Properties of ceramics intrinsically depend on their chemical composition, bonding etc. However, macroscopic properties of ceramics are also governed by microstructure. Since, microstructure itself is governed by processing route, it is important to understand the effect of microstructural features on the properties of ceramics. Invariably, for more than the last fifty years researchers across the globe have established a strong relation between grain size, which by far is one of the important microstructural features and the properties of ceramics. However, to the best knowledge of authors there is no article which consolidates this link. Therefore, this review article presents effect of grain size on the mechanical, thermal, optical, electrical, dielectric and piezoelectric properties of ceramics. However, grain size is not the sole parameter governing the properties of ceramics and therefore effect of grain size in relation with other parameters like: densification level, flaw size, indentation area, phase (crystal structure) formed and the nature of phases segregated to the grain boundaries etc. is also elaborately presented.

For more than the last 50 years, researchers across the globe have studied grain size dependence of various properties of ceramics. While developing the understanding of properties of materials one needs to take into account crystal structure of the material, prevalent at a typical service condition, the manufacturing route by which a material is produced and the microstructure of the material. Microstructure of the material itself depends on the processing route adopted for the manufacturing of materials. Therefore, it is essential to understand that even though the inherent behavior of a material gets governed by the atomic/molecular bonding and its crystal structure, if enough attention is not paid to the manufacturing of the material, desired microstructure required for an application may not be achieved, resulting in inferior macroscopic properties of the materials. It is a well-known fact that ceramics inherently display stronger bonding resulting in high melting point and high modulus of elasticity. However, during processing of ceramics, if due care is not taken in processes such as powder synthesis and shaping and sintering, it is very likely that the ceramics formed may contain defects such as a deviation from stoichiometry, a formation of an undesirable phase (crystal structure), defective microstructure, insufficient densification etc. Moreover, it also needs to be understood that even if high purity ceramic powder consisting of desired phase is synthesized, due care still needs to be taken in order to produce dense polycrystalline ceramics so that they exhibit the desired performance under the operational conditions. While describing the microstructural features of a ceramic, it is customary to specify the average grain size, since grain size indirectly also gives an idea about the concentration of grain boundaries which are the planar defects. Therefore, even though microstructural evolution in the ceramics as a function of various parameters, like sintering time and temperature, pressure applied during pressure assisted sintering, amount of additives added in liquid phase sintering etc. has been studied in each of these cases researchers have correlated these aspects to grain size and have also correlated these parameters to the properties of ceramics through the grain size and other microstructural features. Therefore, this paper presents a detailed review of effect of grain size on the following properties of ceramics: mechanical, thermal, optical, electrical conductivity, dielectric and piezoelectric. Since the majority of the readers of Metal Powder Report work on hard metals and refractory ceramics, the first half of this paper is dedicated to the grain size dependence of mechanical properties of ceramics, whereas the second half of the paper gives glimpses of the effect of grain size on the other properties mentioned above.

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