Controlling and predicting rupture and dewetting of quiescent thin water films, hundreds of microns thick, was studied experimentally. Wax, polycarbonate, steel and aluminium surfaces were immersed in water; the water level was lowered to form thin films above the surfaces. Spontaneous film rupture only occurred on wax, a low-energy surface. Films ruptured at the edges of the other—high-energy—surfaces. Increased surface roughness decreased chances of rupture and dewetting in the film. Introducing large wax or steel protrusions (on the order of millimetres) on smooth surfaces showed films rupturing above the protrusions and adhering to them; further thinning caused rupture and dewetting away from the protrusions. Entrapped air bubbles, injected through the surface and into the film, ruptured as they breached the film surface to form stable holes in the film if it was sufficiently thin. Entrapped air was the best means of rupturing films on all surfaces.

The widespread use of low friction PVD coatings on machine elements is limited by the high costs associated with fulfilling the demands on the surface quality of both the supporting substrate and the counter surface. In this work, an attempt is made at lowering these demands, by adding a sacrificial carbon overcoat to a TaC/aC low friction coating. Both coatings were deposited by planar magnetron DC sputtering, as separate steps in a single PVD-process. Coatings were deposited on substrates of two different surface roughnesses, in order to test the ability of this coating system to function on rougher substrates. Reciprocating ball on disc tests was performed, using balls with two different surface roughnesses. The worn surfaces were investigated using 3-D profilometry and SEM. The ability of the different overcoats to initially reduce the roughness of both the coated surface and the counter surface and to produce stable, low-friction conditions was examined for the different initial roughnesses. The implications for design of efficient run-in coatings for various systems are discussed.

No description available.

At micrometer length scales of material removal, size effect is observed in mechanical micro-cutting where the energy per unit volume i.e. specific cutting energy increases nonlinearly as the uncut chip thickness is reduced from several hundred microns to a few microns (or less). There is no consensus in the literature on the cutting mechanism that causes this size effect. Noticeable discrepancy is also observed in the surface roughness produced at small feeds in micro-turning between the theoretical and the measured roughness. To date, there has been little effort made to develop a detailed process model for micro-cutting to accurately predict the size effect in specific cutting energy, and to develop a fundamental understanding of surface generation at the low feeds typical of micro-cutting processes. The main objective of this thesis is therefore to develop a predictive process model of micro-cutting of ductile metals that is capable of accurately predicting the size effect in specific cutting energy based on strain gradient based material strengthening considerations. In addition, this thesis attempts to explain the discrepancy between the theoretical and measured surface roughness at small feeds in micro-turning via a model that accounts for the size effect due to material strengthening. A coupled thermo-mechanical finite element model formulation incorporating strain gradient plasticity is developed to simulate orthogonal micro-cutting process. The thermo-mechanical model is experimentally validated in orthogonal micro-cutting of a strain rate insensitive aluminum alloy Al5083-H116. The model is then used to analyze the contributions of two major material strengthening factors to the size effect in specific cutting energy: strain gradient and temperature. The effects of cutting edge radius on the specific cutting energy and its role relative to the material length scale arising from strain gradient plasticity are also examined. A surface roughness model for micro-turning that incorporates the effects of kinematic roughness, cutting edge roughness and surface roughening due to plastic side flow is developed and shown to explain the observed discrepancy between the theoretical and measured surface roughness in micro-cutting. In addition, the model is found to accurately capture the increase in surface roughness at very low feeds.

