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Techniques for CollectorsKnoops & Vickers Hardness data

21st Aug 2013 05:31 UTCAnonymous User

Hi All,

Does anyone know where I can download/look-up hardness data for minerals other than the Moh's scale i.e. Knoops or Vickers Hardness testing?

Thanks,

M. Burdett

21st Aug 2013 08:21 UTCBen Grguric Expert

Hi Mark,

For each species in the Handbook of Mineralogy (Vol 1 Sulphides etc) by Anthony et al there seems to be a pdf page available on the web. For example moncheite (http://rruff.info/doclib/hom/moncheite.pdf). This page has Vickers microhardness number (VHN) where available.

Uytenbogaart and Burke (1971) Table for Microscopic Identification of Ore Minerals. Dover Publications, has a table of ore minerals listed in order of increasing VHN.

Cheers,

Ben.

21st Aug 2013 16:33 UTCDonald Peck

With a loading of 100KG, the Vickers scale value can be computed from the Mohs Scale. logVHN = 2.5 (log Mohs) + 1.00

There are probably conversions for other loadings, but I don't know what they are.

21st Aug 2013 23:08 UTCOwen Lewis

Where does that mathematical relationship come from Donald?

My understanding is that Mohs's scale is simply a list of ten nominatad minerals, each numbered 1 - 10 and with a sample of one being able to scratch all the other numbered below it. The 'scale' is irrational; the hardness grades are in no mathematical ratio to each other and no SI unit of measurement can be applied to Mohs's gradings - it is inherently unitless. The saving grace of Mohs's scale is that there are so few points on the scale, so widely separated is terms of rationally measured hardness as to overcome (most?) of the inaccuracies inherent in different specimen qualities, surface finishes, stylus shapes, loadings and angles of attack used in Mohs's testing under field conditions. The more others have tried to refine the use of Mohs's scale the more glaring its limitations become. It is what it is. A pig's ear has real and significant uses - as a pig's ear. But if one tries to use it in place of a silk purse, it's bound to be a failure.

Vickers test is not a scratch test but a test of permanent deformation using a diamond indenter of a prescribed shape, Vickers scale is rational and, though it uses its own units of measurement, these are in a simple mathematical relationship to force applied/area of application in Imperial (and hence to SI) units.

Until a material harder than diamond was developed it was not possible to measure accurately the hardness of diamond (or, more properly, the hardnesses of diamond), this leap forward was first made by the Russians around 1980 using an advanced form of scratch testing on a nanoscale, with a fullerene stylus of prescribed shape, prepared (flat) surfaces of materials under test, carefully metered loading and a normal (90 deg) angle of application of sylus to specimen. This testing quantified for the first time what lapidarists had known for many years that, there is a substantial directional variation in diamond hardness. For the record, this variation in diamond hardness can be more (c. 30GPa) the variation in harness between corundum and talc :-)

Testing of other minerals with nano-scratch testing shows that Mohs's scale and testing method hold true enough (provided it is not 'refined') to Mohs's 8 but above that level the results from Mohs's testing is 'flaky'. Since it's appearance as a man-made gem in the 1970's, CZ has been Mohs rated as 8 or 8.5. Nano-scratch testing proved it to be harder than corundum (27GPa to 23 GPa) and therefore is Mohs 9+.

22nd Aug 2013 16:44 UTCDonald Peck

Owen, I got the formula from Joe Mandarino. I know he pulled it out of the literature, but from where, I don't know. I realize that the Mohs Scale is irrational, but perhaps this works because its plot has a resemblance to a logarithmic curve. In the few instances where I checked it out, the fit was pretty good. :-S

22nd Aug 2013 17:31 UTCDavid Von Bargen Manager

Original research on vickers hardness on Mohs "standards". Hardness can vary widely on which direction it is measured.

http://rruff.info/doclib/MinMag/Volume_28/28-206-718.pdf

22nd Aug 2013 17:47 UTCOwen Lewis

Well, here's a paper with some actual comparative Mohs/Vickers values. http://rruff.info/doclib/MinMag/Volume_28/28-206-718.pdf

I think this shows two things:

1. Hardnesses of Mohs's minerals is in many cases (most particularly Quartz) so uncertain in random testing as to highlight the element of 'wish fulfillment' inherent in interpolating decimal points into Moh's scale.

2. That there can be no true mathematical relationship between the Mohs and Vickers (or any other rational scale of hardness, all of which will correlate). But some keen soul might care to plot the graph (plotting all the ranged Vickers values) and over lay with the proposed logarithmically derived relation.

It should be noted also that even with the minimum Vickers loading of 50 g ised for this testing, the load is too high to to test accurately minerals in the Mohs 1 - 3 range. Mineral testing with a 100kg load must simply be fanciful (destructive) across most if not all the Mohs 1 - 9 range. Though converting to that scale would, in the hardness range that concerns us, so compress the Vickers values so calculated as to cease to show meaningful variations when displayed graphically?

