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Tough hybrid ceramic-based material with high strength
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Scripta Materialia 67 (2012) 744–747 www.elsevier.com/locate/scriptamat
Tough hybrid ceramic-based material with high strength Shuqi Guo,a,⇑ Yutaka Kagawaa,b and Toshiyuki Nishimurac a
b
Hybrid Materials Unit, National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan Research Center for Advanced Science and Technology, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8505, Japan c Sialon Unit, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan Received 31 May 2012; revised 9 July 2012; accepted 30 July 2012 Available online 4 August 2012
This study describes a tough and strong hybrid ceramic material consisting of platelet-like zirconium compounds and metal. A mixture of boron carbide and excess zirconium powder was heated to 1900 °C using a liquid-phase reaction sintering technique to produce a platelet-like ZrB2-based hybrid ceramic bonded by a thin zirconium layer. The platelet-like ZrB2 grains were randomly present in the as-sintered hybrid ceramic. Relative to non-hybrid ceramics, the fracture toughness and flexural strength of the hybrid ceramic increased by approximately 2-fold. Ó 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Zirconium compounds; Hybrid ceramics; Platelet-like grains; Flexural strength; Fracture toughness
The major problem with the use of ceramics as structural materials is their intrinsic brittleness and sensitivity to flaws or defects that result from the absence of macroscopic plastic deformation at low homologous temperatures. To overcome this problem, extrinsic toughening and/or microstructure design are necessary. Extrinsic toughening mechanisms relevant to monolithic and composite ceramics essentially include crack deflection at the crack tip, bridging and/or sliding between crack surfaces and plastic deformation surrounding the crack wake zone [1,2]. Typically, continuous ceramic fiber-reinforced ceramic matrix composites show large damage tolerance and high resistance to failure [3,4], as a result of crack deflection, fiber bridging and interface sliding. In addition, silicon nitride containing elongated grains show considerably improved fracture toughness [5,6], attributed to the crack deflection, elongated grain pullout, elastic bridging and frictional grain bridging. Recently, studies in biological materials showed that composites having strength and toughness of orders of magnitude greater than those of their constituent phases could be fabricated by creating the appropriate composite structure [7–11]. In particular, it is well known that tough and strong nacre consists of 95 vol.% of microscopic platelet-like aragonite layers bonded by a thin layer of organic protein material (5 vol.%). However,
⇑ Corresponding author. Tel.: +81 (0)29 859 2223; fax: +81 (0)29 859 2401; e-mail: [email protected]
the mechanical properties of nacre are considerably superior relative to the properties of its constituents [7– 9]. Based on the structure of nacre, hybrid ceramic materials consisting of Al2O3 platelets and polymers have been fabricated [12,13]. Their fracture toughness (30 MPa m1/2) and strength (200 MPa) are comparable to those of aluminium alloys. However, it is difficult to attain tough hybrid materials with durability, strength and refractoriness comparable to those of a ceramic and/ or metal because of poor refractoriness of the polymer. In this study, we attempted to synthesize, using an in situ liquid-phase reactive sintering technique, a hybrid ceramic comprising randomly dispersed platelet-like grains bonded by a thin metal layer in the grain boundaries. This hybrid ceramic had higher fracture toughness and flexural strength than its constituent phases. Commercially available zirconium (Zr) powder (325 mesh, 98% purity, Kojundo Chemical Laboratory, Japan) and boron carbide (B4C) powder (D50 = 0.8 lm, 98% purity, H.C. Starck GmbH, Germany) were weighed according to the stoichiometry of the following reaction. ð2:2 þ xÞZr þ 0:6B4 C ! 1:2ZrB2 þ ZrC0:6 þ xZr
ð1Þ
Here x = 0, 0.3 or 0.5. The corresponding volume fractions of the individual phases were 59.4% ZrB2 and 40.6% ZrC0.6 for x = 0; 53.4% ZrB2, 39.2% ZrC0.6 and 10.1% Zr for x = 0.3; and 50.0% ZrB2, 34.2% ZrC0.6 and 15.8% Zr for x = 0.5.
