Synthesis and characterization of high-strength ceramic composites in the system of potassium titanate – Metallurgical slag

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CERAMICS INTERNATIONAL

Ceramics International 41 (2015) 13294–13303 www.elsevier.com/locate/ceramint

Synthesis and characterization of high-strength ceramic composites in the system of potassium titanate – Metallurgical slag A.V. Gorokhovskya,b, J.I. Escalante-Garciac, E. Sanchez-Valdesc, I.N. Burmistrova,b,n, D.V. Kuznetsova National University of Science and Technology “MISiS”, 119049 Moscow, Russia b Yuri Gagarin State Technical University of Saratov, 410054 Saratov, Russia c Centre for Research and Advanced Studies (CINVESTAV), 25000 Saltillo, Mexico

a

Received 1 December 2014; received in revised form 24 June 2015; accepted 19 July 2015 Available online 29 July 2015

Abstract Ceramic composites based on different mixtures of the powdered slags from silicomanganese manufacturing and synthetic potassium titanates, were produced and investigated. Green bodies were fabricated by uni-axial compression at 35 and 196 MPa. The sinters were obtained by onestage (1100 1C/1 h) or two-stage (800 1C/1 h and 1100 1C/1 h) thermal regimes. The best mechanical properties were achieved for conditions of 33 wt% of slag, compressed under 196 MPa and treated by the two-stage regime of sintering. Structural investigation of the processes taking place in the produced ceramic materials was carried out by scanning electron microscopy and X-ray diffraction. The resulting products were characterized by a gradient phase composition constituted by perovskite, leucite, potassium hexatitanate, pyrophanite-ilmenite solid solutions, hollanadite-like and pyroxene-like crystalline phases as well as glassy phase of varied chemical composition. The mechanism of the sintering processes was also characterized. & 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Keywords: Slag; Potassium polytitanate; Sintering behavior; Multi-phase ceramic composites; Mechanical strength

1. Introduction High strength ceramics have many applications related to structural, refractory and friction materials. However, conventional ceramic materials of this type are commonly produced by using expensive raw materials, i.e. fine powders of different carbides and nitrides, yttrium stabilized ZrO2, high purity Al2O3, as well as by means of lengthy and complicated sintering processes [1–3]. From this point of view, the development of alternatives to produce high-strength and low cost ceramic composites for high temperature applications is to be considered very important. It is known that glass–ceramic systems are very attractive to produce ceramic composites due to low porosity depending on n Corresponding author at: National University of Science and Technology “MISiS”, 119049 Moscow, Russia. Tel.: þ7 8452 99 87 00. E-mail address: [email protected] (I.N. Burmistrov).

http://dx.doi.org/10.1016/j.ceramint.2015.07.112 0272-8842/& 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

the high fluidity of glass-forming melts contained in the raw materials mixture or appeared during the sintering. Complex matrix–reinforcement interactions, especially for ceramic fiber-reinforced glasses, induce the cracks deviating or absorbing fracture energy; that is why, the combination of powdered glasses and ceramic whiskers is especially promising to produce high-strength ceramic composites [4–9]. Nonetheless, the best reinforcement of ceramic composites produced on the base of a glassy phase containing raw materials requires expensive hot-pressing techniques [6] and it requires the search for alternative solutions. On the other hand, fiber-shaped potassium tetra-, hexa- and octa-titanates (K2Ti4O9, K2Ti6O13, K2Ti8O17, respectively) are considered as attractive reinforcing additives for matrices of diverse nature (polymers, metals, and ceramics) due to its relatively low cost, low friction coefficient and thermal conductivity, as well as high mechanical properties and excellent reflectance in the near infra-red region of radiation [10–12].

