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ZrB2 based novel composite with NiAl as reinforcement phase
International Journal of Refractory Metals & Hard Materials 70 (2018) 56–65
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International Journal of Refractory Metals & Hard Materials journal homepage: www.elsevier.com/locate/IJRMHM
ZrB2 based novel composite with NiAl as reinforcement phase a,⁎
a,b
J.K. Sonber , T.S.R.Ch. Murthy a b c
, K. Sairam
a,b
c
, A. Nagaraj , S. Majumdar
a,b
, V. Kain
MARK a,b
Materials Processing and Corrosion Engineering Division, BARC, Mumbai, India Homi Bhabha National Institute, Mumbai, India Laser and Plasma Technology Division, BARC, Mumbai, India
A R T I C L E I N F O
A B S T R A C T
Keywords: ZrB2 NiAl Composite Hot pressing Mechanical properties Microstructure
ZrB2 based novel composites have been prepared using NiAl as reinforcement phase. Three samples of compositions (a) ZrB2 + 5%NiAl (a) ZrB2 + 10%NiAl (a) ZrB2 + 20%NiAl were prepared by hot pressing at 1700 °C. All the three samples were densified to a density of > 94% of its theoretical value. Addition of NiAl resulted in liquid phase sintering and assisted in densification. The composite was characterized by mechanical property measurement and microstructure analysis. Flexural strength of the composite was found to be in the range of 421 to 438 MPa. Hardness of composite was found to decrease with addition of NiAl. Indentation fracture toughness was found to increase with increasing NiAl content. Microstructural characterization revealed the uniform distribution of NiAl phase in ZrB2 matrix. Grain coarsening was observed in composite with higher NiAl content. Fracture surface analysis revealed that the mode of fracture is transgranular. Microstructure analysis of crack propagation revealed the presence of crack bridging and crack arrest near NiAl phase, which resulted in higher fracture toughness of 6.8 MPa·m1/2. Isothermal oxidation study at 1400 °C revealed that the developed composite has good oxidation resistance.
1. Introduction ZrB2 is a potential material for high temperature applications due to its extremely high melting point, high thermal conductivity, low coefficient of thermal expansion, high hardness, good retention of strength at high temperature and chemical stability [1–2]. It is a candidate material for leading edge applications for space re-entry vehicles [1]. It is used for holding molten metal and as thermo-well tubes in metal processing due to its compatibility with liquid metals [3]. It also finds application as electrode material in Hall-Heroult cell and electric discharge machining due to its good electrical conductivity [4–5]. The major issue in ZrB2 based material is its poor sinterability and poor fracture toughness. Due to strong co-valent bonds and low selfdiffusivity, ZrB2 can be fully densified only at very high sintering temperatures of the order of ~2000 °C and also requires external pressure. Moreover, ZrB2 powder is always coated with zirconium and boron oxides which promotes evaporation and condensation mechanism, that results in mass transfer without densification [1,6]. Several additives have been used to improve densification. Addition of carbon and carbides such as TiC, SiC and B4C assisted in densification by reducing the oxide layer present on the surface of ZrB2 particles [7–10]. Transition metal silicides such as MoSi2, TaSi2, WSi2, TiSi2, CrSi2, ZrSi2 etc. have been reported to improve densification of ZrB2
⁎
[11–16]. In this study, possibility of using aluminide compounds as sintering aid and reinforcement to boride ceramics has been investigated. Though, aluminide compounds are well known for their excellent high temperature properties, there is no literature on the use of these compounds as reinforcement for ZrB2. For the present study, NiAl was chosen as sinter additive and reinforcement. It is expected to assist in densification by formation of liquid phase, during sintering. NiAl is an excellent material for high temperature applications as it has high melting point, good thermal conductivity and excellent oxidation resistance [17]. It is considered a potential material for turbine blade applications [18]. Table 1 shows some of the properties of ZrB2 [2] and NiAl [19–21]. In present study, investigation was carried out to explore the use of NiAl as reinforcement for ZrB2.. Microstructure evolution, mechanical properties and oxidation behavior of ZrB2 + NiAl composites were also studied. 2. Experimental In-house prepared ZrB2 and NiAl powders were used as starting materials. The procedure followed for the preparation of ZrB2 powder is already reported elsewhere [15]. NiAl was prepared by reaction of
Corresponding author. E-mail address: [email protected] (J.K. Sonber).
http://dx.doi.org/10.1016/j.ijrmhm.2017.09.013 Received 30 August 2017; Received in revised form 19 September 2017; Accepted 21 September 2017 Available online 22 September 2017 0263-4368/ © 2017 Elsevier Ltd. All rights reserved.
International Journal of Refractory Metals & Hard Materials 70 (2018) 56–65
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Table 1 Salient properties of ZrB2 [1–2] and NiAl [18–20]. Property
ZrB2
NiAl
Crystal structure Density (gm/cm3) Melting point (°C) Thermal expansion coefficient (°C− 1) Thermal conductivity (Wm− 1 K− 1) Hardness (GPa) Elastic modulus (GPa) Fracture Toughness (MPa·m1/2) Electrical resistivity (Ω·cm)
Hexagonal 6.1 3245 5.9 × 10− 6 57.9 18–22 489 3.3 10 × 10− 6
Cubic 7.16 1638 15.1 × 10− 6 70–80 4.2 193 5.4 9× 10− 6
Fig. 1. XRD pattern ZrB2 and NiAl used for preparation of novel composite.
nickel and aluminum powder at 900 °C temperature in vacuum of 1 × 10− 3 mbar using reaction (1). (1)
Ni + Al → NiAl
The reaction product was in the form of small lumps of 10 mm size which were subjected to grinding operation by vibratory cup mill in order to get fine powder. The powder was then characterized by X-ray diffraction (XRD) analysis and scanning electron microscope (SEM). Fine zirconium diboride and NiAl were wet mixed thoroughly in a planetary ball mill for 5 h using Tungsten carbide ball as grinding media. The mixed powders were dried in a vacuum oven at 150 °C, then filled in a high density graphite die (60 mm cavity) and hot pressed at a temperature of 1700 °C under a pressure of 35 MPa for 2 h in a high vacuum (1 × 10− 5 mbar) chamber. The pellets were ejected from the die after cooling and the density was measured by liquid displacement method. Hot pressed samples were polished to mirror finish using diamond powder of various grades from 15 to 0.25 μm in an auto polisher (laboforce-3, Struers). Polished samples were characterized by XRD (Rigaku Miniflex II diffractometer) using Cu Kα (λ = 1.5405 Å) for phase identification. Microstructural characterization of the polished samples was carried out by scanning electron microscopy- energy dispersive spectroscopy (SEM-EDS) and Electron back scattered diffraction (EBSD). Microhardness was measured on the polished surface at a load of 100 gf and dwell time of 10 s. The indentation fracture toughness (KIC) data were evaluated by crack length measurement of the crack pattern formed around Vickers indents (using 10 Kg load), adopting the model formulation proposed by Anstis et al. [22],
KIC = 0.016(E H)1 2P c3
2
Fig. 2. SEM image of starting powders (a) ZrB2 (b) NiAl.
samples were measured by ultrasonic method as per ASTM C 1419-99a test procedure. Flexural strength was measured by a 3-point bend test at ambient temperature as per ASTM C 1161-02C test procedure. Test specimens of composites were cut using electric discharge machining (EDM) and machined into bar shapes of 1.5 mm × 2.0 mm × 25 mm from hot pressed disk of diameter 60 mm. After 3 point bend test, fracture surface of the broken samples were analyzed by SEM. Isothermal oxidation studies of the composite samples were carried out at a temperature of 1400 °C. Samples with dimension of 10 mm × 10 mm × 5 mm were cut from hot pressed disk by EDM. All the surfaces of the cut sample were ground with emery papers (1/0, 2/ 0, 3/0, 4/0) and finally polished with diamond paste up to 1 μm finish. Oxidation tests were conducted in a resistance heated furnace in air. In order to avoid oxidation during non-isothermal heating, the sample was directly inserted into the hot zone after the furnace reached the set temperature. Samples were placed in an alumina crucible loaded inside the furnace. The samples were oxidized in air for different time intervals (2 h, 4 h and 8 h) at the set temperature of 1400 °C. The samples were carefully weighed to an accuracy of 0.01 mg before and after exposure, to determine the weight change during the oxidation process. The morphology and nature of oxide layer was established by observing the surface in an SEM equipped with EDS. The phases present on the oxidized surface were analyzed by XRD.
