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The use of intermetallic AlxMgy powder to obtain AlMgB14-based materials
Materials Today Communications 22 (2020) 100848
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The use of intermetallic AlxMgy powder to obtain AlMgB14-based materials a
a,
a
b
a
Zhukov I.A. , Nikitin P.Yu. *, Vorozhtsov A.B. , Perevislov S.N. , Sokolov S.D. , Ziatdinov M.H. a b
T
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Tomsk State University, Russia St. Petersburg State Technological Institute (Technical University), Russia
A R T I C LE I N FO
A B S T R A C T
Keywords: Aluminum magnesium boride AlMgB14 Ultra-Hard boride Ceramics Hot pressing Hardness
In this work, AlMgB14-based materials were obtained from powders of the intermetallic compound Al12Mg17 and boron using hot pressing method and mechanical treatment with a planetary mill. The method for producing intermetallic Al12Mg17 powder is presented. The effect of the mechanical treatment on the dispersion of the Al12Mg17 powder was investigated. The phase composition, microstructure, and properties (density, hardness and coefficient of friction) of the AlMgB14-based materials were investigated. The hardness of the obtained sample is 31.9 GPa, and the density is 2.35 g/cm3 (90% of theoretical density). The friction coefficient under dry conditions is 0.38. The friction coefficient under lubricated conditions (LITOL-24 lubricant) is 0.18. The mechanism of AlMgB14 formation was investigated. When using the intermetallic Al12Mg17 powder as a raw precursor of the Al-Mg-B powder mixture, the AlMgB14 phase is formed by direct boration of AlxMg1-xB2 diboride, while the formation of spinel MgAl2O4 phase is due to contamination of the raw boron powder.
1. Introduction A material based on a ternary compound of aluminum, magnesium and boron – AlMgB14 (so-called BAM) has recently been intensively studied. The unique combination of high hardness (27−32 GPa) [1], a low coefficient of friction (COF, 0.08-0.02) [2,3], electrical conductivity, relatively low density and high chemical stability, makes it possible to use materials based on BAM as structural materials in engineering, as well as wear-resistant coatings for the bearing parts of machines (shafts, antifriction bearings), turbines and cutting tools [3–5]. In 1970, Matkovich and Economy [6] reported an orthorhombic single-crystalline AlMgB14 compound for the first time. Higashi et al. clarified the structure of single-crystal AlMgB14. It consists of icosahedra B12 linked by aluminum and magnesium, and the true stoichiometry is close to Al0.75Mg0.78B14 [7]. The production of polycrystalline materials was actively investigated by scientists at the Ames Laboratory (USA) [1,3,8–12] and was first reported in Ref. [1]. For the synthesis of AlMgB14, raw aluminum, magnesium and boron powders were mixed in an atomic ratio of 1:1:14 using mechanical alloying methods (MA) and then sintered by hot pressing methods within the temperature range of 1573–1773 K. Samples with a density of 2.54 g/cm3 (98.06% of the theoretical density of 2.59 g/cm3) and a hardness value of 32−35 GPa were obtained. It was also reported that the addition of 30 wt. % of TiB2 lead to a significant increase in the hardness values, reaching up to 42 GPa. To date, there are many works devoted to the study of the
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structure, mechanical properties, and synthesis methods of materials based on AlMgB14, as well as coatings obtained from those materials [1,5,9,10,13,14]. It is important to note that in all of these works, to obtain AlMgB14, mixtures of raw aluminum, magnesium, and boron powders are used in ratios at or near 1:1:14. The obtained powder mixtures are sintered by high-temperature vacuum sintering [16–18], hot pressing [19–22], spark plasma sintering (SPS) [23–27], or two-step sintering [25,28] methods. The use of metal compounds in a bound form as raw powders can be an alternative to using a mixture of elemental aluminum, magnesium and boron powders. This can improve the efficiency because magnesium does not evaporate from the compound, as when using elemental powders. Magnesium has a low boiling point, which leads to the evaporation of magnesium particles from Al-Mg-B mixtures during the synthesis of AlMgB14 [16]. To compensate for magnesium evaporation during the synthesis, the authors of [16] proposed to use an excess of magnesium in an atomic ratio of Al:Mg:B – 1:6:14. This method allows for a high yield of AlMgB14-based materials, however, it requires the use of high-purity powders and high-temperature synthetic conditions, which significantly increases the labour intensity and fabrication cost. Moreover, the use of intermetallic Al-Mg for the synthesis of BAM can reduce the spinel phase content in sintered samples. It is known that aluminum powder has a dense oxide film on its surface. The oxide film on the intermetallic compound is not dense as that of aluminum. [15]. It is also likely that the AlMgB14 formation mechanism using an
Corresponding author. E-mail address: [email protected] (P.Y. Nikitin).
https://doi.org/10.1016/j.mtcomm.2019.100848 Received 16 September 2019; Received in revised form 11 December 2019; Accepted 14 December 2019 Available online 16 December 2019 2352-4928/ © 2019 Elsevier Ltd. All rights reserved.
Materials Today Communications 22 (2020) 100848
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treated in a planetary mill in an argon atmosphere with rotational frequency of 14 Hz [35]. Steel balls with a diameter of 4.5 mm were used as grinding bodies. The mass ratio of grinding bodies to powder mixture was 3:1. The mechanical treatment was conducted for 3 h. The obtained powder mixture was placed in a 23 mm diameter graphite die with a movable upper plug. Then, simultaneous consolidation and sintering of the mixture was conducted by using a hot pressing method at a temperature of 1400 °C, 30 MPa of pressure and a holding time of 30 min. Samples were heated from room temperature to 800 °C at a heating rate of 200 °C min−1 with a holding at 800 °C for 3 min. Then they were heated to 1400 °C at the heating rate of 200 °C min−1.
intermetallic Al-Mg compound as a raw precursor is facilitated by the direct borating of the intermetallic compound, while when using elemental powders, the formation of the intermetallic compound occurs first, and then it is borated. Varužan Kevorkijan et al. [17] used MgB2 and AlB12 compounds as raw precursors for the synthesis of AlMgB14. However, the AlMgB14 content in the sintered samples was no more than 25 wt. %. This is because for an intensive BAM formation reaction to reach completeness, free boron atoms are needed [17]. The synthesis of AlMgB14-based materials from mixtures of AlB2, MgB2 and boron was conducted in [16]. As a result, materials with a 85 wt. % AlMgB14 phase content were obtained. In this work, a method for obtaining AlMgB14based materials using an intermetallic AlxMgy compound as a raw precursor was proposed. Thus, the purpose of this work is to study the phase composition, structure and properties of AlMgB14-based materials obtained from mixtures of intermetallic AlxMgy and boron powders.
