Ceramic compositions, microstructure

This chapter aims to describe the principles behind the processing, microstructural development and properties of particulate ceramic composites and to illustrate these using experimental results. The main emphasis is on examples where the addition of particulates to a ceramic matrix causes new mechanisms to operate that give an improvement in properties greater than would be expected from a rule of mixtures . The chapter concentrates almost exclusively on structural composites, since this is where most work has been done to date. Particulate nanocomposites are included in the chapter, since the important examples described are currently at the coarse end of the nanoscale , and the principles underpinning their properties seem to be a simple extension of those relevant to the microcomposites with which the rest of the chapter is concerned. 

In the literature, there has been no report about production of functionally graded SiAION ceramics by tape casting. The main advantage of this method with respect to others is that continuous change in composition, microstructure and mechanical properties can be obtained by stacking controlled layer thicknesses of different tape compositions. 

Because the first reports on CNT-ceramic composites date only from 1998, and because only a few teams have worked so far on these novel materials, it could be argued that we are at the infancy of the development of a new class of composite materials. Researches on these materials depend firstly on a better knowledge of the CNTs by their users. Depending on their microstructural characteristics (SWCNTs, individual or in ropes, MWCNTs, diameter, length, number of walls), but also on the synthesis methods which have been used, the properties of CNTs may greatly vary. Notably, the treatments involved in the control of the surface properties and reactivity of the CNTs need to be optimized for a particular form of CNTs synthesized by a particular method. 

As with other ceramic composites, the combination of a- and/or P-sialon with reinforcement agents results in sialon composites. This simple and obvious statement encompasses many factors which must be taken into account for successfully fabricating composites with a designed microstructure and improved properties (Prewo, 1989). For sialon matrix composites, the most important factors are physical compatibility including Young s modulus, elastic strain (Kerans and Parthasarathy, 1991) and thermal expansion coefficient (Sambell etal., 1972a, b), and chemical compatibility between sialon matrix... 

Functionally graded materials (FGMs) are multifunctional materials, which contain a spatial variation in composition and/or microstructure for the specific purpose of controlling variations in thermal, structural or functional properties. Also in the ceramics composites field, a wide range of functionally graded (FG) ceramics are available. Hence, a possible classification of the different classes is made in this chapter. 

The need to develop fibers with better microstructural stability at elevated temperatures and ability to retain their properties between 1000-2000°C. The requirements of fiber properties for strong and tough ceramic composites have been discussed by DiCarlo.83 A small diameter, stoichiometric SiC fiber fabricated by either CVD or polymer pyrolysis, and a microstructur-ally stable, creep-resistant oxide fiber appear to be the most promising reinforcements. 

Fig. 8.15 Changes in interface microstructure in SiC fiber-reinforced BMAS glass-ceramic composites induced by exposure to high temperature oxidizing environments, (a) After tensile stress-rupture experiment at 1100°C, the 90° fibers show a distinct dual layer at the BN coating-fiber interface, (b) After thermal aging for 500 h at 1200°C, a subtle double layer appears at the same site, (c) Near the composite surface, the effects of thermal aging (and oxidation) are more pronounced.24...

---