Preparation of h-BN nano-film coated a-Si3N4 composite particles

by a chemical route

Jingguo Lia,b and Lian Gao*a

aState Key Lab on High Performance Ceramics and Superfine Microstructure, ShanghaiInstitute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, People’s Republic ofChina. E-mail: liangaoc@online.sh.cn

bInstitute of Materials Science and Engineering, Shandong University, Jinan 250061, People’sRepublic of China

Received 22nd October 2002, Accepted 8th January 2003

First published as an Advance Article on the web 29th January 2003

In this study, a-Si3N4 particles were successfully coated with h-BN nano-film by a novel chemical route.

Ammonium biborate was deposited onto the surface of silicon nitride particles to form NH4HB4O7?3H2O–

Si3N4 composite powders by the heterogeneous precipitation method in anhydrous ethanol solution. The

obtained composite powders were dried at room temperature, then nitrided in an ammonia flow at 900 uC and

subsequently crystallized at 1600 uC to obtain h-BN nano-film coated a-Si3N4 composite particles. The

prepared composite particles were characterized by IR, SAM, TEM, HRTEM, TG and DSC techniques. TEM

and HRTEM micrographs showed that a-Si3N4 particles were well coated by BN ultrathin film in thickness of

1 nm.

I. Introduction

Si3N4 ceramic is a promising high temperature structuralmaterial due to its excellent thermomechanical properties.However, this material is very difficult or even impossibleto mechanically form into complex shapes1 because of itsbrittleness and extremely high hardness. This disadvantageleads to high machining cost and hinders its broad applications.Cook et al.2 suggested that this problem can be overcome byintroducing weak interfaces to deflect growing cracks. Hexa-gonal boron nitride has excellent machinability due to itsplate-like structure similar to that of graphite. Moreover,hexagonal boron nitride has unique chemical and physicalproperties such as low density, high melting point, chemicalinertness, and high thermal conductivity. Therefore, it is widelyused as bulk material and thin films in electronic and ceramicapplications.3,4 The addition of hexagonal boron nitride(h-BN) into Si3N4 ceramics matrix has been used to improvethe machinability of Si3N4 ceramics. Kusunose et al.5,6 reportedthat Si3N4–BN nanocomposites showed good machinabilityand excellent mechanical properties at high temperatures.Various synthetic methods for preparation of h-BN such

as direct nitriding of boric acid with ammonia gas and thereaction with nitrogen compounds have been reported.5 Thefirst step of usual methods for preparation of Si3N4–BNcomposites is to obtain Si3N4–boric acid–urea slurry by millingSi3N4 powder with boric acid and urea in water or alcohols.Since boric acid is highly soluble in water and alcohol, theslurry is easy to be uniform. However, drying of the water oralcohol slurry resulted in segregation of the boric acid andformation of hard agglomerates, cemented together by theboric acid.7 To improve the homogeneity of the mixture,various synthetic methods have been developed. One syntheticroute that has recently attracted attention is the use of powdercoating technique. In ceramic processing, powder coatingtechnique is investigated mostly for improving the structuralhomogeneity of products.8–10 The improvement in homoge-neity results in an enhanced sintering of the green compactsand permits less additive for sintering materials to highdensity. Usually the BN films or coatings have been obtainedby chemical vapor deposition (CVD) or by pyrolysis of a

polymeric ceramic precursor.11–13 In spite of the fact thatCVD and pyrolysis of polymeric precursor have some advan-tages over other processes, they are associated with severaldisadvantages. For example, the precursors used in CVDprocesses and pyrolysis of polymeric precursor are toxic,corrosive and expensive.The present paper reports a novel method for preparation

of h-BN nano-film coated a-Si3N4 particles by a chemicalroute. The approach used in this work involves the produc-tion of composite particles with a core-shell structure. Suchcomposite particles can be used as building blocks to preparebulk composites.

