- Email: [email protected]
Advantages and disadvantages of graphite addition on the characteristics of hot-pressed ZrB2–SiC composites
Journal Pre-proof Advantages and disadvantages of graphite addition on the characteristics of hotpressed ZrB2–SiC composites Nasser Pourmohammadie Vafa, Mahdi Ghassemi Kakroudi, Mehdi Shahedi Asl PII:
S0272-8842(19)33575-8
DOI:
https://doi.org/10.1016/j.ceramint.2019.12.086
Reference:
CERI 23739
To appear in:
Ceramics International
Received Date: 15 October 2019 Revised Date:
3 December 2019
Accepted Date: 7 December 2019
Please cite this article as: N.P. Vafa, M.G. Kakroudi, M.S. Asl, Advantages and disadvantages of graphite addition on the characteristics of hot-pressed ZrB2–SiC composites, Ceramics International (2020), doi: https://doi.org/10.1016/j.ceramint.2019.12.086. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
Advantages and disadvantages of graphite addition on the characteristics of hot-pressed ZrB2–SiC composites
Nasser Pourmohammadie Vafa a, Mahdi Ghassemi Kakroudi a∗, Mehdi Shahedi Asl b a b
Department of Materials Science and Engineering, University of Tabriz, Tabriz, Iran
Department of Mechanical Engineering, University of Mohaghegh Ardabili, Ardabil, Iran ∗
Corresponding author: M. Ghassemi Kakroudi ([email protected])
Abstract ZrB2–SiC–graphite composites with 0-35 vol% graphite flakes were densified via hot-pressing route at the temperature of 1800 ºC under the uniaxial pressure of 40 MPa for 1 hour. Consolidation, mechanical properties, and microstructure of hot-pressed composites were investigated by variation of graphite content. By the addition of graphite, the relative density of composites increased, and at this hot pressing condition, fully densified composites were fabricated. The highest flexural strength of 366 MPa was measured for composite containing 7.5 vol% graphite, while the maximum Vickers hardness resulted in 2.5 vol% graphite doped one, and its value was equal to 20.8 GPa. Phase analysis of hot-pressed samples revealed the formation of the Zr3C2 and B4C phases besides the main existing ZrB2, SiC, and graphite phases. The newly carbide phases formed at the surface of ZrB2 grains. The addition of graphite into the ZrB2–SiC composites improved the sintering process and caused a fine-grained microstructure.
Keywords: ZrB2; SiC; Graphite; Mechanical properties; Microstructure.
1. Introduction In recent decades, due to the need for advanced high-temperature engineering materials, much attention has been paid to the ultra-high temperature ceramics (UHTCs). As a member of UHTCs, zirconium diboride due to the combination of properties like the high melting point of 3245 °C, chemical inertness, good electrical and thermal conductivity, and high hardness, is an excellent candidate for applications such as high-performance aircraft and hypersonic flight vehicles. Intense covalent bounding and poor self-diffusion coefficient at the grain boundaries persuade the ZrB2 ceramics to densify at extremely high temperatures alongside an external pressure. Presence of ZrO2 and B2O3 on the surface of powders as oxide impurities, impede the consolidation and causes the grain growth. Hence, to densify these ceramics, sintering techniques along with an applied pressure such as spark plasma sintering or hot pressing, and sintering aids are required [1-5]. Different kind of sintering aids are studied for densification of ZrB2 such as metals [6,7], oxides [8-10], carbides [11-13], nitrides [14,15] and carbon (with various morphologies) [16-19]. Silicon carbide (SiC) is mostly used as an additive and reinforcement for ZrB2-based composites, which eliminates the oxide impurities and hinders the extraordinary grain growth of ZrB2 [1,4]. Up to now, many research has been conducted for the betterment of the densification and flexural strength of the ZrB2-based ceramics. Nowadays, the main challenge about ZrB2-based composites is the amendment of the fracture toughness of these ceramics to manufacture machinable pieces. It has been reported that the addition of graphite flakes because of layered microstructure could improve the toughness of the ZrB2-SiC composites, while it reduces the flexural strength of the composites [20,21]. Hou et al. [18] investigated the influence of the diameter of graphite flakes on the mechanical behavior of ZrB2based ceramics reinforced with nano-sized SiC. Zhou et al. [20] hot pressed ZrB2-SiC-Gr composites with 15 vol% graphite flakes at 1900 ºC. In addition to grain refinement and densification of the composites, they reported that orientation of the Gr flakes causes the anisotropic mechanical behavior like fracture toughness and flexural strength except for hardness. Gr flakes prefer to orientate in a direction that their base planes stand perpendicular to the direction of hot-press. This preferred orientation creates an anisotropic behavior in the fracture toughness and flexural strength of the composites.
Zhi et al. [22] investigated the influence of graphite addition on the hot pressing of ZrB2– graphite ceramics. They found that by addition of graphite, the resistance to the thermal shock and the fracture toughness were improved remarkably, while the flexural strength and microhardness of the composites scaled down. Wang et al. [23] studied the fracture toughness of hot-pressed ZrB2–SiC composites containing Gr, up to the temperature of 1800 °C in a vacuum atmosphere and up to 1300 °C in the air. In vacuum, fracture toughness decreased linearly with the temperature up to 1300 °C, then from 1300 °C to 1600 °C increased, and reduced rapidly at 1800 °C. Also, results revealed that the temperature for the brittle-ductile transition of ZrB2– SiC–Gr composite is 1300 °C. Liu et al. [24] fabricated ZrB2-SiC/Graphite ceramics in a laminated structure by tap-casting and hot-pressed it. They reported that the flexural strengths were 382 MPa in the parallel direction and 429 MPa in the perpendicular direction. The values for indentation strength at first decreased and then by increasing the indentation load increased. Many studies have been conducted for the graphite addition up to 10 vol% in ZrB2 based composites. In the present study, which is the first part of the machinable ZrB2-SiC-Gr project, the graphite flakes addition on the hot pressing of the ZrB2-SiC-Gr composites was studied. Therefore, 0-35 vol% Gr was added into the ZrB2-25 vol% SiC ceramics and consolidation, microstructure, flexural strength, and Vickers hardness of hot-pressed ZrB2-SiC composites was studied.
