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Microstructure and mechanical properties of boron suboxide ceramics prepared by pressureless microwave sintering
Ceramics International ∎ (∎∎∎∎) ∎∎∎–∎∎∎
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Short communication
Microstructure and mechanical properties of boron suboxide ceramics prepared by pressureless microwave sintering Dmytro Demirskyi a,n, Oleg Vasylkiv a,b a b
Nanyang Technological University, 50 Nanyang Avenue, 639798 Singapore National Institute for Materials Science, 102-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan
art ic l e i nf o
a b s t r a c t
Article history: Received 14 May 2016 Received in revised form 20 May 2016 Accepted 23 May 2016
Monolithic boron suboxide ceramic was densified by microwave sintering at 28 GHz using highly stoichiometric B6O powders. High heating rate of 200 °C min 1 was maintained owing to special insulation cell which acts as additional microwave absorber. This insured homogeneous heating during sintering and resulted in dense and crack-free specimens. Room-temperature strength and fracture toughness of bulk B6O ceramics were as high as 400 MPa and 2.8 MPa m1/2, respectively. & 2016 Published by Elsevier Ltd.
Keywords: Microwave heating High-heating rate Boron suboxide Strength
1. Introduction Boron-suboxide-based ceramic composites have been extensively studied as alternatives to boron carbide [1–6]. Singlephase boron suboxide (B6O) possesses high hardness ( 428 GPa), good high-temperature hardness [6], and chemical stability. Owing to these distinctive properties, B6O-based ceramics have excellent potential for use as cutting tools and as superabrasive and high-temperature materials. The consolidation of boron-suboxide-based composites also involves another problem of B6O: the densification of covalent boron-rich compounds, such as α-B and B4C, requires a high temperature or high pressure to activate mass transport in these highly covalent solids. The crystal structure of B6O also affects its densification: the oxygen deficiency in the rhombohedral cell at the 6c position affects the ‘sinterability’ of as-synthesized boron suboxide powder [3]. The results of [3] indicate that spark plasma sintering (SPS) of boron suboxide powders with a low oxygen deficiency level, i.e., high x value in B6Ox, can be employed as a method for obtaining dense boron suboxide ceramics. Microwave sintering (MWS) is another alternative to SPS and conventional sintering. Microwave sintering has the advantages of uniform and rapid heating since the energy is directly coupled into the specimen rather than being conducted into the specimen from an external heat source. Additionally, thermal stresses upon heatn
Corresponding author. E-mail address: [email protected] (D. Demirskyi).
up are minimized because of this unique nature of homogeneous and volumetric heating. In addition, enhanced densification rates and finer microstructures have been reported for microwave sintered materials [7–13]. To our knowledge, pressureless consolidation of B6O has not been reported in the literature, probably because B6O, similar to B4C, would require temperatures exceeding 2100 °C to assist consolidation [14]. A study of Akashi et al., [15] showed that hot-pressed bulk B6O has lower electrical conductivity than that of B4C, but greater thermal conductivity than that of boron carbide which suggests that boron suboxide is more suitable for volumetric microwave heating. Furthermore, Rizzo et al. [16] reported that above 1800 °C the decomposition of boron suboxide, and thus formation of B6Ox phases with decreasing x value, would be initiated. Hence, to lower consolidation temperature we selected to use microwave energy to promote diffusion processes [9,12] in boron suboxide ceramics. The objective of the present work was to densify the boron suboxide ceramic by microwave sintering. A special insulation cell was used to unsure high heating rates, the common feature of MWS, and rapidly heat-up green specimens with size of 40 mm in diameter. The microstructure and mechanical properties of microwave sintered B6O were investigated.
2. Methodology We used boron suboxide powder mass-synthesized or consolidated in our previous studies [3,4,17] using an excess
http://dx.doi.org/10.1016/j.ceramint.2016.05.151 0272-8842/& 2016 Published by Elsevier Ltd.
