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Microstructures and mechanical properties of ZrB2–SiC–Ni ceramic composites prepared by spark plasma sintering
Ceramics International 45 (2019) 16707–16712
Contents lists available at ScienceDirect
Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Short communication
Microstructures and mechanical properties of ZrB2–SiC–Ni ceramic composites prepared by spark plasma sintering
T
Xiaojie Yana, Xiaochao Jinb, Pan Lic, Cheng Houb, Xin Haob, Zhigang Lia,∗, Xueling Fanb,∗∗ a
Institute of Applied Mechanics, College of Mechanical and Vehicle Engineering, Taiyuan University of Technology, Taiyuan, 030024, China State Key Laboratory for Strength and Vibration of Mechanical Structures, School of Aerospace Engineering, Xi'an Jiaotong University, Xi'an, 710049, China c College of Mechanical and Electrical Engineering, North University of China, Taiyuan, 030051, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: ZrB2-SiC-Ni Ultra-high temperature ceramic Spark plasma sintering Microstructure Mechanical properties
ZrB2 based ultra-high temperature ceramic is one of the most promising thermal protection materials that can be used in hypersonic vehicles. Four different ZrB2-25 mol.% SiC–Ni ceramic composites are prepared by spark plasma sintering at 1700 οC, in which the content of Ni are 2, 4, 6 and 8 mol.%, respectively. To make a comparison, ZrB2–SiC ceramic is also sintered with the same method. The densifications and microstructures are carefully checked. Nanoindentation tests are performed to determine the hardness and elastic modulus of sintered specimens. The fracture toughness of ceramics is evaluated by a series of single-edge notched-beam tests. Results show that densities of the ZrB2–SiC–Ni ceramic composites obviously increase with the increase of the content of Ni. The hardness and elastic modulus of the specimens slightly decline, while the fracture toughness significantly increase with the increase of the Ni content. The fracture toughness of ZrB2-25 mol.% SiC-8 mol.% Ni reaches 8.3 MPa m1/2, which is about 1.7 times that of the ZrB2–SiC composites. The results in this paper show that the introduction of Ni can greatly improve the fracture toughness of ZrB2–SiC ceramics.
1. Introduction ZrB2 based ultra-high temperature ceramic, with its high melting point, high strength, high hardness, good electrical and thermal conductivity, has become one of the best candidates for the thermal protective materials [1–4], which can be used for the nose cone and leading edge of hypersonic vehicles. Due to the existence of strong covalent bonds and low self-diffusion coefficients in ZrB2 based ceramics, it is difficult to achieve complete densification. Usually the sintering temperature of ZrB2 ceramics is above 2000 °C. At present, the preparation of ZrB2-based ceramics at lower temperatures has been studied by a few researchers. There are three strategies for low temperature sintering densification: (1) Using a faster sintering technology, sintering temperature and sintering time are the main factors affecting the densification of ceramics. A longer sintering time will cause excessive growth of ceramic grains, which will reduce the densification [5]. (2) Selecting smaller ceramic powder, the smaller powder has larger specific surface area and activity, and promote the rearrangement and mass transfer of particles during sintering, thus achieving densification at lower temperature [6]. (3) Using sintering additives, appropriate additives can promote the removal of
∗
oxides on the surface of powder particles to induce sintering densification [4]. Previous studies show that the introduction of SiC into ZrB2 based ceramics has significantly improved their densifications and bending strength [7–9]. However, the fracture toughness of ZrB2–SiC is still relative low, only 3–4 MPa m1/2 [10]. Therefore, improvement of the fracture toughness of ZrB2–SiC is one of the main focuses of current researches. Recently, metal powders have been introduced into ceramics to form novel ultra-high temperature ceramics [11]. Adding transition metal powders to ceramic matrix can obviously improve the densification and the fracture toughness of the ceramic matrix, due to the metal has good toughness and ductility [12]. In the sintering process, liquid metal phase fills the pores inside the ceramic matrix, which can help to achieve more uniform distribution of ceramic phases. Metal has a good wettability over most ceramics, which can improve the internal structure of ceramics, rearrange particles and grain boundary migration; as a result, fewer macroscopic defects will be formed in the sintering [13–15]. It should be pointed out that some metal additives have negative effects on the sintering process of ceramics. The addition of Fe and Mo
Corresponding author. Corresponding author. E-mail addresses: [email protected] (Z. Li), [email protected] (X. Fan).
