Pursuing enhanced oxidation resistance of ZrB2 ceramics by SiC and WC co-doping

Accepted Manuscript Title: Pursuing enhanced oxidation resistance of ZrB2 ceramics by SiC and WC co-doping Authors: Xiaoqiang Feng, Xin Wang, Yuan Liu, Wei Tian, Min Zhang, Xian Jian, Liangjun Yin, Linbo Zhang, Jianliang Xie, Longjiang Deng PII: DOI: Reference:

S0955-2219(18)30471-0 https://doi.org/10.1016/j.jeurceramsoc.2018.07.041 JECS 12009

To appear in:

Journal of the European Ceramic Society

Received date: Revised date: Accepted date:

26-3-2018 25-7-2018 26-7-2018

Please cite this article as: Feng X, Wang X, Liu Y, Tian W, Zhang M, Jian X, Yin L, Zhang L, Xie J, Deng L, Pursuing enhanced oxidation resistance of ZrB2 ceramics by SiC and WC co-doping, Journal of the European Ceramic Society (2018), https://doi.org/10.1016/j.jeurceramsoc.2018.07.041 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Pursuing enhanced oxidation resistance of ZrB2 ceramics by SiC and WC co-doping Xiaoqiang Feng123, Xin Wang123*, Yuan Liu123, Wei Tian4, Min Zhang123, Xian Jian4, Liangjun Yin4, Linbo Zhang123, Jianliang Xie123, Longjiang Deng123 National Engineering Research Center of Electromagnetic Radiation Control Materials,

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University of Electronic Science and Technology of China, 2006 Xiyuan Road, Chengdu

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611731, P.R. China

State Key Laboratory of Electronic Thin Film and Integrated Devices, University of

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Electronic Science and Technology of China, 2006 Xiyuan Road, Chengdu 611731, P.R.

Key Laboratory of Multi-spectral Absorbing Materials and Structures of Ministry of

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China

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Education, University of Electronic Science and Technology of China, 2006 Xiyuan

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Road, Chengdu 611731, P.R. China

School of Materials and Energy, University of Electronic Science and Technology of

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China, 2006 Xiyuan Road, Chengdu 611731, P.R. China

Corresponding author: [email protected]

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*

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Abstract The oxidative degradation of ZrB2 ceramics is the main challenge for its extensive application under high temperature condition. Here, we report an effective method for codoping suitable compounds into ZrB2 in order to significantly improve its anti-oxidation

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performance. The incorporation of SiC and WC into ZrB2 matrix is achieved using spark plasma sintering (SPS) at 1800 °C. The oxidation behavior of ZrB2-based ceramics is

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investigated in the temperature range of 1000 °C~1600 °C. The oxidation resistance of

single SiC-doped ZrB2 ceramics is improved due to the formation of silica layer on the

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surface of the ceramics. As for the WC-doped ZrB2, a dense ZrO2 layer is formed which

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enhances the oxidation resistance. Notably, the SiC and WC co-doped ZrB2 ceramics with

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relative density of almost 100% exhibit the lowest oxidation weight gain in the process

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of oxidation treatment. Consequently, the co-doped ZrB2 ceramics have the highest

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oxidation resistance among all the samples.

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resistance

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Keywords: Ultra-high temperature ceramics, Zirconium diboride, Co-doping, Oxidation

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1. Introduction Boride, carbide and nitride ceramics, such as TaC, ZrB2, ZrC, HfB2, HfC and HfN, are called as ultra-high temperature ceramics (UHTCs) owing to their high melting temperatures up to 3000 °C [1, 2]. Due to their unique characteristics of high melting

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point, hardness, good thermal shock resistance and excellent oxidation resistance, the UHTCs are considered as promising materials for application in thermal protective

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systems (TPS) [3-6]. Among the UHTC materials, ZrB2 has attracted considerable

attention due to its many advantages, including low theoretical density (6.09 g/cm3), high

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electrical conductivity (10.3×104 S/cm) and thermal conductivity (58.2 W/ (m K)-1) [7-

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10].

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However, ZrB2 starts to undergo oxidation at about 700 °C (reaction 1) and is severely

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degraded at temperatures above 1100 °C. At temperatures below 1100 °C, molten B2O3

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covers the surface of ZrB2 to inhibit further oxidation. However, the anti-oxidation protection fails at 1100 °C due to the rapid evaporation of B2O3 gas (reaction 2) and

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therefore, only a porous and non-protective ZrO2 layer remains on the surface [7].