Please cite this article as: D. Demirskyi, O. Vasylkiv, Microstructure and mechanical properties of boron suboxide ceramics prepared by pressureless microwave sintering, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.05.151i
2
D. Demirskyi, O. Vasylkiv / Ceramics International ∎ (∎∎∎∎) ∎∎∎–∎∎∎
Fig. 1. Schematic drawing illustrating microwave sintering set-up during hightemperature sintering experiments on boron suboxide ceramics.
amorphous boron (aB, 97%, Wako Pure Chemical Industries, Ltd., Japan) and B2O3 (99.0%, Kanto Chemical, Japan) as starting precursors, and 10:1 aB:B2O3 mole ratios. The synthesized B6O powder had a specific star-shape morphology, and a low value of oxygen deficiency (x 40.85 in B6Ox) [3]. B6O powders were cold pressed in the steel die with an inner diameter of 40 mm at 160 MPa and were subjected to cold-isostatic pressing at 300 MPa. This double pressing procedure resulted in specimens of approximately 45% green density. The density for B6O0.9 ceramic – 2.55 g/cm3 was used as theoretical density. The samples were placed in a porous thermally insulating boron nitride crucible set-up (Fig. 1) which was surrounded by the mixture of h-BN and tungsten powder. Unlike ceramic materials microwave interaction with metals in porous powder specimens is restricted to metal surface [18]. In case of tungsten powder [19], the skin depth steadily increases with increase in temperature up to 2000 °C. Hence, the mixture of W with BN acts as microwave susceptor allowing fast heating rate to consolidation temperature. In order to minimize heat loss during MWS process, the BN cell was wrapped using few layers of graphite cloth that is commonly used during SPS for covering the graphite die. The temperature of the samples was measured with a C-type WRe5/26 thermocouple (shielded with molybdenum). The probing point was located at the center of the sample bottom surface. A high power 28 GHz millimeter-wave generator combined with a multi-mode applicator (Fuji Denpa Kogyo, FGS-10-28) with a maximum power of 10 kW was used. Consolidation experiments were carried out under temperature control mode in such a way that the supplied gyrotron power was automatically adjusted to achieve the preset temperature cycle at the probe point. The heating rate was fixed at 50 °C min 1 in the temperature range below 600 °C and it was increased to 200 °C min 1 while heating to sintering temperature of 1700–1900 °C. The cooling rate after the sintering was fixed at 50 °C min 1 down to the temperature of 1200 °C, after which the sample was kept at natural cooling in the applicator. Dwell time at preset sintering temperature was 5 min, with exception at 1300, 1500 or 1900 °C, where no dwell time was used. All experiments were performed in argon gas with a flow rate of 2 L min 1. The sintered specimens were ground with diamond disks with a particle size of up to 0.5 mm. Then, the density of the samples was measured by the Archimedes method using ethanol as a medium in accordance with ASTM B 963–08. The three-point and four-point flexural strength was determined using rectangular blocks cut from sintered specimens with a diameter of 37–39 mm using electric discharge machining. Their lateral surfaces were ground and polished using diamond pastes.
The flexural strength and fracture toughness were measured by a mechanical strength testing equipment Shimadzu AG-X plus system (Shimadzu, Japan) using the three-point bending mode with a span of 16 mm. The dimensions of samples were 2 2.5 20 mm and 2 4 20 mm with a notch of 2 mm depth and 0.1 mm width, respectively. In addition, four-point flexural strength was performed using a 20/10 mm span of the supports and 2 2.5 26 mm. Measurements were performed using and the loading speed was 0.5 mm min 1. Six samples were tested in case of flexural strength measurement and three samples in case of fracture toughness. In addition indentation fracture toughness was calculated with a hardness testing machine from equation KIC ¼0.073 10 9 (P/ cl3/2) [20] using the half-length of the crack (cl) formed around the corners of indentations at loads (P) of 49 N (AVK-A, Akashi Co., Tokyo Japan). P (N) is the applied load and cl (m) is the average half-length of cracks. Twenty measurements were made at different locations of each pellet. Microstructural observations and analyses were carried out on the fracture surfaces and on polished and etched samples by scanning electron microscopy (SEM, SU 8000, Hitachi, Japan). Etching was performed in boiling HNO3 acid for 10 s. X-ray diffraction (XRD) analysis (Rigaku RINT 2500 HLR, Japan) was performed on polished samples to identify the crystalline phases using Cu Kα radiation. XRD phase identification was performed using JADE (MDI) software with enabled background alignment options.