2.3. Characterizations The average particle size of the powders and the mixtures based on them was measured using an ANALYSETTE 22 MicroTec plus apparatus (Fritsch – Germany). X-ray diffraction analysis of the powder mixtures and the sintered samples was performed using a Rigaku diffractometer with CuKα radiation, as well as the Crystallographica Search Match program (Oxford Cryosystems - UK) based on the «Powder Diffraction file» database. The quantitative content of the phases was determined using the Rietveld method. The microstructure of the powders and the sintered samples was determined using a Tescan Vega 3 microscope (Czech Republic) equipped with energy dispersive spectroscopy (EDX). The densities of the sintered samples were calculated using the Archimedes method. Nanohardness was measured using a table top nanoindentation system (CSM Instruments - Sweden) with a load of 100 mN and a dwell time of 10 s. The processing of experimental data, in particular, the determination of nanohardness, is based on the methods of Oliver and Pharr [37]. The indenter of Berkovich was used, 10 indentations were made from different places of the sample. Each value is averaged over the results of several measurements. Results excluding the values of confidence intervals are excluded from the statistical data. The obtained nanohardness values were converted to Vickers microhardness values. COF was measured using an Anton Paar ball-on-disk tester. A zirconia ball with a diameter of 6 mm was used in this work. The applied normal load was 2 N. The tests were performed under dry conditions, or with LITOL-24 (State standard 21150-75) lubricant, with a linear velocity of 0.02 m/s at room temperature in an ambient environment. The chemical analysis of the intermetallic powder was performed using an XRF-1800 wavelength-dispersive spectrometer. To determine the amount of oxygen in the AlxMgy powder and boron powder, a LECO ONH (USA) analyser was used.
2. Material and methods 2.1. Preparation of AlxMgy powder Intermetallic AlxMgy powder was obtained from 99.9% pure magnesium and aluminum ingots. The fusion of aluminum and magnesium was carried out in a graphite crucible in argon. During the first stage, 1 kg of aluminum was melted at a temperature of 720 °C. Then, magnesium was added into the aluminum melt in a mass ratio of Al:Mg – 1:1. The Al:Mg mass ratio was selected based on the atomic ratio of 1:1:14 of the Al-Mg-B powder mixture. Wherein, according to the Al–Mg phase equilibrium diagram [36], the eutectic area remains in the range of 0.4:0.6 of the atomic ratios Al:Mg (Mg:Al). Mixing was carried out using the device described in [29]. The resulting alloy was poured into a steel chill at a temperature of 670 °C. During the casting process, the alloy was subjected to vibration [30]. During the second stage, the obtained alloy was mechanically milled, and then treated in a planetary mill with a rotation frequency of 12 Hz in an argon atmosphere. Steel balls with a diameter of 8.7 mm were used as grinding bodies. The mass ratio of balls to powder mixture was 2:1. The mechanical treatment was conducted for 5 h. After the mechanical treatment, AlxMgy powder (average particle size < d > of ∼20 μm) was selected as the raw precursor for the AlxMgy-B mixture. The selection process was carried out by sieving. 2.2. The process for obtaining AlMgB14 During the next stage, AlxMgy flakes were mixed with 99.9% pure amorphous black boron (average particle size is 2.1 μm). The obtained intermetallic compound was mixed with boron in an atomic AlxMgy:B ratio of 2:14, since the alloy was obtained in a Al:Mg ratio of 1:1. Before mixing, boron powder was annealed in vacuum chamber at a temperature of 150 °C for 2 h. The obtained mixture was mechanically
3. Results 3.1. Phase composition, microstructure, and chemical composition of the AlxMgy powder Fig. 1 shows the XRD pattern (a) and SEM-images of the AlxMgy
Fig. 1. XRD-pattern (a) and SEM micrograph (Det. SE) of the AlxMgy powder after 5 h of milling in a planetary mill (b). 2
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Table 1 Elemental analysis of the AlxMgy powder after 5 h of milling in a planetary mill. Element
Al
Mg
Si
Fe
Ti
O
Result, (at. %)
50.4
48.68
0.18
0.09
0.64
0.01
Fig. 2. Histograms of the particle size distribution for the Al12Mg17-B powder after 1, 2 and 3 h of mechanical milling.
powder (b). It was found that the peaks in the XRD pattern of the intermetallic powder correspond to Al12Mg17 and Al phases. It can be seen that the morphology of the powder consists of individual small and larger particles of irregular shape, which is typical for powders after mechanical milling. The chemical composition is given in Table 1. Metal impurities, such as Si, Fe, Ti, and oxide were found. The presence of metallic impurities may be due to the presence of impurities in the aluminium and magnesium ingots. The presence of Fe may also be due to the milling of steel balls in the planetary mill [38]. However, preliminary studies on the effect of mechanical activation on the phase composition [31] showed that there are no impurity phases in the powder mixture after mechanical activation of the Al-Mg-B powder mixture. Fig. 2 shows histograms of the particle size distribution for the Al12Mg17-B powder mixture after mechanical treatment in the planetary mill. The average particle size after 1 h of mechanical treatment is 0.2 μm, after 2 h is 1.2 μm, and after 3 h is 0.4 μm. In this case, the particle size distribution is unimodal. After an hour of mechanical activation, intensive milling of the powder mixture occurs. After 2 h of MA, agglomeration of nanosized particles occurs under the action of Van der Waals forces. After 3 h of MA, agglomerated particles are again milled. A similar effect was observed in the previous work [31].
