Method for forming a protective film on a slider air bearing surface in a magnetic recording disk file

A method is described for depositing a thin protective carbon film on the air bearing surface of a slider in a magnetic recording disk file. A disk file with a conventional slider and a disk with an essentially carbon protective overcoat is cycled on and off at a frequency such that the slider is maintained in contact with the disk. Alternatively, the disk is rotated at a low constant speed so that the slider is maintained in contact with the disk. In both cases, carbon is transferred from the carbon overcoated disk to the air bearing surface of the slider such that during subsequent operation of the disk file, the disk file has excellent durability, especially when operated at relatively low humidity, as is required in those disk files which use a magnetoresistive sensor on the slider.

TECHNICAL FIELD 
This invention relates to magnetic recording disk files of the type which 
utilize disks with protective carbon overcoats and read or write heads 
supported on air bearing sliders. In particular, the invention relates to 
a method for forming a relatively thin protective carbon film on the air 
bearing surface of a slider to thereby improve the durability of the 
head-disk interface in the disk file. 
BACKGROUND OF THE INVENTION 
In conventional rotating disk files, the read/write transducers (or heads) 
are supported on sliders which ride on cushions or bearings of air above 
the disk surface when the disks are rotating at operating speed. The 
slider is connected to a linear or rotary voice coil actuator by means of 
a relatively fragile suspension. In large capacity magnetic recording disk 
files, there is generally a stack of rigid disks and a number of actuators 
with each actuator supporting a number of sliders. The actuators move the 
sliders generally radially between the disks so that each head may access 
the recording area of a respective disk. In these conventional disk files, 
the slider is biased against the disk surface by a small force from the 
suspension when the disk is not rotating. The slider is thus in contact 
with the disk surface from the time the disk file is turned on until the 
disk reaches a speed sufficient to cause the slider to ride on the air 
bearing. The slider is again in contact with the disk surface when the 
disk file is turned off and the rotational speed of the disk falls below 
that necessary to create the air bearing. 
One type of magnetic recording disk used in disk files is a thin film metal 
alloy or metal oxide disk which has a protective overcoat of essentially 
carbon. The carbon overcoat protects the magnetic layer of the disk from 
corrosion as well as wear caused by contact by the air bearing surface of 
the slider supporting the read/write head. A liquid lubricant such as a 
perfluoroether, is typically applied to the carbon overcoat. 
Protective carbon overcoats for thin film metal alloy disks are well known. 
They are typically formed by sputter deposition from a graphite target, 
and are generally referred to merely as protective carbon overcoats, 
sputtered carbon overcoats, "diamondlike" carbon overcoats, amorphous 
carbon overcoats, or, in the case of those overcoats formed by sputter 
deposition in the presence of a hydrogen containing gas, hydrogenated 
carbon overcoats. Tsai, et al in "Structure and Properties of Sputtered 
Carbon Overcoats on Rigid Magnetic Media Disks," J. Vac Science Technology 
A6(4), July/August 1988, pp. 2307-2314, describe such protective carbon 
overcoats and refer to them as amorphous "diamondlike" carbon films, the 
"diamondlike" referring to their hardness rather than their crystalline 
structure. Assignee' U.S. Pat. No. 4,778,582, issued Oct. 18, 1988, 
describes the prior art of thin film disk amorphous hydrogenated carbon 
overcoats and a sputter deposition process for forming such an overcoat. 
Recent advances in read/write head technology have resulted in the possible 
use of magnetoresistive (MR) read heads in disk files. Unlike conventional 
inductive heads, MR heads, due to the material from which they are 
fabricated, are highly susceptible to corrosion. Accordingly, in order to 
assure proper operation of such heads in disk files, including those which 
use carbon overcoated disks, it is necessary to operate the disk file at 
very low humidity. However, it has been discovered that when disk files 
with carbon overcoated disks are operated at low humidity in conjunction 
with TiC/Al.sub.2 O.sub.3 sliders, the disks wear at an extremely high 
rate. 
SUMMARY OF THE INVENTION 
The present invention is a method for transferring carbon from the carbon 
overcoated disk to the air bearing surface of a slider. In the preferred 
method, the assembled disk file is maintained in an environment having a 
relative humidity (RH) of greater than approximately 30%. The disk file is 
then cycled on and off at a frequency sufficient to maintain the slider in 
contact with the carbon overcoated disk. As a result of these start/stop 
(s/s) cycles, carbon is transferred from the carbon overcoated disk to the 
air bearing surface of the slider to a thickness less than approximately 
50 .ANG.. After the carbon film has been deposited on the air bearing 
surface of the slider in this manner, the disk file can then be maintained 
at relatively low humidity and operated without any loss in durability of 
the disk. This permits MR heads to be used in disk files without any loss 
in disk durability. 