3. Results and discussion Fig. 2 shows microstructure evolution of B6O ceramics with initially star-shaped structure during microwave sintering at the different stages of consolidation process. It is clear that at relatively low temperatures, the covalent B6O is densified by stacking and partial sintering of initial powder particles, which was observed on the fractured surface of the specimen after MWS at 1300 °C. At 1500 °C fracture of ceramic specimen showed some signs of surface diffusion mechanisms, i.e., some roundish and sharp-edged structures were observed. Finally, at temperatures above 1700 °C dense ceramic specimens were observed with density over 94% (Table 1). Slight difference in grain size between specimens was observed; some 2–3 mm grains were present in all consolidated specimens, these are indication of the ongoing grain growth process. Importantly, owing to homogeneous heating during MWS bulk B6O specimens were free of macroscopic cracks, which are usual during high-temperature consolidation of B4C. Fig. 3 illustrates the powder X-ray diffraction pattern for a B6O specimen subjected to MWS at 1850 °C. Every peak was identified as B6O and no trace of B2O3 or other impurities was observed. B6O phase was identified using ICSD data card #50-1505. The lattice parameters of B6O were determined as a¼ 0.538(9) nm, and c¼1.232(7) nm (in hexagonal expression). Different fracture mechanisms are active for our samples (Fig. 4). Breakage of transgranular type is observed for large boron suboxide grains. Small B6O and triple point grains show an intergranular pull out mechanism as boron suboxide grains are extracted from the ‘matrix’. It can be also observed that intergranular sharp-edged grains, especially with size of 1 mm, contribute to B6O pull out through intergranular sliding. This behavior is similar of that reported for boron suboxide [1,4], boron carbide [21–25] or silicon carbide [26], for which twin formation or intergranular fracture is usually reported. In the present study, the large number of striations on the large grains ( 2 mm) may indirectly indicate the formation of twins [4,27]. One should note that, there is no physical basis for twinning to
Please cite this article as: D. Demirskyi, O. Vasylkiv, Microstructure and mechanical properties of boron suboxide ceramics prepared by pressureless microwave sintering, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.05.151i
D. Demirskyi, O. Vasylkiv / Ceramics International ∎ (∎∎∎∎) ∎∎∎–∎∎∎
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Fig. 2. Microstructure evolution during microwave consolidation of boron suboxide ceramics: left image in top row shows initial powder, obtained by procedure described in [18]. Mind the quasi-layered surfaces with sharp-edge shape are clearly visible for ceramics on intermediate stages of consolidation. Bottom row shows structure of boron suboxide after polishing and etching. Table 1. Physical and mechanical properties of B6O ceramics. Composition
Method
T, °C
Dwell time, min
Density, %
Grain size
Room temperature strength, s25 °C, MPa
Fracture toughness, MPa m1/2
B6O
MWS
1700
5
94.2 7 0.6
0.9 7 0.3
286 7 33 (3P)
B6O
MWS
1800
5
96.7 7 0.4
1.1 70.3
3147 13 (3P)
B6O
MWS
1850
5
98.17 0.5
1.4 7 0.5
B6O
MWS
1900
0
96.2 7 0.3
1.3 7 0.4
B6O [3,4] B6O [2] B6O [5] B þ B6O additions [2] B4C [24] B4C [25]
SPS HP SPS HP SPS SPS
1800 – 1900 – 1800 2000
1 – 5 – 5 6
98.6 – 2.52 – 498 99.4
o 1.6 1 0.26 2 2.1 2.55
405 7 15 (3P) 392 7 14 (4P) 340 7 10 (3P) 3307 20 (4P) 3007 25 (3P) 360** (4P) 4107 30 (4P) 345** (4P) 448 7 20 (3P) 540 7 150 (3P)
2.17 0.2* 1.8 70.1Y 2.0* 1.7 70.3Y 2.8 7 0.3* 2.5 7 0.2Y 3.17 0.2* 2.9 7 0.3Y 2.5 7 0.15Y
* ** Y
2 70.2* 4.6 7 0.2Y 1.7 70.25Y
Toughness by SEVNB or SENB. The value is approximated for the zero porosity. Indentation fracture toughness.
