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Single-cycle thermal shock resistance of ZrB2–SiCnp ceramic composites
Available online at www.sciencedirect.com
CERAMICS INTERNATIONAL
Ceramics International 40 (2014) 16665–16669 www.elsevier.com/locate/ceramint
Single-cycle thermal shock resistance of ZrB2–SiCnp ceramic composites Han Wenboa,b,n, Zhou Shanbaob, Zhang Jihongc b
a Materials Science and Engineering Postdoctoral Research Station, Harbin Institute of Technology, Harbin 150001, China Science and Technology on Advanced Composites in Special Environments Laboratory, Harbin Institute of Technology, Harbin 150001, China c Mudanjiang Jingangzuan Boron Carbide Co., Ltd, Mudanjiang 157009, China
Received 26 July 2014; received in revised form 7 August 2014; accepted 7 August 2014 Available online 15 August 2014
Abstract SiC nanoparticles were introduced in a ZrB2 matrix prepared via vacuum hot pressing sintering to improve the thermal shock resistance of ultra-high temperature ZrB2–SiC ceramics, namely, ZrB2-20 vol% nanoparticle SiC composites (ZrB2–20SiCnp). The thermal shock resistance of the material were determined at different temperatures via the water quenching-residual strength test method. The specific temperature of the water bath environment and the influence of different nanoparticle sizes on the thermal shock resistance of the material were also investigated. The results show that the critical thermal shock temperature difference (ΔTc) of the ZrB2–20SiCnp composite is 428 1C, which is 11.2% higher than 385 1C, ΔTc of a SiC microparticle-strengthened ZrB2 composite material. The results from the thermal shock analysis of the ZrB2–SiCnp composite indicate that ΔTc is strongly dependent on the formation of unusual intragranular nanostructures. & 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: B. Composites; B. Nanocomposite; C. Thermal shock resistance; D. Borides
. 1. Introduction Ultra-high temperature ceramics (UHTCs) are a unique class of materials because they can resist extremely high temperatures (4 3000 1C) and can maintain a stable shape in reactive environments. UHTCs are used in extreme environments such as reusable atmospheric re-entry vehicles, hypersonic flight vehicles, and rocket propulsion. The thermal shock resistance of UHTCs is a comprehensive reflection of its mechanical and thermal physical properties in response to thermal conditions [1,2]. When UHTCs are exposed to thermal shock, their strength significantly decreases because of its inherent brittleness. Thus, flaking or even breakage may occur, which greatly limits the use of the material as a structural member and reduces its excellent high-temperature performance. Studies have shown that the thermal shock resistance of UHTCs is one of the important factors in determining the material’s life. Therefore, study on the thermal shock resistance of ceramic
materials has been one of the main directions of studies in the field of high-temperature structural ceramics. This paper attempts to introduce SiC nanoparticles in a ZrB2 matrix to improve the thermal shock resistance of ultra-high temperature ZrB2–SiC ceramics. ZrB2-20 vol% SiCnp composites (ZrB2–20SiCnp) were prepared via vacuum hot pressing sintering. The water quenching-residual strength test method [3–6] was used to test the thermal shock resistance of the material at different temperatures. The specific temperature of the water bath environment and the influence of different nanoparticle sizes on the thermal shock resistance of the material, and the thermal shock properties of the material were also investigated. The relationship between the microstructure and the mechanical of the ZrB2–SiCnp composite was determined. 2. Experimental procedure 2.1. Materials
n
Corresponding author. Tel./fax: þ 86 45186403016. E-mail address: [email protected] (H. Wenbo).