Fig. 3. XRD patterns of the samples sintered by hot pressing the AlxMgy-B powders after 1, 2 and 3 h of mechanical milling.
found in the light regions, which corresponds to spinel MgAl2O4 and Al phases (Table 2). This is consistent with the XRD results. Using the Rietveld method and the EDX results, the AlMgB14 phase content of the sample is ∼ 90%, the AlMgB4 phase content is ∼ 5%, and the MgAl2O4 and Al content is no more than 5%. 3.3. The properties of the sintered materials The density of the sample obtained from the powder mixture after 3 h of mechanical treatment is 2.35 g/cm3 (90.7% of the theoretical density of 2.59 g/cm3). In areas without visible defects, the maximum measured hardness value of the sample is 31.9 GPa, and the minimum is 24 GPa. In other areas, the hardness drops to 12–16 GPa. Wherein, the average grain size of the sample measured from the SEM-image of the fracture surface (Fig. 4c) is 2 μm. The results of the COF measurements under dry and lubricated conditions are presented in Fig. 5. The average COF under dry conditions is 0.38. The average COF in LITOL-24 lubricant is 0.18.
3.2. Phase composition and microstructure of sintered AlMgB14-based samples The XRD patterns of samples obtained by hot pressing Al12Mg17-B powder mixtures after 1, 2 and 3 h of mechanical treatment are shown in Fig. 3. The phase compositions of the samples are dominated by the AlMgB14 phase. A small amount of spinel MgAl2O4, diboride AlxMg1-xB2 and Al phases were also detected. The SEM-images of the sintered sample from the AlxMgy-B powder after 3 h of mechanical milling are shown in Fig. 4. EDX analysis showed that Al, Mg and B are contained in the dark regions, which corresponds to the AlMgB14 phase. In contrast, O, Al and Mg were
4. Discussion The Al12Mg17 phase detected in the intermetallic powder is consistent with the Al-Mg state diagram and corresponds to the eutectic region [34]. The presence of the aluminum phase in the same powder is explained by the fact that in the formed Al12Mg17 phase, there is more magnesium than aluminum; however, when initially melting, the mass ratio of aluminum and magnesium was 1:1. This atomic ratio was 3
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Fig. 4. SEM-images (Det. SE) of the sample sintered by hot pressing the AlxMgy-B powder after 3 h of mechanical milling.
The phase composition of obtained materials are closed to the phase composition of materials, obtained in the works [16,22–24]. AlxMg1-xB2 diborides are intermediate phases in the formation of AlMgB14. To explain the presence of AlMgB4, MgAl2O4, and Al phases in the XRD patterns of the AlMgB14-based samples sintered at a temperature of 1400 °C, additional samples were obtained at temperatures of 600 °C, 800 °C and 1100 °C (Fig. 6). XRD analysis of the samples sintered at a temperature of 600 °C showed that the main phases are Mg0.08Al0.92 and Mg0.815Al0.185B2. A small amount of intermetallic Al12Mg17 was also detected. The main phases of the samples that sintered at a temperature of 800 °C are Mg0.1Al0.9B2 and Mg0.08Al0.92. The high background at small diffraction angles for the samples sintered at 600 °C and 800 °C is due to the presence of amorphous boron. At the same time, as the sintering temperature increases from 600 °C to 800 °C, the degree of crystallinity of the samples increases from 53% to 61%. It is known that the formation of the MgAl2O4 spinel phase occurs most intensely at ∼800 °C [23]. In the XRD pattern of the sample sintered at a temperature of 800 °C, no MgAl2O4 phase was detected. This indicates that the MgAl2O4 phase is formed at temperatures above 800 °C. At a sintering temperature of 1100 °C, the AlMgB14 phase is formed by the boration of AlxMg1-xB2 diborides. At the same time, the spinel phase of MgAl2O4 is formed. Thus, we speculate that the phase formation in the studied Al12Mg17-B powder mixtures system occurs as follows (Fig. 7). Since the melting
Table 2 Results of EDX-analysis of the sample sintered by hot pressing the AlxMgy-B powder after 3 h of mechanical milling. Mark/Element (wt. %)
Al
Mg
B
O
1 2
12.43 11.35
11.29 11.07
76.28 65.17
– 12.41
chosen based on the required atomic ratio of Al:Mg:B of 1:1:14 for the synthesis of AlMgB14, which is close to the stoichiometric ratio. Moreover, free aluminum in the liquid phase provides diffusion-based masstransfer, having a positive effect on the sintering process for AlMgB14based materials. The presence of metal impurities in the obtained powder is consistent with their presence in the raw aluminum and magnesium ingots, which are used to obtain the powders. The increase in the average particle size of Al12Mg17-B powder mixtures with the increase in mechanical treatment time from 1 to 3 h is due to the following. With an increase in mechanical treatment time, the particles pass into the nanoscale range and agglomerate under the action of Van der Waals forces, as seen in the histograms of the particle size distribution (2 h). Then nanosized particles are again milled (3 h). Apparently, further mechanical treatment can enhance the agglomeration of the particles, which could reduce diffusion between the raw powders during the synthesis of AlMgB14 [31].
Fig. 5. Friction coefficient of the AlMgB14 sample: (a) under dry conditions, (b) in LITOL-24 lubricant. 4
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Fig. 7. AlMgB14 formation mechanism. Fig. 6. XRD patterns of AlxMgy-B powder samples after 3 h of mechanical milling and sintered by hot pressing at temperatures of 600, 800 °C and 1100 °C.