(2)
where, E is the Young's modulus, H the Vickers's hardness, P the applied indentation load, and c the half crack length. Young's moduli (E) of the 57
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Table 2 Density and mechanical properties of ZrB2 + NiAl composites prepared by hot pressing at 1700 °C and 35 MPa. Sample
Bulk density (gm/cm3)
Theoretical density (gm/cm3)
Relative density (%)
Hardness (GPa)
KIC (MPa·m1/2)
Flexural strength (MPa)
ZrB2 + 5%NiAl ZrB2 + 10%NiAl ZrB2 + 20%NiAl
5.848 5.845 5.951
6.15 6.21 6.31
95.1 94.1 94.3
23.6 ± 1 23.1 ± 1 15.8 ± 1
5.6 ± 0.1 6.4 ± 0.1 6.8 ± 0.1
438 ± 5 432 ± 5 421 ± 5
o
ZrB2+ 20%NiAl o
# NiAl
o
o
# o
#
Relative Intensity
temperature of 1800 to 2000 °C. Farahbakhsh et al. [7] have added nano sized carbon black powder (10%) as sintering aid to ZrB2 + 20% SiC composite and obtained a density of 99.8% by hot pressing at 1850 °C. From the above discussion, it is evident that temperature of ≥ 1800 °C is required for obtaining the good density (≥ 95%TD). In the present study, NiAl addition is found to be very effective in lowering the sintering temperature to 1700 °C.
o ZrB2
ZrB2+ 10%NiAl
oo
o
o
oo
o
o
o
o o
#
o
o o
# 30
3.3. Phase analysis and microstructure evolution XRD pattern of the dense pellets of ZrB2 + NiAl (5, 10 and 20%) composite samples are shown in Fig. 3. It indicates the presence of only ZrB2 and NiAl phase in all the three samples. It confirms that ZrB2 and NiAl are compatible with each other and do not lead to formation of any reaction product at a temperature of 1700 °C. It was observed that, intensity of NiAl peaks increases with increase of NiAl content from 5 to 20%. Fig. 4 presents the back scattered electron image of ZrB2 + 20% NiAl, that shows the presence of NiAl phase (dark phase) uniformly distributed in ZrB2 matrix (light gray phase). The EDS spectra (Fig. 1(b) & (c)) confirmed that Zr and B are present in the light gray grains whereas Ni and Al are present in the dark phase. It also shows that NiAl phase is acting as a binder and holds ZrB2 grains together. ZrB2 grains are having rounded corners (Fig. 4), which is the signature of liquid phase sintering. Rounded corners indicate solution and reprecipitation mechanism during liquid phase sintering. Fig. 5 presents elemental mapping of Zr, Ni and Al. It shows that Ni and Al are present in dark phase whereas Zr is present in bright phase. Fig. 6 presents the EBSD band contrast image of ZrB2 + NiAl composites along with analyzed results. Fig. 6(a) and (b) shows the different sized grains and Fig. 6(c) and (d) show the grain size distribution plots. Grains of sizes from 1 to 8 μm were seen in ZrB2 + 10% NiAl sample. In ZrB2 + 20% sample, grain coarsening was observed and grains of 2–10 μm were seen. Mean grain diameter increased from 4.6 to 6.9 μm on increasing the NiAl content from 10 to 20 vol%. The observed grain coarsening is due to presence of increased amount of liquid phase during sintering. During liquid phase sintering, grain coarsening occurs due to Ostwald ripening by the dissolution of the finer particles and their precipitation on the larger particles [24]. Fig. 7(a) and (b) presents different grain orientation with different colour coding in ZrB2 + 10%NiAl and ZrB2 + 20%NiAl composites respectively. Fig. 7(c) presents the inverse pole figure indicating the orientation of the grains at different crystallographic directions. It indicated that there is no preferential grain orientation in these composite samples. Fig. 8(a) and (b) present the EBSD phase map of ZrB2 + 10% NiAl and ZrB2 + 20%NiAl composites respectively. It shows the uniform distribution of NiAl phase in ZrB2 matrix.
o
ZrB2+ 5%NiAl o
20
o
40
50
o 60
oo
o 70
o 80
2θ angle (degrees) Fig. 3. XRD pattern of hot pressed ZrB2 + NiAl composites hot pressed at 1700 °C.
3. Results and discussion 3.1. Characterization of starting powders Fig. 1 presents the XRD pattern of the starting powders. It confirms that there is no impurity phase in the starting powders which is in the detection limit of XRD. Fig. 2 presents SEM images of ZrB2 and NiAl powder. It shows that ZrB2 particles are in the range of 2–5 μm. The particles of NiAl are in the range of 20–30 μm. 3.2. Densification Hot pressing conditions and properties of the ZrB2 + NiAl composites are presented in Table 2. Composite with 5 vol% NiAl was densified to 95.08% of theoretical density (TD) at a temperature of 1700 °C and a pressure of 35 MPa. Composites with 10% and 20%NiAl were also hot pressed upto ~ 94% of theoretical density at similar processing conditions. In case of monolithic ZrB2, a near full density (99.8% ρth) was obtained at a higher temperature of 1850 °C under 35 MPa [15]. At 1750 °C, ZrB2 was densified upto only 80% of theoretical density [13]. In the present study, ZrB2 ceramic was densified at a lower temperature of 1700 °C due to formation of liquid NiAl phase. NiAl has melting temperature of 1638 °C. Presence of liquid phase during sintering is well known to enhance densification [23,24]. Formation of liquid phase was confirmed by analyzing the morphology of ZrB2 and NiAl grains in microstructures (discussed in Section 3.3). It is worth to discuss and compare the densification behavior of ZrB2 with various other sinter additives, which are reported in the literature. Addition of 20% SiC resulted in near theoretical density at a temperature of 1900 °C and 32 MPa [25]. Sun et al. [26] have added 25% Nb and obtained 97.2% density at 1800 °C. Addition of 10% Mo was observed to give a density of 98.9% ρth on hot pressing at 1950 °C and 20 MPa [27]. Sciti et al. [11] have used MoSi2 as additive and obtained a density of 98.1% ρth on hot pressing at 1800 °C and 30 MPa. Zhao et al. [28] have used AlB2 as sinter additive and got full density at
3.4. Mechanical properties Vickers hardness and fracture toughness of ZrB2 composites are presented in Fig. 9 and Table 2. Hardness of ZrB2 with 5% and 10%NiAl samples was measured to be 23.6 and 23.1 GPa respectively. Increase of NiAl to 20 vol% was found to decrease the hardness to 15.8 GPa. The decrease in hardness is attributed to lower hardness of NiAl phase (4.2 GPa) [20]. In previous studies, hardness of monolithic ZrB2 has been reported 58
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J.K. Sonber et al.