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However, the use of whiskers is limited due to their high cost, health risks during handling and difficult homogenization within the matrix. Some variations of the technological processes, which allow the manufacturing of relatively cheap modifications of amorphous potassium titanates named polytitanates (PPT) and characterized by varied TiO2/K2O molar ratios, were developed during the last decades (patents: EP1170257, US6036938, US6579619, RU2326051, RU2366609). This kind of product is characterized by particles of non-fibrous (safety) shape of; moreover, further thermal treatment of the latter allows obtaining whiskers-like crystalline potassium titanates. The described concept would allow using the potassium polytitanate precursors as additives to produce materials that require thermal processing, i.e. sintering ceramic composites without the health risks associated to the handling/manufacturing, as the fiber-shaped crystals are formed in situ the ceramic body during the heat treatment. Glass–ceramic or ceramic composites based on the use of potassium titanate precursors and produced by powder technology, have not been well investigated so far. An investigation on ceramic composites of K2Ti6O13 and bioactive (AW) glass–ceramics [13], reported that the obtained materials had improved fracture toughness; however, the bending strength of the composites was lower than that of the monolithic A-W glass–ceramics. The same effect was noted in a publication on the synthesis of glass–ceramic composites in the system based on the mixtures of calcium– silicate–borate bioactive glass and potassium polytitanate [14]; which reported that the chemical interaction of the potassium titanate particles with molten glass promoted crystallization of new phases containing the components of both raw materials (CaTiO3 in this case). From this point of view, the selection of suitable systems based on acceptable combinations of powdered glasses and potassium polytitanate precursors, which, during the sintering, could generate high-strength and refractory crystalline phases by interaction of the components, is a promising route to develop high-strength ceramic composites working at high temperatures. For example, during the sintering, the interaction of the potassium polytitanate (source of K2O and TiO2) and any cheap vitreous raw material, such as metallurgic slag (source of Al2O3, SiO2, CaO and MgO), could promote the formation of high temperature melting phases such as tialite (Al2TiO5), leucite (KAlSi2O6), kalsilite (KAlSiO4) and perovskite (CaTiO3), together with high-strength crystalline whiskers-like potassium titanates. Raw material mixtures based on the combination of cheap powdered metallurgical slags (0.04–0.06 USD/kg) and not so expensive potassium titanates (3–4 USD/kg) could be considered as reasonable alternative to the relatively expensive compounds of nitrides and carbides (3–6 USD/kg), Y2O3 stabilized ZrO2 (10–100 USD/kg) and Al2O3 nanopowders of high purity (10–12 USD/kg); moreover, the proposed route does not involve the complicated and expensive technologies used to produce advanced ceramics based on abovementioned synthetic powders [15]. Such statement could be only reasonable if the ceramics based on PPT-slag raw material mixtures are characterized by

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better thermo-mechanical properties. High strength ceramic composites would be very attractive in the manufacturing of some advanced high-temperature employable ceramic articles for parts of automotive engines, electric and electronic devices, shielding materials for nuclear fusion reactors, tools for melting furnaces of non-ferrous alloys, etc. [16]. The main objective of this work was to investigate the sintering behavior of green bodies obtained with different mixtures of potassium polytitanate precursors and metallurgical slag of silicon manganese production, aiming to obtain ceramic composites of high thermo-mechanical properties. 2. Experimental procedures Previous work [17,18] showed that in spite of different crystallinities, the potassium polytitanates having [TiO2]/[K2O] molar ratios varying within the range of 5–7 were characterized by similar crystallization behavior including completed amorphization at 500–700 1C, following partial transformation into K2Ti4O9 at 900–1000 1C and crystallization of fibershaped K2Ti6O13 near the melting point of the potassium tetratitanate (1104 1C). This work was carried out with two kinds of the potassium polytitanates (PPT), i.e. in the quasi-crystalline (PPT) and amorphous (PTK-6) forms. To produce the quasi-crystalline PPT in the laboratory conditions, the mixtures of raw materials contained (in wt%): powdered TiO2 (20), KOH (20) and KNO3 (60) (all 99% þ purity, Aldrich); these were thermally treated in an alumina crucible at 500 1C for 2 h [17]. The synthesized product was carefully washed with distilled water, filtrated with a Whathman No 42 paper filter and finally dried at 60 1C for 2 h. The obtained PPT powder had a molar ratio of [TiO2]/ [K2O] ¼ 5.25, an average diameter of particles of 5 μm, and a strongly distorted lepidocrocite-like structure [17–19]. The Powdered amorphous potassium polytitanate was a commercial product (PTK-6 trade-mark, Nanocomposite Ltd., Russia) with an average size of 3 mm (agglomerated platy nanoparticles), produced by a thermo-mechanical method using the Ti (OH)4–KOH–H2O raw materials mixture and had chemical composition with a molar ratio of [TiO2]/[K2O]=5.33. The initial experiments were carried out using both of the potassium polytitanate precursors (PPT and PTK-6). The sintering behavior of the raw material mixtures, the phase composition and structure of the ceramic products obtained using both kinds of the potassium polytitanates were found to be identical. Thus, further research work, related to the production of the specimens to measure the exploitation properties, was carried out made with the relatively cheap commercial potassium polytitanate PTK-6. In order to identify the influence of possible variations in the slag chemical composition, two kinds of the slag of silicon manganese production (reported in Table 1) were applied as raw materials in the initial series of the experiments. Each slag was ball milled for 2 h resulting in a particle size distribution in the range of 3–100 μm (average of 35 μm). The slags No 1 and No 2 were produced in different plants (Minera Autlan C. A. , Mexico and Metallurg Scopin, Russia, respectively) and

