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The effect of sintering time, temperature, and graphene addition on the hardness and microstructure of aluminum composites
Abstract
Currently, graphene is used in aluminum matrix composite manufacturing due to its superior mechanical properties. However, few detailed studies exist on the effect of the process conditions such as sintering temperature (TS), time (tS), and a number of graphene nanoplatelets. Therefore, the effects of different sintering times (tS = 60, 120, 180, 300 min), sintering temperatures (TS = 550, 600, 630℃), and graphene addition (0.1, 0.3, 0.5 wt%) on apparent density and hardness were reported in detail in this study. The crystal structure and microstructure of fabricated composites by powder metallurgy method were examined with X-ray diffractometer and scanning electron microscopy. Apparent density and mechanical properties were tested by density meter and micro Vickers hardness tester. The results indicated that the best sintering time, sintering temperature, and graphene addition were determined to be 180 min, 630℃, and 0.1 wt%, respectively, for the best hardness of composite. The hardness of composite increased from 38 to 57 HV when compared with pure aluminum under the best process conditions.
... At higher temperatures, the sintering density of composites decreased for all compositions and durations based on factors including grain growth, thermal expansion, and increased porosity. [43][44][45] In the current study, increasing the sintering temperature (up to 600°C) causes a rise in density and a decrease in porosity due to the superiority of diffusion over thermal expansion and grain growth, but increasing the temperature further (630°C and higher) causes the opposite, i.e., density declines and porosity rises due to the superiority of thermal expansion and grain growth over diffusion. 44,46,47 ...
... [43][44][45] In the current study, increasing the sintering temperature (up to 600°C) causes a rise in density and a decrease in porosity due to the superiority of diffusion over thermal expansion and grain growth, but increasing the temperature further (630°C and higher) causes the opposite, i.e., density declines and porosity rises due to the superiority of thermal expansion and grain growth over diffusion. 44,46,47 ...
... Higher temperatures, on the other hand, provoke grain growth and a loss of hardness. 44 This pattern of rising and decreasing hardness is also presented in literature. 40 The sintering time analysis is presented in Fig. 4b. ...
The purpose of this article is to investigate the effect of various process parameters such as compaction pressure, sintering temperature, and time on the physio-mechanical properties of a powder metallurgy-fabricated composite made of pure aluminium/alumina. Temperatures (580°C, 600°C, and 630°C), periods (1.5, 2, and 2.5 hr), compacting loads (30KN-65KN), and alumina percentages (2, 4, 6, and 8weight percent) are all considered. X-ray diffraction (XRD) and X-ray fluorescence spectroscopy (XRF) studies are carried out to determine the phases present and their proportions. Crystallite size study is performed using XRD data, and the Al+4 weight % alumina composite has the smallest size of any composite tested. For optimization, sintering density, porosity, and microhardness are calculated. Scanning electron microscopy (SEM) is used to analyse the different microstructures. At 600°C, 2 hr of operating time, and 4weight% alumina additions, the highest sintering density and microhardness are found.
... The film oxides high porosity allowed the Cl − ions to pass and react with the substrate, which ultimately degraded the corrosion performance and resulted in a greater corrosion rate. This is due to the fact that density, porosity, and grain growth are all highly associated with Ti content and sintering time [27]. The results demonstrate that sintering time is an important parameter in determining sintering temperature due to the reliance of diffusion on sintering time. ...
... This is due to the fact that sintering time increases and aluminum metal diffuses, decreasing the ability of the alloys surface to resist corrosion. It was previously reported by Gurbuz et al. [27] that the diffusion of particles depends on sintering time. They concluded that increasing the sintering time of aluminum alloys by more than 3 h had an adverse effect on the microstructure due to increased grain growth and more porosity. ...
The present study reports the manufacturing, hardness, and corrosion behavior of Al-20%Ti-6%Cu and Al-15%Ti-6%Cu alloys, which were successfully fabricated by powder metallurgy technique. The microstructure of synthesized alloys was studied, and the corrosion tests of the sintered samples were carried out in a 1.5 M HCl solution at 25 °C using potentiodynamic polarization curves. X-ray diffraction analyses were employed to examine the phase structure of fabricated alloys. Scanning electron microscopy (SEM) was used in combination with energy-dispersive X-ray spectroscopy (EDS) to investigate the surface morphology and the elemental composition of the corroded surface of all manufactured alloys. The results revealed that the hardness increased significantly as the titanium content increased in the alloys, from 15 to 20 wt%. The hardness finding showed that the hardness of Al-20%Ti-6%Cu sintered for 5 h increased in comparison to Al-15%Ti-6%Cu sintered for 3 h. Maximum hardness was reported with the addition of 20 wt% Ti to the alloy after sintering at 550 °C for 5 h. The polarization curve data showed that the corrosion behavior of alloys was improved with the increasing Ti content and also confirmed that alloys do not suffer pitting attacks. Moreover, SEM images revealed that the alloys fabricated with a higher amount of titanium and sintered for 3 h, exhibit an enhancement in resistance to corrosion. Furthermore, these alloys formed a substantial layer of corrosion products, preventing pitting on their corroded surfaces. Therefore, these alloys are a material of choice in many industrial applications, like aerospace and automobiles.
... For example, Gürbüz et al. manufactured graphene/aluminum matrix composites at 550, 600, and 630 °C. The results show that the composites obtained at 630 °C presented enhanced apparent density and hardness compared to those fabricated at lower temperatures [54]. However, higher temperatures can also produce Conversely, both yield and ultimate strengths increased with the increasing volume fraction of activated nanocarbon. ...
... Manufacturing parameters also affect the mechanical properties of the composites, higher sintering and consolidation temperatures will produce stronger and harder samples. For example, Gürbüz et al. manufactured graphene/aluminum matrix composites at 550, 600, and 630 • C. The results show that the composites obtained at 630 • C presented enhanced apparent density and hardness compared to those fabricated at lower temperatures [54]. However, higher temperatures can also produce negative results, such as the formation of undesirable amounts of secondary phases (Al 4 C 3 ), which will reduce the mechanical properties [55,56]. ...
6061 aluminum composites with 0.5 and 1 vol. % graphene nanoplatelets as well as 1 and 2 vol. % activated nanocarbon were manufactured by a powder metallurgy method. Scanning electron microscopy and Raman spectroscopy were used to study the morphology, structure, and distribution of nanocarbon reinforcements in the composite samples. Density Functional Theory (DFT) calculations were performed to understand the aluminum-carbon bonding and the effects of hybridized networks of carbon atoms on nanocarbon aluminum matrix composites. Scanning electron microscopy showed the good distribution and low agglomeration tendencies of nanoparticles in the composites. The formation of secondary phases at the materials interface was not detected in the hot-pressed composites. Raman spectroscopy showed structural changes in the reinforced composites after the manufacturing process. The results from Density Functional Theory calculations suggest that it is thermodynamically possible to form carbon rings in the aluminum matrix, which may be responsible for the improved mechanical strength. Our results also suggest that these carbon networks are graphene-like, which also agrees with the Raman spectroscopy data. Micro-Vickers hardness and compressive tests were used to determine the mechanical properties of the samples. Composites presented enhanced hardness, yield and ultimate strength compared to the 6061 aluminum alloy with no nanocarbon reinforcement. Ductility was also affected, as shown by the reduction in elongation and by the number of dimples in the fractured surfaces of the materials.
... After, compaction specimens were subjected to vacuum sintering. The specimens were sintered at different temperatures 900°C 2 h, 1000°C 1 h, 1000°C 2 h as shown in table 2. sintering was done at varying temperatures and times to analyze porous gaps, grain size, and growth [12]. After sintering, specimens were cooled in atmospheric air at room temperature. ...
... Due to the agglomeration of compounds friction between particles increased. Therefore, the interaction between particles decreased which led to an increase in porosity and a decrease in hardness [12]. The influence of reinforcement (HEA) with titanium clearly can see in hardness values. ...
Owing to its weight-to-strength ratio, titanium is a widely used material, especially in gas turbine engines. It possesses a high melting point and corrosion resistance, however, exhibits poor wear resistance. An improvement in its tribological properties can be accomplished by the addition of a suitable reinforcement in metal matrix composite (MMC). In this research, titanium MMCs were fabricated through mechanical alloying (MA) followed by vacuum arc melting of 95% titanium reinforced with 5% of (AlSi) 0.5 CoFeNi high entropy alloy (HEA). Compaction was later done at 1000 MPa, while specimens were heat-treated at sintering temperatures of 900℃ and 1000℃, with varying sintering times of 1 hour and 2 hours at 10 ⁻⁴ millibar vacuum. Microhardness and sliding wear rate of reinforced HEA specimens exuded improvement when compared to the Ti 900℃ 2hr specimen. Owing to the reinforcement, a reduction in wear rate and more than 5% improvement in microhardness had been observed, at higher sintering temperatures. The improvement was attributed to the synergistic effect of sintering time and temperature during the density and wettability analysis which was supported by the morphological analysis.
... The hardness and compressive strength of composites increased with increasing sintering temperature and observed the highest value at 600℃. Studies by Saboori et al. [270], Baig et al. [240], and Gurbuz et al. [271] also demonstrated that increasing sintering temperature improved the sinterability and resulted in enhanced properties. This can be explained by equation (1), showing the relationship between diffusion and sintering temperature [276]. ...
... Gurbuz et al. [271] also observed an increase in hardness with sintering time up to 3 h and a reduction in hardness of Al-GNP composites beyond 3 h of sintering. The degradation in hardness with prolonged sintering was explained by equation (2), showing that the increase in sintering time increases the radial distance traveled by an atom. ...
