Upgrading superplastic deformation performance of fine-grained alumina by graphite particles

Abstract

The process of ceramic part forming is often accompanied by grain growth, which in turn limits the deformation capabilities. Alumina ceramics are not usually good candidates for superplasticity as the grain growth reduces quickly their deformation performance. In order to impede the grain boundary mobility and to limit the grain growth, zirconia has often been associated with the alumina since the early 80s. In this work, a dense MgO-doped α-alumina containing a small amount of carbon has been produced. The sintering and the deformation behaviour of this material are compared to those of a MgO-doped alumina. The presence of small carbon particles reduces greatly the grain growth during the sintering and during the compression experiments. The superplasticity of the carbon-containing alumina is greatly improved.

Introduction

A growing range of structural ceramics have presented interesting performances in terms of superplastic deformation1, 2, 3, 4, 5. However these ceramics require a submicrometer grain size and a high temperature to exhibit a superplastic behaviour5, 6, 7, 8, 9. The thermal stability of such a grain size is a critical issue and its control is thus an essential step in achieving applicable superplasticity for structural ceramics.

Static and dynamic changes in grain size during deformation contribute to the overall growth which usually reduces continuously the superplastic capacity of the considered material. Different approaches have been experimented in order to control the grain growth during the deformation: (1) MgO has been added to alumina in order to lower the grain boundary mobility through solute drag[3]and (2) ZrO₂ particles have been particularly useful in impeding the grain growth of alumina by second phase pinning2, 3, 7. However the MgO effect is limited and one problem arises with the addition of ZrO₂ particles: their effectiveness is related to the homogeneity of their distribution which can evolve during the deformation of the material.

In the present work, another route was utilized for controlling the grain growth: a MgO-doped α-alumina polycrystal with a homogeneous distribution of graphite-pinning particles was produced. Those particles resulted from the thermal decomposition, during hot-pressing under vacuum, of the polymers used for the atomization of the alumina powder. In order to characterize the effect of those carbon particles, the hot-pressing procedure was also carried out on the calcined MgO-doped α-alumina powder. The microstructure investigations of the two sets of materials are presented and compared to each other. Moreover the compressive deformation behaviour of both materials are examined and compared at T=1400°C in air and under vacuum. The presence of carbon in the form of precipitates results in a remarkable improvement of superplastic behaviour of the α-alumina. Compressive deformations of true strains up to ε=−0.7 were achieved at a strain rate $\text{ε}\text{̇}\text{=10}{}^{\mathrm{-4}}$ s⁻¹ at T=1400°C at low flow stresses (less than σ=40 MPa) with only limited density loss. The correlations between strain rates, stresses and grain sizes are discussed in relation to different deformation mechanisms.