http://dx.doi.org/10.1016/j.ceramint.2014.08.028 0272-8842/& 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
The starting powders used in this study include the following: ZrB2 powder (Northwest Institute for Non-ferrous Metal
H. Wenbo et al. / Ceramics International 40 (2014) 16665–16669
Research, China) with an average particle size of 2 μm (N99%), and β-SiC nanopowder (Kaier Nanotechnology Development Co., Ltd, China) with an average particle size of 30 nm (N98%). The SiC nanopowder was first dispersed in ethanol and ultrasonicated for 1 h. The powder mixture containing ZrB2 and 20 vol% SiC nanoparticles was ball-milled using a ZrO2 ball media and ethanol at 180 rpm for 12 h. All ball-milling processes were performed in polyethylene bottles. After mixing, the resulting slurry was dried via rotary evaporation and then screened. The obtained powder mixtures were hot-pressed at 1900 1C for 30 min at a uniaxial pressure of 30 MPa in an argon atmosphere. 2.2. Thermal shock test and characterization Thermal shock tests were performed using the water quenching method. Every thermal cycling includes three steps: (1) the samples were heated to the test temperatures for 10 min in an oven, (2) the heated samples were immediately dropped in cool water with a temperature of 20 1C, and (3) the surface of samples was examined using a quadruplex magnifier when the sample had the same temperature as that of the water. When a visible failure area was detected, the number of thermal cycles was recorded and is defined as the number of cycles to failure. The microstructural features of the thermal shock test specimens were investigated via scanning electron microscopy (SEM, FEI Sirion, Holland) with simultaneous chemical analysis via energy dispersive spectroscopy (EDS, EDAX Inc). Flexural strength (σ) was tested using the three-point bending method on 3 mm 4 mm 36 mm bars with a 30 mm span and a crosshead speed of 0.5 mm/min. Each specimen was ground and polished using diamond slurries to achieve a 1-μm finish. The edges of all specimens were chamfered to minimize the effect of stress concentration caused by machining flaws. A minimum of five specimens was tested in each experimental condition. The critical temperature difference (ΔTc) is defined as 70% of the room temperature strength, which was determined using linear interpolation of the retained strength values as described in ASTM C1525-04 [7]. 3. Results and discussion Fig. 1 shows the residual strength curve of the thermally shocked ZrB2–20SiCnp composite as a function of thermal shock temperature difference (ΔT). The selected ΔT values were 0, 300, 400, 500, 600, 800, 1000, and 1200 1C. The results indicate that the residual strength decreases with increasing ΔT. The attenuation range of the material strength is significantly small when ΔT is less 400 1C. The residual strength is 850 MPa when ΔT is 400 1C. This strength is equal to the original strength of ZrB2–20SiCnp [8]. The residual strength is 40% of the original strength when ΔT is 500 1C. The results indicate that the critical ΔT (ΔTc) of ZrB2–20SiCnp is between 400 1C and 500 1C. Based on calculations, ΔTc is found to be 428 1C, which is 11.2% higher than that of the SiC microparticle-strengthened ZrB2 composite material w385 1C. The residual strength slowly decreased with increasing ΔT.
1200
ZrB2-20SiC
1000
Flexural strength(MPa)
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800 600 400
Tc=428°C
200 0 0
200 400 600 800 1000 1200 Thermal shock temperature difference(°C)
Fig. 1. Residual strength curve of the thermally shocked ZrB2–20SiCnp composite as a function of ΔT.
Fig. 2 shows the SEM micrographs of the surface of the thermally shocked ZrB2–SiCnp composite for a single cycle at different ΔT values, namely, 300 1C, 500 1C, and 800 1C. When ΔT is 300 1C, a few microcracks were observed on the sample surface (as shown in the arrowhead in Fig. 2a). A long crackle was observed on the sample surface when ΔT is 500 1C. The cracks spread when ΔT was increased to 800 1C, as shown in Fig. 2c. Numerous snowflake oxides were observed on the surface of the sample, as shown in the enlarged photograph (Fig. 2d). The EDS results show that the major composition of the sample is ZrO2. The results indicate that oxidization occurs on the surface of the ZrB2 matrix at this test temperature. From Figs. 1 and 2, we speculate that the thermally shocked ZrB2–SiCnp composite model for a single cycle contains a mixed pattern because of breakage and thermal shock damage. When the actual ΔT is below the ΔTc of the material (e.g., ΔT=300 1C), the crack expands at a stable state because a low thermoelastic strain energy inhibits sudden crack propagation. Although the material still experiences thermal shock damage to a particular extent, the attenuation range of its residual strength is very low. By contrast, when the actual ΔT is higher than the ΔTc of the material (e.g., ΔT=500 1C), the residual strength rapidly decreases because a high thermoelastic strain energy causes sudden crack propagation. The residual strength of the material slightly increases when ΔT 4 1000 1C, as shown in Fig. 1. To investigate this change, the thermally shocked surface of the ZrB2–SiCnp composite for a single cycle at ΔT ¼ 1200 1C was observed via SEM, as shown in Fig. 3. The thermally shocked surface, where severe oxidization occurred at ΔT ¼ 1200 1C, appears loose and had a random pattern. Many submicron ZrO2 particles were observed via SEM and DES, as shown in Fig. 3. Compression is observed in the internal matrix upon cooling because the thermal expansion coefficient of ZrO2 is significantly higher than that of a single-phase ZrB2–SiCnp composite material. Thus, surface crack formation can be minimized, which helps improve residual strength.
