Phase and Microstructural Correlation of Spark Plasma Sintered Hfb2-Zrb2 Based Ultra-High Temperature Ceramic Composites

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Article Phase and Microstructural Correlation of Spark Plasma Sintered HfB2-ZrB2 Based Ultra-High Temperature Ceramic Composites

Ambreen Nisar and Kantesh Balani * ID High Temperature Ceramic Laboratory, Department of Materials Science and Engineering, Indian Institute of Technology Kanpur, Kanpur 208016, India; [email protected] * Correspondence: [email protected]; Tel.: +91-512-259-6194

Received: 31 May 2017; Accepted: 13 July 2017; Published: 26 July 2017

Abstract: The refractory diborides (HfB2 and ZrB2) are considered as promising ultra-high temperature ceramic (UHTCs) where low damage tolerance limits their application for the thermal protection system in re-entry vehicles. In this regard, SiC and CNT have been synergistically added as the sintering aids and toughening agents in the spark plasma sintered (SPS) HfB2-ZrB2 system. Herein, a novel equimolar composition of HfB2 and ZrB2 has shown to form a solid-solution which then allows compositional tailoring of mechanical properties (such as hardness, elastic modulus, and fracture toughness). The hardness of the processed composite is higher than the individual phase hardness up to 1.5 times, insinuating the synergy of SiC and CNT reinforcement in HfB2-ZrB2 composites. The enhanced fracture toughness of CNT reinforced composite (up to a 196% increment) surpassing that of the parent materials (ZrB2/HfB2-SiC) is attributed to the synergy of solid solution formation and enhanced densiﬁcation (~99.5%). In addition, the reduction in the analytically quantiﬁed interfacial residual tensile stress with SiC and CNT reinforcements contribute to the enhancement in the fracture toughness of HfB2-ZrB2-SiC-CNT composites, mandatory for aerospace applications.

Keywords: ultra-high temperature ceramic (UHTC); zirconium diboride (ZrB2); hafnium diboride (HfB2); carbon nanotubes (CNT); spark plasma sintering (SPS); fracture toughness

1. Introduction Carbon-carbon (C–C) composites are attractive materials for hypersonic ﬂight vehicles but they oxidize in air at temperatures >500 ◦C and needs thermal protection systems (TPS) to survive aero-thermal heating [1]. Ultra high-temperature ceramics (UHTCs) are ideal for the use in TPS for the next generation hypersonic vehicles operating above 2000 ◦C in both neutral and oxidizing environment [2]. The UHTC ceramic coatings provides the oxidation protection of C–C composites and should be: (1) well-adhered to provide erosion resistance; (2) resist the diffusion of oxygen through coating and thus, the substrate; (3) mechanically compatible to prevent coating spallation due to the mismatch in the thermal expansion during harsh aero-thermal conditions; and (4) reliability and reproducibility during processing [1,3]. Among UHTC family, zirconium diboride (ZrB2) and ◦ ◦ ◦ hafnium diboride (HfB2) owing to melting points above 3000 C (3245 C and 3380 C, respectively), high electrical (1 × 107 S/m and 9.1 × 106 S/m, respectively) and thermal conductivity (60 W/mK and 104 W/mK, respectively), good chemical inertness and high wear resistance [4] emerge as leading candidate materials for the high-temperature structural applications in aerospace, refractory, and automotive sectors [5]. As a result of the strong covalent bonding and low self-diffusion coefﬁcients of ZrB2 and HfB2, the obtainment of highly dense compacts has often required higher pressure (>20 MPa) and prolonged exposure of 2–3 h in atmosphere-controlled conventional furnaces at sintering temperatures

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>2000 ◦C[5,6]. In recent years, spark plasma sintering (SPS) has emerged as a potential technique for the processing of UHTCs with enhanced densification, fine grain size of a few microns at a much lower sintering temperature ~500 ◦C less than the conventional techniques and time 10–30 min [7–11]. Further, reinforcements such as SiC and TaSi2 not only act as a sintering aid to densify the ZrB2/HfB2 ceramics, but also enhances their mechanical, oxidation, and tribological performances [7,8,12–15]. Several investigations proved that the addition of SiC leads to the enhancement of the oxidation resistance of boride based UHTC [7]. Hot-pressed HfB2-20 vol % SiC and ZrB2-20 vol % SiC ceramics was flown on NASA-funded hypervelocity slender-body flight tests (nose tip radius of 3.5 mm) in 1997 during the SHARP-B1 program (NASA, Washington, WA, USA) and as components of leading-edge strakes in 2000 during the SHARP-B2 program (NASA, Washington, WA, USA), which successfully showed the non-ablating performance of borides ceramics [16]. Although the material HfB2/ZrB2-SiC is currently set as the baseline material based on their high-temperature thermo-mechanical performances [17], but there is still a need to further enhance their fracture strength and toughness. Thus, it is likely to further reinforce these material with nano-scale filler such as carbon nanotube (CNT), graphite and carbon fibers in order to improve its fracture toughness, lower its density, enhances densification and impart an acceptable level of thermal shock making UHTC a preferred candidate for aerospace applications [7,18–21]. A study [7] revealed that the addition of CNT in ZrB2-20SiC ceramic increases the fracture toughness up to ~53% (with 10 vol % of CNT addition) and enhances the oxidation resistance via grain sealing mechanism under 30 s plasma arc jet exposure with a flux of 2.5 MW/m2. It has also been proposed that incorporation of carbon removes the native oxides on the surfaces of the non-oxide ceramics by interfacial reactions and promotes densification [22]. A recent report on ZrB2-SiC with addition of HfB2 (0, 5, 10 and 15 vol %) has shown that flexural strength increases from ~284 MPa to ~356 MPa due to the formation of (Hf,Zr)B2 solid solution [23]. Herein, the idea of solid solution formation of (Hf,Zr)B2 is utilized on HfB2-ZrB2 system (in a 1:1 ratio) and the influence of SiC and CNT reinforcements on sintering of the HfB2-ZrB2 system has been investigated and compared with the established parent materials HfB2-20SiC (vol %) and ZrB2-20SiC (vol %) based UHTC [6,24,25]. To make reliable UHTC coating, it is important to thoroughly understand the physical and chemical properties of the coated materials. In this regard, the present study stems to the bulk material properties which is imperative to study for isolating the substrate effect (C–C, SiC–C composites, etc.). The sintered HfB2-ZrB2-SiC-CNT pellets were characterized in terms of microstructure, mechanical properties (hardness, elastic modulus, and fracture toughness), and the obtained results are correlated with the generation of effective residual stresses in the material during SPS processing.

2. Materials and Method

2.1. Processing of Composites

Commercial powders of HfB2 and ZrB2 (Samics Research Materials, Rajasthan, India, 99.7% pure, particle size <5 µm, irregular shape), SiC (H.C. Starck, Selb, Germany, 99.9% pure, particle size <1 µm, irregular shape) and multi-walled carbon nanotubes (CNT, Nanostructured and Amorphous Materials Inc., Houston, TX, USA, 95% pure, with an outer diameter of 40 nm, wall thickness of 20 nm, and 1–2 µm long) were used as the starting materials. SEM micrographs of the selected borides HfB2 (Figure1a) and ZrB2 (Figure1b), and the reinforcements SiC (Figure1c) and CNT (Figure1d) elicits the morphology and size of the starting powders. The following powder mixtures (1) HfB2-20 vol % SiC (labeled as H20S), (2) ZrB2-20 vol % SiC (labeled as Z20S), (3) HfB2-ZrB2 (in 1:1 ratio)-20 vol % SiC (labeled as HZ20S), and (4) HfB2-ZrB2 (in 1:1 ratio)-20 vol % SiC-6 vol % CNT (labeled as HZ20S6C) are used in the present study. The powder mixtures were dry ball milled with ball to powder weight ratio of 5:2 using tungsten carbide jar and tungsten carbide balls for 8 min at 500 rpm. The ball milling parameters were carefully chosen to avoid any structural damage to the CNT [7]. Coatings 2017, 7, 110 3 of 15 Coatings 2017, 7, 110 3 of 15

