- Email: [email protected]
Fracture toughness and hardness investigation in ZrB2–SiC–ZrC composite
Journal Pre-proof Fracture toughness and hardness investigation in ZrB2–SiC–ZrC composite Arsalan Rezapour, Zohre Balak PII:
S0254-0584(19)31099-5
DOI:
https://doi.org/10.1016/j.matchemphys.2019.122284
Reference:
MAC 122284
To appear in:
Materials Chemistry and Physics
Received Date: 11 April 2019 Accepted Date: 7 October 2019
Please cite this article as: A. Rezapour, Z. Balak, Fracture toughness and hardness investigation in ZrB2–SiC–ZrC composite, Materials Chemistry and Physics (2019), doi: https://doi.org/10.1016/ j.matchemphys.2019.122284. 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 B.V.
Fracture toughness and hardness investigation in ZrB2-SiC-ZrC composite Arsalan rezapour, Zohre Balak* Corresponding Author Email : [email protected] (Zohre Balak)
Department of Materials Science and Engineering, Ahvaz Branch, Islamic Azad University, Ahvaz, Iran.
Abstract Goal of this research is survey the effect of ZrC on fracture toughness and hardness of ZrB230vol%SiC. So, ZrB2-30vol%SiC composites with different amount ZrC (4, 8 and 12 vol %) were SPSed at 1800 °C, for time of 9 minutes and under the pressure of 30 MPa. Microstructural evaluations were done by scanning electron microscopic (SEM). Densification was measured by Arashmidous method. Fracture toughness and hardness were determined by Indentation method and Macro-Vickers respectively. With increase in ZrC amount, SST rises from 1263 °C to 1345 °C and 1389 °C. Increase in ZrC addition up to 8 vol% resulted accompanied with hardness ascent nealy 5 GPa but in higher amount of 12 vol%, aslight decrease was happened.It was cleared with ZrC ascent up to 8vol%, fracture toughness was enhanced from 4.8 MPa m0.5 up to 6.6 MPa m0.5, but with more addition up to 12 vol%, fracture toughness was decreased to 5.9 MPa m0.5. Keywords: Fracture toughness, hardness, ZrB2-30vol%SiC, SPS
1. Introduction It is well-known that the ZrB2-SiC-based composites are the most promising materials for the application in thermal protection systems and high temperature components of future hypersonic aircraft or reentry vehicles due to their high temperature melting and their excellent comprehensive characteristics [1, 2]. Although, similar to the other ceramics materials, their inherent brittleness was still as major obstacle and terminated their wide application. Hence, different groups of researcher [3-19] afforded to enhance the fracture toughness. Z. balak et al. [3] were applied Taguchi experimental design technique to determine the most influential additives and SPS parameters. They examined nine factors (SiC, Cf, MoSi2, HfB2 and ZrC content, milling time of Cf and SPS parameters such as temperature, time and pressure) on four different levels in order to obtain the optimum mixture. The results showed that temperature with 34.7% and SiC with 29.7% have significant effect on fracture toughness. Also, they studied [5] the effect of nano-SiC particles on fracture toughness. Investigation the SEM images of the Vickers indent and their path propagation showed that the deviation and branching of crack are the most important toughening mechanisms in ZrB2-SiC nanocomposites. Studies [8] on SCFs showed increase SCFs from 10 to 40 vol% improves the fracture toughness from 4.1 to 5.1 MPam0.5 by promoting toughening mechanisms. 1
Carbon additives (such as carbon black, graphite, graphene, carbon fiber and carbon nano tube) incorporated in ZrB2 composites were investigated by M. Shahedi et al. [4,6,7,12, 19]. Typically, they fabricated a hot pressed ZrB2–based composite reinforced with 20 vol% SiC and doped with 10 vol% nano–sized carbon black (~15 nm) at 1850 °C under 20 MPa pressure [19]. The addition of nano–sized powder carbon black with porous structure to ZrB2–SiC composite meaningfully led to the manufacture of fully correspondence composites. Also, ZrB2 and ZrB2–20 vol%SiC–10 vol% CNTs composites were hot pressed by Shahedi Asl and co–workers to enhance its fracture toughness [15]. Embedding CNTs into the designed ceramics enhanced the densification (by 4%) and indentation fracture toughness (increased up to ~5.1 MPam0.5). Other researches in Italy [14-16, 18] focused on embedding continuous carbon fiber (CCFs) by applying a combination method of vacuum-bag infiltration and conventional hot pressing without the help of expensive techniques such as CVI or PIP. For example, D. Sciti et al. [14] were infiltrated the homemade 1D SiC preforms by UHTC slurries based on ZrB2. Considerations the all mechanical properties (densification, hardness, flexural strength and fracture toughness) resulted to choose the ZrC as addition in this research among all of them mentioned above. Although carbon additives such as SCFs improves the fracture toughness but frequently accompanied with a loss in flexural strength [20] while ZrC not only improves the flexural strength but also expect to enhance the fracture toughness by acts as grain growth inhibitor. So, besides the SPS parameters, it was selected as variable in this research to improve the fracture toughness. 2. Experimental Commercially available ZrB2 (particle size of 20 μm, purity: 99.5%, Northwest Institute for Non-Ferrous Metal Research, China), SiC ((particle size of 15 μm, purity: 98.7%, Northwest Institute for Non-Ferrous Metal Research, China) and ZrC (particle size of 20 μm, Alfa acer company) powders were used as starting materials. Three composites with different compositions of ZrB2–20 vol% SiC-4 vol% ZrC, ZrB2–20 vol% SiC-8 vol% ZrC and ZrB2–20 vol% SiC-12 vol% ZrC, were fabricated to survey the ZrC effect according to table 1. So, the initial powders were weighed and mixed through highenergy ball milling in ethanol media for 3 hours. Then, each batch was dried on a hot plate stirrer (Heidolph MR 3001 K, Germany) at 150 °C for 120 min to remove the ethanol. Each powder mixture was loaded in a graphite die directly (inner diameter: 40 mm), lined with a graphite foil. The sintering process was carried out using a spark plasma sintering machine (SPS-20T-10, China, Vacuum: 15 Pa) at different sintering parameters which were presented in table 1.
