Chapter 9.3 - Microstructural Control and Mechanical Properties

Abstract

The mechanical properties of ceramics have been tremendously improved through well-organized control of the microstructure elements during the past few decades. This chapter describes such research efforts, taking an example of silicon nitride. For example, it has become possible to realize both high strength and high fracture toughness in silicon nitride through controlling the size, morphology and alignment of the grains simultaneously, though these two properties have been believed to be in antagonistic relationship. At the same time, controlling the chemical phases of grain boundaries gives rise to further significant improvements in the fracture toughness, heat resistance, thermal conductivity, etc. Tremendous progress has also been made in understanding atomic behaviors in the grain boundaries. In addition, we also show how the mechanical properties change when the size, shapes, and orientation of pores as well as grains are controlled. Particular emphasis is placed in unique or improved properties including fracture energy, fracture toughness, strain tolerance (fracture strain), and thermal shock resistance through such porous structure control, even compared with those of the dense materials.