Figure 1. SEM micrographs of the starting powder (a) HfB2, and (b) ZrB2. TEM micrographs of the Figure 1. SEM micrographs of the starting powder (a) HfB2, and (b) ZrB2. TEM micrographs of the reinforcements: (c) SiC and (d) CNT. reinforcements: (c) SiC and (d) CNT. The milled composite powders were then processed via spark plasma sintering (SPS, Dr. Sinter The515S, milled Kanagawa composite, Japan) at powders 1850 °C under were thena vacuum processed (<6 Pa) viawith spark the uniaxial plasma pressure sintering of 30 (SPS, MPa Dr. and Sinter 515S,hold Kanagawa, time of 10 Japan) min at at a 1850heating◦C rate under of 100 a vacuum °C/min during (<6 Pa) the with sintering the uniaxial cycle. Graphite pressure dies of and 30 MPa and holdpunches time were of 10 used min for at afabricating heating rate pellets of 100 of 15◦C/min mm diameter during and the sintering 5 mm thickness. cycle. Graphite The ram dies and punchesdisplacement were data used during for the fabricating SPS sintering pellets cycle of is 15utilized mm to diameter investigate and the 5 densificat mm thickness.ion behavior. The ram displacementThe instantaneous data during densification the SPS of sintering the composites cycle is is utilizedanalyzed to using investigate the formula the [26] densification: behavior. The instantaneous densification of the composites is analyzedℎ푓 using the formula [26]: 휌instantaneous = [ ] (1) ℎ푓 + 푑푓 − 푑푖 " # where, hf, df, and di are the thickness of sintered sample, maximumh f displacement, and instantaneous ρinstantaneous = (1) displacement during sintering, respectively. The final hdensityf + d f −wasdi measured via Archimedes method on a hydrostatic balance using ethanol as an immersion media. where, hf , df , and di are the thickness of sintered sample, maximum displacement, and instantaneous displacement2.2. Phase, duringMicrostructural sintering, and respectively.Mechanical Characterization The final density was measured via Archimedes method on a hydrostaticX-ray diffraction balance using(XRD) ethanolpattern of as SPS an processed immersion HfB media.2-ZrB2-SiC-CNT composites was obtained using Cu Kα radiation (λ = 1.54 Å, PANalytical Empyrean diffractometer, Tokyo, Japan) operating at 2.2. Phase,25 kV Microstructuraland 15 mA with a and scan Mechanical speed of 0.5 Characterization s/step and at a step size of 0.02° in the 2range from 20° to 90°. Further, the structural damage to CNT during processing was evinced using Nd-YAG green laser X-ray diffraction (XRD) pattern of SPS processed HfB -ZrB -SiC-CNT composites was obtained micro-Raman spectroscopy (λ = 532 nm, Princeton Instruments,2 Tokyo,2 Japan, STR Raman, TE-PMT using Cu Kα radiation (λ = 1.54 Å, PANalytical Empyrean diffractometer, Tokyo, Japan) operating at detector) in the backscattering mode with an exposure of 10 s. ◦ ◦ 25 kV andThe 15 microstructure mA with a scan of speedthe ground of 0.5 and s/step polished and atsurface a step (using size of0.1 0.02 m diamondin the 2θ suspension)range from of 20 to ◦ 90 . Further,HfB2-ZrB the2-SiC structural-CNT composites damage was to CNT observed during using processing scanning was electron evinced microscope using Nd-YAG (SEM, green Zeiss, laser micro-RamanOberkochen, spectroscopy Germany, Model (λ = 532EVO nm, 50). The Princeton energy Instruments,dispersive X-ray Tokyo, analysis Japan, (EDX) STR at a Raman,point is used TE-PMT detector)to confirm in the various backscattering phases in mode SPS processed with an exposureHfB2-ZrB2- ofSiC 10-CNT s. composites. To see the structure of Thethe CNTs, microstructure samples were of the fractured ground and and observed polished using surface field emission (using 0.1SEMµ m (FE diamond-SEM, JEOL, suspension) JSM- 7100F, Peabody, MA, USA). of HfB2-ZrB2-SiC-CNT composites was observed using scanning electron microscope (SEM, Zeiss,

Oberkochen, Germany, Model EVO 50). The energy dispersive X-ray analysis (EDX) at a point is used to conﬁrm various phases in SPS processed HfB2-ZrB2-SiC-CNT composites. To see the structure of Coatings 2017, 7, 110 4 of 15 the CNTs, samples were fractured and observed using ﬁeld emission SEM (FE-SEM, JEOL, JSM-7100F, Peabody, MA, USA). Load displacement curves for polished sintered HfB2-ZrB2-SiC-CNT pellets were measured via instrumented micro Vickers indenter of type V-I 51 (MHT, CSM instruments, Peseux, Switzerland) at a loading rate of 4 N/min and holding of 10 s at maximum load of 2 N. Fracture toughness was calculated at a load (P of 49 N) using Vickers indenter via universal hardness testing machine (FH-10, Tinius-Olsen Ltd., Salfords, UK) with a dwell time of 10 s. The crack lengths (c) from the center of indents were measured using SEM and fracture toughness was computed using Anstis’ equation [27]: r E P K = 0.016 (2) IC H c3/2 where E is the elastic modulus and H is the hardness (measured experimentally) of HfB2-ZrB2-SiC-CNT composites. An average of ten values is reported for each composite. For evaluating the fracture toughness, indentation technique is not very suitable when compared with that of SENB method [28,29], but a similar trend of fracture toughness has been reported in literature [27,30]. With reference from the study [31], the indentation technique might not give the true fracture toughness values of the CNT reinforced composite, which may be attributed to minimal radial cracking in comparison to the observation of larger cracks in the composites without CNT reinforcement. The absence of cracking in the CNT reinforced composite via indentation is reasoned to their assisted shear deformation under the indenter [31]. For that matter it is important to be noted here that herein the indentation fracture toughness technique is used only as a “ranking parameter” towards comparing the toughness of processed HfB2-ZrB2-SiC-CNT composites. The measurement of the critical energy release rate (GIC) is calculated using the following equation

1 − ν2 G = K2 (3) IC IC E where ν is the Poisson’s ratio calculated using rule of mixture for all composites [7].

3. Results and Discussion

3.1. Densification of HfB2-ZrB2-SiC-CNT Composites The instantaneous relative density as a function of sintering temperature during SPS processing of HfB2-ZrB2-SiC-CNT composites is presented in Figure2. The addition of SiC in HfB 2 and ZrB2 has led to nearly full densification (100% for H20S, ~99% for Z20S) is attributed to the high thermal conductivity of SiC and its ability to sit near the grain boundary which aids densification, allowing the sintering to be faster and at lower temperatures [7,12]. The densification was found to decrease in HZ20S (from ≤1% in H20S and Z20S to ~4% in HZ20S, see Table1) is commensurate with recent report in literature [23] which shows that the addition of HfB2 in ZrB2 increases porosity due to the occurrence of chemical reaction between them. On the other hand, further addition of CNT (i.e., HZ20S6C), increases the densification to ~99.5% which is attributed to the high thermal conductivity of CNT [12,32]. Moreover, carbon has long been used as a sintering aid for the densification of a ceramics, and it is often proposed that incorporation of carbon removes the native oxides on the surfaces of the non-oxide ceramics by interfacial reactions and promotes the densification [15]. Coatings 2017,, 7,, 110 5 of 15

100100

9080 HZ20S 8060

HZ20S6C 7040 Z20S

H20S 6020

Instantaneous densification (%) densification Instantaneous

500 600 800 1000 1200 1400 1600 1800 Temperature (°C)

Figure 2. Instantaneous relative density plot of HfB2-ZrB2-SiC-CNT composites. Figure 2. Instantaneous relative density plot of HfB2-ZrB2-SiC-CNT composites.

Table 1. Nomenclature, relative densiﬁcation and grain size of HfB2-ZrB2-SiC-CNT composites. Table 1. Nomenclature, relative densification and grain size of HfB2-ZrB2-SiC-CNT composites.

TheoreticalTheoretical ArchimedesArchimedes Density %% RelativeRelative CompositionsCompositions Sample Sample ID ID DensityDensity (g/cc) (g/cc) Density(g/cc) (g/cc) DensiﬁcationDensification HfB2 + 20 vol % SiC H20S 9.04 9.04 100.0 HfB2 + 20 vol % SiC H20S 9.04 9.04 100.0 ZrB2 + 20 vol % SiC Z20S 5.51 5.46 99.0 ZrB + 20 vol % SiC Z20S 5.51 5.46 99.0 HfB2-ZrB22 (1:1) + 20 vol % SiC HZ20S 7.27 6.98 96.0 HfB -ZrB (1:1) + 20 vol % SiC HZ20S 7.27 6.98 96.0 HfB22-ZrB22 (1:1) + 20 vol % SiC + HfB -ZrB (1:1) + 20 vol % SiC HZ20S6C 6.92 6.85 99.5 2 62 vol % CNT HZ20S6C 6.92 6.85 99.5 + 6 vol % CNT

