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Nitrogenation of hafnium carbide powders in AC and DC plasma by Electrical Discharge Assisted Mechanical Milling
Journal of Alloys and Compounds 715 (2017) 192e198
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Nitrogenation of hafnium carbide powders in AC and DC plasma by Electrical Discharge Assisted Mechanical Milling I.S. Aisyah, M. Wyszomirska*, A. Calka, D. Wexler Faculty of Engineering, University of Wollongong, Northfields Ave, Wollongong, NSW, 2522 Australia
a r t i c l e i n f o
a b s t r a c t
Article history: Received 1 March 2017 Received in revised form 26 April 2017 Accepted 27 April 2017 Available online 28 April 2017
Metal carbonitride powders and coatings are well known as superhard materials which are widely applied in many engineering products. Of this group of materials hafnium carbonitride properties are one of the least known due to difficulties with preparation. In this study, the electric discharge assisted mechanical milling (EDAMM) method involving a nitrogen plasma together with mechanical milling was used to synthetise hafnium carbonitride powders within minutes. It was found that difficulties in conventional manufacturing methods, resulting in the formation of only thin layers instead of bulk materials can be overcome by applying the new technique, and HfCxNy powder could be successfully engineered with controlled amount of nitrogen uptake. In this investigation we studied the effects of AC and DC discharges on nitrogen solubility in HfC powder. Microstructure and phase evolution were determined using XRD and SEM, while the nitrogen content in the grains was evaluated by EDS and bulk nitrogen content by CHN analysis. Maximum solubilities of 3.42 wt% and 2.95 wt%N under DC an AC discharge currents, respectively, were obtained. Reaction paths varied depending on processing conditions with HfCxNy formed under DC discharge processing and mixed products containing HfC, several HfCxNy phases and HfN like phase formed after AC processing. After nitrogenation the most highly nitrided powders were subjected to 1 min additional EDAMM processing using high power discharges to allow the formation of large, sintered particles of sufficient size to perform hardness measurements on. Vickers harnesses of fully sintered large particles after 5 and 10 min of EDAMM were 1808 and 2169HV, respectively. © 2017 Elsevier B.V. All rights reserved.
Keywords: Transition metal alloys and compounds Powder metallurgy Nitrogen absorption Hard metals Electro-discharge mechanical alloying Electro-mechano-synthesis
1. Introduction Transition metals carbides are well known for their specific combination of properties such as high hardness [5], high melting temperature, high thermal stability [12] and wear resistance [1]. All these properties make them attractive for cutting tools applications. It has been proven that their mechanical properties, hardness in particular, can be additionally enhanced by introduction of nitrogen into the elementary cell. These properties are strongly dependent on [C]/([C]þ[N]) ratio [2e4]. The carbonitrides, especially of group IV periodic table (Ti, Zr) stimulated growing interest as an alternative material for hard coatings. Conventionally, they can be obtained by several methods such as: magnetron sputtering
* Corresponding author. E-mail addresses: [email protected] (I.S. Aisyah), [email protected]. au (M. Wyszomirska), [email protected] (A. Calka), [email protected] (D. Wexler). http://dx.doi.org/10.1016/j.jallcom.2017.04.291 0925-8388/© 2017 Elsevier B.V. All rights reserved.
[10], self-propagating high-temperature synthesis [6], plasma enhanced magnetron sputtering (PEMS) [4] or high energy ball milling [1]. So far the compounds of TiCN and ZrCN as deposited layers and sintered powders have been extensively investigated [1,2,4,6e9] while severe lack of research on hafnium carbonitride, particularly as a bulk material, exists. HfCN found its application as a part of different kind of hard coatings such as Hf-Ti-N [5] or multicomponent ZrHf(CN) thin films [1] obtained by magnetron sputtering. Properties of bulk samples obtained so far provide some difficulties. For example Yang et. Al [4] investigated the hardness of both carbides and nitrides of titanium and found that it was impossible to obtain non-porous bulk samples, particularly with increasing nitrogen content. Kral et al. [11] stated however, that non-porous samples would be possible to obtain by diffusion annealing of transition metal carbides in a nitrogen atmosphere. Theoretical examination of HfC and HfN miscibility indicated complete solid solubility at temperatures 1400e1800 C under nitrogen pressures of between 30 and 300 atm [1]. Further
I.S. Aisyah et al. / Journal of Alloys and Compounds 715 (2017) 192e198
