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Mechanical properties and thermal shock behavior of hot-pressed ZrB2–SiC–AlN composites
Journal of Alloys and Compounds 475 (2009) 762–765
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Mechanical properties and thermal shock behavior of hot-pressed ZrB2 –SiC–AlN composites Yu Wang ∗ , Jun Liang, Wenbo Han, Xinghong Zhang Center for Composite Materials and Structure, Harbin Institute of Technology, Harbin 150001, China
a r t i c l e
i n f o
Article history: Received 18 June 2008 Received in revised form 29 July 2008 Accepted 1 August 2008 Available online 11 September 2008 Keywords: ZrB2 –SiC–AlN composites Microstructure Mechanical properties Thermal shock resistance
a b s t r a c t ZrB2 -based ceramics containing 20 vol.% SiC and 10 vol.% AlN were prepared by hot pressing two different ZrB2 precursor powders with average particle sizes of 5 m in ZSA (ZrB2 –20 vol.% SiC–10 vol.% AlN) and 2 m in GZSA (ZrB2 –20 vol.% SiC–10 vol.% AlN) ceramics. The microstructures and mechanical properties were investigated. The thermal shock behavior was studied through the methods of retained flexural strength after quenching into water of 20 ◦ C with the temperature differences between 0 and 800 ◦ C. GZSA showed a better resistance to thermal shock in terms of the increased critical temperature difference (TC of GZSA was 408 ◦ C, about 40 ◦ C higher than that of ZSA), since the finer grain size enhanced fracture toughness by promoting crack bridging, which inhibited the propagation of cracks due to thermal stresses. The influence of quenching temperature difference on the fracture toughness was not significant. Comparing with ZSA, the thermal stress fracture resistance parameter, R, and thermal stress damage resistance parameter, R , of GZSA were all increased. © 2008 Elsevier B.V. All rights reserved.
1. Introduction As one of the most promising high temperature ceramic materials, ZrB2 has attracted much attention for demanding elevated temperature applications for its unique combination of high hardness, high melting point, excellent corrosion resistance against molten iron and slags and superb thermal stability [1]. This ceramic has currently been employed as molten metal crucibles and high temperature electrodes, and is also a candidate for thermal protection systems for hypersonic re-entry vehicles [2]. Monolithic ZrB2 has poor oxidation resistance at high temperatures, especially at temperatures above 1100 ◦ C. The addition of SiC is highly beneficial for improving the oxidation resistance of ZrB2 at temperatures above 1200 ◦ C, as a result of the formation of a protective borosilicate glass layer [3,4]. On the other hand, due to the high melting point of the constituents and the presence of oxide impurities on the particle surfaces, the sintering of ZrB2 powders to full density is rather difficult. The introduction of AlN can promote the densification through a marked reduction in porosity by filling of inter-particle voids as a result of formation of a glassy phase causing liquid phase sintering [5,6]. The research work of Han et al. [6] on the ZrB2 –SiC–AlN ceramics has shown that the addition of AlN improved the sinterability, flexural strength and fracture toughness
∗ Corresponding author. Tel.: +86 451 86412613; fax: +86 451 86412613. E-mail address: [email protected] (Y. Wang). 0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2008.08.001
of ZrB2 -based ceramics due to the improvement in densification and inhibition of grain growth. The thermal shock damage is one of the main failure modes for ultra-high temperature ceramics (UHTCs) applied in aerospace and astronautic industries, and the thermal shock resistance is an important performance index to evaluate UHTCs in these applications. Zhang et al. [7] has researched the thermal shock behavior of SiC-whisker-reinforced ZrB2 UHTCs by water quench testing into a boiling water bath. However, to date, no studies have focused on the thermal shock behavior of ZrB2 –SiC–AlN ceramics. In this work, the microstructure, mechanical properties and thermal shock behavior of hot-pressed ZrB2 –SiC–AlN composites, prepared from a single grade of SiC and AlN, and two different ZrB2 precursor powders, were investigated. 2. Experimental procedure Commercially available zirconium diboride, silicon carbide and aluminum nitride powders were selected as raw materials. The two ZrB2 powders both had a purity of >99.5%, but had particle sizes of 5 m and 2 m. SiC and AlN powders had average particle sizes of 1 m and 100 nm, and had purities of 98.7 and 98.1%, respectively. Two types of batches were prepared by hot pressing: (i) one containing coarse (5 m) ZrB2 particles (material ZSA), and (ii) the other containing fine particles (2 m) of ZrB2 (material GZSA), with the same particle sizes of SiC and AlN in both materials. The phase compositions of both ZSA and GZSA ceramics were 70 vol.% ZrB2 , 20 vol.% SiC and 10 vol.% AlN. The starting powder mixtures were ballmilled in a polyethylene bottle for 8 h using WC balls and ethanol as the grinding media, and then dried in a rotating evaporator. The as-processed powder mixtures were hot pressed at 1850 ◦ C for 60 min in argon atmosphere, with an applied uniaxial pressure of 30 MPa using a BN-coated graphite die.
