Tribology and Mechanics of Magnetic Storage Devices

Transcription

1 Tribology and Mechanics of Magnetic Storage Devices

2 Bharat Bhushan Tribology and Mechanics of Magnetic Storage Devices With 577 Figures in 868 Parts Springer-Verlag New York Berlin Heidelberg London Paris Tokyo Hong Kong

3 Bharat Bhushan, Ph.D., M.B.A., P.E. IBM Research Division Almaden Research Center San Jose, California, U.S.A. Library of Congress Cataloging-in-Publications Data Bhushan. Bhara! Tribology and mechanics of magnetic storage devices / Bharat Bhushan. p. cm. Includes bibliographical references. I. Magnetic recorders and recording. 2. Tribology. 3. Magnetic disks. 4. Magnetic tapes. 1. Title. Tk B dc Printed on acid-free paper 1990 Springer-Verlag New York Inc. Softcover reprint of the hardcover 1 st edition 1990 All rights reserved. This work may not be translated or copied in whole or in part without the written permission of the publisher (Springer-Verlag New York, Inc., 175 Fifth Avenue, New York, NY 10010, USA), except for brief excerpts in connection with reviews or scholarly analysis. Use in connection with any form of information and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed is forbidden. The use of general descriptive names, trade names, trademarks, etc., in this publication, even if the former are not especially identified, is not to be taken as a sign that such names, as understood by the Trade Marks and Merchandise Marks Act, may accordingly be used freely by anyone. Typeset by Asco Trade Typesetting, Hong Kong I ISBN-13: DOl: / e-isbn-13:

4 Dedicated to my parents Narain Dass and Devi Vati; my parents-in-law Jagdish Saran and Shakuntla Devi Agarwal; my wife Sudha; my son Ankur; and my daughter Noopur

5 Preface Magnetic recording is presently a $50 billion industry. It spans audio, video, and digital applications in the form of tapes and disks. The industry is expected to grow by a factor of five or more in the next decade. This growth will be accompanied by dramatic improvements in the technology, and the potential exists for magnetic-recording densities to improve by at least one order of magnitude! Magnetic-recording process is accomplished by relative motion between a magnetic head and a magnetic medium. Types of magnetic media for digital recording are: flexible media (tapes and floppy disks) and rigid disks. Physical contact between head and medium occurs during starts and stops and hydrodynamic air film develops at high speeds. Hying heights (mean separation between head and medium) are on the order of 0.1 micrometer comparable to surface roughness of the mating members. Need for higher and higher recording densities requires that surfaces be as smooth as possible and flying heights be as low as possible. Smoother surfaces lead to increased static/ kinetic friction and wear. In the case of magnetic tapes, in order to have high bit capacity for a given size of a spool, we like to use as thin a tape substrate as possible. Thinner tapes are prone to local or bulk viscoelastic deformation during storage. This may lead to variations in head-tape separations resulting in problems in data reliability. Anisotropic mechanical properties and viscoelastic deformations of the floppy disks may result in data-reliability problems. In the case of heads used in conjunction with rigid disks, dynamics of a head are very critical in maintaining head-disk separation. All magnetic media have to be lubricated sufficiently to minimize head and magnetic-medium wear. The lubrication is carried out either topically or in bulk. A fundamental understanding of tribology and mechanics of head-magnetic medium interface is, therefore, very crucial for the future of the fast growing magnetic-recording industry. This book is a systematic compilation of current knowledge of tribology applied to magnetic storage devices and is the first of its kind. Tribology has been very broadly defined here. It covers surface roughness, friction, interface temperatures, wear, lubrication, lubricants, and surface finishing (machining). Long-term reliability problems of the magnetic tapes encountered during handling and storage are discussed in a separate book by this author to be published by Springer-Verlag approximately in The organization of the book is straightforward. Chapter 1 reviews the evolu-