In recent years, aluminium (Al) matrix reinforced with graphene-based nanomaterial (GN) composites (AGNCs) have emerged as a potential candidate for various applications due to their lightweight combined with excellent mechanical, tribological, thermal, and electrical properties. This paper is intended to provide a comprehensive and critical review of the state-of-the-art research activities related to the processing, properties, and applications of AGNCs. The review begins by highlighting the need for “lightweighting” and AGNCs, followed by a short introduction to the structure and properties of GNs. It covers the current challenges in the processing of AGNCs, such as low wettability, interfacial reactions, and agglomeration of GNs. Different consolidation and post-processing techniques used for the fabrication of AGNCs are also presented. Subsequently, the effect of GN addition on properties of AGNCs is critically reviewed. Different micromechanical models used for quantifying the strengthening of AGNCs aided by various strengthening mechanisms are also elucidated. More importantly, based on the reported properties, the potential and promising applications of AGNCs in various industries, such as automobile, aerospace, space, electrical, electronics, and energy systems, have been proposed. Afterwards, the current understanding of AGNCs is thoroughly summarized. Finally, directions for future research in the field of AGNCs have been suggested.
... The hardness of the composites embellishes with the addition of reinforcing particles as it hampers the kinesis of dislocation as shown in Fig. 3. As a result, the higher the number of SiC particles and the more homogeneous their dispersion, the more hurdles to dislocation movement, hence hardness increases with the addition of particles as reported by distinct researchers [18,19]. Due to decreased Pierls stresses, increased disorder agility and the eventual initiation of additional slip systems, the hardness of the composites decreases as the temperature rises [20]. ...
AA7050 aluminium alloy used for the main landing gear link was reinforced with SiC particles utilizing stir casting and uniform dispersion of reinforced particles was analyzed through SEM with EDS mapping. Wear test were performed on pin on disc apparatus by varying the process parameters and experimental runs were designed using response surface methodology. The influence of SiC particles on wear resistance at high temperatures was explored and the findings led to the development of a novel wear equation. The hardness of composites increased due to impediments of dislocation movement, and it declines with an increase in temperature owing to a reduction of Pierls stresses. The formation of a Mechanically Mixed Layer (MML) enhances wear resistance with the inclusion of reinforced particles, and the breakdown of this layer swifts the wear from moderate to severe. The mode of wear was a combination of shearing and abrasive at room temperature, shearing and adhesive until the temperature 200ᵒC, and plastic deformation when the temperature exceeded 200 °C, which was confirmed by worn surface morphology.
... The 6XXX series has all of these. Zinc is the primary alloying element in the 7XXX high-strength Alloy series, which is heat treatable and prone to fatigue [11]. All other aluminium alloys with primary alloying elements, such as lithium, iron, zirconium, molybdenum, and so on, are also included in the 8XXX series. ...
Composites made from metal matrix materials have been widely used in aerospace, automotive applications. Alloys based on aluminium are widely used because of their excellent combination of strength, lightweight, and corrosion resistance. A hybrid composite comprising of aluminium powder and diamond powder was investigated in this study. An alloy based on aluminium was created through powder metallurgy. A hybrid composite of Al and an Al metal matrix reinforced with diamond particles contains 1% diamond and 3% diamond. ASTM standards were used to investigate mechanical and wear properties. The results revealed that 3 weight per cent of diamond reinforced with aluminium increased wear resistance by 60.66% over pure aluminium without diamond. When 3% Diamond is mixed with 97% aluminium, the compressive strength increases by 87.27% compared to pure aluminium. Comparatively, a composite of 97% aluminium and 3% diamond has the maximum hardness value. Examining the surface morphology of hybrid composites was demonstrated via SEM images.
... As illustrated in Fig. 14b, passing the optimal temperature of 600 K, the yield strength of the sintered alloys would be diminished. The existence of optimal temperature in the sintering of metallic products prepared by powder metallurgy (e.g., Al matrix composites) to achieve the best mechanical and physical properties has been thoroughly studied in the literature [67,68]. For example, utilizing the spark plasma sintering process, Liu et al. [69] examined the effect of sintering temperature on the mechanical properties of nanocrystalline Al powders. ...
In the present work, a series of molecular dynamics simulations are conducted to figure out how sintering parameters would influence the atomic structure, sintering mechanisms, and elastoplastic properties of sintered Al-Cu nanoparticulate systems. For this purpose, first, utilizing the atomic potential energy diagrams, the suitable sintering temperatures for the introduced nanoparticles (NPs) have been calculated. NPs are then sintered at the temperatures of 410, 510, 600 and 680 K, and microstructural changes have been probed during the process. Finally, employing a uniaxial tensile test, the sintering temperature and holding time effects on the tensile behavior of the sintered products are studied in detail. Our simulation results indicate a strong correlation between the crystalline structure of the sintered NPs and the process temperature. It is also concluded that the main sintering mechanism at low-temperature conditions is dislocation slip, while at elevated temperatures, the sintering mechanism switches to diffusion-based phenomena. Moreover, it is revealed that increasing the sintering temperature from 410 to 680 K enhances the Young modulus of the samples monotonically, while the greatest yield strength is achieved at 600 K. Similar correlation is also found between the holding time and mechanical properties of the final product.
... Therefore, it can be stated that as the density increases, the ratio of voids and pores in the structure will also decrease. The sintering temperature plays an important role in the sintering process and the structure of composites, a denser structure is obtained at higher sintering temperatures due to higher diffusion rates [31,32]. Equation 5 explains the relationship between sintering temperature and diffusion. ...
In this experimental study, Al7075 matrix composites reinforced with different proportions of MgO were produced by powder metallurgy method. Different sintering temperatures and times were applied in the powder metallurgy production process. In the second stage of the experimental study, firstly, the porosity and hardness measurements of the composite materials were made. Then, microstructure images were taken with SEM and optical microscope, and XRD analyzes were performed. Using the obtained data, the effects of different MgO ratios and different sintering parameters on the structural properties of composite materials were evaluated. As the sintering temperature increased, the density of the composite structure increased and then decreased again. Accordingly, the amount of porosity first decreased and then increased again. Significant size growth occurred in all samples sintered at 600 °C. This change was associated with the high amount of porosity in the same samples. A more stable microstructure was obtained from the samples sintered at 550 °C. Thus, it can be said that the presence of excess MgO particles in the system causes the material quality to deteriorate due to increased microscopic structural problems, wetting rates, intergranular interaction problems between adjacent layers, recovery mechanism and entanglement of voids, and dislocations. Therefore, the ideal rate, time and temperature value for MgO addition should be carefully determined. As a result, it was seen that the sintering temperature of 550 °C gave the most suitable results. The sintering time strengthened the phase volume of the Al7075 alloy, making the compound more stable.
... The trend of variation of hardness for holding time is also positive. This positive gain in the hardness of composites is exhibited due to improved diffusion phenomena at large time intervals [33,34]. Further, the maximum hardness is achieved at a holding time of 10 min, and beyond this, there is a decline in hardness value; it would be due to grain coarsening that started to take place for a longer duration of sintering time [35]. ...
In the present work, Al-TiO2-Gr hybrid composites were fabricated through a sustainable manufacturing approach, i.e., ERS (Electric Resistance Sintering) technique. In this experimental work, sintering is performed in a high-density graphite die, which also works as a heating element. The green compacts kept in the graphite die are sintered in two ways simultaneously (conduction and resistance heating). This facilitated the accomplishment of the sintering at a lower current (300–500 A). The aluminum (Al) was reinforced with 9 wt. % TiO2 (rutile) nanoparticles and 3 wt. % graphite microparticles to synthesize a self-lubricated high wear resistance material. Mechanical properties such as density, hardness, and wear loss of the Al-TiO2-Gr hybrid composite were investigated. Scanning electron microscopy (SEM), Energy-dispersive X-ray spectroscopy (EDS), and X-ray diffraction (XRD) were performed for microstructural investigation. The experiments were performed according to the Taguchi design of the experiment, where three input process parameters (temperature, holding time, and sintering load) were taken to fabricate the Al-TiO2-Gr composite. The sintering temperature of 550 °C resulted in the maximum value of mean sintered density (approx. 2.45 gm/cm3). The holding time of 10 min for the sintering resulted in the maximum value of mean sintered density and mean hardness (HRB 53.5). The mean value of wear loss was found to be minimum for the composites sintered at 600 °C for 10 min. The maximum value of the sintering load (800 N) revealed better density and hardness. Worn surfaces and wear debris were also analyzed with the help of SEM images. The sintering temperature of 600 °C resulted in imparting more wear resistance which was proved by smooth surfaces, micro-cutting, and fewer crates, grooves, and smaller pits.
... Figure 6a shows the maineffect plot for density of the Al alloy composite and the corresponding response is recorded in Table 5. As expected the density of developing composite increases with increase in sintering temperature from 580 °C to 620 °C due to higher diffusion rates [15]. It is to be noted that the density increases with bimodal-1 reinforcement in Al alloy matrix material. ...
Bimodal size SiC particles reinforced Al-Si alloy composites were fabricated by powder metallurgy route. Three different bimodal reinforcements with various volume fractions of micro and nano sized SiC particles were dispersed in Al-11.5%Si-1%Mg matrix by mechanical alloying for 10 h. The composite powders were hot pressed at 580 °C, 600 °C and 620 °C in a vacuum atmosphere with different cooling rates (10 °C/min, 15 °C/min and 20 °C/min). The microstructural changes of bimodal SiC(m–n) in the matrix material were carried out using OM, SEM and TEM with energy dispersive spectroscopy (EDS). The sample experiments were implemented based on Taguchi L9 OA with three parameters of bimodal reinforcement, sintering temperature and rate of cooling respectively. The measurement of mechanical properties specifies that hardness and tensile strength of Al-11.5%Si-1%Mg composite increases with the decrease of nano-SiC concentration in bimodal reinforcement. Moreover, the fine precipitation of Al-Si rich compound as observed at higher cooling rate (20 °C/min) also contributed for the strength improvement of developing composite.
