Creep behaviour of alumina–mullite–zirconia nanocomposites obtained by a colloidal processing route

Abstract

The high-temperature creep behaviour of high-purity alumina (A) and an alumina–mullite–zirconia nanocomposite (AZS) has been studied. The alumina–mullite–zirconia nanocomposite was prepared by using a colloidal processing route (powder–alkoxide mixtures). Creep tests were carried out in air in a 4-point-bending-fixture from 1200 to 1400 °C under constant stresses ranging from 30 to 220 MPa. Creep parameters (stress exponent n and activation energy Q) were correlated with microstructural features in order to determine the dominant creep mechanisms for both materials. The slow crack growth region (SCG), given by pairs of critical stress and the corresponding critical strain rate at the temperatures 1200, 1300 and 1400 °C of both materials was studied. It was found that the creep rate of AZS was two orders of magnitude lower than the creep rate of undoped alumina A. The dominant creep mechanism of A is assumed to be a combination of grain boundary and lattice diffusion controlled creep. The creep mechanism for AZS is different and depends on the temperature. It is supposed that lattice diffusion controlled creep (Nabarro–Herring) is the dominant creep mechanism at 1200 °C, whereas at 1300 °C it is supposed to be grain boundary sliding accommodated by grain boundary diffusion. Comparing the slow crack growth region of both materials, a dramatic improvement was observed. The slow crack growth region of alumina is shifted nearly twice concerning the applied stresses for AZS at the temperatures 1200, 1300 and 1400 °C.