WO1988006334A1

WO1988006334A1 - Surface coating for magnetic head

Info

Publication number: WO1988006334A1
Application number: PCT/US1988/000438
Authority: WO
Inventors: John C. Scott
Original assignee: Akashic Memories Corporation
Priority date: 1987-02-13
Filing date: 1988-02-12
Publication date: 1988-08-25

Abstract

A wear layer is sputtered onto a magnetic recording head (17) of the type which rides upon a cushion of air over a rotating disk surface. Providing a wear layer on the air bearing surface of the head reduces friction between the head and disk thus extending disk and head life. In the preferred embodiment, a 50 A thick chromium layer (19) is sputtered directly onto the air bearing surface followed by a 200 A thick carbon layer (21) sputtered onto the chromium layer. Testing has shown a disk life increase of 50% using a head with such a wear layer.

Description

SURFACE COATING FOR MAGNETIC HEAD

Background of the Invention

This invention relates to magnetic heads used to read from and write to magnetic disks, and more specifically to a magnetic head with a layer of chromium and carbon thereon to reduce friction between the head and the magnetic disk surface during start up or slow down of the disk, thereby increasing disk and head lifetime, and allowing lower torque motors to be used to rotate the disk, an important feature for microdisk drives.

In magnetic recording employing a magnetic head and a rotating magnetic disk, the magnetic head contacts the disk surface when disk rotational velocity is insufficient to generate an air cushion for the magnetic head to ride upon. This situation exists during start up and slow down of the disk. The result is that for each start/stop cycle, the contact between head and disk wears the surface of the disk or head, until eventually the disk's magnetic layer is damaged or the gap of the head is abraided and the disk can no longer be written to or read from accurately. Such failure is frequently experienced in the Winchester technology as the number of start/stop cycles gets large, ie when the disk has been used for a reasonably long length of time, and in fact the friction can become so large as to stop motion of the disk completely. Further, if the correct conditions do not exist between the head and disk, the disk will not rotate even after a very small number of start/stop cycles.

In the case of smaller disk drives the head/disk interface becomes even more important since the head resides on the disk for a longer time before it begins to fly due to the lower velocity of the disk. This is particularly evident in the case of micro-disks where the disk diameter is 130 mm or 95 mm.

Various methods of reducing this frictional force have been attempted, all of the them relating to improvements to the disk. Examples of these are: 1) applying surface lubricants, both wet and dry, to the disk surface; 2) polishing or texturing the disk surface; 3) embedding oxygen in the outer carbon surface of the disk further improving the frictional properties of the carbon surface. [This latter method is the subject of a copending application, seri al number , filed February 3, 1987, entitled "Improved

Protective Layer For Magnetic Disk", by Robert Kobliska, et al, and assigned to Akashic Memories Corporation.]

All of these methods require the addition of costly manufacturing steps which add to the cost of each disk. Problems also exist in maintaining consistent quality of the disk surface.

Summary of the Invention

In accordance with preferred embodiments of the invention, an improved magnetic recording head is provided having a friction reducing surface layer deposited over the magnetic structure of the device. In the preferred mode, this friction reducing layer is sputtered carbon, having a nominal thickness of about 200A. Further, to increase the adhesion of the carbon layer, it is preferred to deposit a layer of chromium on the magnetic structure before the carbon layer is deposited.

Using such a carbon layer has proven to reduce head to disk friction significantly, thereby substantially increasing disk lifetime without substantially increasing manufacturing costs. Brief Description of the Drawings

Figure 1 is a side view of a slider with a magnetic recording head attached having a friction reducing surface according to the invention.

Figure 2 is a perspective view of a disk with a magnetic head mounted thereon supported by a carrier.

Figure 3 is a graph illustrating the frictional forces between a head and a disk before coating the air bearing surface of the head.

Figure A is a graph illustrating the frictional forces between a head and a disk after coating the air bearing surface of the head with a thin layer of chromium followed by a layer of carbon.

Detailed Description of the Invention

Shown in Figure 1 , is a side view of a slider 11 having a base portion 13 bonded to a flexure 15 for attaching the slider to a return arm (not shown) in a typical disk drive. Attached to the base portion 13 is a magnetic transducer, hereinafter called read/write head 17, which is typically constructed of a ferrite or other suitable magnetic material as is well known in the art for reading from, and writing to, a magnetic disk. The head 17 has a gap 20 between its magnetic pole pieces as is also traditional. In addition, in conformance with the known art, the slider has two rails 24 which have air bearing surfaces IS where the slider would normally contact the disk when the disk is not moving. Power for the head 17 is provided by electrical leads 26.

