# Processing of reaction-bonded B4C-SiC composites in a single-mode microwave cavity

Anthony Thuault, Sylvain Marinel, Etienne Savary, Romain Heuguet, Sébastien Saunier, Dominique Goeuriot, Dinesh Agrawal

► To cite this version:

Anthony Thuault, Sylvain Marinel, Etienne Savary, Romain Heuguet, Sébastien Saunier, et al. Processing of reaction-bonded B4C-SiC composites in a single-mode microwave cavity. Ceramics International, 2013, 39 (2), pp.1215-1219. 10.1016/j.ceramint.2012.07.047. emse-01063766

HAL Id: emse-01063766

https://hal-emse.ccsd.cnrs.fr/emse-01063766

Submitted on 23 Aug 2021

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
# Processing of reaction-bonded B₄C–SiC composites in a single-mode microwave cavity

Anthony Thuault<sup>a,*</sup>, Sylvain Marinel<sup>a,b</sup>, Etienne Savary<sup>a,c</sup>, Romain Heuguet<sup>a</sup>, Sébastien Saunier<sup>c</sup>, Dominique Goeuriot<sup>c</sup>, Dinesh Agrawal<sup>b</sup>

<sup>a</sup>CRISMAT Laboratory UMR 6508 CNRS-ENSICAEN-UCBN, 6 Boulevard du Maréchal Juin, 14050 Caen Cedex, France

<sup>b</sup>Materials Research Institute, Materials Research Laboratory Building, The Pennsylvania State University, University Park, PA 16802, USA

<sup>c</sup>Département Mécanique et Procédés d'élaboration, Centre des Sciences des Matériaux et des Structures, Ecole Nationale Supérieure des Mines de Saint-Etienne, 42023 Saint-Etienne Cedex 2, France

In this study, the reaction sintering of boron carbide, which consists in doing reactive infiltration of molten silicon throughout a porous sample made of B₄C and carbon graphite was investigated. Thus, it has been shown that a single-mode microwave cavity can be successfully used to produce reaction-bonded B₄C–SiC composite. A specific package, consisting of a SiC based susceptor and a boron nitride based insulating container, was used to heat up the B₄C–Si system using a single-mode microwaves cavity under an Ar–H₂ atmosphere. Pore-free B₄C–SiC composite successfully produced consists of a mixture of B₄C and polygonal shaped β-SiC within a residual silicon matrix. The indentation technique permits to determine mechanical properties of the samples which are compared to those obtained conventionally. It appears that the average hardness (H≈22 GPa) value is quite constant all along the sample thickness which highlights good homogeneity of the samples obtained. Some aspects of the microstructure are also discussed and compared to those of samples conventionally obtained.

**Keywords:** A. Microwave processing; C. Mechanical properties; D. Carbides; E. Structural applications

## 1. Introduction

Boron carbide (B₄C) ceramic is a refractory material with a high melting point (≈ 2723 K), a high hardness, good mechanical properties, a low specific weight (2.52 g cm⁻³), a high corrosion resistance to chemical agents and a high neutron absorption cross section. B₄C is currently used in many advanced technological application fields [1,2] such as the nuclear industry [3], high-temperature thermoelectricity conversion [4,5] and ballistic protections [6]. As many covalent type carbide ceramics, dense B₄C can be manufactured by a hot-pressing sintering technique at a very high temperature (about 2200 °C), which is a very

costly method limited to the processing of plates or cylindrical samples. Thus, the widespread use of B₄C is restricted by the intrinsic limitations of the hot-pressing technique. Therefore, the Reaction-Bonded Sintering process (RBS-B₄C), which is much more cost-effective, represents an interesting alternative method to produce dense B₄C-based pieces [7]. RBS-B₄C is made by silicon infiltration into a porous preform, made of a B₄C and carbon graphite and shaped by uni-axial pressing. Pure silicon pieces are then put on the top of the preform and the assembly is subsequently heated up to about 1500 °C in order to melt the silicon. The molten silicon infiltrates throughout the boron carbide preform, fills the pores and reacts with graphite. The reaction between silicon and graphite mostly leads to the formation of silicon carbide [8] and, as a result, the composite is mainly made of B₄C grains, SiC grains and there may be some not reacted silicon left [9]. This pressureless and low

*Corresponding author. Tel.: +33 02 31 45 13 77; fax: +33 02 31 45 13 09.

