The kinetics and mechanism of combusted Zr–B–Si mixtures and the structural features of ceramics based on zirconium boride and silicide

Ceramics International 42 (2016) 16758–16765

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The kinetics and mechanism of combusted Zr–B–Si mixtures and the structural features of ceramics based on zirconium boride and silicide Yu.S. Pogozhev a, I.V. Iatsyuk a, A.Yu. Potanin a,n, E.A. Levashov a, A.V. Novikov a, N.A. Kochetov b, D.Yu. Kovalev b a

National University of Science and Technology “MISIS”, SHS Research and Education Centre MISIS-ISMAN, Leninsky prospect, 4, Moscow, 119049 Russia Institute of Structural Macrokinetics and Materials Science, Russian Academy of Sciences, ul. Academica Osipyana, 8, Chernogolovka, Moscow Region, 142432 Russia

b

art ic l e i nf o

a b s t r a c t

Article history: Received 5 May 2016 Received in revised form 14 July 2016 Accepted 22 July 2016 Available online 25 July 2016

The study focuses on investigation of the combustion kinetics and mechanisms, as well as the phase- and structure formation processes, during elemental synthesis of ceramics based on zirconium diboride and silicide doped with aluminum. The effect of the degree of dilution with an inert component and initial temperature T0 on the combustion kinetics of the Zr–Si–Al–B mixture is studied. An increase in T0 in the range of 298–700 K causes a directly proportional rise in the combustion temperature Tc and rate Uc, which demonstrates that staging of the reactions of formation of zirconium boride and silicide remains invariant. The effective activation energy Eeff of the combustion process is 225 kJ/mol, suggesting that the liquid-phase processes have a decisive effect on the reaction kinetics. The interaction of zirconium with boron and silicon runs through the Zr–Si–Al–B melt that is formed in the combustion zone. Staging of chemical transformations during phase and structure formation of SHS products is studied. The primary ZrB2 grains crystallize from the melt in the combustion zone; the ZrSi silicide phase is formed with a delay of no longer than 0.5 s. Compact ceramics with composition ZrB2–ZrSi–ZrSi2–ZrSiAl2 synthesized by forced SHS- pressing showing a great potential for high-temperature applications both as a construction material and as a precursor for ion-plasma deposition of coatings. & 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Keywords: Combustion Kinetics Mechanism Structure formation Zirconium diboride Zirconium silicide

1. Introduction Zirconium diboride (ZrB2)-based composite ceramic is a promising high-temperature construction material for manufacturing aircraft and spacecraft equipment parts that can operate at temperatures above 1500 °C under severe wear conditions. The enabling factors include an extremely high melting point that often is higher than 3000 °C [1,2] and a unique combination of properties: increased high-temperature strength, high hardness, electrical and thermal conductivity, chemical and thermal stability, and relatively low density (
6 g/cm3) [2–11]. Another important factor is that ZrB2 is less expensive compared to its analog with similar properties, hafnium diboride HfB2 [12]. Because of its strong covalent bond, as well as low bulk and grain boundary diffusion in the crystal lattice of zirconium diboride, there is a problem associated with compaction of ZrB2 powder during sintering and hot pressing [13]. Compaction is possible at high pressures and temperatures above 2100 °C [13]. n

Corresponding author. E-mail address: [email protected] (Yu.S. Pogozhev).

