Ceramic materials hardness

FIG. 8-69 Ecc( ntric plii valve shown in erosion-resistant reverse flow direction, Shaded components can he made of hard metal or ceramic materials, Cm 1 li(isy Fislier-R/iseuimini. ... 

Network solids are typically hard and rigid they have high melting and boiling points. Ceramic materials are commonly network solids. 

Aluminum oxide, A1203, is known almost universally as alumina. It exists with a variety of crystal structures, many of which form important ceramic materials (see Section 14.22). As a-alumina, it is the very hard, stable, crystalline substance corundum impure microcrystalline corundum is the purple-black abrasive known as emery. Some impure forms of alumina are beautiful, rare, and highly prized (Fig. 14.25). A less dense and more reactive form of the oxide is y-alumina. This form absorbs water and is used as the stationary phase in chromatography. 

The ultimate goal of assemblies of nanoscale MBBs is to create nanostructures with improved properties and functionality heretofore unavailable to conventional materials and devices. As a result, one should be able to alter and engineer materials with desired properties. For example, ceramics and metals produced through controlled consolidation of their MBBs are shown to possess properties substantially improved and different from materials with coarse microstmctures. Such different and improved properties include greater hardness and higher yield strength in the case of metals and better ductility in the case of ceramic materials [102]. 

Three main properties render clay suitable for making ceramic materials its plasticity when wet, its hardness when dry, and the toughness, increased hardness, and stability that it acquires when fired. The addition of water to dry clay produces a clay-water mixture that, within a narrow range of water content, has plastic properties it is deformed, without breaking or cracking, by the application of an external stress, and it retains the acquired shape when the deforming stress is removed. Wet clay mixtures can, therefore, be modeled, molded, or otherwise made to acquire a shape that will be retained after the forming operations. Water-poor mixtures are not plastic, however, and excess water results in mixtures, known as slips, that are too fluid to retain a shape, as shown in Table 56. 

Hardness Estimation of Minerals, Rocks and Ceramic Materials... 

Although in practice the formation of inner cracks requires a certain threshold loading, for most brittle ceramic materials this threshold is negligibly small (usually less than 1 newton, seen clearly in hardness tests). It is thought that cracks make well defined spheres entirely beneath the contact zone, and that they grow downwards as the load is applied. Such a system presents a complicated elastic-plastic problem. 

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