E-mail addresses: anthony.thuault@ensicaen.fr, thuault.anthony@yahoo.fr (A. Thuault).
temperature process significantly lowers the cost of pro-
duction of B₄C-based products compared to the hot-
pressing technology. To obtain high-quality reaction-
bonded materials, the processing conditions, such as the
starting composition, the powders morphology, and the
thermal conditions, must be accurately determined [9]. The
Microwaves Assisted Processing (MASS) method has been
investigated in order to produce SiC–Si and SiC–(Si–Al)
reaction-bonded composites in a multimode microwaves
cavity [10]. This study has emphasized the fact that
microwave heating can produce identical composites, in
term of mechanical properties, compared to conventional
heating, with substantial processing time reduction. It may
be assumed that some specific features involved by micro-
waves [11], such as fast heating rate, selective heating or
inverse thermal gradient, could have beneficial effects on
the reaction-bonded materials obtained. However, few
studies report the use of microwaves to produce reaction-
bonded B₄C based materials. Most of them concern the
multi-mode microwave processing of Si₃N₄ based material
[12,13] or the microwaves joining of composites [14]. In
addition, it is well established that semiconductors, such as
silicon, silicon/boron carbides, couple very strongly with
microwaves, especially with *H* field [11,15–17]. Therefore,
it is expected that 2.45 GHz *H*-field irradiation can
effectively heat up the B₄C–Si system and provide unique
microstructures with improved mechanical properties. As a
consequence, the aim of this work is to test this method
using a single-mode microwave cavity and to investigate
the feasibility of the RBS-B₄C process. A single mode
microwave cavity, working at 2.45 GHz, was designed to
make RBS-B₄C under Ar–H₂ atmosphere. Phase composi-
tion, microstructure and mechanical properties of the
silicon infiltrated samples were then characterized. Results
are discussed and compared to those usually obtained
using a conventional process.

## 2. Experimental

85 wt% of B₄C (F20 Metabap 5003) and 15 wt% of C
graphite powders (Alfa Aesar 99.9995%, D50~74 µm)
were thoroughly mixed in an agate mortar under dry
condition. An organic binder (Rhodoviol 4%, Prolabo)
was added to the mixture powder and cylindrical samples
(6.5 mm diameter and 5.4 mm thickness) were made using
a 30.6 kN uni-axial load. Green pellets have an apparent
specific weight of about 1.6 g cm⁻³, which is roughly 65%
of the theoretical value of the B₄C–C mixture (≈ 2.47 g
cm⁻³). Cylindrical samples (m ≈ 0.15 g, 6.5 mm diameter
and 3 mm thickness) of Si powder (Alfa Aesar 99.9%,
0.149 mm) were also made using the same process. The
amount of silicon was calculated in order to fully fill the
pores of the B₄C–C preform. Assuming that the silicon
infiltrates throughout the preform and subsequently fully
reacts with graphite to form SiC, the final composition
theoretically expected is made of 56% B₄C, 32.5% SiC and
11.5% Si by weight, and the theoretical density of this final

mixture is about 2.72 g cm⁻³. It must be noted that this
composition is not optimized to reduce the final amount of
residual silicon (to do so, the amount of graphite should be
higher). However, this silicon amount is large enough to
totally fill the pores and thus, it is suitable to investigate
the reactivity between B₄C, C and Si in our experimental
conditions. The microwaves heating system consists
of a microwaves generator (SAIREM GMP 20 KSM,
2.45 GHz), which delivers a variable power up to 2 kW
along a standard R26 rectangular waveguide (section of
86.36 × 43.18 mm²) equipped with two circulators and
ended by a TE₁₀M rectangular microwaves cavity. The
cavity can be tuned in both modes TE₁₀m (*m* = 2 or 3) by
adjusting the length between the coupling iris and the short
circuit piston [18,19]. The temperature was measured using
an infrared pyrometer (Ircon, Modline 5, 350–2000 °C)
vertically positioned on the top of the cavity and focused
on the top of the samples surface [19]. The scheme of the
overall assembly is illustrated in Fig. 1. A cylindrical SiC
susceptor was used to limit the radiation losses from the
sample surface. Moreover, thermal insulation was ensured
using BN powder placed between the SiC susceptor and
the BN crucible, as shown in Fig. 1. The process consists in
(i) putting a silicon cylinder on the top of the B₄C–C
graphite cylinder, (ii) putting the double-layer sample
inside the specifically designed crucible, and (iii) putting
the assembly inside the cavity at the appropriate location.
Then, the sample was heated up by raising the power by
50 W steps every 10 min until 650 W to reach a tempera-
ture of about 1500 °C and a 10 min dwell time was
imposed at the final temperature. The heating was inves-
tigated in the TE₁₀₂ mode at *H* field maximum in order to
minimize the risk of plasma formation and arcing, espe-
cially when semiconducting materials are heated using

