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Materials Science and Engineering A 491 (2008) 338–342

Formation of intragranular nano-structures in micro-sizedceramic composite materials

Chonghai Xu a,b,∗, Deming Sun c

a School of Mechanical Engineering, Shandong Institute of Light Industry, Jinan 250353, PR Chinab School of Mechanical Engineering, Shandong University, Jinan 250061, PR China

c School of Materials Science and Engineering, Shandong Jianzhu University, Jinan 250014, PR China

Received 6 December 2007; received in revised form 3 February 2008; accepted 4 February 2008

bstract

Intragranular nano-structures observed in Al2O3/SiC/(W, Ti)C, Al2O3/SiC/Ti(C, N), Al2O3/(W, Ti)C/RE, Al2O3/Ti(C, N)/RE andl2O3/Cr3C2/(W, Ti)C micro-sized composite ceramic materials were reported. Analysis techniques such as SEM, TEM and EDAX were used

o investigate the morphology and formation mechanism of the intragranular nano-structures and their effects on the mechanical property. It was

ound that grain size of the observed nanometer particles is mainly in the range of tens to hundreds of nanometer, which belongs to the typicalntragranular structure. The intragranular nano-structures are effective in the improvement of mechanical properties of the micro-sized ceramicomposite materials.

2008 Published by Elsevier B.V.

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eywords: Ceramic material; Micro-composite; Intragranular nano-structure

. Introduction

In the past decades, considerable improvements in mechan-cal properties of the single phase ceramic materials have beenchieved by incorporating one or more other components intohe base material to form ceramic matrix composites (CMCs)1–3]. Researches on the micro-sized composite ceramics haveovered varieties of oxide, nitride, carbide and boride ceram-cs [4–6]. However, the corresponding material compositions,rocessing techniques, reinforcing and toughening mechanisms,roperties and applications still need further study. It has beenointed out that nano-ceramic is a strategic way to solve the brit-leness of ceramics, while the formation of nano-structures is theundamental reason for the property improvement of ceramicaterials [7,8]. Generally, the intragranular nano-structures

ppear in nano-sized or nano-micro-sized ceramic compos-te materials or the so-called ceramic matrix nanocomposites

CMNCs) [7,9,10]. However, the intragranular nano-structuresormed in several kinds of Al2O3 series micro-sized ceramicomposites were observed in the present study. The morphology

∗ Corresponding author at: Tel.: +86 531 89631066; fax: +86 531 89631066.E-mail address: xch@sdili.edu.cn (C. Xu).

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921-5093/$ – see front matter © 2008 Published by Elsevier B.V.oi:10.1016/j.msea.2008.02.029

nd formation mechanism of the intragranular nano-structuresnd their effects on the mechanical property were investigated.

. Experimental details

Five kinds of new ceramic materials prepared previ-usly, Al2O3/15%SiC/15%Ti(C, N), Al2O3/15%SiC/15%(W,i)C, Al2O3/45%Ti(C, N)/RE, Al2O3/45%(W, Ti)C/RE,l2O3/10%Cr3C2/20%(W, Ti)C were used for the microstruc-

ural analyses (Table 1) [11–14]. Powder size of the usedaw materials is nearly 0.5–1.5 �m. Pure alumina ceramic wassed for comparison. Microstructures of the materials werebserved with scanning electronic microscope (SEM, modelitachi S-570) and transmission electron microscope (TEM,odel Hitachi H-800) equipped with the energy spectrum and

perated at 175 kV.

. Results and discussion

.1. Results of microstructural analyses

SEM morphology of the fracture face of Al2O3/15%SiC/5%(W, Ti)C ceramic samples is shown in Fig. 1. It is foundhat the added ceramic grains SiC and (W, Ti)C are distributed

C. Xu, D. Sun / Materials Science and Engineering A 491 (2008) 338–342 339

Table 1Compositions and properties of the tested ceramic materials [11–14]

Material composition Flexural strength (MPa) Fracture toughness (MPa m1/2) Hardness (GPa)

Al2O3/15%SiC/15%Ti(C, N) 721 ± 64 5.4 ± 0.2 19.0 ± 0.2Al2O3/15%SiC/15%(W, Ti)C 753 ± 55 5.3 ± 0.2 19.0 ± 0.3Al2O3/45%Ti(C, N)/REa 1010 ± 71 6.1 ± 0.3 19.1 ± 0.2Al2O3/45%(W, Ti)C/REa 853 ± 43 6.0 ± 0.2 19.2 ± 0.2A 9.4 ± 0.5 18.8 ± 0.3A 4.1 ± 0.3 18.0 ± 0.3

volume percent.