Fig. 4. SEM-EDS analysis of ZrB2 + 20%NiAl (a) BSE image of ZrB2 + 20%NiAl (b) EDS pattern of ZrB2 phase (light gray phase) (c) EDS pattern of NiAl phase (dark phase).
presence of these mechanisms was confirmed by microstructural analysis of crack propagation path, which will be discussed in the fractography section (Section 3.5). Fig. 10 presents flexural strength of ZrB2 composites with varying addition of NiAl content. It was measured in the range of 421 to 438 MPa. It was found that increase in liquid phase decreased flexural strength marginally. For monolithic ZrB2, strength has been reported in the range of 355 to 565 MPa. Chamberlain et al. [25] have obtained a strength of 565 MPa for hot pressed ZrB2. For reactively hot pressed ZrB2 a value of 434 MPa was reported [30]. Farahbakhsh et al. [7] have measured the flexural strength of 90% dense monolithic ZrB2 as 355 MPa. Licheri et al. [31] have reported flexural strength of spark plasma sintered ZrB2 to be 399 MPa. Present study composite's flexural strength values are comparable with the literature data of monolithic ZrB2.
in the range of 22–23 GPa [1,2,15,25]. Fahren holtz et al. [29] have reported the hardness of hot pressed ZrB2 as 23 GPa. Sonber et al. [16] have reported the hardness of ZrB2 + 10%CrSi2 to be 19 GPa. Present reported hardness values are comparable with the literature data. Indentation Fracture toughness of ZrB2 + 5%NiAl sample was measured to be 5.6 MPa·m1/2. With increase in NiAl content to 10%, fracture toughness was increased by 14% to 6.4 MPa·m1/2 Further in. crease in NiAl content to 20% has further increased fracture toughness to 6.8 MPa·m1/2. The fracture toughness values obtained in the composite samples are higher than the values reported for monolithic ZrB2 in literature. Fracture toughness of monolithic ZrB2 has been reported as 3.5 MPa·m1/2 [15]. NiAl addition is found beneficial for improvement of the fracture toughness by ~94% compared to monolithic ZrB2. A setback of ceramic is poor fracture toughness; therefore, enhancement of fracture toughness by addition of intermetallic (NiAl) will be worth to explore these composites for aggressive environment applications. In the present study, increase in fracture toughness is due to crack arrest, crack deflection and crack bridging near NiAl grains. The
3.5. Fractography Fig. 11 presents the fracture surfaces of ZrB2 + NiAl composite. It 59
International Journal of Refractory Metals & Hard Materials 70 (2018) 56–65
(a) BSE image
(b) Zr
(c) Ni
(d) Al
Fig. 5. Elemental mapping of Zr, Ni and Al in ZrB2 + 20% NiAl composite (Hot pressed at 1700 °C).
(a) ZrB2 +10%NiAl
(b) ZrB2 + 20% NiAl
(c)
(d)
0.25 Mean grain diameter : 4.6 μm
0.30
0.20
0.25 Area fraction
Area Fraction
J.K. Sonber et al.
0.15 0.10 0.05
Fig. 6. EBSD Band contrast image of (a) ZrB2 + 10%NiAl and (b) ZrB2 + 20%NiAl. Grain size distribution of (c) ZrB2 + 10%NiAl and (d) ZrB2 + 20%NiAl.
Mean grain diameter : 6.9 μm
0.20 0.15 0.10 0.05
0.00
0.00 0
2
4 Grain size (μm)
6
8
0
2
4
6
8
10
Grain size (μm)
sample. It confirmed the presence of crack bridging and crack arrest at the NiAl grains, which explains high fracture toughness. It showed that cracks did not propagate through NiAl grains, which explains that NiAl addition is extremely effective in increasing the resistance to crack
shows the fully dense microstructures. The mode of fracture is seen to be transgranular, which confirms that there are no weak phases on the grain boundaries and the grains are bonded very strongly. Fig. 12 presents the crack propagation path in ZrB2 + 20%NiAl 60
International Journal of Refractory Metals & Hard Materials 70 (2018) 56–65
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Fig. 8. EBSD phase maps of (a) ZrB2 + 10%NiAl and (b) ZrB2 + 20%NiAl.
8
Hardness (GPa)
24
propagation. It also shows that crack propagates across the ZrB2 grains and hence the mode of fracture is transgranular. The cracks were originated at the edge of indentation, which were made for measuring indentation fracture toughness.
7
1/2
Fig. 7. EBSD microstructure of (a) ZrB2 + 10%NiAl and (b) ZrB2 + 20%NiAl (c) inverse pole figure indicating the orientation of the grains at different crystallographic directions.
20 6 16
12
5
3.6. Oxidation study 8
The weight gain data obtained during oxidation at 1400 °C as a function of time for ZrB2 + NiAl samples are presented in Fig. 13. All the three samples have shown continuous weight gain with time. The decrease in rate of oxidation is due to the formation of protective layer. In a previous study [15], oxidation rate was found to be constant at 900 °C in case of monolithic ZrB2. Fig. 14(a) and (b) presents the SEM microstructures of the oxidized surface of ZrB2 + 5%NiAl and ZrB2 + 20%NiAl respectively. It shows the presence of a protective layer. Oxide layer in the ZrB2 + 5%NiAl sample has granular structure.
4 5
10
15
20
NiAl content (Vol. %) Fig. 9. Hardness and fracture toughness of hot pressed ZrB2 + NiAl composites.
61
Fracture toughness, KIC (MPa.m )
28
International Journal of Refractory Metals & Hard Materials 70 (2018) 56–65
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layer formed partially protects the inner ZrB2. SiC addition to ZrB2 has been found to improve its oxidation resistance. The oxide layer in case of ZrB2 + SiC composite consist of ZrO2 and borosilicate glass. The protective oxide layer effectively limits the inward diffusion of oxygen toward the reaction interface [32]. Silvestroni et al. [33]. have reported that TaSi2 reinforced ZrB2 has good oxidation resistance upto 1650 °C due to formation of SiO2 based glassy layer. Lavrenko et al. [34]have reported that ZrB2 + MoSi2 composite has exceptionally high oxidation resistance upto 1600 °C. Sonber et al. [15,16] have reported that addition of TiSi2 or CrSi2 improves the oxidation resistance of ZrB2 by formation of SiO2 rich glassy layer. Ouyang et al. [35] have reported that ZrB2 + 30%SiC + 10%AlN composite has very good oxidation resistance at 1700 °C. Guo et al. [36] have observed that addition of Si improves the oxidation resistance of ZrB2 at 1500 °C whereas Zr addition decreases the oxidation resistance. In a nutshell, the addition of NiAl has helped to enhance the densification by assisting the liquid phase sintering without much deteriorating the mechanical properties (hardness and flexural strength). Presence of NiAl in the composite has helped to enhance the fracture toughness by ~94% due to crack bridging, crack arrest and crack deflection mechanisms. Moreover, the addition of NiAl also enhanced the oxidation resistance of composite by formation of protective oxide layers consisting of ZrO2 and NiAl2O4.
Flexural strength (MPa)
460
440
420
400
380 5
10
15
20
NiAl content (Vol.%) Fig. 10. Flexural strength of hot pressed ZrB2 + NiAl Composites.