examination of the Hf-C-N system involved experiments on HfC and HfN which were arc-melted and then hot-pressed, but due to low diffusion rates homogenous one phase structures could not be obtained [8]. To the best of our knowledge only Cordoba et al. [1] has made a successful attempt to manufacture homogeneous powder material of hafnium carbonitride by high energy ball milling of elemental powders of hafnium and graphite in nitrogen atmosphere. Obtained particles were in the nanometric scale (100e500 nm) but the process required 80e120 min to complete and the product particles varied greatly in size. Despite the abovementioned difficulties in synthetising good-quality powders of hafnium carbonitrides, they found the product was of sufficient quality for applications such as thin layers on cutting tools and metal-gate electrodes obtained by MOCVD method [13]. In this article, we report on the synthesis of HfCxNy in only 10 min via the versatile process of Electrical Discharge Assisted Mechanical Milling (EDAMM), which combines benefits of traditional ball milling with rapid phase transformations induced by electrical discharges [14e21]. In this method, application of lowcurrent, high voltage electrical impulses leads to formation of localized nitrogen plasma which, in combination with mechanical mixing, results in conditions suitable for very high levels of nitrogenation of HfC powders. 2. Materials and methods Hafnium carbide, (HfC) with the particle size <1.25 mm and 99% purity, purchased from Sigma Aldrich, was used as a starting powder. Powder underwent EDAMM processing in a nitrogen plasma under atmospheric pressure. The powder was placed in a hemispherical reaction chamber and was subjected to controlled electrical discharge together with repeated impact of hardened rod end. The reaction chamber, schematically shown on Fig. 1, is custom built specifically for discharge milling. In this experiment both direct (DC) and alternating current (AC) short duration impulses were used to induce conditions suitable for high solubility of nitrogen in the carbide. The operating electrical parameters can be read from Fig. 1. Impulse modes were selected to prevent melting and maintain the sample in powder form throughout the experiment. HfC powder was milled for 0.5, 1, 2, 5 and 10 min in DC and AC
193
modes. The highest reaction rate was achieved when the average discharge power was set to ~0.54 kW. Higher values of the discharge power were found to cause excessive powder particles agglomeration which was accompanied by the decrease in the rate of nitrogen uptake. These nitrogen rich powder products processed for 5 and 10 min were additionally milled at high power (3 kW) for 1 min to promote micro-sintering and formation of the agglomerated particles. As a result, the particles underwent fast chemical reactions but afterwards were micro sintered to sizes big enough to perform hardness measurement on. After EDAMM, samples were characterized by X-ray analysis using Philips Diffractometer equipped with CuKa radiation and graphite monochromator. Phase identification was carried out using Traces software, Version 5.1.0. X-ray diffractograms were indexed and compared using International Centre for Diffraction Data (JCPDS-ICDD) Powder Diffraction Files (PDF). Both nitrogen contents of the products were estimated by Carlo Erba 1106 combustion elemental analysis. Morphology of the powders was characterized using JOEL 7001 FEGSEM scanning electron microscope (SEM), while SEM-energy dispersive spectroscopy (EDS) was performed using a Bruker EDS detection system running in semiquantitative standardless analysis mode. This enabled elemental composition analysis. Microhardness was measured using a Vickers microhardness indentation tester, with 50 g load set for 15 s and in all experiments the final value is an average of 8 indentations. 3. Results and discussion 3.1. X-ray diffraction X-ray diffraction results obtained from HfC powder (ICCD-PDF card no 06-0510) subjected to EDAMM in DC mode for 0.5, 1, 2, 5, 10 min and are shown in Fig. 2. Hafnium (IV) Oxide (PDF 40-1173) can be seen in the XRD plot, being a minor contaminant in starting powder. During EDAMM under DC mode the XRD peaks related to HfO2 gradually disappear and none of them can be noticed in samples processed for 5min. The attributed explanation is the well known process of reduction of metal oxides under high temperature nonoxide environments. After 1 min of EDAMM peaks indexed to HfC
Fig. 1. Electric Discharge Assisted Mechanical Milling (EDAMM) working in a) alternating and b) direct current presented with corresponding oscilloscope images.
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Fig. 2. XRD results obtained from HfC starting powder subjected to EDAMM in DC mode.
shift to the lower 2-theta angle values (from 33,5 to 33,37 for the first peak), consistent with an increase in lattice parameter associated with the dissolution of nitrogen into the HfC and formation of an interstitial solid solution of HfC(N) (Fig. 3 1 min). To the right of the HfC(N) peak an additional overlapping peak associated with a newly formed phase, HfCxNy can be seen. After 2 min EDAMM the solid solution HfC(N) peak has shifted back to the original position. We attribute this to a preference for
Fig. 3. Enlargement of 33,5 2-theta peak of HfC subjected to EDAMM in DC mode.
