Synthesis of Vanadium-Vanadium Carbide in-situ Nanocomposites by High Energy Ball Milling and Spark Plasma Sintering

In the present work, Vanadium-Vanadium Carbide (V-V2C) in-situ nanocomposites were synthesized by mechanically milling vanadium powders with 0.5 wt.% stearic acid. Milled powders were consolidated using spark plasma sintering at 1150, 1250 and 1350 °C for 10 min. Phase and morphology of the milled powders were studied using X-ray diffraction and scanning electron microscopy. X-ray diffraction analysis of 10 h milled powder shows the evolution of amorphous phase. Energy dispersive X-ray spectroscopy studies on milled powder shows the presence of carbon, which could be due to the decomposition of stearic acid during milling. Degree of crystallinity of milled powder was confirmed using the selective area electron diffraction pattern. X-ray diffraction analysis of sintered samples indicate sharp peaks from vanadium and vanadium carbide (V2C), endorsing amorphous to nanocrystalline transformation. Micro-hardness value of sintered samples increases with increasing sintering temperature.


Introduction
Transition metal carbides are an exceptional class of materials with diverse industrial applications such as metallurgy, tooling, electronics, high temperature coatings, catalysts, etc. [1][2][3][4][5][6].Among the metal carbides, carbides of vanadium possess a unique combination of properties like high hardness, high wear resistance, high melting point and metallic conductivity owing to which it finds extensive use in variety of cutting tool applications [7].It is understood from the V-C phase diagram [8] that vanadium carbides with different stoichiometries are possible, VC, V 2 C, V 8 C 7 and V 6 C 5. High hardness of the aforementioned carbides depends on the carbon to metal (C/V) ratio [9].Generally, C/V ratio greater than 0.8 yields higher hardness and in this context V 2 C (0.5) stands next to V 8 C 7 (0.87).While many techniques like have been proposed for developing V 2 C [10][11][12][13][14][15][16], but powder metallurgical (P/M) synthesis of nanostructured vanadium carbide is closely associated with tooling applications.It has also been reported that spark plasma sintering (SPS) can sinter powders close to theoretical density at relatively lower temperatures when compared to conventional sintering techniques [17][18][19].Further, specimens developed using SPS show improved properties in terms of retaining nanostructure, corrosion resistance and improved microstructure [20][21].
In the present work, we have attempted a simple approach to synthesize bulk vanadium-V 2 C in situ nanocomposites by high energy ball milling and SPS.We have also discussed the dispersion strengthening effect of nano-crystalline V 2 C in V matrix by mechanically milling pure vanadium powders with stearic acid (CH 3 (CH 2 )16COOH) as process control agent (PCA) [22][23][24].

Powder synthesis
The starting materials used in this study were vanadium (V) powder with purity 99.5% and particle size equivalent to ~325 mesh procured from Changsha Langfeng Metal Materials Co., Ltd., China.Powders were handled and milled in high pure argon (99.99% purity) environment.Vanadium powders were dry milled with stearic acid (0.5 wt%) as process control agent at 250 rpm for 10 h using high energy planetary ball mill (Retsch, PM 400, Germany).Tungsten carbide vial and balls were used for milling.Ball-to-powder ratio was maintained as 10:1.Milling cycle consists of 10 min milling and 20 min idle.

Spark plasma sintering (SPS)
Milled powders were accurately weighed and transferred into high density graphite die-punch setup.Before transferring the vanadium powder inside the graphite die, inner walls of the die was coated with boron-nitride aerosol spray.Boron nitride spray ensures effective electrical conductivity during sintering and avoids sticking of sample with die-punch setup after sintering.Powders packed inside the graphite die-punch setup was consolidated using SPS (Dr.Sinterlab, SPS Syntex, 511/515S) at 1150, 1250 and 1350 °C with a constant pressure of 3.3 kN for 10 min.

