Fretting wear behavior of cobalt - Based superalloys at high temperature – A comparative study

• Published in: Tribology international 2020-05, Vol.145, p.106155, Article 106155
• Main Authors: Korashy, Ahmed, Attia, Helmi, Thomson, Vince, Oskooei, Saeid
• Format: Article
• Language: English
• Subjects: Aerospace engines; Chromium oxides; Cobalt base alloys; Cobalt-based superalloys; Coefficient of friction; Combustion chambers; Comparative studies; Contact; Damage; Fretting; Friction; Friction reduction; High temperature; Materials selection; Nickel; Nickel base alloys; Oxidation; Raman spectroscopy; Superalloys; Surface oxides; Tribology; Tungsten; Wear
• ISSN: 0301-679X; 1879-2464
• DOI: 10.1016/j.triboint.2019.106155

Superalloys are extensively used in the design of combustor components of aero engines because they can withstand high temperature oxidizing environments, while maintaining their strength. This investigation aims at assessing the tribological characteristic of some combustor components candidate cobalt-based and nickel-based superalloys. Three combinations of the alloys were tested at different operating conditions and temperatures; CP1 (Cobalt 1/Cobalt 2), CP2 (Cobalt 2/Nickel 1) and CP3 (Cobalt 1/Nickel 1). The investigation showed that the increase in temperature from 350 °C to 550 °C was associated with a decrease in the coefficient of friction for all tested contact pairs, a decrease in wear coefficient (wear damage) for CP1, and an increase in wear coefficient for CP2 and CP3. Samples analysis using SEM-EDS and Raman Spectroscopy showed that the least wear coefficient exhibited by CP1 at 550 °C, and by CP2 at 350 °C is due to the development of a compacted protective layer of chromium oxide Cr2O3, known for its favourable tribological characteristic, which contributes to load bearing, precludes the metal-metal contact, and reduces the friction and wear damage. On the other hand, the decrease in friction was associated with an increase in wear coefficient for CP2 and CP3 due to the formation of a porous oxide layer with high tungsten trioxide (WO3) content which makes the oxide softer, decreases friction and increases the wear damage in the oxide layer. Material selection maps are presented to aid aero-engine designers. •The main findings in this investigation on high temperature fretting wear of selected cobalt-based superalloys are:•Temperature increase leads to a decrease in the coefficient of friction for all pairs.•The formation of protective oxide rich in Cr2O3 leads to lower wear rate.•The formation of non-protective oxide rich in WO3 leads to higher wear rate.•Wear maps are provided to help selecting optimum contact pair, based on loads and temperature.