Mechanical Properties of Alumina Matrix Composites due to a Combination of Sr- and Ca-Hexaluminates

Abstract

Alumina matrix composites have been successfully used in a wide range of applications for decades. Even alumina itself provides several desirable properties e.g. strength, chemical and thermal stability together with hardness, its toughness is still to be concerned. An addition of some intermediate phases such as alkali earth hexaluminates can enhance fracture toughness according to their unique morphology. This work investigated a variation in mechanical properties of alumina as a presence of two types of dispersed phases: strontium hexaluminate and calcium hexaluminate. The preformed hexaluminate compounds were prepared from a powder mixture of the alkali earth carbonates and alumina. The preformed hexaluminates were then added into alumina, followed by pressureless sintered at 1600 oC for 2 hours in air. A change in mechanical properties of alumina according to types and amount of additives as well as the ratio of calcium hexaluminate to strontium hexaluminate compounds on was studied. The composites with the relative density of at least 93%TD were achieved. Both hexaluminates resulted in a significant increase in fracture toughness relative to the monolithic alumina while they slightly affected on hardness and strength. When the ratio of calcium hexaluminate to strontium hexaluminate in alumina matrix was considered, a rise in fracture toughness strongly depended on the fraction of calcium hexaluinate. The calcium hexaluminate tended to be more efficient additive in order to strengthen alumina than that of the strontium ones