High temperature characterisation of cordierite–mullite refractory by ultrasonic means

Refractory materials containing cordierite (2MgO·2Al2O3·5SiO2) and mullite (3Al2O3·2SiO2) are used as support in furnaces, because of their low thermal expansion properties (coefficient of thermal expansion (CTE) ≈3–4 × 10−6 K−1) which confer them a very good ability to thermal shock resistance. Composed of two phases presenting very different CTE (1.5–3 × 10−6 for cordierite and 4–6 × 10−6 K−1 for mullite), these materials can develop damage during thermal cycling due to internal stresses. This paper is devoted to the characterisation of the damage generated by this CTE mismatch, thanks to the application of ultrasonic techniques like long bar mode echography and acoustic emission (AE). The combination of these two techniques allows, during the applied thermal cycle (20–1200 °C), to continuously follow both the evolution of the elastic properties (Young's modulus) and the acoustic emission activity generated within the material. The analysis of these two characteristics, which are closely related to the damage evolution, makes it possible to propose a chronology of the mechanisms (damage, expansion) acting during the heating and the cooling stages.