... Similar observations were reported by a researcher in his study, where varied Graphene Nanoplatelet binary Particles (GNPs) in Al matrix along with fixed proportion of silicon nitride, effectively controls the mechanical performance of the composite. Addition of graphene beyond 0.1wt%, considerably reduced the hardness of Al-GNPs composites, due to the agglomeration tendency of GNPs [39]. Excessive particle presence causes higher porosity and lower hardness [40]. ...
This study comparatively investigates the rotational dry sliding wear behaviour of hybrid ceramic reinforced AA6082/3wt.%Boron Nitride (BN)/4wt.%Boron Carbide (B4C) stir cast composite against its alloy by varying weight percentage (2, 4 and 6 wt.%) of Corn Cob Ash. Maximum hardness (70.7 BHN) and impact strength (32 J) was identified at 4 wt.%. The input parameters were applied load (15, 25 and 35 N), sliding distance (750, 1250, and 1750 m) and sliding velocity (1, 2 and 3 m/s), whereas specific wear rate was the response parameter designed based on Taguchi's L27 array. The percentage contribution of each influential input factor towards the performance characteristics of the developed composites were defined using Analysis of Variance. Optimum wear rate was obtained at 15 N load, 750 m sliding distance and 1 m/s sliding velocity combination. Signal to Noise ratio identified applied load as most influential followed by sliding distance and sliding velocity.
... Compaction and sintering by die are used for producing products of high accuracy dimensions with excellent surface finishing. Further operation used for close tolerance and higher accuracy are known as secondary operations [9][10][11]. ...
This research paper describes the synthesis of two solid billets processed by pure aluminium powder & zinc oxide powder, which was passed through parallel channel Pressing die to enhance mechanical properties and texture. Initially a die was designed for compaction, which consist of two die half and two plungers for top and bottom. Die parts were tightening together by using nut and bolts. After compaction, sintering was done using furnace. After that billets were passed first through parallel channel pressing die. Vickers Hardness, density, compressive strength and texture were evaluated before and after passing through parallel channel pressing die. Grain refinement and improvement in mechanical properties were observed by multiple passes of commercial billets of aluminium alloys. The research paper concludes the improvement in properties of billets prepared by PM (Powder Metallurgy) technique. Meanwhile there is lack of literature about PM techniques for aluminium alloys. In future PM will be a significant topic for research and samples made by (PM) Powder Metallurgy Technique will be easier to synthesize. Powder metallurgy is a science used for producing powder and products by mixing with alloys[1]. The major steps in powder metallurgy are powder production, compaction, sintering and secondary operations. Excellence of any product is highly dependent on process quality. Product quality suggest many changes in po wder description[2]. This awareness estimated the significance of producing homogeneous powders by high transparency. The problems like handling are due to its smaller in size and quantity, increased low quality powder & high scrap production. So it is necessary to analyse the quality of product during production. The concept of powder properties, handling and processing must be tied with powder production [2-3]. Various techniques can be implemented for producing fine powders like alloying, plasma, chemical precipitation etc. Plasma technique is one of the exciting techniques for producing ultra-fine powder. The cheapest method for powder producing is mechanical alloying. It is used for producing alloys required high strength and other mechanical properties. Mechanical forces are generally required to convert bulk material into fine powder or size reduction [4-5]. Compaction is most important process in powder metallurgy. This action helps in converting free powders into desired shape and size and provides initial strength to the product before heating. A die can be designed for completion of compaction process. A die can be of single piece or two pieces die. Two plungers are required to press the powder in the die which can be done by hydraulic press or universal testing machine [6-8]. The process of heating a product after compaction without melting is known as sintering. In this process the material atoms diffused and particles fused together and form solid piece. During sintering the material temperature is always less than its melting temperature. The properties of the FCC alloys produced by PM are totally different from the metals made by other conventional methods. A layer of oxide is formed on aluminium surface during sin
... Among the different reinforcement materials, Alumina (Al2O3) is mostly considered as it improves properties like strength, wear resistance [9][10] and corrosion resistance [11] of AMMCs. Among the different fabrication processes, the powder metallurgy process is generally used to fabricate MMCs through steps which directly improve their physical and mechanical properties i.e., through blending, compacting and sintering [1,[12][13][14][15][16][17][18]. ...
In this investigation tribological properties of Aluminium-Alumina metal matrix composites (AMMCs) are developed through developing microstructure and improving physical properties by controlling process parameters of two different powder metallurgy routes. In one route; 5, 10 and 15 weight percent Alumina (Al2O3) powder was manually blended with pure Aluminium (Al) and compacted at 10 ton/inch2 uniform pressure followed by sintering at 400°C, 500°C and 600°C for 30 minutes. Alternately, commercially pure Al powder was oxidized at 500°C, 600°C, 700°C and 800°C for 15, 30 and 45 minutes individually followed by same powder metallurgy process as applied in the first route of AMMCs fabrication like blending, compacting and sintering. Optical micrographs of fabricated AMMCs were taken and corelated with the apparent porosity of fabricated AMMCs as well as with different process parameters and variables like sintering and oxidation temperatures, oxidation duration, and wt. % of Alumina. Tribological properties of all AMMCs were also measured and corelated with the process parameters and variables as well as with the observed microstructure and measured apparent porosity. It is observed that finer grain structures are developed by increasing sintering and oxidation temperatures, and oxidation duration. It is also observed that wear resistance of AMMCs is enhanced by increasing sintering and oxidation temperatures, oxidation duration, and wt. % of Alumina individually; whereas, more enhancement is observed in case of second route of AMMCs fabrication. Therefore, the uniqueness of this investigation is to improve the wear resistance of pure Aluminium by fabricating AMMCs through simply heating pure Al powder at different temperatures followed by powder metallurgy process instead of adding reinforcement material (Alumina powder).
... However, powder consolidation is not a trivial process and requires the optimization of the process parameters to achieve maximum densification (Awotunde et al., 2019;Attarilar et al., 2021a), without causing significant coarsening of the consolidated microstructure or introducing unwanted phases (Koch et al., 2008). Gürbüz et al. (2017) studied the effect of sintering time on the consolidation of Al-0.1 wt% graphene nanoplatelets (GNPs). They reported that the density of the green pressed composite increased up to a sintering time of 180 min, after which the density dropped. ...
Several studies investigating the ball-milling of ductile face-centered cubic metals have reported a so-called in-situ consolidation phenomenon where the milled powder is also consolidated during the milling process. Thus, instead of refined powders or agglomerated particles, the formation of spherical bulk particles of the milled material is reported using a combination of cryomilling and room temperature milling processes. In this study, we studied the effect of the milling vial shape on the in-situ consolidation of a graphene nanoplatelets (GNPs) reinforced aluminum-lithium (Al-Li) matrix nanocomposite for the first time. An in-situ consolidated nanometric Al-Li-GNPs nanocomposite with an average grain size of 48 nm and high hardness of 1.48 GPa was attained after only 8 h of room-temperature milling. The results presented suggest that dense nanostructured composites can be prepared by in-situ consolidation during a one-step milling process and subsequently investigated in order to analyze their mechanical behavior. This allows for the intrinsic mechanical behavior of the synthesized material to be examined without the interference of subsequent high-temperature consolidation processes, thus avoiding unwanted structural changes such as grain growth and second phase formations.
... After the experimentation, they concluded that an increase in sintering temperature leads to reduce porosity and increment in the hardness of the composite. Gürbuz et al. (2017) have studied the effect of variation in sintering temperature on the properties of aluminum composites. Result reveals that the increase in sintering temperature leads to increase in metallurgical and mechanical properties of the composite. ...
Purpose
The purpose of this study is to investigate the effect of the sintering temperature on the microstructural, mechanical and physical properties of Cu-SiC composites.
Design/methodology/approach
The powder metallurgy route was used to fabricate the samples. Cold compaction of powders was conducted at 250 MPa which was followed by sintering at 850°C–950°C at the interval of 50 °C in the open atmospheric furnace. SiC was used as a reinforcement and the volumetric fraction of the SiC was varied as 10%, 15% and 20%. The processed samples were metallurgically characterized by the scanning electron microscope (SEM). Mechanical characterization was done using tensile and Vickers’ micro-hardness testing to check the hardness and strength of the samples. Archimedes principle and Four-point collinear probe method were used to measure the density and electrical resistivity of the samples.
Findings
SEM micrograph reveals the uniform dispersion of the SiC particles in the Cu matrix element. The results revealed that the Hardness and tensile strength were improved due to the addition of SiC and were maximum for the samples sintered at 950 °C. The addition of SiC has also increased the electrical resistivity of the Cu-SiC composite and was lowest for Cu 100% while the relative density has shown the reverse trend. Further, it was found that the maximum hardness of 91.67 Hv and ultimate tensile strength of 312.93 MPa were found for Cu-20% SiC composite and the lowest electrical resistivity of 2.017 µ- Ω-cm was found for pure Cu sample sintered at 950 °C, and this temperature was concluded as the optimum sintering temperature.
Research limitations/implications
The powder metallurgy route for the fabrication of the composites is a challenging task as the trapping of oxygen cannot be controlled during the compaction process as well as during the sintering process. So, a more intensive study is required to overcome these kinds of limitations.
Originality/value
As of the author’s best knowledge, no work has been reported on the effect of sintering temperature on the properties of the Cu-SiC composites which has huge potential in the industries.
... At higher temperatures strength increases because of strong interfacial bonding. Gurbuz et al. [25] prepared Aluminium-graphene nanoplatelets composite by powder metallurgy method. The effect of different sintering times (60,120,180,300min) and different sintering temperatures (550°C,600°C,630°C) are investigated. ...
