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Taguchi design and hardness optimization of ZrB 2 -based compositesreinforced with chopped carbon ber and different additives andprepared by SPS

Z. Balak ⇑ , Mohammad Zakeri, Mohammadreza Rahimipour, Esmael SalahiMaterials and Energy Research Center (MERC), Tehran, Iran

a r t i c l e i n f o

Article history:Received 30 September 2014Received in revised form 13 March 2015Accepted 17 March 2015Available online 3 April 2015

Keywords:ZrB2-based compositesSpark plasma sinteringHardnessTaguchi design

a b s t r a c t

ZrB2–SiC composites with different additives were prepared by spark plasma sintering (SPS). Taguchidesign was applied to explore effective parameters for achieving the highest hardness. Nine factorsincludingcontents of SiC, C f , MoSi 2 , HfB 2 and ZrC, milling timeof C f (M.t) and SPS parameters such as tem-perature, time and pressure in four levels were considered through the Taguchi technique. It has beenconcluded that the most signicant effects on the hardness are related to temperature, MoSi 2 , ZrC, SiCand HfB 2 by 54.7%, 12.3%, 9.1%, 8.2% and 6.7%, relative importance respectively. Pressure by 0.13% andCf by 0.43%, M.t by 0.94 and time by 2% have the least effect on the hardness.

2015 Elsevier B.V. All rights reserved.

1. Introduction

Zirconium diboride (ZrB 2) is an ultra high temperature ceramic(UHTC) that has strong covalent bonding, which gives it a meltingtemperature above 3000 C (3250 C for ZrB 2), high hardness(23 GPa), and high elastic modulus (>500 GPa experimentally,546 GPa by calculation). The bonding also has metallic character,which results in high thermal conductivity (60 W/m K or higher)and electrical conductivity (107 S/m). With this unusualcombination of properties, ZrB 2 shows promise for diverse applica-tions such as cutting tools, molten metal crucibles, and thermalprotection systems for hypersonic aerospace vehicles [1–4] .

Typically, three routes are used to improve the densicationand microstructure of ZrB

2-based ceramics. The rst route is to

use sintering aids such as WC, Si 3N4 , VC and Y 2 O3 [5–7] . The secondroute is to explore innovative sintering techniques for densica-tion, including spark plasma sintering (SPS), reactive SPS, andcombined SPS and self-propagating high-temperature synthesis.Compared to conventional hot pressing, SPS allows higher heatingrates and usually involves only a very short holding time. The thirdroute is to adjust the sintering parameters [8] .

In this article, all of these three routes were used to optimizemicrostructure and hardness. For the rst route, C f with differentcontents and milling times and different contents of MoSi 2 , HfB2 ,SiC and ZrC in different content were chosen as additives. In thesecond route, different temperature (1600–1900 C), pressures(10–40 MPa) and times (4–16 min) were selected as sintering con-ditions. Finally, for the third route, SPS was applied as sinteringmethod. Of course it is clear that the investigation of all theseparameters in a single work may not be possible and accompaniesvery high cost. So, an optimization strategy is necessary to evaluateall of these parameters and the effect of each of them on the hard-ness. The Taguchi method is the best opportunity to eliminatevariations during the design of experiment.

This method uses a special set of arrays called orthogonalarrays. These standard arrays stipulate the way of conducting theminimal number of experiments which could give the full informa-tion of all the factors that affect the performance parameter. Also, itallows independent evaluation of each factor through a small num-ber of runs.

In this paper, we focus on the effect of different additives andSPS conditions on the hardness of ZrB 2 -based ceramics. By apply-ing the Taguchi method, the effect of nine parameters such asSiC, Cf , MoSi 2 , HfB2 and ZrC content, milling time of C f and SPSparameters such as temperature, time and pressure at four levelswas investigated. Effect of each parameter on hardness were evalu-ated and discussed.

http://dx.doi.org/10.1016/j.jallcom.2015.03.131

0925-8388/ 2015 Elsevier B.V. All rights reserved.