Table. 1. Chemical composition and sintering conditions of all composites. Sample Code
ZrC Vol%
Temperature °C 2
Time min
Pressure MPa
10-ZS4ZrC 4-ZS8ZrC 16-ZS12ZrC
4 8 12
1800 1800 1800
9 9 9
30 30 30
After removing the surface layers from the sintered disk by grinding, Microstructure characterization were carried out using scanning electron microscope (SEM: Vega Tescan, Czech Republic) equipped with energy dispersive spectrometer (EDS) on polished surface. The hardness of the samples was measured using a Vickers pyramid indenter and load of 30 Kg. The fracture toughness of the samples was measured based on the indentation method in which, a 30 kg loaded Vickers indenter was used. Its calculation details were describe elsewhere [8]. 3. Results and Discussion 3.1. Microstructure and shrinkage SEM images of the polished surfaces of ZrB2-30 vol% SiC composites with different ZrC contents were shown in Fig. 1. Three distinct phases can be recognized; dark grey, light grey and black regions. EDS and XRD analysis of ZS8ZrC were applied to identify the phases in Figs. 2 and 3. According to EDS analysis, these different phases which indicated by points A, B and C belong to ZrB2, ZrC and SiC phases respectively and correspond with previous [21] research. In addition, XRD result (Fig. 3) confirms the presence of these phases in microstructure and occurring no chemical reaction between them. Using Image Tools Software, grain size of all composites were calculated and beside the relative density and open porosity data which were measured by Arashmidous method, are listed in table 2. It is appears, although carbide additives are introduced as grain growth inhibitor, but here, by increase in ZrC amount, grain coarsening was occurred noticeably except for 4 vol%. Effect of ZrC addition besides the other additives and SPS parameters which were investigated using Taguchi design revealed its positive effect to achieve fine microstructure. Precise inspection showed in this scholarship, the whole carbide additives (ZrC+SiC > 34 vol%) are higher than previous articles[3](< 35 vol%). So, it seems applying high volume percent of carbide additives have destructive effect which is discussed in details section 3.3.
3
ZS4ZrC
ZS8ZrC
20 µm
20 µm
ZS12ZrC
20 µm Fig. 1. SEM images of the polished surfaces of ZrB2-30 vol% SiC composites with different ZrC amounts.
Starting shrinkage temperatures (SST) which were extracted from the SPS curves (timetemperature-displacement) are listed in table 3. With increase in ZrC amount, SST rises from 1263 °C to 1345 °C and 1389 °C[3,22]. It can be explained by its higher melting point (3532 °C ) rather than ZrB2 (3236°C ). In fact, ZrC with high melting and its strong covalent Zr-C bond does not act as a sintering aid like SiC and MoSi2[22].
4
Element A, wt% B, wt% C, wt% 70 / 9 67 / 2 Zr 64/3 Si 25 / 1 B 32/8 35/7 C Fig. 2. SEM image and EDS analysis of ZS8ZrC.
5
Fig. 3. XRD analysis of ZS8ZrC composite.
Table 2. ZrB2 and SiC grains size, relative density and open porosity of all composites. Sample code ZS [23] ZS4ZrC ZS8ZrC ZS12ZrC
grain size, ZrB2 SiC 5.6 3.9 3 1.5 6.1 4 7.5 5.1
Relative Density, % 94.4 93.4 92.2
Open Porosity, % 4.48 6.2 6.7
Table 3. ZrB2 and SiC grains size, relative density and open porosity of all composites. Sample code ZS4ZrC ZS8ZrC ZS12ZrC
Vol% ZrC 4 8 12
Shrinkage Temperature 1263 1345 1389
6
3.2.
Hardness
Hardness against ZrC amount is shown in Fig. 4. Increase in ZrC addition up to 8 vol% resulted accompanied with hardness ascent nealy 5 GPa but in higher amount of 12 vol%, aslight decrease was happened. Based on our knwoldege, the porosity, grain size and secondary phase (type, amount and dispersion) are effective factors on hardness [24], so for better analysis, beside the hardness, are shown in Fig. 4. It was well-known the pores in ceramics have no resistance to applied stress, so materials with more porosity have lower apparent micro-hardness values than the dense counterparts [25]. Typically, Chamberlain et al. [25] prepared ZrB2 ceramics at different temperatures (1900–2150 °C) and 180 min soaking time by the pressureless method and compared the data with hot-pressed ZrB2. They concluded that hardness decreases in pressureless method relative to hot-pressing due to larger grain size in the pressureless materials which originates from its lager grain size. In fact, the larger grain size decreases the frequency with which dislocations encounter grain boundaries, thus reducing the amount of stress required for deformation to occur. In addition, it was cleared that the grain size depends to its variation, can affect the hardness. At the result, there is challenge between two factors: (1) the amount of open porosity percent and (2) grain size variation were obtained in the presence of ZrC. It is clear that the addition of 8 vol% ZrC, impresses the significant ascent and the hardness reaches to 13.4 GPa from 8.3 GPa for 4 vol% ZrC. Tracing the grain size and open porosity variations indicate the rising trend which is wrecking for hardness. So, it can be concluded that the presence of ZrC in matrix is the main factor to improve hardness.
20 16 12 8 4 0
4 vol%ZrC
8 vol%ZrC
12 vol%ZrC
Hardness
8.3
13.4
11.9
Open Porosity, %
4.4
6.2
6.7
SiC grain size
1.5
4
5.1
Fig. 4. Hardness, open porosity and SiC grain size against ZrC content.
7
It can be explained by two reasons; 1) ZrC grains (28 GPa) have higher hardness rather than ZrB2 (25GPa), 2) ZrC can dissolve to ZrB2 lattice up to 4.5 wt% and form solid solution which is well-known as strengthening mechanism. However, slight decrease for 12 vol% ZrC can be originated from no fine dispersion of them at the result of the high amount of carbides addition (totally 42 vol%).
