Self-lubricating bidirectional carbon fiber reinforced smart aluminum composites by squeeze infiltration process

1. Introduction

Smart composites are self-responding advanced materials to environmental changes without altering in its characteristics [1]. These active composites are expected to create a revolution in design and fabrication of engineering components and systems in aerospace, automotive, and defense sectors. Smart metallic composite can be of different types such as self-lubricating, self-healing, super-hydrophobic and shape-memory. Nowadays, self-lubricating smart composites (SLSC) are considered for tribological applications to perform self -activation in critical dry conditions. The deposition of thin solid lubricant film on the engineering material is the common technique used to develop self-lubricating composite. However, limitations like short lifespan of the thin lubricant film during service and the difficulty in the deposition of the thin film on the complex shaped components cause major setback to the technique [2]. Hence, to offer the continuous supply of self-lubrication under service till the degradation of the component is really challenging, and the studies are limited. The SLSC can be developed by integrating self-active reinforcements in the metal, ceramic or polymer matrix. In recent years, high strength carbon fibers have emerged as one of the potential reinforcements in aluminum metal matrix offering the unique combination of properties with low density when blending together. Lancin et al. [3] reported that the formation of brittle interfacial reaction product Al₄C₃ between carbon fiber and the aluminum is the major problem encountered in carbon fiber reinforced aluminum matrix composites (C_fAMC), which affects the final properties of the composite. Several studies have been conducted on the coating of carbon fiber using Cu [4], Ni [5], Al₂O₃ [6], SiC [7], etc. to avoid the detrimental reaction with the molten matrix metal during composite preparation. However, some of the factors like; change in density of the reinforcement, dissolution of the coatings into the matrix thereby formation of brittle products and reduction in ductility, set limitations to the coating process. But based on the fabrication methods of the composite, better adhesion and smooth interface without harmful reactions between the matrix and reinforcement can be obtained.

Different fabrication techniques have been implemented to develop fiber reinforced metal matrix composites like special flux method [8], infiltration [9], powder metallurgy, diffusing bonding [10], etc. Among the above techniques, squeeze infiltration of molten metal into the permeable fibrous preform has been emerged as one of the economical and effective method due to its supremacy in incorporating high reinforcement content. Also, this process overcomes wetting problems, poor interface bonding, and interfacial reactions [11]. Various studies have been carried out using carbon short fibers [12], unidirectional fibers [13] and carbon nanotube [14] reinforced composite. Carbon in the form of graphite [15], graphene [16] and diamonds [17] of particles size in the range of micro and nano were also used for the preparation of aluminum composites. In contrast, the studies on the development of bidirectional satin weave PAN-based carbon fiber fabric reinforced aluminum matrix composites and its application for self-lubrication are finite. Hence, the present study aims to develop bidirectional carbon fiber fabric reinforced Al 6061 composite (C_fAMC) by squeeze infiltration technique with controlled interfacial reaction and to investigate the role of carbon fiber in the aluminum matrix forming a self-lubricating smart composite.

2. Experimental

Al 6061 was chosen as the matrix alloy since it is one of the most widely used wrought alloys for the application in aerospace, automotive, naval and defence sectors. Al 6061 alloy of composition Al-1.2 Mg-0.8 Si (wt%) supplied by Sargam Metals, India was used. Satin weave PAN-based 3 K bidirectional carbon fiber fabric was used as the reinforcement, having 3000 carbon fibers in each tow. The average diameters of the fibers were 7 μm with a density of 1.8 g/cm³. Bidirectional C_f/Al composite was fabricated by squeeze infiltration technique with optimized process parameters. Besides, Al alloy was processed by squeeze casting to obtain a balance during comparison of characterization with the composite. The carbon fiber mat was cut into pieces and stacked together to fabricate the preform of dimension 8 cm length, 3 cm width and 1 cm height. The layers of the carbon fiber mat in the preform were stitched together with the help of carbon fiber strands, and then the preform was preheated to 350 °C for 1 h. Meanwhile, Al 6061 alloy was melted at 780 °C and pure Mg of 1 wt% was added to the melt to enhance the wettability during infiltration. Finally, the liquid Al was poured into the preheated die (220 °C) containing the carbon fiber preform, and a squeeze pressure of 40 MPa was applied over the molten Al to infiltrate the preform. The composite was solidified under pressure, and it was maintained for 3 min.