Fig. 3. XRD pattern of dense B6O ceramics after MWS at 1800 °C for 5 min.
occur under flexural loading in these covalent ceramics, due to higher magnitude of stresses required for twinning to occur. Fracture was mainly intergranular for B6O ceramic consolidated
at 1700 °C. An increase in transgranular fracture was observed with increase in consolidation temperature and mean grain size of dense ceramic specimens. Interestingly, samples consolidated at 1850 °C and 1900 °C exhibited some sharp-edged grains during fracture similar to that observed in (Fig. 2) for boron suboxide at intermediate stages of the microwave sintering process. This difference in fracture mechanisms correlates with mechanical properties of boron suboxide ceramics. If the pull out of the B6O grain is not realized mainly transgranular fracture is observed, small triple point grains promote deflection of the propagating crack, hence a higher toughness, but lower strength is observed. B6O consolidated at 1850° C had slightly higher strength than at other temperatures, 405 7 15 (3P) and 392 7 14 (4P). When comparing two testing methods, we should note that, since only six samples were tested for each ceramic, it is clear that the difference in strength can be treated as minor, since it is well within the measurement error. In case of ceramic consolidated at 1700 and 1800 °C lower toughness and strength can be attributed to different fracture mechanisms and slightly ‘weaker’ grain boundaries, if compared to B6O consolidated at 1850 °C. Furthermore, we should notice, that density also plays important role in determining fracture
Please cite this article as: D. Demirskyi, O. Vasylkiv, Microstructure and mechanical properties of boron suboxide ceramics prepared by pressureless microwave sintering, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.05.151i
4
D. Demirskyi, O. Vasylkiv / Ceramics International ∎ (∎∎∎∎) ∎∎∎–∎∎∎
Fig. 4. Typical microstructures of fractured surfaces of boron suboxide ceramics after flexural strength test at room temperature. Notice the sharp-edged structures that originate from initial powder morphology are still present in dense ceramics (see black arrows).
mechanism (and thus strength): specimens consolidated at 1800 °C for 5 min and at 1900 °C without dwell time had similar level of density ( 96%) and different values of mean grain size, but showed similar fracture behavior (Fig. 4). According to [4,27], the room-temperature bending strength s25 °С in boron-rich ceramics is affected by the grain size. Hence, s25 °С of 300–500 MPa is expected when the grain size of B4C or B6O is below 10 mm, which is also consistent with findings of the present study. In terms of fracture toughness, KIC obtained by SENB method generally exceeds that of corresponding values derived using indentation method. Similar situation was observed in [28] for the B6O–TiB2 ceramic composites. In terms of hardness values boron suboxide showed hardness of 29–33 GPa at load of 49 N which is in agreement with values obtained in [3]. Table 1 shows that the data for the room-temperature strength of boron suboxide are similar to that of boron carbide, although a wide range of values has been reported for monolithic boron carbide ceramics (200–600 MPa) [23]. Study [23] suggests and the strength to toughness ratio for monolithic B4C depends strongly on consolidation conditions. This should be also genuine for B6O ceramics obtained in the present study. This is the first attempt to consolidate boron suboxide using pressureless sintering. Hence further optimization of processing parameters may lead to increase of mechanical performance of bulk B6O (or B4C) in the future, which is considered as a next step in ongoing research. In summary, boron suboxide (B6O) powder with peculiar starshape morphology was consolidated by pressureless sintering using microwave heating to promote densification. High heating rate of 200 °C min 1 was maintained owing to special insulation cell which uses tungsten and BN mixture as additional microwave absorber. This insured homogeneous heating during microwave
sintering and resulted in dense and crack-free specimens. The bulk B6O ceramics showed a comparable strength to that of monolithic boron carbide ceramics (290–420 MPa). Best strength to toughness ratio was obtained when boron suboxide was sintered at 1850 °C for 5 min with strength and toughness of 400 MPa and 2.8 MPa m1/2, respectively.
Acknowledgements We thank to Dr. Toshiyuki Nishimura (NIMS) for the use of the flexural strength measurement facility.
