Investigation of the Scuffing Mechanism under Starved Lubrication Conditions Using Macro, Meso, Micro and Nano Analytical Techniques

Studies to better understand scuffing began approximately sixty years ago, when it was postulated that scuffing occurs when a critical temperature of the surface is reached, at which desorption of surface films occurs. An extension to this hypothesis was proposed suggesting interaction of chemically active species with the metal surfaces, notably oxygen and thus scuffing would be associated with the rate of oxide formation and destruction. In the 1950???s another major development in the field of tribology was started which recognized and understood elastohydrodynamic lubrication (EHL) became the focus of study and it was assumed that a system is in danger of scuffing when the thickness of the fluid film between the surfaces. Recent fundamental scuffing studies related to air conditioning compressors under dry sliding conditions were performed by Sheiretov, who proposed a process leading to subsurface failure, which eventually led to scuffing. Further studies carried by Yoon focusing on scuffing under starved lubrication conditions (typical conditions for air conditioning compressors), suggested that scuffing was related to shear failure of the bulk material which was caused by formation of macroscopic adhesions at the sliding interface. In these recent studies the effect of surface topography on scuffing has not been treated thoroughly. Further more their approach was based on macro -tribological experiments and meso-to-micro analysis methods. This was intentional since the surface roughness of typical engineering surfaces is large with Ra values from 0.1 to 1 mm, thus the previous researchers were seeking average effects in their studies. Also earlier studies did not examine the progression leading to scuffing. In the present study we investigate, first; the progression of surface topography and respective change in surface parameters with tribological testing leading to scuffing; second, changes in chemical composition of the surface and near surface layers of the sample at the micro and nano-meter scales, and investigate its relation to scuffing, this study differs from the ones carried out by its predecessors (Sheiretov, Yoon) in that it tracks topographical and chemical changes of the interface surface of the (softer) aluminum disc as it progresses form its virgin state to its scuffed state. Furthermore the analyses are carried out using macro to nanometer scales and techniques. To our knowledge this is the first such comprehensive study that will investigate the use of techniques that are typically used in semi -conductors and MEMS applications, whose surfaces are by several orders smoother than the engineering surfaces that will be used in this study. A shoe-on-disc geometry, which is used to carry out the experiments consists of a steel shoe on aluminum disc and in an approximate simulation of a swash-plate/shoe contact in an automotive swash--plate compressor. The High Pressure Tribometer (HPT) is used to simulate the progression of wear conditions. The refrigerant used in this case is R410A with a POE lubricant. Once the experimental samples have been tested on the HPT, two sets of analytical tests are run, the first to measure the surface roughness and extract the statistical parameters, and the second to analyze chemical composition of the surfaces and the nano-meter range sub-surfaces. Based on the study it is suggested that the use of the micro to nano meter range scales in analyzing engineering surfaces is appropriate and correlates well to the macro to meso scale wear results. Thus the goal of this thesis Is to investigate the evolution of both the topographical and chemical characteristics as the surface of an aluminum alloy is worn out in a swash plate compressor using macro to nano-scales.Air Conditioning and Refrigeration Project 12