Materials Science and Engineering A 419 (2006) 290–296 Hardness and fracture toughness of mullite–zirconia composites obtained by slip casting L.B. Garrido a,∗ , E.F. Aglietti a , L. Martorello b , M.A. Camerucci b , A.L. Cavalieri b a CETMIC, C. Centenario y 506, C.C. 49, B1897 ZCA, M.B. Gonnet, Argentina b INTEMA, Fac. Ingenier´ ıa-UNMdP, Av. J.B. Justo 4302, 7600 Mar del Plata, Argentina Received 17 December 2004; received in revised form 21 November 2005; accepted 2 January 2006 Abstract Mullite–ZrO 2 composites were obtained by reaction-sintering using three alumina to zircon weight ratios (45.5/54.5, 51.7/48.3 and 35.1/64.9 wt.%). Green compacts were made by slip casting of stabilized aqueous suspensions of alumina and zircon mixtures and then were reaction-sintered from 1450 to 1600 ◦ C under different firing conditions. The phase evolution was followed by XRD, the densification degree was calculated from densities and the microstructures were studied by SEM. Hardness, H and fracture toughness, K IC were measured by Vickers inden- tation. Effects of both alumina/zircon ratios and sintering conditions on the H, K IC and the microstructure were examined. Inter- and intragranular mechanisms of fracture were observed. Mainly a phase transformation toughening was assumed although crack deflection and microcracking mechanisms cannot be discarded. © 2006 Elsevier B.V. All rights reserved. Keywords: Ceramics; Indentation; Mechanical properties 1. Introduction Dispersed zirconia particles, added as a second phase to mullite materials, enhance their thermomechanical properties mainly by transformation toughening [1,2] and also by other mechanisms such as microcracking or crack deflection. Mullite–ZrO 2 composites can be made from alumina and zircon mixtures [3]. Without sintering aids or stabilizing oxides, the densification is achieved at 1450–1500 ◦ C and the complete mullitization occurs at temperatures near 1600 ◦ C [4]. Previous studies [4,5] have shown that typical microstructural features of the composites are important factors to determine their final properties. In order to ensure good mechanical properties, slip casting is used as a conventional method to obtain dense, fine and homogeneous microstructures free of agglomerates and packing variations in the green compact. The conventional indentation techniques are widely used for characterizing intrinsic mechanical parameters in order to com- pare and select brittle materials. The advantages of this method ∗ Corresponding author. Tel.: +54 221 4840247; fax: +54 221 4710075. E-mail addresses: lgarrido@cetmic.unlp.edu.ar (L.B. Garrido), andcamer@fi.mdp.edu.ar (M.A. Camerucci). include the small size of the test specimen, the easy specimen preparation and the simplicity of the test [6]. When the hard indenter comes into contact with the specimen surface, which causes irreversible deformation and a residual impression, it is possible to measure the hardness (H) of the material. The anal- ysis of the cracks emerging from the corners of the impression provides quantitative information on fracture toughness (K IC ) [7]. It is well known that, both H and K IC , depend on the microstructure, which is strongly influenced by the processing and heating schedule. The aim of the present work is to study the variation of the H and K IC mechanical parameters as a function of the microstruc- tures of mullite–zirconia composites. These composites were reaction-sintered from alumina–zircon mixtures and their tough- ening mechanisms analyzed. 2. Experimental Starting powders were commercially available alumina (- Al 2 O 3 , A-16SG Alcoa Inc., USA) and zircon (ZrSiO 4 , Mahlw- erke Kreutz, Mikron, Germany) with unimodal narrow particle size distributions (average size particle: 0.6 and 1.8 m and BET specific surface areas: 9.5 and 4.1 m 2 /g, respectively). 0921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.01.035