Abstract:
A wear gauge is provided for simulating the wear of a magnetic recording component at its interface with a magnetic storage medium comprising a wear gauge block having a wear surface that substantially approximates the geometrical dimensions and contour of the working surface of the magnetic recording component, wherein the wear surface has a coating of transparent material. A test method comprises mounting the wear gauge in a test fixture, positioning a tape and adjusting the overwrap angle at edges of the wear surface, choosing the desired tape tension, speed and runtime, and running the tape, preferably unidirectionally, in operational contact with the wear surface for the desired runtime. The wear surface is inspected under white light illumination to observe and record locations of interferometric color changes on the wear surface. The color changes are correlated to thickness changes of the transparent coating on the wear surface.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     This invention relates to wear gauges, and more particularly, to a wear gauge for measuring tape head wear in a magnetic tape recording system and to a method of measuring tape head contact pressure distribution and wear.  
         [0003]     2. Description of the Related Art  
         [0004]     In magnetic recording systems, the mechanical interaction of the magnetic head and the recording medium is an important factor determining the performance and reliability of the system. Ideally, the head is in contact or near contact with the moving recording medium to optimize read/write processes. The contact at the head/medium interface results in wear which is a major concern affecting performance and lifetime of both head and medium.  
         [0005]     In magnetic tape and disk recording systems, considerable effort has been made to determine the wear resistance of magnetic head materials in contact with the recording media. As recording density increases, knowledge of the head wear characteristics becomes of increased importance, since even very small dimensional changes at the surface of the head can contribute to performance degradation of the recording system.  
         [0006]     A common procedure for determining head wear is to run the head for an extended period in operative contact with the moving magnetic medium. For example, U.S. Pat. No. 4,091,654 discloses a method for testing abrasion of a recording head. According to this patent, the smoothness of a simulated head made of polished steel or other material is measured before and after a given amount of contact with a recording surface using a fiber optic emitter/detector to measure light reflected from the surface of the head. However, this type of test does not measure the wear rate or identify specific wear regions on the head.  
         [0007]     U.S. Pat. No. 3,753,093 discloses a device for determining the wear rate of a simulated recording head in contact with a moving magnetic tape. The surface of the simulated head contacting the tape has deposited thereon strips of magnetic alloy similar to the alloy of an actual recording head. During the tape transport operation, the increase of the electrical resistance of magnetic alloy strip is monitored to measure the rate of wear of the magnetic alloy strip. This type of test requires a complex procedure for depositing the magnetic alloy strips and electrical contacts and does not provide a good measure of the microscopic wear profile.  
         [0008]     There is an ongoing need for a wear gauge and method of use providing a simple and inexpensive tool for evaluating head wear and head/media compliance in magnetic recording systems. The present invention provides an improved wear gauge and method of use addressing this need.  
       SUMMARY OF THE INVENTION  
       [0009]     In accordance with the principles of the present invention, there is disclosed a wear gauge for simulating the wear of a magnetic recording head at its interface with a magnetic storage medium comprising a wear gauge block having a wear surface that substantially approximates the geometrical dimensions and contour of the working surface of the magnetic recording head, wherein the wear surface has a coating of transparent material having appropriate wear characteristics.  
         [0010]     A method of testing the wear of a wear gauge simulating a magnetic recording head at the interface of the recording head with a magnetic recording medium is disclosed. The method of testing comprises mounting a wear gauge having a wear surface in a test fixture, positioning a tape and adjusting the wrap or overwrap angle at edges of the wear surface, choosing the desired tape tension, speed and runtime, running the tape preferably unidirectionally in operational contact with the wear surface for the desired runtime, inspecting the wear surface under white light illumination, recording the observed locations of interferometric color changes on the wear surface, converting color changes to thickness changes of a transparent coating on the wear surface, and running profilometer scans across the wear surface if desired.  
