Research Reports of Nagaoka National College of Technology, Vol. 47, 2011

PaperStructure and Oxidation Behavior of Carbon/Ceramic
Composite Material Synthesized by Spark PlasmaSintering
Shinsuke HOSHII1, Akira KOJIMA2 and Masahiro GOTO3

1 Technical Support Center for Education and Research, Nagaoka National College of Technology
2 Department of Chemistry and Materials Science, Gunma National College of Technology
3 Gunma Prefectural Gunma Industrial Technology Center

Using coke, together with SiC or B4C powders as raw materials, carbon/ceramic composites wereprepared by a spark plasma sintering process of a short duration. The oxidation resistance of the composite
containing B4C powder was higher than that of the composite containing SiC powder. The temperature atwhich the weight decrease of the composite containing 10 wt% B4C powder began shifted to thehigh-temperature side compared with that of the carbon material composed only of coke. Approximately
44% of the initial weight of the specimen remained even after thermogravimetric analysis in air up to 1000oC. By X-ray diffraction analysis, diffraction peaks corresponding to B2O3 were observed. This finding
indicates that B4C in the composite is oxidized to B2O3, which then forms an oxidation-resistant film,
leading to an increase in the oxidation resistance of the composite.
Key Words: composite, ceramic, rapid baking, spark plasma sintering, oxidation behavior

1. Introduction
Spark plasma sintering (SPS) enables high-speed
sintering at low temperature and suppresses crystal
growth in sintered bodies. Therefore, SPS has beenused for the preparation and study of variousmaterials1)-3). The authors have been carrying out
research on the preparation and property evaluationof carbon materials by SPS. On the basis of theresults obtained thus far, it was confirmed thatcarbon materials can be synthesized without theaddition of any binders in an extremely shortsintering time by SPS and that the synthesizedcarbon materials have good properties equivalent or
superior to those of commercially available artificial
graphite materials4),5). In this study, carbon/ceramiccomposite materials using few processes in a shorttime by SPS were synthesized. Furthermore, athermogravimetric analysis of the synthesizedcomposite material was carried out to examine its
oxidation behavior in air. In addition, the crystalstructure of the material was analyzed using an
X-ray diffraction apparatus, and the oxidation
resistance mechanism of carbon/ceramic composite
materials synthesized by SPS was clarified.

2. Experimental Method
As materials of the carbon/ceramic composite,coke powder (purity of 99.8% or more, average
particle size of 10-15 ȝm), SiC powder (purity of98% or more, average particle size <125 ȝm) andB4C powder (purity of 99% or more, average particle
size <105 ȝm) were used. The powder mixture
obtained by adding a predetermined amount of SiC― 25 ―Shinsuke HOSHII, Akira KOJIMA and Masahiro GOTO

powder or B4C powder to the coke powder was used
as a raw material. The proportion of SiC powder orB4C powder added was 2, 6 or 10 wt% with respect
to the weight of the coke powder. The raw powder
containing a predetermined amount of SiC powder
or B4C powder was placed in the SPS apparatus(DR. SINTER SPS-1050, Izumi TechnologyCompany Ltd.) and sintered at 1800oC while
applying a pressure of 60 MPa in vacuum to theprepared carbon/ceramic composite with a diameterof 20 mm and a thickness of 5 mm. The bulk density
and Shore hardness of the obtained composite
material were measured. To examine the oxidation
behavior of the composite material, athermogravimetric analysis of a specimen of thecomposite material with dimensions of 5×5×5 mm3
was carried out in air using a thermogravimetricapparatus (TG/DTA-6300, Seiko Instruments Inc.)
up to 1000oC. The conditions of thethermogravimetric analysis were an airflow rate of
50 ml/min and a temperature increase at a rate of
10oC/min. Furthermore, the crystalline structure of
the composite material was analyzed using an X-ray
diffraction apparatus (RINT-22VK/PC, RigakuCorporation).

Table 1 Bulk density and shore hardness of carbon/
ceramic composites and carbon material.
Amount of ceramic
powder added / wt%
2SiC
6102
B4C
610Carbon material

Bulk density/ g cm-3
1.87
1.89
1.94
1.93
1.98
1.99
1.77

Shore
hardness
6263
62
64
64
6459

Weight loss /%-200204060(d)
(c)

(a)

80(b)

100
0200

400

600

800 1000

Temperature /㷄Fig.1 Weight loss curves of carbon material and carbon/
SiC composites : (a)carbon material, (b)composite(SiC 2 wt%), (c)composite (SiC 6 wt%), and (d)composite (SiC 10 wt%).

-20

3. Results
SPS was completed in 18 min, andcarbon/ceramic composites were synthesized in 2
hours including the cooling time. All thecarbon/ceramic composite specimens were black.
They were highly firmly structured without
powdering or breakage. Table 1 summarizes thedensities and Shore hardnesses of the specimens.
The hardness of the composite material increases as
the amount of SiC or B4C powder added increases.
The bulk density of the composite material
containing 10 wt% SiC powder is 1.94 g/cm3 andthat containing 10 wt% B4C powder is 1.99 g/cm3.
The Shore hardness of the composite material isalmost constant regardless of the amount of SiC or
B4C powder added. It was 62 with 10 wt% SiC
powder and 64 with 10 wt% B4C powder. The bulk
density and Shore hardness of the carbon material
composed only of coke are 1.77 g/cm3 and 59,

Weight loss /%02040(d)
(c)

60(a)

80(b)

100
0200

400 600 800 1000
Temperature /㷄Fig.2 Weight loss curves of carbon material and carbon/
B4C composites : (a)carbon material, (b)composite(B4C 2 wt%), (c)composite (B4C 6 wt%), and (d)composite (B4C 10 wt%).