As cast specimens of alloy and infiltrated composites were used for characterization. The microscopic images were observed using Leica DMRX optical microscope, JEOL scanning electron microscope (SEM), and JEOL-JEM-2100 transmission electron microscope (TEM) wherein last two were equipped with Oxford EDS. The extracted C_f from the infiltrated composite was analyzed using Philips X-ray diffraction with CuK_α radiation. Thermogravimetric analysis (TGA) was conducted in Hitachi STA7300 (TG) equipment at a heating rate of 10 °C/min. The Archimedes principle was used to calculate the density of alloy and infiltrated composites. The hardness and compression measurements were carried out using Zwick Brinell hardness and Instron machines respectively. The test was conducted at room temperature. During hardness testing, a load of 62.5 kg was applied by a 2.5 mm ball indenter. The compression samples were prepared based on ASTM-E9 specification having 6 mm diameter and 9 mm length. High carbon steel tool was used for machining the samples. The dry wear tests were carried out in room temperature using DUCOM (TR20LE) pin-on-disc apparatus at two conditions; constant sliding velocity of 2 and 4 m/s with different loads of 20, 30, 40 and 50 N and at constant load of 30 and 50 N with different sliding velocities of 1, 2, 3 and 4 m/s with 1500 m as sliding distance for the duo. The wear rate was calculated based on the weight loss of the pin having a diameter of 6 mm and a length of 30 mm. The counter surface disk used for pin-on-disk wear testing was EN31 high carbon alloy steel. Electrochemical and Immersion tests were conducted to analyze the corrosion characteristics of alloy and composite. Electrochemical studies were done using potentiodynamic electrochemical workstation of CH Instruments (Model CHI680) having three-electrode cell configuration. For all the experiments, Al alloy or composite were used as the working electrode, platinum was used as the auxiliary electrode and calomel as the reference electrode. The working electrode under study was dipped in 3.5 wt% NaCl solution and the applied scan rate was 1 mV/s. In immersion test method, the material under study (alloy and composite) having a dimension of 0.22 cm³ was immersed in 3.5 wt% NaCl solution for 7, 14 and 21 days and measured the weight loss of the material after the exposure to NaCl. Nitric acid was used to remove the corrosion products from the corroded samples.

3. Result and discussion

3.1. Structural characteristics

The SEM images of as received and surface treated carbon fibers are shown in Fig. 1. As received carbon fiber shows a glossy surface finish (Fig. 1(a)) that originated from the polymer coatings. For the smooth weaving of carbon fiber mat and to evade the static charges due to the friction between carbon fibers, normally polymer coating is applied on the surface of the carbon fibers [18]. This kind of coating inhibits the wettability between the metal matrix and reinforcement during infiltration. Hence, the carbon fiber is surface treated in acetone and distilled water to remove the coating used for sizing the fibre and dried in an oven. The treated carbon fiber reveals an uneven surface finish due to the removal of coating (Fig. 1(b)).

The macrograph of the C_f mat preform and the C_f/Al 6061 infiltrated composite are shown in Fig. 2. It is clear from the macrograph that the developed composite is free from porosities due to the solidification process that takes place under pressure and also it provokes the concept of selectively reinforced composite (SRC). The application of selective infiltration leads to the development of advanced functionally graded composite materials, through which the location specific properties can be achieved.

