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The tribology of microelectromechanical systems (MEMS) and the wear mechanism of materials used to fabricate these devices are of current interest to researchers. Most work to date has concentrated on layers of these materials and little data are available on real, fully functional MEMS. This thesis investigates both the surface and wear properties of a self-assembled monolayer (SAM) coated MEMS test structure fabricated at Sandia National Laboratories, USA. Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), atomic force microscopy (AFM) and transmission electron microscopy (TEM) were used to study the surface chemistry, morphology and bulk structure of the MEMS gears and substrate. Wear behaviour of the MEMS was investigated using a diamond tipped AFM cantilever operated under different loads. Regarding the MEMS test structure, SEM and AFM images show an island-like morphology on the polycrystalline silicon (polysilicon) gear and silicon nitride substrate surfaces but it is not associated with columnar growth according to SEM and TEM cross-sections investigations. AFM wear studies show this island-structure undergoes greater wear than the surrounding areas and will be worn flat at the early stage of wear. XPS and AFM force curve results confirm the presence of the octadecyltrichlorosilane (OTS) SAM on the MEMS surface. The presence of OTS increases the wear resistance of the MEMS surface. Compared with the uncoated LPCVD polysilicon layer and CVD silicon nitride layer, wear resistance of the MEMS gear improves by 2 times while that of the substrate improves by up to 3 times. The wear mechanisms of the selected materials used for MEMS manufacture in the load range from 10 to 70 muN are discussed. Determination of specific wear amounts as a function of maximum contact pressure provides useful information on determining the wear mechanisms of these materials.

Capillary electrophoresis (CE) has proved to be a very efficient separation method in aqueous media. This work reports developments in nonaqueous capillary electrophoresis (NACE) for samples from the lubricants industry. Two commonly used additives in lubricants, zinc dialkyldithiophosphate and the metallic detergent alkylsalicylate, were analysed. Method development included selection of solvent, pH, and electrolyte type and concentration. Improvements in the separation were obtained by altering the injection conditions and the electric field strength. Hexadimethrine bromide (HDB) was shown to be essential in controlling the electroosmotic flow (EOF), and its concentration was optimised. Best separation was seen when analyte dilution prior to injection was carried out with methanol, resulting in sample stacking. Analysis of ZDDPs showed that separation occurred through migration of free dithiophosphate ions. Calcium alkylsalicylate showed similar results where the free alkylsalicylate ions migrated. Identification was achieved through peak patterns and relative migration times. Adequate separation conditions were identified and then coupling of NACE to mass spectrometry was attempted. Direct infusion MS was first utilised with great success in identification of the two additives. Using a solution of methanol and ammonium acetate it was possible to identify free dithiophosphate ions from the ZDDP and free alkylsalicylate ions from magnesium alkylsalicylates. Only the molecular ions were obtained in all instances making identification easier. NACE-MS was carried out successfully; however, sample dilution and adequate interfacing were problematic. Analysis of formulated lubricating oil was performed by NACE, NACE-MS and MS. Degradation products were identified for ZDDPs, where sulphur atoms were replaced with oxygen leading to the formation of phosphates. NACE has been shown to be a technique of potential for the analysis of lubricant additives, and in combination with mass spectrometry it could be very powerful.

This study explores the wear and oxidation resistance of novel CrAlYN/CrN nanoscale multilayer coatings grown by a combined high power impulse magnetron sputtering (HIPIMS) / unbalanced magnetron (UBM) sputtering process. Firstly, it is found that a HIPIMS substrate etching pre-treatment prior to the coating deposition effectively removes contaminants from the substrate surface and provides the base for epitaxial growth. Subsequently, an improvement in the adhesion and the tribological performance of CrAlYN/CrN is observed compared to coatings pre-treated with a conventional cathodic arc pre-treatment. Secondly, the effect of the deposition conditions on the microstructure and the subsequently arising coating properties is investigated. A variation in the energy of ions impinging at the substrate during the coating growth permits a change in the microstructure from polycrystalline columnar to partially single crystalline. Depending on the microstructure, CrAlYN/CrN reveals a maximum high hardness of 51 GPa and extremely low wear rated of 10[-17] m3N[-1]m[-1]. Modifications of the microstructure from nano-layered columnar to near-monolithic columnar are discovered with a change in the chemical composition. These structural changes only have a mild effect on the mechanical and tribological performance. Finally, the oxidation performance of CrAlYN/CrN is extensively investigated. Annealing in air leads to simultaniously occurring processes with (i) the thermal degradation of the nanolayered CrAlYN/CrN into Cr[2]N and AIN and (ii) the surface oxidation with the formation of corundum-type oxides and at long exposure times (iii) diffusion of substrate elements through the coating. The observed phase transformation into Cr[2]N and AIN may be promoted by the diffusion of the substrate element Cr into the coating. The results suggest that the oxidation performance of CrAlYN/CrN can be improved if the coating is grown under high energetic bombardment. The coating is prevented from cracking and therefore protects the substrate material more effectively from oxidative attack. Furthermore, it is found that the oxidation resistance can also be improved when changing the chemical composition. Highly Al-containing coatings form Cr[2]O[3]-Al[2]O[3] oxides which protect the underlying more effectively than (Cr,Al)[2]O[3] oxides grown on low Al-containing CrAlYN/CrN.