http://dx.doi.org/10.1016/j.ceramint.2016.07.157 0272-8842/& 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Additionally, ZrB2 exhibits poor mechanical properties and low resistance to high-temperature oxidation because of ablation of material. Doping with various silicides (e.g., MoSi2, ZrSi2, and TaSi2) enhances the strength of ZrB2-based ceramics by increasing its density and reducing grain size [14–16]. When fabricating ceramics, silicides decrease temperature and pressure exposure duration during hot pressing [17,18] or spark plasma sintering [19]. The main mechanism of protection against oxidation at temperatures above 1000 °C is the formation of SiO2 and ZrO2 barrier layers on the surface of zirconium boride–silicide ceramics, which slows down the diffusion of oxygen into the material [20– 22]. Boron in the ceramic facilitates the formation of B2O3 oxide, which efficiently heals fractures formed in the oxide layer because of the difference between the thermal expansion coefficients during high-temperature oxidation [20–22]. In addition, doping with aluminum is a promising method as it increases the resistance of these ceramics to high-temperature oxidation and corrosion because of the formation of an Al2O3 film [23–25]. ZrB2-based ceramics doped with MoSi2 and ZrSi2 were synthesized by hot pressing at pressures up to 30 MPa and temperatures below 1850 °C [15,16]. The presence of the liquid disilicide phase during hot pressing contributed to the process of shrinkage,

Yu.S. Pogozhev et al. / Ceramics International 42 (2016) 16758–16765

grain ordering, removal of oxides from the surface of ZrB2 particles, and increased packing density of the particles. Depending on the disilicide content (20–40 at%), the relative density of the samples ranged between 94–99%. It should be mentioned that the maximum disilicide concentration was 40 at% because exceeding this threshold significantly deteriorated the mechanical properties of the ceramics because of the formation of large amounts of the SiO2–B2O3 glassy phase at high temperatures [15,16]. Self-propagating high-temperature synthesis (SHS) is one of the promising ways to produce ceramics in the Zr–B–Si and Zr–Si– B–Al systems. However, combustion and structure formation in these systems have not previously been studied. Bertolino [26] investigated combustion of mixtures in the Zr–Si system that included seven compounds; two of those (ZrSi and Zr5Si4) exist in the low- (α) and high-temperature (β) forms. The Zr5Si3 compound is stable only at high temperatures. The main SHS products included ZrSi2, and α- and β-ZrSi. For zirconium-rich reactionary mixtures, the high-temperature phase Zr5Si3 was always the main component of the product. There were no Zr5Si4, Zr3Si2, Zr2Si, and Zr3Si compounds among the synthesis products despite the fact that their formation enthalpies are higher than those of ZrSi and ZrSi2 and do not substantially differ from that of Zr5Si3 [26]. The force SHS pressing technique is used to produce compact ceramics [27]. The features of combustion of the Mo–Si–B mixtures and the mechanisms of phase and structure formation of cathodes/targets for magnetron sputtering of high-temperature coatings [23–25] were studied in [28,29]. This study focuses on investigations of the kinetics and mechanisms of combustion during elemental synthesis of ceramic materials in the Zr–Si–Al–B system and the processes of phase and structure formation of products for high-temperature applications.

2. Materials and methods Commercially available zirconium powders (PTsrK-1 grade) with an average grain size of
10–15 mm; ASD-1 aluminum (dispersity of
50 mm); and amorphous black boron (grade B-99A) with an average grain size of
0.1 mm were used as initial components of the reaction mixture. Silicon powder was prepared by grinding KEF-4.5 monocrystals (orientation 100) followed by sifting fractions with grain sizes smaller than 45 mm. The granulometric composition of zirconium powder includes 85% of the fine fraction (grain sizes up to 20 mm), whereas the remaining 15% are coarse fractions of up to 50 mm in size. Silicon powder is bidispersed and contains virtually equal amounts of fine (up to 15 mm) and coarse (15–45 mm) fractions. Aluminum is polydisperse and contains virtually no particles smaller than 20 mm in size. The reaction mixture was diluted with an inert component (the final product) [30] because of the high temperature (over 2800 K) and combustion rate (over 5 cm/s) of the mixtures, as well as to prevent aluminum evaporation. The powdered final product was produced in the following way: a quaternary mixture was prepared, then the SHS reaction was carried out in this mixture, yielding a porous sinter cake, which was subsequently subjected to crushing and milling. The composition of the reactionary mixture was calculated to obtain zirconium diboride and silicide in a 1:1 ratio. Concentration of the inert dilutant (X) was varied within 20–40%. The content of the aluminum dopant was 5.6 wt%, which corresponded to 10 at% and was limited by the application scope of these high-temperature ceramic materials as cathodes for sputtering heat-resistant coatings. At higher aluminum concentrations within the target as it is sputtered in the nitrogen-containing medium, aluminum nitride AlN with a hexagonal crystal structure is formed in the

