technologies

Article Reactive Spark Plasma and Mechanical Properties of Diboride– Diboride Ultrahigh Temperature Ceramic Solid Solutions

Karthiselva N. S. and Srinivasa Rao Bakshi * Department of Metallurgical and Materials Engineering, Indian Institute of Technology Madras, Chennai 600036, India; [email protected] * Correspondence: [email protected]; Tel.: +91-44-2257-4781

Academic Editor: Sandip P. Harimkar Received: 31 July 2016; Accepted: 6 September 2016; Published: 9 September 2016 Abstract: Ultrahigh temperature ceramics (UHTCs) such as diborides of zirconium, tantalum and their composites are considered to be the candidate materials for thermal protection systems of hypersonic vehicles due to their exceptional combination of physical, chemical and mechanical properties. A composite of ZrB2-TiB2 is expected to have better properties. In this study, an attempt has been made to fabricate ZrB2-TiB2 ceramics using mechanically activated elemental powders followed by reactive spark plasma sintering (RSPS) at 1400 ◦C. Microstructure and phase analysis was carried out using X-ray diffractometer (XRD) and electron microscopy to understand microstructure evolution. Fracture toughness and hardness were evaluated using indentation methods. Nanoindentation was used to measure elastic modulus. Compressive strength of the composites has been reported.

Keywords: ball milling; reactive spark plasma sintering; borides; UHTCs; solid solution

1. Introduction

Zirconium diboride (ZrB2) and (TiB2) have very high melting points above 3000 ◦C, high hardness (>25 GPa), elastic modulus (>500 GPa), good electrical (1.0 × 107 S/m) and thermal (60–120 W·m−1·K−1) conductivities [1,2] and chemical stability. These physical and chemical characteristics are mandatory requirements in applications such as thermal protection systems of re-entry vehicles, crucibles and rocket nozzle [3–5]. However, fabricating fully dense compacts using conventional sintering techniques requires long processing time (>2 h) and high temperatures (>1800 ◦C). Apart from sintering related challenges, mechanical and oxidation resistance also needs to be improved. In order to address these issues various sintering additives such as SiC, Si3N4, TiSi2, MoSi2, TiB2, CNT, and Graphene have been used [6–12]. TiB2 possesses similar (hexagonal, space group P6/mmm) and properties as ZrB2 [13]. It has been reported that ZrB2-TiB2 system exhibits a complete solid solution [10,13]. Mroz [14] showed that (Ti,Zr)B2 solid solutions exhibited higher mechanical properties than TiB2 or ZrB2. Aviles et al. [13] fabricated Ti1-xZr-B solid solutions using mechanically induced self-sustaining reactions (MSR). They have reported that solid solutions can be used to fabricate raw materials for technological applications. Inagaki et al. [15] have synthesized Zr0.5Ti0.5B2 solid solution using hot ◦ pressing and spark plasma sintering at 1600–2200 C. Li et al. [16] have fabricated TiB2/ZrB2/SiC compacts using spark plasma sintering at 1700 ◦C with 40 MPa applied pressure. It was reported that solid solution formation refined the microstructure and improved mechanical properties. ◦ Chakraborty et al. [17] have fabricated TiB2-ZrB2 based ceramics using hot pressing at 2200 C in Ar atmosphere. They have reported that TiB2 completely dissolves into ZrB2 and forms a solid solution. It was also reported that mechanical and wear properties improved with addition of TiB2 in ZrB2.

Technologies 2016, 4, 30; doi:10.3390/technologies4030030 www.mdpi.com/journal/technologies Technologies 2016, 4, 30 2 of 11

In recent times, reactive spark plasma sintering (RSPS) has been identified as an important sintering method to fabricate ultrahigh temperature ceramics UHTCs [18–20]. Recently, we have ◦ fabricated TiB2, ZrB2 and TiB2-CNT composite using RSPS at temperatures as low as 1400 C[3,12]. Simultaneous synthesis and densification is one of the advantages of this technique. It is a very rapid process, and, hence, fine-grained bulk ceramic compacts can be fabricated. Previous research on ZrB2-TiB2 system shows that, in order to fabricate dense ceramic solid solutions, high temperatures as high as 2000 ◦C is required. ◦ However, we have fabricated ZrB2-TiB2 composites using spark plasma sintering at 1500 C having improved properties [10]. Composites were obtained using commercially available TiB2 and ZrB2 powders. There is no report available on fabrication of ZrB2-TiB2 using elemental powders followed by RSPS. Thus, the objective of the present work is to fabricate ZrB2-TiB2 solid solution at temperatures as low as possible. Furthermore, the solid solution formation and its effect on mechanical properties are presented.

2. Materials and Methods Commercially pure titanium (Ti) (−325 mesh, 99.5% metal basis, Alfa Aesar, Chennai, India), zirconium (Zr) (99% pure sponge fines, <30 µm in size obtained from Nuclear Fuel Complex, Hyderabad, India), and (B) (amorphous powder, LobaChemie, Mumbai, India) were ball milled in a planetary ball mill (Fritsch Pulverisette-5, Germany) for 8 h. Elemental powders were taken in stoichiometric ratio and to produce ZrB2, TiB2 and solid solutions. Milling was carried out using vials and balls (10 mm in diameter). Toluene was used as a process controlling agent. Composition details are given in Table1. In order to find the onset of reaction between elemental powders, Differential thermal analysis (DTA) was carried out. Milled powders were heated from room temperature to 1400 ◦C at a heating rate of 50 ◦C /min.