3.2 Fracture Toughness For fracture toughness evaluation in relation with sintering parameters and ZrC content, SEM images of cracks induced for all composites were taken. Fig. 5 shows SEM image of induced cracks by Vickers indent in sample 13, typically. Blue continues and red dashed lines indicate the vertical and horizontal diameters of indent and crack path propagation respectively and applied to calculate the fracture toughness.
13-12ZrC-1650-9-30
Fig.5. SEM image of induced Vickers indent and its cracks in sample 13.
Fig. 6 exhibits the fracture toughness, open porosity and SiC grain size of ZrB2-30 Vol% SiC composites with different ZrC amounts. It seems ZrC up to 8 vol%, rises the fracture
8
toughness from 4.8 MPa m0.5 to 6.6 MPa m0.5 while for 12 vol% indicates descending trend and it reaches to 5.9 MPa m0.5. Fracture toughness is mainly affected from two agents; Firstly the grain size of secondary phase and secondly the open porosity percent based on previous research [3] and both of them are listed in table 1. Increase in secondary phase grain size (SiC grain size) leads to occur diminution in fracture toughness.
10 8 6 4 2 0
4 vol%ZrC
8 vol%ZrC
12 vol%ZrC
Fracture Toughness
4.8
6.6
5.9
Open Porosity, %
4.4
6.2
6.7
SiC grain size
1.5
4
5.1
Fig. 6. Fracture toughness, open porosity and SiC grain size of ZrB2-30 Vol% SiC composites with different ZrC amounts
Coarse SiC grains size have no resist against the crack path propagation and will be fractured during the interaction with them. It means, in presence of these SiC grains, no toughening mechanisms such as crack deflection or bridging active [3, 5]. SEM images of crack path propagation of two samples 7 and 16 with 8 vol% and 12 vol% ZrC are presented in Fig. 7 typically which admit this issue. Induced crack in sample 7 (Fig. 7a-c) with lower ZrC rather than sample 16 (Fig. 7d-f) goes through more meandrous path and further toughening mechanisms occur which correlate with their obtained fracture toughness (Fig. 7). Besides, it seems, some SiC grains fracture in sample 16 in the result of coarsening while in sample 7, make the crack deflection, bridging and branching appear. These toughening mechanisms make the length of crack in indentation technique increased, which leads to fracture toughness ascent [26]. In sample 7 (Fig. 7b and c) crack deflected with higher angle in comparison with sample 16 (Fig. 7d-f) and reveal it has microstructure with more toughness.
9
a) Sam. 7 Crack Propagation
Branching
deflection
Bridging
b) Sam. 7 Crack Propagation
Branching
Bridging
Fig. 7. SEM images of crack path propagation of a), b) and c) sample 7, d), e) and f) sample 16 10
c) Sam. 7
d) Sam. 16 Crack Propagation
Accumulation
Continuum lattice of grain as ring
Continuous Fig. 7. Fig. SEM images of crack path propagation of a), b) and c) sample 7, d), e) and f) sample 16 11
e) Sam. 16 Crack Propagation
SiC Grain Fracture
f) Sam. 16 Crack Propagation
SiC Grain Fracture
Continuous Fig. 7. SEM images of crack path propagation of a), b) and c) sample 7, d), e) and f) sample 16 12
Open porosity is the second affecting agent on fracture toughness and acts as crack arresting or trapping which is well-known as another toughening factor. Of course, it is useful to some extent (5-10%). Under the conditions that the microstructure has no grain refinement and enough densification, presence of more open porosity (more than 10%) not only improves the fracture toughness but also make it to decrease. Type, amount and dispersion of additives are another affecting factors as well as secondary grain size and open porosity. The best fracture toughness, 6.6 MPa m0.5 was obtained for 8 vol% ZrC while it has higher SiC grain size than the composite with 4 vol% ZrC. One reason can be related to its higher open porosity and another, it is possible to reflect from carbon dissolution in ZrB2 crystal lattice. The phase diagram for the Zr–B–C system shows that, the solubility limit for ZrC in ZrB2 is 4.5 wt% [27]. Hence, it seems the carbon solubility in ZrB2 crystal lattice and the formation of interstitial solid solution can introduce as third reason for improving the fracture toughness. Formation the solid solution is well-known as strengthening mechanism and here it hypothesizes such phenomena rises the grain resistance against the crack propagation path and finally makes the fracture toughness enhances. However, this trend cannot be found for higher ZrC content (12 vol %) which may be attributed to SiC grain size happening as well as solubility limit of ZrC. In the high amount of ZrC and its more presence in matrix (due to its limit solubility) besides the 30 vol% SiC addition, not only the well dispersion of secondary (SiC and ZrC grains) occur but also impress the agglomeration and grain growth happen by interconnecting the adjacent grains. These incorporations lead to form the continuum lattice of grain as ring or their accumulation (Fig. 7d). Finally, it was concluded that the best synergetic effect of SiC and ZrC on fracture toughness achieves during applying their appropriate amount with well dispersion.
