Scuffing of cast iron and Al390-T6 materials used in compressor applications

Abstract

Scuffing can lead to catastrophic failure in engineering components, but the surface mechanism that leads to scuffing is not well understood. In this experimental study, the surface and subsurface changes on gray cast iron and Al390-T6 were investigated under starved lubrication (a mixture of R410A refrigerant combined with a polyolester lubricant) and pure sliding. Controlled tribological experiments were conducted using a high-pressure tribometer to simulate the contact conditions found in typical air conditioning compressors. Tribological tests were conducted on several sets of tribo-pairs consisting of gray cast iron disks and gray cast iron pins, for periods of 25, 50, and 75% of the time-to-scuff in order to investigate the progression of surface and subsurface changes leading to scuffing. The evolution of the chemical composition in the topmost surface layer of the disks was investigated as the disk/pin tribo-pairs progressed towards scuffing. Important changes in chemical composition were observed on the disks that had scuffed. Specifically, a marked decrease in manganese compounds was measured, which indicates a significant loss of the protective coating that was applied to the disks. Vickers microhardness tests were also performed on the disks at all stages, and showed a reduction in the means of hardness measurements as the disk neared the scuffed stage, in agreement with other studies. Similar scuffing experiments were also performed with tribo-pairs of Al390-T6/52100 steel; surface and subsurface changes were also characterized using chemical analysis and cross-section scanning electron microscope (SEM). In this case, depletion of silicon, which is embedded in the aluminum matrix to increase the hardness of the alloy compounds, was observed at scuffing. This observed reduction in hardness not only agrees with the cast iron hardness study, but also was consistent with the chemical analysis of the cast iron where a chemical component associated with hardening is depleted at the instance of scuffing.