Thermo-mechanical Characterisation of Emission-Reduced Refractories (Project Area: Testing Technology)
E. Skiera, J. Linke, J. Malzbender, C. Thomser, R. Steinbrech
The aim of the DFG SPP “FIRE” is the development of novel, carbon-reduced and carbon-free refractory materials. Within the framework of this project the work is focussed on the thermo-mechanical characterisation of newly-developed refractories, materials which are developed in other subprojects. Investigations on the mechanism of crack growth and crack resistance as well as thermal shock experiments are carried out.
Conventionally materials are tested in downward thermal shocks with cooling in water or air. In this project the implementation of electron beam material test facilities, which are well known from the fusion research, offers an innovative method for thermal shock tests in an upward mode. Moreover results will be compared to application relevant thermal shock tests of refractories in molten metal, which are performed at RWTH Aachen.
Characterization of crack growth with the wedge splitting test
Controlled crack propagation experiments were carried out using a wedge splitting test (WST) using pre-notched specimens of cubic shape (20 mm edge length). The crack growth was observed in-situ by optical (LM) and electron microscopy (SEM) on a polished side face of the specimen. The crack development was continuously monitored and documented in digital micrographs (see Figure 1). Specially developed image correlation tools helped to visualize the fracture path (see Figure 1). From the measured data thermal shock parameters have been derived. Two refractory materials from TU Freiberg, consisting of an almost pure alumina (99 % Al2O3), and of an alumina with titania and zirconia additives (AZT: 95 % Al2O3, 2.5 % TiO2, 2.5 % ZrO2) have been investigated. Both materials were sintered at 1600°C and had a maximum grain size of about 1 mm. An almost straight crack path was observed in alumina, whereas in AZT it is kinked and shows branches (see Figure 1). Initial results imply that the thermal shock resistance is higher in AZT compared to the almost pure alumina, which has to be confirmed in further ongoing tests.
Thermal shock test by electron beam facilities
The electron beam facility JUDITH 2 (Juelich Divertor Test Facility in Hot Cells) was used for thermal shock experiments (see Figure 2). Single as well as cyclic thermal shock experiments were carried out. For comparison with the carbon-free ceramics, the behaviour of an MgO-C material with a carbon-content of 10 % was also investigated. Power density dependent, thermally induced surface erosion was observed for MgO-C.
Initial thermal shock experiments for the carbon-free materials demonstrated a charging of the samples that led to a shielding of the electron beam. After successful modification of the sample preparation procedure, several experiments with varying parameters have been realized. An area of 4 cm² was heated up to temperatures of more than 1600 °C with an absorbed power density of about 38 MW/m². For pulse durations of 15 ms the AZT material shows no deterioration in a single thermal shock test, but in cyclic thermal shock experiments cracking occurs after approximately 10 pulses (see Figure 2).