2. Experimental procedure The starting powders used to prepare ZrB2-SiC-Gr composites were, commercially ZrB2 powder (mean particle size: < 2 µm, purity: 99%, Leung Hi-tech Co., China), α-SiC powder (mean particle size: 0.7 µm, purity: > 99%, Carborundum Universal Limited, China) and Graphite flakes (mean diameter: < 5 µm, thickness: < 0.5 µm, purity: > 98%, Pack Sang Scrap Co, India). To get the powder mixture with composition of ZrB2–SiC containing 0-35 vol% Gr, the starting powders were weighed in an appropriate ratio. In all the composites the ZrB2 to SiC volume ratio was 75:25. The composition of the ZrB2-SiC-Gr composites has been shown in Table 1. At first, to intercept the particles from adhesion and agglomeration, the SiC powder and Gr flakes were dispersed for 30 min in ethanol, separately, by an ultrasonic agitator (Mercury, Turkey). Then
ZrB2 powder was added to the SiC and Gr suspension. The obtained slurry homogeneously mixed in a polyethylene cup with ZrO2 balls in ethanol media for 60 min with 200 rpm. Then the obtained slurries were dried for 8 h in an evaporator (Alfa D500, Iran) at the temperature of 110 ºC. After drying, the resultant powder mixtures were screened with mesh 100. The composites were densified in a rectangular graphite mold with the dimension of 10×25 mm, which was sealed with graphite sheets as a barrier to sticking of the sample to the mold. Powder mixtures whit designated weights were charged into the die to get the samples with the final thickness of 10 mm after hot pressing. The composites were hot-pressed in a vacuum chamber (Shenyang Weitai Science & Technology Development Co., Ltd., China) with a pressure of 5×10-2 Pa. The heating rate up to the temperature of 1000 °C was 10 °C/min. For the removal of volatiles, the samples were held for 15 min at 1000 °C. Afterward, the chamber was heated again up to the final temperature of the hot pressing process, which was 1800 °C, at the same rate of heating. At the temperature of 1800 °C, a 40 MPa uniaxial pressure was loaded on the sample and remained for 60 min. In the end, the chamber was chilled to the 300 °C by water circulation and then naturally up to the room temperature. Finally, the hot-pressed samples were removed and grinded to separate the adhesive graphite sheets. An automatic polisher system (Sanat Ceram, Iran) was used to polish the surface of the samples with diamond paste. Phase analysis, morphology and particle distribution of the raw materials and prepared powder mixtures, were done by XRD patterns (λ = 1.54 Å, Cu lamp, 40 kV, 30 mA, Philips PW1730) and SEM (Mira3 Tescan, Czech Republic). The bulk density of the specimens was measured according to the ASTM B328 by the Archimedes method. The theoretical density of composites was calculated based on the role of mixtures, where densities of ZrB2, SiC, and Gr were considered as 6.11 g/cm3 [1], 3.21 g/cm3 [1] and 2.16 g/cm3 [20] respectively. Samples were cut into specimens with dimensions of 3×4×25 mm for the 3-point bending test. 3-point flexural strength of the composites was measured using a universal testing machine (Zwick Roell SP600, Germany) with a span of 20 mm and a loading rate of 0.05 mm/min. The flexural strength of the composites was evaluated according to the Eq. 1:
=
(1)
Where
shows the flexural strength in MPa, P is the force in N, l, b, and h are the span, width,
and thickness of the specimen in mm. Vickers microhardness of the samples evaluated by employing a Vickers indentor (Zwick Roell, Germany) on the polished surfaces of the samples with a load of 9.8 N for 15 s. X-ray diffraction with a step size and detecting time of 0.05 and 1 s, respectively, was used for phase analysis of the hot-pressed samples. Xpert high score plus software (2.2b (2.2.2)) was used for the analysis of the resulted patterns. The polished surface and fracture cross-section of the composites were investigated utilizing scanning electron microscopy with the beneficiary of EDS (Digital X-Ray Processor, DXP-X10P).
Table 1. The composition of the ZrB2-SiC-Gr composites.
Composite
vol% ZrB2
vol% SiC
vol% Gr
ZS
75.00
25.00
0.00
ZSG2.5
73.13
24.38
2.50
ZSG5
71.25
23.75
5.00
ZSG7.5
69.38
23.13
7.50
ZSG10
67.50
22.50
10.00
ZSG15
63.75
21.25
15.00
ASG20
60.00
20.00
20.00
ZSG25
56.25
18.75
25.00
ZSG30
52.50
17.50
30.00
ZSG35
48.75
16.25
35.00
3. Results and discussion The phase analysis of as-received ZrB2, SiC powders, and Gr flakes (not shown here) indicated the presence of lonely hexagonal ZrB2, SiC, and Graphite. It should be mentioned that there are little peaks of zirconium oxide and boron oxide beside the sharp peaks of ZrB2 in the XRD pattern of ZrB2 due to the existence of oxide impurities on the surface of the starting powders. SEM micrographs of starting powders (not shown here) emphasized the particle size of the powders as supplier’s reports. The SEM micrograph and XRD pattern of ZrB2-SiC powder mixture containing 10 vol% Gr is represented in Fig. 1. As can be seen in Fig. 1a, the fine SiC
particles surrounded the ZrB2 particles, and Gr flakes are distributed among ZrB2 particles. According to the XRD pattern (Fig. 1b), the only detected phases were zirconium diboride, silicon carbide, and graphite. Also, oxides of zirconium and boron relevant to the ZrB2 powder were detected with poor intensity.
Fig. 1. (a) SEM micrograph and (b) XRD pattern of ZrB2-SiC powder mixture containing 10 vol% Gr (ZSG10).
The values of bulk, theoretical and relative density, open, closed, and total porosity are reported in Table 2. The values of the mean relative density of samples hot-pressed at 1800 °C are displayed in Fig. 2. As can be seen, there are several trends in the densification of the samples (the relative density).
Table 2. The values of bulk, theoretical and relative density, open, closed, and total porosity of hot-pressed composites. Composite
bulk density (g/cm3)
ρ composite (g/cm3)
Relative density (%)
Total porosity (%)
Closed porosity (%)
Open porosity (%)
ZS
5.25±0.03
5.39
97.40±0.52
2.60±0.52
2.09±0.47
0.51±0.25
ZSG2.5
5.18±0.01
5.31
97.68±0.22
2.32±0.22
0.77±1.17
1.56±1.24
ZSG5
5.15±0.01
5.23
98.41±0.20
1.59±0.21
0
1.59±0.21
ZSG7.5
5.16±0.02
5.15
100.14±0.20
0
0
0
ZSG10
5.06±0.03
5.07
99.87±0.56
0.13±0.56
0
0.13±0.56
ZSG15
4.82±0.01
4.92
98.01±0.11
1.99±0.11
0.48±0.90
1.52±0.82
ZSG20
4.68±0.01
4.76
98.35±0.30
1.65±0.30
0.95±0.31
0.70±0.02
ZSG25
4.66±0.05
4.60
101.32±0.98
0
0
0
ZSG30
4.49±0.05
4.45
101.06±1.06
0
0
0
ZSG35
4.14±0.03
4.29
96.48±0.74
3.52±0.74
1.53±0.51
1.99±0.52
Fig. 2. The relative density of hot-pressed ZrB2-SiC-Gr composites at 1800 °C.
At this hot pressing temperature, the relative density of 97.40% was obtained for ZrB2-SiC sample (ZS). The total porosity is 2.6%, where most of the porosities are in closed kind, and
0.51% of the porosities are open. The microstructure of fracture cross-section of the specimen ZS is depicted in Fig. 3. According to Fig. 3, a few closed porosities remained in the specimen ZS. The fracture type at this composite is intergranular, which is due to the poor sintering of the ceramic. As can be seen, the average grain diameter for this sample is about 5 µm which is attributed to the placement of SiC grains at the grains boundaries of ZrB2. Fine SiC particles at the grain boundaries act as a second phase and inhibit the grain boundary movement and thus hinder the grain growth of ZrB2. The residual closed porosities could be the result of oxides on the surface of starting powders like B2O3 and ZrO2. Also, probable agglomeration of submicron SiC particles can cause the formation of closed porosities in the microstructure of the composites [12]. Also, in the previous study, it is conducted that SiC reacts with ZrO2 at the surface of the ZrB2 particles according to reaction 1 [10]. 3SiC + 2 ZrO2 = 2ZrC + 3 SiO (g) + CO (g)
(Reaction
1)
Fig. 3. The microstructure of fracture cross-section of ZrB2- 25 vol% SiC (a:SE and b:BSE modes).
The XRD pattern related to the fracture surface of ZS sample is shown in Fig. 4. Based on this pattern, the detected phases are ZrB2, SiC, ZrO2. Besides the main sharp peaks, tiny peaks of B4C and Zr3C2 were observed in the pattern. B4C could be resulted as a reaction of oxide impurities of ZrO2 and B2O3 with free C of SiC powder according to reaction 2 [25]. Also, Zr3C2 may be formed according to reaction 3. ZrO2 + 2B2O3 + 10C = ZrC + B4C + 8CO (g)
(Reaction 2)
3ZrO2 + 4SiC = Zr3C2 + 2CO + 4SiO
(Reaction 3)
Fig. 4. The XRD pattern related to the fracture surface of the hot-pressed ZS sample.
