Composites: Part B 43 (2012) 1564–1569

Contents lists available at SciVerse ScienceDirect

Composites: Part B

journal homepage: www.elsevier .com/locate /composi tesb

Effect of deflocculants on hardness and densification of YSZ–Al2O3 (whiskers &particulates) composites

Muhammad Abdullah a,⇑, Jamil Ahmad a, Mazhar Mehmood a, H. Waqas a, M. Mujahid b

a National Center for Nanotechnology and Department of Metallurgy and Materials Engineering, Pakistan Institute of Engineering and Applied Sciences, PO Nilore, Islamabad, Pakistanb School of Chemical & Materials Engineering (SC&ME), National University of Sciences and Technology, H-12, Islamabad, Pakistan

a r t i c l e i n f o

Article history:Received 17 April 2011Received in revised form 15 June 2011Accepted 18 August 2011Available online 26 August 2011

Keywords:A. Ceramic-matrix composites (CMCs)B. HardnessB. MicrostructureD. FractographyD. Electron microscopy

1359-8368/$ - see front matter � 2011 Elsevier Ltd. Adoi:10.1016/j.compositesb.2011.08.015

⇑ Corresponding author. Tel.: +92 51 2207380; fax:E-mail address: mabhutta_99@yahoo.com (M. Abd

a b s t r a c t

In the present study effect of deflocculants like P-Aminobenzoic Acid (PABA) and Cetyltrimethyl ammo-nium bromide (CTAB) on densification and hardness of 3 mol.% Yttria-stabilized ZrO2 (abridged asYSZ) + Al2O3 (whiskers or particulates) composite have been studied. Maximum hardness & density wereachieved at 1 wt% of CTAB or PABA, while further addition (5, 10 and 15 wt%) had no significant affect onthe aforementioned properties. It was also observed that alumina addition in form of particulates onlyimproved the density while its addition in form of whiskers significantly increased the hardness ofYSZ + alumina composite. The maximum hardness achieved was more than 14 GPa in case of sample con-taining alumina in form of whiskers.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction prepared and subsequently they were transformed into alumina

Due to better mechanical properties zirconia (ZrO2) nanocom-posites have found widespread applications in the field of struc-tural & bio-ceramics [1]. The properties of zirconia can further beimproved through its unique transformation tougheningmechanism [2–4].

Along with excellent corrosion resistance and good toughnessthe pure zirconia has less hardness as compare to other ceramicslike Al2O3, SiC, and Si3N4 [5]. The hardness of conventionally avail-able YSZ is 12 GPa [6]. To further improve the hardness of YSZwithout affecting its other properties, there is a need of some rein-forcement material. Alumina (Al2O3) is fairly inert and possessesexcellent mechanical properties. Therefore, it may be a potentialcandidate to form composite with zirconia. For the last few dec-ades, lot of work has been focused on synthesis and characteriza-tion of zirconia–alumina composite to obtain better mechanicalproperties than both of its constituent oxides [7].

Different techniques for production of alumina whiskers havebeen reported like hydrothermal [8], carburization and pyrolysis[9], and Vapor phase growth [10]. However, hydrothermal tech-nique as reported by Li et al. [8] is unique due to low temperaturesynthesis, cost economical, less time consuming and requires com-mon precursor salts. Using hydrothermal technique, whiskers ofAmmonium Aluminum Carbonate Hydroxide (AACH) were

ll rights reserved.

+92 51 2208070.ullah).

whiskers in situ during sintering.Some works on use of deflocculating agents like CTAB [11] and

PABA [12,13] are reported, however a systematic study of usingdifferent concentration of these deflocculating agent in zirconia–alumina composite still lacks.

Reinforcement geometry is also an important factor, which mayaffect the properties of synthesized composite. It has been reportedthat whiskers reinforced ceramic composites gave better mechani-cal properties [14]. However, alumina whiskers reinforced zirconiacomposite have been rarely reported. Only note worthy work is doneby Nevarez-Rascon et al. [15] but unfortunately the whiskers usedby them were of high aspect ratio (1400 approx.) which resulted intosevere agglomeration. The presence of large agglomerates may leadto low sintered density [16] and also affect the mechanical proper-ties of structural ceramics. In the present work, emphasis has beenfocused on the prevention of agglomeration. For this purpose, thewhiskers of relatively lower aspect ratio have been used. Further-more, to prevent the formation of agglomerates, deflocculatingagents like CTAB and PABA [11,13,17] have also been used. Improve-ment in density and hardness of the zirconia alumina composites byusing these deflocculating agents has also been reported.

