US 3661570 A
A process is described for providing a magnetic transducer material useful for the signal heads of a recorder-reproduced system. The process results in a material for a head core which consists of a majority or first phase of an alloy of iron, silicon and aluminum with an abrasion resistant second phase disposed in the grain boundaries throughout the first phase material.
Description (OCR text may contain errors)
I Umted States Patent [151 3,661,570
Moss May 9, 1972 [541 MAGNETIC HEAD MATERIAL FOREIGN PATENTS OR APPLIcATIoNs METHOD 109,085 10/1956 U.S.S.R.
 Inventor: Herbert lrwIn Moss, Yardley, Pa. OTHER PUBLICATIONS  Asslgnee: RCA Corpormmn, Goetzel, C. G.; Treatise On Powder Metallurgy, Vol. 1 pg. 1-  Filed: Apr. 3, 1970 61nterscience Publishers, lnc., New York 1949) TN 695 G6 I PP N05 25,465 Primary Examiner-Carl D. Quarforth Assistant Examinen-R. E. Schafer 52 us. CI ..75/206, 29/1825, 29/603,
75/212, 75/213, 75/2l4,148/104,179/100.2 C  Int. Cl. ..B22f 1/00  ABSTRACT  Field of Search ..75/212,214, 206, 213; A process is described for providing a magnetic transducer 29 1 25, 03; 14 104, 105; 179/1002 C material useful for the signal heads of a recorder-reproduced system. The process results in a material for a head core which  References Cited consists of a majority or first phase of an alloy of iron, silicon and aluminum with an abrasion resistant second phase UNITED STATES PATENTS disposed in the grain boundaries throughout the first phase material. 2,864,734 12/1958 Adams et a1. ..75/216 X 2/1959 Adams et al..
7 Claims, 2 Drawing Figures PATENTEDMAY 91912 -T|P PROTRUSTION,mfls
lllllll lll| b 100 200 300 400 500 000 700 000 9001000 -TIME, hours Fig. 2
INVENTORA Herbert J, Moss ATTORNEY MAGNETIC HEAD MATERIAL METHOD The magnetic record-playback heads used with magnetic tape systems in video recorders are a small but vitally important component in determining total system performance. Good performance at high frequencies depends upon several factors: a very short magnetic gap; close spacing between tape and head; and a high relative velocity between tape and head. To achieve the close spacing between tape and head, the tape is in direct physical contact with the head. This leads to considerable head wear because of the abrasive action between tape and head, which in turn limits head life.
Presently many magnetic beads are fabricated from materials known as Alfecon or Sendust consisting primarily of iron, silicon, and aluminum. This material is prepared by a vacuummelting and casting process in which the final average grain size is about 350 microns. The abrasive action which takes place between tape and head typically limits these heads to approximately l50 hours of useful life.
An object of the present invention is the provision of a process for providing magnetic head material which exhibits increased resistance to wear, and hence longer life, and exhibits little or no degradation in desired magnetic properties.
In accordance with one embodiment of the invention, a magnetic transducer head material is provided having a first phase of a magnetic alloy of iron, silicon and aluminum and a hard abrasion resistant second phase in the grain boundaries of the first phase. A granular sample of the first phase alloy is placed in a mill with a solution of a non-aqueous solvent and water. The contents of the mill are milled for a time sufficient to provide a particle size of the alloy no greater than 44 microns and to form a given amount of an oxide of the alloy on the surface of the particles of the alloy. The particles are separated from the non-aqueous solvent and water solution and dried. Then, the particles are hot pressed for a time sufficient to form a substantially theoretically dense compacted body from the particles.
REFERRING TO THE ACCOMPANYING DRAWING;
FIG. 1 is a perspective view, exemplifying a transducer head configuration with energizing coil.
FIG. 2 is a graphic illustration showing relative transducer head wear as a function of operating time.
In FIG. 1 there is shown a magnetic transducer 1, having pole pieces 2 and 3 arranged in confronting relation to form a gap 4. The width of the gap 4 for video recordplayback applications, for example may be in the range of 1-5 microns. An aperture 5 is provided through the transducer to facilitate the use of a coil or winding 6 for energizing the transducer 1. The geometric configuration of the transducer head or core of FIG. 1 is by way of example only. The invention and hence further discussion is directed to the process of providing the material from which the transducer is formed.
