Low wear closure glass for magnetic tape heads

Abstract: A low wear closure glass for use in multitrack magnetic tape heads has been prepared by combining lead borosilicate glass with borosilicate glass in a 60/40 weight percent ratio. The glass formed from the combination exhibits compatibility with ferrite and has reduced wear. Controlling the anneal temperature regulates the residual stress of the closure glass produced, and thus, provides some control over the wear characteristics of the closure glass.

Description: The subject invention generally relates to the closure glass used in multitrack magnetic tape heads and, more particularly, to a new glass formulation with improved wear properties for use as the closure glass. Description of the Prior Art Lead oxide based glasses are commonly used for the closure glass in multitrack tape heads because of their compatibility with Nickel-Zinc (NiZn) ferrite. A typical example of a lead oxide based glass in common use for multitrack tape heads is IBM 400 glass. Table 1 details the composition IBM 400 glass. IBM 400 glass has an annealing point (glass viscosity = 10¹³ poise) of 435°C, a softening point (glass viscosity = 10 7.6 poise) of 530°C, a working point (glass viscosity = 10⁴ poise) of 695°C and a coefficient of thermal expansion equal to 70*10⁻⁷ ± 3.0*10⁻⁷/°C. IBM 400 glass has superior properties in terms of its compatibility with NiZn ferrite; however, the glass has a high wear rate compared to that of the ferrite in the magnetic tape head. Borosilicate based glasses have higher hardness and better wear attributes compared to lead oxide glasses. Borosilicate glass having Corning® glass code 7740 has an annealing point of 565°C, a softening point of 821°C, a working point of 1252°C, and a coefficient of thermal expansion equal to 32.5*10⁻⁷/°C. Lead borosilicate glass having Corning® glass code 7570 has an annealing point of 363°C, a softening point of 440°C, a working point of 560°C, and a coefficient of thermal expansion equal to 84*10⁻⁷/°C. However, neither borosilicate glass having Corning® glass code 7740 nor lead borosilicate glass having Corning® glass code 7570 are acceptable for bonding with NiZn ferrite. U.S. Patent 4,638,387 to Mukasa et al. discloses the use of borosilicate glass as an impregnating material for filling fine voids in a sliding contact member used in conjunction with a magnetic tape head. U.S. Patent 4,069,360 to Yanagisawa et al. discloses the use of a polysilicate layer, which may be borosilicate glass, coated with a lubricant to protect and reduce frictional wearing of the recording disc on which it is coated and the surface of the magnetic recording head. U.S. Patent 4,170,032 to Yokoyama et al. discloses a magnetic head wherein borosilicate glass is sputter deposited in the gap area. U.S. Patent 4,110,804 to Castrodale et al. discloses the use of lead borosilicate glass for bonding parts of the transducer in a read/write and tunnel erase magnetic head assembly. None of the above-patents show or discuss the use of borosilicate based glass as a closure glass in a multitrack tape head. Prior art multitrack tape heads suffer from excessive wear of the closure glass. When the closure glass wears down at a faster rate than the ferrite, the tape head becomes unreliable. The superior wear characteristics of borosilicate based glasses have not been applied in multitrack tape heads because of their incompatibility with NiZn ferrite. SUMMARY OF THE INVENTION The present invention is defined in the attached claims. It is therefore an object of this invention to provide a multitrack tape head with improved wear characteristics which utilizes a borosilicate based glass composition. It is another object of this invention to provide a glass composition compatible with NiZn ferrite which is a combination of borosilicate glass and lead borosilicate glass. It is yet another object of this invention to provide a closure glass in which the wear characteristics can be regulated by adjusting the anneal cycle. According to the invention, lead borosilicate glass having Corning® glass code 7570 (3% wt SiO₂, 11% wt Al₂O₃, 12% wt B₂O₃, 0% wt Na₂O, 74% wt Pbo) has been combined with borosilicate glass having Corning® glass code 7740 (81% wt SiO₂, 2% wt Al₂O₃, 13% wt B₂O₃, 4% wt Na₂O, 0% wt Pbo) to form a new glass which has improved attributes for use as a closure glass. Experiments were performed from which it was determined that an optimum weight percentage ratio of lead borosilicate glass to borosilicate glass was a 60/40 ratio. The 60/40 ratio of lead borosilicate glass to borosilicate glass produced the best results in terms of melting temperature, ferrite compatibility, stress and wear. A powdered mixture of 60% by weight lead borosilicate glass and 40% by weight borosilicate glass was put on the surface of a slotted write closure slab (a ferrite slab with slots cut therein). The slab was then placed in an oven and the glass was melted in a procedure called "glassing". Glass bubbles were then removed using a "hipping" procedure. Finally, the glass was annealed. Experiments were performed in which it was determined that 510°C was the optimum anneal temperature. It was determined that the residual stress in the glass, which is a function of the anneal cycle, can affect the wear characteristics of the glass produced. Wear bar samples were prepared from the slabs and were tested using a media abrasivity test procedure. BRIEF