Optical microstructures of the BD C_f/Al 6061 infiltrated composite are shown in Fig. 3. The tiny interspaces of the carbon fiber fabric are completely infiltrated by liquid aluminum, which shows the effect of squeeze pressure on wettability between the matrix and reinforcement. Uniform distribution of carbon fibers is observed in the matrix aluminum due to the use of an anticipatory preform having homogeneous arrangements of bidirectional fibers (Fig. 3(a)). In addition, squeeze infiltration did not make fiber agglomeration and damage in preform. Perfect adhesion between the carbon fiber and aluminum is perceived in the microstructure. The cross-section of the C_f/Al composite in Fig. 3(b) shows the depth of penetration of molten aluminum into the multiple stacks of fiber mat preform and the infiltration is observed between each fiber. Moreover, a single layer of carbon fiber mat in the preform contains fibers arranged in the transverse and longitudinal direction. Twenty layers of such carbon fiber mat are successfully infiltrated. The junction between the transverse and longitudinal alignment of woven carbon fiber fabric is clearly seen in the microstructure (Fig. 3(a)). While polishing the composite, longitudinally oriented fibers on the oblique plane get polished and its appearance resembles with elliptical shape cross section, which is observed in the microstructure (Fig. 4).

The SEM image (Fig. 5(a)) of the squeeze infiltrated composite reveals the intimate interfacial bonding between the aluminum and carbon fiber. The matrix-reinforcement interface is free from defects with adequate wetting behavior. Fig. 5(b) shows the SEM images of the extracted carbon fiber from the infiltrated composite. The surface of the extracted fiber is smooth without remarkable interfacial reactions, which specify the precision of squeeze infiltration process parameters. Suzuki et al. [19] reported that the excess development of brittle reaction phase Al₄C₃ in the Al-carbon fiber composite cause notch shape damages on the surface of the fiber and results in the degradation of properties, which is very limited in this system.

The TEM examination of interface between C_f and Al in the composite is shown in Fig. 6(a), in which the existence of oxide layers like Al₂O₃ and MgAl₂O₄ spinel are observed and confirmed by TEM-EDS (Al₂O₃ (at.%): Al-42.57, O-57.43; MgAl₂O₄ (at.%): Al-35.56, O-56.95, Mg-7.48). The formation of oxide layers at the interface region is due to the reaction between Al alloy and the oxygen present in the surface of the C_f preform, in addition these oxide layers may inhibit the direct reaction of Al with C_f. Fig. 6(b) shows the HRTEM image of oxide layer having crystalline lattice planes with an interplanar distance d₀₁₂ of 0.34 nm, which corresponds with Al₂O₃ structure. Both orthorhombic and trigonal structures of alumina are detected from the interface. Major portion of the oxide layer found at the interface is Al₂O₃. The perfect arrangement of atoms in Al₂O₃ lattice can be seen from the deconstructed TEM image using FFT (Fast Fourier Transform) in Gatan Microscopy suite software.

Very limited formation of Al₄C₃ crystals having a lath shape is observed (Fig. 7(a)) in the oxide-matrix interface region, which is confirmed by TEM-EDS (Al₄C₃ (at.%): Al-81.59, C-18.41) and selected area diffraction analysis (SAD) with an interplanar distance d₁₀₇ of 0.23 nm. Observations in higher magnification show single twinned regions in Al₄C₃ crystals (Fig. 7(b)) with a cross section of about 7 nm and it is also evident from the auxiliary electron diffraction (additional spots marked as symmetrical quadrilateral) in SAD pattern (Fig. 7(c)). The difference in the thermal expansion coefficient between Al₄C₃ and matrix aluminum cause compressive strain in the carbide crystals during solidification, which create surface twinning in Al₄C₃ [20]. Very few precipitates of intermetallic phase Mg₂Si (Mg₂Si (at.%): Mg-67.74, Si-32.26) is observed at the composite interface region, which forms from the reaction of Mg and Si present in the Al 6061 alloy (Fig. 8).