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coating, which intensively oxidizes in air at temperatures above 700 °C, yielding N2 and volatile nitrogen oxides NO, N2O, N2O3, and NO2 [31]. The powders were mixed in a stainless steel ball mill using hard-alloy grinding bodies. Wet mixing in isopropanol was used to prevent spontaneous ignition of zirconium powder and brushing of aluminum. The final mixtures were dried in a vacuum drying oven at 50 °C. The macrokinetic parameters of the combustion process were studied in a laboratory-scale SHS reactor according to previously reported procedures [28,30] using cylinder-shaped samples (10 mm in diameter and 18 mm in height, with 60% relative density). The combustion temperature (Tc) was measured using W–Re thermocouples (Ø 100 mm) inserted into holes in the sample. The combustion rate was measured by high-speed video recording using a Panasonic WV-BL600 video camera at 15-fold magnification. The phase composition of the combustion products was determined by X-ray diffraction (XRD) analysis using monochrome Cu-Kα radiation. Pointwise recording (in step-by-step scanning mode) was used within the range of angles 2θ ¼10C110° with an exposure time per point of 4 s The sequence of phase transformations in the combustion wave was studied by dynamic X-ray diffraction [28,32] using an LKD-41 one-coordinate position-sensitive detector. The XRD patterns were recorded within the 2θ angle range of 24–62°. The exposure time was 0.5 s. The experiments were carried out in helium atmosphere. The dynamics of structural transformations in the combustion wave were studied using the method of stopped combustion front (SCF) by quenching in a copper wedge [27,30] followed by scanning electron microscopy (SEM) and electron microprobe analysis using a Hitachi S-3400N microscope equipped with a NORAN energy-dispersive X-ray spectrometer.

3. Results and discussion Fig. 1 shows the experimental dependence of the combustion temperature (Tc) and rate (Uc) on the initial temperature (T0) of the

Fig. 1. Dependence of the combustion temperature Tc (a) and rate (b) of the mixtures with different degrees of dilution with the final product on the initial temperature T0.

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process for the mixtures diluted with the final product at X¼ 20– 40%. The experimental Tc value for the undiluted mixture at T0 equal to room temperature was 2803 K, which is higher than the recommended measurement range of a W–Re thermocouple. Contrariwise, the Tc value measured using a radiation pyrometer was to some extent underestimated (2394 K) because of the loss of radiation as it passed through the quartz glass of the reaction chamber. For the mixtures diluted with the final product, an increase in the initial temperature in the range under study results in a directly proportional rise in Tc and Uc, which indicates that the combustion mechanism and the stages of the chemical reactions with the formation of zirconium boride and silicide are the same. An increase in T0 of 400 K leads to a Tc increase of 150–200 K in average. Uc is the parameter most sensitive to changes in T0 [30]; the Uc values increase threefold with rising T0. This dependence is more pronounced for the composition with dilution X ¼20%. The linear shape of the Tc(T0) and Uc(T0) functions is typical of SHS processes that are not accompanied by complete melting of the products. However, the resulting dependences do not inform the mode under which these reactions occur (i.e., whether it is the attached mode when the chemical reactions run parallel or the detached mode when the reactions are separated in time [28,33]). If the reactions run in the detached mode, the thermograms contain several heat release peaks. Fig. 2 shows the combustion wave temperature profiles for the minimally diluted mixture with X¼ 20%. They have similar shapes. All thermograms contain only one temperature peak regardless of T0, which indirectly indicates that parallel reactions of zirconium with boron and silicon occur. The results of semi-quantitative XRD analysis of the samples synthesized in the laboratory-scale SHS reactor are summarized in Table 1. ZrB2 content decreases as the degree of dilution rises from 20 to 40%, whereas the total content of the silicide phases ZrSi and ZrSi2 remains virtually constant, although the ratio between the silicide phases changes. Hence, a rise in X leads to an increase in ZrSi concentration from 28 to 49% and a decrease in ZrSi2 content from 20 to 6%. For a mixture with dilution X¼40%, there is no ZrSi2 phase among the products when T0 is increased to 674 K. It is most likely that at higher Tc values the saturation of ZrSi with silicon from the zirconium-poor melt takes more time. Furthermore, the formation of a ternary compound ZrSiAl2 was observed at higher X values, although an increase in T0 had virtually no effect on the phase composition of the synthesis products. The effective activation energy of the combustion process Eeff was determined by plotting the semi-log graph showing the combustion rate as a function of combustion temperature: ln(Uc/