3.2. Phase Analysis of HfB2-ZrB2-SiC-CNT Composites 3.2. Phase Analysis of HfB2-ZrB2-SiC-CNT Composites The XRD pattern of HfB2-ZrB2-SiC-CNT composites shown in Figure 3 indicates the retention of the startingThe XRD phases pattern even of HfB after2-ZrB SPS2-SiC-CNT processing. composites Due to the shown poor in X Figure-ray reflecting3 indicates abil theity retention, SiC peak of theintensity starting appears phases low even [22] after in SPS the processing. XRD pattern Due (Fig tou there poor3). In X-ray addition, reﬂecting no peak ability,-shift SiC in peak SiC intensityphase is appearsobserved low in all [22 the] in processed the XRD pattern SPS pellet (Figure when3). Incompared addition, to no that peak-shift of starting in SiC powders, phase is whi observedch confirms in all thethat processed SiC has not SPS formed pellet when any comparedsolid solution to thats with of starting HfB2/ZrB powders,2 up to which the processing conﬁrms that temperature SiC has not of ◦ formed1850 °C. any HZ solid20S and solutions HZ20S6C with HfBcomposites2/ZrB2 up show to the peak processing broadening temperature and merging of 1850 as C. shown HZ20S in and the HZ20S6Cmarked region, composites Figure show3 indicating peak broadening the formation and of merging partial solid as shown-solution in theas few marked peaks region, corresponding Figure3 indicatingto the parent the phases formation (HfB of2/ZrB partial2) were solid-solution also observed. as few Formation peaks corresponding of HfB2-ZrB2 so tolid the solution parent may phases be (HfBattributed2/ZrB to2) their were nearly also observed. equal lattice Formation parameters of HfB (a =2 -ZrB3.1672 Åsolid , c = solution3.53 Å for may ZrB be2, and attributed a = 3.165 to Å their and nearlyc = 3.51 equal Å for lattice HfB2; parameterscalculated using (a =3.167 Xpert Å, highc = 3.53score Å plus for ZrBsoftware2, and, 3.0.0.123,a = 3.165 ÅPANalytical, and c = 3.51 Tokyo, Å for HfBJapan2;) calculated [33,34]. In usingliterature, Xpert Post high et al score [35] plushas also software, reported 3.0.0.123, the complete PANalytical, mutual Tokyo, solubility Japan) in borides. [33,34]. In literature, Post et al. [35] has also reported the complete mutual solubility in borides.

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Merged peaks solid solution formation ZrB Merged peaks solid solution formation ZrB2  HfBHfB2 SiCSiC 2  2     HZ20S6CHZ20S6C pelletpellet             HZ20S6CHZ20S6C powderpowder    HZ20SHZ20S pelletpellet              HZ20SHZ20S powderpowder              Z20SZ20S pelletpellet          Z20SZ20S powderpowder               H20SH20S pelletpellet     Intensity (a.u.)       H20S powder Intensity (a.u.)          H20S powder         SiCSiC powderpowder            ZrB powder       ZrB22 powder           HfBHfB2 powderpowder          2 2020 3030 4040 5050 6060 7070 8080 9090 2 2

Figure 3. XRD spectra of initial composite powders as well as SPS processed HfB2-ZrB2-SiC-CNT Figure 3. 3. XRD spectra of of initial initial composite composite powders powders as as well well as as SPS SPS processed processed HfB HfB22-ZrB-ZrB22-SiC-CNT-SiC-CNT composites.composites.

The D, G, and G’ peaks in the Raman spectra (see Figure 4) confirm the presence of CNTs in the The D,, G,, and G’ peaks in the Raman spectra (see FigureFigure4 4)) conﬁrmconfirm thethe presencepresence ofof CNTsCNTs in in the the ballball milled milled and and sintered sintered HfB HfB22--ZrBZrB22--SiCSiC--CNTCNT pellets. pellets. Raman Raman peaks peaks for for pristine pristine CNT CNT powders powders are are ball milled and sintered HfB2-ZrB2-SiC-CNT pellets. Raman peaks for pristine CNT powders are observed at 1347.0 cm−−11 (D-peak, arises from disorder and defects), 1585.6 cm−−11 (G-peak, corresponds observed atat 1347.01347.0 cmcm−1 ((DD--peak,peak, arises arises from disorder and defects),defects), 1585.61585.6 cmcm−1 ((GG--peak,peak, corresponds toto CC––CC stretchingstretching modemode fromfrom doublydoubly degenerateddegenerated phononphonon EE2g2g atat thethe BrillouinBrillouin zonezone center)center) andand atat to C–C stretching mode from doubly degenerated phonon E2g at the Brillouin zone center) and at 2732.4 cm−−11 (G’-peak, corresponds to the second-order phonon counter part of G) [36]. The peak shift 2732.4 cm − 1(G’(G’-peak,-peak, corresponds corresponds to tothe the second second-order-order phonon phonon counter counter part part of G of) [36]G)[. 36The]. peak The peakshift of D (1350.2 cm−−11 and 1353.8 cm−−11), G (1588.7 cm−−11 and 1593.8 cm−−11) and G’ (2735.7 cm−−11 and 2741.9 shiftof D of(1350.2D (1350.2 cm cmand−1 1353.8and 1353.8 cm ), cm G− (1588.71), G (1588.7 cm and cm− 11593.8and 1593.8 cm ) cmand− 1G’) and (2735.7G’ (2735.7 cm and cm −2741.91 and cm−−11) peaks to higher wave numbers, respectively, in the ball milled and SPS pellets, indicate the 2741.9cm ) peaks cm−1 )to peaks higher to higherwave numbers, wave numbers, respectively, respectively, in the in ball the mill balled milled and andSPS SPSpellets, pellets, indicate indicate the generation of compressive stresses in the CNTs during ball milling [7,18] and after sintering. The thegeneration generation of compressive of compressive stresses stresses in the in CNTs the CNTs during during ball ballmilling milling [7,18] [7 and,18] andafter aftersintering. sintering. The development of compressive stress in the CNT is attributed to the thermal contraction of the ceramic Thedevelopment development of compressive of compressive stress stress in the inCNT the is CNT attributed is attributed to the thermal to the thermal contraction contraction of the ceramic of the matrix during cooling (discussed in the later Section 3.5). The defect to graphitic peak ratio, ID/IG ceramicmatrix during matrix cool duringing (discussed cooling (discussed in the later in S theection later 3.5). Section The defect3.5). Theto graphitic defect to peak graphitic ratio, peakID/IG remains similar for pristine CNT powder (=1.06) as well as ball milled powder (=1.01). However, ID/IG ratio,remainsI / similarI remains for pristine similar CNT for pristine powder CNT(=1.06) powder as well (=1.06) as ball asmilled well powder as ball milled (=1.01). powder However, (=1.01). ID/IG decreasesD toG 0.79 for the SPS processed pellet, which elicits the conversion of CNTs to graphite-like However,decreases Ito/ 0.79I decreases for the SPS to 0.79processed for the pellet, SPS processed which elicits pellet, the which conversion elicits the of conversionCNTs to grap ofhite CNTs-like to structures [37]D .G Similar observation is also reported for TaC-CNT ceramic composite [32]. graphite-likestructures [37] structures. Similar observation [37]. Similar is observation also reported is also for reportedTaC-CNT for ceramic TaC-CNT composite ceramic [32] composite. [32].

-1 1593.8 cm -1 1593.8 cm -1 2741.9 cm -1 -1 2741.9 cm 1353.81353.8 cm cm-1 HZ20S6C pellet IIDD//IIGG == 0.790.79 HZ20S6C pellet

-1 1588.71588.7 cm cm-1 -1 -1 1350.21350.2 cm cm-1 2735.72735.7 cm cm-1

HZ20S6C powder IIDD//IIGG == 1.011.01 HZ20S6C powder

-1 -1 -1 1585.61585.6 cm cm 1347.01347.0 cm cm-1

Intensity (a.u.)

Intensity (a.u.) -1 2732.42732.4 cm cm-1

ID/IG = 1.06 ID/IG = 1.06PristinePristine CNTCNT DD-peak-peak GG-peak-peak D'D'-peak-peak G'G'-peak-peak 12001200 15001500 18001800 21002100 24002400 27002700 30003000 -1 RamanRaman shiftshift ((cmcm-1))

FigureFigure 4. 4. RamanRaman spectraspectra showing showing DD,, GG,, D’D’,, andand G’G’ peakpeak ofof CNTCNT inin CNTCNT reinforcedreinforced composite composite powder powder asas well well as asas a aa pellet. pellet.pellet.

