Acta metall, mater. Vol. 41, No. 6, pp. 1647-1652, 1993 0956-7151/93 $6.00 + 0.00 Printed in Great Britain. All rights reserved Copyright © 1993 Pergamon Press Ltd MICROSTRUCTURE AND MECHANICAL PROPERTIES OF MULLITE-ZIRCONIA REACTION-SINTERED COMPOSITES R. TORRECILLAS 1, J. S. MOYA 2, S. DE AZA 2, H. GROS 3 and G. FANTOZZI 3 qnstituto Tecnol6gico de Materiales, 33428 Llanera Asturias, Spain, 2Instituto de Cerfimica y Vidrio CSIC, Arganda del Rey, 28500 Madrid, Spain and 3GEMPPM INSA de Lyon, 69621 Villeurbanne Cedex, France (Received 25 November 1991; in revised form 7 December 1992) Abstract--The flexural strength, fracture toughness (Klc), creep behaviour and thermal shock of mullite~irconia and mullite-zirconia-alumina composites obtained by reaction-sintering of zircon + alumina mixtures have been studied in the temperature interval ranging from room temperature to 1400°C. The results are discussed in terms of the microstructural features of the reaction-sintered composites. 1. INTRODUCTION Mullite and mullite-based composites are presently considered as potential engineering materials for high temperature applications. However, only few works have focused on the study of the high temperature mechanical behaviour of this group of structural materials [1-3]. Mullite-zirconia based composites obtained by reaction sintering of zircon + alumina mixtures are nowadays considered of high techno- logical interest, because of the low cost of the starting materials and processing route compared with other alternative routes (i.e. sol-gel, solid state sintering, etc.) [4-9]. The present study was undertaken to determine the thermomechanical behaviour of reaction-sintered mullite-zirconia and mullite-zirconia-alumina com- posites, and to establish correlations with the micro- structure in terms of distribution and structure of the phases. 2. EXPERIMENTAL PROCEDURE 2. I. Processing The following have been used as starting materials: (i) Zircon fine powdert (Na20, 0.05; MgO, 0.01; CaO, 0.09; Fe203, 0.07) with 1/zm average particle size. (ii) s-Alumina powder:~ (Na2O, 0.01; MgO, 0.1; CaO, 0.02; Fe203, 0.03) with 0.5 #m mean particle size. Zircon + alumina mixtures have been formulated according to the following equations 2ZrSiO4 + 3A1203 ~ 3A1203 2SIO2 + 2ZrO2 labelled MZ tCeraten SA, Getafe Madrid, Spain. ~Alcoa CT 3000 SG, Germany. 2ZrSiO4 + 4A1203 , 3A1203 2SiO: + AI203 q- 2ZrO 2 labelled MZA. The mixtures have been attrition milled for 2 h in isopropyl alcohol media using alumina balls. The slips have been subsequently spray dried into spheri- cal agglomerates with a diameter ranging from 50 to 150/~m. These powders were CIP at 200 MPa into blocks of 50 x 50 x 10 mm dimensions. Blocks of MZ were fired at 1600°C/4 h and the MZA one at 1580°C/4 h. Bars of 6×4x40mm, 3x4x40mm and ~b 30 x 3 mm sizes were sawn from these blocks for mechanical tests. The tensile surface was polished down to 6/~m. 2.2. Mechanical tests Flexural strength measurements have been carried out on 4-point bending tests (35 and 10 mm out and inner span) at a crosshead-speed of 0.1 mm/min at a temperature in the range from 20 to 1400°C. Fracture toughness values have been determined by 4-point bending test as a function of loading rate, in the same temperature range. The sample dimensions were 6 x 4 x 40 mm. A notch of 0.3 mm width was ma- chined at low cutting speed to a relative depth a/w ~, 0.4. Creep tests have been carried out in air on 4-point bending system (36 mm outerspan and 18 mm inner span) on 3 x 4 x 40 mm bars. The creep strain has been calculated from the deflection using Hollenberg method [5]. The stress and temperature dependence of the creep rate is determined with the following equation = A (tr/G)" exp( - Q/R T) where ~ is the steady state creep rate, A a constant dependant on microstructure and composition of the sample, G the shear modulus, T the absolute tempera- ture, Q the activation energy, R the gas constant, a 1647