⇑ Corresponding author. Tel.: +98 0611 4452040.E-mail address: Zbalak1983@gmail.com (Z. Balak).

Journal of Alloys and Compounds 639 (2015) 617–625

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2. Experimental

2.1. Materials

Six basic raw materials including ZrB 2 (20 l m), SiC (25 l m), MoSi 2 (25 l m),HfB2 (30 l m), ZrC (20 l m) and C f (T800, diameter 5 l m) were used to producethe composites.

2.2. Experimental design

Selection of control factors is an important stage of Taguchi applications anddesign of the factors is generally set by experimenter’s experience. It is known thatSiC, Cf , MoSi 2 , HfB2 and ZrC content, milling time of C f and SPS parameters such astemperature, time and pressure are among the most important factors in hardness[9–18] . Of course it is necessary to explain it, these factors were selected not only inorder to reach best densication and hardness but to reach optimize conditions forpreparing one composite with best relative density, hardness, exural strength,toughness, oxidation and thermal shock resistance. On the other hand, not all nineselected factors have a positiveeffect onhardnessand they were chosen for anothergoal. For example, C f has inuence on hardness but its effect is negative and it waschosen due to its good inuence on toughness according to unpublished data. Inthis study, all of these factors were chosen as input variables and their impactson the response variable (Hv) were investigated. Four combinations of variables(levels) were selected for each of the studied factors. The used factors and levelsare presentedin Table1 . The level ofeachfactor and the values ofthe tested factorswere chosen based on previous researches.

In this study, L 32 orthogonal array was chosen to determine the optimum con-ditions. With respect to the L 32 orthogonal array ( Table 2 ), conditions for prepara-tion of each sample are shown in Table 3 .

2.3. Manufacturing and characterization

The powders corresponding to 32 different compositions, according to Table 3 ,were mixed by wet ball-milling at 200 rpm for 3 h in a zirconia bottle, using zirco-nia balls and ethanol as media. The mixtures were then dried. The powder mixturewas put into graphite die lines with graphite foil which has an inner diameter of 50 mm and sintered using SPS apparatus (SPS-20T-10, China). The sintering wasperformedat Differenttemperatures (between1600 C and 1900 C), different pres-sures (between 10 MPa and 40 MPa) and different holding times (between 4 minand 16 min) based on Taguchi design ( Table 2 ) in vacuum. Sintered 50 mm diame-ter pellets were produced with different thicknesses of 2.8 mm and 6.5 mm relatedto their compositions and SPS conditions. After removing the surface layer fromtheobtained disks by grinding, the bulk density was measured according to ASTM C

373-88. Then the phase composition was determined by X-ray diffraction (XRD,Siemens, D500) using Cu Ka radiation on polished cross-sectioned composite.Scanning electron microscopy (Sigma/VP, Zeiss) on an instrument equipped withbackscattered electron imaging (BSE) was performed to observe the microstruc-tures of the composites. Macro-hardness (Hv30.0) was measured by a Vickersindenter with 30 kg as applied load for 20 s on polished sections.

3. Results and discussion

In this study, to measure the density, open porosity percent wasused instead of relative density due to phase transformations. Forhardness evaluation, a Vickers indenter with 1 kg as applied loadfor 20 s was used, at rst. Since the most of composites had sixconstituents, micro-hardness data, was variable. Thus the indenterwith 1 kg as applied could not cover all of phases. So, the data

obtained couldnot indicate the real hardness of samples and there-fore were not reliable. Therefore, the Macro-hardness (Hv30.0) wasapplied to measure hardness.

Table 1

Process factors and their levels used in the experiments.