formation of HfCxNy over HfC(N) because of the high energy conditions present. Nitrogen appears more eager to form a chemical bond with the HfC substrate rather than remain dissolved as a solid solution in the HfC lattice. Note that the HfCxNy peaks are to the right of both the HfC(N) and the original HfC peaks. They are also located to the left of predicted position for HfN (111) peak, which in Fig. 3 would be located at a 2-theta value of 33.93. These results suggest that a new compound of HfCxNy was formed with a d spacing value in between those two compounds. Summing up, HfC subjected to 2 min milling time is a two phase material consisting of both HfC and HfCxNy whereas longer milling times lead to the presence of only the carbonitride phase, with one composition, also indicated by the symmetry of the peaks for long milling times. In the case of EDAMM working in alternating current mode a different transformation mechanism (Fig. 4) is observed. Peaks related to HfO2 are present, even after 10 min of milling, a result attributed to the lower temperature operative under AC mode, being insufficient to lead to a complete HfO2 reduction reaction. Here, formation of a new HfC1-xNx phase can be observed, while the HfC peak slightly loses intensity but do not disappear, even after 10min of milling. HfC1-xNx shows up on the diffraction pattern as an asymmetric peak. We attribute this to the fact that the product comprises more than one phase within that composition. The asymmetry of the peaks is attributed to the overlapping of these different phases and is also possibly due to the formation of HfN phase (06-0516), which appears as a 33.93 2-theta peak for it's (111) reflection, indicated on Fig. 5. Concluding, as can be seen on Fig. 5, there appear to be several phases; HfC, HfC1-xNx but also HfN present in the final product of milling under AC mode, a result consistent with the SEM results described below. Taking into account that the discharge energy under AC and DC modes is the same, it is suggested that HfC particles are heated to much higher temperatures under DC discharge which generates highly localized Joule heating of particles in the vicinity of a very narrow electrical discharge area as it passes through the powder. In the case of AC discharge, a much larger area of the powder is uniformly heated to much lower temperature. This would also mean that under AC mode HfC powder when heated to lower temperature should experience lower nitrogenation rate.
Fig. 4. XRD results obtained from HfC starting powder subjected to EDAMM in AC mode.
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Fig. 5. Enlargement of 33,5 2-theta peak of HfC subjected to EDAMM in AC mode.
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Microscopy under both secondary and backscattered electron imaging. Results of DC mode experiments can be seen in Fig. 6 aed where it initially causes breaking of the particles. Their irregular shape and smaller sizes indicate that mechanical forces applied by the rod's end resulted in rapid fracturing of powder particles during the first 2 min. The rapid nitrogen intake within first 2 min can be attributed to fast fracturing of powder particles and creation of fresh, clean surfaces that rapidly absorb the nitrogen ions. Further milling creates a wide range of particles sizes. After 5 min, however, clumps of particles are observed, comprising smaller particles apparently electrostatically bonded to the larger ones, forming large agglomerates with rough and porous surfaces. Further milling for 10 min results in further breakage and simultaneous agglomeration of the particles. During the AC mode milling the discharge is more uniform and less localized and the difference in the particle sizes appears not as large as in DC mode (Fig. 7 aed). Final milling in higher electrical conditions (high intensity plasma) was applied to both AC and DC processed powders for 1 min, which resulted in dramatic increases in the size of the particles up to 100 mm. We attribute this to an increase in plasma discharges between particles that induce particle softening together with surface welding. We believe this process is similar to spark plasma sintering (SPS) on a micro-scale. The particles processed in DC mode were later examined by the EDS analysis and used for hardness measurements (Fig. 8).
3.3. EDS analysis 3.2. SEM analysis The impact of the rod end together with an electric discharge during EDAMM represents a complex process. The evolving particle microstructures were examined using Scanning Electron
Chemical composition examination by the EDS analysis, combined with SEM-backscattered imaging, allowed estimation of the elemental distribution in the particles after 10 min of EDAMM under DC and AC modes (Figs. 9 and 10, respectively). It can be seen
Fig. 6. SEM secondary electron images of HfC a) starting powder and after b)2, c)5 and d)10 min of EDAMM in DC mode.
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Fig. 7. SEM secondary electron images of HfC a) starting powder and after b)2, c)5 and d)10 min of EDAMM in AC mode.
that estimates of the nitrogen content in the grains subjected to direct current vary between 2.5 and 4.8 wt% and there are significantly lighter areas, consistent with the presence of higher atomic number elements contents, visible in regions between the grains. EDS analysis revealed the presence of Fe in those areas, most probably originating from the milling media. It can be also noted that there are quite pronounced differences in colour of the grains which exceed differences attributable to changes in orientation, and these appear likely related to variations in chemical composition. Grains identified as having more nitrogen are darker whereas
an increased amount of carbon is associated with lighter colour of the grains. Processing of the stating powder in AC mode on the other hand, results in the estimated nitrogen contents varying from 3.5 to 9 wt %. Grains appear much larger compared to processing in DC mode, and the Hf/C/N ratios show a large distribution. It has to be born in mind that the carbon and nitrogen content cannot be precisely determined with the EDS analysis because of the detecting errors when quantifying light elements. The EDS analysis is only used here as a tool to observe the differences in the
Fig. 8. Mechanism of micro-sintering in high energy EDAMM processing developed for hardness measurements on powders.
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local chemistry and to give general overview of the elemental distribution. Differences in bulk nitrogen uptake are presented by the CHN analysis below. 3.4. CHN analysis
Fig. 9. SEM backscattered image with results of EDS spot analysis of HfC cross-section after 10 min EDAMM in DC mode.