Conclusion ZrB2-30vol%SiC composites with 4, 8 and 12 vol % ZrC additions were SPSed at 1800 °C, for time of 9 minutes and under the pressure of 30 MPa. Microstructural investigations, densification, hardness and fracture toughness were evaluated. The results were obtained as follow: With increase in ZrC amount, SST rises from 1263 °C to 1345 °C and 1389 °C. Increase in ZrC addition up to 8 vol% resulted accompanied with hardness ascent nealy 5 GPa but in higher amount of 12 vol%, aslight decrease was happened.It was cleared with ZrC ascent up to 8vol%, fracture toughness was enhanced from 4.8 MPa m0.5 up to 6.6 MPa m0.5, but with more addition up to 12 vol%, fracture toughness was decreased to 5.9 MPa m0.5
13
Reference [1] Sujuan Li, Chuncheng Wei, Weiwei Wang, Peng Wang, Yuying Liu, Shuang Li, Guangwu Wen, Fracture toughness and R-curve behavior of laminated ZrB2–SiC/SiCw ceramic, Journal of Alloys and Compounds, Volume 784, 5 May 2019, Pages 96-101 [2] R. V. Krishnarao, G. Madhusudan reddy and V.V. Bhanuprasad, Joining and machining of (ZrB2-SiC) and (Cf-SiC) based composites, 4th International Conference and Expo on Ceramics and Composite Materials, May 14-15, 2018 | Rome, Italy, Res. Rev. J Mat. Sci. 2018, Volume 6 [3] Zohre Balak, Mehdi Shahedi Asl, Mahdi Azizieh, Hosein Kafashan, Raziye Hayati, Effect of different additives and open porosity on fracture toughness of ZrB2–SiC-based composites prepared by SPS, Ceramics International 43 (2017) 2209–2220 [4] 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 44 (2018) 7334-7348 [5] Rozbe Eatemadi, Zohre Balak, Investigating the effect of SPS parameters on densification and fracture toughness of ZrB2-SiC nanocomposite ,Cer. Int., Volume 45, Issue 4, March 2019, Pages 4763-4770 [6] 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, 716 (2018) 99-106. [7] M Shahedi Asl, Microstructure, hardness and fracture toughness of spark plasma sintered ZrB2-SiC-Cf composites, Ceramics International 43 (2017) 15047-15052. [8] Sadegh Karimirad, Zohre Balak, Characteristics of spark plasma sintered ZrB2-SiC-SCFs composites, Ceramics International, Volume 45, Issue 5, 1 April 2019, Pages 6275-6281 [9] Yuhang Bai Mengyong Sun Mingxing Li Shangwu Fan Laifei Cheng, Improved fracture toughness of laminated ZrB2-SiC-MoSi2 ceramics using SiC whisker, Ceramics International Volume 44, Issue 8, 1 June 2018, Pages 8890-8897 [10] Wei-MingGuoa Yang Youa Guo-Jun Zhangb Shang-HuaWua Hua-TayLin, Improvement of fracture toughness of ZrB2–SiC composites with carbon interfaces, Journal of the European Ceramic Society, Volume 35, Issue 6, June 2015, Pages 1985-1989 [11] Yehong Cheng Yang Lyu Shanbao Zhou Wenbo Han, Non-axially aligned ZrB2-SiC/ZrB2SiC-graphene short fibrous monolithic ceramics with isotropic in-plane properties, Ceramics International, Volume 45, Issue 3, 15 February 2019, Pages 4113-4118 [12] Shahedi asl M., Ghassemi kakroudi M., Noori R., Hardness and toughness of hot pressed ZrB2-SiC composites consolidated under relatively low pressure, Journal of alloys and compounds, 2015, 619, 481-487. [13] Shahedi asl M., Ghassemi kakroudi M., Abedi kondolaji R., Nasiri H., Influence of graphite nano-flakes on densification and mechanical properties of hot-pressed ZrB2–SiC composite. Ceramics International, 2015, 41, 5843-5851. [14] D. Sciti, A. Natali Murri, V.Medri, L. Zoli, Continuous C fibre composites with a porous ZrB2 Matrix, Materials and Design 85 (2015) 127–134 [15] L. Zoli, D. Sciti, Efficacy of a ZrB2–SiC matrix in protecting C fibres from oxidation in novel UHTCMCmaterials,Mater. Des. 113 (2017) 207–213 [16] Antonio Vinci, Luca Zoli, Diletta Sciti, Cesare Melandri, Stefano Guicciardi, Understanding the mechanical properties of novel UHTCMCs through random forest and regression tree analysis, Materials and Design 145 (2018) 97–107 14
[17] Yang, F., X. Zhang, J. Han and S. Du, (2009). Characterization of hot-pressed short carbon fiber reinforced ZrB2–SiC ultra-high temperature ceramic composites, J. of Allo. and Comp., 472: 395–399 [18] S. Failla, P. Galizia, L. Zoli, A. Vinci, D. Sciti, Toughening effect of non-periodic fiber distribution on crack propagation energy of UHTC composites, Journal of Alloys and Compounds 777 (2019) 612-618 [19] I. Farahbakhsh, Z. Ahmadi, M. Shahedi Asl, “Densification, microstructure and mechanical properties of hot pressed ZrB2–SiC ceramic doped with nano–sized carbon black, Ceram. Int. 43 (11) (2017) 8411–8417. [20] Z. Balak, Mohammad Zakeri, Application of Taguchi L32 orthogonal design to optimize flexural strength of ZrB2-based composites prepared by spark plasma sintering, Int. Journal of Refractory Metals and Hard Materials 55 (2016) 58–67 [21] Z. Balak , M. Zakeri, Effect of HfB2 on microstructure and mechanical properties of ZrB2–SiC-based composites, Int. Journal of Refractory Metals and Hard Materials 54 (2016) 127–137 [22] Z. Balak, M. Zakerib, M. R. Rahimipourb, E. Salahi, H. Kafashan, Investigation of Effective Parameters on Densification of ZrB2-SiC Based Composites Using Taguchi Method, ACERP: Vol. 2, No. 2, (Spring 2016) 7-15 [23] Nima Akhlaghi, Zohre Balak and Ebrahim Najafi Birgani, Milling time and sintering parameters optimization to fabricate ZrB2-30 vol% SiC composite with highest thermal shock resistance, Mater. Res. Express 6 (2019) 055606 [24] Z. Balaka,n, M.Zakeria, M.R.Rahimipura, E.Salahia, H.Nasiri, Effect of open porosity on flexural strength and hardness of ZrB2-based composites, Ceramics International 41 (2015) 8312–8319 [25] [12] Adam L. Chamberlain, William G. Fahrenholtz, Gregory E. Hilmas, Pressureless sintering of zirconium diboride, J. Am. Ceram. Soc. 89 (2) (2006) 450–456. [26] Mengran Caoa, Shubin Wanga, Wenbo Han, Influence of nanosized SiC particle on the fracture toughness of ZrB2-based nanocomposite ceramic, Materials Science and Engineering A 527 (2010) 2925–2928 [27] Rudy, E., "Ternary phase equilibria in transition-metal-boroncarbon- silicon systems", part V, compendium of phase diagram data, (1969), 562-581.
