Study of the Nanocomposite cBN/TiC-SWCNTs by Field Actived Sparck Plasma Sintering Process

Commercially available ultra fine powder of TiC (<3 µm, 99.8 %purity, Sumitomo Sitix, Co. Ltd., Japan, cBN (<5 µm, 99.8 % purity, Nihon New Metals Co. Ltd., Japan) and NC produced by HIPCO process with diameter of 1.0 nm (IFW-TU-Dresden-Germany) were used as the reinforcement materials. The mixture containing cBN_x-TiC_1-x (x=0.8) and with addition of 0.1 wt % of NC nanocomposites were prepared by wet milling in anhydrous alcohol for 3 h.

To obtain homogenized and fine powder mixtures, the powder mixtures of cBN–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 NC was carried out to minimize the agglomerate of the added NC. Firstly; the weighed NC were immersed into acetone for about 20 h, and then were ultrasonically dispersed for 4 h.

Secondly, the treated NC (3 vol. %) were mixed with the former ball-milled blend (cBN_x-TiC_1-x (x=0.8) by magnetic agitation for 8 h. Again, a ball milling was applied to the slurry cBN_x-TiC_1-x (x=0.8) at a speed of 250 RPM for 08 h for further mixing.

Finally, the powder mixtures with dispersed NC were dried by rotary evaporator under vacuum condition and were sieved to 70 mesh.

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

cBN_x-TiC_1-x (x=0.8)-0.1wt%. NC (here after, it is referred as BTNC).

The nanocomposites were sintered by FASPS technology. The two sintering schedules are shownin Figure 1. In the sparck plasma sintering process, temperature profile and piston displacement or shrinkage the displacement velocity is not presented here (Figure 1).

Figure 1.Variation of die displacement or shrinkage, temperature and applied pressure in dependence on the heating time during the FASPS the sintered samples

The resulting BTNC ultrafine powder mixtures were hot-pressed in graphite dies (inner diameter of 20 mm) coated with graphite shit lubricant at 1800 °C in vacuum.

The applied pressure of 75 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 and sintered in the vacuum (1 Pa) at various temperatures (1800 °C) using an FASPS apparatus (Lab. Sinter, FASPS-1050, Sumitomo Coal Mining Co. Ltd., Germany) (Table 1). A constant heating rate of 120 °C/min was employed, while the applied pressure was 75 MPa. The on/off time ratio of the pulsed current was set to 10/2 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 FASPS 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.

Figure 2.Sintered samples in the die diameter of 20 mm of BT and BTNC before and after gold metallization for microscopic observations. a- BT before metallization, b-BTNC before metallization, c-BT after etching and gold metallization, d- BTNC after etching and gold metallization.

Density of the sintered samples was measured by the Archimede’s using the densimeter type Micromiritics Accupyc 1330. The micro hardness 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 mesuearement of the producing failler.

In this study, 06 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 6 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 ²⁹. The wear resistance and the friction coefficient will be performed in the near future. The hardness (H), the elastic modulus (E) and the toughness (K_IC) of the fabricated samples were measured under ambient conditions using the instrumented Vickers indentation method (ZwickRoell, 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 measurement 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

Where P is the indentation load. Through the combination of equations (1) and (2), the linear relation between P and c^3/2 was obtained:

P = Ac^3/2 + B (A= K_IC / 0.075) …...(3)

A linear regression analysis was applied to the relations of P and c^3/2 by the least square method.

To obtain the values of A and B, where A was the slope, B was the intercept.

In addition, a high determination coefficient (R²) was obtained through the linear regression model. Hence, when combined with the linear regression model, IF was shown to be an effective method in the evaluation of fracture toughness for its convenience and material saving ³⁰.

(Figure 3) shows the variation of die displacement or shrinkage, temperature and applied pressure in dependence on the heating time during the FASPS sintering. We can conclude that addition of the NC in the nanocomposite ceramics matrix tools improved the sintering behavior of the cBN _x-TiC_1-x (x=0.8) ceramics matrix nanocomposites (CMNCs) tools.

Figure 3.X-ray diffraction spectrum on the top surface of the sintered samples using a 20-mm diameter die, a) BT, b) BTNC (CMNCs) tools.

