Right) GNP/Ti2AlC.
24
Fig. 9. COF vs. lateral displacement curves obtained by a nano-scratch test performed at 300 C with a load of 7000 µN.
25
Fig. 10. Wear volume loss after low load (7000 µN) high temperature (300 C) scratch test. (Inset) SPM image of the scratch profile used to calculate the wear volume loss.
26
(a)
(b) Fig. 11. SEM images of the scratch obtained at RT with an 8 N load (a) Ti2AlC, (b) GNP/Ti2AlC. 27
400
Flexural Stress (MPa)
350
GNP/Ti2AlC
300 250 200 150 100
Ti2AlC
50 0 0.000
0.001
0.002
0.003
0.004
0.005
0.006
0.007
Flexural Strain
Fig. 12. Flexural stress-strain curves obtained from three-point bending test.
28
(a)
(b) 29
(c) Fig. 13. SEM images of the fractures surface after three point bending test: (a) Ti2AlC, (b) GNP/ Ti2AlC, and (c) SEM image showing GNP sintered with Ti2AlC nano-laminates.
30
Fig. 14. Schematic is representing the bonding site of 2D graphene with 3D Ti2AlC.
Table 1. Nano mechanical properties of Ti2AlC and GNP/Ti2AlC True Elastic
Material
Load
Temperature (C)
Ti2AlC
7000 µN
25
553.27
88.4
29.9
2.46
GNP/ Ti2AlC
7000 µN
25
431.48
59.04
29.35
7.51
Ti2AlC
7000 µN
300
213.78
48.67
8.47 3.02
GNP/ Ti2AlC
7000 µN
300
240.14
19.37
Ti2AlC
8N
25
176.26
9.52
14.09 1.94 6.06 0.84
GNP/ Ti2AlC
8N
25
108.32
3.5
5.53
31
Modulus (Et, GPa)
Hardness (H, GPa)
0.44
Table 2. Bulk mechanical properties obtained from three-point bending test Material
Flexural stress
Flexural Strain
(MPa)
Energy adsorbed (MJ/m3)
Ti2AlC
127.67
0.0045
0.17
GNP/ Ti2AlC
376.78
0.0063
0.74
32