Y. Wang et al. / Journal of Alloys and Compounds 475 (2009) 762–765
763
Fig. 1. SEM micrographs of polished cross sections of (a) ZSA and (b) GZSA composites.
Bulk density and apparent porosity of the sintered samples were measured by using Archimedes’ method in deionized water, and the theoretical density was calculated by applying a volumetric rule of mixtures. The microstructures of polished and fractured surfaces were analyzed by scanning electron microscopy (SEM, FEI Sirion 200, Philips, Holland) and energy dispersive spectroscopy (EDS, EDAX Inc., USA). Flexural strength () was measured by using a conventional three point loading fixture. The dimensions of all tested specimens were 36 mm × 4 mm × 3 mm, with a loading span of 30 mm and a crosshead speed of 0.5 mm min−1 at room temperature. Fracture toughness (KIC ) was evaluated by a single-edge notched beam (SENB) test on 22 mm × 2 mm × 4 mm test bars with a 16 mm span and a machine crosshead speed of 0.05 mm min−1 . All surfaces of the specimens were ground and polished using diamond abrasives down to a 0.5 m finish. The edges of all the specimens employed in flexural strength testing were chamfered to minimize the effect of stress concentration due to machining flaws. Thermal shock resistance was assessed by measuring the reduction in flexural strength produced by rapid quenching of test specimens from elevated temperature. The samples were heated for 15 min at a selected temperature in an electric resistance furnace and then dropped into a water bath of 20 ◦ C. Retained flexural strength and fracture toughness were tested after thermal shock for both composites.
3. Results and discussion 3.1. Microstructure characteristics and mechanical properties The polished cross-section micrographs of ZSA and GZSA ceramics (Fig. 1) showed a uniform distribution of reinforcing particles in the ZrB2 -matrix. Analysis by EDS (not shown here) indicated that the gray matrix was ZrB2 and the dark gray particles were a SiC–AlN, which was in reasonable agreement with results reported in the literature [6]. The formation of a solid solution was consistent with the SiC–AlN phase diagram [8]. When a combination of relative density (Table 1) and SEM analysis was used, no indication of porosity was found in ZSA or GZSA billets, which showed that hot-pressing way was effective for the densification of ZSA or GZSA ceramics.
Table 1 Mechanical properties of ZSA and GZSA composites (relative density, flexural strength and fracture toughness KIC ) Materials
Relative density (%)
(MPa)
KIC (MPa m1/2 )
ZSA GZSA
98.7 100
749 ± 53.3 831 ± 12.4
4.6 ± 0.3 5.6 ± 0.5
Fig. 2 shows the fracture surfaces of ZSA and GZSA ceramics, which exhibited a mixed mechanism of inter- and intra-granular fracture. The grain size distributions in GZSA and ZSA were 2–3 m and 3–5 m, respectively, and the average grain size in GZSA (Figs. 1b and 2b) was ∼2.5 m, much smaller than the mean size in ZSA ceramics (∼4.5 m, see Figs. 1a and 2a), owing to the grain refinement of matrix. Relative density and mechanical properties of ZSA and GZSA composites were listed in Table 1. The relative density of GZSA is 100%, higher than that of ZSA with 98.7%. The flexural strength of GZSA was 831 MPa, approximately 11% higher than ZSA with 749 MPa. From statements above, it can be concluded that smaller matrix grain size led to higher relative density and flexural strength. From Table 1, it was found that the fracture toughness of GZSA is about 20% higher than that of ZSA. This should be also contributed to the grain refinement, since the grain refinement can increase the grain boundary area, lengthened the crack propagation path. Apparently, the improvement of fracture toughness is advantageous to the thermal shock resistance [9,10]. 3.2. Thermal shock behavior The critical temperature difference of materials after severe quenching was an important index to evaluate the thermal shock resistance [11]. According to ASTM standard C1525-04, the critical
Fig. 2. SEM micrographs of the fracture surface of (a) ZSA and (b) GZSA ceramics.