         [0011]     The above as well as additional objects, features, and advantages of the present invention will become apparent in the following detailed description. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]     For a fuller understanding of the nature and advantages of the present invention, as well as the preferred mode of use, reference should be made to the following detailed description read in conjunction with the accompanying drawings. In the following drawings, like reference numerals designate like or similar parts throughout the drawings:  
         [0013]      FIG. 1  is an end view, not to scale, of a bi-directional flat contour read/write tape recording head simulated by an embodiment of the wear gauge of the present invention;  
         [0014]      FIG. 2  is an end view, not to scale, depicting the separation of the tape from the transducing surface of the flat contour recording head of  FIG. 1 ;  
         [0015]      FIG. 3   a  is a perspective view, not to scale, of an embodiment of the wear gauge of the present invention for use with a magnetic tape recording system;  
         [0016]      FIG. 3   b  is a end view, not to scale, of the wear gauge of  FIG. 3   a  showing a magnetic tape interfacing with the gauge;  
         [0017]      FIG. 4  is a schematic chart showing a method of wear testing employing the wear gauge of the present invention;  
         [0018]      FIG. 5  is a view of the top surface of the wear gauge of the present invention viewed with white light after interaction with a magnetic tape;  
         [0019]      FIG. 6  is a profilometer scan of the top surface of the wear gauge of  FIG. 5  after interaction with a magnetic tape;  
         [0020]      FIG. 7   a - c  are perspective views, not to scale, of the wear gauge of the present invention at various stages of fabrication by a first method; and  
         [0021]      FIG. 8   a - d  are top and side views, not to scale, of a wafer during steps in fabrication of the wear gauges of the present invention by a second method. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0022]      FIG. 1  illustrates a prior art bi-directional read-while-write flat contour tape recording head  100 . Rowbar substrates  102  and  104  of a wear resistant material, such as the substrate ceramic typically used in magnetic disk drive heads, are mounted in carriers  105  and  106  fixed at a small angle α with respect to each other. The ceramic rowbar substrates  102  and  104  are provided with flat transducing surfaces  108  and  110  and a row of transducers at the surfaces of gaps  112  and  114 . Electrical connection cables  116  and  118  connect the transducers to the read/write channel of the associated tape drive. To control the overwrap angle θ of the tape  120  at edges  122  and  124 , outriggers  126  and  128  lapped at the desired wrap angle are provided. The wrap angle going onto the flat transducing surface is usually between ⅛ degree and 4.5 degrees. The rows of transducers are protected by closures  130  and  132  made of the same or similar ceramic as the rowbar substrates  102  and  104 .  
         [0023]      FIG. 2  illustrates flat transducing surface  108  of the flat contour head  100  of  FIG. 1 . As the tape  120  moves from left-to-right or from right-to-left over the flat transducing surfaces  108 , the tape separation from the transducing surface is different in different zones across the surface. At the edges  122  and  222 , the overwrap angle results in bending of the tape to conform to the flat transducing surfaces in a narrow “compression zone”  204  where the tape is in contact with the edges  122  and  222 , the compression zone having a effective length of about 0.1-10 microns, increasing to 15-45 microns over the life of the head. By scraping (skiving) the air from the surface of the moving tape, a vacuum forms between the tape and the flat transducing surface holding the tape in contact with the transducing surface; however, bending of the recording tape due to the overwrap results in separation of the tape from the transducing surface in a “canopy zone”  206  for a distance that depends on the wrap angle, the tape thickness and the tape tension and speed. For typical values of tape tension and tape thickness and wrap angles in the range of ½-2 degrees, the canopy zone distance is in the range of 20-200 microns. In a “tack-down zone”  208 , the vacuum between the tape and the transducing surface is sufficient to overcome this separation and the tape  120  is in contact or near contact with the flat transducing surface  108 . In the flat contour head  100 , the transducers are positioned at the gaps  112  and  114  in the tack-down zones  208  where the tape is in contact or near contact.  
