Steven T. Patton

Effect of diamondlike carbon coating and surface topography on the performance of metal evaporated magnetic tapes

In this study, metal evaporated (ME) tapes with and without diamondlike carbon (DLC) coating were tested for friction, wear, and magnetic performance. Performance of flat DLC ME tape is compared with two non-DEC ME tapes (one being flat and the other with waviness). By using a commercial VCR as a magnetometer and the Wallace equation, changes in the rms head output level were correlated to changes in head-to-tape spacing as tape wear occurred during play/rewind cycling tests. Interface stability and recording performance at a 0.6 /spl mu/m recording wavelength were measured to bit level resolution using a dropout counter. Some pause mode and environmental testing were also performed. Tribological performance and magnetic reliability of ME tape was affected by waviness of the ME tape surface as well as by the presence of a DLC coating. Waviness of one kind of non-DEC ME tape caused poor magnetic performance and localized wear of the tape surface, but loose wear debris could escape contact areas by settling into valleys on the tape surface. In the case of flat non-DLC ME tape, loose wear debris could not escape from contact areas leading to early failure. DLC coating prevented catastrophic abrasive wear and reduction in head-to-tape spacing or intimate contact during cycling tests, but could not significantly improve the durability of ME tape. DLC coating reduces asperity compliance as compared to non-DEC tapes. All ME tapes failed by lateral crack formation (cracks often propagated through coating defects) driven by longitudinal tape tension or flexing of the tape in the transport. In pause mode, DLC coating improved magnetic reliability and tribological performance by preventing the heads from contacting the metallic magnetic layer. Based on this study, a flat tape must be used for magnetic considerations and DLC coating is needed to prevent abrasive wear and intimate contact. Reducing the number of coating defects and optimization of the toughness of magnetic and DLC coatings are key parameters for future ME tapes. At fixed temperature, high relative humidity leads to severe tape instability and high dropout frequency. At high specific humidities, lower temperature leads to severe tape instability and high dropout frequency. Bearing ratio curves for the tapes determine the onset of high friction and tape instability at high humidities.

Micromechanical and tribological characterization of alternate pole tip materials for magnetic recording heads

Pole tip recession or PTR (relative wear of the pole tip with respect to the air bearing surface) causes signal loss when using inductive heads. Loss of signal caused by spacing between a head gap and the recording medium is magnified in high-density short wavelength recording. Nickel iron (NiFe) is the most commonly used pole material. NiFe is softer than the head substrate material (typically NiZn ferrite or Al2O3TiC) which leads to PTR as a result of differential wear of the materials. Alternate pole tip materials which are more wear resistant and superior in magnetic properties (such as high saturation magnetization), as compared with NiFe, need to be developed. In this research, NiFe, cobalt zirconium tantalum (CoZrTa) and iron aluminum nitride (FeAlN) materials were studied. In the first phase of this study, micromechanical characterization of the three pole tip materials, the alumina (Al2O3) insulating under/overcoat and gap material and the Al2O3TiC substrate was conducted using a depth-sensing nanoindenter. The nanohardness of NiFe, CoZrTa and Al2O3 are similar and about one half that of FeAlN, and the hardness of the Al2O3TiC substrate is about twice that of FeAlN. Microscratch studies showed that the critical load required to cause failure of the NiFe and CoZrTa films are similar and about one fourth that of FeAlN, and the critical load for FeAlN is comparable with that of the Al2O3 and Al2O3TiC substrate. Thus, FeAlN is superior in mechanical properties to NiFe and CoZrTa. In the second phase of this study, dummy tape heads fabricated with the three pole materials were run against metal particle (MP) tape in a linear tape drive. The PTR was measured by atomic force microscope (AFM) imaging before and after the sliding tests. Any nonuniformities in the thin-film region gets removed in the first few kilometres of sliding. FeAlN poles exhibited a low (∼10 nm) and constant PTR over 1 000 km of tape sliding distance, whereas the NiFe and CoZrTa poles exhibited growth in recession to about 30 and 40 nm, respectively, over the same sliding distance. The superior wear resistance and high saturation magnetization of FeAlN are ideal for high-density thin-film inductive heads.

Pole tip recession studies of hard carbon‐coated thin‐film tape heads

Hard carbon coatings were deposited by cathodic arc and direct ion beam deposition techniques on thin‐film Al2O3–TiC heads and by the latter technique on thin‐film Ni–Zn ferrite heads. Functional accelerated tests were conducted against metal particle tapes in a linear tape drive. Ion beam carbon coatings on Ni–Zn ferrite and Al2O3–TiC heads substantially reduced the pole tip recession observed with uncoated heads. Cathodic arc carbon coated Al2O3–TiC heads performed better than uncoated heads, but were less effective than the ion beam coating. Pole tip recession increased only if carbon was removed from the pole tip. This suggests that coating effectiveness is determined by its adherence to the pole tip. In two‐wide pole tip heads, wear of the pole adjacent to the substrate was less than that of the other pole. Coatings withstood accelerated tests and may meet life time requirements of future heads.

