Ceramic Composites Research in Japan

Dateline: 06/18/00

By Laurel M. Sheppard

The Annual Meeting of the Ceramic Society of Japan was held earlier this year in mid March in Sendai, north of Tokyo. The 600+ papers presented at this meeting fell into several categories: high temperature and engineering ceramics, bioceramics, electronic ceramics, traditional ceramics, glasses and photonic ceramics, ceramics processing, porous ceramics, and characterization methods. The first category covered ceramic matrix composites.

As in other countries, Japanese researchers have done much research on how to make SiC/SiC composites. One method uses a reaction bonding process that makes 2D-SiC fiber fabrics using a two-step coating process. First the fabrics are coated with porous SiC by a polymer impregnation and pyrolysis treatment, followed by pyrolitic carbon and beta-SiC by chemical vapor infiltration. These composites attain densities of 90% or higher and show non-catastrophic failure behavior. Three dimensional composites can be made using similar processes. Other composites being studied include Si₃N₄/SiC, C/BN, Si₃N₄-BN, and Si₃N₄-TiN for wear resistance and oxidation resistance applications.

SiC can be used to reinforce other materials. ZrB₂-based particulate composites have been formed from reactive hot pressing Zr, Si and B₄C powders in situ at 1900 degrees C and under 30 MPa for 60 minutes in an argon atmosphere. The following mechanical properties were obtained: a bending strength of 506 MPa, a fracture toughness around 4 MPa-m ^½ and a Vickers hardness of around 21 GPa. This strength is double that of unreinforced ZrB₂.

Such in situ reactions can produce better mechanical properties since the microstructures are more homogeneous. This is especially true for BN composites. SiC-BN composites have been made using an in situ reaction between Si₃N₄, B₄C and C during hot pressing. Strengths and densities were high, as shown in Table 1 for selected composites; a composite with around 54 vol% BN showed a strength of 343 MPa.

Table 1. Properties of Selected BN Composites

Relative density (TD%)	Young’s modulus (GPa)	Flexural strength (MPa)	Vickers hardness (GPa)	Strain to failure (X10-3)
95	107	342	2.7	3.2
95	107	369	2.7	3.4
94	81	246	1.8	3.0
97	240	588	8.7	2.4
97	317	420	7.6	1.9

Source: National Industrial Research Institute of Nagoya

TiC-TiB₂-CrB composites have been made by hot pressing a mixture of TiC and CrB₂. When the raw material content was 65mol% of TiC and 35 mol% of CrB₂, and the hot pressing temperature was 1300 degrees C, a bending strength of 550 MPa was obtained even with more than 10% of porosity.

Oxide/oxide composites are also under investigation. Cordierite-ZrO₂ composites have been made, with mechanical properties improved by increasing ZrO₂ content. An increase to 25 wt% leads to a fracture strength of 290 MPa and a toughness of 3.5 MPa-m^½ . Alumina/YAG (yttrium aluminum garnet) composites have been synthesized using a novel method called pulse electric current sintering. The wear resistance of these materials depends on the YAG content: above 40% or none added at all results in large wear. Other oxide composites under investigation include Al₂O₃/LaPO₄, ZrO₂/LaPO₄, and spinel/mica (see Table 2.)

Table 2. Mechanical Properties of Spinel/Mica Composites

Sintering temperature degrees C	Flexure strength MPa	Vickers hardness GPa	Fracture toughness MPa-m½	Strength MPa
1300	293	12	1.53	175
1300	233	12	1.48	175
1200	203	12	1.13	147
1400	274	11	1.64	189
1400	242	11	1.72	199
1300	266	8.8	1.79	206

Oxides are being combined with carbides as well. SiC/ZrO₂ composites have been made with 95% densities. Beta-spodumene reinforced with ZrB₂ particles are being developed for cookware for induction heating stoves. A combination of a polymer precursor and oxide precursor has been used to make C-SiC-Al₂O₃ or C-SiC-SiO₂ composites.

Ceramics are not the only materials being reinforced. Metal matrix composites have also received some interest. For instance, high speed steel has been reinforced with long alumina fibers. This is a laminated composite, with alternating sheets of steel (with a thickness of 300 microns) and unidirectional fibers hot pressed at 1000 to 1200 degrees C for 1-2 h under a pressure of 20-40 MPa in argon. Densities ranging from 94 to 98% were achieved. On the other hand, ceramics have been reinforced with metal, specifically Al-Si₃N₄ by infiltrating molten aluminum into porous Si₃N₄ using hot isostatic pressing or Al₂O₃ reinforced with nickel particles.

Despite this progress, challenges remain before these materials find wide use. Japanese researchers are facing similar problems as others working in this area: high price of raw materials, oxidation issues, and end user resistance. But Japan may become the leader if the country’s track record in other advanced ceramics (like electronics) is any indication.

About the Author

Laurel M. Sheppard is President of Lash Publications International (www.lashpublications.com) and Technical Editor of Ceramic Industry (www.ceramicindustry.com). She has a B.S. in ceramic engineering and has written numerous articles on ceramic technology and manufacturing, as well as a market report on ceramic matrix composites for Business Communications Co. (www.buscom.com)

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