The EDS analysis revealed the presence of only Zirconium and oxygen indicating the presence of ZrO2 layer. The oxide layer in the ZrB2 + 20%NiAl sample was analyzed to contain Al, Ni, O and Zr. Fig. 15 presents the XRD pattern of all the three composites after oxidation at 1400 °C. It revealed that ZrO2 is major oxidation product in ZrB2 + 5%NiAl. In composites with higher amount of NiAl phase, NiAl2O4 was observed as major oxidation product in addition to ZrO2. Following reactions are possible during the oxidation process.
2 5ZrB2 + O2 → 2 5ZrO2 + 2 5B2 O3
(4)
2NiAl + 5 2O2 → 2NiO + Al2O3
(5)
NiO + Al2O3 → NiAl2O4
(6)
4. Conclusions Salient conclusions of the study are summarized below: 1. Nickel Aluminide (NiAl) is found to be an effective sintering aid for densification of ZrB2. Addition of 5–20%NiAl assisted to bring down the hot pressing temperature to 1700 °C from 1850 °C for monolithic ZrB2. 2. NiAl melted during hot pressing and resulted in increased densification due to liquid phase sintering. Microstructures revealed that
Monteverde et al. [32] have carried out oxidation study at 1350 °C and found that monolithic ZrB2 has poor oxidation resistance. The oxide
Fig. 11. Fracture surface of ZrB2 + NiAl composite (a) ZrB2 + 5%NiAl (b) ZrB2 + 10%NiAl (c) ZrB2 + 20%NiAl.
62
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Fig. 12. Crack propagation path in ZrB2 + 20%NiAl composite sample.
3. 4. 5. 6.
7.
layer consisting of ZrO2 and NiAl2O4.
ZrB2 and NiAl are compatible with each other and does not lead to the formation of any reaction product. NiAl phase was found to be uniformly distributed throughout the microstructure. Little grain coarsening was observed by increasing the liquid phase content in the composite from 10 to 20%. Flexural strength of the composite was measured in the range of 421 to 438 MPa. Addition of NiAl was also found to increase the fracture toughness to 6.8 MPa·m1/2 (~ 94% enhanced compared with monolithic ZrB2). Crack bridging and crack arrest were observed at NiAl phase explaining the high fracture toughness The developed composite were found to have excellent oxidation resistance at 1400 °C in air due to formation of protective oxide
Suggestions for further work In future study, precise mechanism of liquid phase sintering in this system can be studied by the use of sintering kinetics studies [37].
Acknowledgement Author's wish to thank Mr. P.V. Durgaprasad, RSD, BARC for his help in measuring flexural strength for this study.
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International Journal of Refractory Metals & Hard Materials 70 (2018) 56–65
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o ZrO2
ZrB2 + 5% NiAl
0.8
# NiAl2O4 #
0.6
Relative Intensity
o 0.4
0.2
o#
4
6
o
#
o
o o
oo
o #
o oo
o
o
ZrB2+10%NiAl
o
# oo
o
o
o
8
Time (hour)
20
o o o o
30
40
ZrB2+5%NiAl
o
o
o
Fig. 13. Specific weight gain with time during oxidation of ZrB2 + NiAl composites at 1400 °C.
#
# o
# o 2
# oo #
0.0 0
ZrB2+20%NiAl
# o
ZrB2 + 20% NiAl
2
Specific Weight Gain (Kg / m )
ZrB2 + 10% NiAl
o
#o 50
o o o# 60
o 70
o 80
2θ angle (degrees) Fig. 15. XRD pattern of ZrB2 + NiAl samples oxidized at 1400 °C.
References [1] W.G. Fahrenholtz, G.E. Hilmas, Refractory diborides of zirconium and hafnium, J. Am. Ceram. Soc. 90 (5) (2007) 1347–1364. [2] M.L. Bauccio, ASM Engineered Materials Reference book, ASM International, United States of America, 1994. [3] C. Mroz, Annual Mineral review: zirconium diboride, Am. Ceram. Soc. Bull. 74 (1995) 164–165. [4] H.M. Zaw, J.Y.H. Fuh, A.Y.C. Nee, L. Lu, Formation of a new EDM electrode material using sintering techniques, J. Mater. Process. Technol. 89–90 (1999) 182–186. [5] A.K. Khanra, B.R. Sarkar, B. Bhattacharya, L.C. Pathak, M.M. Godkhindi, Performance of ZrB2-Cu composite as an EDM electrode, J. Mater. Process. Technol. 83 (2007) 122–126. [6] J.K. Sonber, A.K. Suri, Synthesis and densification of ZrB2: review, Adv. Appl. Ceram. 110 (6) (2011) 321–334. [7] I. Farahbakhsh, Z. Ahmadi, M.S. Asl, Densification, microstructure and mechanical properties of hot pressed ZrB2–SiC ceramic doped with nano-sized carbon black, Ceram. Int. 43 (2017) 8411–8417. [8] E.W. Neuman, G.E. Hilmas, W.G. Fahrenholtz, Processing, microstructure, and mechanical properties of zirconium diboride-boron carbide ceramics, Ceram. Int. 43 (9) (2017) 6942–6948. [9] S. Zhou, Z. Wang, X. Sun, J. Han, Microstructure, mechanical properties and thermal shock resistance of zirconium diboride containing silicon carbide ceramic toughened by carbon black, Mater. Chem. Phys. 122 (2–3) (2010) 470–473. [10] S.K. Mishra, S.K. Das, Sintering and microstructural behaviour of SHS produced zirconium diboride powder with the addition of C and TiC, Mater. Lett. 59 (27) (2005) 3467–3470. [11] D. Sciti, F. Monteverdea, S. Guicciardia, G. Pezzottib, A. Bellosia, Microstructure and mechanical properties of ZrB2–MoSi2 ceramic composites produced by different sintering techniques, Mater. Sci. Eng. A 434 (1–2) (2006) 303–309. [12] D. Sciti, L. Silvestroni, G. Celotti, C. Melandri, S. Guicciardi, Sintering and mechanical properties of ZrB2–TaSi2 and HfB2–TaSi2 ceramic composites, J. Am. Ceram. Soc. 91 (10) (2008) 3285–3291. [13] J.K. Sonber, T.S.R.Ch. Murthy, C. Subramanian, R.C. Hubli, R.K. Fotedar, A.K. Suri, Effect of WSi2 addition on densification and properties of ZrB2, Adv. Appl. Ceram. 113 (2) (2014) 114–119. [14] S. Guo, T. Nishimura, Y. Kagawa, Low-temperature hot pressing of ZrB2-based ceramics with ZrSi2 additives, Int. J. Appl. Ceram. Technol. 8 (6) (2011) 1425–1435. [15] J.K. Sonber, T.S.R.Ch. Murthy, C. Subramanian, S. Kumar, R.K. Fotedar, A.K. Suri, Investigations on synthesis of ZrB2 and development of new composites with HfB2 and TiSi2, Int. J. Refract. Met. Hard Mater. 29 (2011) 21–30. [16] J.K. Sonber, T.S.R.Ch. Murthy, C. Subramanian, N. Krishnamurthy, R.C. Hubli, A.K. Suri, Effect of CrSi2 and HfB2 addition on densification and properties of ZrB2, Int. J. Refract. Met. Hard Mater. 31 (2012) 125–131. [17] R.D. Noebe, R.R. Bowman, M.V. Nathal, Physical and mechanical properties of the B2 compound NiAl, Int. Mater. Rev. 38 (1993) 193–232. [18] R. Darolia, NiAl alloys for high-temperature structural applications, JOM 43 (3) (1991) 44–49. [19] D.B. Miracle, Overview no. 104 the physical and mechanical properties of NiAl, Acta Metall. Mater. 41 (3) (1993) 649–684. [20] G.K. Dey, Physical metallurgy of nickel aluminides, Sadhana 28 (1 & 2) (2003) 247–262. [21] A. Michalski, J. Jaroszewicz, M. Rosinski, D. Siemiaszko, NiAl–Al2O3 composites
Fig. 14. SEM microstructure of ZrB2 + NiAl composite after oxidation at 1400 °C for 8 h (a) ZrB2 + 5%NiAl (b) ZrB2 + 20%NiAl.