By the addition of Gr up to 10 vol%, the relative density exhibited a neat increment and fully densified composites achieved for 7.5 and 10 vol% of Gr. The SEM micrograph of the fracture
surface of the composite containing 10 vol% Gr (ZSG10) is demonstrated in Fig.5. As can be seen, the SEM micrograph of this sample shows a fully densified microstructure, and unlike the ZS composite, which showed an intergranular mode of fracture, this composite has a transgranular type of fracture. Based on Table 2, there is about 0.5% porosity at this composites, which is in the closed type. According to the XRD pattern of this sample (Fig. 6), Zr3C2, B4C, and C peaks were detected next to the ZrB2 and SiC phases. Owning to the addition of Gr, and based on Reaction 2, more Zr3C2 and B4C phases formed compared to the ZS sample was detected. Zr3C2 has a hexagonal lattice same as the ZrB2 and Zr3C2 formes at the surface of the ZrB2 particles. SEM micrograph of the polished surface composite containing 10 vol% Gr (ZSG10) is depicted in Fig. 7. There are three kinds of grains in the microstructure of this sample. According to the EDS analysis, dark grains are related to the SiC, gray areas are for ZrB2, and the bright zone is related to the Zr3C2 phase formed via reaction of Gr and SiC with ZrO2 on the surface of ZrB2 grains.
Fig. 5. The fracture surface microstructure of ZrB2-SiC-10 vol% Gr composite (a:SE and b:BSE modes).
Fig. 6. The XRD pattern of the fracture surface of ZrB2-SiC-10 vol% Gr composite (ZSG10).
Fig. 7. The polished surface micrograph of hot-pressed ZrB2-SiC-Gr composite with 10 vol% Gr (a:SE and b:BSE modes).
For more Gr amounts, the relative density descends and for composite containing 35 vol% Gr (ZSG35) 95.75% of theoretical density was achieved. Agglomeration of Gr flakes could be the reason for 4.25% remaining porosity at the specimen. Unlike the ZS sample which most of the porosities are closed one, in this sample (ZSG35) the number of open porosities is more than closed type. The fracture surface microstructure of this sample (ZSG35) is shown in Fig. 8. As can be seen, the Gr flakes have a preferred orientation and orienteered in the direction parallel to the hot pressing direction. For Gr content more than
10 vol% the Gr flacks linked together and oriented perpendicular to the hot pressing direction and a layered microstructure created.
Fig. 8. The microstructure of the fracture surface of ZrB2-SiC-35 vol% Gr composite (a:SE and b:BSE modes).
Fig. 9. The XRD pattern of the fracture surface of ZrB2-SiC-35 vol% Gr composite.
Fig. 10 demonstrates the 3-point flexural strength of ZrB2-SiC-Gr composites hot-pressed at 1800 °C. Because of the dramatical decrease of strength at 15 vol% gr, the flexural strength can be divided into two zones. Zone 1 with Gr content up to 10 vol% and zone 2 with 10-35 vol% Gr. Flexural strength of sample ZS is about 302 MPa, and up to 10 vol% Gr addition improved the flexural strength due to the reduction of grain size and porosities. Also, the formation of Zr3C2 and B4C phase could improve the densification and strength of these ceramics. The topmost value of flexural strength related to the sample ZSG10 and equal to 366 MPa. By surcharge of Gr more than 10 vol%, the flexural strength gets a descending trend and decreases severely. For Gr contents of more than 25 vol%, the flexural strength reaches the values of less than 200 MPa. Weak sinterability of Gr flake and its agglomeration could be the reason for decreasing the flexural strength by increasing Gr.
Fig. 10. The 3-point flexural strength of ZrB2-SiC-Gr composites.
The measured Vickers hardness values for hot-pressed ZrB2-SiC-Gr composites with various Gr content is exhibited in Fig. 11. For sample ZS, the microhardness of 19.47 GPa was achieved, and by doping of 2.5 vol% Gr (ZSG2.5), the Vickers hardness reached the highest value of 20.84 GPa. This increase is due to the effect of Gr on the grain refinement and newly formed ZrC and B4C phases. Because of the soft nature of Gr, by enhancement of Gr vol% the Vickers hardness starts to decrease. For composite containing 35 vol Gr (ZSG35), the Vickers hardness of 2.15 GPa was measured. Fig. 12 shows the SEM
micrograph of the polished surface of composite ZSG35 containing 10 and 35 vol% Gr. The Vickers indentation site is marked which is exactly on the graphite flakes.
Fig. 9. The Vickers hardness values of ZrB2-SiC-Gr composites.
Fig. 10. The polished surface micrograph of hot-pressed ZrB2-SiC-Gr composite with 35 vol% Gr (a:SE and b:BSE modes).
According to the microstructural observations and the trends of mechanical properties such as flexural strength and Vickers hardness, a model is suggested for these composites (Fig. 13). For the Gr contents of less than 10 vol%, the graphite flacks and SiC submicron particles distribute homogenously between ZrB2 particles, which inhibits the ZrB2 grain growth and results in a finegrained microstructure. For the Gr contents more than 10 vol%, the graphite flacks link together to and form chains that surround ZrB2 particles. During the hot-pressing process, these graphite chains are oriented perpendicular to the direction of the hot-press and have a preferred orientation. The linked orientated Gr create a layered microstructure for the ZrB2-based composites [26-30].
Fig. 13. Suggested model for microstructure of hot-pressed composites with different Gr contents.
According to the results obtaind from density measurement and mechanical properties including hardness and flexural strength, there are advantegaes and disadvantages for graphite addition. In ZrB2-SiC composites, addition of grapgite improves the consolidation of composites due to the reduction behavior and nearly full densified composites were obtained. Up to the 10 vol% graphite, the properties of the composites were improved while addition of more than 15 vol% graphite due to the poor sinterability of Gr. Advantage of graphite addition is the reduction of oxide impurities and improving the consolidation of the ZrB2-based composites. Poor
sinterability of the graphite which cause the low hardness and flexural strength of the composites, is the disadvantage of the Gr [31-36]. 4. Conclusions In the present research, ZrB2-25 vol% SiC composites with 0-35 vol% Gr addition were hotpressed at 1800 ºC under 40 MPa of mechanical pressure for 1 h. The consolidation, microstructure, and mechanical behavior of hot-pressed composites were investigated by graphite content. The relative density of composites enhanced by Gr addition and for composites containing 7.5 and 10 vol% Gr, 100% of theoretical density was obtained. The highest flexural strength of 366 MPa was measured for composite containing 7.5 vol% Gr, while the maximum Vickers hardness has resulted for 2.5 vol% Gr, and its value was 20.8 GPa. Phase analysis of hot-pressed samples revealed the formation of the ZrC and B4C phases besides the main existing ZrB2, SiC and Gr phases. Addition of Gr into the ZrB2-SiC composite improved the sintering process, and ZrB2 grains showed a transgranular type of fracture.
Acknowledgment The authors would like to acknowledge the Advanced Ceramic Research Group (ACRG) of the University of Tabriz, Iran, for its helpful support.
References
[1] M. J. Gasch, D. T. Ellerby, S. M. Johnson, "Ultra high temperature ceramic composites," in Handbook of ceramic composites, USA, Springer, 2005, pp. 197-224. [2] W. G. Fahrenholtz, G. E. Hilmas, I. G. Talmy, and J. A. Zaykoski, "Refractory diborides of zirconium and hafnium," Journal of the American Ceramic Society, vol. 90, pp. 1347-1364, 2007. [3] K. Upadhya, J.M. Yang, and W.P. Hoffman, "Materials for ultrahigh temperature structural applications," American Ceramic Society Bulletin, vol. 76, no. 12, pp. 51-56, 1997. [4] S. Q. Guo, "Densification of ZrB2-based composites and their mechanical and physical properties: A review," Journal of the European Ceramic Society, vol. 29, no. 6, p. 995–1011, 2009.