This paper interprets the effect of sintering parameters like sintering time and sintering temperature as well as various sintering methods on distinct properties of the material. The variation of Physical, mechanical, and Tribological behaviour depending on sintering temperature, time and method based on various aluminium metal matrix composites have been investigated. The advantages of aluminium metal matrix composites are high strength to weight ratio, high wear resistance, and erosion resistance, etc. Aluminium Metal matrix composites have vast applications in various fields like structural, automobile, and aviation industries. The optimum value of sintering parameters and choice of sintering methods has a major role in getting these required properties of aluminium metal matrix composites prepared by the powder metallurgy process.
... HV1000B Vickers hardness experiments showed that Al-30 SiC having a volume fraction of 0.5% graphene additive exhibited the highest hardness result. Gurbuz et al. [21] investigated the effect of sintering temperature, time, and graphene additive fraction on the hardness and microstructure of graphene-reinforced aluminum composites. They stated that they achieved the best hardness value in these composites at a temperature of 630 • C and a sintering time of 180 min in a volume fraction of 0.1%. ...
This study aimed to investigate experimentally the repeated low-velocity impact behaviors of SiC reinforced aluminum 6061 metal-matrix composites for different volume fractions and energy levels. In addition, the hardness variations were measured by the Vickers hardness tests from the impacted and impact-free cross-sections of the particle reinforced metal-matrix composites. Low-velocity impact tests were applied to composite samples manufactured by powder metallurgy (in 10, 20, and 30% volume fractions) at two total energy levels (15 and 60 J as single) and in repetitions equal to the sum of these energy levels (5 + 5 + 5 and 20 + 20 + 20 J as repeated). As a result, in increasing the impact number for all volume fractions, the total contact time was shortened and the peak contact force increased, whereas both the permanent central deflection and the absorbed energies reduced. Hence, these variations obtained under repeated impacts (5 + 5 + 5 and 20 + 20 + 20 J) revealed that metal-matrix composites showed a tougher behavior with an increase in the impact numbers from 1st to 3rd, particularly because of the strain hardening effect. Furthermore, an increase in volume fraction from 10 to 30% resulted in an increase in the impact strength under all repeated and single impacts despite changing deformation and damage mechanisms due to increasing the strain hardening effect particle fractures. The hardness was affected by the volume fraction and increased as the volume fraction increased in both the impacted and impact-free zones. The repeated impact increased the impacted zone hardness more than the single impact for all volume fractions. Additionally, the hardness of the impacted zone under 20 + 20 + 20 J repeated impact was measured as the highest value in the 30% volume fraction. Therefore, metal-matrix composites can behave harder with the strain hardening effect under repeated impacts.
... The samples were maintained at that pressure for 30 mins for effective distribution of load throughout the surface before ejecting it from the die. After the compaction, all the green samples were placed in a muffle furnace at 630°C for 3 h [32]. The samples were kept in the furnace itself till they attained the ambient temperature. ...
Materials, with high strength applications, are always high in demand for the industry. Metal matrix composites play an important role in providing these materials. In the present study, Aluminium (Al)-graphene nanoplatelets (GNP) nano composites (Al-GNP) were fabricated through the route of powder metallurgy. Pure Al was used as matrix material and GNP (C-750) was used as reinforcement. The process consists of different steps viz dry ball milling followed by compaction and sintering. Compression and hardness test were performed on the formed samples. Four types of composites were fabricated with different proportions of GNP (0.1 wt%, 0.25 wt%, 0.5 wt% and 1 wt%) and the results were compared with pure Al made with the similar process. Morphological characterization like XRD, SEM and FTIR of the formed samples was also performed. It was found that both the hardness and compressive stress were increased up to a certain proportion of GNP in Al-GNP. The maximum hardness achieved was 23 HV with 0.25 wt% GNP which was 98.2 % more than the hardness value of pure Al. Similarly, the maximum compressive stress achieved was 109.33 MPa which was 59.18% high than the compressive stress of pure Al.
... This phenomenon has also been reported in the sintering process of other composites. Wang and Gürbüz et al. [21,22] observed in B4C/Al and Graphene/Al composites that in an appropriate temperature range, increasing the sintering temperature and prolonging the sintering time, respectively, will significantly increase the density of the composite, thereby improving the performance. However, excessively increasing the sintering temperature leads to a significant decrease in material properties. ...
In this paper, six-layer AlN/Al gradient composites were prepared by a spark plasma sintering process to study the influences of sintering temperature and holding time on the microstructure and mechanical properties. The well-bonded interface enables the composite to exhibit excellent thermal and mechanical properties. The hardness and thermal expansion properties of the composite exhibit a gradient property. The hardness increased with the volume fraction of AlN while the CTE decreased as the volume fraction of AlN. The thermal expansion reaches the lowest value of 13–14 ppm/K, and the hardness reaches the maximum value of 1.25 GPa, when the target volume fraction of AlN is 45%. The simulation results show that this gradient material can effectively reduce the thermal stress caused by the mismatch of the thermal expansion coefficient as a transmitter and receiver (T/R) module. This paper attempts to provide experimental support for the preparation of gradient Al matrix composites.
... When applied as rotor plates, drive shafts in aerospace engine, improved wear resistance of aluminum alloys is usually achieved by fabrication of aluminum matrix composites (AMC) [30,31]. The manufacturing process of AMC takes long period, which consists of heating, soaking, and cooling [32,33]. Defects and reinforcement agglomeration also remain as challenges in the fabrication process of AMC [34]. ...
In this study, Inconel 625 coatings were successfully deposited on 6061 aluminum alloy using a high-pressure cold spray process. The microstructure, mechanical and tribological properties of the coatings were systematically studied. The cold sprayed Inconel 625 coatings had a low porosity level due to the severe plastic deformation of the splats. EBSD analyses revealed that grain refinement occurred within the coatings and substrates (near the substrate-coating interface), which could be attributed to strain accumulation and fragmentation process. Good adhesion strength of above 57.0 MPa was achieved between the coatings and substrates. Nano-hardness and micro-hardness values were higher than those of bulk Inconel 625 and were uniform along the thickness of the coatings. The effects of normal load and sliding velocity on tribological properties of cold sprayed Inconel 625 coatings were investigated by sliding wear tests. The wear results demonstrated that both increased sliding velocity and normal load led to higher specific wear rates, which could result from the combined effects of tribo-film delamination, adhesive wear, abrasive wear, and thermal softening. Lower coefficients of friction of the coatings measured under 5 N compared to those measured under 2 N with the same sliding velocity could be explained by transition of wear mechanisms as well as larger coverage of tribo-film on the wear tracks.
The present study reports the investigation of the influence of naphthalene–camphor binary solvent and the densification process on the pore structure formation of α‐alumina monoliths using the freeze‐casting technique with a freezing temperature control mechanism. Freeze‐casting technique allowed a controlled production of monoliths with high porosity (65%–70%) and distinct pore morphologies based on the adjustment of solvent composition (hypoeutectic, eutectic, and hypereutectic) and sintering time. In general, the longer the sintering time, the greater the average pore diameter (8–10 µm), the larger the grain size (∼319–596 nm), the higher the material density (∼1450–1650 kg/m ³ ), the lower the open porosity (∼60%–49%), the higher the closed porosity (5%–18%), and the greater the compressive strength of the monoliths (hypoeutectic: 23.56 MPa; eutectic: 4.81 MPa; hypereutectic: 23.49 MPa). The study also reveals that the solvent composition directly influenced the alignment of the pore structure, as the pores of the final monolith resemble frozen solvent crystals. Hydraulic tortuosity varied from 1.30 to 1.53 for selected samples. Finally, solvent composition and sintering time proved to be important parameters that should be considered in the manufacturing process of porous ceramic structures via freeze casting.
In this study, the synthesis-structure-property relationship of graphene-reinforced Al matrix nanocomposites was investigated. The Al-Li-GNPs nanocomposite was synthesized to attain both high strength and good ductility. The incorporation of GNPs as a reinforcement in the Al-based matrix provided a nanocomposite structure for an integrated strengthening effect. To promote plasticity and maintain good ductility, the nanocrystalline Al matrix was alloyed with Li to reduce its stacking fault energy and promote additional deformation mechanisms. The compressive yield strength (CYS) increased from 88 MPa for the starting Al to 403 MPa for the Al-Li-GNPs nanocomposites with 1.0 wt% GNPs. Fracture analysis indicated that the synthesized nanocomposite exhibited a ductile nature and significant plastic deformation. Based on microscopic analysis, the enhanced strength of the Al-Li-GNPs nanocomposite was attributed to grain refinement, load transfer, and strain hardening. The good ductility, on the other hand, was attributed to dislocation slipping, the formation of stacking faults, and twinning.
Aluminum matrix composites (AMCs) developed with micro/nano-reinforcements emerge as an attractive candidate for innumerable applications, including automotive, aerospace, electronics, biomedical, and many more, owing to their high strength-to-weight ratio and outstanding tribological, mechanical, electrical, and thermal characteristics. This work aims to offer a review of the state of the art of research in the processing, fabrication, properties, and potential applications of AMCs. The review starts with an emphasis on light-weighted AMCs, followed by a brief discussion of the hybrid metal matrix composite structure and micro/nano-reinforcement. This review also includes an in-depth assessment of manufacturing processes and parametric factors that regulate the properties of AMCs. It also highlights the challenges that are currently encountered when processing AMCs, such as limited wettability, reinforcement agglomeration, and interfacial reactions, before analyzing the effect of adding micro/nano-reinforcements on the attributes of AMCs. In addition to the stated characteristics, the most feasible and novel applications of AMCs have been envisioned. Lastly, new research directions in the field of AMCs have been recommended and critically discussed.