3.2. XRD analysis

The XRD diffractogram of extracted fibers from the infiltrated composite using NaOH solution is shown in Fig. 9. The extraction is done by immersing the composite specimen in an aqueous solution containing 10 g of NaOH in 100 ml of water followed by filtration. The major diffraction peaks observed in the extracted sample belongs to C and precipitated phases like Al₁₂Mg₁₇, MgAl₂O₄ spinel and Al₂O₃. The reaction between Al and its alloying element Mg leads to the formation of intermetallic phase Al₁₂Mg₁₇, which helps in enhancing the mechanical properties of the material. Besides, the externally added Mg helps in reducing the surface tension of molten aluminum during its infiltration into the preform by accumulating oxygen and oxide present in the infiltration front. MgAl₂O₄ spinel forms from the above process and it enhances the matrix/reinforcement wettability. The higher thickness of the oxide layer at the infiltration front reduces the wettability and leads to the incomplete infiltration of molten metal into the preform. The reaction between aluminum and carbon cause the brittle Al₄C₃ phase. The formation of Al₄C₃ in this study is controlled by the squeeze infiltration technique and its optimized process parameters like temperature, pressure and dwell time. The formation and growth of Al₄C₃ phase increases, if the melt temperature is high and the solidification rate is slow. But the high heat transfer coefficient of squeeze infiltration technique increases the cooling rate, so the possibility of Al₄C₃ formation is lowered.

3.3. Thermal and physical properties

The TGA curve of surface treated carbon fiber is shown in Fig. 10. The carbon fiber is stable during heating in air atmosphere till 600 °C and the rapid decomposition occurs after that. Instability of carbon fiber above 600 °C is due to the oxidation of carbon, as the temperature increases the rate of fiber oxidation also increases simultaneously. Hence, the preform preheating is carried out below the carbon fiber decomposition temperature in air atmosphere followed by metal infiltration. Higher temperature can be used if the preheating is done in inert or vacuum atmosphere. The carbon fiber fabric reinforced aluminum composite showed a low density of 2.16 g/cm³ compared to matrix Al alloy (2.7 g/cm³). The reduction in the density of the composite is due to the incorporation of low density carbon fiber (1.8 g/cm³) in the matrix alloy. The content of carbon fiber in the infiltrated composite is 63 vol.%.

3.4. Mechanical characteristics

Carbon fiber reinforced composite exhibited superior hardness properties in both top surface (105 BHN) and cross section (93 BHN) when compared to base alloy (56 BHN). The presence of hard and stiff carbon fiber in the matrix shares the load generated from the indenter and being a continuous fiber it acts as a barrier to matrix dislocation. A slight reduction of hardness is observed in the cross section of the composite due to the orientation of the fiber having longitudinal and transverse alignment. The average compressive strength obtained for BD C_f/Al 6061 composite is 290 MPa, which is considerably higher than unreinforced Al alloy (223 MPa). The compression specimen is taken in the vertical direction with respect to carbon fiber preform plane direction. The supplement of bidirectional carbon fiber in the matrix enhance the compressive strength of the composite by holding the load in both the directions due to the nature of fiber orientation, thereby restrains the matrix plastic flow behavior. Fig. 11(a) and (b) shows the compressive stress-strain curve and SEM fractograph of the compression tested sample of C_f/Al composite. The rise and fall in the stress after barreling is due to the resistance provided by the longitudinal fibers against deformation. The above statement is clear from the SEM image, in which one of the longitudinal layers gets compressed when load is applied and gets fractured after withstanding the compression load, the next transverse layer slides off after the delamination of fiber from the matrix during compressive loading.

3.5. Tribological characteristics

The wear rate of Al 6061 alloy (squeeze cast) and BD C_f/Al composite at constant sliding velocity with different loads is shown in Fig. 12(a). The wear rate of Al alloy at 2 m/s increases enormously with increase in load due to the generation of higher temperature at higher load, which leads to thermal softening and plastic shear deformation of the pin material. The unreinforced alloy experienced a galling seizure above the load of 40 N followed by heavy noise, vibration and transfer of pin material to the counter surface, consequently, the tests were stopped. Mild wear to sever wear transition occurs at 30-40 N loads in Al alloy whereas the composite shows a mild wear regime even up to 50 N. The wear rate of C_f/Al composite is phenomenally lower compared to Al alloy due to the incorporation of carbon fiber thereby suppressing the transition to a severe wear rate. The curves of the composite at 2 m/s and 4 m/s overlapped each other due to the spacing of ordinate scale while considering the weight loss of alloy, which is much higher when compared to composite. Fig.12b shows the extensive graph of the composite at 2 m/s and 4 m/s, and it is clear from the graph that wear rate increases negligibly with increase in load due to the intensity of heat generated. Fig. 12(c) shows the wear rate of C_f/Al composite at constant load with different sliding velocities. In this case also, composite shows extreme wear resistance. However, an imperceptible amount of wear loss is experienced in the composite with increase in sliding velocity. The incorporation of carbon fiber in the matrix develops a self-lubricating (SL) tribolayer on the sliding surface, which intercepts the contact of matrix alloy to counter surface (disk). Additionally, the carbon fiber in the composite conveys the major portion of stress concentration developed during friction thereby holding the plastic flow of soft matrix.

