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Scripta Materialia 63 (2010) 185–188

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Interface structure and formation mechanism of BN/intergranularamorphous phase in pressureless sintered Si3N4/BN composites

Yongfeng Li,a,* Ping Liu,a Xiangdong Wang,b Haiyun Jin,c

Jianfeng Yanga and Guanjun Qiaoa,**

aState Key Laboratory for Mechanical Behavior of Materials, Xi’an Jiaotong University, Xi’an 710049, People’s Republic of ChinabSchool of Science, Xi’an Jiaotong University, Xi’an 710049, People’s Republic of China

cSchool of Electrical Engineering, Xi’an Jiaotong University, Xi’an 710049, People’s Republic of China

Received 8 January 2010; revised 12 March 2010; accepted 16 March 2010Available online 18 March 2010

The interface structures formed by the BN/intergranular amorphous phase and the Si3N4/intergranular amorphous phase inpressureless sintered Si3N4/hexagonal-BN composites doped with Y2O3 and Al2O3 were investigated by high-resolution transmis-sion electron microscopy. Compared with the direct Si3N4/glass bonding in the Si3N4/amorphous phase interface, an ordered tran-sition layer between hexagonal-BN and the glass phase, including a turbostratic-BN zone and an amorphous-BN zone, was foundfor the first time. In addition, a chemical reaction mechanism for the formation of BN/amorphous phase interfaces was suggested.� 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Keywords: Interface structure; Si3N4/hexagonal-BN composites; Pressureless sintering; High-resolution transmission electron microscopy

Both boron nitride and silicon nitride are wellknown as important ceramic materials for applicationsin many engineering fields because of their high-temper-ature resistance, chemical inertness and poor wettabilityby many glass and metal melts [1,2]. Such properties ascorrosion resistance to molten metal and thermal shockresistance could be greatly improved by introducing hex-agonal boron nitride (h-BN) in Si3N4 matrices [3,4]. Inaddition, the cleavage behavior of plate-like h-BN en-dowed Si3N4/h-BN composites with good machinabilityif the h-BN content was sufficiently high [5–8]. Si3N4/h-BN composites are usually hot pressed via a chemicalroute to homogeneously disperse fine h-BN particlesinto Si3N4 matrix [5–8] because of the poor sinterabilityof h-BN. Although various processing techniques [9–12]have been developed to fabricate Si3N4/BN compositesfor structural and functional applications, pressurelesssintering is still a desirable research goal.

Experimental evidence for the poor sinterability of h-BN comprises its strong covalent nature and plate-likestructure. Although it has been repeated in the litera-

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* Corresponding author. Tel.: +86 29 82665221; fax: +86 2982663453; e-mail: lyf6111@gmail.com

** Corresponding author. Tel.: +86 29 82665221; fax: +86 2982663453; e-mail: gjqiao@mail.xjtu.edu.cn

ture, the current understanding of the interface structurethat controls the sintering behavior and mechanicalproperties of pressureless sintered Si3N4/BN compositesis poor. Many efforts have been made to understand thestructure and chemistry of grain boundary amorphousfilms for many different Si3N4 materials [13–15]. Directatomic resolution images have been obtained to illus-trate how rare-earth atoms bond to the interface be-tween the intergranular phase and matrix grains inSi3N4 ceramics [14,15]. In contrast, much less informa-tion is known about the interfacial structure formedby BN grains and the intergranular amorphous phase(IAP) in pressureless sintered Si3N4/BN composites.

In this paper, Si3N4 ceramics with 25 vol.% h-BNwere fabricated by pressureless sintering with 10 wt.%Y2O3 and 5 wt.% Al2O3 as sintering additives. The inter-face structures of BN/IAP and Si3N4/IAP were investi-gated by high-resolution transmission electronmicroscopy (HR-TEM). The chemical reaction mecha-nism for the formation of BN/IAP interface was alsoinvestigated in detail.

a-Si3N4 powder (over 95% a-Si3N4 content and1.5 wt.% oxygen content, mean particle size 0.5 lm)and h-BN powder (98% purity, mean particle size1.5 lm), comprising 25 vol.% of the volume of a-Si3N4

and h-BN powders, were used as starting materials, with

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Figure 2. STEM, EDS, and HR-TEM images of pressureless sinteredSi3N4/BN composites. (a) STEM image of the BN/IAP interface; (b)the corresponding EDS patterns; (c) a typical HR-TEM image of theBN/IAP interface.

