Diamond abrasives> not created equal

J

JD Spydo

Joined
Nov 20, 2004
Messages
4,827
About 5 years ago I think it was BLADE magazine that did an article on Diamond abrasives for use in Diamond sharpening tools. It pointed out that there were 3 types of Diamonds. Monocrystalline, Polycrystalline and a third type that I honestly don't remember the name of. But the article was very quick to point out that "MONOCRYSTALLINE" diamonds were the best for use in abrasives and other tool applications

I have noticed that DMT and other top notch companies only use MONOCRYSTALLINE diamonds in their sharpening tools. I would like some feedback from you guys/gals as to what you all know about this debate of diamond types? Also about good sources for diamond sharpening and maintenance tools. Also about what possible other roles diamonds could play in the machine tool field.

I bet I'm not the only one who has given thought to this :)
 
I've only heard of monocrystalline and polycrystalline, don't know about a third type. The explanation given on the DMT site as to why mono- is better than poly- sounds right to me. I would be interested to read feedback from people who have polycrystalline stones to see if it's really true. From their website:

MONOCRYSTALLINE Diamond
(a strong, single crystal)

Uniform and precisely sized Monocrystalline Diamond is permanently bonded to nickel on a precision ground plate. Approximately two thirds of each crystal is embedded in the nickel with one third exposed. Because Diamonds are the hardest material known, the working abrasive points (top third of the diamond) will wear very slowly to give a long product life. Extremely pure, DMT® diamond crystals permit a single layer of diamond to perform for years. Competitors claims of multiple layers of diamond only indicate poor quality diamond that quickly wears.

POLYCRYSTALLINE Diamond

Our competition uses this fragmented compound that breaks apart and wears away easily with use. Sharpeners made with Polycrystalline Diamond will wear out and need replacement.
Click to expand...

I've never tried any other brand of diamond stones than DMT but I can vouch for their quality. I got a small portable DMT coarse stone back around the mid-80's and it's still perfectly serviceable, although it has not had anything like daily use. I have had the Duo-Sharp stone for a couple of years now and so far it's been great. Lots of room to sharpen and it cuts really quick. I probably won't have to replace it ever, because I usually touch up the edge before it really gets dull and needs a lot of sharpening. Once in a while I get a dull knife from somebody and that's when I'm really glad to have a diamond stone.
 
Polycrystalline crystals are less stable than monocrystaline because polycrystalline crystals have multiple crystals that only have a partial surface anchored to the nickel plate allowing the exposed crystals attached to it to break off (crystals resemble the old rock sugar sold on a stick); where as, a monocrystalline crystal is a single diamond secured in the nickel plate.

OK, end of chemistry lesson. :D

I prefer DMT and EZE products myself.

I have a EZE diamond sportsman's steel that I've used hard for 20 years and still works great.

The DMT benchstone DuoSharp F/C I have seems capable of very heavy pressure sharpening without damage.

My Lansky Diamond Hones have, for the most part, worn out, after about 15 years of moderate use.
 
Marketing hype.

polycrystalline diamond made by explosion synthesis lacks cleavage planes and is not as easy to fracture, has a good stock removal rate and very high pressure resistance.

monocrystaline are made by heat and temp or are natural bort and fracture easily along cleavage planes so can't take as much pressure but this fracture makes the grains self-sharpening.

Each comes in several grades. Here are 5 types offered by one company:

http://www.microdiamant.com/diamondpowder/diatypes.htm (click the "DP" "FG" etc. for a page on each type)
 
yuzuha said:
Marketing hype.

polycrystalline diamond made by explosion synthesis lacks cleavage planes and is not as easy to fracture, has a good stock removal rate and very high pressure resistance.

monocrystaline are made by heat and temp or are natural bort and fracture easily along cleavage planes so can't take as much pressure but this fracture makes the grains self-sharpening.

Each comes in several grades. Here are 5 types offered by one company:

http://www.microdiamant.com/diamondpowder/diatypes.htm (click the "DP" "FG" etc. for a page on each type)
Click to expand...

Yuzuha, are you sure about that, or did you turn switch polycrystalline and monocrystalline around? In general, monocrystalline diamonds are more difficult to make. I know for sure that, polycrystalline diamonds are useless for micro-avil applications, because they don't withstand a fraction of the pressure that monocrystalline diamonds of high purity possess (micro-anvil experiments are conducted at pressures of 2-3 GPa, or roughly 20.000 to 30.000 atmospheres on the anvil).
 
Yes, but those diamond anvil experiments use large gem grade diamonds oriented so they are not applying the force along a cleavage plane. Abrasives are rarely gem grade and oddly shaped with no alignment of cleavage planes. The lower quality monocrystaline diamonds break easily along the cleavage planes and so are often used in resin bonded diamond grinding wheels where that fracturing provides sort of a self-sharpening action and their lower price makes them economical in such wheels.

The explosion made polycrystalline diamonds in the link I provided do not have weak cleavage planes and so are not as fragile, and are equally hard in all directions (regular diamond crystals are harder in one direction than another so have a preffered orientation, but that doesn't help much when they are randomly oriented as in pastes or plates). Though, they can also be manufactured to be more friable (polycrystalline can be made with precision microfractures, like Dupont Myolex, to have an even, self-sharpening fracture, which make them good for precision diamond paste grinding, though these are twice as expensive as other grades).

The place I linked to provides 5 different types of diamond powder with different characteristics and prices, so it really depends on exactly what you buy.

Here is a composite pic I made of the diamonds on a DMT plate http://members.cox.net/~yuzuha/dplproc.jpg (the lines on the right and distortion on the bottom are artifacts from combining multiple images) Very irregular shapes and orientations.
 