This research developed a CrN/NbN coating with promising properties for tribological applications where corrosion plays a big role. Building on this knowledge base a novel approach to the surface treatment was selected, in which multiple layers, each chosen for a specific purpose, were optimised to combine the best properties of each. In this approach a careful consideration of macro- and microstructure of each layer is required in order to extract the good properties of each layer while eliminating the negative ones. It was shown in the work that, if such consideration is neglected, a catastrophic failure may follow. For example poor adhesion may cause a total failure of the coating. As the number of layers and interfaces increase a good understanding on the structure and the properties of each layer becomes very important as the number of parameters and possible combinations increase many times. In this whole work the intention was to take a very practical approach to the coating. The objective was to combine different approaches, such as duplex treatments and multi layering and investigate the specific interactions that are not otherwise apparent. The results of this work show that such an approach is viable and should lead to excellent results as long as the wear mechanisms of the coating are understood and the coating is correctly engineered for the application.

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No description available.

The effect of oxides on the topographies of various random engineering surfaces and steady-state thermal contact resistances between oxidefree and oxidized metallic surfaces in contact in high vacua have been investigated. The results of many previous investigations indicated that such problems cannot be dealt with without a detailed study of the surface topographies, resulting in accurate surface characterisation. The distributions of the real micro-contact zones and heat transfer phenomena across interfaces could 'then be predicted. Cylindrical specimens normally mild steel EN3B and commercially pure copper of nominal area 4.908 x 10-4 m2 were oxidized under controlled environmental conditions and subsequently pressed into contact at their flat faces. The oxide film thickness measurements were taken using a high resolution Stereoscan electron microscope. Surface topographies were quantified before and after oxidation to determine the effects of oxide films upon their topological characteristics. Following a detailed analysis, two theories concerning the problem of predicting thermal contact resistances of oxidized joints having Gaussian distribution of surface heights have been developed. The developed theories establish the surface parameters measured to design a predictable thermal joint and apply for thin films of the order of naturally occurring oxides. From a statistical analysis of the experimental measurements for freshly-assembled contacts, an empirical expression (R = 66.0 p-0.945 a-0.128 X0.0346) has been established relating the loading pressure, mean roughness of'the contacting surfaces and oxide film thickness to the thermal resistance of the contacts in high vacua. Finally a method of producing joints with low thermal contact resistance (and vice versa) is suggested.

The topic of this PhD is to investigate materials interfaces under the application of com-pressive forces and dynamic friction. Friction studies are important in applications for high-speed machining and ballistic penetration modelling, two areas where it is important to understand the behaviour of rapidly moving interfaces. Gaining insight into the velocity dependence of the effective tangential force, and its time-evolution, under various external loads is also of particular interest. It is important to understand on an atomic and/or molec-ular level the fundamentals of tribological processes. Some of the processes investigated in this thesis include plastic deformation due to high compression, the response of materials when sliding occurs in terms of temperature variation across the interface and its relation-ship with atomic diffusion. Moreover, the materials dependence on operating conditions of temperature, loading and dynamic friction are factors that ultimately determine the design of tribological systems. In the last few years it has been shown that materials properties depend on the size, as smaller specimens are relatively stronger than larger ones. This thesis is aiming to em-ploy state of the art numerical and theoretical methods, which are vital to give a significant insight and understanding of the fundamental issues concerning dynamic friction of tribo-logical processes at the atomic scale. The mechanical behaviour is investigated in detail to reveal an accurate theoretical description of the frictional force at metallic surfaces. Special consideration is taken into account for the mechanism that causes dissipation in the form of heat. The strong deformation when materials undergo dynamic friction causes energy to dissipate away from the interface at a high rate. Additionally, investigation of the plastic deformation and its variation under conditions prevalent at high speed sliding is carried out. Knowledge of the yield point under these conditions is important to obtain accurate constitutive models for the shear stresses. In-vestigating how the material strength varies under sliding friction and obtaining accurate evaluation of the stresses involved has proved difficult and time consuming. This is primar¬ily attributed to the fact that experiments are difficult to conduct and expensive facilities are required. This thesis focuses on aspects of this complex process with the aid of molecular dynamic simulations.