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l2O3/10%Cr3C2/20%(W, Ti)C 562 ± 46l2O3 296 ± 37

a RE means rare earth element. The content of the reinforcement phases is in

early uniformly inside the Al2O3 matrix with the grain size of–3 �m, which is the typical characteristic of the micro-sizederamic composite material.

Microstructural morphologies of the developed micro-sizederamic materials under TEM are described as following. Inl2O3/15%SiC/15%(W, Ti)C ceramic material, most of theiC particles are dispersed uniformly in the alumina matrix,hile small amount of them exist inside the alumina grainsith the grain size less than 200–300 nm (Fig. 2, indicated by

rrow). They belong to the so-called intragranular structures.he case has also been observed in Al2O3/15%SiC/15%/Ti(C,) ceramic material where the representative nano-sized particle

s indicated by arrow in Fig. 3. The existence of the sub-icrometer or nanometer sized grains inside the matrix grainsill improve the flexural strength and fracture toughness of theaterial with the similar toughening mechanism of intragran-

lar fracture happened frequently in ceramic nano-composites9,15,16].

In the rare earth reinforced Al2O3/45%(W, Ti)C/RE ceramicn Fig. 4(a), some black spherical or particulate-like compounds

ess than 100nm (indicated by arrows) exist inside the alumina

atrix grains besides the main phases of Al2O3 and (W, Ti)C. Itas been pointed out [13,14] that rare earth elements can react

ig. 1. SEM morphology of fracture face of Al2O3/15%SiC/15%(W,Ti)Ceramic.

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Fig. 2. TEM morphology of Al2O3/15%SiC/15%(W,Ti)C ceramic.

ith one or several chemical elements among W, Fe, Cr, Mond Ni to form the corresponding complex compounds suchs rare earth inclusions or rare earth intermetallics. Therefore,he added rare earth elements can work to gather the impu-ity elements. As a result, the ceramic interfaces are purifiednd then the binding strength can be increased. Dislocations arelso found to exist near the small grains in Fig. 4(a) which sug-ests that they have obvious pinning effects to the dislocations,

hile the dislocation-based strengthening mechanism is one of

he effective strengthening mechanisms in CMNCs [10,17]. Theimilar case happened either in Al2O3/45%Ti(C, N)/RE ceramicaterial where small grains (indicated by arrow) are less than

Fig. 3. TEM morphology of Al2O315%SiC/15%/Ti(C,N) ceramic.

340 C. Xu, D. Sun / Materials Science and E

Fig. 4. TEM morphologies of (a) Al2O3/(W,Ti)C/RE, (b) Al2O3/Ti(C,N)/RE,and (c) Al2O3/10%Cr3C2/20%(W,Ti)C ceramics.

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ngineering A 491 (2008) 338–342

00 nm(Fig. 4(b)). However, these small grains exist inside theeinforcement phase Ti(C, N) which is different from that inl2O3/45%(W, Ti)C/RE ceramic.Fig. 4(c) gives the typical TEM morphology of the intra-

ranular grains in Al2O3/10%Cr3C2/20%(W, Ti)C ceramicomposite. These grains are less than 50 nm with three kindsf different setup. According to the results of EDAX spectrumnalysis and other analyses [11], the white grains indicated byetter C mainly consist of Al with small amount of W, Cr and Tilements, the black grains indicated by letter A mainly consistf Cr accompanied with small amount of W, Al and Ti elements,hile the grey grains indicated by letter B are mainly composedf W and Ti elements. It can be determined that the black grainsre mainly Cr3C2, while the grey and white grains are mainlyW, Ti)C and Al2O3, respectively. The existence of these intra-ranular grains in the matrix will undoubtedly play an importantole in the reinforcement of ceramic materials. Dislocation mor-hologies in Al2O3/10%Cr3C2/20%(W, Ti)C ceramic materialre shown in Fig. 5 (a–d) where the typical nano-sized par-icles and their interaction with dislocations are indicated byrrows. They can help to understand the effects of these kindsf intragranular nano-structures.