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Table 1 Chemical compositions of the slags of silicon manganese production used in the experiments. Slag no

No 1 No 2

Content of the oxide (wt%) Na2O

K2O

CaO

MgO

MnO

Al2O3

Fe2O3

SiO2

SO3

0.7 0.4

0.6 2.4

18.6 12.5

12.8 10.0

15.0 18.8

8.1 10.5

2.0 1.8

40.2 41.8

2.0 1.8

The compressive strength (σc) of the sintered samples was evaluated by a hydraulic press (Controls model 65-L1301) with a cell of 15 kN following ASTM C773-88 standard test method and using cubic (1  1  1 cm3) specimens. The bending strength (four-points testing, σb) was estimated in accordance with ASTM C1161-94 for the specimens of 4  3  45 mm3 using a hydraulic press (MTS 810) with a cell of 10 kN and calculated from the equation: σ b ¼ 3Px=4W h2 ;

had some differences in their chemical composition; slag No 1 had higher [CaO] and [MgO] but lower [MnO] and [Al2O3] as well as a slightly higher content of the crystalline phase. The main series of experiments carried out to define an optimal regime of sintering was performed using the slag No 1. An additional series of the specimens based on the slag No 2 and characterized with the same weight ratio of slag: PTK-6, was implemented and investigated using the optimal experimental conditions. The thermal behavior of raw materials was characterized by differential scanning calorimetry (DSC, BAHR STA503 and STA 449 F3 Jupiter) on powdered samples of 20 mg heated at 10 1C/min. Mixtures of raw materials were prepared with different slag contents of 0, 33, 50, 66 and 100 wt%, these were homogenized by joint milling for 30 min in a porcelain container with alumina balls; 2% of powdered dextrin was added as a lubricant. The obtained mixtures were used to produce green body tablet shaped specimens by compression in stainless steel molds using static pressures of 35 or 196 MPa; the dimensions varied depending on the requirements of various standards used for the characterization. The green bodies were treated in an electrical furnace with either one of two regimes: 1) Regime I, one stage, heating at 5 1C/min up to the sintering temperature of 1100 1C, soaking for 2 h and cooling down to room temperature at 10 1C/min; 2) Regime II, two stage, heating up to 800 1C at 10 1C/min and soaking for 1 h, further heating up to 1100 1C at 10 1C/min and soaking for 1 h, then cooling down to room temperature at 10 1C/min. The Regime I, designed based on previous research [17], demonstrated that thermal treatment of compacts based on potassium polytitanate at 1100 1C allowed sintering of high strength materials with a structure comprised by interlaced fiber-shaped crystals of potassium hexatitanate. The Regime II was established considering the DSC results obtained in this research. The phase composition of samples was determined by X-ray diffractometry (XRD, Philips, Model X´Pert- MPD); CuKα radiation with a nickel filter was used operating 40 kV and 100 mA. Powdered samples were scanned in continuous mode at 10–601/2θ at a rate of 0.021θ/s. The reflection positions and relative intensities were compared to the International Center for Diffraction Data (ICDD-2008). The morphology of the products and their chemical compositions were determined by Scanning Electron Microscopy (SEM, Philips XL30ESEM) equipped with an energy dispersive spectrometer (EDS, EDAX Pegasus) for microanalysis.

