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Ceramics International 40 (2014) 10767–10776www.elsevier.com/locate/ceramint

Effect of B4C, MoSi2, nano SiC and micro-sized SiCon pressureless sintering behavior, room-temperature mechanical properties

and fracture behavior of Zr(Hf)B2-based composites

Alireza Abdollahin, Mehri Mashhadi

Faculty of Materials & Manufacturing Processes, Malek-e-Ashtar University of Technology, P.O. Box 15875-1774, Tehran, Iran

Received 15 January 2014; received in revised form 12 March 2014; accepted 12 March 2014Available online 24 March 2014

Abstract

In the present paper, ZrB2–HfB2 composite was developed via a pressureless sintering method and SiC, MoSi2 and B4C powders were used asadditives. In order to produce composite samples, ZrB2 is first milled for 2 h, afterwards the reinforcing particles were added. The mixture wasformed by cold isostatic press (CIP) and, after the pyrolysis, it was sintered at 2100 1C and 2150 1C. In order to compare the effect of differentadditives on pressureless sintering behavior of ZrB2–HfB2 composite, the shrinkage percentage of samples was measured both before and afterthe sintering, and the microstructure of samples was examined using scanning electron microscopy (SEM), equipped with EDS spectroscopy.Bending test and resonance frequency method were used to measure the strength and the elastic modulus of the samples, respectively. The resultsshow that samples containing MoSi2 and SiC nanoparticles had the maximum flexural strength and elastic modulus. Moreover, the flexuralstrength and elastic modulus of the samples increased as the sintering temperature rose from 2100 1C to 2150 1C, suggesting that a temperatureincrease leads to an improved sintering process.& 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Keywords: A. Sintering; B. Nanocomposites; C. Mechanical properties; D. Borides

1. Introduction

Nowadays there is a strong need in the aerospace industryto have new materials which along with functioning in oxidi-zing and corrosive atmospheres, withstand temperatures above2000 1C for long hours. Ultra high temperature ceramics(UHTCs), known as advanced materials, are the best optionto meeting this demand. Using UHTC in aerospace industryincludes a wide range of thermal protection materials in thehypersonic aerospace vehicles or the re-usable atmosphericre-entry vehicles, specific components of propulsion, nose cap,sharp leading edge, etc. This group of materials includestransition metal borides, carbides, and nitrides such as ZrB2,HfB2, ZrC, TaC, HfC, HfN, etc. that have such characteristics

10.1016/j.ceramint.2014.03.06614 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

g author at: Tel.: þ98 9127363731; fax: þ98 22215451.sses: alirezaabdollahi1366@gmail.com (A. Abdollahi),hoo.com (M. Mashhadi).

as high melting point, high level of hardness, and relativelygood oxidation resistance [1,2].Among ultra high temperature ceramics, ZrB2-based com-

posites, due to properties such as high melting point (above3200 1C), good thermal and electrical conductivity, high levelof hardness and strength and chemical stability at hightemperature, are used more in aerospace industry [3–5]Pressureless sintering is one of the simplest methods to

produce ZrB2-based composites. After preparing the rawmaterials, the part is formed by a single-action press and thenit is pressed immediately by cold isostatic press (CIP). Finally,it is sintered at 2000–2200 1C. Sintering aids are used tofacilitate the process of producing ZrB2-based composites andimprove their characteristics. Generally, the additives can bedivided into three categories: (1) those which make a liquidphase, (2) those which produce solid solution, and (3) reactiveagents [6]. These additives include metals such as Ni and Fe orceramics such as SiC, Si3N4, B4C, MoSi2, etc. [7].

Fig. 1. Standard bending test samples produced in this research.

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The studies of the additives effects on composition andmicrostructure of ZrB2-based composites suggest that meltingpoint, production process, grain size, and secondary phasedistribution are the elements influencing the strength, oxidationresistance, and other characteristics. B4C, for example, is akind of additive, used to react with oxide impurities duringsintering process, which in turn improves the productionprocess [7,8].

