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SiC composites containing carbon nanotubes and oxide additives based on organoelementoxanes. Preparation by spark plasma sintering
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SiC composites containing carbon nanotubes and oxide additives based on organoelementoxanes. Preparation by spark plasma sintering Anna A. Zabelinaa,b,∗, Galina I. Shcherbakovaa, Pavel P. Faikovb, Evgeny V. Zharikovc SSC RF JSC “State Research Institute for Chemistry and Technology of Organoelement Compounds”, Entusiastov highway 38, 105118, Moscow, Russia D. Mendeleev University of Chemical Technology of Russia, Miusskaya sq, 125047, Moscow, Russia c Prokhorov General Physics Institute of the Russian Academy of Sciences, Vavilova str. 38, 119991, Moscow, Russia a
b
A R T I C LE I N FO
A B S T R A C T
Keywords: Silicon carbide Composite material Organomagnesiumoxane-yttriumoxanealumoxane oligomer Multi-walled carbon nanotubes Spark plasma sintering
Samples of a SiC composite with an oxide additive of eutectic composition produced from the organomagnesiumoxane-yttriumoxane-alumoxane oligomer (OMYA) and multi-walled carbon nanotubes (MWCNTs) were prepared by the spark plasma sintering (SPS). It is shown that the combined introduction of an oxide additive derived from OMYA together with MWCNTs significantly increases the fracture toughness and strength of the composite.
1. Introduction Recently, durable ceramic composite materials that can withstand high temperatures (1500 °C) and aggressive environments are widely used in various fields (aviation and space technology, production of armor ceramics). SiC - based materials meet these requirements. They have high fracture toughness, high strength, wear resistance, resistance to frequent temperature changes, chemical inertness with respect to corrosive environments, high thermal conductivity, etc. High temperature (above 2000 °C) is required to obtain a composite material based on SiC that causes serious technological difficulties. In order to reduce the sintering temperature and make the material denser, the additives are introduced. As a rule, different oxides (such as Al2O3, Y2O3, RE2O3, MgO, etc.) serve as sintering additives [1] which bring about conditions for sintering SiC composites in the presence of a liquid phase. However, individual oxides in contact with SiC, upon reaching the sintering temperature of silicon carbide, interact with SiC that accompanied with the release of gases (CO, SiO, etc.) which significantly increases the porosity of the material [2,3]. A higher stability of the oxide components and a lower reactivity with SiC can be ensured by the combined introduction of oxides (Y2O3–Al2O3, Dy2O3– Al2O3, Al2O3– Y2O3– SiO2, etc.) [4–11]. At combined introduction of Y2O3 and Al2O3 to silicon carbide in different ratios, the best composite characteristics were obtained when not single
3:5 phase (i. e. yttrium-aluminum garnet Y3Al5O12, or YAG) was formed in the melt, but two different phases [4]. At a molar ratio of Y2O3/ Al2O3 = 2 : 3 the Y4Al2O9 (YAM) phase is formed in addition to yttrium aluminum garnet [12] and the formation of a eutectic liquid phase (YAG + YAM) with a low melting point occurs. As a result, the density of the ceramic composite increases and its mechanical characteristics improve. The most common ways to fabricate the sintering additives are mechanical mixing of commercial oxide powders [3,5,8–10,13,14] and the sol-gel method [7,15–18]. However, both methods are multistage (in terms of time, the whole process takes up from a day or more); both methods require significant energy and labor costs, and, do not guarantee a uniform mutual distribution of oxide phases. Organoelement oligomers are a new class of unsurpassed precursors for producing high-temperature, oxidation-resistant, high-strength nanostructural ceramic composites: ceramic fibers, matrices, protective coatings, etc. They allow the use of a fundamentally differing "polymer technology" for the production of ceramics by pyrolysis of ceramicforming oligomers [19,20]. The advantages of precursors based on ceramic-forming oligomers are the absence of uncontrolled impurities, the achievement high compatibility of components, the ability to simulate micro- and macrostructures of ceramics at the stage of synthesis of ceramic-forming oligomer, and the possibility of fabrication nano-ceramic products of sophisticated geometry without using excessively high temperatures
∗ Corresponding author. SSC RF JSC “State Research Institute for Chemistry and Technology of Organoelement Compounds”, Entusiastov highway 38, 105118, Moscow, Russia. E-mail address: [email protected] (A.A. Zabelina).
https://doi.org/10.1016/j.ceramint.2019.09.269 Received 4 July 2019; Received in revised form 21 September 2019; Accepted 27 September 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Anna A. Zabelina, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.09.269
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nanotubes, on the one hand, increase the flexural strength of the material, but, on the other hand, they reduce the value of microhardness, since the interfacial interactions of the matrix grains weaken, because the CNTs braid the grains and prevent them from interacting with each other. Therefore, the amount of CNTs in the composite material usually does not exceed 10 wt % [25]. Of greatest interest is the joint introduction of oxide additives and multi-walled carbon nanotubes [15,23]. In this case, oxide additives were prepared by the polymer sol – gel method [26] and introduced as a nano-dispersion. An important feature of Ref. [15] is that SiC-composite with 6 vol % of MWCNT prepared by SPS contained the oxide additives in low concentrations (1 vol %. Y3Al5O12 and 1 vol % MgAl2O4), which were introduced both individually, and together, that ultimately allowed nearly twice increase in fracture toughness and strength. The purpose of present work is to obtain by plasma sintering the SiC composite comprising both multiwall carbon nanotubes and a complex oxide additive of eutectic composition synthesized from organoelementoxanes by a new method.
