Diamond Slurry Sprays, Lubricity & Particle Characteristics: How To Put The Ultimate Edge On Razors, Knives & Tools

Diamond Slurry Sprays, Lubricity & Particle Characteristics:

How To Put The Ultimate Edge On Razors, Knives & Tools

By John H. Tischler

Friday, December 24, 2010

Introduction

While I’m sure that you all understand the concept of friction, many of you probably are unfamiliar with the term “lubricity.”  Lubricity is defined in the Merriam-Webster dictionary as: the capacity for reducing friction. You can also think of it as the ability of a lubricant (i.e. oil, water, Teflon, etc.) to reduce friction between two objects.  Just for your information, the study of lubricity is part of the larger science of Tribology – the study of interacting surfaces in relative motion.  While this information may be very interesting to some and boring to others, it’s critical for anyone who wishes to truly understand blade sharpening and polishing.  So if you belong to this determined group, or wish to, read on.

Lubricity & The Pursuit Of The Perfect Edge

Since the pursuit of the perfectly keen blade is our agenda here, let’s apply the principles of lubricity to the honing and stropping of fine blades.  Assume we have a flat surface covered with a liquid containing particles that are harder than steel.  Let us further assume that we are moving a steel blade over that surface, using just the weight of the blade itself for downward pressure.  As the lubricity of the liquid increases, the friction between the abrasive surface, and the steel contacting that surface, decreases.  This decrease in friction reduces the ability of the abrasive to remove metal.  The degree to which an abrasive removes material is often referred to as the “cutting power” of the abrasive.  

Since cutting power is what we all want from our abrasive surfaces, we should always want to keep lubricity to a minimum, right?  Well, that depends.  Have you ever tried honing a blade on a dry waterstone?  How about on a dry DMT diamond plate?  If you have, you have already experienced the effects of very low lubricity – quite a bit of metal is removed (not always where you want it to be) and the stone or plate is soon clogged with worn metal fragments.  This makes the abrasive surface an inconsistent cutter – leaving some spots that will cut harshly and others where cutting power is virtually non-existent.  Further, this lack of lubricity also makes for a very rough edge – something you can see under a microscope and, certainly, feel against your skin.  

The Role Of Lubricants In Creating Better Edges

It is for these reasons that we generally use a lubricant (something which increases lubricity) when we are honing.  Lubricants reduce the friction between the abrasive and the blade – reducing cutting power, but producing a more consistent “cut” and a smoother edge.  The lubricant also helps to carry away the metal (and “spent” abrasive particles) removed by the abrasive in action, helping to prevent the “clogging” of the honing or stropping surface which occurs when the surface is too dry.  Lastly, using a lubricant extends the service life of an abrasive’s surface.

Proper lubrication is most important when you are using an abrasive that is considerably harder than the piece you are attempting to sharpen or polish – and they don’t come any harder than diamond.  Diamonds, natural or synthetic, are the hardest substance on earth.  Hardness isn’t the same thing as toughness – you could take a hammer and easily smash an engagement ring (PLEASE don’t try this at home); it just means that, when moved against a softer material (which is everything else on earth other than another diamond) with sufficient pressure, the diamond will cut into that material and remove some of it.  The depth and shape of this cut, and the amount of material removed, are all impacted by the shape of the diamond, the pressure applied to the workpiece against the diamond and, last but not least, the lubricity of the solution standing between it and the softer material.

Water is the most common and least expensive lubricant.  It also has extremely low lubricity and viscosity (a measure of the resistance of a fluid under stress, often known as “thickness”) – which is one of the reasons why it’s not used to lubricate the moving parts in car and truck engines.  It does, however, have qualities that are very desirable when honing a razor, fine knife or tool.  It is plentiful and inexpensive, is easily cleaned up and does a fair job of carrying away worn metal and spent abrasive, ensuring a more consistent “cut” and extending the life of abrasive materials.  It also aids in producing a less jagged edge than you would otherwise produce using a dry surface, although not nearly as smooth as one would get with a superior lubricant.