(1)

B2O3  l   B2O3  g 

(2)

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5 ZrB2  c  + O2  g  =ZrO2  c  +B2O3  l  2

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Several methods have been reported to improve the oxidation resistance of ZrB2, and

all of these strategies aim to form an anti-oxidation protective layer on the surface of ZrB2. These methods can be divided into three groups: (1) forming a continuous glass layer; (2) densifying the porous ZrO2 layer to inhibit oxygen diffusion; (3) combining the two effects to increase the energy barrier for oxygen diffusion. In the first method, researchers 3

usually use silicon-containing additives (i.e. SiC, MoSi2 and Si3N4) [11-15], since SiO2 has higher viscosity, higher melting temperature and lower vapor pressure than B2O3 [15]. In the second method, WC is one of the typical dopants. The oxidation product WO3 is present as liquid phase in ZrO2 and promotes densification of the porous ZrO2 layer via

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liquid phase sintering [16, 17]. In the third method, both SiC and WC are used to improve the oxidation resistance of ZrB2 ceramics [5]. The oxidation mechanism of single-doped

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ZrB2 ceramics has been well studied, but the oxidation mechanism of co-doped ZrB2

ceramics is still unclear due to the complex interaction among the various oxidation

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products.

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Herein, we investigate the co-doping approach to improve the oxidation resistance of

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ZrB2 ceramics. The morphology, composition and structural evolution of the oxidation

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products are studied carefully to clarify the oxidation mechanism of ZrB2-SiC-WC

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compounds. For comparison, the oxidation behaviors of pure ZrB2 ceramics and single-

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doped ZrB2 ceramics are also studied.

2. Experimental procedure

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2.1 Sample preparation and characterization Raw materials including ZrB2, SiC and WC are commercial chemicals. ZrB2 (10-15

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μm, 99.5 %) and SiC (1.6-1.7 μm, 99.5 %) powders were purchased from Eno Material, China. WC (2.9-3.1 μm, 99.5 %) powders were obtained from Kelong Chemical Reagent Factory, China. The as-designed compositions of synthesized powders are given in Table 1. The powder mixtures are ball-milled in ethanol for 1 h. These powders are then dried

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for 24 h at 80 °C. ZrB2 ceramic composites are prepared by spark plasma sintering (SPS 4-6-20, Chen Hua) using a graphite mould at 1800 °C for 5 min under 40 MPa of uniaxial pressure in vacuum. The furnace is heated up to the sintering temperature at the heating rate of 100 °C/min. After holding the sintering temperature for 5 min, the furnace is

Composition

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naturally cooled down to room temperature.

ZrB2(vol%)

SiC(vol%)

WC(vol%)

Pure ZrB2

100

0

0

ZS10

90

10

ZS20

80

ZS30

70

ZW3

96.57

ZW7 ZW11

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20

0 0 0

0

3.43

93.02

0

6.98

89.35

0

10.65

20

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30

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ZS20W5

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Sample

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Table 1 List of as-designed ZrB2-based ceramics. ZrB2 (90 vol%)-SiC(10 vol%)

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composite is named as ZS10, similarly, ZrB2 (96.57 vol%)-WC(3.43 vol%) composite is named as ZW3. Other sample is defined in the similar way shown in the table 1.

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The sintered density of the specimens is determined using the Archimedes method with

deionized water as the immersion medium. The theoretical density of each specimen is calculated based on the volume fractions of ZrB2, SiC and WC in the ceramics. Crystalline phases, microstructures and chemical compositions of ZrB2 ceramics are characterized by using X-ray diffraction (XRD, XRD-7000, Shimadzu), scanning 5

electron microscopy (SEM, JSM-7600F, JEOL) and energy-dispersive spectroscopy (EDS, NORAN SYSTEM 7, Thermo Scientific), respectively.