Tc) as a function of 1/Tc [30]. The Eeff value was calculated using the formula Eeff ¼2Rtgα, where R is the universal gas constant, whereas tgα was determined graphically (Fig. 3). Based on linear approximation, Eeff ¼225 kJ/mol, indicating a significant influence of the processes taking place in the melt on the kinetics of combustion. The resulting data indicated that the interaction of zirconium with boron and silicon occurs through the eutectic Zr–Si melt (Tmelt ¼ 1370 °C) and that primary crystallization of the ZrB2 phase starts in the oversaturated melt. If the eutectic Al–Si (Tmelt ¼ 577 °C) is present, the aluminum increases the volume of the Zr–Si–Al melt. The dynamics of phase transformations in the combustion wave were studied by dynamic X-ray diffraction using the mixture not diluted with the final product (X ¼0) at T0 ¼ Troom as an example. Fig. 4 shows the results as a selective sequence of X-ray diffraction patterns recorded during real-time combustion of a sample. In the range τ ¼0.0–0.5 s, the XRD pattern contains only lines belongs to the initial reagents (Fig. 4a). Peaks belonging to aluminum disappear immediately after the combustion front passes through at τ ¼1.0 s (Fig. 4b), which indicates that aluminum has melted; the intensities of Zr and Si lines decrease almost twofold. Fine fractions of Zr and Si powders probably melt at this time. Meanwhile, the first lines belonging to zirconium diboride ZrB2 corresponding to the crystallographic planes (001) and (101) emerge, indicating that the reaction of formation of zirconium diboride from the Zr–Si–Al melt has started, although the fraction of this melt is small at this point. E.A. Levashov et al. [28] demonstrated that the characteristic dissolution time of a 0.1 mm boron particle in a Si or Al–Si melt is extremely short (10  6 s). The solid-phase reaction between zirconium and boron at such a low activation energy Eeff ¼ 225 kJ/mol is unlikely. The lines corresponding to zirconium silicide (ZrSi) appear in the XRD pattern after another 0.5 s (Fig. 4c). It should be mentioned that the absence of two heat release peaks on the thermograms (Fig. 2) is caused by the small difference between the adiabatic combustion temperature values (Tcad) of the Zr þ2B and Zrþ Si mixtures. For the first mixture (for the formation of ZrB2) Tfad ¼2570 K, whereas for the second one (ZrSi), Tfad ¼2380 K. In this case, the peak of heat release for the formation of ZrB2 overlaps the peak for the formation of ZrSi, especially as the time interval between them is relatively short. The independent line belonging to Zr (110) is still present at this time point (τ ¼1.5 s), indicating that the largest zirconium particles (with oxidized surfaces) still exist in the solid state. At τ ¼ 2 s, the XRD pattern contains lines belonging to the ZrB2 and ZrSi phases (Fig. 4d). No independent aluminum-containing

T, K

2500 2250 2000 1750 1500 1250 1000 750

T0 = 711 К T0 = 506 К T0 = 293 К

500 250 15

17,5

τ, s 20

22,5

25

Fig. 2. Characteristic combustion wave temperature profiles for the mixture with the degree of dilution with the final product X ¼ 20% at different T0 values.

