high purity boron nitride thin films prepared by the pdcs route

201 (2007) 7822–7828www.elsevier.com/locate/surfcoat

Surface & Coatings Technology

High purity boron nitride thin films prepared by the PDCs route

Hussein Termoss a, Bérangère Toury a,⁎, Arnaud Brioude a, Jacques Dazord a,Jacques Le Brusq b, Philippe Miele a,1

a Laboratoire des Multimatériaux et Interfaces UMR 5615-CNRS, Université Lyon 1, Université de Lyon, 69622 Villeurbanne Cedex, Franceb Laboratoire de Physique de la Matière Condensée et Nanostructures UMR 5586-CNRS, Université Lyon 1, Université de Lyon,

69622 Villeurbanne Cedex, France

Received 13 October 2006; accepted in revised form 8 March 2007Available online 16 March 2007

Abstract

Hexagonal boron nitride (h-BN) thin films (b10 μm) were successfully obtained on various substrates (graphite-standard and HOPG, quartzand SiC) using the preceramic polymer route. Thin films were formed using precursor solutions of poly(2,4,6-trimethylamino)borazine(polyMAB) as a source of BN. Various preparation conditions were used (i.e. solvent, precursor nature and concentration, substrate and depositionmethod) and their impact on final BN film quality measured. Surface morphology was observed by Scanning Electronic Microscopy (SEM) andAtomic Force Microscopy (AFM). Presence of BN material was confirmed by infrared and Raman spectroscopies and the structure observed byHigh Resolution Transmission Electronic Microscopy (HRTEM). The chemical composition of samples analyzed by X-ray PhotoelectronSpectroscopy (XPS) gives a B/N ratio close to 1. Boron nitride films were also prepared using borazine (B3N3H6) as precursor. Initial results arepresented and compared with those obtained from polyMAB solutions.© 2007 Elsevier B.V. All rights reserved.

Keywords: Boron nitride; Polymer Derived Ceramics; Dip-Coating; Ceramic thin films; Borazine

1. Introduction

Boron nitride is a very promising ceramic which exists in fourdifferent phases: cubic (c-BN), hexagonal (h-BN), wurtzite (w-BN) and turbostratic (t-BN), the two former being the mostimportant. Thin h-BN coatings find applications in diverse fields.They are used to modify the interface in fiber-reinforcedcomposites to improve fiber pullout and prevent interfacialreaction [1]. h-BN is also used as protective coating againsthumidity, oxidation or corrosion [2,3]. In addition, BN isemployed as an insulating film in various electrical structures[4]. Another application is the use of BN thin film for solidlubrification. In fact, the graphitic sheets present in the hexagonalpolymorph confer good lubrification properties to the ceramicwhich do not degrade in air even at high temperatures [5]. The BNcoatings can be deposited using different methods [6]. Literaturereports the use of sputtering [7], evaporation, ion beam, pulse

⁎ Corresponding author. Tel.: +33 472 43 36 12; fax: +33 472 44 06 18.E-mail address: [email protected] (B. Toury).

1 Also with the Institut Universitaire de France (IUF).

0257-8972/$ - see front matter © 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.surfcoat.2007.03.016

plasma [8] and more recently atomic layer deposition [9].However, themost commonly usedmethod remains the ChemicalVapor Deposition (CVD) or Infiltration (CVI) from BCl3–NH3

system [10]. Although the CVD technique offers an effectivepathway for depositing a uniform layer of ceramic on a variety ofsubstrates, it is associatedwith several disadvantages as well. Onemajor disadvantage is the impurity of the final ceramicwhichmaycontain, for example, an excess of boron or oxygen and/or B2O3

[5,6]. High cost, scale-up problems and need to use substrateswith simple geometry are other disadvantages inherent to thistechnique. Two alternative routes have been reported forgenerating BN coatings, i.e. carbon reduction of boric acidsolution in a nitrogen atmosphere [11,12] and polymer derivedceramics (PDCs) route. The PDCs method is i) easy to implementbecause it uses liquid state polymers and ii) versatile because ofthe large variety of available precursors [13]. Previous worksreport the use of different kinds of precursors, i.e. dibromoborane-dimethyl sulphide, (CH3)2S BHBr2 [14], tris(alkylamino)borane,B(NHR)3 [3,15] and borazine and derivatives [16–20] as a BNsource to obtain the coatings by a liquid-forming process. It isimportant to note that most of the above cited works were carried