15
S0254-0584(19)31099-5
DOI:
https://doi.org/10.1016/j.matchemphys.2019.122284
Reference:
MAC 122284
To appear in:
Materials Chemistry and Physics
Received Date: 11 April 2019 Accepted Date: 7 October 2019
Please cite this article as: A. Rezapour, Z. Balak, Fracture toughness and hardness investigation in ZrB2–SiC–ZrC composite, Materials Chemistry and Physics (2019), doi: https://doi.org/10.1016/ j.matchemphys.2019.122284. 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 B.V.
Fracture toughness and hardness investigation in ZrB2-SiC-ZrC composite Arsalan rezapour, Zohre Balak* Corresponding Author Email : [email protected] (Zohre Balak)
Department of Materials Science and Engineering, Ahvaz Branch, Islamic Azad University, Ahvaz, Iran.
Abstract Goal of this research is survey the effect of ZrC on fracture toughness and hardness of ZrB230vol%SiC. So, ZrB2-30vol%SiC composites with different amount ZrC (4, 8 and 12 vol %) were SPSed at 1800 °C, for time of 9 minutes and under the pressure of 30 MPa. Microstructural evaluations were done by scanning electron microscopic (SEM). Densification was measured by Arashmidous method. Fracture toughness and hardness were determined by Indentation method and Macro-Vickers respectively. With increase in ZrC amount, SST rises from 1263 °C to 1345 °C and 1389 °C. Increase in ZrC addition up to 8 vol% resulted accompanied with hardness ascent nealy 5 GPa but in higher amount of 12 vol%, aslight decrease was happened.It was cleared with ZrC ascent up to 8vol%, fracture toughness was enhanced from 4.8 MPa m0.5 up to 6.6 MPa m0.5, but with more addition up to 12 vol%, fracture toughness was decreased to 5.9 MPa m0.5. Keywords: Fracture toughness, hardness, ZrB2-30vol%SiC, SPS
1. Introduction It is well-known that the ZrB2-SiC-based composites are the most promising materials for the application in thermal protection systems and high temperature components of future hypersonic aircraft or reentry vehicles due to their high temperature melting and their excellent comprehensive characteristics [1, 2]. Although, similar to the other ceramics materials, their inherent brittleness was still as major obstacle and terminated their wide application. Hence, different groups of researcher [3-19] afforded to enhance the fracture toughness. Z. balak et al. [3] were applied Taguchi experimental design technique to determine the most influential additives and SPS parameters. They examined nine factors (SiC, Cf, MoSi2, HfB2 and ZrC content, milling time of Cf and SPS parameters such as temperature, time and pressure) on four different levels in order to obtain the optimum mixture. The results showed that temperature with 34.7% and SiC with 29.7% have significant effect on fracture toughness. Also, they studied [5] the effect of nano-SiC particles on fracture toughness. Investigation the SEM images of the Vickers indent and their path propagation showed that the deviation and branching of crack are the most important toughening mechanisms in ZrB2-SiC nanocomposites. Studies [8] on SCFs showed increase SCFs from 10 to 40 vol% improves the fracture toughness from 4.1 to 5.1 MPam0.5 by promoting toughening mechanisms. 1
Carbon additives (such as carbon black, graphite, graphene, carbon fiber and carbon nano tube) incorporated in ZrB2 composites were investigated by M. Shahedi et al. [4,6,7,12, 19]. Typically, they fabricated a hot pressed ZrB2–based composite reinforced with 20 vol% SiC and doped with 10 vol% nano–sized carbon black (~15 nm) at 1850 °C under 20 MPa pressure [19]. The addition of nano–sized powder carbon black with porous structure to ZrB2–SiC composite meaningfully led to the manufacture of fully correspondence composites. Also, ZrB2 and ZrB2–20 vol%SiC–10 vol% CNTs composites were hot pressed by Shahedi Asl and co–workers to enhance its fracture toughness [15]. Embedding CNTs into the designed ceramics enhanced the densification (by 4%) and indentation fracture toughness (increased up to ~5.1 MPam0.5). Other researches in Italy [14-16, 18] focused on embedding continuous carbon fiber (CCFs) by applying a combination method of vacuum-bag infiltration and conventional hot pressing without the help of expensive techniques such as CVI or PIP. For example, D. Sciti et al. [14] were infiltrated the homemade 1D SiC preforms by UHTC slurries based on ZrB2. Considerations the all mechanical properties (densification, hardness, flexural strength and fracture toughness) resulted to choose the ZrC as addition in this research among all of them mentioned above. Although carbon additives such as SCFs improves the fracture toughness but frequently accompanied with a loss in flexural strength [20] while ZrC not only improves the flexural strength but also expect to enhance the fracture toughness by acts as grain growth inhibitor. So, besides the SPS parameters, it was selected as variable in this research to improve the fracture toughness. 2. Experimental Commercially available ZrB2 (particle size of 20 μm, purity: 99.5%, Northwest Institute for Non-Ferrous Metal Research, China), SiC ((particle size of 15 μm, purity: 98.7%, Northwest Institute for Non-Ferrous Metal Research, China) and ZrC (particle size of 20 μm, Alfa acer company) powders were used as starting materials. Three composites with different compositions of ZrB2–20 vol% SiC-4 vol% ZrC, ZrB2–20 vol% SiC-8 vol% ZrC and ZrB2–20 vol% SiC-12 vol% ZrC, were fabricated to survey the ZrC effect according to table 1. So, the initial powders were weighed and mixed through highenergy ball milling in ethanol media for 3 hours. Then, each batch was dried on a hot plate stirrer (Heidolph MR 3001 K, Germany) at 150 °C for 120 min to remove the ethanol. Each powder mixture was loaded in a graphite die directly (inner diameter: 40 mm), lined with a graphite foil. The sintering process was carried out using a spark plasma sintering machine (SPS-20T-10, China, Vacuum: 15 Pa) at different sintering parameters which were presented in table 1.