The sample is already fully densified to 98 % at T= 1800 °C at the beginning of the heating with a fast displacement of the dies and remains until the end of the heating, the temperature sensor records the displacement of the dies from T= 400 °C. The NC enhanced sample has better sintering behavior and densification than NC free.

(Figure 1) shows also the thermodilatometric measurements up to 1800 °C under vacuum in the FASPS chamber for different compositions. This experiment was performed in order to optimize the sintering temperature. As can be seen on the graph, the starting shrinkage temperature slightly increases with addition of the NC. At T= 400 °C we beginning the measurements. We will compare the dilatometric curves versus–heating time with versus temperature in the future experiments.

In all two samples, XRD analysis shows that the crystallite size of the consolidated material is ~26 nm, which is about one-half the initial grain size (~56 nm) of powder. All samples are analyzed by XRD (Figure 3), and where appropriate by FESEM.. The standard XRD spectra for several phases of interest herein for the reference purpose ²⁶. The Bravais structure of cBN, hBN, TiC, bundles of NC and TiB₂ are illustrated in the Figure 4.

Figure 4.Bravais structure of the construction of (CMNCs) tools, a-cBN, b-hBN, c-TiC, and the bundles of NC with hexagonal lattice and d-TiB2

XRD analysis of the samples indicates that the only phases formed in the sample without NC are titanium carbide TiC (5 µm), with a cubic crystal structure and the transformation of the cBN to hBN crystal structure. The addition of 0.1 wt % NC has considerable effect on XRD pattern. (intensity of the XRD spectrum of the nanocomposite, which indicates that the reaction between the TiC, cBN powders and NC did happen during the sintering process in the system cBN-TiC-NC.

The X-ray diffraction investigations were carried out with diffractometer (Philips 1710) using CoKα radiation. The X-ray diffraction phase analysis and profile of diffraction line analysis were applied. After sintering of the investigated samples the following new phases (Figure 3) were formed TiB₂ in the cBN-TiC system, and TiB₂ in cBN_x-TiC_1-x (x=0.8)-0.1 wt % NC ceramics matrix nanocomposite (CMNCs) tools system. The shape of diffraction lines is a powerful tool for fine microstructure characterization. The line half-width contains information about average crystalline size and lattice defect density which can be treated as internal stored energy ²⁷. When we compare the intensities of the tree pics located at 32, 42.5 and 50 degrees corresponding to TiB₂, TiC and cBN to hBN, respectively. The highest intensity is observed for the TiB₂ phase when the samples is reinforced by NC (graphitic pic located at 26 degrees) (Figure 3a and Figure 3b).The addition of NC by 0.1 wt% move the reaction of more boron atoms to react with TiC and produced TiB₂ phase for additional consolidation of the (CMNCs) tools.

All three samples processed at 75 MPa are 95-98% density of cBN (ρ = 3.45 g/cm3), with balance hBN (ρ = 2.1 g/cm3). The appearance of hBN under such a high pressure is surprising.

A possible explanation is that densification via plastic deformation under high pressure is incomplete, forming micro-pores at “triple junctions «between cBN grains, which then become favorable sites for nucleation and growth of the lower density hBN.

Such behavior is most likely to occur during heat -up of the sample under high pressure. The larger amount of hBN in the sample processed at 75 MPa may be attributed to the same cause. Because of their pressure, densification via plastic deformation is less complete, leaving larger micro-pores at triple junctions of cBN that allow more hBN to form. Plastic deformation accompanied by recrystallization at points of contact between neighboring cBN particles under high pressure is a possible explanation.

However, as noted above, fully dense phase-pure cBN is not achieved able under the designated processing conditions; a small fraction of hBN is invariably formed. To eliminate cBN decomposition during FASPS, it will be necessary to investigate the use of sintering aids. The additions of TiC to cBN prevent its decomposition into hBN, probably by forming a thin surface-passivation film of Tic-base compounds. Moreover, by controlling reactions between TiC and cBN phases, fully dense composite structures can be obtained, comprising high fractions of super hard cBN particles cemented together with hard TiB₂composites.By the addition of 0.1 wt% of NC we obtain super hard ceramics matrix nanocomposites (CMNCs) tools. When such composite structures contain residual un-reacted TiC, there is the prospect of enhanced toughness, while retaining high hardness, stiffness, To summarize, fully dense nanocrystalline cBN-TiC can be produced by FASPS processing, but the formation of a minor fraction of hBN seems unavoidable.