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[22]
[23] [24] [25] [26] [27]
[28] [29]
[30]
zirconium diboride, J. Eur. Ceram. Soc. 29 (2009) 3401–3408. [31] R. Licheri, C. Musa, R. Orrù, G. Cao, D. Sciti, L. Silvestroni, Bulk monolithic zirconium and tantalum diborides by reactive and non-reactive spark plasma sintering, J. Alloys Compd. 663 (2016) 351–359. [32] F. Monteverde, A. Bellosi, Oxidation of ZrB2-based ceramics in dry air, J. Electrochem. Soc. 150 (11) (2003) B552–B559. [33] L. Silvestroni, H.J. Kleebe, Critical oxidation behavior of Ta-containing ZrB2 composites in the 1500–1650 °C temperature range, J. Eur. Ceram. Soc. 37 (5) (2017) 1899–1908. [34] V.O. Lavrenko, A.D. Panasyuk, O.M. Grigorev, O.V. Koroteev, V.A. Kotenko, Hightemperature (to 1600 °C) oxidation of ZrB2–MoSi2 ceramics in air, Powder Metall. Met. Ceram. 51 (1–2) (2012) 102–107. [35] G. Ouyang, P.K. Ray, M.J. Kramer, M. Akin, High-temperature oxidation of ZrB2–SiC–AlN composites at 1600 °C, J. Am. Ceram. Soc. 99 (3) (2016) 808–813. [36] W.M. Guo, X.J. Zhou, G.J. Zhang, Y.M. Kan, Y.G. Li, P.L. Wang, Effect of Si and Zr additions on oxidation resistance of hot-pressed ZrB2–SiC composites with polycarbosilane as a precursor at 1500 °C, J. Alloys Compd. 471 (1–2) (2009) 153–156. [37] J.D. Bolton, A.J. Gant, Microstructural development and sintering kinetics in ceramic reinforced high speed steel metal matrix composites, Powder Metall. 40 (1997) 143–151.
produced by pulse plasma sintering with the participation of the SHS reaction, Intermetallics 14 (2006) 603–606. G.R. Anstis, P. Chantikul, B.R. Lawn, D.B. Marshall, A critical evaluation of indentation techniques for measuring fracture toughness: I, direct crack measurements, J. Am. Ceram. Soc. 64 (1981) 533–538. W.D. Kingery, H.K. Bowen, D.R. Uhlmann, Introduction to Ceramics, Wiley, United States of America, 1976. M.W. Barsoum, Fundamentals of Ceramics, CRC Press, 2002. A.L. Chamberlain, W.G. Fahrenholtz, G.E. Hilmas, High strength zirconium diboride based ceramics, J. Am. Ceram. Soc. 87 (6) (2004) 1170–1172. X. Sun, W. Han, P. Hu, Z. Wang, X. Zhang, Microstructure and mechanical properties of ZrB2–Nb composite, Int. J. Refract. Met. Hard Mater. 28 (2010) 472–474. H. Wang, D. Chen, C.N. Wang, R. Zhang, D. Fang, Preparation and characterization of high-toughness ZrB2/Mo composites by hot pressing process, Int. J. Refract. Met. Hard Mater. 27 (2009) 1024–1026. J. Zhao, H.T. Liu, J.X. Liu, G.J. Zhang, ZrB2 ceramics doped with AlB2, Ceram. Int. 40 (6) (2014) 8915–8920. W.G. Fahrenholtz, G.E. Hilmas, A.L. Chamberlain, J.W. Zimmerman, Processing and characterization of ZrB2-based ultra-high temperature monolithic and fibrous monolithic ceramics, J. Mater. Sci. 39 (2004) 5951–5957. A.L. Chamberlain, W.G. Fahrenholtz, G.E. Hilmas, Reactive hot pressing of
65
Contents lists available at ScienceDirect
International Journal of Refractory Metals & Hard Materials journal homepage: www.elsevier.com/locate/IJRMHM
ZrB2 based novel composite with NiAl as reinforcement phase a,⁎
a,b
J.K. Sonber , T.S.R.Ch. Murthy a b c
, K. Sairam
a,b
c
, A. Nagaraj , S. Majumdar
a,b
, V. Kain
MARK a,b
Materials Processing and Corrosion Engineering Division, BARC, Mumbai, India Homi Bhabha National Institute, Mumbai, India Laser and Plasma Technology Division, BARC, Mumbai, India
A R T I C L E I N F O
A B S T R A C T
Keywords: ZrB2 NiAl Composite Hot pressing Mechanical properties Microstructure
ZrB2 based novel composites have been prepared using NiAl as reinforcement phase. Three samples of compositions (a) ZrB2 + 5%NiAl (a) ZrB2 + 10%NiAl (a) ZrB2 + 20%NiAl were prepared by hot pressing at 1700 °C. All the three samples were densified to a density of > 94% of its theoretical value. Addition of NiAl resulted in liquid phase sintering and assisted in densification. The composite was characterized by mechanical property measurement and microstructure analysis. Flexural strength of the composite was found to be in the range of 421 to 438 MPa. Hardness of composite was found to decrease with addition of NiAl. Indentation fracture toughness was found to increase with increasing NiAl content. Microstructural characterization revealed the uniform distribution of NiAl phase in ZrB2 matrix. Grain coarsening was observed in composite with higher NiAl content. Fracture surface analysis revealed that the mode of fracture is transgranular. Microstructure analysis of crack propagation revealed the presence of crack bridging and crack arrest near NiAl phase, which resulted in higher fracture toughness of 6.8 MPa·m1/2. Isothermal oxidation study at 1400 °C revealed that the developed composite has good oxidation resistance.
1. Introduction ZrB2 is a potential material for high temperature applications due to its extremely high melting point, high thermal conductivity, low coefficient of thermal expansion, high hardness, good retention of strength at high temperature and chemical stability [1–2]. It is a candidate material for leading edge applications for space re-entry vehicles [1]. It is used for holding molten metal and as thermo-well tubes in metal processing due to its compatibility with liquid metals [3]. It also finds application as electrode material in Hall-Heroult cell and electric discharge machining due to its good electrical conductivity [4–5]. The major issue in ZrB2 based material is its poor sinterability and poor fracture toughness. Due to strong co-valent bonds and low selfdiffusivity, ZrB2 can be fully densified only at very high sintering temperatures of the order of ~2000 °C and also requires external pressure. Moreover, ZrB2 powder is always coated with zirconium and boron oxides which promotes evaporation and condensation mechanism, that results in mass transfer without densification [1,6]. Several additives have been used to improve densification. Addition of carbon and carbides such as TiC, SiC and B4C assisted in densification by reducing the oxide layer present on the surface of ZrB2 particles [7–10]. Transition metal silicides such as MoSi2, TaSi2, WSi2, TiSi2, CrSi2, ZrSi2 etc. have been reported to improve densification of ZrB2
⁎
[11–16]. In this study, possibility of using aluminide compounds as sintering aid and reinforcement to boride ceramics has been investigated. Though, aluminide compounds are well known for their excellent high temperature properties, there is no literature on the use of these compounds as reinforcement for ZrB2. For the present study, NiAl was chosen as sinter additive and reinforcement. It is expected to assist in densification by formation of liquid phase, during sintering. NiAl is an excellent material for high temperature applications as it has high melting point, good thermal conductivity and excellent oxidation resistance [17]. It is considered a potential material for turbine blade applications [18]. Table 1 shows some of the properties of ZrB2 [2] and NiAl [19–21]. In present study, investigation was carried out to explore the use of NiAl as reinforcement for ZrB2.. Microstructure evolution, mechanical properties and oxidation behavior of ZrB2 + NiAl composites were also studied. 2. Experimental In-house prepared ZrB2 and NiAl powders were used as starting materials. The procedure followed for the preparation of ZrB2 powder is already reported elsewhere [15]. NiAl was prepared by reaction of
Corresponding author. E-mail address: [email protected] (J.K. Sonber).