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Please cite this article as: D. Demirskyi, O. Vasylkiv, Microstructure and mechanical properties of boron suboxide ceramics prepared by pressureless microwave sintering, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.05.151i
Contents lists available at ScienceDirect
Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Short communication
Microstructure and mechanical properties of boron suboxide ceramics prepared by pressureless microwave sintering Dmytro Demirskyi a,n, Oleg Vasylkiv a,b a b
Nanyang Technological University, 50 Nanyang Avenue, 639798 Singapore National Institute for Materials Science, 102-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan
art ic l e i nf o
a b s t r a c t
Article history: Received 14 May 2016 Received in revised form 20 May 2016 Accepted 23 May 2016
Monolithic boron suboxide ceramic was densified by microwave sintering at 28 GHz using highly stoichiometric B6O powders. High heating rate of 200 °C min 1 was maintained owing to special insulation cell which acts as additional microwave absorber. This insured homogeneous heating during sintering and resulted in dense and crack-free specimens. Room-temperature strength and fracture toughness of bulk B6O ceramics were as high as 400 MPa and 2.8 MPa m1/2, respectively. & 2016 Published by Elsevier Ltd.
Keywords: Microwave heating High-heating rate Boron suboxide Strength
1. Introduction Boron-suboxide-based ceramic composites have been extensively studied as alternatives to boron carbide [1–6]. Singlephase boron suboxide (B6O) possesses high hardness ( 428 GPa), good high-temperature hardness [6], and chemical stability. Owing to these distinctive properties, B6O-based ceramics have excellent potential for use as cutting tools and as superabrasive and high-temperature materials. The consolidation of boron-suboxide-based composites also involves another problem of B6O: the densification of covalent boron-rich compounds, such as α-B and B4C, requires a high temperature or high pressure to activate mass transport in these highly covalent solids. The crystal structure of B6O also affects its densification: the oxygen deficiency in the rhombohedral cell at the 6c position affects the ‘sinterability’ of as-synthesized boron suboxide powder [3]. The results of [3] indicate that spark plasma sintering (SPS) of boron suboxide powders with a low oxygen deficiency level, i.e., high x value in B6Ox, can be employed as a method for obtaining dense boron suboxide ceramics. Microwave sintering (MWS) is another alternative to SPS and conventional sintering. Microwave sintering has the advantages of uniform and rapid heating since the energy is directly coupled into the specimen rather than being conducted into the specimen from an external heat source. Additionally, thermal stresses upon heatn
Corresponding author. E-mail address: [email protected] (D. Demirskyi).
up are minimized because of this unique nature of homogeneous and volumetric heating. In addition, enhanced densification rates and finer microstructures have been reported for microwave sintered materials [7–13]. To our knowledge, pressureless consolidation of B6O has not been reported in the literature, probably because B6O, similar to B4C, would require temperatures exceeding 2100 °C to assist consolidation [14]. A study of Akashi et al., [15] showed that hot-pressed bulk B6O has lower electrical conductivity than that of B4C, but greater thermal conductivity than that of boron carbide which suggests that boron suboxide is more suitable for volumetric microwave heating. Furthermore, Rizzo et al. [16] reported that above 1800 °C the decomposition of boron suboxide, and thus formation of B6Ox phases with decreasing x value, would be initiated. Hence, to lower consolidation temperature we selected to use microwave energy to promote diffusion processes [9,12] in boron suboxide ceramics. The objective of the present work was to densify the boron suboxide ceramic by microwave sintering. A special insulation cell was used to unsure high heating rates, the common feature of MWS, and rapidly heat-up green specimens with size of 40 mm in diameter. The microstructure and mechanical properties of microwave sintered B6O were investigated.
2. Methodology We used boron suboxide powder mass-synthesized or consolidated in our previous studies [3,4,17] using an excess
http://dx.doi.org/10.1016/j.ceramint.2016.05.151 0272-8842/& 2016 Published by Elsevier Ltd.
