Alumina–MWCNT composites: microstructural characterization and mechanical properties

ABSTRACT In the present work, Al2O3–multiwalled carbon nanotube (MWCNT) composites have been developed by both conventional sintering and spark plasma sintering (SPS) and their microstructures, mechanical properties and wear behavior have been investigated. Further, the influence of various other parameters such as the sintering time, sintering temperature, MWCNT loading level and processing technique adopted for development of the composites has also been analyzed. The powder metallurgy route was selected for development of Al2O3–0.2, 0.5, 0.8, 3, 5 vol% MWCNT composites using both conventional sintering and SPS. For conventionally sintered Al2O3–MWCNT composites, it has been found that both the hardness and relative density of the composites decreased up to a loading level of 0.2 vol% of MWCNTs, followed by a continuous increase with the addition of MWCNTs to the Al2O3 matrix, attaining a maximum value in the case of Al2O3–3 vol% MWCNT composite. The wear behavior of conventionally sintered composites also exhibits significant improvement with increase in sintering time. The SPSed Al2O3–MWCNT composites show a much higher relative density and better mechanical and tribological properties as compared to conventionally sintered Al2O3–MWCNT composites.


Introduction
The substantial progress in ceramic-based nanocomposites (CMNCs) is playing a vital role in broadening the range of areas in which ceramics can be applied. Alumina (Al 2 O 3 )-based composites are potential engineering materials possessing superior mechanical as well as tribological properties. Monolithic ceramics suffer from inherent brittleness, poor creep resistance and low fracture toughness. Considerable number of attempts have been made over the last few decades to develop ceramic-matrix composites (CMCs) with better mechanical properties as compared to monolithic ceramics [1,2]. CMCs have low weight, high hardness, and superior thermal and chemical resistance, and they have emerged in recent years, as an attractive choice for a wide range of applications. Since Niihara introduced the concept of nanocomposites in 1991 [3], the addition of nanofillers as a reinforcement phase has become one of the most promising methods of improving the mechanical properties of CMNCs. Carbon nanotubes (CNTs) have emerged as potentially attractive nanofillers for CMNCs. In the present work, multiwalled carbon nanotubes (MWCNTs) have been used as reinforcement for the development of Al 2 O 3 -MWCNT composites. The density of MWCNTs is~2.6 gm/cc, and their specific surface area lies in the range of 200-400 m 2 /g. The tensile strength of MWCNTs ranges between 10 and 60 GPa whereas its modulus lies in the range of 0.3-1 TPa. It has high thermal conductivity of 3000 W/m K and exceptional electrical conductivity in the range of 10 6 -10 7 S/m [4]. Al 2 O 3 is one of the most commonly used ceramic materials due to its extremely high hardness (15)(16)(17)(18)(19)(20)(21)(22), high oxidation resistance and good chemical stability. Among the various engineering ceramics, Al 2 O 3 is one of the most cost-effective and economically viable materials. Al 2 O 3 possesses an extremely high melting point of~2071°C and its density lies in the range of 3.75-3.95 gm/cc. Apart from these attributes, its bulk modulus is~324 GPa, its Young's modulus is~413 GPa and its compressive strength lies in the range of 2000-4000 MPa. The fracture toughness of Al 2 O 3 is~5 MPa√m and its coefficient of thermal expansion is 10.9 × 10 −6 /K [5]. Although Al 2 O 3 has several excellent functional properties, its applications are limited due to its low fracture toughness. Significant efforts have been made to improve the fracture toughness of Al 2 O 3 by the addition of nanofillers or the use of new sintering processes such as spark plasma sintering (SPS) [6,7]. Despite the tremendous efforts exerted in the area of composites, however, uniform dispersion of the nanofillers in the ceramic matrix is still a key challenge. The difficulties associated with the homogeneous distribution of nanofillers and reproducible development of materials possessing enhanced mechanical properties could be considered the major hindrance in the area of nanocomposites [8,9]. Renewed interest in CMCs was observed with the discovery and commercial availability of carbonaceous nanofillers such as CNTs and graphene [10,11]. The outstanding functional characteristics and exceptional mechanical properties of CNTs make them an attractive choice as nano-reinforcements to improve the fracture toughness of brittle ceramics.
