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YB2C2: A machinable layered ternary ceramic with excellent damage tolerance
Scripta Materialia 124 (2016) 86–89
Contents lists available at ScienceDirect
Scripta Materialia journal homepage: www.elsevier.com/locate/scriptamat
Regular Article
YB2C2: A machinable layered ternary ceramic with excellent damage tolerance Guorui Zhao a,b, Jixin Chen a,⁎, Yueming Li a,b, Meishuan Li a a b
Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China University of Chinese Academy of Sciences, Beijing 100039, China
a r t i c l e
i n f o
Article history: Received 13 April 2016 Received in revised form 29 May 2016 Accepted 26 June 2016 Available online xxxx Keywords: YB2C2 Layered ceramic Damage tolerance Machinability
a b s t r a c t High purity bulk YB2C2, a member of ternary rare earth metal diborodicarbides, was synthesized by an in situ hotpressing method using YH2, B4C, and graphite as initial materials. Physical and mechanical properties of YB2C2, such as Young's modulus (207 GPa), shear modulus (87 GPa), bulk modulus (116 GPa), Vickers hardness (4.2 ± 0.3 GPa), three-point flexural strength (460 ± 10 MPa), and fracture toughness (4.6 ± 0.1 MPa·m1/2) were characterized. The as-received YB2C2 ceramic possesses typical layered structure, excellent damage tolerance and easy machinability. © 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
Ultrahigh-temperature ceramics (UHTCs) based on transition metal borides or carbides are regarded as the materials of choice for nose and sharp leading edges of hypersonic vehicles [1–3]. Among the family of UHTCs, ZrB2-based materials have been widely investigated owing to their high melting point, chemical inertness and no phase transformations in the solid state. However, the intrinsic brittleness, poor thermal shock resistance and machinability have greatly hampered their applications as structural components. Hence, searching for new UHTCs with high damage tolerance, good thermal shock resistance and easy machinability is of great urgency. In recent years, a series of layered ternary ceramics, i.e., MAX phases [4,5] have been extensively studied. MAX phases possess a layered crystal structure composed of alternately stacking of M-X octahedron slabs and weakly bonded A slabs. Due to this unique crystal structure and chemical bonds characteristics, MAX phases exhibit many excellent properties, such as high damage tolerance, good thermal shock resistance and machinability [5]. In terms of MAX phases, their damage tolerance mainly stems from the weak bondings among adjacent layers [6]. These results give us a hint that the ceramics with layered crystal structure and alternately stacking of weak bondings and strong bondings among the layers, or, in other words, weak bonds in adjacent layers exist together with the strong covalent bonds in the layers, are probably damage tolerant, thermal shock resistant and readily machinable. Recently, Zhou et al. predicted that YB2 [7], YB4 [8], YB6 [9], and YbB6 [10] are damage tolerant even “ductile” ceramics for ultrahigh-
⁎ Corresponding author at: 72 Wenhua Road, Shenyang 110016, China. E-mail address: [email protected] (J. Chen).
http://dx.doi.org/10.1016/j.scriptamat.2016.06.041 1359-6462/© 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
temperature applications through first-principles calculations and experimental investigations. For YB6 and YbB6, their “ductility” is underpinned by the anisotropic chemical bonding within the crystal structure, that is, the coexistence of strong σ bond connecting the B6 octahedra and the weak banana bond within the B6 octahedron [9,10]. These works open the door to design next generation UHTCs for applications in hypersonic vehicles. ReB2C2 (Re_Sc, Y, lanthanides and actinides) phases are a class of layered rare earth metal diborodicarbides [11]. For YB2C2 (Re_Y), boron and carbon atoms form infinite, planar, two-dimensional (2-D) networks which alternate with 2-D sheets of Y atom along the c direction. Within the B2C2 layer, each atom is bonded to three other atoms so as to form fused four- and eight-membered rings. Each four-membered ring is made of two boron and two carbon atoms in opposite positions, whereas each eight-membered ring contains four boron and four carbon atoms with B-C contacts [12,13]. For YB2C2, weak Y-B and Y-C bonds between Y layer and B2C2 layer exist together with the strong B-C covalent bonds in the layers among the B-C plane [12]. Its crystal structure and chemical bonding properties are highly similar to MAX phases, YbB6 and others damage tolerant ceramics. Furthermore, the melting point of material is an essential factor for ultrahigh temperature applications. However, the melting point or decomposition temperature of YB2C2 has not been reported till now. In previous works, YB2C2 always appeared when Y2O3 was used as sintering aid of B4C. Goldstein et al. [14] found that YB2C2 still existed when heating the mixture of B4C and Y2O3 at 2180 °C for 120 min. This means that the decomposition temperature of YB2C2 is at least higher than 2180 °C. Therefore, the unique structure and properties features of YB2C2 render it as a promising damage tolerant material for ultrahigh temperature applications.
