Mechanical High-Temperature Properties and Damage Behavior of Coarse-Grained Alumina Refractory Metal Composites.

Anja Weidner,Tilo Zienert,Yvonne Ranglack-Klemm,Horst Biermann,Christos G Aneziris

doi:10.3390/ma12233927

Abstract

The present study provides the mechanical properties of a new generation of refractory composites based on coarse-grained Al2O3 ceramic and the refractory metals Nb and Ta. The materials were manufactured by refractory castable technology and subsequently sintered at 1600 °C for 4 h. The mechanical properties and the damage behavior of the coarse-grained refractory composites were investigated at high temperatures between 1300 and 1500 °C. The compressive strength is given as a function of temperature for materials with two different volume fractions of the refractory metals Ta and Nb. It is demonstrated that these refractory composites do not fail in a completely brittle manner in the studied temperature range. The compressive strength for all materials significantly decreases with increasing temperature. Failure occurred due to the formation of cracks along the ceramic/metal interfaces of the coarse-grained Al2O3 particles. In microstructural observations of sintered specimens, the formation of tantalates, as well as niobium oxides, were observed. The lower compressive strength of coarse-grained Nb-Al2O3 refractory composites compared to Ta-Al2O3 is probably attributed to the formation of niobium oxides. The formation of tantalates, however, seems to have no detrimental effect on compressive strength.

Highlights

Ceramic–metal composites benefit from the combination of a high melting point, hardness and the chemical stability of ceramics, with the high toughness and ductility of metals
The aim of the present paper is to investigate a new generation of coarse-grained refractory composites [39] based on pre-synthesized coarse grains of a ceramic (Al2 O3 ) and refractory metals (Nb, Ta)
High-temperature mechanical properties were determined for the new class of coarse-grained horizontal curve develops in the further course of the test, i.e., there are almost no further relaxation refractory composites, based on Al2 O3 and 11 or 21 vol % of refractory metals Ta and Nb, respectively

Summary

Introduction

Ceramic–metal composites benefit from the combination of a high melting point, hardness and the chemical stability of ceramics, with the high toughness and ductility of metals. The upper limit of the application temperature of such composites is restricted by the melting point of the metal, the reaction between the metal and ceramic particles, the chemical interaction with the environment, or the thermal mismatch between the ceramic and the metallic phases. The use of refractory metals with a high melting point could make such composites applicable at even higher temperatures [1]. Niobium and tantalum exhibit a similar coefficient of thermal expansion (CTE) as alumina in a wide temperature range [4]. Kirby et al [4] showed that, between 1000 and 2000 ◦ C, a linear increase of CTE occurred for all three materials, with 25–34 × 10−6 ·K−1 for alumina, 24–30 × 10−6 ·K−1 for niobium, and 20–25 × 10−6 ·K−1 for tantalum.

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