Automotive and aircraft industries are advancing swiftly, creating a constant need for innovative and trustworthy materials. Aluminum composites (aluminum matrix composites [AMCs]) exhibit enhanced mechanical and tribological behaviors when contrasted to their conventional equivalents and as a result have superior potential to be widely accepted for automotive and aircraft engineering and other component applications. This study aims to provide a thorough and critical analysis of the most recent research initiatives concerning the processing, characteristics, and applications of AMCs. It covers the recent advancements in the aluminum-based composites reinforced with SiC, TiC, and graphene, fabrication methods, and mechanical properties of AMCs. Graphene nanoplatelets are many times stronger and yet lighter than steel and other metals, and thus a good contender for reinforcing them. However, the homogeneous distribution of graphene into the metal or aluminum is a challenging aspect for material researchers. The fabrication techniques for AMCs for achieving homogeneous distribution of graphene are critically reviewed. The mechanical properties, specifically microhardness, wear behavior, and tensile strength of aluminum-based composites, are reviewed and analyzed. Finally, a way forward for fostering further development in this area has been discussed.
Magnesium (Mg) composites reinforced with carbon-based nanomaterial (CBN) often exhibit low density, enhanced strength, good conductivity, improved wear resistance, and excellent biocompatibility when compared to current industry Mg alloys. This review aims to critically evaluate recent developments in Mg-CBN composites and is divided into five sections: First, a brief introduction to Mg-CBN composites is provided, followed by a discussion of different fabrication techniques for these composites, including powder metallurgy, casting, friction stir processing, and selective laser melting. A particular focus is on the current processing challenges, including dispersion strategies to create homogeneous Mg-CBN composites. The effect of processing on the quantifying disorder in CBNs and distinguishing different sp2 carbon materials is also highlighted. Then, the effect of CBN on various properties of Mg-CBN composites is thoroughly analyzed, and the strengthening efficiency of CNTs and graphene in the Mg matrix is examined. Finally, the potential applications of Mg-CBN composites in various industries are proposed, followed by a summary and suggestions for future research directions in the field of Mg-CBN composites.
In this study, the elastic properties of aluminium nanocomposite representative volumetric element (RVE) reinforced with GNP have been analysed. Pure aluminium is lightweight and has low strength which is not suitable for various aerospace applications. Adding graphene to aluminium gives a highly strengthened nano-matrix. A 3D multiscale finite element (FE) representative volumetric element (RVE) has been developed to estimate the mechanical behaviour of GNP-reinforced aluminium graphene nanocomposite (AGNC). The factors influencing the behaviour of AGNC have been investigated with different weight fractions (wt%), sizes and orientations of GNP. The Young’s modulus of AGNC is enhanced by increasing the wt% of GNP and reducing the size of GNP in the aluminium matrix. The Young’s modulus of AGNC with 1% wt% has been enhanced two times and yield strength by five times than pure Al matrix. In the case of different sizes of GNP, the strength of 15-nm-diameter GNP AGNC enhanced two times and medium-sized GNP, i.e. 30 nm has shown a great combination of strength and ductility. After that different orientations have also influenced the mechanical properties and enhancement shown in layered orientation compared to different angles of GNP.Graphical abstract
In this study, it was aimed to investigate the microstructure, hardness and
wear behavior of graphene nanoplate (GNP) reinforced composites with Al 99.9
matrix produced by powder metallurgy. Different temperatures and times were
applied in the sintering process. The hardness values of the composites
increased as the sintering temperature and time increased. The hardness
values decreased with the increase of GNP reinforcement ratio. The wear
losses decreased depending on the increase in sintering temperature and
time. With the increase in the GNP reinforcement ratio, reductions in wear
losses were recorded. It has been concluded that the GNP reinforcement
element in the composite structure reduces the friction coefficient and wear
losses by having some lubricating effect. It was observed that the neck and
bonding formation between Al 99.9 matrix grains improved with increasing
sintering temperature and time. It was concluded that with the development
of intergranular bonds, the porosity in the composite structure decreased
and the mechanical properties increased.
Al4032/Bimodal-B4C composites were synthesized via different milling methods, namely low speed ball milling (SLBM), high speed ball milling (SHBM) and two speed ball milling (TSBM). The Al4032 powder particles were shifted to flakes and some agglomerated B4C were slowly dispersed in flakes during SLBM. SHBM generated severe cold welding of Al4032 flakes and stronger collision, resulting in agglomerated zones within the Al matrix grain. While, TSBM processed samples showed the well coordination of bimodal B4C dispersion in matrix material and better interfacial bonding between Al4032/B4C. The composite powders synthesized at varying concentration of B4C (2.5, 5 and 7.5 wt.%) were hot pressed at 620 °C for 60 min in a vacuum furnace.The results demonstrated that the Al4032 composite with the dispersion of 5 wt.% of bimodal B4C through long term SLBM followed by short term SHBM achieved better hardness and tensile strength values of 87.32 HV and 219.36 MPa.
This work intended to manufacture the Al6061 alloy, Al6061-B4C, and Al6061-B4C-graphene composites with various B4C and graphene contents (B4C: 1 wt.%, 3 wt.%, 6 wt.%, 9 wt.%, 12 wt.%, 15 wt.%, 30 wt.%; graphene: 0.15 wt.%, 0.30 wt.%, 0.45 wt.%) via the induction heat treatment process and powder metallurgy route. This paper also discussed the influence of the heat treatment process and B4C/graphene amount on the wear rate, mass loss, friction coefficient, compressive strength, hardness, density, and microstructure of Al6061 hybrid composites. According to the tribological test results, wear rate and friction coefficient decreased from 1 × 10–7 mm3/(Nm), 0.43 (Al6061 alloy) to 0.8 × 10–8 mm3/(Nm), 0.22 (Al6061-30B4C-0.15-graphene) because of the graphene/B4C addition and the induction heat treatment process. Maximum hardness (238 ± 4 HV) and compressive strength (571 ± 6 MPa) were detected at induction heat treated and sintered Al6061-30B4C-0.15-graphene composite, which increased by ~ 150% and ~ 126% compared with Al6061 alloy. The present results indicated that the induction heat treatment and addition of homogeneously dispersed graphene/B4C particles improved the tribological and mechanical properties of Al6061 hybrid composites.
The advancements in the material science is the need of the hour to generate efficient and lightweight materials for diverse technological fields, e.g., automobile, aerospace, naval, and defence. Herein, we have attempted to develop an Al-GNP based composite to achieve superior mechanical properties. A variety of Al-GNP composites were developed by varying the levels of GNPs from 0 wt% to 1 wt%. With the help of ball-milling based powder metallurgy technique, a Al-GNP composite with superior mechanical properties has been fabricated. Fascinatingly, the developed material exhibit uniform dispersion of GNPs with intact graphitic properties and excellent compressive/tensile strength. Out of developed composites, Al- 0.5 wt% GNP was the best in enhancing the mechanical properties of Al. An enhancement of ∼70% and 135% was observed in the compressive strength and UTS of the pure Al metal due to addition of 0.5 wt% GNPs.
In recent years, graphene has been used as a reinforcing element to develop thermal, tribological, and mechanical properties of aluminum matrix composites due to its extraordinary properties. Although there are many studies on graphene-reinforced aluminum and its mechanical properties in the literature, the number of detailed corrosion studies with the electrochemical impedance spectroscopy (EIS) method for graphene-reinforced aluminum is few. Pure aluminum and aluminum-graphene composites with various contents (0.1 and 0.5 wt.%) were fabricated via the powder metallurgy route. The microstructures and corrosion resistances of the prepared samples were investigated by the scanning electron microscope and electrochemical impedance spectroscopy. The corrosion experiments were performed in 3.5 wt.%NaCl solution at 28 °C with saturated calomel reference electrode and platinum auxiliary electrode. From the EIS corrosion test results, the AlGr05 sample had maximum corrosion resistance of 2.985 Ω cm2. The Tafel polarization method results showed that the AlGr05 sample (0.0051 mA/cm2) had the lowest corrosion flow rate. Also, the SEM images of the corrosion samples showed that the highest cohesion of constituent particles, the least amount of corrosion, and particle separation were observed at Al-0.5 wt.%graphene. In conclusion, the highest corrosion resistance among specimens was obtained at Al-0.5 wt.% graphene composite.
In the present work, aluminum matrix composites reinforced with graphene nanoplatelets (GNPs: 0.15-0.45wt.%) and silicon dioxide (SiO2: 1, 3, 6, 9wt.%) were produced by the powder metallurgy method. Hardness, compressive strength, density, friction coefficient, and wear rate of the prepared specimens were examined. According to the experimental results, the best compressive strength (~ 390 MPa), density (~ 2.66 g/cm3), hardness (~ 62 HV), the lowest porosity (~ 1.3%), friction coefficient (~ 0.19 for a load of 10 N), and wear rate (~ 0.003 mm3/Nm for a load of 5 N) were detected et al.-6SiO2-0.15graphene composite. Compared to pure Al, the compressive strength, hardness, and wear resistance of Al-6SiO2-0.15graphene composite were improved by ~ 110%, ~ 106%, and ~ 107, respectively. Hence, it may be concluded that SiO2 has excellent wear resistance and graphene has remarkable strength, good solid lubricating properties for Al-based composites.
This paper includes a comprehensive review of composites filed with aluminium metal matrix. The literature study includes all aspects of the preparation methods, mechanical properties, tribological properties, erosion properties and thermal properties of corrosion and cavitation. Recent years, owing to the huge demand in various industrial applications, aluminium graphene/CNT nano-composites was gaining greater momentum. The Fabrication of metal matrix composites, strengthening mechanisms in metal matrix composites, mechanical behavior of all composites and evaluation of mechanical properties were discussed below in detail. In fact these newly developed materials have high strength, high stiffness and high hardness, whereas these materials are difficult to be machined by conventional machining processes. Keywords: Aluminium, MMC, Carbon Nanotubes, Graphene, Fabrication.
