Thermal Actuator Sliders in Hard Disk Drives

Thermal flying height control (TFC) slider has been introduced to hard disk drives to achieve an ultra-low and stable flying height (FH). A thermal actuator is embedded inside the TFC slider to purposely produce a localised thermal protrusion at around the read/write element. By applying an electric current to the heater, the thermal protrusion can be used to compensate almost all static FH loss. In addition, it can significantly reduce the instability induced by the short range adhesion forces in the head-disk interface, as only a small area of protrusion is at a very low spacing.

The effects of air bearings on thermal actuation efficiency and the capability in following disk waviness of the TFC sliders, and their inherent relations are explored. It is found that the air bearing plays an important role in achieving high thermal actuation efficiency and low flying height modulation (FHM) of TFC sliders. The ABS design strategies and concepts for TFC sliders are proposed based on the underlying mechanism. The results show that both excellent thermal actuation efficiency and strong capability in following disk waviness can be achieved through proper arrangements of the air bearing pressure distribution on the air bearing surface (ABS) of TFC sliders.

Fig. 1 shows the 3-D finite element (FE) model of a TFC slider which includes the read/write element, substrate, upper and lower poles, write coil, photoresist, upper and lower shield, and the heater. A thermal-structural analysis is performed using Ansys based on the above FE model to obtain the temperature distribution and the thermal deformation of the slider.

Fig. 2 shows a typical iteration process of the coupled-field analysis for the steady state of a TFC slider. The temperature on the disk surface is set as a constant of 300 K. At the beginning of the iteration, a constant heat transfer coefficient of 6.0e5 W/mK is applied on the whole ABS, while heat transfer coefficients on the ABS for the following iterations are dependent on the air bearing and heat transfer model. It can be seen that a total of four iterations is enough in this study for the FH, protrusion, and temperature of read element to reach their respective convergent values. Fig. 3 plots the thermal protrusion profiles under the applied heating power of 15, 25, and 35 mW, respectively.

The investigation of the physics of the heat transfer process from the slider of a hard disk drive to its disk is also performed. A generalized heat transfer model, which incorporates various molecular dynamics models, is proposed to solve the heat transfer problem in thin film bearings at head-disk interface. The proposed model considers the impact of molecular collisions between film molecules and solid surfaces, which plays an important role in the heat transfer of thin film bearings and is expected to improve the accuracy of predicting the thermal protrusion caused by the heating of the slider at the head-disk interface of hard disk drives.

From Fig. 4, we can also see that the effect of ξ is significant at low slider-disk spacing and becomes inconsiderable as the slider-disk spacing increases. It is estimated that its influence is negligible only at very high slider-disk spacing of above 1 m. Therefore, the factor ξ is a crucial parameter in determining the heat transfer across the head-disk interface. In the old model, this factor is unity. The old model will underestimate the heat flux from the slider to the disk due to the fact that this factor should be in the range of 0.8–0.85 for air molecule. Therefore, it is necessary to set a more realistic value to improve the simulation accuracy of predicting the heat flux at the head-disk interface of hard disk drives.

Other parameters, such as T and ω which are air temperature and temperature exponent of the coefficient of viscosity, respectively, also have contributions to the heat flux especially when the hard disk drive operates in a wide range of temperatures.

For more information about this research, please refer to the following paper(s):

• W. D. Zhou, B. Liu, S. K. Yu, W. Hua, and C. H. Wong, “A generalized heat transfer model for thin film bearings at head-disk interface,” Applied Physics Letters 92, 043109, 2008.
• B. Liu, S. K. Yu, W. D. Zhou, C. H. Wong, and W. Hua, “Low flying-height slider with high thermal actuation efficiency and small flying-height modulation caused by disk waviness,” IEEE Transactions on Magnetics 44, pp. 145, 2008.