2. Experimental procedure

2.1. Preparation of AACH whiskers

Alumina whiskers were produced in situ from transformation ofAmmonium Aluminum Carbonate Hydroxide (AACH) whiskers

Table 1Types of samples prepared.

M. Abdullah et al. / Composites: Part B 43 (2012) 1564–1569 1565

during sintering. The AACH whiskers were produced by hydrother-mal technique [8].

[90%YSZ + 10%Al2O3(w)]100�x + [CTAB]x Where[90%YSZ + 10%Al2O3(p)]100�x + [CTAB]x x = 0, 1, 5, 10, 15[90%YSZ + 10%Al2O3(w)]100�x + [PABA]x w = whisker[90%YSZ + 10%Al2O3(p)]100�x + [PABA]x p = particulate

2.2. Synthesis & characterization of YSZ + Al2O3 composite

Total 20 types of samples in Table 1 in the form of 04 serieswere prepared.

For preparation of these samples the measured amounts ofdeflocculates like Cetyltrimethyl ammonium bromide (CTAB) or P-Aminobenzoic Acid (PABA) were dissolved in 99.9% analar gradeethanol. The starting materials for this experiments were AACHwhiskers synthesized through hydrothermal technique as men-tioned in Section 2.1, High-puritya-Al2O3 particulates (Make: Alma-tis, Germany; purity > 99.99%; Average particle size �0.2 lm) andZrO2 + 3 mol.% Y2O3 (thereafter abbreviated as TZ-3Y; Make: Terio,China; purity > 99.99%; Average particle size �0.2 lm). The mea-sured amounts of these particulates or whiskers were dispersed sep-arately in solution containing measured amount of deflocculant andwere sonicated for 30 min. The AACH whiskers were added on thebasis of balancing total Aluminum content. These solutions werethen mixed with each other and magnetically stirred at 40 �C over-night to get dried powders i.e. YSZ and alumina powder mixture alsocontaining CTAB/PABA.

This dry mixture was thermally characterized by using MettleToledo, TGA/SDTA851 up to the temperature of 1000 �C with heatingrate of 20 �C/min in air. On the basis of TGA results, the powder wascalcined at 650 �C to remove organic deflocculants and other volatilespecies such as NH4+, OH�, HCO3� [8] and residual water vapors. Thecalcined mixture was ground for 1/2 h and pressed uniaxially at aload of 5 � 103 kg in a disk steel die to make pellets of dia10 mm.The prepared green pellets were sintered in a high temperature(Carbolite HTF-18/8) furnace at a temperature of 1450 �C in air fordwelling time of 2 h at heating and cooling rate of 2 �C/min to getYZS-alumina composites.

The density of sintered pellets was measured in ethanol throughArchimedes method using high accuracy (10�6 gm) weighingbalance.

The Vicker hardness of sintered pellets was measured by usingVicker Microhardness testers (Series 401MVD WOLPERT Group) atroom temperature. Four samples per composition and approxi-mately 10–15 indents per measurement were made and the averagehardness was calculated. The separation between the neighboringindents was kept more than four diagonal lengths of indentationimpression as per ASTM standard of ASTM C 1327-99 [18]. For hard-ness testing, a load of 1 Kg was applied for 15 s.

Phase analysis of the samples at different processing steps wasperformed by X-ray diffraction using Bruker D8 X-ray diffractome-ter with Cu Ka radiation in h–2h mode. The microstructural studywas carried out using JEOL-JSM5910 scanning electron microscope(SEM) at 20 kV. Before electron microscopy, the samples were goldcoated using SPI-Module sputter coater in order to avoid chargingduring exposure to electron beam.

3. Results

3.1. AACH whiskers, alumina powder & tetragonal zirconia

Fig. 1a and b shows the SEM micrographs of as prepared AACHwhiskers. From here whiskers with approximate diameter of 100–200 nm and with length of several micrometers can be observed.Fig. 2 shows the SEM image of as received a-Al2O3 particulatescluster of roughly spherical morphology.

Fig. 3a and b shows the XRD patterns of as prepared AACH whis-kers and the pure AACH whiskers calcined at 1450 �C. The as pre-pared whiskers show the pattern similar to AACH as reported in

literature [8]. While the XRD pattern of the whiskers sintered at1450 �C is similar to that of a-Al2O3 which confirms that AACHhas completely transformed to a-Al2O3 after sintering [8].

