AMBIENT-TEMPERATURE MECHANICAL RESPONSE OF ALUMINA-FLUOROMICA LAMINATES

Flexural delamination experiments were used to evaluate the mechanical performance of thermochemically stable alumina-fluoromica laminates. Hot-pressed, precracked laminate specimens, in which two MgAl2O4-spine l-coated alumina substrates were separated by a thin layer of fluoroph logopite (KMg3(AlSi3)O10F2), were tested in four-point flexure at room temperature. Two types of mechanical response were observed: steady-s tate delamination and brittle failure. Microstructural analysis showed that the delamination response was associated with fine (less-than-or -equal-to 5 mum) grains of the mica; the brittle response occurred whe n the mica interphase consisted of large (>30 mum) grains that bridged the interphase. The steady-state strain-energy release rate (G(ss)) m easured on the graceful, delaminating beams was 9.1+/-0.4 J.m-2 for ra ndomly oriented approximately 5-mum grains but only 2.8+/-0.2 J.m-2 fo r approximately 1-mum grains that were aligned with easy-cleavage plan es parallel to the laminate interfaces. The results suggested that deb onding of the specimens occurred via cleavage of the mica grains. Obse rvation of delamination cracks confirmed this point: propagation occur red within the fluoromica interphase rather than along the spinel/alum ina or spinel/fluorophlogopite interfaces. The mechanical feasibility of laminate specimens without the protective spinel coating on the sub strate containing the notch was also tested to address an issue relate d to the preparation of alumina fiber/mica interphase/alumina matrix c omposites. The delamination response again occurred for the case of a fine-grained mica interphase.