[5] M.M. Mokhayer, M.G. Kakroudi, S.S. Milani, H. Ghiasi, N.P. Vafa, "Investigation of AlN addition on the microstructure and mechanical properties of TiB2 ceramics," Ceramics International, vol. 45, no. 13, pp. 16577-16583, 2019. [6] C. N. Sun, T. Baldridge and M. C. Gupta, "Fabrication of ZrB2–Zr cermet using laser sintering technique," Materials Letters, vol. 63, pp. 2529-2531, 2009. [7] X. Sun, W. Han, Q. Liu, P. Hu and C. Hong, "ZrB2-ceramic toughened by refractory metal Nb prepared by hot-pressing," Materials and Design, vol. 31, pp. 4427-4431, 2010. [8] X. Zhang, W. Li, C. Hong and W. Han, "Microstructure and mechanical properties of ZrB2-based composites reinforced and toughened by zirconia," International Journal of Applied Ceramic Technology, vol. 5, no. 5, pp. 499-504, 2008. [9] N. Pourmohammadie Vafa, Behzad Nayebi, M. Shahedi Asl, M. Jaberi Zamharir, M. Ghassemi Kakroudi, "Reactive hot pressing of ZrB2-based composites with changes in ZrO2/SiC ratio and sintering conditions. Part I: Mechanical behavior," Ceramics International, vol. 42, pp. 2724-2733, 2016. [10] N. Pourmohammadie Vafa, M. Shahedi Asl, M. Jaberi Zamharir, M. Ghassemi Kakroudi, "Reactive hot pressing of ZrB2-based composites with changes in ZrO2/SiC ratio and sintering conditions. Part I: densification behavior," Ceramics International, vol. 41, p. 8388–8396, 2015. [11] M. Jaberi Zamharir, M. Shahedi Asl, N. Pourmohammadie Vafa, M. Ghassemi Kakroudi, "Significance of hot pressing parameters and reinforcement size on densification behavior of ZrB2– 25 vol% SiC UHTCs," Ceramics International, vol. 41, pp. 6439-6447, 2015. [12] M. Jaberi Zamharir, M. Shahedi Asl, M. Ghassemi Kakroudi, N. Pourmohammadie Vafa, M. Jaberi Zamharir, "Significance of hot pressing parameters and reinforcement size on sinterability and mechanical properties of ZrB2–25 vol% SiC UHTCs," Ceramics International, vol. 41, pp. 96289636, 2015. [13] M. Shahedi Asl, M. Ghassemi Kakroudi, "Fractographical assessment of densification mechanisms in hot pressed ZrB2–SiC composites," Ceramics International, vol. 40, p. 15273–15281, 2014. [14] F. Monteverde and A. Bellosi, "Beneficial effect of AlN as sintering aid on microstructure and mechanical properties of hot-pressed ZrB2," Advanced Engineering Materials, vol. 5, no. 7, pp. 508512, 2003. [15] M. Shahedi Asl, B. Nayebi, Z. Ahmadi, P. Pirmohammadi, M. Ghassemi Kakroudi, "Fractographical characterization of hot pressed and pressureless sintered SiAlON-doped ZrB2–SiC composites," Materials Characterization, vol. 102, pp. 137-145, 2015. [16] M. Shahedi Asl, M. Ghassemi Kakroudi, "Characterization of hot-pressed graphene reinforced ZrB2–SiC composite," Materials Science & Engineering A, vol. 625, p. 385–392, 2015. [17] M. Shahedi Asl, M. Ghassemi Kakroudi, R. Abedi Kondolaji, H. Nasiri, "Influence of graphite nanoflakes on densification and mechanical properties of hot-pressed ZrB2–SiC composite," 5843–5851,
vol. 41, p. 5843–5851, 2015. [18] Y. Hou, P. Hu, X. Zhang and K. Gui, "Effects of graphite flake diameter on mechanical properties and thermal shock behavior of ZrB2–nanoSiC–graphite ceramics," International Journal of Refractory Metals and Hard Materials, vol. 41, pp. 133-137, 2013. [19] W. Zhi, W. Zhanjun and S. Guodong., "Fabrication, mechanical properties and thermal shock resistance of a ZrB2–graphite ceramic," International Journal of Refractory Metals and Hard Materials, vol. 29, pp. 351-355, 2011. [20] S. Zhou, Z. Wang, W. Zhang, "Effect of Graphite Flake Orientation on Microstructure and Mechanical Properties," Journal of Alloys and Compounds, vol. 485, p. 181–185, 2009. [21] X. Zhang, Z. Wang, X. Sun, W. Han, C. Hong, "Effect of graphite flake on the mechanical properties of hot pressed ZrB2–SiC ceramics," Materials Letters, vol. 62, p. 4360–4362, 2008. [22] W. Zhi, W. Zhanjun, S. Guodong,, "Fabrication, mechanical properties and thermal shock resistance of a ZrB2–graphite ceramic," Int. J. Refract. Met. Hard Mater., vol. 29, pp. 351-355, 2011. [23] L. Wang, J. Liang, G. Fang, "High temperature fracture behavior of ZrB2–SiC–graphite composite in vacuum and air," Journal of Alloys and Compounds, vol. 619, pp. 145-150, 2015. [24] X. Liu, C. Wei, L. Feng, J. Niu, Z. Yang, "R-curve behavior of laminated ZrB2-SiC/Graphite ceramics in two different directions," Ceramics International, vol. 43, no. 8, pp. 6612-6617, 2017. [25] B. Nayebi, M. Shahedi Asl, M. Ghassemi Kakroudi, I. Farahbakhsh,M. Shokouhimehr, "Interfacial phenomena and formation of nano-particles in porous ZrB2–40 vol% B4C UHTC," Ceramics International, vol. 41, p. 379–387, 2015. [26] M. Ghassemi Kakroudi, M. Dehghanzadeh Alvari, M. Shahedi Asl, N. Pourmohammadie Vafa, T. Rabizadeh, "Hot pressing and oxidation behavior of ZrB2–SiC–TaC composites," Ceramics International, https://doi.org/10.1016/j.ceramint.2019.10.093, 2019. [27] H. Istgaldi, M. Shahedi Asl, P. Shahi, B. Nayebi, Z. Ahmadi, "Solid solution formation during spark plasma sintering of ZrB2–TiC–graphite composites," Ceramics International, https://doi.org/10.1016/j.ceramint.2019.09.287, 2019. [28] M. Shahedi Asl, B. Nayebi, A. Motallebzadeh, M. Shokouhimehr, " Nanoindentation and nanostructural characterization of ZrB2–SiC composite doped with graphite nano-flakes," Composites Part B: Engineering, vol. 175, 107153, 2019. [29] M. Shahedi Asl, B. Nayebi, M. Ghassemi Kakroudi, M. Shokouhimehr, " Investigation of hot pressed ZrB2–SiC–carbon black nanocomposite by scanning and transmission electron microscopy," Ceramics International, vol. 45, p. 16759– 16764, 2019. [30] M. Vajdi, F. Sadegh Moghanlou, E. Ranjbarpour Niari, M. Shahedi Asl, M. Shokouhimehr, "Heat transfer and pressure drop in a ZrB2 microchannel heat sink: A numerical approach," Ceramics International, vol. 46, p. 1730-1735, 2020.
[31] M. Khoeini, A. Nemati, M.Zakeri, M. Shahedi Asl, " Pressureless sintering of ZrB2 ceramics codoped with TiC and graphite," International Journal of Refractory Metals and Hard Materials, vol. 81, p. 189-195, 2019. [32] M. Shahedi Asl, B. Nayebi, M.Shokouhimehr, " TEM characterization of spark plasma sintered ZrB2–SiC–graphene nanocomposite," Ceramics International, vol. 44, p. 15269-15273, 2018. [33] M. Shahedi Asl, M. Jaberi Zamharir, Z. Ahmadi, S. Parvizi, "Effects of nano-graphite content on the characteristics of spark plasma sintered ZrB2–SiC composites," Materials Science and Engineering: A, vol. 716, p. 99-106, 2018. [34] S. Parvizi, Z. Ahmadi, M. Jaberi Zamharir, M. Shahedi Asl, " International Journal of Refractory Metals and Hard Materials, vol. 75, p. 10-17, 2018. [35] M. Shahedi Asl, B. Nayebi, Z. Ahmadi, M. Jaberi Zamharir, M. Shokouhimehr, " Effects of carbon additives on the properties of ZrB2–based composites: A review," Ceramics International, vol. 44, p. 7334-7348, 2018. [36] M. Shahedi Asl, " A statistical approach towards processing optimization of ZrB2–SiC–graphite nanocomposites. Part I: Relative density," Ceramics International, vol. 44, p. 6935-6939, 2018.