Characterization techniques
Both powder and sintered samples were subjected to X-ray diffraction (Rigaku Ultima, Japan) analysis using Cu-Kα radiation (λ = 1.54 Å) at a scan rate of 4°min -1 .Crystallite size of milled powder was calculated using Scherrer equation.Williamson-Hall plot was used to calculate the lattice strain in milled and sintered samples by plotting 4*Sinθ in abscissa and β hkl *Cosθ in ordinate, where β hkl is obtained from the following relation, where β r = peak broadening (in radians) due to combined effects of crystallite size and lattice strain, β i = peak broadening (in radians) due to instrumental error and β o = peak broadening (in radians) due to processing of powders.
Further, the morphology and chemical composition of both powder and sintered samples were confirmed from SEM-EDS (Hitachi SU6600, FESEM) and TEM (FEI, Tecnai, USA) studies.TEM micrographs were recorded at an accelerating voltage of 300 keV.The density of sintered sample was determined using Archimedes principle.Micro-hardness test (Wilson Hardness Tester, Vickers 402 MVD) was done with a load of 300 g and 10 sec dwell time.

X-ray Diffraction Analysis
X-ray diffraction pattern of powder and sintered samples can be seen from Fig. 1.Premixed pattern, Fig. 1 a, shows a sharp diffraction pattern corresponding to vanadium from (011), ( 002) and (112) planes.After 10 h milling sharp vanadium peaks are completely replaced by diffused peaks, Fig. 1 b, indicating amorphisation of vanadium powders.Possible reason for the amorphisation of powder particles could be the ballistic decomposition of stearic acid during mechanical milling [25].Absence of surfactant (stearic acid) among the vanadium particulates during milling must have distorted the vanadium lattice locally.Moreover, decomposition of stearic acid must have incorporated interstitial (carbon) impurity in vanadium lattice.When these carbon atoms penetrate into the interstitial sites of vanadium localized lattice distortions could have initiated [26].When these localized distortion reaches some critical value, long-range ordering of vanadium lattice gets destroyed leading to solid state amorphisation of vanadium powders.Egami et.al. has reported that atomic size ratio between the solute and solvent atoms can decide the minimal concentration of solute atoms (carbon) required to destroy the long-range ordering of a BCC lattice [27].In addition to it, BCC lattice can lose its long-range ordered structure by solid state reaction (MA) if its atomic ratio lies between 0.66 to 1.31 [28].In our case, the atomic ratio between vanadium (0.135 nm) and carbon (0.07 nm) was found to be ~1.93.Both decomposition of stearic acid and the atomic ratio between vanadium and carbon atoms must have complimented each other in destroying the long-range ordering of vanadium lattice during milling.X-ray diffraction pattern of vanadium powders SPS at three different temperatures, 1150, 1250 and 1350 °C, is presented in Fig. 1 c-e.X-ray pattern corresponding to SPS samples at 1150, 1250 and 1350 °C reveal sharp peaks from vanadium and vanadium carbide.Sharp peaks from V and V 2 C confirms the amorphous to nano-crystalline transformation attributed by Joule heating during SPS.Possible reasons for the additional sharp peaks from V 2 C could be due to the rearrangement and solid state inter-diffusion among vanadium and carbon atoms during sintering [29].Rearrangement of atoms must have encouraged X-rays with more planes for diffraction leading to additional peaks in the diffraction pattern.Table I clearly shows the effect of sintering temperature on the crystallite size and lattice strain of the sintered samples.Crystallite size (Table I) calculated from the sintered diffraction pattern endorses nano-crystalline nature of the sintered samples.Increasing sintering temperature from 1150 to 1350 °C increases the crystallite size and decreases the lattice strain owing to thermal treatment during sintering.Increase in the crystallite size of the sintered samples with increasing sintering temperature could be due to the lattice expansion during sintering.Decrease in the lattice strain (Table I) with increasing sintering temperature indirectly speaks about the stress relaxation phenomenon during sintering.
Tab.I Crystallite size and lattice strain of mechanically milled powders and spark plasma sintered samples.