∗∗
https://doi.org/10.1016/j.ceramint.2019.05.151 Received 17 January 2019; Received in revised form 6 May 2019; Accepted 14 May 2019 Available online 20 May 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Ceramics International 45 (2019) 16707–16712
X. Yan, et al.
improve the densification behavior of pressureless sintered ZrB2 composites. However, the excessive Fe addition would accelerate the degradation of mechanical properties of ZrB2 ceramics [16]. Similar agglomeration phenomenon can be also found in ZrB2-based ceramics with Ti as additive [17]. In addition, high content of Ni as additive in ZrB2 ceramic would affect its mechanical properties at high temperature, due to forming low melting phase [18]. Therefore, choosing a reasonable content of metal is one of the key factors for preparing metal toughened ceramics. As one of the transition metals, Ni has high melting point (1455 οC), relative low density (9.8 g/cm3). In addition, the thermal conductivity (90 W/(m·K)) of Ni is quite close to that of ZrB2–SiC ceramic [19]. Therefore, it is worth introducing Ni into ZrB2–SiC ceramic to prepare a novel ZrB2–SiC–Ni ultra-high temperature ceramic. In this work, ultrahigh temperature ceramics are prepared by spark plasma sintering, as it is one of the most effective densification methods [20,21]. The paper is organized as follows. In Section 2, materials preparation and experimental procedures are introduced. In Section 3, results of nanoindentation and single-edge notched-beam tests are detailed discussed. Finally, conclusions are summarized in Section 4. 2. Materials and experimental procedures 2.1. Materials preparation Nanosized ZrB2 (purity > 98%, 500 nm, Beijing Forsman Scientific Co.Ltd., China), SiC (purity > 99%, 40 nm, Nanjing Emperor Nano Material Co. Ltd., China) and Ni (purity > 99.5%, 80 nm, Nanjing Emperor Nano Material Co. Ltd., China) powders are used as the raw materials. The microstructures of raw powder (ZrB2, SiC, Ni) are observed by SEM in Fig. 1. The Four different ZrB2–SiC–Ni ultra-high temperature ceramics are sintered, in which the content of Ni are 2, 4, 6 and 8 mol.%, respectively. To make a comparison, ZrB2–SiC ceramic without Ni is also prepared. In the preparation process, powders are mixed firstly, and then are ball-milled for 12 h using ZrO2 ball media. After mixing, the obtained powders are dried by rotary evaporation and then screened. Next, the resulting powders are put in a cylindrical graphite die coupled with carbon paper at room temperature with a pressure of 10 MPa. Then, the pre-pressed samples are sintered under vacuum in a spark plasma sintering apparatus (LABOX-325S, Sinter Land Inc., Japan). A two-step method is adopted in the sintering process [22]. A heating rate of 200 οC/min is first employed until the temperature reaches 1400 οC and keeps it at 1400 οC for 1 min. And then, the temperature increases to 1700 °C with the same heating rate of 200 ο C/min and holds it at 1400 οC for 10 min. Last, the samples are cooled at 100 °C/min to room temperature. The uniaxial pressure is kept at 40 MPa during the sintering process. The five different specimens are denoted as ZS, ZSN2, ZSN4, ZSN6 and ZSN8, respectively, according to the content of additive Ni. 2.2. Characterization and mechanical tests After polished by sand paper, bulk densities of the sintered specimens are determined by Archimedes’ method. The microstructural
Fig. 2. The typical sintering curve of ZSN2.
features of the surface of the specimens are characterized by a scanning electron microscopy (SEM, FEI Quanta 400) with the distribution of chemical element analysis by energy dispersive spectroscopy (EDS). Crystalline phases are detected by XRD (X-ray diffractometer, Bruker AXS Inc., Germany) using Cu Kα radiation. Hardness (H) and Elastic modulus (E) of the samples are determined using Berkovich indentation (G200, Agilent Technologies). Indentation tests are using a strain rate of 0.05 s−1 and a maximum indentation depth of 1500 nm. When indenter tip reaches the maximum indentation depth, it is held for 10 s. Five data points are tested in each experimental condition. Continuous stiffness method (CSM) is adopted in indentation tests, which is using Oliver-Pharr method through the loaddisplacement curves to calculate the stiffness [23]. Here, the Hardness and Elastic modulus are calculated by [24],
F Ac
(1)
Ac = 24.56hc2
(2)
H=
where F is the load, hc is the contact depth, Ac is the contact area that is between indenter tip and specimen.
hc = h − ε
F S
(3)
where hc is the contact depth, h is indentation depth, ε is a constant of 0.75 for the Berkovich tip. S is the contact stiffness that is the slope of load vs displacement.
