View Full Version : multiple hardness grades for diamond?

03-20-2013, 07:36 PM
I was talking to a glass artist, a journeyman carpenter, jack-of-all-trades kind of guy. He seemed very knowledgeable about things, so I asked him about how to cut the mineral crystal that looks like glass in the front of a wood stove. He said to use a soft grade of diamond wheel in a saw. I told him I'd never heard of varying grades of hardness on diamonds, in fact they are all no. 10 on the Mohs scale as far as I knew. He grinned knowingly and told me oh no, there are different grades of hardness on industrial diamond saw blades.

This true?


03-20-2013, 09:00 PM
Hardness of an abrasive wheel is usally the hardness of the binder, not the abrasive, afaik.

'soft' binders release abrasive faster so the wheel stays sharp, for grinding really hard things that will wear down the diamond quickly.

03-20-2013, 09:18 PM
yep... what Moons says. I have four different Diamond saw blades for Green Concrete, Stone, Asphalt and Cured Concrete. The difference is primarily the binder, then the size and concentration of the diamonds

Jaakko Fagerlund
03-21-2013, 05:59 AM
There are different structures in the diamonds and some of them are better than the others, but 'better' here depends on the application.

But in grinding etc. common use, the binder makes the thing work.

03-21-2013, 08:21 AM
It's also possible he was referring to the grit, which is the size of the diamonds in the binding compound or plating, something like sand paper.

03-21-2013, 09:22 AM
this doesn't apply to some guy cutting concrete, but your question got me googling ..... Cubic zirconia which of course you told your girl friend is diamond is 8 mohs vs 10 and will pick up tiny scratches over time. Synthetic diamonds can have a different hardness depending on the % of Nitrogeon impurity. How do they tell? measure with something harder - ultrahard fullerite. A man made carbon structure harder than diamond. Diamond is the hardest naturally occuring substance, not the hardest substance.

but, as best I was able to google :), actual diamond is all 10 Mohs.

03-21-2013, 11:12 AM
This isn't what your friend was talking about but there are different grades of diamond hardness. There is such a thing as "superhard" diamonds that can be up to twice as hard as regular diamond. Russia has enough superhard diamonds to last the world supply for a thousand years. One of the major problems is that they are also super strong and very difficult to process into grit. They are also located in a geographical area very difficult to access and would be very expensive to mine.


The MOHS scale is an arbitrary scale and the reason that diamond is a 10 is because it was selected as the upper reference point. There is no actual correlation mathematically between the various levels on the scale.

03-21-2013, 02:21 PM
The MOHS scale is an arbitrary scale and the reason that diamond is a 10 is because it was selected as the upper reference point. There is no actual correlation mathematically between the various levels on the scale.

Quite so. The mohs scale is arbitrary and a diamond (Mohs 10) is several times harder than the next reference point, corundum (Mohs 9).

I've read a few references to these newly discovered "superhard" diamonds but I don't understand the mechanism behind it. They're all covalent carbon-carbon bonds and the structure is always a face centered cubic right? How could some have stronger bonds and therefore be harder than others?

If anyone has an explanation I'm interested.


03-21-2013, 05:01 PM
It isn't the same structure. They have very recently duplicated the structure in a lab. The superhard diamonds are thought to have been formed by an asteroid impact. There are no geologic processes known capable of creating the required pressure but an asteroid impact certainly may. It is a nanoscale structure at the molecular level such as C-60 which is what they used in the lab.


03-21-2013, 05:23 PM
...but is it a diamond then? in the link you supplied they do not call it a diamond. graphite isn't diamond either, same element, different structure. That these Russion rocks CANNOT be used as gems suggest they are not a diamond structure that cleaves etc. If you google "ultrahard fullerite" that I noted above it is the same idea, man made soccer ball shaped carbon structures but they don't call them diamonds. One thing the Ruskie rocks would do though is displace diamond as the hardest NATURAL occuring material

I think the secretive Ruskies are calling the super hard diamonds as a marketing ploy. Like new ultra Tide lol

03-21-2013, 05:26 PM
There surely are different 'grades' of diamonds but when referring to 'soft' or 'hard' diamond cutting blades it refers to the binding matrix that supports the diamonds more than the properties of the diamonds themselves though the grit size and other factors have a bearing on it. Diamond cutting blades rely on erosion to expose new diamonds and remain 'sharp'. A 'harder' blade would be used on an abrasive material because it resists erosion which would wear out a 'softer' blade in short order. I once wore out a test blade in just four meters of cutting when they usually would last ten times that.

I specialise in cutting concrete and those segmented-tooth concrete cutting blades are of no use for cutting glass. I have tried a few times and the glass just breaks. For glass I would suggest a continuous-rim blade such as is used for cutting glazed tiles. I could have asked our local Tyrolit rep for his recommendation today if I had seen the thread earlier.

03-21-2013, 10:22 PM
There are several differences possible in diamond grit wheels and cutters. First, there is regular diamond as found in nature although the industrial quality diamonds are usually dark or black in colour. That is caused by impurities including non-crystalline carbon. There are also amorphous diamond coatings that have no large scale crystalline structure but are still considered to be diamond because of the hardness which is as high or even harder than regular diamond. If it is carbon and at least as hard as cubic diamond then it is considered to be a type of diamond. There is also polycrystalline diamond which is used for cutting tools. There is also a rare hexagonal form of diamond as well as several recently discovered high pressure phase changes that are unstable.

Even regular diamond is unstable. Diamonds are NOT forever. Regular diamond is actually Metastable and degrades back to graphite very slowly. The rate at which this happens is directly correlated with temperature. At room temp it takes millennia to degrade significantly. At 1500 C it takes minutes even in an inert atmosphere.

There are also differences in the amount or concentration of grit vs binder. The higher the concentration the harder the wheel and the longer it lasts. Diamond wheels should be used with coolant to prevent thermal degradation. This is especially so when grinding iron alloys since diamond (carbon) is soluble in iron.

03-22-2013, 01:10 AM
Since there's some discussion here of grinding with diamond wheels- I've wondered whether there's a slow SFM where diamond would work well with steel. I know it's not recommended because diamond is soluble in iron, but is there a point where the problem begins, or is it simply there and active at all times?

03-22-2013, 01:20 AM
It is temperature related. That means the "instantaneous" temperature at the point of contact. That will be very high even at slow speed although it will take longer to wear out. It will of course take longer to grind too. I'm not sure if the two are linearly related.