Materials Chemistry and Physics 67 (2001) 263–266 Densification of mullite–SiC nanocomposite sol–gel precursors by pressureless sintering K.G.K. Warrier ∗ , G.M. Anilkumar Regional Research Laboratory, Council of Scientific and Industrial Research, Trivandrum 695019, India Abstract Mullite–SiC nanocomposite has been synthesised based on a nanoprecursor route and sintered to high density through pressureless sintering technique. The approach has been first to evolve a method to obtain high density, fine-grained mullite matrix phase through a sol–gel seeded route. Nanosize SiC particles (∼200 nm) were dispersed in a seeded mullite precursor sol to obtain mullite-coated SiC particles, which were further compacted and sintered to hybrid composites, resulting in distribution of SiC particles at the inter- and intra-mullite grain positions. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Sol–gel; Nanocomposites; Mullite; Silicon carbide 1. Introduction Ceramic matrix composites have shown enhanced me- chanical properties and microstructural features making them a class of materials of expanding applications [1,2]. The ceramic nanocomposites generally involve one phase in the nanosize range. The dispersed second phase prob- ably controls the inherent flaws at the grain boundaries, and reduces the residual stress at the interface of the ma- trix. When alumina dispersed with silicon carbide fine particles, excellent mechanical properties were observed. For example, a composite containing 5 vol.% nanosize sil- icon carbide in alumina matrix had increase of fracture strength from 320 to 1050 MPa, and fracture toughness from 3.2 to 4.7 MPa m 1/2 [3]. The proposed mechanism for such increase in toughness has been attributed to par- ticle bridging, crack pinning, crack deflection and stress induced micro-cracking [3,4].The primary microstructural requirement of such successful composite is minimum ag- glomeration of the second phase silicon carbide particles in the matrix and uniform distribution of such particles. The major ceramic systems investigated for the matrix are alumina, mullite, silica, zirconia and silicon nitride [5,6]. Similarly, a few of the second phase materials reported are, silicon carbide, silica, magnesia, titania, and sometimes nanosize metals such as silver, nickel, iron and cobalt [7,8]. The usual method of preparation of such composites is by mechanical mixing of the constituents followed by consoli- ∗ Corresponding author. E-mail address: warrier@csrrltrd.ren.nic.in (K.G.K. Warrier). dation and sintering, hot pressing or hot isostatic pressing [9]. More recently, scientific developments are reported for synthesis of such composites by techniques other than hot pressing and use of precoated precursor second phases [10]. Sol–gel methods are reported to be very effective in the synthesis of such composites in view of the advantages of achieving excellent dispersion of the second phase particles as well as possibility of sintering at lower temperature [11]. The major approaches reported for synthesis of nanocom- posites using sol–gel methods are mixing different compo- nents of the composite as sol particles, introduction of the nanophase component in sol containing the matrix phase, infiltration of the sol containing the second phase particles through a porous preform of the matrix phase, and use of precoated second phase particles [12]. Mullite has been considered a very good candidate ma- terial for high temperature ceramic applications in view of its high creep resistance, chemical stability, mechanical strength and relatively low thermal expansion coefficient and thermal shock resistance [13]. The low toughness limits its use in many areas of structural applications. Second phase zirconia particles are known to toughen the mullite. Incor- poration of nanosize silicon carbide particles, which have near similar thermal expansion properties, is indicated to improve the toughness of mullite [14]. Synthesis and characterisation of mullite–SiC nanocom- posite synthesised through the sol–gel route was employed. The silicon carbide particles were coated by a mullite precur- sor and were introduced as coated precursors in the mullite matrix. The sintering and densification of such composites are described. 0254-0584/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved. PII:S0254-0584(00)00447-8