2.2 Oxidation Tests

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Before the oxidation treatment, all the bulk specimens are polished using abrasive paper and are ultrasonically cleaned in ethanol afterwards. The oxidation process is

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conducted in a tabular furnace with molybdenum disilicide (MoSi2) as heating element. All ZrB2-based ceramics are oxidized at 1000 °C, 1200 °C, 1400 °C and 1600 °C in air. Subsequently, the oxidized ZrB2-based ceramics are embedded in epoxy resin and the

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cross-sections of ZrB2-based ceramics are polished by a polisher (LaboForce-100,

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Struers). Thermogravimetric analyses (TGA) of ZrB2-based ceramics are carried out

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using a thermal gravimetric analyzer (STA 449C, Netzsch). Specimens are heated at 10 °C/min up to 1400 °C in air atmosphere at a flow rate of 20 ml/min to obtain non-

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isothermal thermo gravimetric curve. Specimens are also heated at 40 °C/min up to

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1200 °C and held for 2 h in air at the flow rate of 20 ml/min to obtain isothermal thermo

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gravimetric curve.

3. Results and discussion

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3.1 Microstructure of the as-sintered ZrB2-based ceramics The addition of SiC and WC has a significant influence on the microstructure of ZrB2-

based ceramics. It is found that pure ZrB2 and the ZrB2 composites differ from each other in terms of their microstructure and density. The measured relative densities of all ZrB2 composites are higher than that of pure ZrB2, which is helpful for the oxidation resistance. 6

Compared with the single- (SiC or WC) doped samples, SiC and WC co-doped ZrB2 composites have the highest relative density of up to almost 100%, indicating the feasibility of co-doping approach using SPS technology. Compared with the normal pressureless sintering method, the density of pure ZrB2 is

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increased by 20% and the relative density value is 72% on average, which is still relatively low. Next, SEM observations are applied to confirm the microstructure of ZrB2-based

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ceramics and understand the density changes. As shown in Fig. 1-a, the pure ZrB2 bulk

has a large number of porosities in the size range of 1 µm ~ 9 µm resulting in relatively

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lower density. After SiC and WC doping, the porosity of ZrB2 composite decreases with

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the pore size range of 0.5 µm ~ 3 µm, as shown in Fig. 1-b and Fig. 1-c, respectively.

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This change in porosity obviously favors the increase in relative density. From Fig. 1-b,

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it can be seen that SiC particles in darker color are fairly uniformly distributed in ZrB2

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matrix and fill up the pore space between grain boundaries of ZrB2 particles, leading to decrease in ZrB2 porosity. Samples with higher amounts of doped SiC (10 vol%, 20 vol%

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and 30 vol%) have greater relative densities of 85%, 93% and 95%, respectively.

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Similarly, all the ZrB2-WC composites containing 3 vol%~11 vol% of WC display higher relative density. It is found that the optimal doping ratio is 6.98 vol% of WC (ZW7 sample), showing the highest value of 97%. The relative densities of ZW3 and ZW11 are

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80% and 87%, respectively. The WC particles are also uniformly distributed in ZrB2 matrix as shown in Fig. 1-c. The ZrB2 particles are connected to each other closely. The reason is that WC plays a role in eliminating the oxygen contamination in ZrB2 matrix [18]. Thus, doping with WC favors the densification of the ZrB2 ceramics to improve the

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relative density. Importantly, after SiC and WC co-doping, the as-designed composite of ZrB2-20vol%SiC-5vol%WC (ZS20W5) has the relative density of almost 100% and the morphology is shown in Fig. 1-d. Both SiC and WC particles are uniformly distributed in ZrB2 matrix. Moreover, the pores between the grain boundaries and impurities in the ZrB2

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matrix are eliminated due to the complementary effect of SiC and WC, which is helpful

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to improve the oxidation resistance.

Fig. 1. Surface morphology of ZrB2-based ceramics: (a) pure ZrB2, (b) ZS30, (c) ZW11,

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(d) ZS20W5.

The XRD patterns of ZrB2-based ceramics shown in Fig. 2 confirm the hexagonal phase of ZrB2 (PDF #34-0432) as the matrix. The XRD pattern of the ZrB2-SiC composite

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of ZS10 is similar to that of pure ZrB2. The SiC peaks are not observed for the ZS10 sample because of the small amount of SiC. On the other hand, some SiC diffraction peaks are recorded for the ZrB2-SiC composites of ZS30 and ZS20W5, which confirms the existence of doped SiC. However, WB and ZrC are indexed after adding WC into the

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which suggests that the added WC forms a solid solution with ZrB2 [16].