Equally MoSi2, reacting with impurities on the surface ofZrB2 particles, improves the composite production process bycreating intergranular phases [9,10], In other words, refractorymetal silicides (MoSi2, TiSi2, and HfSi2) can form liquid phaseduring the sintering process. Furthermore, these liquid phaseformers can fill the voids among matrix grains, favor the mattertransport, and then accelerate the densification process [11].

SiC has the following effects on the secondary phase inZrB2-based composites: it improves both the thermal shockresistance and the oxidation resistance, while increasingmechanical properties, particularly fracture toughness andflexural strength [12–15]. SiC also makes the productionprocess better since the oxide impurities on the surface ofZrB2 particles such as B2O3, ZrO2, SiO2, and carbon, whichare produced during powder manufacturing process, lead todisruption in the production process. Silicon carbide createsintergranular liquid phase by reacting with these oxides,improving the production process. Since these oxides createglass phases during the sintering process (which softens athigh temperature), their removal does improve the mechanicalproperties [16,17].

HfB2 is also usually used with silicon carbide in ultra hightemperature composites because it has very high refractoriness.Additionally, the presence of HfB2 improves oxidation resis-tance. Sintering process of HfB2 powder becomes better in thepresence of carbon, boron, silicon, and silicon carbide [18–21].

Studies conducted on ZrB2-based composites show thatZrB2–HfB2 composite is the best option for using in nosecap and leading edges of hypersonic aerospace and re-usableatmospheric re-entry vehicles [8]. Therefore, the present researchis focused on the effect of nano SiC, micro-sized SiC, MoSi2, andB4C additives on mechanical properties and pressureless sinteringbehavior of ZrB2–HfB2 composite.

2. Experimental

In the present paper, ZrB2–HfB2 ultra high temperaturecomposite was produced by the pressureless sintering methodand SiC (at nano and micro-sized scale), MoSi2, and B4C powderswere used as additives. In order to produce the composite samples,ZrB2 powder was first milled with ethanol and zirconia balls for2 h in a planetary ball mill. The ball-to-powder weight ratio and therotational speed were defined 1:10 and 200 rpm respectively. ThenHfB2, B4C, and MoSi2 particles were blended with milled ZrB2

powder and finally nano SiC and micro-sized SiC particles wereadded separately. The mixture was cold pressed at 90 MPa as thebending test standard samples (56� 12� 12 mm dimensions) andthen was formed by CIP at 2000 bar (200 MPa) to increase thegreen compact’s strength. After preparing the samples, they were

pyrolyzed in an argon atmosphere at 1000 1C to remove resin. Thepressureless sintering process was also conducted in the argonatmosphere at 2100 1C and 2150 1C for 1 h. Table 1 shows thename and compositions of samples produced in this research. Fig.1shows the bending test samples after sintering process at 2100 1Cand 2150 1C.In order to compare the effects of MoSi2, B4C, nano SiC, and

micro-sized SiC on pressureless sintering behavior of ZrB2–

HfB2 composite, the shrinkage percentage of samples wasmeasured before and after sintering and the microstructure ofthe samples was examined using scanning electron microscopy(SEM), equipped with EDS spectroscopy. Also, the matrixgrain size was investigated with a Clemex image analyzer. Tomeasure the samples’ strength, the four-point flexural test wasperformed according to ASTM C1161 standard. Considering thefact that elastic modulus is an inherent material property, it wasmeasured by means of a resonance frequency method.

3. Results and discussion

3.1. Weight changes and shrinkage percentage of samplesafter sintering at 2100 1C and 2150 1C

Weight changes after sintering are very significant and play animportant role in explaining samples’ sinterability. Measuring theweight and dimension of samples before and after sintering makesit possible to calculate the weight changes and shrinkagepercentage. Weight change and shrinkage percentage of samplesafter sintering at 2100 1C and 2150 1C are shown in Figs. 2 and 3respectively.Comparison of the data, presented in Figs. 2 and 3, indicates

that the shrinkage percentage of the samples, containing nanoSiC, is higher than the ones, containing micro-sized SiC.It proves improvement sintering behavior of ZrB2 in thepresence of nano SiC reinforcing phase, which is a result ofhigher surface energy and, thus, better sinterability of nano SiCthan micro-sized SiC particles [17]. Another noteworthy pointin Fig. 3 is that the shrinkage percentage of the samples,containing MoSi2, is higher than that of those with B4C, showingthat compared to B4C particles, MoSi2 has a better effect onsintering and densification. Generally, the use of molybdenum(Mo) or MoSi2 additives in ZrB2-based composites decreases the

Table 1Name and composition of samples produced in this research.