Please cite this article as: D. Demirskyi, O. Vasylkiv, Microstructure and mechanical properties of boron suboxide ceramics prepared by pressureless microwave sintering, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.05.151i
2
D. Demirskyi, O. Vasylkiv / Ceramics International ∎ (∎∎∎∎) ∎∎∎–∎∎∎
Fig. 1. Schematic drawing illustrating microwave sintering set-up during hightemperature sintering experiments on boron suboxide ceramics.
amorphous boron (aB, 97%, Wako Pure Chemical Industries, Ltd., Japan) and B2O3 (99.0%, Kanto Chemical, Japan) as starting precursors, and 10:1 aB:B2O3 mole ratios. The synthesized B6O powder had a specific star-shape morphology, and a low value of oxygen deficiency (x 40.85 in B6Ox) [3]. B6O powders were cold pressed in the steel die with an inner diameter of 40 mm at 160 MPa and were subjected to cold-isostatic pressing at 300 MPa. This double pressing procedure resulted in specimens of approximately 45% green density. The density for B6O0.9 ceramic – 2.55 g/cm3 was used as theoretical density. The samples were placed in a porous thermally insulating boron nitride crucible set-up (Fig. 1) which was surrounded by the mixture of h-BN and tungsten powder. Unlike ceramic materials microwave interaction with metals in porous powder specimens is restricted to metal surface [18]. In case of tungsten powder [19], the skin depth steadily increases with increase in temperature up to 2000 °C. Hence, the mixture of W with BN acts as microwave susceptor allowing fast heating rate to consolidation temperature. In order to minimize heat loss during MWS process, the BN cell was wrapped using few layers of graphite cloth that is commonly used during SPS for covering the graphite die. The temperature of the samples was measured with a C-type WRe5/26 thermocouple (shielded with molybdenum). The probing point was located at the center of the sample bottom surface. A high power 28 GHz millimeter-wave generator combined with a multi-mode applicator (Fuji Denpa Kogyo, FGS-10-28) with a maximum power of 10 kW was used. Consolidation experiments were carried out under temperature control mode in such a way that the supplied gyrotron power was automatically adjusted to achieve the preset temperature cycle at the probe point. The heating rate was fixed at 50 °C min 1 in the temperature range below 600 °C and it was increased to 200 °C min 1 while heating to sintering temperature of 1700–1900 °C. The cooling rate after the sintering was fixed at 50 °C min 1 down to the temperature of 1200 °C, after which the sample was kept at natural cooling in the applicator. Dwell time at preset sintering temperature was 5 min, with exception at 1300, 1500 or 1900 °C, where no dwell time was used. All experiments were performed in argon gas with a flow rate of 2 L min 1. The sintered specimens were ground with diamond disks with a particle size of up to 0.5 mm. Then, the density of the samples was measured by the Archimedes method using ethanol as a medium in accordance with ASTM B 963–08. The three-point and four-point flexural strength was determined using rectangular blocks cut from sintered specimens with a diameter of 37–39 mm using electric discharge machining. Their lateral surfaces were ground and polished using diamond pastes.
The flexural strength and fracture toughness were measured by a mechanical strength testing equipment Shimadzu AG-X plus system (Shimadzu, Japan) using the three-point bending mode with a span of 16 mm. The dimensions of samples were 2 2.5 20 mm and 2 4 20 mm with a notch of 2 mm depth and 0.1 mm width, respectively. In addition, four-point flexural strength was performed using a 20/10 mm span of the supports and 2 2.5 26 mm. Measurements were performed using and the loading speed was 0.5 mm min 1. Six samples were tested in case of flexural strength measurement and three samples in case of fracture toughness. In addition indentation fracture toughness was calculated with a hardness testing machine from equation KIC ¼0.073 10 9 (P/ cl3/2) [20] using the half-length of the crack (cl) formed around the corners of indentations at loads (P) of 49 N (AVK-A, Akashi Co., Tokyo Japan). P (N) is the applied load and cl (m) is the average half-length of cracks. Twenty measurements were made at different locations of each pellet. Microstructural observations and analyses were carried out on the fracture surfaces and on polished and etched samples by scanning electron microscopy (SEM, SU 8000, Hitachi, Japan). Etching was performed in boiling HNO3 acid for 10 s. X-ray diffraction (XRD) analysis (Rigaku RINT 2500 HLR, Japan) was performed on polished samples to identify the crystalline phases using Cu Kα radiation. XRD phase identification was performed using JADE (MDI) software with enabled background alignment options.