where P is the applied load until fracture, x is the distance between the support points (40 mm), W and h are the thickness and height of the ceramic rods. 10 specimens were tested for each of the tests, these were produced with Accutom precision cut-off machine and grinding-polishing machine Tegramin-20. To measure resistance to thermal shock, six ceramic specimens (4  3  45 mm3) from each group of composites were heated in an electrical furnace up to 1100 1C and then immersed in water at 23 7 2 1C (heating–cooling cycle). The number of cycles required to cause failure was recorded for each composite. Those specimens that survived more of 15 heating–cooling cycles were then tested to examine bending strength degradation due to the thermal shock. The water resistance (WR) was investigated by a powder method (Russian standard GOST 10134-62). 10 g of powdered material with particle size fraction of 0.1–0.3 mm was treated at 98 1C in 100 ml of distilled water for 5 h. The results were expressed as the average value of weight percentage losses after three repetitions. The apparent density of sintered specimens was measured by the pycnometric method; the density was measured again after the samples were crushed and milled to a size below 35 μm. The density differences before and after milling were used to estimate the percentages of closed porosity (P), through the relation [18] P ¼ 100  ðd m –do Þ=d o ; where do and dm are the densities of the sinters before and after milling respectively. The experimentally associated error to these measurements was evaluated as 7 0.15%. 3. Results The investigation of the thermal behavior of the compacted raw material mixtures indicated (Fig. 1) that the slag had a temperature of glass transition Tg  715 1C (slag No 1) and  698 1C (slag No 2). The slag phase composition consisted of a vitreous phase (slag No 2) or vitreous phase with a small amount of forsterite (Mg2SiO4, slag No 1); at 900–1000 1C the slags crystallized forming pyroxene-like solid solutions with some remaining forsterite (compositions containing the slag, Fig. 2). The endothermic peak at 1100 1C probably appears due to formation of eutectic composition between forsterite and crystalline phases (including pyroxenes) that were formed in the interval 900–1000 1C. Both of the potassium polytitanates used did not show significant thermal effects up to the melting point of crystalline

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Fig. 1. DSC curves for the raw materials used to produce the ceramic composites.

K2Ti4O9, (1104 1C, Fig. 1), similar to the results of our previous research [18]. The compressive strength of sinters based on the slag No 1 produced from green bodies fabricated at 35 MPa in the main experimental series (slag No 1 and PTK-6), was low (Table 2). The resulting phase composition (Fig. 3) could be considered as a mixture of crystalline phases that would form in sinters based on pure raw materials (Fig. 2), i.e. pyroxene and forsterite from the slag particles, and K2Ti6O13 from the potassium polytitanate particles; a small amount of leucite was identified as a result of some interaction between the raw materials. On the other hand, the sinters based on green bodies fabricated at 196 MPa displayed very attractive mechanical properties which did not depend on the kind of the metallurgical slag used in the batch (Table 2); nonetheless, these depended on the regime of the thermal treatment and fraction of the raw materials used. The composites produced by Regime I showed acceptable but not so high mechanical strength due to a high porosity and the absence of a continuous ceramic structure, similarly to the sinters from green bodies fabricated at 35 MPa (micrograph in Fig. 3). Figs. 4 and 5 shows micrographs of composites fabricated by Regime II; the produced materials showed multiphase structures characterized with areas of different phases and chemical compositions. The areas located along the arrow pointed in Fig. 4 (composition based on 33% slag content in the batch) are further magnified and shown as the micrographs in Fig. 5. Fig. 6 presents the X-ray diffractograms of various sinters as a function of the raw material composition and regime of the thermal treatment. Under Regime I, the composites showed similar phase composition, regardless of the slag content; four principal crystalline phases were noted: perovskite, pyrophanite-like, pyroxene-like and leucite, the relative intensity of their reflections depended on the slag contents. Increased slag contents promoted higher contents of pyrothanite and reduced contents of perovskite; the contents of pyroxenes and leucite remained fairly constant. For specimens treated under Regime

Fig. 2. XRD patterns of the raw materials before and after their thermal treatment at 1100 1C for 1 h (marked as (T)). A – slags (No 1 and No 2), B – potassium polytitanates (laboratory grade PPT and commercial PTK-6).

II, the slag content had a strong influence on the phase composition. 33% slag content resulted in composites with perovskite and hollandite-like as the main crystalline phases, with a small amount of the potassium hexatitanate and pyroxenes. For 50% slag, the composites had pyrophanitelike crystals as the main crystalline phase; leucite, perovskite and hollandite-like phases also formed. On the other hand, the mixtures with 66% slag resulted in perovskite and pyroxenelike crystalline phases as the main crystalline components, altogether with leucite and pyrothanite. The EDS analysis of various crystals present in different areas of the obtained composites, allowed describing the distribution of the crystalline phases in the structures reported in , as follows. The bright areas contained mainly perovskite, regardless the slag content; for 33% slag these areas also contained hollandite-like crystalline phase and some quantity of potassium hexatitanate. The crystals of leucite and pyroxene-like crystalline phase appeared in dark colored areas; whereas the pyrophanite-like crystalline phase formed mainly in the border of bright and dark areas.