CHN combustion analysis (Fig. 11) revealed bulk nitrogen content in the samples obtained by EDAMM. Nitrogen uptake was found to increase with increasing milling time and reach maximum values of 3.42 and 2.98 wt%N in DC and AC modes, respectively. Extrapolation of obtained data to longer milling times indicates only slight increase of nitrogen content. As can be observed, under direct current plasma, nitrogenation of HfC is more favorable and we attribute this to the fact that this environment provides sufficient energy for dissolution and migration of nitrogen into the powder. 3.5. Sintering and Vickers microhardness
Fig. 10. SEM backscattered images with results of EDS analysis of HfC cross-section after 10 min EDAMM in AC mode.
As mentioned in the experimental part, the usage of discharge power up to 3 kW can be used to promote excessive powder particle agglomeration. This phenomenon was used to form fully sintered, high density particles for microhardness investigations. For the hardness tests samples nitrogenated in DC mode for 5 and 10 min were selected. X-ray diffraction results (not shown) obtained from these samples showed sets of peaks which can be indexed to HfCxNy phases, consistent with EDS results. Vickers microhardness measurements performed by the indentation method revealed significant increases in hardness after HfC milled in nitrogen plasma after 5 and 10 min (Fig. 12). The average values obtained from 8 indentations were 1808 and 2169HV with the standard deviation values of 108.48 and 151.83, respectively. The reason for the increased hardness is thought to be caused by the higher lattice strain with the presence of the nitrogen. 4. Conclusions
Fig. 11. Nitrogen uptake of HfC subjected to EDAMM in DC and AC mode.
We have found that HfCxNy compounds in powder form can be produced in as short a time as 2e5 min using EDAMM processing in nitrogen. Direct Current processing for very short periods (<1 min) results in nitrogen dissolution in HfC, DC processing for longer periods (1 min and above) results in formation of the compound HfCxNy, with near complete transformation after 5 min. Alternating current mode processing resulted in incomplete transformation with formation of a range of products including HfC, HfC1-xNx plus formation of a compound of composition close to HfN. Maximum solubilities of 3.42 wt% and 2.95 wt%N were obtained under DC an AC discharge currents, respectively. A higher rate of nitrogenation occurred in powders processed under DC discharge
Fig. 12. Hardness indentations in HfCxNy product obtained after 5 (left) and 10 min (right) EDAMM under DC mode.
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compared to AC. It was found that the reaction rate is dependent on time and, although carbonitrides can be produced in as little as 2e5 min, further nitrogen intake still occurs after longer milling times. Only slight nitrogen intake would occur after 10min. This result represents a significant advantage over most conventional synthesis techniques as it enables the synthesis of high nitrogen content Hf(C,N) precursor powders suitable for postprocessing via surfacing engineering or hot consolidation techniques. Acknowledgements This investigation was supported by funding from the Australian Research Council Discovery Grant No. DP130101390. The authors acknowledge use of the facilities and the assistance of Dr. Mitchell Nancarrow and Tony Romeo at the UOW Electron Microscopy Centre.
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References [1] J.M. Cordoba, M.J. Sayagues, M.D. Alcal, F.J. Gotor, Monophasic nanostructured powders of niobium, tantalum, and hafnium carbonitrides synthesized by a mechanically induced self-propagating reaction, J. Am. Ceram. Soc. 90 (2) (2007) 381e387. [2] K. Aigner, W. Lengauer, D. Rafaja, P. Ettmayer, Lattice parameters and thermal expansion of Ti(CxNa1-x), Zr(CxN1-x), Hf(CxN1-x) and TiN1-x from 298 to 1473K as investigated by high-temperature X-ray diffraction, JALCOM 215 (1994) 121e126. [3] R. Wei, E. Langa, C. Rincon, J.H. Arps, Deposition of thick nitrides and carbonitrides for sand erosion protection, Surf. Coat. Technol. 201 (2006) 4453e4459. [4] Q. Yang, W. Lengauer, T. Koch, M. Scheerer, I. Smid, Hardness and elastic properties of Ti(CxN1-x), Zr(CxN1-x) and Hf(CxN1-x), JALCOM 309 (2000) L5eL9. [5] U. Konig, Deposition and properties of multicomponent hard coatings, Surf. Coat. Technol. 33 (91) (1987) 103. [6] C.L. Yeh, Y.D. Chen, Direct formation of titanium carbonitrides by SHS in nitrogen, Ceram. Int. 31 (2005) 719e729. [7] K. Maweja, L.A. Cornish, N. Can, Transformation and alloying mechanisms in
