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In Materials Science , hardness is the characteristic of a Solid material expressing its resistance to permanent deformation. Hardness can be measured on the Mohs Scale or various other scales. There are three principal Operational Definition s of hardness: #Scratch hardness #Indentation hardness #Rebound, dynamic or absolute hardness SCRATCH HARDNESS In Mineralogy , ''hardness'' commonly refers to a material's ability to penetrate softer materials. An object made of a ''hard'' material will scratch an object made of a ''softer'' material. Scratch hardness is usually measured on the Mohs Scale Of Mineral Hardness . Pure Diamond is the hardest known natural mineral substance and will scratch any other material. Diamond is therefore used to cut other diamonds; in particular, higher-grade diamonds are used to cut lower-grade diamonds. The hardest substance known today is Aggregated Diamond Nanorods , with a hardness 1.11 times diamond. Estimates from proposed molecular structure indicate the hardness of Beta Carbon Nitride should also be greater than diamond (but less than Ultrahard Fullerite ). This material has not yet been successfully synthesized. In the , a material 40 times harder than diamond. It is a superhard molecular rod, comprised of Acetylene units. INDENTATION HARDNESS Primarily used in Engineering and Metallurgy , indentation hardness seeks to characterise a material's hardness, i.e. its resistance to permanent, and in particular Plastic , deformation. It is usually measured by loading an indenter of specified geometry onto the material and measuring the dimensions of the resulting indentation. There are several alternative definitions of indentation hardness, the most common of which are:
There is, in general, no simple relationship between the results of different hardness tests. Though there are practical conversion tables for hard steels, for example, some materials show qualitatively different behaviours under the various measurement methods. Hardness increases with decreasing Grain Size . This is known as the Hall-Petch Effect . However, below a critical grain-size, hardness decreases with decreases grain size. This is known as the inverse Hall-Petch effect. For measuring hardness of nanograined materials, Nanoindentation is used. REBOUND HARDNESS Also known as ''dynamic'' or ''absolute hardness'', rebound hardness measures the height of rebound of an indenter dropped onto a material using an instrument known as a Scleroscope . One scale that measures rebound hardness is the Bennett Hardness Scale . REFERENCES
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