Microstructural development and the evolution of defects in constrained and sinter-forged ceramics

Abstract

In this study, the microstructural evolution of uniaxial loaded, or constrained sintered YSZ compacts and the growth of cracks and defects in constrained ceramic films is reported. It is shown that a uniaxial load applied to a YSZ compact will lead to a modified densification. In addition to a modified densification, the pore structure is shown to exhibit an anisotropic modification from the applied load, the magnitude of anisotropy increasing with load, temperature, and localized density. Further investigation of this anisotropic micro-pore behavior with artificial controlled sized pores shows that there is a critical pore size at which the behavior changes from that of nano-pores to that of micro-pores.The growth of cracks and holes in constrained YSZ films is investigated by introducing artificial defects with a Focused Ion Beam into green centrifuge-cast YSZ films. This technique for studying defect growth allows micron scale defects to be introduced into a fragile green film for sintering studies. Stress concentrations near defects will lead to increased damage and crack growth in the films. Channel cracks are observed in the films with introduced defects, and are absent in films without defects. Sintering cracks are observed in some samples with high interfacial friction and large thickness. From analyzing the percent of each crack growth, small crack to large crack behavior transition point is observed.Currently, most densification models assume isotropic sintering behavior. It is shown in this study that assumption is inaccurate. This research has increased the knowledge base of anisotropic microstructure evolution, an area that is currently not very well understood but applicable to many important systems.