Mechanism and microstructural evolution of combustion synthesis of ZrB2–Al2O3 composite powders

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CERAMICS INTERNATIONAL

Ceramics International 41 (2015) 5790–5797 www.elsevier.com/locate/ceramint

Mechanism and microstructural evolution of combustion synthesis of ZrB2–Al2O3 composite powders G.Q. Xiaon, Y.L. Fu, Z.W. Zhang, A.D. Hou College of Materials and Mineral Resources, Xi'an University of Architecture and Technology, Xi'an 710055, China Received 6 November 2014; received in revised form 22 December 2014; accepted 3 January 2015 Available online 12 January 2015

Abstract Mechanism of combustion synthesis (CS) of ZrB2–Al2O3 composite powders was systematically analyzed by a combustion front quenching method (CFQM). The microstructural evolution during the CS process was investigated by field-emission scanning electron microscopy (FESEM) equipped with energy dispersive X-ray spectrometer (EDS). The combustion temperature and wave velocity were measured by the data acquisition system. Moreover, the phase constituents of the final product were examined by X-ray diffraction (XRD). The thermal behaviors of the stoichiometic powders under the thermal exposure were characterized using differential scanning calorimetry (DSC) and thermogravimetric (TG). The results showed that the combustion reaction started from the melting of the B2O3 and Al particles, which was followed by the formation of ZrO2–B2O3–Al solution. The ignition temperature of this system was determined to be around 800 1C. B and Al2O3 were then precipitated from the solution. As the CS reaction proceeded, Zr and Al2O3 were produced by the reaction between ZrO2 particles and Al and precipitated from the solution. ZrB2 could then be formed by the direct reaction between Zr and B. Finally, the ZrB2–Al2O3 composite powders were obtained. Furthermore, a model corresponding to the dissolution–precipitation mechanism was proposed. & 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Keywords: ZrB2–Al2O3 composite powders; Combustion synthesis; Mechanism; Combustion front quenching method

1. Introduction ZrB2 is an excellent high-temperature structural ceramic due to its high melting point (3245 1C), high hardness (23 GPa) and strength, as well as excellent corrosion and wear resistance. Therefore, it is usually applied to re-entry space shuttles, rocket nozzle inserts, and thermal protection systems of hypersonic vehicles [1,2].One of the main problems of ZrB2, however, is the poor sinterability [3]. On the other hand, Al2O3 is usually employed in firebricks, crucibles, porcelain, cutting tools etc., relying on its high hardness, good thermal stability and corrosion resistance [4,5]. But, its application is limited owing to the inherent brittleness. To retain the advantages of ZrB2 and Al2O3 and overcome their weaknesses, the ZrB2–Al2O3 composite powders are desired based on the synergy mechanism.

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Corresponding author. Tel.: þ86 29 82202700. E-mail address: [email protected] (G.Q. Xiao).

http://dx.doi.org/10.1016/j.ceramint.2015.01.007 0272-8842/& 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

This composite can be successfully utilized in the ceramic materials and carbon-containing refractory and abrasives [6]. The ZrB2–Al2O3 composite powders have been successfully fabricated by conventional methods, including pressureless sintering [7], reactive sintering [8,9], direct mixing [10,11], melt direct reaction [12,13], microwave synthesis [6], and mechanochemical synthesis [14]. All these synthetic routes are based on complicated processes and high energy consumption, along with inevitable introducing of impurities. Combustion synthesis (CS) technique exhibits advantages in high efficiency, low cost, and high purity of products, which has been applied recently [15]. Currently several efforts have been carried out to research the CS of ZrB2–Al2O3 composite powders [16–22]. The powders prepared by the CS method showed a better sinterability by comparing with the powders obtained by replacement reaction and carbothermic reduction [16,17]. The grain growth of borides was inhibited to some extent by the addition of a little Ti (5 wt%) [18]. However, the process to synthesize nanostructured ZrB2–Al2O3 powders by combining the CS and mechanochemical method was