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7823H. Termoss et al. / Surface & Coatings Technology 201 (2007) 7822–7828

out to coat fibres, oxide powders or lead to thick coatings (15 to100 μm) on planar substrates. Only one study addresses thin filmdeposition on dense substrate by spin coating [21]. But in thatcase, the film produced is boron rich with a B/N ratio of 0.75. Inthe present work, we report the preparation of nanocrystallizedBN films of high purity with a B/N ratio close to 1. The films areuniform and their thicknesses are less than 10 μm. Variation ofpreparation conditions (i.e. solvent, precursor nature andconcentration, kind of substrate, deposition method) influencesthe surfacemorphology and this is studied bySEMandAFM.Thecrystallinity of the resulting films is investigated by TEM and (IR,Raman) spectroscopies. The chemical composition is studied byXPS.

2. Experimental procedure

All experiments are performed in a pure argon atmosphere andanhydrous conditions using standard vacuum-line and Schlenktechniques. Coatings are performed in a glove box purged withargon.

2.1. Sample preparation

2,4,6-tri(methylamino)borazine (MAB), [(CH3)(H)N]3B3N3H3 is prepared by reaction of an excess of methylamineand 2,4,6-trichloroborazine, itself synthesized from BCl3 andNH4Cl [22]. Synthesis procedures have already been describedin the literature [23]. Poly[2,4,6-tri(methylamino)borazine](pMAB) is prepared by thermal polycondensation of MABunder an argon atmosphere as previously published [24].Chemical and structural data obtained on these compoundscorrespond to the reported data. Borazine, B3N3H6 wasobtained by reaction of NaBH4 with (NH4)2(SO4) in tetraglymefollowing a procedure previously published [25].

MAB and polyMAB are solubilized in freshly distillateddichloromethane or toluene (5, 10 or 20% by weight) whereasborazine was used pure. Precursor solutions are then dip-coatedon different substrates: graphite (cx2120 UP-20 from CARBO-NIX®), silicon carbide (SiC-6H), Highly Ordered PyroliticGraphite (HOPG) and quartz. All the substrates are dehydratedunder vacuum prior to use by heating at 500 °C for 5 h. Graphitesubstrate surface is polished. Samples coated with MAB orpolyMAB solutions are first heated in a tubular furnace withammonia according to the following procedures: from 20 to45 °C (20 °C/h) with a first dwell time of 1 h and from 45 to1000 °C (60 °C/h) with a final dwell time of 1 h. For this step,dichloromethane is used as solvent. From 20 to 115 °C (20 °C/h)with a first dwell time of 1 h, and from 115 to 1000 °C (60 °C/h)with a final dwell time of 1 h, toluene is used as solvent. Allsamples (except quartz) are treated at a final high temperature of1800 °C (heating rate 100 °C/h) under nitrogen conditions.

For the borazine derived samples, a different process is used:heating under argon from −15 to 100 °C (heating rate 30 °C/h)in situ in the experimental equipment then transferred to thefurnace under nitrogen from 20 to 1000 °C (heating rate 60 °C/h)with a dwell time of 1 h, increased to 1400 °C (heating rate100 °C/h) with a final dwell time of 1 h.

2.2. Instrumentation

Scanning electron microscopy is performed with a HitachiS800 machine operating at 150 kV. Transmission electronmicrographs are obtained with a Topcon (model EMB-002B)microscope operating at 200 kV. IR spectra are recorded with aFTIR Nicolet Magna 550 spectrometer using KBr pellets. TheRaman scattering measurements are collected with an XY Dilortriple spectrometer followed by Nitrogen cooled CCD (ChargeCoupled Device) multichannel detector.

The coating surface morphology is analyzed with anatomic-force microscope (Molecular Imaging, Picoscan 5), incontact mode. XPS data are recorded using a NANOSCAN100 (Cameca-Riber) apparatus with AlKα X-ray line at1486.6 eV.