1359-6462/$ - see front matter Ó 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.scriptamat.2012.07.042
S. Guo et al. / Scripta Materialia 67 (2012) 744–747
The required amounts of Zr and B4C were wet ballmilled for 24 h in SiC media with alcohol as a solvent, and the resulting slurry was subsequently dried. The obtained powder mixtures were hot-pressed in graphite dies to tablets of an average size of 21 mm 25 mm 3.5 mm. The compacted powders were heated to 1900 °C under a pressure of 20 MPa in a flowing Ar atmosphere at a heating rate of 15 °C min1. After hot pressing at 1900 °C for 60 min, the electric power was shut off to allow the sample to cool to room temperature. The load was removed when the die temperature dropped below 1700 °C. The densities of the resulting hybrid ceramics were evaluated by the Archimedes method. The theoretical densities of the hybrid ceramics were calculated according to the rule of mixture. X-ray diffraction (XRD) was used to identify the crystalline phases present in the asprepared hybrid ceramics. The microstructure of the hybrid ceramics was characterized by field emission scanning electron microscopy (FE-SEM). The flexural strength was measured using a four-point bending test fixture (inner span 10 mm, outer span 20 mm) at room temperature. The bending test was performed on specimens 25 mm 2.5 mm 2 mm in size, using an Autograph testing system (AG-50KNI, Shimadzu, Kyoto, Japan) with a crosshead speed of 0.5 mm min1. At least 12 specimens were used in each measurement. After testing, the fracture surface was examined by SEM. In addition, the room-temperature fracture toughness was determined with the same four-point bending test fixture with a crosshead speed of 0.05 mm min1 using a singleedge notched beam (SENB) specimen with a 0.8 mm deep and 0.15 mm thick notch. The fracture toughness was obtained as given by Simpson [14]. The XRD patterns of the reactive sintered hybrid ceramics are presented in Figure 1. For x = 0, only ZrB2 and ZrCx phases, and no excess Zr phase, were detected in the hybrid ceramic. Note that the main intensity peaks of ZrC were observed in the wide range 33° < 2h < 70°, and not only in the range 33° < 2h < 34°, suggesting that the zirconium carbide formed might be ZrCx. For x = 0.3 or 0.5, Zr phase was detected in addition to ZrB2 and ZrCx phases. Hence, ZrB2 and ZrCx phases coexist with Zr phase, forming the intergranular metal phase-containing hybrid ceramics. In addition, the lattice parameter of the ZrCx is determined
745
˚ for x = 0, a = 4.672 A ˚ for x = 0.3, to be a = 4.670 A ˚ for x = 0.5. Obviously, the calculated and a = 4.677 A lattice parameter of ZrCx for three compositions dis˚ , PDF #035agrees with that of ZrC (a = 4.693 A 0784), but is nearly identical to that of the cubic ˚ , PDF #065-9886). PresumZrC0.6 phase (a = 4.672 A ably, the ZrCx phase present in the hybrid ceramics is ZrC0.6 phase. The microstructure of the as-synthesized hybrid ceramics consists of platelet-like ZrB2 grains (dark contrast), equiaxed ZrC0.6 grains (bright grey contrast) and Zr phase (white contrast) at the grain boundaries, as shown in Figure 2b. On the other hand, when x = 0 (Fig. 2a), as a result of the complete reaction between the reactants according to the reaction (1), the Zr phase was not distinguishable and only platelet-like ZrB2 grains and equiaxed ZrC0.6 grains were observed. This is consistent with the XRD patterns in which peaks of Zr phase are absent. Thus, it is presumed that the assynthesized material (x = 0) is a pure ceramic composite with no metal phase, i.e. a non-hybrid ceramic. The measured grain sizes of ZrB2 and ZrC0.6 are summarized in Table 1. The ZrB2 and ZrC0.6 grains were coarsened with increased of amount of Zr. In addition, some pores were observed at the grain boundaries for all the samples prepared (indicated by arrows in Fig. 2). A relative density >98% is obtained for the three compositions of hybrid ceramics (Table 1). Plots of the room-temperature flexural strength and fracture toughness as a function of the amount of Zr in the hybrid ceramics are presented in Figure 3. It was found that the flexural strength strongly depended on the amount of Zr. The non-hybrid ceramic (x = 0) without the Zr phase has a flexural strength of 542.1 ± 100.6 MPa, which is comparable to that of sin-
(a) ZrB2
ZrC0.6
(b)
(b)
ZrB2
ZrC0.6
Zr
Zr
Intensity (a.u.)