Fig. 1. A fragment of the ceramic-forming skeleton of OMYA.
and pressures. The polymer-ceramic technology based on the use of organoelementoxanes developed by the authors [21] makes it possible to obtain ceramic precursors with the required molar ratio of components. Ceramic-forming oligomers are synthesized at low temperatures for 2–3 h. Moreover, the pyrolysis of polymers leads to the formation of oxide additives of perfect nanostructure. Complex ceramic - forming organomagnesiumoxane-yttriumoxanealumoxane oligomer (OMYA) hydrolytically stable in air, soluble in organic solvents has been first synthesized recently [21] by co-condensation of acetoxyacetate-alkoxyalumoxane-stabilized yttrium acetylacetonate hydrate (асас)3Y۰2,5Н2О and magnesium acetyl acetonate (асас)2Mg. A fragment of the OMYA structure is shown in Fig. 1. The atoms of the elements forming this structure are distributed uniformly in a certain order in the space. To obtain ceramics one should only remove functional organic groups, for example, by hydrolysis or heat treatment. The resulting oxide additive has a structure containing Al, Y and Mg with a given molar ratio of Al: Mg and Al: Y. Oxide additives are usually introduced into the composite in the amount of 7–30 vol %. In Refs. [13,14], a combination of oxide additives were introduced in the MgO – Al2O3 – Y2O3 system in the amount of 25 and 30 vol %. They formed the eutectic composition MgO·Al2O3 – Y2O3·5Al2O3 (MgO - 44.04 mol.%, Y2O3 - 7.86 mol.%, Al2O3 - 48.1 mol.%) at a temperature of 1775° С [14] and Al2O3 = 77.88 mol. %, Y2O3 = 12.09 mol. %, MgO = 10.03 mol. % at a temperature of 1770° С [22]. The eutectic composition was introduced in order to minimize the temperature of liquid phase appearance during sintering, and for more uniform distribution of the additive in the bulk of the base material, due to the minimum size of eutectic crystallites. The obtained SiC ceramics had higher strength, but showed a decrease in chemical resistance. In Ref. [18] Al2O3, Y2O3, and MgO powders were used as activating additives, they were mixed in proportions corresponding to the compositions of Y3Al5O12 yttrium-aluminum garnet and MgAl2O4 magnesium aluminate spinel. These powders were added to SiC in an amount from 5 to 20 wt %. According to the authors of Ref. [18], the optimal amount of the activating additive is 10–15 wt %, its decrease leads to incomplete wetting of SiC grains by oxides and, as a result, to a decrease of mechanical properties. On the other hand, increasing the concentration of the additive to 20 wt % results in the decrease of material hardness. The introduction of carbon nanotubes (CNT) is another promising modern approach to the preparation of high performance SiC composites. For example, the authors of Ref. [24] showed that with the introduction of CNTs, an improvement in flexural strength and fracture toughness by about 10% is observed compared to pure sintered silicon carbide. Authors of Ref. [25] have shown that the strength of the CNT/ SiC material with three-point bending averaged 234 ± 18.9 GPa, which significantly exceeds the strength value of the pure SiC material (150 GPa). As follows from the literature analysis, the carbon
2. Experimental Silicon carbide of f1000 qualification with an average grain size of 5–10 μm, which was subjected to grinding to a grain size of 2–3 μm in a vibratory mill in ethyl alcohol, was used as a feedstock. Then, silicon carbide was additionally purified in hydrofluoric acid HF to remove free-flowing steel grinding media and SiO2 impurity. Bayer's multi-walled carbon nanotubes (MWCNT) in the amount of 1.6 and 10 vol % with external and internal diameters of 13 and 4 nm, respectively, 1 μm in length and bulk density of 0.13–0.15 g/cm3 were used as one of reinforcing components. The original MWCNTs were in bundles, which required their processing in an ultrasonic disperser in a solution of polyvinyl alcohol [29]. MWCNTs were dispersed on a Sonoplus HD 3100 disperser with a capacity of 50%, dispersion time 10–20 min, and pulse duration 1–1.5 s. The complex oxide additive (MYA) of the eutectic composition Al2O3 = 77.88 mol. %, Y2O3 = 12.09 mol. %, MgO = 10.03 mol. % with a melting point of 1770°С was another reinforcing component selected after Ref. [22]. To obtain MYA of the selected composition, the organomagnesiumoxane-yttriumoxane-alumoxane oligomer (OMYA) was synthesized with molar ratios of the components Al: Mg = 6.43 and Al: Y = 15.35, followed by heat treatment at 750 °C in air. Thermogravimetric analysis (TGA) was carried out on a Mettler Toledo TGA/SDTA 851 installation during heating in air to 1100°С at a rate of 10°С/min. Diffractometric studies were carried out in a divergent ZeemanBolin beam on Shimadzu XRD-6000 vertical X-ray diffractometer at ambient temperature with CuKα radiation (λKαav = (2λKα1+ λKα2)/ 3 = 1.54178 Å. The crystalline phases were identified using ICDD PDF2 Release 2003 data. The surface morphology of the composite material was studied on scanning electron microscope Quanta 250 and their elemental composition was studied on Philips SEM 505 microscope equipped with a Sapphire Si(Li) SEM10 energy dispersive detector using Micro Capture SEM3.0 M image capture system. Samples of the composite material were sintered using the method of spark plasma sintering (SPS) [27,28] on the HP D 25 installation with direct heating by FCT Systeme, Germany. The sintered SiC composite samples were ground on an EcoMet 250 grinding and polishing machine (Buehler, Germany). The microhardness of the composite was measured on Micromet 5114 by indentation the Vickers pyramid into the previously prepared section of the sample. The microhardness of the sample was determined under a load of 300 g and a load time of 15 s. The densities of composite samples were measured using 2
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Fig. 2. Thermogram of organomagnesiumoxane-yttriumoxane-alumoxane oligomer (OMYA).