A feature of the invention to be discussed involves decreasing the grain size of a first phase material thus increasing the grain boundary area. An effective increase in material hardness might be expected due to the increased hardness that is usually observed in the vicinity of grain boundaries in a polycrystalline material. A further contribution to effective material hardness arises when a hard, more abrasion-resistant second phase is incorporated into the grain boundaries of the first phase material.
A phase may be defined as a homogeneous portion of matter that is physically distinct and mechanically separable. The first or primary phase is that phase which is present to the greatest extent, while the second phase makes up less than fifty percent of the total volume.
In the process of providing the material for magnetic heads to be discussed, it is preferable that the second phase is present predominantly in the grain boundaries between adjacent grains of the primary phase rather than in the interior of the grains of the first phase. In the process to be described, the second phase is a thin layer of an oxide of the first phase substantially surrounding each grain of the first phase matrix.
Throughout the discussion of the transducer head material 1 the first or primary phase material is a magnetic alloy. Described for the use of the first phase are ternary alloys of iron, silicon and aluminum of weight percent ranges which in,- clude the magnetic alloys more specifically known as Sendust and Alfecon. However other magnetic alloys which should be suitable for the first phase material are: binary alloys of ironnickel having for example 45-75 wt. nickel, which are known as Permalloy; binary alloys of iron-aluminum with up to 16 wt. aluminum, for example Alfenol; ternary alloys of iron-nickel-copper with for example 77 wt. nickel, 5 wt. copper and the balance iron, an example of which is known as Mumetal; quaternary alloys of iron-nickel-copper-molybdenum with for example 77 wt. nickel, 5 wt. copper, 4 wt. molybdenum and the balance iron.
The term effective hardness" is used to describe the actual wear resistance of a fabricated magnetic head when in contact with a magnetic tape material, since there may be-only a small measurable difierence in hardness between heads of different wear properties when tested with a standard laboratory hardness testing device, such as a Rockwell Hardness Tester.
A preferred method of providing the required second phase grain boundary material is to permit the magnetic alloy to oxidize during the process of particle size reduction prior to consolidation. The oxide is harder and more abrasion-resistant than the alloy. This will be described in more detail later. The second phase must meet at least three requirements. The quantity of second phase grain boundary material, i.e. the volume percent, must not be so great as to adversely influence the magnetic properties of the head. For video signal transducer applications the particle size of the second phase material preferably should not exceed for example 0. 1 micron, so that any grain boundaries which may occur in the gap area do not alter gap definition. The second phase material must not adversely react with the magnetic alloy at least up to the consolidation temperature so that the magnetic properties of the magnetic alloy are not degraded.
A preferred method is herein described for consolidating the magnetic alloy containing the dispersed second phase material. The method used to effect this consolidation or densification, includes what is known in the prior art as hotpressing or pressure-sintering. The hot-pressing technique permits the attainment of almost theoretically dense material with essentially no grain growth.
Hot pressing of magnetic ceramic materials such as, ferrite, for magnetic head purposes is known in the art. The prior art, however, points out that a final average grain size greater than 30 microns, and preferably 50 microns or greater, must be obtained to achieve a desirable decrease in wear of video magnetic heads.
By contrast it has been found that a decrease in the average grain size of a magnetic alloy of iron, aluminum and silicon and incorporation of a hard abrasion resistant second phase in the grain boundaries, has decreased wear of the transducer. In the case of ferrite transducers, hot pressing is used to circumvent certain problems inherent in the use of single crystal ferrite such as, difficulty of growing single crystal ferrite with proper stoichiometry, limited composition of ferrite, and magnetic and mechanical anisotropy.
Details of a preferred method of making the magnetic head material will now be described, which includes the preparation of a ternary magnetic alloy of iron, silicon and aluminum and attrition of the alloy to particle sizes less than a given size, for example, 44 microns. The formation of a surface coating of oxide on the individual alloy particles or grains is preferably effected during the attrition. The final body of material for the transducer is then produced by hot-pressing.