Aluminium-copper composite is an alternative material for replacing Al alloys in automotive parts. One of strategies on improvement of Al-Cu composite properties is by correct processing method such as a combination of mechanical alloying and powder metallurgy. In this study, the effects of compaction pressure and sintering temperature on structural properties and microstructural properties of mechanically-alloyed Al-Cu-graphite composite were discussed. Elemental powder of Al, Cu and graphite were milled in a planetary ball milling for 10 h. Then, the as-milled composite was cold-compacted for 300, 600, 900 and 1100 MPa and undergo sintering at 400, 450, 500 and 550°C. As-milled and sintered Al-Cu-graphite composite were characterized for phase identification, structural properties, microstructural and density. The result showed that after milling, the composite consists of starting materials but after sintering new phases was formed. Cu2O and CuO co-exist at 500°C and Al2Cu started to form at 550°C as Cu2O diminished. The morphology of sintered Al-Cu-graphite showed has denser structure with the presence of Al-rich and Cu-rich regions. The increment of the green density was resulted from increased of compaction pressure and sintering temperature. At higher sintering temperature (550°C), sintered density was reduced due to the presence of Al2Cu that slower the diffusion process during sintering.
Few-layered-graphene (FLG)-reinforced Al-4 wt.% Cu matrix composites were produced via the powder metallurgy (PM). FLG was incorporated into the matrix via a mechanical alloying (MA) process conducted for 5, 7 and 9 h in a planetary ball mill. The mechanically alloyed (MA'ed) powders were consolidated by uniaxial pressing and pressureless sintering. Properties of the Al-4Cu-xFLG composites were examined via Differential Scanning Calorimetry (DSC), X-ray Diffraction (XRD), Optical Microscopy (OM), Scanning Electron Microscopy (SEM), Energy Dispersive X-Ray Spectroscopy (EDX), Archi-medes method, microhardness, compressive and wear tests. According to the mechanical characterization, FLG addition relatively improved the hardness, whereas it caused the decline of compressive strength. However, the specific wear ratio of the same sample increased by two times compared to the Al-4Cu.
Recently, graphene nanoplatelets (GNPs) or silicon nitride (Si3N4) may be employed as a reinforcing element in the Al2024 aluminium alloy matrix due to the solid lubricant property of GNPs and high compressive strength of Si3N4. However, there is no article related to the investigation of the mechanical properties of Al2024–Si3N4–graphene hybrid composites. In this article, Si3N4 or Si3N4/graphene binary particle-reinforced Al2024 aluminium-based composites were produced by powder metallurgy route and induction heat-treatment process. The effects of induction heat treatment (a 500°C sintering temperature and a 35 MPa pressure), Si3N4 (1–12 wt%) and graphene (0.15–0.45 wt%) contents on the microstructure and mechanical strength of Al-based composites were examined. According to the test results, the micro Vickers hardness improved from ~94.8 HV (Al2024 alloy) to ~105.2 HV (Al2024–9Si3N4) and ~108.5 HV (Al2024–9Si3N4–0.15GNPs). Similarly, the compressive strength was enhanced from ~361 MPa (Al2024 alloy) to ~510 MPa (Al2024–9Si3N4) and ~610 MPa (Al2024–9Si3N4–0.15GNPs). The compressive strength of sintered and induction heat-treated Al2024–9Si3N4–0.15GNPs composite improved by ~8.9% compared with the conventional sintered same specimen’s strength. In conclusion, the induction heat-treatment process and Si3N4/graphene addition significantly enhanced the mechanical strength of Al2024 hybrid composites.
The thermal stability of polycrystalline tin selenide (SnSe) is essential for its long-term applications. In this study, the thermo-mechanical stability of hot compacted polycrystalline SnSe and SnSe/graphene nanocomposite is evaluated. All samples were prepared using a combination of mechanical alloying and hot compaction under an argon atmosphere. A severe bloating behavior was observed in the pristine SnSe hot compacted disks after post densification annealing. Consequently, these disks exhibited volume expansion and a drop in density, resulting in the formation of pores and cracks within the sample, which significantly degraded the electrical performance with consecutive thermal cycling. Interestingly, the bloating behavior was reduced upon the incorporation of graphene within the SnSe matrix. However, this reduction was limited to samples with a homogeneous distribution of graphene. Dilatometer measurements showed that the as-compacted SnSe/graphene nanoplatelets (GNP) sample in which graphene was milled for 4 h exhibited less hysteresis than the pristine SnSe sample. Accordingly, the electrical performance of the SnSe/GNP samples was more stable than the pristine SnSe after consecutive thermal cycles. The obtained results indicate that homogeneously distributed graphene plays a significant role in improving the thermal stability of the SnSe-based nanocomposite.
Recently, aluminum matrix composites have been fabricated by pure zirconia (ZrO2) or pure graphene in the aluminum matrix because of good solid lubricant property of graphene and high compressive strength of ZrO2. Nevertheless, there is no study on the effect of both ZrO2 and graphene reinforced aluminum composite. In this current work, the tribological behaviors of Al-ZrO2 and Al-ZrO2-graphene composites with various contents (ZrO2: 1–12wt.%; graphene: 0.15–0.45wt.%) were investigated under different loads (5 and 10 N) via the pin-on-disk wear test unit. The density, porosity, hardness, and compressive strength were investigated by the Archimedes’ principle kit, Vickers hardness test unit, and universal test machine, respectively. According to the test results, the micro-Vickers hardness, porosity, and compressive strength enhanced from 30 ± 1.2 HV, 7%, 186 ± 4 MPa (pure Al) to 75 ± 2 HV, 3.7%, 490 ± 4 MPa (Al-9%ZrO2-0.15%graphene), respectively. Similarly, the lowest friction coefficient (0.18 under a 10 N load), the mass loss (0.011 g under a 5 N load), and wear rate (0.0031 mm3/(Nm) under a 5 N load) were obtained at the Al-9%ZrO2-0.15%graphene composite. The mechanical strength and tribological behaviors of Al hybrid composites deteriorated in the case of over 9wt.%ZrO2 and 0.15wt.%graphene contents due to the agglomerations of graphene and ZrO2 nanoparticles. Therefore, it may be concluded that graphene is an excellent solid lubricator, and ZrO2 has a remarkable wear resistance for Al hybrid composites.
The severe deterioration of the interface is a crucial factor to restrict the performance improvement of composites. In order to understand the effect of interfacial structure on the mechanical properties and failure modes of the composites, the carbon fiber reinforced aluminum matrix (Cf/Al) composites were fabricated by pressure infiltration process at different fabrication temperatures and Ni coating modification. The micro-indentation test is used to determine the mechanical properties of the matrix between fiber bundles to infer the content of the Al4C3. The results indicate that compared with Al alloy, the hardness and modulus of the matrix between the carbon fiber bundles can be significantly increased by 38.5% and 36.5%, while the matrix between Ni-coated fiber bundles remains unchanged. This is due to the Ni coating preventing the carbon fiber against reaction with liquid aluminum during processing the composite, which reduces the generation of brittle carbides. In addition, it was revealed for the first time that the oblique/axial brittle fracture modes of the fibers occurred simultaneously due to severe interfacial reaction, and the brittle failure mechanism was clarified. The presence of Ni coating on the carbon fiber significantly improves the bending strength and interlaminar shear strength (ILSS) of the composite by approximately 81% and 86%, respectively. The Ni coating optimizes the interfacial bonding, so that the interface can absorb more fracture energy under loads. This work can provide a pratical route to prepare and applicate the high-quality continuous fiber reinforced Al matrix composites.
The strengthening effect of composites is rather limited in comparison with the excellent properties of graphene due to difficulty in acquiring strong interfacial bonding. To enhance the interfacial bonding and reduce the interface mismatch between the matrix and reduced graphene oxide (rGO), a novel strategy in this study is proposed through generating hybrid layered double oxides (LDO) nanoparticles on rGO ([email protected]). The 2024Al composites with heterogeneous structure were constructed by ball milling and spark plasma sintering (SPS), which was reinforced by flake-like [email protected] zones contained [email protected] in the Al matrix with fine grain size of ~ 1μm. The yield strength, elongation and fracture energy of 1 vol.% [email protected]/Al composite with heterogeneous microstructure were 69.6%, 63.9% and 140.5% higher than those of the composite reinforced by uniformly distributed 0.67 vol.% graphene oxide (GO), respectively, achieving an improvement in the strength-ductility synergy of the fabricated [email protected]/Al composite. The rationally spatial arrays of [email protected] and [email protected] zones are beneficial for promoting the synergistic strengthening of Orowan, solid solution, thermal mismatch and load transfer and simultaneously toughening the composite through enhanced crack deflection and bridging effects. The proposed method offers a promising route for fabricating composite with optimized and improved material properties by coupling interface and heterogeneous structure.
AISI 304 quality stainless steel has become an
indispensable material for daily use thanks to its high
corrosion resistance, high heat conductivity, formability
and visual properties [1-2]. Stainless steel flat materials are
produced with hot rolling, annealing and cold rolling after
the casting process and then it is prepared for service
processes by ensuring annealing and surface passivation.
In this study, the effect of annealing temperature, time and
the difference of acidification processes on the corrosion
resistance of the material is investigated.