Fig. 4a–c shows the XRD pattern of samples containing YSZ; theas received YSZ powder shows the mixed phases of monoclinicalong with tetragonal. But when this mixed phase zirconia was sin-tered at a temperature of 1450 �C, then complete tetragonal phasewas developed. This tetragonal phase is onward responsible forimprovement in properties of composite through transformationtoughening mechanism [2]. Fig. 4a–c shows the XRD pattern ofthe sintered pellet of ZrO2 + Al2O3; here the peaks of alumina with-in the peak of zirconia confirm that both oxides have maintainedtheir inertness. The very small peaks of alumina are due to its smallquantity as well as low X-ray reflection cross-section of alumina ascompare to zirconia.

3.2. Densification and hardness of 90%YSZ + 10%Al2O3(P &W) + [15]ureor [PABA] composite

Fig. 5a and b shows the effect of adding different concentrationsof CTAB or PABA on hardness of 90%YSZ + 10%Al2O3 composite. Theaverage vicker hardness, Fig. 5a for the samples containing aluminawhiskers as reinforcement is 14.44 GPa (maximum) while for alu-mina particulates it is 13.72 GPa at a concentration of 1 wt% CTAB.The values of average hardness are 14.34 GPa and 13.65 GPa for alu-mina whiskers and particulates reinforcements respectively at aconcentration of 1 wt% PABA. These values are higher as comparedto previously reported [15] for 10 wt% alumina whiskers and pureYSZ. These results revealed that the hardness decreases as theamount of the deflocculating agents is increased.

Fig. 6a and b shows the relative densities of the 90%YSZ + 10%A-l2O3 (w or P) composite samples containing CTAB or PABA. The den-sity increases as the deflocculating agent was added. At aconcentration of 1 wt% CTAB the samples containing alumina whis-kers shows the relative density of 93% while samples containing alu-mina particulates shows 95.5% relative density. On the other handfor the samples with 1 wt% PABA shows the density of 91% and95% for whiskers and particulates containing samples respectively.Here also the relative density first increases with addition of defloc-culating agent up to 1 wt% CTAB or PABA but with further increase inconcentration of deflocculating agent the density decreases.

Fig. 7a–d shows the SEM micrographs of the sintered pellets ofAl2O3 particulates and Al2O3 whiskers in the matrix of YSZ with1 wt% CTAB. In all the pellets crack free microstructure with theaverage grain size of approximately 0.3 lm were observed. Toexamine the morphology and to determine the elemental compo-sition of whisker, the pellets were fractured and observed underSEM and analysed by EDS attached with the microscope. The frac-tographs at low and higher magnifications along with their EDSanalysis are shown in Fig. 8a and b.

4. Discussion

The difference in density of whiskers and particulates contain-ing composite (Fig. 6a and b) suggests that whiskers resist com-plete shrinkage during sintering and very small voids or porosityremains inside the composite which decreases its density.

Fig. 1. (a and b): SEM images of AACH whiskers synthesized by hydrothermal process (a) at low and (b) high magnification.

Fig. 2. Morphology of as received a-Al2O3 particulate.

10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90

311

331

240

221

220

200

111

110

1010

018

220

(b)

(a)

300

116

104

214

113

024

012

Inte

nsity

(a.u

.)

2θ (degree)

110

Fig. 3. (a and b): X-ray diffraction pattern of (a) as prepared AACH and (b) sinteredAACH whiskers.

30 60 90

(c)

(b)

•••••••

•

∗∗∗∗∗∗∗∗∗

∗∗∗

∗

• α−Al2O3 ∗ Tetragonal ZrO2

116

214

300

02411

3

110

104

012

114

301

004 213

220103 21

1

20220

011

2

110

002

101 (a)

Inte

nsity

(a.u

.)

2θ θ (degree)

Fig. 4. (a–c): X-ray diffraction of (a) as received YSZ, (b) sintered YSZ and (c)YSZ + Al2O3 (whiskers) pellets.

1566 M. Abdullah et al. / Composites: Part B 43 (2012) 1564–1569

The increase in density and hardness, Fig. 5a and b, with addi-tion of CTAB and PABA is due to uniform dispersion of whiskersand particles. The density of the compacts depends upon many fac-tors like applied load; morphology, size & shape of the reinforce-ment; state of die wall lubrication; and agglomeration [12]. Thedensity can also be improved due to better dispersion of reinforce-ment. If the dispersion of the reinforcement increases then the

sample will have uniform shrinkage during sintering and this re-sult in decreasing the porosity and improving the density as ob-served in Fig. 6a and b.