http://dx.doi.org/10.1016/j.ijrmhm.2017.09.013 Received 30 August 2017; Received in revised form 19 September 2017; Accepted 21 September 2017 Available online 22 September 2017 0263-4368/ © 2017 Elsevier Ltd. All rights reserved.
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Table 1 Salient properties of ZrB2 [1–2] and NiAl [18–20]. Property
ZrB2
NiAl
Crystal structure Density (gm/cm3) Melting point (°C) Thermal expansion coefficient (°C− 1) Thermal conductivity (Wm− 1 K− 1) Hardness (GPa) Elastic modulus (GPa) Fracture Toughness (MPa·m1/2) Electrical resistivity (Ω·cm)
Hexagonal 6.1 3245 5.9 × 10− 6 57.9 18–22 489 3.3 10 × 10− 6
Cubic 7.16 1638 15.1 × 10− 6 70–80 4.2 193 5.4 9× 10− 6
Fig. 1. XRD pattern ZrB2 and NiAl used for preparation of novel composite.
nickel and aluminum powder at 900 °C temperature in vacuum of 1 × 10− 3 mbar using reaction (1). (1)
Ni + Al → NiAl
The reaction product was in the form of small lumps of 10 mm size which were subjected to grinding operation by vibratory cup mill in order to get fine powder. The powder was then characterized by X-ray diffraction (XRD) analysis and scanning electron microscope (SEM). Fine zirconium diboride and NiAl were wet mixed thoroughly in a planetary ball mill for 5 h using Tungsten carbide ball as grinding media. The mixed powders were dried in a vacuum oven at 150 °C, then filled in a high density graphite die (60 mm cavity) and hot pressed at a temperature of 1700 °C under a pressure of 35 MPa for 2 h in a high vacuum (1 × 10− 5 mbar) chamber. The pellets were ejected from the die after cooling and the density was measured by liquid displacement method. Hot pressed samples were polished to mirror finish using diamond powder of various grades from 15 to 0.25 μm in an auto polisher (laboforce-3, Struers). Polished samples were characterized by XRD (Rigaku Miniflex II diffractometer) using Cu Kα (λ = 1.5405 Å) for phase identification. Microstructural characterization of the polished samples was carried out by scanning electron microscopy- energy dispersive spectroscopy (SEM-EDS) and Electron back scattered diffraction (EBSD). Microhardness was measured on the polished surface at a load of 100 gf and dwell time of 10 s. The indentation fracture toughness (KIC) data were evaluated by crack length measurement of the crack pattern formed around Vickers indents (using 10 Kg load), adopting the model formulation proposed by Anstis et al. [22],
KIC = 0.016(E H)1 2P c3
2
Fig. 2. SEM image of starting powders (a) ZrB2 (b) NiAl.
samples were measured by ultrasonic method as per ASTM C 1419-99a test procedure. Flexural strength was measured by a 3-point bend test at ambient temperature as per ASTM C 1161-02C test procedure. Test specimens of composites were cut using electric discharge machining (EDM) and machined into bar shapes of 1.5 mm × 2.0 mm × 25 mm from hot pressed disk of diameter 60 mm. After 3 point bend test, fracture surface of the broken samples were analyzed by SEM. Isothermal oxidation studies of the composite samples were carried out at a temperature of 1400 °C. Samples with dimension of 10 mm × 10 mm × 5 mm were cut from hot pressed disk by EDM. All the surfaces of the cut sample were ground with emery papers (1/0, 2/ 0, 3/0, 4/0) and finally polished with diamond paste up to 1 μm finish. Oxidation tests were conducted in a resistance heated furnace in air. In order to avoid oxidation during non-isothermal heating, the sample was directly inserted into the hot zone after the furnace reached the set temperature. Samples were placed in an alumina crucible loaded inside the furnace. The samples were oxidized in air for different time intervals (2 h, 4 h and 8 h) at the set temperature of 1400 °C. The samples were carefully weighed to an accuracy of 0.01 mg before and after exposure, to determine the weight change during the oxidation process. The morphology and nature of oxide layer was established by observing the surface in an SEM equipped with EDS. The phases present on the oxidized surface were analyzed by XRD.
(2)
where, E is the Young's modulus, H the Vickers's hardness, P the applied indentation load, and c the half crack length. Young's moduli (E) of the 57
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Table 2 Density and mechanical properties of ZrB2 + NiAl composites prepared by hot pressing at 1700 °C and 35 MPa. Sample
Bulk density (gm/cm3)
Theoretical density (gm/cm3)
Relative density (%)
Hardness (GPa)
KIC (MPa·m1/2)
Flexural strength (MPa)
ZrB2 + 5%NiAl ZrB2 + 10%NiAl ZrB2 + 20%NiAl
5.848 5.845 5.951
6.15 6.21 6.31
95.1 94.1 94.3
23.6 ± 1 23.1 ± 1 15.8 ± 1
5.6 ± 0.1 6.4 ± 0.1 6.8 ± 0.1
438 ± 5 432 ± 5 421 ± 5
o
ZrB2+ 20%NiAl o
# NiAl
o
o
# o
#
Relative Intensity
temperature of 1800 to 2000 °C. Farahbakhsh et al. [7] have added nano sized carbon black powder (10%) as sintering aid to ZrB2 + 20% SiC composite and obtained a density of 99.8% by hot pressing at 1850 °C. From the above discussion, it is evident that temperature of ≥ 1800 °C is required for obtaining the good density (≥ 95%TD). In the present study, NiAl addition is found to be very effective in lowering the sintering temperature to 1700 °C.