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Table 1 The results of XRD analysis of the samples synthesized in a laboratory-scale SHS reactor. X, % Combustion parameters

Phase composition ZrSi

ZrB2

20

30

40

T0 ¼ 293 K Tc ¼2268 K Uc ¼ 2.99 cm/s T0 ¼ 711 K Tc ¼2403 K Uc ¼ 8.98 cm/s T0 ¼ 293 K Tc ¼2145 K Uc ¼ 1.56 cm/s T0 ¼ 658 K Tc ¼2323 K Uc ¼ 4.38 cm/s T0 ¼ 293 K Tc ¼1890 K Uc ¼ 0.71 cm/s T0 ¼ 674 K Tc ¼2110 K Uc ¼ 2.40 cm/s

ZrSi2

ZrSiAl2

Mass fraction, %

Lattice parameter, nm

Mass fraction, %

Lattice parameter, nm

Mass fraction, %

Lattice parameter, Mass fracnm tion, %

Lattice parameter, nm

52

A ¼ 3.165

28

20

–

C ¼3.526 A ¼ 3.166

28

–

–

43

C ¼3.527 A ¼ 3.166

42

7

A ¼ 3.894 C ¼9.006

43

C ¼3.527 A ¼ 3.167

43

A ¼ 3.742 B ¼14.621 C ¼3.696 A ¼ 3.742 B ¼14.619 C ¼3.696 A ¼ 3.707 B ¼14.592 C ¼3.737 A ¼ 3.710 B ¼14.570 C ¼3.742

–

53

A ¼3.761 B¼ 9.906 C¼ 3.752 A ¼3.765 B¼ 9.915 C¼ 3.749 A ¼3.768 B¼ 9.926 C¼ 3.754 A ¼3.766

7

A ¼ 3.895 C ¼9.000

40

C ¼3.529 A ¼ 3.166

47

7

A ¼ 3.893

39

C ¼3.527 A ¼ 3.165

49

12

C ¼8.998 A ¼ 3.891

19

8

7

B¼ 9.932 C¼ 3.754 A ¼3.764 B¼ 9.932 C¼ 3.756 A ¼3.764 B¼ 9.932 C¼ 3.754

C ¼3.526

A ¼ 3.746 B ¼14.556 C ¼3.703 –

6

–

C ¼8.993

8,25

Х = 40 %

-ln(Uc/Tc) 8 7,75

α

7,5 7,25 7

Х = 30 %

Х = 20 %

Eeff = 2R×tgα = 225 kJ/mol

6,75 6,5

1/Tc×104, K-1

6,25 4,3

4,4

4,5

4,6

4,7

4,8

4,9

5

5,1

5,2

5,3

Fig. 3. Semi-logarithmic dependence of the combustion rate of Zr–Si–Al–B mixtures at different dilutions with the final product on reverse combustion temperature.

phase has been detected among the combustion products by dynamic X-ray diffraction because aluminum partially evaporates and partially dissolves in ZrSi at Tc ¼ 2800 K (at X¼ 0) [34]. It was found that the diffraction line intensities gradually decrease and the lines shift towards smaller angles when proceeding from the heating zone to the post-reaction zone. This is caused by the thermal effect of crystal lattice expansion in the resulting phases [35]. The results of the experiments on combustion wave quenching followed by SEM and EDS of the characteristic zones of the SCF sample elucidate the dynamics of structural transformations for the mixture with maximum dilution (X¼40%). The combustion front could not be quenched for the samples at X¼20% and 30% even at the minimal angle at the copper wedge apex (3°) because of the high heat release during combustion. The microstructure and phase composition of an inert diluent (the powdered final product) are shown in Fig. 5. Its structure includes faceted ZrB2 grains distributed over a ZrSi ceramic matrix, which is represented by two areas with different aluminum content. According to the EDS analysis, the aluminum content in the

