Materials Science and Engineering A 396 (2005) 352–359

An investigation of a thin metal film covering on HPHTas-grown diamond from Fe–Ni–C system

Bin Xu a, ∗, Mu-sen Lib, Jian-jun Cuib, Jian-hong Gongb, Shu-hua Wanga

a Department of Materials Science and Engineering, Shandong Institute of Architecture and Engineering,47 He Ping Road, Jinan, Shandong 250014, PR China

b College of Materials Science and Engineering, Shandong University, 73 Jing Shi Road, Jinan, Shandong 250061, PR China

Received 8 October 2004; received in revised form 25 January 2005; accepted 3 February 2005

Abstract

Scanning Raman spectroscopy, scanning auger microprobe, electron probe microanalysis, X-ray powder diffraction and transmissionelectron microscopy were used to investigate a thin metal film covering on HPHT as-grown diamond from Fe–Ni–C system. In differentregions of the film, the fine structure of carbon and iron atoms are different. It is clear that the microstructure and composition in the innerp have beeng formations atg ted to thed h at hightP

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art of the film near diamond are different from those in other regions of the film. The fine structures of carbon and iron atomsreatly transformed. Crystalline graphite and amorphous carbon were not found in the inner part near diamond. The diamondhould be directly associated with carbide. There exist parallel relationships between the crystal faces of (Fe,Ni) and Fe3C. We suggest thraphite could not be directly catalyzed into diamond structure in the molten film, and the carbon for diamond growth may be relaecomposition of Fe3C under the effect of (Fe,Ni) phases in the inner part, which can play an important role under diamond growt

emperature and high pressure.ublished by Elsevier B.V.

eywords:High temperature and high pressure; Diamond; Graphite; Thin metal film covering on diamond; Catalysis

. Introduction

Synthetic diamond can be made by a variety of methods,ut the static pressure technique that graphite discs are placedlternatively with catalyst discs in a cell assembly is mostlysed in industry. One of the most important characteristicsnd basic phenomena during diamond growth is that therelways exists a metal film covering on a growing diamond,

solating the diamond from graphite, through which graphitetructure is broken and makes a transition to diamond at highemperature and high pressure (HPHT)[1–3]. In addition,he structures and morphologies of the nanoscale particlesound in diamond single crystals are closely associated withhose of the film at room temperature and atmospheric pres-ure[4], indicating that the microstructures of the film shoulde closely linked with diamond growth. Therefore, the study

∗ Corresponding author. Tel.: +86 531 5282887; fax: +86 531 6367282.E-mail address:xubin@sdai.edu.cn (B. Xu).

on the film may be of great significance to explain diamgrowth mechanism. However, probably because of thetreme difficulty for sample preparation of the film, few invtigations on the thin film have been reported so far, especfor the film formed using an iron-based alloy as a cataCompared with traditional nickel-based alloy catalyst,iron-based alloy catalyst accords with the criterion forlecting suitable catalyst in high dissolution ability of carbin the catalyst[3]. Therefore, the study and developmenthe iron-based alloy catalyst should be enhanced.

It is almost impossible to in situ study the molten filmHPHT, but much information of diamond growth at HPcan be remained in the film at room temperature and ampressure when the cell assembly is cooling rapidly afteishing the synthetic process. According to our previous sit can be proposed that graphite cannot directly transformdiamond at HPHT, but it may be catalyzed step by step ifilm mainly due to the fact that diamond has been not foin the metal film up to now[5]. Thereby, it is necessary

921-5093/$ – see front matter. Published by Elsevier B.V.

oi:10.1016/j.msea.2005.02.005

B. Xu et al. / Materials Science and Engineering A 396 (2005) 352–359 353

investigate the film divided into the different regions. Thepresent paper reports about the results obtained from the dif-ferent regions in the thin metal film from Fe–Ni–C system forthe first time. The function of the film on diamond formationand growth is analyzed briefly. This study would be helpfulto obtain high-quality diamond single crystals.

2. Experimental

Diamond single crystals were synthesized in a cubic anvilapparatus under a high temperature of approximately 1300◦Cand a high pressure of 5.3 GPa using graphite as the carbonsource and Fe75Ni25 alloy manufactured by powder metal-lurgical technology as the molten catalyst. As starting mate-rials, graphite discs (purity 99.9%) were placed alternativelywith the alloy discs, forming a cell assembly. After keepingthe cell assembly at temperature of 1300◦C for 8 min, thetemperature was decreased rapidly by turning off the electricpower, and then the pressure was unloaded. After this, thecell assembly was taken out from the cubic anvil apparatusfor quenching. Finally, the metal films at room temperatureand atmospheric pressure from Fe–Ni–C system could beobtained.