In this study, the influence of multilayer graphene content on the green and sintered properties of the multilayer graphene/Copper nanocomposites was investigated. Flake powder metallurgy, as a new production method, was employed to prepare the multilayer graphene reinforced copper matrix nanocomposites. Results showed that the increase in agglomeration content inhibited particle-particle contact during the sintering process and therefore sintered density decreased with increasing the multilayer graphene content. The green density of 8.46 g/cm3 was found for the monolithic Cu sample, which was 16.4% higher than that of the 5 wt% MLG/Copper nanocomposites. The high conductivity value (78.5 IACs) was obtained with 0.5 wt% the multilayer graphene reinforced nanocomposites. The electrical conductivity of sintered 5 wt% the multilayer graphene/Copper nanocomposites was 61.48 IACs. When the amount of the multilayer graphene particles as higher than 3 wt%, the decreasing rate in hardness significantly increased. The decreasing rate in the hardness of the multilayer graphene/Copper nanocomposites can be attributed to decrease in density and the non-homogeneous distribution of multilayer graphene particulates in Cu matrix. © 2015, The Korean Institute of Metals and Materials and Springer Science+Business Media Dordrecht.
Polycrystalline NiFe 2 O 4 was prepared by solid state reaction from nano size powder of NiO and Fe 2 O 3 which were synthesized by wet chemical method. The inverse spinel single phase of the sample has been confirmed by the X-ray diffraction patterns. SEM micrographs of the samples revealed that the grain size increases and the porosity decreases with the increase in sintering temperature and has great influence on the magnetic properties of NiFe 2 O 4. Enhancement of real part of initial permeability (µ′) as a function of frequency has been observed with the increase in sintering temperature. Temperature dependence real part of initial permeability has been observed at various sintering temperature and gives the manifestation of Hopkinson effect. Variation of Curie temperature (T c) has been found with the variation of sintering temperature.
In recent years, graphene has attracted considerable research interest in all fields of science due to its unique properties. Its excellent mechanical properties lead it to be used in nano-composites for strength enhancement. This paper reports an Aluminum–Graphene Nanoplatelets (Al/GNPs) composite using a semi-powder method followed by hot extrusion. The effect of GNP nano-particle integration on tensile, compressive and hardness response of Al is investigated in this paper. It is demonstrated that 0.3 wt% Graphene Nanoplatelets distributed homogeneously in the matrix aluminum act as an effective reinforcing filler to prevent deformation. Compared to monolithic aluminum (in tension), Al–0.3 wt% GNPs composite exhibited higher 0.2% yield strength (+14.7%), ultimate tensile strength (+11.1%) and lower failure strain (−40.6%). Surprisingly, compared to monolithic Al (in compression), Al–0.3 wt% GNPs composite exhibited same 0.2% compressive yield strength and lower ultimate compression strength (−7.8%), and lower failure strain (−20.2%). The Al–0.3 wt% GNPs composite exhibited higher Vickers hardness compared to monolithic aluminum (+11.8%). Scanning electron microscopy (SEM), Energy-Dispersive X-ray Spectroscopy (EDS) and X-ray diffraction (XRD) were used to investigate the surface morphology, elemental percentage composition, and phase analysis, respectively.
In recent years, reducing friction and wear-related mechanical failures in moving mechanical systems has gained increased attention due to friction's adverse impacts on efficiency, durability, and environmental compatibility. Accordingly, the search continues for novel materials, coatings, and lubricants (both liquid and solid) that can potentially reduce friction and wear. Despite intense R&D efforts on graphene for a myriad of existing and future applications, its tribological potential as a lubricant remains relatively unexplored. In this review, we provide an up-to-date survey of recent tribological studies based on graphene from the nano-scale to macro-scale, in particular, its use as a self-lubricating solid or as an additive for lubricating oils.
Effect of graphene nanoplatelets (GNPs) addition on mechanical properties of magnesium–10wt%Titanium (Mg–10Ti) alloy is investigated in current work. The Mg-(10Ti + 0.18GNPs) composite was synthesized using the semi powder metallurgy method followed by hot extrusion. Microstructural characterization results revealed the uniform distribution of reinforcement (Ti + GNPs) particles in the matrix, therefore (Ti + GNPs) particles act as an effective reinforcing filler to prevent the deformation. Room temperature tensile results showed that the addition of Ti + GNPs to monolithic Mg lead to increase in 0.2% yield strength (0.2% YS), ultimate tensile strength (UTS), and failure strain. Scanning Electron Microscopy (SEM), Energy-Dispersive X-ray Spectroscopy (EDS) and X-Ray Diffraction (XRD) were used to investigate the surface morphology, elemental dispersion and phase analysis, respectively.
Composites of graphene platelets and powdered aluminum were made using ball milling, hot isostatic pressing and extrusion. The mechanical properties and microstructure were studied using hardness and tensile tests, as well as electron microscopy, X-ray diffraction and differential scanning calorimetry. Compared to the pure aluminum and multi-walled carbon nanotube composites, the graphene–aluminum composite showed decreased strength and hardness. This is explained in the context of enhanced aluminum carbide formation with the graphene filler.Highlights► We investigated the mechanical properties of aluminum and aluminum nanocomposites. ► Graphene composite had lower strength and hardness compared to nanotube reinforcement. ► Processing causes aluminum carbide formation at graphene defects. ► The carbides in between grains is a source of weakness and lowers tensile strength.
We measured the elastic properties and intrinsic breaking strength of free-standing monolayer graphene membranes by nanoindentation in an atomic force microscope. The force-displacement behavior is interpreted within a framework of nonlinear elastic stress-strain response, and yields second- and third-order elastic stiffnesses of 340 newtons per meter (N m(-1)) and -690 Nm(-1), respectively. The breaking strength is 42 N m(-1) and represents the intrinsic strength of a defect-free sheet. These quantities correspond to a Young's modulus of E = 1.0 terapascals, third-order elastic stiffness of D = -2.0 terapascals, and intrinsic strength of sigma(int) = 130 gigapascals for bulk graphite. These experiments establish graphene as the strongest material ever measured, and show that atomically perfect nanoscale materials can be mechanically tested to deformations well beyond the linear regime.
We report the measurement of the thermal conductivity of a suspended single-layer graphene. The room temperature values of the thermal conductivity in the range approximately (4.84+/-0.44)x10(3) to (5.30+/-0.48)x10(3) W/mK were extracted for a single-layer graphene from the dependence of the Raman G peak frequency on the excitation laser power and independently measured G peak temperature coefficient. The extremely high value of the thermal conductivity suggests that graphene can outperform carbon nanotubes in heat conduction. The superb thermal conduction property of graphene is beneficial for the proposed electronic applications and establishes graphene as an excellent material for thermal management.
Graphene reinforced bulk titanium matrix composites (TMCs) were successfully fabricated via powder metallurgy approach. 0.5 wt.% graphene nanoflakes (GNFs) and Ti6Al4V mixture powders were prepared by a wet process. The composites were then consolidated using hot isostatic pressing (HIP) with a pressure of 150 MPa at 700 °C followed by isothermal forging with a forging ratio of 3 at 970 °C. The microstructure and mechanical properties of TMCs had been investigated by optical microscopy, SEM, TEM and static tensile tests. Microstructure observation illustrated a uniform distribution of graphene in the composite and in-situ formed TiC particles at the metallurgical interface between titanium matrix and graphene. Compared with the unreinforced titanium matrix, the 0.5 wt.% GNFs reinforced composite exhibits significantly improved strength without losing ductility, which demonstrates that GNFs could actually act as superb reinforcements in TMCs.
Graphene nanoplatelets (GNPs)/silicone rubber composites were prepared with the assistance of the Flacktek SpeedMixer. A scanning electron microscope (SEM), transmission electron microscopy (TEM), Raman spectra, Fourier-transform infrared spectra (FTIR), and X-ray photoelectron spectra (XPS) were carried out to characterize the structure of graphene nanoplatelets. An electronic universal testing machine, laser thermal conductivity analysis (LFA), thermogravimetric analysis (TGA), and a scanning electron microscope (SEM) reveal the effects of GNP loading content on the thermal conductivity, electrical, and mechanical properties of the composites. The results show that the GNPs present a homogeneous dispersion in silicone rubber and the thermal conductivity of composites exhibits improving from 0.16 to 0.26. W. /. (m. ·. K) (an increase of 53.1%) and the tensile strength varies from 0.240 to 0.608. MPa (an increase of 153%) with the addition of a low content (0-8%) of GNPs. In addition, the thermal stability of silicone rubber composites is significantly enhanced.
As one of the most important engineering materials, aluminum alloys have been widely applied in many fields. However, the requirement of enhancing their mechanical properties without sacrificing the ductility is always a challenge in the development of aluminum alloys. Thanks to the excellent physical and mechanical properties, graphene nanoflakes (GNFs) have been applied as promising reinforcing elements in various engineering materials, including polymers and ceramics. However, the investigation of GNFs as reinforcement phase in metals or alloys, especially in aluminum alloys, is still very limited. In this study, the aluminum alloy reinforced by GNFs was successfully prepared via powder metallurgy approach. The GNFs were mixed with aluminum alloy powders through ball milling and followed by hot isostatic pressing. The green body was then hot extruded to obtain the final GNFs reinforced aluminum alloy nanocomposite. The scanning electron microscopy and transmission electron microscope analysis show that GNFs were well dispersed in the aluminum alloy matrix and no chemical reactions were observed at the interfaces between the GNFs and aluminum alloy matrix. The mechanical properties׳ testing results show that with increasing filling content of GNFs, both tensile and yield strengths were remarkably increased without losing the ductility performance. These results not only provided a pathway to achieve the goal of preparing high strength aluminum alloys with excellent ductilitybut they also shed light on the development of other metal alloys reinforced by GNFs.
Graphene nanosheet (GNS)/aluminum nitride (AlN) composites were prepared by hot-pressing and effects of GNSs on their microstructural, mechanical, thermal, and electrical properties were investigated. At 1.49 vol% GNSs content, the fracture toughness (5.09 MPa m1/2) and flexural strength (441 MPa) of the composite were significantly increased by 30.17% and 17.28%, respectively, compared to monolithic AlN. The electrical conductivity of the composites was effectively enhanced with the addition of GNSs, and showed a typical percolation behavior with a low percolation threshold of 2.50±0.4 vol%. The thermal conductivity of the composites decreased with the addition of GNSs.