         [0024]      FIG. 3   a  is a perspective view of an embodiment of a wear gauge  300  suitable for simulating and measuring the tape-head contact pressure distribution and head surface wear of a flat contour tape head  100  of the type shown in  FIGS. 1 and 2 . As a result of the variations in tape separation from the transducing surface  108  in different zones the contact pressure distribution and resulting wear of the transducing surface  108  varies across the surface. Use of the wear gauge  300  provides a simple means to measure this non-uniform wear and to derive the contact pressure distribution in this complex wear situation.  
         [0025]     Wear gauge  300  is a dummy flat profile head comprising a wear gauge block  301  having a wear surface  302  and air skiving edges  304  and  306 . The wear gauge is made of a hard wear resistant material, preferably the ceramic AlTiC used as a substrate for magnetic recording heads. The wear surface  302  of the dummy head has substantially the same dimensions and edge geometry as the transducing surface (acting surface or working surface)  108  of the tape head  100 . The surface  302  has a thin coating  308  of transparent material, preferably SiO 2  having a thickness in the range of 50-250 nm. Alternatively, other transparent coatings, including, but not limited to, Al 2 O 3 , amorphous carbon and sapphire may be used. The transparent coating material is preferably chosen to wear more rapidly than the working surface. Thickness changes of the coating  308  due to tape induced wear are measured by observation of optical interference of light reflected from the front and back surfaces of the coating or by surface profilometry measurements across selected regions of the surface.  
         [0026]      FIG. 3   b  illustrates a test fixture  320  showing an end view of the wear gauge  300  mounted in a carrier  322  with a magnetic tape  324  interfacing with the gauge. The tape is shown moving from left to right (indicated by the arrow  326 ) across the coated wear surface  302 . Movement of the tape from right to left may also be used as may bidirectional movement. The overwrap angle θ is controlled to a precisely fixed value in the range of ⅛ to 4.5 degrees, and preferably 1 degree, by means (not shown) well known to the art.  
         [0027]     Although, the wear gauge described in this embodiment is designed to simulate the wear of a flat contour tape head, a wear gauge according to the principles of this invention can be designed having a contoured wear surface to simulate a contoured tape head. A wear gauge to simulate a tape head having a cylindrical contoured transducing surface is same as wear gauge  300  except that instead of wear surface  302  being flat, the wear surface has a cylindrical contour having curvature in the direction of the tape motion across the head. As with the case for a flat wear surface, the contoured wear surface has a thin coating  308  of transparent material. Alternatively, wear gauges according to the principles of the invention can also be made to simulate wear of other components that interact mechanically with a moving tape, including, but not limited to rollers and guides. Suitable wear gauges for simulating wear of other components may be made by providing a thin coating of transparent material on the working surfaces of the components that interface with the moving tape.  
         [0028]     A test method  400  according to an embodiment of the invention is illustrated schematically in  FIG. 4 . The test comprises mounting the wear gauge  300  in the test fixture  320  (step  410 ), positioning the tape  324  with respect to the wear surface  302  so that the overwrap angle θ at edges  304  and  306  is precisely 1 degree (step  420 ), setting the desired tape tension, speed and runtime (step  430 ), and running the tape in one direction at a desired tension and speed for a fixed time period (step  440 ), preferably about 10 hours or more. Unidirectional motion of the tape for runtimes of several hours duration may be achieved by several methods. For tests using a tape cassette of limited tape length, the tape is run to the desired stopping location, is stopped and the head is retracted and the is tape rewound and stopped again. The head is reengaged with the tape and the tape started. This procedure is repeated until sufficient unidirectional runtime is accumulated. Alternatively, instead of retracting the head to disengage it from the tape for rewinds, the tape may be lifted away from the head using movable rollers or by air pressure. Another alternative test method is to use an endless loop of tape to run the wear test. Disadvantages of this method is the need to prepare suitable loops of each tape type to be wear tested and the abrasivity of the tape decreases rapidly during the first few hundred cycles.  
