<p>Hard coatings are often used for protection of tool surfaces due to coating properties like low friction and high wear resistance. Even though many of the hard coatings have been tested for wear, it is important to try new wear test setups to fully understand tribological mechanisms and the potential of hard coatings. Few experiments have been performed with dual-coated systems where the sliding contact surfaces are coated with the same, or different, hard coating. The dual-coated system could be the solution to many new technical devices and perhaps a further improvement of conventional coated systems.</p><p>In this thesis, the wear tests of dual-coated systems were performed in dry reciprocating sliding mode at room temperature. This, quite off the ordinary, wear test setup was performed to study selected hard coatings and set focus on wear mechanisms in forthcoming future surface coating application areas like MEMS and orthopedic implants.</p><p>Wear tests of four different PVD hard coatings, CrN, TiAlN, WC/C and diamond-like coating (DLC) were performed in a slider-on-flat-surface (SOFS) tribo-tester with reciprocation sliding mode at room temperature and dry sliding with TiAlN coated counter body. Wear mechanisms and the amount of wear were estimated, by investigation of the wear scars produced in SOFS, by means of scanning electron microscopy (SEM), atomic force microscopy (AFM) and optical profilometer (OP).</p><p>Typical wear mechanisms found for coated surfaces in reciprocation sliding contact were crack formation, surface flattening for shorter sliding distance, elongation of surface defects, debris and thin film formation. Two types of film formation were found: tribo-oxidation film and formation of a self-lubrication film. The tribo-oxidation was the most evident for CrN and the formation of a self-lubrication film was revealed for DLC, where smearing of asperities were the initiation of the process. The DLC coatings showed lowest friction coefficient and worn volume of all the selected hard coatings.</p><p>Adhesion measurements were performed for all coatings by AFM. Both the unworn and worn surface of each coating were investigated and two coatings, DLC and TiAlN, showed low adhesion forces, which indicated promising properties for small scale devices like MEMS and NEMS with coated, non-sticking, surfaces.</p> / Grundutbildningsprogram: Civilingenjör Bred IngångInriktning: Civilingenjör Maskinteknik och Materialteknik

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.

As alumina plays ever more important roles in advanced technologies, such as substrates for in vivo biological sensors, catalysts for water purification and components of novel fuel devices, it is exposed to various environments. These environments lead to wear and degradation due to chemical and mechanical forces. Macroscale tribological tests including the scratch test and indentation are unable to analyze nanoscale properties due to their limited sensitivity. Today, nanotribologists incorporate three main tools to analyze nanoscale tribological properties: atomic force microscopy (AFM), the nanoindentor and surface force apparatus. Of these, AFM is the instrument of choice due to the sensitivity to and diversity of surface properties analyzed in a single setting. Through adhesion measurements and scratch testing under known loads the AFM was used to analyze the effect of OH- on the degradation of the alumina surface. Fourier transform infrared spectroscopy (FTIR) and AFM were also used to analyze the stability of self-assembled monolayers (SAMs) on the alumina surface. Through scanning a region of the surface with a defined force in an aqueous environment, the tribochemical surface properties were found. The pressure required to induce degradation of the first O-Al layer (~2 �) ranged from 3.10 GPa at a pH 3 to 1.58 GPa at pH 10. Further analysis of adhesion across the surface and within a defect region exposed significant changes in the forces of adhesion as the alumina surface experienced wear. At pH 3, 5, 7, and 10 the adhesion was approximately double that of the defect free surface. Two well known molecules were examined for their ability to protect the surface from bond rupture due to the catalytic effect of OH-. SAMs of octadyclephosphonic acid (OPA) and Octadycletrichlorosilane (OTS) were assembled on the alumina surface. FTIR spectra indicated well ordered monolayers formed from each molecule. The exposure of OPA to aqueous environments of pH >7 lead to the degradation of the SAM. OTS, on the other hand, demonstrated significantly more resilience to degradation as indicated through FTIR and AFM analysis.

Thesis (doctoral)--Helsinki University of Technology, 2001. / Includes bibliographical references. Also available on the World Wide Web.

Thesis (Ph.D.) -- University of Texas at Arlington, 2008.

Thesis ( Ph.D. ) -- University of Texas at Arlington, 2008.

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