Tout comme la dureté des métaux et de leurs alliages est mesurée de manière pratique à l'aide des tests de dureté Rockwell, Brinell et Vickers, la dureté des minéraux est mieux déterminée à l'aide du test de dureté de Moh.
The hardness test is all about determining the resistance of the mineral to be scratched. Hardness is basically the characteristic of the material instead of being its fundamental physical property.
Different types of materials are measured for their hardness using various hardness testing methods.
L'échelle de dureté de Moh a été développée par un minéralogiste allemand nommé Friedrich Mohs en 1812.
Ten minerals of different hardness were selected by him that ranged from being very soft ones (talc) to the hardest mineral (diamond). All of the minerals are easy to obtain with the exception of a diamond.
Moh’s hardness test holds primary importance when it comes to identifying mineral specimens.
Ten reference minerals ranging from hardness on a scale of 1 to 10 are used to compare the resistance of a mineral that has to be scratched.
The test is greatly used because most of the mineral specimen is very close to the hardness of minerals provided in Moh’s hardness scale.
Moh’s hardness scale
Moh’s hardness scale is demonstrated using the following table
Making comparisons of hardness
The resistance of the material to be scratched is referred to as “hardness”. For conducting a test, a sharp point of one specimen is placed on an unmarked surface attempting to make a scratch to another specimen.
While observing the darkness of two specimens, there can be four situations you might witness:
- Specimen A tends to be harder than specimen B if specimen A scratches specimen B.
- If specimen B doesn’t get scratched by specimen A, specimen B tends to be harder than specimen A.
- If both the specimen is relatively ineffective while they scratch one another, it is possible that two specimens are of equal hardness.
- If there are three specimen A, B, and C and specimen A cannot be scratched by specimen C while it can be scratched by specimen B, it is expected that specimen A has the hardness between the hardness of specimen B and C.
The testing procedure of Moh’s hardness
Moh’s hardness testing procedure consists of the following steps
- Begin by identifying and locating an unscratched and smooth surface for the testing procedure.
- Hold the specimen of unknown hardness with one hand against the top of the table firmly so that the surface to be tested is accessible and exposed. By placing it on the top of the table, you can have a firm grip on your specimen, and you’ll be able to hold it motionless so that it will not affect the findings of the test.
- In your other hand, hold one of the specimens of standard hardness. Now, select the flat surface of the unknown specimen and place it against the specimen of known hardness.
- Press the point of the standard hardness specimen firmly against the unknown specimen. Then, drag the point of the standard hardness specimen towards the surface of the unknown specimen.
- The surface of the unknown specimen should be examined. Brush away any powder or any mineral fragments with the help of your finger. Examine whether a scratch was produced from the test? A scratch will not be some kind of a mark that wipes away on the surface. Instead, it would be a distinctive groove cut made on the surface of the mineral that shouldn’t be confused with a mineral residue or powder.
- You can conduct the test twice to confirm your findings.
Some great tips for Moh’s hardness test
To achieve viable results every time you conduct Moh’s hardness test, a few tips can be followed. Some of these are discussed below:
- Some specimens actually contain impurities that might interfere with your test results. If the information from your tests or the results is not conclusive, try testing it again. It is quite possible that an impurity might be embedded in one of your tested specimens.
- You have to be very careful while handling the granular or small specimen as some hard materials can also be very brittle. It is possible that one of your specimens might start crumbling or breaking rather than scratching.
- The specimen shouldn’t be rubbed back and forth against another specimen as it may not produce any mark and alter the results. The test should be performed gently with the goal of cutting a scratch with a single determined motion.
- This type of testing shouldn’t be done on fine furniture. Rather, this test should be performed on a workbench or lab table with a protective covering or durable surface.
- For a handy reference, a list of minerals in their relative order of hardness can be very beneficial. You can get a list of potential minerals quickly by determining the hardness of a specific specimen.
Toughness, strength, and hardness
While conducting Moh’s hardness test, you should always keep in mind that your purpose is to test “the resistance to scratching”.
There’s a possibility that the specimen might fail in other ways while performing the test. They might deform, crumble or break instead of scratching.
Due to the lack of toughness, hard materials are expected to break when they are subjected to stress. The crumbliness or deformation of materials can be due to a lack of strength when they are subjected to stress.
Uses for Moh’s hardness test
Moh’s hardness tests are extensively used for determining the relative hardness of specimens of minerals.
These tests are performed as a part of mineral identification procedures in the classroom, laboratory or in the field when other tests are not available or when the easily identified materials are being examined.
Hardness tests are essential to determine the suitability of materials that are used in industry for various purposes, including a specific end-use application or a specific industrial process.
To confirm and test whether the hardening treatments such as tempering, annealing, case hardening or work hardening have been done to specification, Moh’s hardness test is ideal to be conducted in manufacturing processes.
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