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Processing, microstructure and mechanical properties of HfB2-ZrB2-SiC composites: Effect of B4C and carbon nanotube reinforcements
International Journal of Refractory Metals & Hard Materials 81 (2019) 111–118
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International Journal of Refractory Metals & Hard Materials journal homepage: www.elsevier.com/locate/IJRMHM
Processing, microstructure and mechanical properties of HfB2-ZrB2-SiC composites: Effect of B4C and carbon nanotube reinforcements Ambreen Nisara,b, a b
⁎,1
T
, Mohammad Mohsin Khana,1,2, Shipra Bajpaia, Kantesh Balania
High Temperature Ceramic Laboratory, Department of Materials Science and Engineering, Indian Institute of Technology Kanpur, Kanpur 208016, UP, India School of Engineering, The University of British Columbia, Kelowna V1V 1V7, Canada
A R T I C LE I N FO
A B S T R A C T
Keywords: Ultra-high temperature ceramic (UHTC) Carbon nanotubes (CNT) Spark plasma sintering (SPS) Zirconium diboride (ZrB2) Hafnium diborides (HfB2)
The fully dense HfB2-ZrB2-SiC composites were processed using spark plasma sintering (SPS) at 1850 °C. The effect of reinforcements (B4C and CNT) on the densification as well as mechanical properties were investigated and compared (with monolithic) in the present study. The study showed that the addition of B4C and CNT were not only beneficial for the densification but also towards enhancing the mechanical properties (hardness, elastic modulus, and fracture toughness) of HfB2-ZrB2-SiC composites. The augmentation in the mechanical properties establish the synergy between solid solution formation (with the equimolar composition of HfB2/ZrB2) and the reinforcements (SiC, B4C, and CNT). The highest increase in the indentation fracture toughness with the reinforcements of B4C as well as CNT is > 3 times (~13.8 MPam0.5 when it is 3–4 MPam0.5 for monolithic ZrB2/ HfB2) on HfB2-ZrB2-SiC composites, which is attributed to the crack deflection and pull-out mechanisms. An increase in the analytically quantified interfacial compressive residual stresses in the composites during SPS processing with the synergistic addition of reinforcements (SiC, B4C, and CNT) and its effect on the indentation fracture toughness has also been addressed.
1. Introduction Transition metal diborides such as hafnium diboride (HfB2) and zirconium diboride (ZrB2) owing to their high melting temperature (3380 °C and 3245 °C, respectively), high thermal conductivity (104 W/ m/K and 60 W/m/K, respectively), and resistance to chemical as well as wear attack are referred to as ultra-high temperature ceramics (UHTCs) [1]. UHTCs are discernible choice for several advanced structural applications such as armor, cutting tool, molten metal containment, leading edges, and propulsion system in hypersonic re-entry vehicles etc. [2–4]. However, the use of ZrB2 and HfB2 as a structural material has considerably been limited due to their poor sinterability, and low fracture toughness (3–4 MPam0.5) [2]. In order to achieve good densification and lower the sintering temperature (> 500 °C than that using other conventional sintering techniques), spark plasma sintering (SPS) has proven to be the best choice. In this technique, the densification is stimulated by the combined application of the pulsed electric field as well as the pressure which leads to evolution of a fine and homogeneous microstructure. Recent studies have shown that the reinforcements such as silicon
nitride (Si3N4) [5], silicon carbide (SiC) [2,4,6,7], boron carbide (B4C) [8,9], graphene nanoplatelets (GNP) [10], carbon nanotubes (CNT) [2–4,6,7,11,12] etc. could effectively increase the sinterability facilitating a fairly dense and uniform microstructure which influence the strength, oxidation resistance and other characteristics. Addition of 20 vol% SiC in diborides (i.e. H20S/Z20S) has already been reported as the baseline UHTC based on their high temperature thermo-mechanical performance [1,11], but still, there is a scope to further enhance their mechanical properties (hardness, fracture toughness etc.). Thus, there is a need to add more components to the system (making the material with ternary or quaternary phase) to perceive their effect on the thermo-mechanical performance. In this regard, B4C known as the third hardest natural material is chosen as a reinforcement to the baseline materials (H20S/Z20S) due to low density (2.52 g cm-3), high hardness (35–45 GPa), and abrasion resistance [13]. Weng et al. [14] showed that the addition of 2 vol% B4C in H20S has resulted the fracture toughness of ~7 MPam0.5. Reinforcement of CNT in the UHTC system has also shown to improve its fracture toughness (from ~3 MPam0.5 for monolithic ZrB2 to 8.4 MPam0.5 with 10 vol% of CNT in ZrB2), apart from enhancing the densification (nearly 100%) and imparting an
⁎
Corresponding author. E-mail address: [email protected] (A. Nisar). 1 Have equally contributed as first authors. 2 Current address: National Institute of Technology, Srinagar 190006, India. https://doi.org/10.1016/j.ijrmhm.2019.02.014 Received 24 September 2018; Received in revised form 10 February 2019; Accepted 17 February 2019 Available online 19 February 2019 0263-4368/ © 2019 Elsevier Ltd. All rights reserved.
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present study. The powder mixtures were dry ball-milled using tungsten carbide jars and balls for 8 min at the speed of 500 rpm with the ball to powder weight ratio as 5:2. The chosen parameters are as per the previous studies [2,7,11] which confirms that CNT had no obvious structural damage. The ball milled composite powders were then processed using spark plasma sintering (SPS, Dr. Sinter 515S, Kanagawa, Japan) at maximum holding temperature of 1850 °C for 10 min with the heating rate of 100 °C min–1 under vacuum (< 6 Pa) and uniaxial pressure of 30 MPa. Graphite dies, and punches were utilized for fabricating pellets of 15 mm diameter and thickness of 3 mm. The ram displacement data recorded during SPS sintering cycle was utilized to calculate the instantaneous densification behavior of HfB2-ZrB2-SiC composites using the formula:
acceptable level of thermal shock making UHTC a preferred candidate for aerospace applications [4,7,11]. Nisar et.al [2] delineated the contribution of residual stresses as well as reinforcement of SiC (i.e. 20 vol % in ZrB2) and CNT (i.e. 10 vol% in ZrB2) to be ~12%, 14%, and 55%, respectively towards enhancing the fracture toughness of ZrB2 ceramic. A recent report by authors [7] on the HfB2-ZrB2 system (with equimolar concentration of HfB2 and ZrB2 forming solid solution during SPS processing) with SiC and CNT reinforcement has shown to enhance the fracture toughness up to 196% (from ~5.2 MPam0.5 to 10.2 MPam0.5) has been attributed to the synergy of solid solution formation and complete densification. Generally speaking, the notion behind choosing all these reinforcements (i.e. SiC, B4C, and CNT) is to remove the surface oxide from the ceramic during sintering which inhibited the sintering process (detailed explaination in Section 3.1). Herein, the idea is to appropriately sinter/densify (HfB2/ZrB2-SiC) with B4C and CNT addition and to observe its effect on the processing and mechanical properties. In this paper, the fully dense (> 99%) HfB2-ZrB2-SiC composites with B4C and CNT reinforcements were fabricated using SPS at 1850 °C. The sintered HfB2-ZrB2-SiC composites with B4C and CNT reinforcements were characterized in terms of phase, microstructure and mechanical properties (hardness, elastic modulus, and fracture toughness). The effective residual stresses generated during the SPS processing of the composite material were, then, correlated with the mechanical properties.
