Tribological insights of nickel- and cobalt-based alloys in extreme conditions

Nickel- and cobalt- based alloys are widely used as structural components in demanding environments due to their excellent stability (i.e., resistance to mechanical and chemical degradation) at elevated temperatures. These superalloys were primarily developed to meet the demand of jet engine and industrial gas turbine engine blades operating at extreme temperature ranges. Unfortunately, while intensive alloy and process development activities for these high-temperature alloys have been performed over the last few decades, their tribological behavior (i.e., friction and wear) has received little attention. With the increasing demand in temperature and spread of application of these alloys to other static and dynamic components in the engine, there is a clear need for a better understanding of their tribological behavior.

The main purpose of this study was to critically investigate the friction and wear behavior of Co- and Ni- based materials under low and high temperature environmental conditions. The ultimate goal was to identify the underlying interfacial processes leading to the observed tribological behavior. Thus, a series of studies on the friction and wear behavior of Ni-based and Co-based superalloys was conducted using a custom build high temperature fretting wear apparatus.In addition, ex situ analysis was performed on the worn surfaces using XPS, AFM, and cross-sectional SEM imaging of the near-surface region. The results showed a clear correlation between the third body formation process (e.g. oxide layer formation, transferfilms) and the tribological behavior of the superalloys as a function of temperature. The low friction and wear of these material systems at elevated temperatures is attributed to the formation of a lubricious ‘glaze layer’. Depending on the contact conditions, stable lubricous oxides will form above a critical temperature and provide sufficient wear resistance as long as the system continuously operates at these temperatures.