Table 1. Microstructural details and mechanical properties of reaction spark plasma sintered samples.

Sample Vol % Relative Grain Size Nanohardness Elastic Indentation Fracture 1/2 Name TiB2 Density (%) (µm) (GPa) Modulus (GPa) Toughness (MPa.m ) Zr-B 0 97 1.2 ± 0.25 29 ± 2 495 ± 43 3.2 ± 0.6 75Zr-25Ti-B 25 96 0.7 ± 0.30 33 ± 4 496 ± 47 3.5 ± 0.5 50Zr-50Ti-B 50 97 1.5 ± 0.50 34 ± 5 523 ± 52 3.9 ± 0.6 25Zr-75Ti-B 75 98 1.5 ± 0.40 33 ± 6 568 ± 44 3.7 ± 0.6 Ti-B 100 98 0.5 ± 0.20 34 ± 4 658 ± 32 3.4 ± 0.5

RSPS was carried out in spark plasma sintering unit (Model SPS-625, SPS Syntex Inc., Kawasaki, Japan) using 8 h milled powders at 1400 ◦C with a heating rate of 50 ◦C /min and applied pressure of 50 MPa. It should be noted that applied pressure was programmed in such a way that maximum pressure reached at highest sintering temperature. Compacts of 20 mm diameter and 5 mm thickness were fabricated using high-density graphite die. For easy removal of samples, a graphite foil (0.15 mm thickness) was inserted on the top and bottom of the powder bed and inner surface of the die. To understand the sintering behavior, the temperature, ram displacement and chamber pressure were noted as a function of time. Monolithic compacts and composites fabricated were named as Zr-B, 25Zr-75Ti-B, 50Zr-50Ti-B, 75Zr-25Ti-B, Ti-B, as described in Table1. Cross section samples were prepared using electro-discharge machining (EDM). Cross-sectioned samples were grounded using SiC emery sheets followed by polishing utilizing paste. Water immersion technique was used to calculate bulk density. Density of the composites was calculated based on rule of mixture. An X-ray diffractometer (Panalytical, Almelo, The Netherlands) was used for phase identification. The grain size distribution and morphology were investigated using scanning electron micrograpy (FEI Quanta 400, Hillsboro, OR, USA) and InspectF (FEI, Eden Prairie, MN, USA). Nanohardness and elastic modulus were measured using a nanoindenter (Hysitron Triboindenter (TI 950, Hysitron, Eden Prairie, MN, USA). A load of 8000 µN was used and a triangular load pattern Technologies 2016, 4, 30 3 of 11 were investigated using scanning electron micrograpy (FEI Quanta 400, Hillsboro, OR, USA) and InspectF (FEI, Eden Prairie, MN, USA). TechnologiesNanohardness2016, 4, 30 and elastic modulus were measured using a nanoindenter (Hysitron3 of 11 Triboindenter (TI 950, Hysitron, Eden Prairie, MN, USA). A load of 8000 µN was used and a triangular load pattern was used. The Oliver and Pharr model [21] was used for analysis of was used. The Oliver and Pharr model [21] was used for analysis of indentation data. Indentation indentation data. Indentation fracture toughness was evaluated using Vicker’s hardness tester. A fracture toughness was evaluated using Vicker’s hardness tester. A load of 5 kg and dwell time of load of 5 kg and dwell time of 10 s was used and the cracks formed were measured using SEM. The 10 s was used and the cracks formed were measured using SEM. The Anstis method [22] was used to Anstis method [22] was used to calculate indentation fracture toughness using Equation (1): calculate indentation fracture toughness using Equation (1): 1/ 2  E1/2  P  K = 0.016 E P (1) K IC= 0.016    3/ 2  (1) IC HH   C3/2 

Room temperaturetemperature compression compression was was carried carried out out using using a Zwick a Zwick universal universal testing testing machine machine (Zwick, (Zwick,Ulm, Germany). Ulm, Germany). Samples Samples having dimensionshaving dimensions of 6 mm of height 6 mm andheight 3.5 and mm 3.5 width mm werewidth used were for used the forcompression the compression testing. testing.

