He chose ten common minerals minerals that anyone interested in minerals is likely to have in their collections and used them to set up a 'Hardness Scale'! He assigned each of the ten minerals a hardness number from 1 to 10, with 1 being the softest and 10 being the hardest! All other minerals and things that aren't minerals, too can be tested against the Mohs Scale minerals and given a value. What are the advantages and disadvantages of the 'simple' hardness scale as compared to the Mohs hardness scale?
My answer:. Many minerals have a range of hardness! Primarily used in engineering and metallurgy , indentation hardness seeks to characterise a material's hardness; i. It is usually measured by loading an indenter of specified geometry onto the material and measuring the dimensions of the resulting indentation.
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 particle size. This is known as the Hall-Petch effect. However, below a critical grain-size, hardness decreases with decreasing grain size.
This is known as the inverse Hall-Petch effect. For measuring hardness of nanograined materials, nanoindentation is used. It is a "superhard" molecular rod, comprised of acetylene units. It is important to note that hardness of a material to deformation is dependent to its microdurability or small-scale shear modulus in any direction, not to any rigidity or stiffness properties such as the bulk modulus or Young's modulus. Scientists and journalists often confuse stiffness for hardness [1] [2] , and spuriously report materials that are not actually harder than diamond because the anisotropy of their solid cells compromise hardness in other dimensions, resulting in a material prone to spalling and flaking in squamose or acicular habits in that dimension.
In other words, a claimed hard material should have similar hardness characteristics at any location on its surface. Also known as dynamic hardness , rebound hardness measures the height of the "bounce" of a diamond-tipped hammer dropped from a fixed height onto a material.
The device used to take this measurement is known as a scleroscope. One scale that measures rebound hardness is the Bennett Hardness Scale. In solid mechanics , solids generally have three responses to force , depending on the amount of force and the type of material:. Hardness is not a fundamental material property, so it should be viewed in context with other properties such as strength, elasticity and ductility to understand how these properties relate to each other.
For example, while a material may be hard, it can also be brittle. Material hardness is measured using a hardness test. The value of hardness is dependent on several factors, such as:.
Materials will exhibit different hardness measurements under different types of loading. For instance, a material that performs well under a short-duration, high-impact load may not perform as well when subjected to a continuous long-duration load. Therefore, hardness tests should be carried out under various load conditions to ensure that well-informed engineering and design decisions can be made.
The three main types of hardness are scratch, rebound and indentation hardness. Each of these parameters is measured using different tests that use different sets of tools.
Below, we will discuss these hardness types in detail. During indentation hardness tests, the test subject is subjected to a continuous point load until an impression is formed.
Indentation hardness tests are one of the most common hardness tests performed across several industries as materials are often subjected to continuous loading in real-world applications. This type of hardness is typically measured on a scale known as the Rockwell Scale. Figure 1. Rockwell hardness testing equipment. These scratches are typically narrow deformations caused by the removal of material due to contact with a sharp, harder material.
Scratch hardness tests are usually performed on brittle materials, such as ceramics and minerals because they do not undergo plastic deformation. This type of hardness is quantified on a scale known as the Mohs Scale, which is based on the relative scratch hardness of various materials. On this scale, talc is assigned a value of 1, while diamond is assigned a value of
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