Sample name Composition (wt%)

ZrB2 HfB2 SiCn (40 nm) SiCm (60 μm) MoSi2 B4C

ZSnMH 71 15 10 4 –

ZSmMH 66 15 – 15 4 –

ZSnBH 72 15 10 – 3ZSmBH 67 15 – 15 – 3

Fig. 2. Weight changes of samples after sintering at 2100 1C and 2150 1C.Fig. 3. Shrinkage percentage of samples after sintering at 2100 1C and2150 1C.

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necessary activation energy for densification. Therefore, addingMo or MoSi2 accelerates the diffusion of atoms, improving thedensification process, and consequently increasing the density anddecreasing porosity [19,22,23]. In addition, as Fig. 3 demon-strates, the shrinkage percentage at 2150 1C is higher than at2100 1C, indicating that as the temperature rises, the sinteringprocess improves.

3.2. The effects of sintering temperature and type ofreinforcing phase on mechanical properties

3.2.1. The effect of MoSi2Table 2 contains room-temperature mechanical properties of

the samples. The flexural strength and elastic modulus of thesamples in different sintering temperatures are also comparedwith each other in Figs. 4 and 5. As demonstrated, the ZSnMHsample has the highest strength and modulus at 2150 1C whileZSnBH has the lowest strength and modulus at 2100 1C. Theinvestigations proved that a decrease in SiC particle sizeimproved the flexural strength of ZrB2–SiC composites[6,24]. Results from this study also indicate that as the sizeof SiC particles decreases, the strength and modulus increasein ZrB2–SiC–HfB2–MoSi2 and ZrB2–SiC–HfB2–B4C; i.e. thestrength and modulus of the samples, containing nano SiC, arehigher than the ones with micro-sized SiC; therefore, it can beconcluded that the presence of nano SiC increases mechanicalproperties. What must be considered here is that SiC particlesinhibit grain growth of ZrB2 matrix. The particle size of SiCplays an important role: the smaller the size, the more effective

it becomes, because SiC particles are located in ZrB2 grainboundaries and inhibit grain growth. According to the Hall–Petch mechanism, the strength has an inverse relationship withgrain size; therefore, with a decrease in particle size, strengthincreases [25]. In other words, by impeding the movement ofgrain boundaries, SiC nanoparticles inhibit grain growth of thematrix [26], consequently decreasing the grain size (Table 3).Therefore, according to the Hall–Petch relationship, thestrength of the samples containing nano SiC is greater thanthe ones, containing micro-sized SiC.One reason for the increase in the strength of samples with

SiC nanoparticles could be crack deflection (Figs. 6 and 7).These track changes are due to the deformation of grainboundaries caused by nanoparticles’ penetration, ultimatelyleading to a decrease in the crack energy and an increase inthe composite strength. Fig. 7 shows the presence of SiCnanoparticles in matrix grain boundaries as well as inside thegrains.Another reason for the strength growth of the samples with

SiC nanoparticles could be the presence of SiC nanoparticlesin crack path, which either deflects or impedes the crack(Figs. 6 and 7). In both cases, by lessening the crack energy,nanoparticles build up the composite strength. In other words,the strength of samples with SiC nanoparticles could beexplained by the interaction between crack and SiC particles.As the crack meets SiC particles, two cases can occur. Thecrack may either break SiC particles or bypass them [24]. Inthe former, the crack must have a large amount of energy, thenthe fracture would be transgranular, leading to a decrease in

Fig. 4. Flexural strength of sintered samples at 2100 1C and 2150 1C.

Fig. 5. Elastic modulus of sintered samples at 2100 1C and 2150 1C.

Table 3Matrix grain size of samples calculated with Clemex image analyzer.

Sample name Sintering temperature (1C)

2100 2150

ZSnMH 26.429 (mm) 34.172 (mm)ZSmMH 31.571 (mm) 37.143 (mm)ZSnBH 36.189 (mm) 46.270 (mm)ZSmMH 40.02 (mm) 50 (mm)

Table 2Room-temperature mechanical properties of samples.