hf ⎡ ⎤ ρinstantaneous = ⎢ hf + df − di ⎥ ⎣ ⎦
(1)
where, hf, df, and di are the thickness of the sintered sample, maximum displacement, and instantaneous displacement during sintering, respectively. The final density of the as-processed pellets was measured using the Archimedes method (with ethanol as an immersion medium) on a hydrostatic balance. 2.2. Phase, microstructural and mechanical characterizations
2. Materials and method Phase analysis of the initial powders, composite powders as well as SPS processed pellets was carried out using Rich-Seifert 2000D diffractometer operated at 25 kV and 15 mA at a scan speed of 0.5̊ s–1 and a step size of 0.02̊ using Cu Kα radiation (λ = 1.54 Å, PANanalytical, Empyrean diffractometer, Tokyo, Japan) in the 2θ range of 20̊ to 90̊. Further, to study the structural damage in the CNT, micro-Raman spectroscopy (Princeton Instruments, STR Raman, TE-PMT detector) was carried out using Nd-YAG green laser (λ = 532 nm) with a laser power of 12.5 mW in the backscattered mode. The fractured micrographs of HfB2-ZrB2-SiC composites were observed using field-emission scanning electron microscope (FESEM, JEOL, JSM-7100F, MA, USA) in the backscattered mode. Hardness and elastic modulus of the processed compacts were measured using instrumented indentation technique (MMT, CSM Instruments, Switzerland) on the polished surface (up to 0.1 μm finish using diamond suspension). The loading (and unloading at the same rate) was done at the load of 2 N with the ramp rate of 66.67 mN/s and
2.1. Processing of HfB2-ZrB2-SiC composites Commercial diborides: HfB2 and ZrB2; carbides: SiC and B4C (irregular shaped powders Samics Research Materials, Rajasthan, India) and multi-walled carbon nanotubes (CNT, Nanostructured and Amorphous Materials Inc., TX, USA, with 95% purity, an outer diameter of 40 nm, inner diameter of 20 nm and length of 1–2 μm) were used as the starting materials. SEM micrographs, shown in Fig. 1, elicits the morphology and particle size of the selected diborides HfB2 (Fig. 1a) and ZrB2 (Fig. 1b); and reinforcements B4C (Fig. 1c), SiC (Fig. 1d) and CNT (Fig. 1e). The following compositions: (i) HfB2-6 vol% B4C (labelled as H6B), (ii) ZrB2-6 vol% B4C (labelled as Z6C), (iii) HfB2–20 vol% SiC6 vol% B4C (labelled as H20S6B), (iv) ZrB2-20 vol% SiC-6 vol% B4C (labelled as Z20S6B), (v) HfB2-ZrB2 (1:1 ratio)-20 vol% SiC-6 vol% B4C (labelled as HZ20S6B) and (vi) HfB2-ZrB2 (1:1 ratio)-20 vol% SiC-6 vol % B4C-6 vol% CNT (labelled as HZ20S6B6C) have been chosen in the
Fig. 1. Electron micrographs of the inital powders (a) HfB2, (b) ZrB2, and reinforcements (c) B4C, (d) SiC, and (e) CNT. 112
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dwell of 10 s at maximum load using V-I51Vicker's indenter. The reported value is at least an average of the 15 indents for each composition. Based on the maximum depth (hm) and final depth (hf) of indentation, the plasticity index in terms of elastic recovery (re) and the resistance to deformation (rd) for HfB2-ZrB2-SiC composites and effect of B4C and CNT reinforcement have been calculated using the following relations [7]:
re =
rd =
hm − h f (2)
hm hf
(3)
hm
The indentation fracture toughness (KIC) was measured at the loading of 20 N using the universal hardness testing machine (UTM, FH10, Tinius-Olsen Ltd.) with a dwell time of 10 s. The crack lengths were measured using SEM from the centre of the indents and the fracture toughness was calculated using Anstis equation [15], see Eq. (4):
KIC = 0.016
E P H c 3/2
(4)
where H is the hardness, E is the elastic modulus (both measured experimentally), P is the load utilized and c is the radial crack length. The reported value is an average of at least 10 indents for each composition. As it has been already established in literature that the indentation technique for measuring fracture toughness is not suitable when compared to that of single edge-notched beam (SENB) method [16], but a similar trend in the fracture toughness values have been reported [17]. In this regard, it is to be noted that indentation fracture toughness is used only as a ranking parameter towards comparing the toughness of HfB2-ZrB2-SiC composites. Based on the ratio of half crack length to half indent diagonal length (referred to as c/a which is > 3.5 in the present study) the fracture toughness was employed using formulation proposed by Anstis et al. [15]. Also, the measurement of the critical energy release rate (GIC) is given by the relation: 2 2 ⎡1 − ν ⎤ GIC = KIC ⎥ ⎢ E ⎦ ⎣
(5)
where ν is the Poisson's ratio calculated using the rule of mixture (ROM) for all composites. 3. Results and discussion
Fig. 2. (a): Plot showing a variation of instantaneous densification with SPS processing time of HfB2-ZrB2-SiC composites, (b) Plot of the standard free energy change vs absolute temperature for the reactant and product (using FactSage [18] for Eqs. 6-9).
3.1. Densification of HfB2-ZrB2-SiC composites
5MO2 (s ) + 5B4 C (s ) → 5MB2 (s ) + 6BO (g ) + 4CO (g )
(8)
Based on the ram displacement recorded during SPS processing, the instantaneous densification of HfB2-ZrB2-SiC composites is presented in Fig. 2a. The sintering process comprises of the three distinct steps: (i) compaction (labelled as step I in Fig. 2a) i.e. initial increasing trend due to the rearrangement of powder particles with the application of pressure; (ii) thermal expansion (labelled as step II in Fig. 2a) i.e. intermediate decreasing trend due to expansion in ceramics with the application of temperature; and (iii) shrinkage (labelled as step III in Fig. 2a) i.e. final increasing trend due to the densification/compaction. All the composites showed maximum shrinkage in the final step, crucial in rendering pore closure and complete densification. The complete densification (nearly ~100%) of all composites has been attributed to the reinforcements (SiC, B4C, and CNT) which are very effective in promoting the densification of diborides [7,9,11]. The selected reinforcement act as the sintering aid by removing the surface oxide as per the following reactions:
MO2 (s ) + 5B2 O3 (l) + 5C (s ) → MB2 (s ) + 5CO (g )
(9)
M = Zr, Hf The plot of the standard free energy change against absolute temperature (i.e. Ellingham diagram using FactSage [18]) for reactants and products in the aforementioned reactions is shown in Fig. 2b. It is inferred from the plot that the feasible temperature at which the phase change in these reactions occur is ≥1405.1 °C, 1051.9 °C, 1500.1 °C, and 1792.6 °C, respectively (i.e. when ΔG = 0). Through these calculations it is anticipated that sintering of these composites at 1850 °C ensures the possibility of occurrence of each of the aforementioned reactions. This reveals that the reinforcement of SiC, B4C as well as CNT act as a sintering aid by removing the surface oxides/impurities. Also, these reactions are known to enhance the densification of diborides based ceramics by lowering the diffusion barrier [9]. The full densification of these refractory ceramic composites would govern their mechanical properties, later shown in Section 3.4.
2 2 2 SiC (s ) + O2 (g ) → SiO2 (s ) + CO (g ) 3 3 3
(6)
3.2. Phase analysis of HfB2-ZrB2-SiC composites
SiO2 (s ) + 3C (s ) → SiC (s ) + 2CO (g )
(7)
The comparison of the various phase present in the initial powders 113
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Fig. 4. Fractographs of SPS processed HfB2-ZrB2-SiC composites in backscattered mode (a) H6B, (b) Z6B, (c) H20S6B, (d) Z20S6B, (e) HZ20S6B, (f) HZ20S6B6C. High magnification micrograph showing (g) deformation as well as retention of CNT after SPS processing in HZ20S6B6C sample, (h) crack-deflection, branching and CNT pull-out mechanisms. Fig. 3. (a): XRD spectra of the initial powder, composite powder as well as SPS processed HfB2-ZrB2-SiC composites and, (b) Raman spectra showing signature peaks of CNT in pristine CNT as well as SPS processed HZ20S6B6C sample.
for powder and HZ20S6B6C pellet, called as 2G-peak and is attributed to the overtone of the D-peak. Another band at 2912.7 cm−1 and 2917.0 cm−1 known as (D + G)-peak was also observed. With respect to the pristine CNT, all the Raman bands of the SPS processed HZ20S6B6C are shifted towards the higher wave number indicating the generation of compressive stresses in the CNT, attributed to the thermal contraction of the UHTC matrix during SPS cooling. The ratio of the defect to graphitic peak i.e. ID/IG decreases from 1.04 to 0.90 for the SPS processed HZ20S6B6C sample eliciting the conversion of CNT into graphitic structures. Similar observations have previously been reported by the authors for other UHTC-CNT composites [4,7,11].
as well as SPS processed HfB2-ZrB2-SiC composites are shown in Fig. 3a. The XRD pattern confirms the retention of the starting matrix (i.e. HfB2 and ZrB2) as well as the reinforcements (B4C and SiC). No new phases or any peak shift in SiC and B4C phases is observed in the composites, which denies the formation of any solid solution in between the reinforcements and matrix phase up to 1850 °C. However, the mutual solubility of HfB2-ZrB2 was observed in HZ20S6B as well as in HZ20S6B6C composites elicited from the merged peaks corresponding to the parent phases (HfB2/ZrB2). The mutual solubility of diborides has already been reported in previous literature [7,19]. In order to confirm the presence as well as the structural deformation (if any) in the CNT during SPS processing, Raman spectra of HZ20S6B6C sample was compared to that of the pristine CNT, as shown in Fig. 3b. The spectra presented has been normalized to the highest peak intensity. The D-band is activated in the first order scattering process of sp2 carbons by the presence of in-plane substitutional heteroatomic vacancies, grain boundaries and represents the disorder induced in the CNT, observed at 1341.8 cm−1 and 1349.75 cm−1, respectively, for powder and pellet. The G-peak observed at 1577.7 cm−1 and 1585.1 cm−1 for powder and HZ20S6B6C pellet, respectively, is a tangential band which is related to the graphite tangential E2g Raman active mode where the two atoms in the graphene unit cell vibrate tangentially one against the other. The Raman spectra also exhibit second-order weak band respectively at 2683.7 cm−1 and 2688.7 cm−1
3.3. Microstructural analysis of HfB2-ZrB2-SiC composites Fractured micrograph of HfB2-ZrB2-SiC composites shown in Fig. 4 elicit that all the samples were fully densified after SPS processing at 1850 °C, complementing the result obtained in Fig. 2a. Uniform distribution as well as tight encapsulation of the reinforcements B4C, SiC and CNT in the matrix, were also observed in all composites. Such microstructural features assist in densification by occupying the pore volume and improved mechanical properties (explained later in the following section). The grain size of HfB2 and ZrB2 varies from ~6–15 μm when reinforced with B4C (dark color phase) as shown in Figs. 4a and b. With the reinforcement of SiC, the grain size reduces to ~6–9 μm (Figs. 4c and d). It is important to be mentioned here that SiC appeared brighter than B4C. As seen from the microstructure (Fig. 4e 114
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and 394 GPa, respectively, which further increases to 426 GPa and 417 GPa, respectively, with the reinforcement of SiC. The synergistic effect of the solid solution formation and CNT reinforcement has been comprehended with the further increase in the modulus values to 468 GPa for HZ20S6B and 481 GPa for HZ20S6B6C. An increase in the elastic recovery, re (from 0.15 to 0.26, see Table 1) and the decrease in the resistance to deformation, rd (0.85–0.74, see Table 1) has resulted in the high hardness of HfB2-ZrB2-SiC composites. Further, an increasing trend of H/E confirms that the resistance to plastic deformation increases up to 0.057 (see Table 1) with the synergistic addition of SiC, B4C and CNT reinforcements. All the reinforcements have shown a significant effect on enhancing the fracture toughness of the SPS processed HfB2-ZrB2-SiC composites. The fracture toughness of HfB2 and ZrB2 with 6 vol% of B4C addition were as high as 9.4 MPam0.5 and 8.9 MPam0.5, respectively (~2.3 times for HfB2-2 wt% B4C pressureless sintered at 2200 °C [21]). The fracture toughness of H20S and Z20S (processed at similar SPS conditions to that in the present study) reported in the recent literature [7] is 5.2 MPam0.5 and 5.7 MPam0.5, respectively. Similar to the hardness, the fracture toughness with SiC reinforcement further increases to 11.9 MPam0.5 and 11.3 MPam0.5, respectively, for H20S6B and Z20S6B (see Table 1). This result insinuates the synergistic effect of SiC and B4C towards enhancing the fracture toughness of HfB2/ZrB2 composites. Further, increase in the fracture toughness of HZ20S6B composite to ~12.9 MPam0.5 is attributed to the formation of (Hf, Zr)B2 solid solution. The maximum toughness of 13.8 MPam0.5 for HZ20S6B6C reveals the synergistic interplay of reinforcements (SiC, B4C, and CNT) as toughening agents as well as the solid solution strengthening towards enhancing the fracture toughness of HfB2-ZrB2 system. The reinforcements (SiC, B4C, and CNT) enhance the toughness of the UHTC material via well-established CNT pull-out, crack bridging and crack-deflection mechanisms, shown microstructurally in Figs. 4g and h. The critical energy release rate (GIC, shown in Table 1) is the measure of the energy required to propagate crack in the material. The energy is found to increase from ~215 Jm−2 and ~195 Jm−2, respectively for H6B and Z6B to ~387 Jm−2 for HZ20S6B6C, making it a potential material entailed for the high load tolerance conditions. There are various toughening mechanism in ceramic composite materials which include: crack bridging (see Fig. 4h), micro-cracking and residual stresses, discussed in detail in the following section.
