Mechanically induced self-propagating reaction of vanadium carbonitride

Author's Accepted Manuscript Mechanically induced self-propagating reaction of vanadium carbonitride M.A. Roldán, M.D. Alcalá, C. Real PII: S0272-8842(14)01937-3 DOI: http://dx.doi.org/10.1016/j.ceramint.2014.12.016 Reference: CERI9617 To appear in: Ceramics International Received date: 6 November 2014 Revised date: 1 December 2014 Accepted date: 2 December 2014 Cite this article as: M.A. Roldán, M.D. Alcalá, C. Real, Mechanically induced selfpropagating reaction of vanadium carbonitride, Ceramics International, http://dx.doi.org/ 10.1016/j.ceramint.2014.12.016 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. www.elsevier.com/locate/ceramint 1 MECHANICALLY INDUCED SELF-PROPAGATING REACTION OF VANADIUM CARBONITRIDE M.A. Roldán, M.D. Alcalá, C. Real* Instituto Ciencias de Materiales de Sevilla CSIC-US, Av. Américo Vespucio nº49, 41092-Sevilla (Spain) *Corresponding autor. Abstract Vanadium carbonitrides (VCxN1-x) were prepared via mechanosynthesis from mixtures of elemental vanadium and carbon with different V/C atomic ratios under a nitrogen atmosphere using a high-energy ball mill. We obtained the full composition range of carbonitrides at room temperature. The products were characterized using X-ray diffraction, scanning electron microscopy and electron energy loss spectroscopy. The results showed particle-sized products with high sinterability and very high microhardness. Keywords Mechanosynthesis; Vanadium carbonitride; EELS; Microhardness Introduction Transition metal carbonitrides are important materials because of their unique and outstanding combination of physical properties, such as high melting points, high hardnesses, good thermal and chemical stabilities, and excellent electrical and thermal conductivities, among others. These properties make carbonitrides useful materials for a wide range of applications, such as coatings and cutting tools. Furthermore, carbonitrides are widely being used in the preparation of advanced engineering ceramicbased composites that are employed in several applications in key high-level technologies1-3. Currently, the most common synthesis methods for producing carbonitrides are chemical vapor deposition from organometallic precursors4-9, thermal diffusion3,10, solid-state reactions from precursor oxides and carbides11-13, and self-sustaining hightemperature synthesis from vanadium carbide14. This last procedure appears to be a good alternative to the classic metallurgic methods for synthesizing borides, carbides, nitrides and carbonitrides15-23. It has been established that the physical and chemical properties of these solid solutions depend on their compositions (C/N ratio)3,24-25; however, the aforementioned methods do not allow for the synthesis of these materials 2 over the entire composition range. Our group has previously reported26-28 that mechanical synthesis methods provide good results in the synthesis of these types of compounds, affording particles with nanometric sizes. There are examples in the literature of the synthesis of VC using mechanical processes29-30, as well as examples of other carbonitrides synthesized using self-propagating or combustion reactions induced by milling26-27,31-32. In the present paper, we synthesize vanadium carbonitride from its constituent elements using reactive milling. Reactive milling is an easier, faster and more economical process than the previous methods. The microstructural characterization and hardnesses of the final products are also presented. Experimental Graphite powder (<270 mesh, Fe≤0.4%, Merck), vanadium (99.5% purity, <100 mesh, Aldrich) and nitrogen (99.9%; H2O≤3 ppm, O2 ≤2 ppm and CnHm ≤5 ppm; Air Liquide) were used in this work. Different C-V powder mixtures were milled under 11 bars of high-purity nitrogen gas using a modified planetary ball mill (Fritsch model Pulverisette 7) at a rotation rate of 700 rpm for both the rotation of the supporting disc and the superimposed rotation in the direction opposite to the vial. Six tempered steel balls and 5 g of reactive powder (C-V) were placed in a tempered steel vial (67Rc) for each milling experiment. The volume of the vial was 45 ml. The diameter and weight of the balls were 15 mm and 13.85 g, respectively. The powder-to-ball mass