G.Q. Xiao et al. / Ceramics International 41 (2015) 5790–5797

quite complicated [19,20]. Therefore, in order to optimize the reaction process and the properties of materials, it is worthwhile to explore the reaction mechanism of ZrB2–Al2O3 composite powders during the CS process. To date, the studies on the mechanism of CS of ZrB2–Al2O3 composite have been performed by several research groups. Yu et al. [21] reported that the CS reaction started from the melting of B2O3 and Al which was followed by the formation of Zr and A12O3. And the ZrB2 grains were finally formed by direct reaction between B and Zr. Mishra et al. [22] studied the phase evolution of ZrB2–Al2O3 composite, believing that the reaction started from the melting of all the raw materials, and the intermediate (Zr, Al)B2 was formed during the CS process, and ZrB2 was solidified first during the cooling process. Considering the reaction sequenceand microstructural evolution during CS directly affects the chemical phases and mechanical properties in the final products, it is worth addressing an investigation on this issue. In this work, the microstructural evolution during the CS process to prepare ZrB2–Al2O3 composite powders was investigated to reveal the reaction mechanism of ZrB2–Al2O3 composite by adopting a combustion front quenching method (CFQM). The mechanism of the CS was discussed in particular and a model corresponding to the mechanism was proposed. 2. Experimental procedure The raw materials, zirconia (o3 μm, 99.9% purity), boron oxide (o80 μm, 99.5% purity), aluminum (o25 μm, 99.0% purity) and alumina (o48 μm, 99.0% purity) were used to make the composite powders. The compositions of these mixtures are listed in Table 1. In fact, the combustion wave is incapable to be self-quenched owing to too much heat release for sample 1. So sample 2 (sample 2 with extra A12O3 as the diluents) was used as the starting mixture for the combustion front quenching process. These mixtures were used for the following tests. 2.1. Measurements of temperature–time profile and wave rate of the CS Sample 1 was compressed into a cylinder (20 mm in diameter and 15 mm in height) with a theoretical density of about 57%. A thermocouple pair of W-3% Re versus W-25% Re (0.1 mm in diameter) was inserted into the bottom of the sample along the cylinder axis and linked to the data acquisition system by means of which a temperature–time curve could be recorded. The reaction was carried out in a stainless steel reactor under Ar atmosphere at a pressure of 0.1 MPa at an initial temperature of

25 1C. The compacted sample was ignited by a tungsten wire on its top. Furthermore, the mixtures of sample 1 and sample 2 were compressed, respectively, into a cylinder (20 mm in diameter and 30 mm in height) with a relative density of 5771%. Correspondingly, two pairs of thermocouples were set against the outside of the compact of sample 1 in the axial distance (noted as L) of 21.5 mm to get two temperature–time profiles with a time interval Δt between those two curves. The value of L for the compact of sample 2 was 20.6 mm. Then the reaction wave rate could be evaluated by the value of L/Δt. 2.2. XRD analysis of the combustion–synthesized product The phase compositions of the synthesized ZrB2–Al2O3 composite powders were determined by X-ray diffraction (XRD, D/MAX 2400, Japan) using Cu Kα radiation. The data were collected over a 2θ-range of 3 to 901 using a step size of 0.021. 2.3. Combustion front quenching experiment The schematic diagram of the CFQM test is shown in Fig. 1. Firstly, 14 g of mixtures from sample 2 and 3 g of mixtures from sample 1 were successively poured into the steel die, being compressed to form a cylinder compact (20 mm in diameter and 25 mm in height). The top of this shaped compact (from sample 1) protruded about 2 mm from the die, as shown in Fig. 1. Secondly, the system was ignited according to the method as described above. The combustion wave self-propagated in the compact, and then self-quenched before it reached the bottom of the compact due to the cooling effect of the steel mold. Finally, the microstructural evolution along the axis of the quenched sample was analyzed by field-emission scanning electron microscopy (FESEM, SU 6600, Japan) attached with the energy dispersive X-ray spectrometer (EDS). 2.4. DSC–TG analysis of stoichiometic compositions The thermal effects of the B2O3–Al and ZrO2–Al powders blended in stoichiometic ratio under the thermal exposure were analyzed using differential scanning calorimetry and thermogravimetric (DSC–TG, Q 600, America) by heating a sample

Table 1 Compositions of the initial mixtures. Samples

Sample 1 Sample 2

Compositions/(mol%) ZrO2

B2O3

Al

Al2O3

17% 17%

17% 17%

66% 66%

0 17%

5791

Fig. 1. Schematic diagram of the combustion front quenching test.

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from 25 to 1000 1C at a rate of 20 1C/min under the flowing Ar atmosphere. The samples ranged from 7 to 8 mg in mass. 3. Results and discussion 3.1. Combustion temperature and combustion wave speed The temperature–time curve of sample 1 is shown in Fig. 2. It can be seen that the combustion temperature (Tc) of sample 1 is 1946 1C which is lower than the melting points of Al2O3 (2054 1C) and ZrO2 (3050 1C). The combustion wave velocities of samples are presented in Fig. 3. The average combustion velocity of sample 1 was 4.3 mm/s (Fig. 3(a)); the average

propagating velocity of sample 2 was 2.6 mm/s (Fig. 3(b)), which was much lower than that of sample 1. Moreover, the adiabatic temperatures (Tad) for sample 1 and 2 were found to be 2054 and 1911 1C, respectively. As a result, the combustion wave would be self-quenched successfully by the introduction of Al2O3 as diluents.