2000
Load (mN)
1500
HZ20S6B6C HZ20S6B
1000
Z20S6B H20S6B
500
Z6B H6B
0 0
500
1000
1500
2000
2500
3000
Displacement (nm) Fig. 5. Load vs displacement curves for SPS processed HfB2-ZrB2-SiC composites during instrumented indentation.
and f) no difference in the phase contrast of HfB2 as well as ZrB2 is observed, attributed to the mutual solubility of both at high temperature (1850 °C). Also, the fractograph shown in Fig. 4g elicits that the CNT underwent deformation during SPS processing at 1850 °C and pressure of 30 MPa, however, some of the CNT still retain their structure. The partial deformation of CNT during SPS processing of TaC-CNT composites at 1850 °C has also been reported in the literature [6]. Similar to B4C and SiC, the retained CNT, as well as deformed CNT, sits at the intergranular regions of the UHTC matrix. The deflection and branching of cracks from the edges of the reinforcements (SiC as well as B4C); and CNT pull-out (shown in Fig. 4h) corroborates with the enhanced mechanical properties of the HfB2-ZrB2-SiC composites by mitigating the failure at high load, discussed in the following section. 3.4. Effect of B4C and CNT on the mechanical properties of HfB2-ZrB2-SiC composites The load-displacement curve shown in Fig. 5 using the Oliver-Pharr indentation method was utilized to calculate the hardness, modulus and plasticity index of the HfB2-ZrB2-SiC composites (presented in Table 1). The hardness of H6B and Z6B samples, i.e. ~16.9 GPa and 17.6 GPa, respectively, is comparable to that reported in the literature [20]. With the addition of SiC, the hardness of these samples was observed to further increase to 20.6 GPa and 22.7 GPa, respectively. This increase in the hardness is attributed to the reduction in the grain size, as observed in Fig. 4. In HZ20S6B sample, the hardness further increases to 25.3 GPa, which is attributed to the formation of solid-solution. Further, the synergistic interplay of reinforcements (B4C, SiC, and CNT) as toughening agent (shown later in this section) and mutual solubility of the matrices (HfB2 and ZrB2) led to the highest hardness (~27.2 GPa) of HZ20S6B6C sample. Similar to hardness, the modulus of the SPS processed HfB2-ZrB2-SiC composites increase with the B4C and CNT reinforcements (see Table 1). The elastic modulus of H6B and Z6B composites are 405 GPa
3.5. Effect of residual stress on the mechanical properties of HfB2-ZrB2-SiC composites The mismatch in the thermal expansion coefficient (CTE) of the matrix and reinforced particles generates interfacial residual stress in the material, which ordains the strengthening mechanisms in UHTC composites. The interfacial residual stress (if compressive in nature) lowers the high tensile stress in the matrix, thus, favors crack closure. In the recent report by authors [2], it has been established that the contribution of residual stresses (due to SiC and CNT reinforcements) in enhancing the fracture toughness (of ZrB2) during SPS processing is ~12%. Ignoring the effect of the distribution and size of the
Table 1 Mechanical properties of the SPS processed HfB2-ZrB2-SiC composites. Composites
H (GPa)
H6B Z6B H20S6B Z20S6B HZ20S6B HZ20S6B6C
16.9 17.6 21.6 22.7 25.3 27.2
± ± ± ± ± ±
E (GPa) 1.2 2.1 0.9 1.1 0.6 0.7
405 394 426 417 468 481
± ± ± ± ± ±
7 9 11 8 5 10
re
rd
H/E
KIC (MPam0.5)
GIC (Jm−2)
0.15 0.17 0.19 0.23 0.24 0.26
0.85 0.83 0.81 0.77 0.76 0.74
0.042 0.045 0.051 0.054 0.054 0.057
9.4 ± 1.6 8.9 ± 1.7 11.9 ± 0.7 11.3 ± 0.9 12.9 ± 0.6 13.8 ± 0.4
214.8 195.2 327.0 297.9 348.1 387.3
± ± ± ± ± ±
3.8 4.2 1.6 2.1 1.3 1.0
H = hardness, E = elastic modulus, re, = plasticity index in terms of elastic recovery, rd = resistance to deformation, KIC = fracture toughness and GIC = critical energy release rate. 115
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ε′ = (αr − αm) ΔT
reinforcements (B4C and CNT), the structural stability of HfB2-ZrB2-SiC composites has been computed analytically. The CTE of a composite material is given as [22]:
(α eff )u = fm αm + fr αr +
(αm − αr ) ⎡ 1 1 1⎤ − − 1 1 ⎢ ⎥ K K K − K ⎣ eff m r⎦ Km r
where, Er, Em, νr and νm are the Young's modulus and Poisson's ratio (calculated using ROM [11,28]) of the reinforcement and matrix respectively; ε′ is the thermal expansion misfit strain and ΔT is the temperature at which stresses begin to accumulate (set as 1400 °C) [27,29]. Young's modulus for the reinforced phase, Er is estimated by ROM, utilizing the modulus of matrix (HfB2/ZrB2/equal proportion of both) and composite (Ec). Similarly, the residual stresses (σ0) evaluated using Hsueh's model, tabulated in Table 2 is given by the relation [30]:
(10)
The above Eq. (10) is further modified to calculate both the upper and lower bounds of CTE, considering that Kl ≤ Keff ≤ Ku, where the upper and lower bounds of K can be obtained from Hashin-Shtrikman (HS) bounds [23]:
(α eff )u
σo
Kr (3Km + 4 Gm) = αm − fr (αm − αr ) Km (3Kr + 4 Gm) + 4 fr Gm (Kr − Km )
Km (3Kr + 4 Gr ) Kr (3Km + 4 Gr ) + 4 fm Gr (Km − Kr )
(12)
where, (αeff)u and (αeff)l are the upper and lower bounds of CTE respectively of a given composite whereas α, K, G and f are CTE, bulk modulus, shear modulus and volume fraction with subscript “m” and “r” set for matrix and reinforcement, respectively. The values of ν (taken as 0.12, 0.11, 0.14, 0.18 and 0.17, respectively for HfB2, ZrB2, SiC, B4C and CNT), α (taken as 6.3 × 10−6 K−1, 5.9 × 10−6 K−1, 3.5 × 10−6 K−1, 5.5 × 10−6 K−1 and 2.5 × 10−6 K−1, respectively for HfB2, ZrB2, SiC, B4C and CNT), K (taken as 230 GPa, 229 GPa, 234 GPa, 218 GPa and 190 GPa, respectively for HfB2, ZrB2, SiC, B4C and CNT) and G (taken as 212 GPa, 211 GPa, 41 GPa, 180 GPA and 150 GPa, respectively for HfB2, ZrB2, SiC, B4C and CNT) used for calculations have been taken from the literature [11,13,24,25]. High temperature processing of the UHTC based composite requires the quantification of effective residual stresses (ΔT = 1400 °C [25] where these stresses develop). During SPS processing, the composites underwent cooling from the final sintering temperature (i.e. 1850 °C) to room temperature. Therefore, the matrix (HfB2/ZrB2/HfB2-ZrB2) tends to shrink faster than the reinforcements (SiC, B4C, and CNT) leading to the generation of compressive stresses in the reinforced particles, σr and tensile stresses state in the matrix, σm as evaluated using Taya's model [26] and presented in Table 2. The interfacial compressive stress field in the composite play a vital role in dictating the toughening mechanism in UHTC [27] due to their high operational temperature.