Microscopic analysis
Powder SEM micrograph shows the effect of mechanical milling on the particle size and morphology of vanadium powders (Fig. 2).Extensive reduction in the particle size after milling is evident from Fig. 2. Average particle size of vanadium powders before and after milling was found to be ~9 and ~0.5 μm, respectively.Considerable particle size reduction from SEM micrographs reveals the domination of fracturing phenomenon during milling.The EDS spectra (Fig. 2 d) of milled powder clearly show the co-existence of carbon and vanadium peaks which goes in-congruence with XRD analysis.Thus SEM-EDS analysis confirms the presence of carbon among vanadium powders during milling.TEM micrograph and their corresponding (inset in Fig. 3) selective area electron diffraction (SAED) pattern is shown in Fig. 3. SAED pattern of the 10 h milled powders reveals a diffused ring pattern which confirms the amorphous nature of the vanadium powders.Thus XRD and SAED analysis confirms the amorphisation of vanadium powders after 10 h milling.Fig. 4 shows the SEM micrographs of vanadium powder sintered at 1150, 1250 and 1350 °C.EDS spectra of sintered samples also confirms the coexistence of carbon with vanadium.Apart from the carbon atoms supplied during milling, additional carbon atoms must have penetrated from graphite die and punch to vanadium powders during sintering.As carbon diffusivity in vanadium is reported to be higher at elevated temperatures [30], sintering temperature must have initiated an in situ reaction between carbon and vanadium leading to V 2 C phase formation (Fig. 1 c-e).Thus V 2 C reinforced V nano-composite is achieved with high energy ball milling and SPS.Increasing sintering temperature from 1150 to 1350 °C, growth, fracture and segregation in the needle like structures is evident.To understand the chemical composition of needles, EDS spot scan (Fig. 5) was performed.EDS spectra confirms the presence of carbon and vanadium in the needles.SEM micrograph (Fig. 4 a-c) also shows that sintering temperature must have played a crucial role in deciding the growth of needles.Fig. 6.Effect of sintering temperature on the microhardness of spark plasma sintered samples.Further, sintering temperature is observed to have direct effect on the microhardness of sintered V-V 2 C in-situ nano-composite (Fig. 6).The hardness values of spark plasma sintered V-V 2 C in-situ nano-composite exceeds the microhardness value of V 2 C reported by Lailei and his co-workers [31].Reason for relatively higher micro-hardness is the nanostructured (V-V 2 C) grains and dispersion strengthening of V 2 C in V [32].Relative density of vanadium powders sintered at 1150, 1250 and 1350 °C was found to be 92, 94 and 95 % respectively.

Conclusions
V-V 2 C in situ nanocomposites are synthesized with high energy ball milling and spark plasma sintering.10 h of dry milling vanadium powder with stearic acid as process control agent results in solid state amorphisation.Decomposition of stearic acid during milling causes amorphisation of vanadium powders.XRD, SEM-EDS and TEM-SAED studies confirm amorphisation of vanadium powders during milling.Amorphous to nanocrystalline transformation was observed in the vanadium samples during sintering.Spark plasma sintering of amorphous vanadium powder initiates in situ reaction between vanadium and carbon resulting in V-V 2 C in situ nanocomposites.Sintering temperature chiefly influences the growth of needle like structures.Increasing sintering temperature from 1150 to 1350 °C increases both relative density and microhardness of sintered samples.Thus V-V 2 C in situ nanocomposites with microhardness values comparable V 8 C 7 was synthesized successfully using SPS.

Fig. 1 .
Fig. 1.X-ray diffraction patterns of mechanically milled powders and spark plasma sintered compacts at three different temperatures for 10 minutes.

Fig. 2 .
Fig. 2. SEM images of a) premixed powder, and b) 10 h milled vanadium powder, EDS area scan of c) premixed powder, and d) 10 h milled vanadium powder.

Fig. 5 .
Fig. 5. a) SEM image needle shaped V 2 C particles, and b) EDS scan on needle shaped V 2 C particles.