There are two reasons behind the better dispersion of Al2O3.First and main reason as already mentioned, is due to addition ofdeflocculating agents and other is the use of AACH and its subse-quent transformation into Al2O3 in situ during sintering. CTAB isa well known cationic surfactant. Addition of CTAB increases thezeta potential of Al2O3 [19]. In general, electrostatic repulsive forcebetween the particles increases with the increase in zeta potential.It is assumed that when the alumina whiskers/particulates areadded into the CTAB containing solution then CTAB adsorb onthe surface of the alumina and produces a positive charge layeron the particles surface. During mixing when the secondary parti-cles break then this surfactant immediately coat the newly formedsurfaces and these particles then repel each other. In this way,agglomeration of the particles is prevented and dispersion is im-proved. In fact, the increase in electrostatic repulsive force isresponsible for the dispersion of alumina (Al2O3).

Similarly the addition of PABA also results in improving thedensity and hardness of the composite. The PABA an organic com-pound (with formula (H2NC6H4CO2H) is excellent dispersing agent.Actually, PABA has both acidic and basic groups in its single mole-cule. It is speculated that there is a possibility of intermolecularionization, after which the carboxyl group of PABA specifically ab-sorbs on the surface of AACH particle while the ionized amine

0 2 4 6 8 10 12 14 16

12.6

12.8

13.0

13.2

13.4

13.6

13.8

14.0

14.2

14.4

14.6

14.8

15.0

15.2

15.4 10% Al2O3 (w)+90% ZrO2with CTAB 10% Al2O3 (P)+90% ZrO2with CTAB

Har

dnes

s (G

Pa)

CTAB (wt. %)

(a) (b)

0 2 4 6 8 10 12 14 16

12.6

12.8

13.0

13.2

13.4

13.6

13.8

14.0

14.2

14.4

14.6

14.8

15.0

15.2

15.4 10% Al2O3(w)+90%ZrO2with PABA 10% Al2O3(P)+90%ZrO2with PABA

Har

dnes

s (G

Pa)

PABA (wt. %)

Fig. 5. (a and b): Hardness of pellets (a) [90%YSZ + 10%Al2O3(w)]100�x + [CTAB]x (b) [90%YSZ + 10%Al2O3(w)]100�x + [PABA]x.

0 2 4 6 8 10 12 14 1686

87

88

89

90

91

92

93

94

95

96

Rel

ativ

e D

ensi

ty (%

)

CTAB (wt %)

10% Al2O3(w)+90% ZrO2 with CTAB 10% Al2O3(p)+90% ZrO2 with CTAB

0 2 4 6 8 10 12 14 1686

87

88

89

90

91

92

93

94

95

96

Rel

ativ

e D

ensi

ty (%

)

PABA (wt %)

10% Al2O3(w)+90% ZrO2 with PABA 10% Al2O3(p)+90% ZrO2 with PABA

(a) (b)

Fig. 6. (a and b): Relative density of pellets (a) [90%YSZ + 10%Al2O3(w)]100�x + [CTAB]x (b) [90%YSZ + 10%Al2O3(w)]100�x + [PABA]x.

M. Abdullah et al. / Composites: Part B 43 (2012) 1564–1569 1567

group which has positive charge, is towards solution and this givealumina a net positive charge. So this positive charge due to elec-trostatic repulsion does not let the particles of Al2O3 to agglomer-ate [20] in the matrix of zirconia.

The other factor contributing towards the improvement in theproperties is due to the use of AACH instead of Al2O3, and duringsintering this AACH converts into Al2O3 whiskers. The Al2O3 whis-kers formed from AACH have larger specific surface area and havebetter sorption capacity and supported the dispersion of the Al2O3

whiskers [21].Other important observation that can be made from Fig. 5a and

b and Fig. 6a and b, is the decrease in density and hardness withincreasing the amount of CTAB or PABA above 1 wt% concentration.When the deflocculating agent is added above a certain amountthen it produces small amount of porosity which may results indeteriorating the properties of the material [11]. This can be con-firmed from thermogravimetry (TG) analysis curve at Fig. 10 forthe samples containing 25% deflocculating agent. TG thermogram

of pure AACH in Fig. 10a shows the total weight loss of 63%. Thisis in good agreement with the literature [8] that the AACH (NH4A-l(OH)2CO3) decomposes to Al2O3 according to the followingreaction,