MYA: Al2O3 = 77.88 mol. %, Y2O3 = 12.09 mol.%, MgO = 10.03 mol. % was confirmed. According to the results of scanning electron microscopy (Fig. 4 a, b, c), we can see that after the OMYA pyrolysis, the volume of the sintering additive is divided into individual structural microelements (Fig. 4a): the surface of the heat-treated sample consists of individual microparticles. It should be noted that the results of energy-dispersive X-ray spectroscopy (EDS) (Fig. 4c and d) confirm the formation of the initially chosen oxide eutectic composition (mol.%): Al2O3 - 77.88; Y2O3 - 12.09; MgO - 10.03 with a molar ratio of Al: Y = 6.43; Al: Mg = 15.35. The micrograph shown in Fig. 4 b, demonstrates that the particle size of the additive obtained after pyrolysis of the elementoxane oligomer is from 50 to 210 nm. The sintering additive MYA along with dispersed MWCNTs and crushed silicon carbide were mixed in polyvinyl alcohol in a planetary mill with grinding bodies for 40 min to evenly distribute the components in the suspension, which was further dried under an infrared lamp on a hotplate along with continuous mixing. The resulting powder mixture was additionally homogenized by rubbing through a sieve with a mesh size of 0.25 mm. Then the molding compound was exposed to spark plasma sintering at a temperature of 2000–2200 °C with a heating rate of 450 °C/min under a pressure of 19–22 kN. Samples were held at maximum temperature and pressure from 10 to 15 min. As a result, the composite samples with a diameter of 20 mm and a thickness of 5 mm were obtained. The synthesis occurs under pressure and the action of high-temperature plasma (short-lived spark plasma) arising in the intervals between the particles of material being baked. The sintering process is carried out in argon that prevented the material from oxidation. In fact, the basis of the process is a modified hot-pressing method, in which
Archimedes water immersion technique. The flexural strength (σ) of a composite material was studied by the method of three-point bending at room temperature on Instron 5581 with a loading rate of 1 mm/min. 3. Results and discussion The thermogram of organomagnesiumoxane-yttriumoxane-alumoxane oligomer is presented in Fig. 2 The TGA curve shows that the main weight loss is observed within the temperature range of 200–500°С (the residue is about 36 wt %). On the SDTA curve there is an exothermic effect at 50 °C, which is associated with the splitting of the hydroxyl group OH, and an exothermic effect with a maximum at 300 °C, this effect is related to the heat release during the decomposition of the organic component of the oligomer. A further increase in temperature has no significant effect. This allows heat treatment, with the aim of obtaining an oxide additive at temperatures of 600–800 °C. The separation of functional organic groups occurs therewith, which results in the formation of MYA (X-ray amorphous substance) used in this work as a sintering additive. To confirm the formation of the initially selected composition (Al2O3 = 77.88 mol. %, Y2O3 = 12.09 mol. %, MgO = 10.03 mol. %), it is necessary to see fully formed phases, so the oligomer was calcined to a temperature of 1500° С and investigated by X-ray phase analysis (XRD). X-ray phase analysis showed (Fig. 3) that the pyrolysis of sintering additive MYA at 1500 °C leads to the formation mixture of three different phases: yttrium-aluminum garnet Y3Al5O12 - 11.68 mol. %, magnesium-aluminum spinel MgAl2O4 - 25.04 mol. %, corundum αAl2O3 - 63.28 mol. %. When recalculating the quantity of detected phases in terms of the percentage of oxides in these phases, the formation of the composition of the initially selected sintering additive 3
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Fig. 3. Diffraction patterns of a sample of elementoxane after heat treatment at 1500°С
Fig. 4. The electron micrographs, the elemental composition, the elemental mapping with overlay elements MYA on the same map.
showed that the microstructure of the composite consisted of very tight SiC grains of 20–30 μm size. MWCNTs are distributed along the grain boundaries and, besides, they are incorporated into the SiC grains. The pictures clearly show that the MWCNTs accumulate in the pores, and fill the microcracks in the composite material. Table 1 shows the results of mechanical testing of ceramic composite samples. One can see that introduction of only carbon nanotubes in an amount of 10 vol % into pure silicon carbide leads to decrease the value of the flexural strength. It is most likely due to the fact that nanotubes braid silicon carbide grains, thereby reducing the interfacial interactions between the grains. When introducing only MYA eutectic oxide additive into silicon carbide, the values of the fracture toughness, density and flexural strength increase, which indicates that the additive
electric current is passed directly through the mold and the moldable billet, and not through an external heater. During the sintering process, the sample is heated by direct transmission of short pulses of direct current 8000 A at external mechanical pressure, as a result, the heating cycle time up to 2200 °C is reduced to several minutes. The agglomeration activity of materials increases due to the processes occurring at the points of contact of powder particles, and, as a result, there is no increase in the grain size of the material. The growth of grains and the appearance of equilibrium states are blocked also due to the short time of SPS process, which makes it possible to fabricate materials with previously unattainable structure and properties. Studies of the microstructure of beam sample after flexural strength test with composition SiC +10 vol % MWCNT +0.5 vol % MYA (Fig. 5) 4
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Fig. 5. Electron micrographs of the sample SiC +10 vol % MWCNT +0.5 vol % MYA.