G. Zhao et al. / Scripta Materialia 124 (2016) 86–89
Fig. 1. X-ray diffraction pattern of YB2C2 synthesized by an in situ hot pressing method.
However, there has been little research into the synthesis, properties and application of YB2C2. In this work, high purity bulk YB2C2 was firstly prepared by an in situ hot pressing process. Commercially available yttrium hydride (99%, − 200 mesh, TITD, China), boron carbide powders (99%, −200 mesh, Jingangzhuan, China), and graphite powders (99%, − 200 mesh, Tianyuan, China) were selected as starting materials to synthesize YB2C2 ceramic. These powders were mixed according to the molar ratio of YH2:B4C:C = 2:1:2.9 and milled in a polypropylene jar for 12 h. After that, they were compacted uniaxially under 5 MPa in a graphite mold pre-sprayed with a BN layer. The reactive synthesis process was performed in a furnace using graphite as the heating element in a flowing Ar atmosphere. Green compacts of the mixed powders was heated to 1900 °C with a heating rate of 10 °C/min, held at that temperature for 30 min under a pressure of 30 MPa. Finally, the sample was cooled down to ambient temperature and then the contaminants on the surface were machined off by using a grinding wheel. The phase compositions of the sample were identified using an Xray diffractometer (XRD) with CuKα radiation (Rigaku D/max-2400, Tokyo, Japan). The density of sintered sample was determined by the Archimedes method. The surface and fracture morphologies of the sample were investigated in a SUPRA 35 scanning electron microscope (SEM) (LEO, Oberkochen, Germany) equipped with an energy-dispersive spectroscopy (EDS) system. The polished surface was etched for 30 s in an acid solution consisting of HF, HNO3 and H2O mixed at an equal volume fraction. The grain size of samples was measured by a laser confocal scanning microscope (LCSM) (Olympus LEXT OLS4000, Tokyo, Japan). More than 100 grains are measured to determine the
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average grain size. The Young's modulus, shear modulus, and Poisson's ratio of YB2C2 were evaluated on rectangular bar with dimensions of 3 × 15 × 40 mm3 by the impulse excitation technique using the resonance frequency and damping analyzer (IMCE, Diepenbeek, Belgium). Vickers hardness was determined at loads of 1, 3, 5, 10, 30 N with a dwell time of 15 s by a microhardness tester (Taiming HXD-1000B, Shanghai, China). The three-point flexural strength was evaluated on a universal-testing machine. The size of the samples for the flexural tests is 3 × 4 × 36 mm3 and the crosshead speed is 0.5 mm/min. Fracture toughness was measured using the single edge notched beam method on the specimens with dimensions of 4 × 8 × 36 mm3. The notches were introduced by a diamond-coated wheel slotting. The thickness of the blade is 0.10 mm and the width of the notches is about 0.15 mm. Four-point flexural tests with a crosshead speed of 0.05 mm/min were performed for fracture toughness measurements. To evaluate the damage tolerance of the material, indentations on 3 mm × 4 mm × 36 mm flexural specimens were performed using a Vickers indenter under loads of 50, 100, and 200 N. One diagonal of the indent is parallel to the length of the specimen. The residual strength after indentation was determined by four-point flexural test. In order to study the machinability of YB2C2, a drilling test was carried out at a constant normal force of about 50 N and a speed of 600 rpm, using a 2 mm diameter cemented carbide drill. SEM observation of the drilled surface was performed to investigate the level of damage