[Hammarlund] Graphite is NOT an Abrasive

Sat, 14 Dec 2002 01:15:41 -0500

In Hammarlund Digest #259, it was claimed by one poster that
graphite is abrasive.  No proof or any evidence was given
other than "ask any geologist or gem-ologist".

This poster is totally off-base.

To be abrasive to another substance, a material must be harder
or equal in hardness to that substance.  According to the
"Field Guide to North American Rocks and Minerals," pure
graphite is rarely found in nature, normally being found mixed
with clays and iron oxides.  The pure material has a Moh
hardness of 1.  The clays and oxide are generally harder.

To those not familiar with this method of measuring hardness,
the Moh scale ranks the hardness of different minerals from 1
to 10 with talc being unity and diamond being 10.  A working
scale is also described for those not familiar with the
minerals chosen:

    1. very easily scratched with the fingernail, and has a
        greasy feel to the hand
    2. easily scratched by the fingernail
    3. scratched by copper coin (old pennies)
    4. easily scratched by a knife
    5. scratched with difficulty by a knife
    6. easily scratched by hardened steel file
    7. not scratched by a file but will scratch window glass

Industrial abrasives such as sandpaper (quartz = 7), corundum
(aluminum oxide = 9) and diamond (10) fill out the top end of
the scale.

So graphite is too soft to be an abrasive.  But is it a
lubricant?  Of course; it is a dry film lubricant.  But forget
geology and go lubrication science instead.

To talk about the interactions of surfaces moving with respect
to each other, the correct science is not geology but rather
tribology.  This science includes the study of lubrication,
friction, and wear.

To a tribologist, dry film lubricants fall into three general
categories: long chain molecules and polymers such as Teflon�
(polytetrafluoroethylene), lamella structure materials such as
graphite and molybdenum disulfide, and soft metals such as
lead and Babbit alloys.

Lamella structures materials are those who naturally form thin
platelets which slide easily over each other (low shear between
the basal planes is the technical way of saying this).

Graphite is one of several forms of carbon.  It is composed of
sheets of carbon atoms bonded to each other tightly by
chemical bonds.  But the sheets are held to each other by
quite weak electrical forces (van der Waals forces).  For
those that have never seen a large chunk of pure graphite,
think of a sheet of mica.  You can take a sheet of mica and
separate it into thinner and thinner sheets.  Each sheet is
fairly tough, but the sheets separate easily.  Only with
graphite, however, the forces holding the sheets together is
far weaker still.  A little water vapor, adsorbed from the
surrounding air, reduces these forces still further.  Thus in
a very high vacuum or at very high temperatures, graphite
loses some of its lubricating properties.  [The opposite is
true of another dry film lamella lubricant, molybdenum
disulfide.]

Another way of looking at graphite is as a stack of playing
cards.  Push on the cards sideways and they slide over each
other easily.  One author even described the lubricating
properties of graphite as analogous to walking across a floor
covered by thin flat slippery plates.  They slide so easily it
is difficult to maintain your balance.

Molybdenum disulfide also has a similar atomic structure, only
here the molybdenum bonds to other molybdenum atoms in the
basal plane are exceptionally strong.  So too are the covalent
bonds to the sulfur atoms perpendicular to this plane.  But
the van der Waals forces between the sulfur atoms in one sheet
and the sulfur atoms in the adjacent sheet are even weaker
than the forces holding the graphite sheets together.
Adsorbed water vapor between these sheets does not
help, but instead hinders the lubrication, so molybdenum
disulfide is an even better lubricant in vacuum or at high
temperatures.

The reasons that graphite is used to lubricate locks are
several.  When asked about lubricants, most people think of
hydrodynamic lubricants - liquids that form a continuous
lubricating film between mating surfaces of sufficient
pressure to prevent contact.  Typically high molecular weight
oils are used, those that wet the surfaces and do not
evaporate easily.  Unfortunately oils still evaporate, albeit
slowly (or not so slowly as in WD-40), and they tend to
oxidize over time.  The oxidation makes them thick, gummy, and
can even solidify them.  Evaporation of the lubricant, or
"gumming up the works" are two of the worst things that can
happen in a lock with close fitting components.  Graphite does
not evaporate or oxidize to any appreciable extent over very
long periods of time.

But for all their useful properties, graphite and molybdenum
disulfide are not too useful in lubricating electronic
equipment.  Graphite is a fairly good conductor of electricity
and molybdenum disulfide is slightly conductive (and a
semiconductor).  Both can make a black mess if applied
carelessly, and being dry powders they can also blow about and
migrate too.  This is not what you want to happen inside a
radio with high voltage present.

But to go back to the original question of whether brass gears
need lubrication, the answer is that lubrication will minimize
their wear.  In the case of brass used against steel in gears,
the brass preferentially wears.  In fact, there are other
copper alloys such as the bronzes and leaded bronzes that are
specifically used as bearings with steel.  But with brass
against brass gears, a small amount of lubrication will
minimize fretting and give longer life.  Steel against steel
gears, however, require lubrication to have any reasonable
life.

If you wish to read more about dry film lubrication, I have found the
following website to be useful:

     http://www.dynamiccoatingsinc.com/technical.htm

I am not sure where the "old wives' tale" of graphite being
abrasive started.  I suspect that it may be related to the
fact that regular lubricating oils when severely overheated
and burned produce amorphous carbon that can be abrasive.  But
this, like diamond (which is about as abrasive as you can
get), is an entirely different form of carbon than graphite.

        73,  Dr. Barry L. Ornitz     WA4VZQ     [email protected]