Factors Unit Symbol Level 1 Level 2 Level 3 Level 4

SiC Vol% SiC 5 10 15 20Cf Vol% Cf 0 2.5 5 7.5Milling Time Hr M.t 0 2.5 5 7.5MoSi 2 Vol% MoSi 2 0 2 4 6HfB2 Vol% HfB 2 0 5 10 15ZrC Vol% ZrC 0 5 10 15Temperature C T 1600 1700 1800 1900Pressure MPa P 10 20 30 40Time min t 4 8 12 16

Table 2

Orthogonal array (L 32 ) determined by Taguchi method based on the nine factors infour levels.

Sample Fac.1

Fac.2

Fac.3

Fac.4

Fac.5

Fac.6

Fac.7

Fac.8

Fac.9

1 1 1 1 1 1 1 1 1 12 1 2 2 2 2 2 2 2 23 1 3 3 3 3 3 3 3 3

4 1 4 4 4 4 4 4 4 45 2 1 1 2 2 3 3 4 46 2 2 2 1 1 4 4 3 37 2 3 3 4 4 1 1 2 28 2 4 4 3 3 2 2 1 19 3 1 2 3 4 1 2 3 4

10 3 2 1 4 3 2 1 4 311 3 3 4 1 2 3 4 1 212 3 4 3 2 1 4 3 2 113 4 1 2 4 3 3 4 2 114 4 2 1 3 4 4 3 1 215 4 3 4 2 1 1 2 4 316 4 4 3 1 2 2 1 3 417 1 1 4 1 4 2 3 2 318 1 2 3 2 3 1 4 1 419 1 3 2 3 2 4 1 4 120 1 4 1 4 1 3 2 3 2

21 2 1 4 2 3 4 1 3 222 2 2 3 1 4 3 2 4 123 2 3 2 4 1 2 3 1 424 2 4 1 3 2 1 4 2 325 3 1 3 3 1 2 4 4 226 3 2 4 4 2 1 3 3 127 3 3 1 1 3 4 2 2 428 3 4 2 2 4 3 1 1 329 4 1 3 4 2 4 2 1 330 4 2 4 3 1 3 1 2 431 4 3 1 2 4 2 4 3 132 4 4 2 1 3 1 3 4 2

Table 3Conditions for preparation of each sample.

Sample SiC C f M.t MoSi 2 HfB2 ZrC T P t

1 5 0 0 0 0 0 1600 10 42 5 2.5 2.5 2 5 5 1700 20 83 5 5 5 4 10 10 1800 30 124 5 7.5 7.5 6 15 15 1900 40 165 10 0 0 2 5 10 1800 40 166 10 2.5 2.5 0 0 15 1900 30 127 10 5 5 6 15 0 1600 20 88 10 7.5 7.5 4 10 5 1700 10 49 15 0 2.5 4 15 0 1700 30 16

10 15 2.5 0 6 10 5 1600 40 1211 15 5 7.5 0 5 10 1900 10 812 15 7.5 5 2 0 15 1800 20 413 20 0 2.5 6 10 10 1900 20 4

14 20 2.5 0 4 15 15 1800 10 815 20 5 7.5 2 0 0 1700 40 1216 20 7.5 5 0 5 5 1600 30 1617 5 0 7.5 0 15 5 1800 20 1218 5 2.5 5 2 10 0 1900 10 1619 5 5 2.5 4 5 15 1600 40 420 5 7.5 0 6 0 10 1700 30 821 10 0 7.5 2 10 15 1600 30 822 10 2.5 5 0 15 10 1700 40 423 10 5 2.5 6 0 5 1800 10 1624 10 7.5 0 4 5 0 1900 20 1225 15 0 5 4 0 5 1900 40 826 15 2.5 7.5 6 5 0 1800 30 427 15 5 0 0 10 15 1700 20 1628 15 7.5 2.5 2 15 10 1600 10 1229 20 0 5 6 5 15 1700 10 1230 20 2.5 7.5 4 0 10 1600 20 1631 20 5 0 2 15 5 1900 30 432 20 7.5 2.5 0 10 0 1800 40 8

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Table 4

Results of open porosity percent and hardness of all samples.

