Page 1 of 6 STICTION/FRICTION TEST 1.0 STICTION/FRICTION TEST 1.1 SCOPE Static friction (stiction) and dynamic (running) friction between the air bearing surface of sliders in a drive and the corresponding disk surfaces are real measurable forces resulting from the interactions between those surfaces while in stationary contact, intermittent contact during relative motion and/or while flying. The measurement of these forces both before and after contact start stop cycles is important for drive performance since these forces have two primary impacts on drive performance: 1 - The ability of the drive spindle motor to overcome the frictional resistance of the interface to start the disk rotating. 2 - The loss of data or operation due to a head crash or excessive wear debris generated by slider disk contact. 1.2 TEST DESCRIPTION 1.2.1 Purpose of Test To measure the static and dynamic friction forces of the slider/disk interface both before and after contact start/stop. The control these frictional forces are key technologies for manufacturing drives with successful, long term performance. There are many, often conflicting theories on how to control and change frictional properties of the slider/disk interface. This document deals with test techniques and considerations involving the acquisition of valid data on these frictional forces. To more fully address these test techniques, same traditional categories of tribological terms, conditions and causes are discussed. These are not to be construed as authoritative. IV-87
1.2.2 General Theory 1.2.2.1 Static Friction Stiction Page 2 of 6 The conditions which cause an HGA (Head Gimbal Assembly) slider at rest on the disk to resist lateral movement include phenomena such as jo blocking ; dynamics of lubrication; macro-mechanical interference (asperities); debris; contamination, etc. The static frictional force at the slider/disk interface must be overcome by the torque of the spindle motor. This torque, applied through the disk on one side of the slider and through the actuator, head arm, flexure and gimbal to the other side of the slider creates a force couple on the slider. This force couple acts on the slider to try to rotate the slider on an axis parallel to the disk surface and radially outward from the spindle centerline. As the slider tries to rotate, it is constrained by an opposing force couple created by the downward force (gram load) Of the flexure and the upward force of the disk on the leading edge of the slider. These two force couples intensify until either the slider breaks free or the maximum torque of the motor is established. See figure 1. Historically tests of stiction have been done using either a constrained disk or a constrained baseplate of the HGA measuring the force on the constrained member. While both of these techniques may be useful in tribological studies, for the purpose of this specification all tests are done by measuring the force at the baseplate of the HGA using a device which constrains all 6 degrees of freedom of the baseplate while torque is applied to the disk. Specifically, the Z. height, pitch, roll, yaw, and x-y translation of the baseplate are constrained. Thus, the slider motion allowed is limited to that allowed by the flexure with a clamp baseplate as the disk is forced to rotate. This has been judged to be the more valid testing technique as it more accurately simulates the environment in a drive. 1.2.2.2 Dynamic Friction Friction The conditions which cause dynamic resistance between slider and disk include: gram load, surface speed, gimballing anomalies, flexure and slider resonances, lubrication, surface finish, etc. Measurements of these forces are primarily of value in determining mechanical changes in the head/disk interface either initially or over time due to mechanical interaction. Many tests have been devised by the disk drive community to: 1 - Accelerate the results of contact start/stop over the lifetime of a drive. 2 - Identify changes in manufacturing processes which could affect the mechanical integrity of the slider/disk interface. The test methods in this document therefore include tests addressing both these issues. IV-88
Page 3 of 6 1.2.3 Test Limitations 1.2.3.2 Friction 1.2.3.1 Stiction 1.2.3.1.1 The force (or displacement) transducers and the associated mechanical assemblies used to perform this test are limited by their mechanical resonance frequencies in the speed at which they can detect the force on the HGA baseplate. As a result, the torque on the disk must be increased as slowly as possible. Since most testers are capable of feedback control on spindle speed only (and not torque) in practical terms this requires that the tester be used at the slowest speed possible without cogging. 1.2.3.1.2 Each stiction measurement is for a discrete location of the slider on the disk and may not be typical of the entire disk. This limitation can be minimized by multiple measurements on a single surface. 1.2.3.1.3 Each stiction measurement is affected by a non-repeatable set of conditions such as a thin layer of contamination at the interface or a set of non-specified conditions, i.e., time in position and angular orientation of the slider. 1.2.3.1.4 The maximum measured force {see 1.3.1) may be different from the force at the slider/disk interface due to the influence of the dynamics of the flexure. 1.2.3.2.1 The test involves relating slider/disk motion with the slider in contact with the disk surface. This alters the conditions of the interface during the measurement. 1.3 TEST METHODS 1.3.1 Stiction Test Allow the slider to come to rest on the disk surface for a (TBD) period of time with the specified gram loading and Z-height. Prepare event recording equipment. Cause spindle to begin rotation and note when the measured force stops increasing. The force measured at this point is defined as stiction. IV-89
Page 4 of 6 1.3.2 Friction Test Rotate the disk at a prescribed RPM and measure the force on the baseplate of the HGA during rotation. Record: a. The average force for one complete revolution of the disk. b. The maximum and minimum force for one complete revolution. c. The modulation (envelope) of the force which has no precise accepted definition but in general is the envelope of the force excluding any unusually high or low friction regions of the disk. An alternative is to measure the force which is exceeded only 10% of the time minus the force which is exceeded 90% of the time for one complete revolution. 1.3.3 CSS/Friction Test Cause the disk RPM to cycle repeatedly for a specified number of cycles. The cycle specification shall include: Low cycle speed (RPM) High cycle speed (RPM) Acceleration time (SEC) Deceleration time (SEC) High dwell time (SEC) Low dwell time (SEC) Also specify: total test cycle XXX : interval between tests XXX : track radius Record static & dynamic friction after each XXX cycle. 1.3.4 Drag Test Cause the disk to turn at a constant RPM for a specified time. Record static and dynamic friction at the conclusion. IV-90
Page 5 of 6 1.3.5 Take Off Speed Test By triggering an oscilloscope with the index provided from the spindle controller and putting the friction force output on channel 1, events which are caused by anomalies on the disk will appear at the same location on the screen with each subsequent revolution. Running the spindle at a low speed such as 100 RPM will produce a repeatable trace. Then as the spindle speed is increased beyond takeoff speed, these events can no longer be seen and the trace becomes random with respect to index. (The only forces now being realized are from windage and flexure resonance.) If the speed is now slowly reduced, fixed events will again appear on the screen indicating slider sensing of the disk. This method allows the user to approximate the takeoff and landing RPMs of the slider. 1.4 TEST EQUIPMENT 1.5 NOTES 1.4.1 Force Transducer The force transducer should be able to measure the transmitted forces without affecting the results. Thus, the method of transducing should have a minimal amount of relative movement that it itself contributes. It should be easily/positively calibrated, be free of drift and have a suitable bandwidth for the data to be acquired. 1.4.2 Spindle For stiction measurement the spindle should be capable of smooth transition from zero to non-zero RPM. Its constant speeds should have well controlled Instantaneous Speed Variation (ISV) and well controlled axial and radial T.I.R. 1.4.3 Spin Stand The spin stand should have well defined vibration isolation and geometry to hold the HGA baseplate accurately. 1.5.1 Test Parameters to be Specified. In order to fully define the tests, values for the following test parameters must be specified. Examples are given to assist the user in determining the format of the test procedures. TEST PARAMETERS EXAMPLES Z height.059" Gram load 9.5 grams Ambient temperature 70 deg F +/- 10% Relative humidity 40% RH +/- 10% Head skew angle 16 deg +/- 0.5 deg IV-91
Page 6 of 6 ****DIAGRAM, PG 211. FiGuRE 1** IV-92