Register to reply

Can soft materials scratch hard materials?

by skeleton
Tags: materials, scratch, soft
Share this thread:
Borek
#19
Jan25-14, 03:31 PM
Admin
Borek's Avatar
P: 23,579
That just confirms answer to the original question is "yes".
skeleton
#20
Jan25-14, 04:03 PM
P: 86
MY EXPERIMENT

OK, so I just did a little experiment in my garage. I used a Dremel tool with a few new cotton brushes on various materials. Each time, I started with a clean brush that was free of any grit. I spun it on various materials: steel and wood. The surface area of the cotton brush (4mm x 20mm diameter) was about 15 times larger than the area of the test material (4mm x 4mm).

COTTON ON EQUAL HARDNESS WOOD (BOTH ARE CELLULOSE)

I ran the experiment on the wooden shaft of the shovel. Quickly (5 secs) the wood area got hot (burning coloration on face of wood) and became grooved.

I suspect that the wood material is not harder than the cotton. They are both made of cellulose and lignin, although the wood is more densely packed than the cotton brush. Clearly the micro surface of the wood was being burnt, as evident by both smell and coloration. This process probably weakens the cell wall of the cellulose and lignin - which provides the strength.

SOFT COTTON ON HARD STEEL

The experiment was repeated on steel that was a scratched face of a garden shovel. Running the Dremel tool for a few seconds (5 secs), there was no polishing evident on the steel. However, after about 20 seconds the small test area began to shine (polished). I noticed that the steel area was very hot and the cotton brush (circular) was accumulating metal grit from the steel test area.

Once the steel was sufficiently polished, there rate of steel removal seemed to diminish. This was judged by the the amount of residue developing on the cotton brush. Once polished, the brush did not get any darker with grit. This suggests to me that the originally scratched steel was easy to polish because the cotton fibers were able to snag the micro ridges. Once all the ridges were removed (polished state), then the fibers had nothing further to grab on to. Thus, further polishing subsided.

Perhaps the hard steel material is getting over-heated from the polishing method (friction) while the soft material (circular cotton brush) isn't. The brush has a much larger surface area and its speedy movement through the air allows for better heat dissipation. Steel has a weaker strength when heated. Probably the softer cotton fibers are tearing off micro ridges of steel flakes from the previously roughened steel material (shovel). Once enough residue is removed and impregnated into the face of the cotton brush, then a second process is arising. Eventually steel grit speeds up the abrading process as now the dirty brush rubs against the steel plate.

CONCLUSIONS

I was delighted by the quickness of the polishing/grinding ability of the cotton on the test materials. I suspect a different process is happening here where a soft material can polish a hard material.

This trivial experiment might validate my hypothesis that soft materials can polish hard materials by mechanically removing surface roughness on the hard material.

Note, polishing is perhaps opposite to scratching. In my vocabulary: polishing is the removal of surface ridges, whereas scratching is the creation of surface valleys.

This experiment did not test if a soft material can scratch (make valleys) onto the surface of a hard material.
Crazymechanic
#21
Jan25-14, 04:23 PM
P: 853
i think one thing is clear , every two objects , physical objects that interact tend to wear or change there structure or surface with time.
klimatos
#22
Jan25-14, 08:16 PM
P: 412
I think the question has been fully and properly answered. However, I would like to add a bit of what I think is interesting information. Gemologists would warn you not to confuse hardness with toughness. Diamonds are the hardest of all gem stones, but are not very tough. They are easily fractured along their cleavage planes. Corondum gems (oriental rubies and sapphires) and the second hardest gemstones, but also the toughest. It is extremely difficult to shatter them. Jade is not very hard at all, and is easily carved. However, it is very tough and does not break easily. This makes it ideal for elaborate and very thin carvings.
Baluncore
#23
Jan26-14, 01:38 AM
Sci Advisor
Thanks
P: 1,911
Quote Quote by Skeleton
Note, polishing is perhaps opposite to scratching. In my vocabulary: polishing is the removal of surface ridges, whereas scratching is the creation of surface valleys.
I see the removal of ridges as grinding, scraping or lapping, while “polishing” is the removal of surface contamination to reveal the reflective material surface below.