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sub-stoichiometric titanium carbonitrides - tungsten high energy ball milled powders, Int. J. Refract. Met. Hard Mater. 29 (2011) 312e319. S. Binder, W. Lengauer, P. Ettmayer, J. Bauer, J. Debuigne, M. Bohn, Phase equilibria in the systems Ti-C-N, Zr-C-N and Hf-C-N, JALCOM 217 (1995) 128e136. W. Lengauer, S. Binder, K. Aigner, P. Ettmayer, A. Guillou, J. Debuigne, G. Groboth, Solid state properties of group IVb carbonitrides, JALCOM 217 (1995) 137e147. M. Braic, M. Balaceanu, A. Vladescu, C.N. Zoita, V. Braic, Study of (Zr,Ti)CN, (Zr,Hf)CN and (Zr,Nb)CN films prepared by reactive magnetron sputtering, Thin Solid Films 519 (2011) 4092e4096. C. Kral, W. Lengauer, D. Rafaja, P. Ettmayer, Critical review on the elastic properties of transition metal carbides, nitrides and carbonitrides, JALCOM 265 (1998) 215e233. P. Kieffer, P. Ettmayer, M. Freudhofmeier, About nitrides and carbonitrides and nitride-based cemented hard alloys, Mod. Dev. Powder Metall. Metal Powder Indus. Fed. (1971) 201e214. W. Wang, T. Nabatame, Y. Shimogaki, Dielectric evolution characteristics of HfCN metal-electrode-gated MOS stacks, J. Electrochem. Soc. 154 (2) (2007) G25eG29. D. Bishop, A. Calka, Phase transformations in ilmenite induced by electric discharge assisted mechanical milling, JALCOM 469 (2009) 380e385. A. Calka, D. Oleszak, Synthesis of TiB2 by electric discharge assisted mechanical milling, JALCOM 440 (2007) 346e348. A. Calka, D. Oleszak, N. Stanford, Rapid synthesis of TiCeFe3C composite by electric discharge assisted mechanical milling of ilmenite (FeTiO3) with graphite, JALCOM 459 (2008) 498e500. A. Calka, A. Mosbah, N. Stanford, P. Balaz, Rapid synthesis of Bi and Sb sulfides using electric discharge assisted mechanical milling, JALCOM 455 (2008) 285e288. A. Calka, D. Wexler, B. Monaghan, A. Mosbah, P. Balaz, Rapid reduction of copper sulfide (Cu2S) with elemental Fe and Mg using electrical discharge assisted mechanical milling (EDAMM), JALCOM 486 (2009) 492e496. A. Calka, A. Mosbah, N. Stanford, P. Balaz, Reduction of PbS and Sb2S3 with elemental Fe and Mg in dusty plasma environment created during electrical discharge assisted mechanical milling (EDAMM), JALCOM 467 (2009) 477e484. A. Calka, A. Chowdhury, K. Konstantinov, Rapid synthesis of functional oxides by electric discharge assisted mechanical milling method, JALCOM 536S (2012) S3eS8. S.A. Needham, A. Calka, G.X. Wang, A. Mosbah, H.K. Liu, A new rapid synthesis technique for electrochemically active materials used in energy storage applications, Electrochem. Commun. 8 (2006) 434e438.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Nitrogenation of hafnium carbide powders in AC and DC plasma by Electrical Discharge Assisted Mechanical Milling I.S. Aisyah, M. Wyszomirska*, A. Calka, D. Wexler Faculty of Engineering, University of Wollongong, Northfields Ave, Wollongong, NSW, 2522 Australia
a r t i c l e i n f o
a b s t r a c t
Article history: Received 1 March 2017 Received in revised form 26 April 2017 Accepted 27 April 2017 Available online 28 April 2017
Metal carbonitride powders and coatings are well known as superhard materials which are widely applied in many engineering products. Of this group of materials hafnium carbonitride properties are one of the least known due to difficulties with preparation. In this study, the electric discharge assisted mechanical milling (EDAMM) method involving a nitrogen plasma together with mechanical milling was used to synthetise hafnium carbonitride powders within minutes. It was found that difficulties in conventional manufacturing methods, resulting in the formation of only thin layers instead of bulk materials can be overcome by applying the new technique, and HfCxNy powder could be successfully engineered with controlled amount of nitrogen uptake. In this investigation we studied the effects of AC and DC discharges on nitrogen solubility in HfC powder. Microstructure and phase evolution were determined using XRD and SEM, while the nitrogen content in the grains was evaluated by EDS and bulk nitrogen content by CHN analysis. Maximum solubilities of 3.42 wt% and 2.95 wt%N under DC an AC discharge currents, respectively, were obtained. Reaction paths varied depending on processing conditions with HfCxNy formed under DC discharge processing and mixed products containing HfC, several HfCxNy phases and HfN like phase formed after AC processing. After nitrogenation the most highly nitrided powders were subjected to 1 min additional EDAMM processing using high power discharges to allow the formation of large, sintered particles of sufficient size to perform hardness measurements on. Vickers harnesses of fully sintered large particles after 5 and 10 min of EDAMM were 1808 and 2169HV, respectively. © 2017 Elsevier B.V. All rights reserved.
Keywords: Transition metal alloys and compounds Powder metallurgy Nitrogen absorption Hard metals Electro-discharge mechanical alloying Electro-mechano-synthesis
1. Introduction Transition metals carbides are well known for their specific combination of properties such as high hardness [5], high melting temperature, high thermal stability [12] and wear resistance [1]. All these properties make them attractive for cutting tools applications. It has been proven that their mechanical properties, hardness in particular, can be additionally enhanced by introduction of nitrogen into the elementary cell. These properties are strongly dependent on [C]/([C]þ[N]) ratio [2e4]. The carbonitrides, especially of group IV periodic table (Ti, Zr) stimulated growing interest as an alternative material for hard coatings. Conventionally, they can be obtained by several methods such as: magnetron sputtering
* Corresponding author. E-mail addresses: [email protected] (I.S. Aisyah), [email protected]. au (M. Wyszomirska), [email protected] (A. Calka), [email protected] (D. Wexler). http://dx.doi.org/10.1016/j.jallcom.2017.04.291 0925-8388/© 2017 Elsevier B.V. All rights reserved.