Sintering temperature (1C) 2100 2150

Sample name ZSnMH ZSmMH ZSnBH ZSmBH ZSnMH ZSmMH ZSnBH ZSmBHFlexural strength (MPa) 100.5 67.29 60.355 50.97 110.48 100.93 87.78 70.44Elastic modulus (GPa) 393 364 342 312 417 385 371 365

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strength. As shown in SEM images, this type of fracture hardlyoccurs in samples with SiC nanoparticles. In the latter case,however, the crack does not have sufficient energy to fractureSiC particles and, consequently, either enters the grain bound-ary, bypassing the grain itself, or is impeded as it crashes withSiC particles. This type of fracture, known as intergranular

fracture, builds up the composite strength, observable in SEMimages of the samples with nanoparticles SiC. In other words,studying the SEM images shows that in samples, containing SiCnanoparticles, a crack seldom breaks ZrB2 grains, i.e. transgra-nular fracture hardly occurs. As the crack reaches SiC particles,it either is impeded or is deflected to keep on moving throughgrain boundaries. However, in case of samples with micro-sizedSiC particles, the fracture is usually transgranular (Figs. 8 and 9)and, as a result, the strength of these samples is less than theones containing SiC nanoparticles. Another demonstrated pointin the images of fracture surface is that debonding is very lowdue to stress. This shows a complete sintering process andcreation of a high quality interface in samples containing SiCnanoparticles. Guo [6] also believes that as SiC grain sizedecreases, deboning decreases as well and this leads to theincrease of strength and decrease of fracture toughness.Meanwhile, as shown in Figs. 4 and 5, the strength of the

sintered samples at 2150 1C is higher (regardless of whetherSiC particles are in micro or nanometer range), indicating animprovement in sintering process at 2150 1C. Comparing themicrostructure of the samples shows that in a sample sinteredat 2150 1C, the grain boundaries are no more straight andangular and rather take a curved shape, which is a proof forimprovement of sintering process in this sample. However, thegrain boundaries in the sample, sintered at 2100 1C, remainstraight and angular even after sintering.Fig. 10 shows the image of composite samples after being

fractured under flexural loading. As can be seen, the fracturesurface of samples sintered at 2100 1C with micro-sized SiCparticles is almost flat (less bending) as the crack travelsstraight and through the grains. In fact in this case, the samplehas not experienced much deformation during its fracture.However, the curvature of the fracture surface is greater insamples, which contain SiC nanoparticles and are sintered at2150 1C. The reason behind this phenomenon is continuousdeflection of the crack (as it crashes with SiC nanoparticles)and its passage through the grain boundaries. In other words,in this case, the sample has tolerated severe plastic deformationprior to its final fracture. This confirms higher strength of thesamples with SiC nanoparticles and sintered at 2150 1C. Huet al. [24] reported similar results about ZrB2–SiC fracture.

3.2.2. The effect of B4C particlesBased on the results obtained from the present study, ZrB2–

HfB2–B4C–SiC composites (either at nano or micro-sizedscale) have lower mechanical properties than ZrB2–HfB2–

MoSi2–SiC composites; hence ZSmBH has the lowest strengthand modulus. Although SiC nanoparticles, in comparison with

Fig. 6. Fracture surface of ZSnMH sample (sintered at 2100 1C).

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samples containing micro-sized SiC particles, possess bettermechanical prosperities, even in samples with SiC nanoparti-cles there is lower strength and modulus than in ZrB2–HfB2–

MoSi2–SiC samples. This can be explained by different effectsof MoSi2 and B4C additives on sintering and densification ofZrB2-based composites. According to the conducted studies,MoSi2 additives improve the densification process of ZrB2-based composites more effectively than B4C do; therefore, thedensity is higher in samples containing MoSi2 comparedwith those that contain B4C. However, their porosity is lowerand their grain size is smaller (Table 3). Since the strength ofZrB2-based composites is strongly dependant on the grain size

(according to the Hall–Petch mechanism), samples with MoSi2have better mechanical properties due to their smaller grainsize. Recent studies also show that ZrB2–MoSi2 compositeshave better mechanical properties than ZrB2–B4C composites[10,23,27,28].On the other hand, as aforementioned, grain debonding is

lower in samples with MoSi2 compared to samples containingB4C. This shows that ZrB2–HfB2–MoSi2–SiC compositehas higher interface quality. Since the interface has a strongimpact on elastic modulus [6,29], it can be concluded thatZrB2–HfB2–MoSi2–SiC sample has a higher modulus becauseof its better interface.