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Y. Wang et al. / Journal of Alloys and Compounds 475 (2009) 762–765
Fig. 3. Retained strength ( r ) versus temperature difference (T) for ZSA and GZSA ceramics; the solid lines connect the mean r values at each T and the horizontal dashed lines designate the 30% reductions in flexural strength of ZSA and GZSA.
value of the temperature difference (exposure temperature minus the water quench temperature) was determined by a 30% reduction in flexural strength compared to the average flexural strength of the as-received test specimens. The curves of retained flexural strength ( r ) of GZSA and ZSA composites versus temperature difference (T) were plotted in Fig. 3. The curve for ZSA showed a critical temperature difference of 367 ◦ C and a characteristic drop of flexural strength at that point. This drop was followed by a modest decrease as T increased further. The GZSA ceramics produced a 30% reduction in strength for a T of 408 ◦ C. These results apparently showed that GZSA composites had greater resistance to thermal shock. This was mainly attributed to the improvement of fracture toughness in GZSA ceramics. It was believed that the grain boundaries were critical to the resistance of crack propagation. For GZSA, the finer grain size enhanced fracture toughness by promoting crack bridging and lengthening crack propagation path, which inhibited the propagation of cracks due to thermal stresses. The propagation paths of thermal shock-induced cracks in GZSA ceramics were shown in Fig. 4, and the crack deflection, crack bridging and microcracks could be seen. These mechanisms absorbed crack propagating energy during fracture and led to the enhanced toughness, and thus were beneficial to the improvement of the thermal shock resistance. Fig. 5 shows the variation of the fracture toughness (KIC ) versus the quenching temperature difference (T) to evaluate whether the influence of T on the fracture toughness (KIC ) was significant. As can be seen, GZSA composites exhibited higher fracture toughness values than those of ZSA ceramics for all the investigated Ts. The fracture toughness of GZSA decreased slightly from 5.6 to 5.1 MPa m1/2 as quenching temperature difference increased. For ZSA ceramics, the fracture toughness also degraded slightly except for a sharp degradation at the T of 800 ◦ C. The results indicated that the influence of quenching temperature difference on
Fig. 5. Fracture toughness (KIC ) versus temperature difference (T) for ZSA and GZSA composites.
Fig. 6. SEM micrograph of thermal shock-induced crack propagation on a polished surface of ZSA ceramics at T of 800 ◦ C.
the fracture toughness was not obvious. The reason of the sharp degradation of ZSA at the T of 800 ◦ C was probably that catastrophic cracks originated and propagated unstably. These cracks usually initiated at the grain boundaries and/or defects (Fig. 6). In order to further evaluate the thermal stress crack initiation and propagation behavior of ZSA and GZSA composites, two thermal shock resistance parameters are used according to Hasselman’s model for the thermal shock [9,12,13]: thermal stress fracture resistance parameter, R, and thermal stress damage resistance parameter, R , and they are defined, respectively, as follows: R=
f (1 − ) E˛
R =
2 KIC
f2 (1 − )
Fig. 4. SEM micrographs of thermal shock-induced crack propagation on polished surfaces of GZSA ceramics at T of 800 ◦ C.
(1)
(2)
Y. Wang et al. / Journal of Alloys and Compounds 475 (2009) 762–765
where f is the tensile strength of the material, E is the Young’s modulus, is the Poisson’s ratio, ˛ is the thermal expansion coefficient, and KIC is the fracture toughness. R represents the critical temperature difference, TC , to which a body can be subjected without the initiation of fracture under steady state heat flow or severe transient thermal condition [13]. R decides the resistance to catastrophic crack propagation of ceramics under a critical temperature difference, TC [13,14]. From Table 1, the calculated crack initiation parameter R of GZSA was approximately 10% larger than that of ZSA, which was very close to the experimental result shown in the curves in Fig. 3. For the simplicity of the calculation, the magnitudes of the Poisson’s ratio, the thermal expansion coefficient and the Young’s modulus for ZSA and GZSA were supposed to be identical, respectively. The calculated crack propagation parameter R of GZSA was much larger (more than 20%) than that of the ZSA, implying that the crack propagation was more restricted for GZSA ceramics. Hence, unstable crack propagation occurred more possibly in ZSA ceramics during water quenching, which might further interpret the sharp degradation in fracture toughness of ZSA at the T of 800 ◦ C in Fig. 5. The calculated results above further confirmed that the finer starting particle size of the ZrB2 -matrix had beneficial effect on the thermal shock resistance of ZrB2 –SiC–AlN ceramics. 4. Conclusion Hot-pressed specimens with two different ZrB2 particle sizes were prepared from powder mixtures with the compositions of ZrB2 –20 vol.% SiC–10 vol.% AlN. The fracture surfaces of both materials showed a mixed mechanism of inter- and intra-granular fracture. The flexural strength and fracture toughness of GZSA were approximately 11 and 20% higher than those of ZSA ceramics, respectively, as a result of the grain refinement of the matrix.