3.2. Phase constituents of the combustion–synthesized product XRD pattern of sample 1 is shown in Fig. 4, revealing that there were ZrB2 and Al2O3 phases without any by-product in sample 1.

o

1946 C

2000

Temperature

o

C

3.3. Microstructural evolution during the CS process 1500

SEM photograph of the initial reactants is shown in Fig. 5. The EDS analysis shows that the bright and finer particles at the site of “A” were ZrO2; the spherical ones at the site of “B” were Al; the bright and bigger ones at the site of “C” were B2O3; and the irregular gray lamellar ones at the site of “D” were Al2O3.

1000 500 0 0

100

200

300

400

500

Time (s) Fig. 2. Temperature–time profile of sample 1 during the CS.

Temperature

o

C

2000

o

1938 C 1911 oC

1500 1000

Δt 500 0 10

20

30

20

30

Time (s)

Temperature

o

C

2000

o

1875 C

3.3.1. Melting of B2O3 and Al particles and formation of ZrO2–B2O3–Al solution The change appearing first in the reacting region of the quenched sample was the melting of B2O3 and Al particles, as shown in Fig. 6. The melting points of B2O3 and Al are 450 1C and 660 1C, respectively. Firstly, the Al particles which did not melt were surrounded by the melting B2O3 at the site of “E” (Fig. 6(a)). As the temperature rose, the solution at the site of “F” was formed, while the remaining Al particles at the site of “G” were wrapped by the solution and still did not melt (Fig. 6(b)). The composition of solution at the site of “F” (with B, C, O, Al, Zr, Au in molar ratio of 10.81:18.01:27.13:23.95:13.34:6.76) shows that the ZrO2–B2O3–Al solution was formed by the dissolution of fine ZrO2 particles into the melting B2O3 and Al, which is consistent with the report in the Ref. [23–25].

o

1871 C

1500 1000

Δt

500 0 10

Time (s) Fig. 3. Diagram of the combustion wave velocity test: (a) sample 1 and (b) sample 2.

Fig. 4. XRD pattern of the combustion–synthesized product.

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Fig. 5. SEM photograph of the initial reactants.

Fig. 6. SEM photographs of changes in the reacting region of the quenched sample: (a) B2O3 particles that were melting at the site of “E”, and (b) Al particles were starting to melt and form the ZrO2–B2O3–Al solution at the site of “F”, and the remaining Al particles at the site of “G”.

3.3.2. Formation of plentiful Al2O3 and precipitation of a few ZrB2 particles It can be seen that a large number of tiny white particles at the site of “H” precipitated from the solution (Fig. 7(a)). The EDS analysis result implies that the small particles (with B, C, O, Al, Zr in molar ratio of 7.97:16.83:48.18:10.72:16.30) were B and Al2O3. As the reaction progressed, the Al2O3 grains at the site of “I” started to crystallize and grew up to form columnar crystals, as shown in Fig. 7(b). Meantime, a few of white particles at the

Fig. 7. SEM photographs of formed Al2O3 particles and precipitated ZrB2 particles: (a) precipitation of tiny white particles at the site of “H”; (b) the crystallization and growth of the gray particles at the site of “I”; and (c) bits of white particles at the site of “J” precipitate from the ZrO2–B2O3–Al solution at the site of “K”.

site of “J” precipitated from the ZrO2–B2O3–Al solution at the site of “K”. This can also be observed clearly in Fig. 7(c), where the white fine particles (with B, C, O, Al, Zr, Au in molar ratio of 4.39:18.21:35.52:25.19:11.71:4.97) were ZrB2. 3.3.3. Precipitation of mass ZrB2 and Al2O3 particles from the ZrO2–B2O3–Al solution As the reaction proceeded, Al had melted at the site of “L” and the bigger white particles had precipitated from the ZrO2– B2O3–Al solution at the site of “M”, as shown in Fig. 8(a). The EDS analysis result at the site of “L” (with B, C, O, Al, Si, Zr,