Table 3 Results of elemental analysis of the raw powders.
temperatures of B2O3 and Al12Mg17 are approximately the same (∼450 °C), the solid boron particle is probably covered by two layers of liquid (Fig. 7b). Next, there is a diffusion process for the formation of AlxMgx-1B2 boride crystals (1), which can be seen in the XRD patterns, both at 600 °C and at 800 °C (Fig. 7d). Unreacted liquid appears as an amorphous phase of intermetallic compounds. When increasing sintering temperature, the diffusion between Al, Mg and B increases, and the formation of crystalline phases proceeds to completion resulting in the formation of AlMgB14 (2) (Fig. 7e). The remaining liquid crystallizes in a spinel form (3). This is consistent with the results of elemental analysis of the raw powders of boron and Al12Mg17 (Table 3). It was found that the oxygen content in the raw boron powder is ∼ 1.1 wt. %, the oxygen content in Al12Mg17 powder does not exceed 0.1%. Thus, when using Al12Mg17 intermetallic powder, the formation of MgAl2O4 spinel during the reaction of Al2O3 and MgO occurs to a much lesser extent due to the low oxygen content in the raw Al12Mg17 intermetallic powder. Al12Mg17 + B → AlxMg1-xB2
(1)
AlxMg1-xB2 + B → AlMgB14
(2)
B2O3 + Al + Mg → MgAl2O4 + B
(3)
Powder
Mass fraction of oxygen, %
Boron powder Al12Mg17 before mechanical treatment Al12Mg17 after mechanical treatment
1.1 0.003 0.07
noted that in [2,26,32], water-based lubricant coatings were used, which resulted in the formation of B(OH)3. In this work, LITOL-24 petroleum oil was used as a lubricant. It is used in hinges, gears, plain bearings and rolling bearings. COF values larger than that given in [2,26] are explained by the presence of porosity in the samples. In addition, the authors of [2,3,9,10,33] obtained AlMgB14-TiB2 composite materials, and in accordance with [9] TiB2 has a significant effect on the friction coefficient of AlMgB14-TiB2 due to the formation of a B2O3 oxide film on the surface of the material. In the future, to increase the density of the samples, it seems necessary to optimize the morphology of the Al12Mg17 intermetallic compound powders, which will provide a more dense particle packing during sintering.
5. Conclusions Studies have shown the possibility of synthesizing AlMgB14-based materials from AlxMgy alloy. The proposed method for producing the alloy makes it possible to manufacture a material that can be used as raw precursor for obtaining AlMgB14-based materials. The use of the Al12Mg17 intermetallic powder as a raw precursor provides several
The hardness results are consistent with the hardness results obtained in [1,16,19,20,22,23]. The results of the COF measurements are consistent with those of [33], in which COF measurements were performed under dry conditions at various temperatures. It should be 5
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advantages. Firstly, according to the state diagram of the Al-Mg system, Al12Mg17 corresponds to the eutectic region, being fragile, therefore, when mechanically milling the Al12Mg17-B powder mixture, it is necessary to spend significantly less time to obtain nanosized particles. Secondly, when using Al12Mg17 intermetallic powder, the main source of oxygen for the formation of the spinel phase is boron powder. Finally, such a composition is prone to less oxidation, since Al and Mg are in a bound state. According to the Pilling-Bedworth ratio (PBR) [39], Al has a PBR of ∼ 1.28: the oxide film on the aluminum surface is dense, Mg has a PBR of ∼ 0.8: the oxide film on the magnesium surface has defects. Thus, when using Al12Mg17 intermetallic alloy powder as a raw precursor, the oxide film is destroyed on the surface of Al12Mg17 powder during sintering. The phase composition is mainly represented by the AlMgB14 phase; the XRD results are consistent with the results obtained by other authors. Based on the obtained XRD patterns, the mechanism was proposed for the formation of AlMgB14 and impurity phases. When using an AlxMgy intermetallic compound as raw precursor, AlMgB14 is formed in several stages. During the first stage, the boration of intermetallic compounds to form AlxMg1-xB2 diboride occurs. In the second stage, the resulting diborides are then borated to form AlMgB14. The formation of the spinel phase is associated with the presence of an oxide film on boron powder. To reduce the number of impurity phases, the preliminary annealing of boron at temperatures above 150 °C or other methods of removing the oxide film from the boron surface are necessary. It was found that the AlMgB14-based material produced by hot pressing the powder at a temperature of 1400 °C after 3 h of mechanical treatment possessed a hardness of 31.9 GPa and COF of 0.18. The porosity of the obtained material was ∼ 10%. To reduce the porosity, optimization of the morphology of the powders of a given particle size distribution is necessary.
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Zhukov, V. Promakhov, M. Khmeleva, Ex situ introduction and distribution of nonmetallic particles in aluminum melt: modeling and experiment, JOM 69 (12) (2017) 2653–2657, https://doi.org/10.1007/s11837-017-2594-1. [30] V.V. Promakhov, M.G. Khmeleva, I.A. Zhukov, V.V. Platov, A.P. Khrustalyov, A.B. Vorozhtsov, Influence of vibration treatment and modification of A356 aluminum alloy on its structure and mechanical properties, Metals 9 (1) (2019) 87, https://doi.org/10.3390/met9010087. [31] I.A. Zhukov, M.K. Ziatdinov, Y.A. Dubkova, P.Y. Nikitin, Synthesis of AlMgB14: influence of mechanical activation of Al–Mg–B powder mixture on phase composition of sintered materials, Russ. Phys. J. 61 (8) (2018) 1466–1471, https://doi. org/10.1007/s11182-018-1557-5. [32] J. Chen, J. Cheng, S. Wang, S. Zhu, Z. Qiao, J. Yang, Self-lubricity and wear behaviors of bulk polycrystalline AlMgB14 depending on the counterparts in deionized water, Tribol. 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Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements These studies were funded by Russia Science Foundation (project No. 19-79-10042). Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.mtcomm.2019. 100848. References [1] B.A. Cook, J.L. Harringa, T.L. Lewis, A.M. Russell, A new class of ultra-hard materials based on AlMgB14, Scripta mater. 42 (2000) 597–602. [2] X. Lu, K. Yao, J. Ouyang, Y. Tian, Tribological characteristics and tribo-chemical mechanisms of Al–Mg–Ti–B coatings under water–glycol lubrication, Wear 326 (2015) 68–73, https://doi.org/10.1016/j.wear.2014.12.034. [3] B.A. Cook, J.L. Harringa, J. Anderegg, A.M. Russell, J. Qu, P.J. Blau, A.A. Elmoursi, Analysis of wear mechanisms in low-friction AlMgB14–TiB2 coatings, Surf. Coat. Technol. 205 (7) (2010) 2296–2301, https://doi.org/10.1016/j.surfcoat.2010.09. 007. [4] Y. Tian, G. Li, J. Shinar, N.L. Wang, B.A. Cook, J.W. Anderegg, J.E. Snyder, Electrical transport in amorphous semiconducting AlMgB14 films, Appl. Phys. Lett. 85 (7) (2004) 1181–1183, https://doi.org/10.1063/1.1781738. [5] Y. Tian, A.F. Bastawros, C.C. Lo, A.P. Constant, A.M. Russell, B.A. Cook, Superhard self-lubricating AlMgB14 films for microelectromechanical devices, Appl. Phys. Lett. 83 (14) (2003) 2781–2783, https://doi.org/10.1063/1.1615677. [6] V.I. Matkovich, J. Economy, Structure of MgAlB14 and a brief critique of structural relationships in higher borides, Acta Cryst. Sect. B 26 (5) (1970) 616–621, https:// doi.org/10.1107/S0567740870002868. [7] I. Higashi, T. Ito, J. Less, Refinement of the structure of MgAlB14, Common Met. 92 (2) (1983) 239–246, https://doi.org/10.1016/0022-5088(83)90490-3.