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
S0272-8842(19)33575-8
DOI:
https://doi.org/10.1016/j.ceramint.2019.12.086
Reference:
CERI 23739
To appear in:
Ceramics International
Received Date: 15 October 2019 Revised Date:
3 December 2019
Accepted Date: 7 December 2019
Please cite this article as: N.P. Vafa, M.G. Kakroudi, M.S. Asl, Advantages and disadvantages of graphite addition on the characteristics of hot-pressed ZrB2–SiC composites, Ceramics International (2020), doi: https://doi.org/10.1016/j.ceramint.2019.12.086. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
Advantages and disadvantages of graphite addition on the characteristics of hot-pressed ZrB2–SiC composites
Nasser Pourmohammadie Vafa a, Mahdi Ghassemi Kakroudi a∗, Mehdi Shahedi Asl b a b
Department of Materials Science and Engineering, University of Tabriz, Tabriz, Iran
Department of Mechanical Engineering, University of Mohaghegh Ardabili, Ardabil, Iran ∗
Corresponding author: M. Ghassemi Kakroudi ([email protected])
Abstract ZrB2–SiC–graphite composites with 0-35 vol% graphite flakes were densified via hot-pressing route at the temperature of 1800 ºC under the uniaxial pressure of 40 MPa for 1 hour. Consolidation, mechanical properties, and microstructure of hot-pressed composites were investigated by variation of graphite content. By the addition of graphite, the relative density of composites increased, and at this hot pressing condition, fully densified composites were fabricated. The highest flexural strength of 366 MPa was measured for composite containing 7.5 vol% graphite, while the maximum Vickers hardness resulted in 2.5 vol% graphite doped one, and its value was equal to 20.8 GPa. Phase analysis of hot-pressed samples revealed the formation of the Zr3C2 and B4C phases besides the main existing ZrB2, SiC, and graphite phases. The newly carbide phases formed at the surface of ZrB2 grains. The addition of graphite into the ZrB2–SiC composites improved the sintering process and caused a fine-grained microstructure.
Keywords: ZrB2; SiC; Graphite; Mechanical properties; Microstructure.
1. Introduction In recent decades, due to the need for advanced high-temperature engineering materials, much attention has been paid to the ultra-high temperature ceramics (UHTCs). As a member of UHTCs, zirconium diboride due to the combination of properties like the high melting point of 3245 °C, chemical inertness, good electrical and thermal conductivity, and high hardness, is an excellent candidate for applications such as high-performance aircraft and hypersonic flight vehicles. Intense covalent bounding and poor self-diffusion coefficient at the grain boundaries persuade the ZrB2 ceramics to densify at extremely high temperatures alongside an external pressure. Presence of ZrO2 and B2O3 on the surface of powders as oxide impurities, impede the consolidation and causes the grain growth. Hence, to densify these ceramics, sintering techniques along with an applied pressure such as spark plasma sintering or hot pressing, and sintering aids are required [1-5]. Different kind of sintering aids are studied for densification of ZrB2 such as metals [6,7], oxides [8-10], carbides [11-13], nitrides [14,15] and carbon (with various morphologies) [16-19]. Silicon carbide (SiC) is mostly used as an additive and reinforcement for ZrB2-based composites, which eliminates the oxide impurities and hinders the extraordinary grain growth of ZrB2 [1,4]. Up to now, many research has been conducted for the betterment of the densification and flexural strength of the ZrB2-based ceramics. Nowadays, the main challenge about ZrB2-based composites is the amendment of the fracture toughness of these ceramics to manufacture machinable pieces. It has been reported that the addition of graphite flakes because of layered microstructure could improve the toughness of the ZrB2-SiC composites, while it reduces the flexural strength of the composites [20,21]. Hou et al. [18] investigated the influence of the diameter of graphite flakes on the mechanical behavior of ZrB2based ceramics reinforced with nano-sized SiC. Zhou et al. [20] hot pressed ZrB2-SiC-Gr composites with 15 vol% graphite flakes at 1900 ºC. In addition to grain refinement and densification of the composites, they reported that orientation of the Gr flakes causes the anisotropic mechanical behavior like fracture toughness and flexural strength except for hardness. Gr flakes prefer to orientate in a direction that their base planes stand perpendicular to the direction of hot-press. This preferred orientation creates an anisotropic behavior in the fracture toughness and flexural strength of the composites.
Zhi et al. [22] investigated the influence of graphite addition on the hot pressing of ZrB2– graphite ceramics. They found that by addition of graphite, the resistance to the thermal shock and the fracture toughness were improved remarkably, while the flexural strength and microhardness of the composites scaled down. Wang et al. [23] studied the fracture toughness of hot-pressed ZrB2–SiC composites containing Gr, up to the temperature of 1800 °C in a vacuum atmosphere and up to 1300 °C in the air. In vacuum, fracture toughness decreased linearly with the temperature up to 1300 °C, then from 1300 °C to 1600 °C increased, and reduced rapidly at 1800 °C. Also, results revealed that the temperature for the brittle-ductile transition of ZrB2– SiC–Gr composite is 1300 °C. Liu et al. [24] fabricated ZrB2-SiC/Graphite ceramics in a laminated structure by tap-casting and hot-pressed it. They reported that the flexural strengths were 382 MPa in the parallel direction and 429 MPa in the perpendicular direction. The values for indentation strength at first decreased and then by increasing the indentation load increased. Many studies have been conducted for the graphite addition up to 10 vol% in ZrB2 based composites. In the present study, which is the first part of the machinable ZrB2-SiC-Gr project, the graphite flakes addition on the hot pressing of the ZrB2-SiC-Gr composites was studied. Therefore, 0-35 vol% Gr was added into the ZrB2-25 vol% SiC ceramics and consolidation, microstructure, flexural strength, and Vickers hardness of hot-pressed ZrB2-SiC composites was studied.
2. Experimental procedure The starting powders used to prepare ZrB2-SiC-Gr composites were, commercially ZrB2 powder (mean particle size: < 2 µm, purity: 99%, Leung Hi-tech Co., China), α-SiC powder (mean particle size: 0.7 µm, purity: > 99%, Carborundum Universal Limited, China) and Graphite flakes (mean diameter: < 5 µm, thickness: < 0.5 µm, purity: > 98%, Pack Sang Scrap Co, India). To get the powder mixture with composition of ZrB2–SiC containing 0-35 vol% Gr, the starting powders were weighed in an appropriate ratio. In all the composites the ZrB2 to SiC volume ratio was 75:25. The composition of the ZrB2-SiC-Gr composites has been shown in Table 1. At first, to intercept the particles from adhesion and agglomeration, the SiC powder and Gr flakes were dispersed for 30 min in ethanol, separately, by an ultrasonic agitator (Mercury, Turkey). Then
ZrB2 powder was added to the SiC and Gr suspension. The obtained slurry homogeneously mixed in a polyethylene cup with ZrO2 balls in ethanol media for 60 min with 200 rpm. Then the obtained slurries were dried for 8 h in an evaporator (Alfa D500, Iran) at the temperature of 110 ºC. After drying, the resultant powder mixtures were screened with mesh 100. The composites were densified in a rectangular graphite mold with the dimension of 10×25 mm, which was sealed with graphite sheets as a barrier to sticking of the sample to the mold. Powder mixtures whit designated weights were charged into the die to get the samples with the final thickness of 10 mm after hot pressing. The composites were hot-pressed in a vacuum chamber (Shenyang Weitai Science & Technology Development Co., Ltd., China) with a pressure of 5×10-2 Pa. The heating rate up to the temperature of 1000 °C was 10 °C/min. For the removal of volatiles, the samples were held for 15 min at 1000 °C. Afterward, the chamber was heated again up to the final temperature of the hot pressing process, which was 1800 °C, at the same rate of heating. At the temperature of 1800 °C, a 40 MPa uniaxial pressure was loaded on the sample and remained for 60 min. In the end, the chamber was chilled to the 300 °C by water circulation and then naturally up to the room temperature. Finally, the hot-pressed samples were removed and grinded to separate the adhesive graphite sheets. An automatic polisher system (Sanat Ceram, Iran) was used to polish the surface of the samples with diamond paste. Phase analysis, morphology and particle distribution of the raw materials and prepared powder mixtures, were done by XRD patterns (λ = 1.54 Å, Cu lamp, 40 kV, 30 mA, Philips PW1730) and SEM (Mira3 Tescan, Czech Republic). The bulk density of the specimens was measured according to the ASTM B328 by the Archimedes method. The theoretical density of composites was calculated based on the role of mixtures, where densities of ZrB2, SiC, and Gr were considered as 6.11 g/cm3 [1], 3.21 g/cm3 [1] and 2.16 g/cm3 [20] respectively. Samples were cut into specimens with dimensions of 3×4×25 mm for the 3-point bending test. 3-point flexural strength of the composites was measured using a universal testing machine (Zwick Roell SP600, Germany) with a span of 20 mm and a loading rate of 0.05 mm/min. The flexural strength of the composites was evaluated according to the Eq. 1:
=
(1)
Where
shows the flexural strength in MPa, P is the force in N, l, b, and h are the span, width,
and thickness of the specimen in mm. Vickers microhardness of the samples evaluated by employing a Vickers indentor (Zwick Roell, Germany) on the polished surfaces of the samples with a load of 9.8 N for 15 s. X-ray diffraction with a step size and detecting time of 0.05 and 1 s, respectively, was used for phase analysis of the hot-pressed samples. Xpert high score plus software (2.2b (2.2.2)) was used for the analysis of the resulted patterns. The polished surface and fracture cross-section of the composites were investigated utilizing scanning electron microscopy with the beneficiary of EDS (Digital X-Ray Processor, DXP-X10P).