caused by drilling process. Fig. 1 shows a typical XRD pattern of the sintered sample. Most of the peaks can be indexed using the reflections of YB2C2 (PDF # 65-2830), which indicates the high purity of the as-prepared ceramic. However, weak diffraction peaks (2θ) at about 13.2, 26.6, 29.9, 31.9 and 40.5°cannot be identified, which implies that there maybe a new phase in the asprepared sample. According to the PDF # 65-2830, the density of YB2C2 is 4.35 g/cm3, which is very close to the value (4.22 g/cm3) in this work. The relative density is determined as 97%. The typical backscattered electron SEM image of an etched sample was shown in Fig. 2(a). Few micropores can be observed, indicating a high density of the as-prepared YB2C2. Besides, it showed the presence of two regions: a bright region corresponding to YB2C2 and a dark minority region. Energy dispersive spectroscopic (EDS) analysis indicated that the phase in the dark region consisted of Y (9.8 at.%), B (23.6 at.%), and C (66.6 at.%), which means that the new phase in Y-BC system is an carbon-rich phase. However, the definite stoichiometry of this new phase is unknown by far. The polished surface of YB2C2 observed by LCSM is shown in Fig. 2(b). It can be seen that YB2C2 grains show an equiaxed-like morphology, and the average grain size is about 5 μm. The Young's modulus (E), shear modulus (G) and Poisson's ratio (ν) of the YB2C2 are 207 GPa, 87 GPa and 0.2, respectively. The calculated bulk modulus (B) according to the relationship of B = E / [3(1 − 2ν)] is 116 GPa. Low shear modulus and high Poisson's ratio indicate low shear deformation resistance. Pugh's shear to bulk modulus ratio (G/
Fig. 2. (a) SEM and (b) LCSM micrographs of etched and polished surface, and (c) fractured surface of YB2C2.
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G. Zhao et al. / Scripta Materialia 124 (2016) 86–89
Fig. 3. (a) Vickers hardness of YB2C2 versus indentation load, the insert shows SEM micrographs of a hardness indent (10 N), and (b) indentation load dependence of residual flexural strength of YB2C2.
B) is often used as criteria to distinguish ductility or damage tolerance materials from brittle ones [15]. Low G/B value indicate intrinsic ductile or damage tolerant. For YB2C2, Pugh's shear to bulk modulus ratio (G/B) is 0.748. This value is very close to those of high damage tolerant ceramics like Ti3SiC2 (0.759) [16], and Ti3AlC2 (0.706) [17], indicating that YB2C2 is a possible damage tolerant ceramic. The three-point flexural strength of YB2C2 is 460 ± 10 MPa. The fracture toughness (KIC) of YB2C2 is 4.6 ± 0.1 MPa·m1/2. YB2C2 has high fracture toughness, compared with other transition metal borides, such as YbB6 (3.2 MPa·m1/2) [10], ZrB2 (3.5–4.2 MPa·m1/2) [1] etc. Fig. 2(c) shows a high-magnification SEM micrograph of the fracture surface of YB2C2 after flexural test. A laminated structure can be recognized from its typical cleavage nature. The delamination of YB2C2 grains is mainly caused by crack deflection as cracks penetrating inside grains due to the low shear resistance. In addition, kink bands are also found on the fracture surface, which are shown as insert in Fig. 2(c). According to previous studies on damage tolerant ceramics such as Ti3SiC2 [18], welldocumented damage tolerance and intrinsic toughness were associated with energy dissipating mechanisms such as kinking and delamination. Thus, it is believed that the existence of kink bands may improve the damage tolerance and intrinsic toughness of YB2C2. Fig. 3(a) shows the Vickers hardness of YB2C2 as a function of indentation