σr
=
σm
=
β=
−
(1 − f ) ε′ σm f
+ Em
(13)
2fβε′ (1 − f )(β + 2)(1 + νm) + 3βf (1 − νm)
(14)
1 + νm Er 1 − 2νr Em
H6B Z6B H20S6B Z20S6B HZ20S6B HZ20S6B6C
CTE (×10−6 K−1)
Residual stresses (MPa)
ROM
Hsueh's model σο
6.25 6.72 5.69 6.06 5.88 5.63
HS model UB
LB
6.25 6.72 5.69 6.06 5.88 5.63
4.75 5.12 5.31 4.94 5.12 5.37
397.3 406.1 194.2 205.8 91.7 62.9
σr
322.8 380.6 185.0 192.9 82.7 68.5
−31.2 −28.2 −35.9 −32.5 −39.2 −41.8
αm) ΔT +
1 − 2νr Er
(17)
Fully dense novel composites of HfB2-ZrB2-SiC ceramics with B4C and CNT reinforcements were processed using SPS at 1850 °C. The partial graphitization of CNT into graphitic structure was confirmed by the diminution in the ID/IG ratio (from ~1.04 to ~0.90) and further complemented by the SEM, however, most of the CNT remain intact even after SPS processing. The uniform distribution of the reinforcements (SiC, B4C and CNT) in the SPS processed monolithic UHTC have led to an enhancement in the hardness (from 16.9 GPa in H6B, 17.6 GPa in Z6B, to 21.6 GPa in H20S6B, 22.7 GPa in Z20S6B, 25.3 GPa in HZ20S6B, and 27.2 GPa in HZ20S6B6C) and fracture toughness (from 9.4 MPam0.5 in H6B, 8.9 MPam0.5 in Z6B, to 11.9 MPam0.5 in H20S6B, 11.3 MPam0.5 in Z20S6B, 12.9 MPam0.5 in HZ20S6B, and 13.8 MPam0.5 in HZ20S6B6C). The addition of secondary phases (SiC, B4C, and CNT) generate the beneficial interfacial compressive residual stresses in the material which helps in mitigating the failure at higher load (i.e. enhancement in the fracture toughness). The results unequivocally
Taya's model σm
(αr − 1 + νm (1 + νr ) Em
4. Conclusions
(15)
Table 2 Theoretical calculation of the coefficient of thermal expansion and residual stresses in the SPS processed HfB2-ZrB2-SiC composites. Composites
=
The CTE estimated using ROM matches well with the upper bound of the HS model (see Table 2) which supports the homogeneous distribution of the secondary phases, as evident from the microstructure shown in Fig. 4. It is apparent from Table 2 that the addition of secondary phase introduces interfacial stresses (tensile in the matrix and compressive in the reinforcement) which improves the structural integrity of the UHTC composites by enhancing the fracture toughness (values presented in Table 1). The increase in the compressive stress with increasing the complexity of the UHTC composites from 31.2 MPa (in H6B) and 28.2 MPa (in Z6B) with B4C reinforcement to 35.9 MPa (in H20S6B) and 32.5 MPa (in Z20S6B) with SiC reinforcement to 39.2 MPa (in HZ20S6B) due to the solid solution formation and 41.8 MPa with CNT reinforcement (with addition of secondary phases from B4C to SiC and then CNT) can be clearly seen. This increasing trend in the compressive residual stress in the material lowers the residual tensile stresses (up to 68.5 MPa in HZ20S6B6C, see Table 2) and, thus, favors enhancement in the fracture toughness (up to 13.8 MPam0.5 for HZ20S6B6C, see Table 1). In summary, the effect of microstructural features (distribution of the reinforcements) on enhancing the fracture toughness of the SPS processed HfB2-ZrB2-SiC composites is schematically shown in Fig. 6. As we add the reinforcement from B4C to SiC, the combination of both and then CNT (making the UHTC system more complex), the impediment to the crack propagation increases. The comparison of the microstructural observation (shown in Fig. 4) and mechanical properties (see Table 1) unequivocally establishes the synergy between the solid solution formation (in HfB2-ZrB2 system) and reinforcements (of SiC, B4C and CNT) towards enhancing the fracture toughness of the diboride composites (with hardness of ~27.2 GPa, modulus of 481 GPa and fracture toughness of 13.8 MPam0.5 in HZ20S6B6C). All the reinforcements have contributed to enhance the fracture toughness of the processed composites via crack-deflection, branching and CNT pull-out mechanisms. The results discerned have proved HZ20S6B6C sample as a viable material for application where damage tolerance is an issue.
(11)
(α eff )l = αr + fm (αr − αm )
(16)
ROM = rule of mixture, CTE = coefficient of thermal expansion, HS = HashinShtrikman, UB = upper bound and LB = lower bound. 116
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HfB2/ZrB2
(a)
Monolithic HfB2/ZrB2
(b)
Crack branching and deflection by B4C
Wide crack opening
B4C with B4C
No reinforcement = 0 r KIC
r
KIC
~3-4 MPam0.5 HfB2/ZrB2
-31.2 MPa for H6B 9.4 MPam0.5 for H6B HfB2/ZrB2
(c)
(d)
Crack branching and deflection by B4C and SiC CNT SiC
B4C
SiC B4C
r
KIC
with SiC and B4C -39.2 MPa for HZ20S6B 12.9 MPam0.5 for HZ20S6B
r
KIC
with SiC, B4C and CNT -41.8 MPa for HZ20S6B6C 13.8 MPam0.5 for HZ20S6B6C
Fig. 6. Schematic illustrating the strengthening mechanism in HfB2-ZrB2-SiC composites.
establish the synergy of solid solution formation and reinforcements (B4C and CNT) on the processing and mechanical properties of HfB2ZrB2-SiC ceramics, making them a suitable damage tolerant structural material for re-entry space applications.
110. [8] A. Abdollahi, M. Mashhadi, Effect of B4C, MoSi2, nano SiC and micro-sized SiC on pressureless sintering behavior, room-temperature mechanical properties and fracture behavior of Zr(Hf)B2-based composites, Ceram. Int. 40 (7, Part B) (2014) 10767–10776. [9] L. Weng, et al., The effect of B4C on the microstructure and thermo-mechanical properties of HfB2-based ceramics, J. Alloys Compd. 473 (1–2) (2009) 314–318. [10] M.S. Asl, M.G. Kakroudi, Characterization of hot-pressed graphene reinforced ZrB2–SiC composite, Mater. Sci. Eng. A 625 (2015) 385–392. [11] A. Nisar, et al., Effect of carbon nanotube on processing, microstructural, mechanical and ablation behavior of ZrB2-20SiC based ultra-high temperature ceramic composites, Carbon 111 (2017) 269–282. [12] A. Nisar, K. Balani, Role of interfaces on multi-length scale Wear mechanics of TaCbased composites, Adv. Eng. Mater. 19 (5) (2017) 1600713. [13] Vladislav Domnich, et al., Boron carbide: structure, properties, and stability under stress, J. Am. Ceram. Soc. 94 (11) (2011) 3605–3628. [14] L. Weng, W. Han, C. Hong, Fabrication and thermal shock resistance of HfB 2-SiC composite with B 4 C additives, Mater. Sci.-Pol. 29 (4) (2011) 248–252. [15] G.R. Anstis, et al., A critical evaluation of indentation techniques for measuring fracture toughness: I, direct crack measurements, J. Am. Ceram. Soc. 64 (9) (1981) 533–538. [16] X. Wang, N.P. Padture, H. Tanaka, Contact-damage-resistant ceramic/single-wall carbon nanotubes and ceramic/graphite composites, Nat. Mater. 3 (8) (2004) 539–544. [17] G.D. Quinn, R.C. Bradt, On the Vickers indentation fracture toughness test, J. Am. Ceram. Soc. 90 (3) (2007) 673–680. [18] C.W. Bale, et al., FactSage thermochemical software and databases, Calphad 26 (2) (2002) 189–228. [19] B. Post, F.W. Glaser, D. Moskowitz, Transition metal diborides, Acta Metall. 2 (1) (1954) 20–25. [20] S. Zhu, et al., Microwave sintering of a ZrB2–B4C particulate ceramic composite, Compos. A: Appl. Sci. Manuf. 39 (3) (2008) 449–453. [21] J. Zou, G.-J. Zhang, Y.-M. Kan, Pressureless densification and mechanical properties of hafnium diboride doped with B4C: from solid state sintering to liquid phase sintering, J. Eur. Ceram. Soc. 30 (12) (2010) 2699–2705. [22] B.W. Rosen, Z. Hashin, Effective thermal expansion coefficients and specific heats of composite materials, Int. J. Eng. Sci. 8 (2) (1970) 157–173. [23] H.L. Brown, P.E. Armstrong, C.P. Kempter, Elastic properties of some polycrystalline transition-metal monocarbides, J. Chem. Phys. 45 (2) (1966) 547–549.
