Memory disc lubricated by N-acyl sarcosine derivative

A recording medium lubricated with a thin film of a substituted N-acyl sarcosine. The lubricant is dissolved in a volatile organic solvent (preferably hexane) and coated onto the recording medium. A particularly preferred lubricant is N-oleoyl sarcosine.

BACKGROUND OF THE INVENTION 
The present invention relates to a method for providing lubrication on a 
recording medium and to the lubricated medium produced by such method. 
DESCRIPTION OF THE PRIOR ART 
Memory discs used for mass storage of data in the computer industry are 
typically provided with magnetic or optically readable memory indicia. 
A commercially available metal magnetic memory disc is manufactured by 
stamping a metal blank from a metal strip, mechanically machining the 
blank, and then coating the blank with various layers including at least 
one magnetic memory layer comprising memory indicia. A wear layer is 
deposited upon the magnetic memory layer, and then a lubricating coating 
is applied on top of the wear layer. It is very important that the 
lubricant be long lasting because breakdown of the lubricant can result in 
interference between the disc and read-write head, resulting in a 
so-called "head crash". 
Lubricants for recording media are known in the prior art. However, such 
prior art lubricants each suffer from various deficiencies making them 
less than entirely adequate for their intended purpose. 
A commonly used lubricant for magnetic recording members is described in 
Roller et al U.S. Pat. No. 3,778,308. Roller et al describe their 
lubricant as a thin outer coating of a friction-modifying 
abrasion-resistant perfluoroalkyl polyether. Perfluoroethyl and 
perfluoroisopropyl polyethers are preferred. Such polyethers are 
commercially available under the trade name Fomblin AM 2001. These 
lubricants are dissolved in chlorofluorocarbon solvents, which are being 
phased out of use because of environmental concerns. 
Although the perfluoroalkyl polyether lubricants perform adequately, they 
are very expensive. Accordingly, there is a need to provide a less 
expensive lubricant for recording media having friction, wear, and 
anti-corrosion properties equal to or better than the perfluoroalkyl 
polyethers. 
Huisman U.S. Pat. No. 4,153,754 discloses a magnetic tape having a 
magnetizable coating comprising a binder, magnetizable pigment particles, 
and a dispersing agent. The magnetizable particles may be iron powder, 
Fe.sub.2 O.sub.3 particles or CrO.sub.2 particles having approximate sizes 
of 0.1-1 microns x 0.01-0.2 microns. The dispersing agent may include 
N-acyl sarcosine derivatives and salts thereof. The magnetizable coating 
described by Huisman is unsuitable for use as a lubricant film on a hard 
disc because the magnetizable particles would frictionally abrade the 
sputtered carbon wear layer typically applied to such discs. 
A principal objective of the present invention is to provide a lubricated 
recording medium having improved friction, wear, and anti-corrosion 
properties compared with prior art recording media. 
A related objective of the invention is to provide a method for lubricating 
a recording medium that does not require a halogenated hydrocarbon 
solvent. 
Another objective of the invention is to provide a recording medium 
lubricant that is less expensive than prior art lubricant compositions. 
Additional objectives and advantages of the present invention will become 
apparent to persons skilled in the art from the following detailed 
description. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, there is provided a recording 
medium comprising a solid substrate, a memory layer supported by the solid 
substrate, and an organic lubricant overlying the recording medium. The 
substrate may be metal, glass, or a polymer with aluminum alloys being 
particularly preferred. The memory layer contains memory indicia which may 
be encoded by magnetic or optical means. A particularly preferred magnetic 
memory layer comprises a thin film containing cobalt applied by 
electroless plating deposition or vacuum sputtering. The magnetic memory 
layer may also take the form of iron oxide particles or other magnetizable 
particles suspended in an organic medium. In a preferred embodiment, a 
wear layer, preferably sputtered carbon, overlies the magnetic memory 
layer. 
The organic lubricant comprises a film of a substituted N-acyl sarcosine 
having the formula 
##STR1## 
wherein R is a saturated or unsaturated aliphatic group having at least 12 
carbon atoms. 
The organic lubricant is applied to the recording medium as a solution in a 
volatile organic solvent which can dissolve the N-acyl sarcosine. The 
solution contains less than 2 wt% of the lubricant, preferably less than 1 
wt%. Solutions containing about 0.3-0.7 wt% of the lubricant are 
particularly preferred. 
The lubricant of the invention may also contain a non-polar synthetic oil. 
Some suitable synthetic oils are polyalphaolefins, polybutenes, and 
polyisobutenes. Polyalphaolefins are particularly preferred. The synthetic 
oil is also dissolved in the volatile solvent, preferably in a 
concentration of about 0.1-2 wt% and more preferably about 0.2-1 wt%. The 
synthetic oil is believed to improve corrosion resistance because it is 
hydrophobic. 