1 − vi2 1 1 − v2 = + Er E Ei
Er =
π 2β
S Ac
(4)
(5)
where Er is the reduced modulus which combines the modulus of the indenter and the specimen, Ei and vi are Elastic modulus and Poisson's ratio for Berkovich indenter (Ei = 1140 GPa, vi = 0.07), respectively, E and v are the same physical parameters for the specimen, β = 1.034 is the shape constant of a Berkovich tip.
Fig. 1. The microstructures of raw powder (ZrB2, SiC, Ni) by SEM. 16708
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Fig. 3. The EDS results of ZS: microstructures and distribution of five elements (Zr, O, Si, C, B). (The red arrow is SiC). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
The specimens are machined into rectangular specimens with the dimensions of 2 mm × 3 mm × 20 mm. Fracture toughness (KIC) is evaluated by single-edge notched-beam tests with a loading rate of 0.1 mm/min. KIC is determined by,
KIC =
PS 3 BW 2
a ⋅f ⎛ ⎞ ⎝W ⎠
2 a a πa πa tan f ⎛ ⎞ = ⎡1.88 + 0.75 ⎛ − 0.5⎞ ⎤ sec ⎥ W 2 W 2 W ⎝W ⎠ ⎢ ⎝ ⎠ ⎣ ⎦
(6)
(7)
where P is the failure load; S, B and W are the span (S = 16 mm), width (W = 2 mm) and height (B = 3 mm) of the test specimen, respectively; a a is the notch depth which is 1/3 of the specimen's height; and f W is the correction factor that is related to the interface size.
( )
3. Results and discussion 3.1. Microstructure The typical sintering curve of ZSN2 is shown in Fig. 2. The sintering process is divided into three stages: heating, holding and cooling. During the heating stage, the displacement of tip decreased gradually with the increase of temperature, for the densification is achieved by water vapor evaporation and pore compaction, which is also verified by the sudden increase of vacuum percentage at 600 οC (see Fig. 2). It is
worth noting that displacement curve decreases more obviously after 1400 οC during the heating, which may be caused by the melting of Ni and further filling the pores in ceramic substrate. It is also found that the vacuum curve has a small peak after 1400 οC. The reason of this phenomenon is the evaporation of oxidation product B2O3. This phenomenon is consistent with previous paper [25]. The microstructures of the specimens are illustrated in Figs. 3 and 4. With the increase of Ni content, the densities of specimens obviously increase, while sizes of pores decrease. The relative densities of the five samples reach 92.4%, 92.7%, 93.3%, 94.1%, 94.7%, respectively. The increased relative density is attributed to that the liquid phase of Ni fills the internal pores of ceramics. In addition, the introduction of Ni causes grain boundary migration, which would result in a more uniform distribution of different particles [26]. According to Fig. 3, the dark grains are SiC and the grey ones are ZrB2. It is clearly that all the elements are uniformly distributed, which also means that the materials are well sintered. It is worth noting that Ni is uniformly dispersed in the ceramic matrix (see Fig. 4), and it could be inferred that nano-nickel particles enter ZrB2–SiC and form an inner crystalline structure [27]. Crystalline phases of the specimens are shown in Fig. 5. It can be found that the peak intensity and position of ZrB2 and SiC in the first phase and second phase are almost the same in the five composites with different Ni contents. The modest peak of Ni is also detected and its intensity increases with the increase of Ni content. According to the XRD results, a little amount of ZrO2 is generated since ZrB2 reacts with
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Fig. 4. The EDS results of ZSN6: microstructures and distribution of five elements (Zr, O, Si, C, Ni).
Fig. 5. The XRD analysis of in ZrB2–SiC–Ni ceramics with different Ni contents.
the oxygen impurities on the surface of particles in the sintering process.