Sample Open porosity percent, % Hardness, Hv

Trial 1 Trial 2 Trial 3 Trial 4 Trial 1 Trial 2 Trial 3 Trial 4 Trial 5

1 29.84 28.11 31.4 29.68 34.2 37 34.3 34.6 35.22 0 0 0 0 310.7 309 309 311 3103 1.32 1.74 1.21 1.93 314.5 315.2 319 315.4 315.34 0.68 1.4 1.97 0.7 372 375 365.7 376.3 369.65 1.96 1.98 1.86 1.84 364.3 360.4 360.6 362 360.46 0 0 0 0 341 342.4 339.8 340 341.37 12.8 12.9 11.9 11.83 155.6 144.8 148.3 155.1 1478 10.94 11.95 10.6 10.5 188.2 175.9 176.8 187.8 1909 1.1 0 1.3 0 319 317.7 319.2 316.7 317.410 9.1 9.23 9.6 9.32 130 149.5 148.4 140.3 143.611 3.94 3.95 3.67 3.28 353.8 350.6 351.6 348 349.512 1.63 3.23 4.74 0.44 382.7 381.4 382.3 380 38013 1.15 0.98 0.85 1.18 397.4 391 399.3 400.2 398.214 0.03 1.66 1.34 0.37 383.6 395.9 387.2 398.4 38915 3.31 3.44 2.94 3.8 320.7 327.2 323.4 320.1 326.216 13.52 14.12 14.6 13.24 135.9 136.5 135.4 138.16 136.317 14.15 13.03 14.1 13.14 169.2 168.7 172.2 171.4 166.518 0 0 0 0 266.6 266.9 262.3 260.6 263.519 16.98 17.67 16.9 17.62 115 115.6 113 118.4 113.620 1.40 1.5 1.45 1.34 320.5 320.7 320 318 32021 15.07 15.78 16.4 15.36 113 110.4 113.8 110 112.422 10.74 10.33 10.3 10.27 230.9 230.2 235.2 231 232.323 0.55 0 0.27 0.51 350.6 350.2 348.8 350 35224 0.18 0 0.58 0.48 312.9 296.5 299 292.5 29825 0.48 0 0.61 0.42 340.6 338.4 339.6 338 345.626 7.6 6.05 5.6 6.34 207.3 206.5 211 209 21027 15.6 16.22 15.8 15.15 189 185 198.1 186.2 18628 12.52 12.5 11.6 12.46 158.5 155.9 154.4 154 15529 0.081 0 0.09 0.082 415.2 417.4 415 410.8 41030 6.8 6.60 6.22 6.26 278.8 279.9 285.8 295 29031 3.13 3.44 3.69 3.68 392.4 396.8 394.3 392 38532 6.55 8.3 9.44 9.2 185.5 194 188.6 182.6 185

Fig. 1. SEM images of 5, 9, 11 and 18 composites.

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Table 4 shows the results of the open porosity percent andhardness conducted on the 32 mixtures prepared as suggested bythe Taguchi method. Open porosity percent measurement wasrepeated four times for each mixture are shown as Trial 1–4 inTable 4 . The hardness was obtained by ve correct indents for eachmixture. Results of each indent are shown as Trial 1–5 in Table 4 . Itis apparent that the average hardness in samples 2–6, 9, 11–15, 20,23–25, 29 and 31 which their open porosity percent is less than 3%,is more than 300 Hv except 18. The hardness of 18 is 264 Hvbecause its microstructure is much larger than other sample suchas 5, 9 and 11. Fig. 1 shows the SEM images of 5, 9, 11 and 18 com-posites. These images conrm the hardness results.

Also, according to Table 3 , sample 1 with most open porositypercent, 29.8%, has least hardness, 35 Hv. SEM image of sample 1

is shown in Fig. 2 . With respect to the open porosity percent andhardness data ( Table 4 ), it can be found that they change adverselytogether. On the other hand, by increasing open porosity percent,hardness decreases.

