Tribology of metal evaporated tapes—for improvement of recording density

Increased recording density in video tape recorders and tape drives for data storage has been achieved by the increase in areal recording density and the decrease in tape thickness. Areal recording density can be increased by introducing high performance tapes, like metal evaporated tapes, with superior magnetic characteristics and smooth magnetic surfaces to reduce the spacing loss. However smoother surfaces often produce a higher friction coefficient, which could result in tape damage by the scanning heads and unstable runnability of tapes in VTRs or tape drives. Also thinner tapes show lower mechanical stiffness in general, which could result in damage of the tape edges during tape transportation. Superior durability and runnability are thus required of high performance tape in addition to magnetic characteristics, in spite of the trend towards smoother surface and thinner tapes. Therefore the development of practical new magnetic tapes requires research into their tribology. It was found that the durability and runnability of metal evaporated tapes with smoother surfaces can be improved by DLC coating, and that the edge damage of thinner tapes can be eliminated by decreasing the static friction coefficient, but not the kinetic one. Though the durability and the runnability of metal evaporated tapes themselves have been improved from the tape design point of view, as mentioned above, further improvement may be expected by integrating tape design with that of the VTR/tribo-elements tape drive design and thus further increasing recording density in the future.     

Effect of temperature,humidity and crystal directions on wear of rotary heads

In the advanced tape drive systems which achieve a higher recording density, the wear of rotary heads should be decreased to obtain a higher reproduced output signal by decreasing the ‘Gap depth’ of heads, keeping enough life time. By using the heads made by two kinds of crystal directions of Mn–Zn ferrite and metal evaporated tapes with DLC coating, temperature and humidity dependences of head wear are investigated. The wear rates of both crystal direction heads increase with the decrease in temperature. The wear rate of a (110) head increases very quickly at low temperature, compared with a (100) head. These temperature dependences of head wear are caused by the temperature dependence of the elastic modulus of the tape and the temperature dependence of the friction coefficient between the tape and the head.     

Tribology of helical scan tape drive systems

The future direction of the designs of the metal evaporated tape for higher density recording in the helical scan tape drives which use MR heads was investigated. Static friction force between a tape and a roller guide increases the force with which the tape edge is pressed against the flange of the roller guide and wear debris is produced from the tape edge. The wear debris is driven to the recording area on the tape and increases a rewrite ratio, which indicates the ratio of failure of reading data. A solid lubricant at operating temperature was found to reduce the coefficient of static friction. However, the combination between the solid lubricant and MR heads which are heated by sense current may cause tape damage in dwell tests at low temperature. The increase in a melting temperature of the solid lubricant was found to improve the durability in dwell tests at low temperature.     

Tribology of videotapes

Increased recording density in magnetic recording has been achieved by introducing newly developed magnetic tapes, such as metal particulate tapes and metal-evaporated tapes. As a loss in magnetic recording increases with spacing between a magnetic tape and a magnetic head, these are obliged to contact each other. A tape also rubs tape-transportation elements in videotape recorders (VTRs), such as the stationary drum and guide posts. Thus it is not too much to say that the development of new magnetic tapes for practical application depends on tribological research.

In this paper, tribological analyses for the improvements of durability and runnability of metal particulate tapes and metal-evaporated videotapes are reviewed.     

Recent research of tape/drive tribology

Decreasing track width and tape thickness to increase the volumetric recording density of helical scan-tape-drive systems, which is suitable for higher volumetric recording density, will result in the failure of tracking. The displacement of tape forwarding position, which causes failure of tracking, is caused by static friction coefficient between a tape and a roller guide. It was found that surface roughness and materials of roller guides are very important to reduce static friction coefficient.     

The effect of tape overwrap angle and head radius on head/tape spacing and contact pressure in linear tape recording

Linear tape recording devices typically employ contact recording to minimize head/media magnetic spacing and thus maximize recording density. Successful contact recording is a delicate balance between spacing and contact pressure. If an air bearing develops, spacing will exceed the 100 nm upper limit for current high recording density devices. Conversely, high contact pressure can lead to head wear or tape damage. Most recording heads produced today for linear tape devices consist of one or more cylindrical wafer sections that are bonded together to form a rigid structure. Each individual tape bearing surface is cylindrical with straight edges. The tape is held over the head contact surface by tape guides so that the tape is wrapped sharply around the edges where cylindrical surface and flat surface meet. Overwrap angle is defined as the angle between the tape after it leaves the head and a tangent to the cylindrical head surface at the edge where cylindrical surface and flat surface meet. This paper will use one-dimensional finite element modeling to explore how overwrap angle and bump radius affect head/tape spacing and contact pressure. The effects of tape stiffness, tape tension, and tape velocity will be investigated. Feasible combinations of overwrap angle and bump radius will be presented which minimize head/tape separation, yet keep contact pressure within bounds.     