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Coatings 2017, 7, 110 7 of 15 3.3. Microstructural Analysis of HfB2-ZrB2-SiC-CNT Composites

3.3. MicrostructuralThe surface morphology Analysis of HfB of the2-ZrB SPS2-SiC processed-CNT Composites HfB2-ZrB 2-SiC-CNT composites, Figure5 elucidates the uniform distribution of SiC and CNT phases (black color phase) in the matrix. Both samples, The surface morphology of the SPS processed HfB2-ZrB2-SiC-CNT composites, Figure 5 H20S (Figure5a) and Z20S (Figure5b) show a highly dense structure with no porosity which is in elucidates the uniform distribution of SiC and CNT phases (black color phase) in the matrix. Both accordancesamples, H20S with (Fig thature 5a) measured and Z20S using (Figure Archimedes 5b) show a methodhighly dense (see structure Table1). with However, no porosity despite which having µ µ theis in same accordance initial grain with that size measured(<5 m), ZrB using2 grains Archimedes tend to method grow bigger (see Table (~10–20 1). However,m) when despite compared µ ◦ withhaving HfB the2 (~7–10 same initialm), which grain is size attributed (<5 m), to ZrB the2 similargrains sinteringtend to grow temperature bigger (~10 (~1850–20 C)m) during when SPS 2 2 whencompared diffusivity with HfB of2 the(~7– two10 m), (ZrB which2: 0.21 is cmattributed/s and to HfB the2 :similar 0.16 cm sintering/s [6]) temperature is different. (~1850 Similar °C) to the XRDduring analysis SPS when (Figure diffusivity3), the of formation the two (ZrB of HfB2: 0.212-ZrB cm2/ssolid and HfB solution2: 0.16 was cm2/s evinced [6]) is different. in HZ20S Similar (Figure 5c) andto the HZ20S6C XRD analysis (Figure (Fig5d)ure pellets. 3), the Along formation with of the HfB uniform2-ZrB2 solid distribution solution SiC was sits evinced at the intergranularin HZ20S regions(Figure of5c) HfB and2 HZ20S6Cand ZrB2 matrices (Figure 5d) (tightly pellets. encapsulated Along with within the uniform the matrices, distribution see inset SiC sitsof Figure at the5 a,b) whichintergranular might assistregions in of enhancing HfB2 and the ZrB densification2 matrices (tightly by occupying encapsulated the pore within volume the matrices, and thus, see mechanical inset properties.of Figure 5a As,b) seenwhich from might the assist microstructure in enhancing (Figure the densification5c,d), the (Hf,Zr)B by occupying2 solid the solution pore volume originates and either fromthus, HfB mechanical2 or ZrB 2 properties.grains. HfB As2 grains seen from appeared the microstructu brighter whilere (Fig ZrBures2 grains 5c,d), appeared the (Hf,Zr)B darker2 solid than the solidsolution solution originates which either supports from theHfB mutual2 or ZrB solubility2 grains. HfB of borides2 grains atappeared high temperature. brighter while Similar ZrB2 observation grains appeared darker than the solid solution which supports the mutual solubility of borides at high was reported by Silvestroni et al. for HfB2 reinforced with ZrB2 composites [34]. The porosity observed intemperature. HZ20S (Figure Similar5c) decreases observation with was CNT reported reinforcement by Silvestroni (Figure et5 d) al from for HfB ~4%2 reinforced to ~0.5% (see with Figure ZrB2 5e) composites [34]. The porosity observed in HZ20S (Figure 5c) decreases with CNT reinforcement is due to the ability of CNT to fill the gap and occupy pores [38]. It can also be anticipated that HfB2 has (Figure 5d) from ~4% to ~0.5% (see Figure 5e) is due to the ability of CNT to fill the gap and occupy a negative effect on the densification of ZrB2-SiC composites, which is similar to the results obtained pores [38]. It can also be anticipated that HfB2 has a negative effect on the densification of ZrB2-SiC by Balak et al. [23]. The structure of the CNT is evident from the image shown in Figure5f, and composites, which is similar to the results obtained by Balak et al [23]. The structure of the CNT is similar to SiC, CNT also sits at the intergranular regions of the HfB -ZrB system. Such observations evident from the image shown in Figure 5f, and similar to SiC, CNT2 also 2sits at the intergranular supports enhancement in the mechanical properties of the material by mitigating the failure at higher regions of the HfB2-ZrB2 system. Such observations supports enhancement in the mechanical load, studied in the following section. The EDX results confirms the presence of various phases in properties of the material by mitigating the failure at higher load, studied in the following section.The the HfB -ZrB -SiC-CNT composite (marked point in Figure5e), revealing that the dark gray grains EDX results2 confirm2 s the presence of various phases in the HfB2-ZrB2-SiC-CNT composite (marked are ZrB (not shown here), light phases are of HfB , SiC in Figure5g, along with the formation of point in2 Figure 5e), revealing that the dark gray grains2 are ZrB2 (not shown here), light phases are of HfB2,-ZrB SiC 2insolid Figure solution 5g, along (Figure with 5theh). formation The presence of HfB of2- carbonZrB2 solid in thesolution marked (Fig regionure 5h). in The Figure presence5f is also confirmedof carbon in from the marked the EDX region shown in inFig Figureure 5f 5isi. also confirmed from the EDX shown in Figure 5i.

(a) HfB2 (b) HfB SiC 2 SiC 2 m SiC ZrB2 ZrB2 20 m 20 m

(e) 20 m 20 m HfB2

FigureFigure 5. Cont 5. Cont. .

Coatings 2017, 7, 110 8 of 15 Coatings 2017, 7, 110 8 of 15

(e) HfB2 (f) (h) (i)

SiC (g) ZrB2 CNT 200 nm 20 m

Si Hf Zr C

.) .)

.) C

a.u

Counts ( Counts

Counts ( Counts Counts ( Counts Hf ZrZr

Energy (keV) Energy (keV) Energy (keV)

Figure 5. Back scattered SEM images of the SPS processed HfB2-ZrB2-SiC-CNT composites: (a) H20S Figure 5. Back scattered SEM images of the SPS processed HfB2-ZrB2-SiC-CNT composites: (a) H20S and inset shows that SiC sits at the intragranular regions, (b) Z20S, (c) HZ20S, (d) HZ20S6C, and inset shows that SiC sits at the intragranular regions, (b) Z20S, (c) HZ20S, (d) HZ20S6C, high high magniﬁcation image (e) eliciting mutual diffusion of HfB and ZrB of the marked area in magnification image (e) eliciting mutual diffusion of HfB2 and ZrB2 of the marked2 area in (c) and (f) (c)marked and (f) area marked in (d area) showing in (d) the showing presence the and presence morphology and morphology of the CNT ofremains the CNT intact remains even after intact SPS even g h afterprocessing. SPS processing. EDS of the EDS marked of the region marked confirming region conﬁrming presence of presence (g) SiC, (h of) solid ( ) SiC, solution ( ) solid of HfB solution2 and of

HfBZrB2 2and; and ZrB (i) 2CNT.; and (i) CNT.

3.4.3.4. Mechanical Mechanical Characterization Characterization ofof HfBHfB2--ZrBZrB22-SiC-SiC-CNT-CNT C Compositesomposites TheThe structural structural integrity integrity ofof thethe processed composit compositee is is evaluated evaluated using using instrumented instrumented indentation indentation technique.technique. The The load-displacement load-displacement curves curves (see (see Fig Figureure 6) of HfB HfB2-2ZrB-ZrB2-SiC2-SiC-CNT-CNT composites composites were were utilized to calculate hardness, Young’s modulus and plasticity index of HfB2-ZrB2-SiC-CNT utilized to calculate hardness, Young’s modulus and plasticity index of HfB2-ZrB2-SiC-CNT composites, tabulatedcomposites, in Table tabulated2. The in hardness Table 2. The of the hardness composites of the is composites found to be is higherfound to (up be tohigher 144% (up for to HZ20S6C 144% sample)for HZ20S6C than that sample) calculated than that using calculated rule of mixture using rule (ROM), of mixture see Table (ROM),2. This see is Table due to2. theThis limitation is due to of ROMthe limitationwhich doesn’t of ROM account which for doesn’t the microstructural account for thefeatures microstructural such as porosity, features distribution such as po ofrosity, various distribution of various phases, and interfaces governing the mechanical properties of a composite phases, and interfaces governing the mechanical properties of a composite material. Such a behaviour material. Such a behaviour establishes the synergy of reinforcement (SiC and CNT) and solid solution establishes the synergy of reinforcement (SiC and CNT) and solid solution formation in HfB2-ZrB2 formation in HfB2-ZrB2 system. The hardness of the SPS processed H20S and Z20S is found to be system. The hardness of the SPS processed H20S and Z20S is found to be 23.4 GPa and 23.6 GPa, 23.4 GPa and 23.6 GPa, respectively, is higher than the values reported for hot pressed samples (i.e., respectively, is higher than the values reported for hot pressed samples (i.e., ~20.5 GPa for H20S ~20.5 GPa for H20S and ~18.1 GPa for Z20S) [39]. This is attributed to the highly dense structure of and ~18.1 GPa for Z20S) [39]. This is attributed to the highly dense structure of HfB and ZrB HfB2 and ZrB2 with SiC reinforcement (porosity ≤ 1%) establishing the efficacy of the SPS technique2 2 with SiC reinforcement (porosity ≤ 1%) establishing the efﬁcacy of the SPS technique in getting in getting dense microstructure with superior mechanical properties of HfB2/ZrB2 based composites. denseIn HZ20S microstructure samples, with the formation superior mechanical of a solid solution properties should of HfB further2/ZrB 2 increasebased composites. the hardness, In but,HZ20S samples,porosity the has formation played a dominant of a solid solutionrole and shouldhas marg furtherinally increasedecreased the the hardness, hardness but, to 20.4 porosity GPa. Further, has played a dominantsynergistic role interplay and has of marginally CNT addition decreased which the acted hardness both as to a 20.4 sintering GPa. Further,aid (porosity synergistic decreases interplay to of~0.5%) CNT addition and toughening which acted agent both (shown as a sinteringlater in this aid section); (porosity and decreases (Hf,Zr)B to2 solid ~0.5%) solution and toughening led HZ20S6C agent (shownsample later to show in this highest section); hardness and (Hf,Zr)B (~28.12 GPa)solid in solution the current led HZ20S6C system. Similar sample to to hardness, show highest the elastic hardness (~28.1modulus GPa) showed in the currenta minimum system. value Similar for the to HZ20S hardness, sample the (425 elastic GPa) modulus and maximum showed for a minimumthe HZ20S6C value for the HZ20S sample (425 GPa) and maximum for the HZ20S6C sample (528 GPa) revealing the combined effect of reinforcement (SiC and CNT), decreased porosity, and solid-solution.