To date, extensive research has been conducted to enhance the fracture toughness of ceramic-based materials. Many atomistic simulations have predicted that CNTs possess a capability of enduring significant tensile and compressive forces prior to failure due to their unique morphology and considerable flexibility. The addition of CNTs as nanofillers can not only enhance the hardness and strength of the composites but can also enhance their wear resistance. Due to their closed tubular structures, CNTs form a weak interaction at the matrix interface during the wear process [12]. However, CNT-reinforced CMNCs developed to date have exhibited much lower mechanical performance than expected. This might primarily be attributable to the agglomeration of CNTs and weak interfacial bonding between the nanotubes and the matrix. The effectiveness of CNTs in reducing the wear rate and providing a stable coefficient of friction has been validated experimentally under different loading conditions for carbon-reinforced composites [13,14]. A study on Al 2 O 3 -CNT composites describing the effects of CNTs on their mechanical characteristics and electrical performance has been conducted earlier [15,16] and the effects of CNT addition on the tribological properties have also been reported [17]. Significant enhancement of wear resistance with the addition of CNTs has been observed. However, a detailed understanding of the tribological behavior of Al 2 O 3 -CNT composites will require further research. This paper reports the influence of MWCNT addition on such properties as the density, hardness, fracture toughness and wear behavior of both conventionally sintered and SPSed Al 2 O 3 -MWCNT composites.

Synthesis of MWCNTs
For the fabrication of various Al 2 O 3 -MWCNT composites, a low-pressure chemical vapor deposition (LPCVD) technique was used to synthesize MWCNTs. Due to high van der Waals interactions and a high aspect ratio, achieving uniform dispersion of MWCNTs in ceramic matrices presents major difficulties that can lead to MWCNT agglomeration. To combat this issue, surface modification of MWCNTs has been achieved through acid functionalization. Synthesized MWCNTs were treated with strong oxidizing agents such as H 2 SO 4 and HNO 3 in order to reduce the high van der Waals forces and minimize the MWCNT agglomeration and thus to enhance their dispersion in the Al 2 O 3 matrix. Figure 1 shows a schematic diagram of the LPCVD technique used to synthesize MWCNTs along with the acid functionalization procedure.

Fabrication of Al 2 O 3 -MWCNT composites
The powder processing route was adopted to fabricate Al 2 O 3 -MWCNT composites. It is well known that MWCNTs tend to agglomerate in the host matrix due to their structural morphology, which leads to an ill-constructed interface between the matrix and MWCNTs. Thus, homogenous dispersion of MWCNTs within the ceramic matrix is extremely important in order to impart the desired mechanical properties to composites. The detailed procedures followed for fabrication of Al 2 O 3 -MWCNT composites from their milled-powder mixtures are illustrated in Figure 2.

Consolidation and sintering
For the consolidation of Al 2 O 3 -MWCNT composites, both pressure-free and pressure-assisted sintering routes were selected. Pure Al 2 O 3 and Al 2 O 3 -0.2, 0.5, 0.8, 3 and 5 vol% MWCNT composites were fabricated. In the case of conventional sintering, green compacts were prepared in a uniaxial cold compaction machine under a load of~395 MPa and later sintered at 1650°C for three different holding times of 1, 2 and 3 h in an inert Ar atmosphere. For the pressure-assisted sintering, SPS was carried out using a Dr. Sinter 515S apparatus (SPS Syntex Inc., Kanagawa, Japan) with a pulse on-off ratio of 12:2. The various sintering parameters adopted during the SPS of Al 2 O 3 -MWCNT composites were as follows: Temperature = 1450°C, time = 10 min, heating rate = 100°C/min, pressure = 50 MPa, diameter of graphite die = 15 mm, vacuum in chamber = 6 Pa, ambience = Ar (at flow rate of 2 L/min), voltage = 20 V and current flow =~1200 A.