DESCRIPTION OF THE DRAWINGS The foregoing and other objects, aspects and advantages of the invention will be better understood from the following detailed description of the preferred embodiments of the invention with reference to the accompanying drawings, in which: Figure 1 is an isometric view of a closure section in a multitrack head; Figure 2 is an isometric view of a multitrack tape head; Figures 3-5 are time versus temperature profiles of the oven temperature used for the steps of glassing, hipping, and annealing, respectively; Figure 6 and 7 are a graphical representation of bow stress test results versus the anneal temperature utilized for a glass sample; and Figures 8-10 are plots of the media abrasivity test results for wear bars which use IBM 400 glass, high stress lead borosilicate glass/borosilicate glass, and low stress lead borosilicate glass/borosilicate glass, respectively. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION Referring now to the drawings and, more particularly to Figure 1, there is shown a closure section 10 for a multitrack tape head. A row of magnetic ferrite blocks 12 are supported by a substrate 14. The ferrite blocks 12 and substrate 14 are joined together by the closure glass 16. The width of the ferrite blocks 12 determine the width of the data tracks and the width of the closure glass 16 between the ferrite blocks 12 establishes the separation distance between the data tracks. Figure 2 shows a multitrack tape head 11 where a ferrite wafer 18 having the same length and width as closure section 10 is spaced from the closure section 10 by a transducing gap 20. The transducing gap 20 co-acts with a magnetic medium such as a tape or disc which is being written on. Fabrication of the multitrack head 11 is not specifically important to this invention and may be done according to well known techniques of which U.S. Patent 4,366,518 to Chow et al., 4,396,967 to Argumendo et al., and IBM TDB Vol. 18, No. 6, pp. 1981-1984, 1975 by Franklin et al. are but a few examples. This invention is primarily concerned with the wear characteristics of the closure glass 16 used in the multitrack tape head 11. Lead borosilicate glass is available from the Corning glass company of New York and is sold under Corning® glass code 7570. The lead borosilicate glass is finely powdered and may be applied by dipping, extruding, silk screening, or by other suitable techniques. A slurry of the glass can be prepared by adding a suitable vehicle to the glass such as amyl acetate and 1.2% by weight nitrocellulose. Lead borosilicate glass has a working point equal to 560°C, where the working point is defined by a glass viscosity equal to 10⁴ poise. The coefficient of thermal expansion for lead borosilicate glass is equal to 84*10⁻⁷ in/in/°C. Borosilicate glass is available from the Corning glass company of New York and is sold under Corning® glass code 7740 (which is a low expansion borosilicate glass). Like lead borosilicate glass, the borosilicate glass is available in powder form and can be applied by the techniques discussed above. Borosilicate glass has a working point equal to 1252°C and a coefficient of thermal expansion equal to 33*10⁻⁷in/in/°C. Both lead borosilicate glass and borosilicate glass have high corrosion resistance. The inventors have determined experimentally that a combination of lead borosilicate glass and borosilicate glass will produce a new glass which is suitable for use in multitrack magnetic tape heads. A 70/30 weight percent mixture of lead borosilicate glass to borosilicate glass was compatible with NiZn ferrite and had a melting temperature equal to 870°C. The 70/30 glass had good properties generally but was subject to excessive wear. A 50/50 weight percent mixture of lead borosilicate glass to borosilicate glass had a melting temperature equal to 980°C, too high for NiZn ferrite compatibility. In addition, the 50/50 glass had a tendency to crack because of its highly stressed state when incorporated within the slots of the write closure pieces (i.e., closure glass 16 in Figure 1). A 60/40 weight percent mixture of lead borosilicate glass to borosilicate glass had the optimum characteristics for use as the closure glass 16 in a multitrack tape head. The 60/40 glass had a melting temperature of 920°C and a coefficient of thermal expansion which was suitable for use with NiZn ferrite. Acceptable composition limits are a 55/45 weight percent mixture of lead borosilicate glass to borosilicate glass to a 65/35 weight percent mixture of lead borosilicate glass to borosilicate glass. If the lead borosilicate component exceeds 65%, stress is too great and wear is too high. If the borosilicate component exceeds 45%, stress again is too great and the glassing temperature required for processing causes unacceptable ferrite grain boundary attack. Figures 3 through 5 show the time, temperature, and pressure conditions used for glassing, hipping, annealing the 60/40 glass combination. Figure 3 shows the temperature profile for the step of glassing. The lead borosilicate glass and the borosilicate glass were combined in powder form in a 60/40 weight percent mixture. After being thoroughly mixed, approximately three to four grams of the powder combination was placed on the surface of a slab of NiZn ferrite which had multiple slots cut in the top surface. The slab with the glass powder was placed into an infra-red heating furnace. The glass was melted on