         [0029]     At the end of the test run period the contact footprint and wear rates are revealed by changes in the thickness of the coating  308  of SiO 2  on wear surface  302 . The wear surface  302  is inspected at relatively low magnification under white light illumination incident on the coating  308  (step  450 ). Thickness variations of the coating are observed and located as color changes due to interference effects of light reflected from the top surface of the coating and light transmitted through the coating and reflected from the wear surface  302  (step  460 ). Illumination with light at near normal incidence to the plane of the wear surface is preferred since the optical path of the light through the coating is then simply twice the coating thickness. For normal incidence, the retardation of the light reflected from the back surface of a coating having thickness t is 2 nt where n is the index of refraction of the coating.  
         [0030]     Since the index of refraction of the coating  308  is known or can be measured, the color changes can be readily converted to thickness changes of the coating using considerations of thin film interference theory well known to the art (step  470 ). Both constructive interference and destructive interference effects on various spectral components of the incident white light must be considered in making the conversion from observed color to coating thickness. For a coating having an index of refraction less than the index of the substrate material, the condition for destructive interference is that the retardation 2 nt=λ/2, where λ is the wavelength in air of a spectral component of the light. For constructive interference, the condition is that 2 nt=λ. A chart showing observed color of the reflected light as a function of coating thickness may be generated for a particular coating, such as for example SiO 2 , and the illumination conditions used to inspect the wear gauge. Measurements of the position on the wear surface at which interference colors of the reflected light occur provides a mapping of the variations of coating thickness due to wear. Alternatively, inspection with monochromatic illumination may be used resulting in observation of regions of constructive and destructive interference due to coating thickness changes, however the sensitivity to changes in thickness is reduced due to use of a single wavelength of incident light. Surface profilometry using commercial profilometers as is known to the art is carried out across selected regions of the wear surface  302  to provide quantitative confirmation of the interferometrically derived wear of the coating  308  (step  480 ). Alternatively, optical profilometry measurements of the coating on the wear surface may be done by first coating the surface with a thin gold film (gold flashing) as is known to the art.  
         [0031]      FIG. 5  is a black and white rendering of a view in color of the wear surface of a wear gauge under white light illumination depicting thickness changes of the coating  308  after a wear test using a magnetic tape medium. The wear gauge used for this test was a rowbar of a magnetic recording tape head with the transducing surface coated with a layer of SiO 2  having a thickness of approximately 75 nm. The direction of tape motion, represented by arrow  501 , was from left to right. The narrow bright bands  502  show the wear at the edges  304  and  306  of the wear surface of the wear gauge where the tape overwrap at the surface occurs. The light band  503  was due to the alumina forming the gap in the transducing surface of the ceramic rowbar substrate ( FIG. 1 ). Regions  512  and  514  that correspond to the low contact pressure in the canopy zones  206  discussed above with reference to  FIG. 2  appear blue when viewed in color. The low contact pressure in these regions is due to separation of the tape from the transducing surface in a canopy zone region resulting from tape stiffness resisting bending forced by the tape overwrap. A broad region  516  that corresponds the higher contact pressure in the tack-down zone appears magenta when observed in color. In the transition regions between low contact pressure regions  512  and  514  and the higher contact pressure region  516  gradual changes in the coating thickness can be seen as subtle changes of color under white light inspection. Each change of color corresponds to a change of thickness of the coating allowing for a quantification of the wear by visual inspection.  
         [0032]      FIG. 6  shows an exemplary profilometer trace measured across the wear surface of the wear gauge shown in  FIG. 5 . The direction of tape motion, represented by arrow  601 , was from left to right. In  FIG. 6 , severe wear  610  at the left and right edges of the profile trace correspond to regions of high contact pressure where the tape overwrap occurs. The humps  612  and  614  in the profile trace corresponds low contact pressure in the canopy zone  206  discussed above with reference to  FIG. 2  where separation of the tape from the transducing surface in a canopy zone region results from tape stiffness resisting bending forced by the tape overwrap. The gradual thickness decrease (approximately 10 nm) of the SiO 2  coating measured in going from the top of the hump  612  toward the broad central region  616  corresponding to the tack-down zone  208  (see  FIG. 2 ) may be observed as gradual color changes due to the interference effects discussed herein above during inspection of the wear gauge under white light illumination.  