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ZrB2 matrix. Interestingly, there is no WC phase in the as-prepared ZrB2-WC samples,

Fig. 2. XRD patterns of ZrB2-based ceramics: (a) pure ZrB2, (b) ZS10, (c) ZS30, (d) ZW3, (e) ZW11, (f) ZS20W5.

3.2 Microstructure evolution in the oxidation process 9

3.2.1 Evolution of oxidized surface The microstructure evolution of the oxidized surface of ZrB2-based ceramics is considered. As shown in Fig. 3, the surface of pure ZrB2 is oxidized completely. All surface particles are cracked into small fragments due to the rapid evaporation of B2O3

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and volume change of ZrO2, indicating the poor anti-oxidation performance of pure ZrB2.

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Fig. 3. Surface SEM images of pure ZrB2 after oxidation (a) at 1000 °C for 3 h, (b) larger

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version of (a).

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Different treatment temperatures including 1200 °C, 1400 °C and 1600 °C are adopted

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to investigate the oxidation behavior of both ZS and ZSW samples. The typical results of ZS10 and ZS20W5 are shown in Fig. 4. For all oxidized samples, relatively dark and bright colored regions are found on the surface, which exhibit different morphologies

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and size distributions. The relatively dark substances are observed in Fig. 4-a,b,c due to the existence of SiO2 in ZrB2-SiC composites. After the oxidation of ZS10 at lower temperature of 1200 °C, the dark substances are small sized and distributed uniformly, as shown in Fig. 4-a. With the increase in oxidation temperature up to 1400 °C and

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1600 °C, the dark regions become larger, as shown in Fig. 4-b and 4-c, respectively. It is confirmed that more silica is formed randomly on the surface of ZS10 at higher oxidation temperature. Furthermore, some gaseous bubbles are observed on the surface after oxidation at

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1600 °C for high SiC-content (more than 20 vol%) doped ceramic, as shown in Fig. 4d. The formation of the bubbles is related to the evolution of gas-phase products (i.e. CO,

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CO2) [19]. The elevated temperature causes an increase in oxidation rate and the rapid

growth of silica. Therefore, the coherent silica layer exerts both positive and negative

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effects on the oxidation process. For the positive effect, the silica layer inhibits the

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oxygen diffusion to slow down the oxidation rate. However, the gas-phase products are

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also sealed under the silica layer. As the inner pressure becomes high enough and the

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oxidation of ZrB2 ceramic.

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gas leaks out, the silica layer is destroyed, which increases the risk of continuous

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Fig. 4. Surface SEM images of ZrB2-based composites after oxidation for 3 h: (a) ZS10

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ZS20W5 oxidation at 1600 °C.

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oxidation at 1200 °C, (b) ZS10 oxidation at 1400 °C, (c) ZS10 oxidation at 1600 °C, (d)

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Fig. 5 shows the XRD patterns of ZrB2-based ceramics after oxidation at

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1000 °C~1600 °C for 3 hours. It is found that the major phase is ZrO2 (PDF #37-1484) for these ZrB2-based ceramics. However, minor ZrB2 peaks still remain in ZS30 and ZS20W5 after oxidation at 1000 °C, which suggests that SiC has a significant effect on

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improving the oxidation resistance of ZrB2-based ceramics. Additionally, no WO3 is detected in ZW11 ceramic, as shown in Fig. 5-c, which indicates that most of the WO3 on the surface is evaporated. ZrSiO4 phase appears in ZS20W5 after oxidation at 1400 °C for 3 hours, which can improve the oxidation resistance of the ceramic [11]. The

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formation mechanism of ZrSiO4 is related to active oxidation of SiC and the reaction routes are expressed in Eqs. (3) and (4) [20]. (3)

1 ZrO2 +SiO+ O2 =ZrSiO4 2

(4)

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ZrO2  SiO2 =ZrSiO4

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Fig. 5. XRD patterns of ZrB2-based ceramics after oxidation for 3 h at 1000 °C, 1200 °C,

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1400 °C and 1600 °C: (a) pure ZrB2, (b) ZS20, (c) ZW11, (d) ZS20W5.