[10], self-propagating high-temperature synthesis [6], plasma enhanced magnetron sputtering (PEMS) [4] or high energy ball milling [1]. So far the compounds of TiCN and ZrCN as deposited layers and sintered powders have been extensively investigated [1,2,4,6e9] while severe lack of research on hafnium carbonitride, particularly as a bulk material, exists. HfCN found its application as a part of different kind of hard coatings such as Hf-Ti-N [5] or multicomponent ZrHf(CN) thin films [1] obtained by magnetron sputtering. Properties of bulk samples obtained so far provide some difficulties. For example Yang et. Al [4] investigated the hardness of both carbides and nitrides of titanium and found that it was impossible to obtain non-porous bulk samples, particularly with increasing nitrogen content. Kral et al. [11] stated however, that non-porous samples would be possible to obtain by diffusion annealing of transition metal carbides in a nitrogen atmosphere. Theoretical examination of HfC and HfN miscibility indicated complete solid solubility at temperatures 1400e1800 C under nitrogen pressures of between 30 and 300 atm [1]. Further
I.S. Aisyah et al. / Journal of Alloys and Compounds 715 (2017) 192e198
examination of the Hf-C-N system involved experiments on HfC and HfN which were arc-melted and then hot-pressed, but due to low diffusion rates homogenous one phase structures could not be obtained [8]. To the best of our knowledge only Cordoba et al. [1] has made a successful attempt to manufacture homogeneous powder material of hafnium carbonitride by high energy ball milling of elemental powders of hafnium and graphite in nitrogen atmosphere. Obtained particles were in the nanometric scale (100e500 nm) but the process required 80e120 min to complete and the product particles varied greatly in size. Despite the abovementioned difficulties in synthetising good-quality powders of hafnium carbonitrides, they found the product was of sufficient quality for applications such as thin layers on cutting tools and metal-gate electrodes obtained by MOCVD method [13]. In this article, we report on the synthesis of HfCxNy in only 10 min via the versatile process of Electrical Discharge Assisted Mechanical Milling (EDAMM), which combines benefits of traditional ball milling with rapid phase transformations induced by electrical discharges [14e21]. In this method, application of lowcurrent, high voltage electrical impulses leads to formation of localized nitrogen plasma which, in combination with mechanical mixing, results in conditions suitable for very high levels of nitrogenation of HfC powders. 2. Materials and methods Hafnium carbide, (HfC) with the particle size <1.25 mm and 99% purity, purchased from Sigma Aldrich, was used as a starting powder. Powder underwent EDAMM processing in a nitrogen plasma under atmospheric pressure. The powder was placed in a hemispherical reaction chamber and was subjected to controlled electrical discharge together with repeated impact of hardened rod end. The reaction chamber, schematically shown on Fig. 1, is custom built specifically for discharge milling. In this experiment both direct (DC) and alternating current (AC) short duration impulses were used to induce conditions suitable for high solubility of nitrogen in the carbide. The operating electrical parameters can be read from Fig. 1. Impulse modes were selected to prevent melting and maintain the sample in powder form throughout the experiment. HfC powder was milled for 0.5, 1, 2, 5 and 10 min in DC and AC
193
modes. The highest reaction rate was achieved when the average discharge power was set to ~0.54 kW. Higher values of the discharge power were found to cause excessive powder particles agglomeration which was accompanied by the decrease in the rate of nitrogen uptake. These nitrogen rich powder products processed for 5 and 10 min were additionally milled at high power (3 kW) for 1 min to promote micro-sintering and formation of the agglomerated particles. As a result, the particles underwent fast chemical reactions but afterwards were micro sintered to sizes big enough to perform hardness measurement on. After EDAMM, samples were characterized by X-ray analysis using Philips Diffractometer equipped with CuKa radiation and graphite monochromator. Phase identification was carried out using Traces software, Version 5.1.0. X-ray diffractograms were indexed and compared using International Centre for Diffraction Data (JCPDS-ICDD) Powder Diffraction Files (PDF). Both nitrogen contents of the products were estimated by Carlo Erba 1106 combustion elemental analysis. Morphology of the powders was characterized using JOEL 7001 FEGSEM scanning electron microscope (SEM), while SEM-energy dispersive spectroscopy (EDS) was performed using a Bruker EDS detection system running in semiquantitative standardless analysis mode. This enabled elemental composition analysis. Microhardness was measured using a Vickers microhardness indentation tester, with 50 g load set for 15 s and in all experiments the final value is an average of 8 indentations. 3. Results and discussion 3.1. X-ray diffraction X-ray diffraction results obtained from HfC powder (ICCD-PDF card no 06-0510) subjected to EDAMM in DC mode for 0.5, 1, 2, 5, 10 min and are shown in Fig. 2. Hafnium (IV) Oxide (PDF 40-1173) can be seen in the XRD plot, being a minor contaminant in starting powder. During EDAMM under DC mode the XRD peaks related to HfO2 gradually disappear and none of them can be noticed in samples processed for 5min. The attributed explanation is the well known process of reduction of metal oxides under high temperature nonoxide environments. After 1 min of EDAMM peaks indexed to HfC
Fig. 1. Electric Discharge Assisted Mechanical Milling (EDAMM) working in a) alternating and b) direct current presented with corresponding oscilloscope images.