H. Wenbo et al. / Ceramics International 40 (2014) 16665–16669
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b
5μm
c
20μm
d
20μm
2μm
Fig. 2. SEM micrographs of the thermally shocked surface of the ZrB2–20SiCnp composite for a single cycle, (a) ΔT ¼300 1C, (b) ΔT¼ 500 1C, (c) and (d) ΔT ¼800 1C.
a
b
20μm
1μ m
Fig. 3. SEM micrographs of the thermally shocked surface of the ZrB2–20SiCnp composite for a single cycle at ΔT¼ 1200 1C.
Fig. 4 shows the fractural surface of the ZrB2–SiCnp composite after thermal shock at different ΔT values. The analytical fracture pattern of the ZrB2–SiCnp composite changed from transcrystalline (Fig. 3a) to intercrystalline (Fig. 3b and c) with increasing ΔT. The change is dependent on the formation of intragranular nanostructures during the sintering process of SiC nanoparticles and ZrB2 micropowders [9]. Bulky SiC particles usually have a transgranular structure at the matrix boundary of the crystal grain. Fine SiC nanoparticles have an intragranular structure in the ZrB2 matrix inside of the crystal grain, where numerous interfaces and microcracks are formed because of the difference in the thermal coefficient of expansion of ZrB2 and SiC, as shown in Fig. 4. When the temperature rapidly decreases during thermal shock, the microcracks on the top of the intragranular
structure usually produce tensile stress. The mechanism of thermal shock damage pattern is expressed as follows: E p αp ΔT ð1Þ σs ¼ 1 υp where EP is the elastic module of SiC, GPa; αp is the thermal coefficient of expansion of SiC, 10 6 K 1; υp is the Poisson rate of SiC. We prepared the same component of SiC micro or nanoparticle-reinforced ZrB2 composite materials to evaluate the influence of SiC nanoparticles on the thermal shock resistance of ZrB2–SiCnp. The residual strength rate (R) is expressed as follows: σr R¼ 100 ð2Þ σ0
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H. Wenbo et al. / Ceramics International 40 (2014) 16665–16669
a
b
2μ m
2μ m
c
2μ m Fig. 4. SEM micrographs of the fractural surface of the ZrB2–20SiCnp composite after thermal shock (a) ΔT ¼400 1C, (b) ΔT¼ 800 1C and (c) ΔT¼ 1200 1C.
contrast, the introduction of SiC nanoparticles in a ZrB2 ceramic composite in the same thermal shock test conditions show that the residual strength can be maintained at 85.6% of the initial strength. Moreover, the attenuation level is less than 15%. The residual strength decreases with increasing thermal shock temperature. Thus, the introduction of nanoparticles can improve the thermal shock resistance of the material. The ZrB2–SiCnp composite material has an intragranular structure in the matrix, and internal changes at the nanometer scale affects the matrix grain volume to provide more space when materials undergo rapid cooling. Moreover, the matrix grain can be relatively free from shrinkage and the accumulated thermal elastic strain energy in the material’s internal structure is significantly decreased. Thus, the ability of the material to resist thermal shock is improved. Fig. 5. Residual strength of ZrB2–20SiCp and ZrB2–20SiCnp composites as a function of ΔT.
where σr is the residual strength, MPa; and σ0 is the original strength, MPa. The R values of ZrB2–20SiCnp and ZrB2– 20SiCp after thermal shock are shown in Fig. 5. As shown in Fig. 5, when ΔT o 300 1C, the attenuation degree of the strength of the two composite materials after thermal shock is small. The result indicates that the composites can resist thermal shock damage. However, when ΔT ¼ 400 1C, the strength of the SiC microparticle-reinforced ZrB2 composite material significantly decreased, and the residual strength is only 29.7% of the original strength. The results show that ΔTc cannot exceed 400 1C, and based on Fig. 5, ΔTc is 368 1C. Thus, the results of the present study and those of a previous study [8] are very similar to each other. By
4. Conclusions Based on the single thermal shock test results, the ΔTc of SiC nanoparticle-strengthened ZrB2–SiCnp is 428 1C, which is higher than that of the ZrB2 composite material reinforced with the same content of SiC microparticles (368 1C). Microscopic structure analysis results show that the intragranular structure is the main reason for the improved thermal shock resistance of the ZrB2–SiCnp composite material. The residual strength is also improved because ZrO2, which is found on the surface of the ceramic composite, has higher coefficient of thermal expansion. Thus, pressure is produced in the sample during the cooling process, which hinders crack growth to a particular extent.
