Investigation of the role of oxide scale on Stellite 21 modified with yttrium in resisting wear at elevated temperatures

Abstract

Wear damage caused by sliding contact between metallic components at elevated temperatures during service is a common problem for many mechanical systems. Considerable efforts have been made to reduce such type of wear damage. Recent studies demonstrated that the modification of metallic materials by alloying with oxygen-active elements, such as yttrium, was effective to diminish high-temperature wear. Such improvement in the high-temperature wear resistance by alloying yttrium could benefit from the enhanced oxidation resistance as well as a strengthened oxide film with higher adherence to the substrate. It is logical to expect that a strengthened oxide scale can help to bear the contact stress force, thus further reduce wear. However, how a thin oxide scale can effectively reduce wear is not well evaluated and understood. Clarification of this issue is the objective of this research. Sliding wear performance of yttrium-free and yttrium-containing specimens of Stellite 21 at elevated temperatures, respectively, in air and in an argon environment were investigated using various techniques. Effects of yttrium additions on mechanical properties of the oxide film formed on Stellite 21 were also evaluated. The research demonstrated that a stronger and more adherent oxide scale could markedly benefit the wear resistance of Stellite 21, leading to lower wear of the alloy in air than in the argon environment. An improved oxide scale by yttrium rendered the oxide scale more protective. However, the protective effect of oxide scale decreased under high wearing forces.