3.2. Mechanical properties The hardness and elastic modulus of Nano-indentation of the
samples slightly decrease with the increase of Ni content, as shown in Fig. 6(a). The hardness and elastic modulus of ZS are 20.2 GPa and 453.7 GPa, respectively. The hardness and elastic modulus of ZSN8 are 18.7 GPa and 440.6 GPa, with a decrease of 7.4% and 2.9%, respectively. Generally, the hardness and modulus of Ni (the hardness and elastic modulus of Ni-based alloys are about 8 GPa and 200 GPa, respectively [28]) are lower than ZrB2–SiC ceramic matrix, which results in a decline of those of the sintered ZrB2–SiC–Ni ultra-high temperature ceramics specimens. The fracture toughness results of ZrB2–SiC–Ni ultra-high temperature ceramics are compared with those from previous works [29–31], as shown in Fig. 6(b). The results indicate that Ni additive could improve the facture toughness of ZrB2 ceramics. Fig. 6(b) also shows that the fracture toughness of ZrB2–SiC–Ni ultra-high temperature ceramics obviously increases with the increase of Ni content. The average fracture toughness ZSN8 reaches 8.3 MPa m1/2, which is about 1.7 times that of the ZrB2–SiC ceramics (only 4.9 MPa m1/2). Obviously, the introduction of Ni improves the densification and modifies the macrodefects during sintering, which would further improve the fracture toughness of the ZrB2–SiC–Ni ceramics [32]. The SEM images of the fracture surfaces of the specimens are displayed in Fig. 7. For all the specimens, macroscopic fracture morphologies are typical brittle fracture, just similar with ZS (Fig. 7(a)). The intergranular fracture is the dominate mode for ZS, as shown in Fig. 7(b). It is obvious that many microcracks are observed that result in more fracture energy dissipation in ZSN2 than ZS, as shown in Fig. 7(c),
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Fig. 6. Measured properties of ZrB2–SiC–Ni ceramics with different Ni contents: (a) hardness and elastic modulus, (b) fracture toughness and comparison with previous works.
increase with the increase the Ni content. The fracture toughness of ZrB2-25 mol.% SiC-8 mol.% Ni ultra-high temperature ceramic reaches 8.3 MPa m1/2, which is about 1.7 times that of the ZrB2–SiC composites. Results show that the introduction of Ni can greatly improve the fracture toughness of ZrB2–SiC ceramics by microcracks and the refinement of particles. This work provides a guidance on improving fracture toughness of the ultra-high temperature ceramics by introducing nanosized metal powders. Acknowledgements This work was supported by National Natural Science Foundation of China (Grant Nos. 11472204 and 117110165), and the Fundamental Research Funds for the Central Universities (xzy022019016). References Fig. 7. The SEM images of the fracture surfaces: (a) macro-morphology of ZS; (b), (c), (d) microstructures of ZS, ZSN2 and ZSN8, respectively. (The white arrow is the intergranular fracture interface and the red arrow is the microcrack). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
which could explain the higher fracture toughness of ZSN2. For ZrB2based ceramics, there are two main reasons for microcracks forming: one is the mismatch of thermal expansion coefficients between Ni and ZrB2–SiC ceramic; the other one is that microcracks will occur at the crack tip under external load when the size of metal particles in sintered materials is larger than its critical size. When the main crack extends to Ni particles, the stress concentration at the tip of the main crack will be released due to the existence of micro-cracks, which is the toughening mechanism of microcracks [33]. According to Fig. 7(d), the particle size of ZSN8 is lower than that of ZS, which means that Ni could restrain grain growth to some extent, due to that liquid phase sintering of nickel promotes rearrangement and mass transfer of ceramic particles during sintering [18]. This also could explain the increased fracture toughness of ZSN8. The improvement of fracture toughness of ceramics with Ni addition is attribute to generation of microcracks and the refinement of particles. 4. Conclusion In this work, ZrB2–SiC–Ni ultra-high temperature ceramics with different Ni contents are prepared by spark plasma sintering at 1700 οC. To make a comparison, ZrB2–SiC ceramic is also sintered with the same method. Nanoindentation and single-edge notched-beam tests are performed to determine the mechanical properties. Experiments results indicate that the hardness and elastic modulus of the specimens slightly decline, while the densities and fracture toughness significantly
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16712
Contents lists available at ScienceDirect
Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Short communication
Microstructures and mechanical properties of ZrB2–SiC–Ni ceramic composites prepared by spark plasma sintering
T
Xiaojie Yana, Xiaochao Jinb, Pan Lic, Cheng Houb, Xin Haob, Zhigang Lia,∗, Xueling Fanb,∗∗ a
Institute of Applied Mechanics, College of Mechanical and Vehicle Engineering, Taiyuan University of Technology, Taiyuan, 030024, China State Key Laboratory for Strength and Vibration of Mechanical Structures, School of Aerospace Engineering, Xi'an Jiaotong University, Xi'an, 710049, China c College of Mechanical and Electrical Engineering, North University of China, Taiyuan, 030051, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: ZrB2-SiC-Ni Ultra-high temperature ceramic Spark plasma sintering Microstructure Mechanical properties
ZrB2 based ultra-high temperature ceramic is one of the most promising thermal protection materials that can be used in hypersonic vehicles. Four different ZrB2-25 mol.% SiC–Ni ceramic composites are prepared by spark plasma sintering at 1700 οC, in which the content of Ni are 2, 4, 6 and 8 mol.%, respectively. To make a comparison, ZrB2–SiC ceramic is also sintered with the same method. The densifications and microstructures are carefully checked. Nanoindentation tests are performed to determine the hardness and elastic modulus of sintered specimens. The fracture toughness of ceramics is evaluated by a series of single-edge notched-beam tests. Results show that densities of the ZrB2–SiC–Ni ceramic composites obviously increase with the increase of the content of Ni. The hardness and elastic modulus of the specimens slightly decline, while the fracture toughness significantly increase with the increase of the Ni content. The fracture toughness of ZrB2-25 mol.% SiC-8 mol.% Ni reaches 8.3 MPa m1/2, which is about 1.7 times that of the ZrB2–SiC composites. The results in this paper show that the introduction of Ni can greatly improve the fracture toughness of ZrB2–SiC ceramics.
1. Introduction ZrB2 based ultra-high temperature ceramic, with its high melting point, high strength, high hardness, good electrical and thermal conductivity, has become one of the best candidates for the thermal protective materials [1–4], which can be used for the nose cone and leading edge of hypersonic vehicles. Due to the existence of strong covalent bonds and low self-diffusion coefficients in ZrB2 based ceramics, it is difficult to achieve complete densification. Usually the sintering temperature of ZrB2 ceramics is above 2000 °C. At present, the preparation of ZrB2-based ceramics at lower temperatures has been studied by a few researchers. There are three strategies for low temperature sintering densification: (1) Using a faster sintering technology, sintering temperature and sintering time are the main factors affecting the densification of ceramics. A longer sintering time will cause excessive growth of ceramic grains, which will reduce the densification [5]. (2) Selecting smaller ceramic powder, the smaller powder has larger specific surface area and activity, and promote the rearrangement and mass transfer of particles during sintering, thus achieving densification at lower temperature [6]. (3) Using sintering additives, appropriate additives can promote the removal of
∗
oxides on the surface of powder particles to induce sintering densification [4]. Previous studies show that the introduction of SiC into ZrB2 based ceramics has significantly improved their densifications and bending strength [7–9]. However, the fracture toughness of ZrB2–SiC is still relative low, only 3–4 MPa m1/2 [10]. Therefore, improvement of the fracture toughness of ZrB2–SiC is one of the main focuses of current researches. Recently, metal powders have been introduced into ceramics to form novel ultra-high temperature ceramics [11]. Adding transition metal powders to ceramic matrix can obviously improve the densification and the fracture toughness of the ceramic matrix, due to the metal has good toughness and ductility [12]. In the sintering process, liquid metal phase fills the pores inside the ceramic matrix, which can help to achieve more uniform distribution of ceramic phases. Metal has a good wettability over most ceramics, which can improve the internal structure of ceramics, rearrange particles and grain boundary migration; as a result, fewer macroscopic defects will be formed in the sintering [13–15]. It should be pointed out that some metal additives have negative effects on the sintering process of ceramics. The addition of Fe and Mo
Corresponding author. Corresponding author. E-mail addresses: [email protected] (Z. Li), [email protected] (X. Fan).