This is unfortunate, since the presence of even a small amount of hBN, particularly at nano grain boundaries, must adversely affect fracture toughness. On the other hand, an addition of TiC to cBN provides a route to produce fully dense cBN-based composites with hBN, and thus potentially enhance mechanical performance. The FESEM microstructures of samples with following compositions:

The FESEM microstructures (Figure 5) revealed that the ceramics matrix nanocomposites (CMNCs)) tools has good density of the binder less phases in the final structure of the products. The high magnification representative microstructure of the sample without NC (Figure 5a-b-k-i) consists of large hBN (dark) grains (Figure 5 e-f-h) and TiC (gray) (Figure 5c-d) particle sintered together and remaining not transformed and unreached cBN (Figure 5k-i)The addition of 0.1 wt % N C (Figure 5e-f) to the reaction changed the morphology of hBN from slightly finer grains with near spherical morphology to the large plate like grains. Figure 5e-d, shown the NC in the matrix intra grain boundaries TiC and hBN particle forming the interface with different orientations. The mechanical properties are improved by both phases and grain boundaries. This contributed to the increasing of the Vickers microhardness. In the Figure 5k-i is presented the typical of the loss of NC identified by the pore like structure (canneaux). High magnification images of the starting ultrafine powder of NC and sintered titanium carbide (TiC) are presented in Figure 5a-b and c-d.

Figure 5.High magnification micro structural representative FESEM image of BT and BTNC polished and etched surface for the sintered samples using a 20 mm diameter dieof the sintered samples

The influence of the addition of the reinfort of carbon nnotubes on the relaties density of sintered cBN_x-TiC_1-x (x=0.8) nanocomposites is shown in Table 2. The theoritical density of the nanocomposite used for obtaining relative density was calculated using a rule of mixture, using the densities of two constituent phase (ρ_hBN= 2.1 g/cm3, ρ cBN ₌ 3.45 g/cm3, ρTiC ₌ 4.50 g/cm³, ρ_NC= 2.25 g/cm³) with the given SPS processing parameters, the cBN _x-TiC_1-x (x=0.8)with and without 0.1 wt % NC sample exhibited best densification with relative density greater than 97.5%, with the similar processing parameters with addition ofcarbon nanotubes. The relative density increases with the addition of NC. The cBN_x-TiC_1-x(x=0.8)-0.1 wt % NC nanocomposites at T=1800 °C exhibited relative density of about 98.5%.Depending on the final density to be achieved, the FASPS operating condition were properly chosen, that is, 1800 °C, 75 MPa for 10 min, to obtain a highest relative density for the nanocomposites cBN_x-TiC_1-x (x=0.8), densecBN_x-TiC_1-x (x=0.8)-0.1 wt % NC, TiC for zero compacts porosity, 97.5, 98.06, and 94.06, respectively.

In the Figure 6, the microstructural representation and EDS analysis displays elemental analyses of the various regions of the sintered samples. Secondary electron image, atomic concentration cartographies of B, Ti, N and C are also illustrated. EDS spectra and was used to determine the elemental composition of the different regions in the sample with NC addition and are presented at the Figure 7.

Figure 6.Microstructural representation and EDS maps of BTNC sample analysis displays elemental analyses of the various regions of the sintered samples. Secondary electron image, atomic concentration cartographies of B, Ti, N and C of polished and etched surface for the sintered samples

Figure 7.EDX spectra in different region of BT and BTNC polished and etched surface for the sintered samples using a 20-mm diameter die of the sintered samples

The 1.5 % porosity of the sample cBN_x-TiC_1-x (x=0.8)-0.1 wt % NC exhibited a round microstructure with high ductility, 98.5 %.

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.