H. Wenbo et al. / Ceramics International 40 (2014) 16665–16669
Acknowledgements This work was supported by the National Natural Science Foundation of China (grant no. 50972029), China Postdoctoral Science Foundation funded project (grant no. 2011M501032) and Foundation of Science and Technology on Advanced Composites in Special Environments Laboratory (grant no. 578001261).
References [1] K.Y. Upadhya, W.P. Hoffmann, Materials for ultrahigh temperature structural applications, Am. Ceram. Soc. Bull. 76 (12) (1997) 51–56. [2] W.B. Han, P. Hu, X.H. Zhang, J.C. Han, S.H. Meng., High-temperature oxidation at 1900 1C of ZrB2–xSiC ultrahigh-temperature ceramic composites, J. Am. Ceram. Soc. 91 (10) (2008) 3328–3334.
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[3] Qiang Liu, Wenbo Han, Ping Hu, Microstructure and mechanical properties of ZrB2–SiC nanocomposite ceramic, Scr. Mater. 61 (7) (2009) 690–692. [4] Baoxia Ma, Wenbo Han, Thermal shock resistance of ZrC matrix ceramics, Int. J. Refract. Met. Hard Mater 28 (2) (2010) 187–190. [5] Z.H. Jin, W.J. Luo., Thermal shock residual strength of functionally graded ceramics, Mater. Sci. Eng., A 435 (2006) 71–77. [6] Y.W. Bao, X.H. Wang, H.B. Zhang, Y.C. Zhou., Thermal shock behavior of Ti3AlC2 from between 200 1C and 1300 1C, J. Eur. Ceram. Soc. 25 (2005) 3367–3374. [7] J.W. Zimmermann, G.E. Hilmas, W.G. Fahrenholtz, Thermal shock resistance of ZrB2 and ZrB2–30% SiC, Mater. Chem. Phys. 112 (2008) 140–145. [8] F. Monteverde., L. Scatteia, Resistance to thermal shock and to oxidation of metal diborides-SiC ceramics for aerospace application, J. Am. Ceram. Soc. 90 (2007) 1130–1138. [9] F. Monteverde, C. Melandri, S. Guicciardi, Microstructure and mechanical properties of an HfB2-30 vol% SiC composite consolidated by spark plasma sintering, Mater. Chem. Phys. 100 (2–3) (2006) 513–519.
CERAMICS INTERNATIONAL
Ceramics International 40 (2014) 16665–16669 www.elsevier.com/locate/ceramint
Single-cycle thermal shock resistance of ZrB2–SiCnp ceramic composites Han Wenboa,b,n, Zhou Shanbaob, Zhang Jihongc b
a Materials Science and Engineering Postdoctoral Research Station, Harbin Institute of Technology, Harbin 150001, China Science and Technology on Advanced Composites in Special Environments Laboratory, Harbin Institute of Technology, Harbin 150001, China c Mudanjiang Jingangzuan Boron Carbide Co., Ltd, Mudanjiang 157009, China
Received 26 July 2014; received in revised form 7 August 2014; accepted 7 August 2014 Available online 15 August 2014
Abstract SiC nanoparticles were introduced in a ZrB2 matrix prepared via vacuum hot pressing sintering to improve the thermal shock resistance of ultra-high temperature ZrB2–SiC ceramics, namely, ZrB2-20 vol% nanoparticle SiC composites (ZrB2–20SiCnp). The thermal shock resistance of the material were determined at different temperatures via the water quenching-residual strength test method. The specific temperature of the water bath environment and the influence of different nanoparticle sizes on the thermal shock resistance of the material were also investigated. The results show that the critical thermal shock temperature difference (ΔTc) of the ZrB2–20SiCnp composite is 428 1C, which is 11.2% higher than 385 1C, ΔTc of a SiC microparticle-strengthened ZrB2 composite material. The results from the thermal shock analysis of the ZrB2–SiCnp composite indicate that ΔTc is strongly dependent on the formation of unusual intragranular nanostructures. & 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: B. Composites; B. Nanocomposite; C. Thermal shock resistance; D. Borides
. 1. Introduction Ultra-high temperature ceramics (UHTCs) are a unique class of materials because they can resist extremely high temperatures (4 3000 1C) and can maintain a stable shape in reactive environments. UHTCs are used in extreme environments such as reusable atmospheric re-entry vehicles, hypersonic flight vehicles, and rocket propulsion. The thermal shock resistance of UHTCs is a comprehensive reflection of its mechanical and thermal physical properties in response to thermal conditions [1,2]. When UHTCs are exposed to thermal shock, their strength significantly decreases because of its inherent brittleness. Thus, flaking or even breakage may occur, which greatly limits the use of the material as a structural member and reduces its excellent high-temperature performance. Studies have shown that the thermal shock resistance of UHTCs is one of the important factors in determining the material’s life. Therefore, study on the thermal shock resistance of ceramic