<figure><img src="image_2.png" /><figcaption>Fig. 1. Scheme of the microwaves TE<sub>10m</sub> single-mode cavity, representing the used process and a picture of the home-designed crucible.</figcaption></figure>
microwaves [16]. Afterwards, the assembly was cooled down to RT during 30 min. The entire heating cycle was conducted under a gas flow made of 95% Ar + 5% H₂. The crystalline phases were identified by X-Ray diffraction (XRD) using Cu Kα radiation (Philips X'Pert diffractometer). The sintered samples were coated in a carbon resin (Struers Polyfast), polished (Struers Tegra-Pol 31) and observed by a scanning electron microscopy (Zeiss Supra 55). Chemical composition was checked by SEM and energy dispersive microscopy (EDAX-EDS). Hardness was determined using a Vickers type micro-durometer with an applying force of 1 kgf and Young's modulus was determined using a nanoindenter (MTS XP), equipped with a Berkovich tip, for a 10,000 nm indentation depth.

### 3. Results and discussion

Fig. 2A shows a typical cross-section of a sample obtained using the microwave reaction-bonded process. It is clearly shown that the molten silicon has fully infiltrated the porous body. The apparent density is about

2.6 g cm⁻³, which is very close to the expected value (≈ 2.72 g cm³), assuming a complete reaction between graphite and silicon. The sample dimension changes are about 0% and about -5.1% along respectively, the diameter and the height. Compared to the classic sintering process, which leads to shrinkage values ranged between 15% and 20%, it is observed that the reaction-bonded process is a near net shape technique, which is a great advantage compared to the conventional sintering process. The anisotropy of the samples shrinkage observed is not clearly understood yet. It may be due to the fact that the preform has been uni-axialy pressed. The SEM microstructure (Fig. 2B) reveals, as expected, a composite microstructure consisting of B₄C and SiC grains, embedded within a silicon matrix. The SiC grains have mostly a polygonal shape, which is very similar to the microstructure reported by Hayun et al. [20] in free carbon added reaction-bonded boron carbide composite. Moreover, the microstructure presents very few pores, which confirms that the infiltration process was successfully completed. SEM observations at various locations of the pellets reveal a quite uniform and homogeneous microstructure at large scale throughout the sample. The XRD pattern performed on a crushed sample is shown in Fig. 3. It appears that the sample seems to be made of B₄C, β-SiC and residual silicon. Hayun et al. [20,21] have deeply studied the microstructure of the reaction-bonded boron carbide made using a conventional method. They used the silicon infiltration process in vacuum (10⁻⁵ Torr) using preform of B₄C with or without carbon addition. They reported that the reaction-bonded boron carbide composite microstructure consists of core-rimed boron carbide particles, β-SiC and residual Si. They explained that the molten silicon is saturated with free carbon and boron; hence, a reaction due to a secondary equilibrium of the ternary system occurs. The newly formed equilibrium phase, i.e. B₁₂(B,C,Si)₃, precipitates at the interface with the initial boron carbide particles and forms the rim regions. The rim region can be seen using SEM and/or in