Some suitable solvents are hydrocarbons such as hexane and heptane; lower 
aliphatic alcohols such as methanol, ethanol, and isopropanol; lower 
aliphatic ketones such as acetone; and toluene. Hexane is the particularly 
preferred solvent. 
The organic lubricant should be essentially free of particulate matter. As 
used herein, the term "particulate matter" refers to solid particles 
having greater hardness than the lubricant film of the present invention. 
It is important to avoid such hard solid particles in order to minimize 
frictional abrasion of the wear layer. 
The dilute lubricant solution may be applied onto the recording medium by 
any of several means including spraying, brushing, wiping, or dipping. The 
dipping technique is preferred. 
The solvent is evaporated, leaving a thin lubricant coating. The coating 
has a thickness of less than about 300 angstroms, generally less than 100 
angstroms. A particularly preferred coating has a thickness of 
approximately 20-30 angstroms.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT 
The recording medium and lubricating method of the present invention are 
described below in detail with reference to a particularly preferred 
embodiment. 
The preferred recording medium has a substrate comprising an aluminum 
alloy, preferably an alloy of the 5000 Series. A 5086 (Aluminum 
Association Series) alloy is particularly preferred. The substrate is made 
by blanking from sheet material, then flattening the disc thereby 
obtained. Thereafter, the substrate is rough machined and then fine 
machined or polished to a high level of smoothness, e.g., less than about 
250 angstroms surface roughness, often referred to as a mirror-like 
finish. The polished substrate is treated to activate its surface so that 
coatings deposited on it will have a high level of adhesion. One suitable 
form of activation comprises treatment with acidified metal salt 
solutions, then rinsing to remove excess solution. 
A hard underlayer is applied to the activated aluminum surface as a base 
for a memory layer. The hard underlayer is preferably a nickel-phosphorus 
coating that is deposited from a nickel sulfate-hypophosphite electrolyte. 
Alternatively, the hard underlayer may be sputtered chromium. The hard 
underlayer typically has a thickness of about 10-25 microns. 
A memory layer is applied over the hard underlayer. In the preferred 
magnetic medium, the memory layer comprises a thin film of cobalt or 
similar material which is suitable for magnetic encodation of memory 
indicia. A cobalt-phosphorus thin film is normally applied by electroless 
plating deposition as disclosed in U.S. Pat. No. 3,138,479, the disclosure 
of which is incorporated herein by reference. Other suitable magnetic 
memory materials such as cobalt-chromium thin films may also be employed 
in the memory layer. 
A wear layer or protective layer is applied over the memory layer to 
increase the disc's useful life. The protective layer normally is carbon, 
applied by known sputtering techniques. Other suitable protective layer 
materials are silica, zirconia, magnesia, boron nitride, alumina, titanium 
carbide, and titanium diboride. 
In order to reduce friction and abrasive wear on the protective layer 
during use, the recording medium is lubricated with a substituted N-acyl 
sarcosine. The lubricant is applied to the disc as a thin film. 
In the preferred embodiment, N-oleoyl sarcosine is dissolved in hexane to 
form a dilute (0.5 wt%) solution. A magnetic recording medium is dipped 
into the solution and hexane is evaporated, leaving a thin lubricant film 
overlying the outer wear layer of sputtered carbon. 
The lubricant film of the invention was compared with three different prior 
art lubricants with respect to friction, stiction, and anti-corrosion 
properties. A first prior art lubricant (Lubricant A) was employed as a 
0.5 wt% solution in a chlorofluorocarbon solvent with and without baking 
at an elevated temperature. A second prior art lubricant (Lubricant B) 
consisted of a 0.5 wt% solution of a perfluoroalkyl polyether in a 
chlorofluorocarbon solvent. Lubricant B and the lubricant of the present 
invention do not require any elevated temperature baking. 
All four lubricants were applied to identical 130 mm (51/4 in) diameter 
magnetic hard discs with a test radius of 30 mm (1.2 in). Friction was 
measured at 1 rpm and at 200 rpm. Results are shown in FIGS. 1 and 2. The 
N-oleoyl sarcosine lubricant generally performed best of all four 
compositions tested. 
As used herein the term "stiction" refers to the force in grams required to 
start movement of the test disc, recorded at 1 rpm. Results of the 
stiction test are shown in FIG. 3. Again, the N-oleoyl sarcosine lubricant 
outperformed all others over most of the test range. 
Anti-corrosion properties were tested by exposing test and control discs to 
an 80% relative humidity atmosphere at 60.degree. C. in a closed container 
for one day. Error growth was determined by measuring differences in 
magnetic moment loss, both before and after exposure to the high humidity 
atmosphere. The disc lubricated with N-oleoyl sarcosine showed 
anti-corrosion properties at least comparable to discs lubricated with the 
other lubricants. 
While the invention has been described in terms of preferred embodiments, 
the claims appended hereto are intended to encompass all embodiments which 
fall within the spirit of the invention.