A scratchy situation

[atgcpaul]

Loring's post on what type of wood to face his jaw vises got me thinking about something that I don't fully understanding. Hardness and scratch resistance. Are they the same thing?

For the sake of this discussion, let's say diamond is the hardest material. If I were to rub a sheet of diamond all day long with anything but diamond, I'm not going to scratch it or show any kind of wear, right?

Along the same lines, there's the Janka scale which is used to grade the hardness of wood. Let's say I have a perfectly homogenous block of hard maple and perfectly homogenous block of balsa wood. If I rub the two together all day long, is there going to be any sign of wear on the hard maple?

In both of the situations above, let's assume all materials are pure representations and there are no flaws.

[LCHIEN]

your primary problem here is the understanding of hardness.
The classic measure of hardness for most materials (often metal alloys) is the Rockwell hardness scale.
Its measured by taking a precisely pointed Indenter and pushing on it with a predetermined force and seeing how deep a dent it makes.

The Janka hardness test is similar but for wood flooring. It tries to halfway embed a steel ball into the wood and the resulting force required gives an indication of the hardness.

Now you are describing a hardness test by rubbing two blocks of material together.
In your test, if they were perfectly flat then there would be no wear. You would get no scratches if both surfaces are clean and flat. You would only get scratches if you had some hard, grainy material between them (an abrasive) and increasing force would cause the grains to make deeper impressions on the wood surfaces (both of them, the softer would get a bigger dent or scratch). There's other problems with horizontally moving items such as how much down vs. side force is applied to a moving, rolling, maybe non-spherical abrasive item. The indentation type test is much more reproducible.

So scratch resistance is related to hardness, but I think your test spreads the load across two large flat areas giving a low PSI force and will not show much in the absence of a controlled abrasive and applied force whereas a real hardness test involves a force (pounds) concentrated on a very small area (Square inches) giving a large Pounds/SqIn pressure value that will dent the objects being tested. e.g. 1 pound-force applied on a .01 square inch point will give you 100 PSI. The point is a big amplifier.

[woodturner]

For the sake of this discussion, let's say diamond is the hardest material. If I were to rub a sheet of diamond all day long with anything but diamond, I'm not going to scratch it or show any kind of wear, right?

Maybe.

The "machinist's rule" is that the softer material abrades the harder material. Grit and debris embed more easily in the softer material, and these harder materials can then abrade the harder surface. However, given the hardness of diamond, where would these "harder particles" come from, and what would they be?

If you make a bearing with one side steel and the other brass, the "rule" holds, but I'm not sure it would apply with very hard materials like diamond.

[herb fellows]

If I were to rub a sheet of diamond all day long with anything but diamond, I'm not going to scratch it or show any kind of wear, right?

Although this sounds right, it isn't. I use diamond discs with gemstones, glass, rocks etc. The diamonds will wear down, sometimes by design (diamonds are set to wear away and create new sharper surfaces, much like sandpaper), but I suspect that even if they were a perfectly flat surface, they would anyway.
Diamond is #10 on the mohs scale. This is a scale of hardness.
Glass is about a 5-6.
Diamond will scratch glass, glass will not scratch diamond. However, if you're talking about abrading something on a constant basis, as opposed to just scratching it one time, I think virtually anything will wear down.

[vaking]

Water polishes stone. Water is soft compared to stone but time takes its tall even on a harder surface. So softer material does "scratch" the harder one. Usually scratches are very small but with time there will be many of them and the process will actually result in harder material appear as polished as opposed to scratched. That's what people refer as abrade.

[cabinetman]

Water polishes stone. Water is soft compared to stone but time takes its tall even on a harder surface. So softer material does "scratch" the harder one. Usually scratches are very small but with time there will be many of them and the process will actually result in harder material appear as polished as opposed to scratched. That's what people refer as abrade.

There may be a need for clarification of the terminology used. Assuming specific results depends on many factors. Diamond will cut diamond, and glass will cut glass. For surfaces, "hardness" and "smoothness" are relative. Hardness as compared to impact resistance could be conditional.

For example, you could take a veneer of Ipe and laminate it to Balsa wood, and wind up with a very hard surface, with low impact resistance. With relation to woodworking, "rubbing" two surfaces (to mimic sanding) on an all day long test is futile as we know it takes a degree of surface consistency (grit) to have some predictable result.

A real time application is a wood to wood runner/slide assembly for drawers. Making a wise selection for mating materials, and any treatment to afford the reduction of friction helps. Wear will persist, as factors can be too commonplace to control.

.