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Study of Mechanical Properties of Aluminum Nanocomposites using Auto Ignition Technique



Al2O3 - high ZrO2 nanocomposite was synthesized by auto-ignition technique using metal nitrate and citric acid with a ratio of citrate: nitrate equal to 1:1.5. The precursor zirconium oxynitrite was derived from precipitation of zirconium oxycloride followed by dissolution of gel in nitric acid. The thermal decomposition behaviour of the gel was studied in order to study the decomposition behaviour and to determine the crystallization temperature. The effect of calcination temperature on phase formation of the powder was studied. The composite powder was characterized by X-ray diffraction, surface area analysis, agglomerate size distribution etc. The non-isothermal densification of the compact showed a duplex sintering behaviour. The ZTA compacts were sintered in the temperature range 1400 to 1600oC to study the densification behaviour and phases formed. Higher density (95%) was achieved at higher temperature. The effect of sintering temperature and porosity on the hardness, compressive strength and flexural strength of the ZTA sintered specimens were also studied.

Key words: Al2O3-ZrO2, Auto ignition, Flexural Strength, Compressive Strength

1. Introduction
Alumina - Zirconia composites show the highest mechanical property than the individual components [1]. The improvement in strength and toughness has been explained with respect to shape and volume change of the tetragonal zirconia precipitates during a stress-induced or micro-crack induced transformation to the stable m-ZrO2 [2]. The first mechanism of improvement is due to the low fraction of zirconia grains which enhances strength by inhibiting alumina grain growth [3]. The second mechanism is the metastable retention of the tetragonal grain through chemical stabilization resulting from the limited alumina dissolution in the zirconia [4]. The combination of both mechanisms counters the crack deflection and strengthens the composites. However at low fraction zirconia, the availability of t-phase is not enough to give better toughness. Lange [5] reported that improved strength and toughness could be achieved for ceramic containing equal quantities of t-ZrO2 and -Al2O3. The relative density decreases with increase in zirconia content as well as temperature due to t-m ZrO2 transformation [6]. The Y-ZTA prepared by mixing route showed an inhomogeneous distribution of phases which reduces t-phase. The fabrication of dense and fine grained Al2O3-ZrO2 composites with the ZrO2 grains less than the critical size is achieved by different methods such as sol-gel, hydrothermal, co-precipitation [7, 8, 9]. The self-ignition process seems to be quite simple, does not involve multiple steps and excellent control of stoichiometric homogeneity. K. C. Patil et al [10] reported the variation of zirconia in 3Y-ZTA composites using different nitrate to carbohydrazide molar ratio in combustion technique and found m-ZrO2 phase appears at 15000C. S. Biamino [11] observed that higher sucrose to metal ratio showed higher surface area for the individual synthesis of alumina and zirconia systems. S.T.Aruna et al [12] reported the effect of mixed fuels on the particle size and phase retention at higher temperature for 20 wt% zirconia-alumina composite. K. A. Singh et al [13] demonstrated that crystallite size of zirconia increases with fuel (citric acid) content. However, very few literatures are available on the mechanical property of combustion synthesized ZTA composite with higher zirconia content.
In the present work the Al2O3-high ZrO2 powders were prepared by nitrate-citrate auto ignition method. The citrate to nitrate ratio was kept 1:1.5. The phase evolution, particle size distribution, agglomeration strength, densification behaviour and mechanical properties were studied in detail.

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