Well, after using the Sharpmaker diamond hones to reprofile my Jess Horn dramatically I can assure you they are long lasting. I wore 3 cheap Harbor Freight diamond pads smooth in very short order when I started that reprofiling. My DMT X coarse has seem very limited use so far, but I expect good results from it. My wife's grandfather has a DMT coarse that he has had since 1992 that still cuts extremely well. I'm not sure about the monocrystaline vs. polycrystaline argument, but I trust DMT and Spyderco.
 
yuzuha said:
polycrystalline diamond made by explosion synthesis lacks cleavage planes and is not as easy to fracture, has a good stock removal rate and very high pressure resistance.
Click to expand...

That is very interesting information, is it known which type of mono/poly crystals are used by DMT, Ez-Lap, Spyderco, etc. .

-Cliff
 
dose anybody have an oppinion on these diamond sharpening stones.

they come in a three grade packs for around £22 delivered.

DSC05071.jpg


thankyou.

John.
 
Yuzuha, that is actually not correct. In general, very few materials actually cleave along a "cleavage plane". Most materials actually fracture conchoidally (very much to my own personal dismay). However, diamond does fracture more easily in certain general directions. The weakest plane in diamond is the <111> plane along which diamond has an ideal strength of 95 GPa. Along other directions single crystalline diamonds has been measured to have a fracture strength exceeding 300 GPa. In comparison, polycrystalline samples have a fracture strength of <1-4 GPa. Polycrystalline samples with nano-sized grains have been measured to have larger fracture strengths (closer to 4 GPa) than those with larger grains. This, however, has been attributed to a purity issue. Well prepared single crystals or natural diamonds have a much higher purity than many CVD samples and purity seems to be stongly affecting the fracture strength. It is argued that the reason why nano-grained samples are stronger than polycrystalline samples with larger grains is due to the fact that the effect of the impurity stays confined to the the grain.
Phys. Rev. B. 64: 212103 (2001)
Phys. Stat. Sol. (a) 201: 2553 (2004)
Diamond Relat. Mater. 9: 1734 (2000)
Diamond Relat. Mater. 14: 6 (2005)
Appl. Phys. Lett. 87: 141902 (2005)
The reason why I got so curious about this, is that it would make no sense to me that a single crystal should be weaker than a polycrystal. After all a grain boundary is always a defect. Even in metals a pure single crystal has a vastly higher fracture strength than a polycrystal, which is why I assumed that a large grain structure would be prefereable to a small grain structure, but in metals other things are at play.

But this is all pretty theoretical, and how well this translates to the practical application of grinding remains questionable. Therefore on a more practically note: All high precision micro machining applications rely on grinding wheels with mono-crystalline diamonds.
Frauenhofer Institute Produktions Technologie, Annual Report (2004)
 
All of the diamonds are held in place by nickel plating, which is not really hard.
Bill
 
Quartz and ruby/sapphire will fracture conchoidally, but I've never heard of diamond doing so. The online materials database lists the Fracture Toughness of natural diamond as 3.4 MPa-m&#65405;, synthetic CVD 6 - 10.7 MPa-m&#65405; and polycrystalline as 6 - 8.8 MPa-m&#65405;.

And, we're talking the bulk behavior of randomly oriented grit for which the list the advantages of polycrystalline diamond as: http://www.metallographic.com/Industrial Products/Diamond-Sub.htm
and I found all sorts of links to places that make all kinds of precision grinding wheels out of polycrystalline diamond like http://www.amecsuperabrasives.com/PCD/PCD.cfm

They also make many grades of mono and poly crystalline including metal plated diamond powders that vary in friability. I'm just saying that DMT's marketing statement that monocrystalline diamonds are better than polycrystalline is a nebulous blanket statement that means next to nothing unless you specify exactly what mono powder compared to what specific poly powder and exactly which parameters you're measuring.
 
This thread is getting down into the nitty gritty as far as the properties of diamonds goes... but for us laymen, could somebody who has used both mono- and poly- diamonds let us know which they thought worked better or lasted longer? I think how they perform in regular use would be the point of interest to most of us.
 
Yuzuha, no argument there. I was just getting curious more from a general and in my case work-related point of view.

I think Bill makes a very good point: the longevity of your abrasive is probably more dependend on a good substrate than anything else. I noticed that on my DMT stone the nickle matrix does get somewhat eroded from flattening waterstones.

The question that I have for DMT is, why is the D8XX so expensive and why don't they make it as a 6" version. $89 for a coarse stone is prohibitive for a student buget :(.
 
Tinderbox (UK) said:
dose anybody have an oppinion on these diamond sharpening stones.

they come in a three grade packs for around £22 delivered.

DSC05071.jpg


thankyou.

John.
Click to expand...


try em out, i bought a doublesided plate with 30/60µ grits and im very happy with it it was 50€ or so. dont know if they are mono or poly crystalline though.

if those stones wear out you would atleast have a few blades reprofiled i guess and 22£ aint that much money wasted if they turn out to be crap
 
My impression from use is that polycrystalline diamonds break down in such a way that your coarse and fine plates eventually have the same size crystals .... which means the coarse plate actually becomes less aggressive than the fine, since you have the same size or maybe even smaller diamond abrasive chips now spaced more widely on the plate.

I've read and also seem to find myself that soft steels are a bigger risk to some diamond plates. I assume the problem is tearing diamonds from the nickel that bonds them to the plate. Truing or flattening ceramic rods and sharpening stones on the other hand doesn't seem to have any ill effect, so that kind of argues the same point IMO.

The poorest service I've gotten is from Eze-lap products which seem to lose aggression fairly fast. The inexpensive diamond hones from Harbor Freight have actually served me better. The monocrystalline DMT hones retain aggression much longer. I've also used a large Norton plate that's had plenty of use before and it seems to be an excellent product.
 
You must log in or register to reply here.
Back
Top