186 Y. Li et al. / Scripta Materialia 63 (2010) 185–188

10 wt.% Y2O3 (99.9% purity) and 5 wt.% Al2O3 (99.9%purity) as sintering additives. Wet mixing was performedin anhydrous alcohol for 12 h and the slurry was dried ina rotary evaporator, then sieved to 200 mesh and uni-axiallly pressed to form rectangular bars. The greencompacts were pressureless sintered in a graphite-heaterfurnace (high muti-5000, Fuijidenpa) at 1800 �C for 2 hin N2 gas.

Crystalline phases were identified by X-ray diffrac-tometry (XRD; Rigaku D/MAX-2400) with Cu Ka radi-ation. The fracture microstructures were characterizedby field emission scanning electron microscopy (FE-SEM; JEOL JSM-7000F). The morphology, lattice dis-tance and crystallographic structure were characterizedby HR-TEM using a JEOL JEM-2100F microscopeequipped with energy-dispersive spectroscopy (EDS)using the Cu-grid as the sample holder. The TEM spec-imens were prepared by cutting and grinding the sin-tered specimens to a plate with a thickness of 20 lm,then dimpling and ion beam milling.

Figure 1a shows the XRD pattern of Si3N4/BN com-posites with 25 vol.% h-BN, revealing that the sampleexhibits both b-Si3N4 and h-BN being the dominatephases. Figure 1b shows the FE-SEM micrograph ofSi3N4/BN composites. The FE-SEM image displayedthat Si3N4/BN composites exhibit the irregular shapewith h-BN particles randomly distributing among theb-Si3N4 grains.

Figure 2a and b shows a typical STEM image and thecorresponding EDS patterns of BN/IAP interface inSi3N4/BN composites, respectively. From Figure 2a, atransition layer with a width of about 20 nm was foundbetween h-BN and the glass phase. The EDS analysis(Fig. 2b) reveals that the transition layer consists of Si,O, N, B, Al and Y. The Si concentration decreases fromthe glass phase to the BN zone, and the same trend isobserved for O, Al and Y, whereas the N and B concen-trations tend to increase slowly.

Figure 2c shows a typical HR-TEM image of the BN/IAP interface in Si3N4/BN composites, revealing thatthe width of the transition layer is more than 20 nm,and there are many defects (such as Frank dislocation,stacking faults) in the transition layer. The BN grainsare significantly buckled and their stacking sequencesare disordered, similar to those of turbostratic-BN (t-BN).

It is well known that the crystal structure of b-Si3N4

is almost strain free, whereas the structure of a-Si3N4

contains considerable strain, expressed by lattice distor-tion and the displacement of atoms from the idealizedpositions [2]. Bond lengths in the a-Si3N4 structure arevariable – much more so than in the b structure [16].

Figure 1. XRD pattern and FE-SEM micrograph of pressurelesssintered Si3N4/BN composites.

At high temperature, the bond length of Si–N in a-Si3N4 structure is lengthened, some of the Si–N bondseven breaking because of atomic diffusion.

On the other hand, a-Si3N4 can accommodate oxygenwithin its crystal lattice [17,18]. The oxygen can exist asa surface oxide layer (of composition close to SiO2)[19,20] and also as internal oxygen. During the liquid-phase sintering process, the surface oxide layer of a-Si3N4 reacts with Y2O3, Al2O3 to form liquid silicate.At the same time, the internal oxygen diffuses from theinterior to the outside of the a-Si3N4. As diffusion accel-erates at higher temperatures, the inner strain of the a-Si3N4 is released by means of the breaking and reform-ing of Si–N bonds in each unit cell, resulting in the for-mation of a b-Si3N4 crystal nucleus. These b-Si3N4

nuclei travel through the liquid phase and self-assembleto form b-Si3N4 grains. This process is the so-called dis-solution–diffusion–precipitation mechanism, which iscontrolled by either the reaction or the diffusion.

For the Y2O3–Al2O3–SiO2–Si3N4 liquid phase inpressureless sintered Si3N4/BN composites, the wettabil-ity of the glass phase with Si3N4 grains is very good.From the HR-TEM image of the Si3N4/IAP interfacein Si3N4/BN composites in Figure 3a, it can be seen thatthe interfacial structure is very likely direct Si3N4/glassbonding, which is in agreement with previous findings[13–15].

