Polycrystalline fracture toughness

Conversion of glass to a polycrystalline ceramic is accompanied by increased strength (two to four times), increased fracture toughness (two to four times), increased electrical resistivity (10 times), increased deformation temperature (200-400°C), increased abrasion resistance, and increased thermal shock resistance. All these factors contribute to many applications for glass ceramics dinnerware, cooking utensils, stove tops, radomes, hermetic seals to metals, building materials, and so on. 

These two methods of synthesis have led to the discovery that at compositions around x = 2, a monophase, polycrystalline NASION is very difficult to produce, even though it can be sintered to reasonable mass density ( >3.20 g/cm ) at relatively low temperatures (1100-1375°C). Unfortunately these materials contain a ZtOz second phase, possess poor mechanical strength with a low fracture toughness, have an anomalous thermal expansion behaviour, and are subject to chemical and phase instability during densification . 

P. Hing and G. W. Groves, The strength and fracture toughness of polycrystalline magnesium oxide containing metallic particles and fibers. J. Mater. Sci. 7(4), 427-434 (1972). 

It is experimentally well established that the fracture toughness of a polycrystalline ceramic is appreciably higher than that of single crystals of the same composition. For example, Ki. of single-crystal alumina is about... 

Polycrystalline cubic boron nitride is preferable over single crystal cBN because it can be manufactured in bigger sizes and has higher fracture toughness. However, like its counterpart pcD, it can only be sintered at very high pressures and temperatures with the aid of binders because of its strong covalent bonds. 

An indication of the relative position of various cutting tool materials with respect to their wear resistance and fracture toughness is shown in Fig. 38. Diamond is by far the most wear resistant material, particularly in pure single crystal form, but because single crystal diamond is a brittle material, it does not have the average toughness of the polycrystalline forms - both pcD (cobalt-containing) and CVD diamond (which contains no metal phases). 

The mechanical properties of polycrystalline ZnO ceramics are of special importance for their applications as varistors (see Section 1.4.4.1). When a varistor experiences a high-current pulse, the electrical energy is quickly converted to heat. The inertia of the material, which resists its thermal expansion, and the resonances of the resultant elastic waves in the block, may lead to microcracks and finally to mechanical failure [107]. Characteristic values of mechanical properties of ZnO ceramics are 1.2-1.4 MPa m for fracture toughness, and 100-125 MPa for flexural strength. 

The use of yttria ceramics as standalone materials for structural applications is rather limited by their poor mechanical properties. For example, the commercially available 99.9% polycrystalline yttria CeraDoy (Ceradyne Inc.) has a flexural strength of 99 MPa, a hardness of 5.85 GPa, and a fiacture toughness of 1.4MPam. The hardness and fracture toughness of polycrystalline yttria are virtually grain-size... 

It was observed that the addition of ceria deteriorates the mechanical properties, including the fracture toughness of yttria-stabilized ZTA, although reports exist to the contrary [46] for polycrystalline yttria-stabilized ZTA, in which fracmre toughness is increased by the addition of ceria. 

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