The Princess And The Pea – How Something Very Small Can Be Very Irritating

Even though it is the main topic of this paper, I am going to leave the subject of lubricity for a little while to talk about abrasive particles on the micron and particularly the sub-micron levels.  You will have to trust me that this short diversion is important and that I will return to the topic in a few paragraphs.  Firstly let us quickly try to wrap our brains around the concept of the micron-sized particle.  To put microns in perspective, you may wish to consider that the period at the end of this sentence is approximately 400 microns across; and that one micron is equal to about 1/300^th the diameter of a human hair strand.  Sub-micron level particles are obviously even smaller – so small, in fact that Electron Microscopes are required to see them.

Despite this miniscule size, there is a common complaint about slurries which utilize diamonds smaller than one micron in size – that they leave an edge that is too harsh or “toothy.”  This seems somewhat counter-intuitive when dealing with particles so tiny, but it’s true.  This effect leads many honemeisters (persons skilled in the art of creating perfect edges) to use chromium oxide (a substance which is softer than diamond) pasted strops after using diamond slurry sprays to “soften” the edge.  The reasoning often runs something like this: “Diamonds are a harder substance and thus will cut deeper and rougher scratches than will chromium oxide, therefore honing on chromium oxide is necessary to ‘smooth’ the edge.”   This is only partially correct and is typically a result of using the wrong type of diamond in the wrong way.  

Synthetic Monocrystalline Diamonds (SMD)

Most diamond slurry formulations sold for sharpening fine blades rely on synthetic monocrystalline diamonds (SMD) as the abrasive.  

Popular for their extremely low cost and high durability, SMD diamond slurries are an excellent choice for earlier stage material removal, but have some inherent problems that make them a poor choice for final finishing work.  These problems include, first and foremost, the shape of the particle itself.  It is “blocky” with very sharp, but very few cutting edges.  SMD slurries also suffer from problems with particle shape, such as the existence of needle-like “slivers” and long and flat “plate structures.”  These “strays,“ which often get past even the highest quality Particle Size Distribution (PSD) testing equipment and procedures, lead to SMD’s relatively high “scratch rate” – tiny bits of damage to the blade’s sub-surface.  These scratches are not visible through magnification under a normal microscope (you would need an Electron Microscope to see them), but they can be perceived on an edge that is used for shaving as “harshness.”  

For these reasons, SMD diamond slurries are not the first choice of manufacturers needing to produce the finest finishes.  They are also why I do not recommend the use of SMD diamonds in sub-micron level slurries used for final finishing/polishing.  While it is beyond the scope of this paper to delve deeply into the subject of diamond morphology, I felt that I have to mention it I have read claims by leading competitors that monocrystalline diamonds are more appropriate for finishing because they will leave a less “toothy edge” than polycrystalline diamonds.  It is well known that, in fact, the opposite is true.

Synthetic Polycrystalline Diamonds (SPD)

Synthetic polycrystalline diamonds (SPD) share many attributes of their monocrystalline cousins (particularly hardness), but their structure – formed by a controlled detonation of high explosives, is very different.  Each tiny SPD particle contains even smaller “microcrystallites” whose numerous planes are pointed in different crystallographic directions every 10 to 50 nanometers (a measure significantly smaller than microns) – regardless of the particle size distribution of the parent particle. The hardness of each individual microcrystallite can be compared with that of synthetic monocrystalline diamond. Due to the greater number of cutting edges and greater surface area simultaneously contacting the workpiece, SPD-based slurries remove more metal than SMD slurries and are much less likely to cause subsurface damage to the metal ^±. But all of these good qualities come at a price – with polycrystalline diamonds costing much more than monocrystalline preparations. 

Lubricity Revisited

Earlier in this paper, I described the characteristic problems of using abrasives with little or no lubricant – inconsistent cutting, harsh cutting and clogging of the abrasive surface.  Considering what we have learned about the “stray” particles in SMD slurries, is it any wonder that these problems would be compounded by using a solution with very low lubricity – particularly when you are using particles with an inherent tendency to “gouge?”  Does scratching not worsen in an absence of lubrication?