In this experiment, the cross-sections of the films were ex-a cope( ), aP d aJ A),r omt cro-s rgoni onu sa ope(

3. Results and discussion

3.1. Raman spectra of the thin metal film

Fig. 1a shows a pit left by an as-grown diamond singlecrystal on graphite matrix, and the microstructure surround-ing the pit is that of the cross-sectional fracture of the thinmetal film. A magnified image corresponding toFig. 1a canbe seen fromFig. 1b, in which four points indicate four micro-regions on the cross-section of the film, respectively.Fig. 2illustrates Raman spectra on the four micro-regions as shownin Fig. 1b. According to the literature[6,7], the Raman bandsin diamond, graphite crystal and amorphous carbon have beencharacterized as 1332, 1581 and 1355 cm−1, respectively. Soit can be known that there exist graphite crystal (1581 cm−1)and amorphous carbon (1357 cm−1) in regions 2–4 in the film(Fig. 2b and d). From regions 4 to 2 in the film, the contentof graphite decreases gradually. An interesting phenomenonis that there are no pure carbon structures such as graphitecrystal and amorphous carbon in one region adjacent to theas-grown diamond, as shown inFig. 2a. This is related tothe formation of diamond and will be studied and discussedfurther below.

3.2. Auger differential coefficient spectra of carbon andi

thet thes hapesc sen-t thed es ofc witht theg r

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mined using a JAX-840 type scanning electron microsSEM), a Rerinishaw RM2000 type of Raman (RamanHI-610 type of scanning auger microprobe (SAM) anXA-8800R type of electron probe microanalysis (EPMespectively. The films, which were carefully taken out frhe graphite matrix under observation of an optic micope, were ground into powders and thinned by an aon-milling in turn, until they were suitable for examinatising a D/max-� C type X-ray diffraction (XRD) apparatund a Philips CM-30 type transmission electron microscTEM).

ig. 1. (a) SEM image of a pit left by an as-grown diamond. (b) A magetal film (1, 2, 3 and 4 indicate four micro-regions in the film).

ron atoms of the thin metal film

If there is oxygen or other impurities in a sample,ransition energy of Auger electrons of carbon atoms inample should be changed, but their Auger spectrum sould not be evidently changed, especially their repreative spectra[8]. Auger spectra can be used to defineifferent structures of carbon. So, Auger spectrum shaparbon atoms in the thin metal film can be comparedhe structure of carbon in the as-grown diamond and inraphite matrix from the cell assembly.Fig. 3 shows Auge

SEM image corresponding to (a) showing the cross-sectional fracture

354 B. Xu et al. / Materials Science and Engineering A 396 (2005) 352–359

Fig. 2. Raman spectra of the thin metal film (a, b, c and d indicate 1, 2, 3 and 4 micro-regions shown inFig. 1b, respectively).

differential coefficient spectra of carbon atoms for the dia-mond (Fig. 3a), four micro-regions in the film (Fig. 3b–e)and the graphite (Fig. 3f). Fig. 3a represents transitions ofAuger electrons of carbon atoms at 251.0 and 272.0 eV fordiamond, andFig. 3f is representative Auger spectrum fortransitions of Auger electrons of carbon atoms at 261.5 and282.5 eV of the graphite, which are different from those ofdiamond and pure graphite crystal, respectively[9], maybebecause of the electrical conductivity of the sample, i.e., thecell assembly.

Among the four (regions 1–4) micro-regions in the film,two representative transitions of Auger electrons of carbonatoms are 251.0 and 276.5 eV for region 1 (Fig. 3b), 254.0and 276.5 eV for region 2 (Fig. 3c), 246.5 and 269.0 eV forregion 3 (Fig. 3d), as well as 258.5 and 276.5 eV for re-gion 4 (Fig. 3e). Compared with those of diamond, onlythe transitions 251.0 and 276.5 eV for the inner part (re-gion 1) are approximate to those of diamond (251.0 and272.0 eV). This fact indicates that only in the inner part,the fine structures of carbon bear a close relationship withthose of the diamond. Compared with Auger spectrum shapesshown inFig. 3c–e, the Auger spectrum shape inFig. 3bhas been greatly changed, which suggests that the fine struc-tures of carbon in the inner part have greatly been trans-formed.

