Microstructure, strength and fracture characteristics of a free-standing plasma-sprayed alumina

Plasma spraying, commonly used to apply wear and heat-resistant barriers, can be adapted to produce free-standing bulk ceramic parts. In this,work the strength and fracture characteristics of a bulk plasma-sprayed alumina material with a highly aligned quasi-layered microstructure and about 14% porosity, are investigated for both the as-sprayed and annealed material conditions. It is found that strength and toughness for fracture normal to grain (splat) alignment are three times higher than for fracture parallel to the alignment. Subsequent annealing increases strength and toughness further by a factor of about three in both directions. The porosity change on annealing is low, despite a 9% volume shrinkage. Both material conditions exhibit pronounced R-curve behaviour. The marked strength and fracture anisotropy is clearly a consequence of the pronounced alignment of microstructure itself; and the occurrence of a splat-internal microcrack sub-structure in the as-sprayed condition. The fracture path observed in the as-sprayed material is primarily transgranular (i.e. through the splats), and not along splat interfaces as previously reported for plasma-sprayed ceramics. (C) 1997 Elsevier Science Limited.