materials has been one of the main directions of studies in the field of high-temperature structural ceramics. This paper attempts to introduce SiC nanoparticles in a ZrB2 matrix to improve the thermal shock resistance of ultra-high temperature ZrB2–SiC ceramics. ZrB2-20 vol% SiCnp composites (ZrB2–20SiCnp) were prepared via vacuum hot pressing sintering. The water quenching-residual strength test method [3–6] was used to test the thermal shock resistance of the material at different temperatures. The specific temperature of the water bath environment and the influence of different nanoparticle sizes on the thermal shock resistance of the material, and the thermal shock properties of the material were also investigated. The relationship between the microstructure and the mechanical of the ZrB2–SiCnp composite was determined. 2. Experimental procedure 2.1. Materials
n
Corresponding author. Tel./fax: þ 86 45186403016. E-mail address: [email protected] (H. Wenbo).
http://dx.doi.org/10.1016/j.ceramint.2014.08.028 0272-8842/& 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
The starting powders used in this study include the following: ZrB2 powder (Northwest Institute for Non-ferrous Metal
H. Wenbo et al. / Ceramics International 40 (2014) 16665–16669
Research, China) with an average particle size of 2 μm (N99%), and β-SiC nanopowder (Kaier Nanotechnology Development Co., Ltd, China) with an average particle size of 30 nm (N98%). The SiC nanopowder was first dispersed in ethanol and ultrasonicated for 1 h. The powder mixture containing ZrB2 and 20 vol% SiC nanoparticles was ball-milled using a ZrO2 ball media and ethanol at 180 rpm for 12 h. All ball-milling processes were performed in polyethylene bottles. After mixing, the resulting slurry was dried via rotary evaporation and then screened. The obtained powder mixtures were hot-pressed at 1900 1C for 30 min at a uniaxial pressure of 30 MPa in an argon atmosphere. 2.2. Thermal shock test and characterization Thermal shock tests were performed using the water quenching method. Every thermal cycling includes three steps: (1) the samples were heated to the test temperatures for 10 min in an oven, (2) the heated samples were immediately dropped in cool water with a temperature of 20 1C, and (3) the surface of samples was examined using a quadruplex magnifier when the sample had the same temperature as that of the water. When a visible failure area was detected, the number of thermal cycles was recorded and is defined as the number of cycles to failure. The microstructural features of the thermal shock test specimens were investigated via scanning electron microscopy (SEM, FEI Sirion, Holland) with simultaneous chemical analysis via energy dispersive spectroscopy (EDS, EDAX Inc). Flexural strength (σ) was tested using the three-point bending method on 3 mm 4 mm 36 mm bars with a 30 mm span and a crosshead speed of 0.5 mm/min. Each specimen was ground and polished using diamond slurries to achieve a 1-μm finish. The edges of all specimens were chamfered to minimize the effect of stress concentration caused by machining flaws. A minimum of five specimens was tested in each experimental condition. The critical temperature difference (ΔTc) is defined as 70% of the room temperature strength, which was determined using linear interpolation of the retained strength values as described in ASTM C1525-04 [7]. 3. Results and discussion Fig. 1 shows the residual strength curve of the thermally shocked ZrB2–20SiCnp composite as a function of thermal shock temperature difference (ΔT). The selected ΔT values were 0, 300, 400, 500, 600, 800, 1000, and 1200 1C. The results indicate that the residual strength decreases with increasing ΔT. The attenuation range of the material strength is significantly small when ΔT is less 400 1C. The residual strength is 850 MPa when ΔT is 400 1C. This strength is equal to the original strength of ZrB2–20SiCnp [8]. The residual strength is 40% of the original strength when ΔT is 500 1C. The results indicate that the critical ΔT (ΔTc) of ZrB2–20SiCnp is between 400 1C and 500 1C. Based on calculations, ΔTc is found to be 428 1C, which is 11.2% higher than that of the SiC microparticle-strengthened ZrB2 composite material w385 1C. The residual strength slowly decreased with increasing ΔT.
