A viscoelastic analysis of crown sensitivity in hard disk drive systems

As the flying height of the magnetic recording head becomes increasingly smaller to achieve higher linear recording density, more research efforts are focused on the head disk interface (HDI). Advanced slider designs are needed to ensure flying heights are less than 50 nm. At the same time, the reliability and stability of these ultralow flying sliders has become an important issue. Crown, which can be generated at both the head gimbal assembly (HGA) level and file level, plays a critical role in the flying attitude of the slider and tribology in HDI.;Dynamic and transient experiments are carried out to obtain the viscoelastic properties of three adhesives. The creep master curves are constructed based on time-temperature superposition, and shift factors are fitted with the Williams-Landel-Ferry equation.;The HGA is first modeled as a purely elastic trilayer system. The set of displacement equations initially suggested by Suhir is solved by a new method. Laplace transformation is employed to solve the coupled integral equations, and the exact solution for shear stress is obtained. The equation for thermal deflection in an HGA is then derived. Elastic calculations are performed to study the effects of material properties and geometry on thermal bending of the slider.;The major objective of this thesis is to develop a viscoelastic theory for crown sensitivity, which is defined as the crown change per degree temperature change. The elastic-viscoelastic correspondence principle is employed to the elastic solution. The viscoelastic thermal bending is obtained in the Laplace transform domain. In general, the viscoelastic solution thus obtained must be transformed back into the time domain by numerical inversion. The derived viscoelastic equation for deflection is capable of incorporating arbitrary thermal history. Two asymptotic limiting cases are given in which the solutions are explicit in the time domain. The developed solution scheme is applied to calculate the crown sensitivity with the measured adhesive properties, and the results are compared with those directly from crown sensitivity measurements. A satisfactory agreement is achieved. On average, the crown sensitivity for the Black Max adhesive is 0.5 nm/°C, and 0.015 nm/°C for the Ablebond adhesive. Effects of adhesive thickness and bond length on crown sensitivity are also investigated.;In an HGA, the ceramic slider is attached to the stainless steel suspension by a thin layer of adhesive. The temperature fluctuations inside modern magnetic recording disk files range from 20 to 80°C. The mismatch of the thermal expansion coefficients between the stainless steel and the ceramic gives rise to thermal bending, or crown, as a result of temperature variation. The magnitude of the thermal bending is governed by the properties of the polymeric adhesive. Because the adhesive is a viscoelastic material, both time and temperature are coupled in the evolution of crown.;The finite element method (FEM) is adopted to further study issues related to the HDI and HGA. An incremental form of the viscoelastic constitutive equation is proposed for FEM modeling of thermal bending. The FEM equations for HDI simulation are summarized, and the potential angular creep of the slider is studied.