3. Results Results and and Discussion Discussion

3.1. Mechanical Milling and Thermal Analysis Figure1 1shows shows 0.5 0.5 h ball h ball milled milled elemental elemental powders. powders. It shows It thatshows no impuritythat no materialimpurity signatures material weresignatures observed were other observed than startingother than elemental starting powders.elemental However,powders. presenceHowever, of presence small amount of small of amountmonoclinic of monoclinic zirconium dioxidezirconium (ZrO dioxide2) was (ZrO observed.2) was Itobserved. should be It notedshould that be amorphousnoted that amorphous boron was boronused, and,was hence,used, and, it was hence, not observed it was not in theobserved x-ray diffractogram.in the x-ray diffractogram. From Figure 2From, slight Figure peak shifting2, slight peakand broadening shifting and are broadening observed afterare observed 8 h of ball after milling. 8 h of Thisball milling. is due to This the millingis due to induced the milling lattice induced strain latticeand reduction strain and in crystallitereduction size.in crystallite Reactions size. among Reactions elemental among Zr-B elemental and Ti-B Zr-B is highly and exothermicTi-B is highly in exothermicnature. However, in nature. TiB 2However,and ZrB2 TiBpeaks2 and were ZrB not2 peaks observed were not after observed 8h of milling. after 8h Toluene of milling. was Toluene used as wasa process used controllingas a process medium controlling and medium it prevented and temperatureit prevented risetemperature and reaction rise betweenand reaction the powders.between theHowever, powders. monoclinic However, ZrO monoclinic2 peak intensity ZrO2 peak decreased intensity and decreased small tungsten and small carbide tungsten (WC) peak carbide observed (WC) peakafter 8observed h milling. after 8 h milling.

Figure 1. X-ray diffractogram of 0.5 h ball milled elemental powders.

Differential scanning calorimetry plot of 8 h mechanically milled elemental powders is shown Differential scanning calorimetry plot of 8 h mechanically milled elemental powders is shown in in Figure 3. The first exothermic peak observed between 900 and 1000 °C corresponds to α phase to β Figure3. The first exothermic peak observed between 900 and 1000 ◦C corresponds to α phase to β phase transition of Ti and Zr. The second exothermic peak which occurs in the range of 1000–1300 °C phase transition of Ti and Zr. The second exothermic peak which occurs in the range of 1000–1300 ◦C corresponds to formation of ZrB2 and TiB2. Thus, 1400 °C was chosen as sintering temperature to corresponds to formation of ZrB and TiB . Thus, 1400 ◦C was chosen as sintering temperature to fabricate compacts. It should be noted2 that2 the onset of exothermic reaction varies with addition of fabricate compacts. It should be noted that the onset of exothermic reaction varies with addition of Zr or Ti. Zr or Ti.

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Figure 2. XRD of 8 h ball milled elemental powders. Figure 2. XRD of 8 h ball milled elemental powders.

Figure 3. Differential scanning calorimetry (DSC) traces of elemental powder mixtures milled for 8 h. Figure 3. Differential scanning calorimetry (DSC)(DSC) traces of elemental powder mixtures milled for 8 h. 3.2. Simultaneous Synthesis and Densification 3.2. Simultaneous Synthesis and DensificationDensification During RSPS, a sudden increase in punch displacement was observed in the temperature range During RSPS, a sudden increase in punch displacement was observed in the temperature range of 550During–700 °C. RSPS, During a sudden this time, increase an an in punchintense displacement flash of light was appeared observed on inthe the top temperature and bottom range of the of of 550–700◦ °C. During this time, an an intense flash of light appeared on the top and bottom of the 550–700die punches.C. During This kind this time,of phenomenon an an intense was flash obse of lightrved appearedduring our on earlier the top studies and bottom as well. of the High die die punches. This kind of phenomenon was observed during our earlier studies as well. High punches.exothermic This reaction kind of between phenomenon the elemental was observed powders during is the our reason earlier studiesfor this as occurrence well. High of exothermic the flash, exothermic reaction between the elemental powders is the reason for this occurrence of the flash, reactionand sudden between displacement. the elemental To understand powders isthis the ph reasonenomenon, for this the occurrence sample displacement, of the flash, temperature, and sudden and sudden displacement. To understand this phenomenon, the sample displacement, temperature, displacement.time and pressure To understandduring RSPS this were phenomenon, analysed. Figu there sample4 shows displacement, the plot of displacement temperature, behaviour time and of time and pressure during RSPS were analysed. Figure 4 shows the plot of displacement behaviour of pressure8 h milled during elemental RSPS powders were analysed. during Figure RSPS.4 The shows sample the plot displacement of displacement reported behaviour in the present of 8 h milled study 8 h milled elemental powders during RSPS. The sample displacement reported in the present study elementalincludes contribution powders during from RSPS.graphi Thete dies, sample punches, displacement and spacers. reported Tamburini in the present et al. [23] study reported includes a includes contribution from graphite dies, punches, and spacers. Tamburini et al. [23] reported a contributionmethod to subtract from graphite this complex dies, punches, displacement, and spacers. and it Tamburinican be found et al.elsewhere. [23] reported However, a method in the to method to subtract this complex displacement, and it can be found elsewhere. However, in the subtractpresent study, this complex it is difficult displacement, to remove and this it contribution can be found from elsewhere. sample However,displacement in the due present to following study, present study, it is difficult to remove this contribution from sample displacement due to following itreasons. is difficult In the to remove present this study, contribution we have from used sample RSPS displacementin which self-propagating due to following high reasons. temperature In the reasons. In the present study, we have used RSPS in which self-propagating high temperature presentsynthesis study, reaction we have (SHS) used induced RSPS sample in which displacement self-propagating occurs high and temperature this facilitates synthesis faster reactiondensification. (SHS) synthesis reaction (SHS) induced sample displacement occurs and this facilitates faster densification. inducedDue to the sample excessive displacement heat released occurs during and thisthis facilitates event, graphitic faster densification. die, punches Dueand spacers to the excessive may expand heat Due to the excessive heat released during this event, graphitic die, punches and spacers may expand releasedand during during cooling this it event, may graphiticcontract. Thus, die, punches it is very and difficult spacers to may subtract expand from and the during data. cooling it may and during cooling it may contract. Thus, it is very difficult to subtract from the data. contract.It can Thus, be observed it is very that, difficult there to are subtract four distinct from the stages data. in the plot; (1) initial sintering stage (2) SHS It can be observed that, there are four distinct stages in the plot; (1) initial sintering stage (2) SHS reactionIt can region be observed (3) additional that, there sintering are fourstage distinct (4) minor stages densification in the plot; stage. (1) initial In the sintering initial stage, stage reaction region (3) additional sintering stage (4) minor densification stage. In the initial stage, (2)gradual SHS reactionshrinkage region due to (3) particle additional rearrangement sintering stage was (4)observed. minor densificationDuring the second stage. stage, In the sudden initial gradual shrinkage due to particle rearrangement was observed. During the second stage, sudden stage,displacement gradual change shrinkage observed due to in particle all the rearrangement samples. This waswasobserved. due to highly During exothermic the second reaction stage, displacement change observed in all the samples. This was due to highly exothermic reaction suddenbetween displacement the milled powders. change observed in all the samples. This was due to highly exothermic reaction between the milled powders. betweenThe theadiabatic milled temperature powders. of TiB2 and ZrB2 is above 3000 K, which is above the threshold limit The adiabatic temperature of TiB2 and ZrB2 is above 3000 K, which is above the threshold limit of 1800 K value of SHS reaction to occur [24]. During the exothermic reaction, a very high of 1800 K value of SHS reaction to occur [24]. During the exothermic reaction, a very high temperature is released, which is sufficient to convert milled powders to products. Similar temperature is released, which is sufficient to convert milled powders to products. Similar