765
The critical temperature difference of thermal shock resistance for GZSA was found to be higher than that for ZSA experimentally as much as 40 ◦ C. Comparing with ZSA, the thermal stress fracture resistance parameter, R, and thermal stress damage resistance parameter, R , of GZSA were all increased. Such superior thermal shock resistance was attributed to the remarkable improvement of the fracture toughness by grain refinement. The influence of quenching temperature difference on the fracture toughness was not significant. The results in this work indicated that grain refinement of the ZrB2 -matrix was a potential method for improving the thermal shock resistance of ZrB2 –SiC–AlN ceramics. Acknowledgements This work was supported by Program for New Century Excellent Talents in University (NCET-05-0346), the National Natural Science Foundation of China (50602010) and the National Defence foundation (9140A12030106HT01). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
F. Monteverde, S. Guicciardi, A. Bellosi, Mater. Sci. Eng. A 346 (2003) 310–319. K. Upadhya, J.M. Yang, W.P. Hoffmann, Am. Ceram. Soc. Bull. 58 (1997) 51–56. W.G. Fahrenholtz, J. Am. Ceram. Soc. 90 (2007) 143–148. M.M. Opeka, I.G. Talmy, J.A. Zaykosky, J. Mater. Sci. 39 (2004) 5887–5904. F. Monteverde, A. Bellosi, Adv. Eng. Mater. 7 (2003) 508–512. W.B. Han, G. Li, X.H. Zhang, J.C. Han, J. Alloys Compd. 471 (2009) 488–491. X.H. Zhang, L. Xu, S.Y. Du, W.B. Han, J.C. Han, Scr. Mater. 59 (2008) 55–58. W.R. Smith, R.W. Missen, Theory and Algorithms 19 (1980) 1–10. D.P.H. Hasselman, J. Am. Ceram. Soc. 53 (1970) 490–495. T.N. Tiegs, P.F. Becher, J. Am. Ceram. Soc. 70 (1987) 109. M.I. Nieto, R. Martinez, L. Mazerolles, C. Boudin, J. Eur. Ceram. Soc. 24 (2004) 2293–2301. [12] D.P.H. Hasselman, J. Am. Ceram. Soc. 52 (1969) 600–604. [13] D.P.H. Hasselman, Am. Ceram. Soc. Bull. 49 (1970) 1033–1037. [14] Q.C. Zhang, Mechanical Properties of Ceramic Materials, Chinese Science Press, Beijing, 1987.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Mechanical properties and thermal shock behavior of hot-pressed ZrB2 –SiC–AlN composites Yu Wang ∗ , Jun Liang, Wenbo Han, Xinghong Zhang Center for Composite Materials and Structure, Harbin Institute of Technology, Harbin 150001, China
a r t i c l e
i n f o
Article history: Received 18 June 2008 Received in revised form 29 July 2008 Accepted 1 August 2008 Available online 11 September 2008 Keywords: ZrB2 –SiC–AlN composites Microstructure Mechanical properties Thermal shock resistance
a b s t r a c t ZrB2 -based ceramics containing 20 vol.% SiC and 10 vol.% AlN were prepared by hot pressing two different ZrB2 precursor powders with average particle sizes of 5 m in ZSA (ZrB2 –20 vol.% SiC–10 vol.% AlN) and 2 m in GZSA (ZrB2 –20 vol.% SiC–10 vol.% AlN) ceramics. The microstructures and mechanical properties were investigated. The thermal shock behavior was studied through the methods of retained flexural strength after quenching into water of 20 ◦ C with the temperature differences between 0 and 800 ◦ C. GZSA showed a better resistance to thermal shock in terms of the increased critical temperature difference (TC of GZSA was 408 ◦ C, about 40 ◦ C higher than that of ZSA), since the finer grain size enhanced fracture toughness by promoting crack bridging, which inhibited the propagation of cracks due to thermal stresses. The influence of quenching temperature difference on the fracture toughness was not significant. Comparing with ZSA, the thermal stress fracture resistance parameter, R, and thermal stress damage resistance parameter, R , of GZSA were all increased. © 2008 Elsevier B.V. All rights reserved.