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Fig. 2. XRD results obtained from HfC starting powder subjected to EDAMM in DC mode.
shift to the lower 2-theta angle values (from 33,5 to 33,37 for the first peak), consistent with an increase in lattice parameter associated with the dissolution of nitrogen into the HfC and formation of an interstitial solid solution of HfC(N) (Fig. 3 1 min). To the right of the HfC(N) peak an additional overlapping peak associated with a newly formed phase, HfCxNy can be seen. After 2 min EDAMM the solid solution HfC(N) peak has shifted back to the original position. We attribute this to a preference for
Fig. 3. Enlargement of 33,5 2-theta peak of HfC subjected to EDAMM in DC mode.
formation of HfCxNy over HfC(N) because of the high energy conditions present. Nitrogen appears more eager to form a chemical bond with the HfC substrate rather than remain dissolved as a solid solution in the HfC lattice. Note that the HfCxNy peaks are to the right of both the HfC(N) and the original HfC peaks. They are also located to the left of predicted position for HfN (111) peak, which in Fig. 3 would be located at a 2-theta value of 33.93. These results suggest that a new compound of HfCxNy was formed with a d spacing value in between those two compounds. Summing up, HfC subjected to 2 min milling time is a two phase material consisting of both HfC and HfCxNy whereas longer milling times lead to the presence of only the carbonitride phase, with one composition, also indicated by the symmetry of the peaks for long milling times. In the case of EDAMM working in alternating current mode a different transformation mechanism (Fig. 4) is observed. Peaks related to HfO2 are present, even after 10 min of milling, a result attributed to the lower temperature operative under AC mode, being insufficient to lead to a complete HfO2 reduction reaction. Here, formation of a new HfC1-xNx phase can be observed, while the HfC peak slightly loses intensity but do not disappear, even after 10min of milling. HfC1-xNx shows up on the diffraction pattern as an asymmetric peak. We attribute this to the fact that the product comprises more than one phase within that composition. The asymmetry of the peaks is attributed to the overlapping of these different phases and is also possibly due to the formation of HfN phase (06-0516), which appears as a 33.93 2-theta peak for it's (111) reflection, indicated on Fig. 5. Concluding, as can be seen on Fig. 5, there appear to be several phases; HfC, HfC1-xNx but also HfN present in the final product of milling under AC mode, a result consistent with the SEM results described below. Taking into account that the discharge energy under AC and DC modes is the same, it is suggested that HfC particles are heated to much higher temperatures under DC discharge which generates highly localized Joule heating of particles in the vicinity of a very narrow electrical discharge area as it passes through the powder. In the case of AC discharge, a much larger area of the powder is uniformly heated to much lower temperature. This would also mean that under AC mode HfC powder when heated to lower temperature should experience lower nitrogenation rate.
Fig. 4. XRD results obtained from HfC starting powder subjected to EDAMM in AC mode.
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Fig. 5. Enlargement of 33,5 2-theta peak of HfC subjected to EDAMM in AC mode.
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Microscopy under both secondary and backscattered electron imaging. Results of DC mode experiments can be seen in Fig. 6 aed where it initially causes breaking of the particles. Their irregular shape and smaller sizes indicate that mechanical forces applied by the rod's end resulted in rapid fracturing of powder particles during the first 2 min. The rapid nitrogen intake within first 2 min can be attributed to fast fracturing of powder particles and creation of fresh, clean surfaces that rapidly absorb the nitrogen ions. Further milling creates a wide range of particles sizes. After 5 min, however, clumps of particles are observed, comprising smaller particles apparently electrostatically bonded to the larger ones, forming large agglomerates with rough and porous surfaces. Further milling for 10 min results in further breakage and simultaneous agglomeration of the particles. During the AC mode milling the discharge is more uniform and less localized and the difference in the particle sizes appears not as large as in DC mode (Fig. 7 aed). Final milling in higher electrical conditions (high intensity plasma) was applied to both AC and DC processed powders for 1 min, which resulted in dramatic increases in the size of the particles up to 100 mm. We attribute this to an increase in plasma discharges between particles that induce particle softening together with surface welding. We believe this process is similar to spark plasma sintering (SPS) on a micro-scale. The particles processed in DC mode were later examined by the EDS analysis and used for hardness measurements (Fig. 8).