<figure><img src="image_3.png" /></figure>

<figure><img src="image_4.png" /><figcaption>Fig. 2. (a) Cross section of a typical RB B₄C-SiC sample processed using microwaves and (b) its typical microstructure observed using SEM.</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>Fig. 3. XRD pattern of a crushed RB B₄C-SiC composite sample.</figcaption></figure>
the XRD pattern, in which a slight split of the ‘B₄C’ peaks is observed. In reaction-bonded sample obtained using microwave sintering, the XRD pattern does not reveal any peak split (Fig. 3). This confirmed that the composite is mainly made of B₄C, β-SiC and residual silicon. It may be assumed that the rapid heating generated by microwaves and the fact that the infiltration process has been done under an Ar–H₂ gas flow at atmospheric pressure may prevent the secondary reaction. Young’s modulus was calculated by nano-indentation along the z-axis of the sample and plotted against z (depth) in Fig. 4. Measurements were carried out as a matrix 12 × 4 (vertical × horizontal) with an indentation depth of 10,000 nm to obtain large imprints in order to test an area which has a quite homogeneous composition. So, for each depth values, four values of Young’s modulus and hardness were obtained and measurement points were separated by 400 µm. Finally, it appears that the average Young’s modulus is about 309 ± 4 GPa. In addition, the hardness was determined using a Vickers type micro-durometer along the z-axis of the sample and plotted against z in Fig. 4. Measurements were carried out as a matrix 9 × 4 and the measurement points were separated by 500 µm. Results show an average hardness value of 22 ± 0.5 GPa. It is noted that these mechanical properties are constant throughout the sample, which confirms the microstructure homogeneity and the uniform composition throughout the sample. Because of (i) the obtained microstructure slightly different from the one obtained by conventional process [20,21], since the core-rimed region has not been observed and because of (ii) the quite large residual silicon phase amount (estimated by calculations to, at least 12.7 vol% if we assumed that all the graphite has reacted but, as the composition was not experimentally determined, it is possible that this amount is underestimated), the average value of Young’s modulus is slightly lower than the one obtained using the conventional process, which is about 360 GPa [22]. However, the average hardness value

obtained in the experiment ($H \approx 22$ GPa) is comparable to the reported value obtained by a conventional reaction-bonded process (about 20–22 GPa) [21]. It can also be noted that those mechanical properties can be improved by adjusting the initial composition to reduce the residual amount of silicon.

### 4. Conclusion

An original assembly made of a silicon carbide susceptor and a boron nitride based insulating crucible, was designed to produce reaction-bonded boron carbide samples, using a TE₁₀₂ single-mode microwaves cavity. The heating process was successfully carried out, under a flowing Ar–H₂ atmosphere, without noticing any undesirable effects such as arcing or plasma formation. In this study, we have successfully produced a pore free reaction-bonded B₄C–SiC composite, owing to the good silicon infiltration throughout the preform. The final microstructure results in a mixture of B₄C and polygonal shaped β-SiC in a residual silicon matrix. The mechanical properties are comparable to those obtained conventionally and the average hardness value (about 22 GPa) is quite constant all along the sample z-axis. In order to produce larger samples, experiments in a multimode microwave system are in progress.

### Acknowledgments

Prof. Sylvain Marinel gratefully acknowledges the support of the government agencies: the DGA governmental institution, under contract "project ERE# 2011.60.052.00.470.7501", and the Lower-Normandy Region Council (France). The authors want to thank explicitly Dr. François Barthelemy, from the DGA agency, for his valuable support.

### References

[1] A.K. Suri, C. Subramanian, J.K. Sonber, T.S.R.C. Murthy, Synthesis and consolidation of boron carbide—a review, International Materials Review 55 (2010) 4–40.

[2] F. Thévenot, Boron carbide—a comprehensive review, Journal of the European Ceramic Society 6 (1990) 205–225.

[3] K. Reinmuth, A. Lipp, H. Knoch, K.A. Schwetz, Boron carbide as neutron absorbent, Journal of Nuclear Materials 124 (1984) 175–184.

[4] C. Wood, High-temperature thermoelectric energy conversion—II Materials Survey, Energy Conversion and Management 24 (1984) 331–343.

[5] H. Werheit, Boron rich solids—a chance for high-efficiency high-temperature thermoelectric energy conversion, Materials Science and Engineering B 29 (1995) 228–232.