After sintering, the pressure was removed and the samples were allowed to cool naturally in the furnace until they attained room temperature. Figure 3 shows the SPS profile used for the fabrication of various Al 2 O 3 -MWCNT composites.
The Archimedes' method was used to determine the bulk density of various composites, and the rule of mixtures was followed to calculate their theoretical density. The density of Al 2 O 3 was assumed to be 3.95 g/cc and the density of MWCNT to be 2.6 g/  cc. The morphology of the fabricated composites was analyzed under an optical microscope and SEM.

Characterization techniques
Several techniques were used to characterize the synthesized MWCNTs, milled Al 2 O 3 -MWCNT powder mixtures and sintered Al 2 O 3 -MWCNT composites. The xray diffraction (XRD) of the MWCNTs, milled powder mixtures and fabricated composites was conducted using a Panalytical PW 3040 X'Pert MPD X-ray diffractometer using CuKα radiation (λ = 0.15415 nm). A JEOL JEM-2100 high-resolution transmission electron microscope (HRTEM) at an acceleration voltage of 200 keV was used to analyze the morphologies of the blended powder mixtures. The morphologies of the sintered composites were analyzed using a Zeiss Axio Scope.A1 optical microscope, a Nova NanoSEM 450/FEI field emission scanning electron microscope (FESEM) and a JEOL-JSM -6480LV scanning electron microscope (SEM), both of the units enabled with energy-dispersive X-ray (EDX) analysis systems. In order to determine the thermal stability of various powder mixtures, differential scanning calorimetry (DSC) and thermo-gravimetric analysis (TGA) were conducted using a Netzsch STA 409C Simultaneous Thermal Analyzer at a heating rate of 10°C /min in an Ar atmosphere. A Malvern Nano Zetasizer ZS system was used for the particle size analysis.

Mechanical testing
The mechanical properties of pure sintered Al 2 O 3 and various Al 2 O 3 -MWCNT composites were investigated.
For pure Al 2 O 3 and various Al 2 O 3 -based composites, the hardness values were measured on a polished cross-section using a Vickers microhardness tester for 10 s. Loads of 100 and 500 gf were applied to conventionally sintered and SPSed composites, respectively. An average of five indents was considered for each sample. The sizes of the conventionally sintered and SPS samples were 5 mm × 10 mm and 5 mm × 15 mm, respectively. The single-edge notched beam method under ambient conditions was adopted to determine the fracture toughness of the composites. Due to sample size restrictions, the indentation fracture toughness testing technique was adopted and carried out at different loads to create a notched crack at the indented point. A notched crack with the length l and diameter 2a was formed by the application of stresses in the range of 500-2000 kgf, with the force applied till a complete fracture of the sample was achieved. A crosshead speed of 0.05 mm/min and a maximum span length of 10 mm were set for the toughness test. For a particular indentation load (P), corresponding hardness (H) values were recorded from the point of notch initiation until the complete fracture of the sample. The K IC values were determined by the Shetty equation using the Palmqvist crack model [18,19]: In order to investigate the wear mechanism and determine the wear performance of pure Al 2 O 3 and various Al 2 O 3 -based composites, the dry sliding wear test was carried out using a DUCOM TR208-M1 ballon-plate tribometer at a sliding speed of 20 rpm and sliding time of 10 min. The wear test was done under a normal load of 1 kgf using a diamond indenter with a diameter of 2 mm and wear tracks 6 mm in diameter were formed on the surfaces of various samples. The variations in the wear rate and wear depth with respect to the sliding time were investigated. SEM was used to characterize the morphologies of the worn surfaces and the wear debris obtained from the wear tracks.   [20]. Figure 5 shows HRTEM images of various Al 2 O 3 -MWCNT powder mixtures. As the loading level of MWCNTs was very low in the case of the Al 2 O 3 -0.2 vol % MWCNT powder mixture, no prominent MWCNTs were visible in Figure 5(a). However, in Figure 5(b-e), both MWCNTs and Al 2 O 3 nanoparticles can be clearly observed in the milled powder mixtures. MWCNTs with hollow cores can be easily seen adhering to Al 2 O 3 particles. Both the Al 2 O 3 and MWCNTs were blended by ball milling in the desired volume fractions for a period of 30 min in order to obtain uniformly dispersed powder mixtures. The HRTEM images reveal that the graphitic structure of MWCNTs was well preserved during blending and that the short milling duration did not cause deterioration of the structure of MWCNTs. The presence of nanostructured Al 2 O 3 particles in the blended powder mixture is confirmed by the images. Due to its high brittleness, micron-sized Al 2 O 3 was reduced to the nanometric domain within a short duration of milling.

