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Mohs Scale of Hardness and Modern Materials

written by: KennethSleight • edited by: Lamar Stonecypher • updated: 6/27/2011

The ancient idea of testing the hardness of a particular rock by trying to scratch it with another rock was standardized in the early 1800s. This system is still in use today due to its ridiculous ease of use even though other systems have been developed that are far more precise.

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    Diamond covered bits Mohs scale was the first system for determining the relative hardness of common minerals. (Although this method was known as far back as 300 B.C. as Theophrastus explained it in his treatise On Stones.) So, who “invented “the Mohs hardness scale? Some people think it is an acronym for “measure of hardness scale," but it was developed in 1812 by Friedrich Mohs (this is why it is never "Moh’s scale" as the name of the inventor is actually Mohs) for on-site determination of the hardness of unknown minerals during project excavation. This scale is still used in a teaching capacity today.

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    The relative hardness test was more than adequate for elementary building needs, but as technology advanced new hardness tests were developed that used a standard scale instead of a relative one. Therefore, Mohs scale is only relevant as a quick reference to determine which type of cutting element should be used.

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    Mohs Scale

    Mohs Scale

    Mohs scale was determined using minerals that were highly available at the time (early 1800’s). The lowest number a mineral can be scratched by and the mineral can scratch determines placement on the scale. Thus an item that can be scratched by apatite (5) and scratch fluorite (4) falls between the two. Here is the actual scale (available in a kit), as it still exists today.

    1. Talc
    2. Gypsum
    3. Calcite
    4. Fluorite
    5. Apatite
    6. Orthoclase Feldspar
    7. Quartz
    8. Topaz
    9. Corundum
    10. Diamond

    In Mohs scale diamond is the hardest mineral rating a 10 while Talc is the lowest with a rating of one. This does not mean that diamond is 10 times harder than talc; in fact, the difference is actually 1600 times based on absolute hardness tests. Because of the relativity of the scale the change from one hardness rating to the next is arbitrary. The difference between corundum and diamond (9 and 10) is 300 percent while the difference between calcite to fluorite (3 and 4) is only 25 percent. This has led to the addition of half points on the Mohs scale for several minerals like chromium, which rates an 8.5 because it will scratch topaz but can be scratched by corundum.

    Extended Mohs Scale

    The new and improved version of Mohs scale has extended the number of known items to 15. The following is the extended scale:

    1. Liquid
    2. Gypsum
    3. Calcite
    4. Fluorite
    5. Apatite
    6. Orthoclase Feldspar
    7. Vitreous pure silica
    8. Quartz
    9. Topaz
    10. Garnet
    11. Fused zirconia
    12. Fused alumina
    13. Silicon carbide
    14. Boron carbide
    15. Diamond

    This is the current field kit used by surveyors to determine the approximate hardness of unknowns in prospective building environments.

    Common Items

    If you don’t happen to carry a Mohs kit around with you on an everyday basis, knowing the hardness of some common items may help you place an unknown item in an approximate place on the scale. Graphite (used as pencil lead) rates a paltry 1.5 on the scale, a fingernail is 2.5, an American coin (penny, nickel, dime, or quarter) rates a 3.5, a pocketknife blade comes in at 5.5, and a hardened steel file is between 7 and 8.

    Carbon Fibers

    There are places where it is inappropriate to use Mohs scale; carbon fiber is one of these places. It is a misnomer to believe that Mohs hardness scale determines strength for all materials, as it only measures relative hardness, which is a measure of the ability of a material to resist abrasion. Many very hard objects are brittle and break easily even as they are resistant to scratching. Carbon fibers can be easily cut with standard steel drill bits and even pocketknives, but this doesn’t mean that they are not strong. In fact, the carbon fiber is so thin that the mesh may actually be tearing and not the actual carbon fibers themselves. In this case, there in a shredding effect that only rips through the structure but doesn’t actually scratch the surface. In addition, carbon fiber is a catch-all for several different chemically bonded materials. Therefore, the hardness of one particular carbon fiber will be different from that of another. Carbon nanotubes would be (if they were measurable) in the same range as diamond because the strength of their structure is comparable.

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    Superhard Materials

    The interest in superhard materials (materials that are harder than diamond) has increased as the need to manufacture items with high tolerances to abrasion has increased. Although coating drill bits and saw blades with diamond is common practice, at high heat diamonds burn. This leads to complications if blades burn out too quickly and need to be replaced. This slows production and adds to cost. A superhard material like nanocrystalline diamond, borazon, or beta-carbon nitride would be far more efficient for cutting steel.

    Another use for these superhard materials is using them for coatings on bearings, gears, or other motor parts to extend the life of these components considerably. As the costs to manufacture these materials decrease, their usage will increase and we will start to see them in commercial uses instead of just the major industrial applications that are currently using them.

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    Simple Acronyms and Mnemonic Devices

    Here are a couple of possible memory devices to help you remember the order of Mohs scale:

    The Girls Can Flirt And Other Queer Things Can Do or,

    The great Captain Fitch acted out quietly to capture dusk

    and my personal favorite,

    Two Girls Coming From Alaska On Quick Trains Carrying Diamonds.

    Even though Mohs hardness scale is ancient by technological standards, it is still a good way to make rough estimates of comparative hardness. As a field tool it is still has value but when precision engineering is necessary, the use of an absolute hardness test (Vickers, Knoop, Meyer, or Brinell) is used.

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    References

    Mineralogical Society of America; Collector's Corner; MOHS' SCALE OF HARDNESS. http://www.minsocam.org/MSA/collectors_corner/article/mohs.htm

    CALCE and the University of Maryland; Material Hardness; 2001. http://www.calce.umd.edu/TSFA/Hardness_ad_.htm

    Diamond covered bits image courtesy of Wanderlinse @ FlickR http://www.flickr.com/photos/wanderlinse/4390218010/sizes/m/in/photostream/

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