1200
ZrB2-20SiC
1000
Flexural strength(MPa)
16666
800 600 400
Tc=428°C
200 0 0
200 400 600 800 1000 1200 Thermal shock temperature difference(°C)
Fig. 1. Residual strength curve of the thermally shocked ZrB2–20SiCnp composite as a function of ΔT.
Fig. 2 shows the SEM micrographs of the surface of the thermally shocked ZrB2–SiCnp composite for a single cycle at different ΔT values, namely, 300 1C, 500 1C, and 800 1C. When ΔT is 300 1C, a few microcracks were observed on the sample surface (as shown in the arrowhead in Fig. 2a). A long crackle was observed on the sample surface when ΔT is 500 1C. The cracks spread when ΔT was increased to 800 1C, as shown in Fig. 2c. Numerous snowflake oxides were observed on the surface of the sample, as shown in the enlarged photograph (Fig. 2d). The EDS results show that the major composition of the sample is ZrO2. The results indicate that oxidization occurs on the surface of the ZrB2 matrix at this test temperature. From Figs. 1 and 2, we speculate that the thermally shocked ZrB2–SiCnp composite model for a single cycle contains a mixed pattern because of breakage and thermal shock damage. When the actual ΔT is below the ΔTc of the material (e.g., ΔT=300 1C), the crack expands at a stable state because a low thermoelastic strain energy inhibits sudden crack propagation. Although the material still experiences thermal shock damage to a particular extent, the attenuation range of its residual strength is very low. By contrast, when the actual ΔT is higher than the ΔTc of the material (e.g., ΔT=500 1C), the residual strength rapidly decreases because a high thermoelastic strain energy causes sudden crack propagation. The residual strength of the material slightly increases when ΔT 4 1000 1C, as shown in Fig. 1. To investigate this change, the thermally shocked surface of the ZrB2–SiCnp composite for a single cycle at ΔT ¼ 1200 1C was observed via SEM, as shown in Fig. 3. The thermally shocked surface, where severe oxidization occurred at ΔT ¼ 1200 1C, appears loose and had a random pattern. Many submicron ZrO2 particles were observed via SEM and DES, as shown in Fig. 3. Compression is observed in the internal matrix upon cooling because the thermal expansion coefficient of ZrO2 is significantly higher than that of a single-phase ZrB2–SiCnp composite material. Thus, surface crack formation can be minimized, which helps improve residual strength.
H. Wenbo et al. / Ceramics International 40 (2014) 16665–16669
a
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b
5μm
c
20μm
d
20μm
2μm
Fig. 2. SEM micrographs of the thermally shocked surface of the ZrB2–20SiCnp composite for a single cycle, (a) ΔT ¼300 1C, (b) ΔT¼ 500 1C, (c) and (d) ΔT ¼800 1C.
a
b
20μm
1μ m
Fig. 3. SEM micrographs of the thermally shocked surface of the ZrB2–20SiCnp composite for a single cycle at ΔT¼ 1200 1C.