According to the above results, it can be concluded that the Vickers hardness has been improved by adding NC and enhanced with the fracture toughness value giving a better ductility for the reinforced NC samples. Figure 8 shows the hardness as a function of the applied indentation load for the same sample. At lower loads, the micro hardness reaches a low hardness a constant value of 47.55, 18 and 35.26 GPa at HV₅₀ for reinforced cBN_x-TiC_1-x (x=0.8)-0.1 wt % NC, TiC, and unreinforced cBN_x-TiC_1-x (x=0.8), nanocomposites with CN, respectively.

Figure 8.Representation the variation of Vickers micro hardness fonction of the indentation leads of the sintered samples using a 20-mm

In the present study, the high hardness of the FASPS synthesized sample containing nanocomposite cBN_x-TiC_1-x (x=0.8)-0.1 wt % NC at T= 1800 °C may be attributed to its high density (98.5% from theoretical).

(Figure 8) present the variation of Vickers micro hardness of cBN _x-TiC_1-x (x=0.8)-0.1 wt % NC nanocomposites with NC reinforcement content at T= 1800 °C with the indentation lead. The microhardness of the nanocomposites increased almost linearly with NC reinforcement and with addition cBN (80 vol. %) content in TiC hard carbide. The hardness of CN (28–30 GPa) is nearly 2 times the previously reported values of hardness of TiC (18 GPa). For the nearly single phase hBN sintered (FASPS) in this investigation, the micro hardness was found to be 30 GPa (measured with indentation load of 300 N), which is higher than the previously reported hardness values for hexagonal in the literature. While the higher hardness of the cBN_x-TiC_1-x (x=0.8)-0.1 wt % NC nanocomposite sample could be due to a minor amount of NC intragranular reinforced TiC grain phase in the nanocompositecBN_x-TiC_1-x (x=0.8).

The highest Vickers micro hardness in the range of about 55 GPa was found for lower loads (10 N). A slight increase in average hardness have been obtained from nanocomposites prepared by FASPS sintering of cBN_x-TiC_1-x (x=0.8)-0.1 wt % NC at T=1800 °C exhibited highest hardness of about 47.55 GPa was found for lower loads (500 N). (Figure 6), Itis considered that cBN 20 vol% TiC with the remaining 0.1 wt% NC act as reinforcements play the major role in the consolidation of the products.

The best product contained cBN_x-TiC_1-x (x=0.8)-0.1 wt % NC which was sintered at 1800 °C, 75 MPa for 10 min. The Vickers hardness of cBN-TiC_1-x (x=0.8) increases with NC incorporation in the matrix (Figure 8).

With this addition in the matrix, the electro discharge among powders may lead to self-heating and purification of the particle surface, resulting in activation of the formation of the nanocomposites.

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.

Figure 9.Correlation between the applied load P and half-length of median crack c3/2of polished and etched surface for the sintered samples using the (IF) method.

As a result, the calculated slope A and intercept B were 0.0827 and 4.2534, respectively. The indentation fracture toughness was calculated to be 12.30 MPa m^1/2 for cBN_x-TiC_1-x (x=0.8) -0.1 wt % NC ceramics matrix (CMNCs) tools with excellent wear resistant will be confirmed.

The addition of NC plays a important binder less role (the ductility) in the propagation of failler in this nanocomposites and thus enhance the fracture toughness in comparison with his higher hardness In addition, a high determination coefficient (R²) of 0.9961 was obtained through the linear regression model ³⁰. IF (indentation fracture) was shown to be an effective method in the evaluation of fracture toughness for its convenience and material saving.

In the Figure 10 is presented the Raman response of the nanocomposites cBN_x-TiC_1-x (x=0.8)-0.1 wt % NC, TiC and cBN_x-TiC_1-x (x=0.8) with laser excited by λ_exc=244 nm. The transition of cBN to hBN are detected by the vibration frequency response by tree vibration mode of the hexagonal lattice longitudinal (LO), transversal (TO) and E_2garround the 1447 for E_2g, 2142 for 2LA(K), 2656 for 2TO(KM) and 2887, 3060 cm^-1for 2LO(T) respectively, according to ²⁸. The reinforced phase of the binder less NC is localized in the typical frequency of G and D mode at 1351and (1524, 1594) cm^-1, respectively. The Raman spectra confirm the XRD investigations.

Figure 10.Raman spectra of the BTNC sintered sample using a 20-mm diameter die.