3.2.2 Evolution of oxidation layer The cross-sectional backscattered electron images of oxidized ZrB2-based ceramics are shown in Fig. 6. No layered structure is found in the oxidized pure ZrB2 and only a small amount of residual B2O3 is found on the surface. 13

As seen from Fig. 6-b, the ZS10 sample is divided into three layers: (1) loose and porous ZrO2 layer with a small quantity of SiO2, (2) SiC-depleted layer, and (3) unoxidized ZrB2-SiC layer. The formation of the SiC-depleted layer is related to the active oxidation of SiC [9, 15, 21, 22]. The thickness of oxide layer reaches up to 145 μm.

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A crack is present between the ZrO2 layer and SiC-depleted layer due to the phase transformation of ZrO2 and residual thermal stresses caused by the mismatch in

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coefficients of thermal expansion between ZrO2 and ZrB2 with the change in temperature. Fig. 6-c shows the cross-sectional micrographs of oxidized ZS20. Compared to ZS10,

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the oxidized layer structure of ZS20 shows the following differences: (1) the silica layer

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of ZS20 is thicker and reaches 26 μm; (2) a small amount of SiC is left in the second layer

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of ZS20, reasonably called SiC-poor layer, and the boundary crack is gone; and (3) the

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overall thickness of the oxidized layer (127 μm) is lower.

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Two oxide layers are observed in the cross-sectional micrographs of oxidized ZS30, as shown in Fig. 6-d. More silica is generated in ZrB2 doped with higher content of SiC,

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which makes the silica layer grow to 60 μm. This layer makes oxygen diffusion more

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difficult. Thus, the thickness of the oxidized layer is reduced to 110 μm.

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Fig. 6. Cross-section images for ZrB2-based ceramics after oxidation for 3 h: (a) pure

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(d) ZS30 oxidation at 1400 °C.

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ZrB2 oxidation at 1000 °C, (b) ZS10 oxidation at 1400 °C, (c) ZS20 oxidation at 1400 °C,

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Fig. 7 shows the cross-sectional optical microscopy images of oxidized ZrB2-WC

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composites. Only a thin oxide layer is observed on the surface when the oxidation temperature is 1200 °C. Three different oxide layers are found on the surface of ZW3 after oxidation at 1400 °C for 3 h. The vapor pressures of WO3, (WO3)2, (WO3)3 and

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(WO3)4 at 1600 °C are 8.25×10-3 Pa, 75.2 Pa, 565 Pa and 20.4 Pa, respectively[16]. According to Reaction 5 [16, 17, 23], the rapid evaporation of WO3 in first layer (outermost layer) results in the porous structure of the oxidized ZW3 samples. In contrast, the second layer has a dense structure, due to the liquid phase sintering of WO3 with ZrO2,

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which acts as a barrier to oxygen diffusion [16]. The third layer is the WC-poor layer. However, the third layer disappears when the oxidation temperature rises to 1600 °C. Excessive doping of WC makes the oxide layer crack due to the huge volumetric expansion of WO3 during oxidation and different thermal expansion coefficients of the

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oxidized surface and the unoxidized ceramic [17]. nWO3  s  =  WO3 n  g 

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(5)

Fig. 7. Cross-section images for ZrB2-WC composites after oxidation for 3 h: (a) ZW3 at

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1200 °C, (b) ZW3 at 1400 °C, (c) ZW3 at 1600 °C, (d) ZW11 at 1400 °C.

From the cross-sectional backscattered electron image and elemental mappings of oxidized ZS20W5 shown in Fig. 8, two oxide layers are observed, similar to the oxidation

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behavior of ZS30. SiC and WC are oxidized gradually and the oxidation products move to the surface, as indicated by the Si and W mapping. The first layer is composed of SiO2, ZrO2 and WO3 and the second layer is SiC/WC-poor layer. The co-doping of SiC and WC leads to a much denser SiC/WC-poor layer compared to the singly doped ZrB2

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composites. The channels for oxygen transportation in SiC-depleted layer are reduced, and then oxygen diffusion is inhibited. Thus, oxidation resistance of the co-doped ZrB2

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is improved. Furthermore, the cracks between the oxide layers almost disappear, which

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is beneficial to the oxidation resistance of ZrB2-based ceramics.