Fig. 7. Fracture surface of ZSnMH sample (sintered at 2150 1C).

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Just as the results from ZrB2–HfB2–MoSi2 compositesshow, in samples, which contain B4C particles sintered at2150 1C, both the strength and the elastic modulus are higher,proving better sintering process at this temperature. Further-more, in samples containing SiC particles, accumulation ofnanoparticles at grain boundaries impedes grain growth and, asa result, the strength increases. Figs. 11 and 12 show thepresence of SiC nanoparticles at grain boundaries. Theseresults exactly match the conclusions made by Zhu et al.[26] as well as Rezai et al. [30]. They dealt with the effect ofmicrostructure and SiC particle size on flexural strength atroom temperature in ZrB2-based ceramics containing 30 vol%SiC. Their observations also indicate that flexural strengthlessens as the grain size grows.

Also it should be noted about B4C particles that theseparticles hardly have any effect on the increase in samples’

strength. Because in both cases (nano SiC and micro-sizedSiC) as B4C particles crash the track, they fracture and do notcause any crack impedance or deflection. This is clearlydemonstrated in Figs. 13 and 14. Studying the images of thesurface fracture in ZrB2–HfB2–B4C–SiC composites showsthat cracking leads to the ZrB2 grain fracture in these samples.In other words, fracture in these samples is transgranular. Thisis one of the reasons for low strength and modulus of thesecomposites, both at nano and micro-scales. This occurs insamples containing SiC micro-particles more severely, whereasin samples with SiC nanoparticles, the crack is impeded as itcrashes these particles, which can be seen in Fig. 12.Fig. 15 shows ZSnBH and ZSmBH samples after fracture

under flexural loading. As can be seen, the fracture surface ofthe ZSmBH sample is completely flat, indicating that the cracktravels straight and through the grains. Consequently there

Fig. 8. Fracture surface of ZSmMH sample (sintered at 2100 1C).

Fig. 9. Fracture surface of ZSmMH sample (sintered at 2150 1C).

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Fig. 10. Fracture mode of ZrB2–HfB2–MoSi2–SiC samples after bending test.

Fig. 12. Fracture surface of ZSnBH

Fig. 11. Fracture surface of ZSnBH

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occurs a premature fracture. In contrast, the fracture surface ofZSnBH sample is almost bent, indicating that the crack deflectswhen the load is applied. In other words, ZSnBH sampleshave tolerated higher plastic deformation prior to theirfracture which confirms higher strength for samples, contain-ing nanoparticles, compared with those containing micro-sizedparticles.

4. Conclusions

In the present paper, ZrB2–HfB2 composite was producedby pressureless sintering. Nano and micro-sized SiC, MoSi2and B4C powders were used as reinforcement. The results fromthe bending test show that samples with MoSi2 and nano SiCsintered at 2150 1C have the highest strength and modulus, the

sample (sintered at 2150 1C).

sample (sintered at 2100 1C).

Fig. 14. Fracture surface of ZSmBH sample (sintered at 2150 1C).

Fig. 13. Fracture surface of ZSmBH sample (sintered at 2100 1C).

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Fig. 15. Fracture mode of ZrB2–HfB2–B4C–SiC samples after bending test.

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main reasons behind which are the improvement of sinteringprocess at 2150 1C as well as the presence of SiC nanoparti-cles. On the other hand, the crack has seldom fractured ZrB2

grains in samples containing SiC nanoparticles (transgranularfracture is hardly seen in these samples). Therefore, it is clearthat the fracture in these samples is intergranular. Additionally,the images of fracture surface of the samples with micro-sizedSiC particles show that the fracture is transgranular, confirminglower strength of these samples.

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