For the pressureless sintering of Si3N4/BN compos-ites, the key point is not the densification of Si3N4 butthe densification of BN. BN ceramics are generally fab-ricated by hot pressing or hot isostatic pressing becauseof their poor sinterability. The densification mechanismof h-BN is usually described as a vitreous sintering pro-cess, and h-BN particles do not participate in the disso-lution–diffusion–precipitation process involved in thesintering of Si3N4 ceramics [1]. The partially ionic natureof B–N bonds [21] induces a net charge on the B and Natoms, which result in the electrophilicity of B atomsand the nucleophilicity of N atoms. The decompositionof BN is expected to take place preferably from the rimof BN shells by the addition of hydroxide ions or O-con-taining groups and protons to B and N atoms, respec-tively [22–24]. Based on this idea, much interest has

Figure 3. HR-TEM, FFT and TEM images of pressureless sinteredSi3N4/BN composites. (a) HR-TEM image of the Si3N4/IAP interface;(b) HR-TEM image of the BN/IAP interface with the c-axis of h-BNparallel to the glass phase (taken from area A marked in Fig. 2c); (c)HR-TEM image of the BN/IAP interface with the c-axis of h-BNperpendicular to the glass phase; (d) FFT image of (b); (e) FFT imageof (c); (f) TEM image of Si3N4/BN composites with intragranular andintergranular BN particles.

Y. Li et al. / Scripta Materialia 63 (2010) 185–188 187

been evinced in the functionalization and solubilizationof BN nanotubes (BNNTs) in the past few years [22–28].After functionalization via mild chemical routes,BNNTs can be introduced into homogeneous aqueousand organic solutions. When being hot pressed, h-BNalso gradually reacts with water, and the phase transi-tion of h-BN ? t-BN ? amorphous-BN (a-BN) can beobserved [29]. From these results, we envisage that h-BN would react with the glass phase at hightemperature.

Figure 3b (taken from area A marked in Fig. 2c) andd shows the HR-TEM and fast Fourier transform (FFT)images, respectively, of the BN/IAP interface in theSi3N4/BN composites. From Figure 3b it can be seenthat, when the c-axis of the h-BN crystal is exposed par-allel to the glass phase, there are two distinct zones atthe interface. At the zone near to the h-BN, the B–N lay-ers are stacked roughly parallel to each other along the(0 0 1) direction with some displacements, and randomrotations about the (0 0 1) direction can be observed;this is the t-BN zone. The HR-TEM image also displayssome Frank dislocations in the t-BN section (indicatedwith an arrow). At the zone near to the glass phase,the B and N atoms are arranged in a nearly randomway; this is the a-BN zone. BN particles were etchedto a large extent at the ends of their tips. The explosivetip-end morphology validates the remarkable reactivityof the open ends. The width of the a-BN zone is about4 nm (as shown in Fig. 3b), while the width of the t-BN zone is larger than 20 nm, as observed in Figure 2c.

The same trend was also observed at the interfacewith the c-axis of h-BN perpendicular to the glass phasein Figure 3c. Figure 3c and e shows the HR-TEM andFFT, respectively, images of a thick BN particle whoseouter shells (with a thickness of less than three basalplanes) have been peeled off. Some traces of the peeledlayers can be distinguished at the interfaces. The outershell of BN was peeled as verified by HR-TEM. Tinyprotruding segments of BN at the interfaces in Figure 3cwere found to stem from the etched BN and made novelY-junctions. The widths of the a-BN and the t-BN aremuch smaller than those of the interface with the c-axisof the h-BN parallel to the glass phase. For the strongin-plane covalent bonding in the direction of c-axis ofh-BN crystal, this type of interface could provide anideal protective structure to keep h-BN from furtherchemical corrosion by the glass phase. The anisotropyof the BN/glass interfacial structure could be observed,due to the anisotropic structure of h-BN.

The differences in the BN/glass interfaces resultedfrom the anisotropic distribution of defects along thedifferent directions of the h-BN particles: there are moredefects on the rim of h-BN particles with the c-axis ofthe h-BN parallel to the glass phase than on the surfacewith the c-axis of the h-BN perpendicular to the glassphase for its anisotropic layered structure. Because ofthe weak bonding between the atomic layers in the direc-tion of c-axis, more defects could be induced, amongwhich the mid-plane defects and the interstitials (e.g.O) would buckle the hexagonal planes [30]. The atomsat defect sites, especially at vacancies and open ends,have larger thermal vibration amplitudes than othersites because these edge atoms are only fixed by otheratoms on one side. Thus the glass phase with polarbonds of Al–O, Y–O and Si–O bonds has a more signif-icant effect on these atoms. At high temperature, BNlayers would deform, more B–O bonds would formand B–N bonds would break, and even large BN parti-cles would decompose into small particles at defect sites.