         [0033]     The profilometry trace of  FIG. 6  can be used together with the view in color of the wear surface under white light illumination shown in  FIG. 5  to correlate the observed colors of the worn coating with the measured thickness of the coating at the same positions on the wear surface. A calibration graph or chart can be generated of observed color versus coating thickness for the particular coating material being used on the wear surface. If a constant well-controlled coating thickness is used in fabricating the wear gauges, a reference chart of coating color versus amount of wear can be generated. Reference to the latter calibration chart allows quantitative wear of the coatings of future wear gauge tests to be directly determined from the observed colors.  
         [0034]     Since the hardness of the SiO 2  coating is less than that of the wear surface of a typical magnetic head, use of the wear gauge of the present invention provides an accelerated wear test. Wear tests having a duration of only a few hours simulate the effects of much longer running times of an actual head in a tape recorder system. This accelerated test is often very desirable when running comparative tests of head wear with different tapes and different running conditions such as tape speeds and tape tension.  
         [0035]     When running accelerated tests, it is useful to have a method to calibrate the accelerated tests with the actual wear of a head under the same operating conditions. With reference to  FIG. 3   b , this calibration may be obtained by running the tape over the coated wear block  301  for a time t 1  when the coating  302  at edge  304  is just worn through and the tape contacts the wear block material. This point may be detected by grounding the substrate through an ammeter. When the tape breaks through the insulating coating, a small triboelectric current flows. A second test is run on an uncoated wear block and the time t 2  for an equal amount of wear to occur at uncoated edge  304  is measured. The ratio t 2 /t 1  of the edge wear times of the uncoated and coated wear blocks gives the acceleration factor which applies to all the wear across the coated wear surface.  
         [0036]     A first method of fabricating the wear gauge  300  may be understood with reference to  FIGS. 7   a - c . A ceramic (AlTiC) wafer is cut to form a rectilinear bar (quad section)  702  having a width  704  generally greater than the desired length of the wear gauge in the direction perpendicular to the tape motion. Narrow slots  705  are cut perpendicular to the end surface  706  of the quad section  702 . The end surface  706  of the quad section is lapped and polished to form a flat wear surface  708  having a roughness preferably in the range 1-4 nm. A row slice  709  parallel to the plane of the wear surface  708  separates a wear gauge  710  block from the quad section  702 . A grinding operation removes unwanted material  711  and forms the air-skiving edges  712  of the wear gauge. The bar is then cut to the desired length. A coating  714  of transparent material is formed on the wear surface  708  by a vacuum deposition process, such as sputter deposition, to complete the wear gauge block.  
         [0037]     A second method of fabricating the wear gauge  300  may be understood with reference to  FIGS. 8   a - d . Starting with a blank wafer  802  of AlTiC, slots  804  are cut to create plateaus  806  having a width preferably in the range of 500-1000 microns and spaced 1-3 mm apart. The top surface  808  of the wafer is then polished to provide flat wear surfaces  810  on the plateaus  806  having a roughness of about 2 nm. The wafer is then sliced along the centers  809  of the slots  804  to create bars having the cross-section shown in  FIG. 8   d . The bars are then sliced into wear blocks  811  having the desired length for use as wear gauges. A coating  812  of transparent material is then vacuum deposited on the wear surfaces  810  to complete the fabrication process. This fabrication method is a low cost, batch process well-suited to mass production of standard wear gauges for the tape industry.  
         [0038]     While the present invention has been particularly shown and described with reference to the preferred embodiments, it will be understood by those skilled in the art that various changes in form and detail may be made without departing from the spirit, scope and teaching of the invention. Accordingly, the disclosed invention is to be considered merely as illustrative and limited only as specified in the appended claims.