J. Plasma Fusion Res. SERIES, Vol.3 (2000) 233-237

Preparation and Properties of the Composites Gharacterized
by Grading SiG Coatings and Graphite Substrates
QUANGUI Guo*, LANG Liu, JINREN Song, JIANGANG Lir and yEXI Her
Institute of Coal Chemistry, Chinese Academy of Sciences, P.O.Box 165, Taiyuan Shanxi, 030001, China
Ilnstitute of Plasma Physics,
Chinese Academy of Sciences, P.O.Box 1126, Hefei, Anhui,23003I, China(Received: 18 January 2000 / Accepted: 1 June 2000)

Abstract
The composites with SiC coatings and graphite substrates are discussed in this paper. Microstructure
observations by SEM and optical microscopes indicate the grading distribution of SiC coating. The
thickness of SiC coatings and the distribution of SiC particles have a significant dependence on thereaction temperatures at which the coatings were formed. Test results show that the composites are ofhigh density, high mechanical strength and low porosity, and the excellent performance of oxidation
resistance at high temperatures up to 1200'C as well.

Keywords:
carbon based composite, SiC coatings, graphite , microsrructure

1. Introduction

method, thermal expansion mismatch between SiC

Graphites and CFC (carbon fiber composite) are
now widely used as plasma-facing materials (PFMs) inlarge fusion devices [-3]. It is well known that carbon
based materials have distinct disadvantages as PFM in
the next generation fusion device, due to the chemicalerosion (CE) t4-91 and radiation enhanced sublimation

coating and graphite substrate will cause the formationof microcracks and even spallation due to poor
adherence ofcoating to carbon substrate [20].

The objective of this study was to develop a newkind of composites with SiC coatings and graphite
substrates for plasma-facing materials using chemicalvapor reaction (CVR) combined with chemical vapor(RES) [0,11]. The CE and RES of graphite in hightemperature plasma environment will result in theserious contamination of plasma and the short lifetimeof PFM. To improve this behavior of graphite, some

infiltration (CVD. Previous efforts by the authors [21]and the other researchers [22,23] demonstrated grading

SiC coatings obtained by the above fabrication methods

were very adherent to graphite materials and had
excellent high temperature oxidation resistance. Toefforts have been made. One way is doping the graphitewith the elements of B, Si and Ti [12-16], another way
is to provide a low-Z, stable refractory material to thesurface of graphite for PFMs. Boron and boron carbide
based coatings have broadly been studied by Linke [17],Bolt [18] and Valentine [9]. SiC is also a low-Z, stablerefractory material with many advantages when applied
as a coating on the surface of carbon-based substrates. It
dose not melt and is stable in an inert atmosphere up todetermine the applicability of the composites as PFMs,

detailed knowledge of their properties andmicrostructure should be ascertained.

2. Experimental
2.1 Materials
The properties of graphite materials which were

29OO K. While for SiC coating prepared by CVD*@2000 by The Japan Society of Plasma
Science and Nuclear Fusion ResearchC o rre sponding author' s e -mail : q g guo @ 2 I cn. c om233

euangui G. et al., Preparation and Properties of the Composites Characterized by Grading SiC

Coatings and Graphite Substrates

Table 1 Basic properties of the graphite substrate
Matend WeGraohite

Density Porosity Bending Compressive Electrical Shore Ash content(g,/cmjt (o/ol shength(Mpat strength(MPa) resistiviw (lrOm) hardness (ppm\1.85

1635

75125055

Table2 The effect of reaction temperature on theemployed as substrates are listed in Table 1.
Graphite substrates were cut into cubic specimensof the size of 10 x 10 x 10 mm, polished and cleaned inweight-gain and the density of the samples

an ultrasonic bath with alcohol.