In the preferred embodiments of the invention, the read/write head has attached to the air bearing surfaces 18, a chromium layer 19 over which is deposited a surface layer 21 of carbon. In the preferred mode, both the chromium layer and the carbon layer are deposited by sputtering. However, it is believed that other methods of deposition can also be used, although sputtering is preferred because it can be accomplished relatively quickly and because the properties of the sputtered carbon surface appear to provide additional benefits over other methods of deposition, for example better wear resistance for the head. In addition, the very small thicknesses of chromium and carbon are well controlled by the sputtering process.

In the preferred mode, the nominal thickness of the sputtered chromium layer 19 is about 50A and the nominal thickness of the sputtered carbon layer is about 200A. However, other thicknesses of these materials may also be used, provided they do not become so large as to significantly increase the separation of the head recording gap from the surface of the disk, thereby decreasing the available magnetic signal. Further, in some instances, it appears that the chromium layer may be eliminated completely depending on the desired lifetime. In terms of thickness variations, it appears that the chromium layer may be increased to ass much as 1000 without seriously affecting signal amplitude. Similarly, the carbon layer can be varied from as low as 100A to as much as 300 A, and still provide significant benefits, although 200A is preferred. Further, it should be clear that the lower limit for the carbon thickness is actually zero, since the head will still be able to read and write without the layer. The wear resistance benefits are just reduced as the carbon thickness is reduced below the nominal.

In order to deposit these layers on the head, the flexure 15 with the head secured thereto was attached to a disk substrate 30 as illustrated in Figure 2, with the rail side out, and the substrate was processed in a Varian Associates MDP-1000 disk sputtering machine. The MDP-1000 has multiple sputtering stations, all of which open into a main vacuum chamber, so that depositions can be; arranged serially, ie. one deposition can be followed directly by another in a separate sputtering station, without leaving the main vacuum of the system. In the preferred mode for depositing the chromium and the carbon in the MDP-1000, the pressure in the main vacuum chamber is first reduced to less than 5×10^-7 Torr, and preferably to below 2×10^-7 Torr, as measured o n a Varian 880 ion gauge, and the system is checked to ensure that the following readings are obtained on a residual gas analyzer such as a Dycor Model M100: oxygen (mass 32) less than 2×10^-9 , water (mass 18) less than 1×10^-7. Similarly, the individual stations are pumped by their individual cryopurπps to achieve a gas pressure therein of less than 2×10^-6 Torr or lower and all cryopurπps are stabilized at a temperature of less than 15ºK. Standard procedures are used to ensure adequate cooling water to the station sputtering magnetrons, and gas lines for nitrogen, argon, and compressed air, are monitored to ensure adequate pressure. The assembly made up of disk 20 with head 11 attached thereto is then moved into the main vacuum chamber via a load lock, and into a heating station. Thin assembly is heated with an induction heater to 150^º C± 50^º C. Then it is moved to a sputtering station. The chromium layer is sputter deposited in the station using an argon flow rate of 17 standard cm /min. ± 10% and argon pressure of less than 15m Torr, with a cathode voltage of 550 volts and a target current of 2.5 amps ± 1054. The t yp i ca l deposition rate is about 1000 A/min., and the sputtering time is generally about 3 seconds, to achieve the nominal 50 A layer of chromium. Those skilled in the art will realize that these values for the voltage, current and time can, of course, be manipulated considerably and still achieve the same thickness of chromium. Similarly, other thicknesses of the carbon layer can be achieved by changing these parameters. In the preferred embodiment, a typical target for the chromium deposition is available from Toyo Soda Manufacturing Co., which is of Grade 4NG purity (≥97% chromium by weight), having less than 250 parts per million of other materials therein, other than oxygen.

