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Thermo-mechanical Characterization of Novel Emission-Reduced Refractories

Development of novel, carbon-reduced and carbon-free refractory materials.

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 focused on the thermo-mechanical characterization 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.

Comparison of visibility of crack path in a micrograph (left) and an image correlation processed picture (right) for an AZT sample.Fig. 1: Comparison of visibility of crack path in a micrograph (left) and an image correlation processed picture (right) for an AZT sample.

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 Fig. 1).
A comparison of the thermo-shock parameters reveals a high thermo-shock resistance of the AZT material in comparison the alumina at room temperature. An implementation of the experiments at elevated temperature (up to 1000 °C) is envisaged in order to understand the crack growth behavior under application relevant conditions. Initial results revealed that it is possible also to acquire good quality load-displacement curves also at elevated temperatures. Next step is the evaluation of the mechanical parameters at elevated temperatures that should permit an extension of the data evaluation.

Thermo-shock test with the electron beam facility

Development of an erosion crater in dependence of the thermo-shock timeFig. 2: Development of an erosion crater in dependence of the thermo-shock time

The electron beam facility JUDITH 2 (Juelich Divertor Test Facility in Hot Cells) was used for thermo-shock experiments (see Fig. 2). Single as well as cyclic thermo-shock experiments were carried out. MgO-C ceramics with different carbon content were tested where an area of 4 cm² was loaded with a power density of 42 MW/m².It was observed that the main failure mechanism was thermally assisted surface erosion, which dependent on the number and length of the thermal shock (Fig. 2). In addition a dependence of the mass loss on the carbon content was detected. Specimens with lower carbon content revealed under identical loading conditions a higher mass loss (inferior thermo-shock stability). The lower the carbon content of the MgO-C, the lower is the thermal conductivity, which will increase the thermally induced stresses leading to a stronger damage of the material.


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