Acknowledgments Authors at Indian Institute of Technology Kanpur (IITK) acknowledge the financial support received from IITK-Space Technology Cell and Indian Space Research Organization (ISRO), Trivandrum, India. Authors also acknowledge the support from the Advanced Centre for Materials Science (ACMS) at IITK for extending characterization facilities (SEM and instrumented indentation testing). KB acknowledges Swarnajayanti fellowship (DST/STF/ETA-02/2016-17). References [1] W.G. Fahrenholtz, G.E. Hilmas, Ultra-high temperature ceramics: materials for extreme environments, Scr. Mater. 129 (2017) 94–99. [2] A. Nisar, S. Ariharan, K. Balani, Establishing microstructure-mechanical property correlation in ZrB2-based ultra-high temperature ceramic composites, Ceram. Int. 43 (16) (2017) 13483–13492. [3] A. Nisar, S. Ariharan, K. Balani, Densification kinetics and mechanical properties of tantalum carbide, Int. J. Refract. Met. Hard Mater. 73 (2018) 221–230. [4] A. Nisar, et al., Oxidation studies on TaC based ultra-high temperature ceramic composites under plasma arc jet exposure, Corros. Sci. 109 (2016) 50–61. [5] I.G. Talmy, J.A. Zaykoski, M.M. Opeka, High-temperature chemistry and oxidation of ZrB2 ceramics containing SiC, Si3N4, Ta5Si3, and TaSi2, J. Am. Ceram. Soc. 91 (7) (2008) 2250–2257. [6] A. Nisar, S. Ariharan, K. Balani, Synergistic reinforcement of carbon nanotubes and silicon carbide for toughening tantalum carbide based ultrahigh temperature ceramic, J. Mater. Res. 31 (6) (2016) 682–692. [7] A. Nisar, K. Balani, Phase and microstructural correlation of spark plasma sintered HfB2-ZrB2 based ultra-high temperature ceramic composites, Coatings 7 (8) (2017)
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Acta Mater. 56 (18) (2008) 5345–5354. [28] W.G. Fahrenholtz, et al., Ultra-High Temperature Ceramics: Materials for Extreme Environment Applications, John Wiley & Sons, 2014. [29] J. Watts, et al., Measurement of thermal residual stresses in ZrB2–SiC composites, J. Eur. Ceram. Soc. 31 (9) (2011) 1811–1820. [30] Y. Chen, K. Balani, A. Agarwal, Do thermal residual stresses contribute to the improved fracture toughness of carbon nanotube/alumina nanocomposites? Scr. Mater. 66 (6) (2012) 347–350.
[24] E. Zapata-Solvas, et al., Mechanical properties of ZrB2- and HfB2-based ultra-high temperature ceramics fabricated by spark plasma sintering, J. Eur. Ceram. Soc. 33 (7) (2013) 1373–1386. [25] M. Mallik, et al., Electrical and thermophysical properties of ZrB 2 and HfB 2 based composites, J. Eur. Ceram. Soc. 32 (10) (2012) 2545–2555. [26] M. Taya, et al., Toughening of a particulate-reinforced ceramic-matrix composite by thermal residual stress, J. Am. Ceram. Soc. 73 (5) (1990) 1382–1391. [27] D. Ghosh, G. Subhash, N. Orlovskaya, Measurement of scratch-induced residual stress within SiC grains in ZrB2–SiC composite using micro-Raman spectroscopy,
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Processing, microstructure and mechanical properties of HfB2-ZrB2-SiC composites: Effect of B4C and carbon nanotube reinforcements Ambreen Nisara,b, a b
⁎,1
T
, Mohammad Mohsin Khana,1,2, Shipra Bajpaia, Kantesh Balania
High Temperature Ceramic Laboratory, Department of Materials Science and Engineering, Indian Institute of Technology Kanpur, Kanpur 208016, UP, India School of Engineering, The University of British Columbia, Kelowna V1V 1V7, Canada
A R T I C LE I N FO
A B S T R A C T
Keywords: Ultra-high temperature ceramic (UHTC) Carbon nanotubes (CNT) Spark plasma sintering (SPS) Zirconium diboride (ZrB2) Hafnium diborides (HfB2)
The fully dense HfB2-ZrB2-SiC composites were processed using spark plasma sintering (SPS) at 1850 °C. The effect of reinforcements (B4C and CNT) on the densification as well as mechanical properties were investigated and compared (with monolithic) in the present study. The study showed that the addition of B4C and CNT were not only beneficial for the densification but also towards enhancing the mechanical properties (hardness, elastic modulus, and fracture toughness) of HfB2-ZrB2-SiC composites. The augmentation in the mechanical properties establish the synergy between solid solution formation (with the equimolar composition of HfB2/ZrB2) and the reinforcements (SiC, B4C, and CNT). The highest increase in the indentation fracture toughness with the reinforcements of B4C as well as CNT is > 3 times (~13.8 MPam0.5 when it is 3–4 MPam0.5 for monolithic ZrB2/ HfB2) on HfB2-ZrB2-SiC composites, which is attributed to the crack deflection and pull-out mechanisms. An increase in the analytically quantified interfacial compressive residual stresses in the composites during SPS processing with the synergistic addition of reinforcements (SiC, B4C, and CNT) and its effect on the indentation fracture toughness has also been addressed.
1. Introduction Transition metal diborides such as hafnium diboride (HfB2) and zirconium diboride (ZrB2) owing to their high melting temperature (3380 °C and 3245 °C, respectively), high thermal conductivity (104 W/ m/K and 60 W/m/K, respectively), and resistance to chemical as well as wear attack are referred to as ultra-high temperature ceramics (UHTCs) [1]. UHTCs are discernible choice for several advanced structural applications such as armor, cutting tool, molten metal containment, leading edges, and propulsion system in hypersonic re-entry vehicles etc. [2–4]. However, the use of ZrB2 and HfB2 as a structural material has considerably been limited due to their poor sinterability, and low fracture toughness (3–4 MPam0.5) [2]. In order to achieve good densification and lower the sintering temperature (> 500 °C than that using other conventional sintering techniques), spark plasma sintering (SPS) has proven to be the best choice. In this technique, the densification is stimulated by the combined application of the pulsed electric field as well as the pressure which leads to evolution of a fine and homogeneous microstructure. Recent studies have shown that the reinforcements such as silicon
nitride (Si3N4) [5], silicon carbide (SiC) [2,4,6,7], boron carbide (B4C) [8,9], graphene nanoplatelets (GNP) [10], carbon nanotubes (CNT) [2–4,6,7,11,12] etc. could effectively increase the sinterability facilitating a fairly dense and uniform microstructure which influence the strength, oxidation resistance and other characteristics. Addition of 20 vol% SiC in diborides (i.e. H20S/Z20S) has already been reported as the baseline UHTC based on their high temperature thermo-mechanical performance [1,11], but still, there is a scope to further enhance their mechanical properties (hardness, fracture toughness etc.). Thus, there is a need to add more components to the system (making the material with ternary or quaternary phase) to perceive their effect on the thermo-mechanical performance. In this regard, B4C known as the third hardest natural material is chosen as a reinforcement to the baseline materials (H20S/Z20S) due to low density (2.52 g cm-3), high hardness (35–45 GPa), and abrasion resistance [13]. Weng et al. [14] showed that the addition of 2 vol% B4C in H20S has resulted the fracture toughness of ~7 MPam0.5. Reinforcement of CNT in the UHTC system has also shown to improve its fracture toughness (from ~3 MPam0.5 for monolithic ZrB2 to 8.4 MPam0.5 with 10 vol% of CNT in ZrB2), apart from enhancing the densification (nearly 100%) and imparting an
⁎
Corresponding author. E-mail address: [email protected] (A. Nisar). 1 Have equally contributed as first authors. 2 Current address: National Institute of Technology, Srinagar 190006, India. https://doi.org/10.1016/j.ijrmhm.2019.02.014 Received 24 September 2018; Received in revised form 10 February 2019; Accepted 17 February 2019 Available online 19 February 2019 0263-4368/ © 2019 Elsevier Ltd. All rights reserved.
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present study. The powder mixtures were dry ball-milled using tungsten carbide jars and balls for 8 min at the speed of 500 rpm with the ball to powder weight ratio as 5:2. The chosen parameters are as per the previous studies [2,7,11] which confirms that CNT had no obvious structural damage. The ball milled composite powders were then processed using spark plasma sintering (SPS, Dr. Sinter 515S, Kanagawa, Japan) at maximum holding temperature of 1850 °C for 10 min with the heating rate of 100 °C min–1 under vacuum (< 6 Pa) and uniaxial pressure of 30 MPa. Graphite dies, and punches were utilized for fabricating pellets of 15 mm diameter and thickness of 3 mm. The ram displacement data recorded during SPS sintering cycle was utilized to calculate the instantaneous densification behavior of HfB2-ZrB2-SiC composites using the formula:
acceptable level of thermal shock making UHTC a preferred candidate for aerospace applications [4,7,11]. Nisar et.al [2] delineated the contribution of residual stresses as well as reinforcement of SiC (i.e. 20 vol % in ZrB2) and CNT (i.e. 10 vol% in ZrB2) to be ~12%, 14%, and 55%, respectively towards enhancing the fracture toughness of ZrB2 ceramic. A recent report by authors [7] on the HfB2-ZrB2 system (with equimolar concentration of HfB2 and ZrB2 forming solid solution during SPS processing) with SiC and CNT reinforcement has shown to enhance the fracture toughness up to 196% (from ~5.2 MPam0.5 to 10.2 MPam0.5) has been attributed to the synergy of solid solution formation and complete densification. Generally speaking, the notion behind choosing all these reinforcements (i.e. SiC, B4C, and CNT) is to remove the surface oxide from the ceramic during sintering which inhibited the sintering process (detailed explaination in Section 3.1). Herein, the idea is to appropriately sinter/densify (HfB2/ZrB2-SiC) with B4C and CNT addition and to observe its effect on the processing and mechanical properties. In this paper, the fully dense (> 99%) HfB2-ZrB2-SiC composites with B4C and CNT reinforcements were fabricated using SPS at 1850 °C. The sintered HfB2-ZrB2-SiC composites with B4C and CNT reinforcements were characterized in terms of phase, microstructure and mechanical properties (hardness, elastic modulus, and fracture toughness). The effective residual stresses generated during the SPS processing of the composite material were, then, correlated with the mechanical properties.