ratio (PBR) was 1/16. The vial was purged with nitrogen gas several times, and then the desired nitrogen pressure (11 bars) was achieved prior to milling. The vial was permanently connected to the gas cylinder during the milling experiments through a rotary valve and a flexible polyamide tube. The pressure was continuously monitored using a SMC solenoid valve (model EVT307-5DO-01F-Q, SMC) connected to an ADAM-4000 series data acquisition system (Esis Pty Ltd.). The self-sustaining reaction inside the vial was detected from the time–pressure value, which was monitored during milling. This record exhibited a peak when the ignition occurred due to the heat generated by the highly exothermic reaction. X-ray powder diffraction patterns were recorded using a Philips X’Pert Pro diffractometer equipped with a Θ/Θ goniometer using Cu Kα radiation (40 kV, 40 mA), a secondary Kβ filter, and an X’Celerator detector. The diffraction patterns were scanned from 10° to 90° (2Θ) at a scan rate of 0.42º min−1. Silicon powder (NIST) was 3 used to correct the XRD peak shifts. The lattice parameter, a, of the VCxN1−x phases was calculated from the entire set of peaks in the XRD pattern using the Lapods computer program assuming a cubic symmetry. The average crystalline size of the powder was estimated using the Scherrer equation. The iron contamination in the milled samples was determined using Fe2+ permanganometry. The formation of carbonitrides was also studied using a high-resolution transmission electron microscope (Philips CM200, Eindhoven) equipped with a super twin objective lens and operated at 200 kV with a LaB6 filament. The TEM was equipped with a parallel electron energy loss (EELS) spectrometer (Model 766-2K, Gatan, München, Germany). The N, C and V core-loss edges were recorded in the diffraction mode with a camera length of 470 mm using a 2-mm spectrometer; the entrance aperture yielded an energy resolution at the zero-loss peak of 1.4 eV. Spectra were recorded for dark current and channel-to-channel gain variation. After subtraction of the background using a standard power-law function, the spectra were deconvoluted for plural scattering with the Fourier-ratio method and normalized to the jump. All of these treatments were performed within the EL/P program (Gatan). Microstructural observations were conducted using scanning electron microscopy (SEM model JSM 5400 Jeol). The SEM images were recorded at 30 kV. For the SEM observations, the powdered samples were dispersed in ethanol and supported on a metallic grid. The different samples were sintered using a pressureless process. The samples were first shaped (green bodies) and then sintered at high temperature. The forming process was performed using cold isostatic pressing at 2000 bars for 5 min. The green bodies were sintered at 1750ºC for 60 min (heating rate of 5°C/min, free cooling) under an inert atmosphere (helium gas, H2O≤3 ppm, O2≤2 ppm and CnHm≤0.5 ppm, Air Liquide) in a horizontal furnace (Thermolyne Type 59300 model no. F-59340-CM). Vickers microhardness measurements (FM-700, Future-Tech, Hardness tester) were conducted on cross sections of pellets under a load of 1 kgf for 15 s. An average hardness was calculated from 20 indents per specimen. Results and discussion Table I presents the different compositions prepared via mechanosynthesis and the required preparation times: 4 Table I. Milling times of the final vanadium carbonitrides. x (VCxN1-x) 0.00 0.25 0.30 0.40 0.50 0.55 0.60 0.75 0.85 0.95 1.00 Combustion Time (min) -- -- 120 61 32 42 53 56 61 55 -- Milling Time (min) 480 300 150 90 60 70 80 80 90 80 240 We can observe two different synthesis mechanisms: the combustion induced by the milling process mechanism (self-auto-propagating reaction), which corresponds to the carbonitrides with compositions of x=0.3 to 0.95, and the diffusion mechanism, which corresponds to the compositions (x=0.25) and (x=0 and 1). Fig. 1 shows the X-ray diffraction patterns for the composition VC0.5N0.5 with different milling times, which was obtained via the combustion mechanism, as an example. The diffraction peaks of vanadium broaden as the milling time increases (prior to 30 min, which is the ignition time); after 32 min of the SHS process, only the diffraction peaks of the carbonitride are observed. After the combustion reaction, the milling continues for another 30 minutes to ensure that the reaction is complete and to homogenize the newly formed material. As shown in Fig. 1, the composition of the sample does not change during this time. 20 25 30 35 40 45 50 55 60 65 70 75 80 60 min. VC0,50N0,50 V 32 min. 30 min. 15 min. 2 θ Fig. 1. X-Ray diffraction patterns of the sample with a nominal composition of VC0.5N0.5 at different milling times. 