Diamond-like Carbon (DLC) film exhibits an extreme hardness, low friction coefficient, chemical stability, heat conductivity, and high resistance. Their properties lead to remarkable applications on industry. In the experiment, we use electrondeposition to deposit the DLC film on Si substrate. Different concentrations of electrolyte were used to deposit on the of silicon substrates with different roughness surface. KOH solution was used to etch and to get the different roughness on the surface of silicon substrates. the morphology of surface were observed by SEM and AFM. Composition and microstructure of the DLC film were characterized by the Raman spectroscopy and XPS, repectively. The optical properties of DLC film were investigated by the N&K analyzer. From the AFM results, the surface morphology observed by KOH etching on the surface of silicon substrates during etching time as 0¡B20¡B40¡B60 min, the surface roughness increased from 2.64 to 14.07 nm. Based on thevariation of surface roughness, the growth rate was observed more quicker than the non etch surface. Moreover, to deposit the DLC film on the alkalinity solution was better then acid solution. However, the ID/IG ratio and the sp2/sp3 ratio obtained from Raman and XPS increase with the roughness surface from 1.09 to 1.82 and 0.985 to 2.15, respectively. It is because that the microstructure of DLC film varies and exchange to graphitization. The mixed the ammonia water and ammonium acetate into acetic acid solution was used to deposit DLC film on Si surface, and film shows with lower ID/IG ratio. Additionally, as the amount of ammonium acetate was varied in the solution, the ID/IG ratio of the films observed as decrease from 1.2 to 0.93 with increasing amount of ammonium acetate 10g to 40g. It was due to the methyl radicals increase in the solution. Besides, we can find the optical band gap decreased with DLC films changing to graphitization.

In this study, a conductive polymer is used as tool electrode in machining the stainless steel surface by electrochemical buffing. Using a very small working current of this conductive polymer, the material of the workpiece is dissolved, and the peaks on the workpiece surface is buffed by the abrasive simultaneously. A mirror-like surface can be achieved with high efficiency using this novel method. In the micro-electrochemical machining experiments, the initial surface roughness of the workpiece is about Rmax = 1.645 £gm, the average speed of electrode 25 mm/sec, the machining time 10 min, the electrolyte temperature 25¢J, and the stroke 10 mm. The variable conditions are given as follows: the sodium nitrate (NaNO3) electrolyte of 0 to 40 wt%, the normal load of 0 to 20 N, and the working current of 0 to 100 mA. Experimental results show that the minimum surface roughness of the workpiece can be achieved to about Rmax = 0.3£gm at the electrolyte concentration of 20 wt%, the working current of 25 mA, and the normal load of 10N, which is selected as the optimum operative parameters in the following. The silicon carbide with average particle size of 9.5£gm is added to conduct the electrochemical buffing experiments. Compared with the micro-electrochemical machining method, results show that the maximum machining depth increases to about two times, and the surface roughness decreases to about 50%. In this condition, the mirror-like surface of the workpiece with the working depth of 1.5£gm and Rmax of 0.15£gm can be achieved.

<p>This study focuses on the wear of piston rings in a hydraulic radial piston motor. The piston ring has to satisfy increasing demands for reliability and longer service life. It has two contacting surfaces, the face and the flank, and operates under a boundary lubrication state.</p><p>This first part of the project aimed to detect and characterise piston ring wear. Measurement by weighing gives an overall value for wear defined as loss of mass. Two-dimensional form and surface roughness measurements show the distribution of wear on the piston ring face in contact with the cylinder bore and the piston ring flank in contact with the piston groove. Three-dimensional analyses, both quantitative and qualitative, allow the wear mechanisms to be identified.</p><p>The wear of piston rings from an actual hydraulic motor was characterised. As well, rig testing was performed in two different test rig set-ups, one simulating the sliding movement of the piston ring and the other the tilting movement at the end of the strokes. Wear during the running-in period was investigated, and the findings indicate that the period when this takes place is of short duration. In the long term, mild wear makes the surfaces smoother than they were when new, resulting in a very low wear coefficient. Significant levels of wear were measured on both contacting surfaces of the piston ring. In cases in which the flank exhibits more wear than the face, the wear on the flank can be reduced by proper design of the piston groove.</p><p>The second part of the project aimed to evaluate use of a textured surface for the cylinder bore counter surface and a coated surface for the piston ring. Three modelling experiments were performed to characterise the friction and wear properties under lean boundary lubrication conditions. Under such conditions, textured surfaces have the advantage of retaining more lubricant and supplying it over a longer time. Stable friction was also a distinctive feature of the textured surface. Use of a coating could also possibly reduce the amount of wear. Though a smooth surface, like a polished one, is hard to beat for a working texture, a coated surface is far ahead of a smooth uncoated one. Different manufactured and commonly used cylinder bore surfaces, including textured ones, were evaluated in the sliding movement test rig. That allowed favourable wear properties, such as lowest wear coefficient, to be determined with the use of a roller burnished surface.</p><p>A final part of the research involved simulating wear on the piston ring face throughout the entire service life of a hydraulic motor. This allowed us to determine the roles of surface roughness and coating in prolonging service life and achieving acceptable and secure piston ring operation. The model is simple and realistic, but still needs to be refined so as to correspond even better to reality.</p>

