M. Burak Toprakoğlu - 030070082 - 12th week

Adhesive Wear: (01:38 – 28.04.2011)

Adhesive wear originates from adhesion between two surfaces that are placed in contact. When two surfaces are brought into contacti asperities of the tho surfaces make physical contact. This “true” contact area is significantly smaller than the apparent surface area of the two contact surfaces. The contact area between the two surfaces is localized to the small regions known as asperities; these asperity-asperity contact regions are referred to as junction. The size of a junction is usually in the range 1-100 µm; the typical size of a junction is 10 µm in diameter. The number of junctions is dependent on the surface roughness and the amount of load that is applied. Under load, bonding between asperities on the two contact surfaces may occur. The amount of deformation at these junctions is also dependent on the number of junctions and the size of the junctions. Under sliding motion, plastic deformation, cracking and fracture can occur in the “true” contact area. Adhesive wear is largely due to fracture of material and transfer of material at the asperity-asperity contact regions. Prior to fracture, plastic deformation and crack formation may cause damage to the contact surfaces.

(Biomedical Materials, Roger Narayan, p.186)

Hydrostatic Lubrication: (01:56 - 28.04.2011)

In hydrostatic lubrication of friction bodies, a pocket or recess is incorporated in one friction body’s loaded surface into which a fluid is forced from outside at constant pressure. A pump outside the bearing generages the lubricant pressure are the most important features of hydrostatic lubrication. The lubricating pocket is normally positioned opposite the external load. The load-carrying capacity of a contact with hydrostatic lubrication is also assured when surfaces are not moving. When the volumetric flow of lubricant into the lubricating pocket is constant, the minimum lubrication film thickness is proportional to the cube root of tne ratio of the average lubricant viscosity in the lubrication gap and the load, i.e, the minimum lubrication film thickness is less dependent on the viscosity and the load than is the case in hydrodynamic lubrication.

Hydrostatic lubrication is mainly used: where the friction partners’ surfaces do not have any metallic contact, i.e, wear may not occur, not even when ramping up and ramping down a machine or at low speed; where as low a friction coefficient as possible must be produced at low speeds; and where, as a result of less effective lubricant entraining velocities in the lubrication gap, the wedge effect cannot produce any bearing lubricating film hydrodinamically.

(Springer Handbook of Mechanical Engineering, 10. Volume, Karl-Heinrich Grote, Erik K. Antonsson, p.313)

Melt Spinning: (20:25 - 28.04.2011)

Melt spinning is used when starting polymer can best be processed by heating to the molten state and pumping through the spinneret, much in the manner of conventional extrusion. A typical spinneret is 6 mm (0,25 in) thick and contains approximately 50 holes of diameter 0,25 mm (0,01 in); the holes are countersunk, so that the resulting bore has an L/D ratio of only 5/1 or less. The filaments that emanate from the die are drawn and simultaneously air cooled before being collected together and spooled onto the bobbin, as shown in figure. Significant extension and thinning of the filaments occur while the polymer is still molten, so that the final diameter wound onto the bobbin may be only 1/10 of the extruded size. Melt spinning is used for polyesters and nylons;since these are the most important synthetic fibers, melt spinning is the most important of the three processes for synthetic fibers. (among melt spinning, wet spinning, dry spinning)

(Fundamentals of Modern Manufacturing: Materials, Processes, and Systems, Mikell P. Groover, p. 274)

Diffusion Welding: (23:34 - 28.04.2011)

Diffusion welding (DFW) is a solid-state welding process that results from the application of heat and pressure, usually in a controlled atmosphere, with sufficient time allowed for diffusion and coalescence to occur. Temperatures are well below the melting points of the metals (about 0.5T_m is the maximum), and plastic deformation at the surfaces is minimal. The primary mechanism of coalescence is solid-state diffusion, which involves migration of atoms across the interface between contacting surfaces. Applications of DFW include the joining of high-strength and refractory metals in aerospace and nuclear industries. The process is used to join both similar and dissimilar metals, and in the latter case a filler layer of a different metal is often sandwiched between the two base metals to promote diffusion. The time for diffusion to occur between the faying surfaces can be significant, requiring more than an hour in some applications.

(Fundamentals of Modern Manufacturing: Materials, Processes, and Systems, Mikell P. Groover, p. 731)