For a fuller understanding of the nature and advantages of the present 
invention, reference should be made to the following detailed description 
taken in conjunction with the accompanying drawing.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
The durability of disks in magnetic recording disk files is determined by 
measuring the number of start/stop cycles before failure of the head-disk 
interface. It has been found that when carbon overcoated thin film disks 
are used with conventional sliders (e.g., a composite TiC/Al.sub.2 O.sub.3 
ceramic), the disk durability is directly related to the relative humidity 
to which the head-disk interface is exposed. As the relative humidity 
decreases below approximately 30%, the disk durability decreases generally 
linearly. During the determination of the relationship between disk 
durability and humidity, it was discovered that frequent start/stop 
cycling of sliders on carbon overcoated disks resulted in a deposition of 
a carbon film on the air bearing surfaces of the sliders. Subsequent 
testing of these disk files showed a significantly improved disk 
durability when operated at low humidity (i.e., less than approximately 
30% RH). Thus a method for improving the disk durability of disk files 
involves transferring carbon from the carbon overcoated disk to the air 
bearing surface of the slider by rotating the disk at a velocity 
sufficient to maintain the slider in contact with the disk surface. 
In the preferred embodiment for carrying out the present invention, the 
assembled disk file is connected to a commercially available disk 
exerciser and the disk file is cycled on and off at a frequency such that 
the sliders are maintained in contact with the carbon overcoated disks. 
The improved durability of such disk files is illustrated in FIG. 1, which 
represents the relationship between the percentage of disks which pass an 
additional 7000 s/s cycles as a function of the number of s/s "burn in" 
cycles. In these disk files liquid perfluoroether lubricant was applied to 
the carbon overcoated disks. Referring to FIG. 1, when five disk files 
were not subjected to s/s burn in cycles, four of the five disk files 
failed to survive an additional 7000 s/s cycles at less than 5% RH. On the 
other hand, when eight disk files were subjected to 400 s/s cycles at 
approximately 30% RH, only two of these eight failed to survive an 
additional 7000 s/s cycles at less than 5% RH. As shown by FIG. 1, the 
transfer of the carbon overcoat to the air bearing surface of the slider 
can be accomplished at various s/s cycles and various relative humidities. 
For example, when the disk files were subjected to 4600 s/s cycles at only 
10% RH, all three of these disk files survived an additional 7000 s/s 
cycles at very low humidity, i.e. less than 5% RH. 
While the preferred embodiment for forming the carbon overcoat on the 
slider air bearing surface is by cycling the disk file through a series of 
s/s cycles such that the slider is maintained in contact with the carbon 
overcoated disk, it is also possible to deposit the carbon film onto the 
slider by running the disk file at a relatively constant slow speed such 
that the slider is maintained in essentially continual contact with the 
carbon overcoated disk. It is also preferable to perform this burn in in a 
nondata band or region of the disk, such as a slider "landing zone" 
located either at the inside or outside diameter of the disk. Other 
nondata regions are bands at the extreme outside diameter (OD) and extreme 
inside diameter (ID) of the disk, which are regions beyond the landing 
zone which are not accessible by the slider during normal operation of the 
disk file. Thus, the disk drive exerciser bypasses the normal drive 
electronics and commands the actuator to move to these OD or ID bands 
prior to the burn in. In this manner, the data regions of the disk are not 
exposed to any excessive wear which may be caused by the burn in. In 
addition, while it is possible to transfer carbon to the air bearing 
surface of the slider to a thickness greater than 50 .ANG., a carbon film 
of excessive thickness on the slider results in a spacing loss between the 
read/write head on the slider and the data on the disk. A 50 .ANG. carbon 
film on the slider is sufficient to prevent damage to the disk and 
provides protection to the MR head if the disk file is maintained at 
relatively low humidity (i.e., less than approximately 30% RH). It has 
been found experimentally that if a TiC/Al.sub.2 O.sub.3 slider is 
subjected to 1,000 s/s cycles on a thin film disk with a 300 .ANG. 
overcoat comprising 80-90 at. % carbon and 10-20 at. % hydrogen, an 
essentially uniform film of .about.50 .ANG. of carbon is formed on the air 
bearing surface of the slider. The protective carbon overcoat on the disk 
in this experiment was an amorphous hydrogenated carbon overcoat formed by 
the sputter deposition of carbon from a graphite target in an Ar-4% 
H.sub.2 gas, in a manner similar to that described in the previously cited 
'582 patent. Thus, the carbon film formed on the slider was also an 
essentially amorphous hydrogenated carbon film. The method of the present 
invention is fully applicable to those thin film disks which have 
protective carbon overcoats containing little or no hydrogen, such as the 
carbon overcoats formed by sputtering in the presence of pure Ar. If such 
disks were used, then the carbon film formed on the slider would also be 
an essentially amorphous carbon film having no substantial amount of 
hydrogen. 
While the preferred embodiments of the present invention have been 
illustrated in detail, it should be apparent that modifications and 
adaptations to those embodiments may occur to one skilled in the art 
without departing from the scope of the present invention as set forth in 
the following claims.