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Table 2 Properties of the investigated ceramic composites (basic compositions produced using different combinations of the amorphous PTK-6 and slag No 1 fabricated at 35 and 196 MPa and their analogs obtained with quasi-crystalline PPT and slag No 2). The errors are reported as standard deviation. Property

Apparent density (g/cm3) Porosity (%)

Compressive strength (MPa)

Bending strength (MPa)

Thermal shock resistance (1100–23 1C cycles) Water resistance (weight losses) (%)

Regime

Content of the slag in the raw materials mixture (wt%) 0

33

50

66

100

II I II I I* II II þ II I I* II II þ

3.1970.07 3.1070.06 2.27 0.1 4.77 0.1 n.d. 8957 65 9177 74 8987 57 3927 61 n.d. 1457 15 1337 11

2.997 0.02 2.757 0.03 4.670.2 6.970.2 11.17 0.2 11207 83 11117 97 10887 64 583767 85721 203716 196713

2.8370.03 2.6170.03 6.570.3 8.670.3 12.170.3 992742 981755 998741 480754 62718 156712 146715

2.9670.02 2.6670.02 7.7 70.3 8.2 70.4 13.270.4 675 763 659 749 669 731 385 752 55 715 n.d. n.d.

2.937 0.02 2.897 0.03 8.67 0.5 9.77 0.5 n.d. 1927 21 1897 18 1817 15 1237 32 n.d. 497 7 n.d.

I II I

415 1.3070.03 1.3770.03

415 0.957 002 1.107 0.03

415 0.6270.03 0.7970.02

6 0.5870.02 0.7770.02

1 0.307 0.03 0.337 0.03

n

Green bodies fabricated at 35 MPa. Green bodies fabricated using quasi-crystalline PPT powder. # Green bodies fabricated using commercial PPT powder and powdered slag No 2. þ

Fig. 3. Micrograph (500  ) and diffractogram of the composite (50% of the slag No 1) fabricated by loading at 35 MPa and sintered with Regime I.

Table 3 presents the data on chemical composition of glassy phase in the dark local areas from composites sintered under Regime II. In the glass–ceramic parts of dark areas, the glassy phase had relatively low TiO2 contents (less of 10%) and high

[SiO2] at 33–45%. Its chemical composition included all other principal oxides from both raw materials. The crystalline fraction of these areas was conformed of leucite (more typical for the external part of dark areas) and pyroxenes (usually located in the central part of these areas). The EDS data from some crystals of the phase identified as pyrophanite-like, showed the presence of Fe and Mn with an atomic ratio of [Fe]:[Mn] varied in different crystals from 0 to 12 and could be considered as ilmenite-pyrophanite solid solution (Mn,Fe)TiO3. The simultaneous use of the EDS and XRD methods also allowed identifying the pyroxene-like phase of complex chemical composition: XY(Si,Al)2O6 (X ¼ Ca, Mg, Na, K; Y ¼ Al, Fe, Ti, Mn). For the hollandite-like crystals, EDS indicated the presence of some MnO, K2O, MgO and TiO2, and, taking into account the ICDD database, the proposed composition can be considered as K1.54Ti7.23(Mn,Mg)0.77O16, where the molar ratio of [Mg]/[Mn] varies in the range of 0.7–6.3. The influence of the slag contents on some exploitation properties of the produced ceramic composites, fabricated at 196 MPa, is reported in Table 2. Regardless of the regime, the use 33–50% of the slag showed better mechanical strength than the compacts based on pure potassium polytitanate. Increasing the slag content up to 66% reduced the mechanical strength; nonetheless, the measured values were significantly higher than those from the material produced with 100% slag. On the other hand, an increase of the slag amount in the raw material mixtures decreased the apparent density and increased water resistance and porosity of the obtained glass–ceramic composites. It is noteworthy that the sinters produced using the mixtures of the commercial potassium polytitanate and both slags (No 1 and 2) were characterized by very similar compressive

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Fig. 4. Scanning electron microscoscopy microstructures (250  ) of the composites produced with different slag contents (slag No 1, Regime II) and fabricated using 196 MPa.

strengths (Table 2). This suggests a weak influence of the chemical composition of the slag on the sintering mechanism taking place in the green bodies during their Regime II (twostage treatment) sintering. Such conclusion was also confirmed by XRD data; the differences in the chemical composition among slags No 1 and No 2 did not significantly influence the phase composition of the sinters using with the same ratio slag: PTK-6 (Fig. 7). The slag No 2 showed reduced [CaO] and

Fig. 5. Scanning electron microstructures (4000  ) from composites obtained using 196 MPa and Regime II. The order of micrographs corresponds to the motion along the arrow marked in Fig. 4 (from PPT to slag particles location).