[6] P.G. Karandikar, G. Evans, S. Song, M.K. Aghajanian, M. Sennett, A review of ceramics for armor applications, advanced ceramics, in: L.P. Franks (Ed.), Armor IV—Ceramic Engineering and Science Proceedings, vol.29, John Wiley and Sons, Inc, Hoboken, NJ, 2009 in.

[7] P. Popper, The preparation of dense self-bonded silicon carbide, Special Ceramics, British Ceramics Research Association, London (1960) 209–219.

[8] A.L. Yurkov, A.M. Starchenko, B.S. Skidan, Reaction sintering of boron carbide, Refractories and Industrial Ceramics 30 (1989) 731–736.

<figure><img src="image_6.png" /><figcaption>Fig. 4. Representation of Young’s modulus (E) and hardness (H) values as a function of the depth z.</figcaption></figure>
[9] S. Hayun, A. Weizmann, M. Dariel, N. Frage, The effect of particle size distribution on the microstructure of boron carbide based reaction bonded composites, International Journal of Applied Ceramic Technology 6 (2008) 498–500.

[10] P.G. Karandikar, M.K. Aghajanian, D. Agrawal, J. Cheng, Microwave assisted (MASS) processing of metal ceramic and reaction bonded composites, Ceramic Engineering and Science Proceedings 27 (2) (2007) 435–446.

[11] M. Oghbaei, O. Mirzaee, Microwave versus conventional sintering—a review of fundamentals, advantages and applications, Journal of Alloys and Compounds 494 (2010) 175–189.

[12] M. Willert-Porada, J. Grosse-Berg, I. Sen, H.S. Park, Microwave sintering and infiltration of highly porous silicon nitride ceramics to form dense ceramics, key engineering materials 287, Advanced Si-Based Ceramics and Composites 287 (2005) 171–176.

[13] T.N. Tiegs, J.O. Kiggans, H.T. Lin, C.A. Wilkens, Comparison of properties of sintered and sintered RBS silicon nitride fabricate by microwave and conventional heating, MRS Proceedings 347 (1994) 501–506.

[14] J.G.P. Binner, J.A. Fernie, P.A. Whitaker, An investigation into microwave bonding mechanisms via a study of silicon carbide and zirconia, Journal of Materials Science 33 (1998) 3009–3015.

[15] E. Savary, F. Gascoin, S. Marinel, Fast synthesis of nanocrystalline Mg₂Si by microwave heating—a new route to nano-structured thermoelectric materials, Dalton Transactions 39 (2010) 11074–11080.

[16] S. Marinel, G. Desgardin, A new inductive furnace based on mocrowave irradiation for growing long YBa₂Cu₃O₇ single-domain bars, Advanced Materials 10 (1998) 1448–1452.

[17] J. Cheng, R. Roy, D. Agrawal, Radically different effects on materials by separated microwave electric and magnetic fields, Materials Research Innovations 5 (2002) 170–177.

[18] E. Savary, S. Marinel, F. Gascoin, Y. Kinemuchi, J. Pansiot, R. Retoux, Peculiar effects of microwave sintering on ZnO based varistors properties, Journal of Alloys and Compounds 509 (2011) 6163–6169.

[19] S. Kumar, S. Marinel, M. Miclau, C. Martin, Fast synthesis of CuCrO₂ delafossite by monomode microwave heating, Materials Letters 70 (2012) 40–43.

[20] S. Hayun, A. Weizmann, M.P. Dariel, N. Frage, Microstructural evolution during the infiltration of boron carbide with molten silicon, Journal of the European Ceramic Society 30 (2010) 1007–1014.

[21] S. Hayun, D. Rittel, N. Frage, M.P. Dariel, Static and dynamic mechanical properties of infiltrated B₄C-Si composites, Materials Science and Engineering, A: Structural Materials: Properties, Microstructure and Processing 487 (2008) 405–409.

[22] S. Hayun, H. Dilman, M.P. Dariel, N. Frage, S. Dub, The effect of carbon source on the microstructure and the mechanical properties of reaction bonded boron carbide, Advanced Sintering Science and Technology: Ceramics Transactions (2010) 27–39.