Results and discussion
The hexagonal spot pattern in Figure 6(a) corresponds to the MWCNTs in the Al 2 O 3 -3 vol% MWCNT powder mixture. The SAD pattern of MWCNTs confirms the sixfold symmetry of the carbon atoms positioned in the graphitic lattice. On the other hand, the faint concentric rings seen in the SAD  pattern of the Al 2 O 3 -5 vol% MWCNT powder mixture in Figure 6(b) correspond to the nanostructured Al 2 O 3 in the powder mixture [21,22]. The thermal stability of Al 2 O 3 -MWCNT powder mixtures was analyzed by DSC/TGA. The endothermic peak at~92°C in the DSC plots of Al 2 O 3 -MWCNT powder mixtures in Figure 8(a) corresponds to the removal of the hydroxyl groups and the evaporation of the absorbed moisture. An exothermic peak corresponding to the combustion of MWCNTs can be seen at~761°C. By comparing the DSC plots of different Al 2 O 3 -MWCNT powder mixtures with different MWCNT loading levels, it is observed that the exothermic peak at~761°C is strongest for the Al 2 O 3 -5 vol% MWCNT powder mixture due to its highest content of MWCNTs, resulting in the release of a larger amount of energy because of the higher degree of MWCNT oxidation. The DF HRTEM images in Figure 7 reveal that MWCNTs were well preserved in the form of tiny bundles between the Al 2 O 3 particles at a loading level of 3 vol%, which suggests that complete decomposition of the MWCNTs entrapped between the Al 2 O 3 particles was not achieved in the case of the Al 2 O 3 -3 vol % MWCNT powder mixture. Thus, no prominent exothermic peak is visible in the DSC plot of the Al 2 O 3 -3 vol% MWCNT powder mixture. It can be noted from the TGA plots of various Al 2 O 3 -MWCNT powder mixtures in Figure 8(b) that the mass loss of the powder mixtures occurs in steps. Initially, at~137°C, the toluene used as a process controlling agent during milling and the residual moisture of the Al 2 O 3 -MWCNT powder mixture evaporate. Thereafter, a mass loss corresponding to the combustion of MWCNTs takes place at~625°C. In the case of the Al 2 O 3 -5 vol% MWCNT powder mixture, a large amount of MWCNTs remain entrapped between the Al 2 O 3 particles. As a result, a comparatively smaller mass loss is observed for the Al 2 O 3 -5 vol% MWCNT powder mixture than for the powder mixtures with lower MWCNT content. It should be noted that the Al 2 O 3 -5 vol% MWCNT composite powder shows more residual mass as compared to the Al 2 O 3 -0.5 vol% MWCNT powder mixture because complete decomposition of the entrapped MWCNTs could not be achieved. The TGA plots suggest that the removal of residual moisture and toluene (at~137°) shows a relatively more dominant effect on the mass loss of various powder mixtures as compared to the decomposition of MWCNTs. Although the highest mass loss is observed for the Al 2 O 3 -0.5 vol% MWCNT powder mixture and the lowest mass loss is observed for the Al 2 O 3 -5 vol% MWCNT powder mixture, it is noteworthy that the patterns of both the DSC and TGA plots are alike for all the powder mixtures, indicating that the thermal behavior of all the Al 2 O 3 -MWCNT powder mixtures is identical [24,25].