3. Results and discussion

The PDCs route offers great advantages over the chemicalvapor deposition technique for the preparation of ceramic withoriginal shapes. Firstly, the starting precursors are liquids andeasily handleable compared to volatiles. Secondly, thistechnique can be defined as a low cost technique; it does notrequire expensive precursors and apparatus. Initially, we choseto use the poly(2,4,6-trimethylamino)borazine (polyMAB)known for its high processability and its shaping ability [26].The polymer is obtained after thermal polycondensation of2,4,6-tri(methylamino)borazine (MAB) at 170 °C [24]. Ele-mental analysis of the polymer gives a chemical formula ofBN1.52C0.68H3.10. The presence of carbon reflects the relativelylow ceramic yield (53.1% at 1000 °C under ammonia), but it isessential to assure good solubilization properties of the polymerin organic solvents. MAB has also been used as a coatingprecursor, and although it presents a ceramic yield lower thanpolyMAB (43%), it is soluble in most of the organic solvents.Precursors are then used in toluene or dichloromethane solutionswith a mass concentration of 5%, 10% and 20% (w/w). Onedroplet of each solution is deposited on high quality graphitesubstrates (200 mm2). Each sample is then heated at 1000 °Cunder ammonia. Ammonolysis is needed for polymer cross-linking and the removal of carbon contaminants. A slowheating rate is required (i.e. 20 °C/h) from ambienttemperature to solvent boiling temperature in order to avoidbubble formation resulting from the solvent evolving duringthermal treatment. If these heating conditions are followed,the surface morphology of the coatings is comparablewhatever the solvent used. Considering its higher boilingtemperature, toluene is more efficient than dichloromethane inthe preparation of smooth and regular coatings. Afterammonolysis, samples are heat treated at higher temperature(1800 °C) under nitrogen for the final crystallization step.

The resulting heat-treated specimens show the formation of athin and transparent layer (lightly white colored) of boron nitride.The film thickness estimated after pyrolysis varied with theprecursor concentration and the amount of solution retained onthe substrate after coating. With a solution of 20% (w/w)polyMAB in toluene, the resulting film thickness can reach 4–

Fig. 1. SEM pictures of samples obtained with polyMAB solution in toluene a) 20% (w/w); b) 10% (w/w) and c) 5% (w/w).

7824 H. Termoss et al. / Surface & Coatings Technology 201 (2007) 7822–7828

5 μm and the coating is cracked due to the intrinsic shrinkageassociatedwith pyrolysis (Fig. 1a). This phenomenonwas alreadyobserved by Kim et al. on BN films over 2 μm in thickness [18].With lower concentrations: 10% and 5% w/w (respectivelyFig. 1b and c), most of the films are crack-free, adherent anddifficult to scrape off. The thickness of these films isestimated close to 1 μm. Coatings obtained with polyMABas precursor generally present smooth surface morphology onwhich the visible defects come from the substrate. In thespecial case of relatively dense graphite substrate (1.85 gcm−3) with low porosity (7%), the small flaws observed on itssurface seem to be difficult to fill up.

Two deposit methods were employed and compared on theabove mentioned type of graphite substrate. In Fig. 2a and b, weshow SEM images of coatings realized by dip-coating and bysimple droplet addition, respectively. Comparison of theseimages with the naked substrate (Fig. 2c) shows that the dip-

Fig. 2. SEM pictures of a) sample obtained by dip-coating; b) sam

coating method allows reproduction of the substrate surfacestate with a homogeneous and single-colored deposit. On thecontrary, the two-tone zones which appear in the case of thesimple droplet deposit technique confirm an irregular and roughsurface.

As we have previously seen, the use of standard graphitesubstrate does not always guarantee a defect free coating. Indeed,another series of coatings was carried out on different types ofsubstrate (Quartz, HOPG and SiC) and coating morphologieswere compared via SEM observations. Typical images of eachsample prepared from polyMAB precursor (10% w/w) arepresented in Fig. 3 and they clearly show different surfacemorphologies. In the cases of the quartz (Fig. 3a) and HighlyOrdered Pyrolytic Graphite (Fig. 3b) substrates, high qualitydeposits with large crack-free areas are observed. As mentionedabove for the standard graphite substrate (Fig. 3c), the coatingseems to reproduce the initial surface state of the substrate before

ple obtained by simple droplet deposit and c) naked substrate.