The carbon deposition is performed in a subsequent station as indicated earlier. For that deposition, the typical cathode voltage is 550 volts, the target current is 5.0 amps ± 10%, and the total sputtering time is about 25 seconds to achieve a total preferred nominal thickness of 200 A. A carbon layer of between 100-300 A, however, is acceptable. Again, the argon pressure is maintained below 15 mTorr with the argon flow rate maintained at about 17 standard cm³/min. In the preferred embodiment, a typical target for the carbon deposition is Spectrographic grade carbon, having a purity of 99.999%, e.g., available from Sputtering Materials, Inc., in Santa Clara, California.

Test results illustrated in Figs. 3 and A demonstrate by way of example the improved friction coefficients between a carbon-coated disk and a head coated with an outer carbon layer (Fig. A), and such a combination without ari outer carbon layer on the head (Fig. 3). The carbon layer on the disk can be obtained as described in U.S. patent application Serial No. 730,778 by John Scott et al, filed May 3, 1985, entitled "Improved Thin Film Magnetic Disk", and assigned to Okashic Memories Corporation, incorporated herein by reference.

The graph of Fig. 3 shows the friction coefficient vs. time using an uncoated slider. In Fig. 3, thefriction coefficient μ, is about 0.17, whereμ, = f₁ /F, f₁ is the absolute friction force measured with the disk rotating at a relative velocity of 15 ± 0.5 inches per second, and the head load F, measured by using a strain gauge attached to the head flexure, is 9.5am + 1054. The coefficient μ₂ = f₂ /F, where f₂ is the relative peak to peak friction force, is about 0.03. The static friction μ₃ = f₃/F, where f₃ is the static friction force, is about 0.19, and is measured during the first rotation of the disk and is taken from the zero position to the maximum value of the force trace.

In comparison, Fig. A showing friction coefficients using a head coated with chromium and carbon in accordance with the preferred embodiment of the invention, reveals that μ₁ is 0.10, μ₂ is less than 0.01, and μ₃ is 0.11. These significant reductions in friction coefficients have resulted in increased disk life, measured in start/stop cycles, of over 50%, or from an average 20,000 cycles before the improvement to an average of 30,000 cycles after the improvement.

Those skilled in the art will appreciate that there are many variations encompassed by the concept of the invention. For example, chromium and carbon deposition may be accomplished using chemical vapor deposition, direct evaporation, or reactive evaporation, and layer thicknesses may be varied.

It should also be appreciated that the method of preparing the appropriate layers is not restricted to use of the Varian MDP-1000, which is merely a convenient vehicle for carrying it out. Also, it should be noted that improvement is obtained in those cases where the material of the head is changed, eg. for monolithic ferrite heads, for minicomposite material heads (barium titanate or calcium titanate), and for titanium carbide heads.

Claims

Claims:What claimed is:

1. A magnetic recording element comprising: a magnetic recording head of the type which rides upon a cushion of air over a rotating disk surface and has an air bearing surface; and a wear layer deposited on said air bearing surface to reduce friction between said head and said disk when said head and said disk come in contact.

2. A recording element as in claim 1 wherein said wear layer comprises carbon.

3. A recording element as in claim 2 wherein said carbon layer is between 100-300 A in thickness.

A. A recording element as in claim 2 wherein said carbon layer is about 200 A.

5. A recording element as in claim A wherein said carbon layer is deposited by sputtering.

6. A recording element as in claim 1 further comprising a chromium layer interposed between said air bearing surface and said wear layer.

7. A recording element as in claim 6 wherein said chromium layer is between 30-100 A thick.

8. A recording element as in claim 6 wherein said chromium layer is about 50 A thick.

9. A recording element as in claim 6 wherein said wear layer comprises carbon.

10. A recording element as in claim 8 wherein said wear layer comprises carbon 200 A thick.

11. A recording element as in claim 6 wherein said air bearing surface has been heated to a temperature of 150º C ± 50°C immediately prior to the sputtering of said chromium layer.

12. A method of depositing a wear layer onto a magnetic recording head, of the typo which rides upon a cushion of air over a rotating disk surface and has an air bearing surface, comprising: sputtering carbon directly on said air bearing surface to form a wear layer between 100-300 A in thickness.

13. The method of in claim 12 further comprising the step of sputtering a chromium layer 30-100 A in thickness onto said air bearing surface to act as art interface between said air bearing surface and said carbon layer.

14. The method of claim 13 further comprising heating said head to a temperature of 150ºC ± 50ºC immediately prior to the sputtering of the chromium layer.