Fig. 4shows Auger differential coefficient spectra of ironatoms on four micro-regions in the film. From the Augerspectra shown inFig. 4b–d, there is only a little shift amongtheir shapes. However, if we checkFig. 4b–d againstFig. 4a,an obvious change in spectrum shape can be observed, whichindicates that the fine structures of iron atoms in the inner parthave greatly been transformed too, corresponding to that ofcarbon atoms in the region.

3.3. Composition and microstructures of the thin metalfilm

Fig. 5 demonstrates composition distributions over thewhole film using EPMA method, which may also illustratethat the microstructures in the inner part are different fromthose of other regions in the film, eutectic structure topog-raphy in the middle region, and some un-solvent graphiteflakes in the region near graphite. In particular, there existlower carbon and more iron compositions in the inner partnear diamond than those in other regions of the film.

X-ray powder diffraction was used to study the phasestructures of the thin metal film. As a result, the whole filmcomposes of five phases, that is, graphite, Fe3C, (Fe,Ni)23C6,(Fe,Ni)fcc and (Fe,Ni)pc, as given inFig. 6. (Fe,Ni)fcc and(Fe,Ni)pc refer to face centered cubic (Fe,Ni) and primitive

B. Xu et al. / Materials Science and Engineering A 396 (2005) 352–359 355

Fig. 3. Auger differential coefficient spectra of carbon atoms for the as-gr , 3 and 4micro-regions shown inFig. 1b, respectively) and for the graphite matrix (f).

cubic (Fe,Ni), respectively. Because the films were takenfrom the graphite matrix, a mass of graphite adhered to thesurface on the powders of the film, although the powderswere disposed by a lift-off process using a magnet, whichwould result in the higher beam peak for graphite crystal(2θ = 26.5200◦) in Fig. 6.

partn ultss c-t( -d s, as

own diamond (a), for the thin metal film (b, c, d and e indicate 1, 2

Because of an obvious difference between the innerear diamond and other regions in the film from the reshown inFigs. 2–5, it was worth identifying phase struures of the inner part. In the region, Fe3C, (Fe,Ni)23C6,Fe,Ni)fcc and (Fe,Ni)pc were identified using TEM with inexing selected area electron diffraction (SAD) pattern

356 B. Xu et al. / Materials Science and Engineering A 396 (2005) 352–359

Fig. 4. Auger differential coefficient spectra of iron atoms for the thin metal film (a, b, c and d indicate 1, 2, 3 and 4 micro-regions shown inFig. 1b, respectively).

Fig. 5. Carbon, iron and nickel distributions in a cross-section of the thin metal film and a backscattered electron pattern (the up-left).

B. Xu et al. / Materials Science and Engineering A 396 (2005) 352–359 357

Fig. 6. X-ray powder diffraction diagram of the whole metal film.

shown inFigs. 7–9. However, graphite crystal and amor-phous carbon were not observed, which is in agreement withthe Raman results inFig. 2. In addition, a significant thingis that there exist the orientation relationships between thecrystal faces of (Fe,Ni)fcc and Fe3C, as well as (Fe,Ni)pcand Fe3C in the region, i.e., (1̄1 1)(Fe,Ni)fcc

//(1 0 0)Fe3C and(2̄ 2 0)(Fe,Ni)pc

//(0 2 0)Fe3C according to corresponding SAD

patterns inFigs. 8b and 9b. It is also necessary to iden-tify the phase structures in the other regions of the film,and to look for the orientation relationships found in the in-ner part. The samples suitable for TEM observation couldbe obtained by changing the direction from an argon ion-milling. As a result, all of five phases found by XRD werealso discovered in other regions in the film, but the paral-lel relationships between the crystal faces of (Fe,Ni)fcc andFe3C, or (Fe,Ni)pc and Fe3C mentioned above have been notfound.

3.4. Brief analyses for the catalysis of the thin metal filmfor diamond growth

It should be particularly elucidated that because the syn-thetic temperature and pressure for super quality diamond isonly within a narrow “V” range[3,10,11], there must existsolid structures in short-range order in the molten film withinthe range[12]. From the evidences mentioned above, thereis an obvious difference between the inner part and other re-gions in the film. Raman and TEM did not find graphite andamorphous carbon in the inner part. Furthermore, only in theinner part exist the parallel relationships between the crystalfaces of (Fe,Ni) and Fe3C, as illustrated inFigs. 8b and 9b.Therefore, the catalytic process of the thin metal film for

Fig. 7. (a) TEM image of (Fe,Ni)23C6 phase in the inner part of the film.