One of the most important physical properties of particulate samples is particle size distribution (PSD).
The PSDs of pure Al 2 O 3 and Al 2 O 3 -MWCNT powder mixtures were determined by dynamic laser scattering, and the dispersion condition was evaluated based on the zeta potential. The PSD of Al 2 O 3 -MWCNT powder mixtures was determined in order to find out the average particle size of the various powder mixtures. The PSD of pure Al 2 O 3 milled for 30 min in Figure 9(a) shows that the average size of the Al 2 O 3 particles is~2.131 μm. It should be noted that the as-received pure Al 2 O 3 has a particle size in the range of~80-140 µm (refer to Figure 4(a)). This confirms that a short period of milling can reduce the particle size of Al 2 O 3 to a very fine size. By comparing all the PSD plots in Figure 9(b-f), it is evident that the average particle size of the Al 2 O 3 -MWCNT powder mixture becomes lower with increase in the volume fraction of MWCNTs in Al 2 O 3 . The reduction in the particle size of the powder mixture corresponds to the increase in the volume fraction of MWCNTs with a very fine size. It should be noted that only a single sharp peak is seen in the PSDs of most of the powder mixtures, whereas two adjacent peaks are seen only in the case of the Al 2 O 3 -0.8 vol% MWCNT powder mixture. This confirms the highly uniform PSD in all the powder mixtures [26].
The XRD plots of pure Al 2 O 3 and MWCNTs are shown in Figure 10(a). Sharp peaks located at 2θ values of 28.27°, 38.31°and 49.06°can be seen in the XRD plot of Al 2 O 3 . The XRD spectrum of MWCNT shows the strongest peak at 2θ~26.2°corresponding to the (0 0 2) plane and a low intensity peak at 2θ~43.9°corresponding to the (1 0 0) plane of MWCNTs. Figure 10 [27].
The emergence of new phases and grain growth occurring in the developed composites has been analyzed by XRD. The formation of new peaks in the XRD spectra enables new phase formation or transformation occurring during sintering of the composites to be determined. Figure 11 The grain pinning effect can be clearly seen in the SPSed samples but is not prominent for conventionally sintered samples. Also, no profound peak shift was observed for the SPSed samples as compared to the conventionally sintered samples. The shift in the Al 2 O 3 peaks seen in the inset images in Figure 11(a,b) corresponds to the diffusion of small C atoms into the Al 2 O 3 lattice. The short sintering duration in the case of SPSed samples restricts diffusion, whereas the longer holding time during conventional sintering enables easy diffusion of C atoms [28]. Figure 12 shows optical micrographs of pure Al 2 O 3 and the various Al 2 O 3 -MWCNT composites developed by conventional sintering and SPS. A dense smooth surface is evident in the optical micrographs of pure Al 2 O 3 in Figure 12(a-d), conventionally sintered at 1650°C with a holding time of 1, 2 and 3 h, and SPSed at 1450°C for 10 min. For the conventionally sintered pure Al 2 O 3 samples, a relative density in the range of~86.9-89.44% was achieved, whereas a relative density of~99.24% was observed for the SPSed pure Al 2 O 3 sample (refer to Figure 15). From the optical micrographs of Al 2 O 3 -MWCNT composites in Figure 12(e-t), it is evident that with the addition of MWCNTs to the Al 2 O 3 -MWCNT composites, the MWCNTs start to agglomerate. The size of the MWCNT agglomerates was found to increase with increase in the MWCNT content. Large agglomerates with sizes in the range of 1100-1700 µm can be observed in the optical micrograph of the Al 2 O 3 -5 vol% MWCNT composite in Figure 12(q-s), whereas in Figure 12 Figure 12(q-t) [29]. Figure 13 shows FESEM images of pure sintered Al 2 O 3 and Al 2 O 3 -3 and 5 vol% MWCNT composites developed by conventional sintering at 1650°C for  durations of 1 and 3 h and SPSed at 1450°C for a dwell time of 10 min. From the micrographs in Figure 13(a,b), the effect of sintering parameters such as dwell time, temperature and processing technique, along with the influence of MWCNT addition on the grain size of Al 2 O 3 , can be analyzed. A significant grain growth in conventionally sintered pure Al 2 O 3 is clearly visible. SPS restricted the grain growth of Al 2 O 3 due to the short sintering duration, however, as can be seen in Figure 13(c). Upon addition of MWCNTs, a remarkable reduction in the Al 2 O 3 grain size can be observed from the micrographs. In Figure 13 Figure 13(g,h). Figure 14(a) shows an SEM image of