loads. With increasing load from 1 to 30 N, the hardness gradually decreases from 7.2 ± 1.7 to 4.2 ± 0.3 GPa. The reduction in hardness with indentation load is due to kinking non-linear elasticity [19]. SEM micrograph of the damage region around the indent at a load of 10 N is shown as the inset of Fig. 3(a). It is seen that the damage is confined in the immediate vicinity of the indent, no cracks initiate and propagate from the diagonals, and grains push out as well as decohesion can be clearly seen in the vicinity of the indent edges. This indentation
morphology is remarkably similar to those observed in the typical MAX phase ceramics [16–18]. To further prove the damage tolerance of YB2C2, the four-point flexural strength as a function of indentation loads is shown in Fig. 3(b). It is seen that below 200 N (indent size, 300 μm), the residual flexural strength (σr) of YB2C2 samples slowly decreases with increasing indentation load. Even under the Vickers contact damage of 200 N, the residual flexural strength still maintains about 80% of the undamaged samples. At that time, the indent size is nearly about 8% of the sample's width. So, it is proven that YB2C2 is damage tolerant and insensitive to the defects produced at indentation load at least 200 N. Besides damage tolerance, easy machinability of ceramic materials is also crucial for ultrahigh temperature applications. Attributed to the combination of unique properties, such as low shear modulus, low hardness and high damage tolerance, YB2C2 can be easily drilled by a cemented carbide tool at a constant normal force of 50 N and at a speed of 600 rpm. A macrograph of the drilled samples with 2 mm diameter holes is presented in Fig. 4(a). The drilled hole does not exhibit large-scale cracking or chipping. Details of the drilled surface are shown in Fig. 4(b). The machined surface is very flat and partially covered with a layer of smeared debris. So, YB2C2 has a good machinability and it can be readily machined by conventional cemented carbide tools. In summary, dense YB2C2 ceramic was successfully fabricated via an in situ hot-pressing method using yttrium hydride, boron carbide, and graphite powders as starting materials at 1900 °C for 30 min under the pressure of 30 MPa. The as-received YB2C2 ceramic possesses typical layered structure, low Young's modulus, low shear modulus, low hardness, excellent damage tolerance and easy machinability. These unique features might endow YB2C2 with great untapped potential for future ultrahigh temperature applications.
Fig. 4. (a) Optical photograph of the drilled YB2C2 sample, and (b) SEM micrograph of the machined surface of the drilled hole.
G. Zhao et al. / Scripta Materialia 124 (2016) 86–89
References [1] W.G. Fahrenholtz, G.E. Hilmas, I.G. Talmy, J.A. Zaykoski, J. Am. Ceram. Soc. 90 (2007) 1347–1364. [2] M.M. Opeka, I.G. Talmy, J.A. Zaykoski, J. Mater. Sci. 39 (2004) 5887–5904. [3] S.M. Zhu, W.G. Fahrenholtz, G.E. Hilmas, Scr. Mater. 59 (2008) 123–126. [4] J.Y. Wang, Y.C. Zhou, Annu. Rev. Mater. Res. 39 (2009) 415–443. [5] M.W. Barsoum, Prog. Solid State Chem. 28 (2000) 201–281. [6] F.Z. Li, B. Liu, J.Y. Wang, Y.C. Zhou, J. Am. Ceram. Soc. 93 (2010) 228–234. [7] Y.C. Zhou, H.M. Xiang, Z.H. Feng, Z.P. Li, J. Mater. Sci. Technol. 31 (2015) 285–294. [8] Y.C. Zhou, H.M. Xiang, Z.H. Feng, Z.P. Li, J. Eur. Ceram. Soc. 35 (2015) 4437–4445. [9] Y.C. Zhou, B. Liu, H.M. Xiang, Z.H. Feng, Z.P. Li, Math. Res. Lett. 3 (2015) 210–215. [10] Y.C. Zhou, X.F. Wang, H.M. Xiang, Z.H. Feng, G.G. Wang, J. Eur. Ceram. Soc. (2016), http://dx.doi.org/10.1016/j.jeurceramsoc.2016.02.053.