Thesis (Ph. D.)--University of Missouri-Columbia, 2000. / Typescript. Vita. Includes bibliographical references (leaves 80-84). Also available on the Internet.

Thesis (M.S.)--West Virginia University, 2002. / Title from document title page. Document formatted into pages; contains viii, 56 p. : ill. (some col.). Includes abstract. Includes bibliographical references (p. 28-29).

This work is focused on the development of a hybrid DSMC/CFD solver to examine hypersonic boundary layer flow over discrete surface roughness. The purpose of these investigations is to identify and quantify the non-equilibrium effects that influence the roughness-induced disturbance field and surface quantities of interest for engineering applications. To this end, a new hybrid framework is developed for high-fidelity hybrid solutions involving five-species air hypersonic boundary layer flow applications. A novel approach is developed for DSMC particle generation at a hybrid interface for gas mixtures with internal degrees of freedom. The appropriate velocity distribution function is formulated in the framework of Generalized Chapman-Enskog Theory, and includes contributions from species mass diffusion, shear stress and heat fluxes (both translational and internal) on the perturbation of the equilibrium distribution function. This formulation introduces new breakdown parameters for use in hybrid DSMC/CFD applications, and the new sampling algorithm allows for the generation of DSMC internal energies from the appropriate non-equilibrium distribution for the first time in the literature. The contribution of the internal heat fluxes to the overall perturbation is found to be of the same order as the stress tensor components, underscoring the importance of DSMC particle generation from the Generalized Chapman-Enskog distribution. A detailed comparison of the transport coefficients is made between the DSMC and CFD solvers, and a general best-fit approach is developed for the consistent treatment of diffusion, viscosity and thermal conductivity for a five-species air gas mixture. The DSMC VHS/VSS model parameters are calibrated through an iterative fitting approach using the Nelder-Mead Simplex Algorithm. The VSS model is found to provide the best fit (within 5% over the temperature range) to the transport models used in the CFD solver. The best-fit five-species air parameters are provided for general use by the DSMC community, either for hybrid applications or to provide improved consistency in general DSMC/CFD applications. This hybrid approach has been applied to examine hypersonic boundary layer flow over discrete surface roughness for a variety of roughness geometries and flow conditions. An (asymmetric) elongated hump geometry and (symmetric) diamond shaped roughness geometry are examined at high and low altitude conditions. Detailed comparisons among the hybrid solution and the CFD no-slip and slip wall solutions were made to examine the differences in surface heating, translational/vibrational non-equilibrium in the flow near the roughness, and the vortex structures in the wake through the Q-criterion. In all cases examined, the hybrid solution predicts a lower peak surface heating to the roughness compared to either CFD solution, and a higher peak surface heating in the wake due to vortex heating. The observed differences in vortex heating are a result of the predicted vortex structures which are highlighted using the Q-criterion. The disturbance field modeled by the hybrid solution organizes into a system of streamwise-oriented vortices which are slightly stronger and have a greater spanwise extent compared to the CFD solutions. As a general trend, it was observed that these differences in the predicted heating by the hybrid and CFD solutions increase with increasing Knudsen number. This trend is found for both peak heating values on the roughness and in the wake. / text