ZrB2 x = 0.5
ZrC0.6
(a) x=0
20
30
40
50
60
70
80
2 (degree)
Figure 1. X-ray diffraction patterns of the reactive sintered ceramics.
Figure 2. Back-scattered electron FE-SEM images of the reactive sintered ceramics: (a) the non-hybrid ceramic (x = 0); (b) the hybrid ceramic (x = 0.5).
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S. Guo et al. / Scripta Materialia 67 (2012) 744–747
Table 1. Density and average grain size of the reactive sintered ceramics. Materials
x = 0.0 x = 0.3 x = 0.5
Amount of Zr (vol.%)
Bulk density (g/cm3)
Relative density (%TD)
0.0 10.1 15.8
6.19 6.21 6.21
99.0 98.9 98.7
12
900
11
Flexural Strength,
fs
(MPa)
1000
800
10
700
9 600
500
KIC
8
400
σfs
7
300
0
5 10 15 Volume Fraction of Zr, (vol.%)
6 20
Fracture Toughness, KIC (MPa m1/2)
gle-phase ZrB2 and ZrB2-based ceramics [15,16]. The improved flexural strength was achieved because the additional Zr metal produced a hybrid material. The strength of the hybrid material further increased as the amount of Zr increased. For x = 0.5, the flexural strength of the hybrid material increased 2-fold. In addition, the strength scattering decreased with increasing Zr metal phase. These improvements are associated with the presence of the intergranular metal phase. The presence of the intergranular metal phase in hybrid ceramics effectively transfers the load to the strong and hard platelet-like ZrB2 grains. In addition, the presence of the metal phase leads to a reduced number of large flaws or defects and the relaxation of stress concentration at the crack tips in hybrid ceramics owing to plastic deformation due to the intrinsic ductility of the metal, hence increasing strength and decreasing strength scattering. The SEM observations of the fracture surface of the hybrid ceramics indicated pullout of a large number of visible platelet-like ZrB2 grains (Fig. 4a). This indicated that the platelet-like ZrB2 grains greatly contributed to the crack deflection and bridging. In addition, a trace of crack bowing around the rod-like ZrB2 grain was observed (Fig. 4b). Thus, in addition to the grain pullout, the rod-like grain bowing can contribute to the increase in crack-growth resistance. Furthermore, the thin hexagonal single platelet-like ZrB2 grains and/or single-crystal rods of several micrometers bond together and reinforce each other. This makes ZrB2 grains difficult to break, and hence gives the material higher strength and toughness. However, only a few rod-like grains, with grain size in the range of 1.86–2.96 lm, were observed in any instance. This suggests that the improvements in the fracture toughness and the strength are can be largely attributed to the presence of the platelet-like ZrB2 grains. Similar to the flexural strength, the fracture toughness of the hybrid ceramics depends on the amount of
Figure 3. Plots of the flexural strength and fracture toughness as a function of the amount of metal for the reactive sintered ceramics.
d
ZrB2 platelet’s size (lm) t
Average grain diameter of ZrC0.6, d (lm)
1.82 ± 0.76 3.65 ± 1.71 6.58 ± 2.83
1.33 ± 0.51 2.23 ± 0.83 4.02 ± 1.39
2.14 ± 0.86 3.72 ± 2.09 7.21 ± 2.68
Zr. The fracture toughness (KIC = 6.63 ± 0.39 MPa m1/2) of the non-hybrid ceramics (x = 0) is comparable with that of single-phase ZrB2 and/or ZrB2-based ceramics [15,16] as well as with that of short carbon fiber- and/or silicon carbide fiber-reinforced ZrB2-based ceramic matrix composites [17–19]. The predominantly intergranular crack path is observed in the non-hybrid ceramic (Fig. 4c). The toughening mechanisms include crack deflection at the platelet-like ZrB2/ZrC0.6 interface, platelet-like grain pullout, elastic bridging and frictional grain bridging. However, the contribution of these mechanisms to the increase in fracture toughness is not the same. The platelet-like grain pullout and frictional grain are believed to be the most important toughening mechanisms. In addition, the hybrid ceramics showed further improved