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Fig. 8. Cross-section and elemental mapping for ZS20W5 after oxidation for 3 h: (a) oxidation at 1400 °C, (b) oxidation at 1600 °C, (c) O elemental mapping, (d) Si elemental

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mapping, (e) Zr elemental mapping, (f) W elemental mapping.

The oxides thickness of ZrB2-based ceramics after oxidation at 1400 °C for 3 h is shown in Table 2. It can be seen that the compositions of ZrB2-based ceramics have an obvious effect on the thickness of oxides. The oxides thickness of ZrB2-based ceramics 17

decreases with the increase in SiC content. The ZS20W5 has the lowest oxides thickness value of 102 μm, which shows that the ZS20W5 has the best performance of antioxidation among the ZrB2-based ceramics we investigated. Fig. 9 shows the diagram of oxidation layers of pure ZrB2, ZrB2-SiC composite, ZrB2-WC composite and ZrB2-SiC-

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WC composite. The doped SiC forms a protective layer of glass phase silica on the surface, while the doped WC forms a denser ZrO2 layer to inhibit the oxygen diffusion. Therefore,

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the co-doping of SiC and WC has a complementary effect on improving the oxidation

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resistance of ZrB2-SiC-WC composite.

thickness of oxides

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Sample

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(oxidized at 1400 °C for 3 h)

ZS10

145 μm 127 μm 110 μm

ZW3

260 μm

ZW7

/

ZW11

280 μm

ZS20W5

102 μm

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ZS30

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ZS20

oxidized completely

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Pure ZrB2

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Table 2 Thickness of oxides for ZrB2-based ceramics.

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Fig. 9. The diagram of oxidation layers of four kinds of ZrB2-based ceramics: (a) pure

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ZrB2, (b) ZrB2-SiC composite, (c) ZrB2-WC composite and (d) ZrB2-SiC-WC composite.

3.3 Thermogravimetric analyses of ZrB2-based ceramics

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The relationship between weight gain and oxidation time for ZrB2-based ceramics

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during isothermal oxidation at 1200 °C is presented in Fig. 10-a,b,c. The curve shape is parabolic, as the oxidation behavior is controlled by the diffusion step and fits Jander model [24-26]. Table 3 presents the detailed analyses of the oxidized fraction of ZrB2-

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based ceramics, which is calculated from oxidation weight gain, as a function of volume content of SiC and WC. It can be seen that the oxidation resistance is significantly affected by the composition of ZrB2-based ceramics. The oxidation fraction of ZrB2based ceramics decreases with the increase in SiC content due to the formation of more

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silica glass on the surface. An optimal content (6.98 vol%) of WC provides the best oxidation resistance of ZrB2-WC ceramics due to the dense oxide layer formed through the liquid phase sintering reaction of ZrO2 and WO3 [16]. Furthermore, the huge volumetric expansion results in great internal stress during the oxidation of WC, which

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extrudes the gas and eliminates some pores in ZrO2 layer. As a result, the diffusion rate of oxygen through ZrO2 layer is slowed down. Excessive doping (10.65 vol%) of WC

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deteriorates the oxidation performance of ZrB2-based ceramics, since the internal stress

exceeds the limit and the protective oxide layer is destroyed. ZrB2 ceramics co-doped

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with SiC and WC (ZS20W5) have the lowest oxidized fraction value of 5%, indicating

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the best anti-oxidation performance. This means that SiC and WC together exert a

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complementary effect to improve the oxidation resistance of ZrB2-SiC-WC composites.

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The weight gain with temperature for ZrB2-based ceramics during non-isothermal

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oxidation up to 1400 °C is given in Fig. 10-d,e,f. No weight change is observed for any of the ZrB2-based ceramics at lower temperatures ranging from 30 °C to 650 °C. The first

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mass change of ZrB2-based ceramics happens at around 650 °C, which is ascribed to the

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initial oxidation of ZrB2-based ceramics. After a relatively rapid weight gain during further heating, a slower weight gain stage occurs with temperature up to 1300 °C. Moreover, pure ZrB2 has the shortest platform time corresponding to the slower weight

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gain stage, indicating the worst oxidation resistance. The sharp increase in weight gain at the final stage is due to the failure of the protective oxide layers. It is noted that the ZS20W5 composites have the lowest oxidized fraction of 1% during the non-isothermal oxidation, which indicates that ZS20W5 has the best oxidation resistance.