3. Results and discussion Fig. 2 shows microstructure evolution of B6O ceramics with initially star-shaped structure during microwave sintering at the different stages of consolidation process. It is clear that at relatively low temperatures, the covalent B6O is densified by stacking and partial sintering of initial powder particles, which was observed on the fractured surface of the specimen after MWS at 1300 °C. At 1500 °C fracture of ceramic specimen showed some signs of surface diffusion mechanisms, i.e., some roundish and sharp-edged structures were observed. Finally, at temperatures above 1700 °C dense ceramic specimens were observed with density over 94% (Table 1). Slight difference in grain size between specimens was observed; some 2–3 mm grains were present in all consolidated specimens, these are indication of the ongoing grain growth process. Importantly, owing to homogeneous heating during MWS bulk B6O specimens were free of macroscopic cracks, which are usual during high-temperature consolidation of B4C. Fig. 3 illustrates the powder X-ray diffraction pattern for a B6O specimen subjected to MWS at 1850 °C. Every peak was identified as B6O and no trace of B2O3 or other impurities was observed. B6O phase was identified using ICSD data card #50-1505. The lattice parameters of B6O were determined as a¼ 0.538(9) nm, and c¼1.232(7) nm (in hexagonal expression). Different fracture mechanisms are active for our samples (Fig. 4). Breakage of transgranular type is observed for large boron suboxide grains. Small B6O and triple point grains show an intergranular pull out mechanism as boron suboxide grains are extracted from the ‘matrix’. It can be also observed that intergranular sharp-edged grains, especially with size of 1 mm, contribute to B6O pull out through intergranular sliding. This behavior is similar of that reported for boron suboxide [1,4], boron carbide [21–25] or silicon carbide [26], for which twin formation or intergranular fracture is usually reported. In the present study, the large number of striations on the large grains ( 2 mm) may indirectly indicate the formation of twins [4,27]. One should note that, there is no physical basis for twinning to
Please cite this article as: D. Demirskyi, O. Vasylkiv, Microstructure and mechanical properties of boron suboxide ceramics prepared by pressureless microwave sintering, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.05.151i
D. Demirskyi, O. Vasylkiv / Ceramics International ∎ (∎∎∎∎) ∎∎∎–∎∎∎
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Fig. 2. Microstructure evolution during microwave consolidation of boron suboxide ceramics: left image in top row shows initial powder, obtained by procedure described in [18]. Mind the quasi-layered surfaces with sharp-edge shape are clearly visible for ceramics on intermediate stages of consolidation. Bottom row shows structure of boron suboxide after polishing and etching. Table 1. Physical and mechanical properties of B6O ceramics. Composition
Method
T, °C
Dwell time, min
Density, %
Grain size
Room temperature strength, s25 °C, MPa
Fracture toughness, MPa m1/2
B6O
MWS
1700
5
94.2 7 0.6
0.9 7 0.3
286 7 33 (3P)
B6O
MWS
1800
5
96.7 7 0.4
1.1 70.3
3147 13 (3P)
B6O
MWS
1850
5
98.17 0.5
1.4 7 0.5
B6O
MWS
1900
0
96.2 7 0.3
1.3 7 0.4
B6O [3,4] B6O [2] B6O [5] B þ B6O additions [2] B4C [24] B4C [25]
SPS HP SPS HP SPS SPS
1800 – 1900 – 1800 2000
1 – 5 – 5 6
98.6 – 2.52 – 498 99.4
o 1.6 1 0.26 2 2.1 2.55
405 7 15 (3P) 392 7 14 (4P) 340 7 10 (3P) 3307 20 (4P) 3007 25 (3P) 360** (4P) 4107 30 (4P) 345** (4P) 448 7 20 (3P) 540 7 150 (3P)
2.17 0.2* 1.8 70.1Y 2.0* 1.7 70.3Y 2.8 7 0.3* 2.5 7 0.2Y 3.17 0.2* 2.9 7 0.3Y 2.5 7 0.15Y
* ** Y
2 70.2* 4.6 7 0.2Y 1.7 70.25Y
Toughness by SEVNB or SENB. The value is approximated for the zero porosity. Indentation fracture toughness.
