Squeeze film effects on dynamic performance of MEMS μ-mirrors-consideration of gas rarefaction and surface roughness

The squeeze film behavior of MEMS torsion mirrors is modeled, analyzed and discussed. Effects of gas rarefaction (first-order slip-flow model with non-symmetric accommodation coefficients, ACs) and surface roughness are considered simultaneously by using the average Reynolds type equation (ARTE). Based on the operating conditions with small variations in film thickness and pressure, the ARTE is linearized. A coordinate transformation, by stretching or contracting the axes by referring to the roughness flow factors, is proposed to transform the linearized ARTE into a diffusion type modal equation. The dynamic coefficients (stiffness and damping coefficients) are then derived and expressed in analytical form. The results show that the tilting frequency (or Γ₀ squeeze number), roughness parameters (γ Peklenik numbers, σ standard deviation of composite roughness) and gas rarefaction parameters (D inverse Knudsen number, ACs) are all important parameters on analyzing the dynamic performance of MEMS torsion mirrors.     

Thin film elastohydrodynamic lubrication—a power-law fluid model

A 1-D modified Reynolds equation for power-law fluid is derived from the viscous adsorption theory for thin film elastohydrodynamic lubrication (TFEHL). The lubricating film between solid surfaces is modeled as three fixed layers, which are two adsorption layers on each surface and a middle layer between them. The comparisons between classical non-Newtonian EHL and non-Newtonian TFEHL are discussed. Results show that the TFEHL model can reasonably calculate the pressure distribution, the film thickness, the velocity distribution and the average viscosity. The thickness and viscosity of the adsorption layer and the flow index influence the lubrication characteristics of the contact conjunction significantly. The film thickness increases with the increase of flow index. As the flow index becomes greater, the dimple in the film shape moves towards the center of the contacts. The effect of flow index produces an obvious difference in the pressure distribution. The greater the flow index, the greater the pressure spike, and the pressure spike tends to move toward the center. The larger the flow index, the more the velocity varies in both the middle layer and adsorption layers along the Z-axis. The greater the thickness and viscosity of the adsorption layer and the flow index, the greater the deviation in central film thickness versus speed between EHL model and TFEHL model produced in the very thin film regime.     

Application of modified molecular gas lubrication equation to the analysis of micromotor bushings

The microtribological phenomena have important effects on microelectro-mechanical systems (MEMS), especially when relative motion is required (e.g. microactuators). From the design concept of self-acting bearings (e.g. the flying head in a magnetic recording system), some step-shaped bushings beneath the rotor of a micromotor are considered and analyzed quantitatively. The modified molecular gas lubrication equation (MMGL) is utilized to analyze the step-shaped bushings. The results show that the coupling effects of roughness and gas rarefaction are significant and important in the design of bushings.     

Surface Roughness Effects in Journal Bearings with Non-Newtonian Lubricants

The static performance of finite journal bearings lubricated with non-Newtonian power law fluids is analyzed by using a control volume method with an Elrod algorithm to solve the average Reynolds equation and determine the cavitation region accurately. The results show that the flow behavior index of power law fluids has an insignificant affect on the load ratios, side flow ratios and cavitation regions, while it significantly affects load capacities and side flow rates. Furthermore, the effects of film thickness ratios, pressure flow factors, shear flow factors, slenderness ratios, eccentricities and inlet pressures on the variations of cavitation regions are also discussed.     

Modeling of Head/Disk Interface—An Average Flow Model

The average flow model is utilized to derive an average Reynolds type equation, which includes the effects of surface roughness, gas rarefaction, and accommodation coefficients (ACs). The related flow factors (pressure flow factors, shear flow factors) are derived in closed form and are expressed as functions of roughness parameters (standard deviation of the composite roughness, Peklenik number and film thickness ratio), Knudsen number, and ACs. Finally, the effects of surface roughness, gas rarefaction, and ACs on the bearing performance of magnetic head sliders with fixed configurations are considered.     

Elastohydrodynamic Lubrication of Circular Contacts at Impact Loading with Generalized Newtonian Lubricants

In this article, pure squeeze elastohydrodynamic lubrication (EHL) motion of circular contacts with power law model lubricant is explored at impact loading. The coupled transient modified Reynolds, the elasticity deformation, and the ball motion equations are solved simultaneously, thus obtaining the transient pressure profiles, film shapes, normal squeeze velocities, and accelerations. The simulation results reveal that the greater the flow index (n), the earlier the pressure spike and the dimple form, while the maximum pressure and the film thickness increase, and the diameter of the dimple, the maximum value of the impact force, the rebounding velocity, and the acceleration decrease. Further, this analysis numerically demonstrates that the contact central pressure for a ball impacting and rebounding from a lubricated surface reached two peaks during the total impact period. As the flow index increases, the primary and the secondary peak increase, and the first and second peaks form earlier; as the total impact time decreases. Moreover, the phase shift between the time of the peak value of the squeeze acceleration and the zero value of the squeeze velocity increase with increasing flow index.     