∗∗
https://doi.org/10.1016/j.ceramint.2019.05.151 Received 17 January 2019; Received in revised form 6 May 2019; Accepted 14 May 2019 Available online 20 May 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Ceramics International 45 (2019) 16707–16712
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improve the densification behavior of pressureless sintered ZrB2 composites. However, the excessive Fe addition would accelerate the degradation of mechanical properties of ZrB2 ceramics [16]. Similar agglomeration phenomenon can be also found in ZrB2-based ceramics with Ti as additive [17]. In addition, high content of Ni as additive in ZrB2 ceramic would affect its mechanical properties at high temperature, due to forming low melting phase [18]. Therefore, choosing a reasonable content of metal is one of the key factors for preparing metal toughened ceramics. As one of the transition metals, Ni has high melting point (1455 οC), relative low density (9.8 g/cm3). In addition, the thermal conductivity (90 W/(m·K)) of Ni is quite close to that of ZrB2–SiC ceramic [19]. Therefore, it is worth introducing Ni into ZrB2–SiC ceramic to prepare a novel ZrB2–SiC–Ni ultra-high temperature ceramic. In this work, ultrahigh temperature ceramics are prepared by spark plasma sintering, as it is one of the most effective densification methods [20,21]. The paper is organized as follows. In Section 2, materials preparation and experimental procedures are introduced. In Section 3, results of nanoindentation and single-edge notched-beam tests are detailed discussed. Finally, conclusions are summarized in Section 4. 2. Materials and experimental procedures 2.1. Materials preparation Nanosized ZrB2 (purity > 98%, 500 nm, Beijing Forsman Scientific Co.Ltd., China), SiC (purity > 99%, 40 nm, Nanjing Emperor Nano Material Co. Ltd., China) and Ni (purity > 99.5%, 80 nm, Nanjing Emperor Nano Material Co. Ltd., China) powders are used as the raw materials. The microstructures of raw powder (ZrB2, SiC, Ni) are observed by SEM in Fig. 1. The Four different ZrB2–SiC–Ni ultra-high temperature ceramics are sintered, in which the content of Ni are 2, 4, 6 and 8 mol.%, respectively. To make a comparison, ZrB2–SiC ceramic without Ni is also prepared. In the preparation process, powders are mixed firstly, and then are ball-milled for 12 h using ZrO2 ball media. After mixing, the obtained powders are dried by rotary evaporation and then screened. Next, the resulting powders are put in a cylindrical graphite die coupled with carbon paper at room temperature with a pressure of 10 MPa. Then, the pre-pressed samples are sintered under vacuum in a spark plasma sintering apparatus (LABOX-325S, Sinter Land Inc., Japan). A two-step method is adopted in the sintering process [22]. A heating rate of 200 οC/min is first employed until the temperature reaches 1400 οC and keeps it at 1400 οC for 1 min. And then, the temperature increases to 1700 °C with the same heating rate of 200 ο C/min and holds it at 1400 οC for 10 min. Last, the samples are cooled at 100 °C/min to room temperature. The uniaxial pressure is kept at 40 MPa during the sintering process. The five different specimens are denoted as ZS, ZSN2, ZSN4, ZSN6 and ZSN8, respectively, according to the content of additive Ni. 2.2. Characterization and mechanical tests After polished by sand paper, bulk densities of the sintered specimens are determined by Archimedes’ method. The microstructural
Fig. 2. The typical sintering curve of ZSN2.
features of the surface of the specimens are characterized by a scanning electron microscopy (SEM, FEI Quanta 400) with the distribution of chemical element analysis by energy dispersive spectroscopy (EDS). Crystalline phases are detected by XRD (X-ray diffractometer, Bruker AXS Inc., Germany) using Cu Kα radiation. Hardness (H) and Elastic modulus (E) of the samples are determined using Berkovich indentation (G200, Agilent Technologies). Indentation tests are using a strain rate of 0.05 s−1 and a maximum indentation depth of 1500 nm. When indenter tip reaches the maximum indentation depth, it is held for 10 s. Five data points are tested in each experimental condition. Continuous stiffness method (CSM) is adopted in indentation tests, which is using Oliver-Pharr method through the loaddisplacement curves to calculate the stiffness [23]. Here, the Hardness and Elastic modulus are calculated by [24],
F Ac
(1)
Ac = 24.56hc2
(2)
H=
where F is the load, hc is the contact depth, Ac is the contact area that is between indenter tip and specimen.
hc = h − ε
F S
(3)
where hc is the contact depth, h is indentation depth, ε is a constant of 0.75 for the Berkovich tip. S is the contact stiffness that is the slope of load vs displacement.