6 viii Preface tion of the tribology, its economic significance, and brief descriptions of physics of magnetic recording and various magnetic storage systems. Chapter 2 discusses the statistical theory of surface roughness, methods of characterization of solid surfaces, and physico-chemical characteristics of surface layers. Chapter 3 is devoted to the elastic and plastic real area of contacts that occur when two solid surfaces are placed in contact. Both analyses and experimental data are presented. Chapter 4 introduces phenomena of friction encountered when the two surfaces in contact slide against each other and discusses mechanisms of friction and physical and chemical properties that control friction in magnetic storage devices. Chapter 5 is devoted to the interface temperature generated from the frictional energy input. The impact of temperature rise on the performance of the head-medium interface is discussed. In Chapter 6, different wear modes are first discussed, then wear mechanisms relevant for magnetic storage devices are identified and representative results are presented. Chapter 7 presents different types of wear tests needed in the development of both heads and media. Methods of accelerating wear during screening tests are presented. Chapter 8 reviews the different regimes of lubrication, different types of solid and liquid lubricants, and mechanisms of lubrication for magnetic media. In Chapter 9, the theory of hydrodynamic/elastohydrodynarnic air lubrication for a slip-flow regime and for rough surfaces relevant to modem magnetic media is presented. Techniques for measurement of the air film thickness in magnetic media and some results are also presented. Chapter 10 discusses the technology of surface finishing of ceramics, commonly used for construction of heads. I have tried, wherever possible, to discuss theories and types of experimental measurements that can be used in corroborating theories and in developing our understanding. Emphasis has been on the fundamental understanding of the subject matter before proceeding to a diversity of practical applications. I have presented ample experimental data, relevant properties of materials and surfaces, to make this book useful to engineers and scientists working in the industry. The book is intended for three types of readers: graduate students of tribology and mechanics, research workers who are active or intend to become active in this field, and practicing engineers who have encountered a tribology or reliability problem and hope to solve it as expeditiously as possible. Most of the theories of tribology presented in this book are very general and are applicable to other than magnetic-storage systems; for example, theories of friction presented in this book are applicable to any interface. This book should serve as an excellent text for a graduate course in tribology. I wish to thank my wife Sudha, my son Ankur, and my daughter Noopur, who have been very forbearing during the years when I spent long days and nights in conducting the research and preparing this book. They provided the lubrication necessary to minimize friction and wear. I would also like to thank IBM Corporation for providing typing support. Los Gatos, California BHARAT BHUSHAN

7 Contents Preface... vii 1. Introduction... I. I Definition and History of Tribology... I. I. I Definition.... I. I.2 History... I 1.2 Industrial Significance of Tribology Physics of Magnetic Recording Basic Principle Vertical Recording Methods of Encoding Binary Information I.3.4 Design Considerations Recording Density Reproduced Signal Amplitude... II Signal-to-Noise Ratio Magnetic Data-Storage Systems History of Magnetic Recording Storage Hierarchy Examples of Modem Data-Storage Systems Tape Drives Floppy Disk (Diskette) Drives Rigid Disk Drives Head Materials Permalloys Mu-Metal and Hy-Mu 800B Sendust Alloys Alfenol Alloys Amorphous Magnetic Alloys Ferrites Some Examples of Head Constructions Media Materials Flexible Media Rigid Disks Functional Requirements Manufacturing Processes of Magnetic Media Particulate Tapes... 51

8 x Contents Particulate Floppy Disks Rigid Disks References Solid Surface Characterization The Nature of Surfaces Statistical Analysis of Surface Roughness Average Roughness Parameters Probability Distribution and Density Functions Surface Height Distribution Function Texture Descriptors Distribution and Statistics of the Asperities and Valleys Practical Considerations in Measurement of Roughness Parameters Long- and Short-Wavelength Filtering Measuring Length Measurement of Surface Roughness Mechanical Stylus Method Surface Mapping Measurement of Circular Surfaces Relocation Replication Sources of Errors Optical Methods Taper-Sectioning Method Light-Sectioning Method Specular Reflection Methods Diffuse Reflection (Scattering) Methods Speckle Pattern Optical Interference Methods Digital Optical Profiler Fluid Methods Electrical Methods I Electron Microscopy Methods Reflection Electron Microscopy Integration of Backscattered Signal Stereomicroscopy Scanning Tunneling Microscopy Atomic Force Microscopy Comparison of Measurement Methods Measurement of Isolated Asperities Optical Methods Glide Test Methods Physico-Chemical Characteristics of Surface Layers Deformed Layer Bielby Layer Chemically-Reacted Layer Physisorbed Layer Chemisorbed Layer Surface Tension, Surface Energy, and Wetting

9 Contents xi Methods of Surface Characterization 151 References Contact between Solid Surfaces Physical Properties of Polymers Physical States of Polymers Complex Modulus and Compliance Creep and Relaxation Behavior Temperature and Frequency Effects Apparent and Real Area of Contact Analysis of the Real Area of Contact Elastic Contact Limit of Elastic Deformation Optimization of Mechanical Properties and Surface Roughness Parameters of Magnetic Media Calculations of the Real Areas of Contact of Typical Particulate Magnetic Tapes Experimental Evidence of Elastic Contacts in Magnetic Tapes Changes in Contact Area Because of Tape-Surface Wear Calculations of the Real Area of Contact of Typical Magnetic Rigid Disks Optimization of Relative Young's Moduli of Thin-Film Composite Structure Measurement of the Real Area of Contact Review of Measurement Techniques Electrical-Contact Resistance Optical Techniques Ultrasonic Technique Neutrographic Technique Paints and Radioactive Traces Comparison of Different Measurement Techniques Calculation for Overestimation of the Contact Area by Various Optical Techniques Feasibility of Phase-Contrast Microscopy Selection of Optimum Measurement Technique Measurement of Typical Magnetic Tapes Test Apparatus and Procedure Results and Discussion References Friction Introduction Need for Controlled Friction Friction Theories The Deformation (Hysteresis) Friction The Adhesional Friction Real Area of Contact Adhesion Strength of Contacts Experimental Evidence of Tabor's Classical Theory of Adhesion

