Acta mater. 49 (2001) 811–816 www.elsevier.com/locate/actamat CERAMIC NANOCOMPOSITES OBTAINED BY SOL–GEL COATING OF SUBMICRON POWDERS A. BOULLE, Z. OUDJEDI, R. GUINEBRETIE ` RE, B. SOULESTIN and A. DAUGER² Science des Proce ´de ´s Ce ´ramiques et Traitements de Surfaces, UMR CNRS 6638, ENSCI, 47 Avenue Albert Thomas, 87065 Limoges, France ( Received 3 September 1999; received in revised form 3 November 2000; accepted 6 November 2000 ) Abstract—MgAl 2 O 4 –ZrO 2 nanocomposites were fabricated by conventional sintering of composite powders obtained by sol–gel coating of a submicron spinel powder. In the composite powder the zirconia grains remain narrow sized and completely tetragonal even after being heat treated at temperatures where a free xerogel is completely monoclinic. The sintered material exhibits a dense, fine and highly homogeneous micro- structure. The zirconia nanoparticles are located at both inter- and intragranular positions and exhibit heteroep- itaxial relationships with the surrounding crystals. Tetragonal zirconia seems to be stabilised by an interface effect. Both the scale of the microstructure and the fraction of intragranular grains were controlled by adjusting the mean grain size of spinel grains before coating and sintering conditions.  2001 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. Keywords: Sol–gel; Coating; Nanocomposites; Microstructure; Grain growth 1. INTRODUCTION The spinel MgAl 2 O 4 is a well-known refractory oxide showing no phase transition up to its melting point (2105°C). Sintering of the spinel MgAl 2 O 4 is gov- erned by volume diffusion [1, 2]. It is thus impossible to obtain a dense material by means of conventional sintering at temperatures below 1700°C. Moreover, significant grain growth strongly inhibits sintering kinetics [3]. However, almost dense spinel can be achieved below 1600°C by hot pressing [4, 5] or by liquid phase sintering with CaO additions [6]. Another attractive feature of MgAl 2 O 4 is its ability to superplastic deformation at high temperature [5, 7]. Hindering grain growth by a second phase addition such as zirconia will thus promote both sintering at lower temperature [8–14] and the ability to achieve superplastic deformation [15–18]. The benefits of the presence of metastable tetra- gonal zirconia particles in ceramic materials are well known [19–23]. In more recent years, great attention has been paid to composite materials containing a nanosized second phase [15–17, 24–26]. The mechan- ical properties of the so-called nanocomposites are significantly increased in comparison with microcom- posites [15, 24]. Moreover, the presence of intragran- ular second phase particles results in a significant ² To whom all correspondence should be addressed. Tel.: +33-5-5545-2222; fax: +33-5-5579-0998. 1359-6454/01/$20.00  2001 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. PII:S1359-6454(00)00366-9 toughness increase on the one hand [15, 18, 25, 26] and in enhanced superplastic ductility on the other hand [15, 18]. Since a nanoparticle is an almost perfect single crystal, the cracks propagate essentially at the inter- face between the particles and the surrounding grains [25]. Great improvement in mechanical properties is thus expected from a clean interface [25], and a for- tiori from a semi-coherent interface. Moreover, such interfaces are believed to stabilise the tetragonal phase in the case of ZrO 2 particles [27, 28]. The sol–gel process has been reported to be a method of choice for the elaboration of nanocompos- ites [29]. In this paper we present a method based on sol–gel coating of submicron powders, leading to dense and fine grained MgAl 2 O 4 –ZrO 2 nanocompos- ites containing both inter- and intragranular tetragonal zirconia particles. Structural and microstructural aspects are discussed. The effects of the sintering con- ditions and of the morphology of the spinel powders are also considered. 2. EXPERIMENTAL PROCEDURE 2.1. Characterisation 2.1.1. X-ray diffraction (XRD). The experiments were carried out on a high resolution X-ray diffrac- tion set-up. A four reflections monochromator pro- vides high resolution (divergence = 12 arcsec, relative wavelength spread = 1.410 -4 ). The instru-