2NH4AlðOHÞ2CO3 ! Al2O3 þ 2NH3 þ 2CO2 þ 2H2O

While for the samples with YSZ + AACH + CTAB or PABA theamount of precursor materials used for making the mixture wereAACH = 0.818 gm (which converts into 0.3 gm Al2O3(W));YSZ = 2.7 gm and CTAB/PABA = 0.88 gm. So the total weight remainshould be 3 gm (i.e. 68.213% of the total weight) but this comes outto be 69.7% and 71.3% for the samples containing CTAB and PABArespectively. This means that out of the added 25%, approximately1.5% and 3% weight of CTAB and PABA respectively is not removedand remains within the composite even at high temperature.Approximately 3–4% combustion products of CTAB remains evenat high temperatures [22]. So when relatively large amount ofdeflocculating agent was added then the remaining reaction prod-

Fig. 7. (a–d): SEM micrographs at low and high magnification of the sintered pellets of (a and b) [90%YSZ + 10%Al2O3(p)]99 + [CTAB]1 (c and d) [90%YSZ + 10%Al2O3(w)]99 + [CTAB]1.

0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 9.00 10.00

keV

001

0

100

200

300

400

500

600

700

800

900

1000

Cou

nts

OK

a

AlK

aZ

rLl

ZrL

a

0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 9.00 10.00

keV

004

0

100

200

300

400

500

600

700

800

900

1000

Cou

nts

OK

a

AlK

aZ

rLl

ZrL

a

(a)

(b)

Fig. 8. (a and b): (a) Fractograph and EDS of composite (b) magnified image identifying the presence of whisker in composite matrix.

1568 M. Abdullah et al. / Composites: Part B 43 (2012) 1564–1569

0 200 400 600 800 10003035404550556065707580859095

100105

(c)(b)

(a)

Wei

ght (

%)

Temperature (°C)

[90 % ZrO2(3Y) +10 % Al2O3(w)]75+ [PABA]25

[90 % ZrO2(3Y) +10 % Al2O3(w)]75+ [CTAB]25

Pure AACH

Fig. 10. (a–c): TGA of (a) pure AACH and [90%YSZ + 10%Al2O3(W)]75with (b) CTABand (c) PABA composite.

Fig. 9. SEM micrographs of vickers hardness indentation, the arrow marksexplained in text.

M. Abdullah et al. / Composites: Part B 43 (2012) 1564–1569 1569

ucts produce low density regions and results in deterioration of den-sity and hardness. Similarly the maximum hardness achieved forCTAB is higher than the value achieved from PABA. The reason isobvious from Fig. 10 that as more mass of the PABA as compare toCTAB remains within the samples after sintering, so the CTAB pro-duces better results than PABA.

It is observed that hardness of the samples with Al2O3 whiskers ishigher than the samples with equal amount of Al2O3 particulates.This improvement in the mechanical properties of whiskers rein-forced composites is due to crack deflection and crack bridging ef-fects [23] offered by the added whiskers. This appreciation can besupported by Fig. 9, SEM image of one arm of a vicker hardness in-dented impression in whiskers containing composite. From herethe crack deflection (zigzagged cracks, arrow marks) is evident.

5. Conclusions

Mechanical properties of YSZ + Al2O3 nanocomposite can be in-creased by careful selection of deflocculant. The addition of CTAB

has better effect on the density and hardness of the ZrO2 + Al2O3

composite as compare to PABA addition. The maximum hardnessfor CTAB (1 wt%) was 14.44 GPa at a composition of [90%YS-Z + 10%Al2O3(w)]99 + [CTAB]1 while maximum hardness of14.34 GPa was obtained for [90%YSZ + 10%Al2O3(w)]99 + [PABA]1.The density of Al2O3 particulates containing samples was higherthan samples with Al2O3 whiskers, while hardness of Al2O3 whis-kers containing samples was more than Al2O3 particulates contain-ing samples.

Acknowledgment

Authors would like to thank Higher Education Commission(HEC), Pakistan for the financial support of this research.

References

[1] Liang X, Qiu Y, Zhou S, Hu X, Yu G, Deng X. Preparation and properties of dentalzirconia ceramics. J Univ Sci Technol B 2008;15(6):764–8.