dark gray area is
5.4%, whereas its content in the light gray area is much lower (less than 1%). It should be mentioned that the ZrB2 grains are mainly located in the Al-rich areas, which indirectly confirms the assumption that ZrB2 crystallizes from the Zr–Si–Al–B eutectic melt as zirconium dissolves in it. The synthesis products also contain a small amount of zirconium oxide (ZrO2), which was present in the initial zirconium powder and is additionally formed in the combustion wave by interacting with oxygen impurities [36]. The microstructures of the SCF sample recorded in different regions of the quenched combustion front are shown in Fig. 6. The initial reaction mixture (Fig. 6a,b) contains light-colored zirconium grains up to 20 mm in size, dark gray fragmentary silicon grains up to 50 mm in size, and round-shaped aluminum grains up to 20 mm in size. Highly dispersed boron grains are placed in the dark gray matrix. Zirconium particles dissolving in the Al–Si and Si melts can be seen in the combustion front (Fig. 6c,d). As a result, hardened regions with composition close to that of zirconium aluminosilicide (ZrSiAl2) and the equimolar silicide ZrSi are detected in the SCF. ZrB2 grains cannot be seen directly in the combustion front at

2θ, degrees

Zr (110) ZrB2 (110) ZrSi (061)

ZrB2 (002)

ZrSi (200)

(c) τ = 1.5 s

ZrSi (061)

ZrB2 (102)

(d) τ = 2.0 s ZrSi (200) ZrB2 (200) ZrB2 (002)

ZrB2 (001)

Si (311) Zr (110)

Si (220) Zr (102)

ZrB2 (101) ZrSi (131)

ZrB2 (001)

Si (311) Zr (110)

Al (200) Si (220) Zr (102)

(b) τ = 1.0 s ZrB2 (101)

Zr (002)

Zr (100)

Si (111)

ZrB2 (001)

Zr (101)

Al (111)

Zr (100) Zr (002)

Si (111)

(a) τ = 0.0-0.5 s

ZrB2 (101) ZrSi (131)

I, counts Zr (101)

I, counts

ZrSi (021) ZrB2 (100) ZrSi (130) ZrSi (111)

Yu.S. Pogozhev et al. / Ceramics International 42 (2016) 16758–16765

ZrSi (021) ZrB2 (100) ZrSi (130) ZrSi (111)

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2θ, degrees

Fig. 4. A selective sequence of XRD patterns recorded by dynamic X-ray diffraction of combustion of the mixture not diluted by the final product, X ¼ 0.

Fig. 6g shows the microstructure of the final product that includes faceted ZrB2 grains of different dispersion, light gray regions corresponding to ZrSi silicide and dark gray regions

this magnification because they are extremely small. However, ZrB2 grains have been identified beyond the combustion front (in the post-combustion zone) and are shown with an arrow (Fig. 6f).

Zr(Si.Al)

ZrO2

a

ZrSi

ZrB2

70000

ZrB2 b

60000

ZrSi

Intensity

50000

ZrO2

40000 30000 20000 10000 0 20

25

30

35

40

45

50

55

60

65

70

75

80

85

2θ, degrees Fig. 5. The microstructure (a) and phase composition (b) of the inert diluent (the powdered final product).

Yu.S. Pogozhev et al. / Ceramics International 42 (2016) 16758–16765

a

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b

Product

Al

c

Zr

Si

ZrSiAl2

d

Reaction mixture

Products

ZrSi

Combustion front e

Zr

ZrB2

ZrSiAl2

f

ZrSi

ZrB2

ZrSiAl2

g

Fig. 6. The microstructures in different regions of the SCF sample: a,b – initial mixture; c,d – combustion front; e,f – post-combustion zone; g – final product.