If the surface contamination is abrasive then polishing with a soft cloth may scratch the base material with the contaminant.

If the surface contamination is a chemical product of the base material reacting with the environment, then polishing will erode the weathered material and there will be a progressive loss of base material due to repetition of surface corrosion.

Where the contamination is tightly bound to the surface, polishing will require a fine abrasive, which can cause many fine scratches in the surface.
Lok
#24
Jan27-14, 02:51 PM
P: 462
Quote Quote by skeleton View Post
I was delighted by the quickness of the polishing/grinding ability of the cotton on the test materials. I suspect a different process is happening here where a soft material can polish a hard material.

This trivial experiment might validate my hypothesis that soft materials can polish hard materials by mechanically removing surface roughness on the hard material.

Note, polishing is perhaps opposite to scratching. In my vocabulary: polishing is the removal of surface ridges, whereas scratching is the creation of surface valleys.
Soft materials will definitely affect hard materials, i insist on affect as scratching is a bit much.
Mainly the surface atoms, molecules or grains are not held as strongly as in the bulk of a material so even hard materials can be chipped of. (of course small chips)
Ultra fine Polishing looks like atomic scratching as the energy requirements of polishing tend to be similar to the specified materials boil off energy. Usually the ultra-fine polishing mediums are not necessarily as hard as one might expect (rust is a nice one, 5 on Mohs).
Diamond is with most Carbon atoms bonded to other 4 Carbons, yet the surface atoms do not have that luxury and are bonded only to 2 or 3 atoms, this does not make it necessarily a weaker bond as other effects can come into play but it enhances its reactivity. Ex. diamond will quickly abrade when milling Iron/steel without cooling. Carbon reacts chemically with iron. This is not uncommon in wear, it will look like abrasion but it could be a reaction.
On the same note wood which is quite soft abrades iron/steel quickly (The wood-milling industry prefers diamond in a nonferrous mesh for speed cutting wood).

Of course speed is important and temperature too but for cotton and steel, it is the steel that is harder at cottons max temperature (cotton begins to dehydrate around 180-200 deg C). So at best your polishing is just the "boiling off" abrading of some of the irons grains or surface atoms.
russ_watters
#25
Jan27-14, 03:06 PM
Mentor
P: 22,297
Quote Quote by Borek View Post
That just confirms answer to the original question is "yes".
I suppose then it depends on how you define "scratch". It seems to me the laymens definition is very broad. But in materials science, "wire drawing" has a different name because it is a different process. Breaking something with kinetic energy is not the same as wearing it with friction.

In any case, I'm not familiar with using cloth to polish glass, again except using a definition that really just means cleaning. That said, I remember in school polishing hard metals with a buffing wheel and buffing compound: which contained hard materials for abrasion.

The OP is asking whether the stylus he bought will scratch his cell phone screen. The answer is no.
russ_watters
#26
Jan27-14, 07:09 PM
Mentor
P: 22,297
Quote Quote by Crazymechanic View Post
So you're saying that if I would use a smooth cloth to rub a diamond or steel I wouldn't cause some wear or change in the surface , assuming the action would go on long enough.?
Correct (for steel at least, the cloth must be clean)

People pass diamonds down through generations. Have you ever seen or even heard of a diamond in an engagement ring becoming worn-down?
Borek
#27
Jan28-14, 02:12 AM
Admin
Borek's Avatar
P: 23,579
Quote Quote by russ_watters View Post
People pass diamonds down through generations. Have you ever seen or even heard of a diamond in an engagement ring becoming worn-down?
That can just mean they wear down much slower, it doesn't mean they don't wear out at all.


Register to reply

Related Discussions
Materials Science vs. Materials Engineering Academic Guidance 6
What is an acoustically soft/hard scatterer? General Physics 2
Majoring in ChemE or Materials Science...Is it hard to get a job with only a BS? Career Guidance 1
Detail for Hard Water and Soft Help Chemistry 1
What soft compressible materials safely work with wiring Electrical Engineering 3