strength characteristics of a composite reinforced with CNT, the ceramic matrix should be fine-crystalline and practically non-porous. Such matrix will transmit well the mechanical load on the CNT, as well as protect the CNT from interaction with the environment, in particular with atmospheric oxygen. The reciprocal simultaneous action of both MWCNTs and additives in sufficient quantities makes it possible to obtain a high-strength fine-crystalline material. Apparently, the combined introduction of 10 vol % MCNTs +0.5 vol % MYA produces a synergistic effect. When the SiC grains are wetted with MYA oxide melt, the boundary layer grows on the surface of the primary grains uniformly in all directions. During sintering of the composite the Y, Al, and Mg diffuse from the oxide melt into grains. The main driving force of the compaction process is the sliding of particles along the boundaries of each other in the liquid phase. We also believe that the formation of a synergistic effect was influenced by a specific method of producing oxide additives, which allows them to be obtained in the form of ultrafine highly active particles. In addition, the use of SPS technique makes it possible to distribute even a small amount of the additive over a larger surface of SiC grains compared, for example, with hot pressing. Since high-temperature plasma is formed during sintering directly at the SiC grain boundaries, the conductive MWCNTs are distributed in thin layers of the liquid phase and facilitate to the runoff of stresses in the material during synthesis. Under the joint influence of plasma, high temperature, and pressure, the MWCNTs fill the structural defects in the sintering material by bonding between individual SiC grains. In general, the synergistic effect consists of three components: 1
makes the material denser. The joint introduction of MWCNT and MYA (sample No. 5) provides the combined effect of MWCNT and additives, which increases the fracture toughness by 1.5 times and the flexural strength by 1.6 times. Sample No. 6 has the highest values. It comprises the increased high amount of MCNTs — 10 vol % and MYA additive in the amount of 0.5 vol % The fracture toughness of specimen No. 6 exceeds the fracture toughness of pure silicon carbide almost twice, and its flexural strength is 1.9 times higher. One of the main tasks was to obtain a dense sintered material with a high level of strength. Even when using such a high-tech method as SPS, it is not possible to obtain a material with a density close to theoretical without the use of additives. The introduction of MWCNTs allows increasing crack resistance, while the strength and density of the material are slightly reduced. On the other hand, the introduction of sintering oxide additives, even without MWCNTs, improves the characteristics of the sintered material. However, only the combined use of additives and a reinforcing component allows one to obtain dense and durable composite samples with a high level of crack resistance. It should be noted that the weight of the composite material practically does not increase, since the high mass of oxides is balanced by very light MWCNTs. By comparison of samples of SiC +10 vol % MCNTs (sample № 2) and SiC + 10 vol % MCNTs +0.5 vol % MYA (sample № 6), one can see that the flexural strength increases by almost 2.5 times, the fracture toughness and microhardness value also increase, due to the introduction of only 0.5 vol % MYA. For the maximum realization of the
Table 1 Mechanical properties of the samples. №
Material
Load (g)
Microhardness (GPa)
Density (g/cm3)
Fracture toughness (MPa·m1/2)
Flexural strength (МPа)
1 2 3
SiC SiC + 10 vol % MWCNTs SiC + 0.5 vol % MYA SiC +1 vol % MWCNTs +0.5 vol % MYA SiC+6 vol % MWCNTs +0.5 vol % MYA SiC+10 vol % MWCNTs + 0.5 vol % MYA
300 300 300
32.8 23.7 30.3
2.96 3.09 3.16
3.77 4.26 4.84
180 137 220
300 300 300
35.4 33.8 31.9
3.20 3.23 3.22
5.12 5.75 6,24
190 290 345
4 5 6
5
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- features of the sintering additive, 2 - features of the sintering method, 3 - distribution of MWCNTs in thin layers of the liquid oxide phase formed during sintering.
[10]
4. Conclusions
[11]
The silicon carbide composite material, reinforced with various concentrations of multi-walled carbon nanotubes (0, 1, 6, 10 vol%) together with the additive of 0.5 vol % based on organomagnesiumoxane-yttriumoxane-alumoxane oligomer was obtained by spark plasma sintering. Synthesized activating additive generates a triple oxide eutectic system (Al2O3 = 77.88 mol. % - Y2O3 = 12.09 mol. % MgO = 10.03 mol. %) during heat treatment. The SiC content in the obtained samples is not less than 89.5 vol %. The fabricated samples of the SiC composite have high enough mechanical properties at a low content of sintering additive, which is seen by a comparison with other works. Introduction of sintering additive MYA in the amount of 0.5 vol % significantly improves the properties of the composite. The introduction of sintering additives in small quantities, on the one hand, allows to preserve the unique properties of silicon-carbide ceramics, in particular, the high temperature of products operation, and, on the other hand, the quantity of these additives allows to improve the sintering process. Thus, the paper demonstrated the preparation of strong and crackresistant material with a density close to theoretical for silicon carbide (3.21 g/cm3). A small amount of additives allows to achieve the properties of the composite, primarily high-temperature strength, and resistance to thermal shock, will be close to the outstanding properties of pure silicon carbide. There are further prospects for achieving higher characteristics by adjusting the amount of sintering additive and the ratio of SiC –MYA- MWCNT in the composite.
[12]
[13]
[14]
[15]
[16]
[17]
[18]
[19]
Declaration of competing interest [20]
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.
[21]
Acknowledgments The work was supported by the Russian Foundation for Basic Research, project no. 17-03-00331 A.