5 For the sample with a composition of x=0.25, the reaction follows a diffusion mechanism similar to that for VC and VN. Fig. 2 presents the X-ray diffraction patterns for this sample at different milling times. The analysis of the XRD patterns indicates that the V peaks broaden with increasing milling time due to the refinement of the crystallite size, the formation of defects, and microstrains in the milled samples. Furthermore, the diffraction patterns begin to exhibit peaks corresponding to vanadium carbonitride. After 3 h of milling, the VCN peaks are the most prominent peaks in the diffraction pattern. Further increasing the milling time to 5 h led to a continuous increase in the intensities of the VCN diffraction peaks; the considerable broadening of the VCN diffraction peaks suggests that this phase was obtained with a very refined microstructure and a high level of microstrains. 20 30 40 50 60 70 80 VC0,25N0,75 V metal 1 h 2 h 3 h 4 h 5 h 2Θ Fig. 2. X-ray diffraction patterns of the sample with a nominal composition of VC0.25N0.75 at different milling times. The sample with a composition corresponding to x=0.3 is special due to a combustion reaction during the milling process, but the required ignition time is higher than the ignition time for the other compositions. Moreover, the final product is not only carbonitride because a mixture of products is obtained (VCxN1-x, V2N and V2C), as shown in Fig. 3. 6 20 30 40 50 60 70 80 V2C/ V2N VCxNy 2Θ Fig. 3. X-Ray diffraction pattern of the final product of the sample with a nominal composition of VC0.3N0.7. The literature indicates that the self-propagating reaction induced by ball milling is similar to the self-sustaining high-temperature synthesis (SHS), in which the nomenclature is the same as that proposed by Yen33 (MSR), and it can be divided into three steps. 1. An activation period in which the particle size decreases, the number of defects increases and, in some cases, a small amount of product is formed via the diffusion process. 2. An ignition period in which a very high local temperature is reached, the reaction propagates for the whole sample and the product is formed instantly. 3. After the ignition period, a partial combustion occurs, which means that extra milling time is required to complete the reaction and to homogenize the formed product. On the other hand, according to data in the literature26-27,32-37 for reactions that follow an MSR process, the condition is always an adiabatic temperature above 2000 K, and in the same family of compounds, this reaction mechanism typically occurs in a determined range of compositions. Therefore, in a review of this type of grindinginduced reaction, Takacs36 commented that the high ignition time is ocassionally caused by small amounts of reaction products formed via diffusion at the beginning of grinding, which inhibit the reaction as inert additives; thus, when combustion does not occur before a minimum time, the reaction proceeds via diffusion. On studies of SHS, Yeh et al.38-40 found that during the formation of carbonitrides of Ti, Nb and Ta, after combustion, a propagation front is generated that runs across the sample and that the 7 front extension decreases as the carbon content increases. In this type of reaction, nitrogen is incorporated in the second stage of the process, and as the front extension decreases, a smaller amount of nitrogen is incorporated. The maximum incorporation of nitrogen occurs for C/N = 1. For this reason, and taking into account that for compositions with C/N > 0.7, the quantity of metal available for the reaction is less, it becomes difficult to prepare samples with nitrogen-deficient compositions using this procedure. Yeh et al.12 applied SHS to obtain vanadium carbonitride, VCxN1-x, for compositions over x = 0.3; within the range of x = 0.3 to x = 0.7, they observed that afterburning occurs along