Table 1. The composition of the ZrB2-SiC-Gr composites.
Composite
vol% ZrB2
vol% SiC
vol% Gr
ZS
75.00
25.00
0.00
ZSG2.5
73.13
24.38
2.50
ZSG5
71.25
23.75
5.00
ZSG7.5
69.38
23.13
7.50
ZSG10
67.50
22.50
10.00
ZSG15
63.75
21.25
15.00
ASG20
60.00
20.00
20.00
ZSG25
56.25
18.75
25.00
ZSG30
52.50
17.50
30.00
ZSG35
48.75
16.25
35.00
3. Results and discussion The phase analysis of as-received ZrB2, SiC powders, and Gr flakes (not shown here) indicated the presence of lonely hexagonal ZrB2, SiC, and Graphite. It should be mentioned that there are little peaks of zirconium oxide and boron oxide beside the sharp peaks of ZrB2 in the XRD pattern of ZrB2 due to the existence of oxide impurities on the surface of the starting powders. SEM micrographs of starting powders (not shown here) emphasized the particle size of the powders as supplier’s reports. The SEM micrograph and XRD pattern of ZrB2-SiC powder mixture containing 10 vol% Gr is represented in Fig. 1. As can be seen in Fig. 1a, the fine SiC
particles surrounded the ZrB2 particles, and Gr flakes are distributed among ZrB2 particles. According to the XRD pattern (Fig. 1b), the only detected phases were zirconium diboride, silicon carbide, and graphite. Also, oxides of zirconium and boron relevant to the ZrB2 powder were detected with poor intensity.
Fig. 1. (a) SEM micrograph and (b) XRD pattern of ZrB2-SiC powder mixture containing 10 vol% Gr (ZSG10).
The values of bulk, theoretical and relative density, open, closed, and total porosity are reported in Table 2. The values of the mean relative density of samples hot-pressed at 1800 °C are displayed in Fig. 2. As can be seen, there are several trends in the densification of the samples (the relative density).
Table 2. The values of bulk, theoretical and relative density, open, closed, and total porosity of hot-pressed composites. Composite
bulk density (g/cm3)
ρ composite (g/cm3)
Relative density (%)
Total porosity (%)
Closed porosity (%)
Open porosity (%)
ZS
5.25±0.03
5.39
97.40±0.52
2.60±0.52
2.09±0.47
0.51±0.25
ZSG2.5
5.18±0.01
5.31
97.68±0.22
2.32±0.22
0.77±1.17
1.56±1.24
ZSG5
5.15±0.01
5.23
98.41±0.20
1.59±0.21
0
1.59±0.21
ZSG7.5
5.16±0.02
5.15
100.14±0.20
0
0
0
ZSG10
5.06±0.03
5.07
99.87±0.56
0.13±0.56
0
0.13±0.56
ZSG15
4.82±0.01
4.92
98.01±0.11
1.99±0.11
0.48±0.90
1.52±0.82
ZSG20
4.68±0.01
4.76
98.35±0.30
1.65±0.30
0.95±0.31
0.70±0.02
ZSG25
4.66±0.05
4.60
101.32±0.98
0
0
0
ZSG30
4.49±0.05
4.45
101.06±1.06
0
0
0
ZSG35
4.14±0.03
4.29
96.48±0.74
3.52±0.74
1.53±0.51
1.99±0.52
Fig. 2. The relative density of hot-pressed ZrB2-SiC-Gr composites at 1800 °C.
At this hot pressing temperature, the relative density of 97.40% was obtained for ZrB2-SiC sample (ZS). The total porosity is 2.6%, where most of the porosities are in closed kind, and
0.51% of the porosities are open. The microstructure of fracture cross-section of the specimen ZS is depicted in Fig. 3. According to Fig. 3, a few closed porosities remained in the specimen ZS. The fracture type at this composite is intergranular, which is due to the poor sintering of the ceramic. As can be seen, the average grain diameter for this sample is about 5 µm which is attributed to the placement of SiC grains at the grains boundaries of ZrB2. Fine SiC particles at the grain boundaries act as a second phase and inhibit the grain boundary movement and thus hinder the grain growth of ZrB2. The residual closed porosities could be the result of oxides on the surface of starting powders like B2O3 and ZrO2. Also, probable agglomeration of submicron SiC particles can cause the formation of closed porosities in the microstructure of the composites [12]. Also, in the previous study, it is conducted that SiC reacts with ZrO2 at the surface of the ZrB2 particles according to reaction 1 [10]. 3SiC + 2 ZrO2 = 2ZrC + 3 SiO (g) + CO (g)
(Reaction
1)
Fig. 3. The microstructure of fracture cross-section of ZrB2- 25 vol% SiC (a:SE and b:BSE modes).
The XRD pattern related to the fracture surface of ZS sample is shown in Fig. 4. Based on this pattern, the detected phases are ZrB2, SiC, ZrO2. Besides the main sharp peaks, tiny peaks of B4C and Zr3C2 were observed in the pattern. B4C could be resulted as a reaction of oxide impurities of ZrO2 and B2O3 with free C of SiC powder according to reaction 2 [25]. Also, Zr3C2 may be formed according to reaction 3. ZrO2 + 2B2O3 + 10C = ZrC + B4C + 8CO (g)
(Reaction 2)
3ZrO2 + 4SiC = Zr3C2 + 2CO + 4SiO
(Reaction 3)
Fig. 4. The XRD pattern related to the fracture surface of the hot-pressed ZS sample.