Table. 1. Chemical composition and sintering conditions of all composites. Sample Code
ZrC Vol%
Temperature °C 2
Time min
Pressure MPa
10-ZS4ZrC 4-ZS8ZrC 16-ZS12ZrC
4 8 12
1800 1800 1800
9 9 9
30 30 30
After removing the surface layers from the sintered disk by grinding, Microstructure characterization were carried out using scanning electron microscope (SEM: Vega Tescan, Czech Republic) equipped with energy dispersive spectrometer (EDS) on polished surface. The hardness of the samples was measured using a Vickers pyramid indenter and load of 30 Kg. The fracture toughness of the samples was measured based on the indentation method in which, a 30 kg loaded Vickers indenter was used. Its calculation details were describe elsewhere [8]. 3. Results and Discussion 3.1. Microstructure and shrinkage SEM images of the polished surfaces of ZrB2-30 vol% SiC composites with different ZrC contents were shown in Fig. 1. Three distinct phases can be recognized; dark grey, light grey and black regions. EDS and XRD analysis of ZS8ZrC were applied to identify the phases in Figs. 2 and 3. According to EDS analysis, these different phases which indicated by points A, B and C belong to ZrB2, ZrC and SiC phases respectively and correspond with previous [21] research. In addition, XRD result (Fig. 3) confirms the presence of these phases in microstructure and occurring no chemical reaction between them. Using Image Tools Software, grain size of all composites were calculated and beside the relative density and open porosity data which were measured by Arashmidous method, are listed in table 2. It is appears, although carbide additives are introduced as grain growth inhibitor, but here, by increase in ZrC amount, grain coarsening was occurred noticeably except for 4 vol%. Effect of ZrC addition besides the other additives and SPS parameters which were investigated using Taguchi design revealed its positive effect to achieve fine microstructure. Precise inspection showed in this scholarship, the whole carbide additives (ZrC+SiC > 34 vol%) are higher than previous articles[3](< 35 vol%). So, it seems applying high volume percent of carbide additives have destructive effect which is discussed in details section 3.3.
3
ZS4ZrC
ZS8ZrC
20 µm
20 µm
ZS12ZrC
20 µm Fig. 1. SEM images of the polished surfaces of ZrB2-30 vol% SiC composites with different ZrC amounts.
Starting shrinkage temperatures (SST) which were extracted from the SPS curves (timetemperature-displacement) are listed in table 3. With increase in ZrC amount, SST rises from 1263 °C to 1345 °C and 1389 °C[3,22]. It can be explained by its higher melting point (3532 °C ) rather than ZrB2 (3236°C ). In fact, ZrC with high melting and its strong covalent Zr-C bond does not act as a sintering aid like SiC and MoSi2[22].
4
Element A, wt% B, wt% C, wt% 70 / 9 67 / 2 Zr 64/3 Si 25 / 1 B 32/8 35/7 C Fig. 2. SEM image and EDS analysis of ZS8ZrC.
5
Fig. 3. XRD analysis of ZS8ZrC composite.
Table 2. ZrB2 and SiC grains size, relative density and open porosity of all composites. Sample code ZS [23] ZS4ZrC ZS8ZrC ZS12ZrC
grain size, ZrB2 SiC 5.6 3.9 3 1.5 6.1 4 7.5 5.1
Relative Density, % 94.4 93.4 92.2
Open Porosity, % 4.48 6.2 6.7
Table 3. ZrB2 and SiC grains size, relative density and open porosity of all composites. Sample code ZS4ZrC ZS8ZrC ZS12ZrC
Vol% ZrC 4 8 12
Shrinkage Temperature 1263 1345 1389
6
3.2.
Hardness
Hardness against ZrC amount is shown in Fig. 4. Increase in ZrC addition up to 8 vol% resulted accompanied with hardness ascent nealy 5 GPa but in higher amount of 12 vol%, aslight decrease was happened. Based on our knwoldege, the porosity, grain size and secondary phase (type, amount and dispersion) are effective factors on hardness [24], so for better analysis, beside the hardness, are shown in Fig. 4. It was well-known the pores in ceramics have no resistance to applied stress, so materials with more porosity have lower apparent micro-hardness values than the dense counterparts [25]. Typically, Chamberlain et al. [25] prepared ZrB2 ceramics at different temperatures (1900–2150 °C) and 180 min soaking time by the pressureless method and compared the data with hot-pressed ZrB2. They concluded that hardness decreases in pressureless method relative to hot-pressing due to larger grain size in the pressureless materials which originates from its lager grain size. In fact, the larger grain size decreases the frequency with which dislocations encounter grain boundaries, thus reducing the amount of stress required for deformation to occur. In addition, it was cleared that the grain size depends to its variation, can affect the hardness. At the result, there is challenge between two factors: (1) the amount of open porosity percent and (2) grain size variation were obtained in the presence of ZrC. It is clear that the addition of 8 vol% ZrC, impresses the significant ascent and the hardness reaches to 13.4 GPa from 8.3 GPa for 4 vol% ZrC. Tracing the grain size and open porosity variations indicate the rising trend which is wrecking for hardness. So, it can be concluded that the presence of ZrC in matrix is the main factor to improve hardness.
20 16 12 8 4 0
4 vol%ZrC
8 vol%ZrC
12 vol%ZrC
Hardness
8.3
13.4
11.9
Open Porosity, %
4.4
6.2
6.7
SiC grain size
1.5
4
5.1
Fig. 4. Hardness, open porosity and SiC grain size against ZrC content.
7
It can be explained by two reasons; 1) ZrC grains (28 GPa) have higher hardness rather than ZrB2 (25GPa), 2) ZrC can dissolve to ZrB2 lattice up to 4.5 wt% and form solid solution which is well-known as strengthening mechanism. However, slight decrease for 12 vol% ZrC can be originated from no fine dispersion of them at the result of the high amount of carbides addition (totally 42 vol%).