fracture toughness and flexural strength with increasing intergranular metal phase (Fig. 3). It is evident that the increase in fracture toughness was associated with the presence of the intergranular metal phase as well as with the coarseness of grains with amount of Zr (Table 1). The fracture toughness of the hybrid ceramics increased by 40% and 55% compared with the non-hybrid ceramics, and the corresponding volume fractions of the intergranular metal phase were 10% and 16%. This indicates that even though the intergranular metal phase present in the microstructure of the studied materials is not significant in terms of volume, it greatly contributes to the fracture toughness and flexural strength of these hybrid ceramics. The crack propagation behavior of Vickers indentation cracks in the hybrid ceramics was observed by FE-SEM (Fig. 4d). It was seen that crack propagation behavior differed from that of the non-hybrid ceramics (Fig. 4c). In the hybrid ceramics, a large number of microcracks formed in the frontal process zone ahead of the Vickers indentation edge, whereas a main long crack is observed in the non-hybrid ceramic. In addition, microcracking behavior was observed only within the ZrC0.6 grains and the microcracks were arrested within the ZrC0.6 grains without propagation to the grain boundaries and/or to the neighbouring platelet-like ZrB2 grains. Obviously, the presence of intergranular metal phase plays an important role in retaining the simultaneous increase in flexural strength and fracture toughness. It is well known that metals enable energy dissipation and relaxation of stress concentration ahead of the crack tips by means of plastic deformation or the formation of non-connected microcracks, thus indicating their intrinsic ductility and insensitivity to flaws. The function of this ductile phase in the hybrid ceramics was evident by the formation of “uncracked ligaments” between a main crack and microcracks that initiate ahead of it. The toughening by crack bridging can result
S. Guo et al. / Scripta Materialia 67 (2012) 744–747
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Crack Propagation Direction
(a)
(c)
Indent tip Crack tip
(b)
(d)
Indent tip
Figure 4. SEM micrographs of the fracture surfaces (a, b) and crack propagation behavior (c, d) for the reactive sintered ceramics; (a, b, d) the hybrid ceramic (x = 0.5) and (c) the non-hybrid ceramic (x = 0).
from the uncracked metal ligaments where there are microcracks. Thus, compared with the non-hybrid ceramic, the additional toughening mechanisms in the hybrid ceramics include microcracking as well as crack bridging by the uncracked metal ligaments. In addition, the microcracking is associated with residual radial tensile internal stresses in the ZrC0.6 grains and residual compressive stresses in the ZrB2 grains. These different stresses originate from the thermal expansion misfit of ZrB2 (5.9 ppm °C1), ZrC0.6 (7.42 ppm °C1) and Zr (5.7 ppm °C1) upon cooling from the processing temperature. In summaries, the combination of high flexural strength and high fracture toughness is the most distinct feature of this hybrid ceramic material when compared to other platelet-reinforced ceramic-based composites. Another feature of this hybrid ceramic is its superior mechanical properties relative to its constituent phases. In addition, although the intergranular metal phase present in the microstructure of the studied materials is not significant in terms of the volume, it greatly contributes to the high fracture toughness and fracture strength of the hybrid ceramics. Furthermore, the fabrication route has potential to be applied for other hybrid ceramics provided the appropriate powders are available. The process to produce this hybrid ceramic is simple and inexpensive, and it can be readily applicable to other hybrid ceramic materials as well. [1] A.G. Evans, J. Am. Ceram. Soc. 73 (1990) 187. [2] R.O. Ritchie, Int. J. Fracture. 100 (1999) 55.