Tribology in ultra-high density tape drive systems: State of the art and future challenges

Advanced ultra-high density tape drives require ever increasing volumetric recording densities which places a premium on the tribology of the head-to-tape interface. For these drives, ultrathin smooth tapes would be in contact or near contact with the recording head at high relative velocities. Minimum friction, near zero wear with stable magnetic performance over a wide range of operating conditions is required for adequate performance. There are considerable tribological challenges related to both recording heads and media to be used in these systems. Some of the many challenges in the design of these drives are: attaining low air bearing surface wear, attaining and maintaining low pole tip recession, developing durable tape media, and developing dimensionally stable substrates with high modulus for ultra-thin tapes.

Environmental effects on the streaming mode performance of metal evaporated and metal particle tapes

A commercial Hi-8 video cassette recorder was instrumented to measure friction force between the rotary heads and tape, rms head output, and signal dropouts to sub /spl mu/s duration. Streaming (play) mode experiments using metal evaporated (ME) and metal particle (MP) tapes were performed at design tension under equilibrium and nonequilibrium conditions inside of an environmental chamber at various temperature and specific humidity (SH=ratio of the weights of water vapor to dry air in the mixture). Interface stability and recording performance at a 0.6 /spl mu/m recording wavelength were measured to bit level resolution using a dropout counter and changes in rms head output were correlated to changes in head-to-tape spacing using the Wallace equation. ME tape performed best at moderate SH (from 0.009 to 0.013) in the operating temperature range of 15.6 to 32.2/spl deg/C, whereas MP tape performed best at low temperature. At a given temperature, higher SH increased normal and friction forces and decreased head-to-tape spacing due to spontaneous water meniscus formation between contacting and noncontacting asperities on the head, tape, and debris particle surfaces. Dropout frequency and interface stability were sensitive to both SH and temperature. Humidity dependence was governed by the relative size difference between wear debris particles and spontaneously formed water menisci. Competing mechanisms of increased lubricant mobility and spontaneously formed water menisci of smaller radii of curvature at higher temperature governed temperature dependence. A model based on capillary condensation of water vapor onto surfaces in a sliding contact is developed to explain experimental data. An expression for meniscus force consisting of both Laplace and surface tension contributions is developed for nanometer size contact spots and wear debris particles. The model predicts that meniscus force will increase and head-to-tape spacing will decrease with increasing SH which was observed experimentally. Four lubrication regimes are defined for wear debris particles passing through the contact interface. Both SH and extent of deformation of a particle determine whether menisci will form around the particle, and presence or absence of associated meniscus forces explains trends in dropout frequency data. The model allows determination of meniscus height from which minimum values of Kelvin radius and relative humidity (RH) in the contact interface can be calculated. Proximity and intimate contact of the surfaces increases RH and SH in the contact interface over that maintained in the environmental chamber to near their saturation values.

Friction, wear and magnetic performance of metal evaporated and particulate magnetic tapes:

Abstract Metal evaporated (ME), metal particle (MP) and barium ferrite (BaFeO) magnetic tapes are leading candidates for ultra-high-density magnetic tape recording applications. Using a commercial video cassette recorder as a magnetometer and the Wallace equation, changes in the r.m.s. head output level were correlated to changes in head-to-tape spacing as tape wear occurred during play/rewind cycling tests. Interface stability and recording performance at a 0.6 μm recording wavelength were measured to bit level resolution using a drop-out counter. Some pause mode testing was done for comparison with streaming mode experiments. Methodologies to measure head and tape wear were developed and applied to worn specimens. Development of the experimental apparatus with nanometer vertical and submicrosecond temporal resolutions has enabled unprecedented understanding of the interplay of friction, wear and surface topography in a sliding contact. Waviness of ME tape and high roughness of BaFeO tape led to poor high-density recording performance as compared to the excellent performance of the flat MP tape. Interface stability generally improved as the tapes were used and worn smooth by the rotary heads, and head-to-tape spacing was reduced by about 10 nm over 1000 play/rewind cycles for the tapes. ME tape showed the least durability of all of the tapes, and damage areas initiated at high points or bumps on the tape surface were connected by lateral cracks (driven by longitudinal tension) across the tape width at tape failure after about 800 play/rewind cycles. MP tape performance improved gradually through 1000 play/rewind cycles. High asperities on the virgin tapes were severed off the tapes during the record pass, and those that remained on the tape surface increased friction force and tape wear by three-body abrasion early on in play/rewind cycling tests. Lower friction and virtually no wear were observed later in cycling tests when loose wear debris were no longer on the tape surface and the wear mechanism was adhesive. The wear coefficient in the streaming mode was larger than that in the pause mode due to abrasive particles in the contact interface in the streaming mode. MP and BaFeO tapes caused head stains on the metal core and glass surfaces of composite metal-in-gap heads. Anisotropic high tape stiffness led to asymmetrical head wear and head contours with large radii of curvature.