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isothermal oxidation

non-isothermal oxidation

Pure ZrB2

22%

21%

ZS10

11%

7%

ZS20

8%

5%

ZS30

6%

2%

ZW3

14%

12%

ZW7

8%

3%

ZW11

9%

9%

ZS20W5

5%

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Sample

1%

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Table 3 Oxidized fraction for ZrB2-based ceramics.

Fig.10. Mass increase of ZrB2-based ceramics, (a) isothermal oxidation of ZrB2-SiC

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under 1200 °C, (b) isothermal oxidation of ZrB2-WC under 1200 °C, (c) isothermal oxidation of ZrB2-SiC-WC under 1200 °C, (d) non-isothermal oxidation of ZrB2-SiC, (e) non-isothermal oxidation of ZrB2-WC, (f) non-isothermal oxidation of ZrB2-SiC-WC.

4. Conclusion 21

In this work, ZrB2-based ceramics are prepared by Spark Plasma Sintering at 1800 °C under argon atmosphere. The compositions of ZrB2-based ceramics have an obvious effect on the oxidation behavior. SiC improves the oxidation resistance of ZrB2 ceramics by forming a silica protective layer on the surface of the ceramics, while WC improves

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the oxidation resistance of ZrB2 ceramics by forming a dense internal oxidation layer. The co-addition of SiC and WC results in the highest density of almost 100%, the thinnest

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oxides thickness and the minimal oxidized fraction for ZrB2-20vol%SiC-5vol%WC

composite, indicating the best oxidation resistance performance. The co-doped SiC and

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WC have a positive and complementary effect on the oxidation resistance of ZrB2-based

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ceramics due to the formation of protective SiO2 layer and the densification of ZrO2 layer.

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Acknowledgment

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This work was supported by the NSFC (Grant Nos. 51702041) and the Open Foundation of Key Laboratory of Multi-spectral Absorbing Materials and Structures,

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Ministry of Education (ZYGX2016K009-3).

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References [1] M.M. Opeka, I.G. Talmy, J.A. Zaykoski, Oxidation-based materials selection for 2000°C + hypersonic aerosurfaces: Theoretical considerations and historical experience, J. Mater. Sci., 39 (2004) 5887-5904.

IP T

[2] M.J. Gasch, D.T. Ellerby, S.M. Johnson, Ultra High Temperature Ceramic Composites, in: N.P. Bansal (Ed.) Handbook of Ceramic Composites, Springer US,

SC R

Boston, MA, 2005, pp. 197-224.

[3] S.-Q. Guo, J.-M. Yang, H. Tanaka, Y. Kagawa, Effect of thermal exposure on strength

U

of ZrB2-based composites with nano-sized SiC particles, Compos. Sci. Technol., 68

N

(2008) 3033-3040.

A

[4] M.M. Opeka, I.G. Talmy, E.J. Wuchina, J.A. Zaykoski, S.J. Causey, Mechanical,

M

Thermal, and Oxidation Properties of Refractory Hafnium and zirconium

ED

Compounds, J. Eur. Ceram. Soc., 19 (1999) 2405-2414. [5] F. Monteverde, L. Silvestroni, Combined effects of WC and SiC on densification and

PT

thermo-mechanical stability of ZrB2 ceramics, Mater. Des., 109 (2016) 396-407.

CC E

[6] F. Monteverde, S. Guicciardi, A. Bellosi, Advances in microstructure and mechanical properties of zirconium diboride based ceramics, Mater. Sci. Eng. A, 346 (2003) 310319.

A

[7] W.G. Fahrenholtz, G.E. Hilmas, I.G. Talmy, J.A. Zaykoski, Refractory Diborides of Zirconium and Hafnium, J. Am. Ceram. Soc., 90 (2007) 1347-1364.