[22]
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SiC composites containing carbon nanotubes and oxide additives based on organoelementoxanes. Preparation by spark plasma sintering Anna A. Zabelinaa,b,∗, Galina I. Shcherbakovaa, Pavel P. Faikovb, Evgeny V. Zharikovc SSC RF JSC “State Research Institute for Chemistry and Technology of Organoelement Compounds”, Entusiastov highway 38, 105118, Moscow, Russia D. Mendeleev University of Chemical Technology of Russia, Miusskaya sq, 125047, Moscow, Russia c Prokhorov General Physics Institute of the Russian Academy of Sciences, Vavilova str. 38, 119991, Moscow, Russia a
b
A R T I C LE I N FO
A B S T R A C T
Keywords: Silicon carbide Composite material Organomagnesiumoxane-yttriumoxanealumoxane oligomer Multi-walled carbon nanotubes Spark plasma sintering
Samples of a SiC composite with an oxide additive of eutectic composition produced from the organomagnesiumoxane-yttriumoxane-alumoxane oligomer (OMYA) and multi-walled carbon nanotubes (MWCNTs) were prepared by the spark plasma sintering (SPS). It is shown that the combined introduction of an oxide additive derived from OMYA together with MWCNTs significantly increases the fracture toughness and strength of the composite.
1. Introduction Recently, durable ceramic composite materials that can withstand high temperatures (1500 °C) and aggressive environments are widely used in various fields (aviation and space technology, production of armor ceramics). SiC - based materials meet these requirements. They have high fracture toughness, high strength, wear resistance, resistance to frequent temperature changes, chemical inertness with respect to corrosive environments, high thermal conductivity, etc. High temperature (above 2000 °C) is required to obtain a composite material based on SiC that causes serious technological difficulties. In order to reduce the sintering temperature and make the material denser, the additives are introduced. As a rule, different oxides (such as Al2O3, Y2O3, RE2O3, MgO, etc.) serve as sintering additives [1] which bring about conditions for sintering SiC composites in the presence of a liquid phase. However, individual oxides in contact with SiC, upon reaching the sintering temperature of silicon carbide, interact with SiC that accompanied with the release of gases (CO, SiO, etc.) which significantly increases the porosity of the material [2,3]. A higher stability of the oxide components and a lower reactivity with SiC can be ensured by the combined introduction of oxides (Y2O3–Al2O3, Dy2O3– Al2O3, Al2O3– Y2O3– SiO2, etc.) [4–11]. At combined introduction of Y2O3 and Al2O3 to silicon carbide in different ratios, the best composite characteristics were obtained when not single
3:5 phase (i. e. yttrium-aluminum garnet Y3Al5O12, or YAG) was formed in the melt, but two different phases [4]. At a molar ratio of Y2O3/ Al2O3 = 2 : 3 the Y4Al2O9 (YAM) phase is formed in addition to yttrium aluminum garnet [12] and the formation of a eutectic liquid phase (YAG + YAM) with a low melting point occurs. As a result, the density of the ceramic composite increases and its mechanical characteristics improve. The most common ways to fabricate the sintering additives are mechanical mixing of commercial oxide powders [3,5,8–10,13,14] and the sol-gel method [7,15–18]. However, both methods are multistage (in terms of time, the whole process takes up from a day or more); both methods require significant energy and labor costs, and, do not guarantee a uniform mutual distribution of oxide phases. Organoelement oligomers are a new class of unsurpassed precursors for producing high-temperature, oxidation-resistant, high-strength nanostructural ceramic composites: ceramic fibers, matrices, protective coatings, etc. They allow the use of a fundamentally differing "polymer technology" for the production of ceramics by pyrolysis of ceramicforming oligomers [19,20]. The advantages of precursors based on ceramic-forming oligomers are the absence of uncontrolled impurities, the achievement high compatibility of components, the ability to simulate micro- and macrostructures of ceramics at the stage of synthesis of ceramic-forming oligomer, and the possibility of fabrication nano-ceramic products of sophisticated geometry without using excessively high temperatures
∗ Corresponding author. SSC RF JSC “State Research Institute for Chemistry and Technology of Organoelement Compounds”, Entusiastov highway 38, 105118, Moscow, Russia. E-mail address: [email protected] (A.A. Zabelina).
https://doi.org/10.1016/j.ceramint.2019.09.269 Received 4 July 2019; Received in revised form 21 September 2019; Accepted 27 September 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Anna A. Zabelina, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.09.269
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nanotubes, on the one hand, increase the flexural strength of the material, but, on the other hand, they reduce the value of microhardness, since the interfacial interactions of the matrix grains weaken, because the CNTs braid the grains and prevent them from interacting with each other. Therefore, the amount of CNTs in the composite material usually does not exceed 10 wt % [25]. Of greatest interest is the joint introduction of oxide additives and multi-walled carbon nanotubes [15,23]. In this case, oxide additives were prepared by the polymer sol – gel method [26] and introduced as a nano-dispersion. An important feature of Ref. [15] is that SiC-composite with 6 vol % of MWCNT prepared by SPS contained the oxide additives in low concentrations (1 vol %. Y3Al5O12 and 1 vol % MgAl2O4), which were introduced both individually, and together, that ultimately allowed nearly twice increase in fracture toughness and strength. The purpose of present work is to obtain by plasma sintering the SiC composite comprising both multiwall carbon nanotubes and a complex oxide additive of eutectic composition synthesized from organoelementoxanes by a new method.