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vol. 3, (2001) Book 1 [in Russian]. [37] W.C. Oliver, G.M. Pharr, An improved technique for determining hardness and elastic modulus using load and displacement sensing indentation experiments, J. Mater. Res. 7 (6) (1992) 1564–1583. [38] T.L. Lewis, B.A. Cook, J.L. Harringa, A.M. Russell, Al2MgO4, Fe3O4, and FeB impurities in AlMgB14, Mater. Sci. Eng. A 351 (1-2) (2003) 117–122, https://doi.org/ 10.1016/S0921-5093(02)00835-3. [39] N.B. Pilling, R.E. Bedworth, The oxidation of metals at high temperatures, J. Inst. Met 29 (1923) 529–591.
https://doi.org/10.1016/j.apsusc.2015.03.195. [34] P. Chartrand, A.D. Pelton, Critical evaluation and optimization of the thermodynamic properties and phase diagrams of the Al-Mg, Al-Sr, Mg-Sr, and Al-Mg-Sr systems, J. Phase Equilibria Diffus. 15 (6) (1994) 591–605, https://doi.org/10. 1007/BF02647620. [35] I. Zhukov, P. Nikitin, A. Vorozhtsov, Structure, Phase Composition, and Properties of Ceramics Based on AlMgB14, Obtained From Various Powders, in Characterization of Minerals, Metals, and Materials 2019, Springer, Cham, 2019, pp. 45–49, https://doi.org/10.1007/978-3-030-05749-7_5. [36] N.P. Lyakishev, State DiAgrams of Binary Metal Systems Mashinostroenie, Moscow
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The use of intermetallic AlxMgy powder to obtain AlMgB14-based materials a
a,
a
b
a
Zhukov I.A. , Nikitin P.Yu. *, Vorozhtsov A.B. , Perevislov S.N. , Sokolov S.D. , Ziatdinov M.H. a b
T
a
Tomsk State University, Russia St. Petersburg State Technological Institute (Technical University), Russia
A R T I C LE I N FO
A B S T R A C T
Keywords: Aluminum magnesium boride AlMgB14 Ultra-Hard boride Ceramics Hot pressing Hardness
In this work, AlMgB14-based materials were obtained from powders of the intermetallic compound Al12Mg17 and boron using hot pressing method and mechanical treatment with a planetary mill. The method for producing intermetallic Al12Mg17 powder is presented. The effect of the mechanical treatment on the dispersion of the Al12Mg17 powder was investigated. The phase composition, microstructure, and properties (density, hardness and coefficient of friction) of the AlMgB14-based materials were investigated. The hardness of the obtained sample is 31.9 GPa, and the density is 2.35 g/cm3 (90% of theoretical density). The friction coefficient under dry conditions is 0.38. The friction coefficient under lubricated conditions (LITOL-24 lubricant) is 0.18. The mechanism of AlMgB14 formation was investigated. When using the intermetallic Al12Mg17 powder as a raw precursor of the Al-Mg-B powder mixture, the AlMgB14 phase is formed by direct boration of AlxMg1-xB2 diboride, while the formation of spinel MgAl2O4 phase is due to contamination of the raw boron powder.
1. Introduction A material based on a ternary compound of aluminum, magnesium and boron – AlMgB14 (so-called BAM) has recently been intensively studied. The unique combination of high hardness (27−32 GPa) [1], a low coefficient of friction (COF, 0.08-0.02) [2,3], electrical conductivity, relatively low density and high chemical stability, makes it possible to use materials based on BAM as structural materials in engineering, as well as wear-resistant coatings for the bearing parts of machines (shafts, antifriction bearings), turbines and cutting tools [3–5]. In 1970, Matkovich and Economy [6] reported an orthorhombic single-crystalline AlMgB14 compound for the first time. Higashi et al. clarified the structure of single-crystal AlMgB14. It consists of icosahedra B12 linked by aluminum and magnesium, and the true stoichiometry is close to Al0.75Mg0.78B14 [7]. The production of polycrystalline materials was actively investigated by scientists at the Ames Laboratory (USA) [1,3,8–12] and was first reported in Ref. [1]. For the synthesis of AlMgB14, raw aluminum, magnesium and boron powders were mixed in an atomic ratio of 1:1:14 using mechanical alloying methods (MA) and then sintered by hot pressing methods within the temperature range of 1573–1773 K. Samples with a density of 2.54 g/cm3 (98.06% of the theoretical density of 2.59 g/cm3) and a hardness value of 32−35 GPa were obtained. It was also reported that the addition of 30 wt. % of TiB2 lead to a significant increase in the hardness values, reaching up to 42 GPa. To date, there are many works devoted to the study of the
⁎
structure, mechanical properties, and synthesis methods of materials based on AlMgB14, as well as coatings obtained from those materials [1,5,9,10,13,14]. It is important to note that in all of these works, to obtain AlMgB14, mixtures of raw aluminum, magnesium, and boron powders are used in ratios at or near 1:1:14. The obtained powder mixtures are sintered by high-temperature vacuum sintering [16–18], hot pressing [19–22], spark plasma sintering (SPS) [23–27], or two-step sintering [25,28] methods. The use of metal compounds in a bound form as raw powders can be an alternative to using a mixture of elemental aluminum, magnesium and boron powders. This can improve the efficiency because magnesium does not evaporate from the compound, as when using elemental powders. Magnesium has a low boiling point, which leads to the evaporation of magnesium particles from Al-Mg-B mixtures during the synthesis of AlMgB14 [16]. To compensate for magnesium evaporation during the synthesis, the authors of [16] proposed to use an excess of magnesium in an atomic ratio of Al:Mg:B – 1:6:14. This method allows for a high yield of AlMgB14-based materials, however, it requires the use of high-purity powders and high-temperature synthetic conditions, which significantly increases the labour intensity and fabrication cost. Moreover, the use of intermetallic Al-Mg for the synthesis of BAM can reduce the spinel phase content in sintered samples. It is known that aluminum powder has a dense oxide film on its surface. The oxide film on the intermetallic compound is not dense as that of aluminum. [15]. It is also likely that the AlMgB14 formation mechanism using an
Corresponding author. E-mail address: [email protected] (P.Y. Nikitin).