In our next work, it will be discussed in detail and nine parame-ters will be optimized to reach maximum hardness and less openporosity percent.

The data were entered to Qualitek-4 software (version 14.5).The results of ANOVA (analysis of variance) analysis are given inTable 5 . It is clear that Temperature with 54.7%, MoSi 2 with12.3%, ZrC with 9.08%, SiC with 8.16% and HfB 2 with 6.7% and havethe most portions on the open porosity, respectively. These resultsindicated that the temperature of SPS and MoSi 2 are very impor-tant parameters on hardness.

Time with 2.04%, M.t with 0.94%, C f with 0.42% and pressurewith 0.12% have the least impact on hardness. Also, it is clear that

error of DOE (design of experiment), is 5.48% which is acceptable.This error originates from some factors which have negligibleeffect on these properties (hardness and open porosity) such asCf , M.t and pressure. In the other hand, selecting un-effective fac-tors cause makes an error. So, factor selecting, is a very importantstep in DOE. If the step is not done carefully, DOE error will be high(more than 15%) andso, it will not be acceptable. Standard analysismethod was used to investigate the effect of each parameter onhardness.

3.1. Effect of SiC, C f and M.t on hardness

Fig.3 shows effect of SiC, C f content and M.t hardness. It can beobserved that hardness increases by increasing SiC content(Fig. 3 a). SiC increases the hardness by 3 mechanisms; rstly, SiCimproves the sinterability of ZrB 2-based ceramics by formation of intergranular liquid phases [9] which decreases the porosity.Since, the pores in ceramics have no resistance to applied stress,materials with more porosity have lower apparent microhardnessvalues than the dense counterparts [10] . In this study, it is clearlyindicated that porosity decreases by SiC increasing ( Fig. 3 a).Secondly, it is well-known that SiC acts as inhibitor and restrainsgrain growth [11] . Chamberlain et al. [12] prepared ZrB 2 ceramicsat different temperatures (1900–2150 C) and 180 min holdingtime by the pressureless method and compared the data withhot-pressed ZrB 2 . They concluded that hardness decreases in pres-sureless method relative to hot-pressing due to larger grain size inthe pressureless materials. The larger grain size decreases the fre-quency with which dislocations encounter grain boundaries, thusreducing the amount of stress required for deformation to occur.

Thirdly, SiC particles have more hardness than ZrB 2 particles.So, dispersion hardening of ZrB 2 with the harder SiC phaseincreases ZrB 2-base ceramics with higher SiC content. Hwanget al. [13] prepared ZrB 2–SiC composites with different SiC content(0 vol%, 5 vol%, 11 vol% and 22 vol%) by hot pressing at 1650 C.They found that relative density and hardness increase by increas-ing SiC content.

It can be seen that the hardness does not change more byincreasing C f at rst (until 5 vol%) and then decreases slightly(Fig. 3 b). Firstly, hardness reduction due to C f addition is reectedfrom the C f effect on densication. Secondly, according to Guofounding’s [14] , Cf increases the SiC grain and so, decreases thehardness. Fig. 4 shows the SEM images of composites 6 and 12.Although, composite 12 has a lower temperature (1800–1900 C)and time (4–12 min) SPS than composite 6, it is clearly observedthat SiC grains in composite 12 are larger than composite 6 dueto more C f . The addition of ber hindered from breaking apartthe agglomeration of SiC particles during the mixing process. Asa result, clusters of SiC particles were fused together during hotpressing to form larger SiC particles. Furthermore, the hindering

effect is enhanced with increasing ber volume fraction [14] .Totally, we can conclude that for C f as low as 5 vol%, the hardnessis not changed.

Fig. 3 c shows that M.t improves hardness until 5 h and thendecreases it.