o ZrB2
ZrB2+ 10%NiAl
oo
o
o
oo
o
o
o
o o
#
o
o o
# 30
3.3. Phase analysis and microstructure evolution XRD pattern of the dense pellets of ZrB2 + NiAl (5, 10 and 20%) composite samples are shown in Fig. 3. It indicates the presence of only ZrB2 and NiAl phase in all the three samples. It confirms that ZrB2 and NiAl are compatible with each other and do not lead to formation of any reaction product at a temperature of 1700 °C. It was observed that, intensity of NiAl peaks increases with increase of NiAl content from 5 to 20%. Fig. 4 presents the back scattered electron image of ZrB2 + 20% NiAl, that shows the presence of NiAl phase (dark phase) uniformly distributed in ZrB2 matrix (light gray phase). The EDS spectra (Fig. 1(b) & (c)) confirmed that Zr and B are present in the light gray grains whereas Ni and Al are present in the dark phase. It also shows that NiAl phase is acting as a binder and holds ZrB2 grains together. ZrB2 grains are having rounded corners (Fig. 4), which is the signature of liquid phase sintering. Rounded corners indicate solution and reprecipitation mechanism during liquid phase sintering. Fig. 5 presents elemental mapping of Zr, Ni and Al. It shows that Ni and Al are present in dark phase whereas Zr is present in bright phase. Fig. 6 presents the EBSD band contrast image of ZrB2 + NiAl composites along with analyzed results. Fig. 6(a) and (b) shows the different sized grains and Fig. 6(c) and (d) show the grain size distribution plots. Grains of sizes from 1 to 8 μm were seen in ZrB2 + 10% NiAl sample. In ZrB2 + 20% sample, grain coarsening was observed and grains of 2–10 μm were seen. Mean grain diameter increased from 4.6 to 6.9 μm on increasing the NiAl content from 10 to 20 vol%. The observed grain coarsening is due to presence of increased amount of liquid phase during sintering. During liquid phase sintering, grain coarsening occurs due to Ostwald ripening by the dissolution of the finer particles and their precipitation on the larger particles [24]. Fig. 7(a) and (b) presents different grain orientation with different colour coding in ZrB2 + 10%NiAl and ZrB2 + 20%NiAl composites respectively. Fig. 7(c) presents the inverse pole figure indicating the orientation of the grains at different crystallographic directions. It indicated that there is no preferential grain orientation in these composite samples. Fig. 8(a) and (b) present the EBSD phase map of ZrB2 + 10% NiAl and ZrB2 + 20%NiAl composites respectively. It shows the uniform distribution of NiAl phase in ZrB2 matrix.
o
ZrB2+ 5%NiAl o
20
o
40
50
o 60
oo
o 70
o 80
2θ angle (degrees) Fig. 3. XRD pattern of hot pressed ZrB2 + NiAl composites hot pressed at 1700 °C.
3. Results and discussion 3.1. Characterization of starting powders Fig. 1 presents the XRD pattern of the starting powders. It confirms that there is no impurity phase in the starting powders which is in the detection limit of XRD. Fig. 2 presents SEM images of ZrB2 and NiAl powder. It shows that ZrB2 particles are in the range of 2–5 μm. The particles of NiAl are in the range of 20–30 μm. 3.2. Densification Hot pressing conditions and properties of the ZrB2 + NiAl composites are presented in Table 2. Composite with 5 vol% NiAl was densified to 95.08% of theoretical density (TD) at a temperature of 1700 °C and a pressure of 35 MPa. Composites with 10% and 20%NiAl were also hot pressed upto ~ 94% of theoretical density at similar processing conditions. In case of monolithic ZrB2, a near full density (99.8% ρth) was obtained at a higher temperature of 1850 °C under 35 MPa [15]. At 1750 °C, ZrB2 was densified upto only 80% of theoretical density [13]. In the present study, ZrB2 ceramic was densified at a lower temperature of 1700 °C due to formation of liquid NiAl phase. NiAl has melting temperature of 1638 °C. Presence of liquid phase during sintering is well known to enhance densification [23,24]. Formation of liquid phase was confirmed by analyzing the morphology of ZrB2 and NiAl grains in microstructures (discussed in Section 3.3). It is worth to discuss and compare the densification behavior of ZrB2 with various other sinter additives, which are reported in the literature. Addition of 20% SiC resulted in near theoretical density at a temperature of 1900 °C and 32 MPa [25]. Sun et al. [26] have added 25% Nb and obtained 97.2% density at 1800 °C. Addition of 10% Mo was observed to give a density of 98.9% ρth on hot pressing at 1950 °C and 20 MPa [27]. Sciti et al. [11] have used MoSi2 as additive and obtained a density of 98.1% ρth on hot pressing at 1800 °C and 30 MPa. Zhao et al. [28] have used AlB2 as sinter additive and got full density at
3.4. Mechanical properties Vickers hardness and fracture toughness of ZrB2 composites are presented in Fig. 9 and Table 2. Hardness of ZrB2 with 5% and 10%NiAl samples was measured to be 23.6 and 23.1 GPa respectively. Increase of NiAl to 20 vol% was found to decrease the hardness to 15.8 GPa. The decrease in hardness is attributed to lower hardness of NiAl phase (4.2 GPa) [20]. In previous studies, hardness of monolithic ZrB2 has been reported 58
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Fig. 4. SEM-EDS analysis of ZrB2 + 20%NiAl (a) BSE image of ZrB2 + 20%NiAl (b) EDS pattern of ZrB2 phase (light gray phase) (c) EDS pattern of NiAl phase (dark phase).
presence of these mechanisms was confirmed by microstructural analysis of crack propagation path, which will be discussed in the fractography section (Section 3.5). Fig. 10 presents flexural strength of ZrB2 composites with varying addition of NiAl content. It was measured in the range of 421 to 438 MPa. It was found that increase in liquid phase decreased flexural strength marginally. For monolithic ZrB2, strength has been reported in the range of 355 to 565 MPa. Chamberlain et al. [25] have obtained a strength of 565 MPa for hot pressed ZrB2. For reactively hot pressed ZrB2 a value of 434 MPa was reported [30]. Farahbakhsh et al. [7] have measured the flexural strength of 90% dense monolithic ZrB2 as 355 MPa. Licheri et al. [31] have reported flexural strength of spark plasma sintered ZrB2 to be 399 MPa. Present study composite's flexural strength values are comparable with the literature data of monolithic ZrB2.
in the range of 22–23 GPa [1,2,15,25]. Fahren holtz et al. [29] have reported the hardness of hot pressed ZrB2 as 23 GPa. Sonber et al. [16] have reported the hardness of ZrB2 + 10%CrSi2 to be 19 GPa. Present reported hardness values are comparable with the literature data. Indentation Fracture toughness of ZrB2 + 5%NiAl sample was measured to be 5.6 MPa·m1/2. With increase in NiAl content to 10%, fracture toughness was increased by 14% to 6.4 MPa·m1/2 Further in. crease in NiAl content to 20% has further increased fracture toughness to 6.8 MPa·m1/2. The fracture toughness values obtained in the composite samples are higher than the values reported for monolithic ZrB2 in literature. Fracture toughness of monolithic ZrB2 has been reported as 3.5 MPa·m1/2 [15]. NiAl addition is found beneficial for improvement of the fracture toughness by ~94% compared to monolithic ZrB2. A setback of ceramic is poor fracture toughness; therefore, enhancement of fracture toughness by addition of intermetallic (NiAl) will be worth to explore these composites for aggressive environment applications. In the present study, increase in fracture toughness is due to crack arrest, crack deflection and crack bridging near NiAl grains. The
3.5. Fractography Fig. 11 presents the fracture surfaces of ZrB2 + NiAl composite. It 59
International Journal of Refractory Metals & Hard Materials 70 (2018) 56–65
(a) BSE image
(b) Zr
(c) Ni
(d) Al
Fig. 5. Elemental mapping of Zr, Ni and Al in ZrB2 + 20% NiAl composite (Hot pressed at 1700 °C).
(a) ZrB2 +10%NiAl
(b) ZrB2 + 20% NiAl
(c)
(d)
0.25 Mean grain diameter : 4.6 μm
0.30
0.20
0.25 Area fraction
Area Fraction
J.K. Sonber et al.