On the importance of elasticity of materials in the wear of magnetic heads

An analysis of deformations of magnetic tape extended on a magnetic head is presented. The tape deformations are regarded as proportional to contact pressure exerted by the tape, which is of great consequence in the wear of magnetic heads sliding against such tapes. To calculate the distribution of tape deformations, the theory of thin elastic shells and the theorem on minimum potential energy are used. From comparison of these distributions with experimentally obtained surface profiles of worn heads, it is concluded that the wear process of magnetic heads is by nature connected with the elasticity of the materials both of tape and of head.     

Failure Mechanisms of Advanced Metal Evaporated Tape in an Advanced Linear Tape Drive

Demand for increased data storage has resulted in the development of various types of magnetic tape. To achieve higher recording density, tape manufacturers are developing thin-film tapes, such as advanced metal evaporated (AME) tape, for use in linear tape drives. The structure of AME tape is fundamentally different from metal particulate (MP) tape. The goal of this study was to determine the methods and mechanisms associated with failure of AME tapes as well as to investigate the effect of tape cupping and initial edge quality on tape durability in an advanced linear tape drive. It is shown that AME tape exhibits a slightly lower coefficient of friction than the MP. The negatively cupped AME tape demonstrated a greater value of lateral tape motion peak-to-peak amplitude than both of the positively cupped AME samples as well as the MP tape sample. It was found that poor initial tape edge condition plays a large role in debris generation. For these reasons, positively cupped AME tapes with good initial relative edge contour are recommended for use in linear tape drives. The dominant mechanism of failure for the AME samples is adhesive wear resulting in removal of the DLC overcoats and sublayers.

     

Head/Tape Spacing Measurements for Digital Linear Tape Drives Using Three-Color Interferometry

The spacing of the head/tape interface in a digital linear tape (DLT) drive was measured using replica glass heads and three-color interferometry. A modified load-unload method was used to calibrate the light intensity. Class heads with different island widths and zero and nine degree skew angle were used to investigate the effect of head contour on the head/tape spacing. In addition, thin and thick metal particle tapes were evaluated to study the effect of tape compliance, tape speed and tape tension. “Tape tenting” was observed for heads with a narrow center island.     

The measurement of in-situ wear of video heads

A method of measuring in-situ wear of video heads which determines the variation in wear along the surface of the video head is described. Knoop diamond indentations are positioned along the head, either side of the magnetic gap, enabling wear rate to be monitored at several positions. Virgin tape is run over the heads in the form of standard E180 cassettes and the wear occurring at each indentation position is derived from the reduction in length of the main diagonal. Wear rate is plotted against the position of the indentations. This describes the variation in wear along the head and therefore the conformation of the tape to the head. The technique has been used to study the wear of video heads with three types of tape sample: two iron-oxide-based formulations, with either Al₂O₃ or Cr₂O₃ added as head cleaning agents, and one chromium-dioxide-based tape. Little difference was found between the performances of the iron oxide tape samples. The chromium dioxide sample, however, produced wear some five to six times greater than the iron oxide tapes.     

Metal Core Recession and Head Stain Studies of MIG Heads Sliding against Cobalt-Doped Gamma Iron Oxide and Metal Particle Tapes

Metal-in-gap (MIG) heads are commonly used for high-density magnetic recording. Metal core recession and head stains increase the gap between the tape and the head, resulting in signal loss. In this study, accelerated sliding wear tests of Co-γFe₂O₃, and metal particle (MP) tapes against MIC heads made of three different amorphous and, nanocrystalline metals were conducted under various operating conditions. Metal core recess and propensity for head staining were measured. The degree of tape contact with a recessed core was also measured by pressing the tape against a glass slide with Cr grids and using an optical interference technique. The authors found that the core recess was about the same for all three core metals. Core recess by MP tape was larger than that, by oxide tape. The tape speed appeared to have little effect on the recess value. The authors also found that significant recess may occur during initial contouring of the head surface; however, after sliding for about 250 km, core recess reaches a steady-stale value which may be either higher or lower than the initial values. The mechanism of core recession was studied. The authors believe that core recession occurs as a consequence of the debris trapped, between the tape and the core, in addition to that caused by some tape contact. Formation of head stains was observed in all combinations. The stains fanned on the metal cores were heaviest for CoNbZr metal, followed by CoTaZr and FeTaC. Co-γFe₂O₃ tape produced a more severe stain than the MP tape. The apparent roughening of head rubbing surface observed for CoNbZr heads sliding against Co-γFe₂O₃, tape was due to the formation of stains.     