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The adiabatic temperature of TiB2 and ZrB2 is above 3000 K, which is above the threshold limit of 1800 K value of SHS reaction to occur [24]. During the exothermic reaction, a very high temperature is released,Technologies which 2016, is4, 30 sufficient to convert milled powders to products. Similar phenomena was reported5 of 11 byTechnologies us earlier 2016 [3,, 4,]. 30 The Zr-B sample showed this phenomenon at ~300 s, while the Ti-B samples5 of 11 phenomena was reported by us earlier [3,4]. The Zr-B sample showed this phenomenon at ~300 s, showed this at 440 s. From Figure3, it was observed that exothermic reaction was observed in the while the Ti-B samples showed this at 440 s. From Figure 3, it was observed that exothermic reaction temperaturephenomena range was reported of 1000–1300 by us◦ C,earlier but, [3,4]. during The RSPS, Zr-B SHSsample induced showed sudden this phenomenon displacement at occurred~300 s, whilewas observed the Ti-B samplesin the temperature showed this range at 440 of s. 1000–13From Figure00 °C, 3, but,it was during observed RSPS, that SHS exothermic induced reactionsudden in the temperature range of 550–700 ◦C. This indicates that effect of current, applied pressure played wasdisplacement observed occurredin the temperature in the temperature range of range 1000–13 of 550–70000 °C, but, °C. duringThis indicates RSPS, SHSthat effectinduced of current,sudden a role in enhanced kinetics during RSPS. In the third stage, additional densification was observed displacementapplied pressure occurred played in athe role temperature in enhanced range kineti of 550–700cs during °C. RSPS. This indicatesIn the third that stage,effect ofadditional current, where plastic flow is the main densification mechanism. In our earlier study, it was showed that applieddensification pressure was playedobserved a rolewhere in plasticenhanced flow kineti is thecs main during densification RSPS. In the mechanism. third stage, In ouradditional earlier plastic flow controlled the densification [3]. It should be noted that additional densification was not densificationstudy, it was was showed observed that whereplastic plasticflow controlled flow is the the main densification densification [3]. mechanism. It should beIn notedour earlier that observed in Ti-B and for Zr-B. The magnitude of additional densification is observed to decrease with study,additional it was densification showed that was plastic not observedflow controlled in Ti-B theand densification for Zr-B. The [3]. magnitudeIt should beof notedadditional that the increase in vol. % of TiB . It indicates that addition of TiB modifies the sintering mechanism. additionaldensification densification is observed wasto 2decrease not observed with the in increase Ti-B and in vol. for %Zr-B.2 of TiB The2. It magnitudeindicates that of additionadditional of In the fourth stage, minor densification occurs. densificationTiB2 modifies isthe observed sintering to mechanism. decrease with In thethe fourthincrease stage, in vol. minor % of densification TiB2. It indicates occurs. that addition of TiB2 modifies the sintering mechanism. In the fourth stage, minor densification occurs.