For completness, diamond harbess it in the approximate range of 4 to 5 times as hard as corundum.

22nd Aug 2013 19:41 UTCDavid Von Bargen Manager

Diamonds can display a much greater range in hardness depending upon direction (up to possibly a factor of 100) - otherwise they could never be cut and polished. Bond strength is actually higher in some directions in graphite (but some other directions are extremely low)

http://www.minsocam.org/ammin/AM38/AM38_108.pdf

22nd Aug 2013 20:24 UTCGord Howe

Owen;

I think you pretty much nailed it. I have seen a chart somewhere (I can't seem to locate it at the moment) that compares Mohs to Brinell hardness. The gap between the Mohs values is very evident on the chart, and as you say, there can't be a mathematical relation between the two, only random points on the chart. If I can find it I'll surely post it.

Gord

22nd Aug 2013 20:46 UTCRob Woodside 🌟 Manager

Taking logs and plotting them tends to straighten things out and gives the impression that there is a simple relation. Check out the error bars on a log-log plot.

23rd Aug 2013 01:54 UTCHenry Barwood

Don, are you certain that formula says 100Kg? I have no weights that would not break the stage at that loading. I haven't done any microhardness testing in years, but it is one of those neglected techniques in RLM.

23rd Aug 2013 02:02 UTCOwen Lewis

David Von Bargen Wrote:

-------------------------------------------------------

> Diamonds can display a much greater range in

> hardness depending upon direction (up to possibly

> a factor of 100) - otherwise they could never be

> cut and polished. Bond strength is actually higher

> in some directions in graphite (but some other

> directions are extremely low)

>

> http://www.minsocam.org/ammin/AM38/AM38_108.pdf

David,

Many thanks for the link. I've kept a copy of that pdf.

Grinding/sawing a diamond is a different propositiion to scratch testing one or another of the crystal planes. You will have noted that the reference paper does not give any absolute hardness measurements for diamond. Though there were earlier (and rather inaccurate) attempts, the hardness diamond was only truly established once materials harder than diamond became available.

The absolute hardnesses for the 110 and 111 planes for type IIa diamond are approx 137 and 167 GPa respectively. This Russian work was published in 1997 by the American Institute of Physics. Sadly, I can't give you a link to it but if you would like to read the relevant page I can PM you a copy of the 1997 publication.

Best,

Owen

23rd Aug 2013 03:55 UTCDavid Garske

For the historical record, much research was done on directional diamond hardness at the University of Michigan in the 1960's and earlier.

Dave

23rd Aug 2013 12:47 UTCOwen Lewis

David (Garske),

Yes, and well before that too. This is essentially a study of the relative weaknesses in the diamond crystal structure and their exploitation to cut and polish a crystal. The very first diamond cut, the point cut, was simply the improvenent of a natural octahedral crystal to produce a cleaved octahedon that would better reflect the light. The first true cut, the table cut was developed in the 1400's. In this, the upper point of a cleaved octahedron was (over many weeks) sawn off to produce a polished table parallel to the stones girdle and give an additional and better angled reflecting plane. During the 15- and 1600s it was discovered how (with great skill and difficulty) to add a few more reflecting facets. Further improvement to the light handling properties of diamond gems then had to wait for science and engineering advances in the second half of the 1800's, when the next major steps forward were made. With the engineering largely solved the way was open, in 1919, to design and execute the theoretically 'Ideal' cut, still overwhelmingly used today. But some of us (me included) think that the Old European Cut of the late 1800's - 1920's and in stones of warm colour grades is the most desirable of all.

That is a completely different study to the determination of the absolute hardness of diamond, In the latter, there is no seeking out of and exploiting lines of weakness in the crystal. Rather, known loads are applied at 90 deg to the crystal planes via a stylus of prescribed profile and material (harder than diamond) to determine the load per test area required to produce a permanent plastic deformation of the surface without any chipping or cracking. These studies could not begin until the 1970's/80's and were not published in English before 1997 AFAIK.

In respect of diamond, the Russian study shows that that the true hardness of diamond was only about one half of the previous and generally accepted value of 261 GPa. The gross inaccuracy of this earlier accepted value resulted from having to use one diamond to determine the absolute hardness of another diamond. The 'front line' of accuracy on hardness testing has now moved on to establish the absolute hardness of the new super-hard materials. The basic problem remaining that one can only accurately determine hardness if your testing instrument is harder than the material under test.

23rd Aug 2013 17:24 UTCDonald Peck

Henry, the formula that was quoted to me was for VHN₁₀₀. I have never made a microhardness measurement, so I have no experience with it. I agree that 100Kg is a lot of mass.

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