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Magnetic fluids are synthetic materials that combine in a single medium the properties of fluids and magnetic materials. The action of a magnetic field on the magnetic fluid allows the design of new types of bearings and also provides conventional ones with new properties. This work presents a contribution to the study of the lubrication of linear isothermal non-conformal contacts with magnetic fluids under the action of a non uniform magnetic field perpendicular to the plane of the contact. The fluid is assumed to present extrinsic superparamagnetism. The regime of lubrication considered is the isoviscous-rigid. The analytical approach adopted considers small perturbations of the classical analytical solution already known for this type of contact. The main objectives of the work are: To establish the equations and assumptions that are necessary to treat the proposed lubrication problem To analyze the impact of the change in the apparent viscosity of the magnetic fluid on the thickness of the lubricating lm due to the presence of the magnetic field ° To develop an experimental technique able to deal with the problems that arise with the use of magnetic fluids in the lubrication of a two-disc machine The mathematical treatment of the proposed problem brings together the analytical methods adopted in fluid lm lubrication to the analysis of long isothermal hydrodynamic bearings and those from the magnetic-liquid fluid dynamics (ferrohydrodynamics) usually adopted to study the flow of magnetic fluids that present extrinsic superparamagnetism. The result is a coherent system of equations supported by a well defined set of assumptions. The solution obtained allows the study of the eventual change of the lm thickness that can be achieved by the application of a magnetic field across the contact. The work also discusses the experimental technique necessary to study the problem in a two-disc machine that runs with minute amounts of lubricant, applies a magnetic field across the contact and measures the lm thickness by a capacitance method. The work concludes by showing theoretically that the lm thickness can be either increased or reduced depending on the magnetic and viscous properties of the fluid and the intensity of the magnetic field. These conclusions are supported by experiments with a magnetic oil in a two-disc machine.

Digitized by Kansas Correctional Industries

High performance machines such as gas turbine engines demand efficient solid lubricants at high temperature and in vacuum. The current conventional solid lubricants need to be further improved. This research evaluates carbon based solid lubricants using a high vacuum, high temperature pin-on-disc tribometer. The objectives of this research were to develop an understanding of the tribological properties of solid lubricant coatings under extreme operating conditions, and to determine whether using a carbon based solid lubricant would be acceptable for use in those conditions. Experimentally, two solid lubricant coatings on tungsten carbide substrate were tested against two different materials. The coatings were carbon based and molybdenum disulfide based. The other materials were 440C stainless steel and tungsten carbide. The temperature, pressure, and relative humidity are independent variables. The results showed that the carbon based coating increases friction and wears out quickly due to high temperature, high vacuum, and low humidity. Abrasive wear is the dominating mechanism. At elevated temperatures and in dry environment, the carbon based coating underwent significant oxidation and phase transformation. This research is beneficial for future design and development of solid lubricants for aerospace applications, as well as other industries requiring lubricants that must operate in extreme conditions. This thesis includes five chapters. Chapter I is an introduction to tribology and to the materials being used in this research. Chapter II describes the motivation and objectives behind this research. Chapter III discusses the experimental procedure and further explains the materials used. Chapter IV presents and discusses the results obtained. Chapter V discusses the major conclusions obtained from the results and offers some future work that may be conducted concerning this research.

Inaug.-diss.--Eidgenössische technische hochschule, Zürich. / Lebenslauf.

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