131 Introduction

Structural ceramics may be classified in many ways, but for this chapter structural ceramics will be limited to (1) monolithic materials developed for application in power generation systems, for example, gas turbines and diesels engines [1-3], and certain biomedical applications, for example, balls for artificial hips [4-7], and (2) ceramic matrix composites (CMC) being developed for power generation systems, for example, gas turbines and for industrial applications such as hotgas filters in advanced coal-fired plants or circulating fans in very aggressive environmental conditions [8]. In most engineering applications where warranty issues play an important role, nondestructive evaluation (NDE) methods need to be in place to assess the condition of components at scheduled or unscheduled opportunities. For ceramics to be economically competitive, processing costs must be kept low while at the same time assure the user of a reliable product. One way to reduce costs is to improve yield. Yield can be improved through rejection of parts in the early processing steps prior to sintering, machining, and proof testing. There are several steps in the processing of most ceramic materials. Table 13.1 lists the steps often used to produce monolithic structural ceramics. These several processing steps can introduce "flaws" in the materials, not all failure causing. The critical flaw size is dependent upon the fracture toughness. Table 13.2 shows the wide range of material fracture toughness values of various ceramic materials, both monolithics and composites. Table 13.2 also notes the critical flaw sizes for many of these materials. It is the relatively small flaw sizes, 10-100 ^m, in monolithic materials that have provided challenges to the NDE community. Ceramic composites with higher fracture toughness have critical flaw sizes that are much larger and the failure modes are completely different than for monolithics. Another important aspect about ceramic materials, from an NDE standpoint, is that the design methodology is based on probability statistics [9, 10]. This means that there is going to be variability in the base material itself.

0 0

Post a comment