Friction and Wear of Metal Particle, Barium Ferrite and Metal Evaporated Tapes in Rotary Head Recorders

The friction and wear mechanisms of particulate (metal particle and barium ferrite) and metal evaporated (ME) magnetic tapes were investigated. We conducted tests on these tapes in contact with metal-in-gap (MIG) video heads, using a rotary head recorder in the still (pause) mode. We measured signal degradation and friction during the tests. We conducted chemical and surface analyses of the interface components after the tests. We found discernible differences between the tribological behavior ofparticulate tapes, and that of the ME tape. The particulate tapes exhibited a more stable friction and head output than the ME tape. We attributed this to a cleaner contact region, due to effective action of the head cleaning agents (HCAs) found in the particulate tapes. The particulate tapes exhibited wear lifetime longer by an order of magnitude, than that of the ME tape. Mild continuous adhesive wear occurred on particulate tapes followed by catastrophic failure. Tape fatigue possibly led to the catastrophic failure. On the ME tape surface, damage initiated at high points or bumps, which resulted in localized delaminations of the tape coating. This led to a catastrophic removal of the entire magnetic coating over the rubbing track. The major difference between the particulate and ME tapes was that signal dropouts concurrent with increases in friction, which resulted from debris accumulation on the video head, preceded the catastrophic failure in the case of ME tapes. We investigated the running-in process of the video head. We found that the durability of a tape and the initial head output increased, and the initial friction force on a tape decreased, as the head ran-in with the tape. We attributed this result to the tape forming a favorable contour on the head rubbing surface. Deposits on the head surface consisted of binder for the particulate tapes, and lubricant and the magnetic coating for ME tape. Tape materials transferred preferentially to the recessed metal core and the recessed glass of the MIG head.

Environmental effects on the pause mode performance of metal‐evaporated and metal‐particle tapes

A commercial Hi‐8 VCR was instrumented to measure the friction force between the rotary heads and tape and rms head output. Pause mode experiments using metal‐evaporated (ME) and metal‐particle (MP) tapes were performed at design tension under equilibrium conditions inside of an environmental chamber at various temperatures and specific humidities (SH is the ratio of the weights of water vapor to dry air in the mixture). ME tape performed well only at high humidity [on the order of 80% relative humidity (RH)] in the operating temperature range of 15.6–32.2 °C. Under all conditions, lubricant was removed from the ME tape surface, which resulted in increased signal amplitude (due to decreased head medium spacing) and friction during individual experiments. For fixed temperature, increased SH reduced both the lubricant depletion rate and the number and magnitude of spacing loss signal dropouts. The thicker water film maintained on the interface components acted as an additional replenished lubricant, which r...

Effect of Interchanging Tapes and Head Contour on the Durability of Metal Evaporated, Metal Particle and Barium Ferrite Magnetic Tapes

Interchanging metal evaporated in 120-minute cassettes (ME-120) and particulate [metal particle (MP) and barium ferrite (BaFeO)] tapes in a tape recorder caused excessive head and tape wear while a tape formed its preferred head shape in these experiments. The fundamental cause of this phenomenon was that the tapes formed different head shapes during tape recorder operation. To extend the usable lifetime of a rotary head recorder, and to minimize the extent of tape damage, as well as to preserve the integrity of the recorded information, the authors recommend avoiding the practice of interchanging ME-120 and particulate tapes in a tape recorder. The interchanging of ME-180 (180-minute cassette) tape with ME-120 and particulate tapes is not recommended either, but apparently does not lead to excessive head and tape wear. Thin ME-180 tape was inefficient in significantly changing the head contour. This study suggests that the head contour is a determining factor for the performance of a tape in a tape drive. In the authors study of tape compliance using ME tapes, the highly compliant ME-180 tape gave more uniform head wear, lower friction and slightly better durability than the lower compliance ME-120 tape.

Tribological evaluation of the streaming mode performance of metal evaporated and metal particle tapes

A commercial Hi-8 VCR was instrumented to measure the friction force between the rotary heads and tape, rms head output and signal dropouts to sub-/spl mu/s durations. Streaming mode experiments at design tension using metal evaporated (ME) and metal particle (MP) tapes were performed in which the tapes were subjected to repeated play/rewind cycling. Long wavelength waviness of the ME tape surface and a large number of sharper asperities on the MP tape surface affected the magnetic and tribological performance of the tapes. The friction force and head output generally increased, and the dropout frequency (number of dropouts/min) generally decreased as the tapes were worn smooth. Head-to-tape spacing and dropout frequency decreased more for MP tape, as compared to ME tape. On the ME tape surface, damage initiated at high points or bumps, which resulted in weakening of the metal coating at these locations. Cracks initiated at these locations grew (driven by longitudinal tension) laterally across the tape width and connected localized damage areas at tape failure after 840 play/rewind cycles. MP tape performance improved gradually through 1000 play/rewind cycles. Based on this study, ME tape is less durable than MP tape, and reduced waviness of the ME tape surface could further improve ME tape performance.