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ZrB2

ZrSi

ZrSiAl2 / ZrSi2

Fig. 7. Microstructure of the compact ceramic based on zirconium diboride and zirconium silicide.

corresponding to Al-rich ZrSiAl2. It should be mentioned that phase coarsening takes place when proceeding from the postcombustion zone to the final product. Therefore, the obtained results demonstrate that the ZrB2 diboride phase is formed in the combustion zone via crystallization from the Zr–Si–Al–B melt, whereas the ZrSi silicide phase emerges with a delay of less than 0.5 s. Composite targets/cathodes based on zirconium boride and silicide were fabricated using forced SHS-pressing technology [27], which is based on sequential combustion and pressing of hot synthesis products under conditions of quasi-isostatic compression in sand. The phase composition of the compact synthesis products includes the ZrB2, ZrSi, and ZrSi2 phases and the ternary compound ZrSiAl2 with tetragonal crystal lattice (structural type tI8/1) and lattice parameters a¼ 0.3893 nm and c¼ 0.8998 nm. Their concentrations are 48, 33, 5, and 10%, respectively. The presence of high-temperature zirconium disilicide (ZrSi2) is probably related to deeper chemical transformation. Monosilicide ZrSi becomes saturated with silicon from the zirconium-poor melt during slow cooling. The XRD data are in good agreement with the results of structural studies of compact ceramics. The microstructure of the fracture is shown in Fig. 7. The main structural components are faceted ZrB2 grains with an average size of 2–3 mm distributed in the monosilicide ZrSi matrix, which also contains disilicide ZrSi2 and the ternary aluminum-based compound ZrSiAl2. The specific density of the obtained compact ceramics is 5.3 g/ cm3; the residual porosity is no higher than 3%. It is also characterized by a hardness HV ¼ 17.1 GPa that is comparable to that of white and black ceramics, which does not exclude the possibility of using it as a promising construction material. The resulting disc-shaped targets/cathodes based on zirconium diboride and silicide, 115 mm in diameter and 6–8 mm thick, are used in the magnetron sputtering of coatings. Oxidation of these coatings is expected to yield self-assembling Al2O3–ZrO2–SiO2 oxide layers characterized by increased erosion, corrosion, and abrasion resistance.

4. Conclusions

1. The effect of the degree of dilution with an inert component and the initial temperature on the combustion kinetics of the reaction mixture Zr–Si–Al–B has been studied. An increase in T0 leads to a directly proportional rise in Tc and Uc, thus indicating that the staging of the chemical reactions yielding of zirconium boride and silicide is invariant within the T0 range under study (298–700 K). Uc is a most sensitive parameter to changes in T0: its values grow threefold on average as the initial temperature increases. 2. The effective activation energy of the combustion process has

been determined (225 kJ/mol); it suggests that the reaction of zirconium with boron and silicon via the Zr–Si–Al–B melt formed in the combustion zone has a decisive effect on the reaction kinetics. 3. The sequence of chemical transformations during combustion has been studied using the methods of dynamic X-ray diffraction and stopped combustion front. The primary ZrB2 grains are formed in the combustion zone by crystallization from the melt, whereas the silicide ZrSi phase is formed with a delay no more than 0.5 s. 4. Compact ceramic with composition ZrB2–ZrSi–ZrSi2–ZrSiAl2 that is promising for high-temperature applications has been produced by forced SHS pressing.

Acknowledgment This study was supported by the Russian Science Foundation (Project no. 15-19-00203) for the purpose of synthesis of ceramic materials for nanocomposite and functional gradient coatings with enhanced erosion, corrosion, and abrasion resistance, as well as by the Ministry of Education and Science of the Russian Federation in the framework of Increase Competitiveness Program of NUST “MISiS” among the World-Leading Scientific Educational Centers in 2013-2020 (Project no. K2-2016-002) for the purpose of studying the kinetics and mechanisms of combustion processes.

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