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Fig. 7. X-ray diffractograms for the sinters obtained with 33% slag No 1 and No 2 using Regime II.

4. Discussion

Fig. 6. XRD patterns of the ceramic composites produced with different slag contents (marked in wt%) and sintered by different thermal regimes.

Table 3 Chemical composition (EDS data) of the vitreous phase in the dark areas of the ceramic composites produced by Regime II (Fig.4) and based on the mixtures of slag No 1 and commercial PPT. The data is reported as a range of variation determined by 8 measurements in different dark areas. Oxide

TiO2 K2O Na2O CaO MgO MnO Al2O3 Fe2O3 SiO2 SO3

Slag content (%) 33

66

2.6–6.4 9.1–11.2 1.3–1.6 7.5–11.6 7.3–8.4 8.9–10.7 9.5–10.3 2.5–2.7 41.4–42.9 1.9–2.2

0–2.1 0.6–9.2 0.8–1.1 16.9–18.4 6.9–11.9 10.2–12.4 10.3–10.5 0.7–1.8 40.7–41.8 1.8–1.9

increased [MnO] and [Al2O3] (Table 1); therefore the resulting sinters showed slightly lower [CaTiO3] and higher contents of pyrophanate, leucite and pyroxene, while the hollandite remained the main crystalline phase. In any case, the results reported in Table 2 showed that the Regime II (two-stage treatment) promoted ceramics with higher mechanical strength, lower density and lower porosity, in comparison with those materials obtained by sintering with Regime I (one-stage treatment).

The results showed that the fabrication of the green bodies required relatively high compression (196 MPa) in order to obtain ceramic composites of high mechanical properties. The low compacting pressure of 35 MPa resulted in high porosity of green body and only promoted a limited PPT-slag interface. As a result, the interaction between two kinds of raw material particles during the thermal treatment was limited too; the sinters did not have a continuous matrix and were characterized by the presence of crystalline phases typical for ceramic products based on the pure raw materials (Fig. 3) as well as with high porosity and low mechanical properties (Table 2). However, increasing the pressure up to 196 MPa favored obtaining more compact green bodies and promoted intensive chemical interaction among the raw material particles during the sintering. The sinters thus obtained had a much more dense structure, regardless the thermal regime applied; the phase composition was different relative to the crystallization of pure raw materials and the mechanical properties were very attractive. Nevertheless, there is some difference in the sintering processes taking place by applying the one-stage and twostage regimes of the thermal treatment (Regimes I and II respectively), which promote difference in the structure and properties of the sintered ceramic materials. The one-stage Regime I promoted some peculiarities in the sintering behavior of the green body compacts. It is well known that the powdered slag has a trend to surface crystallization [20–23], therefore, the fast heating up to 1100 1C favored intensive crystallization of the surface layer in the slag particles. For that reason, the melt forming at T 4715 1C in the slag particles by melting of glassy phase limited the possibility of its interaction with the potassium titanate particles to dissolve them and promote following formation of pyrophanite, perovskite and hollandite-like crystals, which bear the components of both raw materials (Ca and Ti; Mn, Mg and K, Ti; respectively). At high temperature, such contact conditions would promote only a weak diffusion of ions (Mn2 þ , Mg2 þ from the slag particles, and K þ from the potassium polytitanate) which supports the crystallization of