3.2. Effect of MoSi 2, HfB 2 and ZrC on hardness

Effect of MoSi 2 , HfB2 and ZrC on hardness is shown in Fig. 5 . It isclearly observed that MoSi 2 increases hardness strongly at rst(2 vol%) and then the curve is nearly smooth ( Fig. 5 a). MoSi 2 is veryeffective sintering aid and improves sinterability of ZrB 2-basedceramics by liquid phase formation [15] , as shown in Fig. 3 a. So,due to open porosity reduction by increasing MoSi 2 , and respect

to the fact that the porosities have no resistance to applied stress,hardness increases.

Fig. 2. SEM image of composite 1.

Table 5

Hardness ANOVA analysis results for all parameters.

Fac. DOF(f)

Sumof Sqrs(s)

Variance(V)

F-ratio(F)

Pure sum(s 0 )

Percent P (%)

SiC 3 138493.4 46164.5 80 136763.1 8.168Cf 3 8927 2975.7 5.2 7196.7 0.428M.t 3 17494.4 5831.5 10.1 15764.1 0.94MoSi 2 3 207941.2 69313.7 120.2 206210.9 12.309HfB2 3 114177.3 38,059 66 112,447 6.712ZrC 3 153864.3 51288.1 88.9 152,134 9.08Temp. 3 917843.3 305947.7 5 30.5 916,113 54.709Press. 3 3864.2 1288 2.2 2133.9 0.128Time 3 35,927 1197.7 20.7 34196.7 2.044Error 5.48Total 100

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It is observed that HfB 2 decreases hardness initially and thenincreases it ( Fig. 5 b). HfB 2 has two-sided behavior. Open porositypercent increases by increasing HfB 2 due to its higher meltingpoint and so has negative effect on hardness. But, on the otherhand, HfB 2 has more hardness than ZrB 2 , and due to dispersingharder particles in matrix, is expected to increase hardness andhave a positive effect on hardness. At the result, there is challengebetween two factors: (1) the amount of open porosity percentcaused by HfB 2 , and (2) hardness was obtained by HfB 2 particles.It can be seen, the former factor is dominate up to 10 vol% HfB 2and hardness decreases by increasing HfB 2 . For 15vol% HfB 2

second factor is dominated. Finally, we can conclude, for the sameopen porosity percents, increasing HfB 2 improves the hardness.

The effect of ZrC on hardness is shown in Fig. 5 c. It can beobserved that ZrC improves the hardness. It can be reected fromthree mechanisms: (1) ZrC improves densication up to 10 vol%,according to Fig. 5 c which is accordance with other researchesfounding [11,16] . (2) According to other studies [11,16] ZrC is verygood grain growth inhibitor which is completely in accordancewith our results. SEM images of composites 4, 18, 5, 32, 29 and15 are shown in Fig. 6 . By comparing composites 4 and 18, 5 and32, 29 and 15 together, it is clearly observed that grains size of

(a) SiC

0

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H a r d n e s s ,

H v

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(b) Cf

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(c) M.t

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1 2 3 4 1 2 3 4

1 2 3 4

Level

H a r d n e s s ,

H v

0

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i t y

%

Fig. 3. Effect f (a) SiC, (b) C f and (c) M.t on hardness for combinations of variables (levels) as dened in Table 1 .

SiCSiC

Fig. 4. SEM images of composites 6 and 12.

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ZrB2 (gray grain) and SiC (black grain) decrease by increasing ZrCcontent at same sintering temperatures. In addition by comparingthe reduction amount of grain size in each temperature, we canconclude that ZrC is more effective at higher temperatures due tomore grain growth which occurs. So, by using ZrC, it is possibleto have nearly same grain size at higher temperature rather thanlower sintering without ZrC addition. For example, composite 5have nearly same grain size in comparison by composite 15 whileit is sintering temperature and time is higher ( Fig. 6 ). The additionof the second phase leads to a signicant decrease in the grain sizeof the main material. This is due to the fact that the second phaseconstitutes an additional obstacle (together with pores and impuri-

ties) to grain-boundary migration. According to Zener [16] , inclu-sions in a material are capable of strongly affecting boundary andsurface-diffusion parameters, which ultimately govern collectiverecrystallization.