Fig. 3. SEM pictures of samples obtained with polyMAB solution in toluene (10% w/w) deposited on a) quartz; b) HOPG; c) standard dense graphite and d) SiC.


the deposit. The surface corresponding to layer deposits on theSiC substrate (Fig. 3d) is totally crackedwith no smooth area. Thisseries of results is not surprising considering the roughness of eachsubstrate. It is well-known that the roughness of quartz andHOPGsurfaces can achieve the atomic scale whereas graphite and SiCsubstrates are no more than a few nanometers. The formation ofcracks also depends on the thickness of the film deposited. As aconsequence, the use of SiC and graphite substrates imposes alimitation of the deposit thickness. Moreover, the use of quartz islimited by its low thermal stability (1000 °C), which does notallow the full crystallization of the ceramic layer.

In order to study the roughness of those high quality depositedfilms, Atomic Force Microscopy (AFM) is performed on severalsamples. AFM images of the substrate (standard graphite) and ofthe sample prepared from polyMAB (10%w/w) deposited on thesame kind of substrate are shown in Fig. 4a and b,respectively. The presence of a homogenous and regularcoating is shown in Fig. 4b. The fine-grained morphology ofthe film is confirmed by the average grain size of around

500 nm and the average roughness just a few tens ofnanometers. It is important to note that the layer uniformityallows sample surface observation unlike previous works inwhich no AFM images are presented.

Presence of boron nitride ceramic was confirmed by infraredand Raman spectroscopies. In Fig. 5a, a characteristic BN FT-IRspectrum is shown, and the sample was obtained from BNpowder scratched from the coating. The strong band at1379.8 cm−1 and the weak one at 803.0 cm−1 correspond toB–N stretching and B–N–B bending vibration modes,respectively [12]. Fig. 5b shows a typical Raman spectrumobtained on a ~8 μm thick BN coating, the unique signalappearing at 1366 cm−1 represents the E2g mode peak of BN[27]. The Full Width at Half Maximum (FWHM) of this peak is13 cm−1 and the distinct Raman shift compared to hexagonalpowder signal confirms the graphitic structure of the films [28].Moreover the value of the FWHM is comparable with the oneobserved in the case of h-BN microcrystals [29], assuring thegood crystallinity of the sample.

Fig. 4. AFM images obtained on a) standard graphite substrate and b) a typical BN coating.


This point was confirmed by the observations of the coatingmicrostructure by High Resolution Transmission ElectronicMicroscopy (HRTEM). A representative HRTEM micrograph(Fig. 6) shows the (002) lattice fringes of the well-knownhexagonal phase of BN. The orientation of the planes isdiscontinuous and no preferred direction is observed. Besidescrystalline areas with resolved planes characteristic of BN witha hexagonal structure, small areas of turbostractic BN can alsobe observed. In t-BN, the staking of the sp2 layers are randomand are rotated randomly along the c axis [30]. Indeed, HRTEMobservations clearly show the successful achievement of amixture of h-BN and t-BN. t-BN is currently produced frompMAB precursors [31], and these observations are confirmed bythe electron diffraction pattern performed on the sample (insert

Fig. 5. a) FT-IR and b) Raman spectra of a typical coating.

of Fig. 6). The several thin rings observed corresponding to(002), (100) and (004) planes show that the films are well-crystallized with particles randomly distributed.