https://doi.org/10.1016/j.mtcomm.2019.100848 Received 16 September 2019; Received in revised form 11 December 2019; Accepted 14 December 2019 Available online 16 December 2019 2352-4928/ © 2019 Elsevier Ltd. All rights reserved.
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treated in a planetary mill in an argon atmosphere with rotational frequency of 14 Hz [35]. Steel balls with a diameter of 4.5 mm were used as grinding bodies. The mass ratio of grinding bodies to powder mixture was 3:1. The mechanical treatment was conducted for 3 h. The obtained powder mixture was placed in a 23 mm diameter graphite die with a movable upper plug. Then, simultaneous consolidation and sintering of the mixture was conducted by using a hot pressing method at a temperature of 1400 °C, 30 MPa of pressure and a holding time of 30 min. Samples were heated from room temperature to 800 °C at a heating rate of 200 °C min−1 with a holding at 800 °C for 3 min. Then they were heated to 1400 °C at the heating rate of 200 °C min−1.
intermetallic Al-Mg compound as a raw precursor is facilitated by the direct borating of the intermetallic compound, while when using elemental powders, the formation of the intermetallic compound occurs first, and then it is borated. Varužan Kevorkijan et al. [17] used MgB2 and AlB12 compounds as raw precursors for the synthesis of AlMgB14. However, the AlMgB14 content in the sintered samples was no more than 25 wt. %. This is because for an intensive BAM formation reaction to reach completeness, free boron atoms are needed [17]. The synthesis of AlMgB14-based materials from mixtures of AlB2, MgB2 and boron was conducted in [16]. As a result, materials with a 85 wt. % AlMgB14 phase content were obtained. In this work, a method for obtaining AlMgB14based materials using an intermetallic AlxMgy compound as a raw precursor was proposed. Thus, the purpose of this work is to study the phase composition, structure and properties of AlMgB14-based materials obtained from mixtures of intermetallic AlxMgy and boron powders.
2.3. Characterizations The average particle size of the powders and the mixtures based on them was measured using an ANALYSETTE 22 MicroTec plus apparatus (Fritsch – Germany). X-ray diffraction analysis of the powder mixtures and the sintered samples was performed using a Rigaku diffractometer with CuKα radiation, as well as the Crystallographica Search Match program (Oxford Cryosystems - UK) based on the «Powder Diffraction file» database. The quantitative content of the phases was determined using the Rietveld method. The microstructure of the powders and the sintered samples was determined using a Tescan Vega 3 microscope (Czech Republic) equipped with energy dispersive spectroscopy (EDX). The densities of the sintered samples were calculated using the Archimedes method. Nanohardness was measured using a table top nanoindentation system (CSM Instruments - Sweden) with a load of 100 mN and a dwell time of 10 s. The processing of experimental data, in particular, the determination of nanohardness, is based on the methods of Oliver and Pharr [37]. The indenter of Berkovich was used, 10 indentations were made from different places of the sample. Each value is averaged over the results of several measurements. Results excluding the values of confidence intervals are excluded from the statistical data. The obtained nanohardness values were converted to Vickers microhardness values. COF was measured using an Anton Paar ball-on-disk tester. A zirconia ball with a diameter of 6 mm was used in this work. The applied normal load was 2 N. The tests were performed under dry conditions, or with LITOL-24 (State standard 21150-75) lubricant, with a linear velocity of 0.02 m/s at room temperature in an ambient environment. The chemical analysis of the intermetallic powder was performed using an XRF-1800 wavelength-dispersive spectrometer. To determine the amount of oxygen in the AlxMgy powder and boron powder, a LECO ONH (USA) analyser was used.
2. Material and methods 2.1. Preparation of AlxMgy powder Intermetallic AlxMgy powder was obtained from 99.9% pure magnesium and aluminum ingots. The fusion of aluminum and magnesium was carried out in a graphite crucible in argon. During the first stage, 1 kg of aluminum was melted at a temperature of 720 °C. Then, magnesium was added into the aluminum melt in a mass ratio of Al:Mg – 1:1. The Al:Mg mass ratio was selected based on the atomic ratio of 1:1:14 of the Al-Mg-B powder mixture. Wherein, according to the Al–Mg phase equilibrium diagram [36], the eutectic area remains in the range of 0.4:0.6 of the atomic ratios Al:Mg (Mg:Al). Mixing was carried out using the device described in [29]. The resulting alloy was poured into a steel chill at a temperature of 670 °C. During the casting process, the alloy was subjected to vibration [30]. During the second stage, the obtained alloy was mechanically milled, and then treated in a planetary mill with a rotation frequency of 12 Hz in an argon atmosphere. Steel balls with a diameter of 8.7 mm were used as grinding bodies. The mass ratio of balls to powder mixture was 2:1. The mechanical treatment was conducted for 5 h. After the mechanical treatment, AlxMgy powder (average particle size < d > of ∼20 μm) was selected as the raw precursor for the AlxMgy-B mixture. The selection process was carried out by sieving. 2.2. The process for obtaining AlMgB14 During the next stage, AlxMgy flakes were mixed with 99.9% pure amorphous black boron (average particle size is 2.1 μm). The obtained intermetallic compound was mixed with boron in an atomic AlxMgy:B ratio of 2:14, since the alloy was obtained in a Al:Mg ratio of 1:1. Before mixing, boron powder was annealed in vacuum chamber at a temperature of 150 °C for 2 h. The obtained mixture was mechanically
3. Results 3.1. Phase composition, microstructure, and chemical composition of the AlxMgy powder Fig. 1 shows the XRD pattern (a) and SEM-images of the AlxMgy
Fig. 1. XRD-pattern (a) and SEM micrograph (Det. SE) of the AlxMgy powder after 5 h of milling in a planetary mill (b). 2
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Table 1 Elemental analysis of the AlxMgy powder after 5 h of milling in a planetary mill. Element
Al
Mg
Si
Fe
Ti
O
Result, (at. %)
50.4
48.68
0.18
0.09
0.64
0.01
Fig. 2. Histograms of the particle size distribution for the Al12Mg17-B powder after 1, 2 and 3 h of mechanical milling.
