An experimental study of the dimple/gimbal interface in a hard disk drive
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- Li, L., Fanslau, E. & Talke, F. Microsyst Technol (2011) 17: 863. doi:10.1007/s00542-010-1215-5
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The contact of the dimple/gimbal interface in a hard disk drive was studied experimentally. Two types of dimples with different surface roughness and several types of gimbal materials were investigated. The load–displacement curves for the contact of the dimple and the gimbal exhibit hysteresis, which is related to the plastic deformation of the asperities, during the first few load/unload cycles. The roughness of dimple and gimbal samples was measured before and after load–unload testing using an AFM. The plasticity index was determined based on the roughness measurement. The results show that the roughness and plasticity index of the non-polished dimple decrease with the number of load/unload cycles significantly, i.e., the contact surface becomes smoother than the original surface due to plastic deformation. The roughness and plasticity index of the laser-polished dimple change slightly before and after the load–unload test.
Several studies have been performed in the past to investigate the mechanics of the dimple/gimbal interface. Li et al. (2009) numerically studied the dimple/gimbal interface by modeling the contact as a spherical shell contacting a rigid flat. They investigated the effect of dimple/gimbal geometry and material properties on the onset of plastic deformation at the dimple/gimbal interface. Lee et al. (2009) investigated slip between a dimple and a flexure for the special case of ramp loading. They developed a finite element (FE) model to investigate the slip motion between dimple and gimbal during ramp contact. In their model, the base of the suspension arm was fixed and an impact force was applied to the suspension lift-tab to simulate contact with the ramp.
Zheng et al. (2010) investigated the normal load between the dimple/gimbal interface during a non-operational shock as a function of dimple design parameters and preload. Raeymaekers et al. (2010) investigated fretting wear of the dimple/gimbal interface and found that fretting wear is highly dependent on the normal load. They calculated energy dissipation and plasticity index and found that these parameters were a good measure for resistance to fretting wear. Li et al. (2010a) studied the load process of two different types of dimples against a rigid sapphire gimbal in a nano-indenter. They found that the asperities of the dimple surface deform plastically and are “flattened” after a number of load/unload cycles.
Following the previous work of Li et al. (2010a), we investigate in this paper the load/unload process and deformation of two types of dimples with different surface roughness for several types of gimbal material using a modified nano-indenter.
2 Experimental set-up
Mechanical properties and roughness of dimples and gimbals
85 ± 5
Laser polished dimple
25 ± 2
6 ± 1
85 ± 5
Gold coated gimbal
108 ± 4
The controlled normal load (limited to no more than 9 mN due to the loading range of the nano-indenter) and the corresponding displacement of the dimple were measured using the transducer of the nano-indenter. All experiments were carried out at room temperature of 20–25°C and relative humidity of 40–60%. Each experiment was performed on a new dimple and a new contact zone of the gimbal. All experiments were performed under dry condition. Each load/unload cycle was completed in 10 s (5 s for loading and 5 s for unloading). There is one thing needs to be noticed that the load/unload process, which was used in present work, is not the same as the park loading/unload process in HDDs. We want to investigate the contact and deformation of dimple/gimbal interface by using the load/unload process in a modified nano-indenter.
3 Results and discussion
Kogut and Etsion (2003) assumed that a contact is elastic if ψ < 1.4; that a contact is elastic–plastic if 1.4 < ψ < 8; and that a contact is plastic if ψ > 8. A small plasticity index implies that a surface is smooth and that the asperities are more difficult to deform plastically than in the case that the plasticity index is high. The plasticity index was indentified based on the roughness measurements and material properties shown in Table 1.
Figure 8b shows an SEM image of a laser polished dimple after 200 load/unload cycles. As can be seen from the figure, very few particles are observed at the dimple surface.
Both dimple types exhibit substantial hysteresis during the first few load/unload cycles. Plastic deformation and hysteresis disappears with subsequent load/unload cycles (elastic shakedown). The plastically deformed asperities may cause wear of dimple/gimbal interface.
A gold coated gimbal interface shows large plastic deformation in the first few load/unload cycle.
The surface roughness and plasticity index of a non-polished “rough” dimple decrease with an increase in the number of load unload cycles.
The surface roughness and plasticity index of a laser polished dimple changes little with the number of load/unload cycles.
We would like to thank Mr. Hanya-san of NHK International Corp. for his interest. We also would like to thank Prof. Etsion for his helpful comments and discussions throughout this project. L. Li thanks the China Scholarship Council (CSC) and Prof. G. Zhang from Harbin Institute of Technology, for supporting his Ph.D. studies at UCSD.
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