Coatings 2017, 7, 110 9 of 15

sample (528 GPa) revealing the combined effect of reinforcement (SiC and CNT), decreased porosity, Coatings 2017 7 and solid-solution., , 110 9 of 15

Figure 6. 6. LoadLoad vs. displacement vs. displacement curves curvesfor HfB2 for-ZrB HfB2-SiC2-ZrB-CNT2-SiC-CNT composites compositesobserved during observed instrumented during instrumentedindentation. indentation.

Table 2. 2. MechanicalMechanical properties properties of of HfB HfB22-ZrB-ZrB22--SiC-CNTSiC-CNT composites. Here, Here, ROM ROM = = rule rule of mixture,

E == elastic modulusmodulus,, rree = elastic recovery, recovery, rd == resistance resistance to deformation, KICIC == fracture fracture toughness and

GICIC == critical critical energy energy release release rate. rate.

Hardness (GPa) Hardness (GPa) e d IC K0.5IC IC −G2 IC SampleSample ID ID E (GPa) r re r rKd (MPa·m ) G (J·m ) ROMROM ExperimentalExperimental (MPa·m0.5) (J·m−2) H20S 21.6 ± 0.2 23.4 ± 1.9 456 ± 10 0.560 0.440 5.2 ± 0.5 57.1 ± 3.3 H20S 21.6 ± 0.2 23.4 ± 1.9 456 ± 10 0.560 0.440 5.2 ± 0.5 57.1 ± 3.3 Z20S 19.2 ± 0.3 23.6 ± 2.1 503 ± 13 0.656 0.344 5.7 ± 0.4 63.0 ± 2.1 Z20S 19.2 ± 0.3 23.6 ± 2.1 503 ± 13 0.656 0.344 5.7 ± 0.4 63.0 ± 2.1 HZ20SHZ20S 20.420.4± 0.3 ± 0.3 20.420.4± ±4.9 4.9 425425 ±± 0707 0.658 0.658 0.342 0.3428.7 ± 0.8 8.7 ± 0.8176.8176.8 ± 4.8 ± 4.8 HZ20S6CHZ20S6C 19.519.5± 0.3 ± 0.3 28.128.1± ±1.2 1.2 528528 ±± 0808 0.666 0.666 0.335 0.33510.2 ± 10.20.3 ± 0.3192.6192.6 ± 1.6 ± 1.6

Taking hardness of HfB2 = 18 GPa [40,41], ZrB2 = 15 GPa [5], SiC = 28 GPa [42], and CNT = 15 GPa Taking hardness of HfB = 18 GPa [40,41], ZrB = 15 GPa [5], SiC = 28 GPa [42], and CNT = 15 GPa (along radial direction) [43].2 2 (along radial direction) [43]. Based on the maximum depth (hm) and final depth (hf) of indentation, the plasticity index (re) i.e. Based on the maximum depth (hm) and ﬁnal depth (hf) of indentation, the plasticity index (re) i.e., the elastic recovery and the resistance to deformation (rd) for HfB2-ZrB2-SiC-CNT composites have the elastic recovery and the resistance to deformation (r ) for HfB -ZrB -SiC-CNT composites have been calculated using the following expression: d 2 2 been calculated using the following expression: ℎm − ℎf 푟e = (4) hmℎ−m hf re = (4) hm ℎf 푟 = (5) d ℎ hmf rd = (5) The increase in the elastic recovery (0.560–0.666,hm see Table 2) and decrease in the resistance to deformationThe increase (0.440 in–0.335, the elastic see Table recovery 2) has (0.560–0.666, resulted in see the Table higher2) and hardness decrease and in m theodulus resistance of the toHZ206C deformation composite. (0.440–0.335, see Table2) has resulted in the higher hardness and modulus of the HZ206CThe composite.fracture toughness of the SiC reinforced HfB2 and ZrB2 was observed to be 5.2 MPa·m0.5 and 0.5 0.5 5.7 MPaThe·m fracture, respectively, toughness which of the SiCis higher reinforced than HfBthe 2valuesand ZrB reported2 was observed in literature to be for 5.2 SPS MPa processed·m and 5.7H20S MPa (3.35·m0.5 , MPa respectively,·m0.5) and which Z20S is higher (3.91 MPa than·m the0.5 values) samples reported (the in toughness literature for was SPS measured processed using H20S (3.35indentation MPa·m technique0.5) and Z20S at a (3.91load of MPa 49 ·N)m0.5 [44]) samples. The fracture (the toughness toughness was further measured increased using to 8.7 indentation MPa·m0.5 0.5 techniquewith SiC addition at a load and of 49 10.2 N) [MPa44].· Them0.5 fracturewith synergistic toughness addition further of increased SiC and to CNT 8.7 MPaboth·m in HfBwith2-ZrB SiC2 0.5 additionsystem revealing and 10.2 the MPa synergy·m with of SiC synergistic and CNT as addition a toughening of SiC andagent CNT [7] and both (Hf, inZr)B HfB22 solid-ZrB2 solutionsystem revealing the synergy of SiC and CNT as a toughening agent [7] and (Hf,Zr)B2 solid solution in enhancing the toughness. The critical energy release rate (GIC, see Table2), which reckons the energy Coatings 2017, 7, 110 10 of 15 required to propagate a crack in the material, increases from 57.1 J·m−2 for H20S, to 63.0 J·m−2 for Z20S, to 176.8 J·m−2 for HZ20S and to 192.6 J·m−2 for HZ20S6C. Such a behavior can mitigate the failure at higher load by arresting the crack path making HZ20S6C a promising material where damage tolerance is an issue.

3.5. Effect of Residual Stress on the Mechanical Properties of HfB2-ZrB2-SiC-CNT Composites The mechanism which governs the toughness of a composite material depends on: Tendency of crack bridging; micro-cracking due to misfit between coefficient of thermal expansion (CTE) which introduces stress field and residual stress due to difference in CTE of reinforcement (SiC and CNT); and matrix, which creates local compressive stress field in the matrix, thus, decreasing the stress intensity factor. Ignoring the effect of size and distribution of secondary phases (SiC and CNT), the generation of interfacial residual stresses in the material has been analytically computed. The linear CTE of a composite (assuming that each phase to be isotropic), computed by Levin, Rosen, and Hashin [45], is provided as:

(α − α ) 1 1 1 (α ) = α + α + 1m 2r − − eff u fm1 m fr r 1 1 (6) − Keff Km Kr Km Kr where 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. Considering that KL ≤ Keff ≤ Ku, the upper (KL) and lower (Ku) bounds of bulk modulus can be obtained from Hashin-Shtrikman (HS) bounds [46]. Under such conditions, the above Equation (6) is modiﬁed to calculate both the upper (αeff)u and lower bounds (αeff)l of CTE in the following manner:

Kr (3Km + 4 Gm) (αeff)u = αm − fr (αm − αr) (7) Km (3Kr + 4 Gm) + 4 frGm(Kr − Km)

Km (3Kr + 4 Gr) αe f f = αr + fm (αr − αm) (8) l Kr (3Km + 4 Gr) + 4 fm Gr(Km − Kr)