Most diamond slurry sprays, such as the ones produced by current market leaders, use carriers (the solution in which the abrasive particles are suspended) with very low lubricity – depending primarily on de-ionized water to reduce friction.  You can see this in the way these product atomize (disperse in a spray) – producing an extremely fine mist, much like tap water (A bit too fine, in my opinion.  At a distance from the strop of 5”, one of these products produced a rough circle 8” in diameter – leaving more slurry on the table than on the strop.  Sprayed closer, the slurry is too “concentrated” in one spot.).  They also tend to dry quite quickly on a 100% hard wool felt strop – also an indication of a primary dependence on water as a lubricant.  You can also feel the almost complete absence of viscosity in the “draw” or friction perceived when stropping a blade against a hard wool felt strop treated with these products.  And since it they are often used dry, that friction is enhanced – at least until the abraded metal starts clogging the surface.  

So after I strop a razor on hard wool felt sprayed with products of this type, used wet or dry, I do find that stropping on chromium oxide is necessary before use – as the blade coming off a strop sprayed with it does tend to leave a harsh edge.  If you are working with knives, you probably want a harsher edge – especially at sub-micron levels, as the microscopic scratches may produce better cutting action – you just might want to pay less for it.  

How To Get The Ultimate Edge For Less Money

We have customers all over the world looking to put the best possible edges on razors, knives and fine tools.  In response to these needs, Little Stropper^TM has formulated products that use the less expensive synthetic monocrystalline diamonds in our 1 and 2 micron slurries (where a higher scratch rate matters less and may even be desirable), and the more costly synthetic polycrystalline diamonds (SPD) in our .50 micron and .25 micron slurries.   Since the sub-micron level slurries tend to be used for finishing and polishing, we felt that using synthetic polycrystalline diamonds, with their much lower scratch rates, was worth the extra expense.  Additionally In all of our formulations, we use significantly higher levels of water-soluble lubricants than you will find in the leading brands. So you will feel less friction when using our products, but you will get a better and smoother edge.  You will also use less of the product, as we add special ingredients designed to keep the stropping surface moist for long periods of time.

Another thing you will appreciate is our prices. Even our SPD formulations, which utilize a much more expensive diamond than do our SMD products, are priced lower than the leading brand.  Depending on the formulation used, our products ounce-for-ounce cost up to 35% less than the current market leaders.  We are announcing the following products, which will be available for sale in January of 2011:

• 2.0 Micron Premium Synthetic Monocrystalline Diamond Slurry Spray (2 oz size)
• 1.0 Micron Premium Synthetic Monocrystalline Diamond Slurry Spray (2 oz size)
• .50 Micron Premium Synthetic Monocrystalline Diamond Slurry Spray (2 oz size)
• .50 Micron Premium Synthetic Polycrystalline Diamond Slurry Spray (2 oz size)
• .25 Micron Premium Synthetic Polycrystalline Diamond Slurry Spray (2 oz size)
• SlurryLube^TM – an economically priced diamond slurry extender and lubricant (2 oz size)

Beware Competitors Bearing Carats

A word to the wise – be wary of diamond slurry manufacturers that attempt to distract you from the price of their product by focusing on the number of carats of diamonds in the bottle.  “Stuffing” more and more diamond powder into a bottle of slurry, while certainly providing a rationale for charging a high price, is unlikely to improve the product’s performance.  The additional diamond particles just end up being wasted.  Our products are formulated with a focus on performance and results – not on carat weight.  So you end up paying for effectiveness, not powdered diamond.

Conclusion

I have used quite a bit of science and jargon in this paper – I do apologize for this and have tried to keep it to a minimum.  But most of you reading this (and thank you for coming this far with me) are not Tribologists.  You are folks who, like me, want a smooth close shave every day.  You want knives that will cut cleanly without slipping.  You want chisels and other bladed tools that will remove material reliably without “skipping” off surfaces or damaging your creations.  You also want to spend less time and money getting there.  

If any of the foregoing describes you, please try our Little Stropper^TM line of Premium Diamond Slurries and blade finishing products.  You will find them at www.littlestropper.com/store in January 2011.  If you have questions, please contact us at info@littlestropper.com.

John H. Tischler is President of Vintage Shaving Shoppe, LLC. and Little Stropper^TM brands.  He can be contacted at jtischler@littlestropper.com .

References:

± Tumavitch, Nicholas J., The Evolution of Diamond for Optical Component Finishing, Recent developments in the production of synthetic diamond abrasives have resulted in an array of varieties suitable for optical finishing applications., 2005.