2.2 Microstructure characterization
The microstructure of the composites with grading
SiC coating and graphite substrates was analyzed with ascanning electron microscope. The phase composition ofReaction temperature(oC)

1600r700

1800

Weight-gain(%)

7.31

20.95

7.26

Density of the sampl{g/cm3)

1.94

2.t5

r.94

the coatings was determined by XRD patterns. Anand the density of the composites can be illustrated by

optical microscope was used to determine the thicknessof the coatings and the distribution region of SiC

the formation mechanism of SiC. At the reaction
temperature of 1600'C, due to the low pressure and therelatively low diffusion rate of Si (g) into the interior of
the substrates, the formation of SiC is mainly on theparticles.

surface and the penetration of Si (g) into the interior isvery limited. At 1700"C, SiC is not only formed at thesurface, but also at the center of the sample by thediffusion of Si through the inner pores. At higher
temperature of 1800"C, due to the rapid reaction
between Si and carbon substrate, the primarily formed
SiC layer at the surface of substrate acts as a diffusion
barrier against Si-mass transfer by the gas phase,resulting in the small weight-gain of the samples and therelatively low density of the composites.

2.3 Properties testing
The weight of the samples before and after coating
was determined with an analytical balance. Volume ofsamples and the composites was obtained using theArchimedes law. The density of the composites wascalculated. For the samples with SiC coatings, thecompressive strength was tested and oxidation resistance

was performed in a muffle furnace in static air at1200'c.

3. Results and Discussion
3.1 Preparation of the composites

3.2 Phase composition and macrostructure ofThe composites of SiC coatings and graphiteby the following ways: when cubic graphite samples

the coatings X-ray diffraction method wasused to determine the phase compositionsof the coatings.

were heated to the temperatures ranged from 1600-

Because no graphite diffraction peaks and only SiC

1800"C, vaporized silicon was formed and it reached thecrystal diffraction peaks were observed, as shown in
Fig. 1, from which it can be concluded that the wholesurface of graphite substrate was covered with SiC

substrates were obtained in a high-temperature furnace

surface of graphite and the chemical reaction occurred:

Si + C -+ SiC. The reaction occurred not only at thesurface, but also in the interior of substrates due to theinfiltration of Si (g) through the pores, thus thecomposites of grading SiC coatings and graphite

particles. Fig. 2 was surface photography of thecomposites by the optical microscope prepared in thetemperature of 1700'C. It revealed that the coatings on
the surface of graphite substrate were mainly composed

substrates were prepared.

The composites with the density of 1.88-2.28 glof SiC particles in the size of 5-10 pm.

cm3 were prepared depending on the temperatures. As

Transversal sections of the composites wereindicated in Table 2, the reaction temperature has asignificant influence on the weight-gain and the densityof the samples.
The effects of the temperature on the weight-gain

observed by scanning electron microscope and optical

microscope to characterize the microstructure of thecoatings. SEM images and optical microscopeobservation verified the microstructure of the sradine234

Quangui G. et al.' Preparation and Properties ofthe Composites Characterized by Grading SiC Coatings and Graphite SubstratesIt)

Fig. 1 XRD pattern of the coatings of the composites

Fig.3 Grading distribution of SiC coatings of thecomposites in the transversal section a-SEM
image of the composite with the density of 1.94 gl
cm3; b-optical microscope image of the composite
with the density of 2.05 g/cm3

Fig.2 Surface photography of SiC coating observed by
an optical microscopeof pm; while for the composites with the density ofcoatings possess the following characteristics: the2.14-2.28 g/cm3, the thickness of the coatings is up tohundreds pm, and the distribution region front of SiC
reaches the center of the composites and almost allpores of substrates are filled with the SiC particles (seesurface layers of SiC with the thickness ranged fromFig.4).

several pm to hundreds pm; the distribution of SiC
particles along the distance from surface to the interiorof substrates with the various penetration depths fromseveral pm to the center of the composites, depending

The porosity is calculated assuming that the true
density of graphite and SiC is 2.2 glcm3 and 3.21 glcm3

distribution of SiC coatings on graphite substrate, whichwere shown in Fig. 3. The grading distribution of SiC

respectively. A linear decrease for the porosity and alinear increase for the compressive strength with thedensity can be seen in Fig. 5. For the composites withthe density of 2.28, the porosity is only 6.85 and theon the siliconization temperatures.