Low-Friction Characteristics of Nanostructured Surfaces on Silicon Carbide for Water-Lubricated Seals

This article reports a novel way of in situ formation of ionic liquids (ILs) that are used as lubricant additives for steel/steel contacts. The ILs can be generated simply in base oils by mixing small molecules of 2-oxazolidinone, triglyme, or tetraglyme with lithium bis(trifluoromethane sulfonyl) imide in appropriate molar ratios at room temperature. The said ILs can be formed in polyether, polyester, and polyurea grease and show better solubility in these base oils than commonly used ILs such as 1-methyl-3-hexylimidazolium hexafluorophosphate (L-P106). They can significantly improve the friction-reducing and antiwear properties of the base oils.     

The phase transformation and crystallization kinetics of (1 − x)Li2O–xNa2O–Al2O3–4SiO2 glasses

The phase transformation and crystallization kinetics of (1 − x)Li₂O–xNa₂O–Al₂O₃–4SiO₂ glasses have been studied by using differential thermal analysis (DTA), X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and electron diffraction (ED) analysis. The crystallization temperature at the exothermic peak increases from 1171 to 1212 K when the Na₂O content increases from 0 to 0.6 mol. The crystalline phase is composed of spodumene crystallization when the Na₂O content increases from 0 to 0.6 mol. The activation energy of spodumene crystallization decreases from 444.0 ± 22.2 to 284.0 ± 10.8 kJ mol⁻¹ when the Na₂O content increases from 0 to 0.4 mol. Moreover, the activation energy increases from 284.0 ± 10.8 to 446.0 ± 23.2 kJ mol⁻¹ when the Na₂O content increases from 0.4 to 0.6 mol. The crystallization parameters m and n approach 2, indicating that the surface nucleation and two-dimensional growth are dominant in (1 − x)Li₂O–xNa₂O–Al₂O₃–4SiO₂ glasses.     

Crystallization Behavior and Microstructure of Silica-Free 5K<Subscript>2</Subscript>O-45CaO-50P<Subscript>2</Subscript>O<Subscript>5</Subscript> Bioglass

The crystallization behavior and microstructure of silica-free 5K₂O-45CaO-50P₂O₅ (KCP) bioglass have been studied using differential thermal analysis (DTA), X-ray diffraction (XRD), scanning election microscopy (SEM), transmission electron microscopy (TEM), and selected area electron diffraction (SAED). The activation energy for the KCP bioglass crystallization is found to be 337.4 kJ/mol using a nonisothermal method. The crystalline phases of the glass surface determined by XRD are KCa(PO₃)₃, 4CaO·3P₂O₅, and β-Ca(PO₃)₂ when the KCP bioglass is crystallized at 903 K for 4 hours. The crystalline phase of the powder samples determined by XRD is β-Ca(PO₃)₂ when silica-free KCP glasses crystallized at 873 to 1073 K for 8 hours. Crystallization starts at the surface of the KCP bioglass and then proceeds toward the interior of the glass matrix. The morphology of β-Ca(PO₃)₂ is a fibrillar shape 20 to 180 nm in length and 17 to 20 nm in diameter, with an aspect ratio ranging from 1.0 to 10.6.     

Crystal Structure and Dielectric Characterization of a (SrTiO3/BaTiO3)n Multilayer Film Prepared by Radio Frequency Magnetron Sputtering

(SrTiO₃/BaTiO₃) _n multilayer films with Pt bottom and top electrodes have been prepared by a double target radio frequency (RF) magnetron sputtering, and their dielectric properties have been characterized as a function of temperature, frequency, bias voltage, and applied voltage. The X-ray diffraction (XRD) pattern reveals that the deposited (SrTiO₃/BaTiO₃) _n multilayer films have a designed modulation. The interfaces within the multilayer films appear smooth and dense without any microcracks, and the adhesion is very good. The dielectric constant of the (SrTiO₃/BaTiO₃) _n multilayer film increases with increasing layer number (n), and the leakage current density is less than 1 × 10⁻⁸ A·cm⁻² for the applied voltage less than 5 V for the 450-nm-thick (SrTiO₃/BaTiO₃) _n multilayer films. The remanent polarization (P _r ) and the coercive field (E _c ) of the 350-nm-thick (SrTiO₃/BaTiO₃)₄ multilayer films are 7 μC·cm⁻² and 60 kV·cm⁻¹, respectively, exhibiting ferroelectricity. (SrTiO₃/BaTiO₃)₄ multilayer films have a high E _c and a lower P _r , as compared with the bulk BaTiO₃ single crystal. The 450-nm-thick (SrTiO₃/BaTiO₃)₄ multilayer films have a leakage current density-voltage characteristic, which makes them suitable for application in DRAMs capacitors.