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 1234567891011121314.

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 15. 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 (Mo₂C-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 Mo₂C-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 Mo₂C-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 Mo₂C-TiC-based hard nanomaterials.

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 wave forms 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 wave forms or adopted apparatus were reported for the first time. It should be noted that the names ECAS30, and field activated sintering technique (FAST) 31 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 32. 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 (Mo₂C)_{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 (Mo₂C)_{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.

Commercially available powders of TiC (<2 µm, 99.8 % purity, Sumitomo Sitix, Co. Ltd., Japan, Mo₂C (<2 µm, 99.8 % purity, Nihon New Metals Co. Ltd., Japan) and SWCNTs with diameter of 1.2 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 (Mo₂C)_{1_x}–(TiC)_x (2≤x≤4) and with additionof 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 Mo₂C–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 (Mo₂C–TiC) by magnetic agitation for 8 h. Again, a ball milling was applied to the slurry (Mo₂C–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.

Based on previous sintering tests, the composition ratio of the composites was designed as follows (vol.%):

(Mo₂C)_{1_x}–(TiC)_x (2≤x≤4)/ 1 Wt% SWCNTs (here after, it is referred as MTC).

The (NCMC’s) were sintered by SPS methods (Figure 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 resulting MTC ultrafine powder mixtures were hot-pressed in graphite dies (inner diameter: 20 mm) coated with graphite lubricant at (1700–1800 °C) in vacuum.

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 2). 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 Figure 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 (HV₃₀) and 490 N (HV₅₀), 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 53:. 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:

H_v = (1.8544*F)/(2a)²(1)

the fracture toughness was also calculated by indentation fracture (IF) method according to the equation:

K_IC = (0.16H_va^1/2)(c/a)^-3/2(2)

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 R², 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.

Several quantities of TiC ultrafine powder were added to the SPS sintered dense M_1-xT_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 Mo₂C, 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_0.8T_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_0.8T_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^1/2in 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.