Fig. 1. A fragment of the ceramic-forming skeleton of OMYA.
and pressures. The polymer-ceramic technology based on the use of organoelementoxanes developed by the authors [21] makes it possible to obtain ceramic precursors with the required molar ratio of components. Ceramic-forming oligomers are synthesized at low temperatures for 2–3 h. Moreover, the pyrolysis of polymers leads to the formation of oxide additives of perfect nanostructure. Complex ceramic - forming organomagnesiumoxane-yttriumoxanealumoxane oligomer (OMYA) hydrolytically stable in air, soluble in organic solvents has been first synthesized recently [21] by co-condensation of acetoxyacetate-alkoxyalumoxane-stabilized yttrium acetylacetonate hydrate (асас)3Y۰2,5Н2О and magnesium acetyl acetonate (асас)2Mg. A fragment of the OMYA structure is shown in Fig. 1. The atoms of the elements forming this structure are distributed uniformly in a certain order in the space. To obtain ceramics one should only remove functional organic groups, for example, by hydrolysis or heat treatment. The resulting oxide additive has a structure containing Al, Y and Mg with a given molar ratio of Al: Mg and Al: Y. Oxide additives are usually introduced into the composite in the amount of 7–30 vol %. In Refs. [13,14], a combination of oxide additives were introduced in the MgO – Al2O3 – Y2O3 system in the amount of 25 and 30 vol %. They formed the eutectic composition MgO·Al2O3 – Y2O3·5Al2O3 (MgO - 44.04 mol.%, Y2O3 - 7.86 mol.%, Al2O3 - 48.1 mol.%) at a temperature of 1775° С [14] and Al2O3 = 77.88 mol. %, Y2O3 = 12.09 mol. %, MgO = 10.03 mol. % at a temperature of 1770° С [22]. The eutectic composition was introduced in order to minimize the temperature of liquid phase appearance during sintering, and for more uniform distribution of the additive in the bulk of the base material, due to the minimum size of eutectic crystallites. The obtained SiC ceramics had higher strength, but showed a decrease in chemical resistance. In Ref. [18] Al2O3, Y2O3, and MgO powders were used as activating additives, they were mixed in proportions corresponding to the compositions of Y3Al5O12 yttrium-aluminum garnet and MgAl2O4 magnesium aluminate spinel. These powders were added to SiC in an amount from 5 to 20 wt %. According to the authors of Ref. [18], the optimal amount of the activating additive is 10–15 wt %, its decrease leads to incomplete wetting of SiC grains by oxides and, as a result, to a decrease of mechanical properties. On the other hand, increasing the concentration of the additive to 20 wt % results in the decrease of material hardness. The introduction of carbon nanotubes (CNT) is another promising modern approach to the preparation of high performance SiC composites. For example, the authors of Ref. [24] showed that with the introduction of CNTs, an improvement in flexural strength and fracture toughness by about 10% is observed compared to pure sintered silicon carbide. Authors of Ref. [25] have shown that the strength of the CNT/ SiC material with three-point bending averaged 234 ± 18.9 GPa, which significantly exceeds the strength value of the pure SiC material (150 GPa). As follows from the literature analysis, the carbon
2. Experimental Silicon carbide of f1000 qualification with an average grain size of 5–10 μm, which was subjected to grinding to a grain size of 2–3 μm in a vibratory mill in ethyl alcohol, was used as a feedstock. Then, silicon carbide was additionally purified in hydrofluoric acid HF to remove free-flowing steel grinding media and SiO2 impurity. Bayer's multi-walled carbon nanotubes (MWCNT) in the amount of 1.6 and 10 vol % with external and internal diameters of 13 and 4 nm, respectively, 1 μm in length and bulk density of 0.13–0.15 g/cm3 were used as one of reinforcing components. The original MWCNTs were in bundles, which required their processing in an ultrasonic disperser in a solution of polyvinyl alcohol [29]. MWCNTs were dispersed on a Sonoplus HD 3100 disperser with a capacity of 50%, dispersion time 10–20 min, and pulse duration 1–1.5 s. The complex oxide additive (MYA) of the eutectic composition Al2O3 = 77.88 mol. %, Y2O3 = 12.09 mol. %, MgO = 10.03 mol. % with a melting point of 1770°С was another reinforcing component selected after Ref. [22]. To obtain MYA of the selected composition, the organomagnesiumoxane-yttriumoxane-alumoxane oligomer (OMYA) was synthesized with molar ratios of the components Al: Mg = 6.43 and Al: Y = 15.35, followed by heat treatment at 750 °C in air. Thermogravimetric analysis (TGA) was carried out on a Mettler Toledo TGA/SDTA 851 installation during heating in air to 1100°С at a rate of 10°С/min. Diffractometric studies were carried out in a divergent ZeemanBolin beam on Shimadzu XRD-6000 vertical X-ray diffractometer at ambient temperature with CuKα radiation (λKαav = (2λKα1+ λKα2)/ 3 = 1.54178 Å. The crystalline phases were identified using ICDD PDF2 Release 2003 data. The surface morphology of the composite material was studied on scanning electron microscope Quanta 250 and their elemental composition was studied on Philips SEM 505 microscope equipped with a Sapphire Si(Li) SEM10 energy dispersive detector using Micro Capture SEM3.0 M image capture system. Samples of the composite material were sintered using the method of spark plasma sintering (SPS) [27,28] on the HP D 25 installation with direct heating by FCT Systeme, Germany. The sintered SiC composite samples were ground on an EcoMet 250 grinding and polishing machine (Buehler, Germany). The microhardness of the composite was measured on Micromet 5114 by indentation the Vickers pyramid into the previously prepared section of the sample. The microhardness of the sample was determined under a load of 300 g and a load time of 15 s. The densities of composite samples were measured using 2
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Fig. 2. Thermogram of organomagnesiumoxane-yttriumoxane-alumoxane oligomer (OMYA).