10 xii Contents Chemical Effects in Adhesion Surface Free Energy Theory of Adhesion Grain Boundary Effects on Adhesion Macroscopic Theory of Adhesion for Polymers in the Rubbery State Sources of Stiction MeniscusIYiscous Effects Microcapillary-Evacuation Effect Changes in Surface Chemistry Summary Role of Physical Properties in Magnetic Tapes Description of Reciprocating Friction Test Apparatus and Test Procedure Data Analysis Description of Portable Reciprocating Friction Test Apparatus and Test Procedure Effect of Temperature Tape-Drive Tests Effect of Curing Effect of Particle Loading Effect of Magnetic-Particle Distribution Effect of Surface Roughness Effect of Humidity Effect of Sliding Velocity Friction Reduction by High-Frequency Oscillations Summary Role of Interlayer Pressure in Magnetic Tapes Measurement of Creep Compliance and its Relationship to Friction Description of Creep Apparatus Test Procedure Results and Discussions Relationship between Creep Compliance and Friction I Friction of Tapes with and without Rough Backside Effect of Tape Relaxation Summary Role of Chemical Properties in Magnetic Tapes Polyurethane Chemistry Thermomechanical Performance of Coatings Chemical Changes of Coatings Experimental Procedure Results and Discussions Effect of Binder Hydrolysis on the Drive Performance Measurement of Stiction Summary Role of Physical and Chemical Properties in Rigid Disks I Description of Various Techniques to Measure Friction/Stiction in Disk Drives E ~ f of e cdisk t and Head Slider Material Properties Effect of Surface Roughness of Disk and Head Slider

11 Contents xiii Effect of Contact Start-Stops Effect of Liquid Lubricant Film Effect of Organic Adsorbants Effect of Humidity and Temperature Effect of Rest Period Effect of Head Slider Area Effect of Head Slider Load Effect of Drive Acceleration/Velocity Methods to Reduce Friction/Stiction Summary References Interface Temperature of Sliding Surfaces Introduction Thermal Analysis... '" Sliding of Equally Rough Surfaces (Low Stress) Independent (Flash) Temperature Rise (e r) of an Asperity Contact Steady-State Interaction Temperature Rise (e,) Partition of Heat Average Transient Temperature of an Interface Sliding of a Rough Surface on a Smooth Surface (Low Stress) Steady-State, Independent (Flash) Asperity-Contact Temperature Rise (e r) Steady-State Interaction Temperature Rise (e i) Partition of Heat Average Transient Temperature of an Interface Transient Conditions Temperature Variation Perpendicular to the Sliding Surface Summary Application of Analysis to Particulate Magnetic Media Measurement of Thermophysical Properties of Heads and Medium Materials Magnetic-Head Materials Magnetic Medium Materials Analytical Predictions for Particulate Magnetic Tapes Rough-Rough Surface Condition Rough-Smooth Surface Condition Magnetic Particle (with no Coating of Polymeric Binder) in Contact with the Head Surface Transient Conditions Temperature Variation Perpendicular to the Sliding Surface Summary Temperature Measurement of Particulate Magnetic Tapes Tape Transport System Infrared Measurement System Operating Equations for the Infrared Microscope Spot Size, Detector Response Time, and Temperature Error Interval of the Infrared Microscope AGA Thermovision

12 XIV Contents Determination of the Tape Radiative Properties Friction Force Measurements Temperature Measurements Correlation of Predicted and Measured Asperity-Contact Temperatures Summary References Wear Mechanisms Types of Wear Mechanics Adhesive Wear Abrasive Wear Fatigue Wear Rolling Contact Fatigue Rolling/Sliding Contact Fatigue Sliding Contact Fatigue Static Fatigue Impact Wear Erosion Percussion Corrosive Wear Electrical-Are-Induced Wear Fretting and Fretting Corrosion Head Wear Head Body Wear Wear Mechanisms of Tape Heads Rigid Disk Head Sliders Magnetically Dead Layer in Ferrites Friction Polymers Thin-Film Read-Write Gap Wear Effect of Head/Tape Properties and Operating Parameters on Tape-Head Wear Head Material Hardness Head Material Grain Size Magnetic Particles Tape-Surface Roughness Isolated Asperities on Tape Surface Multiple Passes Tape Tension Tape Stiffness Sliding Speed Humidity Medium Wear Tape Wear Effect of Number of File Passes Effect of PVC Effect of Environment Summary Rigid Disk Wear Particulate Disks Thin-Film Disks