hf ⎡ ⎤ ρinstantaneous = ⎢ hf + df − di ⎥ ⎣ ⎦
(1)
where, hf, df, and di are the thickness of the sintered sample, maximum displacement, and instantaneous displacement during sintering, respectively. The final density of the as-processed pellets was measured using the Archimedes method (with ethanol as an immersion medium) on a hydrostatic balance. 2.2. Phase, microstructural and mechanical characterizations
2. Materials and method Phase analysis of the initial powders, composite powders as well as SPS processed pellets was carried out using Rich-Seifert 2000D diffractometer operated at 25 kV and 15 mA at a scan speed of 0.5̊ s–1 and a step size of 0.02̊ using Cu Kα radiation (λ = 1.54 Å, PANanalytical, Empyrean diffractometer, Tokyo, Japan) in the 2θ range of 20̊ to 90̊. Further, to study the structural damage in the CNT, micro-Raman spectroscopy (Princeton Instruments, STR Raman, TE-PMT detector) was carried out using Nd-YAG green laser (λ = 532 nm) with a laser power of 12.5 mW in the backscattered mode. The fractured micrographs of HfB2-ZrB2-SiC composites were observed using field-emission scanning electron microscope (FESEM, JEOL, JSM-7100F, MA, USA) in the backscattered mode. Hardness and elastic modulus of the processed compacts were measured using instrumented indentation technique (MMT, CSM Instruments, Switzerland) on the polished surface (up to 0.1 μm finish using diamond suspension). The loading (and unloading at the same rate) was done at the load of 2 N with the ramp rate of 66.67 mN/s and
2.1. Processing of HfB2-ZrB2-SiC composites Commercial diborides: HfB2 and ZrB2; carbides: SiC and B4C (irregular shaped powders Samics Research Materials, Rajasthan, India) and multi-walled carbon nanotubes (CNT, Nanostructured and Amorphous Materials Inc., TX, USA, with 95% purity, an outer diameter of 40 nm, inner diameter of 20 nm and length of 1–2 μm) were used as the starting materials. SEM micrographs, shown in Fig. 1, elicits the morphology and particle size of the selected diborides HfB2 (Fig. 1a) and ZrB2 (Fig. 1b); and reinforcements B4C (Fig. 1c), SiC (Fig. 1d) and CNT (Fig. 1e). The following compositions: (i) HfB2-6 vol% B4C (labelled as H6B), (ii) ZrB2-6 vol% B4C (labelled as Z6C), (iii) HfB2–20 vol% SiC6 vol% B4C (labelled as H20S6B), (iv) ZrB2-20 vol% SiC-6 vol% B4C (labelled as Z20S6B), (v) HfB2-ZrB2 (1:1 ratio)-20 vol% SiC-6 vol% B4C (labelled as HZ20S6B) and (vi) HfB2-ZrB2 (1:1 ratio)-20 vol% SiC-6 vol % B4C-6 vol% CNT (labelled as HZ20S6B6C) have been chosen in the
Fig. 1. Electron micrographs of the inital powders (a) HfB2, (b) ZrB2, and reinforcements (c) B4C, (d) SiC, and (e) CNT. 112
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dwell of 10 s at maximum load using V-I51Vicker's indenter. The reported value is at least an average of the 15 indents for each composition. Based on the maximum depth (hm) and final depth (hf) of indentation, the plasticity index in terms of elastic recovery (re) and the resistance to deformation (rd) for HfB2-ZrB2-SiC composites and effect of B4C and CNT reinforcement have been calculated using the following relations [7]:
re =
rd =
hm − h f (2)
hm hf
(3)
hm
The indentation fracture toughness (KIC) was measured at the loading of 20 N using the universal hardness testing machine (UTM, FH10, Tinius-Olsen Ltd.) with a dwell time of 10 s. The crack lengths were measured using SEM from the centre of the indents and the fracture toughness was calculated using Anstis equation [15], see Eq. (4):
KIC = 0.016
E P H c 3/2
(4)
where H is the hardness, E is the elastic modulus (both measured experimentally), P is the load utilized and c is the radial crack length. The reported value is an average of at least 10 indents for each composition. As it has been already established in literature that the indentation technique for measuring fracture toughness is not suitable when compared to that of single edge-notched beam (SENB) method [16], but a similar trend in the fracture toughness values have been reported [17]. In this regard, it is to be noted that indentation fracture toughness is used only as a ranking parameter towards comparing the toughness of HfB2-ZrB2-SiC composites. Based on the ratio of half crack length to half indent diagonal length (referred to as c/a which is > 3.5 in the present study) the fracture toughness was employed using formulation proposed by Anstis et al. [15]. Also, the measurement of the critical energy release rate (GIC) is given by the relation: 2 2 ⎡1 − ν ⎤ GIC = KIC ⎥ ⎢ E ⎦ ⎣
(5)
where ν is the Poisson's ratio calculated using the rule of mixture (ROM) for all composites. 3. Results and discussion
Fig. 2. (a): Plot showing a variation of instantaneous densification with SPS processing time of HfB2-ZrB2-SiC composites, (b) Plot of the standard free energy change vs absolute temperature for the reactant and product (using FactSage [18] for Eqs. 6-9).
3.1. Densification of HfB2-ZrB2-SiC composites
5MO2 (s ) + 5B4 C (s ) → 5MB2 (s ) + 6BO (g ) + 4CO (g )
(8)
Based on the ram displacement recorded during SPS processing, the instantaneous densification of HfB2-ZrB2-SiC composites is presented in Fig. 2a. The sintering process comprises of the three distinct steps: (i) compaction (labelled as step I in Fig. 2a) i.e. initial increasing trend due to the rearrangement of powder particles with the application of pressure; (ii) thermal expansion (labelled as step II in Fig. 2a) i.e. intermediate decreasing trend due to expansion in ceramics with the application of temperature; and (iii) shrinkage (labelled as step III in Fig. 2a) i.e. final increasing trend due to the densification/compaction. All the composites showed maximum shrinkage in the final step, crucial in rendering pore closure and complete densification. The complete densification (nearly ~100%) of all composites has been attributed to the reinforcements (SiC, B4C, and CNT) which are very effective in promoting the densification of diborides [7,9,11]. The selected reinforcement act as the sintering aid by removing the surface oxide as per the following reactions:
MO2 (s ) + 5B2 O3 (l) + 5C (s ) → MB2 (s ) + 5CO (g )
(9)
M = Zr, Hf The plot of the standard free energy change against absolute temperature (i.e. Ellingham diagram using FactSage [18]) for reactants and products in the aforementioned reactions is shown in Fig. 2b. It is inferred from the plot that the feasible temperature at which the phase change in these reactions occur is ≥1405.1 °C, 1051.9 °C, 1500.1 °C, and 1792.6 °C, respectively (i.e. when ΔG = 0). Through these calculations it is anticipated that sintering of these composites at 1850 °C ensures the possibility of occurrence of each of the aforementioned reactions. This reveals that the reinforcement of SiC, B4C as well as CNT act as a sintering aid by removing the surface oxides/impurities. Also, these reactions are known to enhance the densification of diborides based ceramics by lowering the diffusion barrier [9]. The full densification of these refractory ceramic composites would govern their mechanical properties, later shown in Section 3.4.
2 2 2 SiC (s ) + O2 (g ) → SiO2 (s ) + CO (g ) 3 3 3
(6)
3.2. Phase analysis of HfB2-ZrB2-SiC composites
SiO2 (s ) + 3C (s ) → SiC (s ) + 2CO (g )
(7)
The comparison of the various phase present in the initial powders 113
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Fig. 4. Fractographs of SPS processed HfB2-ZrB2-SiC composites in backscattered mode (a) H6B, (b) Z6B, (c) H20S6B, (d) Z20S6B, (e) HZ20S6B, (f) HZ20S6B6C. High magnification micrograph showing (g) deformation as well as retention of CNT after SPS processing in HZ20S6B6C sample, (h) crack-deflection, branching and CNT pull-out mechanisms. Fig. 3. (a): XRD spectra of the initial powder, composite powder as well as SPS processed HfB2-ZrB2-SiC composites and, (b) Raman spectra showing signature peaks of CNT in pristine CNT as well as SPS processed HZ20S6B6C sample.
for powder and HZ20S6B6C pellet, called as 2G-peak and is attributed to the overtone of the D-peak. Another band at 2912.7 cm−1 and 2917.0 cm−1 known as (D + G)-peak was also observed. With respect to the pristine CNT, all the Raman bands of the SPS processed HZ20S6B6C are shifted towards the higher wave number indicating the generation of compressive stresses in the CNT, attributed to the thermal contraction of the UHTC matrix during SPS cooling. The ratio of the defect to graphitic peak i.e. ID/IG decreases from 1.04 to 0.90 for the SPS processed HZ20S6B6C sample eliciting the conversion of CNT into graphitic structures. Similar observations have previously been reported by the authors for other UHTC-CNT composites [4,7,11].