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that the gray particles at the site of “N” (with O, Al, Au in molar ratio of 40.13:52.95:6.92) were Al2O3 and the white ones at the site of “O” (with B, C, O, Al, Zr, Au in molar ratio of 15.90:6.77:22.56:31.11:17.74:5.92) were ZrB2. 3.3.4. Combustion–synthesized product SEM photograph of the combustion–synthesized product is shown in Fig. 9. Layered ZrB2–Al2O3 composite powders with some micro-pores can be seen in the whole morphology (Fig. 9(a)). As can be seen from the amplified photograph (Fig. 9(b)), spherical ZrB2 particles (with B, C, O, Al, Zr, Au in molar ratio of 3.25:27.85:30.53:4.11:28.18:6.10) had an average diameter of about 1.4 μm, and Al2O3 grains (with C, O, Al, Si, Au in molar ratio of 38.86:27.55:19.84:4.48:9.27) existed around the ZrB2 particles. 3.4. Mechanism of the CS 3.4.1. Thermal effect characterization The thermal effect characterization of different systems is shown in Fig. 10. There were four characteristic signals in the DSC–TG curve of B2O3–Al system, as illustrated in Fig. 10(a). The first two endothermic peaks in the range of 100–200 1C were considered to be associated with the dehydration of B2O3, which corresponded with a significant weightlessness in the same temperature range. This is consistent with the report in the Ref. [26]. The third endothermic signal at about 664 1C was clearly linked with the melting of Al. Additionally, there

Fig. 8. SEM photographs of precipitation of mass ZrB2 and Al2O3 particles: (a) formation of ZrB2 particle at the site of “M”; (b) formation of gray particle and the white ones; and (c) magnified photo of particles gray particle at the site of “N” and the white ones at the site of “O”.

Au in molar ratio of 8.87:12.95:33.04:29.12:1.24:9.60:5.18) implies that B phase was first formed by the thermite reaction in the ZrO2–B2O3–Al solution. Moreover, the EDS analysis result of the site of “M” (with B, O, Al, Zr, Au in molar ratio of 26.30:11.25:3.42:51.24:7.78) indicates that the enrichment region of Zr was formed by aluminothermic reduction in the ZrO2–B2O3–Al solution. Therefore, ZrB2 is speculated to precipitate from the solution by a direct reaction between B and Zr. It can be inferred that Al2O3 was primarily precipitated from the ZrO2–B2O3–Al solution. As the reaction proceeded, a large number of ZrB2 crystals precipitated from the solution, as shown in Fig. 8(b) and (c). The EDS analysis results indicate

Fig. 9. SEM photograph of the combustion–synthesized product: (a) whole morphology, and (b) spherical ZrB2 particles in ZrB2–Al2O3 composites.

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o

858 C

TG

0.25

Weight (%)

o

100

and the results are displayed in Fig. 11. It can be seen that: (1) the phase constituents in unreacted and preheated regions were almost the same as the reactants, as shown in Fig. 11(a) and (b); (2) Al2O3 phases increased slightly with the decreasing of Al and ZrO2 phases, as described in Fig. 11(c); (3) as the reaction advanced, the ZrB2 and Al2O3 phases progressively increased and a small amount of unreacted Al and ZrO2 phases were also defected, as shown in Fig. 11(d) and (e).

Temperature Difference ( C/mg)

0.30

102

98

0.20

96 94

0.15

DSC

0.10

o

161 C

92

o

664 C

0.05

800

0.00 1000

126

90 0

200

400

600

5795

o

0.25

TG

100

Weight (%)

961 C 90

0.20 0.15

DSC

80

o

o

0.10 70 o

667 C 60

0

200

400

600

0.05 800

Temperature Difference ( C/mg)

Temperature ( C)

1000

o

Temperature ( C) Fig. 10. DSC–TG curves at a heating rate of 20 1C/min: (a) B2O3–Al system, and (b) ZrO2–Al system.

was an exothermic signal starting at 766 1C, with the peak position at about 858 1C. Hence, the ignition temperature of the thermite reaction was around 800 1C. By the thermodynamic calculation, the adiabatic temperature of the B2O3–Al system is 2054 1C, which is in agreement with the melting point of Al2O3. In this case, it can be deduced the final product would partly melt. Therefore, the reaction equation in the B2O3–Al system can be written as: B2O3 (s)-B2O3 (l)

(1)

Al (s)-Al (l)

(2)

B2O3(l) þ 2Al (l)-2B(s) þ Al2O3(l)

(3)

