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bustion depends in turn mainly on the type and quantity the Institute of M aterials, M inerals and M ining.of fuel used (i.e. the higher the molecular size and weight of the fuel the greater the generation of gas). As the distance between adjacent metal ions in the solution increases as a result of the larger size of the fuel molecule, the interaction of the hydrolysed metal ions will decrease, resulting in

toughened alumina.18 Zirconia is also known to exhibit excellent mechanical properties and superior fracture tough-Nanocrystalline MgAl 2 O 4 and ZrO 2 -MgAl 2 O 4 ness because of its unique phase transformations occurring powders were synthesised by combustion and convenover stress or temperature.19 According to Fujita et al., an tional solid state reaction routes. T he synthesised addition of 20 wt-%ZrO 2 to MgAl 2 O 4 followed by hot powders were processed, dry pressed, and sintered for pressing may increase fracture toughness and bend strength 3 h at temperatures ranging f rom 1550 to 1625°C. T he to 6•3 MPa m1/2 and 400 MPa respectively.10-12 In general, sintered pellets were then characterised in terms of ZrO 2 -MgAl 2 O 4 composite material exhibits better thermal phase (XRD), microstructure (SEM), relative density, shock resistance and chemical inertness than pure MgAl 2 O 4 , apparent porosity, water absorption, hardness, three and in the form of a honeycomb structure this composite point bend strength, and f racture toughness. T he XRD material nds application for molten metal ltration of studies revealed that ZrO 2 was present in tetragonal superalloys and cast steels.20 form in the case of combustion synthesised powders The most common method used for preparation of (CSP), whereas in powders obtained by solid state ceramic powders involves the solid state reaction of comreaction (SSP) it was present in the monoclinic ponent oxides at elevated temperatures. This method suVers form. T his study also revealed that the addition of from several limitations and may not produce a material ZrO 2 improved the mechanical properties of sintered with compositional homogeneity.21-28 It is also known that MgAl 2 O 4 samples: 20 wt-%ZrO 2 -MgAl 2 O 4 composites powders with particle size å 200 nm can be sintered to prepared f rom CSPs and conventional SSPs and sinproduce materials with ne grained microstructure and tered at 1625°C for 3 h had f racture toughness of 5•96 desirable properties. In order to produce ZrO 2 -MgAl 2 O 4 and 4•33 MPa m 1 / 2 and three point bend strength of 269 and 98 MPa respectively. Higher sintered density, composites with good compositional homogeneity, they the presence of tetragonal zirconia as a major phase, have also been prepared by methods including coprecipiand the ner microstructure are probably responsible tation,21 vapour phase oxidation,22 spray pyrolysis,23 freeze for the superior mechanical properties exhibited by drying,24 and sol-gel processing.25 Recently, Hwang and sintered CSP materials as compared with the sintered Kim have adopted the reaction sintering method for the SSPs. BCT /577 preparation of 10 wt-% cubic stabilised zirconia/spinel composites with relative density of around 98%, fracture tough-T he authors are in the Ceramic Materials Division, ness of 12 MPa m1/2, and strength of 240 MPa from a International Advanced Research Centre for Powder pressed compact mixture of monoclinic zirconia (baddeley-Metallurgy and New Materials, Balapur PO, ite) and Mg-Al metal...