The magnetic alloy may be first formed into a bulk form or ingot by for example known vacuum-melting and casting processes, from the constituent elements in the proper proportion, to give the most desirable magnetic properties. Suitable weight percent ranges for the iron, silicon, aluminum alloy of the first phase are 6-12 percent silicon, 4-9 percent aluminum and the balance substantially iron, the combined silicon andaluminum constituting a maximum weight percent of about 17 percent of the alloy mixture. However for descriptive purposes of a preferred embodiment the composition utilized for the first phase is 85.3 wt. iron, 9.5 wt. silicon, and 5.2 wt. aluminum which is known as Alfecon. The outside surface of the ingot is ground away to remove any impurities introduced from the mold. The cast ingot is then cut into slices approximately 0.1 inch thick. The slices of alloy are cleaned in concentrated hydrochloric acid or several minutes, rinsed, and dried. These slices are then crushed in a steel mortar and pestel to provide granules that will pass through a 20 mesh screen (840 microns). The granular alloy material is then placed in a ball mill preferably of steel with steel balls and milled in the presence of a non-aqueous solvent such as ethyl alcohol, isopropyl alcohol, or acetone for a period of for example l2 to 16 hours. After milling, the powder is dried in a vacuum chamber at room temperature. At this point, various particle size ranges are separated by means of sieves. The particle size range used in the preferred embodiment of this invention is less than 20 microns, with the average particle size, for example, 16 microns. However, particle size ranges from 20 to 30 microns, and above have been used to provide relative amounts of improved wear of the magnetic heads. Where the second phase is provided by oxidation it appears preferable to utilize first phase Alfecon particles or grains of 44 microns or less.
It has been observed that the formation of an oxide layer around each particle of the magnetic alloy takes place during the milling operation, if there is a controlled amount of water present in solution with the non-aqueous solvent. It is therefore preferable that the non-aqueous solvent be miscible with water. During this process a certain degree of oxidation of the magnetic alloy takes place and the individual alloy particles are substantially surrounded by an oxide of the alloy. In addition, oxide not attached to the individual magnetic alloy particles become homogeneously dispersed throughout the alloy powder and end-up in grain boundaries during densification. The advantage of using a non-aqueous solvent with a controlled addition of water rather than water-free solvent is that the degree of oxide formed in the former case is considerably increased over the latter case. This is evidenced by microscopic observations of hot pressed bodies formed from material prepared in both manners. In addition, a comparison of the wear properties of magnetic heads fabricated from these materials, show that longer operating life is obtained from heads fabricated from material which was milled in a nonaqueous solvent and water solution. It has been found that a preferred solution of non-aqueous solvent and water is one which contains no less than 80 volume percent alcohol and no more than 20 volume percent water. Introducing the abrasionresistant material in this manner provides a substantially uniform distribution of the material around each alloy particle.
Preferably the amount of second phase should not exceed about 1 volume percent. The determining factor is the magnetic performance of the head structure, i.e., its ability to provide an adequate signal-to-noise ratio and not require excessive drive currents.
Another method of introducing the oxide phase is to expose the desired small particle size magnetic alloy to the air or to accelerate the oxidation process by heat treating the small particle magnetic alloy in air at several hundred degrees centigrade. If other techniques of material attrition known in the metallurgical or ceramic industry are used to obtain small particle size magnetic alloy, the non-aqueous solvent and water technique can still be used. In this case, little grinding action need take place, and the use of a ball mill would serve primarily to suspend the particles in the non-aqueous solvent-water solution to provide a uniform formation of oxide on the surface of the magnetic alloy particles.
A further operation in the process involves the densification of a sample of the small particle of oxidized Alfecon by means of vacuum hot-pressing or hot-pressing in an ambient of inert gas such as argon. A sufficient sample is weighed into a die to provide a densified right-cylindrical specimen with a diameter-to-height ratio of about 2. Samples with diameters of 0.5, l and 2 inches have been prepared. Die and rams may be constructed of a molybdenum alloy and are heated by means of a molybdenum-wound resistance furnace. The die is lined with a tight-fitting cylindrical high purity graphite insert which has a wall thickness of 0.25 inch and an inside diameter of 0.5 inch. Graphite is used as the insert material because of its lubricity, machinability, and inertness toward the Alfecon. The use of a supported graphite die in this fashion permits greater than normal pressures to be used during hot pressing without graphite failure. All graphite dies may also be used provided the wall thickness is sufiicient to withstand the pressures employed during hot pressing. An alternative technique of lining the molybdenum die, to prevent reaction between the alloy and the die and to permit easy ejection after hot pressing, would be to use a graphite cloth similar to that available from the Union Carbide Corporation and trade-named Grafoil." The top and bottom ram faces are separated from the Alfecon material by mean of 0.075 inch thick graphite or vitreous carbon discs. Here again, graphite cloth could be used. The die is operated in a floating manner to provide more uniform density throughout the sample. The die and furnace are contained in a water-cooled hot-pressing vacuum chamber which can be evacuated to at least 10' Uniaxial pressure is applied to the top ram by means of a conventional hydraulic press through a bellows sealed plunger.