Experimental Study of Lubricant Depletion in Heat Assisted Magnetic Recording over the Lifetime of the Drive

Heat assisted magnetic recording (HAMR) is a promising choice to overcome the superparamagnetic limit in magnetic recording and further increase the areal recoding density of hard disk drive. However, it is expected that HAMR causes lubricant depletion problem on disk surface under the high temperature in the heating assisted writing process. Experimental studies of the lubricant depletion under HAMR conditions are still very limited so far. Lubricant depletion over the lifetime of the drive still remains unaddressed. In this study, a self-developed HAMR tester is introduced. The methods to control the repeatability of laser heating temperature, to determine laser heating temperature, and to adjust laser heating temperature are explained. Laser heating time in the test is correlated with that in the drive. Lubricant depletion is determined quantitatively and the relationship between lubricant depletion depth and laser heating time is established. Then, lubricant depletion depth over the lifetime of the drive is predicated. It is found that almost all lubricant on the disk surface will be depleted over the lifetime of the drive.     

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-minule 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 fiction and slightly better durability than the lower compliance ME-120 tape.     

A study of three-body particle generation and subsequent pole tip recession in linear tape recording heads

The Pole Tip Recession (PTR) and transferred material (stain) are major causes of magnetic spacing losses in magnetic recording system. The recorded signal amplitude is only independent of the data being read if the spacing is zero. Thus, the level and more specifically the variation in head media spacing with device life must be a minimum to maximise signal output and minimised errors. It was the purpose of this research to isolate and identify the mechanisms responsible for pole tip recession using the Linear Tape Open format as an experimental platform, but the results have implications for any head where the tape-bearing surface is Al₂O₃/TiC (AlTiC).All experiments were conducted within a matrix of pressure and humidity, which encompassed the system operating extremes. Atomic force microscopy (AFM) was used to analyse the surface topography of the heads and monitor the development of PTR after 100, 1000 and 5000 passes of tape. Auger electron spectroscopy (AES) was employed to analyse the chemical changes on the surface of the heads after 5000 passes of tape and X-ray Photoelectron Spectroscopy (XPS) was used to identify the chemical changes that occurred at the head surfaces. Optical Microscopy was employed to identify the head surface changes before and after wear. Environment was found to have a significant influence on the head/tape interface. Head wear and PTR were highest at high temperature and humidity.Wear between the head and tape was found to transform the surface layers on the TiC grains in the tape-bearing surface to TiO₂. This process results in the production of TiO₂ fragments that become trapped in the recessed pole tip region, acting as three-body abrasive particles. The presence of Ti on the surface of head thin film region confirmed that the three-body particles originated from the head AlTiC ceramic. The TiO₂ (thickness and possible areal coverage) increased with the water content increase, wear of head increased in the high water content condition.     

Pole tip recession in sandwich heads incorporating a FeTaN soft magnetic track

Pole tip recession (PTR) contributes to a growth in signal loss during the lifetime of sandwich recording heads used in helical scan recording. In this paper results are presented for PTR, measured by AFM, resulting from the cycling of Zr–CaTiO₃/FeTaN sandwich heads against commercial ME and MP tapes in Hi-8 recorders, at 30°C/90% rh and 22°C/80% rh. PTR is seen to be within acceptable levels for Hi-8 recording. Particles adhered to the soft magnetic track, and responsible for head staining, are investigated. An unusual wear pattern on the soft magnetic material is noted.     

The tribology of flexible magnetic recording media—the influence of wear on signal performance

The tribology of magnetic recording systems is different to conventional tribology in many important aspects and an understanding of this branch of the science is still in its infancy. This paper presents a review of one specific aspect of the tribology of flexible tape storage systems, that is wear of heads and media and the effect on signal performance. Based on the work of this author's group and others and with reference to classical tribological principles, the various mechanisms of wear have been identified in particulate media, metal evaporated media and various recording head types and have been discussed in terms of the effect of signal degradation and error or drop out production. From this review, the wear mechanisms and surface and subsurface parameters which should have the most influence on the future development of media and systems are identified.     