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leucite (KAlSi2O6) in the molten slag particles, while ilmenitepyrothanite (Mn,Mg,Fe)TiO3 solid solutions and perovskite crystals (CaTiO3) appear in the border of the liquid phase from the slag particles and potassium titanate. In the ceramic composites obtained by the Regime I the flow of K þ from the PPT into the molten slag is limited due to the fast formation of a ceramic layer consisting of the abovementioned high melting crystalline phases (Tm.p. 41800 1C) located near the border of the raw material particles. That is why; the bright areas of the sinters obtained by the Regime I are characterized with relatively high [K2O], which does not promote formation of hollandite-like crystals. At the same time, the pyroxenes crystallized in the slag particles zone, independently of the diffusion flows. On the other hand, taking into account the DSC data (Fig. 1), it is possible to note that during the first stage of the Regime II (heating up to T1 ¼ 800 1C and soaking for 1 h), such soaking temperature (T1) is higher than that of the glass transition (715 1C) but lower than that of surface and bulk crystallization for the slag (921 and 961 1C, respectively). As a result, due to an absence of significant crystallization processes, a molten glassy phase appeared during the soaking at 800 1C, which intensively dissolved the potassium titanate, forming a new liquid phase of gradient chemical composition. The two diffusion flows are typical for this liquid phase: monovalent K þ cations rapidly diffusing into the central part of

the slag particles supporting, by cooling, the subsequent formation of KAlSi2O6 together with pyroxenes and remaining glassy phase; while the Ca, Mn and Fe- ions slowly diffused in the melt formed in the area of previous location of PPT particles as a result of partial PPT dissolution in the molten slag. This favored obtaining the Ca, Mg and Mn rich areas which tended to crystalize during the second stage of the thermal treatment (soaking at 1100 1C), forming a well crystallized ceramic layer constituted by pyrophanite-like and perovskite crystals (close to the initial location of slag particles) and hollandite-like crystalline phase located at a greater distance from previous slag particles location. At the same time, the central part of the light areas, initially occupied by potassium titanate particles, due to low contents of ions diffused from the silicate melt, can crystallize forming K2Ti6O13. Fig. 8 shows a schematic representation of the phase evolution taking place within the green bodies based on different raw material mixtures thermally treated in accordance with Regime II. The micrographs reported in Fig. 5 confirm this mechanism. Nevertheless, it is possible to note that in the sinters obtained by Regime II, the molten slag particles can be considered as traps to collect K þ ions diffused from the PPT zones. At the same time, the diffusion flows of Ca, Mn and Mg ions into the areas initially occupied by PPT particles favors obtaining a high strength ceramic matrix especially well-developed in the compositions containing 66% of PPT.

Mixture containing: slag 66 %, PPT 33% SiO2 Al2O3

2+

Mg Ca2+ Mn2+

SiO2 Al2O3

TiO2 K+

Mg2+ Ca2+ Mn2+ Raw materials

Pyroxenes Glass

KAlSi2O6 Glass

(Mn,Mg)TiO3 CaTiO3 K1,54Ti(Mn,Mg)Ti8O16

KAlSi2O6 Glass

Pyroxenes Glass

Ceramic composite (product)

Mixture containing: slag 33%, PPT 66% K+ TiO2

SiO2 Al2O3 Mg2+ Ca2+ Mn2+

K+ TiO2

Raw materials K1,54Ti8O16 K2Ti6O13

(Mn,Mg)TiO3 CaTiO3

13301

KAlSi2O6 Pyroxenes Glass

(Mn,Mg)TiO3 CaTiO3

K1,34Ti8O16 K2Ti6O13

Ceramic composite (product) Fig. 8. Phase evaluation in the raw material mixtures characterized with different compositions.