The values of ν (taken as 0.12, 0.11, 0.14 and 0.17, respectively for HfB2, ZrB2, SiC, and CNT), −6 −1 −6 −1 −6 −1 −6 −1 α (taken as 6.3 × 10 K , 5.9 × 10 K , 3.5 × 10 K , and 2.5 × 10 K , respectively for HfB2, ZrB2, SiC, and CNT), K (taken as 230 GPa, 229 GPa, 234 GPa, and 190 GPa, respectively for HfB2, ZrB2, SiC, and CNT) and G (taken as 212 GPa, 211 GPa, 41 GPa, and 150 GPa, respectively for HfB2, ZrB2, SiC, and CNT) used for calculations have been taken from the literature [4,6,7]. Since, the composites are being cooled from the final densification temperature (1850 ◦C) to room temperature; the matrix (HfB2, ZrB2, and equal proportions of both) tends to shrink faster than the reinforced particles (SiC and CNT) leading to compressive stress in the reinforced particles, σr and tensile stress state in the matrix, σm as evaluated using Taya’s model [47], tabulated in Table3. The quantification of such thermal residual stresses is necessitated at high temperature during processing (taken as ∆T = 1400 ◦C[6]) where these stresses develop. (1 − f ) ε0 σ = − σ (9) r f m 2 f βε0 σm = + Em (10) (1 − f )(β + 2)(1 + νm) + 3β f (1 − νm) 1 + ν E β = m r (11) 1 − 2νr Em 0 ε = (αr − αm) ∆T (12) where, Er, Em, νr and νm are the Young’s modulus and Poisson’s ratio (calculated using ROM [7,17]) of the reinforcement and matrix respectively; ε0 is the thermal expansion misfit strain and ∆T is the Coatings 2017, 7, 110 11 of 15 temperature at which stresses begin to accumulate (set as 1400 ◦C) [48,49]. Young’s modulus for the reinforced phase, Er is estimated by ROM, utilizing modulus of matrix (HfB2/ZrB2/equal vol % of both) and composite (Ec) as presented in Table2.

Table 3. Theoretical calculation of the coefﬁcient of thermal expansion (CTE), modulus and residual stresses. Here, ROM = rule of mixture, HS = Hashin-Shtrikman, U.B. = upper bound, L.B. = lower bound, Em = elastic modulus of the matrix, Er = elastic modulus of reinforcement, σm = residual stress of the matrix, and σr = residual stress of the reinforcement.

Modulus CTE (×10−6/K) Residual Stress (MPa) (GPa) Composition HS Model Hsueh’s Model Taya’s Model ROM E E m r σ U.B. L.B. 0 σm σr H20S 5.74 5.74 5.73 433 360 950.5 669.1 −29.2 Z20S 5.42 5.42 5.40 366 555 878.5 806.3 −32.8 HZ20S 5.58 5.59 5.58 394 185 591.3 214.5 −40.2 HZ20S6C 5.36 5.36 5.34 485 650 176.4 139.0 −42.8

It is elucidated from Table3 that the addition of secondary phase (SiC and CNT) introduce an interfacial stress (compressive in the reinforcement and tensile in the matrix), which provides enhanced structural integrity and toughness [12]. The increase in the interfacial compressive stress in the reinforcement (from 29.2 MPa and 32.8 MPa, respectively in the parent material H20S and Z20S, to 40.2 MPa in HZ20S, and to 42.8 MPa in HZ20S6C) lowers the high tensile stress in the matrix (from 669.1 MPa, and 806.3 MPa, respectively in H20S and Z20S, to 214.5 MPa in HZ20S and to 139.0 MPa in HZ20S6C) and thus, favors in enhancing the fracture toughness (from 5.2 MPa·m0.5 and 5.7 MPa·m0.5, respectively in the base material H20S and Z20S, to 8.7 MPa·m0.5 in HZ20S and to 10.2 MPa·m0.5 in HZ20S6C). The schematic illustration of the effect of SiC and CNT reinforcement on the microstructural evolution of HfB2, ZrB2, and HfB2-ZrB2 (1:1) system is elucidated in Figure7. The propagation of the crack and its closure is restricted (for enhancement in the toughness) is attributed to the highly dense microstructure (≤1% porosity) and presence of SiC in H20S (Figure7a) and Z20S (Figure7b) which dissipate the energy via crack deflection mechanism, microstructurally shown in Figure7e [ 7,12]. Analogous to solid solution strengthening mechanism in metal alloys, the solid solution formation of HfB2-ZrB2 (in HZ20S and HZ20S6C samples, see Figure7c,d) create a local stress field which then mitigates the crack propagation along with the active bending and branching mechanism of SiC particle (Figure7e), leading to an increase in the toughness when compared with H20S and Z20S (see Table2). Further, the solid solution samples (HZ20S and HZ20S6C samples) show the presence of fine grains (Figure5c–e) when compared with parent materials (H20S and Z20S samples, Z20S sample showed very large grains, see Figure5a,b). The fine grains may serve as the energy-dissipation zone supporting the higher indentation toughness in solid solution samples [50]. Apart from this, the presence of CNT in the HZ20S6C sample enhances the fracture toughness via well-established CNT pull-out, deflection and branching mechanisms (shown in Figure7f). Hence, it can be anticipated that the synergy of reinforcements (SiC and CNT) which act as a sintering aid and toughening along with the solid solution strengthening of (Hf,Zr)B2 alleviate the crack propagation making HZ20S6C sample a viable structural material for aerospace application. Coatings 2017, 7, 110 11 of 15 reinforced phase, Er is estimated by ROM, utilizing modulus of matrix (HfB2/ZrB2/equal vol % of both) and composite (Ec) as presented in Table 2. Table 3. Theoretical calculation of the coefficient of thermal expansion (CTE), modulus and residual stresses. Here, ROM = rule of mixture, HS = Hashin-Shtrikman, U.B. = upper bound, L.B. = lower

bound, Em = elastic modulus of the matrix, Er = elastic modulus of reinforcement, σm = residual stress

of the matrix, and r = residual stress of the reinforcement.

CTE (×10−6/K) Modulus (GPa) Residual Stress (MPa) Composition HS Model Hsueh’s Model Taya’s Model ROM Em Er U.B. L.B. σ0 σm σr H20S 5.74 5.74 5.73 433 360 950.5 669.1 −29.2 Z20S 5.42 5.42 5.40 366 555 878.5 806.3 −32.8 HZ20S 5.58 5.59 5.58 394 185 591.3 214.5 −40.2 HZ20S6C 5.36 5.36 5.34 485 650 176.4 139.0 −42.8

It is elucidated from Table 3 that the addition of secondary phase (SiC and CNT) introduce an interfacial stress (compressive in the reinforcement and tensile in the matrix), which provides enhanced structural integrity and toughness [12]. The increase in the interfacial compressive stress in the reinforcement (from 29.2 MPa and 32.8 MPa, respectively in the parent material H20S and Z20S, to 40.2 MPa in HZ20S, and to 42.8 MPa in HZ20S6C) lowers the high tensile stress in the matrix (from 669.1 MPa, and 806.3 MPa, respectively in H20S and Z20S, to 214.5 MPa in HZ20S and to 139.0 MPa in HZ20S6C) and thus, favors in enhancing the fracture toughness (from 5.2 MPa·m0.5 and 5.7 MPa·m0.5, respectively in the base material H20S and Z20S, to 8.7 MPa·m0.5 in HZ20S and to 10.2 MPa·m0.5 in HZ20S6C). The schematic illustration of the effect of SiC and CNT reinforcement on the microstructural evolution of HfB2, ZrB2, and HfB2-ZrB2 (1:1) system is elucidated in Figure 7. The propagation of the crack and its closure is restricted (for enhancement in the toughness) is attributed to the highly dense microstructure (≤1% porosity) and presence of SiC in H20S (Figure 7a) and Z20S (Figure 7b) which dissipate the energy via crack deflection mechanism, microstructurally shown in Figure 7e [7,12]. Analogous to solid solution strengthening mechanism in metal alloys, the solid solution formation of HfB2-ZrB2 (in HZ20S and HZ20S6C samples, see Figure 7c,d) create a local stress field which then mitigates the crack propagation along with the active bending and branching mechanism of SiC particle (Figure 7e), leading to an increase in the toughness when compared with H20S and Z20S (see Table 2). Further, the solid solution samples (HZ20S and HZ20S6C samples) show the presence of fine grains (Figure 5c–e) when compared with parent materials (H20S and Z20S samples, Z20S sample showed very large grains, see Figure 5a,b). The fine grains may serve as the energy-dissipation zone supporting the higher indentation toughness in solid solution samples [50]. Apart from this, the presence of CNT in the HZ20S6C sample enhances the fracture toughness via well-established CNT pull-out, deflection and branching mechanisms (shown in Figure 7f). Hence, it can be anticipated that the synergy of reinforcements (SiC and CNT) which act as a sintering aid and toughening along with the solid solution strengthening of (Hf,Zr)B2 alleviate the crack propagation Coatings 2017, 7, 110 12 of 15 making HZ20S6C sample a viable structural material for aerospace application.