1. Introduction As one of the most promising high temperature ceramic materials, ZrB2 has attracted much attention for demanding elevated temperature applications for its unique combination of high hardness, high melting point, excellent corrosion resistance against molten iron and slags and superb thermal stability [1]. This ceramic has currently been employed as molten metal crucibles and high temperature electrodes, and is also a candidate for thermal protection systems for hypersonic re-entry vehicles [2]. Monolithic ZrB2 has poor oxidation resistance at high temperatures, especially at temperatures above 1100 ◦ C. The addition of SiC is highly beneficial for improving the oxidation resistance of ZrB2 at temperatures above 1200 ◦ C, as a result of the formation of a protective borosilicate glass layer [3,4]. On the other hand, due to the high melting point of the constituents and the presence of oxide impurities on the particle surfaces, the sintering of ZrB2 powders to full density is rather difficult. The introduction of AlN can promote the densification through a marked reduction in porosity by filling of inter-particle voids as a result of formation of a glassy phase causing liquid phase sintering [5,6]. The research work of Han et al. [6] on the ZrB2 –SiC–AlN ceramics has shown that the addition of AlN improved the sinterability, flexural strength and fracture toughness
∗ Corresponding author. Tel.: +86 451 86412613; fax: +86 451 86412613. E-mail address: [email protected] (Y. Wang). 0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2008.08.001
of ZrB2 -based ceramics due to the improvement in densification and inhibition of grain growth. The thermal shock damage is one of the main failure modes for ultra-high temperature ceramics (UHTCs) applied in aerospace and astronautic industries, and the thermal shock resistance is an important performance index to evaluate UHTCs in these applications. Zhang et al. [7] has researched the thermal shock behavior of SiC-whisker-reinforced ZrB2 UHTCs by water quench testing into a boiling water bath. However, to date, no studies have focused on the thermal shock behavior of ZrB2 –SiC–AlN ceramics. In this work, the microstructure, mechanical properties and thermal shock behavior of hot-pressed ZrB2 –SiC–AlN composites, prepared from a single grade of SiC and AlN, and two different ZrB2 precursor powders, were investigated. 2. Experimental procedure Commercially available zirconium diboride, silicon carbide and aluminum nitride powders were selected as raw materials. The two ZrB2 powders both had a purity of >99.5%, but had particle sizes of 5 m and 2 m. SiC and AlN powders had average particle sizes of 1 m and 100 nm, and had purities of 98.7 and 98.1%, respectively. Two types of batches were prepared by hot pressing: (i) one containing coarse (5 m) ZrB2 particles (material ZSA), and (ii) the other containing fine particles (2 m) of ZrB2 (material GZSA), with the same particle sizes of SiC and AlN in both materials. The phase compositions of both ZSA and GZSA ceramics were 70 vol.% ZrB2 , 20 vol.% SiC and 10 vol.% AlN. The starting powder mixtures were ballmilled in a polyethylene bottle for 8 h using WC balls and ethanol as the grinding media, and then dried in a rotating evaporator. The as-processed powder mixtures were hot pressed at 1850 ◦ C for 60 min in argon atmosphere, with an applied uniaxial pressure of 30 MPa using a BN-coated graphite die.
Y. Wang et al. / Journal of Alloys and Compounds 475 (2009) 762–765
763
Fig. 1. SEM micrographs of polished cross sections of (a) ZSA and (b) GZSA composites.