Figure 4. Sintering behavior at 1400 °C with 50 MPa applied pressure and 50 °C/min heating rate. Figure 4. Sintering behavior at 1400 ◦C with 50 MPa applied pressure and 50 ◦C/min heating rate. Figure 4. Sintering behavior at 1400 °C with 50 MPa applied pressure and 50 °C/min heating rate. 3.3. Phase Analysis and Microstructural Characterisation 3.3. Phase Analysis and Microstructural Characterisation 3.3. Phase Analysis and Microstructural Characterisation The XRD patterns of RSPSed samples are shown in Figure 5. Formation of TiB2 and ZrB2 and The XRD patterns of RSPSed samples are shown in Figure5. Formation of TiB 2 and ZrB2 intermediateThe XRD phase patterns TiB wasof RSPSed observed. samples The 75Zr-25Ti-B, are shown in 50Zr-50Ti-B Figure 5. Formation and 25Zr-75Ti-B of TiB 2exhibited and ZrB 2solid and and intermediate phase TiB was observed. The 75Zr-25Ti-B, 50Zr-50Ti-B and 25Zr-75Ti-B exhibited intermediatesolution formation. phase TiBIt is was well-known observed. that The ZrB75Zr-25Ti-B,2-TiB2 system 50Zr-50Ti-B forms a andperfect 25Zr-75Ti-B solid solution exhibited [25]. solid The solid solution formation. It is well-known that ZrB -TiB system forms a perfect solid solution [25]. atomicsolution radius formation. difference It is well-knownbetween Ti andthat ZrZrB is2-TiB only22 system9%.2 Solid forms solution a perfect formation solid solution was reported [25]. The by Theatomicvarious atomic radiusresearch radius difference differencegroups [13,17]. between between Peak Ti Tishiftingand and Zr Zr isand ison onlybroadeningly 9%. 9%. Solid Solid of solution peaks solution was formation formation observed was in was 75Zr-25Ti-B,reported reported by by variousvarious50Zr-50Ti-B research research and groups 25Zr-75Ti-Bgroups [13 [13,17].,17 as]. Peakcompared Peak shiftingshifting to monolithic and broadening TiB2 and of of peaksZrB peaks2. Itwas wasshould observed observed be noted in in75Zr-25Ti-B, that 75Zr-25Ti-B, TiB2 (a 50Zr-50Ti-B50Zr-50Ti-B= 3.029 Å, c and = and 3.228 25Zr-75Ti-B 25Zr-75Ti-B Å) unit cell as as is compared compared smaller than to monolithic ZrB2 (a = 3.170 TiB TiB2 2andÅ,and c ZrB= ZrB3.5302. It2. shouldÅ)[17]. It should be noted be noted that thatTiB2 TiB(a 2 (a == 3.029 Å, Å, c c = = 3.228 3.228 Å) Å) unit unit cell cell is issmaller smaller than than ZrB ZrB2 (a2 =(a 3.170 = 3.170 Å, cÅ, = 3.530 c = 3.530 Å)[17]. Å) [17].

Figure 5. XRD of RSPSed samples. FigureFigure 5.5. XRDXRD of RSPSed samples. samples. Addition of TiB2 in ZrB2 shifted the peaks to higher angles, and this is due to decrease in lattice

parameterAddition due of to TiB dissolution2 in ZrB2 shiftedof Ti in theZr. peaksThe 50Zr-50Ti-B to higher angles, sample and exhibited this is adue complete to decrease solid insolution lattice parameterwhile the 75Zr-25Ti-B due to dissolution and 25Zr-75Ti-B of Ti in Zr. samples The 50Zr-50Ti-B showed sampleresidual exhibited ZrB2 or TiB a complete2. The presence solid solution of TiB whileand ZrO the2 was75Zr-25Ti-B also observed and 25Zr-75Ti-B [3,4]. samples showed residual ZrB2 or TiB2. The presence of TiB and ZrO2 was also observed [3,4].

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Addition of TiB2 in ZrB2 shifted the peaks to higher angles, and this is due to decrease in lattice parameter due to dissolution of Ti in Zr. The 50Zr-50Ti-B sample exhibited a complete solid solution whileTechnologies the75Zr-25Ti-B 2016, 4, 30 and 25Zr-75Ti-B samples showed residual ZrB2 or TiB2. The presence of TiB6 andof 11 ZrO2 was also observed [3,4]. TheThe SEMSEM backscatteredbackscattered electronelectron micrographs of Zr-B, Ti-B andand 50Zr-50Ti-B50Zr-50Ti-B shownshown inin FigureFigure6 6 indicateindicate thatthat dense, dense, pore-free pore-free microstructure microstructure can becan obtained. be obtained. Brighter Brighter regions inregions 50Zr-50Ti-B in 50Zr-50Ti-B represent ZrBrepresent2 rich phase,ZrB2 rich and phase, a darker and phase a darker corresponds phase corresponds to the TiB2 torich the phase. TiB2 rich phase.

FigureFigure 6.6. SEM back scattered electron (BSE) imagesimages showing the microstructure ( a) Zr-B; (b) Ti-B; andand ((cc)) 50Zr-50Ti-B.50Zr-50Ti-B.