(a) (b) HfB2 (a) SiC SiC ZrB2 SiC

pores

Coatings 2017, 7, 110 12 of 15 Bigger ZrB2 grains No Porosity Porosity ~1% ZrB (c) (d) 2 Figure 7. Cont. ZrB2 pores SiC SiC

HfB -ZrB HfB2-ZrB2 2 2 solid solution) solid solution)

HfB2

Porosity ~4% Porosity <1% (e) (f) HfB2 CNT pull-out

Crack deflection CNT bridging

100 m 100 m ZrB2 200 nm

Figure 7.7. Schematic illustrating the effect of microstructural evolution on the fracture toughness of HfB2-ZrB2-SiC-CNT:HfB2-ZrB2-SiC-CNT: (aa)) H20S,H20S, ((bb)) Z20S,Z20S, ((cc)) HZ20SHZ20S andand ((dd)) HZ20S6CHZ20S6C composites.composites.

4. Conclusions

Novel HfB2-ZrB2-SiC-CNT composites were densified to >96% of relative density using SPS at Novel HfB2-ZrB2-SiC-CNT composites were densified to >96% of relative density using SPS at 1850 ◦°C. The results showed that HfB2 and ZrB2 diffuses mutually to form (Zr,Hf)B2 solid solution, 1850 C. The results showed that HfB2 and ZrB2 diffuses mutually to form (Zr,Hf)B2 solid solution, which enhances the fracturefracture toughnesstoughness ofof thethe materialmaterial (HZ20S:(HZ20S: 8.78.7 MPaMPa··mm0.50.5) when compared with H20S (5.2 MPa MPa··mm0.50.5) )and and Z20S Z20S (5.7 (5.7 MPa MPa·m·m0.50.5). The). The reinforcement reinforcement of CN of CNTT (as (asHZ20S6C) HZ20S6C) and andhas hasled towards achieving the toughest ceramic (10.2 MPa·m0.5) in0.5 HfB2-ZrB2 system. The hardness of the led towards achieving the toughest ceramic (10.2 MPa·m ) in HfB2-ZrB2 system. The hardness ofprocessed the processed composites composites is higher is higherthan the than individual the individual phase hardness phase hardness (assessed (assessed using ROM), using ROM),which whichestablishes establishes the synergy the synergy of SiC and of SiC CNT and in enhancingCNT in enhancing the mechanical the mechanical performance performance of HfB2-ZrB of2 composites (HZ20S6C), attributed to the (Zr,Hf)B2 solid solution strengthening and enhanced HfB2-ZrB2 composites (HZ20S6C), attributed to the (Zr,Hf)B2 solid solution strengthening and enhanceddensification densification with CNT withreinforcement CNT reinforcement.. The study Thereveals study that reveals the reinforcement that the reinforcement generate interfacial generate residual stresses during processing which, then governs the strength of HfB2-ZrB2-SiC-CNT interfacial residual stresses during processing which, then governs the strength of HfB2-ZrB2-SiC-CNT composites, making it mandatory for re-entryre-entry spacespace structuresstructures wherewhere damagedamage tolerancetolerance isis anan issue.issue. Acknowledgements: Authors at Indian Institute of Technology Kanpur (IITK) acknowledge the financial Acknowledgments: Authors at Indian Institute of Technology Kanpur (IITK) acknowledge the financial support support received from IMpacting Research INnovation and Technology (IMPRINT) grant from the Department received from IMpacting Research INnovation and Technology (IMPRINT) grant from the Department of Space, andof Space, Ministry and ofMinistry Human of Resource Human Resource Development Development (MHRD), ( GovernmentMHRD), Government of India. Authorsof India. acknowledgeAuthors acknowledge technical technical assistance from Advanced Centre of Materials Science (ACMS) at IIT Kanpur (for SEM and instrumented indentation facility).

Author Contributions: Ambreen Nisar designed and performed the experiments, analyzed the data and wrote the paper. Kantesh Balani conceived the idea, helped designing the experiment and guided in organizing and editing the paper.

Conflicts of Interest: The authors declare no conflict of interest.

Coatings 2017, 7, 110 13 of 15 assistance from Advanced Centre of Materials Science (ACMS) at IIT Kanpur (for SEM and instrumented indentation facility). Author Contributions: Ambreen Nisar designed and performed the experiments, analyzed the data and wrote the paper. Kantesh Balani conceived the idea, helped designing the experiment and guided in organizing and editing the paper. Conﬂicts of Interest: The authors declare no conﬂict of interest.

References

1. Corral, E.L.; Loehman, R.E. Ultra-high-temperature ceramic coatings for oxidation protection of carbon–carbon composites. J. Am. Ceram. Soc. 2008, 91, 1495–1502. [CrossRef] 2. Balani, K.; Gonzalez, G.; Agarwal, A.; Hickman, R.; O’Dell, J.S.; Seal, S. Synthesis, microstructural characterization, and mechanical property evaluation of vacuum plasma sprayed tantalum carbide. J. Am. Ceram. Soc. 2006, 89, 1419–1425. [CrossRef] 3. Levine, S.R.; Opila, E.J.; Halbig, M.C.; Kiser, J.D.; Singh, M.; Salem, J.A. Evaluation of ultra-high temperature ceramics foraeropropulsion use. J. Eur. Ceram. Soc. 2002, 22, 2757–2767. [CrossRef]

4. Zapata-Solvas, E.; Jayaseelan, D.D.; Lin, H.T.; Brown, P.; Lee, W.E. Mechanical properties of ZrB2- and HfB2-based ultra-high temperature ceramics fabricated by spark plasma sintering. J. Eur. Ceram. Soc. 2013, 33, 1373–1386. [CrossRef] 5. Fahrenholtz, W.G.; Hilmas, G.E.; Talmy, I.G.; Zaykoski, J.A. Refractory diborides of zirconium and hafnium. J. Am. Ceram. Soc. 2007, 90, 1347–1364. [CrossRef]

6. Mallik, M.; Kailath, A.J.; Ray, K.; Mitra, R. Electrical and thermophysical properties of ZrB2 and HfB2 based composites. J. Eur. Ceram. Soc. 2012, 32, 2545–2555. [CrossRef] 7. Nisar, A.; Ariharan, S.; Venkateswaran, T.; Sreenivas, N.; Balani, K. Effect of carbon nanotube on processing,

microstructural, mechanical and ablation behavior of ZrB2–20SiC based ultra-high temperature ceramic composites. Carbon 2017, 111, 269–282. [CrossRef] 8. Nisar, A.; Ariharan, S.; Venkateswaran, T.; Sreenivas, N.; Balani, K. Oxidation studies on tac based ultra-high temperature ceramic composites under plasma arc jet exposure. Corros. Sci. 2016, 109, 50–61. [CrossRef] 9. Gild, J.; Zhang, Y.; Harrington, T.; Jiang, S.; Hu, T.; Quinn, M.C.; Mellor, W.M.; Zhou, N.; Vecchio, K.; Luo, J. High-entropy metal diborides: A new class of high-entropy materials and a new type of ultrahigh temperature ceramics. Sci. Rep. 2016, 6, 37946. [CrossRef][PubMed] 10. Anselmi-Tamburini, U.; Kodera, Y.; Gasch, M.; Unuvar, C.; Munir, Z.A.; Ohyanagi, M.; Johnson, S. Synthesis and characterization of dense ultra-high temperature thermal protection materials produced by ﬁeld activation through spark plasma sintering (SPS): I. Hafnium diboride. J. Mater. Sci. 2006, 41, 3097–3104. [CrossRef] 11. Wang, H.; Wang, C.-A.; Yao, X.; Fang, D. Processing and mechanical properties of zirconium diboride-based ceramics prepared by spark plasma sintering. J. Am. Ceram. Soc. 2007, 90, 1992–1997. [CrossRef] 12. Nisar, A.; Ariharan, S.; Balani, K. Synergistic reinforcement of carbon nanotubes and silicon carbide for toughening tantalum carbide based ultrahigh temperature ceramic. J. Mater. Res. 2016, 31, 682–692. [CrossRef] 13. Nisar, A.; Balani, K. Role of interfaces on multi-length scale wear mechanics of tac-based composites. Adv. Eng. Mater. 2017, 19.[CrossRef] 14. Sciti, D.; Silvestroni, L.; Celotti, G.; Melandri, C.; Guicciardi, S. Sintering and mechanical properties of

ZrB2–TaSi2 and HfB2–TaSi2 ceramic composites. J. Am. Ceram. Soc. 2008, 91, 3285–3291. [CrossRef] 15. Musa, C.; Orrù, R.; Sciti, D.; Silvestroni, L.; Cao, G. Synthesis, consolidation and characterization of

monolithic and SiC whiskers reinforced HfB2 ceramics. J. Eur. Ceram. Soc. 2013, 33, 603–614. [CrossRef] 16. Bull, J.; Kolodziej, P.; Salute, J.; Keese, D. Design, Instrumentation and Preﬂight Testing of a Sharp Ultra-High Temperature Ceramic Nosetip; NASA TM-1998-112229; NASA: Washington, DC, USA, October 1998. 17. Fahrenholtz, W.G.; Wuchina, E.J.; Lee, W.E.; Zhou, Y. Ultra-High Temperature Ceramics: Materials for Extreme Environment Applications; John Wiley & Sons: Hoboken, NJ, USA, 2014. 18. Yadhukulakrishnan, G.B.; Rahman, A.; Karumuri, S.; Stackpoole, M.M.; Kalkan, A.K.; Singh, R.P.; Harimkar, S.P. Spark plasma sintering of silicon carbide and multi-walled carbon nanotube reinforced zirconium diboride ceramic composite. Mater. Sci. Eng. A 2012, 552, 125–133. [CrossRef] Coatings 2017, 7, 110 14 of 15