Details of the actual hot pressing procedure are as follows. The hot press chamber containing the loaded die is evacuated, and the temperature of the die is increased to 600 C and held at this temperature for at least 1 hour. This serves to expel adsorbed gases and moisture from the undensified sample and assures the attainment of the highest possible density during hot-pressing. The sample is then pressed for approximately 1 minute at a pressure of 15,000 psi. This pressure is then released and a pressure of 2,000 to 3,000 psi is applied while the temperature of the die and its contents is increased to 950-l,000 C. The rate of heating from 600 C. to the hot pressing temperature is about 20 C./minute. A S-minute soak period at temperature permits thermal equilibration to take place. In the case of samples larger than 0.5 inch in diameter, longer periods of soak time may be needed for thermal equilibration. A pressure of 15,000 psi is then applied for a period of 1 hour. At the end of the 1 hour period the pressure is released, the electric power to the furnace is turned off, and the die is allowed to cool to room temperature.
The resulting sample of densified material is easily ejected from the die by applying a force to the top ram. This sample exhibits a density which is greater than 99.9 percent of the theoretical density. An almost void or pore free material is essential for video magnetic heads to maintain good gap definition during operation and high frequency response. For video applications, a porosity which is no greater than about 1 percent and preferably not greater than 0.1 percent should be provided. From a microscopic examination of a polished and etched surface the presence of a second phase appears to be distributed in the grain boundaries with no grain growth of the first phase. The oxide second phase, which is harder than the magnetic alloy has been observed to result in increased wear for magnetic heads fabricated from this material and used, for example, in quadruplex broadcast video recorders.
FIG. 2 shows test results for video heads fabricated from hot pressed Alfecon of various particle size ranges. The plots 11-13 are for Alfecon grain or particle size ranges which are respectively, less than 44 microns, 20-30 microns and 10-20 microns. In FIG. 2, tip protrusion relative to a standard vacuum-cast head is plotted as a function of time when tested in a standard high-frequency color broadcast video tape recorder. The tip protrusion as a function of time for the standard vacuum cast head (plot 10) is also included. The observed decrease in wear as the particle size of the Alfecon is decreased is believed due to an increase in grain boundary area and the concomitant increase in the amount of second phase material exposed on the wear surface of the head. Experiments performed in development of the invention showed, that video heads fabricated from small grained (less than 44 microns, 20-30 microns, or -20 microns) hot pressed Alfecon in which there is little or no second phase material disposed in the grain boundaries, wear at a rate comparable to heads made from vacuum cast Alfecon material (plot 10, FIG. 2). Thus, there appears to be little or no wear advantage accruing to a head structure fabricated from second phase free Alfecon, formed either by means of a hot press technique or a conventional sintering technique. The iron-aluminum-silicon alloy material fabricated without a second phase addition will exhibit a hardness of about 48 Rockwell C, whereas the same alloy composition containing second phase additions as taught by the present invention will have a hardness of at least 50 Rockwell C. Hot pressed heads with the described first and second phases show little or no degradation in magnetic properties when compared to vacuum cast Alfecon heads. Another advantage inherent in the present invention is the decrease in eddy current losses at high frequencies, due to the electrically insulating character of the second phase material. Eddy currents are dependent upon the frequency of the applied signal and the resistivity of the head material. In the case of the material described, the grain size and the presence of a second-phase material in the grain boundaries serves to increase the resistivity of the Alfecon.
As described, the first phase of the magnetic alloy is subjected to an environment which produces a hard second phase in and about the grain boundaries of the alloy. This may be enhanced by elevated temperatures or treatment in a solution, which accelerates formation of an oxide surface on the alloy particles. It is believed that such procedures for provision of the second phase, produce a predominantly chemical formation and bond between the first and second phase materials. This formation appears to afford greater coherency with and bonding to the individual first phase particles, and somewhat uniform distribution of the second phase.
