RESEARCH
classes of two-dimensional structure: liquid, plastic, solid. "Solid" monolayers are found to provide lowest coefficients of friction as well as greatest durability and wear-prevention. Mixed monolayer films containing both polar additives and hydrocarbon oil occur only when structures of the oil molecule and the hydrocarbon portion of t h e polar compound are geometrically similar. This permits strong adlineation, or intermolecular cohesion, according to NRL. While the load-carrying capacity of mixed films is much less than that of pure films, mixed monolayers occur frequently in lubrication practice, says Zisman. This is especially true in dynamic machinery where there is not enough tame between formation and destruction of a protective film to allow adsorption equilibrium to be reached. Zisman feels that this explains why load-carrying and wear-preventive properties of chemical additives to oils measure less in dynamic testing devices than in static ones. • Monolayer Films Are Strong. Friction at the boundary of two rubbing solids is due to adhesion between high spots on both surfaces. An adsorbed film serves basically a mechanical function by greatly decreasing the number of contacting rough places. A "solid" film only one ten-millionth of an inch thick on glass can withstand repeated traverses on a rubbed path by a steel ball under loads as high as 100,000 pounds per square inch, the NRL group reports. It continues to separate the rubbing surfaces, reduces coefficient of friction to 0.05, and reduces wear by a factor of 10,000. Some of the highest rough spots can always penetrate adsorbed films on rubbing surfaces, Zisman explains. For steel on glass, junctions formed between these two materials shear at points of contact, and damage is negligible until many traverses of a single rubbed path are made. Steel rubbing against steel results in strong adhesion a n d cold welding. These junctions do not shear just a t contact points but often tear relatively deep in the metal surface. Larger particles of metal which are thus torn off adhere to a sliding surface and plow through the protective monolayer. In general, then, t h e protective value of such monolayers is less for rubbing surfaces which alloy or form strong welds, NRL maintains. 62
C&EN
SEPT.
23,
1957
Chemical requirements for a good, polar wear-preventive adsorbed as a two-dimensional solid monolayer have been drawn up by Zisman and his coworkers to include t h e following four points: • Intermolecular cohesion must h e high; molecules must adsorb in oriented array, a n d paraffinic structures with one or more chemically bonded halogen atoms are desirable. • Polar terminal groups of t h e molecules must adhere strongly to t h e rubbing surface. • Easy slippage a n d low affinity b e tween films on contacting surfaces must be assured; outermost terminal groups of oriented molecules should form a surface having lowest possible free surface-energy (methyl or perfluoromethyl groups are ideal) . • Rate of diffusion to adsorbing surface must not be too slow, a n d molecules should not be too bulkv.
Hydrofhermal Sapphires Sapphires g r o w n in aqueous solution seem f r e e of strain, might thus b e used in optics
•moACs I » # NATIONAL I \ 3 L MEETING
Taking
a
cue
from its h y d r o t h e r ] process for PhysWi* synthetic quartz, Inorganic Bell Telephone Chemistry L a b s has grown synthetic sapphires ( c o r u n d u m — a modification o f A l 2 O a ) a s large as s / 4 X V 4 X V 4 i n c h . U n like flame fusion ( t h e p r e s e n t commercial p r o c e s s ) a n d m e l t t e c h n i q u e s , t h e n e w p r o c e s s gives crystals t h a t s e e m essentially f r e e of s t r a i n , say R . A. L a u dise a n d A. A . Ballman. S u c h crystals m a y p r o v e useful i n lenses, prisms, a n d o t h e r optical items. Some n a t u r a l s a p p h i r e is used for such t h i n g s now—but a t a price—because it transmits l i g h t well into the infrared a n d , e s p e c i a l l y , t h e v a c u u m ultraviolet. A b r a s i v e s u s e m o s t of the c o m m e r c i a l c o r u n d u m m a d e now. H y d r o t h e r m a l p r o c e s s e s like this one exploit t h e f a c t t h a t normally insoluble c h e m i c a l s c a n b e c o m e slightly soluble if p r e s s u r e a n d t e m p e r a t u r e go h i g h e n o u g h . F o r instance, the Bell p r o c ess o p e r a t e s a t a b o v e 4 0 0 ° C . and 30> 0 0 0 p.s.i., L a u d i s e told the D i v i s i o n of ma
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