Dynamics and Stability of Thermal Flying-height Control Sliders in Hard Disk Drives (2012)

As a recent development to further reduce the flying height of a magnetic head in hard disk drives (HDDs) to nanometers, thermal flying-height (TFC) control technology is now widely applied in the HDD industry because it enables consistent read/write spacing, increased storage density and improved HDD reliability. The fast development of TFC technology presents new challenges to head designers because of the complicated structure of a TFC head, the thermo-mechanical-coupling effects and tribology issues arising at nanometer read/write spacing.A steady-state TFC solver dedicated to obtaining the steady-state flying attitude of a TFC slider is developed in this thesis. This solver uses a finite volume based solver (CML static solver) to solve the generalized Reynolds equation and obtain the pressure and spacing fields in the air bearing and a commercial coupled-field solver (ANSYS) to obtain the stress and strain fields due to internal heating. An iterative procedure is adopted to consider the cooling effect of the air bearing on the heater-induced protrusion. Accuracy of the solver is verified by drive-level magnetic tests on several combinations of air bearing and heater designs. TFC sliders' performances under different ambient conditions are investigated based on the TFC solver. It is found that the thermal actuation efficiency of a TFC slider increases with altitude because of the weakened cooling and reduced air bearing stiffness at the transducer area at a higher altitude. In addition, a TFC slider maintains a more consistent read/write spacing at different humidity levels, compared with a non-TFC slider, because the thermal actuation is able to compensate part of the pressure loss caused by water condensation. A TFC slider's flying height in air-helium mixtures is shown to be a highly nonlinear function of the fraction of helium in the gas mixture due to the combined effects of the gas mean free path, viscosity and heat conductivity. These results provide general guidelines for heater and ABS designers to reduce a TFC slider's sensitivity to ambient conditions and improve HDD reliability.A touchdown numerical model for predicting TFC sliders' dynamics at touchdown and over-pushed conditions is developed and implemented based on the CML dynamic simulator. It extends the solution of the time-varying generalized Reynolds equation to near-contact and contact conditions using a statistical multi-asperity approach. Various interfacial forces are considered by use and further development of a sub-boundary lubrication model to capture important tribological effects occurring at touchdown. This model is able to predict a TFC slider's unstable dynamics at the beginning of touchdown, which has been discovered in many related experimental studies. The effects of different head-disk interface factors are investigated using this numerical model. It is found that the suspension is actively involved in the TFC slider's bouncing vibrations and has a significant influence on the excited second air bearing pitch mode. It is also shown that adhesion force serves as an essential factor in exciting the second air bearing mode whereas other interfacial forces only affect details of the slider's bouncing behaviors. By changing the interfacial properties, namely, the interface roughness and lubricant thickness, the variation of interfacial forces with spacing reduction differs, which leads to very different touchdown patterns. With a rougher interface profile the slider smoothly transfers from a flying stage to a sliding stage. With a smoother interface profile the slider experiences a flying-bouncing-sliding transition. With the smoothest interface the slider goes through a flying-bouncing-surfing-sliding transition. The touchdown behaviors predicted by the numerical simulator are correlated with experiments conducted on industry-provided head parts with the same ABS and suspension design. Similar touchdown stages and excited modes are also discovered in the experiments. Though experiments showed a slider spectrum with richer frequency components, the modes missed from the numerical simulations are recovered by conducting a harmonic analysis on a full HGA model with air bearing included.The different touchdown dynamic patterns predicted here result in significant differences in the successful touchdown detection, which is very important for realizing reliable read/write operations, and therefore this work provides guidelines for head disk interface (HDI) optimization. The general approach proposed here is also applicable to studies on the effects of other important HDI factors, such as air bearing geometric features, heater-induced protrusion profiles, and suspension design parameters, and on the slider's touchdown dynamics behaviors, which will assist in obtaining solutions to performance and reliability issues in current hard disk drives.