the sample after combustion. For x = 0.5, the formed carbonitride presented the highest nitridation value, and nitrogen-deficient carbonitrides were obtained for x values between 0.5 and 0.7. For x = 0.3, the products were a nitrogen-deficient carbonitride and V2N because there was less C available and because V2N is not soluble with carbide phase, and thus, the reaction does not proceed toward formation of the compound. In the case of MSR reactions, both carbon and nitrogen react together with the metal; thus, preparing nitrogen-deficient compositions is feasible using MSR reactions. These two methods are in some ways parallel, but they may also be complementary for the preparation of these materials. Our results are in good agreement with the literature14, 38-40, in which the limit of x = 0.25 is commonly used to prepare compounds using these processes (MRS and SHS). Similarly, with the composition x = 0.3, the formation of other compounds is detected, as also observed for Nb, Ta, Hf and V. The crystallite sizes of the different samples were estimated using the Scherrer equation, and these values are presented in Table II. The lattice constants of the VCN cubic unit cell were determined from the entire set of peaks in the diffraction patterns, which were recorded from 10 to 90 °2θ for all samples. Least-squares fittings of the XRD peaks were performed using the Lapoud program.37, and the results are presented in Table II. Table II also presents the lattice parameters calculated using Vegard’s law for the different compositions. The comparison of these values with the experimentally calculated values with the Lapoud program reveals some deviation, indicating that the obtained compounds are deformed and/or, in many cases, the obtained compounds are not stoichiometric, being deficient in nitrogen, as previously observed by Yeh et al .14 in their study of this family of compounds obtained using SHS. 8 The level of iron contamination was calculated from chemical analysis of the total iron content by titration and revealed that contamination by this metal occurred during grinding, although it never exceeded 4%. The corresponding percentages of iron and the values are included in Table II, which indicate that the samples obtained by combustion present values of approximately 1% and increase to near 4% in samples obtained by diffusion. This result and the degree of crystallinity of the obtained samples represent one of the advantages of the combustion mechanism. The combustion mechanism also has an economic advantage because of shorter milling times. Table II. Lattice parameter (a), crystallite size, maximum strain and iron contamination of the final product. Sample a(Å) L. Vegard a (Å) DDC (nm) Microstrains (10-3) %Fe VC0.25N0.75 4.1457 4.1388 8.25 1.252 3.5 VC0.30N0.70 4.1471 -- 18.30 0.692 2.1 VC0.40N0.60 4.1496 4.146 163.50 0.242 0.9 VC0.50N0.50 4.1521 4.148 127.05 0.182 0.8 VC0.55N0.45 4.1536 4.143 112.65 0.153 0.8 VC0.60N0.40 4.1547 4.144 148.35 0.146 1.2 VC0.75N0.25 4.1586 4.153 157.30 0.133 0.7 VC0.85N0.15 4.1612 4.156 158.15 0.128 0.6 VC0.95N0.05 4.1638 4.158 150.00 0.137 0.7 The characterization was completed by measuring energy loss spectra in the transmission electron microscope. Fig. 4 shows the nitrogen and C K-edges for the different prepared samples. The data in the literature for such compounds, particularly titanium carbonitride41-43, show that the carbon K-edge of a carbide is identified by two peaks centered at approximately 283 eV (π bond) and 292 eV (σ bond), in which the first is of higher intensity than the second; the free amorphous carbon is identified by a peak at 285.6 eV and a broader and higher intensity peak at 296 eV. Finally, the carbonitride presents the same peaks as the carbide, 283 eV and 292 eV, but in this case, the second peak was slightly more intense than the first peak because of the inverse relationship for the orbital occupation density involved with respect to the carbide. Figure 4 shows that • The composition x = 0.25 has a spectrum with the peak at 286 eV and a higher intensity peak at 296 eV; according to the literature41-43, this behavior indicates the presence of amorphous carbon or a nitride with the amorphous carbon on a 15 18. Anselmi-Tamburini U, Munir Z.A. 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