3.3. Effect of SPS condition hardness

Fig. 7 shows the effect of SPS conditions on hardness. It isobserved that increasing temperature, obviously improves thehardness. Also, according to Table 4 data and Fig. 5 a, it can be seen,that temperature has the highest inuence on hardness and openporosity percent rather than other parameters. In fact, it isreected from two factors: (1) the signicant effect of temperatureon open porosity percent and (2) grain size. It is well-known that

by increasing temperature, grain size increases and depending onits amount may affect hardness. If this amount be noticeable, there

will be challenge between porosity percent reduction and grainsize as result of increasing temperature.

Hardness signicantly increases from 1600 C to1700 C atrst,and then the curve is nearly smooth and nally increases notice-ably in the range of 1800–1900 C,as shown in Fig. 7 . By comparingopen porosity curve with hardness ( Fig. 7 a), it is clearly observedthis hardness behavior is related to its densication manner. Toexplain it, two composites series were chosen: rst, 16, 27, 17and 11 (without MoSi 2); second, 28, 15, 3, 24. In each series, effectof temperature (1600–1900 C) is investigated.

In rst series, it can be found that open porosity percent isapproximately constant ( 14%) by increasing temperature from

1600 C to 1800 C, and then decreases to 3.5% ( Table 3 ). So, wecan conclude critical temperature for densication is 1900 C. Insecond series, open porosities percent in 28, 15, 3, 24 compositesare 12%, 3%, 1.7% and 0.3% respectively ( Table 3 ) and so reductionin open porosity percent occurs in 1700 C which is different torst series. This is related to MoSi 2 content in each series. In rstseries, there is no MoSi 2 and so the only parameter which affectsopen porosity percent is temperature while in second series, thereis MoSi 2 in all composites. Therefore, in this series, in addition of temperature, MoSi 2 causes open porosity percent reduction.Since, MoSi 2 is very effectiveness sintering aid which acts in tem-peratures more than 1700 C [17] , the critical densication tem-perature is 1700 C, in this series. So, two noticeable arises inhardness (1700 C, 1900 C) are related to the two reductions in

open porosity percent (1700 C, 1900 C). Also, it is concluded thatgrain size changing range in this article has little effect on the

(a) MoSi2

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i t y

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Fig. 5. Effect of (a) MoSi 2 , (b) HfB 2 and (c) ZrC at different levels ( Table 1 ) on hardness.

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hardness except composite 18. In order to investigate the effect of grain size on hardness, composites with most and least grain sizewere measured by Optika Vision Pro-Video and it was found com-posite 18 have the most ( 8 l m) grain size ( Fig. 6 ) between allcomposites and although its porosity is zero, has hardness below300 Hv which originates from its high SPS temperature and time(Table 2 ).

After composite 18, composite 4 have the most grain size,6 l m, ( Fig. 6 ). The smallest grain size belongs to composite 29,2 l m. Therefore, by ignoring composite 18, the maximum differ-

ence grain size in not more than 4 l m and which cannot over-come open porosity reduction.

However, additives such as SiC, ZrC and HfB 2 have inuence on

hardness, as mentioned in Section 3.2 . It means, it is possible that

composites with more open porosity percent have more hardnesseffects related to composition. For example, composite 22( 332 Hv) with 10.5% porosity is harder than composite 32( 190 Hv) with 8% porosity due to its more ZrC and HfB 2 .

Pressure does not have any signicant effect on hardness andopen porosity, as shown in Fig. 7 b which is in accordance by otherresearches [18] .