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[11] J. Bauer, J.F. Halet, J.Y. Saillard, Coord. Chem. Rev. 178 (1998) 723–753. [12] S. Khmelevskyi, P. Mohn, J. Redinger, H. Michor, Supercond. Sci. Technol. 18 (2005) 422–426. [13] O. Reckeweg, F.J. DiSalvo, Z. Naturforsc. B 69 (2014) 289–293. [14] A. Goldstein, Y. Yeshurun, A. Goldenberg, J. Eur. Ceram. Soc. 27 (2007) 695–700. [15] S.F. Pugh, Philos. Mag. 45 (1954) 823–843. [16] Y.C. Zhou, Z.M. Sun, Mater. Res. Innov. 2 (1999) 360–363. [17] X.H. Wang, Y.C. Zhou, J. Mater. Sci. Technol. 26 (2010) 385–416. [18] T. El-Raghy, A. Zavaliangos, M.W. Barsoum, S. Kalidindi, J. Am. Ceram. Soc. 80 (1997) 513–516. [19] A. Murugaiah, M.W. Barsoum, S.R. Kalidindi, T. Zhen, J. Mater. Res. 19 (2004) 1139–1148.
Contents lists available at ScienceDirect
Scripta Materialia journal homepage: www.elsevier.com/locate/scriptamat
Regular Article
YB2C2: A machinable layered ternary ceramic with excellent damage tolerance Guorui Zhao a,b, Jixin Chen a,⁎, Yueming Li a,b, Meishuan Li a a b
Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China University of Chinese Academy of Sciences, Beijing 100039, China
a r t i c l e
i n f o
Article history: Received 13 April 2016 Received in revised form 29 May 2016 Accepted 26 June 2016 Available online xxxx Keywords: YB2C2 Layered ceramic Damage tolerance Machinability
a b s t r a c t High purity bulk YB2C2, a member of ternary rare earth metal diborodicarbides, was synthesized by an in situ hotpressing method using YH2, B4C, and graphite as initial materials. Physical and mechanical properties of YB2C2, such as Young's modulus (207 GPa), shear modulus (87 GPa), bulk modulus (116 GPa), Vickers hardness (4.2 ± 0.3 GPa), three-point flexural strength (460 ± 10 MPa), and fracture toughness (4.6 ± 0.1 MPa·m1/2) were characterized. The as-received YB2C2 ceramic possesses typical layered structure, excellent damage tolerance and easy machinability. © 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
Ultrahigh-temperature ceramics (UHTCs) based on transition metal borides or carbides are regarded as the materials of choice for nose and sharp leading edges of hypersonic vehicles [1–3]. Among the family of UHTCs, ZrB2-based materials have been widely investigated owing to their high melting point, chemical inertness and no phase transformations in the solid state. However, the intrinsic brittleness, poor thermal shock resistance and machinability have greatly hampered their applications as structural components. Hence, searching for new UHTCs with high damage tolerance, good thermal shock resistance and easy machinability is of great urgency. In recent years, a series of layered ternary ceramics, i.e., MAX phases [4,5] have been extensively studied. MAX phases possess a layered crystal structure composed of alternately stacking of M-X octahedron slabs and weakly bonded A slabs. Due to this unique crystal structure and chemical bonds characteristics, MAX phases exhibit many excellent properties, such as high damage tolerance, good thermal shock resistance and machinability [5]. In terms of MAX phases, their damage tolerance mainly stems from the weak bondings among adjacent layers [6]. These results give us a hint that the ceramics with layered crystal structure and alternately stacking of weak bondings and strong bondings among the layers, or, in other words, weak bonds in adjacent layers exist together with the strong covalent bonds in the layers, are probably damage tolerant, thermal shock resistant and readily machinable. Recently, Zhou et al. predicted that YB2 [7], YB4 [8], YB6 [9], and YbB6 [10] are damage tolerant even “ductile” ceramics for ultrahigh-
⁎ Corresponding author at: 72 Wenhua Road, Shenyang 110016, China. E-mail address: [email protected] (J. Chen).
http://dx.doi.org/10.1016/j.scriptamat.2016.06.041 1359-6462/© 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
temperature applications through first-principles calculations and experimental investigations. For YB6 and YbB6, their “ductility” is underpinned by the anisotropic chemical bonding within the crystal structure, that is, the coexistence of strong σ bond connecting the B6 octahedra and the weak banana bond within the B6 octahedron [9,10]. These works open the door to design next generation UHTCs for applications in hypersonic vehicles. ReB2C2 (Re_Sc, Y, lanthanides and actinides) phases are a class of layered rare earth metal diborodicarbides [11]. For YB2C2 (Re_Y), boron and carbon atoms form infinite, planar, two-dimensional (2-D) networks which alternate with 2-D sheets of Y atom along the c direction. Within the B2C2 layer, each atom is bonded to three other atoms so as to form fused four- and eight-membered rings. Each four-membered ring is made of two boron and two carbon atoms in opposite positions, whereas each eight-membered ring contains four boron and four carbon atoms with B-C contacts [12,13]. For YB2C2, weak Y-B and Y-C bonds between Y layer and B2C2 layer exist together with the strong B-C covalent bonds in the layers among the B-C plane [12]. Its crystal structure and chemical bonding properties are highly similar to MAX phases, YbB6 and others damage tolerant ceramics. Furthermore, the melting point of material is an essential factor for ultrahigh temperature applications. However, the melting point or decomposition temperature of YB2C2 has not been reported till now. In previous works, YB2C2 always appeared when Y2O3 was used as sintering aid of B4C. Goldstein et al. [14] found that YB2C2 still existed when heating the mixture of B4C and Y2O3 at 2180 °C for 120 min. This means that the decomposition temperature of YB2C2 is at least higher than 2180 °C. Therefore, the unique structure and properties features of YB2C2 render it as a promising damage tolerant material for ultrahigh temperature applications.
G. Zhao et al. / Scripta Materialia 124 (2016) 86–89
Fig. 1. X-ray diffraction pattern of YB2C2 synthesized by an in situ hot pressing method.
However, there has been little research into the synthesis, properties and application of YB2C2. In this work, high purity bulk YB2C2 was firstly prepared by an in situ hot pressing process. Commercially available yttrium hydride (99%, − 200 mesh, TITD, China), boron carbide powders (99%, −200 mesh, Jingangzhuan, China), and graphite powders (99%, − 200 mesh, Tianyuan, China) were selected as starting materials to synthesize YB2C2 ceramic. These powders were mixed according to the molar ratio of YH2:B4C:C = 2:1:2.9 and milled in a polypropylene jar for 12 h. After that, they were compacted uniaxially under 5 MPa in a graphite mold pre-sprayed with a BN layer. The reactive synthesis process was performed in a furnace using graphite as the heating element in a flowing Ar atmosphere. Green compacts of the mixed powders was heated to 1900 °C with a heating rate of 10 °C/min, held at that temperature for 30 min under a pressure of 30 MPa. Finally, the sample was cooled down to ambient temperature and then the contaminants on the surface were machined off by using a grinding wheel. The phase compositions of the sample were identified using an Xray diffractometer (XRD) with CuKα radiation (Rigaku D/max-2400, Tokyo, Japan). The density of sintered sample was determined by the Archimedes method. The surface and fracture morphologies of the sample were investigated in a SUPRA 35 scanning electron microscope (SEM) (LEO, Oberkochen, Germany) equipped with an energy-dispersive spectroscopy (EDS) system. The polished surface was etched for 30 s in an acid solution consisting of HF, HNO3 and H2O mixed at an equal volume fraction. The grain size of samples was measured by a laser confocal scanning microscope (LCSM) (Olympus LEXT OLS4000, Tokyo, Japan). More than 100 grains are measured to determine the