From Figure 3b–e it can also be observed that thereare many BN grains with nanometer dimensions in theglass zone (indicated with a circle). These nanosizedBN grains would improve the viscosity of glass phase,which would result in the suppression of mass transportand the inhibition of Si3N4 grain boundary migration.They could perhaps also restrain both the a ? b-Si3N4

phase transformation and grain growth. On the otherhand, except for the nanosized BN particles in rawmaterials, the nanosized BN grains could also act asthe heterogeneous nucleation sites of b-Si3N4 grains,which are linked by the newly formed B–O bonds. Theseintergranular BN particles would change into intragran-ular BN particles, whereas micron-sized BN particleswere dispersed at the grain boundaries of b-Si3N4 asintergranular BN particles, as shown in Figure 3f.

From the above results, it was found that BN particlesdo react with the glass phase at high temperature.Although the chemical reaction is thermodynamicallypossible, it has slow kinetics in BN particles as a resultof the strong B–N 2pp–2pp bonds that envelope the outersurface. The role of the glass phase is thus crucial toweaken B–N covalent bonds and to accelerate the chem-ical reaction. Based on our experimental observations, a

Figure 4. Molecular orbital representation of the attack of electro-philic and nucleophilic groups on BN particles followed by decompo-sition [23] (M = Y3+, Al3+ and Si4+).

188 Y. Li et al. / Scripta Materialia 63 (2010) 185–188

chemical reaction mechanism between BN particles andthe glass phase is proposed in Figure 4. In our case, nucle-ophilic O atoms come from the [AlO4] tetrahedron andthe [YO6] octahedron in the glass phase, the surface oxidelayer and the internal oxygen of a-Si3N4, whereas the elec-trophilic Y3+, Al3+ and Si4+ central ions would attack theN. In addition, the N atoms of the [SiN4] tetrahedron in a-Si3N4 could also attack B to form B–N coordinate-cova-lent bonds. With the nucleophilic attack of O on B and theelectrophilic attack of M (M = Y3+, Al3+ and Si4+) on N,the newly formed B–O and N–M coordinate-covalentbonds would weaken the B–N bonds in BN grains. TheB–N bonds would thus be lengthened and become de-formed, the BN structure would buckle and finally theB–N bonds would break, which would cause the largerBN particles to break down into small BN segments. Thisproposed chemical reaction mechanism agrees well withthe EDS patterns (in Fig. 2b) and the observed morphol-ogy of the BN/IAP interface (in Fig. 3). The smaller thesize of the BN particles, the higher their specific surfaceand the stronger the effects of the chemical reaction, thusnanosized BN particles would be good for the densifica-tion of Si3N4/BN composites, as verified by Kusunoseet al. [5,6,11].

In summary, the interface structures of BN/IAP andSi3N4/IAP in pressureless sintered Si3N4/h-BN compos-ites doped with Y2O3 and Al2O3 were investigated byHR-TEM. During the liquid-phase sintering process,with chemical reactions between Y2O3, Al2O3 and the sur-face oxide layer of a-Si3N4, the inner strain of the a-Si3N4

would be released by means of the breaking and reformingof Si–N bonds in each unit cell, resulting in the formationof a b-Si3N4 crystal nucleus. These b-Si3N4 nuclei wouldtravel through the liquid phase and self-assemble into b-Si3N4 grains. The structure of the Si3N4/IAP interface isvery likely direct Si3N4/glass bonding. With regard tothe BN/IAP system, with the nucleophilic attack of Oon B and the electrophilic attack of M (M = Y3+, Al3+

and Si4+) on N, BN particles could also react with theglass phase by forming B–O and N–M coordinate-cova-lent bonds. However, the strong covalent nature andplate-like structure of h-BN would keep the h-BN grainsfrom further chemical corrosion by the glass phase. Thusan ordered transition layer including two different BNzones – a t-BN zone and a-BN zone – was observed atthe BN/IAP interface. The smaller the size of the BN par-ticles, the stronger the effects of the chemical reaction,thus nanosized BN particles could be homogeneously dis-persed into the Si3N4 matrix. Such a chemical reactioncould promote the densification of Si3N4/BN nanocom-posites. It is thus possible that Si3N4/BN composites with

high strength and good machinability could be fabricatedby pressureless sintering.

This work was supported by National NaturalScience Foundation of China (No. 50772086) and Na-tional High-Tech R&D Program of China (863 Pro-gram) (No. 2007AA03 Z558).

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