The friction coefficient of Al 6061 alloy and BD C_f/Al composite at constant sliding velocity with different loads and at constant load with different sliding velocities are shown in Fig. 13(a) and (b). In both the conditions composite shows very low friction coefficient than unreinforced alloy due to the self-lubrication of carbon fiber in the friction surface during sliding, thereby soothe the friction condition. Small fluctuations in the friction coefficient of composite is also observed due to the variation in the frictional layer content.

The SEM images of the worn surface of Al alloy at constant sliding velocity with different loads are shown in Fig. 14. Shallow and wider grooves (marked as G) with the material flow (MF) are experienced in Al alloy rotated at 2 m/s velocity with 20 N load (Fig. 14(a)). At the load of 30 N, severe shearing is noticed in the alloy coupled with deeper grooves (marked as DG), surface deformation, and wedges of the damaged material (marked as W) along the sliding direction. The surface delamination of the material causes wedges and its size increases due to the propagation of the surface cracking (arrow marked). During the wear mechanism of alloy, frictional forces generated under different loads creates cyclic stresses on the sliding surface of the pin, which increases the temperature of the sliding surface. Under the cyclic thermal stresses, the alloy loses its toughness and yield strength and undergoes heavy plastic flow of the material.

The SEM micrographs of the worn surface of C_f/Al composite at constant velocity with different loads are shown in Fig. 15. The worn surface of the composite seems to be smoother compared to alloy and is free from cracks and delamination. The formation of SL-tribolayer also known as the mechanically mixed layer (MML) on the sliding surface can be perceived from the micrograph. It is clear from the SEM images that there is no debonding of carbon fiber from the matrix during friction, which shows good interfacial bonding. Even at higher load, the composite shows excellent wear resistance and all the wear tracks look similar. The carbon fiber orientations in the sliding direction improve the wear resistance of the composite. A similar observation was reported by Wang et al. [21] in the investigation on Al₂O_3f/SiC_p/Al hybrid composite. The worn surfaces of the composite at constant load with different sliding velocities are shown in Fig. 16. In this condition also, the formation of SL-tribolayer is observed on the sliding surface, and the worn surfaces are flat compared to the alloy. The worn surfaces show small granules in-between and above the fibers, these granules are from plastically deformed aluminum during wear, and later it contributes to the formation of MML. Initially, SL-tribolayer or MML forms as a large film, and later it splits and spreads over the friction surface (Fig. 16(b)).

Superior wear resistance of the C_f/Al composite than the unreinforced alloy is due to the evolution of SL-tribolayer. The formation of the SL-tribolayer on the sliding surface of the composite is evident in Fig. 17(a). EDS analysis of the SL-tribolayer formed above the worn surface of the C_f/Al composite shows the presence of carbon, aluminum, chromium, iron, and oxygen (Fig. 17(b)). The existence of oxidation reaction and other elements confirms the lubricating layer or MML [22]. The PAN based carbon fiber has strong tendency to perform the self-lubricating phenomenon due to its turbostratic structure, in which the planes of aromatic sheets (similar to graphite) are randomly oriented (Fig. 18(a)). In the structure of graphite, one carbon atom in the aromatic sheets sits in the center of each hexagon of the adjacent planes, and it is repeated (Fig. 18(b)). The carbon atoms in the aromatic sheet are connected by strong covalent bond and the planes are connected by weak van der waals bond. The debonding of weak van der waals force allows the planes to slip over one another easily; thereby carbon achieves its self-lubricating character [23,24]. Richard Tilley [25] stated that the adsorption of water vapor on the carbon surface weakens the bonding force between the planes. Because of the random orientation in turbostratic structure, the distance between its planes of aromatic sheets is longer when compared to graphite structure. This again weakens the vander waals force and makes the planes more slippery, thereby enhance the self-lubrication feature. Besides, during wear, carbon fiber interacts with moisture and makes the bonding of planes fragile. As a result, the surface planes of the carbon fiber slides comfortably during wear and later it joins together to form a solid SL-film.