3.2 Fracture Toughness For fracture toughness evaluation in relation with sintering parameters and ZrC content, SEM images of cracks induced for all composites were taken. Fig. 5 shows SEM image of induced cracks by Vickers indent in sample 13, typically. Blue continues and red dashed lines indicate the vertical and horizontal diameters of indent and crack path propagation respectively and applied to calculate the fracture toughness.
13-12ZrC-1650-9-30
Fig.5. SEM image of induced Vickers indent and its cracks in sample 13.
Fig. 6 exhibits the fracture toughness, open porosity and SiC grain size of ZrB2-30 Vol% SiC composites with different ZrC amounts. It seems ZrC up to 8 vol%, rises the fracture
8
toughness from 4.8 MPa m0.5 to 6.6 MPa m0.5 while for 12 vol% indicates descending trend and it reaches to 5.9 MPa m0.5. Fracture toughness is mainly affected from two agents; Firstly the grain size of secondary phase and secondly the open porosity percent based on previous research [3] and both of them are listed in table 1. Increase in secondary phase grain size (SiC grain size) leads to occur diminution in fracture toughness.
10 8 6 4 2 0
4 vol%ZrC
8 vol%ZrC
12 vol%ZrC
Fracture Toughness
4.8
6.6
5.9
Open Porosity, %
4.4
6.2
6.7
SiC grain size
1.5
4
5.1
Fig. 6. Fracture toughness, open porosity and SiC grain size of ZrB2-30 Vol% SiC composites with different ZrC amounts
Coarse SiC grains size have no resist against the crack path propagation and will be fractured during the interaction with them. It means, in presence of these SiC grains, no toughening mechanisms such as crack deflection or bridging active [3, 5]. SEM images of crack path propagation of two samples 7 and 16 with 8 vol% and 12 vol% ZrC are presented in Fig. 7 typically which admit this issue. Induced crack in sample 7 (Fig. 7a-c) with lower ZrC rather than sample 16 (Fig. 7d-f) goes through more meandrous path and further toughening mechanisms occur which correlate with their obtained fracture toughness (Fig. 7). Besides, it seems, some SiC grains fracture in sample 16 in the result of coarsening while in sample 7, make the crack deflection, bridging and branching appear. These toughening mechanisms make the length of crack in indentation technique increased, which leads to fracture toughness ascent [26]. In sample 7 (Fig. 7b and c) crack deflected with higher angle in comparison with sample 16 (Fig. 7d-f) and reveal it has microstructure with more toughness.
9
a) Sam. 7 Crack Propagation
Branching
deflection
Bridging
b) Sam. 7 Crack Propagation
Branching
Bridging
Fig. 7. SEM images of crack path propagation of a), b) and c) sample 7, d), e) and f) sample 16 10
c) Sam. 7
d) Sam. 16 Crack Propagation
Accumulation
Continuum lattice of grain as ring
Continuous Fig. 7. Fig. SEM images of crack path propagation of a), b) and c) sample 7, d), e) and f) sample 16 11
e) Sam. 16 Crack Propagation
SiC Grain Fracture
f) Sam. 16 Crack Propagation
SiC Grain Fracture
Continuous Fig. 7. SEM images of crack path propagation of a), b) and c) sample 7, d), e) and f) sample 16 12
Open porosity is the second affecting agent on fracture toughness and acts as crack arresting or trapping which is well-known as another toughening factor. Of course, it is useful to some extent (5-10%). Under the conditions that the microstructure has no grain refinement and enough densification, presence of more open porosity (more than 10%) not only improves the fracture toughness but also make it to decrease. Type, amount and dispersion of additives are another affecting factors as well as secondary grain size and open porosity. The best fracture toughness, 6.6 MPa m0.5 was obtained for 8 vol% ZrC while it has higher SiC grain size than the composite with 4 vol% ZrC. One reason can be related to its higher open porosity and another, it is possible to reflect from carbon dissolution in ZrB2 crystal lattice. The phase diagram for the Zr–B–C system shows that, the solubility limit for ZrC in ZrB2 is 4.5 wt% [27]. Hence, it seems the carbon solubility in ZrB2 crystal lattice and the formation of interstitial solid solution can introduce as third reason for improving the fracture toughness. Formation the solid solution is well-known as strengthening mechanism and here it hypothesizes such phenomena rises the grain resistance against the crack propagation path and finally makes the fracture toughness enhances. However, this trend cannot be found for higher ZrC content (12 vol %) which may be attributed to SiC grain size happening as well as solubility limit of ZrC. In the high amount of ZrC and its more presence in matrix (due to its limit solubility) besides the 30 vol% SiC addition, not only the well dispersion of secondary (SiC and ZrC grains) occur but also impress the agglomeration and grain growth happen by interconnecting the adjacent grains. These incorporations lead to form the continuum lattice of grain as ring or their accumulation (Fig. 7d). Finally, it was concluded that the best synergetic effect of SiC and ZrC on fracture toughness achieves during applying their appropriate amount with well dispersion.
