Abstract
The incorporation of a ductile second phase into a monolithic ceramic is a well-established toughening technique. This study is concerned with the toughening of alpha-alumina/20 vol% iron ceramic matrix composites. The main emphasis has been to investigate the effect of the morphology of the iron on the toughness of the composite material. The fabrication process employed was that of hot pressing. Manipulation of the starting powders has resulted in the formation of hot pressed materials with morphologically different microstructures (a difference in the distribution of the iron particles throughout the alumina matrix). The fracture toughness of the materials has been measured using indentation, double torsion and double cantilever beam methods. The indentation technique was found to be inappropriate for the finite measurement of fracture toughness due to the non-ideal crack patterns produced with each indentation event. All three tests, however, found the composite materials to be tougher than the parent matrix and comparatively accurate fracture toughness values were produced by the double torsion (DT) and double cantilever beam (DCB) testing techniques. The toughest composite was found to be the one which possessed a combination of long-crack (a network distribution of iron particles) and short-crack (discrete iron particles) toughening mechanisms. The DCB test was developed in this study to enable the in situ observation of crack/particle interactions at the microscopic level. Observation of the interactions was made possible by performing the tests on a straining stage which was mounted inside a scanning electron microscope. The observed toughening mechanisms for each composite were isolated and incorporated into a numerical analysis. This analysis allowed specific energy values to be allocated to each type of toughening mechanism. It was found that the energies for the intrinsic toughness of the matrix, debonding and plastic deformation were in the correct proportion to each other and the energy attributed to plastic deformation compared favourably with a theoretically derived value.