3.4.3. Analysis of the mechanism of the CS Based on the above results, it can be concluded that the combustion process gave rise to the melting of B2O3 and Al, and then the dissolution–precipitation process occurred. After the melting of B2O3 and Al, the ZrO2 particles started to dissolve into the molten B2O3 and Al mixture, resulting in the formation of ZrO2–B2O3–Al solution. The DSC analysis results indicate the ignition temperature of this ternary system should be around 800 1C, but the research by Mishra et al. [27] shows that was about 1230 1C. The difference in the ignition temperature may be due to the difference in particle size, heating rate etc. And then B and Al2O3 phases precipitated from the ZrO2– B2O3–Al solution. With the increase of temperature, an enrichment region of Zr phase formed gradually in the solution. Subsequently, ZrB2 could be formed by direct reaction between B and Zr phases. It is worthwhile to point out that the formation of Al2O3 grains had priority over that of ZrB2 grains based on the comprehensive analysis by SEM, DSC and XRD. Accordingly, a model corresponding to the dissolution–precipitation mechanism is shown in Fig. 12. It is possible that the model will vary with mixture ratio and particle size of the reactants. However, the investigation results of microstructural evolution during the CS process can dynamically display the details of the changes in the reaction process and provide the solutions to some applied problems as well as a guide for other similar system. Therefore, it is significant for the study in the future.

For the ZrO2–Al system, DSC result revealed two representative signals, as shown in Fig. 10(b). The first one was an endothermic signal at about 667 1C which was related to the melting of Al. The second one was an exothermic signal at around 961 1C, indicating that the thermite reaction between ZrO2 and Al had taken place. TG curve was nearly flat meaning a constant mass during the reaction process. Accordingly, the equation for the reaction in the ZrO2–Al system can be expressed as: Al(l) þ 3ZrO2(s)-2α-A12O3(s) þ 3[Zr]

(4)

3.4.2. Phase constituents of different regions in quenched sample In order to illustrate the CS mechanism more clearly, it is necessary to further analyze the phase constituents of the different regions besides the reacted region (Fig. 4) in quenched sample,

Fig. 11. XRD patterns for the quenched sample in different regions: (a) reactant region; (b) preheated region; (c) combustion wave front region; (d) combustion wave intermediate region; and (e) combustion wave terminal region.

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Fig. 12. A model corresponding to the mechanism of CS: (a) initial rectants; (b) melting of B2O3; (c) formtion of ZrO2–B2O3–Al solution; (d) volatilization of B2O3 and precipitation of the B and Al2O3 phases; (e) crystallization of Al2O3 particles and precipitation of the Zr and Al2O3 phases; (f) formation of massive ZrB2 and Al2O3 as well as undissolved Al particles; and (g) final products.

4. Conclusions The reaction mechanism of ZrB2–Al2O3 composite powders had been studied by XRD, DSC, SEM and EDS as well as thermal analysis. The combustion reaction mechanism was proposed, and a model corresponding to the dissolution–precipitation mechanism was drawn. The combustion reaction in the ZrO2–B2O3–Al system began with the melting of B2O3 and Al particles. The primary reaction was the formation of Al2O3 from Al and B2O3, which was followed by the reaction between Al and ZrO2, and then B reacted with Zr to form the product ZrB2. The ignition temperature of reaction is prophesied to be around 800 1C. In addition, the formation of only ZrB2 and Al2O3 phases in final product was feasible when the starting mixture of ZrO2, B2O3, and Al was in a molar ratio of 3:3:11.5. Acknowledgments This work is supported by the National Natural Science Foundation of China (No. 51272203) and the International Cooperation Project in Shaanxi Province (No. 2014KW10-05). The authors would also like to give their special thanks to postgraduate student Donovan Lui and Stephen Romanelli at the University of Central Florida for their kind help in language correction. References [1] S.K. Mishra, S.K. Das, V. Sherbacov, Fabrication of Al2O3-ZrB2 in situ composite by SHS dynamic compaction: a novel approach, Compos. Sci. Technol. 67 (2007) 2447–2453. [2] S.H. Jung, H.C. Oh, J.H. Kim, S.C. Choi, S.H. Lee, H.D. Kim, Pretreatment of zirconium diboride powder to improve densification, J. Alloys Compd. 548 (2013) 173–179. [3] S.K. Mishra, S.K. Das, Sintering and microstructural behaviour of SHS produced zirconium diboride powder with the addition of C and TiC, Mater. Lett. 59 (2005) 3467–3470. [4] J.P. Si, J.C. Doo, CVD synthesis of Al2O3 nanotubular structures using a powder source, Ceram. Int. 40 (2014) 7923–7929.

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