Conclusion ZrB2-30vol%SiC composites with 4, 8 and 12 vol % ZrC additions were SPSed at 1800 °C, for time of 9 minutes and under the pressure of 30 MPa. Microstructural investigations, densification, hardness and fracture toughness were evaluated. The results were obtained as follow: With increase in ZrC amount, SST rises from 1263 °C to 1345 °C and 1389 °C. Increase in ZrC addition up to 8 vol% resulted accompanied with hardness ascent nealy 5 GPa but in higher amount of 12 vol%, aslight decrease was happened.It was cleared with ZrC ascent up to 8vol%, fracture toughness was enhanced from 4.8 MPa m0.5 up to 6.6 MPa m0.5, but with more addition up to 12 vol%, fracture toughness was decreased to 5.9 MPa m0.5
13
Reference [1] Sujuan Li, Chuncheng Wei, Weiwei Wang, Peng Wang, Yuying Liu, Shuang Li, Guangwu Wen, Fracture toughness and R-curve behavior of laminated ZrB2–SiC/SiCw ceramic, Journal of Alloys and Compounds, Volume 784, 5 May 2019, Pages 96-101 [2] R. V. Krishnarao, G. Madhusudan reddy and V.V. Bhanuprasad, Joining and machining of (ZrB2-SiC) and (Cf-SiC) based composites, 4th International Conference and Expo on Ceramics and Composite Materials, May 14-15, 2018 | Rome, Italy, Res. Rev. J Mat. Sci. 2018, Volume 6 [3] Zohre Balak, Mehdi Shahedi Asl, Mahdi Azizieh, Hosein Kafashan, Raziye Hayati, Effect of different additives and open porosity on fracture toughness of ZrB2–SiC-based composites prepared by SPS, Ceramics International 43 (2017) 2209–2220 [4] 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 44 (2018) 7334-7348 [5] Rozbe Eatemadi, Zohre Balak, Investigating the effect of SPS parameters on densification and fracture toughness of ZrB2-SiC nanocomposite ,Cer. Int., Volume 45, Issue 4, March 2019, Pages 4763-4770 [6] 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, 716 (2018) 99-106. [7] M Shahedi Asl, Microstructure, hardness and fracture toughness of spark plasma sintered ZrB2-SiC-Cf composites, Ceramics International 43 (2017) 15047-15052. [8] Sadegh Karimirad, Zohre Balak, Characteristics of spark plasma sintered ZrB2-SiC-SCFs composites, Ceramics International, Volume 45, Issue 5, 1 April 2019, Pages 6275-6281 [9] Yuhang Bai Mengyong Sun Mingxing Li Shangwu Fan Laifei Cheng, Improved fracture toughness of laminated ZrB2-SiC-MoSi2 ceramics using SiC whisker, Ceramics International Volume 44, Issue 8, 1 June 2018, Pages 8890-8897 [10] Wei-MingGuoa Yang Youa Guo-Jun Zhangb Shang-HuaWua Hua-TayLin, Improvement of fracture toughness of ZrB2–SiC composites with carbon interfaces, Journal of the European Ceramic Society, Volume 35, Issue 6, June 2015, Pages 1985-1989 [11] Yehong Cheng Yang Lyu Shanbao Zhou Wenbo Han, Non-axially aligned ZrB2-SiC/ZrB2SiC-graphene short fibrous monolithic ceramics with isotropic in-plane properties, Ceramics International, Volume 45, Issue 3, 15 February 2019, Pages 4113-4118 [12] Shahedi asl M., Ghassemi kakroudi M., Noori R., Hardness and toughness of hot pressed ZrB2-SiC composites consolidated under relatively low pressure, Journal of alloys and compounds, 2015, 619, 481-487. [13] Shahedi asl M., Ghassemi kakroudi M., Abedi kondolaji R., Nasiri H., Influence of graphite nano-flakes on densification and mechanical properties of hot-pressed ZrB2–SiC composite. Ceramics International, 2015, 41, 5843-5851. [14] D. Sciti, A. Natali Murri, V.Medri, L. Zoli, Continuous C fibre composites with a porous ZrB2 Matrix, Materials and Design 85 (2015) 127–134 [15] L. Zoli, D. Sciti, Efficacy of a ZrB2–SiC matrix in protecting C fibres from oxidation in novel UHTCMCmaterials,Mater. Des. 113 (2017) 207–213 [16] Antonio Vinci, Luca Zoli, Diletta Sciti, Cesare Melandri, Stefano Guicciardi, Understanding the mechanical properties of novel UHTCMCs through random forest and regression tree analysis, Materials and Design 145 (2018) 97–107 14
[17] Yang, F., X. Zhang, J. Han and S. Du, (2009). Characterization of hot-pressed short carbon fiber reinforced ZrB2–SiC ultra-high temperature ceramic composites, J. of Allo. and Comp., 472: 395–399 [18] S. Failla, P. Galizia, L. Zoli, A. Vinci, D. Sciti, Toughening effect of non-periodic fiber distribution on crack propagation energy of UHTC composites, Journal of Alloys and Compounds 777 (2019) 612-618 [19] I. Farahbakhsh, Z. Ahmadi, M. Shahedi Asl, “Densification, microstructure and mechanical properties of hot pressed ZrB2–SiC ceramic doped with nano–sized carbon black, Ceram. Int. 43 (11) (2017) 8411–8417. [20] Z. Balak, Mohammad Zakeri, Application of Taguchi L32 orthogonal design to optimize flexural strength of ZrB2-based composites prepared by spark plasma sintering, Int. Journal of Refractory Metals and Hard Materials 55 (2016) 58–67 [21] Z. Balak , M. Zakeri, Effect of HfB2 on microstructure and mechanical properties of ZrB2–SiC-based composites, Int. Journal of Refractory Metals and Hard Materials 54 (2016) 127–137 [22] Z. Balak, M. Zakerib, M. R. Rahimipourb, E. Salahi, H. Kafashan, Investigation of Effective Parameters on Densification of ZrB2-SiC Based Composites Using Taguchi Method, ACERP: Vol. 2, No. 2, (Spring 2016) 7-15 [23] Nima Akhlaghi, Zohre Balak and Ebrahim Najafi Birgani, Milling time and sintering parameters optimization to fabricate ZrB2-30 vol% SiC composite with highest thermal shock resistance, Mater. Res. Express 6 (2019) 055606 [24] Z. Balaka,n, M.Zakeria, M.R.Rahimipura, E.Salahia, H.Nasiri, Effect of open porosity on flexural strength and hardness of ZrB2-based composites, Ceramics International 41 (2015) 8312–8319 [25] [12] Adam L. Chamberlain, William G. Fahrenholtz, Gregory E. Hilmas, Pressureless sintering of zirconium diboride, J. Am. Ceram. Soc. 89 (2) (2006) 450–456. [26] Mengran Caoa, Shubin Wanga, Wenbo Han, Influence of nanosized SiC particle on the fracture toughness of ZrB2-based nanocomposite ceramic, Materials Science and Engineering A 527 (2010) 2925–2928 [27] Rudy, E., "Ternary phase equilibria in transition-metal-boroncarbon- silicon systems", part V, compendium of phase diagram data, (1969), 562-581.
15