1 − vi2 1 1 − v2 = + Er E Ei
Er =
π 2β
S Ac
(4)
(5)
where Er is the reduced modulus which combines the modulus of the indenter and the specimen, Ei and vi are Elastic modulus and Poisson's ratio for Berkovich indenter (Ei = 1140 GPa, vi = 0.07), respectively, E and v are the same physical parameters for the specimen, β = 1.034 is the shape constant of a Berkovich tip.
Fig. 1. The microstructures of raw powder (ZrB2, SiC, Ni) by SEM. 16708
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Fig. 3. The EDS results of ZS: microstructures and distribution of five elements (Zr, O, Si, C, B). (The red arrow is SiC). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
The specimens are machined into rectangular specimens with the dimensions of 2 mm × 3 mm × 20 mm. Fracture toughness (KIC) is evaluated by single-edge notched-beam tests with a loading rate of 0.1 mm/min. KIC is determined by,
KIC =
PS 3 BW 2
a ⋅f ⎛ ⎞ ⎝W ⎠
2 a a πa πa tan f ⎛ ⎞ = ⎡1.88 + 0.75 ⎛ − 0.5⎞ ⎤ sec ⎥ W 2 W 2 W ⎝W ⎠ ⎢ ⎝ ⎠ ⎣ ⎦
(6)
(7)
where P is the failure load; S, B and W are the span (S = 16 mm), width (W = 2 mm) and height (B = 3 mm) of the test specimen, respectively; a a is the notch depth which is 1/3 of the specimen's height; and f W is the correction factor that is related to the interface size.
( )
3. Results and discussion 3.1. Microstructure The typical sintering curve of ZSN2 is shown in Fig. 2. The sintering process is divided into three stages: heating, holding and cooling. During the heating stage, the displacement of tip decreased gradually with the increase of temperature, for the densification is achieved by water vapor evaporation and pore compaction, which is also verified by the sudden increase of vacuum percentage at 600 οC (see Fig. 2). It is
worth noting that displacement curve decreases more obviously after 1400 οC during the heating, which may be caused by the melting of Ni and further filling the pores in ceramic substrate. It is also found that the vacuum curve has a small peak after 1400 οC. The reason of this phenomenon is the evaporation of oxidation product B2O3. This phenomenon is consistent with previous paper [25]. The microstructures of the specimens are illustrated in Figs. 3 and 4. With the increase of Ni content, the densities of specimens obviously increase, while sizes of pores decrease. The relative densities of the five samples reach 92.4%, 92.7%, 93.3%, 94.1%, 94.7%, respectively. The increased relative density is attributed to that the liquid phase of Ni fills the internal pores of ceramics. In addition, the introduction of Ni causes grain boundary migration, which would result in a more uniform distribution of different particles [26]. According to Fig. 3, the dark grains are SiC and the grey ones are ZrB2. It is clearly that all the elements are uniformly distributed, which also means that the materials are well sintered. It is worth noting that Ni is uniformly dispersed in the ceramic matrix (see Fig. 4), and it could be inferred that nano-nickel particles enter ZrB2–SiC and form an inner crystalline structure [27]. Crystalline phases of the specimens are shown in Fig. 5. It can be found that the peak intensity and position of ZrB2 and SiC in the first phase and second phase are almost the same in the five composites with different Ni contents. The modest peak of Ni is also detected and its intensity increases with the increase of Ni content. According to the XRD results, a little amount of ZrO2 is generated since ZrB2 reacts with
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Fig. 4. The EDS results of ZSN6: microstructures and distribution of five elements (Zr, O, Si, C, Ni).
Fig. 5. The XRD analysis of in ZrB2–SiC–Ni ceramics with different Ni contents.
the oxygen impurities on the surface of particles in the sintering process.