powder (b). It was found that the peaks in the XRD pattern of the intermetallic powder correspond to Al12Mg17 and Al phases. It can be seen that the morphology of the powder consists of individual small and larger particles of irregular shape, which is typical for powders after mechanical milling. The chemical composition is given in Table 1. Metal impurities, such as Si, Fe, Ti, and oxide were found. The presence of metallic impurities may be due to the presence of impurities in the aluminium and magnesium ingots. The presence of Fe may also be due to the milling of steel balls in the planetary mill [38]. However, preliminary studies on the effect of mechanical activation on the phase composition [31] showed that there are no impurity phases in the powder mixture after mechanical activation of the Al-Mg-B powder mixture. Fig. 2 shows histograms of the particle size distribution for the Al12Mg17-B powder mixture after mechanical treatment in the planetary mill. The average particle size after 1 h of mechanical treatment is 0.2 μm, after 2 h is 1.2 μm, and after 3 h is 0.4 μm. In this case, the particle size distribution is unimodal. After an hour of mechanical activation, intensive milling of the powder mixture occurs. After 2 h of MA, agglomeration of nanosized particles occurs under the action of Van der Waals forces. After 3 h of MA, agglomerated particles are again milled. A similar effect was observed in the previous work [31].
Fig. 3. XRD patterns of the samples sintered by hot pressing the AlxMgy-B powders after 1, 2 and 3 h of mechanical milling.
found in the light regions, which corresponds to spinel MgAl2O4 and Al phases (Table 2). This is consistent with the XRD results. Using the Rietveld method and the EDX results, the AlMgB14 phase content of the sample is ∼ 90%, the AlMgB4 phase content is ∼ 5%, and the MgAl2O4 and Al content is no more than 5%. 3.3. The properties of the sintered materials The density of the sample obtained from the powder mixture after 3 h of mechanical treatment is 2.35 g/cm3 (90.7% of the theoretical density of 2.59 g/cm3). In areas without visible defects, the maximum measured hardness value of the sample is 31.9 GPa, and the minimum is 24 GPa. In other areas, the hardness drops to 12–16 GPa. Wherein, the average grain size of the sample measured from the SEM-image of the fracture surface (Fig. 4c) is 2 μm. The results of the COF measurements under dry and lubricated conditions are presented in Fig. 5. The average COF under dry conditions is 0.38. The average COF in LITOL-24 lubricant is 0.18.
3.2. Phase composition and microstructure of sintered AlMgB14-based samples The XRD patterns of samples obtained by hot pressing Al12Mg17-B powder mixtures after 1, 2 and 3 h of mechanical treatment are shown in Fig. 3. The phase compositions of the samples are dominated by the AlMgB14 phase. A small amount of spinel MgAl2O4, diboride AlxMg1-xB2 and Al phases were also detected. The SEM-images of the sintered sample from the AlxMgy-B powder after 3 h of mechanical milling are shown in Fig. 4. EDX analysis showed that Al, Mg and B are contained in the dark regions, which corresponds to the AlMgB14 phase. In contrast, O, Al and Mg were
4. Discussion The Al12Mg17 phase detected in the intermetallic powder is consistent with the Al-Mg state diagram and corresponds to the eutectic region [34]. The presence of the aluminum phase in the same powder is explained by the fact that in the formed Al12Mg17 phase, there is more magnesium than aluminum; however, when initially melting, the mass ratio of aluminum and magnesium was 1:1. This atomic ratio was 3
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Fig. 4. SEM-images (Det. SE) of the sample sintered by hot pressing the AlxMgy-B powder after 3 h of mechanical milling.
The phase composition of obtained materials are closed to the phase composition of materials, obtained in the works [16,22–24]. AlxMg1-xB2 diborides are intermediate phases in the formation of AlMgB14. To explain the presence of AlMgB4, MgAl2O4, and Al phases in the XRD patterns of the AlMgB14-based samples sintered at a temperature of 1400 °C, additional samples were obtained at temperatures of 600 °C, 800 °C and 1100 °C (Fig. 6). XRD analysis of the samples sintered at a temperature of 600 °C showed that the main phases are Mg0.08Al0.92 and Mg0.815Al0.185B2. A small amount of intermetallic Al12Mg17 was also detected. The main phases of the samples that sintered at a temperature of 800 °C are Mg0.1Al0.9B2 and Mg0.08Al0.92. The high background at small diffraction angles for the samples sintered at 600 °C and 800 °C is due to the presence of amorphous boron. At the same time, as the sintering temperature increases from 600 °C to 800 °C, the degree of crystallinity of the samples increases from 53% to 61%. It is known that the formation of the MgAl2O4 spinel phase occurs most intensely at ∼800 °C [23]. In the XRD pattern of the sample sintered at a temperature of 800 °C, no MgAl2O4 phase was detected. This indicates that the MgAl2O4 phase is formed at temperatures above 800 °C. At a sintering temperature of 1100 °C, the AlMgB14 phase is formed by the boration of AlxMg1-xB2 diborides. At the same time, the spinel phase of MgAl2O4 is formed. Thus, we speculate that the phase formation in the studied Al12Mg17-B powder mixtures system occurs as follows (Fig. 7). Since the melting
Table 2 Results of EDX-analysis of the sample sintered by hot pressing the AlxMgy-B powder after 3 h of mechanical milling. Mark/Element (wt. %)
Al
Mg
B
O
1 2
12.43 11.35
11.29 11.07
76.28 65.17
– 12.41
chosen based on the required atomic ratio of Al:Mg:B of 1:1:14 for the synthesis of AlMgB14, which is close to the stoichiometric ratio. Moreover, free aluminum in the liquid phase provides diffusion-based masstransfer, having a positive effect on the sintering process for AlMgB14based materials. The presence of metal impurities in the obtained powder is consistent with their presence in the raw aluminum and magnesium ingots, which are used to obtain the powders. The increase in the average particle size of Al12Mg17-B powder mixtures with the increase in mechanical treatment time from 1 to 3 h is due to the following. With an increase in mechanical treatment time, the particles pass into the nanoscale range and agglomerate under the action of Van der Waals forces, as seen in the histograms of the particle size distribution (2 h). Then nanosized particles are again milled (3 h). Apparently, further mechanical treatment can enhance the agglomeration of the particles, which could reduce diffusion between the raw powders during the synthesis of AlMgB14 [31].