19. Tian, W.-B.; Kan, Y.-M.; Zhang, G.-J.; Wang, P.-L. Effect of carbon nanotubes on the properties of ZrB2–SiC ceramics. Mater. Sci. Eng. A 2008, 487, 568–573. [CrossRef] 20. Wang, Z.; Hong, C.; Zhang, X.; Sun, X.; Han, J. Microstructure and thermal shock behavior of

ZrB2–SiC–graphite composite. Mater. Chem. Phys. 2009, 113, 338–341. [CrossRef] 21. Balani, K.; Bakshi, S.R.; Mungole, T.; Agarwal, A. Ab-initio molecular modeling of interfaces in tantalum-carbon system. J. Appl. Phys. 2012, 111, 063521–063527. [CrossRef] 22. Liu, L.; Ye, F.; Zhang, Z.; Zhou, Y. Microstructure and mechanical properties of the spark plasma sintered TaC/SiC composites. Mater. Sci. Eng. A 2011, 529, 479–484. [CrossRef]

23. Balak, Z.; Zakeri, M. Effect of HfB2 on microstructure and mechanical properties of ZrB2–SiC-based composites. Int. J. Refract. Metals Hard Mater. 2016, 54, 127–137. [CrossRef]

24. Hu, P.; Guolin, W.; Wang, Z. Oxidation mechanism and resistance of ZrB2–SiC composites. Corros. Sci. 2009, 51, 2724–2732. [CrossRef] 25. Squire, T.H.; Marschall, J. Material property requirements for analysis and design of UHTC components in hypersonic applications. J. Eur. Ceram. Soc. 2010, 30, 2239–2251. [CrossRef] 26. Kumar, A.; Biswas, K.; Basu, B. On the toughness enhancement in hydroxyapatite-based composites. Acta Mater. 2013, 61, 5198–5215. [CrossRef] 27. Anstis, G.R.; Chantiklul, P.; Lawn, B.R.; Marshall, D.B. A critical evaluation of indentation techniques for measuring fracture toughness: I, dircet crack measurements. J. Am. Ceram. Soc. 1981, 64, 533–538. [CrossRef] 28. Awaji, H.; Sakaida, Y. V-notch technique for single-edge notched beam and chevron notch methods. J. Am. Ceram. Soc. 1990, 73, 3522–3523. [CrossRef] 29. Wang, X.; Padture, N.P.; Tanaka, H. Contact-damage-resistant ceramic/single-wall carbon nanotubes and ceramic/graphite composites. Nat. Mater. 2004, 3, 539–544. [CrossRef][PubMed] 30. Chantikul, P.; Anstis, G.R.; Lawn, B.R.; Marshall, D.B. A critical evaluation of indentation techniques for measuring fracture toughness: II, strength method. J. Am. Ceram. Soc. 1981, 64, 539–543. [CrossRef] 31. Sheldon, B.W.; Curtin, W.A. Nanoceramic composites: Tough to test. Nat. Mater. 2004, 3, 505–506. [CrossRef] [PubMed] 32. Bakshi, S.R.; Musaramthota, V.; Virzi, D.A.; Keshri, A.K.; Lahiri, D.; Singh, V.; Seal, S.; Agarwal, A. Spark plasma sintered tantalum carbide–carbon nanotube composite: Effect of pressure, carbon nanotube length and dispersion technique on microstructure and mechanical properties. Mater. Sci. Eng. A 2011, 528, 2538–2547. [CrossRef] 33. Rudy, E. Ternary Phase Equilibria in Transition Metal-Boron-Carbon-Silicon Systems. Part 5. Compendium of Phase Diagram Data; AD0689843; DTIC: Washington, WA, USA, 30 April 1969.

34. Silvestroni, L.; Sciti, D.; Bellosi, A. Microstructure and properties of pressureless sintered HfB2-based composites with additions of ZrB2 or HfC. Adv. Eng. Mater. 2007, 9, 915–920. [CrossRef] 35. Post, B.; Glaser, F.W.; Moskowitz, D. Transition metal diborides. Acta Metall. 1954, 2, 20–25. [CrossRef] 36. Bakshi, S.R.; Musaramthota, V.; Lahiri, D.; Singh, V.; Seal, S.; Agarwal, A. Spark plasma sintered tantalum carbide: Effect of pressure and nano-boron carbide addition on microstructure and mechanical properties. Mater. Sci. Eng. A 2011, 528, 1287–1295. [CrossRef] 37. Dresselhaus, M.S.; Jorio, A.; Hofmann, M.; Dresselhaus, G.; Saito, R. Perspectives on carbon nanotubes and graphene Raman Spectroscopy. Nano Lett. 2010, 10, 751–758. [CrossRef][PubMed]

38. Sonber, J.K.; Murthy, T.S.R.C.; Subramanian, C.; Krishnamurthy, N.; Hubli, R.C.; Suri, A.K. Effect of CrSi2 and HfB2 addition on densiﬁcation and properties of ZrB2. Int. J. Refract. Metals Hard Mater. 2012, 31, 125–131. [CrossRef] 39. Marschall, J.; Erlich, D.C.; Manning, H.; Duppler, W.; Ellerby, D.; Gasch, M. Microhardness and high-velocity

impact resistance of HfB2/SiC and ZrB2/SiC composites. J. Mater. Sci. 2004, 39, 5959–5968. [CrossRef] 40. Gasch, M.; Ellerby, D.; Irby, E.; Beckman, S.; Gusman, M.; Johnson, S. Processing, properties and arc jet oxidation of hafnium diboride/silicon carbide ultra high temperature ceramics. J. Mater. Sci. 2004, 39, 5925–5937. [CrossRef]

41. Licheri, R.; Orrù, R.; Musa, C.; Locci, A.M.; Cao, G. Consolidation via spark plasma sintering of HfB2/SiC and HfB2/HfC/SiC composite powders obtained by self-propagating high-temperature synthesis. J. Alloys Compd. 2009, 478, 572–578. [CrossRef]

42. Asl, M.S.; Kakroudi, M.G.; Noori, S. Hardness and toughness of hot pressed ZrB2–SiC composites consolidated under relatively low pressure. J. Alloys Compd. 2015, 619, 481–487. [CrossRef] Coatings 2017, 7, 110 15 of 15

43. Zhu, C.; Guo, W.; Yu, T.X.; Woo, C.H. Radial compression of carbon nanotubes: Deformation and damage, super-elasticity and super-hardness. Nanotechnology 2005, 16, 1035. [CrossRef] 44. Hu, C.; Sakka, Y.; Byungkoog, J.; Tanaka, H.; Nishimura, T.; Shuqi, G.; Grasso, S. Microstructure and

properties of ZrB2–SiC and HfB2–SiC composites fabricated by spark plasma sintering (SPS) using TaSi2 as sintering aid. J. Ceram. Soc. Jpn. 2010, 118, 997–1001. [CrossRef] 45. Rosen, B.W.; Hashin, Z. Effective thermal expansion coefﬁcients and speciﬁc heats of composite materials. Int. J. Eng. Sci. 1970, 8, 157–173. [CrossRef] 46. Brown, H.L.; Armstrong, P.E.; Kempter, C.P. Elastic properties of some polycrystalline transition-metal monocarbides. J. Chem. Phys. 1966, 45, 547–549. [CrossRef] 47. Taya, M.; Hayashi, S.; Kobayashi, A.S.; Yoon, H.S. Toughening of a particulate-reinforced ceramic-matrix composite by thermal residual stress. J. Am. Ceram. Soc. 1990, 73, 1382–1391. [CrossRef] 48. Ghosh, D.; Subhash, G.; Orlovskaya, N. Measurement of scratch-induced residual stress within sic grains in

ZrB2–SiC composite using micro-Raman Spectroscopy. Acta Mater. 2008, 56, 5345–5354. [CrossRef] 49. Watts, J.; Hilmas, G.; Fahrenholtz, W.G.; Brown, D.; Clausen, B. Measurement of thermal residual stresses in

ZrB2–SiC composites. J. Eur. Ceram. Soc. 2011, 31, 1811–1820. [CrossRef] 50. Zhang, C.; Gupta, A.; Seal, S.; Boesl, B.; Agarwal, A. Solid solution synthesis of tantalum carbide-hafnium carbide by spark plasma sintering. J. Am. Ceram. Soc. 2017, 100, 1853–1862. [CrossRef]

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