13 Contents xv 6.4 Head-Medium Wear Studies using Autoradiographic Techniques Head-Tape Interface Details of the Tape Drive and Neutron Activation of Ferrite Heads Preparation of Radioactive-Ferrite Standards Exposures of Tapes on X-Ray Films Optical Densitometer Measurements of X-Ray Films Measurement of the Average Mass of Radioactive Material on Tape and Drive-Component Surfaces Results and Discussions Head-Disk Interface Head-Tape Wear Studies In-Situ SEM Description of Miniwear Test Apparatus Results and Discussions Summary Tape-Path Component Wear Description of Accelerated Flange Wear and Friction Test Apparatuses Wear and Friction Data Using Different Flange Materials Wear Data using Different Magnetic Tapes Correlation of Accelerated Wear Data with Tape Drive References Measurement Techniques of Head and Medium Wear Head and Tape Wear Accelerated Head Wear Test Screening Wear Resistance of Head Materials and Abrasivity of Tapes Screening Abrasivity of Magnetic Particles Screening Head Contours Tape Durability Tests Loop Test Loose Debris Test Adherent Debris Test Abrasion Test Head Slider and Rigid Disk Wear Accelerated Wear Test Accelerated Durability Test Accelerated CSS Test Accelerated F1yability Test Accelerated Lubricant Persistence Studies Wear Measurement Techniques Accelerated Corrosion Test EC Test ABE Test References Lubrication Mechanisms and Lubricants Regimes of Lubrication Solid Lubrication Fluid Lubrication

14 xvi Contents Hydrostatic Lubrication Hydrodynamic Lubrication Elastohydrodynamic Lubrication Mixed Lubrication Boundary Lubrication Types of Lubricants Solid Lubricants Soft Materials Hard Materials Liquid Lubricants Principal Classes of Lubricants Physical and Chemical Properties of Lubricants Methods of Lubrication in Magnetic Media Internal Versus Topical Lubrication Solid Versus Liquid Lubrication History of Lubricants for Magnetic Media Tapes and Floppy Disks Rigid Disks Particulate Disks Thin-Film Disks Mechanisms of Lubrication in Magnetic Media Tapes and Floppy Disks Lubricant Kinetics Lubrication by Fatty Acid Esters Effect of Humidity Rigid Disks Shear Effect on Lubricant Performance Stability Studies of Magnetic Tape and Floppy Disk Lubricants Experimental Procedures Materials Evaluated Catalytic Effect of Magnetic Oxides Oxidation Model Iron Oxide-Lubricant Interaction Effect of Contaminants on Oxidation Stability Effect of Additives on Oxidation Stability Effect of Volatility on Oxidation Stability Summary and Outlook Volatility Studies of Rigid Disk Lubricants I Experimental Procedure Volatilization Model Results and Discussion References Analysis and Measurement of Hydrodynamic Air Films Lubrication Analysis I Head-Tape Interface Governing Equations Finite Difference Solution Finite Element Solution

15 Contents xvii Head-Floppy Disk Interface Single-Sided Recording Dual-Sided Recording Head-Rigid Disk Interface Governing Equations Numerical Approach Analysis of Two-Rail Taper-Flat-Type Slider Analysis of Shaped-Rail Slider Analysis of Zero-Load Slider Analysis of Transverse Pressurized Contour (TPC) Slider Surface Roughness Effects on Hydrodynamic Lubrication Lubrication Equations for Rough Surfaces Sheer-Film Analysis Squeeze Film Analysis Analysis of Head-Tape Interface Analysis of Head-Rigid Disk Interface Role of Surface Wear Measurement of Air-Film Thickness Principles of Measurement Techniques Optical Interference Technique Capacitance Technique Laser Doppler Interferometry Measurement Results Head-Tape Interface Head-Rigid Disk Interface References Surface Finishing of Ceramic Head Materials Grinding and Lapping Processes Grinding Lapping Free Abrasive Fixed Abrasive Residual Stresses Measurement Techniques Results Discussion Specific Energy Grinding Lapping Interface Temperatures Grinding Analysis Experimental Measurements Lapping Surface Finish and Morphology Surface Finish Morphology Grinding

16 xviii Contents Lapping Chips Discussion Alternate Methods of Finishing Ferrites Cherno-Mechanical Lapping Post Annealing Ni-Zn Ferrite Mn-Zn Ferrite Lapping with Soft Abrasive Precision Crack-Off Technique References Appendix A: Techniques for Measuring Mechanical Properties of Thin Films Appendix B: Surface Analytical Techniques Appendix C: Accelerated Friction and Wear Tests Name Index Subject Index