FF

ig. 8. (a) TEM image of (Fe,Ni)fcc and Fe3C in the inner part of the film. (b) Coe3C [01̄ 2], indicating (1̄1 1̄)(Fe,Ni)fcc

//(1 0 0)Fe3C.

(b) Corresponding SAD pattern from [1 2 5] zone axis of (Fe,Ni)23C6.

rresponding SAD pattern from a multiple zone axis of (Fe,Ni)fcc [2 1 1] and

358 B. Xu et al. / Materials Science and Engineering A 396 (2005) 352–359

Fig. 9. (a) TEM image of (Fe,Ni)pc and Fe3C in the inner part of the film. (b) Corresponding SAD pattern from a multiple zone axis of (Fe,Ni)pc [1 1 1] andFe3C [4 01̄], indicating (̄2 2 0)(Fe,Ni)pc

//(0 2 0)Fe3C.

diamond formation at HPHT can be understood as follows.The graphite structure in the molten film has changed dur-ing carbon diffusion from graphite matrix to growing dia-mond. However, it is not directly catalyzed into diamondstructure in the film. Furthermore, it is not possible that thecarbon sources for diamond growth come from the pure car-bon structures, but from the other phases such as iron car-bides or sosoloids in the inner part, because only iron car-bides and sosoloids were found in the region. A possiblemode of catalyzing graphite to diamond is closely relatedto the Fe3C decomposition because of more carbon in Fe3Cthan that in (Fe,Ni)23C6 or (Fe,Ni) phase. The second ev-idence to support the standpoint mentioned above is lowercarbon and more iron content in the inner part than those inother regions of the film (Fig. 5), which would result fromthe decomposition of Fe3C in the inner part at a momentunder HPHT. From the present studies, Fe3C or metastablecarbide decomposition could control diamond formation iniron-containing alloy solvent[13], and the stability of carbideFe3C will decrease with nickel addition[11]. In addition, non-variant reactions of the eutectic type L(Fe,Ni)fcc + Fe3C + Dand L (Fe,Ni)fcc + Fe3C take place at HPHT from Fe–Ni–Csystem[14,15], and the temperature of the binary eutecticis higher than that of the ternary eutectic at the compositionup to more iron and less carbon[16] (L and D refer to them herea dc ondf

earsi bond.I aceso hatt car-ba ingdb tomsi om-p

4. Conclusions

(1) An obvious difference between the inner part near HPHTas-grown diamond and other regions in the thin metalfilm from Fe–Ni–C system has been observed by Ra-man, SAM, TEM and EMPA. Only in the inner part,graphite and amorphous carbon have not been found; thefine structures of iron and carbon atoms have greatly beentransformed; there exists a parallel relationship betweenthe crystal faces of (Fe,Ni) and Fe3C. Hence, the innerpart can play an important role in diamond formation andgrowth at HPHT.

(2) Graphite could not be directly catalyzed into diamondstructure in the thin metal film under diamond growth atHPHT. A possible mode of diamond growth can be sug-gested as follows. The carbon atoms for diamond growthare closely related to the decomposition of Fe3C in theinner part at HPHT; (Fe,Ni)fcc and (Fe,Ni)pc in short-range order may play a role of catalyzing phases duringthe Fe3C decomposition. The stability of carbide Fe3Cin short-range order will decrease under the effect of thecatalyzing phases.

Acknowledgement

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elt and diamond, respectively). So it is possible that tre (Fe,Ni)fcc and Fe3C in the molten film before diamonrystallization, which would be advantageous to diamormation.

It is usually thought that few electrons exchange appn iron carbides because of the covalent bond and metaln view of the parallel relationships between the crystal ff (Fe,Ni) and Fe3C mentioned, it could be suggested t

here exists a linking for the outermost layer electrons ofon and iron atoms between Fe3C and (Fe,Ni)fcc or Fe3Cnd (Fe,Ni)pc in short-range order in the inner part duriamond growth. We guess that (Fe,Ni)fcc or (Fe,Ni)pc maye the catalyzing phases, which could absorb carbon a

n Fe3C in short-range order at HPHT, promoting its decosition.

This work was supported by the Natural Science Fouion of China, under Grant Nos. 50371048 and 5037203

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