Bulk density and apparent porosity of the sintered samples were measured by using Archimedes’ method in deionized water, and the theoretical density was calculated by applying a volumetric rule of mixtures. The microstructures of polished and fractured surfaces were analyzed by scanning electron microscopy (SEM, FEI Sirion 200, Philips, Holland) and energy dispersive spectroscopy (EDS, EDAX Inc., USA). Flexural strength () was measured by using a conventional three point loading fixture. The dimensions of all tested specimens were 36 mm × 4 mm × 3 mm, with a loading span of 30 mm and a crosshead speed of 0.5 mm min−1 at room temperature. Fracture toughness (KIC ) was evaluated by a single-edge notched beam (SENB) test on 22 mm × 2 mm × 4 mm test bars with a 16 mm span and a machine crosshead speed of 0.05 mm min−1 . All surfaces of the specimens were ground and polished using diamond abrasives down to a 0.5 m finish. The edges of all the specimens employed in flexural strength testing were chamfered to minimize the effect of stress concentration due to machining flaws. Thermal shock resistance was assessed by measuring the reduction in flexural strength produced by rapid quenching of test specimens from elevated temperature. The samples were heated for 15 min at a selected temperature in an electric resistance furnace and then dropped into a water bath of 20 ◦ C. Retained flexural strength and fracture toughness were tested after thermal shock for both composites.
3. Results and discussion 3.1. Microstructure characteristics and mechanical properties The polished cross-section micrographs of ZSA and GZSA ceramics (Fig. 1) showed a uniform distribution of reinforcing particles in the ZrB2 -matrix. Analysis by EDS (not shown here) indicated that the gray matrix was ZrB2 and the dark gray particles were a SiC–AlN, which was in reasonable agreement with results reported in the literature [6]. The formation of a solid solution was consistent with the SiC–AlN phase diagram [8]. When a combination of relative density (Table 1) and SEM analysis was used, no indication of porosity was found in ZSA or GZSA billets, which showed that hot-pressing way was effective for the densification of ZSA or GZSA ceramics.
Table 1 Mechanical properties of ZSA and GZSA composites (relative density, flexural strength and fracture toughness KIC ) Materials
Relative density (%)
(MPa)
KIC (MPa m1/2 )
ZSA GZSA
98.7 100
749 ± 53.3 831 ± 12.4
4.6 ± 0.3 5.6 ± 0.5
Fig. 2 shows the fracture surfaces of ZSA and GZSA ceramics, which exhibited a mixed mechanism of inter- and intra-granular fracture. The grain size distributions in GZSA and ZSA were 2–3 m and 3–5 m, respectively, and the average grain size in GZSA (Figs. 1b and 2b) was ∼2.5 m, much smaller than the mean size in ZSA ceramics (∼4.5 m, see Figs. 1a and 2a), owing to the grain refinement of matrix. Relative density and mechanical properties of ZSA and GZSA composites were listed in Table 1. The relative density of GZSA is 100%, higher than that of ZSA with 98.7%. The flexural strength of GZSA was 831 MPa, approximately 11% higher than ZSA with 749 MPa. From statements above, it can be concluded that smaller matrix grain size led to higher relative density and flexural strength. From Table 1, it was found that the fracture toughness of GZSA is about 20% higher than that of ZSA. This should be also contributed to the grain refinement, since the grain refinement can increase the grain boundary area, lengthened the crack propagation path. Apparently, the improvement of fracture toughness is advantageous to the thermal shock resistance [9,10]. 3.2. Thermal shock behavior The critical temperature difference of materials after severe quenching was an important index to evaluate the thermal shock resistance [11]. According to ASTM standard C1525-04, the critical
Fig. 2. SEM micrographs of the fracture surface of (a) ZSA and (b) GZSA ceramics.
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Fig. 3. Retained strength ( r ) versus temperature difference (T) for ZSA and GZSA ceramics; the solid lines connect the mean r values at each T and the horizontal dashed lines designate the 30% reductions in flexural strength of ZSA and GZSA.
value of the temperature difference (exposure temperature minus the water quench temperature) was determined by a 30% reduction in flexural strength compared to the average flexural strength of the as-received test specimens. The curves of retained flexural strength ( r ) of GZSA and ZSA composites versus temperature difference (T) were plotted in Fig. 3. The curve for ZSA showed a critical temperature difference of 367 ◦ C and a characteristic drop of flexural strength at that point. This drop was followed by a modest decrease as T increased further. The GZSA ceramics produced a 30% reduction in strength for a T of 408 ◦ C. These results apparently showed that GZSA composites had greater resistance to thermal shock. This was mainly attributed to the improvement of fracture toughness in GZSA ceramics. It was believed that the grain boundaries were critical to the resistance of crack propagation. For GZSA, the finer grain size enhanced fracture toughness by promoting crack bridging and lengthening crack propagation path, which inhibited the propagation of cracks due to thermal stresses. The propagation paths of thermal shock-induced cracks in GZSA ceramics were shown in Fig. 4, and the crack deflection, crack bridging and microcracks could be seen. These mechanisms absorbed crack propagating energy during fracture and led to the enhanced toughness, and thus were beneficial to the improvement of the thermal shock resistance. Fig. 5 shows the variation of the fracture toughness (KIC ) versus the quenching temperature difference (T) to evaluate whether the influence of T on the fracture toughness (KIC ) was significant. As can be seen, GZSA composites exhibited higher fracture toughness values than those of ZSA ceramics for all the investigated Ts. The fracture toughness of GZSA decreased slightly from 5.6 to 5.1 MPa m1/2 as quenching temperature difference increased. For ZSA ceramics, the fracture toughness also degraded slightly except for a sharp degradation at the T of 800 ◦ C. The results indicated that the influence of quenching temperature difference on
Fig. 5. Fracture toughness (KIC ) versus temperature difference (T) for ZSA and GZSA composites.