II. Experimental procedure

Si3N4 powder (E-10 grade, Ube Industries, Tokyo, Japan) withmean particle size of 300 nm and specific surface area (BET)of 9.8 m2g21 was used as the core particle. Ammoniumbiborate hydrate (NH4HB4O7?3H2O) was selected as BNprecursor due to its solubility in water and it can provideinterior nitrogen source during its thermal decomposition.First, 10 g of Si3N4 powder was ball milled for 8 h usinganhydrous ethanol (C2H5OH) as the liquid medium to obtainsuspension. Then, the ammonium biborate saturated aqueoussolution containing 9.8 g NH4HB4O7?3H2O was dripped intothe vigorously stirred Si3N4 anhydrous ethanol suspensionat room temperature and the final C2H5OH–H2O molar ratiowas controlled to be 10 : 1. Ammonium biborate hydrate wasprecipitated on the surface of Si3N4 particles to form Si3N4–NH4HB4O7?3H2O composite particles due to its insolubilityin the anhydrous ethanol. After the separation from itsmother solution, Si3N4–NH4HB4O7?3H2O composite particleswere washed with anhydrous ethanol and then dried at roomtemperature. Thus 14.9 g of Si3N4–NH4HB4O7?3H2O wasobtained, one can calculate the content of NH4HB4O7?3H2O inweight is 32.89 wt%. The B content analyzed by the volume-trically neutralizing method is 6.52 wt%, which is near to thevalue of 6.23 wt% calculated from 67.11 wt% Si3N4–32.89 wt%NH4HB4O7?3H2O. Finally, Si3N4–NH4HB4O7?3H2O compo-site particles were put into quartz crucibles and were nitrided at

628 J. Mater. Chem., 2003, 13, 628–630 DOI: 10.1039/b210385k

This journal is # The Royal Society of Chemistry 2003

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900 uC for 8 h to obtain BN-a-Si3N4 composite particles in theflow of NH3 gas using a tube furnace. The flow rate of NH3 gaswas 1000 ml min21. The samples were taken out from the tubefurnace after cooling down to the room temperature in the flowof NH3 gas and then crystallized at 1600 uC for 2 h in theN2 gas.TG and DSC were recorded on a Netzsch STA-449C

apparatus. The sample was placed in the cell and then heated ina N2 flow at a rate of 3.3 uC min21. Infrared (IR) spectroscopicinvestigation was carried out on a Bruker Vector 22 spectro-meter using the KBr pellet method. BN nano-film coateda-Si3N4 composite particles were also characterized using aMicrolab-310F scanning auger microscope (SAM) usingindium particles as templates. The morphologies and micro-structures of composite particles were observed by usingtransmission electron microscopy (TEM) (JEM-200CX) andhigh resolution transmission electron microscopy (HRTEM)(JEM-2010).

III. Results and discussion

(1) TG and DSC analysis of Si3N4–NH4HB4O7?3H2O

TG and DSC techniques were used to analyze the thermalbehavior of the 67.11 wt% Si3N4–32.89 wt% NH4HB4O7?3H2Osample and the results are shown in Fig. 1. Three weight losssteps are found on the TG curve from 100–400 uC andcorresponding endothermic peaks are observed at 122.7 uC,187.1 uC and 330 uC, respectively, from its DSC curve. Thepure NH4HB4O7?3H2O powder is generally accompanied witha weight loss about 39.0 wt% during its complete decom-position. In the mixture 67.11 wt% Si3N4–32.89 wt%NH4HB4O7?3H2O, the theoretical value of the weight loss is12.8 wt%. The experimental weight loss is 12% in the tem-perature range from 100 to 400 uC, which is close to 12.8 wt%.The whole weight loss can be attributed to the desorption ofthe small amount of adsorbed water and ethanol, the loss ofhydration water and releasing of ammonia by the reaction:

NH4HB4O7?3H2O (s) A 2B2O3 (s)1 NH3 (g) 1 4H2O (g) (1)

The second weight loss stage of 2 wt% between 400 uC and900 uC is assigned to the desorption of residual ammonia, theevaporation of small amounts of B2O3 as well as the slowreaction of B2O3 with NH3 into BN. An endothermic peak near800 uC is found, which may be contributed to the formation ofthe small amount of BN. The shape of TG curve shown inFig. 1 is very similar to that of TG curve reported by Lopezet al.14

(2) IR studies and SAM analysis

Fig. 2 shows infrared (IR) spectrum of BN nano-film coateda-Si3N4 composite particles, were prepared by nitridation ofSi3N4–NH4HB4O7?3H2O at 900 uC for 8 h and subsequentlycrystallization at 1600 uC for 2 h. A strong peak at 1388 cm21

and a weaker band at 816 cm21 are observed, which isattributed to the B–N stretching vibrations15 and B–N–Bbending vibrations,16 respectively. The results of IR spectrumsuggest the existence of BN in the composite powder afternitridation and crystallization. The bands in the 1100–450 cm21

region are attributed to Si3N4,16 and the relative intensity of

1388 cm21 in our composite powder is much higher than thathave been reported,16 indicating the relatively high content ofBN. Based on the quantitatively chemical analysis, the B contentin composite powder before and after nitridation and crystal-lization is 6.52 wt% and 5.98 wt%, respectively. The 5.98 wt%corresponds to a composition of 86.3 wt% Si3N4–13.7 wt%BN in the h-BN coated a-Si3N4 composite powder. The slightdecrease of B content suggests the evaporation of a smallamount of B element occurred during the nitriding andcrystallizing process.SAM was employed to study the surface composition of BN

nano-film coated a-Si3N4 composite particles. Generally, theescape depth of Auger electrons is about 1 nm. So, SAMspectrum can be used to characterize the elemental compositionof the material surface. Fig. 3 shows the SAM spectrum ofBN nano-film coated a-Si3N4 composite particles over an areaof 5 mm 6 5 mm. The BN nano-film coated a-Si3N4 compositeparticles have a strong boron peak at about 171 eV and two

Fig. 1 TG and DSC curves of Si3N4–NH4HB4O7?3H2O composite,where the weight content of NH4HB4O7?3H2O is about 32.89 wt%.