3.6. Corrosion analysis

3.6.1. Electrochemical method

The potentiodynamic polarization curves (Fig. 19) show the corrosion potential (E_corr) value of the alloy (-0.87 V) and the composite (-0.76 V). The more positive E_corr obtained for composite is due to the potential difference at the Al-C_f interface. From the Tafel extrapolation, it is found that the corrosion current density (i_corr) of the composite (15 μA/cm²) is slightly higher than the unreinforced alloy (7 μA/cm²), indicating a comparable corrosion characteristic of the composite with the alloy. The anodic polarization curve of the alloy exhibits a small slope, may be due to the formation of the strong passive oxide layer Al₂O₃.

3.6.2. Immersion test

Table 1 shows the corrosion rate of Al alloy and the C_f/Al composite during immersion test. The corrosion rate of the alloy is negligible, and the trend is reduced with increase in exposure time due to the formation of strong passive oxide layer. When the exposure time keeps on increasing, cracks are forming in the oxide layer and cause further corrosion. The corrosion rate of the carbon fiber reinforced aluminum composite is higher compared to the alloy because of galvanic corrosion, and the trend is increased with increase in exposure time.

Fig. 20 shows the surface topography of the corroded samples of alloy immersed for 7, 14 and 21 d. The formation of the oxide layer (Al₂O₃) on the surface of the Al alloy increases with increase in exposure time. The carbon fiber reinforced aluminum composite is more susceptible to galvanic corrosion owing to the noble potential of carbon fiber, which acts as a passive region. Al 6061 alloy act as an active region from which the electron flow and material loss take place. From the Fig. 21 it can be observed that the formation of the oxide layer Al₂O₃ is less in the composite compared to the alloy due to the volume ratio of the cathode (carbon fiber) and anode (alloy). In addition, the corrosion of aluminum creates cracks, pits, and fiber pull-out in the composite. The morphology of the oxide layer is similar in alloy and composite, and EDS confirms its presence.

4. Conclusions

The high volume fraction of bidirectional carbon fiber fabric was uniformly distributed in aluminum 6061 alloy using squeeze infiltration technique to form a self-lubricating smart composite. The major conclusions of the present investigation are as follow:

(1) Complete penetration of liquid metal between every carbon fiber in the preform is observed in composite with perfect adhesion between the carbon fiber and aluminum. Elimination of porosities without fiber damage and fiber agglomeration is maintained in the composite by the use of optimized process parameters during squeeze infiltration.

(2) The surface of the extracted carbon fiber from the infiltrated composite is found to be smooth without any damage, showing remarkable control over interfacial reaction product Al₄C₃. The major diffraction peaks observed in the XRD analysis were C, MgAl₂O₄ spinel, Al₂O₃, and Al₁₂Mg₁₇.

(3) HRTEM studies have shown the presence of Al₂O₃ and MgAl₂O₄ spinel at the interface of C_f/Al composite, thereby protecting the fiber from the deleterious interfacial reaction of Al₄C_3. However, very few traces of Al₄C₃ crystals are observed at the interface.

(4) C_f/Al composite provide a lower density of 2.16 g/cm³ and enhancement in hardness and compressive strength compared to squeeze cast Al 6061 alloy.

(5) Composite exhibits remarkable wear resistance than unreinforced squeeze cast Al 6061 alloy due to self-lubricating phenomenon of carbon fiber, thereby developing tribolayer (MML) on the friction surface. Galvanic corrosion is observed in the C_f/Al composite with the formation of the crack.

Acknowledgements

The authors are grateful to the CSIR for funding and the Director and Members of Materials Science and Technology Division, CSIR-NIIST, Trivandrum for their support and encouragement.

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