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average grain size. The Young's modulus, shear modulus, and Poisson's ratio of YB2C2 were evaluated on rectangular bar with dimensions of 3 × 15 × 40 mm3 by the impulse excitation technique using the resonance frequency and damping analyzer (IMCE, Diepenbeek, Belgium). Vickers hardness was determined at loads of 1, 3, 5, 10, 30 N with a dwell time of 15 s by a microhardness tester (Taiming HXD-1000B, Shanghai, China). The three-point flexural strength was evaluated on a universal-testing machine. The size of the samples for the flexural tests is 3 × 4 × 36 mm3 and the crosshead speed is 0.5 mm/min. Fracture toughness was measured using the single edge notched beam method on the specimens with dimensions of 4 × 8 × 36 mm3. The notches were introduced by a diamond-coated wheel slotting. The thickness of the blade is 0.10 mm and the width of the notches is about 0.15 mm. Four-point flexural tests with a crosshead speed of 0.05 mm/min were performed for fracture toughness measurements. To evaluate the damage tolerance of the material, indentations on 3 mm × 4 mm × 36 mm flexural specimens were performed using a Vickers indenter under loads of 50, 100, and 200 N. One diagonal of the indent is parallel to the length of the specimen. The residual strength after indentation was determined by four-point flexural test. In order to study the machinability of YB2C2, a drilling test was carried out at a constant normal force of about 50 N and a speed of 600 rpm, using a 2 mm diameter cemented carbide drill. SEM observation of the drilled surface was performed to investigate the level of damage caused by drilling process. Fig. 1 shows a typical XRD pattern of the sintered sample. Most of the peaks can be indexed using the reflections of YB2C2 (PDF # 65-2830), which indicates the high purity of the as-prepared ceramic. However, weak diffraction peaks (2θ) at about 13.2, 26.6, 29.9, 31.9 and 40.5°cannot be identified, which implies that there maybe a new phase in the asprepared sample. According to the PDF # 65-2830, the density of YB2C2 is 4.35 g/cm3, which is very close to the value (4.22 g/cm3) in this work. The relative density is determined as 97%. The typical backscattered electron SEM image of an etched sample was shown in Fig. 2(a). Few micropores can be observed, indicating a high density of the as-prepared YB2C2. Besides, it showed the presence of two regions: a bright region corresponding to YB2C2 and a dark minority region. Energy dispersive spectroscopic (EDS) analysis indicated that the phase in the dark region consisted of Y (9.8 at.%), B (23.6 at.%), and C (66.6 at.%), which means that the new phase in Y-BC system is an carbon-rich phase. However, the definite stoichiometry of this new phase is unknown by far. The polished surface of YB2C2 observed by LCSM is shown in Fig. 2(b). It can be seen that YB2C2 grains show an equiaxed-like morphology, and the average grain size is about 5 μm. The Young's modulus (E), shear modulus (G) and Poisson's ratio (ν) of the YB2C2 are 207 GPa, 87 GPa and 0.2, respectively. The calculated bulk modulus (B) according to the relationship of B = E / [3(1 − 2ν)] is 116 GPa. Low shear modulus and high Poisson's ratio indicate low shear deformation resistance. Pugh's shear to bulk modulus ratio (G/
Fig. 2. (a) SEM and (b) LCSM micrographs of etched and polished surface, and (c) fractured surface of YB2C2.
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G. Zhao et al. / Scripta Materialia 124 (2016) 86–89
Fig. 3. (a) Vickers hardness of YB2C2 versus indentation load, the insert shows SEM micrographs of a hardness indent (10 N), and (b) indentation load dependence of residual flexural strength of YB2C2.