[3] W.C. Tu, F.E. Lange, A.G. Evans, J. Am. Ceram. Soc. 79 (1996) 417. [4] D.S. Beyerle, S.M. Spearing, F.W. Zok, A.G. Evans, J. Am. Ceram. Soc. 75 (1992) 2719. [5] P. Sajgalik, J. Dusza, M.J. Hoffman, J. Am. Ceram. Soc. 78 (1995) 2619. [6] S.Q. Guo, N. Hirosaki, Y. Yamamoto, T. Nishimura, H. Tanaka, Ceram. Int. 30 (2004) 635. [7] A.P. Jackson, J.F.V. Vincent, R.M. Turner, J. Mater. Sci. 25 (1990) 3173. [8] S. Kamat, X. Su, R. Ballarini, A.H. Heuer, Nature 405 (2000) 1036. [9] R.Z. Wang, Z. Suo, A.G. Evans, N. Yao, I.A. Aksay, J. Mater. Res. 16 (2001) 2485. [10] P.K. Nalla, J.H. Kinney, R.O. Ritchie, Nat. Mater. 2 (2003) 164. [11] H. Peterlik, P. Roschger, K. Klaushofer, P. Fratzl, Nat. Mater. 5 (2006) 52. [12] L.J. Bonderer, A.R. Studart, L.J. Gauckler, Science 319 (2008) 1069. [13] E. Munch, M.E. Launey, D.H. Alsem, E. Saiz, A.P. Tomsia, R.O. Ritchie, Science 322 (2008) 1516. [14] L.A. Simpson, J. Am. Ceram. Soc. 57 (1974) 151. [15] W.G. Fahrenholtz, G.E. Hilmas, I.G. Talmy, J.A. Zaykoski, J. Am. Ceram. Soc. 90 (2007) 1347. [16] S.Q. Guo, J. Euro, Ceram. Soc. 29 (2009) 995. [17] F. Yang, X. Zhang, J. Han, S. Du, Mater. Design. 29 (2008) 1817. [18] F. Yang, X. Zhang, J. Han, S. Du, J. Alloys Compd. 472 (2009) 395. [19] L. Silvestroni, D. Sciti, C. Melandri, S. Guicciardi, J. Euro, Ceram. Soc. 30 (2010) 2155.
Scripta Materialia 67 (2012) 744–747 www.elsevier.com/locate/scriptamat
Tough hybrid ceramic-based material with high strength Shuqi Guo,a,⇑ Yutaka Kagawaa,b and Toshiyuki Nishimurac a
b
Hybrid Materials Unit, National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan Research Center for Advanced Science and Technology, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8505, Japan c Sialon Unit, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan Received 31 May 2012; revised 9 July 2012; accepted 30 July 2012 Available online 4 August 2012
This study describes a tough and strong hybrid ceramic material consisting of platelet-like zirconium compounds and metal. A mixture of boron carbide and excess zirconium powder was heated to 1900 °C using a liquid-phase reaction sintering technique to produce a platelet-like ZrB2-based hybrid ceramic bonded by a thin zirconium layer. The platelet-like ZrB2 grains were randomly present in the as-sintered hybrid ceramic. Relative to non-hybrid ceramics, the fracture toughness and flexural strength of the hybrid ceramic increased by approximately 2-fold. Ó 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Zirconium compounds; Hybrid ceramics; Platelet-like grains; Flexural strength; Fracture toughness
The major problem with the use of ceramics as structural materials is their intrinsic brittleness and sensitivity to flaws or defects that result from the absence of macroscopic plastic deformation at low homologous temperatures. To overcome this problem, extrinsic toughening and/or microstructure design are necessary. Extrinsic toughening mechanisms relevant to monolithic and composite ceramics essentially include crack deflection at the crack tip, bridging and/or sliding between crack surfaces and plastic deformation surrounding the crack wake zone [1,2]. Typically, continuous ceramic fiber-reinforced ceramic matrix composites show large damage tolerance and high resistance to failure [3,4], as a result of crack deflection, fiber bridging and interface sliding. In addition, silicon nitride containing elongated grains show considerably improved fracture toughness [5,6], attributed to the crack deflection, elongated grain pullout, elastic bridging and frictional grain bridging. Recently, studies in biological materials showed that composites having strength and toughness of orders of magnitude greater than those of their constituent phases could be fabricated by creating the appropriate composite structure [7–11]. In particular, it is well known that tough and strong nacre consists of 95 vol.% of microscopic platelet-like aragonite layers bonded by a thin layer of organic protein material (5 vol.%). However,
⇑ Corresponding author. Tel.: +81 (0)29 859 2223; fax: +81 (0)29 859 2401; e-mail: [email protected]
the mechanical properties of nacre are considerably superior relative to the properties of its constituents [7– 9]. Based on the structure of nacre, hybrid ceramic materials consisting of Al2O3 platelets and polymers have been fabricated [12,13]. Their fracture toughness (30 MPa m1/2) and strength (200 MPa) are comparable to those of aluminium alloys. However, it is difficult to attain tough hybrid materials with durability, strength and refractoriness comparable to those of a ceramic and/ or metal because of poor refractoriness of the polymer. In this study, we attempted to synthesize, using an in situ liquid-phase reactive sintering technique, a hybrid ceramic comprising randomly dispersed platelet-like grains bonded by a thin metal layer in the grain boundaries. This hybrid ceramic had higher fracture toughness and flexural strength than its constituent phases. Commercially available zirconium (Zr) powder (325 mesh, 98% purity, Kojundo Chemical Laboratory, Japan) and boron carbide (B4C) powder (D50 = 0.8 lm, 98% purity, H.C. Starck GmbH, Germany) were weighed according to the stoichiometry of the following reaction. ð2:2 þ xÞZr þ 0:6B4 C ! 1:2ZrB2 þ ZrC0:6 þ xZr
ð1Þ
Here x = 0, 0.3 or 0.5. The corresponding volume fractions of the individual phases were 59.4% ZrB2 and 40.6% ZrC0.6 for x = 0; 53.4% ZrB2, 39.2% ZrC0.6 and 10.1% Zr for x = 0.3; and 50.0% ZrB2, 34.2% ZrC0.6 and 15.8% Zr for x = 0.5.