Finally, the chemical bonding within a specimen wasinvestigated by XPS as shown in the general survey (Fig. 7a).Photoelectron peaks from B1s, N1s, O1s and C1s are clearlyrecognized in the spectrum. The intense B1s component at190.1 eV (Fig. 7b) corresponds to an atomic circle surroundingthe boron atom consisting of only nitrogen atoms, similar to thatoccurring in pure h-BN (theoretical value: 190.3 eV [32]). Thisenergy is higher than the energy of B–C bonds. Literaturereports the peak bending energies of B–C bonds in B4C andB3C at 188.4 [33] and 189.5 eV [34] respectively. Nitrogenatoms related to B-N bonds are characterized by the main peakat 397.5 eV for the N1s (Fig. 7c). This value is very close to thatgiven for BN in literature (397.9 eV [32]). Shifts observed forboth values (B1s and N1s) can be explained by the charge effect,as BN displays an isolating behavior. Furthermore, theasymmetry of the N1s peak (around 400 eV) supports thishypothesis. The chemical composition calculated by thedeconvolution procedure gives the following formula:B1.00N1.03C0.20O0.06. This result points to the incorporation ofa very weak amount of oxygen in the material due to BNxOy

species. This may be due to the high reactivity of the precursor

Fig. 6. High-Resolution TEM micrograph of BN film and Electron DiffractionPattern of the sample.

Fig. 7. XPS of BN film prepared from polyMAB precursor and coated on standard graphite substrate; a) general survey, b) B1s peak and c) N1s peak.


towards oxygen [35]. However, oxygen percentage remains lowand the B/N ratio is close to 1, in agreement with the formationof hexagonal BN, such as with a slight excess of nitrogen. Thecarbon signal can arise from different sources such ascontamination, substrate or impurities in the material (due tothe high C concentration in the starting precursor). However, the

Fig. 8. SEM pictures of samples obtained with pu

asymmetric shape of the peak C1s is typical of carbon pollution[35] and this feature is emphasized by a recent study led by ourlaboratory on fibers prepared with the same starting precursorand similar heating treatments [36]. In this study, only twosamples obtained at 1000 °C and 1500 °C were analyzed due tothe impossibility of performing elemental analyses on BN

re borazine; a) surface and b) cross section.


ceramics owing to its refractory behavior. The results obtainedfor the 1000 °C and 1500 °C samples are B1.00N0.97C0.07H0.23

and B1.00N0.94, respectively. Based on these compositions, it is,therefore, reasonable to consider that the sample prepared at1800 °C is free of carbon and hydrogen.

To expand this study, we led another series of experimentsusing pure borazine B3N3H6 as precursor; the ceramic yieldbeing very high (about 92.6%). In previous works, BN coatingswere obtained from polyborazylene (borazine derivativepolymer) as precursor. Moreover, the polymer is solid andneeds to be solubilized in THF (10 to 30% w/w) to be coated[18,19]. In our laboratory, we use pure borazine monomer liquidto impregnate the graphite substrate. In order to restrict the highvolatility of the compound, the substrate is cooled to −15 °Cprior to the monomer coating and the latter is polymerized insitu on the substrate from low temperature (−15 °C) to 100 °C(heating rate 0.5 °C/min). The sample was then placed in atubular furnace and heated up to 1400 °C, under a nitrogen flow.Examination of the specimens shows a white layer of BN up to amaximum of 5 μm thick. Smooth, homogeneous and well-covered coatings were successfully obtained. In spite of the5 μm thickness, no crack is observed (Fig 8a). Furthermore, incomparison with coatings obtained from polyMAB, originalgraphite flaws have been successfully filled up giving a smoothsurface. The important texturing seen on the surface is linked tothe high degree of ceramic crystallization. The SEM pictureobtained of the cross section (Fig. 8b) shows a tightly adherentcoating in which BN layers are clearly visible, confirming thatthe ceramic is well crystallized.

4. Conclusions

This paper presented the successful preparation of severalboron nitride thin films from polyMAB precursor in solution.Convincing results have been obtained on very smoothsubstrates like High Ordered Pyrolytic Graphite or quartzusing precursor in 10% by weight solution in toluene. Filmsobtained using this method present a thickness of around 1 μmwith no visible surface defects. The ceramic structural andchemical analyses have shown the formation of stoichiometricBN with a mixture of hexagonal and turbostractic structures.Moreover, initial experiments carried out with pure borazine asprecursor have led to high quality BN coating of around 5 μmthickness, and these results represent an interesting feature forsolid lubrification.

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high purity boron nitride thin films prepared by the pdcs route

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