Microstructure of Y2O3 doped Al2O3–ZrO2 eutectics grown by the laser floating zone method

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Abstract

Al2O3–ZrO2 eutectics containing 9 mol% Y2O3 (with respect to zirconia) were produced by directional solidification using the laser floating zone (LFZ) method. The eutectic microstructures were investigated as a function of the growth variables. Using a solidification-axis thermal gradient of 600 °C/mm a homogeneous, colony-free, interpenetrating lamellar microstructure was obtained for growth rates less than 50 mm/h. Higher growth rates produce cellular structures. Colonies grew with the [0001] alumina and the [110] zirconia axis parallel to the growth direction. The uniform lamellar microstructure obtained at low growth rates is stable during thermal treatment at 1500 °C.

Introduction

Micro-composite material systems can be fabricated via the solidification of eutectic melts. These composites are highly homogeneous, present strongly bonded phases as well as a high density of interfacial area together with short interphase distances. The combination of these properties produces an improvement of the material properties, particularly excellent mechanical behavior and thermal stability up to near the melting temperature.1, 2

Al2O3–ZrO2 melt-grown eutectics are surprisingly tough at room temperature and up to near the eutectic point.3 As they are very resistant to grain growth and creep, they are candidates for applications where a combination of high strength and toughness at high temperature and chemical inertness are needed. In a previous work the relationship between the mechanical properties and the microstructure in these compounds was studied.4 The microstructure was a very decisive aspect in determining the mechanical properties of the sample. In fact, the colony was the critical defect in these samples. The microstructure resulting from different solidification procedures using either the Bridgman,5 floating zone melting,6, 7 edge-defined film-fed growth,8 or micro-pulling down methods,9 has been studied previously. Recently, the production of large plates of Al2O3–ZrO2 eutectic composites by laser zone melting has been reported.10 However, because of the difficulty in controlling and reproducing experimental conditions the relationship between processing parameters and microstructure is still uncertain.

When solidified directionally they usually show a columnar colony microstructure; colonies are formed by faceted alumina grains grown along the [0001] sapphire c-axis which contain a very fine and regular dispersion of ZrO2 rods. The grains are surrounded by a coarse granular microstructure.11 The colony size decreases as the growth rate increases,12 and it is replaced by a lamellar microstructure near the rod edge in rod shaped samples.3, 11 Understanding this transition is very important in order to improve the material properties. Unfortunately, the situation is far from being clear. For example, Courtright et al.7 reported that the microstructure changed from fibrous to lamellar as the growth rate increased. However, Echigoya et al.6 and Lee et al.9 found that the degenerated lamellar microstructure was obtained for slow growing rates.

In this work, rods of Al2O3–ZrO2(9 mol% Y2O3) were grown using the laser floating zone (LFZ) method. The principal goal was to search for growth parameters to achieve a homogeneous, colony-free microstructure that is expected to provide better mechanical properties than the colony structure owing to the smaller size of the defects.4, 13 The orientation relationship between the component phases and the thermal stability as a function of the different microstructures were also studied.

Section snippets

Experimental procedure

Starting materials were commercially available Al2O3 (99.99%, Aldrich), ZrO2 (99+%, Alfa) and Y2O3 (99.99%, Aldrich) powders. The powders were milled in a vibratory micro-mill (model MM2000, Retsch, Haan, Germany) with alumina components, fired in air at 1000 °C for 1 h, hand-milled in an agate mortar and mixed into the desired compositions. The composition of the samples was 62 mol% Al2O3–34.5 mol% ZrO2–3.5 mol% Y2O3. It was formulated along the liquidus line between the eutectic point of the

Results and discussion

Determination of the component phases was carried out using both Raman spectroscopy and X-ray diffraction (XRD) techniques. The eutectic samples are composed of α-alumina and cubic zirconia phases.

Conclusions

Al2O3–ZrO2(9 mol% Y2O3) melt-growth composites were produced by the laser float zone method. The microstructure was studied as a function of the processing parameters and thermal gradient; growth rate and rod radius values for optimal crack free, homogeneous, interpenetrating microstructures were obtained. Thermal annealing experiments indicate that the interpenetrating coupled microstructure is stable during thermal treatments at high temperature. Crystallography studies determined the

Acknowledgements

We gratefully acknowledge support from the Spanish Ministry of Science and Technology (CICYT MAT2000–1495 and MAT2000–1533-C03–02). N.R. Harlan also thanks the Ford Foundation (USA) for a fellowship. Dr M.C. Sanchez is acknowledged for his help in the XRD measurements.

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    Ford foundation Post-Doctoral Fellow, USA, present address: CETENASA, Noain, Navarra, Spain.

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