Session BB9.5

4:30 PM BB9.5
MICROSTRUCTURE AND DEFORMATION CHARACTERISTICS OF PLASMA SPRAY-FORMED 2/1 MULLITE CERAMICS UPON ANNEALING. Udo Steinhauser, Wolfgang Braue, Juergen Goering, Bernd Kanka, Hartmut Schneider, German Aerospace Research Establishment (DLR), Materials Research Institute, Cologne, GERMANY; Ekkehard H. Lutz, LWK Plasmakeramik GmbH., Gummersbach, GERMANY.

Mullite-based plasma-spray formed ceramics (MPC) exhibit both a marked property anisotropy and a non-linear stress-strain behavior due to their typical porous laminar grain structure. The effects of annealing on the microstructure-property anisotropy and the macroscopic deformation behavior of a 2/1 mullite composition has been addressed through the combined approach of mechanical testing and SEM, TEM, XRD, and DSC investigations. Upon heat treatments between 1000$^{\circ}$C and 1700$^{\circ}$C the rather low MOR(20 MPa) and Young's modulus (35 GPa) levels in the as-sprayed state show a significant increase by a factor of the order of 2 which is even more pronounced in other plasma ceramics. Simultaneously, the load-displacement characteristics gradually change from a non-linear, damage-tolerant response for as-sprayed mullite to a linear behavior emphasizing the effects of a more rigid microstructure after sintering of adjoining mullite lamellae upon the heat treatment. Despite their brittleness however, the structural integrity of annealed MPC's upon thermal shock is maintained due to substantial microcrack formation. Burner rig and combustion chamber tests employing bulk MPC's as well as thin MPC-based protective layers on ceramic substrates confirm that the excellent thermal stability of MPC's is retained upon annealing in reducing and oxidizing atmospheres employing different flame compositions.