3.3. EDS analysis 3.2. SEM analysis The impact of the rod end together with an electric discharge during EDAMM represents a complex process. The evolving particle microstructures were examined using Scanning Electron
Chemical composition examination by the EDS analysis, combined with SEM-backscattered imaging, allowed estimation of the elemental distribution in the particles after 10 min of EDAMM under DC and AC modes (Figs. 9 and 10, respectively). It can be seen
Fig. 6. SEM secondary electron images of HfC a) starting powder and after b)2, c)5 and d)10 min of EDAMM in DC mode.
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Fig. 7. SEM secondary electron images of HfC a) starting powder and after b)2, c)5 and d)10 min of EDAMM in AC mode.
that estimates of the nitrogen content in the grains subjected to direct current vary between 2.5 and 4.8 wt% and there are significantly lighter areas, consistent with the presence of higher atomic number elements contents, visible in regions between the grains. EDS analysis revealed the presence of Fe in those areas, most probably originating from the milling media. It can be also noted that there are quite pronounced differences in colour of the grains which exceed differences attributable to changes in orientation, and these appear likely related to variations in chemical composition. Grains identified as having more nitrogen are darker whereas
an increased amount of carbon is associated with lighter colour of the grains. Processing of the stating powder in AC mode on the other hand, results in the estimated nitrogen contents varying from 3.5 to 9 wt %. Grains appear much larger compared to processing in DC mode, and the Hf/C/N ratios show a large distribution. It has to be born in mind that the carbon and nitrogen content cannot be precisely determined with the EDS analysis because of the detecting errors when quantifying light elements. The EDS analysis is only used here as a tool to observe the differences in the
Fig. 8. Mechanism of micro-sintering in high energy EDAMM processing developed for hardness measurements on powders.
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local chemistry and to give general overview of the elemental distribution. Differences in bulk nitrogen uptake are presented by the CHN analysis below. 3.4. CHN analysis
Fig. 9. SEM backscattered image with results of EDS spot analysis of HfC cross-section after 10 min EDAMM in DC mode.
CHN combustion analysis (Fig. 11) revealed bulk nitrogen content in the samples obtained by EDAMM. Nitrogen uptake was found to increase with increasing milling time and reach maximum values of 3.42 and 2.98 wt%N in DC and AC modes, respectively. Extrapolation of obtained data to longer milling times indicates only slight increase of nitrogen content. As can be observed, under direct current plasma, nitrogenation of HfC is more favorable and we attribute this to the fact that this environment provides sufficient energy for dissolution and migration of nitrogen into the powder. 3.5. Sintering and Vickers microhardness
Fig. 10. SEM backscattered images with results of EDS analysis of HfC cross-section after 10 min EDAMM in AC mode.
As mentioned in the experimental part, the usage of discharge power up to 3 kW can be used to promote excessive powder particle agglomeration. This phenomenon was used to form fully sintered, high density particles for microhardness investigations. For the hardness tests samples nitrogenated in DC mode for 5 and 10 min were selected. X-ray diffraction results (not shown) obtained from these samples showed sets of peaks which can be indexed to HfCxNy phases, consistent with EDS results. Vickers microhardness measurements performed by the indentation method revealed significant increases in hardness after HfC milled in nitrogen plasma after 5 and 10 min (Fig. 12). The average values obtained from 8 indentations were 1808 and 2169HV with the standard deviation values of 108.48 and 151.83, respectively. The reason for the increased hardness is thought to be caused by the higher lattice strain with the presence of the nitrogen. 4. Conclusions
Fig. 11. Nitrogen uptake of HfC subjected to EDAMM in DC and AC mode.
We have found that HfCxNy compounds in powder form can be produced in as short a time as 2e5 min using EDAMM processing in nitrogen. Direct Current processing for very short periods (<1 min) results in nitrogen dissolution in HfC, DC processing for longer periods (1 min and above) results in formation of the compound HfCxNy, with near complete transformation after 5 min. Alternating current mode processing resulted in incomplete transformation with formation of a range of products including HfC, HfC1-xNx plus formation of a compound of composition close to HfN. Maximum solubilities of 3.42 wt% and 2.95 wt%N were obtained under DC an AC discharge currents, respectively. A higher rate of nitrogenation occurred in powders processed under DC discharge
Fig. 12. Hardness indentations in HfCxNy product obtained after 5 (left) and 10 min (right) EDAMM under DC mode.
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compared to AC. It was found that the reaction rate is dependent on time and, although carbonitrides can be produced in as little as 2e5 min, further nitrogen intake still occurs after longer milling times. Only slight nitrogen intake would occur after 10min. This result represents a significant advantage over most conventional synthesis techniques as it enables the synthesis of high nitrogen content Hf(C,N) precursor powders suitable for postprocessing via surfacing engineering or hot consolidation techniques. Acknowledgements This investigation was supported by funding from the Australian Research Council Discovery Grant No. DP130101390. The authors acknowledge use of the facilities and the assistance of Dr. Mitchell Nancarrow and Tony Romeo at the UOW Electron Microscopy Centre.
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