a conventionally sintered Al 2 O 3 -3 vol% MWCNT composite developed at 1650°C with a holding time of 3 h, along with elemental maps of Al, O and C. A white colored lump of Al 4 C 3 can be seen lying on the surface of the composite. In the SPSed Al 2 O 3 -3 vol% MWCNT composite, rod-like Al 4 C 3 structures can be seen on its surface. Al 4 C 3 is formed by a reaction of MWCNTs with Al 2 O 3 . The diameter of these rods is~270 nm. The high pressure applied during SPS along with the short sintering time allowed the formation of these rod-like structures. On the other hand, in the case of conventionally sintered Al 2 O 3 -3 vol% MWCNT composites, the Al 4 C 3 structures are irregular in shape and are not nanostructured due to the prolonged sintering time [30].
As ceramics require very high sintering temperatures, achieving near full density of the composites without damaging the structure and morphology of the MWCNTs is one of the most important challenges during the development of CMNCs. The highest level of densification in Al 2 O 3 -MWCNT composites can be achieved at an optimum loading level of the nanofiller, as a very high loading level of MWCNTs would lead to their agglomeration in the Al 2 O 3 matrix resulting in poor densification of the CMNCs. Agglomerates of MWCNTs at the grain boundaries lead to abnormal grain growth and poor densification of the composites. On the other hand, a very low nanofiller loading level can leave many of the pores unfilled and result in a lower relative density of the composites. Therefore, an optimum nanofiller loading level results in the highest level of densification in CMNCs [31]. Figure 15  -MWCNT composites was found to improve when MWCNTs were introduced into the Al 2 O 3 matrix in the range of 0.5-3 vol%. However, increasing the loading level of the MWCNTs beyond 3 vol% led to a decrease in the relative density of the  [32]. Figure 16 shows  MWCNT composites. It is noteworthy that in the case of conventionally sintered composites, the hardness of all the composites was higher as compared to pure Al 2 O 3 samples except for Al 2 O 3 -0.2 vol% MWCNT composites, whereas in the case of SPSed composites, this variation was not observed. The addition of MWCNTs up to 3 vol% results in an increase in the hardness of Al 2 O 3 -MWCNT composites. The highest hardness was observed in the case of Al 2 O 3 -3 vol% MWCNT composites irrespective of the sintering technique adopted. The hardness of conventionally sintered Al 2 O 3 -3 vol% MWCNT composites developed by conventional sintering at 1650°C for 3 h was found to be~4.109 GPa, whereas SPSed Al 2 O 3 -3 vol% MWCNT composite showed a hardness value of~8.38 GPa. A drop in the hardness of the composites was observed when the concentration of MWCNTs was increased to 5 vol%, which can be attributed to agglomeration of the nanofiller in the Al 2 O 3 matrix. MWCNTs tend to agglomerate due to their high aspect ratio and strong van der Waals interactions, leading to poor densification of the composites and ultimately decreasing the hardness value. The addition of an optimum loading of 3 vol% MWCNTs results in homogeneous distribution of MWCNTs in the Al 2 O 3 matrix, which effectively restricts grain growth by grain boundary pinning. MWCNTs present around the grain boundaries can effectively reduce the atomic diffusion coefficient and prevent grain growth during sintering. A finer Al 2 O 3 grain size results in higher hardness of the composites. The grain refinement effect of MWCNTs is also evident from the XRD plots of Al 2 O 3 -MWCNT composites in Figure 11 [33]. The tribological properties of Al 2 O 3 -MWCNT composites were investigated using a ball-on-plate tribometer. Figure 17 shows the wear rate of various Al 2 O 3 -MWCNT composites and clearly suggests that the variations in wear rate of the Al 2 O 3 -MWCNT composites follow a similar trend, irrespective of the sintering technique adopted. The wear rate of conventionally sintered Al 2 O 3 -MWCNT composites decreases continuously as the concentration of MWCNTs is increased up to 3 vol%. The improved wear behavior of Al 2 O 3 -MWCNT composites as compared to monolithic Al 2 O 3 is due to a protective tribofilm formed on the wear track by MWCNTs, which provides the composites with effective wear resistance [34]. With the addition of MWCNTs above 3 vol%, however, the wear rate shows a sudden increase due to heterogeneous agglomeration and clustering of MWCNTs in the sintered composite. In