MYA: Al2O3 = 77.88 mol. %, Y2O3 = 12.09 mol.%, MgO = 10.03 mol. % was confirmed. According to the results of scanning electron microscopy (Fig. 4 a, b, c), we can see that after the OMYA pyrolysis, the volume of the sintering additive is divided into individual structural microelements (Fig. 4a): the surface of the heat-treated sample consists of individual microparticles. It should be noted that the results of energy-dispersive X-ray spectroscopy (EDS) (Fig. 4c and d) confirm the formation of the initially chosen oxide eutectic composition (mol.%): Al2O3 - 77.88; Y2O3 - 12.09; MgO - 10.03 with a molar ratio of Al: Y = 6.43; Al: Mg = 15.35. The micrograph shown in Fig. 4 b, demonstrates that the particle size of the additive obtained after pyrolysis of the elementoxane oligomer is from 50 to 210 nm. The sintering additive MYA along with dispersed MWCNTs and crushed silicon carbide were mixed in polyvinyl alcohol in a planetary mill with grinding bodies for 40 min to evenly distribute the components in the suspension, which was further dried under an infrared lamp on a hotplate along with continuous mixing. The resulting powder mixture was additionally homogenized by rubbing through a sieve with a mesh size of 0.25 mm. Then the molding compound was exposed to spark plasma sintering at a temperature of 2000–2200 °C with a heating rate of 450 °C/min under a pressure of 19–22 kN. Samples were held at maximum temperature and pressure from 10 to 15 min. As a result, the composite samples with a diameter of 20 mm and a thickness of 5 mm were obtained. The synthesis occurs under pressure and the action of high-temperature plasma (short-lived spark plasma) arising in the intervals between the particles of material being baked. The sintering process is carried out in argon that prevented the material from oxidation. In fact, the basis of the process is a modified hot-pressing method, in which
Archimedes water immersion technique. The flexural strength (σ) of a composite material was studied by the method of three-point bending at room temperature on Instron 5581 with a loading rate of 1 mm/min. 3. Results and discussion The thermogram of organomagnesiumoxane-yttriumoxane-alumoxane oligomer is presented in Fig. 2 The TGA curve shows that the main weight loss is observed within the temperature range of 200–500°С (the residue is about 36 wt %). On the SDTA curve there is an exothermic effect at 50 °C, which is associated with the splitting of the hydroxyl group OH, and an exothermic effect with a maximum at 300 °C, this effect is related to the heat release during the decomposition of the organic component of the oligomer. A further increase in temperature has no significant effect. This allows heat treatment, with the aim of obtaining an oxide additive at temperatures of 600–800 °C. The separation of functional organic groups occurs therewith, which results in the formation of MYA (X-ray amorphous substance) used in this work as a sintering additive. To confirm the formation of the initially selected composition (Al2O3 = 77.88 mol. %, Y2O3 = 12.09 mol. %, MgO = 10.03 mol. %), it is necessary to see fully formed phases, so the oligomer was calcined to a temperature of 1500° С and investigated by X-ray phase analysis (XRD). X-ray phase analysis showed (Fig. 3) that the pyrolysis of sintering additive MYA at 1500 °C leads to the formation mixture of three different phases: yttrium-aluminum garnet Y3Al5O12 - 11.68 mol. %, magnesium-aluminum spinel MgAl2O4 - 25.04 mol. %, corundum αAl2O3 - 63.28 mol. %. When recalculating the quantity of detected phases in terms of the percentage of oxides in these phases, the formation of the composition of the initially selected sintering additive 3
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Fig. 3. Diffraction patterns of a sample of elementoxane after heat treatment at 1500°С
Fig. 4. The electron micrographs, the elemental composition, the elemental mapping with overlay elements MYA on the same map.
showed that the microstructure of the composite consisted of very tight SiC grains of 20–30 μm size. MWCNTs are distributed along the grain boundaries and, besides, they are incorporated into the SiC grains. The pictures clearly show that the MWCNTs accumulate in the pores, and fill the microcracks in the composite material. Table 1 shows the results of mechanical testing of ceramic composite samples. One can see that introduction of only carbon nanotubes in an amount of 10 vol % into pure silicon carbide leads to decrease the value of the flexural strength. It is most likely due to the fact that nanotubes braid silicon carbide grains, thereby reducing the interfacial interactions between the grains. When introducing only MYA eutectic oxide additive into silicon carbide, the values of the fracture toughness, density and flexural strength increase, which indicates that the additive
electric current is passed directly through the mold and the moldable billet, and not through an external heater. During the sintering process, the sample is heated by direct transmission of short pulses of direct current 8000 A at external mechanical pressure, as a result, the heating cycle time up to 2200 °C is reduced to several minutes. The agglomeration activity of materials increases due to the processes occurring at the points of contact of powder particles, and, as a result, there is no increase in the grain size of the material. The growth of grains and the appearance of equilibrium states are blocked also due to the short time of SPS process, which makes it possible to fabricate materials with previously unattainable structure and properties. Studies of the microstructure of beam sample after flexural strength test with composition SiC +10 vol % MWCNT +0.5 vol % MYA (Fig. 5) 4
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Fig. 5. Electron micrographs of the sample SiC +10 vol % MWCNT +0.5 vol % MYA.