By the addition of Gr up to 10 vol%, the relative density exhibited a neat increment and fully densified composites achieved for 7.5 and 10 vol% of Gr. The SEM micrograph of the fracture
surface of the composite containing 10 vol% Gr (ZSG10) is demonstrated in Fig.5. As can be seen, the SEM micrograph of this sample shows a fully densified microstructure, and unlike the ZS composite, which showed an intergranular mode of fracture, this composite has a transgranular type of fracture. Based on Table 2, there is about 0.5% porosity at this composites, which is in the closed type. According to the XRD pattern of this sample (Fig. 6), Zr3C2, B4C, and C peaks were detected next to the ZrB2 and SiC phases. Owning to the addition of Gr, and based on Reaction 2, more Zr3C2 and B4C phases formed compared to the ZS sample was detected. Zr3C2 has a hexagonal lattice same as the ZrB2 and Zr3C2 formes at the surface of the ZrB2 particles. SEM micrograph of the polished surface composite containing 10 vol% Gr (ZSG10) is depicted in Fig. 7. There are three kinds of grains in the microstructure of this sample. According to the EDS analysis, dark grains are related to the SiC, gray areas are for ZrB2, and the bright zone is related to the Zr3C2 phase formed via reaction of Gr and SiC with ZrO2 on the surface of ZrB2 grains.
Fig. 5. The fracture surface microstructure of ZrB2-SiC-10 vol% Gr composite (a:SE and b:BSE modes).
Fig. 6. The XRD pattern of the fracture surface of ZrB2-SiC-10 vol% Gr composite (ZSG10).
Fig. 7. The polished surface micrograph of hot-pressed ZrB2-SiC-Gr composite with 10 vol% Gr (a:SE and b:BSE modes).
For more Gr amounts, the relative density descends and for composite containing 35 vol% Gr (ZSG35) 95.75% of theoretical density was achieved. Agglomeration of Gr flakes could be the reason for 4.25% remaining porosity at the specimen. Unlike the ZS sample which most of the porosities are closed one, in this sample (ZSG35) the number of open porosities is more than closed type. The fracture surface microstructure of this sample (ZSG35) is shown in Fig. 8. As can be seen, the Gr flakes have a preferred orientation and orienteered in the direction parallel to the hot pressing direction. For Gr content more than
10 vol% the Gr flacks linked together and oriented perpendicular to the hot pressing direction and a layered microstructure created.
Fig. 8. The microstructure of the fracture surface of ZrB2-SiC-35 vol% Gr composite (a:SE and b:BSE modes).
Fig. 9. The XRD pattern of the fracture surface of ZrB2-SiC-35 vol% Gr composite.
Fig. 10 demonstrates the 3-point flexural strength of ZrB2-SiC-Gr composites hot-pressed at 1800 °C. Because of the dramatical decrease of strength at 15 vol% gr, the flexural strength can be divided into two zones. Zone 1 with Gr content up to 10 vol% and zone 2 with 10-35 vol% Gr. Flexural strength of sample ZS is about 302 MPa, and up to 10 vol% Gr addition improved the flexural strength due to the reduction of grain size and porosities. Also, the formation of Zr3C2 and B4C phase could improve the densification and strength of these ceramics. The topmost value of flexural strength related to the sample ZSG10 and equal to 366 MPa. By surcharge of Gr more than 10 vol%, the flexural strength gets a descending trend and decreases severely. For Gr contents of more than 25 vol%, the flexural strength reaches the values of less than 200 MPa. Weak sinterability of Gr flake and its agglomeration could be the reason for decreasing the flexural strength by increasing Gr.
Fig. 10. The 3-point flexural strength of ZrB2-SiC-Gr composites.
The measured Vickers hardness values for hot-pressed ZrB2-SiC-Gr composites with various Gr content is exhibited in Fig. 11. For sample ZS, the microhardness of 19.47 GPa was achieved, and by doping of 2.5 vol% Gr (ZSG2.5), the Vickers hardness reached the highest value of 20.84 GPa. This increase is due to the effect of Gr on the grain refinement and newly formed ZrC and B4C phases. Because of the soft nature of Gr, by enhancement of Gr vol% the Vickers hardness starts to decrease. For composite containing 35 vol Gr (ZSG35), the Vickers hardness of 2.15 GPa was measured. Fig. 12 shows the SEM
micrograph of the polished surface of composite ZSG35 containing 10 and 35 vol% Gr. The Vickers indentation site is marked which is exactly on the graphite flakes.
Fig. 9. The Vickers hardness values of ZrB2-SiC-Gr composites.
Fig. 10. The polished surface micrograph of hot-pressed ZrB2-SiC-Gr composite with 35 vol% Gr (a:SE and b:BSE modes).
According to the microstructural observations and the trends of mechanical properties such as flexural strength and Vickers hardness, a model is suggested for these composites (Fig. 13). For the Gr contents of less than 10 vol%, the graphite flacks and SiC submicron particles distribute homogenously between ZrB2 particles, which inhibits the ZrB2 grain growth and results in a finegrained microstructure. For the Gr contents more than 10 vol%, the graphite flacks link together to and form chains that surround ZrB2 particles. During the hot-pressing process, these graphite chains are oriented perpendicular to the direction of the hot-press and have a preferred orientation. The linked orientated Gr create a layered microstructure for the ZrB2-based composites [26-30].
Fig. 13. Suggested model for microstructure of hot-pressed composites with different Gr contents.
According to the results obtaind from density measurement and mechanical properties including hardness and flexural strength, there are advantegaes and disadvantages for graphite addition. In ZrB2-SiC composites, addition of grapgite improves the consolidation of composites due to the reduction behavior and nearly full densified composites were obtained. Up to the 10 vol% graphite, the properties of the composites were improved while addition of more than 15 vol% graphite due to the poor sinterability of Gr. Advantage of graphite addition is the reduction of oxide impurities and improving the consolidation of the ZrB2-based composites. Poor
sinterability of the graphite which cause the low hardness and flexural strength of the composites, is the disadvantage of the Gr [31-36]. 4. Conclusions In the present research, ZrB2-25 vol% SiC composites with 0-35 vol% Gr addition were hotpressed at 1800 ºC under 40 MPa of mechanical pressure for 1 h. The consolidation, microstructure, and mechanical behavior of hot-pressed composites were investigated by graphite content. The relative density of composites enhanced by Gr addition and for composites containing 7.5 and 10 vol% Gr, 100% of theoretical density was obtained. The highest flexural strength of 366 MPa was measured for composite containing 7.5 vol% Gr, while the maximum Vickers hardness has resulted for 2.5 vol% Gr, and its value was 20.8 GPa. Phase analysis of hot-pressed samples revealed the formation of the ZrC and B4C phases besides the main existing ZrB2, SiC and Gr phases. Addition of Gr into the ZrB2-SiC composite improved the sintering process, and ZrB2 grains showed a transgranular type of fracture.
Acknowledgment The authors would like to acknowledge the Advanced Ceramic Research Group (ACRG) of the University of Tabriz, Iran, for its helpful support.
References
[1] M. J. Gasch, D. T. Ellerby, S. M. Johnson, "Ultra high temperature ceramic composites," in Handbook of ceramic composites, USA, Springer, 2005, pp. 197-224. [2] W. G. Fahrenholtz, G. E. Hilmas, I. G. Talmy, and J. A. Zaykoski, "Refractory diborides of zirconium and hafnium," Journal of the American Ceramic Society, vol. 90, pp. 1347-1364, 2007. [3] K. Upadhya, J.M. Yang, and W.P. Hoffman, "Materials for ultrahigh temperature structural applications," American Ceramic Society Bulletin, vol. 76, no. 12, pp. 51-56, 1997. [4] S. Q. Guo, "Densification of ZrB2-based composites and their mechanical and physical properties: A review," Journal of the European Ceramic Society, vol. 29, no. 6, p. 995–1011, 2009.
