Bernhard Kanka

Aluminosilicate fiber/mullite matrix composites with favorable high-temperature properties

Abstract Oxide-based ceramic matrix composites with a highly porous mullite matrix and Nextel™ 720 alumino silicate fibers have been fabricated by infiltrating filaments with a mullite precursor slurry, and by subsequent one-dimensional (1D) and two-dimensional (2D)-winding up the fiber bundles on mandrels. The green bodies were pressureless sintered in air at 1300°C. These composites which require no fiber/matrix interface are characterized by favorable damage tolerance and bending strengths of 160 MPa at room temperature and up to temperatures of 1200°C. These properties make it an excellent low-cost choice for combustion chamber liners, diffusor rings and other thermal protection systems for high temperature applications in oxidizing environment.

A New Concept for Thermal Protection of All-Mullite Composites in Combustion Chambers

Abstract A new thermal protection concept for all-mullite composite shingles based on a thermally-sprayed mullite layer is described. Because of the insufficient thermal long-term stability of the Nextel™ 720 fibers in the 1273 plus regime, Nextel™ 720 fiber-based ceramic composites are protected by a flame-sprayed mullite coating in order to prevent the composite from thermal degradation in service. The protection layer is deposited on the front side of the ceramic shingle facing the hot gas stream. Front and back sides of the shingles are cooled through film and convection cooling, respectively. Reducing both the composite material and the protection layer to a single phase (mullite) system is a simple, but highly efficient approach to keep thermal and elastic misfit strains at the interface at reasonably low levels. Due to the porous grain texture intrinsic to thermally-sprayed materials, thermal conductivity of the protection layer is low, yielding a considerable thermal insulation effect depending on the layer thickness and the particular heat flow scenario of the combustion chamber. The microstructure/property relationship of the thermal protection layer and its interaction with the underlying composite are discussed focussing on the constraints of real combustion chamber operation conditions.

Temperature-Induced Fibre/Matrix Interactions in Porous Alumino Silicate Ceramic Matrix Composites

Abstract The thermal stability of alumino-silicate fibre (Nextel 720)/porous mullite matrix composites was investigated in the temperature range between 1300 and 1600°C. In the as-prepared state the fibres consist of mullite plus α-Al 2 O 3 , while the porous mullite matrix includes minor amounts of a SiO 2 -rich glass phase. Temperature-controlled reactions between the silica-rich glass phase of the matrix and α-Al 2 O 3 at the rims of the fibres to form mullite have been observed. At the end of this process, virtually all glass phase of the matrix is consumed. Simultaneously, alumina-free layers about 1 μm thick are formed at the periphery of the fibres. The mullite forming process is initiated above about 1500°C under short time heat-treatment conditions (2 h) and at much lower temperature (1300°C) under long-term annealing (1000 h). Subsequent to annealing below the thermal threshold, the composite is damage tolerant and only minor strength degradation occurs. Higher annealing temperatures, however, drastically reduce damage tolerance of the composites, caused by reaction-induced gradually increasing fibre/matrix bonding. According to this study, the thermal stability of alumino silicate (Nextel 720) fibre/mullite matrix composites ranges between 1500°C in short-term and 1300°C in long-term heat-treatment conditions.