Investigations Of Non-isothermal Gas Film Hydroynamic Characteristics And Solid Lubrication Coatings For High Temperature Micro Gas Bearings

In recent years, power MEMS device have become one of the research hotspots due to their advantages of small size, light weight, altra high energy and power density. Compared with batteries, internal combustion engine generators can not only produce high power density but also high energy density, which makes micro gas turbines operate at higher temperature and rotation speed in smaller size. High speed micro gas bearings are the main supporting parts in high speed micro gas turbines, and thus to study their hydrodynamic characteristics and solid lubrication coatings is very significant. Since the gas film is in micron scale and working speed and temperature is under extreme conditions, the isothermal assumpion is not valid. Additionally, the solid lubrication coatings made by Cold spray is still in the attempt stage and the wear-resistant materials are not considered in pervious study. In this dissertation, the above two aspects are detailedly investigated with numerical and experimental methods.Firstly, the hydrodynamic behaviors of wedge-shaped gas films in micro gas bearings are investigated comprehensively. The Reynolds equation, modified Reynolds equation, Navier-Stokes equations and energy equation are adopted to mainly analyze the effects of the wedge factor and wall temperature on the gas film lubrication hydrodynamics in a wedge-shaped micro channel under the symmetrical and asymmetrical wall temperature conditions. Furthermore, the different gas film lubrication characteristics among the previous different governing equations are in-detailed examined. The gas temperatures profiles are parabolic with a peak skewed toward the horizontal wall and enery equations hardly have any effects on the gas film hydrodynamic behaviors. As the wall temperature is elevated, the load capacity obtained by the Reynolds and modified Reynolds equation increases while that by the Navier-Stokes equation decreases. Squeeze increases the gas film pressure, but the gas backflow greatly reduces the gas film pressure. The vertical flow across the gas film determines the angle between velocity vector and tilt wall, and thus it dominates the gas film squeeze. As the gas backflow does not occur, the vertical flow reduces the gas film pressure. But as the gas backflow occurs, the vertical flow increases the gas film pressure. The axymmetrical wall temperature can reduce the gas film squeeze, and thus the gas film pressure decreases. As the horizontal wall is adiabatic and the tilt wall temperature is constant, the heat accumulation on the horizontal wall enlarges the rarefaction effect and the elevated gas film temperature weakens the gas compressibility, so the gas film pressure significantly declines.Secondly, the finite difference lattice Boltzmann model?FDLBM? is employed to investigate the hydrodynamic behaviors of wedge-shaped gas-lubricated film. And the results are compared to the macroscopic methods. In order to verify the FDLBM in the compressible flows on the mesoscopic level, the compressible Couette and Poiseuille flows are firstly simulated and the results are compared with the analytical, the NSE solutions and the references. As for the compressible wedge-shaped gas film flow, the continuum assumption in the MREE and NSE methods predicts an overvalued gas film pressure due to the overvalued compressibility at high Knudsen numbers. Though the gas backflow is not captured by the FDLBM, the film pressure reduces significantly. The hybrid codes will use Navier-Stokes solvers for the low Knudsen number parts and the FDLBM for the high Knudsen number areas to study the multiscale issue in the micro gas bearings.Thirdly, based on the first-order modified Reynolds equation, the effects of the different gaseous lubricant species and the extreme temperature difference between the compressor side and the turbine expander side on the steady characteristics of micro gas journal bearings are mainly investigated. For a given lubricant, the gas viscosity and rarefaction effect dominate the magnitude of load capacity. Among the considered gaseous lubricants, CO₂ gas can significantly improve the stability of gas bearings due to its larger molar mass. The larger temperature difference between the compressor side and the turbine expander side gives rise to the lower pressure of gas film. Moreover, the larger temperature difference can make the peak of the pressure profiles along the axial direction severely skewed toward the higher temperature end and it is unfavorable to the stability of gas bearings. By comparing the film pressures, it is found that the changes of eccentricity ratio and rotation speed hardly have any influences on the thermal creep, while the shorter bearing can enhance the thermal creep. The thermal creep slightly elevates the load capacity, compared with the case without the thermal creep. Additionally, the thermal creep extremely degrades the stability of gas bearings in the case that the rotation speed is more than 106 rpm, the eccentricity ratio is more than 0.75 and the bearing length is more than 110 ?m.Finally, the cold gas dynamic spraying method is proposed to create self-lubricating coatings incorporating Al₂O₃, Ni and Cu. Here, Al₂O₃ is the wear-resistant material, and Ni and Cu are the bonding materials. Different from the previous study, this experiment is focused on the wear-resistant component but not the lubricating material, and the spraying gas is air. The pure Al₂O₃ particle cannot be built up on the Al and Ti-6Al-4V substrates, while pure Ni or Cu can only be built up on the Al substrate but cannot be piled on the Ti-6Al-4V substrate. Spraying 20wt% Al₂O₃?70±5?m?and 80wt% Ni mixture powders on the Al substrate can form a 500?m thickness solid coating and the adhesion trace is very clear. Spraying 20wt% Al₂O₃?70±5?m?+ 40wt% Ni + 40wt% Cu on the Ti-6Al-4V substrate can form a 500?m thickness solid coating and the content of Al₂O₃ atomic can reach up to 13.11%, but the adhesion trance is not clear.