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Thus, the ceramic composites containing 33 and 50% of the slag and sintered by Regime II displayed high thermal and mechanical resistance. It is noteworthy that the variation of the slag chemical composition did not influence exploitation properties of the sinters produced. In comparison with the commercial ceramic materials produced by traditional sintering technologies; the compressive strength of the obtained sinters (990–1100 MPa) is higher than ceramics of alumina porcelain (450–500 MPa) and only lower than ceramics based on yttrium stabilized zirconia and alumina nanoceramics (1500– 2500 MPa). The bending strength of the obtained composites (150–200 MPa) exceeds the typical characteristics of alumina porcelain (40–50 MPa), cordierite and alumina titanate ceramics (50–60 MPa), recrystallized SiC ceramics (90–110 MPa), and is lower only compared to expensive ceramics based on yttrium stabilized zirconia nanoscale Al2O3 of high purity as well as reaction bonded silicon nitride (800–900, 300–350 and 250–280 MPa, respectively) [3,5,6]. The high thermal shock resistance of the obtained ceramics is especially attractive; the materials based on mixtures containing 50 and 66 wt% of PPT reduced their bending strength after 15 heating–cooling (1100–23 1C) cycles only from 146 to 123 MPa and from 196 to 162 MPa, respectively Such thermo-mechanical properties are typical for the previously investigated ceramics based on pure PPT [24]. The structure of this ceramics is formed by mutually interlaced K2Ti6O13 fiber shaped crystals and characterized by high fracture toughness due to a tendency of shear strain deformation. The matrix of the ceramic composites obtained with [PPT]Z 50 wt% had a similar structure (Fig. 5) and promoted a high resistance to micro-cracks propagation. It is known that the properties that affect thermal and mechanical stress resistance are elasticity, strength, coefficient of expansion, Poisson's ratio, and, in some cases, thermal conductivity, diffusivity, or emissivity [3–6,25]. The multiphase ceramic composites produced in this work have a very specific reactively formed structure consisting of different, sometimes mutually combined, crystals of different shapes and properties and include areas with gradient phase composition resulting in gradient elastic characteristics. It should be emphasized that the theoretical models describing thermo-mechanical behavior of such non-homogeneous materials are complicated and the actual stresses that take place in their structure are too complex to calculate. Generally, it is possible to assume that the chemical interaction of the components avoid stress concentration factors which can improve thermomechanical properties. In any case, the mechanism of failure for such materials has not been investigated yet and would be considered as a subject of further research taking into account very attractive mechanical properties shown by the materials obtained in this work. 5. Conclusions 1. The mixtures of slag, from the silicon manganese manufacturing, and quasi-amorphous potassium polytitanate allow producing the high strength ceramic composites (compressive strength of up to 1120 MPa).

2. In order to achieve high strength, the fabrication of ceramic green bodies required pressures of up to 196 MPa. The higher mechanical properties of the ceramic composites in the investigated system can be obtained by two stage sintering. The first stage of sintering at T ¼ 700 1C favors partial dissolution of PPT particles in the molten slag and formation of the melt characterized by gradient chemical composition without the significant crystallization processes. The second stage of sintering including heating and soaking at 1100 1C promotes crystallization of previously formed liquid phase resulting in multiphase ceramic composite containing perovskite, hollandite, pyrophaniteilmenite, leucite and pyroxene-like crystalline phases in different proportions depending the slag contents in the raw materials mixture. 3. The materials characterized with the best mechanical properties can be produced with the mixtures consisting of 33–50% of slag. Their structure has a ceramic matrix consisting of the layers formed by K2Ti6O13, hollandite-like K1.54Ti7.23(Mn,Mg)0.77O16, pyroxene (CaTiO3) and pyrophanite-like (Mn,Mg.Fe)TiO3 solid solutions as well as glass–ceramic inclusions consisting of KAlSi2O6 and pyroxene-like crystals dispersed in the remain glassy matrix. The composites based on the raw material mixtures with slag contents lower than 50 wt% are characterized with glass–ceramic matrix and lower mechanical properties. 4. The variation chemical composition of the metallurgical slag, by using two sources, did not significant influence the structure and properties of the ceramic composites obtained.

Acknowledgment The authors gratefully acknowledge the financial support of the Ministry of Education and Science of the Russian Federation in the framework of Increase Competitiveness Program of NUST «MISiS»(№ К3-2014-066) and financial support of the Ministry of Environment and Mineral Resources of Mexico (Project CONACYT- SEMARNAT-2002-C01-0011). References [1] I.K. Cherian, M.D. Lehigh, I. Nettleship, W.M. Kniven, Stereological observation of platelet-reinforced mullita- and zirconia-matrix composites, J. Am. Ceram. Soc. 79 (1996) 3273–3281. [2] X. Huang, P.S. Nicholson, Mechanical properties and fracture toughness of α-Al2O3-platelet-reinforced Y–PSZ composites at room and high temperatures, J. Am. Ceram. Soc. 76 (1993) 1294–1301. [3] R. Warren (Ed.), Ceramic–matrix composites, Chapman & Hall, NY, 1992. [4] F.L. Matthews, R.D. Rowlings, Composite Materials: Engineering and Science, Chapman & Hall, London, 1994. [5] K.K. Chawla, Ceramic Matrix Composites, Chapman & Hall, London, 1993. [6] A.R. Boccaccini, Glass and glass–ceramic matrix composite materials, J. Ceram. Soc. Jpn. 109 (2001) 99–109. [7] E. Bernardo, G. Scarinci, Sintering behaviour and mechanical properties of Al2O3 platelet-reinforced glass matrix composites obtained by powder technology, Ceram. Int. 30 (2004) 758–791.

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