3.2. Mechanical properties The hardness and elastic modulus of Nano-indentation of the
samples slightly decrease with the increase of Ni content, as shown in Fig. 6(a). The hardness and elastic modulus of ZS are 20.2 GPa and 453.7 GPa, respectively. The hardness and elastic modulus of ZSN8 are 18.7 GPa and 440.6 GPa, with a decrease of 7.4% and 2.9%, respectively. Generally, the hardness and modulus of Ni (the hardness and elastic modulus of Ni-based alloys are about 8 GPa and 200 GPa, respectively [28]) are lower than ZrB2–SiC ceramic matrix, which results in a decline of those of the sintered ZrB2–SiC–Ni ultra-high temperature ceramics specimens. The fracture toughness results of ZrB2–SiC–Ni ultra-high temperature ceramics are compared with those from previous works [29–31], as shown in Fig. 6(b). The results indicate that Ni additive could improve the facture toughness of ZrB2 ceramics. Fig. 6(b) also shows that the fracture toughness of ZrB2–SiC–Ni ultra-high temperature ceramics obviously increases with the increase of Ni content. The average fracture toughness ZSN8 reaches 8.3 MPa m1/2, which is about 1.7 times that of the ZrB2–SiC ceramics (only 4.9 MPa m1/2). Obviously, the introduction of Ni improves the densification and modifies the macrodefects during sintering, which would further improve the fracture toughness of the ZrB2–SiC–Ni ceramics [32]. The SEM images of the fracture surfaces of the specimens are displayed in Fig. 7. For all the specimens, macroscopic fracture morphologies are typical brittle fracture, just similar with ZS (Fig. 7(a)). The intergranular fracture is the dominate mode for ZS, as shown in Fig. 7(b). It is obvious that many microcracks are observed that result in more fracture energy dissipation in ZSN2 than ZS, as shown in Fig. 7(c),
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Fig. 6. Measured properties of ZrB2–SiC–Ni ceramics with different Ni contents: (a) hardness and elastic modulus, (b) fracture toughness and comparison with previous works.
increase with the increase the Ni content. The fracture toughness of ZrB2-25 mol.% SiC-8 mol.% Ni ultra-high temperature ceramic reaches 8.3 MPa m1/2, which is about 1.7 times that of the ZrB2–SiC composites. Results show that the introduction of Ni can greatly improve the fracture toughness of ZrB2–SiC ceramics by microcracks and the refinement of particles. This work provides a guidance on improving fracture toughness of the ultra-high temperature ceramics by introducing nanosized metal powders. Acknowledgements This work was supported by National Natural Science Foundation of China (Grant Nos. 11472204 and 117110165), and the Fundamental Research Funds for the Central Universities (xzy022019016). References Fig. 7. The SEM images of the fracture surfaces: (a) macro-morphology of ZS; (b), (c), (d) microstructures of ZS, ZSN2 and ZSN8, respectively. (The white arrow is the intergranular fracture interface and the red arrow is the microcrack). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
which could explain the higher fracture toughness of ZSN2. For ZrB2based ceramics, there are two main reasons for microcracks forming: one is the mismatch of thermal expansion coefficients between Ni and ZrB2–SiC ceramic; the other one is that microcracks will occur at the crack tip under external load when the size of metal particles in sintered materials is larger than its critical size. When the main crack extends to Ni particles, the stress concentration at the tip of the main crack will be released due to the existence of micro-cracks, which is the toughening mechanism of microcracks [33]. According to Fig. 7(d), the particle size of ZSN8 is lower than that of ZS, which means that Ni could restrain grain growth to some extent, due to that liquid phase sintering of nickel promotes rearrangement and mass transfer of ceramic particles during sintering [18]. This also could explain the increased fracture toughness of ZSN8. The improvement of fracture toughness of ceramics with Ni addition is attribute to generation of microcracks and the refinement of particles. 4. Conclusion In this work, ZrB2–SiC–Ni ultra-high temperature ceramics with different Ni contents are prepared by spark plasma sintering at 1700 οC. To make a comparison, ZrB2–SiC ceramic is also sintered with the same method. Nanoindentation and single-edge notched-beam tests are performed to determine the mechanical properties. Experiments results indicate that the hardness and elastic modulus of the specimens slightly decline, while the densities and fracture toughness significantly
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