.2. Discussion

Usually, the intragranular nano-structures appear in nano-ized or nano-micro-sized composite ceramic materials.owever, several kinds of intragranular nano-structures haveeen observed in Al2O3 series micro-sized ceramic compos-te materials in the present study. It seems that some of theano-particles result from the fining effect of long time ballilling of several tens of hours of the used ceramic raw materi-

ls, while the other nano-sized particulates are the solid solutionsr the products of the reinforcement compounds that are precipi-ated or separated from the matrix grains. When the dislocationsre encountered with the nano-particles, they can be locked orinned by them so that they cannot penetrate across the grains.hese dislocations can release the tensile residual stresses exist-

ng in the matrix and thus improve the strength of ceramics [17].hile when the cracks propagate to meet the nano-particles,

hey will be deflected or bowed dramatically. These processesan absorb an amount of fracture energy, which benefits signifi-antly for the improvement of mechanical properties especiallyhe flexural strength and fracture toughness of the micro-sizederamic composite materials. This can be verified by the exper-mental results in Table 1 and those results reported in theiteratures [9,10,17].

The formation of intragranular nano-structures plays anmportant role in the toughening and strengthening of ceramicomposites. As a matter of fact, the formation of the intra-ranular structures depends on both the migration speed ofhe matrix grain boundary Vm = BmFm and the migration speedf the second phase Vp = BpFp [18,19], where V, B and F

eans the migration speed, migration probability and migra-

ion driving force, the subscript m and p indicates that of theatrix and the second phase, respectively. When the migra-

ion speed of the second phase is less than that of the matrix,

C. Xu, D. Sun / Materials Science and Engineering A 491 (2008) 338–342 341

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Fig. 5. Dislocations in Al2O3

he intragranular structure will be formed. In the preparation ofl2O3/10%Cr3C2/20%(W, Ti)C ceramic, most of the intragran-

lar nano-structures are formed by Cr3C2 and (W, Ti)C, whileew of them are formed by alumina. The difference results fromhe low migration speed of Cr3C2 and (W, Ti)C since only smallmount of second phases are incorporated into the matrix which

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Cr3C2/20%(W,Ti)C ceramic.

eads to low migration probability. Therefore, some Cr3C2 andW, Ti)C particles in nanometer size are trapped into the alumina

atrix grains to form the intragranular nano-structures.After the formation of the intragranular nano-structures,

he interfaces between the intragranular nano-particles and theatrix grains are called sub-interfaces, while the other inter-

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42 C. Xu, D. Sun / Materials Science a

aces are called main interfaces. On the one hand, since thexistence of nano-particles inside the micrometer sized matrixrains, high residual stress will be formed at the sub-interfacehich can result in large amount of sub-grain boundaries andotential microcracks. The formation of sub-grain boundariesan fine the matrix grains and then improve the flexural strengthf the ceramic material according to the Hall–Petch equation18,20]. In fact, the formation of sub-grain boundaries or micro-racks can make the matrix grains be at a potential differentiationtate, i.e., the so-called “nano-sized effect”. On the other hand,he intragranular nano-particles can work with dislocations by

eans of the locking or pinning way to increase the fracturenergy and thus to improve the fracture toughness of the ceramicaterials.

. Conclusions

Although the intragranular nano-structures appear usuallyn nano-sized or nano-micro-sized ceramic composite materi-ls, several kinds of intragranular nano-structures have beenbserved in Al2O3 series micro-sized ceramic composites. Somef the nano-sized particles result from the fining effect of longime ball milling of raw ceramic powders, while the other nano-ized particulates are the solid solutions or the products of the

einforcement compounds that are precipitated or separated fromhe matrix grains. The mechanical properties of the tested micro-ized ceramic materials are obviously increased as a result of theormation of the intragranular nano-structures.

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ngineering A 491 (2008) 338–342

cknowledgements

This work was supported by the National Natural Scienceoundation of China (50405047), Natural Science foundation ofhandong Province (Y2005F04) and Jinan Young Star Plan ofcience and Technology (08108).

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