Effect of time on hardness is shown in Fig. 7 c. It is clear thathardness increases strongly by increasing time until 8 min andthen curve is approximately smooth. It is related to the open poros-ity reduction due to time increasing as shown in Fig. 7 c. Also, it canbe found that increasing grain size due to time has less inuenceon hardness rather than open porosity percent reduction except

composite 18 as mentioned above.

Fig. 6. SEM images of composites 4 (15 vol% ZrC), 18 (0 vol% ZrC) sintered at 1900 C, 16 min; 5 (10 vol% ZrC, 16 min), 32 (0 vol% ZrC, 8min) sintered at 1800 C; 29 (15 vol%ZrC) and 15 (0 vol% ZrC) sintered at 1700 C, 12 min.

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3.4. Optimization

Optimum conditions to reach the best densication and hard-ness are shown in Table 6 . In order to reach best densicationand hardness together (in one composite), the multiple criteria(OEC) of Taguchi method is used. The optimum conditions areshown in Table 6 . It is clear that the optimum level of SiC, MoSi 2 ,ZrC, temperature and time to reach best densication and hardnessare same. So, their optimum level to reach best densication &hardness together will be same, too. Optimum C f level is differentin open porosity and hardness and for best densication & hard-ness, optimum level of densication is obtained by OEC due tomore weights (80–20%) that we gave to open porosity percentresponds rather than hardness responds. These weights are givenbased on open porosity and hardness important for us. For M.t,HfB2 and pressure, the level which both maximum densication

and hardness is available, is obtained by OEC.

4. Conclusion

4.1. It has been concluded that the most signicant effects on thehardness are related to temperature, MoSi 2 , ZrC, SiC andHfB2 by 54.7%, 12.3%, 9.1%, 8.2% and 6.7%, respectively.

4.2. Pressure by 0.13% and C f by 0.43%, M.t by 0.94% and time by2% have the least effect on the hardness.

4.3. By increasing the temperature, time, MoSi 2 and SiC fromlevel 1 to 4, hardness improved continuously while byincreasing HfB 2 from level 1 to 4, hardness decreasedconsistently.

4.4. The optimum condition, to reach the most densication is(levels from Table 1 are indicated in parentheses) T (4), t(4), P (3), SiC (4), C f (2), M.t (2 or 3), MoSi 2 (4), HfB 2 (1 or2) and ZrC (3).

4.5. The optimum condition, to reach the most hardness is

(levels from Table 1 are indicated in parentheses) T (4), t

(a) Temp.

0

50

100

150

200

250

300

350

Level

H a r d n e s s ,

H v

0

3

6

9

12

15

P o r o s

i t y

%

(b) Press.

0

50

100

150

200

250

300

350

Level

H a r d n e s s ,

H v

0

3

6

9

12

15

P o r o s

i t y

%

(c) Time

0

50

100

150

200

250

300

350

1 2 3 4 1 2 3 4

1 2 3 4

Level

H a r d n e s s ,

H v

0

3

6

9

12

15

P o r o s

i t y

%

Fig. 7. Effect of (a) temperature, (b) pressure and (c) time at various levels ( Table 1 ) on hardness.

Table 6

Hardness, open porosity percent and porosity & hardness optimum levels as dened in Table 1 .

Factor SiC C f M.t MoSi 2 HfB2 ZrC T P t

Optimum level, open porosity 4 2 2, 3 4 1,2 3 4 3 4Optimum level, hardness 4 3 3 4 1 3 4 2, 3 4Optimum level, porosity & hardness 4 2 3 4 1 3 4 3 4

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(4), P (2 or 3), SiC (4), C f (3), M.t (3), MoSi 2 (4), HfB 2 (1) andZrC (3).

4.6. The optimum condition, to reach the most densication &hardness is (levels from Table 1 are indicated in parenthe-ses) T (4), t (4), P (3), SiC (4), C f (2), M.t (3), MoSi 2 (4), HfB 2and ZrC (3).

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