Characterization of fretting wear of cobalt-based superalloys at high temperature for aero-engine combustor components

Cobalt-based superalloys are used in aero-engine combustor components due to their high strength and wear resistance at elevated temperatures. The objective of this investigation is to study the role of the surface oxide formed during the fretting process on the tribological behavior of this class of materials, at temperatures up to 550 °C. The investigation, which was carried out using a specially designed tribometer, showed that the wear coefficient and the coefficient of friction (COF) generally decrease with the increase of temperature at different work rates. When the work rate was fixed at relatively high value (350 mJ/s), the wear coefficient was found to decrease monotonically with temperature, while the COF experienced a peak at 200 °C. Analysis of the specimen fretted at 200 °C showed that the hard wear debris promoted abrasive wear and caused an increase in the COF. In addition, the formed oxide was thin, non-continuous and contained mainly cobalt monoxide (CoO), which contributed to the increase in the COF. Analysis of the specimen fretted at 550 °C showed that the wear damage decreased due to the formation of a continuous thick protective oxide containing mainly chromium (III) oxide (Cr2O3), known for its good tribological characteristics. It has also been observed that delamination wear took place within this oxide layer. When the oxide composition and thickness inside and outside the fretted area were compared, it was concluded that the real contact temperature exceeded 600 °C due to the frictional heat.