B) is often used as criteria to distinguish ductility or damage tolerance materials from brittle ones [15]. Low G/B value indicate intrinsic ductile or damage tolerant. For YB2C2, Pugh's shear to bulk modulus ratio (G/B) is 0.748. This value is very close to those of high damage tolerant ceramics like Ti3SiC2 (0.759) [16], and Ti3AlC2 (0.706) [17], indicating that YB2C2 is a possible damage tolerant ceramic. The three-point flexural strength of YB2C2 is 460 ± 10 MPa. The fracture toughness (KIC) of YB2C2 is 4.6 ± 0.1 MPa·m1/2. YB2C2 has high fracture toughness, compared with other transition metal borides, such as YbB6 (3.2 MPa·m1/2) [10], ZrB2 (3.5–4.2 MPa·m1/2) [1] etc. Fig. 2(c) shows a high-magnification SEM micrograph of the fracture surface of YB2C2 after flexural test. A laminated structure can be recognized from its typical cleavage nature. The delamination of YB2C2 grains is mainly caused by crack deflection as cracks penetrating inside grains due to the low shear resistance. In addition, kink bands are also found on the fracture surface, which are shown as insert in Fig. 2(c). According to previous studies on damage tolerant ceramics such as Ti3SiC2 [18], welldocumented damage tolerance and intrinsic toughness were associated with energy dissipating mechanisms such as kinking and delamination. Thus, it is believed that the existence of kink bands may improve the damage tolerance and intrinsic toughness of YB2C2. Fig. 3(a) shows the Vickers hardness of YB2C2 as a function of indentation loads. With increasing load from 1 to 30 N, the hardness gradually decreases from 7.2 ± 1.7 to 4.2 ± 0.3 GPa. The reduction in hardness with indentation load is due to kinking non-linear elasticity [19]. SEM micrograph of the damage region around the indent at a load of 10 N is shown as the inset of Fig. 3(a). It is seen that the damage is confined in the immediate vicinity of the indent, no cracks initiate and propagate from the diagonals, and grains push out as well as decohesion can be clearly seen in the vicinity of the indent edges. This indentation
morphology is remarkably similar to those observed in the typical MAX phase ceramics [16–18]. To further prove the damage tolerance of YB2C2, the four-point flexural strength as a function of indentation loads is shown in Fig. 3(b). It is seen that below 200 N (indent size, 300 μm), the residual flexural strength (σr) of YB2C2 samples slowly decreases with increasing indentation load. Even under the Vickers contact damage of 200 N, the residual flexural strength still maintains about 80% of the undamaged samples. At that time, the indent size is nearly about 8% of the sample's width. So, it is proven that YB2C2 is damage tolerant and insensitive to the defects produced at indentation load at least 200 N. Besides damage tolerance, easy machinability of ceramic materials is also crucial for ultrahigh temperature applications. Attributed to the combination of unique properties, such as low shear modulus, low hardness and high damage tolerance, YB2C2 can be easily drilled by a cemented carbide tool at a constant normal force of 50 N and at a speed of 600 rpm. A macrograph of the drilled samples with 2 mm diameter holes is presented in Fig. 4(a). The drilled hole does not exhibit large-scale cracking or chipping. Details of the drilled surface are shown in Fig. 4(b). The machined surface is very flat and partially covered with a layer of smeared debris. So, YB2C2 has a good machinability and it can be readily machined by conventional cemented carbide tools. In summary, dense YB2C2 ceramic was successfully fabricated via an in situ hot-pressing method using yttrium hydride, boron carbide, and graphite powders as starting materials at 1900 °C for 30 min under the pressure of 30 MPa. The as-received YB2C2 ceramic possesses typical layered structure, low Young's modulus, low shear modulus, low hardness, excellent damage tolerance and easy machinability. These unique features might endow YB2C2 with great untapped potential for future ultrahigh temperature applications.
Fig. 4. (a) Optical photograph of the drilled YB2C2 sample, and (b) SEM micrograph of the machined surface of the drilled hole.
G. Zhao et al. / Scripta Materialia 124 (2016) 86–89
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