FractureFracture surfacessurfaces ofof RSPSedRSPSed samplessamples areare shownshown inin FigureFigure7 7.. It It can can be be clearly clearly observed observed that that grain grain sizesize isis lessless thanthan micronmicron sizesize inin Ti-BTi-B comparedcompared toto allall otherother compositions.compositions. TheThe intermediateintermediate productproduct TiBTiB actedacted asas aa graingrain pinningpinning agentagent andand arrestedarrested graingrain growth.growth. InIn allall otherother cases,cases, itit isis evidentevident thatthat densificationdensification andand graingrain growthgrowth occurredoccurred simultaneously.simultaneously. However, thethe averageaverage graingrain sizesize ofof allall thethe samples is less than 2 µm. It is reported that TiB2 dissolves into ZrB2 along the phase boundary and samples is less than 2 µm. It is reported that TiB2 dissolves into ZrB2 along the phase boundary and restrictsrestricts graingrain growthgrowth [[26].26]. TheThe measuredmeasured graingrain sizessizes areare tabulatedtabulated inin TableTable1 1.. It It should should be be noted noted that that fabricationfabrication ofof dense,dense, fine-grainedfine-grained solidsolid solutionsolution compactscompacts ofof UHTCsUHTCs isis aa challengingchallenging task.task. TheThe bulkbulk densitydensity ofof allall the the samples samples were were measured measured and and presented presented in Tablein Table1. In 1. the In presentthe present study, study, we show we show that morethat more than 95%than dense95% dense compacts compacts of UHTC of UHTC solid solution solid solution can be preparedcan be prepared at low temperature at low temperature of 1400 ◦ ofC using1400 °C the using RSPS the method. RSPS method.

3.4.3.4. Mechanical CharacterisationCharacterisation IndentationIndentation fracturefracture toughnesstoughness waswas measuredmeasured forfor allall thethe RSPSedRSPSed compactscompacts andand thethe valuesvalues areare plottedplotted inin Figure Figure8. 8. It isIt observedis observed that that high high indentation indentation fracture fracture toughness toughness values values are exhibited are exhibited by solid by solutionsolid solution compositions compositions compared compared to monolithic to monolithic samples. samples. This clearly This indicatesclearly indicates that a solid that solution a solid formationsolution formation has a prominent has a effect prominent on mechanical effect properties.on mechanical Figure 9properties. shows the representativeFigure 9 shows Vicker’s the indentationrepresentative and Vicker’s the generated indentation cracks and showing the generated crack tortuosity. cracks showing crack tortuosity.

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Figure 7. Fracture surfaces of RSPSed samples (a) Zr-B; (b) 75Zr-25Ti-B; (c) 50Zr-50Ti-B; (d) 25Zr-75Ti-B; andFigure (e) Ti-B. 7. Fracture surfacessurfaces ofof RSPSedRSPSed samples samples ( a()a) Zr-B; Zr-B; (b ()b 75Zr-25Ti-B;) 75Zr-25Ti-B; (c )(c 50Zr-50Ti-B;) 50Zr-50Ti-B; (d )( 25Zr-75Ti-B;d) 25Zr-75Ti-B; and (e) Ti-B.

FigureFigure 8. 8. GrainGrain size size and and indentation indentation fractu fracturere toughness toughness of of RSPSed RSPSed samples. samples. Figure 8. Grain size and indentation fracture toughness of RSPSed samples. FigureFigure 99b,db,d show crack branchingbranching and deflectiondeflection phenomena.phenomena. The 50Zr-50Ti-B showed high fracturefractureFigure toughness toughness 9b,d show compared compared crack tobranching to ot otherher compositions. compositions. and deflection Considering Considering phenomena. the the differentThe different 50Zr-50Ti-B coeffici coefficientent showed of of thermal thermal high fracture toughness compared to other compositions.−6 −−16 Considering−1 the different coeffici−6ent−1 of −thermal6 −1 expansionexpansion (CTE) (CTE) of of ZrB ZrB2 2(α(αa a= =6.66, 6.66, αcα =c 6.93= 6.93 × 10× 10 K ) Kand )TiB and2 ( TiBαa =2 6.36,(αa = α 6.36,c = 9.30αc ×= 10 9.30 K×),10 residualK ), −6 −1 −6 −1 stressresidualexpansion toughening stress (CTE) toughening ofcan ZrB also2 (α bea can= 6.66,an also important αc be = 6.93 an important ×toughe 10 Kning) tougheningand mechanism. TiB2 (αa mechanism.= 6.36, Residual αc = 9.30 stresses Residual × 10 can K stresses ),generate residual can microcracksgeneratestress toughening microcracks during can cooling during also bestage cooling an ofimportant RSPS. stage ofThese toughe RSPS. mi Theseningcrocracks mechanism. microcracks dissipate Residual dissipate strain energy stresses strain energyof can main generate of crack main andcrackmicrocracks enhance and enhance fractureduring fracture coolingtoughness toughness stage [26]. of BasedRSPS. [26]. on BasedThese the onenergymicrocracks the energyprinciple, dissipate principle, when strain solid when energysolution solid solutionof forms main Ti formscrack+, is + replacedTiand+, isenhance replaced with fractureZr with+, and Zrtoughness the+, internal and the [26]. energy internal Based may energyon theincrease. energy may increase.Moreover, principle, Moreover, solid when solution solid solid solution reaction solution forms occurs reaction Ti ,at is + theoccursreplaced interphase; at with the interphase; Zrhence,, and due the hence,to internal reduced due energy tomovement reduced may increase. of movement grain boundary,Moreover, of grain grainsolid boundary, solutionsize reduction grain reaction size occursreduction occurs and at improvesoccursthe interphase; and the improves mechanical hence, the due properties mechanical to reduced [16]. properties movement [16 of]. grain boundary, grain size reduction occurs and improves the mechanical properties [16].