1359-6462/$ - see front matter Ó 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.scriptamat.2012.07.042
S. Guo et al. / Scripta Materialia 67 (2012) 744–747
The required amounts of Zr and B4C were wet ballmilled for 24 h in SiC media with alcohol as a solvent, and the resulting slurry was subsequently dried. The obtained powder mixtures were hot-pressed in graphite dies to tablets of an average size of 21 mm 25 mm 3.5 mm. The compacted powders were heated to 1900 °C under a pressure of 20 MPa in a flowing Ar atmosphere at a heating rate of 15 °C min1. After hot pressing at 1900 °C for 60 min, the electric power was shut off to allow the sample to cool to room temperature. The load was removed when the die temperature dropped below 1700 °C. The densities of the resulting hybrid ceramics were evaluated by the Archimedes method. The theoretical densities of the hybrid ceramics were calculated according to the rule of mixture. X-ray diffraction (XRD) was used to identify the crystalline phases present in the asprepared hybrid ceramics. The microstructure of the hybrid ceramics was characterized by field emission scanning electron microscopy (FE-SEM). The flexural strength was measured using a four-point bending test fixture (inner span 10 mm, outer span 20 mm) at room temperature. The bending test was performed on specimens 25 mm 2.5 mm 2 mm in size, using an Autograph testing system (AG-50KNI, Shimadzu, Kyoto, Japan) with a crosshead speed of 0.5 mm min1. At least 12 specimens were used in each measurement. After testing, the fracture surface was examined by SEM. In addition, the room-temperature fracture toughness was determined with the same four-point bending test fixture with a crosshead speed of 0.05 mm min1 using a singleedge notched beam (SENB) specimen with a 0.8 mm deep and 0.15 mm thick notch. The fracture toughness was obtained as given by Simpson [14]. The XRD patterns of the reactive sintered hybrid ceramics are presented in Figure 1. For x = 0, only ZrB2 and ZrCx phases, and no excess Zr phase, were detected in the hybrid ceramic. Note that the main intensity peaks of ZrC were observed in the wide range 33° < 2h < 70°, and not only in the range 33° < 2h < 34°, suggesting that the zirconium carbide formed might be ZrCx. For x = 0.3 or 0.5, Zr phase was detected in addition to ZrB2 and ZrCx phases. Hence, ZrB2 and ZrCx phases coexist with Zr phase, forming the intergranular metal phase-containing hybrid ceramics. In addition, the lattice parameter of the ZrCx is determined
745
˚ for x = 0, a = 4.672 A ˚ for x = 0.3, to be a = 4.670 A ˚ for x = 0.5. Obviously, the calculated and a = 4.677 A lattice parameter of ZrCx for three compositions dis˚ , PDF #035agrees with that of ZrC (a = 4.693 A 0784), but is nearly identical to that of the cubic ˚ , PDF #065-9886). PresumZrC0.6 phase (a = 4.672 A ably, the ZrCx phase present in the hybrid ceramics is ZrC0.6 phase. The microstructure of the as-synthesized hybrid ceramics consists of platelet-like ZrB2 grains (dark contrast), equiaxed ZrC0.6 grains (bright grey contrast) and Zr phase (white contrast) at the grain boundaries, as shown in Figure 2b. On the other hand, when x = 0 (Fig. 2a), as a result of the complete reaction between the reactants according to the reaction (1), the Zr phase was not distinguishable and only platelet-like ZrB2 grains and equiaxed ZrC0.6 grains were observed. This is consistent with the XRD patterns in which peaks of Zr phase are absent. Thus, it is presumed that the assynthesized material (x = 0) is a pure ceramic composite with no metal phase, i.e. a non-hybrid ceramic. The measured grain sizes of ZrB2 and ZrC0.6 are summarized in Table 1. The ZrB2 and ZrC0.6 grains were coarsened with increased of amount of Zr. In addition, some pores were observed at the grain boundaries for all the samples prepared (indicated by arrows in Fig. 2). A relative density >98% is obtained for the three compositions of hybrid ceramics (Table 1). Plots of the room-temperature flexural strength and fracture toughness as a function of the amount of Zr in the hybrid ceramics are presented in Figure 3. It was found that the flexural strength strongly depended on the amount of Zr. The non-hybrid ceramic (x = 0) without the Zr phase has a flexural strength of 542.1 ± 100.6 MPa, which is comparable to that of sin-
(a) ZrB2
ZrC0.6
(b)
(b)
ZrB2
ZrC0.6
Zr
Zr
Intensity (a.u.)