the case of conventionally sintered composites, moreover, the wear rate was found to be dependent on the sintering duration. Al 2 O 3 -MWCNT composites sintered at 1650°C for 1 h show a relatively higher wear rate, whereas composites sintered at the same temperature for 3 h show a relatively lower wear rate. This can be attributed to a higher level of densification in the case of the composites sintered for 3 h. SPSed Al 2 O 3 -MWCNT composites show a much lower wear rate as compared to conventionally sintered composites. However, the trend in the variation of wear rate with respect to the loading level of MWCNTs is similar to that of conventionally sintered composites [35].
The plots in Figure 18 show variations in wear depth with respect to time for various Al 2 O 3 -MWCNT composites. For conventionally sintered composites, it is evident from Figure 18  The higher wear depth observed for the monolithic Al 2 O 3 sample corresponds to the removal of rough surface asperities due to its comparatively lower degree of densification and lubricity [36].
The SEM images in Figure 19 show the wear tracks of sintered pure Al 2 O 3 and Al 2 O 3 -MWCNT composites. It is evident from the images that the width of the wear tracks is significantly reduced by increasing the sintering duration from 1 to 3 h. Composites sintered for 3 h display relatively smoother wear tracks, moreover, with hardly any noticeable grain pull-out as compared to composites sintered for 1 h. However, a large area of grain pull-out and severe damage to the wear surfaces, with traces of wear grooves and large residual wear debris on the wear tracks, can be observed for Al 2 O 3 -MWCNT composites sintered for 1 h (Figure 19(a,d,g,j,m)). When the MWCNT content is increased from 0.5 to 5 vol% in the Al 2 O 3 matrix, abrasive sliding wear occurs resulting in a greater amount of Al 2 O 3 grain pull-out. The width of the wear tracks decreased with increase in the sintering duration of the composites from 1 to 3 h, although the sintering temperature did not vary. For both 1 and 3 h sintered composites, a minimum wear track width is observed in the case of Al 2 O 3 -3 vol% MWCNT composites. The lower tangential frictional force between the composite surface of the wear track and the ball reduced the grain pull-out due to the formation of a protective tribofilm by the MWCNTs. MWCNTs embedded in the unpolished surfaces of the composites are dislodged and scattered on the wear track during the wear test to form a protective lubricating tribofilm. Moreover, the rolling effect of MWCNTs on the reduction of abrasion and wear rates cannot be ignored. MWCNTs also contribute to bridging the grains to protect against crack propagation due to their higher aspect ratio during micro-chipping and grain pull-out for improving the wear resistance of the composites [37]. In the SPSed samples, the width of the wear tracks decreases with increase in the MWCNT content in the Al 2 O 3 matrix due to the shorter sintering duration of SPS. The MWCNTs remain well preserved and assist in the effective lubrication of the Al 2 O 3 matrix, which results in an overall improvement of the wear resistance of the composites. sintered Al 2 O 3 -3 vol% MWCNT composites developed by conventional sintering at 1650°C for 2 and 3 h, respectively. It is clear from the SEM images that the sintering duration plays an important role in the conservation of MWCNTs in the Al 2 O 3 matrix, since the wear debris collected from the 3-h sintered composite shows mostly Al 2 O 3 particles with a negligible accumulation of MWCNTs. Longer sintering duration leads to better densification of the composites, making pull-out of MWCNTs difficult during the sliding wear test. This is also confirmed by the variations in wear rate in Figure 17 and the variations in wear depth in Figure 18.  Figure 20(c). A large number of faceted Al 2 O 3 grains can be seen in the SEM image in Figure  20(d). However, MWCNTs are not found in the wear debris of the SPSed Al 2 O 3 -3 vol% MWCNT composite. This is due to the higher level of densification obtained for SPSed composites, making pull-out of MWCNTs difficult [38]. The variations in fracture toughness values (K IC ), determined using the notch indentation fracture toughness method for various sintered Al 2 O 3 -MWCNT composites, are shown in Figure 21   in the formation of an interlinked web-like structure of long nanotubes that weakens the nanofiller-matrix interface [39].
To investigate the various toughening mechanisms, the fractured surfaces of conventionally sintered and SPSed Al 2 O 3 -MWCNT composites were observed under SEM. The SEM images in Figure 22 clearly indicate that crack bridging by MWCNTs is the main toughening mechanism of the Al 2 O 3 -MWCNT composites. When MWCNTs embedded in Al 2 O 3 -MWCNT composites encounter a crack, they bridge the crack wake and effectively obstruct its propagation. The pull-out of MWCNTs also contributes to toughening of the composites. Both MWCNT pull-out and crack bridging by the MWCNTs can be seen in the SEM images in Figure 22. The high aspect ratio of MWCNTs leads to a longer crack-wake bridging zone and improves the toughness of the composites. During crack propagation, an initial uncoiling of MWCNTs occurs in the crack wake, and when the crack propagates further, the uncoiled MWCNTs stretch elastically, serving as stretched MWCNT bridges instead of conventional frictional pull-out bridges. The MWCNTs are responsible for the interfacial strengthening as they tend to bridge the grains due to their high aspect ratio and hence impede the crack propagation. During deformation, MWCNTs can absorb energy through their highly flexible elastic behavior and increase the fracture toughness [40]. Additionally, MWCNTs can act as pinning points to stop grain boundary movements occurring under stress. Thus, MWCNTs embedded in the grains pin the Al 2 O 3 grains together and strengthen the grain boundaries. As a result, these MWCNT-strengthened grain boundaries lead to a changed fracture mode, from intergranular in pure Al 2 O 3 to transgranular in Al 2 O 3 -MWCNT composites. The smaller diameter of MWCNTs allows them to become embedded in the grains during grain growth, and their elongated shape enables them to link various grains together in order to form bridges. The sliding of concentric tubes of MWCNTs allows them to extend to significantly longer than their original length without breaking. MWCNTs can be stretched to a great extent before disintegrating during crack propagation and hence contribute to the bridging effect and toughening mechanism [41].

Conclusions
The effects of various sintering parameters such as sintering temperature, dwell time, sintering pressure and variations in nanofiller concentrations were analyzed for various conventionally sintered and SPSed Al 2 O 3 -MWCNT composites. It was found that addition of MWCNTs at significantly lower loading levels of up to 3 vol% remarkably enhances the mechanical and tribological properties of Al 2 O 3 -based composites. However, any further addition of MWCNTs into the Al 2 O 3 matrix leads to the formation of complex clusters and their agglomeration in the host matrix, with resulting deterioration of the properties of the composites. The major conclusions drawn from the present research work are as follows: (1) Good-quality MWCNTs were synthesized at optimized conditions using the LPCVD technique. The synthesized MWCNTs comprised concentric cylindrical graphene layers with an interlayer spacing of 0.34 nm. The outer diameter of the MWCNTs was found to bẽ 12 nm and the inner diameter~3.3 nm.