Fig. 5. Friction coefficient of the AlMgB14 sample: (a) under dry conditions, (b) in LITOL-24 lubricant. 4
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Fig. 7. AlMgB14 formation mechanism. Fig. 6. XRD patterns of AlxMgy-B powder samples after 3 h of mechanical milling and sintered by hot pressing at temperatures of 600, 800 °C and 1100 °C.
Table 3 Results of elemental analysis of the raw powders.
temperatures of B2O3 and Al12Mg17 are approximately the same (∼450 °C), the solid boron particle is probably covered by two layers of liquid (Fig. 7b). Next, there is a diffusion process for the formation of AlxMgx-1B2 boride crystals (1), which can be seen in the XRD patterns, both at 600 °C and at 800 °C (Fig. 7d). Unreacted liquid appears as an amorphous phase of intermetallic compounds. When increasing sintering temperature, the diffusion between Al, Mg and B increases, and the formation of crystalline phases proceeds to completion resulting in the formation of AlMgB14 (2) (Fig. 7e). The remaining liquid crystallizes in a spinel form (3). This is consistent with the results of elemental analysis of the raw powders of boron and Al12Mg17 (Table 3). It was found that the oxygen content in the raw boron powder is ∼ 1.1 wt. %, the oxygen content in Al12Mg17 powder does not exceed 0.1%. Thus, when using Al12Mg17 intermetallic powder, the formation of MgAl2O4 spinel during the reaction of Al2O3 and MgO occurs to a much lesser extent due to the low oxygen content in the raw Al12Mg17 intermetallic powder. Al12Mg17 + B → AlxMg1-xB2
(1)
AlxMg1-xB2 + B → AlMgB14
(2)
B2O3 + Al + Mg → MgAl2O4 + B
(3)
Powder
Mass fraction of oxygen, %
Boron powder Al12Mg17 before mechanical treatment Al12Mg17 after mechanical treatment
1.1 0.003 0.07
noted that in [2,26,32], water-based lubricant coatings were used, which resulted in the formation of B(OH)3. In this work, LITOL-24 petroleum oil was used as a lubricant. It is used in hinges, gears, plain bearings and rolling bearings. COF values larger than that given in [2,26] are explained by the presence of porosity in the samples. In addition, the authors of [2,3,9,10,33] obtained AlMgB14-TiB2 composite materials, and in accordance with [9] TiB2 has a significant effect on the friction coefficient of AlMgB14-TiB2 due to the formation of a B2O3 oxide film on the surface of the material. In the future, to increase the density of the samples, it seems necessary to optimize the morphology of the Al12Mg17 intermetallic compound powders, which will provide a more dense particle packing during sintering.
5. Conclusions Studies have shown the possibility of synthesizing AlMgB14-based materials from AlxMgy alloy. The proposed method for producing the alloy makes it possible to manufacture a material that can be used as raw precursor for obtaining AlMgB14-based materials. The use of the Al12Mg17 intermetallic powder as a raw precursor provides several
The hardness results are consistent with the hardness results obtained in [1,16,19,20,22,23]. The results of the COF measurements are consistent with those of [33], in which COF measurements were performed under dry conditions at various temperatures. It should be 5
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advantages. Firstly, according to the state diagram of the Al-Mg system, Al12Mg17 corresponds to the eutectic region, being fragile, therefore, when mechanically milling the Al12Mg17-B powder mixture, it is necessary to spend significantly less time to obtain nanosized particles. Secondly, when using Al12Mg17 intermetallic powder, the main source of oxygen for the formation of the spinel phase is boron powder. Finally, such a composition is prone to less oxidation, since Al and Mg are in a bound state. According to the Pilling-Bedworth ratio (PBR) [39], Al has a PBR of ∼ 1.28: the oxide film on the aluminum surface is dense, Mg has a PBR of ∼ 0.8: the oxide film on the magnesium surface has defects. Thus, when using Al12Mg17 intermetallic alloy powder as a raw precursor, the oxide film is destroyed on the surface of Al12Mg17 powder during sintering. The phase composition is mainly represented by the AlMgB14 phase; the XRD results are consistent with the results obtained by other authors. Based on the obtained XRD patterns, the mechanism was proposed for the formation of AlMgB14 and impurity phases. When using an AlxMgy intermetallic compound as raw precursor, AlMgB14 is formed in several stages. During the first stage, the boration of intermetallic compounds to form AlxMg1-xB2 diboride occurs. In the second stage, the resulting diborides are then borated to form AlMgB14. The formation of the spinel phase is associated with the presence of an oxide film on boron powder. To reduce the number of impurity phases, the preliminary annealing of boron at temperatures above 150 °C or other methods of removing the oxide film from the boron surface are necessary. It was found that the AlMgB14-based material produced by hot pressing the powder at a temperature of 1400 °C after 3 h of mechanical treatment possessed a hardness of 31.9 GPa and COF of 0.18. The porosity of the obtained material was ∼ 10%. To reduce the porosity, optimization of the morphology of the powders of a given particle size distribution is necessary.
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