[5] M.M. Mokhayer, M.G. Kakroudi, S.S. Milani, H. Ghiasi, N.P. Vafa, "Investigation of AlN addition on the microstructure and mechanical properties of TiB2 ceramics," Ceramics International, vol. 45, no. 13, pp. 16577-16583, 2019. [6] C. N. Sun, T. Baldridge and M. C. Gupta, "Fabrication of ZrB2–Zr cermet using laser sintering technique," Materials Letters, vol. 63, pp. 2529-2531, 2009. [7] X. Sun, W. Han, Q. Liu, P. Hu and C. Hong, "ZrB2-ceramic toughened by refractory metal Nb prepared by hot-pressing," Materials and Design, vol. 31, pp. 4427-4431, 2010. [8] X. Zhang, W. Li, C. Hong and W. Han, "Microstructure and mechanical properties of ZrB2-based composites reinforced and toughened by zirconia," International Journal of Applied Ceramic Technology, vol. 5, no. 5, pp. 499-504, 2008. [9] N. Pourmohammadie Vafa, Behzad Nayebi, M. Shahedi Asl, M. Jaberi Zamharir, M. Ghassemi Kakroudi, "Reactive hot pressing of ZrB2-based composites with changes in ZrO2/SiC ratio and sintering conditions. Part I: Mechanical behavior," Ceramics International, vol. 42, pp. 2724-2733, 2016. [10] N. Pourmohammadie Vafa, M. Shahedi Asl, M. Jaberi Zamharir, M. Ghassemi Kakroudi, "Reactive hot pressing of ZrB2-based composites with changes in ZrO2/SiC ratio and sintering conditions. Part I: densification behavior," Ceramics International, vol. 41, p. 8388–8396, 2015. [11] M. Jaberi Zamharir, M. Shahedi Asl, N. Pourmohammadie Vafa, M. Ghassemi Kakroudi, "Significance of hot pressing parameters and reinforcement size on densification behavior of ZrB2– 25 vol% SiC UHTCs," Ceramics International, vol. 41, pp. 6439-6447, 2015. [12] M. Jaberi Zamharir, M. Shahedi Asl, M. Ghassemi Kakroudi, N. Pourmohammadie Vafa, M. Jaberi Zamharir, "Significance of hot pressing parameters and reinforcement size on sinterability and mechanical properties of ZrB2–25 vol% SiC UHTCs," Ceramics International, vol. 41, pp. 96289636, 2015. [13] M. Shahedi Asl, M. Ghassemi Kakroudi, "Fractographical assessment of densification mechanisms in hot pressed ZrB2–SiC composites," Ceramics International, vol. 40, p. 15273–15281, 2014. [14] F. Monteverde and A. Bellosi, "Beneficial effect of AlN as sintering aid on microstructure and mechanical properties of hot-pressed ZrB2," Advanced Engineering Materials, vol. 5, no. 7, pp. 508512, 2003. [15] M. Shahedi Asl, B. Nayebi, Z. Ahmadi, P. Pirmohammadi, M. Ghassemi Kakroudi, "Fractographical characterization of hot pressed and pressureless sintered SiAlON-doped ZrB2–SiC composites," Materials Characterization, vol. 102, pp. 137-145, 2015. [16] M. Shahedi Asl, M. Ghassemi Kakroudi, "Characterization of hot-pressed graphene reinforced ZrB2–SiC composite," Materials Science & Engineering A, vol. 625, p. 385–392, 2015. [17] M. Shahedi Asl, M. Ghassemi Kakroudi, R. Abedi Kondolaji, H. Nasiri, "Influence of graphite nanoflakes on densification and mechanical properties of hot-pressed ZrB2–SiC composite," 5843–5851,
vol. 41, p. 5843–5851, 2015. [18] Y. Hou, P. Hu, X. Zhang and K. Gui, "Effects of graphite flake diameter on mechanical properties and thermal shock behavior of ZrB2–nanoSiC–graphite ceramics," International Journal of Refractory Metals and Hard Materials, vol. 41, pp. 133-137, 2013. [19] W. Zhi, W. Zhanjun and S. Guodong., "Fabrication, mechanical properties and thermal shock resistance of a ZrB2–graphite ceramic," International Journal of Refractory Metals and Hard Materials, vol. 29, pp. 351-355, 2011. [20] S. Zhou, Z. Wang, W. Zhang, "Effect of Graphite Flake Orientation on Microstructure and Mechanical Properties," Journal of Alloys and Compounds, vol. 485, p. 181–185, 2009. [21] X. Zhang, Z. Wang, X. Sun, W. Han, C. Hong, "Effect of graphite flake on the mechanical properties of hot pressed ZrB2–SiC ceramics," Materials Letters, vol. 62, p. 4360–4362, 2008. [22] W. Zhi, W. Zhanjun, S. Guodong,, "Fabrication, mechanical properties and thermal shock resistance of a ZrB2–graphite ceramic," Int. J. Refract. Met. Hard Mater., vol. 29, pp. 351-355, 2011. [23] L. Wang, J. Liang, G. Fang, "High temperature fracture behavior of ZrB2–SiC–graphite composite in vacuum and air," Journal of Alloys and Compounds, vol. 619, pp. 145-150, 2015. [24] X. Liu, C. Wei, L. Feng, J. Niu, Z. Yang, "R-curve behavior of laminated ZrB2-SiC/Graphite ceramics in two different directions," Ceramics International, vol. 43, no. 8, pp. 6612-6617, 2017. [25] B. Nayebi, M. Shahedi Asl, M. Ghassemi Kakroudi, I. Farahbakhsh,M. Shokouhimehr, "Interfacial phenomena and formation of nano-particles in porous ZrB2–40 vol% B4C UHTC," Ceramics International, vol. 41, p. 379–387, 2015. [26] M. Ghassemi Kakroudi, M. Dehghanzadeh Alvari, M. Shahedi Asl, N. Pourmohammadie Vafa, T. Rabizadeh, "Hot pressing and oxidation behavior of ZrB2–SiC–TaC composites," Ceramics International, https://doi.org/10.1016/j.ceramint.2019.10.093, 2019. [27] H. Istgaldi, M. Shahedi Asl, P. Shahi, B. Nayebi, Z. Ahmadi, "Solid solution formation during spark plasma sintering of ZrB2–TiC–graphite composites," Ceramics International, https://doi.org/10.1016/j.ceramint.2019.09.287, 2019. [28] M. Shahedi Asl, B. Nayebi, A. Motallebzadeh, M. Shokouhimehr, " Nanoindentation and nanostructural characterization of ZrB2–SiC composite doped with graphite nano-flakes," Composites Part B: Engineering, vol. 175, 107153, 2019. [29] M. Shahedi Asl, B. Nayebi, M. Ghassemi Kakroudi, M. Shokouhimehr, " Investigation of hot pressed ZrB2–SiC–carbon black nanocomposite by scanning and transmission electron microscopy," Ceramics International, vol. 45, p. 16759– 16764, 2019. [30] M. Vajdi, F. Sadegh Moghanlou, E. Ranjbarpour Niari, M. Shahedi Asl, M. Shokouhimehr, "Heat transfer and pressure drop in a ZrB2 microchannel heat sink: A numerical approach," Ceramics International, vol. 46, p. 1730-1735, 2020.
[31] M. Khoeini, A. Nemati, M.Zakeri, M. Shahedi Asl, " Pressureless sintering of ZrB2 ceramics codoped with TiC and graphite," International Journal of Refractory Metals and Hard Materials, vol. 81, p. 189-195, 2019. [32] M. Shahedi Asl, B. Nayebi, M.Shokouhimehr, " TEM characterization of spark plasma sintered ZrB2–SiC–graphene nanocomposite," Ceramics International, vol. 44, p. 15269-15273, 2018. [33] M. Shahedi Asl, M. Jaberi Zamharir, Z. Ahmadi, S. Parvizi, "Effects of nano-graphite content on the characteristics of spark plasma sintered ZrB2–SiC composites," Materials Science and Engineering: A, vol. 716, p. 99-106, 2018. [34] S. Parvizi, Z. Ahmadi, M. Jaberi Zamharir, M. Shahedi Asl, " International Journal of Refractory Metals and Hard Materials, vol. 75, p. 10-17, 2018. [35] M. Shahedi Asl, B. Nayebi, Z. Ahmadi, M. Jaberi Zamharir, M. Shokouhimehr, " Effects of carbon additives on the properties of ZrB2–based composites: A review," Ceramics International, vol. 44, p. 7334-7348, 2018. [36] M. Shahedi Asl, " A statistical approach towards processing optimization of ZrB2–SiC–graphite nanocomposites. Part I: Relative density," Ceramics International, vol. 44, p. 6935-6939, 2018.
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: