Effect of Single Walled Carbon Nanotubes (SWCNTs) Addition during Field Activated Sparck Plasma Sinter (FASPS) in the Ceramics Matrix Nanocomposites (Mo2C)1-x(TiC)x (2≤x≤4): Physical, Mechanical Properties and Sintering Behaviour

Nanocomposites are wear resistant materials used in cutting tool applications. The materials are composed of ultrafine powder hard phase grains surrounded by a tough binder phase carbon nanotubes (Mo2C)1-x(TiC)x (2≤x≤4)//1Wt% SWCNTs. Composite bicarbide Mo2C-TiC was rapidly synthesised and simultaneously consolidated by Field activated sintering technique (spark plasma sintering) at which the extensive volume expansion occurred as a function of the volumic fraction from 20 to 40 vol.% of TiC powders and 1 Wt.% of SWCNTs as reinforcement of the CMNC’s. The sintered powder mixture was examined by XRD patterns, the morphology of the obtained phase was observed by SEM and the phase compositions in different regions were analyzed by EDX. The composites were processed using Field Activated Sintering Technique, spark plasma sintering (SPS) at temperatures in the range of 1700-1800°C with addicting of SWCNTs. The effects of SWCNTs addition on phases morphology, microstructure hardness and fracture toughness of the nanocomposite were investigated. The best product contained 1.0 Wt.% SWCNTs from (Mo2C)1-x(TiC)x , x= 0.2 which was sintered at 1700°C, 70 MPa for 10 min, M0.8T0.2/ 1 Wt% SWCNTs exhibit a better density, highest hardness and a good ductility. Relative densification was achieved 99.5 % from the theoretical and a good mechanical properties like hardness and fracture toughness (KIC=5.6 Mpa m) are enhanced. The results were confirmed using Raman spectroscopy.

Abstract Nanocomposites are wear resistant materials used in cutting tool applications. The materials are composed of ultrafine powder hard phase grains surrounded by a tough binder phase carbon nanotubes (Mo 2 C) 1-x (TiC) x (2≤x≤4)//1Wt% SWCNTs. Composite bicarbide Mo 2 C-TiC was rapidly synthesised and simultaneously consolidated by Field activated sintering technique (spark plasma sintering) at which the extensive volume expansion occurred as a function of the volumic fraction from 20 to 40 vol.% of TiC powders and 1 Wt.% of SWCNTs as reinforcement of the CMNC's. The sintered powder mixture was examined by XRD patterns, the morphology of the obtained phase was observed by SEM and the phase compositions in different regions were analyzed by EDX. The composites were processed using Field Activated Sintering Technique, spark plasma sintering (SPS) at temperatures in the range of 1700-1800°C with addicting of SWCNTs. The effects of SWCNTs addition on phases morphology, microstructure hardness and fracture toughness of the nanocomposite were investigated. The best product contained 1.0 Wt.% SWCNTs from (Mo 2 C) 1-x (TiC) x , x= 0.2 which was sintered at 1700°C, 70 MPa for 10 min, M 0.8 T 0.2 / 1 Wt% SWCNTs exhibit a better density, highest hardness and a good ductility. Relative densification was achieved 99.5 % from the theoretical and a good mechanical properties like hardness and fracture toughness (K IC =5.6 Mpa m 1/2 ) are enhanced. The results were confirmed using Raman spectroscopy.

Introduction
Nanomaterials (defined as being in the size range 1-100 nm in at least one dimension) have been the subject of extensive research in recent years due to their extraordinary properties over their conventional counterparts. For more than two decades, the topic of nanomaterials' development has been widely investigated by many researchers aiming at exploring their potential and finding suitable applications. Ultimately, they proved to be beneficial in several applications in areas like surface engineering, drug delivery, analytical chemistry, bio encapsulation, Nano composite as well as in electronic, magnetic, optical, and mechanical devices.
These applications do stimulate a great deal of research interest amongst institutions and companies aiming at capitalizing on their potential. However, there are still a number of difficulties when it comes to processing of these nanomaterials especially in fabricating final products as these nanomaterials may lose their crystallite size along the path of processing. These problems have to be tackled before the potential of nanoparticles or nanomaterials can be fully realized [1]. The problem of undesirable grain growth is persistent in the sintering/consolidation processes of several advanced materials that usually gain their importance and thus potential properties from their reduced crystallite sizes.
The composite (Mo 2 C-TiC) based hard metals or cemented carbides represent an important class of materials used in a wide range of industrial applications which primarily include cutting/drilling tools and wear resistant components. The introduction and processing of nanostructured composite made of Mo 2 C-TiC based carbon nanotubes and their subsequent consolidation to produce dense components have been the subject of several investigations. One of the attractive means of producing this class of materials is by mechanical alloying technique. However, one of the challenging issues in obtaining the right end-product is the possible loss of the nanocrystallite sizes due to the undesirable grain growth during powder sintering step. The present paper highlights some key issues related to powder synthesis and sintering of composite Mo 2 C-TiC -based nanostructured materials using mechanical alloying. The path of directly consolidating the powders using nonconventional consolidation techniques will be addressed and some light will be shed on the advantageous use of such techniques. Carbon nanotubes-bonded hard nanomaterials will be principally covered in this work along with an additional exposure of the use of other binders in the Mo 2 C-TiC-based hard nanomaterials. Effect of Single Walled Carbon Nanotubes (SWCNTs) Addition during Field Activated Sparck Plasma Sinter (FASPS) in the Ceramics Matrix Nanocomposites (Mo 2 C) 1-x (TiC) x (2≤x≤4): Physical, Mechanical Properties and Sintering Behaviour In order to summarize some of the relevant information reported so far, the various RS process used for successful synthesis of this hard nanocomposites are historically listed in Table 1 along with the corresponding reported applied current characteristics and the apparatus adopted. It can be clearly seen that, as already mentioned, diverse current waveforms and/or apparatus are adopted for the same process. In the last column of Table 1 the references indicate where, at our best knowledge, the process name/acronym, and for each process name/acronym, the different applied current waveforms or adopted apparatus were reported for the first time. It should be noted that the names ECAS [2], and field activated sintering technique (FAST) [3] were introduced to designate the consolidation methods based on the application of a electric field/current regardless their waveforms, while pulse electric current sintering (PECS) was used by some authors as a generalization of RS methods which involve the application of a pulsed electric current [4]. It should be pointed out once more that the applied characteristics and/or the apparatus adopted were in some cases not specified. Therefore, Table 1 should not be considered as comprehensive of all possible combinations of process name, applied current and apparatus adopted in this research field.
In this work the studies of (Mo 2 C) 1_x -(TiC) x (2≤x≤4)// 1 Wt% SWCNTs based NCMC 's, sintered at low pressures (70 MPa-provided by field activated sintering technique (SPS) are presented. A fine (Mo 2 C) 1_x -(TiC) x (2≤x≤4)// powder grade was chosen as the matrix material The use of only 20 vol.% of TiC with 1 Wt% SWCNTs reinforcements aims to increase physical and mechanical resistance of the(NCMC's). Moreover, the relatively small content of dispersed phase decreases the probability of direct contacts between their particles. The (NCMC's) sintering temperature is higher than 1700 °C. In the case of low temperature SPS sintering process weak bonding between the TiC grains may yield to low micro hardness and high fracture toughness.

nm (I have synthesized by Laser ablation at IFW-TU-Dresden-Germany in 2002)
were used as the raw materials were used as starting materials. The mixture containing (Mo 2 C) 1_x -(TiC) x (2≤x≤4) and with addition of 1 Wt% SWCNTs, (NCMC's) were prepared by wet milling in anhydrous alcohol for 3 h..
To obtain homogenized and fine powder mixtures, the powder mixtures of Mo 2 C-TiC were ball-milled at a high speed of 200 RPM for 12 h by using WC balls (diameter: 3 mm) and ethanol as the milling media.
Preliminary treatment of SWCNTs was carried out to minimize the agglomerate of the added SWCNTs. Firstly; the weighed SWCNTs were immersed into acetone for about 20 h, and then were ultrasonically dispersed for 3 h.
Secondly, the treated SWCNTs (3 vol.%) were mixed with the former ball-milled blend (Mo 2 C-TiC) by magnetic agitation for 8 h. Again, a ball milling was applied to the slurry (Mo 2 C-TiC/1 Wt% SWCNTs) at a speed of 200 RPM for 10 h for further mixing.
Finally, the powder mixtures with dispersed SWCNTs were dried by rotary evaporator under vacuum condition and were sieved to 60 meshes.
The (NCMC's) were sintered by SPS methods (Fig.1). The tree sintering schedules are shown. In the spark plasma sintering process, temperature profile and punish displacement or shrinkage the displacement velocity is not presented here ( Figure.1b). The applied pressure of 70 MPa was adjusted to the powder at room temperature and kept constant throughout the hot pressing process. The pressure was applied at the beginning of the sintering process because high green density is favorable for better densification rate by reducing the pores prior to the densification during heating. The heating rate was about 10 °C/min and the dwelling time at terminal temperature was 60 min. The temperature was measured by an infrared pyrometer through a hole opened in the graphite die. Furthermore, for monitoring densification process, the shrinkage of the powder compact was measured by a displacement sensor during the hot pressing.
The dimensions of the finally hot pressed samples were about 20 mm in diameter and 3 mm in thickness.
The mixtures were loosely compacted into a graphite die of 20 mm in diameter after calculation of the weight according to the theoretical density of the compounds and sintered in the vacuum (1 Pa) at various temperatures (1700-1800 °C) using an SPS apparatus (Lab. SINTER, SPS-1050, Sumitomo Coal Mining Co. Ltd., GERMANY) ( Table 1). A constant heating rate of 100°C/min was employed, while the applied pressure was 70 MPa. The on/off time ratio of the pulsed current was set to 12/2 in each run. The maximum current reached approximately 3000 A during sintering.
The soaking time at high temperatures was within 10 min. The upper ram of the SPS apparatus was fixed, while the displacement of the shifting lower press ram was recorded in order to analyze the synthesis and sintering. The sintered samples are presented in the Fig.2.
Density of the sintered samples was measured by the Archimede's using the densimeter type Micromiritics Accupyc 1330. The microhardness at the top was measured by a diamond Vickers hardness tester (MVK-H1, Meter-Mitutoyo, Japan).
The indentation loads, ranging from 10 to 500 N, were applied for 15 s for each measurement. The fracture toughness was measured using the Vickers indentation by the measurement of the producing failler.
In this study, 04 samples for each sintering process were fabricated to obtain an average relative density and hardness.
Young's modulus of the composites was determined by ultrasonic wave transition method measuring the velocity of ultrasonic sound waves passing through the material using an ultrasonic flaw detector (Panametrics Epoch III). The hardness and the fracture toughness were determined by the Vickers indentation method applying load of 294 N (HV 30 ) and 490 N (HV 50 ), by a Future Tech FLC-50VX hardness tester. For each sample 4 indentations were made and the stress intensity factor K IC was calculated from the length of Palmqvist cracks which developed during a Vickers indentation test using E. Rocha-Rangel's equation [39]:. The wear resistance and the friction coefficient will be performed in the near future. The hardness (H), the elastic modulus (E) and the fracture toughness (K IC ) of the fabricated samples were measured under ambient conditions using the instrumented Vickers indentation method (Zwick Roell, ZHU 2.5 apparatus).
The impression diagonal (2a) was measured, and the hardness values were calculated according to the following relation: the fracture toughness was also calculated by indentation fracture (IF) method according to the equation: Where H v was the Vickers hardness, a was the half-length of the indentation diagonal and c was the half-length of the median crack generated by indentation. Generally, the fracture toughness measured by IF method were fluctuating values with relatively large deviations due to the phase distribution and measurements errors of calculation. Thus a linear regression model was applied to get a reliable value of indentation fracture toughness.  To obtain the values of A, B and R 2 , a series of indentation loads (10 N, 50 N, 100 N, 300, 500 N) were applied to get the relations of P and c 3/2 .

XRD Analysis of Sintered M 1-X T x (2≤x≤4) /1Wt%
SWCNTs, (NCMC's) XRD analysis of the samples indicates that the only phases formed in the sample without SWCNTs are titanium carbide TiC, with a cubic crystal structure and Mo 2 C with orthorhombic crystal structure. The addition of 1Wt% SWCNTs reinfort has considerable effect on XRD pattern. XRD spectrum of the (NCMC's), which indicates that the reaction between the TiC powders and SWCNTs did happen during the sintering process. It is considered that disordered carbons on wall defects and the open ends of the SWCNTs serve as carbon sources for interfacial reaction. The sintered samples consisted mainly Mo 2 C-TiC of matrix, as major phases, and some secondary phases, such as SWCNTs appeared depending on the sintering temperature. X-ray diffraction patterns of sample M 1-x T x (2≤x≤4) /1Wt% SWCNTs, M 0.8 T 0.2 /1Wt% SWCNTs, (NCMC's) sintered at 70 MPa pressure were enhanced because they are reinforced with the SWCNTs phases, TiC and SWCNTs with higher intensity, the SWCNTs content at the sintering temperature of 1700 °C for 10 min at 70 MPa, which was reduced to 10 % when sintered at pressure because it reacted with TiC to produce TiC-SWCNTs , in the TiC grain new intergranular reinforcement slightly increased.
These results suggest that TiC grains grew preferentially with the basal planes rotating towards the loading direction of SWCNTs.

SEM Microstructural Observation of Sintered M 1-X T x (2≤x≤4) /1Wt% SWCNTs, (NCMC's)
The SEM microstructures of samples with following compositions: The SEM microstructures (Fig. 3) revealed that the nanocomposite has good density of the binderless phases in the final structure of the products. The high magnification representative microstructure of the sample without SWCNTs consists of large Mo 2 C (dark) grains and TiC (gray) particle sintered together (Fig. 3c-d-e-f-h). The addition of 1 Wt % SWCNTs (Fig.3e-f) to the reaction changed the morphology of TiC from slightly finer grains with near spherical morphology to the large plate like grains. Fig.3e-d, showed the SWCNTs into TiC particle grain and no in the grain boundaries forming the interface with different orientations. The mechanical properties are induced by both phases and grain boundaries. This is a contribution to the increasing of the microhardness. These steps are characteristics of crystal growth with interface-controlled growth mechanism [41]. In the Fig.3g is presented the typical of the loss of SWCNTs identified by the pore like structure (holes) in addition to unreacted High magnification images of the starting ultrafine powder of SWCNTs and sintered titanium carbide (TiC) are presented in Figure.3a-b. In the Figure 4, the microstructural representation and EDS analysis displays elemental analyses of the various regions of the sintered samples. Secondary electron image, atomic concentration cartographies of Mo, Ti, and C.

Relative Density of the Sintered M 1-X T x (2≤x≤4) / 1Wt% SWCNTs, (NCMC's)
The variation of the relatives density of sintered M 1-x T x (2≤x≤4) /1Wt% SWCNTs, (NCMC's) with SWCNTs reinforcement is shown in Table.2. The theoretical density of the composite used for obtaining relative density was calculated using a rule of mixture, using the densities of two constituent phase (ρ Mo2C= 4.53 g /cm 2 , ρ TiiC = 4.50 g /cm 2 , ρ SWCNTs = 2.25 g/cm 2 ) with the given SPS processing parameters, the M 1-x T x// /1Wt% SWCNTs with x=2, sample exhibited best densification with relative density greater than 99.5%, with the similar processing parameters with variation of TiC/SWCNTs content. The relative density increased with increasing of the reinforcement of 1.0 wt% SWCNTs. The M 0.8 T 0.2 /1Wt% SWCNTs, (NCMC's) at T=1700°C exhibited relative density of about 99.5%. Depending on the final density to be achieved, the SPS operating condition were properly chosen, that is, 1700 °C, 70 Mpa for 10 min, to obtain a highest relative density for the (NCMC's), M 0.8 T 0.2 /1Wt% SWCNTs, dense M 0.6 T 0.4 , TiC for zero compacts porosity, 99.5, 95.06, and 92.04, respectively.
Also, the easy sledding of their walls when attached by weak van der Waals force of coalesced MWCNTs can probably increases the relative density. The density of the sintered samples was determined using the Archimedes helium immersion method.

Microhardness and Fracture Toughness (K IC ) of the Sintered M 1-X T x (2≤x≤4) /1Wt% SWCNTs,(NCMC's)
According to the above results, it can be concluded that the Vickers hardness has been improved by adding SWCNTs and enhanced with the fracture toughness value giving a better ductility for the reinforced SWCNTs samples. Figure 6 shows the hardness as a function of the applied indentation load for the same sample. At lower loads, the microhardness reaches a low hardness a constant value of 33.6, 14.8 and 20.6 GPa for reinforced M 0.8 T 0.2 /1Wt% SWCNTs, unreinforced TiC, and M 0.6 T 0.4 , (NCMC's) respectively In the present study, the high hardness of the SPS synthesized sample containing (NCMC's), M 0.8 T 0.2 /1Wt% SWCNTs at T= 1700 °C may be attributed to its high density (99.5% from theoretical). The microhardness in the range of about 2.5-3.5 GPa was found for lower loads (10 N). A slight increase in average hardness has been obtained from (NCMC's) prepared by sintering M 0.8 T 0.2 /1Wt% SWCNTs at T=1700°C exhibited highest hardness of about 33.6 GPa (Fig.6). It is considered that, 40 vol% TiC and the remaining SWCNTs act as reinforcements and plays the major role in the consolidation of the products.
The best product contained M 0.8 T 0.2 /1Wt% SWCNTs which was sintered at 1700 °C, 70 MPa for 10 min. The hardness of the M 0.8 T 0.2 /1Wt% SWCNTs increases with increasing of TiC and SWCNTs content (Fig.6).
With this, the electro discharge among powders may lead to self-heating and purification of the particle surface, resulting in activation of the formation of the (NCMC's).
The corresponding SEM micrographs around the indentation marks at the top surfaces show the lateral cracks extension from the indentation mark (Fig. 7). The relevant data were listed in Table 3.In Figure. 9, a linear regression analysis was applied to the relations of P and c 3/2 by the least square method.   The addition of SWCNTs play a important binderless role (the ductility) in the propagation of failler in this (NCMC's) and thus enhance the fracture toughness in comparison with his higher hardness In addition, a high determination coefficient (R 2 ) of 0.9881 was obtained through the linear regression model [40].
IF (indentation fracture) was shown to be an effective method in the evaluation of fracture toughness for its convenience and materials saving.

Raman Spectroscopy of M 1-X T x (2≤x≤4) /1Wt% SWCNTs, (NCMC's)
In the Figure10 and11is presented the Raman response of the (NCMC's), M 0.8 T 0.2 /1Wt% SWCNTs, TiC and M 0.6 T 0.4 . The TiC and Mo 2 C vibration response are detected around the 1256 and 1144 cm -1 respectively. The reinforced phase of the binderless SWCNTs is localized in the typical frequency of G mode, D mode and Hg (7) at (1828, 1597), (1338, 1367) and 1451 cm -1 , respectively. The pics of the Raman spectra have confirm the XRD investigations.

Conclusions
Several quantities of TiC ultrafine powder were added to the SPS sintered dense M 1-x T x (2≤x≤4)/1Wt% SWCNTs (MTC), (NCMC's) at 1700°C under a pressure of 70 MPa for 10 min in pure Ar atmosphere protection. Phase analysis using XRD and EDX indicated that binderless fines powders Mo 2 C, TiC and un-reacted binder phase SWCNTs were the main products. Microstructural observations by SEM showed that the effect of SWCNTs as binder phase make interface into TiC grains and good physical (relative density) and mechanical properties (Vickers hardness, young's modulus and fracture toughness) of (NCMC's) are obtain for x=0.2, M 0.8 T 0.2 /1Wt% SWCNTs, SPS produced samples. The loss of SWCNTs during or conversion of SWCNTs to DWCNTs or MWCNTs the sintering processes are marqued by the holes like structure in the SEM pictures.
The challenge is to find efficiency methods for mixing of SWCNTs with the matrix composite. I suggest that SWCNTs will be metalized before mixing and sintering for minimizing their loss at high temperature.
The best density 99.5 % and ductility of the nanocomposites M 0.8 T 0.2 /1Wt% SWCNTs are obtained with the addition of the binderless reinforts phases of SWCNTs and TiC.
The fracture toughness are enhanced (ductile (NCMC's) K IC =5.6 Mpa m 1/2 in comparison with his highest Vickers hardness. The Raman responses confirm the XRD investigations of the sintered samples.
Furthers measurements will be carried out with the nanoindentation technics, friction coefficient and wear resistance. Also furthers studies also will be performed with variation of the graphite die diameter, to control the SWCNTs content in the (NCMC's), MTC.
By Field Activated Sintering Technique (SPS), the extensive volume expansion as a function of the pressure will occur.