article posted 21 March 2016
Carsten Blaeß is at the German Standards Bureau...
Crack healing in glass matrix composites
Carsten Blaeß*, Stefan Reinsch & Ralf Müller
Bundesanstalt für Materialforschung und –prüfung (BAM), Richard-Willstätter-Str. 11, 12489 Berlin, Germany
Comprehensive work was done to achieve SOFC sealing glasses matching a complex property combination including precisely tuned CTE, long-term thermal stability of micro structure, chemical inertness, appropriate sealing temperatures and electrical resistivity. More recently, emphasis was progressively focused on mechanical stability and cyclability. In this context, crack initialization and healing is of utmost importance. Both features may be contrarily affected by micro structure, e.g. the crystal volume fraction.
This work therefore aims to study basic influences of micro structure on crack initialization, propagation and healing. For this sake, glass matrix model composites mimicking sealant microstructures have been synthesized using inert filler particles of different size, shape and volume fraction. A matrix glass was chosen possessing minor crystallization tendency and minor glass - filler chemical interaction because of the prolonged annealing procedures required for large crack healing. Cracks are generated by Vickers indention or during heating or cooling of composite - rigid substrate sandwich samples. Crack healing was observed after various thermal treatments by means of light microscopy or in situ by means of heating stage ESEM experiments. The poster will present first results showing that crack healing not only depends on crack size but also on crack geometry. Thus, radial cracks, caused by Vickers indentation can initially heal to a certain extent before the crack is broadened while rounding and bulging up of the four Vickers quadrants both as a result of minimizing the free glass surface area. Thus, healing of much larger planar cracks, caused by thermal stresses during cooling, was completed well before healing of Vickers indentation caused cracks.