Fig. 6. SEM micrograph of thermal shock-induced crack propagation on a polished surface of ZSA ceramics at T of 800 ◦ C.
the fracture toughness was not obvious. The reason of the sharp degradation of ZSA at the T of 800 ◦ C was probably that catastrophic cracks originated and propagated unstably. These cracks usually initiated at the grain boundaries and/or defects (Fig. 6). In order to further evaluate the thermal stress crack initiation and propagation behavior of ZSA and GZSA composites, two thermal shock resistance parameters are used according to Hasselman’s model for the thermal shock [9,12,13]: thermal stress fracture resistance parameter, R, and thermal stress damage resistance parameter, R , and they are defined, respectively, as follows: R=
f (1 − ) E˛
R =
2 KIC
f2 (1 − )
Fig. 4. SEM micrographs of thermal shock-induced crack propagation on polished surfaces of GZSA ceramics at T of 800 ◦ C.
(1)
(2)
Y. Wang et al. / Journal of Alloys and Compounds 475 (2009) 762–765
where f is the tensile strength of the material, E is the Young’s modulus, is the Poisson’s ratio, ˛ is the thermal expansion coefficient, and KIC is the fracture toughness. R represents the critical temperature difference, TC , to which a body can be subjected without the initiation of fracture under steady state heat flow or severe transient thermal condition [13]. R decides the resistance to catastrophic crack propagation of ceramics under a critical temperature difference, TC [13,14]. From Table 1, the calculated crack initiation parameter R of GZSA was approximately 10% larger than that of ZSA, which was very close to the experimental result shown in the curves in Fig. 3. For the simplicity of the calculation, the magnitudes of the Poisson’s ratio, the thermal expansion coefficient and the Young’s modulus for ZSA and GZSA were supposed to be identical, respectively. The calculated crack propagation parameter R of GZSA was much larger (more than 20%) than that of the ZSA, implying that the crack propagation was more restricted for GZSA ceramics. Hence, unstable crack propagation occurred more possibly in ZSA ceramics during water quenching, which might further interpret the sharp degradation in fracture toughness of ZSA at the T of 800 ◦ C in Fig. 5. The calculated results above further confirmed that the finer starting particle size of the ZrB2 -matrix had beneficial effect on the thermal shock resistance of ZrB2 –SiC–AlN ceramics. 4. Conclusion Hot-pressed specimens with two different ZrB2 particle sizes were prepared from powder mixtures with the compositions of ZrB2 –20 vol.% SiC–10 vol.% AlN. The fracture surfaces of both materials showed a mixed mechanism of inter- and intra-granular fracture. The flexural strength and fracture toughness of GZSA were approximately 11 and 20% higher than those of ZSA ceramics, respectively, as a result of the grain refinement of the matrix.
765
The critical temperature difference of thermal shock resistance for GZSA was found to be higher than that for ZSA experimentally as much as 40 ◦ C. Comparing with ZSA, the thermal stress fracture resistance parameter, R, and thermal stress damage resistance parameter, R , of GZSA were all increased. Such superior thermal shock resistance was attributed to the remarkable improvement of the fracture toughness by grain refinement. The influence of quenching temperature difference on the fracture toughness was not significant. The results in this work indicated that grain refinement of the ZrB2 -matrix was a potential method for improving the thermal shock resistance of ZrB2 –SiC–AlN ceramics. Acknowledgements This work was supported by Program for New Century Excellent Talents in University (NCET-05-0346), the National Natural Science Foundation of China (50602010) and the National Defence foundation (9140A12030106HT01). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
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