[2] Garvie RC, Hannink RH, Pascoe RT. Ceramic steel. Nature1975;258(5537):703–4.

[3] Kelly JR, Denry I. Stabilized zirconia as a structural ceramic: an overview. DentMater J 2008;24(3):289–98.

[4] Heuer AH, Lange FF, Swain MV, Evans AG. Transformation toughening: anoverview. J Am Ceram Soc 1986;69(3):1–4.

[5] Santos Inei dos, Teixeira LHP, Strecker Kurt, Elias CN. Effect of Al2O3 additionon the mechanical properties of biocompatible ZrO2–Al2O3 composites. MaterSci Forum 2006;530–531:575–80.

[6] Zhang Y, Chen J, Hu L, Liu W. Pressureless-sintering behavior of nanocrystallineZrO2–Y2O3–Al2O3 system. Mater Lett 2006;60(17–18):2302–5.

[7] Nikolay D, Kollenberg W, Deller K, Oswald M, Tontrup C. Manufacturing andproperties of ZTA-ceramics with nanoscaled ZrO2. CFI-Ceram Forum Int2006;83(4):E35–7.

[8] Li J, Li W, Nai X, Bian S, Liu X, Wei M. Synthesis and formation of aluminawhiskers from hydrothermal solution. J Mater Sci 2010;45(1):177–81.

[9] Patel M, Padhi BK. Formation of alumina whiskers using Al2(SO4)3 andaluminium isopropoxide. J Mater Sci Lett 1991;10(21):1243–5.

[10] Yamai I, Saito H. Vapor phase growth of alumina whiskers by hydrolysis ofaluminum fluoride. J Cryst Growth 1978;45:511–6.

[11] Liu D, Yan Y, Lee K, Yu J. Effect of surfactant on the alumina dispersion andcorrosion behavior of electroless Ni–P–Al2O3 composite coatings. Mater Corros2009;60(9):690–4.

[12] Vishista K, Gnanam FD. Effect of deflocculants on the densification andmechanical properties of sol–gel alumina. Mater Lett 2004;58(11):1665–70.

[13] Guimarães FAT, Silva KL, Trombini V, Pierri JJ, Rodrigues JA, Tomasi R, et al.Correlation between microstructure and mechanical properties of Al2O3/ZrO2

nanocomposites. Ceram Int 2009;35(2):741–5.[14] Zhang X, Xu L, Du S, Han J, Hu P, Han W. Fabrication and mechanical properties

of ZrB2–SiCw ceramic matrix composite. Mater Lett 2008;62(6–7):1058–60.[15] Nevarez-Rascon A, Aguilar-Elguezabal A, Orrantia E, Bocanegra-Bernal MH.

Al2O3(w)–Al2O3(n)–ZrO2 (TZ-3Y)n multi-scale nanocomposite: an alternative fordifferent dental applications? Acta Biomater 2010;6(2):563–70.

[16] Ozkan N, Briscoe BJ. Characterization of die-pressed green compacts. J EurCeram Soc 1997;17(5):697–711.

[17] Hirata Y, Haraguchi I, Ishihara Y. Particle-size effects on colloidal processing ofoxide powders. J Mater Res 1992;7(9):2572–8.

[18] Standard ASTM C1327-99. Standard test method for vickers indentationhardness of advanced ceramics; 1991.

[19] Chen L, Wang L, Zeng Z, Zhang J. Effect of surfactant on the electrodepositionand wear resistance of Ni–Al2O3 composite coatings. Mater Sci Eng A2006;434(1–2):319–25.

[20] Kitayama M, Pask JA. Formation and control of agglomerates in aluminapowder. J Am Ceram Soc 1996;79(8):2003–11.

[21] Fu G-f, Zhang J-x, Liu J, Wang Z-f. Wear behavior of Al–Si alloy matrixcomposites reinforced by [gamma]–Al2O3 decomposed from AACH. TransNonferrous Metals Soc 2006;16(5):1159–62.

[22] Kondrashova N, Vasil’eva O, Val’tsifer V, Astaf’eva S, Strel’nikov V. Preparationof mesoporous silicon dioxide with high specific surface area. Russ J ApplChem 2009;82(1):1–5.

[23] Carter DH, Hurley GF. Crack deflection as a toughening mechanism in SiC-whisker-reinforced MoSi2. J Am Ceram Soc 1987;70(4):C-79–81.