as well as SPS processed HfB2-ZrB2-SiC composites are shown in Fig. 3a. The XRD pattern confirms the retention of the starting matrix (i.e. HfB2 and ZrB2) as well as the reinforcements (B4C and SiC). No new phases or any peak shift in SiC and B4C phases is observed in the composites, which denies the formation of any solid solution in between the reinforcements and matrix phase up to 1850 °C. However, the mutual solubility of HfB2-ZrB2 was observed in HZ20S6B as well as in HZ20S6B6C composites elicited from the merged peaks corresponding to the parent phases (HfB2/ZrB2). The mutual solubility of diborides has already been reported in previous literature [7,19]. In order to confirm the presence as well as the structural deformation (if any) in the CNT during SPS processing, Raman spectra of HZ20S6B6C sample was compared to that of the pristine CNT, as shown in Fig. 3b. The spectra presented has been normalized to the highest peak intensity. The D-band is activated in the first order scattering process of sp2 carbons by the presence of in-plane substitutional heteroatomic vacancies, grain boundaries and represents the disorder induced in the CNT, observed at 1341.8 cm−1 and 1349.75 cm−1, respectively, for powder and pellet. The G-peak observed at 1577.7 cm−1 and 1585.1 cm−1 for powder and HZ20S6B6C pellet, respectively, is a tangential band which is related to the graphite tangential E2g Raman active mode where the two atoms in the graphene unit cell vibrate tangentially one against the other. The Raman spectra also exhibit second-order weak band respectively at 2683.7 cm−1 and 2688.7 cm−1
3.3. Microstructural analysis of HfB2-ZrB2-SiC composites Fractured micrograph of HfB2-ZrB2-SiC composites shown in Fig. 4 elicit that all the samples were fully densified after SPS processing at 1850 °C, complementing the result obtained in Fig. 2a. Uniform distribution as well as tight encapsulation of the reinforcements B4C, SiC and CNT in the matrix, were also observed in all composites. Such microstructural features assist in densification by occupying the pore volume and improved mechanical properties (explained later in the following section). The grain size of HfB2 and ZrB2 varies from ~6–15 μm when reinforced with B4C (dark color phase) as shown in Figs. 4a and b. With the reinforcement of SiC, the grain size reduces to ~6–9 μm (Figs. 4c and d). It is important to be mentioned here that SiC appeared brighter than B4C. As seen from the microstructure (Fig. 4e 114
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and 394 GPa, respectively, which further increases to 426 GPa and 417 GPa, respectively, with the reinforcement of SiC. The synergistic effect of the solid solution formation and CNT reinforcement has been comprehended with the further increase in the modulus values to 468 GPa for HZ20S6B and 481 GPa for HZ20S6B6C. An increase in the elastic recovery, re (from 0.15 to 0.26, see Table 1) and the decrease in the resistance to deformation, rd (0.85–0.74, see Table 1) has resulted in the high hardness of HfB2-ZrB2-SiC composites. Further, an increasing trend of H/E confirms that the resistance to plastic deformation increases up to 0.057 (see Table 1) with the synergistic addition of SiC, B4C and CNT reinforcements. All the reinforcements have shown a significant effect on enhancing the fracture toughness of the SPS processed HfB2-ZrB2-SiC composites. The fracture toughness of HfB2 and ZrB2 with 6 vol% of B4C addition were as high as 9.4 MPam0.5 and 8.9 MPam0.5, respectively (~2.3 times for HfB2-2 wt% B4C pressureless sintered at 2200 °C [21]). The fracture toughness of H20S and Z20S (processed at similar SPS conditions to that in the present study) reported in the recent literature [7] is 5.2 MPam0.5 and 5.7 MPam0.5, respectively. Similar to the hardness, the fracture toughness with SiC reinforcement further increases to 11.9 MPam0.5 and 11.3 MPam0.5, respectively, for H20S6B and Z20S6B (see Table 1). This result insinuates the synergistic effect of SiC and B4C towards enhancing the fracture toughness of HfB2/ZrB2 composites. Further, increase in the fracture toughness of HZ20S6B composite to ~12.9 MPam0.5 is attributed to the formation of (Hf, Zr)B2 solid solution. The maximum toughness of 13.8 MPam0.5 for HZ20S6B6C reveals the synergistic interplay of reinforcements (SiC, B4C, and CNT) as toughening agents as well as the solid solution strengthening towards enhancing the fracture toughness of HfB2-ZrB2 system. The reinforcements (SiC, B4C, and CNT) enhance the toughness of the UHTC material via well-established CNT pull-out, crack bridging and crack-deflection mechanisms, shown microstructurally in Figs. 4g and h. The critical energy release rate (GIC, shown in Table 1) is the measure of the energy required to propagate crack in the material. The energy is found to increase from ~215 Jm−2 and ~195 Jm−2, respectively for H6B and Z6B to ~387 Jm−2 for HZ20S6B6C, making it a potential material entailed for the high load tolerance conditions. There are various toughening mechanism in ceramic composite materials which include: crack bridging (see Fig. 4h), micro-cracking and residual stresses, discussed in detail in the following section.
2000
Load (mN)
1500
HZ20S6B6C HZ20S6B
1000
Z20S6B H20S6B
500
Z6B H6B
0 0
500
1000
1500
2000
2500
3000
Displacement (nm) Fig. 5. Load vs displacement curves for SPS processed HfB2-ZrB2-SiC composites during instrumented indentation.
and f) no difference in the phase contrast of HfB2 as well as ZrB2 is observed, attributed to the mutual solubility of both at high temperature (1850 °C). Also, the fractograph shown in Fig. 4g elicits that the CNT underwent deformation during SPS processing at 1850 °C and pressure of 30 MPa, however, some of the CNT still retain their structure. The partial deformation of CNT during SPS processing of TaC-CNT composites at 1850 °C has also been reported in the literature [6]. Similar to B4C and SiC, the retained CNT, as well as deformed CNT, sits at the intergranular regions of the UHTC matrix. The deflection and branching of cracks from the edges of the reinforcements (SiC as well as B4C); and CNT pull-out (shown in Fig. 4h) corroborates with the enhanced mechanical properties of the HfB2-ZrB2-SiC composites by mitigating the failure at high load, discussed in the following section. 3.4. Effect of B4C and CNT on the mechanical properties of HfB2-ZrB2-SiC composites The load-displacement curve shown in Fig. 5 using the Oliver-Pharr indentation method was utilized to calculate the hardness, modulus and plasticity index of the HfB2-ZrB2-SiC composites (presented in Table 1). The hardness of H6B and Z6B samples, i.e. ~16.9 GPa and 17.6 GPa, respectively, is comparable to that reported in the literature [20]. With the addition of SiC, the hardness of these samples was observed to further increase to 20.6 GPa and 22.7 GPa, respectively. This increase in the hardness is attributed to the reduction in the grain size, as observed in Fig. 4. In HZ20S6B sample, the hardness further increases to 25.3 GPa, which is attributed to the formation of solid-solution. Further, the synergistic interplay of reinforcements (B4C, SiC, and CNT) as toughening agent (shown later in this section) and mutual solubility of the matrices (HfB2 and ZrB2) led to the highest hardness (~27.2 GPa) of HZ20S6B6C sample. Similar to hardness, the modulus of the SPS processed HfB2-ZrB2-SiC composites increase with the B4C and CNT reinforcements (see Table 1). The elastic modulus of H6B and Z6B composites are 405 GPa
3.5. Effect of residual stress on the mechanical properties of HfB2-ZrB2-SiC composites The mismatch in the thermal expansion coefficient (CTE) of the matrix and reinforced particles generates interfacial residual stress in the material, which ordains the strengthening mechanisms in UHTC composites. The interfacial residual stress (if compressive in nature) lowers the high tensile stress in the matrix, thus, favors crack closure. In the recent report by authors [2], it has been established that the contribution of residual stresses (due to SiC and CNT reinforcements) in enhancing the fracture toughness (of ZrB2) during SPS processing is ~12%. Ignoring the effect of the distribution and size of the
Table 1 Mechanical properties of the SPS processed HfB2-ZrB2-SiC composites. Composites
H (GPa)
H6B Z6B H20S6B Z20S6B HZ20S6B HZ20S6B6C
16.9 17.6 21.6 22.7 25.3 27.2
± ± ± ± ± ±
E (GPa) 1.2 2.1 0.9 1.1 0.6 0.7
405 394 426 417 468 481
± ± ± ± ± ±
7 9 11 8 5 10
re
rd
H/E
KIC (MPam0.5)
GIC (Jm−2)
0.15 0.17 0.19 0.23 0.24 0.26
0.85 0.83 0.81 0.77 0.76 0.74
0.042 0.045 0.051 0.054 0.054 0.057
9.4 ± 1.6 8.9 ± 1.7 11.9 ± 0.7 11.3 ± 0.9 12.9 ± 0.6 13.8 ± 0.4
214.8 195.2 327.0 297.9 348.1 387.3
± ± ± ± ± ±
3.8 4.2 1.6 2.1 1.3 1.0
H = hardness, E = elastic modulus, re, = plasticity index in terms of elastic recovery, rd = resistance to deformation, KIC = fracture toughness and GIC = critical energy release rate. 115
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ε′ = (αr − αm) ΔT
reinforcements (B4C and CNT), the structural stability of HfB2-ZrB2-SiC composites has been computed analytically. The CTE of a composite material is given as [22]:
(α eff )u = fm αm + fr αr +
(αm − αr ) ⎡ 1 1 1⎤ − − 1 1 ⎢ ⎥ K K K − K ⎣ eff m r⎦ Km r
where, Er, Em, νr and νm are the Young's modulus and Poisson's ratio (calculated using ROM [11,28]) of the reinforcement and matrix respectively; ε′ is the thermal expansion misfit strain and ΔT is the temperature at which stresses begin to accumulate (set as 1400 °C) [27,29]. Young's modulus for the reinforced phase, Er is estimated by ROM, utilizing the modulus of matrix (HfB2/ZrB2/equal proportion of both) and composite (Ec). Similarly, the residual stresses (σ0) evaluated using Hsueh's model, tabulated in Table 2 is given by the relation [30]:
(10)
The above Eq. (10) is further modified to calculate both the upper and lower bounds of CTE, considering that Kl ≤ Keff ≤ Ku, where the upper and lower bounds of K can be obtained from Hashin-Shtrikman (HS) bounds [23]:
(α eff )u
σo
Kr (3Km + 4 Gm) = αm − fr (αm − αr ) Km (3Kr + 4 Gm) + 4 fr Gm (Kr − Km )
Km (3Kr + 4 Gr ) Kr (3Km + 4 Gr ) + 4 fm Gr (Km − Kr )
(12)
where, (αeff)u and (αeff)l are the upper and lower bounds of CTE respectively of a given composite whereas α, K, G and f are CTE, bulk modulus, shear modulus and volume fraction with subscript “m” and “r” set for matrix and reinforcement, respectively. The values of ν (taken as 0.12, 0.11, 0.14, 0.18 and 0.17, respectively for HfB2, ZrB2, SiC, B4C and CNT), α (taken as 6.3 × 10−6 K−1, 5.9 × 10−6 K−1, 3.5 × 10−6 K−1, 5.5 × 10−6 K−1 and 2.5 × 10−6 K−1, respectively for HfB2, ZrB2, SiC, B4C and CNT), K (taken as 230 GPa, 229 GPa, 234 GPa, 218 GPa and 190 GPa, respectively for HfB2, ZrB2, SiC, B4C and CNT) and G (taken as 212 GPa, 211 GPa, 41 GPa, 180 GPA and 150 GPa, respectively for HfB2, ZrB2, SiC, B4C and CNT) used for calculations have been taken from the literature [11,13,24,25]. High temperature processing of the UHTC based composite requires the quantification of effective residual stresses (ΔT = 1400 °C [25] where these stresses develop). During SPS processing, the composites underwent cooling from the final sintering temperature (i.e. 1850 °C) to room temperature. Therefore, the matrix (HfB2/ZrB2/HfB2-ZrB2) tends to shrink faster than the reinforcements (SiC, B4C, and CNT) leading to the generation of compressive stresses in the reinforced particles, σr and tensile stresses state in the matrix, σm as evaluated using Taya's model [26] and presented in Table 2. The interfacial compressive stress field in the composite play a vital role in dictating the toughening mechanism in UHTC [27] due to their high operational temperature.