Measurement and Origin of Tape Edge Damage in a Linear Tape Drive

Integrity of the magnetic tape edge is the key to maintaining high performance of modern tape drives. Damage to the tape edge under normal drive operation results in the change in tape dimensions and debris generation, both leading to degradation in the reproduction of the recorded signal. The objective of the present study is to develop a methodology for evaluation of tape edge quality and to apply the methodology to monitor tape edge degradation under normal drive operation. Optical microscopy, atomic force microscopy and scanning electron microscopy are employed to study and quantify the quality of the tape edge. AFM measure-ments were made on both individual tape layers and the tape reel. An edge quality measurement technique is used to quantify the damage to tape edge. A technique for the tape lateral motion measurement is used to study the effect of continuous sliding on tape guiding. A lateral force measurement technique is developed to measure the force exerted by the tape edge on the guide flange. The effect of normal drive operation on tape edge quality and on tape guiding in a linear tape drive is studied. It is shown that two edges of a factory-slit tape are imperfect and different, with cracking of the magnetic coating occurring at one edge. Under normal drive operation, one edge experiences more wear with larger amount of debris produced. This larger debris generation occurs on the edge with cracks developed during manufacturing. A possible mechanism of tape edge wear under normal drive operation is proposed.     

Wear of magnetic materials and audio heads sliding against magnetic tapes

The friction and wear of various materials and audio heads sliding against magnetic tapes were studied. Magnetic materials such as Permalloy, Sendust and hot-pressed ferrite (HPF) and some other non-magnetic materials were used as material specimens. Three types of audio heads for cassette tape recorders were used as the head specimens. Their magnetic cores were made from ordinary Permalloy, hard Permalloy and HPF. An experiment using a conical diamond slider was carried out to study the wear resistance of various materials. The specific wear rates of the various material specimens and components of the heads such as the core, shield plate and epoxy resin were measured using the Knoop indentation technique. Wear did not occur uniformly over the surfaces of the Permalloy heads. This irregular wear was examined in detail and its origin is discussed. The specific wear rates of Permalloy and HPF are of the order of 10^?5 mm³ N^?1 m^?1 and 10^?6 mm³ N^?1 m^?1 respectively and that of epoxy resin is of the order of 10^?4 – 10^?5 mm³ N^?1 m^?1. The coefficient of friction of an HPF head is about 0.3, while that of both types of Permalloy head is about 0.7. It is concluded that the mechanism of wear by magnetic tape is not entirely due to the abrasive action of the magnetic powder in the tape but is also partially adhesive in nature.     

Development of a ta-C Wear Resistant Coating with Composite Interlayer for Recording Heads of Magnetic Tape Drives

The effect of two different surface modification methods on the wear life of the coating of magnetic tape drive heads has been studied. In this research, the heads were coated with 10 nm tetrahedral amorphous carbon (ta-C) film using filtered cathodic vacuum arc (FCVA) technique. The surface of the heads was pretreated by bombarding with energetic carbon ions or by developing a Si–Al–C composite interlayer before deposition of the coating. The coated heads were tested at a real head/tape interface of a tape drive. Surface characterization and tribological behavior of the head coatings with and without surface modification has been studied by transmission electron microscopy (TEM), Auger electron spectroscopy (AES), and scanning electron microscopy (SEM). The results reveal that the ta-C coating without any surface modification is not durable and the coating fails due to delamination. Pre-treating the head surface with energetic carbon ions improves the durability of the coating, especially on the head read/write elements; however, the coating of the head ceramic substrate is still partially delaminated. The application of a Si–Al–C composite interlayer is shown to be able to solve the delamination problem effectively and increase the wear life of the coating up to six times in comparison with the sample pretreated with carbon ions. The formation of strong chemical bonds between the head surface and the overcoat is found to be an important factor in improving the durability of the ta-C head coating.     

Microtribological studies of two-phase Al2O3–TiC ceramic at low contact pressure

The two-phase Al₂O₃–TiC ceramic (AlTiC) has many applications. One of the most common uses of AlTiC is for data recording heads where it is used as a bearing surface to support the magnetic sensing elements. This is one of the examples where the ceramic can be used in MEMS. Using Linear Tape Open (LTO) drive and metal particle (MP) tape media as the experimental platform; the wear of the AlTiC at very low loads and for very smooth surfaces has been studied.X-ray photoelectron spectroscopy (XPS), Auger electron Spectroscopy (AES) and Atomic Force Microscopy (AFM) were employed to analyse the AlTiC surface changes during wear at a variety of environmental conditions. Under all experimental conditions, the results showed the TiC phase of AlTiC to have been oxidized to form a surface layer. This gave rise to classical oxidational wear of that phase; with the delamination of the TiO₂ to form pullouts on the AlTiC surface and subsequent three-body abrasive wear particles were produced. The rate of oxidation of the TiC and hence the rate of production of the three-body wear particles increases with atmospheric water vapour content. In the experimental system chosen for this investigation, this results in an increase in differential wear, and hence pole tip recession of the magnetic metal poles of the recording heads. Pole tip recession was shown to correlate with increase in oxidation rate and also increase with atmospheric water vapour content.The wear of the Al₂O₃ phase was probably due to micro-adhesive wear with a wear rate much lower than that of the TiC phase.