Fig. 2 IR spectrum of BN nano-film coated Si3N4 composite particlesprepared by nitridation at 900 uC for 8 h and subsequent crystallizationat 1600 uC for 2 h.

Fig. 3 SAM spectrum of BN nano-film coated Si3N4 compositeparticles prepared by nitridation at 900 uC for 8 h and subsequentcrystallization at 1600 uC for 2 h.

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strong nitrogen peaks at about 362 eV and 380 eV. The strengthof the Si peak at about 80 eV is so weak that it can hardly beseen, which indicates that BN nano-film prevents Augerelectrons of Si from escaping from the surface of the compositeparticles and the surfaces of the a-Si3N4 particles weresuccessfully covered by BN.

(3) Morphology observation by TEM and HRTEM

Fig. 4 shows the representative TEM and HRTEM micro-graphs of h-BN coated a-Si3N4 particles. The lattice fringeshaving an interplanar spacing of 0.33–0.34 nm are observedfrom Fig. 4(b). This interplanar spacing value is basically equalto values reported for hexagonal boron nitride (h-BN), whichconfirms the existence of h-BN. Moreover, it can be observedthat the a-Si3N4 particles are well surrounded with h-BN thinfilm in thickness of about 1 nm. The thickness of BN thin filmcan be adjusted by ratio of Si3N4 powder to NH4HB4O7

saturated aqueous solution. As shown in Fig. 4, the as-prepared composite particles have a unique core-shell structureconsisting of a-Si3N4 particles (core) and h-BN nano-film(shell). The pure BN grains possibly exist in the compositepowder, however, none were observed by HRTEM. Based onthe experimental results and the preparation process ofcomposite particles, we assume typical formation processesof the core-shell structure as illustrated in Fig. 5.Ammonium biborate is transformed to boron oxide due to

decomposition and dehydration during the heating process asdescribed by eqn. (1). In addition, it also provides an interiorammonia source during its thermal decomposition, which is

also effective for its nitridation reaction. As the heatingtemperature increases, boron oxide becomes a vitreous layer onthe surface of a-Si3N4 particles at about 600 uC. This vitreoussurface layer can be slowly nitrided according to the followingreaction equation:

B2O3 (s) 1 2NH3 (g) A 2BN (s) 1 3H2O (g) (2)

The nitridation starts on the uppermost surface and then itproceeds to the interior of the vitreous layer. So, it is importantto have enough holding time to enable B2O3 to be nitridedcompletely. The nitrided powders were subjected to crystal-lization of BN into well-crystallized h-BN at 1600 uC for 2 h,the lattice fringes with an interplanar spacing of 0.33–0.34 nmare clearly observed as shown in Fig. 4(b). Unfortunately,X-ray diffraction (XRD) is not sharp enough to detect h-BNat such low dimensions (thickness of 1 nm). The h-BN coatedSi3N4 powder was not sintered at 1700 uC for 2 h, and thepossible transformation of h-BN into c-BN as well as thermalstability of the composite powder at higher temperature needfurther investigation.

IV. Conclusions

We have described a novel method for preparing BN nano-filmon the surface of Si3N4 particles and characterized compositeparticles with TEM, HRTEM, IR, SAM, TG and DSC. IRanalytical results confirm the formation of BN after nitridationand crystallization at a higher temperature. SAM investigationshows that the peak of Si at 80 eV basically disappears, whichsuggests the surface of Si3N4 particle was covered by BN thinfilm. TEM and HRTEM micrographs show that the surfaceof Si3N4 particles was well coated with h-BN nano-film at athickness of about 1 nm. This method is a much simpler andmore inexpensive technique than usual CVD processes andpyrolysis of polymeric precursor.

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Fig. 4 (a) TEM and (b) HRTEMmicrographs of BN nano-film coatedSi3N4 composite particles prepared by nitridation at 900 uC for 8 h andsubsequent crystallization at 1600 uC for 2 h.

Fig. 5 Schematic drawing of procedures for preparing BN nano-filmcoated a-Si3N4.

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