0.15 0.10 0.05
Fig. 6. EBSD Band contrast image of (a) ZrB2 + 10%NiAl and (b) ZrB2 + 20%NiAl. Grain size distribution of (c) ZrB2 + 10%NiAl and (d) ZrB2 + 20%NiAl.
Mean grain diameter : 6.9 μm
0.20 0.15 0.10 0.05
0.00
0.00 0
2
4 Grain size (μm)
6
8
0
2
4
6
8
10
Grain size (μm)
sample. It confirmed the presence of crack bridging and crack arrest at the NiAl grains, which explains high fracture toughness. It showed that cracks did not propagate through NiAl grains, which explains that NiAl addition is extremely effective in increasing the resistance to crack
shows the fully dense microstructures. The mode of fracture is seen to be transgranular, which confirms that there are no weak phases on the grain boundaries and the grains are bonded very strongly. Fig. 12 presents the crack propagation path in ZrB2 + 20%NiAl 60
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Fig. 8. EBSD phase maps of (a) ZrB2 + 10%NiAl and (b) ZrB2 + 20%NiAl.
8
Hardness (GPa)
24
propagation. It also shows that crack propagates across the ZrB2 grains and hence the mode of fracture is transgranular. The cracks were originated at the edge of indentation, which were made for measuring indentation fracture toughness.
7
1/2
Fig. 7. EBSD microstructure of (a) ZrB2 + 10%NiAl and (b) ZrB2 + 20%NiAl (c) inverse pole figure indicating the orientation of the grains at different crystallographic directions.
20 6 16
12
5
3.6. Oxidation study 8
The weight gain data obtained during oxidation at 1400 °C as a function of time for ZrB2 + NiAl samples are presented in Fig. 13. All the three samples have shown continuous weight gain with time. The decrease in rate of oxidation is due to the formation of protective layer. In a previous study [15], oxidation rate was found to be constant at 900 °C in case of monolithic ZrB2. Fig. 14(a) and (b) presents the SEM microstructures of the oxidized surface of ZrB2 + 5%NiAl and ZrB2 + 20%NiAl respectively. It shows the presence of a protective layer. Oxide layer in the ZrB2 + 5%NiAl sample has granular structure.
4 5
10
15
20
NiAl content (Vol. %) Fig. 9. Hardness and fracture toughness of hot pressed ZrB2 + NiAl composites.
61
Fracture toughness, KIC (MPa.m )
28
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layer formed partially protects the inner ZrB2. SiC addition to ZrB2 has been found to improve its oxidation resistance. The oxide layer in case of ZrB2 + SiC composite consist of ZrO2 and borosilicate glass. The protective oxide layer effectively limits the inward diffusion of oxygen toward the reaction interface [32]. Silvestroni et al. [33]. have reported that TaSi2 reinforced ZrB2 has good oxidation resistance upto 1650 °C due to formation of SiO2 based glassy layer. Lavrenko et al. [34]have reported that ZrB2 + MoSi2 composite has exceptionally high oxidation resistance upto 1600 °C. Sonber et al. [15,16] have reported that addition of TiSi2 or CrSi2 improves the oxidation resistance of ZrB2 by formation of SiO2 rich glassy layer. Ouyang et al. [35] have reported that ZrB2 + 30%SiC + 10%AlN composite has very good oxidation resistance at 1700 °C. Guo et al. [36] have observed that addition of Si improves the oxidation resistance of ZrB2 at 1500 °C whereas Zr addition decreases the oxidation resistance. In a nutshell, the addition of NiAl has helped to enhance the densification by assisting the liquid phase sintering without much deteriorating the mechanical properties (hardness and flexural strength). Presence of NiAl in the composite has helped to enhance the fracture toughness by ~94% due to crack bridging, crack arrest and crack deflection mechanisms. Moreover, the addition of NiAl also enhanced the oxidation resistance of composite by formation of protective oxide layers consisting of ZrO2 and NiAl2O4.
Flexural strength (MPa)
460
440
420
400
380 5
10
15
20
NiAl content (Vol.%) Fig. 10. Flexural strength of hot pressed ZrB2 + NiAl Composites.
The EDS analysis revealed the presence of only Zirconium and oxygen indicating the presence of ZrO2 layer. The oxide layer in the ZrB2 + 20%NiAl sample was analyzed to contain Al, Ni, O and Zr. Fig. 15 presents the XRD pattern of all the three composites after oxidation at 1400 °C. It revealed that ZrO2 is major oxidation product in ZrB2 + 5%NiAl. In composites with higher amount of NiAl phase, NiAl2O4 was observed as major oxidation product in addition to ZrO2. Following reactions are possible during the oxidation process.
2 5ZrB2 + O2 → 2 5ZrO2 + 2 5B2 O3
(4)
2NiAl + 5 2O2 → 2NiO + Al2O3
(5)
NiO + Al2O3 → NiAl2O4
(6)
4. Conclusions Salient conclusions of the study are summarized below: 1. Nickel Aluminide (NiAl) is found to be an effective sintering aid for densification of ZrB2. Addition of 5–20%NiAl assisted to bring down the hot pressing temperature to 1700 °C from 1850 °C for monolithic ZrB2. 2. NiAl melted during hot pressing and resulted in increased densification due to liquid phase sintering. Microstructures revealed that
Monteverde et al. [32] have carried out oxidation study at 1350 °C and found that monolithic ZrB2 has poor oxidation resistance. The oxide
Fig. 11. Fracture surface of ZrB2 + NiAl composite (a) ZrB2 + 5%NiAl (b) ZrB2 + 10%NiAl (c) ZrB2 + 20%NiAl.
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Fig. 12. Crack propagation path in ZrB2 + 20%NiAl composite sample.
3. 4. 5. 6.
7.
layer consisting of ZrO2 and NiAl2O4.
ZrB2 and NiAl are compatible with each other and does not lead to the formation of any reaction product. NiAl phase was found to be uniformly distributed throughout the microstructure. Little grain coarsening was observed by increasing the liquid phase content in the composite from 10 to 20%. Flexural strength of the composite was measured in the range of 421 to 438 MPa. Addition of NiAl was also found to increase the fracture toughness to 6.8 MPa·m1/2 (~ 94% enhanced compared with monolithic ZrB2). Crack bridging and crack arrest were observed at NiAl phase explaining the high fracture toughness The developed composite were found to have excellent oxidation resistance at 1400 °C in air due to formation of protective oxide
Suggestions for further work In future study, precise mechanism of liquid phase sintering in this system can be studied by the use of sintering kinetics studies [37].
Acknowledgement Author's wish to thank Mr. P.V. Durgaprasad, RSD, BARC for his help in measuring flexural strength for this study.
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o ZrO2
ZrB2 + 5% NiAl
0.8
# NiAl2O4 #
0.6
Relative Intensity
o 0.4
0.2
o#
4
6
o
#
o
o o
oo
o #
o oo
o
o
ZrB2+10%NiAl
o
# oo
o
o
o
8
Time (hour)
20
o o o o
30
40
ZrB2+5%NiAl
o
o
o
Fig. 13. Specific weight gain with time during oxidation of ZrB2 + NiAl composites at 1400 °C.
#
# o
# o 2
# oo #
0.0 0
ZrB2+20%NiAl
# o
ZrB2 + 20% NiAl
2
Specific Weight Gain (Kg / m )
ZrB2 + 10% NiAl
o
#o 50
o o o# 60
o 70
o 80
2θ angle (degrees) Fig. 15. XRD pattern of ZrB2 + NiAl samples oxidized at 1400 °C.
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