the slab, thereby filling in the slots, by heating the slab to 920°C during a forty five minute ramp interval, then holding the temperature at 920°C for two hours, then decreasing the temperature to 710°C during a two hour interval, then decreasing the temperature to 345°C during a four hour interval, and finally returning the oven to room temperature. Figure 4 shows the process of "hipping" was performed under both controlled temperature and pressure conditions. Hipping is a procedure designed to remove any glass bubbles present in the glass. Upon removal from the infra-red furnace used in glassing, the glassed, ferrite slabs were placed in an isostatic furnace. During a thirty minute interval the furnace temperature was increased from room temperature to 740°C. The gas pressure for the furnace during the thirty minute ramp was initially 1000 pounds per square inch (psi). The pressure increases as a function of temperature and was approximately 2000 psi at 740°C. This temperature and pressure were maintained for thirty minutes at which time the temperature was reduced to 510°C and the pressure reduced to 1750 psi over a ten minute interval, and the temperature and pressure were held constant for an hour before the isostatic furnace was cooled down. Figure 5 shows the anneal heating cycle which is performed in the infra-red furnace. Annealing is a heating and slow cooling process used to toughen the glass and reduce brittleness. The glassed and hipped, ferrite slabs are placed in the infra-red furnace and the temperature of the furnace is ramped to 510°C from room temperature over a thirty minute interval, then the furnace temperature is held constant for one hour, then the furnace is cooled to 405°C at a cooling rate equal to 14°C ± 2°C/minute over an eight minute time interval and the furnace is held constant at 405°C for three minutes before it is finally cooled down. Figure 6 shows experimental bow stress results for the 60/40 glass combination where various anneal temperatures were used. All samples used in the experiment were crack free. The bow stress test is performed by taking the glassed, hipped, and annealed substrate and removing the excess glass from the top of the slots by grinding. In addition, a large slot is ground into the back side of the substrate. If a profile trace is now taken from edge to edge of the substrate across the glass slots, a measure of stress is obtained. The magnitude of the bow being proportional to the resultant stress of the ferrite/glass laminated structure. Bow measurements were taken on ferrite samples of 11 and 22 mm in length. Data shown in Figure 6 has been normalized to the 11 mm sample. From Figure 6 it can be determined that annealing the 60/40 lead borosilicate to borosilicate glass at 510°C yields a glass with the minimum residual stress. The 510°C optimum anneal temperature was implemented in the anneal cycle shown in Figure 5. Figure 7 shows experimental bow stress results for 65/35 and 55/45 glass combinations where various anneal temperatures were used. The bow stress test was performed as discussed above. Bow measurements were taken on ferrite samples 11 mm in length. Figure 7 indicates some cracking occurs with these combinations under higher stress conditions. These combinations of lead borosilicate and borosilicate glass should be considered the outer limits for an acceptable closure glass. Wear bar samples were prepared from the glassed, hipped, and annealed ferrite slabs which resulted from the procedures shown in Figures 3 through 5. Wear bar samples are sections of the slab which are cut and milled to have a surface which has alternating ferrite and glass portions. A sample under test was then worn with PDM Sonoita recording media by passing the tape over the corner of the test sample. This test produces a flat where the tape wears the corner away. A profile trace taken along the worn flat will show the relative height of both the ferrite and glass. From this information, the relative wear rates of both ferrite and glass can be determined. Figures 8 through 10 show wear bar profile results obtained using the abrasivity test discussed above for wear bars having IBM 400 glass, wear bars having high stress 60/40 lead borosilicate to borosilicate glass (i.e., glass annealed at 480°C), and wear bars having low stress 60/40 lead borosilicate to borosilicate glass (i.e., glass annealed at 510°C), respectively. The results for the IBM 400 glass wear bar shown in Figure 8 is used as a reference and its scale is 5µ'' along the ordinate by 1000µ'' along the abscissa. The scale in both Figures 9 and 10 are 2µ'' by 1000µ''. In comparison to the IBM 400 glass, the 60/40 lead borosilicate to borosilicate glass wore much less. Contrasting Figures 9 and 10, it can be seen that the wear characteristics of the 60/40 glass are related to the residual stress in the glass. Figure 9 shows that the higher the stress in the 60/40 glass the less the wear rate, while Figure 10 shows that at low stress, the 60/40 glass wear is approximately the same as NiZn ferrite which is the desired property for the closure glass in a multitrack tape head. While the invention has been described in terms of its preferred embodiment where specific time-temperature-pressure glassing, hipping, and annealing conditions are utilized, those skilled in the art will recognize that variations in these conditions can be practiced within the scope of the appended claims.