[8] W. Li, Y. Zhang, X. Zhang, C. Hong, W. Han, Thermal shock behavior of ZrB2–SiC ultra-high temperature ceramics with addition of zirconia, J. Alloy Compd., 478

23

(2009) 386-391. [9] P. Hu, W. Guolin, Z. Wang, Oxidation mechanism and resistance of ZrB2–SiC composites, Corros. Sci., 51 (2009) 2724-2732. [10] S.-Q. Guo, Densification of ZrB2-based composites and their mechanical and

IP T

physical properties: A review, J. Eur. Ceram. Soc., 29 (2009) 995-1011. [11] D. Gao, Y. Zhang, C. Xu, Y. Song, X. Shi, Oxidation kinetics of hot-pressed ZrB2–

SC R

SiC ceramic matrix composites, Ceram. Int., 39 (2013) 3113-3119.

[12] W.G. Fahrenholtz, The ZrB2 Volatility Diagram, J. Am. Ceram. Soc., 88 (2005) 3509-

U

3512.

N

[13] D. Sciti, M. Brach, A. Bellosi, Long-term oxidation behavior and mechanical

M

53 (2005) 1297-1302.

A

strength degradation of a pressurelessly sintered ZrB2–MoSi2 ceramic, Scr. Mater.,

ED

[14] J. Han, P. Hu, X. Zhang, S. Meng, W. Han, Oxidation-resistant ZrB2–SiC composites at 2200°C, Compos. Sci. Technol., 68 (2008) 799-806.

PT

[15] F. Monteverde, A. Bellosi Oxidation of ZrB2-Based Ceramics in Dry Air, J.

CC E

Electrochem. Soc., 150 (2003) B552-B559. [16] S.C. Zhang, G.E. Hilmas, W.G. Fahrenholtz, Improved Oxidation Resistance of

A

Zirconium Diboride by Tungsten Carbide Additions, J. Am. Ceram. Soc., 91 (2008) 3530-3535.

[17] S.C. Zhang, G.E. Hilmas, W.G. Fahrenholtz, Oxidation of Zirconium Diboride with Tungsten Carbide Additions, J. Am. Ceram. Soc., 94 (2011) 1198-1205. [18] H.-B. Ma, J. Zou, P. Lu, J.-T. Zhu, Z.-Q. Fu, F.-F. Xu, G.-J. Zhang, Oxygen

24

contamination on the surface of ZrB2 powders and its removal, Scr. Mater., 127 (2017) 160-164. [19] S. Gangireddy, S.N. Karlsdottir, S.J. Norton, J.C. Tucker, J.W. Halloran, In situ microscopy observation of liquid flow, zirconia growth, and CO bubble formation

IP T

during high temperature oxidation of zirconium diboride–silicon carbide, J. Eur. Ceram. Soc., 30 (2010) 2365-2374.

SC R

[20] D. Gao, Y. Zhang, J. Fu, C. Xu, Y. Song, X. Shi, Oxidation of zirconium diboride– silicon carbide ceramics under an oxygen partial pressure of 200Pa: Formation of

U

zircon, Corros. Sci., 52 (2010) 3297-3303

N

[21] P.A. Williams, R. Sakidja, J.H. Perepezko, P. Ritt, Oxidation of ZrB2–SiC ultra-high

M

(2012) 3875-3883.

A

temperature composites over a wide range of SiC content, J. Eur. Ceram. Soc., 32

ED

[22] W.G. Fahrenholtz, Thermodynamic Analysis of ZrB2–SiC Oxidation: Formation of a SiC-Depleted Region, J. Am. Ceram. Soc., 90 (2007) 143-148.

PT

[23] K.N. Marushkin, A.S. Alikhanyan, J.H. Greenberg, V.B. Lazarev, V.A. Malyusov,

CC E

O.N. Rozanova, B.T. Melekh, V.I. Gorgoraki, Sublimation thermodynamics of tungsten trioxide, J. Chem. Thermodynamics, 17 (1985) 245-253

A

[24] J.H. Sharp, G.W. Brindley, B.N.N. Achar, Numerical Data for Some Commonly Used Solid State Reaction Equations, J. Am. Ceram. Soc., 49 (1966) 379-382.

[25] J.H. Sharp, S.A. Wentworth, Kinetic analysis of thermogravimetric data, Anal. Chem., 41 (1969) 2060-2062. [26] K.-C. Chou, X.-M. Hou, Kinetics of High-Temperature Oxidation of Inorganic

25

A

CC E

PT

ED

M

A

N

U

SC R

IP T

Nonmetallic Materials, J. Am. Ceram. Soc., 92 (2009) 585-594.

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