Layered materials with high strength and flaw tolerance based on alumina and aluminium titanate

Laminates in which high strength external layers and flaw tolerant internal layers with similar compositions are combined, can provide improved mechanical behaviour in relation to monolithic materials with the same composition as the layers. The limitation of this design, in which no residual stresses are present, is the difficulty in co-sintering layers with large microstructural differences in the green state. This work describes a new method to obtain laminates constituted by layers with large differences in terms of grain size starting from green bodies with similar microstructures. The approach is based on the effect of small amounts of titania as agents for alumina grain growth enhancement. Starting from fine grained green bodies that combined alumina layers with composite layers made of mixtures of alumina and titania, additional “in situ” formed layers constituted by large (≅20–30 μm) alumina grains were found after sintering contiguous to the composite layers. The thickness of the “in situ” formed layers reached up to 200 μm, depending on the thermal treatment (1450–1550 °C). The fracture behaviour of the laminates and the monoliths was studied, using stable Single Edge V Notched Beam (SEVNB) tests, in terms of work of fracture and the critical stress intensity factor in mode I, KIC. The large grain sized alumina layers reinforced the laminates by crack branching and bridging.