Fig. 3. XRD pattern of dense B6O ceramics after MWS at 1800 °C for 5 min.
occur under flexural loading in these covalent ceramics, due to higher magnitude of stresses required for twinning to occur. Fracture was mainly intergranular for B6O ceramic consolidated
at 1700 °C. An increase in transgranular fracture was observed with increase in consolidation temperature and mean grain size of dense ceramic specimens. Interestingly, samples consolidated at 1850 °C and 1900 °C exhibited some sharp-edged grains during fracture similar to that observed in (Fig. 2) for boron suboxide at intermediate stages of the microwave sintering process. This difference in fracture mechanisms correlates with mechanical properties of boron suboxide ceramics. If the pull out of the B6O grain is not realized mainly transgranular fracture is observed, small triple point grains promote deflection of the propagating crack, hence a higher toughness, but lower strength is observed. B6O consolidated at 1850° C had slightly higher strength than at other temperatures, 405 7 15 (3P) and 392 7 14 (4P). When comparing two testing methods, we should note that, since only six samples were tested for each ceramic, it is clear that the difference in strength can be treated as minor, since it is well within the measurement error. In case of ceramic consolidated at 1700 and 1800 °C lower toughness and strength can be attributed to different fracture mechanisms and slightly ‘weaker’ grain boundaries, if compared to B6O consolidated at 1850 °C. Furthermore, we should notice, that density also plays important role in determining fracture
Please cite this article as: D. Demirskyi, O. Vasylkiv, Microstructure and mechanical properties of boron suboxide ceramics prepared by pressureless microwave sintering, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.05.151i
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D. Demirskyi, O. Vasylkiv / Ceramics International ∎ (∎∎∎∎) ∎∎∎–∎∎∎
Fig. 4. Typical microstructures of fractured surfaces of boron suboxide ceramics after flexural strength test at room temperature. Notice the sharp-edged structures that originate from initial powder morphology are still present in dense ceramics (see black arrows).
mechanism (and thus strength): specimens consolidated at 1800 °C for 5 min and at 1900 °C without dwell time had similar level of density ( 96%) and different values of mean grain size, but showed similar fracture behavior (Fig. 4). According to [4,27], the room-temperature bending strength s25 °С in boron-rich ceramics is affected by the grain size. Hence, s25 °С of 300–500 MPa is expected when the grain size of B4C or B6O is below 10 mm, which is also consistent with findings of the present study. In terms of fracture toughness, KIC obtained by SENB method generally exceeds that of corresponding values derived using indentation method. Similar situation was observed in [28] for the B6O–TiB2 ceramic composites. In terms of hardness values boron suboxide showed hardness of 29–33 GPa at load of 49 N which is in agreement with values obtained in [3]. Table 1 shows that the data for the room-temperature strength of boron suboxide are similar to that of boron carbide, although a wide range of values has been reported for monolithic boron carbide ceramics (200–600 MPa) [23]. Study [23] suggests and the strength to toughness ratio for monolithic B4C depends strongly on consolidation conditions. This should be also genuine for B6O ceramics obtained in the present study. This is the first attempt to consolidate boron suboxide using pressureless sintering. Hence further optimization of processing parameters may lead to increase of mechanical performance of bulk B6O (or B4C) in the future, which is considered as a next step in ongoing research. In summary, boron suboxide (B6O) powder with peculiar starshape morphology was consolidated by pressureless sintering using microwave heating to promote densification. High heating rate of 200 °C min 1 was maintained owing to special insulation cell which uses tungsten and BN mixture as additional microwave absorber. This insured homogeneous heating during microwave
sintering and resulted in dense and crack-free specimens. The bulk B6O ceramics showed a comparable strength to that of monolithic boron carbide ceramics (290–420 MPa). Best strength to toughness ratio was obtained when boron suboxide was sintered at 1850 °C for 5 min with strength and toughness of 400 MPa and 2.8 MPa m1/2, respectively.
Acknowledgements We thank to Dr. Toshiyuki Nishimura (NIMS) for the use of the flexural strength measurement facility.
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Please cite this article as: D. Demirskyi, O. Vasylkiv, Microstructure and mechanical properties of boron suboxide ceramics prepared by pressureless microwave sintering, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.05.151i