strength characteristics of a composite reinforced with CNT, the ceramic matrix should be fine-crystalline and practically non-porous. Such matrix will transmit well the mechanical load on the CNT, as well as protect the CNT from interaction with the environment, in particular with atmospheric oxygen. The reciprocal simultaneous action of both MWCNTs and additives in sufficient quantities makes it possible to obtain a high-strength fine-crystalline material. Apparently, the combined introduction of 10 vol % MCNTs +0.5 vol % MYA produces a synergistic effect. When the SiC grains are wetted with MYA oxide melt, the boundary layer grows on the surface of the primary grains uniformly in all directions. During sintering of the composite the Y, Al, and Mg diffuse from the oxide melt into grains. The main driving force of the compaction process is the sliding of particles along the boundaries of each other in the liquid phase. We also believe that the formation of a synergistic effect was influenced by a specific method of producing oxide additives, which allows them to be obtained in the form of ultrafine highly active particles. In addition, the use of SPS technique makes it possible to distribute even a small amount of the additive over a larger surface of SiC grains compared, for example, with hot pressing. Since high-temperature plasma is formed during sintering directly at the SiC grain boundaries, the conductive MWCNTs are distributed in thin layers of the liquid phase and facilitate to the runoff of stresses in the material during synthesis. Under the joint influence of plasma, high temperature, and pressure, the MWCNTs fill the structural defects in the sintering material by bonding between individual SiC grains. In general, the synergistic effect consists of three components: 1
makes the material denser. The joint introduction of MWCNT and MYA (sample No. 5) provides the combined effect of MWCNT and additives, which increases the fracture toughness by 1.5 times and the flexural strength by 1.6 times. Sample No. 6 has the highest values. It comprises the increased high amount of MCNTs — 10 vol % and MYA additive in the amount of 0.5 vol % The fracture toughness of specimen No. 6 exceeds the fracture toughness of pure silicon carbide almost twice, and its flexural strength is 1.9 times higher. One of the main tasks was to obtain a dense sintered material with a high level of strength. Even when using such a high-tech method as SPS, it is not possible to obtain a material with a density close to theoretical without the use of additives. The introduction of MWCNTs allows increasing crack resistance, while the strength and density of the material are slightly reduced. On the other hand, the introduction of sintering oxide additives, even without MWCNTs, improves the characteristics of the sintered material. However, only the combined use of additives and a reinforcing component allows one to obtain dense and durable composite samples with a high level of crack resistance. It should be noted that the weight of the composite material practically does not increase, since the high mass of oxides is balanced by very light MWCNTs. By comparison of samples of SiC +10 vol % MCNTs (sample № 2) and SiC + 10 vol % MCNTs +0.5 vol % MYA (sample № 6), one can see that the flexural strength increases by almost 2.5 times, the fracture toughness and microhardness value also increase, due to the introduction of only 0.5 vol % MYA. For the maximum realization of the
Table 1 Mechanical properties of the samples. №
Material
Load (g)
Microhardness (GPa)
Density (g/cm3)
Fracture toughness (MPa·m1/2)
Flexural strength (МPа)
1 2 3
SiC SiC + 10 vol % MWCNTs SiC + 0.5 vol % MYA SiC +1 vol % MWCNTs +0.5 vol % MYA SiC+6 vol % MWCNTs +0.5 vol % MYA SiC+10 vol % MWCNTs + 0.5 vol % MYA
300 300 300
32.8 23.7 30.3
2.96 3.09 3.16
3.77 4.26 4.84
180 137 220
300 300 300
35.4 33.8 31.9
3.20 3.23 3.22
5.12 5.75 6,24
190 290 345
4 5 6
5
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- features of the sintering additive, 2 - features of the sintering method, 3 - distribution of MWCNTs in thin layers of the liquid oxide phase formed during sintering.
[10]
4. Conclusions
[11]
The silicon carbide composite material, reinforced with various concentrations of multi-walled carbon nanotubes (0, 1, 6, 10 vol%) together with the additive of 0.5 vol % based on organomagnesiumoxane-yttriumoxane-alumoxane oligomer was obtained by spark plasma sintering. Synthesized activating additive generates a triple oxide eutectic system (Al2O3 = 77.88 mol. % - Y2O3 = 12.09 mol. % MgO = 10.03 mol. %) during heat treatment. The SiC content in the obtained samples is not less than 89.5 vol %. The fabricated samples of the SiC composite have high enough mechanical properties at a low content of sintering additive, which is seen by a comparison with other works. Introduction of sintering additive MYA in the amount of 0.5 vol % significantly improves the properties of the composite. The introduction of sintering additives in small quantities, on the one hand, allows to preserve the unique properties of silicon-carbide ceramics, in particular, the high temperature of products operation, and, on the other hand, the quantity of these additives allows to improve the sintering process. Thus, the paper demonstrated the preparation of strong and crackresistant material with a density close to theoretical for silicon carbide (3.21 g/cm3). A small amount of additives allows to achieve the properties of the composite, primarily high-temperature strength, and resistance to thermal shock, will be close to the outstanding properties of pure silicon carbide. There are further prospects for achieving higher characteristics by adjusting the amount of sintering additive and the ratio of SiC –MYA- MWCNT in the composite.
[12]
[13]
[14]
[15]
[16]
[17]
[18]
[19]
Declaration of competing interest [20]
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.
[21]
Acknowledgments The work was supported by the Russian Foundation for Basic Research, project no. 17-03-00331 A.
[22]
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