Claims: A low wear closure glass for use in magnetic tape heads comprising in combination a lead-boro-silicate glass and a boro-silicate glass, characterised in that the composition of the lead-boro-silicate glass is as follows (in wt %) SiO₂ 3% Al₂O₃ 11% B₂O₃ 12% NaO 0% PbO 74% and the composition of the boro-silicate glass is as follows (in wt %) SiO₂ 81% Al₂O₃ 2% B₂O₃ 13% NaO 4% PbO 0% with the lead-boro-silicate glass being present in the range of 65 to 55 percent by weight and the boro-silicate glass being present in the range of 35 to 45 percent by weight. A low wear closure glass as claimed in Claim 1 characterised in that the lead-boro-silicate glass is present at approximately 60 percent by weight and the boro-silicate glass is present at approximately 40 percent by weight. A multitrack tape head having a closure glass produced by the following process: mixing lead borosilicate glass according to claim 1 at a weight percent ranging from 65 to 55 percent with borosilicate glass according to claim 1 at a weight percent ranging from 35 to 45 percent, said mixing step producing a glass mixture; applying said glass mixture to a slotted ferrite slab; glassing said ferrite slab by heating said slotted ferrite slab with said glass mixture to a temperature sufficient to melt said glass mixture, said glassing step producing a glassed ferrite slab; hipping said glassed ferrite slab by subjecting said glassed ferrite slab to sufficient temperature and pressure conditions to remove any bubbles present in said glass ferrite slab, said hipping step producing a glassed, hipped ferrite slab; and annealing said glassed, hipped ferrite slab at a selected temperature which toughens glass present on said slab. A multitrack tape head as recited in claim 3 wherein said lead borosilicate glass used in said mixing step is present at approximately sixty percent by weight and said borosilicate glass used in said mixing step is present at approximately forty percent by weight. A multitrack tape head as recited in claim 3 wherein said selected temperature in said annealing step is chosen according to a desired stress level for said glass present on said slab. A multitrack tape head as recited in claim 3 wherein said selected temperature in said annealing step is between 500°C and 520°C. A method for making a closure glass for a multitrack tape head, comprising the steps of: mixing lead borosilicate glass according to claim 1 at a weight percent ranging from 65 to 55 percent with borosilicate glass according to claim 1 at a weight percent ranging from 35 to 45 percent, said mixing step producing a glass mixture; applying said glass mixture to a slotted ferrite slab; glassing said ferrite slab by heating said slotted ferrite slab with said glass mixture to a temperature sufficient to melt said glass mixture, said glassing step producing a glassed ferrite slab; hipping said glassed ferrite slab by subjecting said glassed ferrite slab to sufficient temperature and pressure conditions to remove any bubbles present in said glass ferrite slab, said hipping step producing a glassed, hipped ferrite slab; and annealing said glassed, hipped ferrite slab at a selected temperature which toughens glass present on said slab.

Inventors: DAUGHENBAUGH GERALD ALLEN, RIGBY EUGENE BERTRAND, DAUGHENBAUGH, GERALD ALLEN, RIGBY, EUGENE BERTRAND
Applicants: IBM, INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1990
Main classification: C03C3
IPC codes: C03C8, G11B5, C03C3