Fig. 4 shows the fractural surface of the ZrB2–SiCnp composite after thermal shock at different ΔT values. The analytical fracture pattern of the ZrB2–SiCnp composite changed from transcrystalline (Fig. 3a) to intercrystalline (Fig. 3b and c) with increasing ΔT. The change is dependent on the formation of intragranular nanostructures during the sintering process of SiC nanoparticles and ZrB2 micropowders [9]. Bulky SiC particles usually have a transgranular structure at the matrix boundary of the crystal grain. Fine SiC nanoparticles have an intragranular structure in the ZrB2 matrix inside of the crystal grain, where numerous interfaces and microcracks are formed because of the difference in the thermal coefficient of expansion of ZrB2 and SiC, as shown in Fig. 4. When the temperature rapidly decreases during thermal shock, the microcracks on the top of the intragranular
structure usually produce tensile stress. The mechanism of thermal shock damage pattern is expressed as follows: E p αp ΔT ð1Þ σs ¼ 1 υp where EP is the elastic module of SiC, GPa; αp is the thermal coefficient of expansion of SiC, 10 6 K 1; υp is the Poisson rate of SiC. We prepared the same component of SiC micro or nanoparticle-reinforced ZrB2 composite materials to evaluate the influence of SiC nanoparticles on the thermal shock resistance of ZrB2–SiCnp. The residual strength rate (R) is expressed as follows: σr R¼ 100 ð2Þ σ0
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H. Wenbo et al. / Ceramics International 40 (2014) 16665–16669
a
b
2μ m
2μ m
c
2μ m Fig. 4. SEM micrographs of the fractural surface of the ZrB2–20SiCnp composite after thermal shock (a) ΔT ¼400 1C, (b) ΔT¼ 800 1C and (c) ΔT¼ 1200 1C.
contrast, the introduction of SiC nanoparticles in a ZrB2 ceramic composite in the same thermal shock test conditions show that the residual strength can be maintained at 85.6% of the initial strength. Moreover, the attenuation level is less than 15%. The residual strength decreases with increasing thermal shock temperature. Thus, the introduction of nanoparticles can improve the thermal shock resistance of the material. The ZrB2–SiCnp composite material has an intragranular structure in the matrix, and internal changes at the nanometer scale affects the matrix grain volume to provide more space when materials undergo rapid cooling. Moreover, the matrix grain can be relatively free from shrinkage and the accumulated thermal elastic strain energy in the material’s internal structure is significantly decreased. Thus, the ability of the material to resist thermal shock is improved. Fig. 5. Residual strength of ZrB2–20SiCp and ZrB2–20SiCnp composites as a function of ΔT.
where σr is the residual strength, MPa; and σ0 is the original strength, MPa. The R values of ZrB2–20SiCnp and ZrB2– 20SiCp after thermal shock are shown in Fig. 5. As shown in Fig. 5, when ΔT o 300 1C, the attenuation degree of the strength of the two composite materials after thermal shock is small. The result indicates that the composites can resist thermal shock damage. However, when ΔT ¼ 400 1C, the strength of the SiC microparticle-reinforced ZrB2 composite material significantly decreased, and the residual strength is only 29.7% of the original strength. The results show that ΔTc cannot exceed 400 1C, and based on Fig. 5, ΔTc is 368 1C. Thus, the results of the present study and those of a previous study [8] are very similar to each other. By
4. Conclusions Based on the single thermal shock test results, the ΔTc of SiC nanoparticle-strengthened ZrB2–SiCnp is 428 1C, which is higher than that of the ZrB2 composite material reinforced with the same content of SiC microparticles (368 1C). Microscopic structure analysis results show that the intragranular structure is the main reason for the improved thermal shock resistance of the ZrB2–SiCnp composite material. The residual strength is also improved because ZrO2, which is found on the surface of the ceramic composite, has higher coefficient of thermal expansion. Thus, pressure is produced in the sample during the cooling process, which hinders crack growth to a particular extent.
H. Wenbo et al. / Ceramics International 40 (2014) 16665–16669
Acknowledgements This work was supported by the National Natural Science Foundation of China (grant no. 50972029), China Postdoctoral Science Foundation funded project (grant no. 2011M501032) and Foundation of Science and Technology on Advanced Composites in Special Environments Laboratory (grant no. 578001261).
References [1] K.Y. Upadhya, W.P. Hoffmann, Materials for ultrahigh temperature structural applications, Am. Ceram. Soc. Bull. 76 (12) (1997) 51–56. [2] W.B. Han, P. Hu, X.H. Zhang, J.C. Han, S.H. Meng., High-temperature oxidation at 1900 1C of ZrB2–xSiC ultrahigh-temperature ceramic composites, J. Am. Ceram. Soc. 91 (10) (2008) 3328–3334.
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