The distribution of SiC coating is closely relatedwith the density of the composites. For the composites
with the density of 1.88-1.95 g/cm3. the thickness of thecompressive strength is 166 Mpa, showing a decrease of
57Vo of porosity and an increase of l22%o of strength incoatings is about several pm, and the distribution of SiC

comparison with that of graphite substrates. Theis limited to the depths of several tens pm; for thecomposites with the density of 1.96-2.1O g/cm3, thedecrease of porosity is also an evidence that the SiC has

thickness of the coatings is about several tens pm, andthe distribution reqion of SiC is increasins to hundreds

attributed the volume expansion accompanied with thereaction of Si and carbon.

formed in the interior of the substrates, which can be23s

Substrates
Quangui G. et al., Preparation and Properties ofthe Composites Characterized by Grading SiC Coatings and Graphite012345678910
Fig. 4 The distribution of SiC particles in the interior of
the composites with the density of 2.22 glcm3

aiddim tine(hq"r)Fig. 6 Weight-loss of the composite and pure graphite
as a function of oxidation time at 1200'C

16g'*
E't*
E
6tn

12E

mechanical strength have been improved to some extent.

8g10$,*
tr.q

graphite substrates would not deteriorate greatly, incomparison with the doped graphites [24]. On thecontrary, some basic properties such as porosity ands

3.4 Oxidation resistanceFig. 6 showed the weight-loss of the composite

Bowith the density of 2.15 g/cm3 and pure graphite as afunction of oxidation time at 1200"C. Pure graphite wasburn out in about 3.3 hours, while the weigth-loss for60
2.1z.z2.3

Densty(g/cnr1

the composites is only about l07o at the end of lOhours.

Fig. 5 The curves of compressive strength and porosity
change with the density of the composites

substrate was greatly improved by the grading SiC

It is obvious that the oxidation behavior of graphite
coating.

The high performance of oxidation resistance of thecomposite was closely related with its microstructures:. the good adherence of SiC coatings to graphite substrate because of the in-situ formation of SiC by the reaction of Si with carbon of the substrate;. the grading distribution of SiC coatings from outer surface to the interior provides a grading expansion behav-

3.3 Physical and mechanical properties of thecomposites Fig. 5 depicts the dependenceof porosity and the compressive strength
on the density of the composites.
The microstructure characteristics of thecomposites are preferable for their application as

ior that have relaxed the thermal mismatch strain to ensure the integrality of the coatings and the crack-freeplasma-facing materials because of their high density

and small porosity. It can be anticipated that themicrostructure even after thermal shock;. during the exposure of the composite to the oxidative
atmosphere, a silica layer has formed and acts as a diffusion barrier against the oxygen transfer reaching the surface and the interior of substrate, thus rapid oxidation ofgraphite at high temperatures was highly restrained.

behavior of chemical sputtering and hydrogen isotope
retention can be greatly improved because the wholesurface including the surface of pores was covered by
SiC particles, which exhibit thermal and chemical
stability as low-Z materials under plasma environment.
In addition, the formation of SiC in the composites dose
not destroy the structure of graphite skeleton, highthermal conductivity and thermal shock resistance of

4. Conclusion
The composites prepared in this paper were236

Quangui G. et al', Preparation and Properties of the Composites Characterized by Grading SiC Coatings and Graphite Substrates

characterized by grading SiC coatings and graphite
substrates. The thickness of SiC coatings and thedistribution of SiC particles were dependent on thereaction temperatures at which the coatings wereformed. The prepared composites are of high density,high strength and low porosity, and the excellent
performance of oxidation resistance at temperatures up
to 1200'C as well. Further research of investigations on
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