In order to develop high strength metal–matrix composites with acceptable ductility, bulk nanostructured aluminum–matrix composites reinforced with graphene nanoflakes were fabricated by cryomilling and hot extrusion processes. Microstructure and mechanical properties were characterized and determined using transmission electron microscopy, electron dispersion spectroscopy, as well as static tensile tests. The results show that, with an addition of only 0.5 wt% graphene nanoflakes, the bulk nanostructured aluminum/graphene composite exhibited increased strength and unsubdued ductility over pure aluminum. Besides, the mechanical properties of the composites with higher content of graphene nanoflakes were also measured and investigated. Above 1.0 wt% of graphene nanoflakes, however, this strengthening effect sharply dropped due to the clustering of graphene nanoflakes. Furthermore, the optimal addition of graphene nanoflakes into the nanocrystalline aluminum matrix was calculated and discussed.
The Mg-1%Al-1%Sn-0.18% graphene nanoplatelets (GNPs) composite is fabricated by semi powder metallurgy method followed by hot extrusion. Microscopic observation revealed the uniform distribution of GNPs in the matrix. The addition of 0.18. wt% GNPs to Mg-1wt%Al-1wt%Sn alloy lead to increase in tensile strength (i.e., from 236 to 269. MPa). The increase in strength of the composite could be due to high specific surface area, superior nano-filler adhesion and two-dimensional structure of GNPs. © 2014 The Korean Society of Industrial and Engineering Chemistry.
The Mg-Al-graphene nanoplatelets (GNPs) nano-composites were synthesized using the powder metallurgy method. The effect of Al-GNPs hybrids addition in to pure Mg was examined through tensile and Vicker hardness tests. The GNPs content was kept constant (0.18 wt.%) and Al content was varied from 0.5 wt.% to 1.5 wt.%. The increase in Al content led to increase in 0.2%YS, UTS and failure strain (%). However for Al content exceeding over 1 wt.%, the failure strain(%) started to decrease. The best improvement was achieved with 1 wt.% Al (Mg-1.0Al-0.18GNPs). Mechanical strength of synthesized composites proved to be better than Mg-Al-CNTs and Mg-ceramic composites.
The effects of different sintering temperatures, namely 400, 500 and 600 degrees C, on the mechanical properties of four aluminum-graphite alloys were reported. Different percentages of exfoliated graphite nanoplatelets particles (xGnP) were added to pure aluminum by using the powder metallurgy technique to produce Al-0 wt.%xGnP, Al-1 wt.%xGnP, Al-3 wt.%xGnP, and Al-5 wt.%xGnP. The density, fracture surface, compression, and hardness measurements were carried out to report the mechanical properties of the different aluminum-xGnP alloys. Combined data indicated that the Vickers hardness and compressive strength increase, on the other hand, the density decreases with increasing the graphite content in the Al alloys.
Investigations in graphene of controlling friction and wear of Ni3Al matrix self-lubricating composites (NMSC) are needed for moving mechanical assemblies. The friction and wear behaviors of NMSC with graphene nanoplatelets (GNPs) against Si3N4 ball are tested using a constant load of 10N and a constant speed of 0.2m/s from room temperature (RT) to 600 °C. Tribological test results have revealed that small amounts of GNPs in the NMSC are able to drastically reduce the friction coefficients and wear rates over the effective operating range (RT - 400 °C). A possible explanation for these results is that the refinement of grains accompanying the slippage of laminated sheets between GNPs could provide a source of the stress dissipation and form the GNP protective layer during sliding process, leading to the reduction of wear rates as well as the friction coefficients. It is concluded that GNPs hold great potential applications as an effective solid lubricant for moderate loads and stress, and can be easily used for the preparation of the self-lubricating composites in the future.
10–90 vol.% of flake graphite is combined with aluminum powders to form aluminum–graphite composites via vacuum hot pressing process. The results show that the relative density of powder composites achieves up to 99% for composites containing 10–90 vol.% graphite. With the increase of flake graphite from 10 to 90 vol.% in the composites, thermal conductivity increase from 324 to 783 W/m K; the coefficients of thermal expansion (CTE) in direction parallel to basal planes of graphite flakes decrease from 16.9 to −2.5 ppm/K, and the CTE perpendicular to basal plane decreases from 15.2 to 10.1 ppm/K. The parallel arrangements of graphite flakes and aluminum lead the thermal conductivity values of composites to agree well with parallel model estimations. The CTE of composites in a-axis agree with rule of mixture estimation representing proportional contribution of the two phases. Along the c-axis of graphite in composites, the CTE are consistent with Turner model estimations indicating that the graphite flake could absorb the expansion from the aluminum phase.
Graphene has a high fracture strength of 125 GPa, making it an ideal reinforcement for composite materials. Aluminum composites reinforced with graphene nanosheets (GNSs) were fabricated for the first time through a feasible methodology based on flake powder metallurgy. The tensile strength of 249 MPa was achieved in the Al composite reinforced with only 0.3 wt.% GNSs, which is 62% enhancement over the unreinforced Al matrix. The relevant strengthening mechanisms involved in the GNS/Al composites were discussed along with experimental procedure.
It is extremely difficult to effectively incorporate and disperse graphene nanoplatelets into metal matrix by traditional processing methods. In this paper, a novel nanoprocessing method that combines liquid state ultrasonic processing and solid state stirring is presented for fabrication of bulk metal-graphene nanoplatelets nanocomposites. The obtained graphene nanoplatelets reinforced Mg-based metal matrix nanocomposite shows a uniform dispersion of graphene nanoplatelets and dramatically enhanced properties. This novel nanoprocessing route has great potential for the production of ultrahigh performance metal matrix nanocomposites.
Al2O3 is a major reinforcement in aluminum-based composites, which have been developing rapidly in recent years. The aim of this paper is to investigate the effect of alumina particle size, sintering temperature and sintering time on the properties of Al–Al2O3 composite. The average particle size of alumina were 3, 12 and 48μm. Sintering temperature and time were in the range of 500–600°C for 30–90min. A correlation is established between the microstructure and mechanical properties. The investigated properties include density, hardness, microstructure, yield strength, compressive strength and elongation to fracture. It has been concluded that as the particle size of alumina is reduced, the density is increased followed by a fall in density. In addition, at low particle size, the hardness and yield strength and compressive strength and elongation to fracture were higher, compared to coarse particles size of alumina. The variations in properties of Al–Al2O3 composite are dependent on both sintering temperature and time. Prolonged sintering times had an adverse effect on the strength of the composite.
Fully dense graphene nanosheet(GNS)/Al 2 O 3 composites with homogeneously distributed GNSs of thicknesses ranging from 2.5 to 20 nm have been fabricated from ball milled expanded graphite and Al 2 O 3 by spark plasma sintering. The percolation threshold of elec-trical conductivity of the as-prepared GNS/Al 2 O 3 composites is around 3 vol.%, and this new composite outperforms most of carbon nanotube/Al 2 O 3 composites in electrical con-ductivity. The temperature dependence of electrical conductivity indicated that the as-pre-pared composites behaved as a semimetal in a temperature range from 2 to 300 K.
Silicon nitride based composites with different amount (1, 3 or 5 wt%) of carbon nanotubes have been prepared by using hot isostatic pressing. Composites with 1, 5 or 10 wt% carbon black and graphite have been manufactured, in comparison. Optimisation of the manufacturing processes has been performed, providing intact carbon nanotubes during high temperature sintering. In the case of 1 or 3 wt% carbon nanotube addition, the increase of gas pressure during sintering resulted in an increase of bending strength. It was found that microstructure features achieved by properly designed sintering parameters are the main responsible factors for the strength improvements. Scanning electron microscopy showed that carbon nanotubes have a good contact to the surface of silicon nitride grains. The electrical properties of silicon nitride matrices with carbon addition may change essentially.
We have achieved mobilities in excess of 200,000 cm2 V −1 s−1 at electron densities of ∼2 ×1011 cm−2 by suspending single layer graphene. Suspension ∼150 nm above a Si/SiO2 gate electrode and electrical contacts to the graphene was achieved by a combination of electron beam lithography and etching. The specimens were cleaned in situ by employing current-induced heating, directly resulting in a significant improvement of electrical transport. Concomitant with large mobility enhancement, the widths of the characteristic Dirac peaks are reduced by a factor of 10 compared to traditional, nonsuspended devices. This advance should allow for accessing the intrinsic transport properties of graphene.
The excellent mechanical properties of carbon nanotubes are being exploited in a growing number of applications from ballistic armour to nanoelectronics. However, measurements of these properties have not achieved the values predicted by theory due to a combination of artifacts introduced during sample preparation and inadequate measurements. Here we report multiwalled carbon nanotubes with a mean fracture strength >100 GPa, which exceeds earlier observations by a factor of approximately three. These results are in excellent agreement with quantum-mechanical estimates for nanotubes containing only an occasional vacancy defect, and are approximately 80% of the values expected for defect-free tubes. This performance is made possible by omitting chemical treatments from the sample preparation process, thus avoiding the formation of defects. High-resolution imaging was used to directly determine the number of fractured shells and the chirality of the outer shell. Electron irradiation at 200 keV for 10, 100 and 1,800 s led to improvements in the maximum sustainable loads by factors of 2.4, 7.9 and 11.6 compared with non-irradiated samples of similar diameter. This effect is attributed to crosslinking between the shells. Computer simulations also illustrate the effects of various irradiation-induced crosslinking defects on load sharing between the shells.