ZrB2 x = 0.5
ZrC0.6
(a) x=0
20
30
40
50
60
70
80
2 (degree)
Figure 1. X-ray diffraction patterns of the reactive sintered ceramics.
Figure 2. Back-scattered electron FE-SEM images of the reactive sintered ceramics: (a) the non-hybrid ceramic (x = 0); (b) the hybrid ceramic (x = 0.5).
746
S. Guo et al. / Scripta Materialia 67 (2012) 744–747
Table 1. Density and average grain size of the reactive sintered ceramics. Materials
x = 0.0 x = 0.3 x = 0.5
Amount of Zr (vol.%)
Bulk density (g/cm3)
Relative density (%TD)
0.0 10.1 15.8
6.19 6.21 6.21
99.0 98.9 98.7
12
900
11
Flexural Strength,
fs
(MPa)
1000
800
10
700
9 600
500
KIC
8
400
σfs
7
300
0
5 10 15 Volume Fraction of Zr, (vol.%)
6 20
Fracture Toughness, KIC (MPa m1/2)
gle-phase ZrB2 and ZrB2-based ceramics [15,16]. The improved flexural strength was achieved because the additional Zr metal produced a hybrid material. The strength of the hybrid material further increased as the amount of Zr increased. For x = 0.5, the flexural strength of the hybrid material increased 2-fold. In addition, the strength scattering decreased with increasing Zr metal phase. These improvements are associated with the presence of the intergranular metal phase. The presence of the intergranular metal phase in hybrid ceramics effectively transfers the load to the strong and hard platelet-like ZrB2 grains. In addition, the presence of the metal phase leads to a reduced number of large flaws or defects and the relaxation of stress concentration at the crack tips in hybrid ceramics owing to plastic deformation due to the intrinsic ductility of the metal, hence increasing strength and decreasing strength scattering. The SEM observations of the fracture surface of the hybrid ceramics indicated pullout of a large number of visible platelet-like ZrB2 grains (Fig. 4a). This indicated that the platelet-like ZrB2 grains greatly contributed to the crack deflection and bridging. In addition, a trace of crack bowing around the rod-like ZrB2 grain was observed (Fig. 4b). Thus, in addition to the grain pullout, the rod-like grain bowing can contribute to the increase in crack-growth resistance. Furthermore, the thin hexagonal single platelet-like ZrB2 grains and/or single-crystal rods of several micrometers bond together and reinforce each other. This makes ZrB2 grains difficult to break, and hence gives the material higher strength and toughness. However, only a few rod-like grains, with grain size in the range of 1.86–2.96 lm, were observed in any instance. This suggests that the improvements in the fracture toughness and the strength are can be largely attributed to the presence of the platelet-like ZrB2 grains. Similar to the flexural strength, the fracture toughness of the hybrid ceramics depends on the amount of
Figure 3. Plots of the flexural strength and fracture toughness as a function of the amount of metal for the reactive sintered ceramics.