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Figure 9. Representative Vickers indentation pattern showing cracks and toughening mechanisms. Figure 9. Representative Vickers indentation pattern showing cracks and toughening mechanisms. Figure(a, c9.) Vicker’sRepresentative indentation Vickers pattern indentation of (a) Ti-B; (patternc) 75Zr-25Ti-B. showing Crack cracks branching, and toughening deflection showed mechanisms. in (Figurea,c) Vicker’s 9. Representative indentation Vickers pattern indentation of (a) Ti-B; ( cpattern) 75Zr-25Ti-B. showing Crack cracks branching, and toughening deflection mechanisms. showed in (a,c) (Vicker’sb) Ti-B; ( dindentation) 75Zr-25Ti. pattern of (a) Ti-B; (c) 75Zr-25Ti-B. Crack branching, deflection showed in (ba,)c Ti-B;) Vicker’s (d) 75Zr-25Ti. indentation pattern of (a) Ti-B; (c) 75Zr-25Ti-B. Crack branching, deflection showed in (b) Ti-B; (d) 75Zr-25Ti. (b) Ti-B;Figure (d) 75Zr-25Ti.10 shows the room temperature compressive strength of RSPSed compacts. The Figure50Zr-50Ti-B 1010 shows showssample the theshowed room room temperature high temperature compressive compressive comprestrength, strengthssive which strength of clearly RSPSed ofindicates compacts. RSPSed that Thecompacts. the 50Zr-50Ti-B solid The Figure 10 shows the room temperature compressive strength of RSPSed compacts. The sample50Zr-50Ti-Bsolution showed formationsample high compressiveshowed has a pronounced high strength, compressive effect which on mestre clearlychanicalngth, indicates whichproperties. thatclearly Figure the indicates solid 11 shows solution that the formationthehigh solid 50Zr-50Ti-Btemperature sample compressive showed strength high compressivedata of Zr-B and stre 50Zr-50Ti-Bngth, which samples. clearly There indicates was no significantthat the solid hassolution a pronounced formation effect has ona pronounced mechanical properties.effect on me Figurechanical 11 shows properties. the high Figure temperature 11 shows compressive the high solutionchange formation observed has in both a pronounced compositions effect and machine on mechanical reached itsproperties. safety limit. Figure It can be11 saidshows that the the high strengthtemperature data compressive of Zr-B and strength 50Zr-50Ti-B data samples. of Zr-B and There 50Zr-50Ti-B was no significant samples. changeThere was observed no significant in both temperaturesolid solution compressive may exhibit strength better high data temperature of Zr-B and strength. 50Zr-50Ti-B samples. There was no significant compositionschange observed and in machine both compositions reached its safetyand machine limit. It reached can be said its safety that the limit. solid It solution can be said may that exhibit the change observed in both compositions and machine reached its safety limit. It can be said that the bettersolid solution high temperature may exhibit strength. better high temperature strength. solid solution may exhibit better high temperature strength.

Figure 10. Room temperature compressive strength of RSPSed samples.

Figure 10. Room temperature compressive strength of RSPSed samples. Figure 10. Room temperature compressive strength of RSPSed samples.

Figure 11. High temperature compressive strength of RSPSed samples.

Figure 11. High temperature compressive strength of RSPSed samples. Figure 11. High temperature compressive strength of RSPSed samples.