σr
=
σm
=
β=
−
(1 − f ) ε′ σm f
+ Em
(13)
2fβε′ (1 − f )(β + 2)(1 + νm) + 3βf (1 − νm)
(14)
1 + νm Er 1 − 2νr Em
H6B Z6B H20S6B Z20S6B HZ20S6B HZ20S6B6C
CTE (×10−6 K−1)
Residual stresses (MPa)
ROM
Hsueh's model σο
6.25 6.72 5.69 6.06 5.88 5.63
HS model UB
LB
6.25 6.72 5.69 6.06 5.88 5.63
4.75 5.12 5.31 4.94 5.12 5.37
397.3 406.1 194.2 205.8 91.7 62.9
σr
322.8 380.6 185.0 192.9 82.7 68.5
−31.2 −28.2 −35.9 −32.5 −39.2 −41.8
αm) ΔT +
1 − 2νr Er
(17)
Fully dense novel composites of HfB2-ZrB2-SiC ceramics with B4C and CNT reinforcements were processed using SPS at 1850 °C. The partial graphitization of CNT into graphitic structure was confirmed by the diminution in the ID/IG ratio (from ~1.04 to ~0.90) and further complemented by the SEM, however, most of the CNT remain intact even after SPS processing. The uniform distribution of the reinforcements (SiC, B4C and CNT) in the SPS processed monolithic UHTC have led to an enhancement in the hardness (from 16.9 GPa in H6B, 17.6 GPa in Z6B, to 21.6 GPa in H20S6B, 22.7 GPa in Z20S6B, 25.3 GPa in HZ20S6B, and 27.2 GPa in HZ20S6B6C) and fracture toughness (from 9.4 MPam0.5 in H6B, 8.9 MPam0.5 in Z6B, to 11.9 MPam0.5 in H20S6B, 11.3 MPam0.5 in Z20S6B, 12.9 MPam0.5 in HZ20S6B, and 13.8 MPam0.5 in HZ20S6B6C). The addition of secondary phases (SiC, B4C, and CNT) generate the beneficial interfacial compressive residual stresses in the material which helps in mitigating the failure at higher load (i.e. enhancement in the fracture toughness). The results unequivocally
Taya's model σm
(αr − 1 + νm (1 + νr ) Em
4. Conclusions
(15)
Table 2 Theoretical calculation of the coefficient of thermal expansion and residual stresses in the SPS processed HfB2-ZrB2-SiC composites. Composites
=
The CTE estimated using ROM matches well with the upper bound of the HS model (see Table 2) which supports the homogeneous distribution of the secondary phases, as evident from the microstructure shown in Fig. 4. It is apparent from Table 2 that the addition of secondary phase introduces interfacial stresses (tensile in the matrix and compressive in the reinforcement) which improves the structural integrity of the UHTC composites by enhancing the fracture toughness (values presented in Table 1). The increase in the compressive stress with increasing the complexity of the UHTC composites from 31.2 MPa (in H6B) and 28.2 MPa (in Z6B) with B4C reinforcement to 35.9 MPa (in H20S6B) and 32.5 MPa (in Z20S6B) with SiC reinforcement to 39.2 MPa (in HZ20S6B) due to the solid solution formation and 41.8 MPa with CNT reinforcement (with addition of secondary phases from B4C to SiC and then CNT) can be clearly seen. This increasing trend in the compressive residual stress in the material lowers the residual tensile stresses (up to 68.5 MPa in HZ20S6B6C, see Table 2) and, thus, favors enhancement in the fracture toughness (up to 13.8 MPam0.5 for HZ20S6B6C, see Table 1). In summary, the effect of microstructural features (distribution of the reinforcements) on enhancing the fracture toughness of the SPS processed HfB2-ZrB2-SiC composites is schematically shown in Fig. 6. As we add the reinforcement from B4C to SiC, the combination of both and then CNT (making the UHTC system more complex), the impediment to the crack propagation increases. The comparison of the microstructural observation (shown in Fig. 4) and mechanical properties (see Table 1) unequivocally establishes the synergy between the solid solution formation (in HfB2-ZrB2 system) and reinforcements (of SiC, B4C and CNT) towards enhancing the fracture toughness of the diboride composites (with hardness of ~27.2 GPa, modulus of 481 GPa and fracture toughness of 13.8 MPam0.5 in HZ20S6B6C). All the reinforcements have contributed to enhance the fracture toughness of the processed composites via crack-deflection, branching and CNT pull-out mechanisms. The results discerned have proved HZ20S6B6C sample as a viable material for application where damage tolerance is an issue.
(11)
(α eff )l = αr + fm (αr − αm )
(16)
ROM = rule of mixture, CTE = coefficient of thermal expansion, HS = HashinShtrikman, UB = upper bound and LB = lower bound. 116
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HfB2/ZrB2
(a)
Monolithic HfB2/ZrB2
(b)
Crack branching and deflection by B4C
Wide crack opening
B4C with B4C
No reinforcement = 0 r KIC
r
KIC
~3-4 MPam0.5 HfB2/ZrB2
-31.2 MPa for H6B 9.4 MPam0.5 for H6B HfB2/ZrB2
(c)
(d)
Crack branching and deflection by B4C and SiC CNT SiC
B4C
SiC B4C
r
KIC
with SiC and B4C -39.2 MPa for HZ20S6B 12.9 MPam0.5 for HZ20S6B
r
KIC
with SiC, B4C and CNT -41.8 MPa for HZ20S6B6C 13.8 MPam0.5 for HZ20S6B6C
Fig. 6. Schematic illustrating the strengthening mechanism in HfB2-ZrB2-SiC composites.
establish the synergy of solid solution formation and reinforcements (B4C and CNT) on the processing and mechanical properties of HfB2ZrB2-SiC ceramics, making them a suitable damage tolerant structural material for re-entry space applications.
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Acknowledgments Authors at Indian Institute of Technology Kanpur (IITK) acknowledge the financial support received from IITK-Space Technology Cell and Indian Space Research Organization (ISRO), Trivandrum, India. Authors also acknowledge the support from the Advanced Centre for Materials Science (ACMS) at IITK for extending characterization facilities (SEM and instrumented indentation testing). KB acknowledges Swarnajayanti fellowship (DST/STF/ETA-02/2016-17). References [1] W.G. Fahrenholtz, G.E. Hilmas, Ultra-high temperature ceramics: materials for extreme environments, Scr. Mater. 129 (2017) 94–99. [2] A. Nisar, S. Ariharan, K. Balani, Establishing microstructure-mechanical property correlation in ZrB2-based ultra-high temperature ceramic composites, Ceram. Int. 43 (16) (2017) 13483–13492. [3] A. Nisar, S. Ariharan, K. Balani, Densification kinetics and mechanical properties of tantalum carbide, Int. J. Refract. Met. Hard Mater. 73 (2018) 221–230. [4] A. Nisar, et al., Oxidation studies on TaC based ultra-high temperature ceramic composites under plasma arc jet exposure, Corros. Sci. 109 (2016) 50–61. [5] I.G. Talmy, J.A. Zaykoski, M.M. Opeka, High-temperature chemistry and oxidation of ZrB2 ceramics containing SiC, Si3N4, Ta5Si3, and TaSi2, J. Am. Ceram. Soc. 91 (7) (2008) 2250–2257. [6] A. Nisar, S. Ariharan, K. Balani, Synergistic reinforcement of carbon nanotubes and silicon carbide for toughening tantalum carbide based ultrahigh temperature ceramic, J. Mater. Res. 31 (6) (2016) 682–692. [7] A. Nisar, K. Balani, Phase and microstructural correlation of spark plasma sintered HfB2-ZrB2 based ultra-high temperature ceramic composites, Coatings 7 (8) (2017)
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