Magnetic
layers of hard storage disks are coated
with a thin film (5~20 nm) of sputter-deposited
carbon in order to protect the magnetic
layer from corrosion and abrasive
impacts of the slider of the head.
A thin layer (~1 nm) of PFPE (perfluoropolyether)
is then applied over the carbon overcoat
to further dampen the tribological
impact of the slider. The PFPE lubricant
of topical choice is Z-DOL, the chemical
formula of which may be given as HO-CH2-CF2-O-[-CF2-CF2-O-]m-[-CF2-O-]n-CF2-CH2-OH.
The bulk structure of the slider comprises
a composite mixture of aluminum oxide
and titanium carbide.
It
has been shown that surface defects
of Lewis acidity (electron deficient
sites) ubiquitously present on aluminum
oxide or titanium carbide catalyze
the intramolecular disproportionation
reaction of PFPE molecular chains
as illustrated below. (ref.
1)
The
reaction results in not only fragmentation
of molecular chains, but also generation
of molecular fragments possessing
a fluorocarbonyl end group, which,
on contact with moisture, readily
converts to a strong fluorinated carboxylic
acid. Many adverse consequences haven
been shown to result from such degradation
products.
Design
and synthesis of ultra stable PFPE
lubricant:
X1P
(partially fluorinated hexaphenoxy
cyclotriphosphazene) is often used
as an additive that would increase
the durability of disk drives. (ref.
2) It has been shown that X1P
is a strong nucleophile (electron
donor) and its efficacy results from
passivation of Lewis acid sites responsible
for the PFPE degradation. (ref.
3) It follows that PFPE’s
terminated with nucleophilic end-groups
should exhibit a durability comparable
to a PFPE/X1P mixture. To this end,
we synthesized Z-DPA, PFPE terminated
with dialkylamine end-groups. It has
been shown that magnetic hard disks
lubricated with ZDPA possess an excellent
durability. (ref.
4)
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Reference
—————
1.
P. H. Kasai, Macromolecules, 1992,
25, 6791.
2. D. J. Perettie, W. D. Johnson,
T. A. Morgan, K. K. Kar, G. E. Potter,
B. M. DeKoven, J. Chao, Y. C. Lee,
C. Gao, and M. Russak, Adv. Info.
Storage Proc. Syst. 1, edited by G.
C. Adams, B. Bhushan, D. Miu, and
J. Wickert, 1995, 117.
3. P. H. Kasai, J. Info. Storage and
Proc. Syst. , 1999, 1, 23.
4. P. H. Kasai and V. Raman, Tribology
Letters, 2002, 12, 117. |