respectively. The bulk density and Shore hardness ofthe composite material are higher than those of thecarbon material.
Thermogravimetric analysis was carried out toexamine the oxidation behavior of the composite― 26 ―

Structure and Oxidation Behavior of Carbon/Ceramic Composite Materialmaterial. Fig. 1 shows the results of thethermogravimetric analysis of the composite
material containing SiC powder. For the carbon
material composed only of coke, its weightdecreases at approximately 600oC, and the entire
specimen is oxidized and combusted at
approximately 900oC. The percentage of weightdecrease of the composite material containing SiC
powder at 1000oC in air is 98% (2 wt% SiC), 94% (6wt% SiC) and 90% (10 wt% SiC). Therefore, when
SiC powder was added, no increase in the oxidation
resistance of the composite material was observed.
Fig. 2 shows the results of the thermogravimetricanalysis of the composite material containing B4C
powder. The temperature at which the weightdecrease of the composite material containing B4C
powder began increases; in addition, the composite
material still remains after the thermogravimetricanalysis, indicating an increase in the oxidation
resistance of the material. The percentage of weightdecrease of the composite material containing B4C
powder at 1000oC in air is 88% (2 wt% B4C), 64% (6wt% B4C) and 56% (10 wt% B4C). The oxidation
resistance increases, with increasing amount of B4C.
In the composite material containing 10 wt% B4C
powder, the temperature at which the weightdecrease began significantly increases, andapproximately 44% of the specimen remains evenafter the thermogravimetric analysis up to 1000oC.
Curves (a) and (b) appearing in Fig. 3 show theresults of the X-ray diffraction analysis of thecomposite materials containing 10 wt% SiC and B4C
powders,respectively.
Diffraction
peaks
corresponding to Carbon and SiC are seen in curve(a). Diffraction peaks corresponding to Carbon andB4C are seen in curve (b). No diffraction peaks
corresponding to reaction products are observed.
The specimen of the composite material containing
10 wt% B4C remained after the thermogravimetricanalysis was ground and subjected to X-ray
diffraction
analysis.
A
diffraction
peakcorresponding to B2O3 was observed in curve (c). On
the basis of this result, the increase in the oxidation
resistance of the composite material containing B4C
powder is considered to be a result of the formationof an oxidation-resistant B2O3 film through theoxidation of B4C. In addition, the lattice constant

B2O3
B4C
SiC

(c)

(b)

(a)

10203040

5060

2ș䇭/deg䇭(Cu KĮ)
Fig.3 XRD patterns of carbon/ceramic composites : (a)SiC 10wt%, original, (b)B4C 10 wt%, original, and (c) B4C
10wt % after TGA up to 1000͠.

(d(002)) and crystallite diameter (Lc(002)) of thecomposite material containing 10 wt% B4C powder
were measured. The results indicate that d(002) was
0.3414 nm and Lc(002) was 26 nm. The growth of
the graphite structure was less significant in thecomposite material containing B4C powder than in
the carbon material composed only of coke.

4. Discussion
There are several reports on the structure andoxidation behavior of the carbon composite material
with added SiC or B4C powder6)-8). To prepare suchmaterials, processes such as impregnation or coating
are required, necessitating many preparation
processes and much time for heat treatment. In thisstudy, we succeeded in synthesizing acarbon/ceramic composite material using few
processes and a short sintering time by SPS.
From the results of the thermogravimetricanalysis, the oxidation resistance of the composite
material with added B4C powder was found to beincreased, and no oxidation suppression effect wasobserved in the composite material with added SiCpowder. In general, the oxidation of B4C starts atapproximately 500oC, and B2O3 is formed at
approximately 700oC. This B2O3 serves as anoxidation-resistant film, leading to an increase in theoxidation resistance of the composite material. The― 27 ―

Shinsuke HOSHII, Akira KOJIMA and Masahiro GOTO

oxidation of SiC is considered to start gradually atapproximately 800oC and SiO2 is formed at
1100-1200oC. It is considered that SiO2, whichshows the oxidation suppression effect, was notformed in the composite material containing SiC
powder, at temperatures of up to 1000oC adopted in
the thermogravimetric analysis in this study;
therefore, oxidation resistance was not exhibited. It
is considered that the oxidation suppression effect of
the composite material containing SiC powder may
arise in the high-temperature region of
approximately 1200oC.
In general, a graphite structure accompanying
catalytic graphitization develops in carbon materials
upon adding B4C9). However, in this study, theresults of lattice constant and crystallite size
measurements did not indicate any development of
the graphite structure in the composite materialcontaining B4C powder. Miyazaki et al. reported thatthe addition of a small amount of B4C of 10 wt% orless promotes the development of the graphitestructure10),11). The low graphite crystallinity in thisstudy was considered to be due to the use of SPS,
which suppresses crystal growth. It is consideredthat graphite crystallinity influences the oxidation
resistance of carbon materials. A detailed
examination of the relationship between the effectsof SPS on the crystalline structure and oxidation
resistance should be carried out in the future.

5. Conclusions
Using coke, SiC and B4C powders as raw
materials, carbon/ceramic composite materials weresynthesized by SPS for a short duration. The
oxidation resistance of the composite material
containing B4C powder was higher than that of thecomposite containing SiC powder. The temperature
at which the weight decrease of the composite
material containing 10 wt% B4C powder began
shifted to the high-temperature region comparedwith that of the carbon material composed only ofcoke. Approximately 44% of the specimen remainedeven after thermogravimetric analysis in air up to1000oC. By X-ray diffraction analysis, a diffraction
peak corresponding to B2O3 was observed. Thisfinding indicates that B4C in the composite material
is oxidized to B2O3, which then forms anoxidation-resistant film, leading to an increase in theoxidation resistance of the composite material.

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― 28 ―(Received October 3, 2011)