The hot pressing parameters of temperature, pressure, and time have been described for a preferred method for making wear resistant head material including a first phase of Alfecon. Other combinations of temperature, pressure, and time can be used to hot press the particles to substantially theoretical density. For example, a temperature of 850 C. at 15,000 psi for 1 hour will also generate a fully dense body, as will 950 C. at 7,500 psi and l,050 C. at 3,500 psi for 1 hour. The pressure, temperature and time for hot pressing are interdependent, thus a hot pressing temperature of 750 C. would require about 6 hours at 15,000 psi but only about 1 hour at 25,000 psi to reach full density, whereas Alfecon powder subjected to a temperature and pressure of 1,100 C. and 7,500 psi, respectively, would require 5 to 10 minutes to reach full density. Experiments performed utilizing various hot pressing pressures in the range of about 3,500 psi to 60,000 psi, have been found to provide the desired increased wear resistance of the material when fabricated into a magnetic transducer and tested in a recorder-reproducer system. The above hot pressing parameters pertain to a cylindrical sample 0.5 inch in diameter and 0.25 inch high. Larger diameters and/or higher samples may require greater pressures because of the increased friction between sample and die wall.
Another technique of hot pressing which may also be used tofacilitate the densification of the described material is to cold press the alloy into a suitable shape, embed the cold pressed body in small particle size inert powder which is contained in a suitable die, and then apply heat and pressure, the pressure being transmitted to the alloy by means of the inert powder.
Still another technique is one which results in many hot pressed bodies in the form of thin sheets. The production of thin sheets of magnetic alloy containing the second phase rather than a single cylindrical sample which may be 1 or 2 inches thick has at least two advantages; the elimination of the slicing operation to form thin shapes for subsequent head fabrication and less waste of alloy material. One method of hot pressing thin sheets is as follows. A graphite lined die is prepared and a disc of 0.005 or 0.10 inch-thick graphite foil is placed against the face of the bottom ram. A sufiicient undensified sample of the material is weighed into the die to give the desired thickness of densified material. The material is leveled and pressed at several thousand pounds per square inch. A second disc of graphite foil is now placed on top of the sample and the loading process is repeated. The layering of material and graphite foil is continued until the total height is such as to produce a densified sample height which is approximately one-half the diameter of the samples. Hot pressing this arrangement then follows the procedure described above for a single sample. The presence of the graphite foil between each sheet sample permits easy separation of the sheets when the assembly is removed from the die. This process results in many thin sheets of densified magnetic alloy with second phase whose thickness may be as thin as 0.020 inch.
Following the formation of a sample of the material as described, the material is cut and finished by known techniques in the form of a desired transducer head geometry such as shown in FIG. 1. What is claimed is: 1. A process for providing magnetic transducer head material having a first phase of a magnetic alloy of iron, silicon and aluminum and a hard abrasion resistant second phase in the grain boundaries of said first phase, comprising the steps of:
placing a granular sample of said first phase alloy in a mill with a solution of a non-aqueous solvent and water;
milling the contents of said mill for a time sufficient to provide a particle size of said alloy no greater than 44 microns and to form a complex oxide of iron, silicon, aluminum on the surface of the particles of said alloy with the amount of the oxide being no greater than one volume percent of the alloy;
separating the particles from said non-aqueous solvent and water solution;
drying the particles; and
then hot pressing the particles for a time sufficient to form a substantially theoretically dense compacted body from said particles. 7
2. The invention according to claim 1, wherein; a bulk of said first phase alloy is crushed to provide said granular sample for milling, said sample having a granular size no greater than 840 microns.
3. The invention according to claim 1, wherein following the steps of separating the particles from said milling solution and drying, the particles are passed through a sieve to provide a quantity of particles having a size no greater than 20 microns.
4. The invention according to claim 1, wherein said milling solution is no less than volume percent alcohol and no more than 20 volume percent water.
5. The invention according to claim 1, wherein said nonaqueous solvent is ethyl alcohol.
6. The invention according to claim 1, wherein said hot pressing step includes: molding said particles at a pressure, temperature and for a time to provide a compacted body of said particles which has a porosity less than 1 percent.
7. The invention according to claim 6, wherein said hot pressing is performed utilizing a pressure in the range of 3,500 psi to 60,000 psi and a temperature in the range of 750 C. to 1, 1 00 C.