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Figure 1212 shows shows the the representative representative loading loading vs. displacement vs. displacement profile ofprofile nanoindentation. of nanoindentation. Measured nanohardnessMeasured nanohardness and elastic and modulus elastic modulus is reported is reported in Table 1in. Table It can 1. be It can observed be observed from Tablefrom 1Table that 1 thethat hardnessthe hardness and and elastic elastic modulus modulus increases increases with with increase increase in in TiB TiB2 content.2 content.Figure Figure 1212 showsshows thethe pop-in phenomenon in 50Zr-50Ti-B sample. This phenomenon could happen due to various reasons: reasons: (1) phasephase transformation;transformation; (2) (2) dislocation dislocation generation generation under under the the indenter; indenter; and (3)and critical (3) critical resolved resolved shear stressshear beingstress reachedbeing reached at that particularat that particular load. A similar load. kindA similar of pop-in kind phenomenon of pop-in phenomenon was observed was by Guicciardiobserved by et al.Guicciardi[27] in the et ZrBal. [27]2-SiC in composite. the ZrB2-SiC They composite. have reported They thathave pop-in reported was that observed pop-in when was theobserved indentations when the were indentations made inside were the grains. made Thisinside indicates the grains. at room This temperature indicates at small room scale temperature plasticity hassmall been scale exhibited plasticity by has 50Zr-50Ti-B. been exhibited However, by 50Zr-50Ti-B. more investigations However, needmore toinvestigations be carried out need in orderto be tocarried investigate out in order slip- orto twin-systeminvestigate slip- activation or twin-system under the activation indenter. under The mechanicalthe indenter. properties The mechanical of the compactsproperties are of foundthe compacts to be similar are found to those to reportedbe simila inr to the those literature. reported However, in the thoroughliterature. mechanical However, propertythorough characterizationmechanical property and oxidationcharacterization property and evaluation oxidation has property to be carried evalua outtion ashas future to be work carried to useout theseas future composites work to foruse UHTC these composites applications. for UHTC applications.

Figure 12. Load vs. displacement profile profile of 50Zr-50Ti-B sample.

RSPS reduces sintering time and temperature and hence is an energy efficient method. Licheri RSPS reduces sintering time and temperature and hence is an energy efficient method. et al. [28] have discussed few safety aspects related to RSPS process. Firstly, the die, punches and Licheri et al. [28] have discussed few safety aspects related to RSPS process. Firstly, the die, punches spacers can get damaged due to the very high and rapid energy release in the contained and spacers can get damaged due to the very high and rapid energy release in the contained environment. Secondly, there can be expulsion of powders during SHS reaction. Thirdly, entrapped environment. Secondly, there can be expulsion of powders during SHS reaction. Thirdly, entrapped gases may cause porosity, which may be difficult to remove during later stages of sintering. It was gases may cause porosity, which may be difficult to remove during later stages of sintering. It was recommended that the sudden displacement should be avoided to prevent die damage. However, recommended that the sudden displacement should be avoided to prevent die damage. However, we have observed that there is no die damage when we increase pressure linearly with temperature we have observed that there is no die damage when we increase pressure linearly with temperature instead of applying the load in the beginning. In addition, a lower heating rate of up to 100 °C/min is instead of applying the load in the beginning. In addition, a lower heating rate of up to 100 ◦C/min is found to be safe. We have also shown that the addition of the second phase, such as found to be safe. We have also shown that the addition of the second phase, such as carbon nanotubes, nanotubes, also inhibits the SHS reaction [12]. Thus, it is seen that RSPS can be an attractive low also inhibits the SHS reaction [12]. Thus, it is seen that RSPS can be an attractive low temperature temperature method for fabricating dense, fine-grained compacts of ultrahigh temperature ceramics method for fabricating dense, fine-grained compacts of ultrahigh temperature ceramics and their and their solid solutions. solid solutions.

4. Conclusions Conclusions

(1) Monolithic ZrB2, TiB2 and solid solution compacts were successfully fabricated using RSPS of 8 (1) Monolithic ZrB2, TiB2 and solid solution compacts were successfully fabricated using RSPS of 8 h hball ball milled milled elemental elemental powder powder mixtures mixtures at temperatures at temperatures as low as as low 1400 as◦ C1400 with °C 50 with MPa 50 applied MPa appliedpressure. pressure. The fracture The fracture surface imagessurface showedimages showed fine-grained fine-grained microstructure microstructure (<2 µm). (<2 µm). (2) Two major different densification mechanisms were active during RSPS: (1) SHS reaction (2) (2) Two major different densification mechanisms were active during RSPS: (1) SHS reaction (2) plastic plastic flow aided densification. flow aided densification. (3) XRD results revealed solid solution formation. The 50Zr-50Ti-B exhibited a perfect solid (3) XRD results revealed solid solution formation. The 50Zr-50Ti-B exhibited a perfect solid solution solution while other compositions showed residual phases. while other compositions showed residual phases. (4) Solid solution samples showed high indentation fracture toughness due to various toughening mechanisms such as crack deflection and crack branching.

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(4) Solid solution samples showed high indentation fracture toughness due to various toughening mechanisms such as crack deflection and crack branching. (5) The nanohardness, elastic modulus and compressive strength were improved with solid solution formation. Pop-in phenomenon exhibited by 50Zr-50Ti-B during nanoindentation.

Acknowledgments: Srinivasa Rao Bakshi would like to acknowledge funding from IIT Madras New Faculty Seed Grant (MET/11-12/547/NFSC/SRRB) for carrying out this research. Authors acknowledge help of N. Saibaba, Director of Nuclear Fuel Complex in Hyderabad, India for providing zirconium powder. Authors would like to thank B. S. Murty for providing facilities to carry out the research. Author Contributions: K.N.S. and S.R.B. conceived and designed the experiments; K.N.S. performed the experiments; K.N.S. and S.R.B. analyzed the data; K.N.S. wrote the paper and S.R.B. contributed to writing. All the authors read and accepted the final manuscript. Conflicts of Interest: The authors declare that there is no conflict of interest.

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