d
ZrB2 platelet’s size (lm) t
Average grain diameter of ZrC0.6, d (lm)
1.82 ± 0.76 3.65 ± 1.71 6.58 ± 2.83
1.33 ± 0.51 2.23 ± 0.83 4.02 ± 1.39
2.14 ± 0.86 3.72 ± 2.09 7.21 ± 2.68
Zr. The fracture toughness (KIC = 6.63 ± 0.39 MPa m1/2) of the non-hybrid ceramics (x = 0) is comparable with that of single-phase ZrB2 and/or ZrB2-based ceramics [15,16] as well as with that of short carbon fiber- and/or silicon carbide fiber-reinforced ZrB2-based ceramic matrix composites [17–19]. The predominantly intergranular crack path is observed in the non-hybrid ceramic (Fig. 4c). The toughening mechanisms include crack deflection at the platelet-like ZrB2/ZrC0.6 interface, platelet-like grain pullout, elastic bridging and frictional grain bridging. However, the contribution of these mechanisms to the increase in fracture toughness is not the same. The platelet-like grain pullout and frictional grain are believed to be the most important toughening mechanisms. In addition, the hybrid ceramics showed further improved fracture toughness and flexural strength with increasing intergranular metal phase (Fig. 3). It is evident that the increase in fracture toughness was associated with the presence of the intergranular metal phase as well as with the coarseness of grains with amount of Zr (Table 1). The fracture toughness of the hybrid ceramics increased by 40% and 55% compared with the non-hybrid ceramics, and the corresponding volume fractions of the intergranular metal phase were 10% and 16%. This indicates that even though the intergranular metal phase present in the microstructure of the studied materials is not significant in terms of volume, it greatly contributes to the fracture toughness and flexural strength of these hybrid ceramics. The crack propagation behavior of Vickers indentation cracks in the hybrid ceramics was observed by FE-SEM (Fig. 4d). It was seen that crack propagation behavior differed from that of the non-hybrid ceramics (Fig. 4c). In the hybrid ceramics, a large number of microcracks formed in the frontal process zone ahead of the Vickers indentation edge, whereas a main long crack is observed in the non-hybrid ceramic. In addition, microcracking behavior was observed only within the ZrC0.6 grains and the microcracks were arrested within the ZrC0.6 grains without propagation to the grain boundaries and/or to the neighbouring platelet-like ZrB2 grains. Obviously, the presence of intergranular metal phase plays an important role in retaining the simultaneous increase in flexural strength and fracture toughness. It is well known that metals enable energy dissipation and relaxation of stress concentration ahead of the crack tips by means of plastic deformation or the formation of non-connected microcracks, thus indicating their intrinsic ductility and insensitivity to flaws. The function of this ductile phase in the hybrid ceramics was evident by the formation of “uncracked ligaments” between a main crack and microcracks that initiate ahead of it. The toughening by crack bridging can result
S. Guo et al. / Scripta Materialia 67 (2012) 744–747
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Crack Propagation Direction
(a)
(c)
Indent tip Crack tip
(b)
(d)
Indent tip
Figure 4. SEM micrographs of the fracture surfaces (a, b) and crack propagation behavior (c, d) for the reactive sintered ceramics; (a, b, d) the hybrid ceramic (x = 0.5) and (c) the non-hybrid ceramic (x = 0).
from the uncracked metal ligaments where there are microcracks. Thus, compared with the non-hybrid ceramic, the additional toughening mechanisms in the hybrid ceramics include microcracking as well as crack bridging by the uncracked metal ligaments. In addition, the microcracking is associated with residual radial tensile internal stresses in the ZrC0.6 grains and residual compressive stresses in the ZrB2 grains. These different stresses originate from the thermal expansion misfit of ZrB2 (5.9 ppm °C1), ZrC0.6 (7.42 ppm °C1) and Zr (5.7 ppm °C1) upon cooling from the processing temperature. In summaries, the combination of high flexural strength and high fracture toughness is the most distinct feature of this hybrid ceramic material when compared to other platelet-reinforced ceramic-based composites. Another feature of this hybrid ceramic is its superior mechanical properties relative to its constituent phases. In addition, although the intergranular metal phase present in the microstructure of the studied materials is not significant in terms of the volume, it greatly contributes to the high fracture toughness and fracture strength of the hybrid ceramics. Furthermore, the fabrication route has potential to be applied for other hybrid ceramics provided the appropriate powders are available. The process to produce this hybrid ceramic is simple and inexpensive, and it can be readily applicable to other hybrid ceramic materials as well. [1] A.G. Evans, J. Am. Ceram. Soc. 73 (1990) 187. [2] R.O. Ritchie, Int. J. Fracture. 100 (1999) 55.
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